Electrochemically Assisted Remediation of Pesticides in Soils and

Review pubs.acs.org/CR

Electrochemically Assisted Remediation of Pesticides in Soils and Water: A Review M. A. Rodrigo,*,† N. Oturan,‡ and M. A. Oturan*,‡ †

Department of Chemical Engineering, Faculty of Chemical Sciences & Technologies, University of Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain ‡ Laboratoire de Géomatériaux et Environnement (LGE), Université Paris Est, 5 bd Descartes, 77454 Marne la Vallée Cedex 2, France exacerbated by the vast and increasing use of pesticides worldwide and the absence of remediation technologies that have been tested at full-scale. This article is a critical review of pesticide removal from soils and water using electrochemical technologies. The review begins with a discussion of the origin and fate of pesticides and the primary nonelectrochemical technologies that have been developed for pesticide removal, i.e., technologies for extracting pesticides from soil and technologies for removing these pollutants from flushing CONTENTS waters. Successful results have been obtained by electrokinetic 1. Introduction 8720 flushing of the soil using surfactant solutions or by combining 2. Occurrence and Role of Pesticides in Soils and this technology with bioremediation and/or permeable reactive Groundwater 8720 barriers. For pesticide remediation of waters, many technologies 3. Using Nonelectrochemical Technologies to Treat can deplete pesticide levels and even completely mineralize the Pesticides 8723 organics in the water: electro-Fenton and anodic oxidation 3.1. Biological Processes 8723 using diamond electrodes are the most promising of these 3.2. Nonbiological Technologies 8723 technologies. The primary drawbacks of the scientific studies 3.3. Advanced Oxidation Processes (Excluding that have been reported in the literature to date is that they Electrochemical Processes) 8724 have only been conducted at the lab-scale, and in most cases, 4. Electrokinetic Technologies for Depletion of synthetic media (soils or water) were used at pesticide Pesticides from Soils 8724 concentrations that were much higher than those occurring in 4.1. Electrokinetic Soil Flushing 8727 the natural environment. Therefore, in the near future, 4.2. Bioelectroremediation 8729 significant effort should be devoted to studying the differences 4.3. Permeable Reactive Barriers 8729 between treating real and synthetic soils and water, and studies 4.4. Remarks on Electrochemical Remediation of should be conducted over pollutant concentration ranges that Soils Polluted with Pesticides 8730 are similar to those in actual polluted sites. Considerable effort 5. Use of Electrochemical Technologies for Pestishould also be devoted to scaling up electrochemical cide Removal from Water 8730 technologies to investigate important full-scale considerations 5.1. Anodic Oxidation and Cathodic Dechlorinasuch as cost and operation problems. tion Electrocoagulation Electro-Fenton Process Modifications of the Electro-Fenton Process Remarks on Electrochemical Treatment of Water Polluted with Pesticides 6. Conclusions Author Information Corresponding Authors Notes Biographies Acknowledgments List of Abbreviations References 5.2. 5.3. 5.4. 5.5.

8730 8732 8732 8736

2. OCCURRENCE AND ROLE OF PESTICIDES IN SOILS AND GROUNDWATER Pesticide use dates back to 1000 BC, when the Chinese began to use sulfur as a fungicide. Following that initial use, sulfur and arsenic species were used as pesticides for many centuries before synthetic pesticides were developed after World War II. Two arsenic species have been of particular interest: arsenic trioxide (which was used as a herbicide) and lead arsenate (which was used as an insecticide). This historical use explains why some agricultural and industrial soils have remained polluted with arsenic species until today1 and why so many scientific studies on the remediation of soils and groundwaters polluted with arsenic are still being published: electrochemical technology is investigated in many of these studies. Electrokinetic soil remediation (EKSR) has been demonstrated to

8736 8736 8741 8741 8741 8741 8742 8742 8742

1. INTRODUCTION The occurrence of pesticides in the environment has become a highly significant environmental issue. This problem has been © 2014 American Chemical Society

Received: February 7, 2014 Published: July 1, 2014 8720

dx.doi.org/10.1021/cr500077e | Chem. Rev. 2014, 114, 8720−8745

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exhibit a non-negligible solubility in water. Thus, these compounds penetrate the environment by drift, runoff, drainage, and leaching, first accumulating in the air and the soils and then moving toward the surface and groundwater, producing serious environmental problems.7 These environmental problems can become health concerns because these species are primarily bioaccumulative, can reach drinking water reservoirs, and are particularly difficult to remove in conventional water treatment facilities (WTF). Although the concentrations of these pesticides are not sufficiently high to produce acute health effects, long-term exposure to low pesticide concentrations have not been excluded from causing negative health effects. In addition, these compounds are not easy to handle. The World Health Organization (WHO) estimates that pesticides poison 2−3 million people and cause between 20 000 and 200 000 accidental deaths each year primarily in developing countries. Pesticides can be classified in many different ways depending on their roles (herbicides, insecticides, fungicides, acaricides, etc.), their target rates (broad spectrum or selective), the primary active chemical (organochlorines, organophosphates, carbamates, pyrethroids, triazines, etc.), their mobility in plants (systemic and nonsystemic), or their sources (mineral, organic, and synthetic). Thus, there is a vast array of pollutants that have varying potential impacts on the environment. Table 1 is a simplified classification of pesticides, showing representative examples and their primary applications as pesticides.8 However, the number of pesticides is growing rapidly, because pesticide production is an innovative industrial sector; from the beginning of the industrial production of synthetic pesticides to the present, the composition of pesticides has changed significantly from relatively nonpolar and very persistent pesticides to more polar and less persistent pesticides.9 The pesticide production rate has also increased enormously over this period, from 140 tons in the early 40s to 600 000 tons that were used per year at the turn of the 20th century.10 Current annual pesticide production is estimated at approximately 2.5 million tons.11 The USA, Japan, and France are the primary producers and consumers of pesticides. The toxicity and persistence of pesticides has been a subject of considerable interest for decades, and technologies for pesticide removal from freshwater and soils will definitely remain an active research topic over the near future. In addition, the synergistic effects of pesticide occurrence and the presence of other anthropogenic species from household or industrial use (the so-called “emerging pollutants”) make this problem extremely significant. Thus, new regulations are being developed in most developed countries. In the European Union, pesticide concentrations in drinking water are regulated at a concentration of 100 ng dm−3 for individual compounds and a total concentration of 500 ng dm−3.12 In recent years, many studies have been published on the occurrence of pesticides in different countries in either drinking water reservoirs9,13,14 or produced wastewater.15,16 The primary conclusions of these studies clearly indicate that pesticide concentrations are not negligible, although these concentrations rarely exceed the EU maximum permissible limits. These studies have also shown that pesticide occurrence depends strongly on many parameters, including runoff via atmospheric inputs or urban surface leaching, the season (the pesticide application period used to be from spring to summer), the reservoir type (groundwater or surface water), etc. The

achieve good results for arsenic removal from polluted soils and can remove high pollutant concentrations in very short treatment times.2 These results encourage the potential use of EKSR, not only to remediate soils that were previously polluted when arsenic was the primary pesticide in use but also as emergency responses to recent pollution incidents. In addition, EKSR can be also be used to simultaneously remove other pollutants. A good example of the use of EKSR can be found in the work of Buchireddy and co-workers3 in which removals of almost 70% of arsenic, cupper, and chromium were attained after a 15-day period with an energy requirement of only 2.5 kWh per ton of remediated soil. Electrocoagulation appears to be the most promising technology for arsenic removal from waters because higher removal efficiencies can be obtained than by using conventional coagulation, even in waters with very high arsenic concentrations.4,5 The key factor for successful remediation is to ensure that the arsenic (V) species is present during electrocoagulation: electrolysis has been shown to be a good technology with which to attain this oxidation state.6 Figure 1 shows the results of electrocoagulation tests at different current densities for which good efficiencies for

Figure 1. Evolution of arsenic concentration with applied electrical charge during electrocoagulation process using iron electrodes (▲, 0.5 mA cm−2; ⧫, 3.0 mA cm−2) and aluminum electrodes (Δ, 0.5 mA cm−2; ◊, 3.0 mA cm−2). Adapted with permission from ref 4. Copyright 2011 Elsevier.

arsenic removal in waters were obtained. The figure shows that both iron and aluminum electrocoagulation resulted in up to a 5-fold reduction in the initial arsenic concentration, even for strongly polluted waters (i.e., 20 mg dm−3 As), producing an effluent that met the water quality standards fixed by most environmental agencies (i.e., 10 μg dm−3 As). Following World War II, arsenic-based pesticides began to be progressively substituted by newer organic pesticides. The introduction of the first synthetic pesticide, dichlorodiphenyltrichloroethane (DDT), was a groundbreaking event because of its enormous effectiveness. Up until the 1970s, DDT was largely used to combat mosquitoes and eradicate malaria before its negative effects were discovered. Nowadays, a massive number of chemicals are used as pesticides in agricultural activities. These species help farmers to increase production, either by diminishing external problems (i.e., preventing the occurrence of insects, fungi, viruses, etc.), acting as plant growth regulators, or by preventing the growth of weeds (i.e., acting as herbicides). The significant seasonal variation in the discharge of these chemicals, which have poorly traceable sources, results in a diffuse emission. These chemicals are designed to have considerable chemical stability, are resistant to biodegradation, and, in some cases, 8721


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Table 1. Primary Types of Pesticides8

reservoir type is of particular importance because groundwater can remain a source of pollutants for decades as pesticides have higher residence times and exhibit lower microbial activity in groundwater than surface water. However, other studies have shown there is further cause for alarm. Loos and co-workers17

recently conducted a representative and very interesting study in which 164 groundwater samples from 23 European countries were investigated for the frequency of detection and the maximum concentration of persistent pollutants (including pesticides). This study showed that the occurrence of pesticides 8722


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and successful experiments that were performed using a mixture of pesticides;25 2,4-dichlorophenoxyacetic (2,4-D);26 and alpha-hexachlorocyclohexane.27 In these experiments, efficiencies above 90% were obtained for different soil types, and the operation conditions had to be optimized in every case. Good efficiencies were also reported for the removal of other pesticides using fungal communities.28 However, lower removal efficiencies were reported for experiments on different pesticides. For example, Znad and co-workers obtained less impressive (but still significant) removals for the degradation of s-ethyl dipropylthiocarbamate.29 These results show the potential of using this technology for pesticide removal but also show that the technology is not robust.30 Metabolite production was not assessed in most of these studies, although, as we previously mentioned, metabolites can play a very important role in the environment because they can have even more deleterious effects than the parent pesticide.31,32 Another type of bioprocess (phytoremediation) in which plants are used to degrade, assimilate, metabolize or detoxify pesticides also appears to be a promising technology that is cost-effective and ecologically sound. Several experiments have shown that this technology offers good prospects for treating endosulfan,33 ethion,34 isoproturon and glyphosate35 and lindane.36 In every case, a significant percentage of the pesticides contained in a soil were removed over nonexcessively long periods (less than 10 days) using this technology. Plant uptake and phyto-extraction were the primary mechanisms for pesticide removal.

in groundwater is not negligible in Europe (either in terms of the frequency or concentration) but is a relatively significant problem. Twenty percent of the samples assessed contained at least one pesticide that exceeded the EU limit (100 ng dm−3) and 10% exceeded the total limit on pesticides (500 ng dm−3). The pesticides that exceeded the EU standard most frequently were chloridazon-desphenyl, chloridazon-methyldesphenyl, DMS (N,N-dimethylsulfamid), bentazone, desethylatrazine, desethylterbytylazine, DEET (N,N-diethyl-3-methylbenzamide), and dichlorprop. However, many other pesticides were present at very significant concentrations, including atrazine, simazine, diuron, propazine, and mecoprop, among others. An additional concern is that pesticides are known to be degraded in the environment by biotic and abiotic processes. The resulting metabolites from many degradation processes can be even more toxic than the parent compounds.18,19 Thus, it is very important to establish different natural degradation pathways to characterize the problem properly. A final issue for consideration is that these metabolites and the parent pesticide species can also appear in the sludge of municipal wastewater treatment plants because the removal efficiency for these species in these facilities is usually poor. This sludge is sometimes spread on agricultural lands (a practice that is considered to be an economic recycling method), which incorporates these species into the environment, thereby enabling their accumulation and promoting their negative effects on the environment and society.20 At this point, recently ecotoxicology studies are becoming a very important and useful tool to evaluate the risk associated with pesticides in the environmental compartments21,22 by assessing the destiny and effects of pollutants, and explaining the cause of possible risks and the toxic effects that pesticides may cause.

3.2. Nonbiological Technologies

There are many potential nonbiological technologies. Membrane processes may be applied to pesticide removal,37 in particular, nanofiltration (NF) and low pressure reverse osmosis (LPRO). These technologies offer good prospects for pesticide removal in which sieving (size exclusion by the membrane) is the primary mechanism that controls the retention of pesticides. However, important challenges remain to be overcome including membrane fouling and interactions between the pesticides and the organic matter usually found in the water (polysaccharides, humic acids, fulvic acids, etc.). Thus, it is particularly important to couple membrane processes with other technologies, such as adsorption or advanced oxidation processes (AOPs), to develop an efficient process for pesticide removal. Adsorption can be successfully used to remove pesticides from wastewaters. Srivasta and co-workers38 and Foo and Hameed39 have comprehensively reviewed many conventional and nonconventional adsorbents for this application. In applying this technology, special attention should be paid to the interaction between pesticides and adsorbents and, in particular, to desorption test results, i.e., no pesticide leaching should be observed to ensure that the remediation process is effective.40 The search for new adsorbents and the development of new modified materials is currently a highly relevant subject because adsorbents can be used to greatly enhance the efficiency of the depletion of pesticides in water.41−43 Another critical element of this technology is the regeneration of activated carbon. In addition to conventional thermal regeneration, supercritical desorption (i.e., the implementation of supercritical fluid technology to regenerate activated carbon) has been successfully evaluated since the late 1980s, producing promising results.44 The direct extraction of pesticides from water or a soil matrix is less promising, at least from an

3. USING NONELECTROCHEMICAL TECHNOLOGIES TO TREAT PESTICIDES In recent years, many technologies have been developed to remove pesticides from soils and freshwaters. These technologies include physical, chemical, and biological processes. 3.1. Biological Processes

As with many other environmental issues, bioprocesses are the cheapest technologies and are thus considered as the first choice in treating a soil or fresh water that is polluted with pesticides. A good review in which these soil remediation technologies are described exhaustively was recently published by Megharaj and co-workers;23 however, the review considers the removal of organic pollutants and does not focus specifically on pesticides. This review shows that bioremediation technologies are economically feasible but are not currently robust, and significant attention should be paid to parameters such as the bioavailability of the pollutant, low temperatures, anaerobic conditions, low concentrations of nutrients and cosubstrates, and the microbial communities and their interactions, all of which can significantly limit the efficiency of remediation processes. These conclusions are not in agreement with those presented by Robles-González and coworkers24 in a previous review on slurry bioreactors, in which slurry bioremediation was considered to be an effective ad situ and ex situ technology for the remediation of problematic sites, particularly those containing recalcitrant and toxic pollutants because this technology can be used to eliminate or minimize many of the factors that limit the efficiency of the biodegradation process. The authors reviewed three different 8723


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technology. These modifications can be used to obtain complete or almost complete pesticide removal in an efficient treatment, and a good mineralization efficiency can even be achieved for many technologies, regardless of the particular pesticide being oxidized. All of the assessed pesticides have been shown to be refractory to Fenton-related oxidation technologies. The same behavior has been found for oxidation intermediates, although the efficiency is somewhat lower. Note this problem is not encountered using the single Fenton process, in which pesticide removal is very efficient, unlike mineralization, because many oxidation-refractory intermediates form via the reaction system. Thus, the combination of processes that has been reported in the literature is very successful and is an effective means of finding a suitable technology to remediate water that has been contaminated with pesticides. Research in this field is at a critical stage. Most studies to date have been conducted at the lab-scale using reactor volumes ranging from 40 to 500 dm−3 and synthetic solutions to model waters or wastewaters. Larger reactors have been used to connect the experimental systems for combined technologies; however, these reactors have not been used to obtain information on system issues, such as economic or operational problems, that would arise for large scale systems. Thus, this technology is still at an early stage i.e., the lab-scale, and many improvements are required before the technology can be scaled up to bench and pilot plant levels (in terms of assessing the cost and operation conditions) and, consequently, for commercial demonstrations. Note that several studies have been published on mixtures of compounds or the treatment of pesticides in actual waste matrixes86,87 and that similar conclusions were drawn. In addition, in the treatment of actual matrixes, combining this technology with other complementary treatments, such as conventional bio-oxidation88,89 or membrane bio-oxidation,90 has yielded good results and has also become an active research area. Finally, the detection limit in analytical techniques continues to be a critical problem in assessing technologies for pesticide removal. This problem is encountered in the technological assessment of the removal of many other emerging pollutants, such as personal care or pharmaceutical products, and has become a subject of controversy in the scientific community. Most of the studies published in the literature have focused on pollutant removal at concentrations that are significantly different from typical pollutant concentrations in a reservoir and are only found (in a few cases) in industrial wastes that are produced during the manufacture of these anthropogenic species. However, these studies are the only way of assessing whether intermediates that pose significant hazards to environment are produced and to determine the efficiency of the technology, because the actual process is very rapid and existing analytical technology is not sufficiently robust to measure the nature and concentrations of the intermediates in a rapid way.

economic perspective, although many studies on this subject have been published over recent decades. These studies were motivated by the successful application of this technology in analytical chemistry; however, environmental applications are still at the initial lab-scale stage, and the economic feasibility of this technology is still far removed from commercial demonstrations.45,46 3.3. Advanced Oxidation Processes (Excluding Electrochemical Processes)

Oxidation technologies are very significant for pesticide removal, particularly those technologies in which the hydroxyl radical is used as an oxidant, namely advanced oxidation processes (AOPs), thereby oxidizing organics at a higher efficiency than in conventional oxidation technologies.47,48 AOPs can be classified into four primary groups depending on the mechanism by which the hydroxyl radical forms:49 homogeneous (H2O2/Fe2+ and H2O2/O3), photochemical (H2O2/UV, O3/UV, H2O2/Fe2+/UV, and TiO2/UV), sonochemical, and electrochemical oxidation processes. In addition, combining mechanisms to exploit synergistic effects has recently become an important research topic. Various recent reviews50−52 show that all of these technologies produce good results for the pesticide removal, although the operation conditions must be carefully evaluated to obtain high efficiencies in terms of cost and mineralization (i.e., to ensure that the pesticide is not transformed to organic intermediates but to carbon dioxide), and to prevent the formation of byproducts that may be even more hazardous than the parent pesticides from a toxicological perspective. Likewise, the scavenging of hydroxyl radicals by the constituents of the water matrix should be considered because this mechanism can explain the lower efficiency of AOPs in pesticide removal.53,54 An alternative to using AOPs as single treatment technologies is to combine AOPs with other technologies to minimize cost or enhance efficiency by exploiting complementarities or synergies between technologies. Recent reviews on combining AOPs with nanofiltration,55 biological processes,56 adsorption,39 or photochemical technologies57 show the current level of interest in this area. Hydrogen peroxide-based technologies have attracted considerable attention among AOP technologies because of their simplicity, high efficiency and easy application.58−60 Recently, many reviews have been published on these processes, which are particularly interesting because they can be easily combined61 with electrochemical processes (the subject of this work), as well as with other technologies (in particular photo- and sono-technologies). A good review by Ikehata and co-workers on the use of advanced oxidation processes for the degradation of aqueous pesticides was published in 2006.62 However, many studies have since been published, and the direction of this research field has changed significantly from fundamental Fenton processes to enhanced Fenton processes.63 Table 2 summarizes some of the results that have been published over the last two years64−85 on different technological assessments for diverse pesticides, highlighting the experimental conditions used in each study and the efficiencies (or range of efficiencies) reported for each technology. The efficiency of the Fenton process can be enhanced by combining this process with hydrodynamic cavitation, heterogeneous photocatalysis, or irradiation using sono- or photoenergies, which are current active areas of interest for this

4. ELECTROKINETIC TECHNOLOGIES FOR DEPLETION OF PESTICIDES FROM SOILS In electrokinetic soil remediation (EKSR), a direct current electric field is applied across contaminated soil using electrodes located in the subsurface.91 This current simultaneously initiates many physical processes (heating, changes in viscosity, etc.), electrochemical processes (water oxidation and reduction, etc.), chemical processes (ion exchange, dissolution of precipitates, etc.) and electrokinetic processes (electro8724


8725

technology

Photo-Fenton Heterogeneous PhotoFenton Heterogeneous PhotoFenton

Methomyl/16.22 mg dm−3/synthetic solution and H2SO4, Na2CO3, NaHCO3 or NaOH used to adjust pH

Methomyl/16.22 mg dm−3/synthetic solution and H2SO4, Na2CO3, NaHCO3 or NaOH used to adjust pH

Photo-Fenton + Solanum Nigrum L. weed plants Fenton enhanced by hydrodynamic cavitation Sono-Photo- Fenton

Nanofiltration + photoFenton

Fenton

Heterogeneous Fenton

Heterogeneous PhotoFenton Fenton

Fenton enhanced by hydrodynamic cavitation Fenton

Heterogeneous PhotoFenton Sono-Fenton

Photo-Fenton

Photo-Fenton

Fenton

Photo-Fenton

Heterogeneous Fenton

Sono-Fenton

Photo-Fenton

Methyl parathion/20 mg dm−3/synthetic solution and H2SO4 or NaOH used to adjust pH Methyl parathion/0.2 mM/synthetic solution

Methyl parathion/20−50 mg dm−3/synthetic solution and H2SO4 or NaOH used to adjust pH

Metalaxyl/150 mg dm−3/synthetic solution and H2SO4 used to adjust pH and iron chloride as nutrient

2,6 dimethylaniline/1 mM/synthetic solution with HClO4 (pH 1−4) Dinitro butyl phenol/10−40 mg dm−3/synthetic solution containing pesticide and catalyst 2,4 dinitrophenol/100 mg dm−3/synthetic solution and H2SO4 or NaOH used to adjust pH 2,4 dinitrophenol/100 mg dm−3/synthetic solution and H2SO4 or NaOH used to adjust pH 3-indole butyric acid/0.5 mM/synthetic solution and H2SO4 used to adjust pH Malathion/1−33 mg dm−3/synthetic solution with methanol and NaOH/H2SO4 used to adjust pH

Dichlorvos/20 mg dm−3/synthetic solution and H2SO4 used to adjust pH

Chlorpyriphos/30 mg dm−3/synthetic solution aged for 7 days and with HCl (pH 2−4) Chlorpyriphos/30 mg dm−3 synthetic solution aged for 7 days and with HCl (pH 2−4) Dichlorvos/20 mg dm−3/synthetic solution and NaOH or H2SO4 used to adjust pH

Imazapyr/0.1 mM/synthetic solutions and H2SO4 used to fix pH Imazapyr and imazaquin/0.1 mM/synthetic solutions and H2SO4 used to fix pH

Abamectin/9 mg dm−3/synthetic solution with H2SO4 (pH 2.5) Carbofuran/10−200 mg dm−3 /synthetic solution with H2SO4 (pH 3) 4-chloro 3-methyphenol/0.70 mM/synthetic solution with H2SO4 Chlorophenoxy herbicides/1 mM /synthetic solution

pollutant/concentration range/type of water tested

−3

3+

AlFe-PILC catalyst (14 wt. % iron content/pH 3.8

100-cm3 reactor/ultrasonic horn: 270 W, 20 kHz/60−400 mg dm−3 H2O2/ H2O2: FeSO4 ratio = 1:3 1300-cm3 reactor/[Fe3+] = 0.05, 0.1, 0.2, 0.5, and 1 mM/[H2O2] = 1, 2.5, 5, 10, 20, and 50 mM Fe-ZSM-5 catalyst (5 wt. % iron content/pH 3.8

4-dm−3 reactor/pressure range: 1−8 bar for cavitation/100 mg dm−3 H2O2/ H2O2/FeSO4 ratio = 1:4

Cross filtration unit (42-cm2 membrane area)/1.5-dm3 photo reactor/5-W mercury lamp (254 nm)/Malathion: H2O2 ratio = 1:100/H2O2: Fe(II) ratio = 40:1/pH 3 0.6-dm3 photoreactor/80 mg dm−3 H2O2/2 mg dm−3 Fe2+/180 min/pH 2.8

250-cm3 photo reactor/0.57 kWm−2 sunlight/1 g dm−3 [Fe(III)-ssal]-Al2O3 catalyst/5 mM H2O2/pH 2.5 300-cm3 stirred tank reactor/[Fe3+] = 125 mg dm−3/[H2O2] = 250 mg dm−3/ pH 3 300-cm3 stirred tank reactor/1 g dm−3 Fe3+-Al2O3 (10 wt%)/[H2O2] = 250 mg dm−3/pH 3 10-dm3 stirred tank reactor/0.2 mM Fe2+/0.6 mM H2O2/pH 3

5−10 mM H2O2/1 mmol Fe2+/pH 2

100-cm photoreactor/10 mg dm TiO2/[H2O] = 0.02 M/[Fe ] = 10 mg dm−3/pH 3.5 500-cm3 reactor/maximum power rating of ultrasonic irradiation 270 W/ Operating frequency of US horn 20 kHz/120 min/pH 3/FeSO4:H2O2 ratio = 0.5:1 to 3:1 4-dm3 reactor/Inlet pressure 5 bar/FeSO4:H2O2 ratio = 3:1/dichlorvos: H2O2 ratio = 1:0.8/Treatment time 1 h

3

40-cm3 photoreactor/[H2O2] = 0.01 M/[Fe3+] = 10 mg dm−3/pH 3.5

500-cm3 cylindrical photoreactor with low pressure Hg lamp (253.7 nm)/[Fe3+] = 1 mM/H2O2 10 mM/pH 3/Nominal power of UV lamp: 12 W 500-cm3 cylindrical photoreactor with low pressure Hg lamp (253.7 nm)/[Fe3+] = 1 mM/H2O2 10 mM/pH 3/Nominal power of UV lamp: 12 W 500-cm3 cylindrical photoreactor with low pressure Hg lamp (253.7 nm)/[Fe3+] = 1 mM/10 mM H2O2/pH 3/Nominal power of UV lamp: 12 W

17 W cm−2 UV irradiation/6 mmol dm−3 H2O2/[Fe2+] = 0.5 mmol dm−3/pH 2.5 1-dm3 reactor/300-W US power output/100 mg dm−3 H2O2/[Fe2+] = 20 mg dm−3/pH 3 25-cm3 reactor /0.5 g dm−3 nZVI/3 mM H2O2/pH 6.1

primary operation conditions

Table 2. Nonelectrochemical AOP Technologies for Pesticide Removal from Wastewaters maximum efficiency reported/related remarks

71

100% pesticide degradation for FeSO4:H2O2 ratio = 3:1

81

Complete mineralization in 120 min for [H2O2]/[Fe3+] ratio = 40 100% TOC removal after 240 min of irradiation with 5 g dm−3 catalyst (80% TOC removal for 1 g dm−3 under same conditions) 54% TOC removal after 480 min of irradiation with 5 g dm−3 catalyst (36% TOC removal for 1 dm−3 under same conditions)

82

82

80

79

78

77

76

75

75

74

73

72

98.5% pesticide removal (73.7% TOC removal)

93.8% pesticide degradation (76.6% TOC reduction)

99.1% pesticide removal

99.8% pesticide removal after 3 h of irradiation

97% pesticide removal (16.2% mineralization)


99.6% pesticide removal (diminished to 97% after 5 cycles of reuse) 92.5% pesticide removal

80.3% pesticide removal (27% COD removal)


70

70

67

66

69

68

65

64

ref

100% pesticide degradation in 15 min (63% TOC removal in 60 min) Complete degradation of MCPA in 7 min, 95% COD removal of MCPA in 2 h of irradiation, 80% COD reduction of 2,4-D Complete degradation of imazapyr in 10 min, 80% COD removal in 6 h Complete mineralization of imazapyr in 2 h and 96% mineralization (in terms of COD removal) of imazaquin in 3h Complete degradation in 15 min using solar chamber and in 30 min using sunlight Complete pesticide degradation in 20 min

70% pesticide removal in 60 min (60% mineralization in 180 min) 99% pesticide removal (46% mineralization)

Chemical Reviews Review


100-cm3 photoreactor/2.54 10−7 E s−1 photonic flux/0.5 g dm−3 of 7.2% (w/w) Fe-TiO2 photocatalyst/pH 2.8/45 mM H2O2/30 min irradiation

Almost 100% pesticide degradation

85

osmosis, electromigration, electrophoresis, etc.), which significantly change the soil. The most important processes are described below. • Electromigration is the motion of ions in the water retained in the soil under the action of the electrical field generated between the anode(s) and cathode(s). The anions (negatively charged ions) move toward the anode (the positively charged electrode) at the same time that the cations (the positively charged ions) move toward the cathode (the negatively charged electrode). • Electrophoresis is the motion of charged particles in the soil (or particles that have been added to treat the soil) under the action of the aforementioned electrical field. • Electro-osmosis is the motion of the groundwater in the soil under the action of an electrical field. This water can be the groundwater itself or an aqueous solution that has been added to promote the motion of pollutants: sometimes, water or groundwater that has already been treated is simply fed back to the process. The liquid typically flows from the anode to the cathode. • Electrolysis is a set of reactive processes that take place on anodic and cathodic surfaces that have been placed in the soil, either directly or inside an electrolyte solution in contact with the soil (i.e., an electrolyte well). The most important processes are the oxidation of water on the anodic surface and the reduction of water or the electro-deposition of metals on the cathodic surface. From a soil remediation perspective, the primary consequence of the oxidation of water is the formation of an “acidic front” that moves toward the cathode primarily by electro-migration, thereby releasing the pollutants fixed in the soil, especially by precipitation-dissolution and ionic exchange. Water reduction at the cathode generates a basic front in the opposite direction to the acidic front. Both fronts can be modified by adding suitable reactants to the soil. • Electrical heating increases the soil temperature, especially in the vicinity of the electrodes. This temperature rise is caused by ohmic losses that are generated by large ionic resistances of the soil (i.e., corresponding to a low ionic conductivity of the soil). This increase in temperature is caused by the JouleThompson effect and is proportional to the intensity of the current that flows externally to the soil (between the anodes and cathodes) and the soil resistances can be modeled using Ohm’s law. The temperature rise affects the transport properties of volatile organic pollutants and thus, their desorption and mobility. These processes can be suitably combined by setting optimum configurations and operation conditions in a soil remediation process (i.e., promoting specific processes such as electrochemical soil flushing) to ensure the removal of many inorganic and organic contaminants from soils.92−95 This technology has been assessed for many types of soils and is particularly useful for fine-grained soils with low hydraulic conductivities and large specific surface areas96 for which the technology offers clear advantages over conventional remediation processes. This advantage results from the combined effect of two phenomena in electro-osmosis: the accumulation of a net electrical charge on the surface of a solid that is in contact with an electrolyte solution and the accumulation of a thin counterion layer of the liquid surrounding the solid surface. Note that the electrolytic fluid is electrically neutral beyond this thin layer. However, this thin layer (which is known as the electrical double layer or the Debye layer) in contact with the solid surface has a net charge and is therefore attracted by

Heterogeneous PhotoFenton

84 100% pesticide depletion (50% TOC) 5 mg dm−3 Fe2+/150−350 mg dm−3H2O2/range of illumination times: 10−80 min Solar photo-Fenton

83 100% depletion of three pesticides (93% mineralization in 6 h)

maximum efficiency reported/related remarks primary operation conditions

Thermostatic cylindrical cell/1.4 dm−3/1 mM Fe3+/100 mM H2O2/UV wavelength: 253.7 nm/UV power: 12 W/pH 3 Photo-Fenton

technology pollutant/concentration range/type of water tested

Table 2. continued

Review

Mixture of chlortoluron/carbofuran/bentazon (0.125 mM/ 0.125 mM/0.125 mM)/Synthetic solution with 0.05 M Na2SO4 Pyrimethanil/20 mg dm−3/synthetic solution containing 5 g dm−3 NaCl (some experiments performed without NaCl) Thiacloprid/0.06−0.32 mM/synthetic solution and H2SO4 or NaOH used to adjust pH

ref

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Chemical Reviews

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process (by creating electrolytes wells or reservoirs). In both cases, the electrodes used in the electro-remediation treatment of a soil were arranged in the form of solid or hollow sheets or cylinders. When hollow electrodes were used, the electrode was configured such that a fluid could be introduced or extracted through it (i.e., the electrode acted as a pipe), allowing the flushing fluid (water or an aqueous solution) to be circulated throughout the system during the soil treatment. Among the flushing fluids that are normally used (and that are dosed into the groundwater during treatment), we highlight those fluids that can modify the pH of the soil being treated to compensate for the effects of the acidic and/or basic fronts. These fluids were used in all of the studies reviewed in this work (see Table 3), where the most important reagents were a Na2CO3/NaHCO3 buffer for neutralizing the acid front and acetic acid for neutralizing the basic front and lowering the soil pH. Citric acid has also been used for the latter application96 because of it acts as a chelating and complexing agent. Aqueous surfactant solutions are often used in the flushing fluid because of the polarity of organic pollutants.98,99 These molecules possess hydrophilic and lipophilic groups that “split” the pollutant into charged drops that can migrate through the electrical field, enabling the pollutant to be concentrated or migrate back to the anode (via the electrophoretic flux) or the cathode (i.e., the drops are dragged by the electroosmotic flux). The electrodes can be placed in electrolyte wells, enabling electrokinetic remediation to be used as a final treatment technology because the pollutants are concentrated in the wells and can later be treated in the wells themselves (in situ) or during pumping to the anode wells (ex-situ). Using surfactants in the flushing fluid does not always yield positive results: in several cases, using a surfactant in the flushing fluid worsens the results significantly compared to electrochemical flushing without surfactants. Examples of this behavior were observed in DDT removal from a sandy loam100 and chlorobenzene removal from clay soil.101 This behavior can be attributed to the strong interaction between some surfactants and soils, which results in the formation of less soluble inclusion compounds and decreases the electro-osmotic flowrate.102 However, in other studies, pesticide removal was greatly enhanced by adding surfactants to the flushing fluid composition. This behavior has been observed in the removal of hexachlorobenzene from clays103,104 and chlorobenzene and trichloroethylene from clay loams102 and can be explained in terms of enhanced desorption of the pesticide from soils because of the interactions among the surfactant, the soil and the pollutant. These contradictory results imply that no general conclusions can be drawn on the use of surfactants to improve pesticide mobility because the process involved is highly complex; therefore, each soil and pesticide must be assessed on a case by case basis to evaluate whether it is useful to use surfactants in the flushing fluid. In a promising technology for soil flushing, oxidation of Fenton’s reagent is combined with electrokinetic soil remediation processes in which iron(II) ions and/or hydrogen peroxide are used in the flushing fluid. That is, the flushing fluid is dosed with hydrogen peroxide and/or iron(II) ions, and the electrokinetic process generates a flow that removes the pesticides in the soil. This technology has also been used for soil washing105 but is much more effective in soil flushing.104,106 Over 55% hexachlorobenzene removal has been achieved in less than 15 days, showing that the technology offers good prospects for pesticide removal. The primary drawback of this technology is that the pH profile of the soil is strongly modified

charges of the opposite sign and repelled by charges of the same sign; thus, this portion of the fluid can move within the electrical field that is generated between the electrodes of an electrochemical cell, because it is attracted toward the electrode with the opposite sign. This motion does not depend on pressure gradients and causes groundwater flow in low permeability soils or the flow of water that has been added to flush the soil. The mineral particles that make up a soil are usually negatively charged because of the release of protons or the exchange of aluminum or silicon atoms in the mineral structure by monovalent cations. Thus, the water in the Debye layer is positively charged. For this reason, the electroosmotic flux in an electroremediation treatment normally flows from the anode to the cathode. The magnitude of the electroosmostic flux diminishes as the mean ionic concentration in the fluid increases, i.e., the higher the conductivity, the lower is the electroosmotic flow. This result can be explained by a reduction in the thickness of the electrical double layer and, therefore, in the amount of fluid that migrates by electroosmosis. Note that the ground pore size enhances rather than inhibits this fluid flow. This result is obtained because under these conditions, the higher the amount of liquid in contact with the solid, the higher is the volume of liquid in the Debye layer, which can consequently be mobilized by the action of an electrical field. This result is important, because small pore sizes inhibit the hydraulic flux (i.e., via higher pressure losses), which explains why an electroosmotic flux can be generated in soils whose small pore size do not enable appreciable hydraulic fluxes to develop. 4.1. Electrokinetic Soil Flushing

Soil flushing is the mobilization of the groundwater in the soil and any water (or an aqueous solution of suitable chemicals) that has been added to the soil to drag the pollutants in the soil. This water helps to wash the pollutant out of the soil by promoting the redissolution of precipitates, ionic exchange or by simply dragging the pollutants along. Thus, within a simplified view, the pollutants are transferred from the soil to the water, leaving the soil contaminant-free and simplifying the soil decontamination problem to a wastewater treatment problem. In highly permeable soils, flushing is facilitated by the hydraulic flux resulting from a pressure gradient. There is no advantage to using electrochemical technology in these types of soils because of the associated expense. However, low permeability soils have small hydraulic fluxes, enabling the formation of preferential paths. Thus, techniques based on electro-osmosis can be effective in these soils because the unusual characteristics of this process generate high fluxes under these conditions that can be directed to the treatment site. The electro-osmotic flux typically mobilizes groundwater from the anode to the cathode. The water at the cathode can then be pumped to the anode to begin a new pathway toward the cathode. The wastewater can be treated ex situ at an intermediate stage to remove the pollutants. In some applications described in the literature, the electrodes have been placed directly in contact with the soil to be treated, assuming that the soil humidity ensures optimum electrical conductivity. In other studies, the electrodes were placed in contact with electrolyte solutions and not the soil.97 These electrolytes solutions were connected to the soil that was treated using membranes or devices that enabled the conduction of ions, thereby facilitating the electrochemical 8727


electrokinetic soil flushing + Ultrasound

electrokinetic soil flushing -Fenton electrokinetic soil flushing -PRB (Pd/Fe particles)

electrokinetic soil flushing

bioremediation -electrokinetic soil flushing

electrokinetic soil flushing + bioremediation electrokinetic soil flushing -PRB (Pd/Fe particles)


electrokinetic soil flushing assisted with β-CD

HCB/spiked kaolin/100 mg kg−1 HCB/spiked real highly clayey soil/3.7−9.6 mg kg−1

Molinate/real sandy and silt loam soils spiked with pesticides/47−132 mg kg−1 PCP/spiked heavy clay soil/100 mg kg−1

PCP/real silty clay soil spiked with pesticide/100 mg kg−1 PCP/spiked real highly clayey soil 100 mg kg−1

PCP/spiked kaolin/100 mg kg−1

1,2,4,5 tetrachlorobenzene/ spiked clay/0.18 mmol kg−1

electrokinetic soil flushing assisted with surfactant β- cyclodextrine electrokinetic soil flushing assisted with β-CD/Fenton electrokinetic soil flushing

electrokinetic soil flushing assisted by Tween 80 and SDBS surfactants electrokinetic soil flushing assisted by surfactants Tween 80 and β-cyclodextrine

electrokinetic soil flushing -PBR (adsorption onto charcoal)

8728

lab-scale soil cartridge with electrode chambers/graphite anode/graphite cathode/3.14 A m−2/catholyte fluid: sulfuric acid (to maintain pH at 7 and electro-osmotic flow)/ Sphingobium sp. UG30 used to mineralize PCP soil cartridge with electrode chambers; soil column dimensions: 13.0 × 5.4 × 5.9 cm3/ graphite electrodes/3.14 A m−2/Sphingobium sp. UG30 used to mineralize PCP lab-scale column/PRB with Pd/Fe particles blended with acid-washed quartz sand/30 V/ flushing fluid 0.025 M Na2SO4 and 0.025 M Na2CO3/1 M HAc injected into PBR compartment/graphite electrodes/20 V/5−15 days lab-scale column dimensions: 13.5-cm length, 3.8-cm diameter/graphite electrodes/ electrolyte solution: 0.006 M NaHCO3+0.002 M CaCl2+0.001 M MgCl2/2 V cm−1 lab-scale cylindrical column (length: 9 cm, diameter: 5 cm)/Perforate graphite electrodes/ flushing fluid: 0.025 M Na2CO3/NaHCO3 buffer and 5 mM β-cyclodextrin/9−18 V/5− 11 days

lab-scale (reactor dimensions: 28 × 22 × 10 cm3)/flushing solution 1% β-CD and ferrous sulfate/15% hydrogen peroxide/titanium coated with platinum electrodes/15 days/30 V soil column: 20-cm length and 10-cm diameter/titanium electrodes/anolyte solution: 0.01 M NaOH or 0.01 M Na2CO3/10−15 days/30 V soil column: 20-cm length and 10-cm diameter/titanium electrodes/anolyte solution: 0.01 M NaOH or 0.01 M Na2CO3/10−15 days/30 V/ultrasonic processor maximum power: 200 W soil column dimensions: 20 × 7.5 × 12 cm3/platinum-coated graphite electrodes/flushing fluid 10−12% H2O2 and FeSO4/1.5−2.0 V cm−1/10−20 days lab-scale column/PRB with Pd/Fe particles blended with acid-washed quartz sand/30 V/ flushing fluid: 10 mmol dm−3 TX-100 and 0.025 M Na2CO3/graphite electrodes/10 days lab-scale column/length: 7.5 cm; diameter: 8 cm/300−400 g soil/platinized titanium electrodes/electrolyte: 0.01 NaNO3/7−48 days

lab scale (100 g soil)/graphite anode/graphite cathode/45% soil moisture/anodic flushing solution 0.05 M Na2CO3/NaHCO3 buffer/10 days

lab-scale (100 g soil)/graphite anode/graphite cathode/45% soil moisture/anodic flushing solution: 0.05 M Na2CO3/NaHCO3 buffer/10 days

115

50−64%

dechlorination of PCP from 58 to 78%; 70−81% PCP removed from soil worse results obtained than with EK; this technology is not recommended; combination of β-CD and i-TECB leads to inclusion compounds (very immobile species)

not a remediation study but an investigation on the effect of EK on microorganisms 49% PCP removed and 22.9% PCP recovered as phenol

not a remediation study but investigation of EK effects on microorganisms

101

116

114

112

111

119

106

55%

50−57%

120

120

63% 74%

104

64%

103

103

100

13% using SDBS almost 0% using Tween 80 very disperse 11.4% for spiked kaolin with no surfactant; 11.9% using Tween 80; 32.2% using β-CD only 1.1%

117

113

87%

55%

102

61%

lab-scale column dimensions: 8.3 × 5.1 × 5.3 cm3/445 g of soil/platinum electrodes/ flushing fluid contains nonionic surfactant: Triton X-100 or AML-10 reactor dimensions: 22 × 7x4 cm3/carbon felt anode/stainless steel cathode/0.5 M Na2HPO4 (nitric acid added to cathode)/0.89−3.74 A m−2/22 days/Burkholderia spp. RASC c2 used to mineralize 2,4-D soil column 24 × 10 × 10 cm3/graphite electrodes/0.01 M NaNO3/1 V cm−1/adsorption PBR with bamboo charcoal/operation with periodic polarity reversals/1 V cm−1/10 days lab-scale/cylindrical column dimensions: 15-cm length, 5-cm diameter/titanium electrodes/flushing solution 7500 mg dm−3 of Tween 80 or SDBS/10−30 days

electrokinetic soil flushing assisted by surfactant solutions bioremediation + electrokinetic soil flushing

119

14−41%

lab-scale column/length: 7.5 cm; diameter: 8 cm; 300−400 g soil/platinized titanium electrodes/electrolyte: 0.01 NaNO3/7−48 days


ref 118

maximum efficiency reported/related remarks 30−50% during the first 24 h (89−98% at the end of the treatment)

remarks on operation conditions lab-scale cell/length: 7.5 cm; diameter: 8 cm; 300−400 g soil/platinized titanium electrodes/9 days/electrolyte: 0.01 M NaNO3

technology used


HCB/spiked kaolin/not available HCB/spiked kaolin/100−500 mg kg−1 HCB/spiked kaolin/100−500 mg kg−1

hexachlorobenzene/real polluted soil/55 mg kg−1

hexachlorobenzene/spiked kaolin (clay soil)/50 mg kg−1

DDT/spiked real soil (sandy loam)/1.9 mg kg−1

atrazine/spiked sandy soil and actual polluted soil/0.15−19 mg kg−1 bentazone/real sandy and silt loam soils spiked with pesticides/47−132 mg kg−1 chlorobenzene/spiked real clay loam/0.12 wt.% 2,4-D (2,4-dichlorophenoxyacetic acid)/spiked silt silty-clay loam 2,4-dichlorophenol/spiked sandy loam/100 mg kg−1

pollutant/type of soil/concentration range

Table 3. Electrokinetic Technologies for Pesticide Removal from Soils



ref

101

maximum efficiency reported/related remarks

worse results obtained than with EK; this technology is not recommended technology; combination of β-CD and TCB leads to inclusion compounds (very immobile species)

remarks on operation conditions

1,2,3-trichlorobenzene (TCB)/ spiked clay/0.18 mmol kg−1

lab-scale cylindrical column (length: 9 cm, diameter: 5 cm)/perforate graphite electrodes/ flushing fluid: 0.025 M Na2CO3/NaHCO3 buffer and 5 mM β-cyclodextrin/9−18 V/5− 11 days

technology used

electrokinetic soil flushing assisted with β-CD

pollutant/type of soil/concentration range

Table 3. continued

Chemical Reviews

Review

by the acid and basic fronts, thereby affecting the efficiency of Fenton’s process (which is well-known to be highly dependent on acidic pHs near 3). A final consideration for conditioning fluids is the use of biosurfactants. These reagents are molecules that are produced by microorganisms that act as surfactants in the soil.107,108 Obviously, the environmental compatibility of these reagents is higher than that of the chemicals that are typically used; however, very few studies have been conducted to date on this particular topic. 4.2. Bioelectroremediation

It was recently proposed that bioremediation processes should be combined with electrokinetic processes, either by using suspensions or microorganisms as conditioning fluids or, most likely, by enhancing a conventional remediation process using the mobility promoted by EKSR.109,110 In this novel technology, electrokinetic fluxes direct microorganisms to the desired position in the soil or transport pollutants or nutrients through the soil. The use of bioelectrokinetic processes for pesticide removal is currently at a very early stage of development. Most studies have assessed how electrokinetic processes in the soil can affect the behavior of acclimated microorganisms to oxidize a given pesticide. In particular, significant changes in the physicochemical properties of the soil (pH and moisture content) occur during EKSR that are known to have an important and negative effect on the exposed microbial community,111 enzyme activity and, consequently, pesticide mineralization.112 This effect can be easily minimized by regularly reversing an electric field through the soil. However, bioelectroremediation was used very successfully in a study where over 80% of the pesticide was removed without using electric field reversal, showing that there are huge variations within a single bioremediation experiment.113 To some extent, this study illustrates the use of permeable reactive barriers consisting of biobarriers, because water was drawn to a position in the cell in which bacteria were inoculated into the soil to perform bioremediation. The successful results motivate the possibility of manufacturing these biobarriers on an inert support (fixed culture systems). Once this device is fabricated, it could be placed between the anode and the cathode in the soil, and an electrokinetic process could be used to transport pollutants through the biobarrier. 4.3. Permeable Reactive Barriers

These biobarriers are an example of permeable reactive barriers (PRBs), a very promising soil remediation technology that can enhance the results of electrokinetic treatment in some situations. Biobarriers use barriers created by other remediation technologies (such as adsorption, ionic exchange, chemical reaction, biological oxidation, etc.) in which the groundwater is circulated several times by the proper use of the electrokinetic fluxes. The use of PRBs significantly increases the efficiency of pollutant removal over that of conventional EKSR processes. There is a great variety of reactive barriers, which should be chosen depending on the type and characteristics of organic/ inorganic pollutants. That is, the reactive barrier should be chosen according to the treatability of the pollutant by adsorption, ionic exchange, chemical reaction, biological processes, etc. PBRs have been shown to be effective in the removal of chlorinated pesticides using iron beds as PBRs, and PBRs have been successfully tested for removal of pentachlorophenol (PCP)114 and hexachlorophenol.115 In both cases, using a Pd/ 8729


Chemical Reviews

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remediation of soil polluted with pesticides, economic assessment of the technology is out of the scope in most of the manuscripts found in the literature and taking into account the large number of parameters influencing on results, extrapolation of energy consumptions data obtained in lab-scale setups to full scale applications should be considered as merely speculation at the present moment.

Fe PBR produced significantly better results than those obtained using a single EKSR process. In the first study, the two primary processes, i.e., the pollutant being dragged by electrokinetic fluxes without addition of surfactants and the reduction of PCP to phenol, developed at a good rate. Removals of more than 60% of the initial PCP were obtained in less than 15 days of remediation. In the second study, the combined use of a surfactant and PBRs increased hexachlorobenzene removal in a polluted solid by a factor of 4, and 60% removal was obtained in less than 10 days of remediation testing. Note that other studies have been conducted in which cathodic reduction of chlorinated pesticides has been investigated without the use of PBRs. Reddy and co-workers demonstrated dechlorination of PCP in the 58−78% range116 for electrokinetic treatment of a soil without adding a reducing agent or a surfactant in the flushing fluid and without a PBR, showing that electrokinetic technology offers good prospects for removing chlorinated pesticides from soils. Ma and coworkers117 developed another interesting application of EKSRPBRs to remove pesticides from soils in which charcoal bed PBRs simultaneously adsorbed soluble 2,4-dichlorophenol and Cd at removal efficiencies of 76 and 55%, respectively, after 10.5 days of operation. A final application of electrokinetic fluxes is their use as a fence to contain pollution in a restricted polluted area. The pollutant fluxes can easily be captured by the proper placement of a fence of electrodes, a scheme that has been described for other types of pollutants. However, we were not able to find any studies on using a fence for remediating pesticide pollution.

5. USE OF ELECTROCHEMICAL TECHNOLOGIES FOR PESTICIDE REMOVAL FROM WATER Following the pioneering work of Keenan and Stuart in 1976,121 many studies have been published on using electrochemical oxidative technology to deplete pesticides in water. However, Keenan and Stuart’s initial study was performed at a time when there was little interest in treating organics in polluted wastewater and was followed by an inactive period that lasted until the late 1990s in which few studies were published on the electrochemical degradation of pesticides. However, interest in the use of electrochemical technologies to remediate water reservoirs polluted with pesticides became significant in the late 1990s for the reasons given below. • There was growing interest in using anodic oxidation to treat industrial wastes, which was found to yield good efficiencies, especially with the development of highly efficient anodes, such as boron-doped diamond electrodes. The treatment of the industrial wastes produced during the manufacture of some of these pesticides was a pioneered in that period.122 • The feasibility of combining electrochemical technologies with highly efficient and cheaper chemical technologies for pesticide removal was assessed during the same period.123,124 • Recently, there has been more interest in developing new technologies to reduce pesticides in the environment, which has been motivated by increased demand from society and more restrictive regulations on pesticide levels in water. Over the last two decades, the environmental applications of electrochemical technology have increased significantly. Nowadays, a great diversity of electrochemical technologies can be used to treat wastewater polluted with organic compounds, such as those produced during the electrochemical flushing of a particular soil polluted with pesticides. In the scientific literature, anodic oxidation, cathodic dechlorination, electrocoagulation and electro-Fenton processes appear to be the best options among these techniques for treating pesticide-polluted waters.

4.4. Remarks on Electrochemical Remediation of Soils Polluted with Pesticides

In summary, numerous applications of electrokinetic processes have been reported in the literature. Table 3 is a compilation of the most significant studies on the electrokinetic remediation of pesticides in soils that have been mentioned in this review and highlights the primary operating conditions and the best results obtained. Note, however, that most of these studies were conducted at the lab-scale: thus, it would be injudicious to extrapolate these results to real applications without first performing a scale-up study. Another important consideration is that few studies have been conducted on actual polluted soils,118,103,119 i.e., most studies have been conducted on spiked real or synthetic soils. In some cases, huge differences have been obtained between pesticide removal efficiencies in a synthetic spiked soil and an actual polluted soil. A good example can be found in a study by Yuan and co-workers103 in which the same flushing remediation method using β-cyclodextrine yielded 1% removal in actual polluted soils and 33% removal in spiked kaolin. Likewise, and as was described for nonelectrochemical water treatment technologies, higher pesticide concentrations were used in synthetic soils than those found in actual polluted soils. Hence, these technologies show great potential but are considerably removed from application to actual remediation processes, and significant effort is required to assess these technologies for use with real soils and for scale-up. In addition, many issues still need to be investigated at the lab-scale, especially in terms of obtaining a fundamental understanding of these processes. A very promising study120 has demonstrated significant improvement in the removal efficiency using the irradiation by ultrasound, which enhance the electroosmotic flux and current. Due to the very early stage of the research on electrochemical

5.1. Anodic Oxidation and Cathodic Dechlorination

The treatment of industrial wastewaters by anodic oxidation has been widely studied in recent years. Anodic oxidation offers important advantages over other commonly used technologies (such as ozonation, Fenton process, photochemical processes, etc.). The most important advantage offered by anodic oxidation is the complete mineralization of any organic pollutant. However, despite these advantages, this electrochemical technique has not seen wide application in the treatment of groundwater or surface water polluted with organics, primarily because of the low efficiencies expected for the treatment of low-concentration aqueous wastes. The maximum efficiencies for the anodic oxidation of wastewaters have been obtained for the treatment of wastewaters with pollutant concentrations in the 100 mg/dm3 to 20 g/dm3 range (in COD units) .125 Decreasing the pollutant concentrations to the range of several μg/dm3 could be very expensive, primarily 8730


Chemical Reviews

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for uniquely direct electrolytic processes without the contribution of reductive or oxidative reagents that are electrogenerated in the system. Recall that anodic oxidation can occur via direct and mediated mechanisms. In direct processes, the pollutants that reach the anodic surface are adsorbed onto the electrode surface prior to electron transfer. Hence, the process rate is controlled by the mass transfer rate, resulting in an efficiency that is not very high, particularly in the treatment of wastewater polluted with organics at low concentrations. Conversely, for mediated mechanisms, the oxidation processes occur in the aqueous waste through the action of oxidants that are electrogenerated by the oxidation of the salts. Thus, the process rate is controlled volumetrically, and the efficiency increases significantly. A mediated oxidation mechanism can play an important role in the treatment of wastewater with significant salt concentrations, where the global process is not controlled by mass transfer but by the action of these mediated reagents, resulting in significantly higher efficiencies. Thus, lowconcentration wastewaters can be treated resulting in high current efficiencies. The aforementioned issues show that the expected low pollutant concentrations lead to two very important considerations in the anodic oxidation treatment of pesticides in waters: • the design and operation conditions (i.e., the interelectrode gap, temperature, electrode size, flow conditions, etc.) must be optimized to reduce the mass transfer limitations and to improve other significant features of the electrochemical process (i.e., the cell potential, gas management, etc.), and • suitable electrode materials must be used to enhance the mediated oxidation processes. The first issue is typically studied in scale-up assessments and not at the initial lab-scale stage, which is where current research on the anodic oxidation of pesticides is. Thus, most studies in the literature have focused on assessing electrode properties. There are three different categories of electrode materials: metallic, metal oxides and conductive diamond electrodes. Metallic electrodes are in the first category of electrode materials used for anodic oxidation. This category excludes coagulation − sacrificial electrodes, such as iron or aluminum electrodes, and primarily consists of platinum and platinumcoated electrodes. The use of these electrodes has been evaluated for the removal of chloroxylenol126 to compare with the performance of other types of electrodes: these electrodes perform poorly because they do not promote either direct or mediated processes. However, an interesting study was performed in which titanium electrodes were used to degrade monochrotophos127 by promoting indirect electrolysis. Vlyssides’ group performed studies128−133 in which a different type of electrode was used for the coarse removal of pesticides for a very high concentration of stockpiles, producing acceptable results with high removals in short electrolyses times. These results were attributed to the process conditions, i.e., the process was not mass-transfer-controlled, and the inefficiencies were attributed to different issues that masked the nonoptimum properties of these electrodes. Another interesting process in which metallic electrodes are used involves an iron sacrificial electrode and an oxone solution (i.e., a reagent containing persulfate) to remove pesticides. This novel process has been successfully used to remove 2,4,5,-trichlorophenoxyacetic acid.134

At low pesticide concentrations, metallic electrodes do not enable a significant amount of oxidants to form, unlike DSA (dimensionally stable anode) electrodes that facilitate the production of chlorine from chlorides in the water. Thus, metal oxide and/or mixed metal oxide electrodes are one of the primary types of electrodes that have been assessed for pesticide removal by anodic oxidation, especially for chlorine production in reaction media. These electrodes have been used in many studies to evaluate the removal of atrazine,135−137 chlorpyrifos,138 isothiazolin,139 mecocrop,140 methamidophos,141 linuron,142 thiocarbamate,143 and a mixture of pesticides in actual drainage wastewater.144 In some of these studies, the effect of chlorine oxidation could be clearly observed, particularly when electrodes with high ruthenium and iridium oxide contents were used. Thus, chlorinated oxidants can effectively chemically oxidize pesticides by forming many chlorinated intermediates. These results also explain the typically high efficiencies that are obtained for the oxidation of the raw pesticide and the poorer mineralization efficiencies (i.e., the complete transformation of organic compounds into inorganic carbon) of the water. Important differences have been observed among the different types of electrodes in this category in terms of the mineralization efficiency, which increases when the electrode contains PbO2 or SnO2. This observation has been explained in terms of the formation of active hydroxyl radicals using these electrodes. However, these electrodes are not as robust as those containing iridium, molybdenum, and ruthenium oxides, which limit their potential applications. The third type of anodic electrodes are conductive-diamond electrodes and, particularly, boron-doped diamond anodes (BDDs). Boron-doped diamond films have been used in electrolysis as an emerging and very promising material that exhibits very good properties in the electrochemical treatment of wastewaters polluted with organic compounds, including a considerable chemical and electrochemical stability, and the capability to generate large amounts of hydroxyl radicals from the oxidation of water. These radicals are very powerful oxidants with very low life-times.145,146 Once generated, the hydroxyl radicals react rapidly with the organics in the wastewater and combine with other radicals to form oxygen or react with other compounds to form new oxidant reagents. The presence of these radicals result in higher efficiencies than those obtained using other electrode materials. The generation of large amounts of hydroxyl radicals has resulted in the classification of electrochemical oxidation using conductive diamond films electrodes as an electrochemical advanced oxidation process (EAOP), just like the Fenton process. In addition to hydroxyl-radical oxidation, the global oxidation process that occurs in conductive-diamond anodes is known to be complemented by direct electrooxidation on the surface and also by mediated oxidation using other oxidants electrogenerated on the anodic surface from electrolyte salts. In recent years, oxidants have been produced using most of the ions that are typically present in waters (Cl−, SO42‑, CO32‑, PO43‑, etc.).147−149 Combining these oxidation mechanisms has been shown to increase the current efficiency of this technique for many cases, although several exceptions can be found in the literature150 for which increasing the salt concentration reduced the efficiency. In recent years, electrochemical oxidation with BDD has been studied for wastewaters polluted with a great variety of pesticides. Thus, the oxidation assessments of bupira8731


Chemical Reviews

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mate,151−155 chlorpyrifos,156 chloroxylenol,126 mecoprop,157 methamidophos,141 parathion,158 propham,159 and mixtures of pesticides160 have confirmed that electrochemical oxidation with conductive diamond electrodes can be used to successfully treat pesticides with much higher current efficiencies than other anodic oxidation technologies. Using this type of electrode results in almost complete mineralization of waste during electrolysis, where carbon dioxide is the primary final degradation product.161 Coupling light irradiation or ultrasound to anodic oxidation further improves the results attained by anodic oxidation alone. This improvement results from the increase in the formation of hydroxyl radicals and mass transfer. This efficiency increase was investigated for the removal of atrazine162,136 and chloroxylenol126 using UV light irradiation and for pentachlorophenol removal 163 by dosing of ultrasound. Recall that assessing electrochemical technologies for pesticide removal has the same drawbacks that were previously presented for nonelectrochemical water treatment technologies and soil remediation. Technological tests are typically performed using synthetic solutions that contain much higher pesticide concentrations than those found in the environment and are only representative of industrial effluents. Hence, a typical concentration for all of the assessments is 100 mg pesticide dm−3, which is several fold above the typical concentrations found in groundwater or surface water. Thus, although correct conclusions can be drawn regarding the mineralization of pesticides and intermediates, mass transfer limitations in anodic oxidation processes should be more significant in the treatment of actual groundwater or surface water. Consequently, current efficiencies should be significantly lower than those reported in the literature, although the required applied current charges required should also be much lower because the organic load is smaller. This effect of the pollutant concentration can be clearly observed in Figure 2,164 which shows the changes in the concentration during the anodic oxidation of triclosan solutions (a broad-spectrum antimicrobial agent that is occasionally used as a registered pesticide) for initial triclosan concentrations ranging over 4

orders of magnitude. The same trends are observed over the four ranges of concentration without significant deviations, except that lower charges are required to attain the same change at smaller concentrations. This observation confirms that some of the conclusions that are obtained for high concentration ranges can be extrapolated to low concentration ranges. This concentration effect was noticed some time ago in a study stating that the more significant application of electrochemical technology should not be anodic oxidation but the cathodic dechlorination of chlorinated pesticides.165 Although this cathodic process is not a final treatment, a very significant decrease in the toxicity of chlorinated pesticides is obtained, and the cathodic process may be a good alternative when combined with cheaper treatments.166,167 A final application of electrolytic systems is to modify the adsorption capacity168 of an adsorbent or ion exchange processes169 of a resine using electrochemically assisted processes. Very promising results have been obtained using these technologies. For example, a simple electroactivation of carbon produced an increase of nearly 38% in the adsorption capacity of granular activated carbon (GAC). Table 4 summarizes the results of studies published in recent years on the anodic oxidation of pesticides and includes the experimental conditions and maximum removals obtained. 5.2. Electrocoagulation

Electrocoagulation is another electrochemical technology that is used to treat wastewaters. Using electrochemical cells with iron or aluminum as anodes results in the generation of iron (III) (following the oxidation of electrochemically formed iron(II) by oxygen) or aluminum (III) ions. The hydrolysis of these ions results in the formation of species that can promote the coagulation of colloidal suspensions, the destabilization and break-up of oil-in-water (O/W) emulsions, or the enmeshment of soluble pollutants in insoluble metallic hydroxides that are generated during the speciation of metallic electrodissolved ions. This final mechanism enables this technology to be used for pesticide treatment. The use of this technology has been illustrated using iron electrodes both for metribuzin removal170 in which efficiencies of 89% were attained and for monochrotophos removal127 in which removal efficiencies close to 80% were obtained. In this technology, the pollutant concentration does not limit the energetic efficiency. Hence, this technique can compete successfully with other commonly used techniques such as coagulation-flocculation processes. Electrocoagulation offers important advantages over conventional coagulation: in the electrochemical cell, destabilization (coagulation) and removal by flotation processes occur simultaneously, the addition of reagents is not necessary, a minimum amount of sludge is generated, and the operation costs are significantly lower than those of conventional technologies (in spite of the higher investment cost). Thus, this technology is particularly attractive compared to coagulation for treating industrial wastes. Cheng and coworkers171 used electrochemical technology as a complement to more conventional coagulation and adsorption in the coarse treatment of an actual industrial triazine waste with a very high load of pesticides: the organic load was as high as 98,000 mg dm−3.

Figure 2. Changes in triclosan concentration during electrolysis for four different initial concentration ranges (10−2-102 mg dm−3) in methanol/water (150 A m−2, 0.035 M Na2SO4; filled symbols) or methanol media (150 A m−2, 0.035 M NaCl; unfilled symbols). Adapted with permission from ref 164. Copyright 2013 John Wiley & Sons.

5.3. Electro-Fenton Process

The final category of electrochemical technologies involves using enhanced mediated electrolytic processes to produce 8732


type of anodic oxidation

8733

malathion/commercial pesticide diluted in distilled water with 4% w/v NaCl

linuron/50 mg dm−3/synthetic solutions with 1g dm−3 NaCl

isothiazolin/200 mg dm−3/synthetic solution with 10g dm−3 Na2SO4

fenthion/80,000 mg dm−3 measured as COD/commercial pesticide diluted in distilled water with 4% w/v NaCl

anodic oxidation (metal anode)

anodic oxidation (metal oxide/s anode)





diazinon/80,000 mg dm−3 O2 measured as COD/commercial pesticide diluted in distilled water with 4% w/v NaCl

dimethoate/commercial pesticide diluted in distilled water with 4% w/v NaCl


anodic oxidation (conductive-diamond anode)

demeton-S-methyl/80,000 mg dm−3 O2 measured as COD/commercial pesticide diluted in distilled water with 4% w/v NaCl

chloroxylenol/100 mg dm−3/synthetic solutions with Na2SO4 (pH 3)

chloroxylenol/100 mg dm−3/synthetic solutions with Na2SO4 (pH 3) anodic oxidation (metal anode)


chlorpyrifos/450 mg dm−3 O2 measured as COD/synthetic water prepared by dilution of emulsifiable concentrate DURSBAN 4 and sulfuric acid (pH 2)

bupiramate/230 mg dm /synthetic solution with 2−4% NaCl

−3

chlorpyrifos/450 mg dm−3 O2 measured as COD/synthetic water prepared by dilution of emulsifiable concentrate DURSBAN 4 and sulfuric acid (pH 2)

photoanodic oxidation (metal oxide anode)

photoanodic oxidation (metal oxide anodes) anodic oxidation (metal oxide anode)

anodic oxidation (metal oxide anode)

anodic oxidation (metal anode) anodic oxidation (metal oxide anode) anodic oxidation (conductive-diamond anode) anodic oxidation (metal oxide anode)

azinphos-methyl/commercial pesticide diluted in distilled water with 4% w/v NaCl bupiramate/230 mg dm−3/synthetic solution with 2−4% NaCl

atrazine/20 mg dm−3/synthetic solutions with 0.033 M Na2SO4 or 0.1 M NaCl

atrazine/20 mg dm−3/synthetic solutions with 0.033 M Na2SO4 or 0.1 M NaCl

atrazine/20 mg dm−3/synthetic solutions with 0.033 M Na2SO4

atrazine/20 mg dm−3/synthetic solutions with 0.0125−0.10 M NaCl/also tested with H2SO4, Na2SO4, NaClO4, NaOH and NaNO3


photoelectrochemical filter flow press cell/Ti-Ru0.3Ti0.7O2 mixed oxide electrode/100 A m−2//UV source (254 nm): 200W/300cm3 treated volume/3 h Bench-scale complete-mixed electrochemical tank cell (6 dm3)/TiPt anode/stainless steel cathode/4500−5500 A m−2 single-compartment complete-mixed electrochemical tank cell (75 cm3)/SnO2 anode/Pt cathode/200−600 A m−2 single-compartment complete-mixed electrochemical tank cell (75 cm3)/BDD anode/Pt cathode/200−600 A m−2 single-compartment complete-mixed electrochemical tank cell (150 cm3)/Nb-PbO2 anode/graphite carbon PTFE cathode/5cm2 electrode surface/100−500 A m−2 single-compartment complete-mixed electrochemical tank cell (150 cm3)/BDD anode/graphite carbon PTFE cathode/5-cm2 electrode surface/200 A m−2 single-compartment complete-mixed electrochemical tank cell (100 cm3)/3 cm2 Pt anode/3-cm2 stainless steel cathode/current density: 330 A m−2/pH 3 single-compartment complete-mixed electrochemical tank cell (100 cm3)/3-cm2 BDD anodes/3-cm2 stainless steel cathode/ current density: 330−1500 A m−2/pH 3/ bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500− 5500 A m−2 bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500− 5500 A m−2 bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500− 5500 A m−2 bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500− 5500 A m−2 single-compartment complete-mixed electrochemical tank cell (450 cm3) (cylindrical vessel)/Ti-SnO2-Sb/PbO2 anode/stainless steel cathode/150 A m−2 single-compartment complete-mixed electrochemical tank cell (50 cm3)/Pb-PbO2 and C-PbO2 electrodes/stainless steel cathode/ 0−4000 A m−2/electrolysis times up to 180 min bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500− 5500 A m−2

electrochemical flow cell/Ti-RuxSn1‑xO2 anodes/Ti mesh cathode/ 14-cm2 area/100 A m−2/UV radiation (254 nm): 0.113 W cm−2 electrochemical filter flow press cell/Ti-Ru0.3Ti0.7O2 mixed oxide electrode/100 A m−2/300-cm3 treated volume/3 hs

single-compartment completely mixed electrochemical tank cell/ DSA anode (Ti-Ru0.3Ti0.7O2)/platinum cathode/current densities: 100−1200 A m−2

operation conditions

Table 4. Anodic Oxidation Technologies for the Removal of Pesticides from Waters and Wastewaters maximum efficiency reported/related remarks

92% pesticide degradation for C/PbO2 electrodes and 84% pesticide degradation for Pb/ PbO2 after 30 of minutes electrolysis 40% COD reduction after 2 h of electrolysis

82% pesticide oxidation and 15% COD removal after 300 min of electrolysis

77% COD reduction after 2 h of electrolysis




93% mineralization (after 6 h of electrolysis)


100% COD removal after 6 h of electrolyses at 200 A m−2 (70 °C)

60% COD removal for 2% NaCl and 45% for 3% NaCl in 2 h 74% COD removal using 2% NaCl and 52% COD removal using 3% NaCl after 2 h 76% COD removal after 10 h of electrolyses at 500 A m−2

100% pesticide degradation in less than 1 h using NaCl (33.6% TOC removal) low removal efficiencies in the absence of chlorides (65% COD removal in 2 h of electrolysis

refs

130

142

139

132

130

132

132

126

126

156

138

152

151

129

136

136

162

135



type of anodic oxidation

8734

anodic oxidation (metal anode) anodic oxidation (metal anode) electrolysis with iron electrode and oxone (persulfate-containing reagent)

phosalone/commercial pesticide diluted in distilled water with 4% w/v NaCl

phosphamidon/commercial pesticide diluted in distilled water with 4% w/v NaCl 2,4,5 trichlorophenoxyacetic acid/0.025−0.20 mM/synthetic solution with 0.05 M Na2SO4

propham/0.5 mM/synthetic solutions with different salts (0.05 M Na2SO4 or 0.1 M NaCl or 0.1 M NaNO3 or 0.1 M LiClO4 or 0.5 M K2SO4)

sono-anodic oxidation (conductive-diamond anode) anodic oxidation (conductive-diamond anode)

pentachlorophenol/10 mM/synthetic solution with a 0.1 M Britton-Robinson buffer

parathion/1 mM/synthetic solution with a 0.1 M Britton-Robinson buffer (acetic, phosphoric and boric acids adjusted to the required pH using NaOH)

MCP/300 mg dm−3/synthetic solution with 1−6 g dm−3 NaCl

anodic oxidation (metal anode) anodic oxidation with metal electrodes and H2O2 dosing anodic oxidation (conductive-diamond anode)


monochrotophos (MCP)/commercial pesticide diluted in distilled water with 4% w/v NaCl

MCP/50−300 mg dm−3/synthetic solution with 1−6 g dm−3 NaCl



mixture of parathion, methyl-parathion, malathion and degradation products in actual drainage water/measured as COD 83.3 mg dm−3 O2/drainage water taken from a dump near sea with a high salinity (0.7%) mixture: chlortoluron- carbofuran- bentazon/0.125 mM for both components/ synthetic solution with 0.05 M Na2SO4 or 0.1 M NaClO4

methyl parathion/38,500 mg dm−3 O2 measured as COD/commercial pesticide diluted in distilled water with 4% w/v NaCl

metribuzin/100 mg dm−3/synthetic solution with concentrations of NaCl, Na2SO4, and KCl ranging from 0 to 1.5 g dm−3 electrochemically assisted adsorption (Electro-activation of GAC) anodic oxidation (metal anode)

anodic oxidation (conductive-diamond anode) anodic oxidation (metal anode)

methidathion/1.4 mM/synthetic solution with 2−4% NaCl

methamidophos/80,000 mg dm−3 O2 measured as COD/commercial pesticide diluted in distilled water with 4% w/v NaCl





phosalone/commercial pesticide diluted in distilled water with 4% w/v NaCl

methamidophos/50 mg dm−3/synthetic solutions with Na2SO4 5%

methamidophos/50 mg dm−3/synthetic solutions with 5% Na2SO4

methamidophos/50 mg dm−3


Table 4. continued operation conditions

double-compartment electrochemical cell with a porous membrane: 140 cm3/BDD anode/Pt cathode/2-cm2 electrode size/ US horn (20 kHz): 14 W single-compartment complete-mixed electrochemical tank cell (0.175 cm3)/anode: 12 cm2 Nb-BDD/cathode: Pt/25−400 A cm−2 bench scale single-compartment completely mixed tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500−5500 A m−2 bench scale single-compartment completely mixed tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500−5500 A m−2 single-compartment complete-mixed electrochemical tank cell (50 cm3)/iron anode (sacrificial electrode)/graphite cathode/oxone dosing

three-compartment electrochemical cell: 20 cm3/BDD anode/Pt cathode/0.6 cm2

single-compartment complete-mixed electrochemical tank cell (250 cm3)/60-cm2 carbon felt (cathode)/14.5-cm2 BDD (anode)/anodic current density: 210 A m−2 bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500− 5500 A m−2 single-compartment complete-mixed electrochemical tank cell (500 cm3)/Ti electrodes/50−100 A m−2 single-compartment complete-mixed electrochemical tank cell (500 cm3)/titanium anode/2 mmol min−1 H2O2

bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti-Pt anode/stainless steel cathode/4500− 5500 A m−2 electrochemical flow cell/anode: 60-cm2 titanium coated with platinum and iridium/stainless steel cathode/3100 A m−2

electrochemical cell divided by a porous porcelane pot with a 100cm3 reaction compartment/Pb-PbO2 anode/zirconium plate used as cathode/100−500 A m−2 electrochemical cell divided by a porous porcelane pot with a 100cm3 reaction compartment/Ti-SnO2 anode/zirconium plate as cathode/100−500 A m−2 electrochemical cell divided by a porous porcelane pot with a 100cm3 reaction compartment/BDD anode/zirconium plate as cathode/100−500 A m−2 bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti -Pt anode/stainless steel cathode/4500− 5500 A m−2 single-compartment complete-mixed electrochemical tank cell (100 cm3)/200−600 A m−2 bench-scale single-compartment complete-mixed electrochemical tank cell (6 dm3)/Ti-Pt anode/stainless steel cathode/4500− 5500 A m−2 electroactivation of GAC for 2 h at −200 mV vs SCE


refs

133 134

over 90% pesticide degradation after 10 min of treatment

129

159

163

158

127

127

130

160

144

131,128

168

28% COD removal after 2 h of electrolysis

>65% COD removal after 2 h of electrolysis

complete mineralization after 3 h electrolysis

70% pesticide removal and 60% TOC removal after 1.5 h of electrolysis at 3.0 V vs Ag/AgCl treatment 82.6% pesticide degradation after 300 min of operation

100% degradation pesticide removal but only 20% mineralization

60% MCP degradation

28%

complete depletion of three pesticides in less than 2 h (97% TOC removal after 4 h)

complete depletion of pesticides; treatment produced a single refractory intermediate

80−93% COD after 2 h of electrolysis

38% increase in adsorption capacity of GAC

150

75% COD removal with 2% NaCl and 48% with 4% NaCl 71% COD reduction after 2 h of electrolysis

132

129

141

141

141

>65% COD removal in 2 h of electrolysis

100% pesticide degradation for a specified applied current charge of 0.4 Ah dm−3

40% pesticide oxidation for a specified applied current charge of 1.2 Ah dm−3

80% pesticide degradation for a specified applied current charge of 1.2 Ah dm−3




143 not a removal study but investigation of formation of intermediates

H 2O2 + Fe 2 + → Fe3 + + OH− + ·OH

In contrast to the classical Fenton’s process in which Fenton’s reagent (H2O2 + Fe2+)174 is added, in the electroFenton process, this reagent is electrogenerated in situ (H2O2) or regenerated by electrocatalysis (Fe2+).172 Indeed, H2O2 is formed by the 2-electron reduction of O2 from air that is bubbled through the solution, and ferrous iron is cathodically regenerated from an iron salt that is initially added to the solution and behaves as a catalyst.175 The mixture of hydrogen peroxide and iron ions (which is known as Fenton’s reagent) leads to a massive generation of hydroxyl radicals that can oxidize the organics. Compared to the classical Fenton’s process, the electroFenton process has several major advantages175,172 such as the in situ generation of reagents, which prevents risks associated with its transport, storage and handling; the absence of sludge formation (which occurs in the chemical Fenton’s process); a higher removal rate of organic pollutants because of the continuous regeneration of Fe2+; and the feasibility of overall mineralization that can be obtained at a relatively low cost by optimizing the operation parameters. Carbon-based materials, particularly 3D carbon felt, are the best choice for the cathode in electro-Fenton. The same materials used in anodic oxidation can be used for the anode. Using a conductive diamond anode (BDD) can significantly enhance the effectiveness of the process, because •OH are produced either at the anode surface by oxidation of water or in the bulk solution from the electrocatalytically generated Fenton’s reagent. Consequently, the oxidation and mineralization power of the process is enormously enhanced (Figure 3). This considerable improvement that is obtained by combining electro-Fenton and anodic oxidation using a conductive diamond anode was assessed for atrazine176 in a very recent study, in which an improvement of more than 15% was found for TOC removal under the same operation conditions as when conductive-diamond electrodes were used as anodes in electroFenton processes. In addition, this system was used to remove and even almost completely mineralize cyanuric acid, which has been reported many times to be recalcitrant to •OH.135,177 Many studies have been recently published on using this technology to remove pesticides from wastewater such as amitrole,178 atrazine,176 chlorophenoxyacid herbicides,123,179 clopyralid,180 chlortoluron,181 cloroxylenol,126 2−6-dimethylaniline,73 diuron,19,182 glyphosate,183 imazapyr,67 imazaquin,67 malathion,184 mesotrione,185 metomyl,186 methyl-parathion,187 monochrotophos,127 parathion,184 pentachlorophenol,124 phenylurea herbicides,188 picloram,189 propham,159 sulcotrione,185 and tetra-ethyl-pyrophosphate.184 The process can also efficiently remove pesticides from a mixture83,160,190 or a commercial formulation.186 All of the results have been very

single-compartment complete-mixed electrochemical tank cell (100 cm3)/Ti-RuO2 anode/stainless steel cathode


oxidants in bulk electrolytic media. These technologies are particularly effective for pesticide removal because oxidation occurs over the entire volume of the solution and not only near the electrode surface (as in anodic oxidation processes). Thus, these technologies are not limited by mass transfer and should be much more efficient than anodic oxidation technologies in terms of cost. Among these technologies, the electro-Fenton process is the most commonly used and popular technology172,173,48 in which powerful oxidant hydroxyl radicals (•OH) are produced in an electrochemically assisted Fenton’s reaction involving H2O2 and ferrous iron, according to the following reaction:


type of anodic oxidation pollutant/concentration range/type of water tested

Table 4. continued

Review

thiocarbamate/100−500 mg dm−3/synthetic solution with NaCl (0.5−7.5 M)

refs

Chemical Reviews

8735


Chemical Reviews

Review

acidic pH (probably near 3), whereas a pH of at least 6 is required to promote coagulation. Table 5 summarizes some of the most significant studies on the electro-Fenton process and related processes and electrocoagulation that are discussed in this review and shows the primary operation conditions and the most successful results obtained. 5.5. Remarks on Electrochemical Treatment of Water Polluted with Pesticides

Electrochemical technologies are promising technologies for the removal of pesticides from water, but research in this area is still far from the demonstration or commercial stage and because of that, no rigorous economic assessments could be done nowadays. Most research in this area is still at the lab-scale stage, in which small electrochemical cells are used that produce currents barely over 1 A for electrode surfaces of several cm2. The polluted water used in these studies consists of solutions of pesticides at much higher concentrations than those typically found in the environment and, in the assessment of electrochemical technologies, of matrixes of electrolytes that can significantly affect the results and are not representative of the composition of a typical surface or groundwater. It would not be a good practice to add salts to treat these waters, because this dosing would negatively affect the treated effluent (salinity can also be considered to be a type of pollution that is less hazardous but much more persistent than pesticide pollution); however, a conductive medium is needed to implement electrochemical methods. Reactor design has not been investigated in the aforementioned studies, most of which have focused on singlecompartment completely mixed electrochemical tank cells. This type of cell is not a suitable electrochemical cell for the depletion of pesticides, because it is expected to have higher mass transfer limitations than cells that promote turbulence or that can accommodate electrodes with a higher volumetric area. The use of these cells does not correspond to a bad practice but simply reflects that the research in this area is at an early stage in which these reactors are suitable apparatuses for assessing kinetic behavior. Despite these drawbacks, fairly interesting results can be found in the literature because most of the assessed technologies can completely mineralize the pesticide down to negligible concentrations. In particular, very promising results have been obtained for anodic oxidation using diamond electrodes and electro-Fenton technologies and by combining these technologies. Thus, a real solution to the huge environmental issue of the occurrence of pesticides in the environment is in progress, and more attention should be paid over the next few years to scaling up these technologies. Combining several technologies in the same treatment to explore synergistic effects is currently an active research area, and many new technologies are currently being assessed, including the use of novel microbial fuel cell systems197 or combining these technologies with biological systems to increase biodegradability in other ways.198,199

Figure 3. Evolution of TOC removal during mineralization of 0.1 mM atrazine (filled symbol) and 0.2 mM cyanuric acid (unfilled symbols) aqueous solutions using an electro-Fenton process and Pt anode (■,□); anodic oxidation using BDD anode (▲,Δ); electro-Fenton process using BDD anode (◊,⧫). Adapted with permission from ref 176. Copyright 2012 Springer Science.

positive and clearly indicated complete depletion of the pesticides and high degrees of mineralization, showing that this process yields better results than the anodic oxidation of pesticides. In addition, as was described for anodic oxidation, the irradiation with UV light can further improve the results obtained using electro-Fenton technologies (i.e., the photoelectro-Fenton process). This improvement was clearly shown for cloroxylenol,126 where the irradiation of the electro-Fenton process increased the mineralization from 58 to 91% (after 6 h of electrolysis at 330 A m−2) using a Pt anode and from 82 to 96% using a conductive diamond anode under the same operating conditions. The same behavior, although via a different mechanism, was found for the herbicides 2,4-D and 4,6-dinitro-o-cresol (DNOC) when ultrasound was applied together with the electro-Fenton process to explore synergistic effects.191 5.4. Modifications of the Electro-Fenton Process

Several modifications of the electro-Fenton process have been investigated in recent years and have yielded good results for pesticide treatment. In two of these processes, hydrogen peroxide is dosed as a chemical (i.e., hydrogen peroxide is not produced electrochemically at the cathode) to exploit the anodic reaction. The first process is anodic Fenton treatment, which has been successfully applied to remove diazinon.192,193 In this case, the iron electrode provided Fe(II) ions to the solution to promote Fenton’s reaction (H2O2 was added to the solution) to generate •OH. The second process is peroxielectrocoagulation, which is facilitated using iron as an anode, enabling the production of large amounts iron ions. Iron is electrodissolved to iron(II), which is later oxidized to iron (III): the iron (III) ions that are generated as byproducts act as coagulant reagents and increase the efficiency of the treatment process, while iron (II) catalyzes the decomposition of hydrogen peroxide via Fenton’s reaction to form hydroxyl radicals. Removals as high as 95% were obtained using this technology for metribuzin removal,170 and the technology was also evaluated for the removal of chlorophenoxy and chlorobenzoic herbicides.179,194−196 However, a tradeoff is required for the pH, because Fenton oxidation requires an

6. CONCLUSIONS The conclusions from this study are summarized below. • Pesticide occurrence in soils and water is an emerging and highly significant environmental issue on which the attention of the scientific community is currently focused. The increasing 8736


8737





photoelectro-Fenton using conductivediamond anode

photoelectro-Fenton (metal anode)

electro-Fenton with conductive-diamond anode

electro-Fenton (metal anode)

electro-Fenton (Pt anode)

peroxy-electrocoagulation

chlorophenoxyacid herbicides (several)/synthetic solution with 0.05 M Na2SO4

chlortoluron/0.2 mM/synthetic solution


chlorophenoxyacid herbicides (several)/1.5 mM MCPP 1.2 mM CPMP 1 mM 2.4-D 0.3 mM 2,4,5-T/synthetic solutions


electro-Fenton (conductive-diamond anode)

atrazine/0.1 mM/synthetic solutions with 0.1 M Na2SO4

clopyralid/0.5−3.0 mM/synthetic solution



technology

atrazine/0.1 mM/synthetic solutions with 0.1 M Na2SO4

amitrole/350 mg dm−3/synthetic solution with 0.05 M Na2SO4

pollutant/concentration range/type of water tested single-compartment complete-mixed electrochemical tank cell (100 cm3)/Pt anode, 3 cm2/carbon PTFE 3.1 cm2/1 mM Fe2+/current 0.1−0.45 A single-compartment complete-mixed electrochemical tank cell (250 cm3)/ cathode: 60-cm2 carbon felt/anode: 4.5 cm2 Pt/current: 0.05−1.0 A/pH 3.0/0.1 mM Fe2+ single-compartment complete-mixed electrochemical tank cell (250 cm3)/ cathode: 60-cm2 carbon felt/anode: 25 cm2 Ni-BDD/current: 0.05−1.0 A/pH 3.0/0.1 mM Fe2+ single-compartment complete-mixed electrochemical tank cell (100 cm3)/ working electrode: 15-cm2 mercury pool/platinum counter electrode/ reference electrode: saturated calomel electrode/Fe2+: 2 mM single-compartment complete-mixed electrochemical tank cell (100 cm3)/ Fe anode/Carbon-PTFE cathode/pH 3.0−3.5/current intensities from 0.1 to 0.45 A single-compartment complete-mixed electrochemical tank cell (175 cm3)/ cathode: 70-cm2 carbon felt/anode: Pt grid/pH 3/Fe3+ = 0.1 mM single-compartment complete-mixed electrochemical tank cell (500 cm3)/ working electrode: 60-cm2 carbon felt/ anode: cylindrical Pt grid/0.1 mM Fe3+/pH 3/0.05 M Na2SO4 single-compartment complete-mixed electrochemical tank cell (100 cm3)/ 3-cm2 Pt anodes/3-cm2 stainless steel cathode/current density 330 A m−2; 1 mM Fe3+/current density 330 A m−2/ pH 3 single-compartment complete-mixed electrochemical tank cell (100 cm3)/ 3-cm2 BDD anodes/3-cm2 stainless steel cathode/current density: 330 A m−2/pH 3/1 mM Fe3+/ single-compartment complete-mixed electrochemical tank cell (100 cm3)/ 3-cm2 Pt anodes/3-cm2 stainless steel cathode/current density 330 A m−2/ pH 3/1 mM Fe3+/irradiation with 6-W UVA light (360 nm) single-compartment complete-mixed electrochemical tank cell (100 cm3)/ 3-cm2 BDD anodes/3-cm2 stainless


Table 5. Electro-Fenton and Electrocoagulation Technologies for Pesticide Removal from Waters and Wastewaters ref




126

126

126

126

181

almost complete mineralization (98%) of 0.125 mM chlotoluron after 8 h at 300 mA


180

194

123

176

176

178

almost complete mineralization of 3 mM chloppyralid solution after 4 h

>90% pesticide removal

initial pollutant and all aromatic derivatives disappeared at 200 °C; energy consumption of 8.5 × 10−5 kWh using 100 mL of a 0.5 mM solution of 2,4,5-T

97% TOC removal after 8 h of electrolysis

81% TOC removal after 8 h of electrolysis

complete oxidation of pesticide after 4 h of treatment (85% TOC removal after 6 h of treatment)

maximum efficiency reported/remarks on efficiencies



8738

electro-Fenton (metal oxide/s anode)

sono-electro-Fenton (Pt anode)

2,6 dimethylaniline/1 mM/synthetic solution with HClO4 (pH 1−4)

4,6-dinitro-o-cresol (DNOC)/0.5 mM/synthetic solution with 0.05 M Na2SO4




diuron/0.17 mM/synthetic solution

glyphosate/0.1 mM/synthetic solution with 0.05 M Na2SO4

imazapyr/0.05−0.1 mM/synthetic solution


sono-electro-Fenton (Pt anode)

2,4-dichlorophenoxyacetic acid (2,4-D)/1 mM synthetic solution with 0.05 M Na2SO4

diuron/3−27.6 mg dm−3/synthetic solution with 0.05 M Na2SO4


2,4-dichlorophenoxyacetic acid (2,4-D)/1 mM/synthetic solution

technology

anodic Fenton treatment


diazinon/0.1 mM/synthetic solution with 0.04 M NaCl (KCl, Na2SO4 and NaNO3 solutions were also tested)

Table 5. continued

steel cathode/current density 330− 1500 A m−2/pH 3/1 mM Fe3+/ irradiation with 6-W UVA light (360 nm) two-compartment complete-mixed electrochemical cell with 200-cm3 salt bridge/20-cm2 iron sacrificial anode/ 10-cm2 graphite cathode/hydrogen peroxide added to anode compartment (i.e., not produced electrochemically) single-compartment complete-mixed electrochemical tank cell (150 cm3)/ working electrode: 10-cm2 carbon felt/ reference electrode: saturated calomel/counter electrode: 1 cm2 Pt sheet/1 mM Fe3+/pH 3 single-compartment complete-mixed electrochemical tank cell (250 cm3)/ 20-W US horn ultrasounds/4.5-cm2 cylindrical Pt mesh anode/(15 × 8 cm2) carbon felt cathode/pH 3/0.1 mM Fe3+/applied current: 200 mA single-compartment complete-mixed electrochemical tank cell (5 dm3)/ 600-cm2 Ti/IrO2/RuO2 anodes/900cm2 stainless steel cathodes/current density 5−70 A m−2 (cathodic) single-compartment complete-mixed electrochemical tank cell (250 cm3)/ 4.5-cm2 cylindrical Pt mesh anode/(15 × 8 cm2) carbon felt cathode/pH 3/ 0.1 mM Fe3+/I = 200 mA single-compartment complete-mixed electrochemical tank cell (300 cm3)/ 60-cm2 carbon felt cathode/platinum grid anode/pH 2.8−3/I = 30, 50, 100, and 200 mA/0.2 mM Fe3+ single-compartment complete-mixed electrochemical tank cell (125 cm3)/ 15-cm2 carbon felt cathode/platinum plate anode/pH 2.8−3/100 mA/0.5 mM Fe3+ single-compartment complete-mixed electrochemical tank cell (250 cm3)/ 4.5-cm2 cylindrical Pt mesh anode/60cm2 carbon felt cathode/pH 3/Catalyst: Ag+ Co2+, Mn2+/100, 200, and 500 mA single-compartment complete-mixed electrochemical tank cell (125 cm3)/ 20-cm2 carbon felt cathode/platinum sheet anode/pH 3/60, 100, and 200 mA/0.05, 0.1, 0.5, and 1 mM Fe2+


191

significant enhancement of 2,4-D degradation compared to electro-Fenton process

183

67

97% COD reduction after 3.5 h of electrolyses at 200 mA; comparison with a 500-cm3 photoFenton reactor (low pressure mercury lamp at 254 nm used as UV source): complete mineralization after 2 h of

182

19

191

82% TOC removal after 300 min using 0.5 mM Mn2+

93% COD reduction after passage of charge at 1000 °C

study on toxicity removal (microtox method used to measure toxicity); solution toxicity evolution as a function of evolution of intermediates

complete disappearance of DNOC after 85 min using sono-electro-Fenton process

73

175

93% mineralization of 2.4-D solution at 2000 °C with 60 mA.

100% pesticide degradation (42.6% COD removal)

192

ref

100% concentration decay in the presence of chlorides or sulfates; poor COD removal




8739


electro-Fenton (conductive diamond anode)


mixture of chlortoluro- carbofuran- bentazon/0.125 mM−0.125 mM−0.125 mM/synthetic solution with 0.05 M Na2SO4 or 0.1 M NaClO4

formulation: Mistel GD (cymoxanil + mancozeb + additives)/0.17 mM/synthetic solutions with H2SO4 (pH 3−4)


electrocoagulation + H2O2/UV oxidation

mixture of chlortoluron−carbofuran−bentazon/0.125 mM−0.125 mM−0.125 mM/synthetic solution with 0.05 M Na2SO4

methyl-parathion/0.1−0.2 mM/synthetic solution with 50 mM Na2SO4 (electrolyte assessment for HClO4, HNO3, H2SO4, and HCl)

metribuzin/50−300 mg dm−3/synthetic solution with NaCl 0.01−1 g dm−3

electrocoagulation

electro-Fenton and process similar to electro-Fenton (metal anode)

metomyl/1.23 M/synthetic solution with 0.05 M Na2SO4

metribuzin/50−300 mg dm−3/synthetic solution with 0.01−1 g dm−3 NaCl


malathion/0.34 mM/synthetic solution

technology



imazaquin/0.05−0.1 mM

Table 5. continued

cylindrical flow pass electrochemical cell equipped with bipolar iron sacrificial electrodes/3−18A m−2/pH 6 cylindrical flow pass electrochemical cell equipped with bipolar iron sacrificial electrodes/3−18 A m−2/pH 6/very small doses of H2O2 (Fenton’s reaction not promoted because of tradeoff in pH between electrocoagulation and Fenton’s reaction) single-compartment complete-mixed electrochemical tank cell (250 cm3)/ 60-cm2 carbon felt cathode/4.5-cm2 Pt anode/0.1 mM Fe3+/pH 3/I = 50, 100, and 150 mA single-compartment complete-mixed electrochemical tank cell (250 cm3)/ 60-cm2 carbon felt cathode and 12cm2 Pt grid anode/pH 2.8−3.0/1 mM Fe3+/100 mM H2O2/UV 253.7 nm/12 W/pH 3 single-compartment complete-mixed electrochemical tank cell (250 cm3)/ 60-cm2 carbon felt (cathode)/14.5cm2 BDD (anode)/pH 3/0.2 mM Fe3+/anodic current density: 210 A m−2 single-compartment complete-mixed electrochemical tank cell (150 cm3)/ carbon felt cathode/Pt anode/current intensity: 60−300 mA/pH 3−4/catalyst: 1 mM Fe3+ or Cu2+

two-compartment reactor/5.5-cm2 platinum anode/saturated calomel electrode reference/carbon felt cathode/ pH 3/Fe3+ 2 mM/Eapp = −0.55 V/ SCE single-compartment complete-mixed electrochemical tank cell (250 cm3)/ (6 × 8x0.6 cm3) carbon felt cathode/ cylindrical Pt grid anode/pH 2.8 −3/ 100, I = 200 mA/catalyst: Ag+, Co2+, Cu2+, and Fe3+

single-compartment complete-mixed electrochemical tank cell (125 cm3)/ 20-cm2 carbon felt cathode/platinum sheet anode/pH 3/I = 60, 100, 200 mA/0.05, 0.1, 0.5, and 1 mM Fe2+


160

190

92% COD removal after 5 h using Fe3+ catalyst at 300 mA 80% COD removal after 5 h of electrolysis using Cu2+ catalyst at 300 mA

83

187

170

170

186

184

67

ref

complete depletion of three pesticides in less than 80 min (98% TOC removal after 4 h of treatment)

Highest mineralization efficiency obtained using HClO4 medium; complete (100%) TOC removal obtained after 9 h treatment at 150 mA and pH 3 in HClO4 medium 100% depletion of three pesticides (94% mineralization after 6 h)

enhanced mineralization rate (95% removal after 3 h treatment) by UV irradiation

degradation rate of metomyl in 20-L Lannate depended on catalyst used: Fe3+ >Co2+ > Cu2+ > Ag+ COD removal of metomyl in 20-L Lannate depended on catalyst concentration 89% removal of metribuzin after 3 h treatment

irradiation using 1 mM H2O2 and 0.1 mM Fe2+ 97% COD reduction after 3.5 h of electrolyses at 200 mA; comparison with 500-cm3 photo-Fenton reactor (low pressure mercury lamp at 254 nm used as UV source): 96% mineralization after 3 h of irradiation with 1 mM H2O2 and 0.1 mM Fe2+ 87.3% COD abatement after passage at 6000 °C




8740


electro-Fenton (conductive diamond anode)


sulcotrione/0.1 mM/synthetic solution with 0.05 M Na2SO4

tetra-ethyl-pirophoshate/0.5 mM/synthetic solution


picloram/0.125 mM/synthetic solution with 0.05 M Na2SO4

sulcotrione/0.1 mM/synthetic solution with 0.05 M Na2SO4


phenylurea herbicides (several)/0.17 mM diuron 0.25 mM monuron 0.2 mM fenuron/synthetic solution



pentachlorophenol/0.03 mM/synthetic solutions with H2SO4 (pH 3)

propham/0.5 mM/effects of using different salts tested (0.05 M Na2SO4 or 0.1 M NaCl or 0.1 M NaNO3 or 0.1 M LiClO4 or 0.5 M K2SO4)



electrocoagulation

parathion/0.09 mM/synthetic solution

monochrotophos (MCP)/300 mg dm−3/synthetic solution with 1−6 g dm−3 NaCl

monochrotophos (MCP)/300 mg dm−3/synthetic solution with 1−6 g dm−3 NaCl


formulation: 20-L lannate (metomyl + ethanol + additives)/not available/synthetic solutions with H2SO4 (pH 3−4)

technology electro-Fenton (Pt anode)


formulation: cuprofix (cymoxanyl + zineb + additives)/0.7 mM/synthetic solutions with H2SO4 (pH 3−4)

Table 5. continued

single-compartment complete-mixed electrochemical tank cell (150 cm3)/ carbon felt cathode/Pt anode/current intensity: 60−300 mA/pH 3−4/catalyst: 1 mM Fe3+ or Cu2+ single-compartment complete-mixed electrochemical tank cell (150 cm3)/ carbon felt cathode/Pt anode/current intensity: 60−300 mA/pH 3−4/catalyst: 1 mM Fe3+ or Cu2+ single-compartment complete-mixed electrochemical tank cell/Fe electrodes/93 A m−2/pH 8.5 single-compartment complete-mixed electrochemical tank cell/Fe electrodes/93 A m−2/external addition of 2 mmol H2O2 min−1 two-compartment electrochemical reactor/5.5-cm2 platinum anode/saturated calomel electrode reference/carbon felt cathode/pH 3/[Fe3+] = 2 mM/ Eapp = −0.55 V/SCE single-compartment complete-mixed electrochemical tank cell (125 cm3)/ 10-cm2 carbon felt (cathode)/1-cm2 platinum sheet (anode)/I = 50 mA single-compartment complete-mixed electrochemical tank cell (150 cm3)/ 15 cm2 carbon felt cathode/2-cm2 Pt anode/pH 3/[Fe2+] = 0.2 mM/30− 300 mA 150-cm3 cylindrical cell/50-cm2 carbon felt cathode/cylindrical Pt gauze anode/I = 30, 60, 100, 200, 300, and 500 mA/0.1 mM Fe3+/pH 3 cylindrical electrochemical cell 0.175 dm−3/50-cm2 carbon felt (cathode)/ cylindrical Pt gauze (anode)/I = 300 mA/0.5 mM Fe3+/pH 3 single-compartment complete-mixed electrochemical tank cell (250 cm3)/ cylindrical Pt mesh anode/(14 cm × 5 cm x 0.5 cm) carbon felt cathode/pH 3/0.1 mM Fe2+/100−500 mA single-compartment complete-mixed electrochemical tank cell (250 cm3)/ 25-cm2 BDD anode/(14 cm × 5 cm x 0.5 cm) carbon felt cathode/pH 3/0.1 mM Fe2+/I = 100−500 mA double compartment electrochemical reactor/5.5-cm2 platinum anode/saturated calomel electrode reference/ carbon felt cathode/pH 3/[Fe3+] = 2 mM/Eapp = −0.55 V/SCE


ref

124

almost 100% pesticide degradation; 82% mineralization (TOC removal) after passage of charge at 1500 °C

185

184

92.1% COD reduction after passage of current at 6000 °C

185

159

189

mineralization yields ranged between 70% and 98% after 8 h using different technologies and different currents

mineralization yield values ranged between 60% and 93% after 8 h using different technologies and different currents

mineralization of picloram depended on applied current, Fe3+ and picloram concentrations/95% TOC removal at 8 h under optimal conditions almost 100% pesticide depletion (94% TOC removal after 8 h)

188

184

80.1% COD reduction after passage of charge at 6000 °C

90% COD reduction after 3 h of treatment

127

127

190

190

100% pesticide degradation (66% mineralization after 90 min)

78% pesticide removal

100% COD removal after 2 h using either iron or copper as catalysts (I = 300 mA)

complete pesticide removal using either iron or copper catalysts after 2 h of electrolysis at 300 mA




Chemical Reviews

Review

Biographies

demand for these chemicals and the absence of full-scale efficient remediation technologies means that the problem is worsening and that this subject will certainly remain at the scientific forefront in the near future. In the past, arsenic was the primary pollutant associated with pesticide use: electrocoagulation was a suitable technology for removing arsenic from water, and electrokinetic remediation was a good alternative for arsenic removal from soils. Non-negligible concentrations of many synthetic organic pesticides are currently found in numerous soils and groundwater at concentrations that sometimes exceed governmental recommendations. • The search for new technologies that can be used to remediate this significant problem is a topic of the major relevance. Promising results have been obtained using electrochemical technologies, showing that these technologies may help to solve this problem in the near future. The primary drawbacks of the scientific studies that have been published in the literature to date is that they are still at the lab-scale stage, were conducted using synthetic media (soils or water) and at pesticide concentrations that are much higher than those occurring in the natural environment. Thus, significant attention should be paid over the next few years to the differences between current studies and treating real soils or water at pesticide concentrations that are closer to those found in the environment. Significant effort should also be devoted over the next few years to scaling up these electrochemical technologies to consider important aspects such as cost and operation problems at full scale. • Lab-scale studies have shown that electrokinetic processes can be used to significantly remediate soil, either by transporting pollutants by electro-flushing or by combining these processes with bioprocesses or permeable reactive barriers. Electrokinetic processes are suitable for remediating clay soils but many factors affect the results, and more work must be done to elucidate the fundamental issues involved. The most important issues that should be studied are the effect of surfactants on the flushing of pollutants and the conditions under which electroremediation technologies do not negatively affect microorganisms but instead produce a synergistic effect. • Anodic oxidation, cathodic dechlorination, electrocoagulation, and specially the electro-Fenton process are very promising technologies for pesticide removal from water. Most of these technologies can be used to attain complete pesticide removal and a significant group of these technologies can also achieve complete mineralization of pesticides. The latter consideration is significant because intermediates can have even worse effects than the parent pesticides. Good results obtained in using these technologies means that a solution to the huge environmental issue of occurrence of pesticides in the environment is in progress, and more attention should be devoted in the future to scaling up these technologies to investigate their economic feasibility and problems arising from the scale-up.

Prof. Manuel A. Rodrigo was born in Plasencia (Spain) in 1970. He studied Industrial Chemistry at the University of Valencia, where he graduated at the top of his class in 1993. He also got a Ph.D. degree in Chemical Engineering in the University of Valencia in 1997, working on the development of automation systems for biological nutrient removal processes. In 1997, he joined the University the Castilla La Mancha as assistant professor, being responsible for starting a new research line in electrochemical engineering at the Department of Chemical Engineering. In this first electrochemical stage, his research was focused on the electrolyses of wastewaters polluted with organics. After a first postdoctoral training in the Lab of Prof. Comninellis (EPFL, Switzerland), he started working with diamond electrodes, one of the key topics in his research, in which a large study about the applicability of the technology to the treatment of actual industrial wastewaters has been carried out. In 2000, he got the position of Associate Professor at the University of Castilla La Mancha, and started working on electrocoagulation of wastewaters and also on high temperature PEM fuel cells. Afterwards, oxidants production, microbial fuel cells and soil electro-remediation have also been the focus of his research attention. In 2009 he was promoted to Full Professor of Chemical Engineering at the same University. He maintains strong consultant collaboration with many companies in energy and environmental engineering. He is author of more than 200 papers in referenced journals and books (180 in JCR with more than 4200 cites and during the last four years >500 cites/year), more than 60 technical reports for companies, five patents, and he has supervised ten doctoral theses. Currently, his H-factor is 37. At present, he is the vice-dean of Chemical Engineering in the Faculty of Chemical Sciences & Technologies of the University of Castilla La Mancha. He serves as Chairman of the Working Party of Electrochemical Engineering of the European Federation of Chemical Engineering.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 8741


Chemical Reviews

Review

mechanisms and electrochemical engineering. He has been Associate Editor of the Journal of Environment Engineering and Monitoring from 2006 to 2010. He serves as Editor of Sustainable Environmental Research and as Associate Editor of Environmental Chemistry Letters and is member of Editorial Board of several scientific journals. He has supervised 35 Ph.D. theses, published more than 160 peer-reviewed papers (with more than 3900 citations and H factor 36 (according to ISI web of Science)), three books and seven book chapters, and presented more than 200 contributions including 50 invited conferences to scientific national and international congresses.

Nihal Oturan was born in Izmir, Turkey, in 1959. She obtained a Master Degree in Chemistry 1981, Turkey, a DEA Chemistry of pollution 1985, University Paris VII, and a Ph.D. degree in Macromolecular Chemistry, University Paris XIII in 1990. Since 1991 she worked as a Research Engineer at Université Paris-Est Marne-la-Vallée. She took part of the development of electrochemical advanced oxidation processes with M. A. Oturan, specifically the electro-Fenton process with “carbon-felt cathode”. Currently, she manages the Environmental Chemistry team of the laboratory Géomatériaux et Environnement of IFSA (Francilien Institute of Applied Sciences). Her research activities mainly concern the analysis of organic pollutants by different chromatographic techniques and mass spectroscopy, and the development and application of electroand photo-Fenton processes to treatment of wastewater contaminated with pesticides, synthetic dyes, pharmaceuticals, and personal care products. She participated to the supervision of 19 Ph.D. and 26 Ms.Sci. theses, published more than 70 peer-reviewed papers with more than 2000 citations and H factor 27 (according to ISI web of Science), and presented more than 40 communications in scientific congress.

ACKNOWLEDGMENTS M.A.R. would like to acknowledge Université Paris Est for granting him the position of invited professor, thereby enabling him to write this review. Financial support from the Spanish government throught Project CTM2013-45612-R is gratefully acknowledged. LIST OF ABBREVIATIONS 2,4-D, 2,4-dichlorophenoxyacetic acid AOPs, advanced oxidation processes β-CD, beta-cyclodextrine BDD, boron-doped diamond anode COD, chemical oxygen demand EK, electrokinetic EKSR, electrokinetic soil remediation EU, European Union DDT, dichlorodiphenyltrichloroethane DEET, N,N-diethyl-3-methylbenzamide DMS, N,N-dimethylsulfamid DNOC, 4,6-dinitro-o-cresol DSA, dimensionally stable anode GAC, granular activated carbon HAc, acetic acid HCB, hexachlorobenzene i-TECB, 1,2,4,5-tetrachlorobenzene LPRO, low pressure reverse osmosis MCP, monochrotophos NF, nanofiltration PCB, pentachlorophenol PRBs, permeable reactive barriers PTFE, carbon/polytetrafluoroethylene SDBS, sodium dodecyl benzene sulfonate TCB, 1,2,3-trichlorobenzene TOC, total organic carbon WHO, World Health Organization WTF, water treatment facilities

Mehmet A. Oturan was born in Tunceli, Turkey, in 1950. He obtained a Chemical Engineer degree in 1973, MS degree in 1975 and his Ph.D. degree in Analytical Electrochemistry under the supervision of Prof. A. Yildiz in 1979 at the Hacettepe University in Ankara before becoming Associate Professor at the same university. He undertook a postdoctoral stay in 1981 to complete his studies in electrochemistry at the Université Paris VII-Denis Diderot under supervision of Prof. J. M. Savéant in the field of organic electrochemistry. From 1982 to 1989, he worked as contracted lecturer−researcher at the Université Paris VII-Denis Diderot and researcher in pharmaceutical industry. In 1989, he joined the newly-formed Université de Marne-la-Vallée as Associate Professor. There he developed the electro-Fenton process in its “carbon-felt cathode” version. After obtaining his HDR (ability to supervise researches) degree in 1997, he became full Professor at the Université de Marne-la-Vallée (currently Université Paris-Est) in 2002. At present, Professor of Exceptional Class, he is Headmaster of the Département de Géomatériaux at the IFSA (Institut Francilien des Sciences Appliquées), Head of the laboratory Géomatériaux et Environnement (LGE), leader of the Environmental Chemistry research group, head of the master Géo- Environnement and VicePresident of the French group of pesticides. He is also member of National University Council (31st section), Board member of Scientific Council of Université Paris-Est Marne-la-Vallée and Board member of the Doctoral Scholl SIE (Science, Engineering and Environment) of Université Paris-Est. His research activities mainly focus on the generation of radical species in a catalytic way by means of electrochemical, chemical or photochemical techniques and its use in environmental chemistry including AOPs, chemical kinetics and

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