Treatment of soil-washing effluents polluted with herbicide oxyfluorfen

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Treatment of soil-washing effluents polluted with herbicide oxyfluorfen by combined biosorption -electrolysis Khaoula Chair, Ahmed Bedoui, Nasr Bensalah, Cristina Saez, Francisco Jesus Fernandez Morales, Salvador Cotillas, Pablo Ca#izares, and Manuel Andrés Rodrigo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04977 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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Industrial & Engineering Chemistry Research

1

Treatment of soil-washing effluents polluted with herbicide oxyfluorfen by combined

2

biosorption -electrolysis

3 4

Khaoula Chair1, Ahmed Bedoui1, Nasr Bensalah2, Cristina Sáez3, Francisco J. Fernández-

5

Morales3, Salvador Cotillas4, Pablo Cañizares3, Manuel A. Rodrigo3,*

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1

7

Tunisia

8

2

9

University, 2713 Doha, Qatar.

Department of Chemistry, Faculty of Sciences of Gabes, University of Gabes, Gabes,

Department of Chemistry and Earth Sciences, College of Arts and Science, Qatar

10

3

11

Enrique Costa Building, Campus Universitario s/n, 13071 Ciudad Real, Spain

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4

13

Universitario s/n, 02071 Albacete, Spain

Department of Chemical Engineering, Faculty of Chemical Sciences & Technologies.

Department of Chemical Engineering, School of Industrial Engineering, Campus

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Abstract

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In this work, the treatment of soil-washing effluents polluted with herbicide oxyfluorfen is

17

studied using a combined process consisting of biosorption and electrolysis. Results show that

18

oxyfluorfen is very efficiently removed from synthetic soil by soil washing with SDS. The

19

effluent can be treated by biosorption with fresh activated sludge coming from a municipal

20

wastewater treatment plant and the maximum adsorption capacity of this activated sludge was

21

found to be 18 mg oxyfluorfen g-1 biomass. Biosorption fits well to a type I adsorption

22

isotherm. Effluents of the biosorption process underwent anodic oxidation (AO),

23

photoelectrolysis (PE) and sonoelectrolysis at high (HF-SE) and low frequency (LF-SE). The

24

four technologies were able to mineralize completely the effluent, although important

25

differences arose during the treatment which depend significantly on the application of

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ultrasound (US) or ultraviolet (UV) irradiation and on the release of sulfate from the oxidation

2

of the sodium dodecyl sulfate (SDS): intermediates were removed faster because of the

3

activation of sulfate radicals. Oxyfluorfen and its oxidation intermediates are removed faster

4

than SDS and when they are fully depleted, there are still large concentrations of SDS in the

5

treated solution. This opens the possibility of reusing the soil washing fluid.

6

7

Highlights

8

Biosorption of SWF polluted with oxyfluorfen fits well to Type I isotherm

9

Combined biosorption - electrochemical technologies can treat successfully SWF

10

Release of sulfate by oxidation of SDS with a relevant role in performance of electrolysis

11

Relevant role of the irradiation of UV or application of US in the performance of electrolysis

12 13 14

Keywords

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Oxyfluorfen; biosorption; electrolysis; photoelectrolysis; sonoelectrolysis; soil washing

16 17

1. Introduction

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Herbicides are chemical substances widely used in agriculture to improve productivity of

19

crops1. They have an outstanding importance and without them, crop yield could drop hugely

20

and food prices would increase, at least, in the same ratio. Typically, herbicides are

21

xenobiotic, toxic and difficult to be biodegraded. They are strongly adsorbed to different types

22

of soil, but after a certain period they can be leached into surface or groundwater.

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Consequently, they can cause serious problems to human health and environment.

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Currently, one of the most applied herbicides is oxyfluorfen, which is a diphenyl ether used to

25

control certain annual broadleaf and grassy weeds. Oxyfluorfen is moderately persistent in 2 ACS Paragon Plus Environment

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soil 2-4, with a half-life of about 30 to 40 days. It has low water solubility (0.116 mg dm-3 at 20

2

°C), and low vapor pressure (0.026 mPa at 25 °C). Its runoff may impact seriously water

3

reservoirs

4

system disorders, birth defects, genetic mutation, sterility and even death7. It is removed

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naturally by photodegradation8 and bioremediation9-11 and typically, it is very well-sorbed to

6

most soils12,

7

oxyfluorfen can be leached into surface and groundwater. Then, it is necessary to search for

8

effective methods capable to completely remove this pollutant from soils as fast as possible,

9

before their runoff reaches natural water bodies.

5, 6

and it is associated to serious health problems such as cancer, central nervous

13

. However, the low rate of these processes may explain that a part of

10

Soil washing can face easily the removal of oxyfluorfen from soils, because it is a low-cost,

11

very simple and rapid process, which is operated mild operation conditions 14-16. Surfactants-

12

aided soil washing (SASW) can completely remediate soils contaminated with organic

13

pollutants transferring the pollution of soil to a washing fluid, which becomes a highly

14

polluted wastewater containing a complex mixture of dissolved organics and surfactants,

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colloids and inorganic salts. Recently, biosorption has gained an increasing interest for the

16

removal of organic and inorganic pollutants from different wastewater

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involves the use of biological materials that extract organic pollutants from soils using their

18

ligands or functional groups. The use of biosorbents is a very competitive approach because it

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is inexpensive, green and they have high adsorption capability. The treatment of soils

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contaminated with herbicides by biosorption can transfer the pollutants from soil washing

21

effluents to water that can be easily treated. The resulting wastewater containing herbicides

22

can be treated by advanced oxidation processes. Advanced oxidation processes are one of the

23

most effective processes for cleaning polluted herbicide wastewater19. These processes are

24

highly efficient methods for wastewater treatment due to its ability to mineralize a wide range

25

of recalcitrant organic contaminants by generating highly reactive hydroxyl radicals.


17, 18

. Biosorption


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Particularly, electro-oxidation process has received increasing attention due to its ability to

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reach complete mineralization of organics into carbon dioxide, water, and inorganics ions15, 20,

3

21

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This work aims to investigate the remediation of soils spiked with oxyfluorfen by

5

combination of surfactant-aided soil washing and biosorption on activated sludge. The

6

wastewater produced by soil washing/biosorption of spiked soils is then treated by

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electrochemical anodic oxidation (AO) and electrochemically irradiated oxidation using AO

8

with Boron Doped Diamond (BDD) anodes combined with UV light irradiation (PE) and low

9

and high frequency ultrasounds (LF-SE, HF-SE). This process was previously evaluated for

.

22

10

herbicide 2,4-D

and in this case, the main difference is the much lower solubility of

11

oxyfluorfen, which requires the use of surfactants in the soil washing fluid to drag efficiency

12

the pollutant and produces a waste with a significant concentration of micelles.

13 14

2. Materials and methods

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Chemicals. In this work, clay was selected as model of soil. Oxyfluorfen was selected as a

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model of organic compounds and it was purchased from Sigma-Aldrich. Deionized water

17

was used to prepare all solutions. Sodium Dodecyl Sulfate (Fluka, Spain) used as solubilizing

18

agent while hexane and ethyl acetate (Sigma-Aldrich, Spain) were used as solvent for

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extraction of liquid and solid samples.

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Preparation of synthetic groundwater. Synthetic groundwater was used as the solvent of

21

the soil washing process. It was prepared by dissolving 6.671 g of MgSO4·7H2O, 1.318 g of

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NaCl, 1.302 g of NaNO3, 0.255 g of KI and 2.497 g of CaCO3 in 10 dm3 of deionised water.

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Sodium Dodecyl Sulfate (SDS) from 5 to 20 g dm-3 was added as solubilizing agent.

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Preparation of spiked soil. The preparation of soil has been described in the literature

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elsewhere23-25. Polluted soil was prepared by dissolving oxyfluorfen in hexane and then


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mixing this solution with clay soil. The spiked clay was aerated for 1 day to favor evaporation

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of the hexane. In this way the oxyfluorfen was homogeneously distributed on the clay surface.

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The resulting oxyfluorfen concentration in the soil was 0.5 g kg-1 of soil. Table 1 shows the

4

main characteristics of the soil used in this work.

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Table 1. Classification and composition of soil by USCS. Mineral

%

Glauconite

24

Kaolinite

23

Montmorillonite

20

Quartz

12

Muscovite

8

Feldspar

6

Illite

6

Calcite

1

Smectite

-

Parameters Liquid limit

42

Plastic limit

24

Plasticity index

18

USCS code

CL

Low plasticity clay 6 7 8

Soil washing procedure. Soil washing with SDS solutions was carried out in a 20 dm3 stirred

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tank and operated in discontinuous mode. 2 kg of polluted soil (0.5 g of oxyfluorfen kg-1 of



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soil) was stirred with 8 dm3 SDS solution for 24 hours at a stirring rate of 150 rpm. Then, the

2

mixture was decanted for 12 hours to separate the soil from the aqueous phase.

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Biosorption of soil-washing effluents. Batch biosorption experiments were performed in

4

order to evaluate the potential application of this technology for herbicides removal from

5

liquid samples. In each experiment, fresh activated sludge from a conventional Wastewater

6

Treatment Facility located in Ciudad Real, was added to the soil washing fluid (SWF). Before

7

each test, the batch reactors were purged with nitrogen in order to remove the oxygen from

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the gas phase. Working in this way, the removal of herbicide by means of aerobic oxidation of

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herbicide was avoided. The batch reactors were located in an incubator in order to maintain

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the temperature at about 20ºC. The batch reactors were mixed by means of a magnetic stirrer

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rotating at 100 rpm in order to maintain the biomass suspended, but minimizing mass transfer

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between the liquid and gas phases. The ratio biomass/herbicide used was 20. Once finished

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the adsorption tests, after 6 h of contact time, the biomass was separated from the solutions by

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means of filtration through a glass fiber filter. Liquid phase was then treated by

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electrochemical, sonoelectrochemical and photoelectrochemical processes.

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The herbicide recovered and sorbate uptake was calculated with reference to concentrations

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detected in non-inoculated media. The sorbate uptake was calculated using Eq. (1), where q

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corresponds to the sorbate uptake, V is the liquid volume (L), Ci and Cf are the initial and

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final herbicide concentrations and S is the sorbent amount (g).

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q=

V(Ci -Cf ) S

(1)

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Electrochemical, sonoelectrochemical and photoelectrochemical oxidation of the

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biosorption effluents. Electrochemical oxidation experiments were carried out in a bench-

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scale plant with a single compartment electrochemical flow. Bulk oxidations were performed

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in a single-compartment cell, as described in previous works


24

. BDD and steel electrodes

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were used as anode and cathode, respectively. Characteristic of BDD are as follows: sp3/sp2

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ratio: 225; boron content: 500 ppm; width of the diamond layer: 2.68 µm. For the

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electrochemical flow cell, inlet and outlet were provided for effluent circulation through the

4

reactor; the simulated effluent was stored in a thermo-regulated glass tank (2000 cm3) and

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circulated through the cell using a peristaltic pump at a flow rate of 200 dm3 h−1. The

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electrical current was applied using a DC Power Supply (FA-376 PROMAX). Temperature

7

was kept constant by means of a water bath. In the case of photoelectrochemical process the

8

same procedure was applied combined by UV lamp (VL-215MC (Vilber Lourmat), λ = 254

9

nm, intensity of 930 µW cm-2 and energy 4.43-6.20 eV irradiated 15 watts directly to the

10

quartz cover light). In the sonoelectrochemical processes an ultrasound generator was

11

introduced in the tank reservoir working at 24 kHZ or 100 MHz.

12

Analytical methods. pH and conductivity were measured with an InoLab WTW pH-meter

13

and a GLP 31 Crison conductivimeter, respectively. Samples at different electrolysis,

14

sonoelectrolysis and photoelectrolysis times were filtered with 0.20 µm Nylon filters before

15

analysis. Oxyfluorfen and all intermediates generated were identified by HPLC–UV (Agilent

16

1100 series) (detection limit: 0.01 mg dm-3). The analytical column used was type

17

Phenomenex Gemini 5 lm C18 and it was maintained at room temperature. A gradient elution

18

was applied using the mobile phase formed by 25 mM of formic acid water solution (Solvent

19

A) and acetonitrile (Solvent B). A lineal gradient was obtained by initially running 10% of

20

Solvent B ascending to 100% in 40 min. The injection volume was 20 µL and UV detector of

21

HPLC–UV was performed at 291 nm. Ionic species (NO3-, SO42-, Cl-) were measured by ion

22

chromatography using a Shimadzu LC-20A equipped with a Shodex IC I-524A column

23

(anionic species); mobile phase, 2.5 mM phthalic acid at pH 4.0; flow rate, 1·10-3 dm3 min−1

24

(concentration accuracy: ± 0.5 %). The removal of total organic carbon (TOC) was



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determined with a Multi N/C 3100 Analytik Jena TOC analyzer. The particle size was

2

measured using a Mastersizerhydro 2000SM from Malvern26.

3

3. Results and Discussion

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Production of the soil-washing wastewater. Surfactant-aided soil washing (SASW) is a

5

well-known soil remediation technology. It can be used to transfer pollutants from the

6

contaminated soil to a soil washing fluid (SWF), which has to be further treated with an

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efficient wastewater treatment technology to remove definitely the pollution. Hence, SASW

8

remediates the soil and thus, it transforms the problem of soil remediation into a

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hypothetically easier wastewater treatment problem. Typically, the wastewater generated in

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this process exhibits a complex composition, which depends on the organic and inorganic

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species contained in the soil and also on the composition of the soil washing fluid used to

12

extract the pollutants. For the treatment of soil polluted with non-polar compounds,

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surfactants are typical components of the SWF. Consequently, the effluents obtained after the

14

SASW treatment consist of emulsions of micelles (made of pollutant and surfactant), low

15

concentrations of the pollutant (related with its solubility in water) and varying concentrations

16

of free surfactant (not linked with micelles) that depend on the dose used in the soil washing

17

processes and on the critical micellar concentration. The concentration of surfactant contained

18

in the SWF should be high enough to warrant an efficient transfer of the pollutant from soil to

19

water but low enough for avoiding the production of a highly loaded liquid waste. For this

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reason, in order to reduce the organic load of washing effluents, it is needed to determine the

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optimum amount of surfactant required in each case, suitable to extract the pollutant

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completely but not much larger than this value.

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Taking into account these comments, Figure 1 shows the amount of herbicide extracted from

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the soil as a function of the ratio mass of surfactant / mass of soil used in each soil-washing

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process. 8 ACS Paragon Plus Environment

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120 100

OXY extracted / %

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80 60 40 20 0 0

0.02

0.04

0.06

0.08

0.1

SDS/soil / g g-1

1 2

Figure 1. Oxyfluorfen extracted from soil as a function of the SDS/soil ratio.

3

As it can be observed, oxyfluorfen is not transferred to the liquid phase in the case of using

4

SWF consisting of raw water or low concentrated SDS solutions. In fact, the extraction

5

increases exponentially with the mass ratio SDS/soil. This means that soil has high retention

6

capability and a minimum amount of surfactant must be added to start dragging the herbicide.

7

Based on these results, and in order to attain 100 % oxyfluorfen extraction, it was decided to

8

use the ratio 0.08 g of SDS g-1 of soil to wash the polluted soil (500 mg oxyfluorfen kg-1 of

9

soil). The SWF obtained after 24 h of washing and 1.0 h of sedimentation (to separate solid

10

and liquid phases) contained 74 mg dm-3 of oxyfluorfen, around 2700 mg dm-3 of TOC and

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4500 mg dm-3 of COD (most of the organic content is associated to the surfactant). As

12

expected, the organic load of this effluent is high. However, TOC and COD values are lower

13

than those expected taking into account the high concentrated SDS solution used (20 g SDS

14

dm-3) and thus, this indicates that around 75 % of the SDS used was retained in the soil during

15

the soil washing procedure.

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Biosorption of soil-washing fluids. The effluent obtained from the washing of the polluted

17

soil was filtered. Then, in order to study the herbicide removal by means of biosorption, 9 ACS Paragon Plus Environment


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several tests were carried out at different BHR (biomass to herbicide ratio), within the range

2

from 0 to 35 mg biomass/mg oxyfluorfen, by mixing activated sludge with the SWF for 60

3

min. Figure 2 shows the concentration of oxyfluorfen and COD in the effluent of this short

4

biological treatment. It is important to remark that activated sludge was not acclimated to this

5

herbicide, but just used as taken from the biological reactors of a municipal WWTP.

6 7

Figure 2. COD (●) and oxyfluorfen (○) concentration of the effluent of the biosorption

8

process obtained with different ratio biomass/herbicide

9

There is a clear influence of the BHR on the concentrations of herbicides and COD.

10

Regarding the herbicide, the higher the biomass dosed to the soil-washing fluid, the lower is

11

the oxyfluorfen detected in the effluent after the biological tests. The oxyfluorfen removal fits

12

well to an exponential decay, which explains the successful decrease of the initial

13

concentration of more than six times with the highest BHR tested (down to 10 mg dm-3).

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Regarding the rate of the process, it was observed that in each test most of the oxyfluorfen

15

was removed within 45 minutes (data not shown) and then concentration stabilizes. Taking 10 ACS Paragon Plus Environment

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1

into account that biological culture was not acclimated to this type of wastewater, this fast

2

removal which ends up with a stabilization in the concentration indicates that almost no

3

degradation in the biomass is taking place. Thus, in literature

4

the biodegradation of the oxyfluorfen takes several days. Therefore, the adsorption of

5

oxyfluorfen over the activated sludge seems to be the primary mechanisms for the removal.

6

Regarding the concentration of COD in the effluent of the biological process, a different

7

behavior is observed as compared to the oxyfluorfen evolution. In this case, COD increases

8

slightly with the biomass load dosed. This increase can be explained in terms of the soluble

9

COD of the biomass added, which obviously increases as the ratio biomass/herbicide

27, 28

, it has been reported that

10

increases.

11

The oxyfluorfen adsorbed onto the biomass was determined by mass balance, using the

12

concentration of oxyfluorfen in the solution at the beginning of the experiment and once the

13

steady state was reached. This information is plotted in Figure 3 vs. the BHR.

14 15

Figure 3. Effect of biomass loading on herbicide biosorption. 11 ACS Paragon Plus Environment


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An increase in the EBR from 0.0 to 7.5 mg mg-1 apparently does not produce any herbicide

2

removal, pointing out that a minimum concentration of biomass should be added to produce

3

an efficient removal of the pollutant. From this value on, there is a linear decrease of the

4

concentration of oxyfluorfen with the BHR. To understand better these results, the pseudo-

5

equilibrium isotherm was obtained and plotted in Figure 4. As mentioned before, the contact

6

time between the herbicide and the biomass was very short, ensuring that the pseudo steady-

7

state values reached were only due to the biosorption equilibrium and not associated to

8

biodegradation.

9 10

Figure 4. Equilibrium isotherm at 20º C of herbicide oxyfluorfen onto activated sludge

11

biomass.

12

Experimental data define a typical type I adsorption isotherm. These results were fitted to the

13

Langmuir model obtaining a quite good fitting (r2=0.997). The maximum adsorption capacity

14

was obtained for oxyfluorfen concentrations in solution higher than 40 mg dm-3, and was

15

close to 18.0 mg oxyfluorfen g-1 biomass. This specific biosorption value compares favorably

16

with other results reported in literature for other herbicides

22, 29


. However, when they are

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1

compared with the results obtained in a previous work about 2,4-D published recently by our

2

group, they are quite similar when they are expressed in mmol g-1 biomass. Thus, the

3

maximum adsorption capacity of the activated sludge with 2,4-D was found to be 0.054 mmol

4

g-1 biomass (11.9 mg oxyfluorfen g-1 biomass) while the adsorption capacity with oxyfluorfen

5

was 0.050 mmol g-1 biomass.

6

Electrolysis of the biosorption effluents.

7

Once biosorption has been checked for the removal of oxyfluorfen from SWF, the resulting

8

effluent presented significant concentrations of herbicide and organic matter. Therefore, it is

9

necessary to carry out a more efficient post-treatment that allow a complete removal of the

10

herbicide and a total mineralization of the organic matter. In this context, electrochemical

11

technologies were assessed for the treatment of the effluent from biosorption process. Figure

12

5 shows the changes in pollutant concentration and TOC during the anodic oxidation (AO),

13

photoelectrolysis (PE) and sonoelectrolysis at high (HF-SE) and low frequency (LF-SE) of

14

the effluent of the biosorption process using a BHR of 20.



1.2

OXY / OXY0

1 0.8

a)

0.6 0.4 0.2 0 0

20

40

60

80

100

120

140

Q / Ah dm-3 3500 3000 2500

TOC / TOC0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b)

2000 1500 1000 500 0 0

20

40

60

80

100

120

140

Q / Ah dm-3

1 2

Figure 5. Changes in the Oxyfluorfen concentration (a) and TOC (b) during the

3

electrochemical treatment of the effluent of combined soil washing and bio-adsorption

4

process. (■) AO, (□) PE, (▲) HF-SE. (∆) LF-SE. Current density: 30 mA cm-2; anode: BDD;

5

cathode: SS; UV254 nm: 4 W; US high frequency: 100 MHz and US low frequency: 24 kHz.

6

7

As it can be observed, oxyfluorfen is rapidly and effectively oxidized by the four

8

electrochemical advanced oxidation processes. Applied electric charges below 50 Ah dm-3 are

9

required to decrease the concentration of herbicide more than 3 log-units from the initial 74

10

mg dm-3 down to 0.1 mg dm-3, regardless of the technology evaluated. In fact, data are

11

overlapped in the four tests carried out, suggesting very small differences. These results 14 ACS Paragon Plus Environment

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1

indicates that oxyfluorfen contained in the effluent of the combined SASW-biosorption

2

process can be very easily oxidized, as it was also found in previous studies in which the

3

degradation of SASW effluents without the biological step were carried out15, 20. In those

4

studies, the matrix of species was much simpler than in the present work because, it did not

5

contain the metabolites and nutrients added with the activated sludge during the biosorption

6

stage, which could become a key difference in terms of treatability.

7

When the results of the application of the different EAOP are compared in terms of TOC

8

removal, greater differences arise during the treatment, although mineralization attained at the

9

end of the tests was almost the same for the four technologies, meaning that all EAOPS tested

10

lead to similar degree of treatment at large electric charges passed. The electric charges

11

required to attain the complete mineralization of the organic load are more than the double of

12

those required for oxyfluorfen removal. This means that intermediates and oxidation of other

13

species contained in the effluent of the biosorption process are important and they affected in

14

a different way (or with a different rate) by the different technologies evaluated.

15

It is important to point out that the initial TOC concentration measured (around 2700 mg of C

16

dm-3) is mainly related to the presence of SDS, which is significantly higher than the carbon

17

concentration related to oxyfluorfen (initial oxyfluorfen concentration corresponds to around

18

20 mg C dm-3) and it is also higher than the concentration of soluble metabolites added with

19

the activated sludge during the biosorption process (which totalizes only of few mg dm-3 of

20

C). Thus, initially the differences found between the changes observed in the pollutant

21

removal and mineralization should be explained in terms of the degradation of surfactant

22

molecules and also to the formation of reaction intermediates.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Regarding the surfactant, as it is known, SDS consists of a linear hydrocarbon tail attached to

2

a sulfate group that can be easily attacked and released to the solution. Figure 6 shows the

3

increase in the sulfate concentration during the electrochemical treatments.

4

It is important to remark that initially the effluent of bio-adsorption process contains relevant

5

amount of sulfate related to the presence of MgSO4 in the SWF used (made with synthetic

6

groundwater) and also to the sulfate contained in the mixed liquor of the activated sludge

7

used. Anyway, as expected, the concentration of sulfate increases during the process

8

indicating the attack of SDS molecule occurs, but not relevant differences are observed

9

among the different electrochemical technologies evaluated. In addition, a very important

10

observation that can be made is that transformation of SDS into sulfate is quantitative (100%

11

transformation) and this fact supports the total mineralization of the molecule as the evolution

12

of TOC pointed out before.

13

14


Page 16 of 28

Page 17 of 28

40

∆Sulphate / mmol dm-3

35 30 25 20

a)

15 10 5 0 0

20

40

60

80

100

120

Q / Ah dm-3 100

10

Persulfate / mmol dm-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

b)

0.1

0.01

0.001 0

20

40

60

80

100

120

Q / Ah dm-3

1 2

Figure 6. Concentration of sulfate ions released (a) and of persulfate generated (b) during the

3


4

process. (■) AO, (□) PE, (▲) HF-SE, (∆) LF-SE. Current density: 30 mA cm-2; anode: BDD;

5


6

7

Sulfate is a very important species in the electrolysis of wastewater with diamond anodes,

8

because it is easily oxidized to peroxodisulfate which, in turn can be transformed into sulfate

9

radical by activation

10

19, 21

with (1) other oxidants such as hydrogen peroxide, ozone or even

other persulfate molecules, (2) UV light and (3) Ultrasounds 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Concentrations of sulfate suggests that formation of this species is possible in the system. In

2

fact, the concentration of persulfate was monitored (Figure 6b) and, as observed, it varies

3

during the treatment within 0.03 ± 0.01 mmol dm-3. Regarding the sulfate radical, due to its

4

very short lifetime, its concentration is difficult to be observed directly (only by using

5

mediated methods), although it may have a direct influence on the distribution of

6

intermediates. Obviously, its impact is expected to be higher in the case of irradiated EAOP,

7

where the activation is promoted30. In this point, it is important to remark that the effect of

8

sulfate radicals, generated by the activation of persulfates, on the removal of organics has

9

been previously described by elsewhere31. In fact, differences between the different

10

technologies are greater during the treatment, and this variability supports the interactions of

11

the oxidant with the organic intermediates contained in the SWF which was being treated.

12

Figure 7 shows the changes in the concentration of different ionic species during the photo

13

assisted electrolytic technology (PE). Changes in the other technologies (AO and

14

sonoelectrolysis) are very close to those shown in this Figure and hence they were not

15

included for the sake of clarity.


Page 18 of 28

Page 19 of 28

Chloro-species / mmol dm-3

5 4.5 4 3.5 3 2.5 2

a)

1.5 1 0.5 0 0

20

40

60

80

100

Q / Ah dm-3

5

Nitro-species / mmol dm-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.5 4 3.5 3 2.5

b)

2 1.5 1 0.5 0 0

20

40

60

80

100

Q / Ah dm-3

1 2

Figure 7. Concentration of Chlorine-species (a) and nitro-species (b) during the

3


4

process. (■) AO, (□) PE, (▲) HF-SE, (∆) LF-SE. Current density: 30 mA cm-2; anode: BDD;

5

cathode: SS; UV254 nm: 4 W; US high frequency: 100 MHz and US low frequency: 24 kHz. a)

6

(□) Cl-, (▲) ClO3-, (∆) ClO4-; b) (▲) NO3-, (□) NH4+.

7 8

As it can be observed in part a, chlorine, chlorate and perchlorate were detected in the

9

reaction media. As it is well-known, chlorine is rapidly oxidized to hypochlorite which can be

10

further oxidized during diamond-electrolysis to chlorate and finally to perchlorate as main

11

final product. The concentration of hypochlorite in detected in the reaction system was nil,

12

indicating that once formed it rapidly reacts to form chlorate, to oxidize organic compounds

13

or even to form chloramines by reaction with ammonium ions. In fact, this can help to explain 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

the rapid decrease observed in the concentration of ammonium ions (part b of the figure) just

2

at the beginning of the tests. From these initial values, both ammonium and nitrate

3

concentration increases continuously during electrolysis. This can be related to the oxidation

4

of the metabolites added with the activated sludge during the biosorption process. In a

5

previous work32, it was demonstrated that electrochemical oxidation of organic nitrogen lead

6

directly to the formation of nitrates. These species are cathodically reduced to ammonium,

7

which in turn can react with hypochlorite to form chloramines and finally gaseous nitrogen.

8

Figure 8 shows the changes in the concentration of the main intermediates detected by HPLC,

9

produced during the tests carried out with the four EAOP technologies. Only two

10

intermediates were found and they were always the same (although in different

11

concentrations), indicating that the primary mechanism of SDS-oxyfluorfen solutions

12

degradation is the same, regardless the use or absence of irradiation (UV or US). However,

13

the rates are slightly different and depends on the technology used. Likewise, in all cases they

14

are completely degraded for electrical charges below 40 Ah dm-3. On the other hand, it is

15

important to remark the presence in the solution of xylene. This is not a reaction intermediate

16

but an organic present in the commercial oxyfluorfen suspension used as raw material to

17

pollute the soil. As can be observed, it is rapidly degraded in all cases. These results are very

18

interesting because, in combination with those shown in Figure 5, they inform that there is a

19

possibility of partial treatment in which a significant part of the surfactant can be recovered

20

for further use, instead of being totally transformed into carbon dioxide as the raw herbicide

21

and the intermediates are depleted in a moment in which there are still important amounts of

22

SDS in solution.


Page 20 of 28

Page 21 of 28

Intermediates / area units

80000 70000 60000 50000

a)

40000 30000 20000 10000 0 0

20

40

60

80

Q / Ah dm-3

100

120

100

120


60000 50000 40000

b)

30000 20000 10000 0 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

60

80

Q / Ah dm-3

100000 90000 80000 70000 60000 50000 40000 30000 20000 10000 0

c)

0

1

20

20

40

60

80

100

120

Q / Ah dm-3

2

Figure 8. Profile of the main intermediates (quantified as chromatographic area) during the

3

(■) AO, (□) PE, (▲) HF-SE, (∆) LF-SE of the effluent of the combined soil washing bio-

4

adsorption process. a) i1, b) i2 and c) xylene. Current density: 30 mA cm-2; anode: BDD;

5


6



1

In this point, it is also important to check the behavior of the emulsion of

2

oxyfluorfen/surfactant during the combined treatment, to evaluate the break-up of the

3

emulsion and the depletion of micelles. To assess this point, particle size distribution was

4

measured to the final sample of each process. Figure 9 shows the particle concentration and

5

the mean particle size of the effluent after each single treatment. As it can be observed, the

6

amount of particle does not decrease significantly during biosorption treatment and a higher

7

dispersion in size particle is observed, which can be explained in terms of the addition of

8

sludge flocs. In addition, mean particle size is slightly higher after this treatment. This

9

situation reverses when electrolysis is applied and both the number of particles and the mean size decreases very importantly, being clearly less efficient the LF-SE treatment.

0.2

900

0.18

800

0.16

700

0.14

600

0.12

500

0.1 400

0.08

300

0.06 0.04

200

0.02

100

0

Mean size / µm

10

Particle concentration / % Volume

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

0 Washing BIOsorp 1 2

3AO

PE 4

HF-SE 5

LF-SE 6

11 12

Figure 9. Particle concentration (solid bars) and mean size (discrete points) of the effluent of

13

the different stages used in the combined process (washing+biosorption+electrochemical

14

treatment). Current density: 30 mA cm-2; anode: BDD; cathode: SS; UV254 nm: 4 W; US high

15

frequency: 100 MHz and US low frequency: 24 kHz.

16

17 22 ACS Paragon Plus Environment

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

4. Conclusions

2

From this work, the following conclusions can be drawn:

3

− The use of SDS solutions are required to extract oxyfluorfen during washing process

4

of polluted soil. Relevant amount of SDS are retained in the soil after the treatment.

5

− A rapid removal of oxyfluorfen from soil-washing fluids is attained by biosorption

6

with maximum adsorption capacity around 18 mg oxyfluorfen/g biomass.

7

− A combination of biosorption onto activated sludge and electrochemical processes

8

with diamond anodes can attain the complete removal of oxyfluorfen at not very high

9

current charge applied. For this reason, reuse of SDS solution for the SWF can

10

promising. Higher electrical charges are required to fully degrade SDS and attain

11

complete mineralization.

12

− Oxyfluorfen removal during electrochemical processes does not depend significantly

13

on the US or UV irradiation. However, marked differences are observed in the case of

14

mineralization because intermediates removed are faster oxidized in the case of

15

applying US or UV light. Release of sulfate from the oxidation of SDS, oxidation of

16

this anion to persulfate and activation by US or UV light can help to explain these

17

results.

18

19

Acknowledgements

20

The authors acknowledge funding support from the EU and Spanish Government through the

21

MINECO

22

(Sustertech4ch), FEDER 2007-2013 PP201010 (Planta Piloto de Estación de Regeneración de

23

Aguas Depuradas) and INNOCAMPUS.

Project

CTM2013-45612-R

(Electrotech4pest),


CTM2016-76197-R


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

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Treatment of soil-washing effluents polluted with herbicide oxyfluorfen

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