Micelles as Soil and Water Decontamination Agents - Chemical

Review pubs.acs.org/CR

Micelles as Soil and Water Decontamination Agents Afzal Shah,*,† Suniya Shahzad,† Azeema Munir,† Mallikarjuna N. Nadagouda,‡ Gul Shahzada Khan,§ Dilawar Farhan Shams,∥ Dionysios D. Dionysiou,⊥ and Usman Ali Rana# †

Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan Department of Mechanical and Materials Engineering, Wright State University, Dayton, Ohio 45324, United States § Department of Chemistry, Shaheed Benazir Bhutto University, Sheringal, Dir (Upper), 18000 Khyber Pakhtunkhwa, Pakistan ∥ Department of Environmental Sciences, Abdul Wali Khan University Mardan, 23200 Khyber Pakhtunkhwa, Pakistan ⊥ Environmental Engineering and Science Program, Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012, United States # Sustainable Energy Technologies Center, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia ‡

ABSTRACT: Contaminated soil and water pose a serious threat to human health and ecosystem. For the treatment of industrial effluents or minimizing their detrimental effects, preventive and remedial approaches must be adopted prior to the occurrence of any severe environmental, health, or safety hazard. Conventional treatment methods of wastewater are insufficient, complicated, and expensive. Therefore, a method that could use environmentally friendly surfactants for the simultaneous removal of both organic and inorganic contaminants from wastewater is deemed a smart approach. Surfactants containing potential donor ligands can coordinate with metal ions, and thus such compounds can be used for the removal of toxic metals and organometallic compounds from aqueous systems. Surfactants form host−guest complexes with the hydrophobic contaminants of water and soil by a mechanism involving the encapsulation of hydrophobes into the self-assembled aggregates (micelles) of surfactants. However, because undefined amounts of surfactants may be released into the aqueous systems, attention must be paid to their own environmental risks as well. Moreover, surfactant remediation methods must be carefully analyzed in the laboratory before field implementation. The use of biosurfactants is the best choice for the removal of water toxins as such surfactants are associated with the characteristics of biodegradability, versatility, recovery, and reuse. This Review is focused on the currently employed surfactant-based soil and wastewater treatment technologies owing to their critical role in the implementation of certain solutions for controlling pollution level, which is necessary to protect human health and ensure the quality standard of the aquatic environment.

CONTENTS 1. Introduction 2. Surfactants for monitoring organic contaminants 2.1. General facts about surfactants and their use for PAH removal 2.2. Surfactants for the removal of dyes 2.3. Surfactants for the removal of pesticides and petroleum hydrocarbons 2.4. Surfactants for the removal of volatile organic compounds 2.5. Surfactants for the removal of pharmaceuticals and personal care products 3. Surfactants for the removal of toxic metals 3.1. Surfactants for the remediation of metalcontaminated soil 3.2. Use of biosurfactants in soil washing for the removal of heavy metals 3.3. Synthetic and biosurfactants for the removal of heavy metals from wastewater 4. Practical implementation © 2016 American Chemical Society

5. Conclusion Author Information Corresponding Author Notes Biographies Acknowledgments References

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1. INTRODUCTION Although the progress of a country is related to industrial development, industries are blamed as the main source of discarding life-threatening wastes into aquatic systems. The uncontrolled release of industrial wastewater leads to migration of contaminants to the subsurface soil and groundwater. The presence of polycyclic aromatic hydrocarbons (PAHs), phenols, chlorinated hydrocarbons, dyes, and heavy metal ions in

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Received: February 19, 2016 Published: May 3, 2016 6042

DOI: 10.1021/acs.chemrev.6b00132 Chem. Rev. 2016, 116, 6042−6074

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Figure 1. (a) CMC and micelle formation of surfactant monomers and (b) different shapes of micelles.

but is expensive when used on a large scale. Moreover, electrical processes and activated carbon treatments are applicable only in specific cases at small scale. Removal of heavy metals by micellar solution of synthetic surfactants is a smart approach,16,17 however, to avoid the possibility of secondary pollution. Biosurfactants are more promising due to their environmental friendliness and ability to tolerate a wide variation of temperature, pH, and salt concentration.18,19 The microheterogeneous surfactant solutions can solubilize and encapsulate charged and neutral pollutants in different zones of micellar aggregates. The mechanism involves electrostatic interaction of the negatively charged exterior of micelles with cationic pollutants and vice versa. Small nonpolar organic contaminants can penetrate into the hydrophobic micellar interior while organometallic complexes have been reported to interact by mixed binding mode involving electrostatic interaction of the ionic part with the oppositely charged headgroup of micelles and insertion of the organic part in the palisade layer of the micelle.9 PAHs, dyes, and phenols are lifelong organic pollutants commonly found in soil.11 Although phenols are important for the manufacturing of resins and pharmaceutical products, many members of this class are toxic and listed by the U.S. Environmental Protection Agency (USEPA) as priority pollutants. Like phenols, PAHs also demand special environmental concern owing to their toxicity and persistence. PAHs do not ionize, and hence they accumulate in soils. The USEPA is monitoring PAHs for their toxicity, carcinogenicity, and high frequency of occurrence. PAHs have little solubility in water, and thus they have a propensity for adsorption onto particulate matter in the soil. Biological and chemical remediation techniques are currently used for the enhanced degradation of PAHs. However, biological methods (bioremediation) are limited due to supply of bacterial nutrients, nonoptimal ambient conditions (pH, temperature, etc.), lack of certain bacteria, and physiochemical characteristics of PAHs. Chemical technologies are hindered by decreased soil permeability, toxic byproduct formation, heat and gas production, and oxidant introduction into the soil. Therefore, new techniques for enhanced PAH degradation are being explored, and one such technique employs the use of surfactants for enhancing the solubility of PAHs and thus increasing the possibility of their bioavailability and degradation.1,3,4 The development of surfactant-based water-detoxification technology is expected to provide critical information to the environmentalists and human health stakeholders to apply measures for protecting marine and human life from organic and inorganic contaminants.

industrial wastewater adversely affects the quality of fresh water and poses a serious threat to human beings and water-dwelling species. For the decontamination of aqueous systems, several conventional techniques such as solvent extraction, precipitation, adsorption, and ion exchange are commonly used; however, remediation technologies based on environmentally friendly surfactants excel all others due to their versatility, efficient toxins-removal performance, and compliance with the principles of “green chemistry”.1−4 Surfactants form aggregates (micelles) in the bulk of solvents at critical micelle concentration (CMC),5−9 and such aggregates typically contain 50−100 monomers.10 Each micelle has a tiny, nonpolar hydrocarbon droplet at its interior that can dissolve hydrophobic or relatively less polar solutes including polychlorinated biphenyls (PCBs), PAHs, pesticides, and other organic contaminants.11 The water solubility of hydrophobic contaminants such as PCBs and PAHs is 100−1000 times increased due to micellar solubilization.12 The contaminants-encapsulated micelles are removed through ultrafiltration because of their bigger size compared to the pores of the filter. Hg, Pb, Cr, Cd, Zn, Cu, and Ni are the main metal contaminants that adversely affect water quality, soil ecology, and agricultural production.13,14 Mining industries, metalplating industries, and abandoned disposal sites are the central sources of metallic pollutants.15 The removal of heavy metals from polluted soil is difficult due to their facile absorption by crops, lack of decomposition by microbes, and transformation into more toxic compounds. For the remediation of heavy metals contamination, both physical and chemical methods are used (either separately or in combination) for isolation, immobilization, toxicity reduction, and extraction. Applications of methods vary as every metal has a different reactivity and forms specific compounds that can only be removed using selective methods. Each method on the other hand has its own advantages and limitations due to the complex nature of heavy metal contamination depending upon the nature, type, chemical composition of soil, and concentration of pollutants. The extraction of heavy metals becomes more challenging when organic cocontaminants are present. Solidification of heavy metals to constrain their spread is often used, but due to reversibility it needs constant monitoring. If complete elimination or isolation of metal contamination is difficult, then other approaches are adopted to stop their mobility/ permeability and reduce their toxicity by conversion to lesstoxic forms such as reduction of Cr(VI) to Cr(III). Thermal processes are also employed that have their own limitations with applicability for only volatile metals with high cost. Another method, vitrification, can only be applied to sediments 6043


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2. SURFACTANTS FOR MONITORING ORGANIC CONTAMINANTS

phenanthrene was mineralized by Sphingomonas sp. GF2B within 10 days without surfactant addition. After adding Tween 80 to the reaction matrix, biodegradation of phenanthrene was greatly inhibited and only 33.5% of overall phenanthrene was degraded. However, up to 99.5% of phenanthrene was degraded after the addition of the biosurfactant. The increased rate and extent of phenanthrene biodegradation in the presence of biosurfactant can be explained by phenanthrene’s enhanced solubility and its preferential utilization by the organisms.22 Flotation technique is commonly used in mineral ore processing for separating hydrophobic materials from hydrophilic ones. However, flotation is gaining popularity as a remediation technique for polluted soils because of its effective removal of both organic and inorganic contaminants. Like other soil-cleaning processes, flotation can be improved by the addition of a surfactant. Mouton et al.23 compared nonionic and amphoteric surfactants for the removal of PAHs using a surfactant-enhanced flotation process. Brij 35 and Tween 80 were used as nonionic surfactants, and coamydopropylbetaine (BW) and cocamidopropyl hydroxysultaine (CAS) were used as amphoteric surfactants. For PAH removal, CAS and Tween 80 showed great promise while Brij 35 and BW failed to achieve the desired objectives. The PAH removal efficiency of different surfactants can be judged from the data listed in Table 2. Tween 80 showed the highest percentage of 62% followed by CAS when used at concentrations of 10 and 5 g/kg, respectively.

2.1. General facts about surfactants and their use for PAH removal

Surfactant molecules are amphiphilic in nature since their head and tail have strong affinity for polar and nonpolar solvents. Surfactants reduce the surface tension of solvents as they accumulate at the surface and act as a bridge between the liquid and air. In aqueous solutions surfactants begin to aggregate at the CMC, and the self-assembled aggregates/micelles remain active in dynamic equilibrium with the monomers in bulk solution. Practically no micelles form below the CMC, while above the CMC micelles are formed in different shapes and sizes as shown in Figure 1. Surfactants can be used for enhancing the degradation of organic contaminants. A number of surfactants have been reported for their role in increasing the degradation of PAHs. ́ Rodriguez-Escales et al.3 explored the effect of nonionic surfactants (Gold Crew, BS-400, and Tween 80) on PAHs (mixture of fluorene, phenanthrene, anthracene, and pyrene) contaminated soil with varying granulometry and found that soils with 90% removal of chemical oxygen demand (COD). Results showed that the key process involved in the breakup of emulsions obtained for the remediation of aqueous surfactant wastes was a charge-neutralization mechanism based on variations of zeta potential, pH, and aluminum dose required. Furthermore, the performance of the coagulation process was found to depend strongly on the amount of surfactant and the pH of the soil-washing wastewater.25 Upon further investigation, Lopez-Vizcaino et al.26 determined that electrocoagulation is a better technique for effluent remediation. After conducting several electrocoagulation experiments, it was determined that electrochemical coagulation is successful in treating effluents of a soil-remediation process aided by the use of a surfactant such as sodium dodecyl sulfate (SDS) for the extraction of phenanthrene. Synthetic surfactants such as Tween 80, Brij 35, and SDS have been thoroughly evaluated for their role in remediation of polluted soils. While these surfactants have proved to successfully degrade PAHs, issues involving the nonbiodegradability and toxicity of synthetic surfactants are becoming a concern. Thus, more focus is being placed on biosurfactants. A compost-isolated humic acid-like (cHAL) material has been found to be an effective biosurfactant because of its relatively low CMC in water (0.4 g/L), enabling cHAL to increase the solubility of hydrophobic compounds like phenanthrene in an aqueous system when used at concentrations higher than the CMC.27 Upon comparison of the surfactants SDS and cHAL in regard to the sorption of the soils, PAH desorption from the soil was found to be 10 times greater relative to that with water and three times that compared to SDS. The lower performance of SDS is mainly caused by a higher adsorption rate of the soils. Therefore, biosurfactants, which are more environmentally friendly than synthetic surfactants, have properties similar to synthetic surfactants with respect to the remediation of PAHcontaminated soil and should be preferred over synthetic surfactants. Surfactants can be used for the removal of a wide number of contaminants; however, elimination of polyaromatic hydrocarbons, oils, and pesticides is the most important. The solubilization of water-insoluble contaminants in the micellar solutions demands the synthesis of surfactants associated with low CMC values and greater micellar core (aggregation number). In this regard gemini surfactants having 16 times lower CMC than nongemini surfactants are the best choices.28 Due to lower CMC and higher biodegradability characteristics, synthesis of gemini and biosurfactants from new renewable raw materials is the top priority of surfactant chemists to safeguard the ecological damage from hydrophobic organic contaminants present in industrial wastewater. Most of the synthetic surfactants show resistance to biodegradation, and their low solubility can lead to adverse environmental consequences; to avoid these problems, efforts are made to improve the biodegradability of surfactants by increasing the number of methyl and oxyethylene groups in their structures. Phytoremediation is a bioremediation technique that uses plants to mitigate environmental problems without the need of excavating and disposing of contaminated soil, hence making it an easy and low-cost solution for overcoming soil-pollution

2.2. Surfactants for the removal of dyes

Large amounts of dyes are used in the textile industry during fiber bleaching and dyeing processes.30 Wong et al. estimated that, in the course of production and application, 10−20% of the used dyes go as colored wastewater and 50% of that may reach the ecosystem after treatment.31 The chemical structure of dyes varies from one dye to another, and the presence of certain stable aromatic moieties hinders the degradation of these chemicals in conventional treatment processes.30,32 The waste dyes cause organic filling and toxicity in water bodies even if present in a very small amount. Dye concentrations of p-CH3 ≈ p-Cl > H > pOCH3. However, it decreased when negatively charged substituents such as p-SO3− and p-CO2− were introduced. The dyes have a lower affinity for SDS micelles due to electrostatic repulsion between their sulfonate groups and negatively charged micelles of SDS. The measured pKa values increased above the CMC for both nonionic and anionic micelles, suggesting that respective micelles exhibit higher affinity toward undissociated dye. Several substituents restricted the dyes to more hydrophilic surroundings of the polyoxyethylene shell of nonionic micelles, where they exist in hydrazine tautomeric form. Conversely, dyes containing apolar substituents make some way into the hydrocarbon core of nonionic micelles, where they assume azo tautomeric form. Certain dyes that assume azo tautomeric form in nonionic micelles show spectra typical of the hydrazone tautomeric form in SDS micelles.56

Figure 6. Effects of high rhamnolipid concentrations on the absorbance spectra for aqueous solutions of methylene blue and CTAB. Reprinted with permission from ref 51. Copyright 2009 Springer.

2.3. Surfactants for the removal of pesticides and petroleum hydrocarbons

Pesticides are used for the prevention, control, and/or destruction of pests. A number of human benefits are associated with the use of pesticides; however, drawbacks, especially toxicity, cannot be ignored. Many common pesticides (herein, pesticide is a term generically used for any pest-killing substance, including herbicides, insecticides, etc.) are persistent organic pollutants, and like PAHs their removal from contaminated soils is difficult due to their low water solubility. Thus, surfactant-aided remediation techniques may prove beneficial. Bentazone is a selective herbicide, as it only damages plants that cannot metabolize it. In one study soils contaminated with bentazone were investigated for soil washing using three alkylpolyoxyethylene surfactants containing the same hydrophobic chain but varying numbers of oxyethylene groups.57 The selected surfactants resulted in comparatively good pesticide recovery. Degradation of residual bentazone was accomplished using a photocatalytic treatment of the wastes, where suspended TiO2 particles, in the presence of simulated sunlight irradiation, degraded the bentazone after a predetermined time that was dependent on the concentration and nature of the selected surfactant. The most effective investigated surfactant was Brij 35, which had the fastest abatement time for the bentazone present in the collected waste material.57 Like other chlorinated pesticides, biodegradation of hexachlorocyclohexane (HCH) is limited due to its biopersistence, low solubility, and sorption onto soil surfaces. Thus, HCH biodegradation may be improved by using biosurfactants, in which the effect is quantified by the dissolution, bioavailability, and biodegradation of HCH isomers. In fact, the effect of biosurfactants rhamnolipid, sophorolipid, and trehalose-containing lipid on the biodegradation of HCH was explored. The results revealed that solubilization of the HCH isomers is increased 3−9-fold, with

Figure 7. Effect of adsorbent dosage on adsorption capacity of MB (initial MB concentration 200 mg/L; temperature 298 K; pH value 7.3; and contact time 24 h). Reprinted with permission from ref 53. Copyright 2014 Elsevier.

Oakes et al. investigated the interaction of azo dyes with model surfactants on a molecular level and explained the chemistry of surfactant−dye interaction.54 The variation in azo dye−surfactant interactions was probed by altering the chain length and headgroup of nonionic and ionic surfactants. A series of synthesized model azo dyes were found to adopt hydrazone tautomeric forms in aqueous system. The dyes exhibited a single pKa, which was influenced strongly by the presence of surfactant micelles.55 UV−vis spectroscopy, being sensitive to detect changes in tautomeric forms, was applied for the measurement of pKa values of the dyes.56 The nonionic surfactant primarily used in this study was C12EO5 with SDS as the anionic surfactant. The UV−vis spectrum of 1-arylazo-2naphthol in aqueous medium of pH 10 at 25 °C with a λmax at 484 nm suggested the existence of this dye in typical hydrazone tautomeric form.56 Another dye, N-methylated orange I having a structure closely related to 1-arylazo-2-naphthol, demonstrated a decrease in the absorbance of the main band with simultaneous appearance of an absorption peak in the spectrum at ca. 420 nm via an isosbestic point, indicating the existence of equilibrium between the dye in bulk solution and that solubilized within the micelles. The equilibrium shifts toward solubilized dye upon increase in micelle concentration. Considering a CMC value of 6.5 × 10−5 M and a micelle aggregation number of 100, the concentration of nonionic surfactant was calculated, which indicated complete solubiliza6048


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sophorolipid and rhamnolipid being the most effective surfactants, yielding maximum HCH isomer solubilization at a concentration of 40 μg/mL. Trehalose-containing lipid, on the other hand, yielded peak solubilization at 60 μg/mL. Of the three biosurfactants, the highest solubilization and greatest HCH isomer degradation in the soil was obtained with sophorolipid.57 Tributyltin (TBT, an organotin) is another persistent organic pollutant commonly used in pesticides. Because of its toxicity, remediation of contaminated sites is a priority, and in this regard Mathurasa et al.58 investigated the role of an anionic surfactant for the enhanced removal of TBT. Tributyltin and its metabolites, dibutyltin (DBT) and monobutyltin (MBT), act as polar materials in soils that contain low concentrations of organic carbon and where the major mechanism of adsorption is ion exchange. Sodium dihexylsulfosuccinate (SDHS) above the CMC was found to greatly enhance the desorption of butyltins. However, higher concentrations of SDHS are required, and the amount of TBT desorbed results in a synergistic effect, toxic to bacterial species present in the soil medium. However, SDHS by itself was found to be less toxic, and below the CMC it did not initially increase the TBT desorption but promoted the bacterial degradation of TBT. The soil−TBT bacteria interaction was expected to improve due to the formation of a TBT/SDHS monomer complexation. Thus, anionic surfactants at or below the CMC could be useful for the remediation of TBT-contaminated soils.58 Because both anionic and nonionic surfactants have their potential benefits, Guo et al.59 explored the combined impact of a surfactant mixture using an anionic/nonionic surfactant solution (i.e., sodium dodecylbenzenesulfonate (SDBS) and Tween 80) for p-nitrochlorobenzene (pNCB) remediation of polluted soils. Although pNCB is a vital chemical intermediate used in many organic synthesis processes including pharmaceuticals, pesticides, dyes, and rubber chemicals, both the USEPA and China State Environmental Protection Agency (SEPA) have listed pNCB in the environmental priority pollutants list because of its high toxicity, bioaccumulation, nonbiodegradability, and overall environmental risks. Once in the environment, pNCB gets easily adsorbed by soil particles because of its low vapor pressure and aqueous solubility, resulting in large pNCB storage in soils. However, research on pNCB remediation from contaminated soils is quite limited, and no documentation in the literature except the report of Guo et al.59 has explored the use of surfactant mixtures (e.g., SDBS/Tween 80) for pNCB desorption from soil fractions. Their results demonstrate that surfactant dosage and the SDBS/Tween 80 mass ratio significantly influence water solubility, the distribution constant of soil−water matrix, the desorption of pNCB, and the sorption of soil’s surfactant. The addition of SDBS proved to be more effective when compared to individual Tween 80 due to the formation of mixed micelles. SDBS significantly enhanced water solubility, increased pNCB desorption ratio, and inhibited surfactant adsorption onto soil fractions. Low surfactant dosages (i.e., 99.0%) from 10 mmol/L initial concentration. The monolayer capacity (70.42 mmol/g) of the synthesized adsorbent was found to be higher than (52.63 mmol/g) that of the commercial activated carbon. The recovery of adsorbed TCP by 0.1 M NaOH indicated minimum damage to the adsorption capacity.71 In the search for effective adsorbents, Bikshapathi and co-workers synthesized iron nanoparticle-suspended carbon micro/nanofibers (Fe-ACFs and Fe-CNFs) for the efficient removal of persistent VOCs and carbon tetrachloride (CCl4). Fe(III) ions were segregated and monodispersed with the help of surfactants. SDS was found to produce maximum loading (0.68 mg/g) with uniform distribution of Fe nanoparticles over the surface of CNFs.72 Moreover, unique hybrid membranes were developed by spreading silica nanoparticles within the active layer of composite membrane by means of a mediating surfactant, i.e., Tween 40 (polyoxyethylene sorbitan monopalmitate). Maximum selectivity factor and high permeation flux were observed by the membrane having 2 wt % concentration of mediating surfactant due to an increase in the selective adsorption, hydrophobicity, and even distribution of filler in the synthesized polymeric matrix.73 Devi and Saroha used a zerovalent iron composite material with a magnetic biochar 6050


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(ZVI-MBC) as adsorbent for the remediation of pentachlorophenol (PCP). The combined properties of ZVI particles and biochar in ZVI-MBC were found to cause sequential adsorption, and further dechlorination of PCP in the discharge led to complete elimination of PCP. The aging and leaching tests confirmed the durability and stability of ZVI-MBC.74 Tu and co-workers employed saponins for the remediation of nhexane in a biotrickling filter (BTF). The removal efficiency (RE) was revealed to increase from 56.8% to 62.8% and from 83.3% to 91.3% for BTF2 (without saponins) and BTF1 (with saponins) respectively, when loading rate of n-hexane was decreased from 120.0 to 47.80 g−3 h−1. Upon increasing the gas empty bed contact times (EBCTs) from 7.5 to 30.0 s, the RE increased from 38.3% to 61.4% for BTF2 and from 64.5% to 88.4% for BTF1.1 Furthermore, Painmanakul and co-workers investigated the absorption mechanism of benzene in a small bubble column.75 The presence of lubricant oil and nonionic surfactant was revealed to influence the mass transfer parameters, the interfacial area, and the bubble hydrodynamic mechanism. The benzene solubility was observed to intensify in the absorbents (liquid phase) due to availability of surfactant molecules and oily particles. Moreover, the adsorption efficiency of benzene was decreased by enhanced gas flow rates owing to mechanisms of desorption, along with the mixing power of liquid phase.75 In the adsorption method, care is taken to select solvents associated with no/or low toxicity and reduced volatility. The extensive use of aniline in pharmaceutical products, conducting polymers, dyes, and many other chemicals of current domestic and industrial interest has resulted in its abundant discharge. A number of methods are employed for the removal of aniline from wastewater;76 however, Tanhaei and co-workers developed an efficient and cost-affordable MEUF method for the removal of low molecular weight organic pollutants including aniline with the help of poly(ether sulfone) membrane and an anionic surfactant SDS. The maximum removal of aniline (∼80%) was observed in anionic−nonionic mixed surfactants SDS and Brij 35 due to lower CMC as compared to individual surfactants.77 The same researchers also used the MEUF method for the simultaneous elimination of aniline and nickel from water by using a synthesized polysulfone membrane and commercially available membranes UFX5 and NP010 to highlight the importance of micellar size. The maximum removals of aniline (70%) and nickel (97%) were accomplished by NP010 membrane of smaller pore size than polysulfone and UFX5 membranes. The results revealed that aniline is encapsulated into the micellar core and nickel cations interact with the negatively charged exterior of SDS micelle as shown in Figure 8. The results further revealed that copresence of aniline and nickel causes increase in remediation of both of them as compared to their independent presence in solution.78 Although complete elimination of aniline was not achieved, MEUF showed its promise to be an appropriate technique for sufficient removal of aniline from water. Toluene-containing effluents are also released from various industries, and due to the high toxicity of toluene, its small concentration can paralyze the central nervous system and damage kidneys and liver. Biodegradation of toluene is possible by the use of surfactants due to their potential to enhance the solubility of hydrocarbons (HOCs) in aqueous solutions.79,80 One promising methodology for the abstraction of VOCs from polluted groundwater and soils is surfactant-enhanced

Figure 8. Interaction of aniline molecules and nickel ions with an SDS micelle. Reprinted with permission from ref 78. Copyright 2014 Elsevier.

remediation (SER). This technology enhances the solubility of VOCs by means of reversible surfactants having two redoxactive groups, (Fc14) and (Fc12), above and below their CMC under oxidizing (I2+) and reducing (I+) conditions.81 Li and coworkers determined the solubilization of ethylbenzene, toluene, and benzene by ferrocenyl surfactants. The CMCs of Fc14 and Fc12 in I+ are 0.56 and 0.94 mmol/L. The solubilization of toluene by ferrocenyl surfactants in I+ was observed to be 30% greater than that achieved in I2+ as well as by SDS, CTAB, and Triton X-114. Their solubilization capacity varied in the following order: benzene < toluene < ethylbenzene. The concentrations effects of ferrocenyl surfactants on efficiency of VOCs removal followed the sequence ethylbenzene > toluene > benzene.68 PCBs are moderately volatile compounds that are used as lubricants and coolants in transformers and capacitors because of their nonflammable and insulating properties. Their alarming toxicity level, long-distance transfer, chemical stability, and persistence is an issue of great concern for environmentalists.82 PCBs cover 209 structurally related compounds that are predicted to be listed as major priority chemicals for ultimate extraction until 2025.83 Soil contamination by PCBs is the result of their strong adsorption to soil matrix, chemical stability, and perseverance that impedes their degradation and elimination.84 The pros and cons of current methods for PCBcontaminated sediments and soils are reported in a comprehensive review in which the need for sorting out costeffective and efficient alternatives has been suggested.85 One smart approach for PCB removal from soil is the use of mutual utilization of biosurfactant and chemical−biological treatment process. Viisimaa and co-workers studied the application of microbial surfactant PS-17 and joint chemical−biological treatment, using liquid H2O2 and a natural group of microorganisms devastator to soil polluted with PCB along with insulating oil. This 42-day treatment supported by biosurfactant resulted in the removal of PCB by 47−50%. Improved soil respiration and activity of dehydrogenase were revealed by the combined application of microorganisms, biosurfactant, and oxidizing compounds (shown in Figure 9) in adequate dosages as compared to the application of microbial group alone.86 Similarly, Pleurotus ostreatus was found to cause degradation of PCBs by increasing laccase activity.87 Gomes et al. developed an economical electrodialytic remediation method for PCB-polluted soil using Tween 80 and saponin surfactants and iron nanoparticles for desorption and dechlorination of PCBs. Saponin and chlorine-rich PCB 6051


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of PCB removal was found to be 94.7% in coarse sand.93 Wang and Chen further investigated the influence of foam mobility on the removal of PCBs with different concentrations of surfactant (Triton X-100) and gas contents by two sands. Foam flushing was observed to increase the removal of PCB from 79.4% to 85.1% by solution flushing through grainy sand, and from 64.2% to 79.1% using fine sand. The results reveal that the presence of foam decreases the mobility, increases the sweep efficiency, and consequently enhances the removal of PCBs.94 Phthalic acid esters commonly recognized as phthalates are a group of moderately volatile compounds that primarily function as plasticizers to increase the flexibility and durability of plastics. Phthalates contaminate the environment easily as they leave plastics on heating or exposure to organic solvents. A literature survey reveals that phthalates cause adverse, cumulative “cocktail effects” and their trace amounts can react with other chemicals,95 so the development of techniques for their sensitive detection and removal is imperative. In this regard, mixed hemimicelles solid-phase extraction (MHSPE) holds great promise for efficient extraction, preconcentration, and determination of very small amount of phthalates. Liu and coworkers developed the MHSPE method with mesoporous silica-covered magnetic nanoparticles (Fe3O4/meso-SiO2 NPs) as an adsorbent for the removal of phthalates from contaminated water samples. Owing to the superparamagnetism, even mesopore size (2.8 nm), and larger surface area (570 m2/g) of the adsorbent, this method displayed excellent extraction efficiency for the removal of target phthalates, i.e., di-n-butyl phthalate, butylbenzyl phthalate, di-n-cotyl phthalate, and di(2-ethylhexyl) phthalate.96 Moreover, the method detected 21, 12, 32, and 12 ng/L of the respective phthalates, respectively, and comparative studies showed improved results using different adsorbents. The work of Li and co-workers revealed thatmore effective adsorption of diethyl phthalate from water occurs over SDS-covered nanosized alumina compared to SDS-covered microsized alumina due to smaller size and enhanced surface-binding energy. The hydrophobic nature of mixed hemimicelles further facilitated the adsorption process.97 The potential of micellar solutions of surfactants/biosurfactants for elimination of VOCs is due to their ability to compartmentalize these toxins.98 Estimation of the toxicity and environmental fate of surfactants is required for their in situ elimination applications. The use of cleaning surface-active products in the form of detergents and soaps has already caused a lot of damage to the aquatic environment.99 Hence, the degradation of residual surfactants is required to protect the environment from their own deleterious effects. On the basis of these considerations, Aurora Colomer et al.100 evaluated the cytotoxicity and environmental fate of new lysine-based, pHsensitive surfactants, whose pH sensitivity was able to be tuned by altering their chemical structures using fibroblast and erythrocyte cells. The toxicity for erythrocytes was found to increase by increasing charge density and hydrophobicity of the surfactants, while opposite results were observed for fibroblasts, i.e., the toxicity decreased by increasing the charge density. The biodegradability test (CO2 headspace test) revealed efficient biodegradation of these surfactants under aerobic conditions. Moreover, a study was conducted for determining the effect of anionic (SDS), nonionic (AEO), and cationic (1227) surfactants on the behavior of zebrafish larval. AEO and 1227 were reported to be toxic at 1 μg/mL to larval activity of locomotion, while no substantial effects were observed for SDS. Furthermore, exposure of AEO led to reduced eye size, smaller

Figure 9. Chemical−biological treatment of PCB-polluted soil by the addition of biosurfactant. Reprinted with permission from ref 86. Copyright 2013 Elsevier.

congeners (penta, hexa, hepta, and octachlorobiphenyl) displayed 90% and 96% removal of PCBs, respectively.88 Zhu et al. introduced another efficient method for the treatment of PCB-polluted soil in which the PCBs are first extracted by soil washing using polyoxyethylene lauryl ether (Brij35) and hydroxypropyl-β-cyclodextrin (HPBCD) followed by subsequent degradation with TiO2 photocatalysis. The results predicted that the percentage of extraction can be significantly improved by the degree of PCB chlorination, and consequently HPBCD renders efficient the photocatalytic degradation of PCBs.89 Similarly, Guangping Fan et al. developed an electrokinetic technology associated with engineered nanoparticles for the elimination of PCB-contaminated soil. Three unlike surfactants, SDBS (anionic surfactant), Brij35 (nonionic surfactant), and rhamnolipid (biosurfactant), were introduced independently with nanoPd/Fe stabilized by xanthan gum for improving the solubilization of soil PCBs. Brij35−nanoPd/Fe stabilized by xanthan gum showed the maximum removal efficiency versus rhamnolipid and SDBS. The rhamnolipid and SDBS remained inactive in soil as removal of PCBs was not substantial in all tests.90 Furthermore, a study was conducted with combined surfactants, saponin and S,S-ethylenediaminedisuccinic acid (EDDS), for extraction of PCBs from mixed polluted soil. The higher removal efficiency of these surfactants was due to their synergistic performance, which leads to inhibition of their sorption onto soil. EDDS was found to increase the solubility of PCB in saponin micelles.91 Moreover, lecithin-nanoNi/Fe, a composite material, was synthesized through microemulsion technique and tested on PCB-77 as marked pollutant. Lecithin was observed to be an environmentally friendly biosurfactant that functioned as the constituent part of microemulsion as well as an efficient material for agglomerating organic contaminants. As the synthesized composite material was able to combine the functional properties of bimetal and lecithin, the removal of PCB77 by lecithin-nanoNi/Fe was predicted to occur at a faster rate than that with the blank carrier.92 In situ foam flushing is another soil-remediation methodology in which the mobility of washing agent is controlled by foam. Like surfactant flushing, the mechanism of PCB removal by foam flushing involves solubilization. A study carried out on the combined use of flushing with surfactant solution, foam, and water revealed that this methodology effectively integrates the foam’s mobility control and solution-solubilization properties. The effectiveness 6052


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0.05 g/L in media of pH 7.0−7.5.107 In another study the degradation of nonionic and anionic surfactants (LAS with a 10−13 carbon hydrophobic chain and nonylphenolic compounds (NPEs), i.e., nonylphenol ethoxylates (NPEOs) and nonylphenol (NP)) was investigated in agricultural soil adjusted with sewage sludge. No environment risk for LAS homologues was revealed after application of 7 and 8 days of sewage sludge to the agriculture soil at 22.4 °C (summers) and 12.7 °C (winters), respectively, while possible toxic effects were observed for the NPE compounds within the first 56 days after application of sludge to the soil.108 Moreover, a study was conducted to investigate the extent of biodegradation of nonionic and anionic surfactants with the aid of mutual use of biodegradation and ozonation for removal of surfactants, mainly linear alkylbenzenesulfonates (LAS) and alkylpolyglucosides. The anionic surfactant possessing a benzene ring was revealed to be oxidized earlier than the nonionic surfactant. Reduced mineralization was shown by both surfactants owing to primary attack of ozone on carbon bonds during ozonation, thus validating the efficiency of combined use of biodegradation and ozonation for surfactant remediation.109 Tehrani-Bagha and co-workers determined the efficiency of various ultravioletimproved ozonation methods for synthetic surfactants degradation, i.e., sodium dodecylbenzenesulfonate and a nonylphenol ethoxylate having 40 oxyethylene units. The results revealed at least 2 times decrease in total organic carbon and chemical oxygen demand via UV treatment and ozonation versus each process alone, indicating partial oxidation with reduction in mineralization of the studied surfactant solution.110 Moreover, Pietka-Ottlik et al. investigated a series of four dicephalic cationic surfactants with dissimilar lipophilic chain length (n-C9H19 to n-C15H31) and variety of counterions (methylsulfate, bromide, and chloride) to determine their biodegradability and toxicity. No antimicrobial activity was seen for the tested surfactants by Gram-negative bacteria (Pseudomonas putida and Escherichia coli) and yeasts (Rhodotorula glutinis and Saccharomyces cerevisiae) below 1 mg mL−1 concentration, while moderate activity was noticed by some of them by Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus). The toxicity of these surfactants to Vibrio f ischeri depended upon the chain length of the alkanoyl group, showing EC50 values in the range of 2.6−980 mg/L.111 The research group of Motteran studied the degradation of a nonionic linear alcohol ethoxylate (LAE) surfactant in a fluidized-bed reactor under anaerobic conditions by adding 4.7−107.4 mg/L LAE to 535−882 mg/L synthetic organic substrate. The results revealed 98.5% degradation of LAE.112 The water solubility of fatty acid surfactants can be enhanced by inserting choline as a counterion in comparison to classical potassium and sodium soaps, enabling these required longerchain derivatives to function at ambient temperatures. The degradation ability of choline makes the resultant fatty acid soaps as highly biocompatible, but usage of choline is prohibited in cosmetic products by the European Cosmetic Directive 76/768/EEC, primarily owing to its quaternary ammonium ion classification.113 Thus, Klein and co-workers explored the biodegradation of choline soaps (ChS) having chain lengths of alkyl group as m 1/4 12−18 based on the OCDE 301F standard in order to expedite their applications. Moreover, potassium and sodium derivatives were also studied for a better comparison with common soaps and awareness about the effect of the cation. The relationship between the IC50 value and the hydrophobic chain length was revealed to

body height, and shorter head size in comparison with SDS and 1227. It was found that shorter head size relates to less expression of krox20 due to inhibited cell growth and migration.101 Similarly, Pedrazzani et al. investigated the toxicity of three commercially available and three premanufacture detergents and performed a series of degradation tests, i.e., mutagenicity tests, ready biodegradability test (OECD301F), and Vibrio f ischeri and Daphniamagna toxicity tests. The detergents caused no mutation in bacteria. Moreover, a slight enhancement of micronucleus frequency was induced in A. cepa root cells by a commercial ecolabeled detergent, and DNA destruction was reported by all premanufacture surfactants and one commercial one in human leukocytes. However, further tests are required to evaluate environmental impact and to describe mutagenic and toxicological features as well as degradation of detergents.102 Biodegradation is one of the safest methods for irreversible elimination of surfactants from land and marine environments.103 The biological degradation of surfactants involves microorganisms that consume these compounds and use them as energy and carbon sources. The overall biodegradation process takes place in two steps. The first step involves structural variation and sudden damage to amphiphilicity as the result of breakage of the hydrocarbon chain. The transformation of primary degraded products into water, CO2, and minerals takes place in the second step.104 The biodegradability of various bacterial surfactants and SDS was evaluated in liquid medium and in soil microcosms via evolution of CO2. The results suggested that the degradation potential of biosurfactants is comparatively higher and they are more stable in soil than the synthetic surfactants.105 Similarly, a comparative study was carried out on the use of bioluminescent bacterium (Vibrio f ischeri) for determining the toxicity level of a series of biosurfactants (LBBMA111A, LBBMA168, LBBMA155, LBBMA201, and LBBMA191) and a synthetic surfactant, SDS. The decrease of light emission (EC20) by Vibrio f ischeri was evaluated on exposing it to different concentrations of surfactant. The values of EC20 proved that SDS causes considerably higher toxicity to Vibrio f ischeri than biosurfactants, as it can almost completely inhibit bacterial luminescence at 4710 mg/L concentration after 30 min.99 Karci et al. studied the oxidation of a surfactant nonylphenol decaethoxylate (NP10) by photo-Fenton and H 2O2 /UV−C methods and investigated its degradation products and variations in toxicity level. On the basis of photoluminescence inhibition tests, the photo-Fenton method showed less toxicity than the H2O2/ UV−C method as indicated by 12% inhibition of the former method (nearly equal to that of the untreated pollutant having 10 ± 1.6% inhibition) versus the later one showing 27% inhibition.106 Despite the importance and application of anionic surfactants, mainly linear alkylbenzenesulfonates (LAS) in cosmetic products and detergents, their bioremediation to cope with the pollution content in the ecosystem by conventional activated sludge method is unproductive because of less foam production and low degradation kinetics of microorganisms. Hence, Asok and Jisha isolated 20 different bacteria specific for LAS degradation from surfactantcontaminated soil with the help of enrichment culture technique. Pseudomonas aeruginosa (L12) and Pseudomonas nitroreducens (L9) were determined to have the highest efficiencies (81.81 ± 0.8% and 81.33 ± 0.7%, respectively). These isolates were able to degrade LAS concentration up to 6053


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be nonlinear. Moreover, IC50 values of ChS surfactants were consistent with those of the potassium and sodium soaps. Thus, choline carboxylate surfactants are proved to be harmless and applicable for use in customer end products.114 2.5. Surfactants for the removal of pharmaceuticals and personal care products

Pharmaceuticals and personal care products (PPCPs) are a broad group of emerging contaminants that are described by Arp115 as newly appeared contaminants, old contaminants not analytically determined until recently, contaminants contained in newly identified complexes, or transformation products of other pollutants. PPCPs are among the most important due to their ubiquitous presence in the environment owing to their large-scale production and consumption, incomplete removal in municipal wastewater treatment, and diffusion into soil and drinking water sources.116,117 A comprehensive reconnaissance study by the U.S. Geological Survey (USGS) in 2002 on the prevalence of PPCPs revealed the presence of these contaminants in 80% of the 139 streams in the United States.118 PPCPs in the environment are linked with serious health and ecological toxicity as they are biologically active, are persistent and bioaccumulate in organisms, and have the potential to cause estrogenic effects (therefore also classified as endocrine-disrupting chemicals) and increase bacterial resistance.116,119 Pharmaceuticals are a diverse group of human and veterinary drugs and include anti-inflammatory, antibiotics, antidepressants, lipid regulators, betablockers, antiepileptics, steroids, and impotence drugs using nearly 3000 different substances as ingredients.116 Pharmaceuticals are released either as parent compounds or as transformed metabolites. Personal care products, on the other hand, are a wide range of compounds, e.g., triclosan, bisphenols, parabens, and benzophenones, used in many household disinfectants, detergents, toothpastes, plastics, preservative, sunscreens, and other diverse products and released in unaltered form into the environment.119 Conventional sewage treatment plants fail to completely remove PPCPs due to their recalcitrance or slow degradation and potential microbial toxicity, hence necessitating the use of advanced treatment technologies.117,120 Surfactants have been used to treat PCPs and have shown promise. The available literature in this area, though, is scarce, perhaps due to the newly emerging focus on PPCPs remediation and the little attention surfactants received in PPCPs treatment; particularly, the potential of biosurfactants in this area has not been explored to the best of our knowledge. Surfactant-induced remediation of PPCPs has mainly been centered on extraction to remove them from aqueous or soil phase and adsorption to control their transport and prevent subsurface migration. Extraction enhanced by the use of surfactants has been used for the removal of different PPCPs. Emulsion liquid membrane (ELM) incorporating surfactants is one such method that has gained much interest for removal of PPCPs from water. The method is based on liquid membrane technology for selective permeability of solutes. It comprises emulsion globules having an internal phase (with solute-stripping agent, commonly NaOH) trapped inside a membrane phase (surfactant and a diluent oil) that selectively allow diffusion of target solutes/ pollutants from the external phase (aqueous solution), therefore simultaneously performing extraction and stripping in a single step (Figure 10). Surfactants in ELM are principally

Figure 10. Schematic of typical emulsion liquid membrane process. Reprinted with permission from ref 122. Copyright 2014 Springer.

used to increase the emulsion stability, a major issue with ELM that results in membrane rupture. Surfactants prevent emulsion leakage by serving as barrier between the external and internal phases.121,122 For pharmaceuticals, ELM has been used for extraction and removal of nonsteroidal anti-inflammatory drugs (NSAIDs). Daas and Hamdaoui122 demonstrated a removal of >90% for ibuprofen and ketoprofen with ELM from pure, mineral, and seawater mediums under optimum conditions using SPAN 80 as surfactant, hexane as diluent, and sodium carbonate as the internal phase. The method was claimed to remove almost all ibuprofen from complex water matrices containing high mineral content or salts. In a similar study with ELM using Span 80 as surfactant and potassium chloride as inner aqueous solution, complete extraction of paracetamol (acetaminophen) was reported from aqueous solution under the most favorable experimental conditions.123 Application of ELM method on pharmaceuticals is still progressing, but the method has been widely researched for removal of phenolic PPCPs from the environment. A summary of the results obtained with PPCP removal in ELM is presented in Table 3. An observation of the data reveals that the most efficient surfactant is SPAN 80 (sorbitan monooleate), a nonionic surfactant in the removal of PPCPs using ELM systems. Sodium hydroxide and sodium carbonate have been the preferred internal aqueous phases at various concentrations, while kerosene and hexane are the two commonly used membrane phase or diluents mixed with surfactants. Incorporation of carriers or extractants in the membrane phase has also been studied to promote the solute transfer to the internal phase.129 Although removal in ELM is mainly achieved by entrapping the pollutant/solute in the internal phase that maintains a concentration gradient with the external phase by suppressing the solute, surfactants play the key role of keeping the emulsion intact and eliminating its leakage for a sustained removal. The ELM method is advantageous for its better selectivity, simplicity, and higher mass transfer rates due to large interfacial area for extraction and shorter diffusional path within the emulsion globules. ELM, however, strongly relies on a fine balance between different operational conditions such as 6054


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Table 3. Removal of Phenolic PPCPs from Aqueous Media with Surfactant-Aided ELM Method under Optimum Operating Conditions and ELM Emulsion Mixture emulsion PPCPs

surfactant

int. phase

membrane organic solvent

%η

ref

bisphenol A 4-chlorophenol 4-nitrophenol phenol p-chlorophenol di, tri, and penta chlorophenol propylparaben phenol phenol bisphenol A phenol

Span 80 Span 80 Span 80 Span 80 Span 80 Span 80 Span 80 OP-4 Montan-80

0.5 M NaOH 0.2 M NaOH 0.1 N Na2CO3 0.25 N NaOH 0.1 N Na2CO3 0.25 N NaOH 0.5 M NaOH 2−2.5% NaOH 0.125 M NaOH

hexane heptane carrier: tributyl phosphate hexane kerosene n-hexane carrier: trioctylphosphine oxide kerosene carrier: ionic liquid mixed carrier kerosene carrier: Cyanex 923 kerosene kerosene

98 99 >99 >96 100 99.5 98.3 97.5 92

124 125 126, 127 128 129 130 121 131 132

Table 4. PPCPs Adsorption in Surfactant-Modified Clay System clay

surfactant

target compounds

max. sorp. capacity (mg/g) or max. % removal

ref

montmorillonite montmorillonite

dodecyltrimethyl ammonium bromide (DDDTA) didodecyldimethyl ammonium bromide (DDDMA) hexadecyltrimethyl ammonium bromide (HDTMA)

bentonite

HDTMA phenyltrimethyl ammonium bromide (PTMA) HDTMA HDTMA

151.52 mg/g >90% >90% 88% 100% 100% 10.3 mg/g

23 24

bentonite

bisphenol A p-chlorophenol p-nitrophenol phenol p-chlorophenol 2,4-dichlorophenol phenol phenol

45% 39%

vermiculite bentonite

25

26 27

the cationic surfactant. Better adsorption of NSAIDs with surfactants can be linked to their low surface tension in solution and lipophilic properties.145 Adsorption of both quinolones was believed to be dependent on the concentrations of both the surfactant and pharmaceuticals. Polubesova and co-workers134 also studied surfactantenhanced sorption for removal of tetracycline and sulfonamide, two commonly used antibiotic groups in human and veterinary drugs. They used cationic surfactant benzyldimethylhexadecylammonium (BDMHDA) micelles preadsorbed on montmorillonite clay. They have previously shown the amendment as efficient media for removal of organic contaminants.146 For the antibiotics, experiments in both batch and column/filter system revealed highly efficient removal from 94 to 99.9% from concentrated solutions, 5−50 mg/L in batch system and 10 mg/L in column filter. This removal with micelle−clay complex was also significantly higher than that with activated carbon as sorption media. Soil-dissolved organic matter that could be naturally present further supplemented the removal.134 Anionic sodium dodecyl sulfate (SDS) surfactants used for adsorption of another antibiotic, amoxicillin, from aqueous solution have also shown effectual removal at 87.7% with optimum surfactant concentration of 10 mg/L to adsorb 4 mg/L of amoxicillin at a contact time of 40 min, pH 4, and temperature of 50 °C.147 On the other hand, the presence of nonionic surfactants in soils, such as Brij 35, has been found to accelerate the subsurface transport of oxytetracycline antibiotic by reducing its sorption to soil and increased solubilization by the micelles.148 Besides pharmaceuticals, surfactant-modified clays have mainly been used for remediation of phenolic compounds. The different clays and surfactant amendments used and the corresponding removal for different compounds is summarized in Table 4. The adsorption of phenolic compounds on these

surfactant amounts, conditions of membrane, internal and external phase volumes and concentrations, emulsification speed and time, and pH and is in a further development stage for optimized configuration for various contaminants. Surfactant-induced adsorption has also been studied. Generally, the use of granular activated carbon is a common choice for adsorption because it can effectively remove organic contaminants.133 Its suitability, however, is low for neutral or negatively charged PPCPs in natural water, whereas the coexistence of dissolved organic matter such as humic acids with larger molecules can clog the porous spaces.134 Poor selectivity enhancement for target contaminants with activated carbon as adsorbent, transportation for regeneration, and higher cost are other issues.133,135,136 Various natural and synthetic adsorbents modified with addition of surfactants to enhance selective adsorption of PPCPs have been studied especially with natural montmorillonite, bentonite, kaolinite, and vermiculite clays.137−141 Because of their hydrophilic characteristics, adsorption with these clays in natural form is ineffective for anionic pollutants and nonpolar organic compounds.142 Surfactants are known to modify the surface properties of these materials such as their hydrophilicity/ hydrophobicity or surface charge to support sorption of organic compounds.143 For pharmaceuticals, Hari et al. 144 investigated the adsorption of acetaminophen (NSAID), carbamazepine (antiepileptic/benzodiazepine), and norfloxacin and nalidixic acid (antibiotics/fluoroquinolones) in a natural subsurface material. They applied a cationic surfactant (cetylpyridinium chloride) and an ethoxylated nonionic surfactant (Tergitol NP9) having higher adsorption affinity for subsurface material and soils. The adsorption of acetaminophen and carbamazepine enhanced with the surfactants in congruence with their hydrophobicity, while only nalidixic acid adsorption enhanced at high pH with 6055


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Environmental Protection Agency (USEPA) has listed 13 metals in the priority pollutants with Cu, Pb, Zn, Cr, As, Cd, Ni, and Hg as the frequently persistent ones.153,154 These heavy metals in free or bound forms have high water solubility and mobility and are highly toxic and mutagenic in nature.155−157 Metal contamination also reduces the biodegradation of organic contaminants, thus affecting the physiology and ecology of microbes and, hence, the reduction of soil fertility.156 Surfactants application in remediation of heavy metals has commonly been studied using soil washing/desorption/ extraction, adsorption onto soil, and phytoremediation. Surfactants have also shown potential in other prospective methods such as ultrafiltration (used in protein purification) that can be enhanced by pretreatment of membrane with surfactants or biosurfactants for removal of metals ions. Likewise, surfactant-modified activated carbon exhibits 2−4 times higher capacity for the removal of metals compared to plain carbon.158 In addition, there is a growing scope for biosurfactants use in heavy metals removal due to their effective and superior qualities compsred to the synthetic counterparts. Biosurfactants are characterized by low toxicity, easy biodegradability and digestibility, better biocompatibility, and better stability in a wide range of temperature and pH conditions, ionic strength/salinity, and enhanced foaming properties.

organoclays depends on contact time, pH, and temperature.138,140 Synthetic adsorbents modified with surfactants have also been used to remove PPCPs from complex matrices. CabreraLafaurie et al.135 incorporated a cationic surfactant (cetylpyridinium) together with a transition metal (Cu2+, Ni2+, or Co2+) into a Y-zeolite by overcoming its hydrophilicity to develop better selectivity and uptake of two recurring PPCPs in water, i.e., salicylic acid and carbamazepine. The modification led to an increase in adsorption potential of zeolite from 0.03 to 3.9 mg/ g and a notable increase in selectivity particularly toward salicyclic acid. Bisphenol A (BPA) removal from water with synthetic zeolites modified with HDTMA surfactants has also been achieved. Dong et al.149 used synthesized zeolites from coal fly ash (with no affinity for BPA) and HDTMA and attained a maximum sorption capacity of 114.9 mg/g for BPA compared to the zeolite fly ash. Surfactant-enhanced coagulation-sedimentation process as a sorption technique for removal of pharmaceuticals from aqueous phase was also used.150 SDS as surfactant was used together with aluminum(III) chloride and sodium hydroxide in aqueous solutions for tetracycline antibiotics. Tetracycline was collected as sorbed onto SDS-filled precipitate on the hydrophobic regions as the ion pair of their aluminum chelate and dodecyl sulfate ion. A removal of >99% was achieved at an optimum dose of 80 mg/L SDS and 5 mg/L Al (III) ions in continuous and batch tests with municipal sewage and hospital wastewater samples. Collected tetracycline in precipitate was swiftly photodegraded with ultraviolet radiation at 365 nm. The process was equally effective in removing NSAIDs (ketoprofen, ibuprofen, and mefenamic acid), fluoroquinolones, antidepressants fluoxetine, and antihistamines (chlorpheniramine and diphenhydramine).150 Surfactants have also been used in electrocoagulation flotation (ECF) process for treatment of PPCPs. A cationic surfactant cethyltrimethylammonium bromide (CTAB) has been used to remove ibuprofen, diclofenac, and ketoprofen from water. Removal efficiencies with the ECF process without addition of the surfactant were 44%, 14%, and 10% for ibuprofen, diclofenac, and ketoprofen, respectively, in single NSAID systems and less by 10% in mixed systems, which was related to the total increase in NSAID concentrations and the competitive adsorption between the different compounds. Addition of the surfactant significantly increased the removal from 12% to 97%, 12% to 88%, and 6% to 82% for diclofenac, ibuprofen, and ketoprofen. The amount of surfactant for optimum removal of all NSAIDs was estimated to be equivalent to the molar concentration of NSAIDs.145 However, in real hospital effluent, the removal was hindered significantly by the interference of other competing hydrophobic compounds.145

3.1. Surfactants for the remediation of metal-contaminated soil

Soil washing augmented with surfactants has been extensively studied for metal decontamination. Soil washing with pure water aided by the use of different chelating agents, solvents, and chemical additives can remove toxic metals, ;however, duration of the treatment process and low availability due to interaction with soil particles can be a major limitation. In this case, surfactants have been proved to reduce time requirements and maximize the efficiency and effectiveness of the process. Soil washing with surfactants may include ex situ or in situ treatments. In the former case, soil is excavated and placed in a suitable place followed by washing with surfactant/biosurfactant solution. In in situ washing treatment, soil is first treated with surfactants followed by its relatively stable complexation with oppositely charged metal ions and then movement of the resultant complex due to reduced interfacial tension. The charged surfactants make use of the ion-exchange mechanism as well as by the micelles using electrostatic interaction.152,159 Using soil-washing techniques, metals are permanently removed, recycled, or reduced by volume with less time consumption.160 Torres et al.161 studied surfactant-enhanced soil washing for soil remediation contaminated with high concentrations of As, Cd, Cu, Ni, Pb, and Zn from industrial effluents using 11 different types of surfactants: four anionic, four nonionic, one zwitterionic, and two unknown-charged surfactants. Higher removal was noted for Cu, Ni, and Zn with maximum removal achieved for most metals using the anionic surfactant Texapon N-40, for Cu (83.2%), Ni (82.8%), and Zn (86.6%). Removal with Tween 80 was also high for Cd (85.9%), Zn (85.4%), and Cu (81.5%), while Polafix CAPB achieved 79%, 83.2%, and 49.7% removal for Ni, Zn, and As, respectively. Desorption of metals with surfactants can be further supplemented with the use of additives as complexing agents or with changing the pH conditions. Wen and Marshall162 found that using ethylenediaminedisuccinic acid (EDDS) as an additive with surfactant can

3. SURFACTANTS FOR THE REMOVAL OF TOXIC METALS Excessive levels of heavy metals in soil and water have been a major environmental concern over the past few decades.151 Heavy metals are nonbiodegradable; hence, their presence in aqueous systems causes long-term detrimental effects for humans, animals, aquatic life, plants, and microorganisms. Some of the important sources of heavy metal contaminations are weathering/erosion, mineral extraction processes, metallurgical industries, leather industries, pharmaceutical industries, fertilizer and pesticides industries, and indiscriminate burning of fuels containing high metal content.152 The United States 6056


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synthetic surfactants has been extensively investigated. Mukhopadhyay et al.167 studied a plant-based surfactant from Sapindus mukorossi in comparison to anionic surfactant SDS for the removal of As from Fe-rich soil in column washing. It was noted that natural surfactant had superior removal ability (86%) than the SDS toward As removal. The mechanism of removal was found to be micelle solubilization and desorption with no chemical interaction of the biosurfactant with As. Hence, the biosurfactant in this process can be reused. In another study, Chen et al.168 compared quillaja saponin (a biosurfactant), SDS, and EDTA for the removal of Cu and Ni from kaolin clay suspension and saponin (2000 mg/L) micelles for the removal of Cu (83%) and Ni (85%) metals from soil. Kim and Vipulanandan169 also compared the removal of Pb from soil using biosurfactant and anionic and nonionic surfactants in batch experiments. The removal efficiency of metals was linked to the level of contamination and type and concentration of surfactant. Micellar solution of biosurfactant removed 75% of Pb in basic conditions. The Pb removal was attributed to the presence of a carboxyl group on the surfactant by complexation mechanism. The results also revealed higher micelle partitioning of biosurfactant for Pb. Soil washing has also been researched with biosurfactants alone. Rhamnolipids (produced by Pseudomonas aeruginosa) are the most widely used biosurfactant in this respect. Juwarkar et al.170 evaluated the compatibility of rhamnolipid for the removal of Cd and Pb from artificially contaminated soil in column experiments and showed a removal of 92% and 88%, respectively, from leachable and bound fractions within 36 h. An increase in microbial population was also observed that indicated a reduction in toxicity of the soil, hence approving the use of biosurfactants for toxic metals removal without endangering soil fertility and microflora.170 Similarly, Wang and Mulligan171 investigated rhamnolipid for the removal of As, Cu, Pb, and Zn from an oxidized mine tailing. Rhamnolipid (0.1%) at pH 11 significantly enhanced the removal of As, Cu, Pb, and Zn. Mobilization of As may have occurred due to complexation or micelles formation through metal-bridging mechanism.21 In a similar experiment, sequential extraction procedure showed As mobilization from weakly bound fractions of soil. The capillary electrophoresis (CE) analysis showed that arsenate As(V) could be removed with no significant effect on As redox or methylation reaction. These studies showed that rhamnolipid-based soil washing is enhanced at higher pH and biosurfactant could be useful for the removal of As from highly contaminated soil under alkaline conditions.172 Dahrazma and Mulligan used rhamnolipid (0.5% solution) in a continuous-flow configuration for the removal of Cu, Zn, and Ni from sediments and reported a removal of 37%, 13%, and 27%, respectively. The addition of NaOH (1%) enhanced the efficiency by 4 times.173 A sequential extraction procedure showed that Cu was removed mainly from oxide and hydroxide portions, probably due to the solubilization of the organic components of the sediments.174 Small-angle neutron scattering (SANS) showed that pH had a pronounced effect on the size and aggregates of rhamnolipid micelle in the presence of heavy metals. In basic condition using NaOH (1%), large aggregates (>2000 Å) were formed, whereas in acidic conditions large vesicles (550−600 Å) formed in the presence of 1% NaCl. In both acidic and basic conditions, the sizes of aggregates were fine enough to allow the flow of rhamnolipid solution through a pore size of 200 nm, which showed that

mobilize As, Cd, Cr, Ni, Pb, and Zn as well as organic contaminants in the washing process while Al, Ca, Cu, Fe, and Mn remained in the residual fractions. Multiple ultrasonically aided washing treatments increased the metals removal from soil. When using EDDS at higher pH values, Pb and Zn removal was less efficient, probably due to their anionic hydroxide complex formation. Meanwhile, As and Cu levels were not lowered due to their association with Fe-oxides. The extraction/precipitation and reuse of reagents made the process economical, whereas less water was used for each wash, i.e., 6.6 L compared to 53.3 L. Similarly, Doong et al.163 investigated SDS (anionic), Triton X-100 (nonionic), and cetyltrimethylammonium bromide (cationic) surfactants for metal-removal capacities in the presence of EDTA and diphenylthiocarbazone (DPC) and increasing pH. Desorption of Cd, Pb, and Zn was high with anionic and nonionic surfactants. Cationic surfactants were found with better efficiencies in acidic conditions. Metaldesorption efficiency with EDTA addition was significantly enhanced, and in surfactant−EDTA mixture it was in the order of Cd > Pb > Zn. Metal removal with DPC addition was lower by 2−4 times, whereas as increase in pH decreased metalextraction efficiency of nonionic and anionic surfactants.163 The effect of pH in surfactant-based treatment of heavy metals was further shown by Slizovskiy et al.164 using a cationic (1dodecylpyridinium chloride, DPC), a nonionic (oleyl dimethyl benzyl ammonium chloride, ammonyx KP), and rhamnolipid surfactants. Acidification with citric acid buffer or EDTA significantly enhanced removal efficiency for Zn, Cu, Pb, and Cd up to 95%.164 Surfactants in combination with iodide ligand have also been used. Shin and Barrington165 studied SDS and Triton X-100 for desorption of Cd from contaminated soil, in the presence of iodide ligand. Increased concentration of surfactant and iodide ions enhanced the removal of Cd with no effect on Cu, Zn, and Pb. Triton X-100 and iodide ligand removed 65% and 90% of the Cd from soil I (15 mg/kg) and soil II (1275 mg/kg), respectively, compared to 35% and 70% with SDS-iodide ligand. With SDS/SDS-iodide ligand, Cd was removed from the carbonate and oxide forms, while with Triton X-100, it was removed from the exchangeable fraction of the soil. The research showed that selective and specific combinations of surfactant−ligand remove specific heavy metals from the contaminated soil. Similarly, Chang et al.166 used laurylED3ANa3 (LED3A), anionic surfactant (SDS), and nonionic surfactant (Triton X-100) for Cu and Cd removal from contaminated soil in a wide range of pH conditions (5−11). It was found that LED3A alone removed 40% of the Cd from soil. Addition of SDS enhanced the removal up to 80%. The use of LED3A-SDS and alkaline procedures (pH 11) successfully removed both metals without membrane separation. The efficiency with anionic surfactant may have been due to counter exchange at the micelle surface, complexation with surfactant, or matrix dissolution. Although these results showed enhanced metal removal from soil with surfactants, choosing the most appropriate surfactant or combination for particular metal(s) needs further optimization to enhance the removal efficiency. 3.2. Use of biosurfactants in soil washing for the removal of heavy metals

In recent years, there has been an increasing focus on the use of biosurfactants in soil washing due to toxicity of synthetic surfactants and their long-term resistance to degradation. The efficacy of biosurfactants for soil washing in comparison to 6057


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surfactin/1% NaOH. A series of five batch washes using a formulation of 0.1% surfactin/1% NaOH and 4% sophorolipid/ 0.7% HCl removed Cu (70%) and Zn (100%) from the soil. Sequential extraction procedures showed that carbonate and oxide fractions accounted for over 90% of Zn and 70% of Cu from the organic fraction. The results indicated that Cu could more easily be removed than Zn with multiple washings and low surfactant concentration. The maximum removal of Cu in basic conditions was attributed to the hydrolysis of acidic groups of petroleum components present as cocontamination. The experimental formulations were considered useful for organic cocontaminated metals from soil, and the extraction procedures were deemed helpful to find out the best conditions and formulations from prior soil washing for later application in large-scale soil remediation. The same group also investigated rhamnolipid (0.5%), sophorolipid (4%), and surfactin for the removal of Cu and Zn from sediments. Cu removal was higher with rhamnolipid (65%) compared to 25% and 15% with sophorolipid and surfactin, respectively, whereas higher Zn removal was achieved with sophorolipid (60%) compared to 18% and 6% with rhamnolipid and surfactin, respectively. Ultrafiltration and zeta potential measurements showed metals sorption and consequent complexation with surfactants to form micelles. Sequential extraction procedures before and after washing showed that 90% of Zn exists in carbonate and oxide form while 70% of Cu was from the organic fraction of the sediment. The results showed that rhamnolipid and surfactin could be used for the removal of Cu bound to the organic fraction while the sophorolipid can successfully remove Zn bound to carbonates or oxides in sediments.182 Aşcı̧ et al.183 investigated the effect of concentration of rhamnolipid and Zn ion and pH for the removal of Zn from Na-feldspar (soil component). An increase in Zn concentration decreased the sorption of Zn ions from 67.74% to 19.47%. An increase in rhamnolipid concentration (25 mM) resulted in optimum desorption efficiency for Zn (85.29%), while at optimum pH (6.8), desorption of Zn (83.87%) was noted. An increase in Zn ion (2.2 mM) concentration resulted in maximum desorption of Zn (98.83%) using rhamnolipid (25 mM, pH 6.8) facilitated micelle formation and desorption due to surfactant−metal interaction. Aşcı̧ et al.184 also investigated rhamnolipid for Cd removal from kaolin with the effect of pH and initial Cd ions and rhamnolipid concentration. Under optimum conditions (initial Cd ion concentration was 0.87 mM at rhamnolipid concentration of 80 mM and pH 6.8), Cd removal from kaolin was 71.9%. In a similar experiment, the same authors reported the sorption and desorption phenomena of three different types of soils having different geochemistries. It was concluded that soil mineralogy plays an important role in the soil remediation by rhamnolipid.185 Song et al.186 studied saponin for the removal of Cd cocontaminated with phenanthrene. The phenanthrene desorbed into the micelle while Cd formed a complex with the carboxyl group on the saponin micelle. Enhanced efficiency was found for the removal of phenenthrene and Cd versus Triton X-100 and citric acid. Saponin removed Cd (87.7%) and phenenthrene (76.2%) with no inhibitory effect due the presence of organic contaminants on the Cd removal from the soil. The study also found the Cd on the outside and phenanthrene on the inside of the micelles. Mukhopadhyay et al.187 studied biosurfactant from soapnut fruit and phosphate solution separately and in mixture for the removal of As from soil. As removal with biosurfactant−

metal−rhamnolipid complexation had no pronounced effect on the size of the micelles.175 Rhamnolipid has also been used for removal as well as reduction of heavy metals to less toxic forms. For example, Cr(VI), a more toxic form, was reduced to less-toxic Cr(III) form in a study by Massara et al.,176 who investigated the removal of Cr from contaminated Kaolinite by rhamnolipid. The rhamnolipid removed 25% of Cr(III) from the kaolinite, whereas Cr(IV) removal also increased by a factor of 2. The sequential extraction revealed that Cr(III) was removed mainly from the carbonate and oxide components of the kaolinite. Using rhamnolipid in combination with mixed bacterial culture has also been found to show decent removal. Diaz et al.177 investigated rhamnolipid and mixed bacterial culture of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans for Zn and Fe removal from contaminated soil. Combined use of rhamnolipid and a mixture of bacterial culture improved the removal of Zn and Fe from ∼50% and 19% (obtained with separate treatment with bacteria or rhamnolipid) to 63−70% and 36%, respectively. Rhamnolipid was found to be useful for different soil types. Torrens et al.178 investigated the optimum dose of rhamnolipid for removal of soil-bound Cd from four types of soils in batch and column experiments. Batch experiments showed that rhamnolipid sorption of coarse, loamy soil changes with the concentration of rhamnolipid and the concentration of the rhamnolipid matrix. Surfactant-bound Cd was released from soil to the solution phase. In column experiments saturated flow conditions were applied to four soils. The samples were first treated with KNO3 (3.5−7 mM K+) followed by rhamnolipid washing (5 or 10 mM). The results showed that 15 and 36% of the Cd was removed by the KNO3 washing, while rhamnolipid removed 8−54% of the Cd. These investigations proved the effectiveness of rhamnolipid washing for different types of soil; however, soil with more clay content may cause dispersion and column plugging. Manipulation of ionic strength can lead to increased rhamnolipid activity. However, the main ion (Ca) present in soil and geochemistry was not researched. Besides rhamnolipids, other biosurfactants have also been used. Gao et al.179 investigated the removal of Pb, Cr, and Ni in both batch and column experiments from industrial sludge and natural soil using saponin and sophorolipids biosurfactant. Saponin was more effective than sophorolipids due to the presence of carboxyl groups and better selectivity for mobilization and mainly removed carbonate and Fn-Mn oxide-bound metals. In alkaline conditions, the removal efficiency of Pb, Ni, and Cr was 90−100% using the precipitation method. Gusiatin and Klimiuk180 investigated saponin for the removal of heavy metals from loamy sand, loam, and clay. The highest metal-removal efficiency was obtained for loamy sand (82−90%), loam (67−88%), and silty clay (39−62%) in a single wash. A higher mobility factor was observed for loamy sand and loam compared to silty clay. Redistribution index showed Cu in loamy sand and loam as the most stable in residual and organic fraction. The removal of Cd from silty clay required multiple washings. Mulligan et al.181 compared rhamnolipids, surfactin, and sophorolipids in different formulations for the removal of Zn and Cu. The use of 12% rhamnolipid and 4% sophorolipid/ 0.7% HCl removed 19.5% and 15.8% of Zn, respectively. The highest removal of Cu (>25%) was achieved with 12% rhamnolipid or 2% rhamnolipid/1% NaOH or 0.25% 6058


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that Cd2+ reacted faster to form stable complexes with rhamnolipid with subsequent reduction in its concentration. It was found that rhamnolipid (7.3 mM) removed 92% of Cd2+ (0.72 mM) whereas rhamnolipid (3.9 mM) removed 97% of the Cd2+ (0.36 mM). The calculated Cd−rhamnolipid stability constant (log K = −2.47) was higher than the reported Cd− sediment and Cd−humic acid systems. Recovery of rhamnolipid from the metal complex for quantification was achieved by acid precipitation and centrifugation. Ochoa-Loza et al.196 evaluated the complexation affinity of rhamnolipid with 13 metals including three common metals (Ca, Mg, and K) present in soil and wastewaters. Ion-exchange resin technique was used for the determination of conditional stability constants. The stability constants followed the order Al3+ > Cu2+ > Pb2+ > Cd2+ > Zn2+ > Fe3+ > Hg2+ > Ca2+ > Co2+ > Ni2+ > Mn2+ > Mg2+ > K+, indicating preferential complexation of rhamnolipid with heavy metals. The results showed that rhamnolipid could be applied for the removal of metals from contaminated water, especially where metals are strongly bound to colloidal particles and organic contaminants. The Mulligan research group47 investigated the comparative efficiencies of rhamnolipid and Triton X-100 in foam and solution forms. Distilled water in control experiment could only remove Cd (17.8%) and Ni (18.7%). The rhamnolipid (0.5%) removed 73.2% and 68.1% of Cd and Ni, respectively, in comparison to 65.5% and 57.3% with the foam generated by synthetic surfactant, Triton 100-X. The solution form of rhamnolipid removed 61.7% and 51%, respectively, and Triton X-100 removed 52.8% and 45.2% of Cd and Ni, respectively.197 The effect of pH (6.8, 8, and 10) and surfactant concentration (0.5, 1.0, and 1.5%) investigations showed no significant effect on the removal efficiency of these surfactants.198 Better results could be obtained using rhamnolipid foam technology. However, soil chemistry and metal speciation as well as cost effectiveness could be thoroughly investigated before any field applications. Chen et al.199 researched surfactin, SDS, and Tween-80 for the removal of Hg under the effect of concentration of surfactants and mercury, pH, foam volume, and digestion time. Increased surfactant concentration had a positive effect on Hg removal from soil. Surfactin was used 10× CMC, and the highest removal of Hg was noted. SDS required >10× CMC, and Tween required 12 when no surfactants were present. In the presence of SDBS surfactant, the adsorption increased from 63 to 95% at pH 4−7, then decreased slowly down to 75% at pH greater than 12. Adsorption of Pb2+ on plain OMWCNTs at pH < 7 was slightly more in the presence of TX-100. However, the effect of SDBS on the adsorption of Pb2+ was significantly greater than those of TX-100 and BKC, due to possible complex formation between cationic Pb2+ and anionic surfactant (DBS−).207 The modification of OMWCNTs by SDBS may alter the surfaces for increased sorption of Pb2+ by its partial complexation with SDBS adsorbed over OMWCNTs.208,209 In one particular study, synthetic and natural surfactants were used for the removal of Cu, Ni, Zn, Pb, As, and Cd.206 The average removal percentages of Tween 80, Surfacpol 14104, and Emulgin W600 were found to be 67.1%, 64.9%, and 61.2% for Pb, As, and Cd, respectively, while other surfactants showed exceptional removal of several metals. For example, Texapon N-40 removed 83.2% Cu, 82.8% Ni, and 86.6% Zn, and CAPB eliminated 83.2% Zn, 79.0% Ni, and 49.7% As. Therefore, both synthetic and natural surfactants have great potential for the removal of high concentrations of metals from soils containing metal pollutants. Singh and Turner explored commercially available surfactants for the mobilization of several metals including Al, Fe, Cd, Cu, Mn, Ni, Pb, Sn, and Zn from polluted estuarine sediments.210 Increased concentration of the anionic surfactant SDS, up to and above its CMC, was found to increase the release of metals. However, using sodium taurocholate, a bile acid salt, and Triton X-100, a nonionic surfactant, metal mobilization did not change upon varying the concentration of surfactant. The most efficient surfactant for the mobilization of metals was thus SDS, which helped in the 6060


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Figure 11. General method of (a) ex situ washing and (b) in situ flushing for soil decontamination and recovery of surfactants. Reprinted with permission from ref 215. Copyright 2015 Elsevier.

transform into insoluble form through a precipitating agent such as lime. The precipitate characteristically exists in hydroxide form. Precipitating lime is used to eliminate heavy metal ion such as Mn2+, Cd2+, and Zn2+ in a batch continuous scheme.213 Lime precipitating agent is used for the efficient treatment of inorganic effluents containing metal concentration more than 1 g/L. On the whole, adjusting pH to basic conditions is the main factor that appreciably improves the elimination of heavy metal via the chemical precipitation method. However, chemical precipitation demands huge amount of chemicals to decrease metals to an adequate concentration level. Other shortcomings are unwarranted sludge generation, poor settling, heavy sludge removal cost, aggregation of metal precipitates, and long-lasting ecological impacts of waste elimination. Solids or diffused liquids are separated from a liquid phase with bubble attachment by employing the process of flotation. The bubbles rising to the surface burst, and the attached particles are thrown away from the suspension. The elimination process depends on interfacial chemistry and effectiveness of aggregation. The metal ions extraction efficiency can be improved by employing combined membrane and flotation separation techniques using CTAB as a cationic accumulator and SDS as anionic collectors.214

release of Cd, Ni, and Cu by a mechanism involving metal− surfactant complex formation and denudation of hydrophobic host phases. Thus, surfactants play a key role for the remediation of metal-polluted soils and sediments.210 Remediation becomes more challenging when a site is cocontaminated with both organics and metals. A combination of surfactant and chelating agent can be a useful choice to remove the organics and trace metals. Because synthetic surfactants are mostly toxic to soil microorganisms, Cao et al. used the biosurfactant saponin and biodegradable chelant S,Sethylenediaminedisuccinic acid (EDDS) in combination to treat PCB/trace metals (Pb and Cu)-contaminated soil.211 The maximum desorptions of PCB, Cu, and Pb with respective percentages of 45.7%, 85.7%, and 99.8% were achieved by the addition of 3 g/L saponin and 10 mM EDDS. The synergistic performance was contributed by the marked interaction between saponin and EDDS, and the adsorption of saponin and EDDS onto the soils was inhibited by the other surfactant. Metal complexation and the capability of PCB solubilization was enhanced by EDDS and the saponin micelles, respectively. Thus, soil remediation based on a combination of biosurfactant and chelating agent could be a promising method for the decontamination of soil cocontaminated with organic compounds and metals.211,212 The chemical precipitation method is usually applied for the elimination of heavy metals from inorganic discharge. In strongly alkaline conditions, the solubilized metal ions

4. PRACTICAL IMPLEMENTATION Soil washing is a practically employed method for the decontamination of soil from organic contaminants. In this 6061


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were found to be more effective for extracting carboxylic acids than only ILs. The results demonstrated significantly higher amines and phenolic contaminants removal efficiency of quaternary ammonium-based RTILs than n-octanol. The distribution constant values of phenols and amines were generally a factor of 5−10-fold higher than studied for common 1,3-dialkylimidazolium-based RTILs. However, the use of RTILs is limited as they are expensive, highly viscous, and freely soluble in aqueous systems. The occurrence of estrogens in surrounding waters has drawn a great deal of attention from scientists and the general public.229,230 The main source of estrogens is the seepage from wastewater-treatment plants because of their unfinished removal by the existing sewage management.231,232 The estrogens present in water affect endocrine as well as other physiological systems even at dilute concentration,233,234 so their elimination from wastewaters is an issue of serious concern. A number of methods such as activated carbon adsorption, chlorination, ozonation, ultraviolet irradiation, and membrane separation have been tested for estrogens removal.235−237 However, the degree of elimination (from 99.8%) assemblage. SDS (62 mg L−1) and 20 mg L−1 of PAAH were predictably exercised for confirming the absolute and reproducible assemblage. An anionic surfactant, sodium oleate, was also used, and PAAH was almost completely (>98%) coagulated by using 65 mg L−1 of sodium oleate. Figure 12D demonstrates the amounts of surfactant needed for absolute coagulation of various amounts of PAAH. Figure 13 shows the pH dependence on the recovery of PAAH−SDS and PAAH−sodium oleate complexes. PAAH was quantitatively (>99.6%) collected as a compressed aggregate in the pH range of 2−8, and reclamation was found to be temperatureindependent. The decrease in PAAH recovery above pH 8.5 can be accredited to the decrease in positive charge of PAAH owing to deprotonation of −NH3+ groups. Sodium oleate was available for surfactant−PAAH aggregates formation, but total approximate recovery of >98% was attained only in a limited pH regime of 7−8, due to partial protonation of oleate ions in a medium of pH 8. The presence of an ionic headgroup in the surfactant molecule can lead to greater extraction of positively charged contaminants. Triton X-114 is the most recurrently used anionic surfactant for CPE experimentation. Figure 18b displays a substantial change in absorbance signal by varying the concentration of surfactant. Increasing the quantity of surfactant leads to an increase in its volume. This results in a more dilute extract that lowers the sensitivity with increased concentration of Triton X-114 above 0.13% (w/v). Owing to an increase in surfactant volume, flame atomic absorption spectrometry (FAAS) signal is destroyed. A decrease in extraction efficiency was observed at comparatively lower concentrations due to fewer surfactant molecules to entrap ligand−metal complexes. Thus, for acquiring better enrichment factors and higher extraction efficiency, Triton X-114 concentration of 0.13% (w/v) was chosen. The conjoined benefits of cloud-point methodology (easy, inexpensive, rapid, and safe) and its application as a chromogenic reagent for the reason of being sensitive and selective were utilized for the quick extraction of Ag, Pd, Pb, and Cd from real samples. Moreover, no organic solvent was used, contrary to several other preconcentration methodologies.262 Surfactants and polymers can be combined and used for the removal of metallic toxins. In fragile systems, mixture of nonionic polymer and ionic surfactant result in two break points (T1 and T3) in the surface tension−concentration plot, as shown in Figure 19. T1 represents the critical aggregation concentration (CAC) at which the formation of micelle-like aggregates starts on the polymer chain and T3 denotes CMC.263 After point T1 the interface gets more or less saturated by complexes and SDS develops micelle-like aggregates on polymer chains, thus resulting in polymer− surfactant aggregates (PSAs) at the CAC due to strong electrostatic and hydrophobic forces. As soon as bulk polymers get saturated by surfactant, additional SDS is energetically

Figure 17. pH effect on the CPE of Ln3+ with HDEHP, La3+ (●), Eu3+ (▲), and Lu3+ (■). Reprinted with permission from ref 256. Copyright 2007 Elsevier.

Examining the occurrence of toxic trace elements in biotic fluids is tremendously significant to estimate their environmental exposure.257−261 Conventional liquid−liquid extraction and other traditional approaches of wastewater treatment are time-consuming and workforce-intensive, above and beyond demanding comparatively high purity and large amounts of toxic solvents. In this regard CPE is a practically feasible method for any metal ion to be extracted from mother solution. Improvement in the CPE method entails optimization of some 6065


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concentration range to the bulk polymer forms more PSAs in the solution. After saturation of the polymers within the bulk phase, a further increase in SDS concentration leads to its association with the surface polymer−surfactant complexes. The increase in surface tension can be related to the transformation of polymer−surfactant complexes near the surface into PSAs, which then leave the air−liquid interface to go back into the bulk solution. The peak denoted by point C indicates the concentration of SDS at which major polymer− surfactant complexes from the surface go back into bulk solution.267 Therefore, at peak C, the bulk solution contains the highest amount of PSAs. These aggregates were evidenced to interact with free Zn(II) through electrostatic interactions and formed flocculates that were separated from solution by a 20 μm coarse filter. The adsorption of Zn(II) was found proportional to PSA aggregates in the system, and ∼100% elimination efficiency was recorded at a concentration of SDS corresponding to peak C. The method also showed great promise for the removal of Cr(III) and Cd(II)

Figure 19. Sketched surface tension curves for weak and strong polymer and surfactant systems. Reprinted with permission from ref 263. Copyright 2007 Elsevier.

preferred for further saturation of the surface polymer− surfactant complexes followed by bulk migration to form more PSAs. This migration leads to a partial depletion of the surface, which increases the value of surface tension after a low plateau at point T2. At a higher concentration of SDS, another low plateau appears in the surface tension plot after point T3, because further addition of SDS results in the formation of micelles at and above the CMC rather than moving to the interface. High metal-removal efficiency can be achieved in dilute solutions using surfactant−polymer-enhanced ultrafiltration. Nevertheless, membrane cost, energy consumption, and chemical dosage remain as challenges.264,265 The combined role of cationic polymer, poly(ethylenimine) (PEI), and anionic surfactant, SDS, was explored for the removal of Cr(III), Zn(II), and Cd(II).265 The pH of the medium, salt, and organic contaminants were found to influence the removal efficiency of metals. The elimination efficiency was compared to MEUF and polymer-enhanced ultrafiltration (PEUF). Under constant conditions, increasing SDS concentrations from zero to point A (Figure 20) led to

5. CONCLUSION Water-treatment technologies such as flocculation and coagulation cause secondary pollution. Conventional methods such as solvent extraction and ion exchange also suffer from serious limitations of their own environmental risks, high cost, lack of versatility, and incompatibility for the simultaneous removal of both organic and metal-based contaminants. The use of biosurfactant-based remediation technology can overcome all these limitations and meet the stringent environmental regulations because such surfactants have the ability of satisfying the requirements of stability, integrity, and beauty of natural systems. Critical analysis of the currently used soil and water treatment technologies suggests that surfactants could be the future of wastewater management as their micelles can encapsulate hydrophobic organics in their interior (core) and capture inorganic contaminants in their exterior. A large number of surfactants have proved to be promising for the decontamination of water and soil from heavy metals, dyes, pharmaceuticals, and personal care products. The contaminantremoval efficiency of surfactants is influenced by various factors such as soil chemistry, micellar size, ion-exchange capacity, electrolyte content, nature and concentration of pollutant, pH, and aeration state. By applying micelles of ionic−nonionic surfactants, production of clean water and recovery of valuable metals from industrial wastewater could be achieved with low or no undesirable effect on the environment. On the basis of distribution ratio and extraction percentage, it can be concluded that micelles-based water and soil treatment technology has been a success and can be employed further in the future on a large scale. Biosurfactants as environmentally friendly alternatives have been successfully employed, but large-scale application is still limited due to high costs. Further research on the development and application of cost-affordable biosurfactants can make wastewater-treatment technology more economical and greener.

Figure 20. Surface tension and Zn(II) removal efficiency at 80 ppm 750 K branched PEI and 19 ppm/0.3 mM Zn(II) and various SDS concentrations. Reprinted with permission from ref 267. Copyright 2010 American Chemical Society.

electrostatic binding of SDS to PEI to develop polymer− surfactant complexes at the surface, lowering the surface tension until the formation of 8 PSAs around point A. Beyond this concentration range of SDS, 20% Zn(II) was removed due to chelation between Zn(II) and ammonia groups of PEI. The removal was in agreement with the results obtained from PEUF.266 The plateau in surface tension−concentration plot from A to B indicates that addition of SDS monomers in this

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Dr. Afzal Shah). Notes

The authors declare no competing financial interest. 6066


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Biographies

Cincinnati. He teaches courses and performs research in the areas of water quality, treatment, and monitoring. He is currently one of the editors of Chemical Engineering Journal, Editor of the Journal of Advanced Oxidation Technologies, Special Issue Editor of the Journal of Environmental Engineering (ASCE), and member of the Editorial Boards of several other journals. He is the author or coauthor of over 275 refereed journal publications, and his work received over 10 000 citations with an H factor of 55.

Dr. Afzal Shah is assistant professor in Quaid-i-Azam University (QAU), Islamabad, and visiting professor in International Center for Chemical and Biological Sciences of Karachi University, Pakistan. He earned his Ph.D in 2010 under the supervision of Prof. Rumana Qureshi. He worked as a postdoctoral fellow (2014) in the research group of Prof. Kraatz at the University of Toronto. He is the recipient of several awards of QAU and Pakistan Council of Science and Technology including young scientist award of 2013. His research interests include elucidation of proton-coupled electron transfer reaction mechanism of biologically important molecules, development of sensitive voltammetric methods for the detection of cancer biomarkers, and utilization of micelles of biocompatible polymers and green surfactants as drug-delivery vehicles and environmentalprotection agents.

Dr. Usman Ali Rana completed his Ph.D. in the discipline of Materials Engineering in 2012 under the tutelage of Prof. Maria Forsyth and Prof. Douglas. R. MacFarlane. He is assistant professor in Sustainable Energy Technologies (SET) center, at King Saud University. His work is focused on the subjects of electrolytes and electrocatalyst developments for catalytic water splitting, fuel cell catalyst and membranes, ionic liquids as green solvents, electrochemistry of pharmaceutically important compounds, supercapacitor materials, and lithium ion batteries.

Suniya Shahzad received her M.Phil. (Physical Chemistry) from Quaid-i-Azam University, Islamabad, in 2013. Her Ph.D. work in the research group of Dr. Afzal Shah is focused on the development of nanoparticle-modified electrochemical nanobiosensors for unfolding the DNA binding modes of potential anticancer compounds and investigation of their pH-responsive encapsulation and release from micelles of biodegradable surfactants.

ACKNOWLEDGMENTS The authors are thankful to Higher Education Commission and Quaid-i-Azam University, Islamabad, Pakistan. D.D.D. also acknowledges support from the University of Cincinnati through a UNESCO co-Chair Professor position on “Water Access and Sustainability”.

Azeema Munir received her M.Phil. (Physical Chemistry) from Quaidi-Azam University, Islamabad, in 2013. She is doing her Ph.D. in the research group of Dr. Afzal Shah. Her research interests include electrochemistry of surfactants, nanosensors biotechnology, drug analysis, computational studies, and redox mechanisms of drugs.

REFERENCES (1) Shah, A.; Shah, A. Z.; Mahmood, S.; Ullah, I.; Rehman, Z. Cost Effective Procedures for Extremely Efficient Synthesis of Environmental Friendly Surfactants. Tenside, Surfactants, Deterg. 2013, 50, 160−168. (2) Zahid, A.; Lashin, A.; Rana, U. A.; Al-Arifi, N.; Ullah, I.; Dionysiou, D. D.; Qureshi, R.; Waseem, A.; Kraatz, H.-B.; Shah, A. Development of Surfactant Based Electrochemical Sensor for the Trace Level Detection of Mercury. Electrochim. Acta 2016, 190, 1007− 1014. (3) Rodríguez-Escales, P.; Borràs, E.; Sarra, M.; Folch, A. Granulometry and Surfactants, Key Factors in Desorption and Biodegradation (T. Versicolor) of PAHs in Soil and Groundwater. Water, Air, Soil Pollut. 2013, 224, 1−12. (4) Zhao, B.; Zhu, L.; Li, W.; Chen, B. Solubilization and Biodegradation of Phenanthrene in Mixed Anionic−nonionic Surfactant Solutions. Chemosphere 2005, 58, 33−40. (5) Ullah, I.; Ahmad, K.; Shah, A.; Badshah, A.; Ali Rana, U.; Shakir, I.; Rehman, Z.; Khan. Synthesis, Characterization and Effect of a Solvent Mixture on the CMC of a Thio-Based Novel Cationic Surfactant Using a UV−Visible Spectroscopic Technique. J. Surfactants Deterg. 2014, 17, 501−507. (6) Ullah, I.; Shah, A.; Badshah, A.; Shah, A.; Shah, N. A.; Tabor, R. Surface, Aggregation Properties and Antimicrobial Activity of Four Novel Thiourea-based Non-ionic Surfactants. Colloids Surf., A 2015, 464, 104−109. (7) Ullah, I.; Shah, A.; Khan, M.; Khan, S. Z.; ur-Rehman, Z.; Badshah, A. Synthesis and Spectrophotometric Study of Toxic Metals Extraction by Novel Thio-Based Non-Ionic Surfactant. Tenside, Surfactants, Deterg. 2015, 52, 406−413. (8) Munir, A.; Ullah, I.; Shah, A.; Rana, U. A.; Khan, S. U.-D.; Adhikari, B.; Shah, S. M.; Khan, S. B.; Kraatz, H.-B.; Badshah, A. Synthesis, Spectroscopic Characterization and pH Dependent Electrochemical Fate of Two Non-Ionic Surfactants. J. Electrochem. Soc. 2014, 161, H885−H890. (9) Ahmad, Z.; Shah, A.; Siddiq, M.; Kraatz, H.-B. Polymeric Micelles as Drug Delivery Vehicles. RSC Adv. 2014, 4, 17028−17038. (10) Fendler, J. H. Interactions and Kinetics in Membrane Mimetic Systems. Annu. Rev. Phys. Chem. 1984, 35, 137−157.

Dr. Mallikarjuna N. Nadagouda received his Ph.D. from India in 2003. He is a Physical Scientist at National Risk Management Research Laboratory in Cincinnati, Ohio. He has worked in the areas of nanomaterials and nanotechnology, analytical chemistry, green chemistry, polymer blends, solid coatings, solid-state chemistry, and drug delivery. He has received several Scientific and Technological Achievement Awards (STAA) from the EPA, including the National Risk Management Research Laboratory Goal 1 Award. He is a member of the editorial advisory board of several international journals, has published over 175 papers in reviewed journals with a citation index ∼6000 (H index 36), and holds several patents. He is also adjunct professor at Wright State University. Gul Shahzada Khan received his M.Phil. degree in Chemistry from Quaid-i-Azam University, Islamabad, in 2004 and earned his Ph.D. in organic chemistry with Prof. David Barker from the School of Chemical Sciences, University of Auckland, New Zealand. He is currently working as Assistant Professor and Director of the Office of Research Innovation and Commercialization (ORIC) at Shaheed Benazir Bhutto University, Sheringal Dir (U), Pakistan. His research interests include solution behavior of surfactants, isolation, and synthesis and designing of selective biomolecules. Dilawar Farhan Shams received his M.S. degree in Environmental Engineering from the National University of Sciences and Technology, Pakistan, and earned his Ph.D. in Civil/Environmental Engineering from the University of Auckland, New Zealand, in 2013. After his doctoral studies, he joined the Department of Environmental Sciences, Abdul Wali Khan University Mardan, Pakistan, where he is now an Assistant Professor. His current research interests include biological nitrogen removal and wastewater treatment, occurrence and removal of emerging contaminants, and nanotechnology-enhanced filtration and waste-to-fuel conversion. Dr. Dionysios (Dion) D. Dionysiou is currently a Professor of Environmental Engineering and Science Program at the University of 6067


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