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Journal of Hazardous Materials 306 (2016) 149–174

Contents lists available at ScienceDirect

Journal of Hazardous Materials

j o ur nal ho me pa ge: www.elsev ier .com/ locate / jhazmat

eview

emoval of hydrophobic organic pollutants from soilashing/flushing solutions: A critical review

lément Trellua, Emmanuel Mousseta, Yoan Pechauda, David Huguenota,ric D. van Hullebuscha, Giovanni Espositob, Mehmet A. Oturana,∗

Université Paris-Est, Laboratoire Géomatériaux et Environnement (EA 4508), UPEM, Marne-la-Vallée, 77454, FranceUniversity of Cassino and the Southern Lazio, Department of Civil and Mechanical Engineering, Via Di Biasio, 43, Cassino, 03043 FR, Italy

i g h l i g h t s

The treatment of soil washing solu-tions by AOPs and biological treat-ments is reviewed.Advantages and disadvantages of dif-ferent processes are pointed out.Pollutant removal mechanisms arediscussed according to operatingconditions.Relevant recent advances and futureresearch directions are highlighted.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 18 September 2015eceived in revised form 4 December 2015ccepted 7 December 2015vailable online 9 December 2015

a b s t r a c t

The release of hydrophobic organoxenobiotics such as polycyclic aromatic hydrocarbons, petroleumhydrocarbons or polychlorobiphenyls results in long-term contamination of soils and groundwaters. Thisconstitutes a common concern as these compounds have high potential toxicological impact. Therefore,the development of cost-effective processes with high pollutant removal efficiency is a major challengefor researchers and soil remediation companies. Soil washing (SW) and soil flushing (SF) processes

eywords:oil washing solutionntegrated processesdvanced oxidation processesiological treatmentdsorption

enhanced by the use of extracting agents (surfactants, biosurfactants, cyclodextrins etc.) are conceiv-able and efficient approaches. However, this generates high strength effluents containing large amountof extracting agent. For the treatment of these SW/SF solutions, the goal is to remove target pollutantsand to recover extracting agents for further SW/SF steps. Heterogeneous photocatalysis, technologiesbased on Fenton reaction chemistry (including homogeneous photocatalysis such as photo-Fenton),

Abbreviations: 2,4-D, 2,4-dichlorophenoxyacetic acid; AOP, advanced oxidation process; B30–B35, Brij 30–Brij 35; BAP, benzo[a]pyrene; BDD, boron doped dia-ond; CAS, cocoamidopropyl hydroxysultaïne; CD, cyclodextrin; CMC, critical micellar concentration; CMCD, carboxymethyl-�-CD; COD, chemical oxygen demand; DDE,

ichlorodiphenyldichloroethylene; DDT, dichlorodiphenyltrichloroethane; DSA, dimensionally stable anode; EAOP, electrochemical advanced oxidation process; EF, electro-enton; HOC, hydrophobic organic compound; HPCD, hydroxypropyl-�-CD; HTAB, hexadecyltrimethylammonium bromide; LAS, linear alkylsulfonate; MCD, methyl-�-CD;AP, naphthalene; NAPL, non-aqueous phase liquid; PAH, polycyclic aromatic hydrocarbon; PCB, polychlorobiphenyl; PCP, pentachlorophenol; PHE, phenanthrene; PYR,yrene; RAMEB, randomly methylated-�-CD; SDS, sodium dodecylsulfonate; SF, soil flushing; SOM, soil organic matter; SS, stainless steel; SW, soil washing; SWEP,ethyl(3,4-dichlorophenyl)carbamate; TNT, trinitrotoluene; TOC, total organic carbon; TW80, Tween 80; TX100, Triton X 100.∗ Corresponding author. Fax: +33 149329137.

E-mail addresses: mehmet.oturan@univ-paris-est.fr, oturan@univ-mlv.fr (M.A. Oturan).

ttp://dx.doi.org/10.1016/j.jhazmat.2015.12.008304-3894/© 2015 Elsevier B.V. All rights reserved.

dx.doi.org/10.1016/j.jhazmat.2015.12.008http://www.sciencedirect.com/science/journal/03043894http://www.elsevier.com/locate/jhazmathttp://crossmark.crossref.org/dialog/?doi=10.1016/j.jhazmat.2015.12.008&domain=pdfmailto:mehmet.oturan@univ-paris-est.frmailto:oturan@univ-mlv.frdx.doi.org/10.1016/j.jhazmat.2015.12.008

150 C. Trellu et al. / Journal of Hazardous Materials 306 (2016) 149–174

ozonation, electrochemical processes and biological treatments have been investigated. Main advantagesand drawbacks as well as target pollutant removal mechanisms are reviewed and compared. Promisingintegrated treatments, particularly the use of a selective adsorption step of target pollutants and thecombination of advanced oxidation processes with biological treatments, are also discussed.

C

1

catiumwtntmsTe

© 2015 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1502. Extraction of hydrophobic organic compounds (HOCs) from soil by soil washing/soil flushing (SW/SF) processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

2.1. SF process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512.2. SW process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512.3. Extracting agents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .151

2.3.1. Synthetic surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512.3.2. Biosurfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1522.3.3. Cyclodextrins (CDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1522.3.4. Organic cosolvents and vegetable oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1522.3.5. Other alternative extracting agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .153

3. Treatment of SW/SF solutions by degradation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1533.1. Heterogeneous photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

3.1.1. General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1543.1.2. Removal efficiency and kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1543.1.3. Extracting agent recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

3.2. Technologies based on Fenton reaction chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1583.2.1. General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1583.2.2. Removal efficiency and kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1583.2.3. Extracting agent recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

3.3. Ozone processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1613.3.1. General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1613.3.2. Removal efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

3.4. Electrochemical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1623.4.1. General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1623.4.2. Removal efficiency and kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1623.4.3. Extracting agent recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

3.5. Biological treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1653.5.1. General considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1653.5.2. Removal effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1663.5.3. Influence of extracting agents on biodegradation mechanisms and kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1665. New trends and improvement perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

5.1. Improvements of extracting agent characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1685.2. Combination of oxidation and biological processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1695.3. Combination of selective separation and degradation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

. Introduction

As a consequence of continuous increase of industrial and agri-ultural development, environmental issues and considerationsbout sustainable development are becoming increasingly impor-ant. Nowadays, these topics are considered for decision-makingn industrial, economical and political areas. More stringent reg-lations about environmental pollution are introduced each year,ostly concerning wastes [1], ambient air [2], atmosphere [3] andater [4]. Recently, a greater attention has been paid to soil con-

amination. However, the enforcement and the harmonization ofew rules is still a crucial issue, mainly due to the high variability inhe nature of contaminated soils and the costs of immediate invest-

• the reduction of exposure levels, to be as low as reasonablyachievable;

• the prevention of conventional pollution via the best availabletechniques not entailing excessive costs.

Particularly, the treatment of polluted soils and sites using envi-ronmentally friendly and efficient remediation technologies is amajor challenge. Soil quality is greatly affected by the releaseof hydrophobic organoxenobiotics from chemical, coke, woodand oil industries or by diffuse pollution from agricultural andtransport activities [6]. The persistence of hydrophobic organiccompounds (HOCs) in soils is a matter of significant public, sci-entific and regulatory concerns because of their potential toxicity,mutagenicity, carcinogenicity and ability to be bioaccumulated in

ents involved. These regulation levels are chosen by consideringimultaneously health, economic, environmental and social factors.hus, they have to be in agreement with two basic principles in thenvironmental field [5]:

the food chain [7]. Most of them are persistent in the natural

environment, due to their slow degradation by natural attenua-tion or (photo) chemical/biological processes [8]. HOCs such aspetroleum hydrocarbons, polychlorobiphenyls (PCBs), polycyclic

rdous

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ohe((pcelec

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C. Trellu et al. / Journal of Haza

romatic hydrocarbons (PAHs), polychlorinated dibenzodioxinsnd some pesticides are characterized by:

low solubility in water;high octanol/water partition coefficient (Kow);a high organic carbon/water partition coefficient (Koc),leading to their accumulation in soils by sorption mechanismswith soil organic matter (SOM).

However, these physicochemical parameters vary according toach HOC and have an impact on the mobility and availabilityf HOCs in soils as well as the efficiency of their removal duringhe treatment. Moreover, their respective volatility also influencesheir behavior.

Nowadays, all processes used for soil remediation have at leastne important drawback such as high costs (thermal treatments),igh perturbation of the soil texture (thermal treatments), lowfficiency (pump and treat), long treatment time requirementsbiodegradation processes), or selectivity toward target pollutantsvolatile organic compounds for venting, hydrophilic organic com-ounds for pump and treat) [9]. This is the reason why remediationompanies are still looking for effective, less expensive and morenvironmentally friendly processes. Particularly, during the twoast decades, soil washing/soil flushing (SW/SF) processes usingxtracting agents (surfactants, biosurfactants, cyclodextrins (CDs),osolvents) have shown promising results [10,11].

HOCs are mainly sorbed to SOM [12] or present in non-aqueoushase liquids (NAPLs). Extracting agents are used in order tonhance the solubility, desorption and biodegradation of soil pollu-ants [10]. These substances have shown promising results for thenhancement of techniques dealing with in situ chemical oxida-ion [13]. In fact, extracting agents improve the availability of soilollutants for reactive oxygen species by transferring them intohe aqueous phase. However, this process requires the use of largemounts of reagents [14] and leads to important soil disturbance.his is the reason why some authors have tried to develop an effi-ient method based on pollutant removal from soil matrix by SWex situ process) or SF (in situ process), followed by a treatmenttage of the SW/SF solution [15–17]. This alternative integrated pro-ess aims to disturb as low as possible soil structure and biologicalctivity, while removing and destroying efficiently pollutants withptimized operational costs.

Several authors reviewed the use of surfactants, biosurfactants,Ds and cosolvents for the removal of HOCs from soil by SW/SFrocesses [10,11,18–20]. However, to the best of our knowledge,here is no literature review available on studies already carried outoncerning the treatment of these SW/SF solutions. It is extremelymportant to gather all of the existing information about this topic,s the next step for this new integrated treatment is to determineow it could be applied at industrial scale in the most cost-effectiveay. In this paper, after a non-exhaustive description of extract-

ng agent enhanced SW/SF processes, we focus on an overview ofegradation processes studied for the treatment of SW/SF solutions,articularly heterogeneous photocatalysis, technologies based onenton reaction chemistry (including homogeneous photocataly-is such as photo-Fenton), ozonation, electrochemical processesnd biological treatments. Finally, feasible perspectives and furthertudies necessary to improve a whole integrated treatment for soilemediation using SW/SF processes are discussed.

. Extraction of hydrophobic organic compounds (HOCs)

rom soil by soil washing/soil flushing (SW/SF) processes

Soil is a complex porous and solid matrix, which makes the treat-ent of contamination more difficult. Particularly, HOCs have the

Materials 306 (2016) 149–174 151

ability to bind to SOM [12]. SOM originates from the contribution offresh organic matter by living organisms or from the decompositionof this fresh organic matter by mostly biological processes calledhumification [21]. The integration of the organic matter with soilmineral particles forms the so-called clay-humus complex, whichhas an important influence on soil properties and HOCs sorption[20]. In the following sub-sections, some extraction methods arepresented, including SF and SW processes. The different extractingagents used will also be explored.

2.1. SF process

The SF process is an in situ process, where extracting agentsare used in order to improve the mobility of NAPL by reducinginterfacial tension between NAPL and groundwater [10]. Mobilizedcontaminants can then be recovered in extraction wells. However,SF is preferentially used for light NAPL remediation because pump-ing is easily managed from the surface of the underground watertable. The efficiency of this process in real sites strongly depends onfield characteristics (soil heterogeneity, contaminant nature, NAPLsaturation, etc.) [22]. SF is characterized by the amount of solutionused for flushing. It is normalized by comparing the volume of solu-tion used to the pore volume of the soil. This ratio can range from1 to more than 200 [18]. Fig. 1 represents a schematic view of theSF process.

The electrokinetic surfactant-aided SF [23] can also be used. Inthe classical SF process, the driving force is the pressure gradient,while there is a voltage gradient in the electrokinetic surfactant-aided SF.

The SF process is appealing due to the absence of a preliminaryexcavation step, less disruption to the environment, and reductionof worker exposure to hazardous materials [24].

2.2. SW process

The SW process is an ex situ process, i.e., the soil has to be exca-vated before the treatment. It is operated at a certain solid/liquidratio, in the range 1–100% [18] and most frequently between 5and 40%. Extracting agents are supplied to the system in order toimprove the removal of contaminants sorbed to soil [25]. The SWprocess enhances the contact between extracting agents and soilpollutants, thereby allowing better treatment efficiency monitor-ing and contact time reduction compared to the SF process [18].

2.3. Extracting agents

Extracting aqueous solutions with or without additives areemployed to mobilize HOCs from the soil to the SW solution. Addi-tives are used to increase aqueous solubility of HOCs, since it isthe main controlling removing mechanism. These extracting agentscan reduce the time necessary to treat a site compared to the use ofwater alone. Aside from their extracting and solubilizing abilities,they must be of low ecotoxicity for the soil and biodegradable [10].Besides, they can also lead to the mobilization of heavy metals, asit has been recently reviewed by Mao et al. [11].

2.3.1. Synthetic surfactantsSurfactants are amphiphilic molecules composed of two main

components, the hydrophobic tail group and the hydrophilic headgroup. They are mainly characterized by their chemical structure,hydrophilic-lipophilic balance and critical micellar concentration(CMC). CMC is surfactant concentration above which micelles form

and all additional surfactants added to the solution go to micelles.Below CMC, the surface tension changes strongly with the concen-tration of the surfactant. Above CMC, HOC solubility is stronglyenhanced and the surface tension changes with a much lower

152 C. Trellu et al. / Journal of Hazardous Materials 306 (2016) 149–174

Fig. 1. Scheme of a typical in situ s

Fe

satas

ebofer

ist(Tci[

2

mr

ig. 2. Surface tension and hydrophobic organic compounds (HOCs) solubilitynhancement factor as a function of surfactant concentration.

lope (Fig. 2) [10]. Different mechanisms are involved in surfactant-mended remediation such as the decrease of interfacial tension,he phase transfer of HOC from soil-sorbed to micellar pseudo-queous phase and solubilization of HOCs inside the hydrophobicpace formed by micelles [26].

Among extracting agents, synthetic surfactants have the bestxtraction efficiency [10,27]. However, some of them have lowiodegradability [28] and are affected by precipitation or sorptionnto soil, requiring larger amounts and causing possible damagesor soil harmlessness [20,26]. Moreover, surfactants may also formmulsions with high viscosity that are difficult to manage andemove.

There are four major surfactant categories, which include anion-cs (such as sodium dodecylsulfate (SDS) or linear alkylbenzeneulfonate (LAS)), cationics (such as quaternary ammonium deriva-ives), amphoterics (such as cocoamidopropyl hydroxysultaïneCAS)) and nonionics (such as Brij 35 (B35), Tween 80 (TW80) orriton X 100 (TX100)). Non-ionic surfactants are preferably usedompared to ionic surfactants due to their lower soil sorption abil-ty, higher solubilization capacity and higher cost-effectiveness10].

.3.2. BiosurfactantsBiosurfactants are also amphiphilic compounds able to form

icelles. They have a microbial origin and are produced fromenewable resources. Similarly to synthetic surfactants, biosurfac-

oil flushing (SF) installation.

tants have high extraction efficiency [19,20]. For example, Lai et al.[29] observed that rhamnolipid (RHA) and saponin (biosurfactants)exhibited higher petroleum hydrocarbons removal efficiency fromsoil than the synthetic surfactants TW80 and TX100. Other advan-tages of biosurfactants include higher biodegradability, ecologicalsafety, lower toxicity and the possibility to be produced in situ[19,30]. However, the main issue that currently impairs their usein SW/SF processes is their economically reasonable production[19,31].

2.3.3. Cyclodextrins (CDs)CDs have hydrophilic groups on the external side of their ring,

which can dissolve in water, and an apolar cavity providing ahydrophobic matrix, described as a micro heterogeneous environ-ment [32]. Hydrophobic molecules are accommodated within theapolar cavity to form inclusion complexes as it is shown in Fig. 3.Very large differences are observed in the solubility and/or stabilityof inclusion complexes formed with different compounds [32].

There are three main native CDs consisting of cyclic oligosaccha-rides with six (�-CD), seven (�-CD) or eight (�-CD) glucopyranoseunits linked by �-(1,4) bonds [32,33]. The internal dimension ofthe apolar cavity varies according to the number of glucopyranoseunits (Fig. 3).

There are also many derivative �-CDs such as hydroxypropyl-�-CD (HPCD), methyl-�-CD (MCD) or randomly methylated-�-CD(RAMEB), carboxymethyl-�-CD (CMCD). These CDs have highersolubility (in the range 100–1000 g L−1) compared to native �-CD(18.5 g L−1 at 25 ◦C).

CDs enhanced SW/SF treatments of organic pollutants has beenrecently reviewed [18]. They enhance water solubilization of manyHOCs [34]. �-CD is considered as the most accessible and lessexpensive among the native ones [18]. However, its low solubilityincreases its soil sorption and limits its application for SW/SF exper-iments. Consequently, derivative �-CD were marketed and provedtheir high water-solubility and efficiency. Moreover, derivative CDsexhibit better extracting capacity than the native ones [18].

The analysis of the enhancement of HOCs solubility by usingdifferent surfactants and CDs revealed that CDs have a lower abil-ity to solubilize HOCs than traditional surfactants [35]. They areusually ten times less efficient, depending on the nature of CD andsurfactant used [18].

2.3.4. Organic cosolvents and vegetable oil

The use of organic co-solvents (esters, ketones, alcohols, alky-

lamines and aromatics for example) is no longer considered to bea promising technique. It implies several important drawbacks,such as high costs, risks of handling and storing, toxicity and soil

C. Trellu et al. / Journal of Hazardous Materials 306 (2016) 149–174 153

nclusi

ps1

rHtbnmqtar

2

r[nctifapf

3

gcFsi

Fig. 3. Structure of native cyclodextrins (CDs) and i

ermeability disturbing [19]. Moreover, their effect is usually notignificant until the volume-fraction concentration used is above0% [18].

However, it has been shown that vegetable oil could favorablyeplace costly, toxic and low-biodegradable organic solvents forOCs removal from soil [19]. For instance, more than 90% of the

otal PAHs from a heavily contaminated soil has been removedy using a sunflower oil-soil ratio of 2:1 (v/w) [36]. It is worth toote that further assessment of costs, ecological risks, and treat-ent/disposal of contaminated oil is necessary because of huge

uantities of vegetable oil needed [37]. Moreover, the production ofhe vegetable oil also involves large areas of cultivated fields as wells the use of high amounts of pesticides increasing environmentalisks.

.3.5. Other alternative extracting agentsSome other alternative extracting agents have also been

eported, including the followings: salmon deoxyribonucleic acid16] and humic acids [38] for their amphiphilic properties, soilano-particles (composed mainly by organic carbon and inorganiclay material) due to their adsorption capacities [39], polymers forheir ability to change water viscosity [22], surfactant foams lead-ng to homogeneous displacement fronts [22], gemini surfactantsor their low CMC [11]. However, further studies are necessary tossess their efficiency in different remediation cases. Moreover, theroduction at industrial scale of some of them is still the main issueor their extensive application in soil remediation [11].

. Treatment of SW/SF solutions by degradation processes

Electrochemical advanced oxidation processes (EAOPs), hetero-eneous photocatalysis and technologies based on Fenton reaction

hemistry (including homogeneous photocatalysis such as photo-enton) have been largely applied to the treatment of SW/SFolutions [40–42]. These advanced oxidation processes (AOPs)nvolve the generation in sufficient quantity of strong oxidants, in

on complex formed with HOCs. Adapted from [33].

particular, hydroxyl radical (•OH). The main characteristics of thisradical are [43–45]:

• a very strong oxidizing power: E◦(•OH/H2O) = 2.8 V/SHE;• a non-selective feature toward organic compounds;• a very short average lifetime of few nanoseconds in water.

Hydroxyl radicals have the ability to degrade most of organic andorgano-metallic pollutants [46,47] by dehydrogenation, hydroxyla-tion or redox reactions [48]. The degradation of organic compoundsinvolves the formation of several by-products until total miner-alization, i.e., conversion into carbon dioxide (CO2), water andinorganic ions [49,50]. This is the reason why mineralization ratesare much slower than degradation rates [49]. Furthermore, someby-products more toxic than the parent compound can be formed[51]. Therefore, the identification of these by-products, the under-standing of degradation pathways [52,53] and the evolution of thetreated solution toxicity are also relevant parameters to study [54].

The use of ozone and biological treatments is also discussed inthis part.

The efficiency of each process for SW/SF solutions treatmentmust be assessed as a function of the following specific objectives[15,16,55]:

• target pollutant removal efficiency and kinetics: influence ofoperating conditions and specific SW/SF solution characteristics,particularly high extracting agent concentrations;

• extracting agent recovery (concerning this parameter, someauthors studied the extracting capacity of the recycled solu-tion, while others investigated the amount of extracting agent

analysed at the end of the treatment, which can differs fromextracting capacity);

• operating cost optimization (reagent consumption, treatmenttime, energy consumption, sludge production etc.).

154 C. Trellu et al. / Journal of Hazardous

F(

iWpltledtoottba

3

3

iewttacolh(b

h

ceccowi

ig. 4. Schematic representation of the oxidation mechanism of organic compoundsR) by the photocatalytic process. Adapted from [62].

The study of HOC removal from SW/SF is investigated eithern synthetic SW/SF solutions [55] or in real SW/SF solutions [17].

ith synthetic solutions, chemical substances/pollutants initiallyresent in the solution are totally controlled: concentration of pol-

utants, surfactants/cosolvents, or other initial parameters such ashe presence of dissolved organic matter. The use of simple systemsike synthetic solutions makes the control of selected parametersasier and leads to a better understanding of mechanisms involveduring the treatment. However, it does not reproduce real parame-ers affecting the process efficiency. A lot of synergetic, competingr scavenging effects can occur in real SW/SF solutions, dependingn the presence of different nature and concentration of pollu-ants, SOM, or metallic ions, for example. This is why the abilityo use a process for SW/SF solutions treatment has to be assessedy studying real SW/SF solutions. Therefore, both kinds of methodsre complementary studies.

.1. Heterogeneous photocatalysis

.1.1. General considerationsPhotolysis of organic compounds is an important mechanism

n natural environment (solar photolysis) but this process is notffective enough for industrial applications of highly contaminatedastewater treatment. However, light irradiation coupled with

he addition of a photocatalyst increases the efficiency, owing tohe formation of hydroxyl radicals. Several authors reviewed thepplication of this process for the treatment of hazardous organicompounds in wastewater [56–61]. The process is based on the usef semiconductors exhibiting a band gap region in which no energyevels are available to promote the recombination of an electron andole produced by photoactivation in the solid semiconductor (Eq.1)). This band gap (Eg) extends from the top of the filled valenceand to the bottom of the vacant conduction band.

v + semiconductor → h+ + e− (1)By far, titanium dioxide (TiO2) has been the most largely used

ompared to other semiconductor photocatalysts due to its cost-ffectiveness, inert nature (i.e. stable under different reactiononditions) and photostability (i.e. able to promote reactions effi-

iently upon repetitive use). The description of the degradation ofrganic contaminants by heterogeneous photocatalysis has beenidely reviewed [57,60,62]. This process is schematically described

n Fig. 4.

Materials 306 (2016) 149–174

3.1.2. Removal efficiency and kineticsBy using several kinds of pollutants and extracting agents, it

has been shown that it is possible to achieve complete degradationof target pollutants with degradation kinetic rates in the range of0.1–10 h−1 (Table 1).

A decrease in degradation rates is observed with real SW/SFsolutions. Thus, the process needs to last longer in order to reachthe same efficiency than with synthetic SW/SF solutions [63–65]. Itis particularly due to adverse effects coming from the presence ofSOM. Since hydroxyl radicals have a non-selective reactivity towardorganic compounds, SOM acts as •OH scavenger [66–68].

Most of the studies only investigated the first step of targetpollutant degradation. Further studies about total organic carbon(TOC) and chemical oxygen demand (COD) removal as well astoxicity evolution would be necessary since these measurementsprovide reliable and significant results concerning the efficiency ofthe treatment.

3.1.2.1. Influence of extracting agents. The efficiency of the processtightly relies on the adsorption capacity of target pollutants ontothe photocatalyst [74] because it promotes the oxidation of organ-ics by highly oxidant species such as hydroxyl radicals formed atthe surface of the catalyst (Fig. 4). Adsorption depends on pollu-tant and surfactant properties and concentration as well as surfacecharacteristics of the photocatalyst.

3.1.2.1.1. Surfactant: influence of nature and dose. The use oflow concentrations of chemical surfactant (below or close tothe CMC) improves process efficiency for HOCs. This beneficialeffect has been explained by the following mechanism [74]. First,non-ionic surfactant monomers quickly combine with the mod-ified surface of the catalyst. This comes from interaction withhydrophobic sites on the catalyst surface. Thus, a superficial reac-tive monolayer is formed on the catalyst surface. Finally, the targetpollutant implants into the hydrophobic space among the sur-factant head and the catalyst surface system. This promotes itsavailability to photogenerated oxidizing species [74].

Another mechanism has been described by using cationic sur-factants [77,78]: superficial reactive monolayer formation at thesurface of the catalyst came from electrostatic interactions. Theyalso greatly enhance the adsorption of the target pollutant at lowsurfactant concentration. Moreover, higher photocatalytic degra-dation efficiencies were obtained in solutions containing mixedcationic–nonionic surfactants compared to solutions with singlesurfactants, indicating synergistic effects in complex systems [77].

However, it is important to note that the effect of surfactantson more hydrophilic compounds and further degradation steps(in particular on the oxidation of more hydrophilic oxidation by-products) is less favourable, even at low concentration [78].

Decelerated effects of photocatalysis capacity for HOCs degrada-tion were obtained by increasing surfactant concentration higherthan the CMC (such as in SW/SF solutions) [74]. This is due to thecompetitive partition of target pollutants in the micelle that cannotadhere or react at the surface of catalysts [74,78,79]. A scheme ofthis mechanism is proposed in Fig. 5. It is also ascribed to the surfac-tant degradation, which competes with target HOCs for oxidationby hydroxyl radicals at the catalyst/solution interface.

The nature of the surfactant for concentrations much higherthan the CMC has an important influence [64]. In real SW/SF solu-tions containing methyl(3,4-dichlorophenyl) carbamate (SWEP)residues, the process was more efficient by using SDS comparedto hexadecyltrimethylammonium bromide (cationic surfactant)and poly(oxyethylene) dodecyl ether (C12E8, non-ionic surfac-

tant) [64]. Alkylphenol degradation was also less inhibited withSDS than with non-ionic surfactants for high surfactant concen-trations [75]. In a similar study from the same research groupand at concentration of 10 mM (much higher than CMC), three

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Table 1SW/SF solution treatment by photocatalytic processes.

Pollutant (concentration) Extractingagent(concentration)

Synthetic(S)/real (R)solution-volume

Operating conditions Degradationkinetic rate

Observations Reference

Semiconductor(concentration)

Irradiationlight

pH T (◦C)

NAPa

(0.03 mM)B35 (1.2 mM) S

5 mLTiO2 (0.4 g L−1) Xe lamp

(1500 W) with340 nm cut-offfilter

3 – 2.04 h−1 Compared to solutionswithout B35, degradationkinetic rate is 4 timeslower with B35 and 10times lower in real SW/SFsolution with the same B35concentration

[69]

R5 mL

0.78 h−1

PCPb (0.2 mM) �-CD, MCD,HPCD(0–5 mM)

S35 mL

TiO2 (1.7 g L−1) UV lamp(125 W)

7; 11 19 0.3–5.3 h−1 The higher CDconcentration, the lowerPCP degradation rate

[70]

NAPa (0.2 mM) TX100(0.04–0.13 mM)

S1 L

TiO2 (0.1 g L−1) Solar lightsimulator(50 �W cm−2)

7 30 1.9–4.2 h−1 The higher TX100concentration, the lowerNAPa degradation rate andTX100 recovery

[71]

PCB (53 mg L−1) Soya lecithinand TX100

R1 L

TiO2 (0.5 g L−1) Fluorescentlamp(330–400 nm)(8 W)

– 20 – Low degradation rate(treatment time = 15 days).Better results for PCBdegradation rate with soyalecithin as extracting agent.

[72]

Aromatic compounds B35 (25 mM) S5 mL

TiO2 (0.5 g L−1) Xe lamp(1500 W) with340 nm cut-offfilter

– 55 0.12–3.48 h−1

(for individualpollutants)

Addition ofperoxydisulfates hasimproved treatmentperformances

[65]

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Table 1 (Continued)

Pollutant (concentration) Extractingagent(concentration)

Synthetic(S)/real (R)solution-volume

Operating conditions Degradationkinetic rate

Observations Reference

Semiconductor(concentration)

Irradiationlight

pH T (◦C)

SWEPc residues (17 mg L−1) C12E8; SDS;hexadecyltri-methylammo-nium bromide(8.6 mM)

S; R500 mL

TiO2(0.2–1.0 g L−1)

Hg lamp(125 W) with300 nm cut-offfilter

3.0; 5.6; 8.0 25 – The presence of surfactantslargely affects degradationkinetics. Importantinfluence of the nature ofthe surfactant Bubbling airimproved treatmentperformances

[64]

PHEd (0.16 mM) and PYRe (0.05 mM) MCD (83 mM) S–

TiO2 (1 g L−1) UV(200–280 nm)lamp (150 W)

6 22 – Very slow degradation rate [73]

PCPb

(0.018 mM)TX100(0.023–1.01 mM)

S500 mL

La-B codopedTiO2 (0.4 g L−1)

Visible light(400 W): Xelamp with400 nm cut-offfilter

5.7 – 0.1–3.3 h−1 The higher the TX100concentration, the lowerPCP degradation rate,particularly above the CMCPhotocatalytic degradationkinetics are directly linkedto adsorption properties

[74]

Solar light(39.0–27.5 mW cm−2)

1.8–9.3 h−1

Alkylphenols B35 (1 and10 mM); C12E8(1 and 10 mM);SDS (15 mM);mixtures

S5 mL

TiO2 (0.1 g L−1) Xe lamp(1500 W) with340 nm cut-offfilter

– 55 0.1–4.9 h−1 Important influence of thenature of the surfactant

[75]

R5 mL

TiO2 (0.2 and0.5 g L−1)

0.05–1.86 h−1

PHEd (0.006 mM) TX100(1.6–4.8 mM)

S–

TiO2(0.1–0.5 g L−1)

Monochromatic(254 nm) UVlamp (8 W)

3.6; 8.1 25 1.4–3.5 M−1 min−1 Effect of O2, H2O2 andradical scavengers has alsobeen studied

[76]

a Naphtalene.b Pentachlorophenol.c Methyl(3,4-dichlorophenyl) carbamate.d Phenanthrene.e Pyrene.

C. Trellu et al. / Journal of Hazardous Materials 306 (2016) 149–174 157

eous

noBldabc

tt[eitbhtcTc

eabcrbCtctot

3

tcTttl

Fig. 5. Partition equilibria of target pollutant during the heterogen

on-ionic surfactants have been tested and the following orderf efficiency for bentazone degradation has been obtained [63]:35 (C12E23) > poly(oxyethylene) dodecyl ether (C12E8) > C12E5. The

ower inhibitory effect of B35 has been explained by the biggerimension of its hydrated polar head. It reduces the surfactantdsorption on the TiO2 surface and, therefore, the competitionetween bentazone and B35 for the occupation of active sites onatalyst surface.

3.1.2.1.2. CDs: influence of nature and dose. At low concentra-ions (

1 rdous

tTpt(vdl

tsgihfiafltaf

3

3

scomhitF

F

TFldmaociaf

[

Fasth

ssso

Ft

sd

58 C. Trellu et al. / Journal of Haza

ions below 250 mg L−1, Vargas et al. [84] have not observed anyX100 degradation during a 2 h reaction time, while dibenzothio-hene was fully degraded. The same research group also observedhe successful selective photocatalytic degradation of naphthaleneNAP) in TX100 solutions [71]. The selectivity observed in NAPersus TX100 degradation was due to the higher NAP constantegradation rate. However, a total mineralization of the target pol-

utant was not considered.It has also been reported a decrease of the TX100 degrada-

ion rate when TX100 concentration increase (once the CMC isurpassed) [74]. This is ascribed to the unavailability of the photo-enerated oxidizing species for macromolecule surfactant micellesn the bulk. However, lower concentrations of TX100 led to aigher recovery rate after full degradation of PCP because of the

aster degradation of PCP. Therefore, surfactant recovery possibil-ty strongly relies on specific nature and dose of target pollutantsnd surfactants. Furthermore, some studies dealt with the use ofuoro-surfactants [85,86], which led to their full recovery becausehese kind of surfactants are refractory to oxidation. However, theylso have high toxic properties [87], which make them unsuitableor SW/SF steps.

.2. Technologies based on Fenton reaction chemistry

.2.1. General considerationsTechnologies using Fenton reaction chemistry have been widely

tudied due to the production of large amounts of hydroxyl radi-als for the oxidation of organic pollutants [88]. The effectivenessf these AOPs has been recently reviewed [43,89]. They are basedainly on the use of the Fenton’s reagent, which is a mixture of

ydrogen peroxide (H2O2) and ferrous iron (Fe2+). Fenton reactions the catalytic decomposition of H2O2 by iron salts. It is initiated byhe formation of hydroxyl radicals in accordance with the classicalenton reaction (Eq. (2)) at acidic pH (pH 3) [46,90]:

e2+ + H2O2 → Fe3+ + •OH + OH−(k2 = 63 M−1 s−1) (2)

hen, the process is propagated by the catalytic behavior of thee(III)/Fe(II) couple. It can also be coupled to catalytic processeseading to further ferrous ions production. This process has somerawbacks such as the use of high reagent concentration, involve-ent of parasitic reactions consuming generated hydroxyl radicals

nd formation of process sludge under Fe(OH)3 form [89]. Onef the well-known way of improving the efficiency of Fenton’shemistry is to combine the classical Fenton process with UV-Arradiation (photo-Fenton process) [91–95] or solar light [96]. Thection of photons is complex and could be mainly described by theollowing reaction (Eq. (3)) [97].

Fe(OH)]2+ + hv → Fe2+ + •OH (3)

e(OH)2+ is the pre-eminent form of Fe(III) in the pH range 2.8–3.5nd UV light has the ability to perform the reductive photoly-is of Fe(OH)2+. The benefits are twice: in situ generation of Fe2+

hat catalyse the Fenton reaction (Eq. (2)) and formation of furtherydroxyl radicals leading to the improvement of process efficiency.

A major drawback of the Fenton process is the production ofludge from iron(III) hydroxides precipitation. This has been partlyolved by the development of Fenton-like processes (i.e. the use ofolid iron-containing catalysts such as zeolites, alumina, pyrite orther iron oxides) [98–101].

The use of other metallic ions as catalyst has also been studied.or example, Co, Cu and Mn could be used to act as the catalyst of

he Fenton reaction [102].

Some authors also reported promising results by couplingonochemistry and Fenton’s chemistry for organoxenobiotic degra-ation [103,104]. Further combination of processes have also been

Materials 306 (2016) 149–174

investigated such as sono-photo-Fenton [105] or photo-Fenton-like[106].

3.2.2. Removal efficiency and kineticsResults obtained by different research groups are summarized in

Table 2 . Fenton process has been applied to different kinds of SW/SFsolutions with low efficiency. For example, it has been reported46% removal of 365 mg �PAHs L−1 in a solution with 0.1 g L−1 ofCAS (amphoteric surfactant) by using 1.7 mM of Fe2+ and 17.6 mMH2O2 at pH 4 for the Fenton process [107].

However, Bandala et al. [108] reported the higher efficiency ofthe photo-Fenton process by using SDS as extracting agent: thedegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) after 50 minof treatment was 99% for UV irradiation, Fenton andphoto-Fenton processes, respectively.

3.2.2.1. Influence of extracting agents.3.2.2.1.1. Surfactants: influence of nature and dose. Similarly to

adverse effects from SOM, the presence of high surfactant con-centration has an adverse effect on the efficiency of the Fentonprocess [115]. For example, Yang and Wang [116] reported adecrease in methyl orange degradation rate as surfactant concen-tration increased. Much higher reagent doses are necessary to reachdegradation of target pollutants in SW/SF solutions containing sur-factants and SOM [114]. As hydroxyl radicals act in a non-selectiveway, a significant part is wasted in the reactions with the SOM andsurfactants. This is one of the main limiting factor in determiningthe economic feasibility of the process [115]. Therefore, it could beconcluded that the classical Fenton process is not enough efficientfor the treatment of SW/SF solutions containing high concentra-tions of surfactants. Besides, the use of the photo-Fenton improvesperformances but also increases process cost.

3.2.2.1.2. CDs: influence of nature and dose. A study comparedthe treatment of a SW solution containing trinitrotoluene (TNT)as target pollutant by the photo-Fenton process, with and with-out the use of CD as extracting agent [112]. It has been shownthat it is possible to improve the process by using MCD. The useof 5 mM MCD increased the concentration of TNT in the SW solu-tion by a factor of 2.1; it also improved the photo-Fenton oxidationefficiency. The TNT degradation rate was 1.3 times higher com-pared to the degradation in distilled water, despite the presence ofSOM in the real SF solution. Indeed, MCD reduced the •OH scav-enging effect from non-target organic compounds. This has beenlinked to the formation of a ternary complex (TNT-CD-Fe(II)) [112].The enhanced Fenton degradation of PCBs and PAHs by simulta-neous iron and pollutant complexation with CDs has also beenhighlighted [110]. CMCD is able to remove HOCs from SOM bindingsites while complexing Fe2+ at the same time [110]. The forma-tion of the ternary complex allows the formation of radical speciesclose to the included target molecule [55]. Interestingly, it has alsobeen shown that too low concentrations of iron could lead to adecrease in the pollutant degradation rate. This is ascribed to anisolation of pollutants away from •OH, because only a small frac-tion of CMCD is bounded to iron [110]. The formation of the ternarycomplex depends on the functional group present on the externalshape. Iron is coordinated in different functional groups with eachCD [110]. For example, metal binding is stronger with oxygen inthe carboxyl group of CMCD compared to alcohol groups of native�-CD. This is the reason for which �-CD and �-CD are less ableto form the ternary complex compared to CMCD [110]. Moreover,

Fenton degradation of benzo[a]pyrene (BAP) in the presence of aradical scavenger was improved with HPCD but not in the presenceof RAMEB [111]. This confirms the importance of CD nature. Anal-ysis bringing evidence of the ternary complex formation include

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Table 2SW/SF solution treatment by technologies based on Fenton reaction chemistry.

Process Pollutant(concentration)

Extractingagent(concentration)

Synthetic(S)/real (R)solution-volume

Operating conditions Degradationkinetic rate(h−1)

Remarks Reference

[H2O2](mM)

[H2O2]/[Fe2+] pH Irradiation light

Fenton Anthracene(0.28 mM),BAPa

(0.20 mM)

Biosoft, Sorbax,Igepal,Witcomul,Marlipal(1 g L−1)

S50 mL

150 mM 15 – – – Between 24 and 88%degradation after 48 hCoupling withbiodegradation has alsobeen studied

[109]

Fenton PAH CMCD(0.05–1 mM)

S10 mL

Continuousaddition

– 2.5 – – Several assessments aboutthe influence of scavengersand the formation of aternary complex

[110]

Fenton PCB CMCD(1–10 mM)

S2 mL

86 mM 4.3 – – –

Fenton BAPa (0.02 mMand 10−5 mM)

�-CD, HPCD,RAMEB (5 mM)

S5 mL

1 and 10 5 5.5 – – Influence of organicscavenger and formation ofinclusion complex havebeen studied

[111]

Fenton Creosote oil(1.9 mM for�PAH)

CAS (0.2 mM) S250 mL

1.1–12.1 1.1–11 2-9 – – Maximum of 46% PAHsdegradationHigher efficiency obtainedwith electro-oxidation

[107]

Fenton p-Cresol(0.19 mM)

TW80(0.66 mM)

R500 mL

3 mM 16 – – – Rapid total conversion ofp-Cresol (

160

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Table 2 (Continued)

Process Pollutant(concentration)

Extractingagent(concentration)

Synthetic(S)/real (R)solution-volume

Operating conditions Degradationkinetic rate(h−1)

Remarks Reference

[H2O2] (mM) [H2O2]/[Fe2+] pH Irradiation light

Photo-Fenton TNTb (0.5 mM) None (water) R–

30 mM(continuousaddition)

– 3 UV lamp(150 W)

4.86 Addition of MCD reducedthe inhibitory effect ofhydroxyl radicalscavengers present in theSW solution

[112]

MCD (5 mM) 10.5Photo-Fenton PCB

(6.4 mg L−1)LAS (0.86 mM) R

1.1 L8.6–860 – 2.8 UV lamp

(254 nm, 36 W)– Degradation of different

homologous groups,influence of illuminationsource and Fenton’sreagent dose have beenstudied

[113]

Photo-Fenton PCB(6.4 mg L−1)

LAS (0.86 mM) R–

8.6–860 – 2.8 UV lamp(254 nm, 36 W)

– The use of a perfluorinatedsurfactant has also beenstudied

[114]

Solar photo-Fenton DDTc

(0.05–0.11 mM),DDEd

(0.03–0.05 mM),diesel(230–350 mg L−1)

TX100(2.2–8.9 mM)

R250 mL

18 injection of80 mM every20 min

6.6 2.8 sunlight – High degradationefficiency (>98%)Dissolved organic carbonevolution has also beenstudied

[17]

Fentonandphoto-Fenton

2,4-D(0.02 mM)

SDS (17 mM) R–

0–6 10–60 – – – Much higher efficiency ofthe photo-Fenton process

[108]UV lamp(365 nm,150 W m−3)

a Benzo[a]pyrene.b Trinitrotoluene.c Dichlorodiphenyltrichloroethane.d Dichlorodiphenyldichloroethylene.

rdous

an

3

tabat2HeniattdCFF8t

dwrt

ccv[dnnmetio

frakidt

rllSi(AopTihc

C. Trellu et al. / Journal of Haza

bsorbance determinations [55], fluorescence and nuclear mag-etic resonance spectroscopy [117,118].

.2.2.2. Influence of operating conditions.3.2.2.2.1. Fenton’s reagent dose. As the oxidizable organic mat-

er in the reaction medium is comprised of target pollutants as wells extracting agents and SOM, the Fenton’s reagent dose has toe greatly increased. By using PCB as target pollutant (6.4 mg L−1)nd LAS as surfactant (300 mg L−1), 3 g L−1 of H2O2 has been usedo reach more than 99% of PCB degradation at the end of the

h treatment by the photo-Fenton process [114]. But this high2O2 concentration was not sufficient to reach an efficient min-ralization, which stopped around 19%. 30 g L−1 of H2O2 has beenecessary to reach a mineralization rate of 96%. Therefore, sim-

larly to the classical Fenton process, the photo-Fenton processlso requires the use of large amount of Fenton’s reagent duringhe treatment of SW/SF solution [108]. Another example, morehan 95% degradation of dichlorodiphenyltrichloroethane (DDT),ichlorodiphenyldichloroethylene (DDE), diesel and at least 95%OD removal has been reported after 6 h of treatment by the photo-enton process [17]. However, operating conditions included highenton’s reagent dose: FeSO4 at 12 mM and a sequential addition of0 mM of H2O2 every 20 min (i.e. 1440 mM of H2O2 addition duringhe whole treatment).

The use of CDs as extracting agent favors the efficiency of oxi-ation by Fenton reaction [110], but further comparative studiesould be necessary in order to determine how much Fenton’s

eagent is saved with the use of CDs, compared to the use of syn-hetic surfactants.

It is also well known that the efficiency of the Fenton pro-ess depends on the ratio R = [H2O2]/[Fe] [93,119]. In most of theases, R values used for SW/SF solutions treatment have been pre-iously optimised by other authors. For example, Tang and Huang119] determined the optimal R as 11 for the degradation of 2,4-ichlorophenol in pure aqueous solution. Interestingly, it has beenoticed that the Fenton process could be performed without exter-al iron addition, due to the extraction of iron and other transitionetals from the soil during the SW/SF process [15]. The use of

xtracting agents with high chelating ability could also be inves-igated in order to improve this phenomenon. However, it wouldnvolve the investigation of other issues such as cost and toxicityf the chelating agent, extraction of toxic metals, etc. [15].

3.2.2.2.2. pH. It has been reported that the best pH to per-orm the Fenton process was 2.8 [97]. However Fenton oxidation ofeal SW/SF solutions could be successfully performed without pHdjustment. This is ascribed to the presence of substances able toeep Fe3+ in the solution at near neutral pH (SOM or CDs whent is used as extracting agent) and/or to the formation of acidicegradation by-products (particularly carboxylic acids) allowinghe decrease of the pH around 3 during the treatment [15].

3.2.2.2.3. Irradiation light (for the photo-Fenton process). Inegards to the photo-Fenton process, the choice of the irradiationight is an important parameter. Both UV lamp [114] and sun-ight [42] have been used as irradiation light for the treatment ofW/SF solutions. However, it is not possible to clearly determine thenfluence of the irradiation light since other operating conditions[H2O2], [Fe], R, pH) must be taken into account for the comparison.

study investigated the effect of illumination source wavelengthn the PCBs degradation in SW/SF solutions [113]. Iron(III) com-lexes absorb more the radiation at 254 nm than at 366 and 440 nm.

his allows reaching a higher photoreducing rate of iron(III) toron(II) (Eq. (3)) compared with the other wavelengths. Therefore,igher effectiveness was obtained by using an illumination sourceentered at 254.

Materials 306 (2016) 149–174 161

3.2.3. Extracting agent recoverySeveral studies have been performed on the degradation of

surfactants by Fenton oxidation. For example, non-ionic surfac-tants are more easily degraded than anionic surfactants due tothe formation of a complex Fe(III)-anionic surfactant decreasingthe catalytic abilities of Fe(II) for H2O2 decomposition [120]. It hasalso been observed that the degradation rate of non-ionic surfac-tants depends on the number of ethoxy groups [121]. In the areaof SW/SF solutions treatment, the aim is to avoid the degrada-tion of extracting agents in order to recover them. As regards tothis goal, perfluorinated surfactants have very interesting behavior,because they are highly refractory to oxidation by hydroxyl radi-cals [114]. However, these compounds could not be applied becausethey are considered as toxic and persistent organic pollutants. Byusing TW80 as extracting agent, it has been shown that it is possibleto achieve complete conversion of 20 mg L−1 p-cresol, while only10% of 0.86 g L−1 TW80 was degraded by using Fenton process [15].This is ascribed to different reaction rates with hydroxyl radicals.However a complete conversion of target pollutants into CO2, H2Oand inorganic ions (i.e. mineralization) has not been considered andsome toxic by-products could be formed. Finally, the selectivity ofFenton reaction based process could be improved by using CDs, dueto the formation of a ternary complex Fe(II)-CD-pollutant [55].

3.3. Ozone processes

3.3.1. General considerationsOzone (O3) is a strong oxidant with the fifth highest stan-

dard redox potential (E◦ = 2.07 V/SHE, at 25 ◦C) [44]. Thus, ozoneprocesses have the ability to oxidize many organic compounds[122–125]. Ozone is produced by electrical discharge over sitewhere it will be used, because of its unstable nature. Then, it isbubbled inside the effluent and transferred from the gas phase tothe liquid phase. Low ozone concentration in water is an importantlimitation for the efficiency of ozonation.

The degradation of organic pollutants in water by ozone pro-cesses depends strongly on the solution pH [125]. If the ozonationis developed under acidic conditions, the main pathway in organ-ics degradation is the direct oxidation. Ozone acts in a selectiveway with quite lower kinetic rate constants (for example, in therange 1–103 M−1 s−1 with chlorophenols) than hydroxyl radicals[56,126]. It reacts with functional groups of organic pollutantsthrough electrophilic, nucleophilic, and dipolar addition reactions[127]. At higher pH, the decomposition of ozone by hydroxyl ionsis faster and the indirect process occurs following a complex path-way described by Pera-Titus et al. [56]. The indirect oxidation isdeveloped through hydroxyl radical formation, which reacts imme-diately and non-selectively with the organic matter. Overall, atpH < 4 the direct oxidation dominates; at 4 < pH < 9 both mecha-nisms are present; at pH higher than 9 the indirect pathway is themain reaction [56].

Ozonation can also be combined with homogeneous promotersin presence or absence of UV light (O3/H2O2, O3/H2O2/UV, O3/Fe2+,O3/Fe2+/UV) in order to increase the production of hydroxyl radi-cals [122,123,128]. Moreover, the photolysis of ozone leads to theformation of H2O2 which is further degraded into two •OH underUV-C irradiation [124].

3.3.2. Removal efficiencyOzone is often used for in situ chemical oxidation, particularly

by combining ozone oxidation with a post-biological soil treat-ment [13]. Some data have also been obtained for the treatment

of SW/SF solutions by ozone processes. First, the influence of pHand ozone dose on the efficiency of the treatment of a SW solutioncontaining 200 mg L−1 of PAHs has been studied [129]. It has notbeen observed any significant effect from the pH in the range 3–10.

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62 C. Trellu et al. / Journal of Haza

nly higher doses (more than 500 mg O3 L−1) decreased the amountf PAHs (45–65% removal) in the solution. Similar efficiency haseen obtained for the treatment of SW/SF solutions containing40–600 mg L−1 of chlorophenols [130]. Interestingly, for loweroses (less than 50 mg L−1 of O3 consumed), an increase in totalAH concentration has been observed. This has been attributed tohe better extraction efficiency of PAHs for analysis. Indeed, PAHsrapped in humic substances are released after a mild ozonation129].

The NAP removal in Brij 30 (B30) containing solutions byn ozone process has also been investigated [131]. It has beenbserved that an increase in B30 concentration (in the range00–1000 mg L−1) decreased strongly NAP removal efficiency. Theresence of the surfactant decreases gas–liquid mass transfer foroth NAP and ozone. This directly affects NAP removal mecha-isms i.e. volatilization and ozonolysis (lower ozone transfer fromhe gas to the liquid phase). Therefore, the use of a rotating packeded as ozone contactor is reliable. It allows increasing gas-liquidass transfer coefficient compared with the conventional contac-

ors [131].As only few authors studied the use of ozone processes for the

reatment of SW/SF solutions, it appears difficult to get furthernformation on the influence of operating conditions and naturef the SW/SF solution. No authors investigated extracting agentecovery by using ozone processes.

.4. Electrochemical processes

.4.1. General considerationsIn electrochemical treatments, the application of an electric

urrent or potential between two electrodes in an electrochemi-al reactor induces electron transfer (redox) reactions resulting inirect or indirect destruction of the organic compound [132–135].everal different mechanisms are involved according to operatingonditions [5,45,136–138].

Direct electrolysis results from electron exchange betweenrganic pollutants and the electrode surface without involvementf other substances. This is theoretically possible in the poten-ial region of water stability (low potential). However, adsorptionnteractions between organic compounds and anode material andormation of a polymer layer on the anode surface can lead tohe deactivation of the anode (poisoning effect) [5]. The anodeouling effect could be avoided by performing oxidation in theotential region of water discharge i.e. high anodic potentials. Inhis case, formation of hydroxyl radicals can occur at the surfacef the anode material to form heterogeneous hydroxyl radicalsM(•OH), M being anode material) from oxidation of water. Thiss an EAOP named as direct electro-oxidation or anodic oxidationAO) process [5]. Two classes of anodes have been distinguished inO: “active” and “non-active” electrodes [136]. Particularly, anodesith high oxygen evolution overpotential (called non-active elec-

rodes) allow the accumulation of large amounts of physisorbedydroxyl radicals M(•OH) at the anode surface, leading to the com-lete mineralization of organics to CO2 [5] (Eq. (4)).

+ H2O → M(•OH) + H+ + e− (4)

The production of some other electrochemically generatededox reagents can simultaneously occur with the generation of(•OH). This can come from the oxidation of compounds used

s electrolyte. For example, active chlorine species (Cl2, HOCl,lO− and oxychloro species formed by oxidation of Cl–on anode)

re the most traditional ones used [139,140]. It is named medi-ted oxidation. However, chlorinated organic compounds can alsoe produced, leading to increase wastewater toxicity [141]. Thelectrochemical production of ozone, persulfate, percarbonate, per-

Materials 306 (2016) 149–174

phosphate has also been reported for indirect electrolysis processes[142–144].

The combination of the electrochemistry with Fenton reactionhas been extensively studied during the last decades [43,45]. Thevery popular EAOP called electro-Fenton (EF) allows avoiding maindrawbacks of the classical Fenton process, i.e. reagent’s cost, wast-ing reactions and sludge formation [137]. It is based on the in situelectrocatalytic generation of the Fenton’s reagent (the mixture ofH2O2 and iron (II)) in homogeneous medium [135]. H2O2 is contin-uously supplied to the solution from the two-electron reduction ofdissolved O2 on the cathode, as follows (Eq. (5)):

O2(g)+2H+ + 2e− → H2O2 (5)O2 needed to produce H2O2 can be partially formed at the anode byoxidation of water (particularly with Pt anode) and directly injectedas compressed air [135]. The oxidation power of H2O2 is stronglyenhanced in acidic medium in the presence of a catalytic amount ofFe2+ ions [135]. This comes from the production of hydroxyl radicalsvia the Fenton reaction (Eq. (2)). Ferric iron formed by this reactionis then reduced at the cathode [145] (Eq. (6)).

Fe3+ + e− → Fe2+ (6)This allows catalysing efficiently the Fenton reaction, through

regeneration of ferrous iron (catalyst). Thus, hydroxyl radicalsare continuously produced in the solution to be treated viaelectrochemically assisted Fenton reaction. The degradation ofmany persistent organic pollutants by the EF process has alreadybeen reported [45,146–149]. It can also lead to high TOC andCOD removal from wastewaters, indicating total mineralization oforganics to CO2 [150].

The efficiency of the EF process can be enhanced by irradiat-ing the EF reactor with an UV lamp (photo-EF process) or solarlight (solar photo-EF process) [151,152]. As explained above (seetechnologies based on Fenton reaction chemistry), the interest isdouble: supplementary production of hydroxyl radicals and ferrousiron regeneration enhancement.

3.4.2. Removal efficiency and kineticsElectrochemical processes have been successfully applied to the

treatment of SW/SF solutions by using different operating condi-tions, involving different mechanisms (Fig. 6). This is the reasonfor which a large range of kinetics has been obtained (0.01–2 h−1)(Table 3).

3.4.2.1. Influence of extracting agents. Concentration and nature ofthe extracting agent greatly influence the efficiency of the pro-cess. Particularly, the high carbon content (extracting agents, targetHOCs and SOM) leads to high competition effects for the oxidationwith hydroxyl radicals (Fig. 6). However, similarly to Fenton andphoto-Fenton processes, higher efficiency has been obtained withthe EF process by using CDs as extracting agent due to the formationof the ternary complex HOC-CD-Fe(II) [55].

A variety of behaviors and efficiencies have also been observedaccording to the nature of the surfactant used, due to differentdegradation pathways [156]. For example, the COD removal ofmodel solutions containing alkylbenzyldimethylammonium chlo-rid (a cationic surfactant) was higher than with SDS. The aromaticstructure of the cationic surfactant enhanced the organic loadremoval through the formation of insoluble species, while the gen-eration of oxidation-refractory compounds took place with SDS(aliphatic structure) [156].

Moreover, using continuous treatment, it has been reported

more than 80% of PAHs degradation during the first 18 h oftreatment by electro-oxidation with DSA anode [154]. Thereafter,removal efficiency decreased owing to the passivation of the elec-trode surface due to the formation of a polymeric film on its

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163

Table 3SW/SF solution treatment by electrochemical processes.

Process Pollutant(concentration)

Extracting agent(concentration)

Synthetic (S)/real (R)solution-volume

Operating conditions Degradation kineticrate (h−1)

Remarks Reference

Anode-cathodematerial

Electrolyte(concentration)

Power supply

EF PCP (0.1 mM) HPCD (5 mM) S–

Pt-carbon felt – 10 mA cm−2 0.546 A 5 fold increase in apparent rateconstant of PCP degradation wasobserved with HPCD

[153]

EF PHE(0.1 mM)

HPCD (8.8 mM) S400 mL

Pt-carbon felt Na2SO4(0.15 M)

13.3 mA cm−2 1.56 Toxicity and biodegradabilityevolution and extracting agentrecycling have also been studied

[55]

TW80 (0.6 mM) 0.78AO Creosote oil (1.3 mM

for �PAH)CAS (2.2 mM) S

1 LTi/IrO2-SS

a;Ti/SnO2-SS

aNa2SO4 (0.0035 M) 4.0–23 mA cm

−2 0.90 Recirculation flow rate, retentiontime and continuouselectro-oxidation have beenstudied

[154]

AO Creosote oil(1.5–3.0 mM for�PAH)

CAS (0.22–1.1 mM) S1.5 L

Ti/RuO2-SSa Na2SO4

(0.0035–0.028 M)3.08–12.3 mA cm−2 – Toxicity evolution and COD

removal have also been studied[155]

AO NAP (0.83 mM) CAS (9.1 mM) S1.5 L

Ti/RuO2-SSa Na2SO4 (0.0035 M) 9.23 mA cm

−2 0.90 Oxidation by-products have beenanalysed and degradationmechanisms have been proposed

[107]

PYR (0.27 mM) CAS (9.1 mM) S1.5 L

Ti/RuO2-SSa Na2SO4 (0.0035 M) 9.23 mA cm

−2 0.66

AO PHE SDS, TW80,alkylbenzyl-dimethylammoniumchloride

R600 mL

BDDb-SSa;DSAc-SSa

Na2SO4 (0.021 M) 30 mA cm−2 – Much higher efficiency with BDD2

as anode[156]

AO PHE (0.1 mM) TW80 (7.6 mM) S1.5 L

Graphite–graphite Na2SO4 (0.035 M);NaCl (0.035 M)

5 V – The use of two differentelectrochemical cell configurationhas been compared

[157]

PHE, Anthracene,BAP

TW80, B35,Tyloxapol

R1.5 L

Graphite–graphite;Ti/Pt-Ti

Na2SO4 (0.1 M)

AO Fluoranthene(0.1 mM)

TW80 (7.6 mM) S400 mL

Graphite–graphite Na2SO4 (0.1 M) 5 V 0.050 PAH mixture degradation rate hasalso been studied

[158]

Benzanthracene(0.1 mM)

TW80 (7.6) S400 mL

Graphite–graphite Na2SO4 (0.1 M) 5 V 0.020

PYR (0.1 mM) TW80 (7.6 mM) S400 mL

Graphite–graphite Na2SO4 (0.1 M) 5 V 0.012

AO PHE (0.21 mM) TW80 R400 mL

Graphite–graphite – 5 V – Extracting agent recycling has alsobeen studied

[159]

AO PHE HPCD R400 mL

Graphite–graphite – 5 V 0.19 Extracting agent recycling has alsobeen studied

[160]

AO PYR, Anthracene,Fluoranthene

TW80 and TX100 S and R400 mL

Graphite–graphite Na2SO4 (0.1 M) 5 V – PAH mixture degradation rate andextracting agent recycling havealso been studied

[161]

AO Atrazine SDS R1 L

BDDb-steel – 30 mA cm−2 – PAH mixture degradation rate andextracting agent recycling havealso been studied

[162]

a Stainless steel.b Boron doped diamond.c Dimensionally stable anode.

164 C. Trellu et al. / Journal of Hazardous Materials 306 (2016) 149–174

s, whi

sto[mribofrrc[

msaPutihtt

Fig. 6. Schematized view of mechanisms and competition effect

urface. This fouling phenomenon has been studied by investigatinghe influence of surfactants and anode materials during electro-xidation of phenols [163]. By using graphite as anode, Sripriya et al.163] observed that both cationic (cetyltrimethylammonium bro-

ide) and anionic (SDS) surfactants had an adverse effect on phenolemoval. They adsorb onto the anode surface, leading to a block-ng effect. Using oxide coated titanium anode, negative effects haveeen observed with SDS and TW80, while positive effects have beenbtained with cetyltrimethylammonium bromide. Indeed, this sur-actant leads to block the adsorption of electro-generated cationadical of phenol on the electrode surface and to facilitate the chlo-ine evolution reaction by increasing the surface concentration ofhloride ions through formation of a complex with chloride ions163].

Recently, it has been studied the influence of particle size oficelles (organic compound micro-drop covered by a layer of

urfactant) during the treatment of real SW solution containingtrazine and SDS (anionic surfactant) by AO with BDD anode [162].articularly, it has been reported that the higher the amount of SDSsed during the washing process, the lower the particle size andhe more negative the superficial charge of particles [162]. Thiss an important parameter to take into consideration since stericindrance of the large micelles could prevent their oxidation on

he anode surface [164]. However, organic compounds degrada-ion can still occur through mediated oxidation. Then, it has been

ch can occur during the treatment of SW/SF solutions by EAOPs.

observed a continuous decrease of particle size during electrolysisand a complete mineralization of organic compounds [164].

3.4.2.2. Influence of operating conditions.3.4.2.2.1. Electrode material. The use of active anode material

with low oxygen overpotential such as platinum or graphite leadsto a kinetic degradation rate in the range 0.01–0.2 h−1 (Table 3).Better results have been obtained for EF and AO processes usinganodes with higher oxygen overpotential (rate constants in therange 0.5–2 h−1). These latter EAOPs have the ability to producesignificantly higher amounts of hydroxyl radicals, leading to higherdegradation/mineralization kinetics [146].

In a comparative study, better results have been obtained byusing Ti/SnO2 compared to Ti/IrO2 as anode material [154]. This isexplained by the fact that Ti/SnO2 has a higher O2 overpotential(1.9 V vs SHE in 0.05 M of H2SO4 for Ti/SnO2 versus 1.52 V vs SHEin 0.5 M of H2SO4 for Ti/IrO2 [5]), leading to higher hydroxyl radi-cal accumulation on the anode surface [5,154]. The same trend hasbeen reported with SDS, which was more efficiently degraded andmineralized on boron doped diamond (BDD) anode than on PbO2anode [165]. Similarly, it has also been reported that dimensionallystable anode (DSA) was inefficient for the treatment of SW solutions

containing PHE, 10 g L−1 surfactant and COD of 20,000 mg O2 L−1

[156]. In contrast, BDD anode exhibited much higher efficiencycompared to DSA [156]. Recently, BDD anodes have shown verypromising results, due to their corrosion stability, inert surface and

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C. Trellu et al. / Journal of Haza

igh oxygen overpotential (2.3 V vs SHE in 0.5 M of H2SO4) [5,166].owever, their main drawback is currently their high investmentost.

Regarding the EF process, the choice of the cathode has aignificant influence on the efficiency of the process, particu-arly for H2O2 accumulation and Fe2+ regeneration. Best resultsor practical applications have been obtained using gas diffusionlectrodes (GDEs) and three-dimensional electrodes using carbon-ased porous materials, in particular carbon felt cathode [167].

Other mechanisms than the production of hydroxyl radicals onhe anode surface or in the bulk from the Fenton reaction canlso occur during electrochemical treatments, depending on thehoice of operating conditions. Particularly, oxidation mechanismsnvolved in the electrochemical treatment applied by Sanroman’sroup [158] also include direct electro-oxidation and oxidationediated by oxidants generated during the treatment from salts

ontained in the wastewater. However, lower degradation kineticsave been obtained comparing with EF process and anodic oxi-ation using anode with higher oxygen overpotential (processes

eading to the production of large amount of hydroxyl radicals)Table 3).

Finally, the efficiency of electrochemical processes depends onhe criteria design of the electrolytic cell. This parameter is difficulto compare between the different research groups.

3.4.2.2.2. Nature and dose of the electrolyte. Nature and con-entration of electrolyte is of great importance. It has been shownhat the addition of Na2SO4 was required to reduce energy con-umption and treatment costs [155]. Moreover, it has been reportedhe improvement of PAHs degradation rate by increasing Na2SO4lectrolyte concentration up to 0.1 M [157]. The effectiveness oflectrochemical oxidation can also be influenced by the choice ofhe supporting electrolyte used (K3PO4, Na2SO4 and NaCl), due tohe formation of radical species from the electrolyte (mediatedxidation) [168]. Furthermore, it can also affect the degradationathway: the mechanistic study about NAP and PYR electro-xidation showed that oxidation pathway included both oxidationy hydroxyl radicals produced on the anode surface and chlorineediated electrolysis [107].3.4.2.2.3. Current. The increase of the current density can

mprove the organics degradation rate but decrease the cost-fficiency of the process. The high COD content of SW/SF solutionsan allow to reach high current efficiency [5]. However, too highurrent density can increase the portion of current wasted due tohe increase of secondary reactions (such as oxygen evolution at thenode and H2 evolution at the cathode) [5]. Besides, organics degra-ation efficiency can also decrease when too high potentials arepplied. For example, the optimal current density defined by Trant al. [155] was 9.2 mA cm−2 for the treatment with a Ti/RuO2 anodef creosote oil in the presence of synthetic surfactant. The optimiza-ion of the current density applied to the electrolytic cell is alsoarticularly important during the EF process. A too high potentialavors some waste reactions and can decrease H2O2 accumulationn the solution [45]. Finally, the balance between the efficiency ofhese electrochemical processes and the energy consumption haso be further investigated.

3.4.2.2.4. pH. A pH between 2 and 4 is usually required for theF process, while the optimal pH has been defined as 3.0 [45]. How-ver, in the area of SW/SF solutions, the presence of compoundsuch as SOM or CD can allows keeping Fe3+ in the solution at neareutral pH, therefore leading to the possibility to start the process atH > 4 [55]. Moreover, the accumulation of carboxylic acids duringhe electrochemical treatment of SW/SF solutions can also lead to

ecrease the pH around 3 during the treatment [153,156]. Finally,y using Ti/RuO2 anode for the electro-oxidation of creosote oil inhe presence of CAS, it has not been reported any significant effect

Materials 306 (2016) 149–174 165

arising from initial pH in the range 2–9 [155]. This means that pHhas a low influence on the efficiency of the AO process.

3.4.3. Extracting agent recoveryThe issue of extracting agent recycling by using the EF process

has been studied with synthetic solutions [55]. The influence ofHPCD and TW80 on PHE degradation and recycling possibilities hasbeen compared. It has been shown that the apparent constant rateof PHE degradation was two times lower with TW80 than withHPCD [55]. Moreover, TW80 was much more degraded (50%) thanHPCD (10%) during the EF process. However, the absolute degra-dation rate constant of TW80 is 16 times lower than that of HPCD.This behavior has been explained by two different ways of formingcomplexes between TW80-PHE and CD-PHE. In the latter case, PHEis trapped into the CD cavity and the formation of the ternary com-plex (PHE-HPCD-Fe(II)) allows the production of hydroxyl radicalsclose to the contaminant (PHE) and its direct oxidation. In contrast,in the case of TW80, PHE is trapped into the micelle core, leadingto a lower availability toward oxidizing species. Hydroxyl radicalshave to degrade the micelle before degrading the contaminant [55].Fig. 7 depicts an explicative scheme of this mechanism.

According to this, it seems very difficult to achieve a selec-tive removal of pollutants by using synthetic surfactants since thepollutant is trapped inside the micelle. However, using electro-oxidation, solutions with HPCD or chemical surfactants have beensuccessfully reused, while the target pollutant was degraded in themeantime [159–161]. For example, after 95% removal of 35 mg L−1

PHE by electro-oxidation, the level of PHE removal from spiked con-taminated soil reached with the recovered solution of TW80 (82.4%)was similar to those obtained with a virgin new TW80 (87%) [159].However, further experiments with real aged contaminated soilswould be necessary, since it is much more difficult to remove HOCsfrom these kinds of soils compared to spiked contaminated soils[18]. Moreover, as for the other oxidation processes, it is worth tonote that the total mineralization of target pollutant has not beenconsidered for these recycling strategies.

3.5. Biological treatments

3.5.1. General considerationsBiological treatment is the most used process for wastewater

treatment, particularly due to its cost-effectiveness [169]. Physical,chemical, and microbiological aspects of a particular environmenthave to be taken into consideration for the understanding of bio-logical degradation mechanisms of a given compound. Differentmicroorganisms and degradation pathways are involved depend-ing on operating conditions in reactors, including compositionof the effluent to be treated, oxygen concentration, pH, tem-perature, etc. [170,171]. There are three main pathways for thebiodegradation of organic compounds: aerobic, anoxic and anaer-obic biodegradation.

Although some synthetic chemicals are usually recalcitrant tobiodegradation, some specific microorganisms have evolved anextensive range of enzymes, pathways, and control mechanismsthat are responsible for catabolism of a wide variety of such com-pounds [172]. Thus, biological degradation is a powerful treatmentthat is used to alleviate a lot of environmental pollution issues. Forexample, it has been reported that biological processes are able totreat a variety of organic compounds such as halogenated organics[172], nitroaromatic compounds [173], petroleum hydrocarbons[174], polychlorinated dibenzodioxins [175] or PCBs [176]. More-

over, the optimization of operational parameters such as sorption ofsubstrate on biomass can greatly increase biodegradation effective-ness through the increase of pollutant retention times in reactors[177].

166 C. Trellu et al. / Journal of Hazardous Materials 306 (2016) 149–174

t way

3

paiatsasttefcamic

3m

odTaabioenbpeor

go(

�

Fig. 7. Two different mechanisms for PHE degradation according to two differen

.5.2. Removal effectivenessOnly few authors investigated kinetic studies, reported data at

ilot or real scale and used real SW/SF solutions [178,179]. In therea of biological processes, these kinds of studies are particularlymportant to raise understanding of biodegradation mechanismsnd efficiency. However, some authors investigated biodegrada-ion effectiveness by using batch reactor (Table 4). With differentolution characteristics and microorganism cultures, they obtained

large range of effectiveness. Treatment time necessary to achieveufficient removal effectiveness can reach several weeks. Moreover,he high influence of SW/SF characteristics is highlighted, includinghe nature of target pollutant as well as the nature and the dose ofxtracting agent. Different surfactants may have different influenceor a same bacterial culture. Likewise, biodegradation effectivenessan differ for a same surfactant when different microorganismsre used [180]. Therefore, the adaptation and acclimatization oficroorganisms to the mixture surfactant-target pollutant is an

mportant parameter [180]. Besides, the removal of volatile HOCan also occur through volatilization during aerobic treatment.

.5.3. Influence of extracting agents on biodegradationechanisms and kinetics

Only few authors investigated the influence of extracting agentsn biodegradation mechanisms and kinetics. Therefore, it is prettyifficult to discuss results obtained and to highlight clear trends.he availability of HOCs for microbial degradation can be greatlyffected by their preferential interaction with non-aqueous phasesnd SOM [12]. It has been shown that the effect of surfactant oniodegradation depends on both solubilizing power of the extract-

ng agent and HOCs bioavailability inside the CD inclusion complexr surfactant micelle. Certain authors [178,179] assumed that somextracting agents used (non-ionic surfactants, biosurfactants) doot alter HOC degradation