Surfactant and oxidant enhanced electrokinetic remediation of a PCBs polluted soil

Surfactant-enhanced mobilization of persistent organic pollutants: Potential for soil and sediment remediation and unintended consequences

This review aims to provide an overview of the sources and reactions of persistent organic pollutants (POPs) and surfactants in soil and sediments, the surfactant-enhanced solubilisation of POPs, and the unintended consequences of surfactant-induced remediation of soil and sediments contaminated with POPs. POPs include chemical compounds that are recalcitrant to natural degradation through photolytic, chemical, and biological processes in the environment. POPs are potentially toxic compounds mainly used in pesticides, solvents, pharmaceuticals, or industrial applications and pose a significant and persistent risk to the ecosystem and human health. Surfactants can serve as detergents, wetting and foaming compounds, emulsifiers, or dispersants, and have been used extensively to promote the solubilization of POPs and their subsequent removal from environmental matrices, including solid wastes, soil, and sediments. However, improper use of surfactants for remediation of POPs may lead to unintended consequences that include toxicity of surfactants to soil microorganisms and plants, and leaching of POPs, thereby resulting in groundwater contamination.

Roles of oxidant, activator, and surfactant on enhanced electrokinetic remediation of PAHs historically contaminated soil

Electrokinetic (EK) remediation technology can enhance the migration of reagents to soil and is especially suitable for in situ remediation of low permeability contaminated soil. Due to the long aging time and strong hydrophobicity of polycyclic aromatic hydrocarbons (PAHs) from historically polluted soil, some enhanced reagents (oxidant, activator, and surfactant) were used to increase the mobility of PAHs, and remove and degrade PAHs in soil. However, under the electrical field, there are few reports on the roles and combined effect of oxidant, activator, and surfactant for remediation of PAHs historically contaminated soil. In the present study, sodium persulfate (PS, oxidant, 100 g L-1) or/and Tween 80 (TW80, surfactant, 50 g L-1) were added to the anolyte, and citric acid chelated iron(II) (CA-Fe(II), activator, 0.10 mol L-1) was added to catholyte to explore the roles and contribution of enhanced reagents and combined effect on PAHs removal in soil. A constant voltage of 20 V was applied and the total experiment duration was 10 days. The results showed that the removal rate of PAHs in each treatment was PS + CA-Fe(II) (21.3%) > PS + TW80 + CA-Fe(II) (19.9%) > PS (17.4%) > PS + TW80 (11.4%) > TW80 (8.1%) > CK (7.5%). The combination of PS and CA-Fe(II) had the highest removal efficiency of PAHs, and CA-Fe(II) in the catholyte could be transported toward anode via electromigration. The addition of TW80 reduced the electroosmotic flow and inhibited the transport of PS from anolyte to the soil, which decreased the removal of PAHs (from 17.4 to 11.4% with PS, from 21.3 to 19.9% with PS+CA-Fe(II)). The calculation of contribution rates showed that PS was the strongest enhancer (3.3~9.9%), followed by CA-Fe(II) (3.9~8.5%) (with PS), and the contribution of TW80 was small and even negative (-1.4~0.6%). The above results indicated that the combined application of oxidant and activator was conducive to the removal of PAHs, while the addition of surfactant reduced the EOF and the migration of oxidant and further reduced the PAHs removal efficiency. The present study will help to further understand the role of enhanced reagents (especially surfactant) during enhanced EK remediation of PAHs historically contaminated soil.

A novel electrokinetic remediation with in-situ generation of H2O2 for soil PAHs removal

Electrokinetic-Fenton (EK-Fenton) technology requires a high dose of H₂O₂ to produce •OH radicals, which adds a high cost to the remediation process and raises safety concerns during transportation and storage of H₂O₂. Moreover, the remediation efficiency of the conventional EK-Fenton process is low due to the meaningless consumption of H₂O₂ on the electrodes and the alkaline environment near the cathode. In this work, a modified CMK3-gas diffusion electrode (CMK3-GDE) is fabricated. This cathode can continuously generate H₂O₂, and the cumulative H₂O₂ concentration can reach 0.23 M during 10 days of the test. The utilization of cation exchange membranes (CEMs) efficiently restricts the decomposition of H₂O₂ on the electrodes and prevents the alkalization of the soil near the cathode, resulting in a 13.7-43.2% increase of the removal efficiency of polycyclic aromatic hydrocarbons (PAHs). In this new treatment process, PAHs are mainly oxidized into quinones, ketones, alcohols, and small molecule acids, and all these products have lower toxicities than PAHs. The EK-Fenton/CMK3-GDE-CEM system exhibits excellent remediation efficiency for treating PAHs polluted soil, which could be a sustainable, eco-friendly, and low-cost strategy for soil remediation.

Enhanced remediation of Cr(VI)-contaminated groundwater by coupling electrokinetics with ZVI/Fe3O4/AC-based permeable reactive barrier

Although widely used in permeation reaction barrier (PRB) for strengthening the removal of various heavy metals, zero-valent iron (ZVI) is limited by various inherent drawbacks, such as easy passivation and poor electron transfer. As a solution, a synergistic system with PRB and electrokinetics (PRB-EK) was established and applied for the efficient removal of Cr(VI)-contaminated groundwater. As the filling material of PRB, ZVI/Fe₃O₄/activated carbon (ZVI/Fe₃O₄/AC) composites were synthesized by ball milling and thermal treatment. A series of continuous flow column experiments and batch tests was conducted to evaluate the removal efficiency of Cr(VI). Results showed that the removal efficiency of Cr(VI) remained above 93% even when the bed volume (BV) reached 2000 under the operational parameters (iron/AC mass ratio, 2:1; current, 5 mA). The mechanism of Cr(VI) removal by the PRB-EK system was revealed through field emission scanning electron microscopy images, X-ray diffraction, X-ray photoelectron spectroscopy, Fe²⁺ concentration, and redox potential (E_h) values. The key in Cr(VI) reduction was the Fe²⁺/Fe³⁺ cycle driven by the surface microelectrolysis of the composites. The application of an externally supplied weak direct current maintained the redox process by enhancing the electron transfer capability of the system, thereby prolonging the column lifetime. Cr(VI) chemical speciation was determined through sequential extraction, verifying the stability and safety of the system. These findings provide a scientific basis for PRB design and the in-situ remediation of Cr(VI)-contaminated groundwater.

Recent advances in PCB removal from historically contaminated environmental matrices

Despite being drastically restricted in the 1970s, polychlorinated biphenyls (PCBs) still belong among the most hazardous contaminants. The chemical stability and dielectric properties of PCBs made them suitable for a number of applications, which then lead to their ubiquitous presence in the environment. PCBs are highly bioaccumulative and persistent, and their teratogenic, carcinogenic, and endocrine-disrupting features have been widely reported in the literature. This review discusses recent advances in different techniques and approaches to remediate historically contaminated matrices, which are one of the most problematic in regard to decontamination feasibility and efficiency. The current knowledge published in the literature shows that PCBs are not sufficiently removed from the environment by natural processes, and thus, the suitability of some approaches (e.g., natural attenuation) is limited. Physicochemical processes are still the most effective; however, their extensive use is constrained by their high cost and often their destructiveness toward the matrices. Despite their limited reliability, biological methods and their application in combinations with other techniques could be promising. The literature reviewed in this paper documents that a combination of techniques differing in their principles should be a future research direction. Other aspects discussed in this work include the incompleteness of some studies. More attention should be given to the evaluation of toxicity during these processes, particularly in terms of monitoring different modes of toxic action. In addition, decomposition mechanisms and products need to be sufficiently clarified before combined, tailor-made approaches can be employed.

Phytoremediation of PCB: contaminated Algerian soils using native agronomics plants

Pot cultivation experiments were conducted to assess the phytoremediation potential of two local agronomic plants, namely Avena sativa and Vicia sativa. Several soils with long-standing contamination and different levels of Polychlorinated biphenyl (PCB) contamination were used for this study. The soil samples came from different regions of Algeria and had different physico-chemical parameters. We studied the influence of these parameters on remediation potential of the two tested plants. The removal rate of the seven PCBs (PCB 28, 52, 101, 138, 153, 156 and 180) was examined after 40 and 90 days. The results showed that the presence of the plants reduced significantly the overall PCB content, ranging initially from 1.33-127.9 mg kg¹. After 90 days, the forage plant Vicia sativa allowed us to reach an excess dissipation rate of 56.7% compared to the unplanted control for the most polluted soil. An average dissipation rate of 50% was obtained in the moderately polluted soil. The less contaminated soil had an excess dissipation rate of about 24% for both plants and a predominant dissipation of the low chlorinated PCBs.

Iron-carbon material enhanced electrokinetic remediation of PCBs-contaminated soil

The high toxicity and persistence of polychlorinated biphenyls (PCBs) in the environment demands the development of effective remediation for PCBs-contaminated soils. In this study, electrokinetic (EK) remediation integrated with iron-carbon material (Fe/C) was established and used to remediate PCB28 (1 mg kg^-1) contaminated soil under a voltage gradient of 1 V cm^-1. Effects of Fe/C dosage, soil type, and remediation time were investigated. The operational condition was optimized as 4 g kg^-1 Fe/C, yellow soil, and 14 d-remediation, achieving PCB28 removal efficiency of 58.6 ± 8.8% and energy utilization efficiency of 146.5. Introduction of EK-Fe/C did not significantly affect soil properties except for slight soil moisture content increase and total Fe content loss. Soil electrical conductivity exhibited an increasing trend from anode to cathode attributed to EK-induced electromigration and electroosmosis. EK accelerated the corrosion and consumption of reactive Fe⁰/Fe₃C in Fe/C by generating acid condition. Fe/C in turn effectively prevented EK-induced soil acidification and maintained soil neutral to weak alkaline condition. A synergistic effect between EK and Fe/C was revealed by the order of PCB28 removal efficiency-EK-Fe/C (58.6 ± 8.8%) > EK (37.7 ± 1.6%) > Fe/C (6.8 ± 5.0%). This could be primarily attributed to EK and Fe/C enhanced Fenton reaction, where EK promoted Fe/C dissolution and H₂O₂ generation. In addition to oxidation by Fenton reaction generated ·OH, EK-mediated electrochemical oxidation, Fe/C-induced reduction and migration of Fe/C adsorbed PCBs were all significant contributors to PCB28 removal in the EK-Fe/C system. These findings suggest that the combination of EK and Fe/C is a promising technology for remediation of organics-contaminated soil.

Influence of electrode configuration on electrokinetic-enhanced persulfate oxidation remediation of PAH-contaminated soil

Electrokinetic (EK) remediation combined with in situ chemical oxidation (ISCO) can be applied to low permeability organic contaminated soil. However, the effects of electrode configuration on EK-oxidation remediation remain unclear. In this study, EK-ISCO remediation of real polycyclic aromatic hydrocarbon (PAH)-contaminated soil under different electrode configurations was conducted. The results showed that increasing the number of anodes and electrode pairs in one-dimensional (1D) and two-dimensional (2D) electrode configuration was conducive to migration of oxidants into the system. The change in soil pH after remediation in 2D electrode configuration was not obvious, but the increase of soil electrical conductivity (EC) was higher than that of the 1D electrode configuration. The removal rates of PAHs in 2D electrode configurations (35.9-40.9%) were relatively higher than those of the 1D electrode configurations (0.54-31.6%), and the hexagonal electrode configuration yielded the highest pollutant removal efficiency, reaching 40.9%. The energy consumption under 2D electrode configuration was smaller than that under 1D electrode configuration, and the energy consumption of per gram removed PAHs in the hexagon configuration (66.74 kWh (g PAHs)^-1) was lowest in all electrode configurations. Overall, the results of this study suggest that 2D electrode configuration is better than 1D and hexagonal electrode configuration is an optimal electrode configuration.

Ion exchange membranes enhance the electrokinetic in situ chemical oxidation of PAH-contaminated soil

Electrokinetic in situ chemical oxidation (EK-ISCO) could be used to remediate inorganic/organic-contaminated soil. Oxidizing agents were effectively delivered to the contaminated zones through electromigration and the electroosmosis. However, the cathode may react with oxidants, which would reduce the oxidative effect and lead to low contaminant removal rates. In this study, ion-exchange membranes (IEMs) enhanced EK-ISCO was used to remediate polycyclic aromatic hydrocarbons (PAHs) in contaminated soil. IEMs were installed between the electrode compartment and the soil compartment. The results showed that the IEMs could effectively control pH and the oxidation-reduction potential (ORP) changes in the soil column. Placing a cation-exchange membrane (CEM) at the cathode prevented the S₂O₈^2- from contacting the cathode and reduced the oxidative loss effect, which meant that PAH removal efficiency significantly improved (from 33.1% to 87.1%). Furthermore, there were minimal changes to the soil properties. Maintaining the soil at a low pH also improved the PAH removal efficiency (93.1%), but the physicochemical properties of the soil significantly changed and a large amount of power was consumed (2015 kWh t^-1). This study indicated that placing a CEM at the cathode improved remediation efficiency, and reduced power consumption and the adverse effects on soil properties during EK-ISCO.

Solubility and reactivity of surfactant-enhanced alkaline hydrolysis of organophosphorus pesticide DNAPL

The study presented in this paper evaluated the effectiveness of surfactants in enhancing mass removal of organophosphorus pesticides (OPPs) from soil under highly alkaline conditions and potential for enhancing in situ alkaline hydrolysis for treatment of OPPs, particularly parathion (EP3) and methyl parathion (MP3). In control and surfactant experiments, hydrolysis products EP2 acid, MP2 acid, and PNP were formed in non-stoichiometric amounts indicating instability of these compounds. MP3 and malathion were found to have faster hydrolysis rates than EP3 under the conditions studied. All surfactants evaluated increased solubility of OPPs under alkaline conditions with four nonionic alcohol ethoxylate products providing the greater affect over the polyglucosides, sulfonate, and propionate surfactants evaluated. The alcohol ethoxylates were shown to provide substantial mass removal of OPPs from soil. Hydrolysis rates were typically slower in the presence of surfactant, despite the relatively higher aqueous concentrations of OPPs; this was likely due to micellar solubilization of the OPPs which were therefore less accessible for hydrolysis. The results of this study support the use of surfactants for contaminant mass removal from soil, particularly under alkaline conditions, and may have implications for use of some surfactants in combination with other technologies for treatment of OPPs.

Remediation of soil co-contaminated with decabromodiphenyl ether (BDE-209) and copper by enhanced electrokinetics-persulfate process

This work investigated the influences of citric acid and methyl-β-cyclodextrin (MCD) as enhancing agents during the electrokinetics (EK)-persulfate process on the remediation of soil artificially contaminated with decabromodiphenyl ether (BDE-209) and copper (Cu) with an initial concentration of 50 and 1000 mg/kg, respectively. The results clearly demonstrate the efficiency of the process while at the same time, the distribution of the residual contaminants in soil and the EK parameters were greatly influenced by the presence of persulfate, MCD and citric acid. The results show that there was significant removal of BDE-209 and Cu from the soil. MCD-assisted process gave the highest BDE-209 removal (88.6%) and the third largest Cu removal (54.3%) from the soil. Comparatively, the highest Cu removal (92.5%) and the second largest BDE-209 removal (85.6%) were achieved by the joint application of MCD and citric acid in anolyte during the EK-persulfate treatment. MCD and citric acid could increase soil electrical current and electroosmotic flow during EK. The alkalization of soil near cathode was alleviated by the acidic byproducts of persulfate decomposition which could be transported to the soil by electroosmosis and electromigration. This integration process may provide a green efficient technology for remediating co-contaminated soil.

Application of Fenton process to remove organic matter and PCBs from waste (fuller's earth) contaminated with insulating oil

Polychlorinated biphenyls (PCBs) are carcinogenic to humans and can be found in fuller's earth used for the treatment of used transformer oil. This work describes an optimization of the Fenton process for the removal of contaminants from fuller's earth. The effects of pH (2.5 and 4.0), [H₂O₂] (1.47 and 2.07 mol L^-1), and [Fe²⁺] (1.7 and 40 mmol L^-1) were studied. The Fenton process efficiency was monitored using the decreases in the chemical oxygen demand (COD) and the concentrations of oil and grease, total carbon (TC), PCBs, and H₂O₂. The fuller's earth contaminated with insulating oil presented 35% (w/w) of TC, 34% (w/w) of oil and grease, 297.0 g L^-1 COD, and 64 mg of PCBs per kg. The material could therefore be considered a dangerous waste. After Fenton treatment, using a slurry mode, there was a removal of 55% of COD, 20% of oil and grease, and 20% of TC, achieved at pH 2.5 using 2.07 mol L^-1 of H₂O₂ and 40.0 mmol L^-1 of Fe²⁺. No PCBs were detected in the samples after the Fenton treatment, even using smaller amounts of Fenton reagents (1.47 mol L^-1 of H₂O₂, 1.7 mmol L^-1 of Fe²⁺, pH 2.5). The results indicated that the treated fuller's earth was free from PCB residues and could be disposed of in a simple landfill, in accordance with Brazilian PCB regulations.

Electrochemical degradation of ibuprofen-contaminated soils over Fe/Al oxidation electrodes

Ibuprofen (IBP) is one of the most known non-steroidal anti-inflammatory drugs. Due to the high consumption and the several ways of discharge, both the aquifer and soil matrix were contaminated by IBP. This study examined the degradation of the IBP in a soil matrix over Fe/Al oxidation electrodes in an electrokinetic system with processing fluids of sodium dodecylsulfate (SDS). The preparation of the Fe/Al oxidation electrode was carried out at a calcination temperature of 500, 550, and 600 °C, which accounted for Fe³⁺ coating rate of 3.89 ± 0.03%, 4.62 ± 0.04%, and 4.72 ± 0.04%, respectively. Results indicated the generation of hydroxyl radical was proportional to the coating rate of Fe³⁺ on the electrode. A 200 mg kg^-1 of IBP-spiked soil sample was conducted in an electrokinetic system under a potential gradient of 2 V cm^-1. The experimental parameters included electrode area of 11-33 cm² and treatment time of 5-9 days. The remediation efficiency of IBP in the EK systems coupled with Fe/Al oxidation electrode was 70.1-94.6%, which was highly dependent on H₂O₂ addition, electrode area, and treatment time. Both addition of H₂O₂ and prolonging treatment time significantly enhanced the degradation of IBP. Whereas increasing electrode area was only favorable for removal mechanism of IBP. Five reaction mechanisms were clearly provided in this study. The aluminum plays an electron donner to trigger Fenton-like reaction continuously to produce hydroxyl radicals. This study confirmed that the electrokinetic process coupled with Fe/Al oxidation electrodes is a viable technique for the remediation of IBP-contaminated soil. Make good use of redox characteristic of aluminum to trigger the Fenton-like reaction in Fe²⁺-rich environment is a great success in this study. The use of Fe/Al electrodes effectively expands the application of electrochemical degradation in soil remediation.

An overview of electrokinetic soil flushing and its effect on bioremediation of hydrocarbon contaminated soil

Combination of electrokinetic soil flushing and bioremediation (EKSF-Bio) technology has attracted many researchers attention in the last few decades. Electrokinetic is used to increase biodegradation rate of microorganisms in soil pores. Therefore, it is necessary to use solubilizing agents such as surfactants that can improve biodegradation process. This paper describes the basic understanding and recent development associated with electrokinetic soil flushing, bioremediation, and its combination as innovative hybrid solution for treating hydrocarbon contaminated soil. Surfactant has been widely used in many studies and practical applications in remediation of hydrocarbon contaminant, but specific review about those combination technology cannot be found. Surfactants and other flushing/solubilizing agents have significant effects to increase hydrocarbon remediation efficiency. Thus, this paper is expected to provide clear information about fundamental interaction between electrokinetic, flushing agents and bioremediation, principal factors, and an inspiration for ongoing and future research benefit.

Surfactant-Enhanced Electroosmotic Flushing in a Trichlorobenzene Contaminated Clayey Soil

Remediation of the sites contaminated with organic contaminants, such as chlorobenzenes, remains a challenging issue. Electroosmotic flushing can be a promising approach which is based on mechanism of electrokinetic remediation for removal of organic contaminants from fluids in low-permeability soil. To select an optimum surfactant that can effectively enhance electroosmotic flushing, three common surfactants, Triton X-100 (EK2), Tween 80 (EK3), and a mixture of sodium dodecyl sulfate and Triton X-100 (EK4) buffered with Na₂ HPO₄ /NaH₂ PO₄ solution, were tested. The efficiency of each kind of surfactant was evaluated using a three-dimensional box filled with a clayey soil spiked with 1,2,4-trichlorobenzene, and compared with a test (EK1) without surfactant. The results demonstrated that the buffer solutions efficiently neutralized H⁺ and OH^- produced by electrolysis. EK3 with Tween 80 added in the flushing solution reached the highest electroosmotic permeability of 10^-4  cm² /v/s and achieved a notably high cumulative electroosmotic flow (EOF) of 5067 mL within 6 d, which was 6.3, 3.4, and 4.2 times higher than that in EK1, EK2, and EK4, respectively. There were 420 mL more cumulative EOF obtained after 50 h of electrical application in EK4 than in EK2. The introduction of nonreactive ions can increase the current, thereby benefiting the EOF. Both the higher pH caused by the buffer and the application of nonionic surfactants can make the zeta potential more negative, thereby increasing the EOF. Tween 80 can be recommended as the best flushing solution for removing organic contaminants from sites when electrokinetic remediation is applied.

Electrokinetic-Fenton remediation of organochlorine pesticides from historically polluted soil

Soil contamination by persistent organic pollutants (POPs) poses a great threat to historically polluted soil worldwide. In this study, soils were characterized, and organochlorine pesticides contained in the soils were identified and quantified. Individual electrokinetic (IE), EK-Fenton-coupled technologies (EF), and enhanced EK-Fenton treatment (E-1, E-2, and E-3) were applied to remediate soils contaminated with hexachloro-cyclohexane soprocide (HCH) and dichloro-diphenyl-trichloroethane (DDT). Variation of pH, electrical conductivity, and electroosmotic flow was evaluated during the EK-Fenton process. The IE treatment showed low removal efficiency for HCHs (30.5%) and DDTs (25.9%). In the EF treatment, the highest removal level (60.9%) was obtained for α-HCH, whereas P,P-DDT was the lowest (40.0%). Low solubility of pollutants impeded the HCH and DDT removal. After enhanced EK-Fenton treatment, final removal of pollutants decreased as follows: β-HCH (82.6%) > γ-HCH (81.6%) > α-HCH (81.2%) > δ-HCH (80.0%) > P,P-DDD (73.8%) > P,P-DDE (73.1%) > P,P-DDT (72.6%) > O,P-DDT (71.5%). The results demonstrate that EK-Fenton is a promising technology for POP removal in historically polluted soil.

Application of a crustacean bioassay to evaluate a multi-contaminated (metal, PAH, PCB) harbor sediment before and after electrokinetic remediation using eco-friendly enhancing agents

Electrokinetic (EK) remediation can be a suitable technology for treating contaminated dredged harbor sediment, stored on terrestrial disposal sites. Citric acid (CA) and biosurfactants (rhamnolipids and saponin) were chosen as enhancing agents for simultaneous metal (Cd, Cr, Cu, Pb, Zn) and PAH/PCB removal by EK because of their potential low toxicity with a view to site restoration. Three EK runs were performed using a periodic voltage (1Vcm^-1) and various concentrations of agents. The best combination of CA (0.2molL^-1) and saponin (0.85gL^-1) did not remove high amounts of metals (4.4-15.8%) and provided only slightly better results for PAH and PCB removal (29.2% and 38.2%, respectively). The harbor sediment was highly resistant to metal and organics mobilization and transport because of an aged contamination, a high buffering capacity, a very low hydraulic permeability and a high organic matter content. The efficiency of the EK process was also assessed by measuring the acute toxicity of the EK-treated sediment on E. affinis copepods exposed to sediment elutriates. Fortunately, the use of CA and biosurfactants did not significantly impact on sediment toxicity. Some treated sediment sections, particularly those near the anode compartment, were statistically more toxic than the raw sediment. More particularly, E. affinis copepods were significantly sensitive to low pH values and oxidative conditions, to Cu, and to a lesser extent to Pb amounts. The speciation of these metals probably changed in these pH and redox conditions so that they became more easily leachable and bioavailable. In contrast, toxicity was negatively correlated to PAH and PCB amounts after EK treatment, probably due to the production of oxidized metabolites of PAHs and PCBs.

Simultaneous electrodialytic removal of PAH, PCB, TBT and heavy metals from sediments

Contaminated sediments are remediated in order to protect human health and the environment, with the additional benefit of using the treated sediments for other activities. Common for many polluted sediments is the contamination with several different pollutants, making remediation challenging with the need of different remedial actions for each pollutant. In this study, electrodialytic remediation (EDR) of sediments was found effective for simultaneous removal of heavy metals and organic pollutants for sediments from Arctic regions - Sisimiut in Greenland and Hammerfest in Norway. The influence of sediment properties and experimental settings on the remediation process was studied by employing multivariate analysis. The importance of the variables studied varied with the pollutant and based on these results it was possible to assess removal processes for the different pollutants. Desorption was found to be important for the removal of heavy metals and TBT, while photolysis was significant for removal of PAH, PCB and TBT. In addition, dechlorination was found to be important for the removal of PCB. The highest removal efficiencies were found for heavy metals, TBT and PCB (>40%) and lower removal efficiencies for PAH (<35%).

Performance of electroremediation in real contaminated sediments using a big cell, periodic voltage and innovative surfactants

The present work focused on evaluating the electrokinetic (EK) treatment of real contaminated sediments with toxic metals and polycyclic aromatic hydrocarbons (PAHs), using a big laboratory EK cell, periodic voltage and recently tested non-ionic surfactants. The results indicated that the "day on-night off" application mode of voltage, in conjunction with the selected solubilising agents, favoured the overall EK process. Arsenic, nickel and chromium exhibited the highest removal percentages, obtaining 83%, 67% and 63%, respectively, while zinc and lead attained 54% and 41% at the maximum. Furthermore, in the experiments where the non-ionic surfactants were introduced in the electrolyte chambers, there was a major uniformly removal of PAHs from the entire sediment across the EK cell, indicating the high solubilisation capacity of the enhancing agents. Essentially, transport and in some cases removal of PAHs (particularly from sections adjacent to the electrolyte compartments) also occurred in the unenhanced EK run, mainly due their negative charge, their potential weak bonds to the soil matrix and to the periodic application of voltage. Maximum removal was obtained by the use of Nonidet P40 where app. 1/3 (ca. 6498μg out of 20145μg) of the total initial amount of PAHs were removed from the cell.

Review of chemical and electrokinetic remediation of PCBs contaminated soils and sediments

Polychlorinated biphenyls (PCBs) are manmade organic compounds, and pollution due to PCBs has been a global environmental problem because of their persistence, long-range atmospheric transport and bioaccumulation. Many physical, chemical and biological technologies have been utilized to remediate PCBs contaminated soils and sediments, and there are some emerging new technologies and combined methods that may provide cost-effective alternatives to the existing remediation practice. This review provides a general overview on the recent developments in chemical treatment and electrokinetic remediation (EK) technologies related to PCBs remediation. In particular, four technologies including photocatalytic degradation of PCBs combined with soil washing, Fe-based reductive dechlorination, advanced oxidation process, and EK/integrated EK technology (e.g., EK coupled with chemical oxidation, nanotechnology and bioremediation) are reviewed in detail. We focus on the fundamental principles and governing factors of chemical technologies, and EK/integrated EK technologies. Comparative analysis of these technologies including their major advantages and disadvantages is summarized. The existing problems and future prospects of these technologies regarding PCBs remediation are further highlighted.

Scale-up on electrokinetic remediation: Engineering and technological parameters

This study analyses the effect of the scale-up of electrokinetic remediation (EKR) processes in natural soils. A procedure is proposed to prepare soils based on a compacting process to obtaining soils with similar moisture content and density to those found in real soils in the field. The soil used here was from a region with a high agrarian activity (Mora, Spain). The scale-up study was performed in two installations at different scales: a mock-up pilot scale (0.175m(3)) and a prototype with a scale that was very similar to a real application (16m(3)). The electrode configuration selected consisted of rows of graphite electrodes facing each other located in electrolyte wells. The discharge of 20mg of 2,4-dichlorophenoxyacetic acid [2,4-D] per kg of dry soil was treated by applying an electric potential gradient of 1Vcm(-1). An increase in scale was observed to directly influence the amount of energy supplied to the soil being treated. As a result, electroosmotic and electromigration flows and electric heating are more intense than in smaller-scale tests (24%, 1% and 25%, respectively respect to the values in prototype). In addition, possible leaks were evaluated by conducting a watertightness test and quantifying evaporation losses.

A comprehensive guide of remediation technologies for oil contaminated soil - Present works and future directions

Oil spills result in negative impacts on the environment, economy and society. Due to tidal and waves actions, the oil spillage affects the shorelines by adhering to the soil, making it difficult for immediate cleaning of the soil. As shoreline clean-up is the most costly component of a response operation, there is a need for effective oil remediation technologies. This paper provides a review on the remediation technologies for soil contaminated with various types of oil, including diesel, crude oil, petroleum, lubricating oil, bitumen and bunker oil. The methods discussed include solvent extraction, bioremediation, phytoremediation, chemical oxidation, electrokinetic remediation, thermal technologies, ultrasonication, flotation and integrated remediation technologies. Each of these technologies was discussed, and associated with their advantages, disadvantages, advancements and future work in detail. Nonetheless, it is important to note that no single remediation technology is considered the best solution for the remediation of oil contaminated soil.

CAPSULE

This review provides a comprehensive literature on the various remediation technologies studied in the removal of different oil types from soil.

Soil washing in combination with homogeneous Fenton-like oxidation for the removal of 2,4,4'-trichlorodiphenyl from soil contaminated with capacitor oil

Detoxification by chemical oxidation of polychlorinated biphenyls (PCBs) in contaminated soils is very difficult and inefficient because PCBs typically associate with the solid phase or exist as non-aqueous-phase liquids due to their low solubility and slow desorption rates, and thus, they are difficult to remove from soils by using traditional, water-based elution techniques. Surfactant can enhance washing efficiency of PCBs from contaminated soils. This study used Brij 58, Brij 30, Tween 80, and 2-hydroxypropyl-β-cyclodextrin (HPCD) to solubilize 2,4,4'-trichlorodiphenyl (PCB28) from soil contaminated with capacitor oil into solution. The feasibility of PCB28 oxidation in soil washing wastewater through a Fe(3+)-catalyzed Fenton-like reaction was subsequently examined. Washing with 10 g L(-1) Brij 58 solution showed the highest extraction efficiency (up to 61.5 %) compared with that of the three other surfactants. The total concentration of PCB28 in contaminated soil at 25 °C after 48-h extraction was 286 mg L(-1). In contrast to conditions in which no washing agent was added, addition of the four washing agents decreased the efficiency of PCB28 degradation by the Fenton-like reaction, with the decrease due to addition of 10 g L(-1) Brij 58 solution being the smallest. The optimal concentration of H2O2 for preventing its useless decomposition was found to be 50 mM. The efficiency of PCB28 removal was lower when the initial concentration of PCB28 treated in the Fenton-like reaction was higher. The degradation efficiencies of PCB28 at initial concentrations of 0.1, 10, and 176 mg L(-1) in 10 g L(-1) Brij 58 solution at 25 °C and pH 3.0 and 9 h of reaction using 50 mM H2O2 were 64.1, 42.0, and 34.6 %, respectively. This result indicates that soil washing combined with Fenton-like oxidation may be a practical approach for the remediation of PCB-contaminated soil.

Electrokinetic delivery of persulfate to remediate PCBs polluted soils: Effect of different activation methods

Persulfate-based in-situ chemical oxidation (ISCO) for the remediation of organic polluted soils has gained much interest in last decade. However, the transportation of persulfate in low-permeability soil is very low, which limits its efficiency in degrading soil pollutants. Additionally, the oxidation-reduction process of persulfate with organic contaminants takes place slowly, while, the reaction will be greatly accelerated by the production of more powerful radicals once it is activated. Electrokinetic remediation (EK) is a good way for transporting persulfate in low-permeability soil. In this study, different activation methods, using zero-valent iron, citric acid chelated Fe(2+), iron electrode, alkaline pH and peroxide, were evaluated to enhance the activity of persulfate delivered by EK. All the activators and the persulfate were added in the anolyte. The results indicated that zero-valent iron, alkaline, and peroxide enhanced the transportation of persulfate at the first stage of EK test, and the longest delivery distance reached sections S4 or S5 (near the cathode) on the 6th day. The addition of activators accelerated decomposition of persulfate, which resulted in the decreasing soil pH. The mass of persulfate delivered into the soil declined with the continuous decomposition of persulfate by activation. The removal efficiency of PCBs in soil followed the order of alkaline activation > peroxide activation > citric acid chelated Fe(2+) activation > zero-valent iron activation > without activation > iron electrode activation, and the values were 40.5%, 35.6%, 34.1%, 32.4%, 30.8% and 30.5%, respectively. The activation effect was highly dependent on the ratio of activator and persulfate.

Distribution of free radicals and intermediates during the photodegradation of polychlorinated biphenyls strongly affected by cosolvents and TiO₂ catalyst

Polychlorinated biphenyls (PCBs) pose potential ecological risk because of their high toxicity and carcinogenicity. Photodegradation, which is an important process for the removal of PCBs, is greatly influenced by the cosolvent and catalyst. Hence, it is important to explore their effects on the photodegradation behavior of PCBs. In this study, 2,4,4'-trichlorobiphenyl (PCB28) was selected as a model compound, and the effects of two typical cosolvents, namely acetone and ethanol, and TiO2 catalyst on the distributions of free radicals and intermediates were investigated. Interestingly, the TiO2 catalyst did not promote PCB28 photodegradation. Moreover, the free radical distribution was greatly influenced in the presence of the TiO2 catalyst, while was only slightly affected in its absence by the cosolvent kinds. The main photodegradation pathways are proposed on the basis of the distribution of detected intermediates, which were significantly regulated by both the cosolvent and TiO2 catalyst. The results provide novel insights into the photodegradation of PCBs and may have important implications for choosing cosolvent in desorbing soil PCBs and consequently enhancing PCBs degradation.

Ammonium citrate as enhancement for electrodialytic soil remediation and investigation of soil solution during the process

Seven electrodialytic experiments were conducted using ammonium citrate as enhancing agent to remediate copper and chromium-contaminated soil from a wood-preservation site. The purpose was to investigate the effect of current density (0.2, 1.0 and 1.5 mA cm(-2)), concentration of enhancing agent (0.25, 0.5 and 1.0 M) and remediation times (21, 42 and 117 d) for the removal of Cu and Cr from a calcareous soil. To gain insight on metal behavior, soil solution was periodically collected using suction cups. It was seen that current densities higher than 1.0 mA cm(-2) did not increase removal and thus using too high current densities can be a waste of energy. Desorption rate is important and both remediation time and ammonium citrate concentration are relevant parameters. It was possible to collect soil solution samples following an adaptation of the experimental set-up to ensure continuous supply of ammonium citrate to the soil in order to keep it saturated during the remediation. Monitoring soil solution gives valuable information on the evolution of remediation and helps deciding when the soil is remediated. Final concentrations in the soil ranged from 220 to 360 mg Cu kg(-1) (removals: 78-86%) and 440-590 mg Cr kg(-1) (removals: 35-51%), being within the 500 mg kg(-1) limit for a clean soil only for Cu. While further optimization is still required for Cr, the removal percentages are the highest achieved so far, for a real Cu and Cr-contaminated, calcareous soil. The results highlight EDR potential to remediate metal polluted soils at neutral to alkaline pH by choosing a good enhancement solution.

Electrokinetic delivery of persulfate to remediate PCBs polluted soils: effect of injection spot

Persulfate-based in situ chemical oxidation (ISCO) is a promising technique for the remediation of organic compounds contaminated soils. Electrokinetics (EK) provides an alternative method to deliver oxidants into the target zones especially in low permeable-soil. In this study, the flexibility of delivering persulfate by EK to remediate polychlorinated biphenyls (PCBs) polluted soil was investigated. 20% (w/w) of persulfate was injected at the anode, cathode and both electrodes to examine its transport behaviors under electrical field, and the effect of field inversion process was also evaluated. The results showed that high dosage of persulfate could be delivered into S4 section (near cathode) by electroosmosis when persulfate was injected from anode, 30.8% of PCBs was removed from the soil, and the formed hydroxyl precipitation near the cathode during EK process impeded the transportation of persulfate. In contrast, only 18.9% of PCBs was removed with the injection of persulfate from cathode, although the breakthrough of persulfate into the anode reservoir was observed. These results indicated that the electroosmotic flow is more effective for the transportation of persulfate into soil. The addition of persulfate from both electrodes did not significantly facilitate the PCBs oxidation as well as the treatment of electrical field reversion, the reinforced negative depolarization function occurring in the cathode at high current consumed most of the oxidant. Furthermore, it was found that strong acid condition near the anode favored the oxidation of PCBs by persulfate and the degradation of PCBs was in consistent with the oxidation of Soil TOC in EK/persulfate system.