Electroremediation of a natural soil polluted with phenanthrene in a pilot plant

Unraveling the mechanism of interaction: accelerated phenanthrene degradation and rhizosphere biofilm/iron plaque formation influenced by phenolic root exudates

Phenolic root exudates (PREs) secreted by wetland plants facilitate the accumulation of iron in the rhizosphere, potentially providing the essential active iron required for the generation of enzymes that degrade polycyclic aromatic hydrocarbon, thereby enhancing their biodegradation. However, the underlying mechanisms involved are yet to be elucidated. This study focuses on phenanthrene (PHE), a typical polycyclic aromatic hydrocarbon pollutant, utilizing representative PREs from wetland plants, including p-hydroxybenzoic, p-coumaric, caffeic, and ferulic acids. Using hydroponic experiments, 16S rRNA sequencing, and multiple characterization techniques, we aimed to elucidate the interaction mechanism between the accelerated degradation of PHE and the formation of rhizosphere biofilm/iron plaque influenced by PREs. Although all four types of PREs altered the biofilm composition and promoted the formation of iron plaque on the root surface, only caffeic acid, possessing a similar structure to the intermediate metabolite of PHE (catechol), could accelerate the PHE degradation rate. Caffeic acid, notable for its catechol structure, plays a significant role in enhancing PHE degradation through two main mechanisms: (a) it directly boosts PHE co-metabolism by fostering the growth of PHE-degrading bacteria, specifically Burkholderiaceae, and by facilitating the production of the key metabolic enzyme catechol 1,2-dioxygenase (C12O) and (b) it indirectly supports PHE biodegradation by promoting iron plaque formation on root surfaces, thereby enriching free iron for efficient microbial synthesis of C12O, a crucial factor in PHE decomposition.

Electrokinetic remediation for the removal of heavy metals in soil: Limitations, solutions and prospection

Electrokinetic remediation (EKR) technology is a promising method to remove heavy metals from low permeability soil, because it is environmentally friendly, efficient and economical, and can realize in-situ remediation. In this paper, the basic principles and related physical and chemical phenomena of EKR are systematically summarized, and three limiting problems of EKR technology are put forward: the weak ability of dissolving metals, focusing effect, and energy consumption. There are many methods to solve these technical problems, but there is a lack of systematic summary of the causes of problems and solutions. Based on various enhanced EKR technologies, this paper summarizes the main ideas to solve the limiting problems. The advantages and disadvantages of each technology are compared, which has guiding significance for the development of new technology in the future. This paper also discusses the dissolution of residual heavy metals, which is rare in other articles. The energy consumption of EKR and the remediation effect are equally important, and both can be used as indicators for evaluating the feasibility of new technologies. This paper reviews the influence of various electric field conditions on power consumption, such as renewable energy supply, new electrode materials and electrode configurations, suitable voltage values and functional electrolytes. In addition, a variety of energy consumption calculation methods are also introduced, which are suitable for ohmic heat loss, energy distribution when there is non-target ion competition, and power consumption of specific ions in various metal ions. Researchers can make selective reference according to their actual situations. This paper also systematically introduces the engineering design and cost calculation of EKR, lists the research progress of some engineering cases and pilot-scale tests, analyzes the reasons why it is difficult to apply EKR technology in large-scale engineering at present, and puts forward the future research direction.

MIL-101(Fe) based biomass as permeable reactive barrier applied to EK-PRB remediation of antimony contaminated soil

Numerous studies have demonstrated that electrokinetic-permeable reactive barrier (EK-PRB) can be used for the remediation of heavy metal contaminated soils, and their remediation efficiency is mainly determined by the filler material selected. By growing MIL-101(Fe) in situ on hollow loofah fiber (HLF), a novel material entitled HLF@MIL-101(Fe) was developed. The morphological characteristics and loading conditions were investigated, the adsorption characteristics were analyzed, and finally the synthesized composite material was applied to treat antimony-contaminated soil with EK-PRB as the reaction medium. The results show that MIL-101(Fe) is stably loaded on HLF. The adsorption capacity of Sb(III) can reach up to 82.31 mg g^-1, and the adsorption is in accordance with the quasi-secondary kinetic model, which indicates that chemisorption is dominant. The isothermal adsorption model indicates that the adsorption form of HLF@MIL-101(Fe) is mainly monolayer adsorption with more uniform adsorption binding energy. In the EK-PRB experiment, when ethylenediaminetetraacetic acid (EDTA) is used as the cathodic electrolyte, it can effectively enhance the electromigration and electroosmotic effects, and the overall remediation efficiency of the soil is increased by 38.12% compared with the citric acid (CA) group. These demonstrate the feasibility of HLF@MIL-101(Fe) in collaboration with EK-PRB in the treatment of antimony-contaminated soil.

Electrocoagulation coupled with electrooxidation for the simultaneous treatment of multiple pollutants in contaminated sediments

In situ and simultaneous remediation of a variety of pollutants in sediments remains a challenge. In this study, we report that the combination of electrocoagulation (EC) and electrooxidation (EO) is efficient in the immobilization of phosphorus and heavy metals and in the oxidation of ammonium and toxic organic matter. The integrated mixed metal oxide (MMO)/Fe anode system allowed the facile removal of ammonium and phosphorus in the overlying water (99% of 10 mg/L NH₄⁺-N and 95% of 10 mg/L P disappeared in 15 and 30 min, respectively). Compared with the controls of the single Fe anode and single MMO anode systems, the dual MMO/Fe anode system significantly improved the removal of phenanthrene and promoted the transition of Pb and Cu from the mobile species to the immobile species. The concentrations of Pb and Cu in the toxicity characteristic leaching procedure extracts were reduced by 99% and 97% after an 8 hr operation. Further tests with four real polluted samples indicated that substantial proportions of acid-soluble fraction Pb and Cu were reduced (30%-31% for Pb and 16%-23% for Cu), and the amounts of total organic carbon and NH₄⁺-N decreased by 56%-71% and 32%-63%, respectively. It was proposed that the in situ electrogenerated Fe(II) at the Fe anode and the active oxygen/chlorine species at the MMO anode are conducive to outstanding performance in the co-treatment of multiple pollutants. The results suggest that the EC/EO method is a powerful technology for the in situ remediation of sediments contaminated with different pollutants.

A novel solar-powered electrochemical mineralization system for persistent remediation of uranium-contaminated groundwater

Reduction of the migratory ability of uranium via reduction, co-precipitation or immobilization is a widely used technology for remediation of uranium contaminated groundwater (UCG). However, the re-released uranium due to environmental alterations such as oxidation, acid dissolution, or microbial decomposition limits the long-term effect of UCG remediation. Here, we developed a novel solar-powered electrochemical mineralization (SPEM) system for persistent remediation of UCG under laboratory conditions. The SPEM system incorporates uranium into magnetite crystal to achieve long-term stability of uranium. The effects of photoelectric conversion, subsurface void fraction, groundwater seepage velocity, and electrode configuration on uranium removal were systematically analyzed. The results showed that the remediation system had excellent adaptability to complex water quality and geological conditions, and could remediate large-area contamination. After 12 h of persistent treatment, the system with newly hexagonal two-dimensional electrode configuration (1A6C) reduced uranium concentration by more than 85% in simulated subsurface environment. The mineralized uranium was not re-released within continuous rinsing of treated regions using an acid solution (pH = 3.0), for 370 h. The developed method solely requires metallic iron as a raw material, which has high and long-term efficiency, is eco-friendly, simple, and widely applicable, thus reliable for the remediation of deep UCG.

Enhanced electrokinetically-delivered persulfate and alternating electric field induced thermal effect activated persulfate in situ for remediation of phenanthrene contaminated clay

Delivering persulfate (PS) efficiently into clay is an unsolved challenge. This study proposes a novel strategy with enhanced electrokinetically -delivery PS into clay by using PS for continuously flushing cathode to inhibit water electrolysis at cathode electrode. On this basis, a novel approach of heating soil by alternating current (AC) was used to thermally activate PS in situ. Results show that the mass transfer efficiency of PS by electroosmotic flow is about 20 times that by electromigration. Moreover, when PS was added in the anode chamber, using PS solution continuously flushing cathode created a relatively balanced the influent and effluent flow rates, significantly improving the mass transfer efficiency of PS. Compared to using NaNO₃ solution flushing, a significant increase of 51.7% was achieved, reaching 78.8%, for the phenanthrene (PHE) average degradation rate in soil cell. In contrast, the best overall PHE removal rate was observed, reaching 87.8%, by a cycle strategy of enhanced electrokinetically -delivered PS followed by AC heating applied. Electron paramagnetic resonance spectroscopy analysis showed oxidative radicals (SO₄^∙-/•OH) were the major species responsible for enhanced PHE degradation. These results demonstrate that this cycle strategy is a viable method for remediation of polycyclic aromatic hydrocarbons in clay.

Enhanced remediation of heavy metals contaminated soils with EK-PRB using β-CD/hydrothermal biochar by waste cotton as reactive barrier

Heavy metals in the soil are major global environmental problems. Waste cotton was used to synthesize a novel β-CD/hydrothermal biochar (KCB), which is a low-cost and environment-friendly adsorbent for heavy metal soil remediation. KCB were used as reactive materials of electrokinetic-permeable reactive barrier (EK-PRB) to explore the removal characteristics of heavy metals. FTIR and XPS analysis revealed that KCB contained large numbers of surface functional groups. Adsorption of KCB for Pb²⁺ and Cd²⁺ reached 50.44 mg g^-1 and 33.77 mg g^-1, respectively. Metal ions in contaminated soil were removed by reactive barrier through electromigration, electrodialysis and electrophoresis, the removal efficiency of Pb²⁺ and Cd²⁺ in soil reached 92.87% and 86.19%. This finding proves that KCB/EK-PRB can be used as a cheap and green process to effectively remediate soils contaminated with heavy metals.

In situ removal of cadmium by short-distance migration under the action of a low-voltage electric field and granular activated carbon

Cd pollution in soil is a global environmental issue of great concern. The secondary release and low removal rate of Cd are obstacles to the use of adsorption techniques. To develop a sustainable and effective remediation technique, low-voltage direct current (DC) and granular activated carbon (GAC) were applied for in situ Cd removal. The results showed that a low-voltage gradient was more favourable than a high-voltage gradient for Cd removal. A voltage gradient of 0.2 V cm^-1 acted as a driving force for Cd migration while limiting the side effects caused by DC. As an auxiliary enhancement measure, polarity exchange was effective in maintaining uniform distributions of soil moisture and temperature as well as a stable pH while improving Cd removal by weakening inhibition caused by OH^- generated at the cathodes. The average removal rates of total and bioavailable Cd were 61.05% and 76.96%, respectively. The potential mobility of Cd in soil was assessed by the mobility factor (M_F). The M_F was lowered from 42.66% to 8.96%, indicating that the risks of Cd mobility were reduced to low levels. The energy consumption and utilization efficiency of the method were 5.65 KWh m^-3 and 11.25, respectively. The energy utilization efficiency was significantly higher than the efficiencies of other methods that use DC to improve Cd removal. The results suggested that the in situ removal of Cd by low-voltage DC and GAC was efficient and avoided the secondary release of Cd.

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.

Scale-up of electrokinetic permeable reactive barriers for the removal of organochlorine herbicide from spiked soils

This work aims to shed light on the scale-up a combined electrokinetic soil flushing process (EKSF) with permeable reactive barriers (PRB) for the treatment of soil spiked with clopyralid. To do this, remediation tests at lab (3.45 L), bench (175 L) and pilot (1400 L) scales have been carried out. The PRB selected was made of soil merged with particles of zero valent iron (ZVI) and granular activated carbon (GAC). Results show that PRB-EKSF involved electrokinetic transport and dehalogenation as the main mechanisms, while adsorption on GAC was not as relevant as initially expected. Clopyralid was not detected in the electrolyte wells and only in the pilot scale, significant amounts of clopyralid remained in the soil after 600 h of operation. Picolinic acid was the main dehalogenated product detected in the soil after treatment and mobilized by electro-osmosis, mostly to the cathodic well. The transport of volatile compounds into the atmosphere was promoted at pilot scale because of the larger soil surface exposed to the atmosphere and the electrical heating caused by ohmic losses and the larger interelectrode gap.

Enhancing DNAPL removal from low permeability zone using electrical resistance heating with pulsed direct current

Electrical resistance heating (ERH) has been widely applied for contaminant remediation in heterogeneous sites especially when low permeability zones exist, yet requires high energy input. To address the low energy-efficiency of ERH using conventional alternating current (AC), pulsed direct current (PDC) obtained by current rectification was introduced for heating to enhance dense nonaqueous phase liquid (DNAPL) migration in low permeability zones. Here we showed the proof-of-concept in a lab-scale two-dimensional heterogeneous sand system (40 cm × 30 cm) with trichloroethylene (TCE) DNAPL in the central low permeability zone. Applying PDC achieved faster temperature increase compared to that with the conventional AC of the same voltage gradient. The overall TCE removal efficiency from the cell increased from 79.0% to 89.6% with increasing PDC voltage gradient from 3 to 3.75 V cm^-1, compared to that of 9.4-91.1% with conventional AC. The lowest energy consumption of PDC was 390 kWh kg^-1 at a medium voltage gradient of 3.5 V cm^-1, which was 27.8% lower compared to that with AC at the same voltage gradient. These results suggest that remediation using pulsed direct current is a promising approach to improve the energy-efficiency and effectiveness of ERH.

Removal of inorganic contaminants in soil by electrokinetic remediation technologies: A review

The soil contaminated by inorganic contaminants including heavy metals, radioactive elements and salts has been posing risks for human health and ecological environment, which has been widely paid attention in recent years. The electrokinetic remediation (EKR) technology is recognized as the most potential separation technology, which is commonly used to clean sites that are contaminated with organic and inorganic contaminants. It is the most suitable remediation technology for low permeability porous matrices. The main transport mechanism of pollutants in EKR include electromigration, electroosmosis and electrophoresis, coupled with electrolysis and geochemical reactions. Although arduous endeavors have been carried out to build optimal operating conditions and reveal the mechanism of EKR process, a systematic theoretical foundation hasn't been sorted yet. A comprehensive review on electrokinetic remediation of inorganic contaminants in soil is given in this study, and a more systematic theoretical foundation is sorted out according to the latest theoretical achievements. This theoretical system mainly focuses on the scientific and practical aspects of the application of EKR technology in soil remediation, by which we try to dig into the core of this technology. It contains key motive power of electric phenomena, side effects, energy consumption and supply, and removal of heavy metals, radioactive elements and salts in soil during EKR. In addition, correlations between dehydration, crystallization effect, focusing effect and thermal effect are disclosed; optimal operating conditions for the removal of heavy metals by EKR and EKR coupled with PRB are discussed and sorted out. Also discussed herein is the relationship between energy allocation and energy saving. According to the related findings, some potential improvements are also proposed.

Application of enhanced electrokinetic remediation by coupling surfactants for kerosene-contaminated soils: Effect of ionic and nonionic surfactants

Electrokinetic (EK) by coupling surfactants is an enhanced promising remediation technology to eliminate hydrophobic organic contaminants (HOC_s) from low-permeable soils. It is also applied to remediate kerosene-contaminated soils using anionic (SDS) and non-ionic (Tween 80) surfactants at different concentrations. There was negligible removal efficiency (40%) of kerosene during traditional EK without any enhancement technique. In the present study, the application of 0.005M and 0.01M SDS in EK-SDS-1 and EK-SDS-2 improved the removal efficiency to 50 and 55%, respectively towards the anode. Furthermore, the use of Tween 80 in EK-Tw80-1 and EK-Tw80-2 at 0.1 and 1% concentrations was able to raise kerosene removal gradually from 45% to 52% towards the cathode. These findings suggest that higher concentrations of SDS and Tween 80 contribute to the more effective elimination of kerosene. Thus, in EK-SDS-Tw80-V1.5 and EK-SDS-Tw80-V2, SDS and Tween 80 were used simultaneously at higher concentrations, which led to 63 and 67% kerosene removal, respectively. Considering the maximum removal in EK-SDS-Tw80-V2, the energy consumption in EK-SDS-Tw80-V2 was 178 KWh/m³ due to the higher voltage gradient; whereas without increased voltage in EK-SDS-Tw80-V1.5, this amount was decreased to 84 KWh/m³. It is to be mentioned that the electro-osmotic flow (EOF) played a significant role in minimizing kerosene concentration during the EK process, particularly when combined with surfactants.

Applicability of an anionic-nonionic surfactant in p-cresol contaminated soil washing: Finding the optimal mixing ratio

In this study, the parameters influencing p-cresol removal efficiency in soil washing method were investigated. Primarily, extraction efficiencies of three Tween series surfactants (Tween 20, Tween 60, Tween 80) with 10 mM concentration were compared. Tween 80 showed the best results since its value (55%) was 4% and 13% higher than that of Tween 60 and Tween 20. The impact of mixed surfactant on extraction rate was examined by employing a mixture of Tween 80 and one anionic surfactant (sodium dodecyl sulfate) with different molar ratio as the main washing solution. The results denoted that the molar ratio of 3:2 (SDS:Tween80) could enhance the extraction rate up to 38% compared to using SDS and Tween 80 alone. Regarding the initial p-cresol concentration in the collected sample, the cleanup level (390 mg/kg) could only be achieved using the mixed-surfactant. Thus, the minimum required surfactant concentrations to hit the target level was calculated to be 3.54 g/L of Tween 80 and 2.105 g/L of SDS (molar ratio of 0.27 SDS:Tween80). Studying the role of surfactant concentration indicated that its increment from 10 mM to 20 mM, which is way above all the reagents' critical micelle concentration (CMC), does not affect the removal rate considerably. The same results were obtained comparing the effect of washing time in three different levels (30 min, 60 min and 90 min). However, temperature showed to be a more significant parameter as it could enhance the results up to 20% (for SDS).

Application of enhanced electrokinetic approach to remediate Cr-contaminated soil: Effect of chelating agents and permeable reactive barrier

Enhanced electrokinetic (EK) technique was employed to remediate Cr-contaminated soil using a permeable reactive barrier (PRB) and chelating agents. Synthesized nanomagnetic Fe₃O₄ was used as a reactive material in PRB. Moreover, EDTA and citric acid (CA) were used as chelating agents. Sequential extraction method (SEM) was employed to determine Cr-elimination mechanism during the EK process. The results revealed that EDTA (78% Cr removal) was more effective than CA (54% Cr removal) in eliminating Cr from the contaminated soil during the EK process. The application of PRB in combination with EDTA was able to reduce the Cr removal rate to 70 and 66% by locating PRB in the middle section and near the anode/cathode reservoir, respectively. The use of PRB coupled with EDTA near the anode and cathode led to a more uniform Cr removal from the soil during the EK process. The highest energy consumption was 0.12 KWh during the EK remediation using PRB. Traditional EK remediation could only remove exchangeable and carbonate fractions of Cr. The use of chelating agents led to a significant (more than 90%) increase in Cr removal from the following fractions: exchangeable phase, carbonate phase, and bond to Fe-Mn oxides. In addition to electromigration (EM) mechanism, electroosmotic flow (EOF) played an important role in Cr removal during the EK process, especially when coupled with PRB.

Organochlorine pesticides contaminated soil decontamination using TritonX-100-enhanced advanced oxidation under electrokinetic remediation

Remediation of organochlorine pesticides (OCPs)-contaminated soils is urgently required especially in China. Surfactants have emerged as reliable and efficient co-solvent for the treatment of hardly soluble organic pollutants in contaminated soil. Here, we report the use of TritonX-100 (TX-100) in advanced oxidation under electrokinetic technology (EK) for OCPs removal from a historically contaminated soil from a former pharmaceutical industrial wasteland. Result shows that TX-100 (10%) played a key role in soil remediation. In effect, after a treatment period of 15 days, pollutants washed ranged from 50.68% (4,4'-DDT) to 76.07% (HCB), when TX-100 was used as the electrolyte (EK-TX-100). A simple advanced oxidation of the soil using sodium persulfate (PS) under EK approach (EK-PS) was limited to achieve good removal efficiency of the pollutants; as the result of OCPs' hardly dissolvable nature. The achieved removal efficiency were comprised between 22.62% (2,4-DDT) and 55.78% (1,2,4,5-TCB). With the application of TX-100 as co-solvent (EK-TX-100/PS), the pollutants removal efficiency significantly improved (p < 0.05). The treatment efficiency was shifted and up to 88.05% (1,2,4-TCB) was achieved, while the lowest removal efficiency was 56.36% (4,4'-DDE). We come to the conclusion that the use of TX-100-enhanced advanced oxidation (EK-TX-100/PS) as a reliable treatment for remediating organochlorine contaminated soil.

Electrolysis with diamond anodes of the effluents of a combined soil washing - ZVI dechlorination process

In this work, a new soil washing process in which Soil-Liquid extraction technology is enhanced by adding iron particles (zero valent iron nanoparticles or granules) was investigated to remove clopyralid from spiked soils. This novel approach can be efficiently used to extract chlorinated hydrocarbons from soil and aims to obtain soil-washing wastes with low content of hazardous chlorinated species. The iron particles used were subsequently removed from the treated soil using magnetic fields. Then, the complete mineralization of the produced soil washing effluents was successfully achieved by applying anodic oxidation with diamond anodes in an electrochemical flow cell. Results demonstrated that, opposite to what it was initially expected, no improvements in the efficiency of the electrochemical process were observed by adding iron particles during the soil washing. This behavior is explained in terms of the lower electrochemical reactivity of the dechlorinated derivatives produced. Although results are not as promising as initially expected, it does not mean a completely negative outcome for the use of ZVI during washing, because the hazardousness of the pollutants is rapidly decreased in the initial stages of the soil-washing, opening the possibility for the combination of this technology with other processes, such as biological treatment.

Techno-economic analysis of the scale-up process of electrochemically-assisted soil remediation

This work presents a techno-economic study of the scaling-up of the electrochemically-assisted soil remediation (EASR) process of polluted soil. Four scales have been selected for the study: laboratory, bench, pilot and prototype, with a capacity of treating a volume of soil of 1 × 10^-4, 2 × 10^-3, 0.11 and 21.76 m³, respectively. This study analyses the technical information produced by studies carried out at each scale, and informs about the fixed costs (construction of the electrokinetic remediation reactor, installation of auxiliary services and purchase of analytical equipment) and variable costs (start-up, operation and dismantling of the test) derived from running a test at each of the evaluated scales. The information discussed in based on the experience gained with many evaluations carried out over the last decade at these scales. This information can provide useful guidance for developing a scaling-up of the EASR for many researchers starting on the evaluation of this important environmental remediation technology.

Reversible electrokinetic adsorption barriers for the removal of organochlorine herbicide from spiked soils

This work aims to describe the removal of clopyralid from clay soils using electrokinetically assisted soil flushing (EKSF) coupled with a permeable reactive barrier (PRB), consisting of beds of Granulated Activated Carbon (GAC). To do this, two strategies have been evaluated on bench-scale electroremediation facilities (175 dm³): electrokinetic adsorption barrier (EKAB) and reversible electrokinetic adsorption barrier (REKAB). Likewise, to clarify the contribution of the different mechanisms to remediation process results are compared to those obtained in a reference test (without applying an electric field) and to results obtained in the EKSF of soils polluted with compounds with different polarity and vapour pressure. Results show that during EKAB and REKAB tests, clopyralid is removed from the soil by adsorption in PRB, electrokinetic transport and, very less decisively, by evaporation. The application of polarity reversion attains a higher retention of clopyralid in the activated carbon-PRB and a better regulation of pH because of the neutralization of H⁺ and OH^- generated in the electrolyte wells. After 30 days of operation, the removal of clopyralid by EKAB is 45% while it reaches 57% in the case of REKAB.

Simultaneous removal of polycyclic aromatic hydrocarbon and heavy metals from an artificial clayey soil by enhanced electrokinetic method

Contamination of soil by a mixture of organic and non-organic pollutants due to various anthropogenic and natural causes is one of the most important issues in soil pollution. Generating a single layer of soil is a time-consuming process, and soil is a significant part of the environmental cycle; thus, protection and remediation of soil are of paramount importance. In this study, phenanthrene (PHE) as a representative for polycyclic aromatic hydrocarbon and lead (Pb), zinc (Zn), and nickel (Ni) as heavy metal pollutants were used because of their synergistic effects and abundant presence in soil. Soils with three different organic matter levels (OM0, OM1, and OM2) were prepared, spiked with pollutants, placed in electrokinetic cells, and remediated for 15 days. To improve the remediation process and increase the efficacy of pollutant removal, pH control methods and extracting agents were used. Two non-ionic surfactants, Tween 80 and Brij 35, and one chelating agent, EDTA, were utilized for this purpose. A total of nine tests were performed. The soil was divided into five equal sections at the end of each test, and the pH and concentration of pollutants were analyzed. Results indicated that the highest removal percentage of PHE was accomplished by Brij 35 (61%) in OM0. Increasing organic matter resulted in decreasing PHE removal. In this regard, Tween 80 showed enhanced performance. The order of removal of heavy metals was zinc > nickel > lead, and zinc had the highest removal rate of 61%.

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.

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.

Removal of oxyfluorfen from spiked soils using electrokinetic soil flushing with the surrounding arrangements of electrodes

This work reports the results of a study in which the remediation of soil that undergoes an accidental discharge of oxyfluorfen is carried out by using electrokinetic soil flushing (EKSF). Two different electrode configurations were tested, consisting of several electrodes surrounding an electrode of different polarity (so-called 1A6C, one anode surrounded by six cathodes, and 1C6A, one cathode surrounded by six cathodes). A pilot plant scale was used (with a soil volume of 175dm(3)) to perform the studies. During the tests, different parameters were measured daily (flowrates, pH, electrical conductivity and herbicide concentration in different sampling positions). Furthermore, at the end of the test, a complete post-mortem analysis was carried out to obtain a 3-D map of the pollution, pH and electrical conductivity in the soil. The results demonstrate that electrode arrangement is a key factor for effective pollutant removal. In fact, the 1A6C configuration improves the removal rate by 41.3% versus the 27.0% obtained by the 1C6A configuration after a period of 35days. Finally, a bench mark comparison of this study of soil remediation polluted with 2,4-D allows for significant conclusions about the scale-up and full-scale application of this technology.

Application of electrokinetic soil flushing to four herbicides: A comparison

In this work, four bench-scale plants containing soil spiked with four herbicides (2,4-Dichlorophenoxyacetic acid (2,4-D), oxyfluorfen, chlorsulfuron and atrazine) undergo treatment consisting of an electrokinetic soil flushing (EKSF). Results clearly demonstrate that efficiency of EKSF depends on the chemical characteristic of the pesticide used. The amount of pesticide collected in the anode well is more significant than that collected in the cathode wells, indicating that the electromigration is much more important than drainage by electro-osmotic flux for this application. After 15 d of treatment, the 2,4-D is the pesticide most efficiently removed (95% of removal), while chlorsulfuron is the pesticide more resilient to the treatment. Additionally, volatilization was found to be a process of the major significance in the application of electrokinetic techniques to soil polluted with herbicides and because of that it should always be taken into account in the future design of full-scale processes.

Electrokinetic flushing with surrounding electrode arrangements for the remediation of soils that are polluted with 2,4-D: A case study in a pilot plant

This work aimed to evaluate electrokinetic soil flushing (EKSF) technologies for the removal of 2,4-dichlorophenoxyacetic acid (2,4-D) from spiked soils using an electrode configuration consisting of one cathode surrounded by six anodes (1c6a) and one anode surrounded by 6 cathodes (1a6c). Experiments were conducted for over one month in a bench-scale set-up (175 dm(3) of capacity) that was completely automated and operated at a constant electric field (1.0V cm(-1)). The electrical current, temperature, pH, moisture and pollutant concentration in electrolyte wells were monitored daily, and at the end of the experiments, an in-depth sectioned analysis of the complete soil section (post-mortem analysis) was conducted. Despite the geometric similarity, the two strategies led to very different results mainly in terms of water and herbicide mobilization, whereas pH and conductivity do not depend strongly on the electrode configuration. The volume of water extracted from cathodes with 1a6c is seven times higher than that of the 1c6a strategy. Herbicide was transported to the anode wells by electromigration and then dragged toward the cathode wells by electro-osmotic fluxes, with the first process being much more important. The configuration 1c6a was the most efficient and attained a transfer of 70% of the herbicide contained in the soil to flushing water in 35 days. These results outperform those obtained by the configuration 1a6c, for which less than 8% of the herbicide was transferred to flushing fluids in a much longer time (58 days).

Combined soil washing and CDEO for the removal of atrazine from soils

In this work, it is studied the removal of atrazine from spiked soils by soil washing using surfactant fluids, followed by the treatment of the resulting washing waste by electrolysis with boron doped diamond (BDD) anode. Results confirm that combination of both technologies is efficient for the removal and total mineralization of atrazine. Ratio surfactant/soil is a key parameter for the removal of atrazine from soil and influences significantly in the characteristic of the wastewater produced, affecting not only to the total organic load but also to the mean size of micelles. The higher the ratio surfactant soil, the lower is the size of the particles. Electrolyses of this type of waste attain the complete mineralization. TOC and COD are removed from the start of the treatment but the key of the treatment is the reduction in size of the micelles, which lead to a higher negative charge in the surface and to the faster depletion of the surfactant as compared with the pesticide.

Electro-assisted groundwater bioremediation: fundamentals, challenges and future perspectives

Bioremediation is envisaged as an important way to abate groundwater contamination, but the need for chemical addition and limited bioavailability of electron donors/acceptors or contaminants hamper its application. As a promising means to enhance such processes, electrochemical system has drawn considerable attention, as it offers distinct advantages in terms of environmental benignity, controllability and treatment efficiency. Meanwhile, there are also potential risks and considerable engineering challenges for its practical application. This review provides a first comprehensive introduction of this emerging technology, discusses its potential applications and current challenges, identifies the knowledge gaps, and outlooks the future opportunities to bring it to field application. The need for a better understanding on the microbiology under electrochemical stimulation and the future requirements on process monitoring, modeling and evaluation protocols and field investigations are highlighted.

Geotechnical behaviour of low-permeability soils in surfactant-enhanced electrokinetic remediation

Electrokinetic processes provide the basis of a range of very interesting techniques for the remediation of polluted soils. These techniques consist of the application of a current field in the soil that develops different transport mechanisms capable of mobilizing several types of pollutants. However, the use of these techniques could generate nondesirable effects related to the geomechanical behavior of the soil, reducing the effectiveness of the processes. In the case of the remediation of polluted soils with plasticity index higher than 35, an excessive shrinkage can be observed in remediation test. For this reason, the continued evaporation that takes place in the sample top can lead to the development of cracks, distorting the electrokinetic transport regime, and consequently, the development of the operation. On the other hand, when analyzing silty soils, in the surroundings of injection surfactant wells, high seepages can be generated that give rise to the development of piping processes. In this article methods are described to allow a reduction, or to even eliminate, both problems.

Biological permeable reactive barriers coupled with electrokinetic soil flushing for the treatment of diesel-polluted clay soil

Removal of diesel from spiked kaolin has been studied in the laboratory using coupled electrokinetic soil flushing (EKSF) and bioremediation through an innovative biological permeable reactive barriers (Bio-PRBs) positioned between electrode wells. The results show that this technology is efficient in the removal of pollutants and allows the soil to maintain the appropriate conditions for microorganism growth in terms of pH, temperature, and nutrients. At the same time, EKSF was demonstrated to be a very interesting technology for transporting pollutants, microorganisms and nutrients, although results indicate that careful management is necessary to avoid the depletion of nutrients, which are effectively transported by electro-migration. After two weeks of operation, 30% of pollutants are removed and energy consumption is under 70 kWh m(-3). Main fluxes (electroosmosis and evaporation) and changes in the most relevant parameters (nutrients, diesel, microorganisms, surfactants, moisture conductivity and pH) during treatment and in a complete post-study analysis are studied to give a comprehensive description of the most relevant processes occurring in the soil (pollutant transport and biodegradation).