Combination of bioremediation and electrokinetics for the in-situ treatment of diesel polluted soil: A comparison of strategies

Insight into the effect of electric fields on bioremediation of petroleum-contaminated soil: A micro-ecological response

The voltage gradient plays a crucial role in the process of electro-bioremediation for petroleum-contaminated soil. However, the micro-ecological response mechanisms of relevance have been scarcely documented. This study compared petroleum degradation characteristics, soil physicochemical properties, and bacterial microbiome indicators under 0.5 V cm-1, 1 V cm-1, and 2 V cm-1 conditions to elucidate the interaction mechanism among soil micro-ecological factors. The findings indicated that the treatment at 1 V cm-1 resulted in the most effective synergistic enhancement of electrokinetics and bioremediation, yielding a peak petroleum degradation ratio of 43.54 ± 1.64% over 105 days. The improvement in biodegradation resulted from the direct stimulation of bio-metabolism by higher ratios of "window condition" (RWC, 0.5331) and the indirect sustenance of microbial physiological activity by favorable soil conditions. The 1 V cm-1 voltage gradient either maintained or fostered the soil microbiome's response to the remediation system. The structural equation models (SEMs) demonstrated that variations in microbiome properties across different voltage gradients resulted from the influences of effective current intensity, soil pH, redox potential (Eh), dissolved organic carbon (DOC), and electrical conductivity (EC). Optimizing voltage gradients is a practical approach for developing effective micro-ecosystems to efficiently remediate petroleum-contaminated soil and implement electro-bioremediation in various engineering applications.

Flowing-water remediation simulation experiments of lead-contaminated soil using UCB technology

The flowing-water remediation of contaminated soil was investigated. Urease combined with biochar (UCB) technology was used to handle the Pb2+-contaminated sand column. The results showed that with the continuous increase of pore volume, the concentration of Pb2+ in the leachate undergoes three stages: slow growth, rapid growth, and steady state. With increasing seepage velocity, the concentration of Pb2+ in leachate increased slightly. The residual amount of each section of the sand column gradually decreased with increasing migration distance. The comparative results indicated that the UCB technology had a good solidification effect on Pb2+. This was due to urease-induced CaCO3 precipitation, cementation, and adsorption of Pb2+. Biochar provided more nucleation sites for urease, and some Pb2+ was adsorbed on its surface or diffused into the pores of biochar, or ions exchanged with functional groups on the surface of biochar, which effectively stabilized the free Pb2+.

An overview of electrokinetically enhanced chemistry technologies for organochlorine compounds (OCs) remediation from soil

In recent years, electrokinetic (EK) remediation technology has gained significant attention among researchers. This technology has proven effective in the remediation of low-permeability polluted soil. Organochlorines (OCs) are highly toxic, persistent, bioaccumulative, and capable of long-distance migration. They can also accumulate through the food chain, posing significant environmental risks. This paper provides a review of the reaction mechanism of combining chemical technology with EK remediation for the removal of several typical OCs. Furthermore, the factors influencing the efficiency of EK remediation, such as pH and ζ potential, voltage gradients, electrode materials, electrolytes, electrode arrangements, and soil types, are summarized. The paper also presents an overview of recent advancements in the methods of combining chemical technology with EK remediation for the treatment of OCs contaminated soil. Specifically, the research progress in surfactants-combined EK technology, chemical oxidation-combined EK technology, chemical reduction-combined EK technology, and chemical adsorption-combined EK technology is summarized. These findings serve as a foundation for ongoing and future research endeavors in the field. Further exploration and investigation in this area are essential for advancing the field and improving environmental remediation strategies.

Study on Cu- and Pb-contaminated loess remediation using electrokinetic technology coupled with biological permeable reactive barrier

Although the electrokinetic (EK) remediation has drawn great attention because of its good maneuverability, the focusing phenomenon near the cathode and low removal efficiency remain to be addressed. In this study, a novel EK reactor was proposed to remediate Cu and Pb contaminated loess where a biological permeable reactive barrier (bio-PRB) was deployed to the middle of the EK reactor. For comparison, three test configurations, namely, CG, TG-1, and TG-2, were available. CG considered the multiple enzyme-induced carbonate precipitation (EICP) treatments, while TG-1 considered both the multiple EICP treatments and pH regulation. TG-2 further considered NH4+ recovery based on TG-1. CG not only improved Cu and Pb removals by the bio-PRB but also depressed the focusing phenomenon. TG-1 causes more Cu2+ and Pb2+ to migrate toward the bio-PRB and aggravates Cu and Pb removals by the bio-PRB, depressing the focusing phenomenon. TG-2 depressed the focusing phenomenon the most because Cu2+ and Pb2+ can combine with not only CO32- but PO43-. The removal efficiency of Cu and Pb is 34% and 36%, respectively. A NH4+ recovery of about 100% is attained.

Removal of heavy metals from mine tailings by in-situ bioleaching coupled to electrokinetics

This work utilizes a combined biological-electrochemical technique for the in-situ removal of metals from polluted mine tailings. As the main novelty point it is proposed to use electrokinetics (EK) for the in-situ activation of a bioleaching mechanism into the tailings, in order to promote biological dissolution of metal sulphides (Step 1), and for the subsequent removal of leached metals by EK transport out of the tailings (Step 2). Mine tailings were collected from an abandoned Pb/Zn mine located in central-southern Spain. EK-bioleaching experiments were performed under batch mode using a lab scale EK cell. A mixed microbial culture of autochthonous acidophilic bacteria grown from the tailings was used. Direct current with polarity reversal vs alternate current was evaluated in Step 1. In turn, different biological strategies were used: biostimulation, bioaugmentation and the abiotic reference test (EK alone). It was observed that bioleaching activation was very low during Step 1, because it was difficult to maintain acidic pH in the whole soil, but then it worked correctly during Step 2. It was confirmed that microorganisms successfully contributed to the in-situ solubilization of the metal sulphides as final metal removal rates were improved compared to the conventional abiotic EK (best increases of around 40% for Cu, 162% for Pb, 18% for Zn, 13% for Mn, 40% for Ni and 15% for Cr). Alternate current seemed to be the best option. The tailings concentrations of Fe, Al, Cu, Mn, Ni and Pb after treatment comply with regulations, but Pb, Cd and Zn concentrations exceed the maximum values. From the data obtained in this work it has been observed that EK-bioleaching could be feasible, but some upgrades and future work must be done in order to optimize experimental conditions, especially the control of soil pH in acidic values.

An overview of bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils

The global problem of petroleum contamination in soils seriously threatens environmental safety and human health. Current studies have successfully demonstrated the feasibility of bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils due to their easy implementation, environmental benignity, and enhanced removal efficiency compared to bioremediation. This paper reviewed recent progress and development associated with bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils. The working principles, removal efficiencies, affecting factors, and constraints of the two technologies were thoroughly summarized and discussed. The potentials, challenges, and future perspectives were also deliberated to shed light on how to overcome the barriers and realize widespread implementation on large scales of these two technologies.

Integrated anaerobic-aerobic biodegradation of mixed chlorinated solvents by electrolysis coupled with groundwater circulation in a simulated aquifer

Chlorinated solvents are widespread subsurface contaminants that are often present as complex mixtures. Complete biodegradation of mixed chlorinated solvents remains challenging because the optimal redox conditions for biodegradation of different chlorinated solvents differ significantly. In this study, anaerobic and aerobic conditions were integrated by electrolysis coupled with groundwater circulation for biodegradation of a mixture of chloroform (CF, 8.25 mg/L), 1,2-dichloroethane (DCA, 7.01 mg/L), and trichloroethylene (TCE, 4.56 mg/L). A two-dimensional tank was filled with field sandy and silty-clayed sediments to simulate aquifer conditions, a pair of electrodes was installed between an injection well and abstraction well, and groundwater circulation transported cathodic H₂ and anodic O₂ to produce multiple redox conditions. Microbial community analysis demonstrated that the system constructed a habitat suitable for the co-existence of aerobic and anaerobic microbes. After 50 days of treatment, 93.1%, 100%, and 87.3% of CF, 1,2-DCA, and TCE were removed without observed intermediates, respectively. Combined with compound specific isotope analysis, the degradation of 1,2-DCA and CF was mainly attributed to aerobic oxidation and reductive dechlorination, respectively, and TCE was removed by both aerobic and anaerobic biodegradation. Our findings provide a new and efficient strategy for in situ bioremediation of groundwater contaminated by mixed chlorinated solvents.

Immobilizing of lead and copper using chitosan-assisted enzyme-induced carbonate precipitation

Enzyme-induced carbonate precipitation (EICP) is considered as an environmentally friendly method for immobilizing heavy metals (HMs). The fundamental of the EICP method is to catalyze urea hydrolysis using the urease, discharging CO₃^2- and NH₄⁺. CO₃^2- helps to form carbonates that immobilize HMs afterwards. However, HMs can depress urease activity and reduce the degree of urea hydrolysis. Herein, the potential of applying the chitosan-assisted EICP method to Pb and Cu immobilization was explored. The chitosan addition elevated the degree of urea hydrolysis when subjected to the effect of Cu²⁺ toxicity where the protective effect, flocculation and adsorption, and the formation of precipitation, play parts in improving the Cu immobilization efficiency. The use of chitosan addition, however, also causes the side effect (copper-ammonia complex formation). Two calcium source additions, CaCl₂ and Ca(CH₃COO)₂, intervened in the test tube experiments not only to prevent pH from raising to values where Cu²⁺ complexes with NH₃ but also to separate the urease enzyme and Cu²⁺ from each other with the repulsion of charges. The FTIR spectra indicate that the chitosan addition adsorbs Cu²⁺ through its surface hydroxyl and carboxyl groups, while the SEM images distinguish who the mineral are nucleating with. The findings shed light on the potential of applying the chitosan-assisted EICP method to remedy lead- and copper-rich water bodies.

An Overview of Emerging Cyanide Bioremediation Methods

Cyanide compounds are hazardous compounds which are extremely toxic to living organisms, especially free cyanide in the form of hydrogen cyanide gas (HCN) and cyanide ion (CN−). These cyanide compounds are metabolic inhibitors since they can tightly bind to the metals of metalloenzymes. Anthropogenic sources contribute significantly to CN− contamination in the environment, more specifically to surface and underground waters. The treatment processes, such as chemical and physical treatment processes, have been implemented. However, these processes have drawbacks since they generate additional contaminants which further exacerbates the environmental pollution. The biological treatment techniques are mostly overlooked as an alternative to the conventional physical and chemical methods. However, the recent research has focused substantially on this method, with different reactor configurations that were proposed. However, minimal attention was given to the emerging technologies that sought to accelerate the treatment with a subsequent resource recovery from the process. Hence, this review focuses on the recent emerging tools that can be used to accelerate cyanide biodegradation. These tools include, amongst others, electro-bioremediation, anaerobic biodegradation and the use of microbial fuel cell technology. These processes were demonstrated to have the possibility of producing value-added products, such as biogas, co-factors of neurotransmitters and electricity from the treatment process.

Experimental investigation on a combination of soil electrokinetic consolidation and remediation of drained water using composite nanofiber-based electrodes

A novel electrokinetic geosynthetic (EKG) can be efficient in achieving multiple objectives. In this study, a new EKG as an electrode and a drainage channel in the electro-osmotic consolidation was fabricated by electrospun nanofibers containing graphene nanoparticles (GNs) attached to a carbon fiber substrate. To investigate the effectiveness of the fabricated electrodes in electro-osmotic consolidation and remediation of water drained from the system, an experimental apparatus was constructed while considering loading capability in expanded ranges and applying the electric field, and was filled with copper (Cu)-contaminated kaolinite. Experiments were divided into control (CT) and EKG groups, and three categories, C-EK, ES₁-EK, and ES₂-EK (using carbon fiber, electrospun nanofibers containing 1 wt% GNs, and electrospun nanofibers consisting of 2 wt% GNs, respectively). All the experiments were conducted with the same conditions, loading, drainage condition, and duration. However, EKG experiments were performed by employing the electric field under the vertical pressure in the range of 7-113 kPa, while the CT was conducted without the electric field. According to experimental results, 18 wt% polymethyl methacrylate in the dimethylformamide solvent containing 1 and 2 wt% GNs was selected for making a nanofibrous layer on the carbon fiber. The average diameters of the fibers were 404 ± 36 and 690 ± 62 nm and yielded at 1 and 2 wt% GNs, respectively. The results revealed that using the EKG accelerated kaolinite consolidation. The average degree of consolidation was 68 and 85% in the CT and EKG experiments, respectively. Furthermore, the fabricated electrodes were highly effective as a drainage channel for remediating water drained from the system. Moreover, the highest Cu removal efficiency was obtained in ES₂-EK (97%) and ES₁-EK (92%), respectively. Conversely, the lowest Cu removal efficiency was observed in the C-EK group (85%).

Enhancing soil vapor extraction with EKSF for the removal of HCHs

This paper evaluates the combination of electrokinetic soil flushing (EKSF) with soil vapor extraction (SVE) for the removal of four hexachlorocyclohexane (HCH) isomers contained in a real matrix. Results demonstrate that the combination of EKSF and SVE can be positive, but it is required the application of high electric fields (3 V cm^-1) in order to promote a higher temperature in the system, which improves the volatilization of the HCH contained in the system. Electrokinetic transport is also enhanced with the application of higher electric gradients, but these transport processes are slower than the volatilization processes, which are the primary in this system. Hence collection of species in the electrolyte wells is negligible as compared to the compound dragged with air by the SVE but the temperature increase demonstrates a good performance. Combination of EKSF with SVE can efficiently exhaust the four HCH isomers reaching a removal of more than 90% after 15 days of treatment (20% more than values attained by SVE) but it is required the application of high electric fields to promote a higher temperature in the system (to improve the volatilization) and EK transport (to improve the dragging). 1-D transport model can be easily used to estimate the average pore water velocity and the effective diffusion of each compound under the different experimental conditions tested.

Removal of lindane using electrokinetic soil flushing coupled with air stripping

This paper evaluates the remediation of soil spiked with lindane using a combined treatment consisting of electrokinetic soil flushing (EKSF) with air stripping to elucidate the main processes occurring in the soil when electric fields of 0.75 V cm−1 and 1.50 V cm−1 are applied. The results demonstrate that lindane is efficiently transported to the anodic and cathodic wells using flushing fluids containing sodium dodecyl sulfate (SDS). Additionally, an important amount is volatilized and stripped with the injected air. In the cathodic well, lindane is rapidly transformed into other species because of the strongly alkaline media. These other species are also found in the portions of soil next to this well, confirming the efficient transport of chlorinated organics with SDS. After 14 days of operation, nearly 50% of the spiked lindane can be removed from the soil. Operation with large electric fields does not improve the performance of the treatment technology and results in lower current intensities and electro-osmotic fluxes and in higher evaporated water, despite the water content in the soil matrix, indicating the coexistence of multiple inputs in these processes.

Graphical abstract

Changes of microbial community and activity under different electric fields during electro-bioremediation of PAH-contaminated soil

Electro-bioremediation is a promising technology for remediation of soil contaminated with persistent organic compounds such as polycyclic aromatic hydrocarbons (PAHs). During electro-bioremediation, electrical fields have been shown to increase pollutant degradation. However, it remains unclear whether there is an optimal strength for the electrical field applied that is conductive to the maximum role played by microbes. This study aimed to determine the optimal strength of electric field through the analysis of the effects of different voltages on the microbial community and activity. Four bench-scale experiments with voltages of 0, 1, 2 and 3 V cm^-1 were conducted for 90 days in an aged PAH-contaminated soil. The spatiotemporal changes of the soil pH, moisture content and temperature, microbial biomass and community structure, and the degradation extent of PAHs were researched over 90 days. The results indicated that the total microbial biomass and degradation activity were highest at voltages of 2 V cm^-1. The concentration of total phospholipid fatty acids, used to quantify soil microbial biomass, reached 65.7 nmol g^-1 soil, and the mean degradation extent of PAHs was 44.0%. Similarly, the maximum biomass of actinomycetes, bacteria and fungus also occurred at the voltage of 2 V cm^-1. The Gram-positive/Gram-negative and (cy17:0+cy 19:0)/(16:1ω7+18:1ω7) ratios also showed that the intensity of electric field and electrode reactions strongly influenced the microbial community structure. Therefore, to optimize the electro-bioremediation of PAH-contaminated soil, the strength of electric field needs to be selected carefully. This work provides reference for the development of novel electrokinetically enhanced bioremediation processes.

An approach to the photocatalytic mechanism in the TiO2-nanomaterials microorganism interface for the control of infectious processes

The approach of this timely review considers the current literature that is focused on the interface nanostructure/cell-wall microorganism to understand the annihilation mechanism. Morphological studies use optical and electronic microscopes to determine the physical damage on the cell-wall and the possible cell lysis that confirms the viability and microorganism death. The key parameters of the tailoring the surface of the photoactive nanostructures such as the metal functionalization with bacteriostatic properties, hydrophilicity, textural porosity, morphology and the formation of heterojunction systems, can achieve the effective eradication of the microorganisms under natural conditions, ranging from practical to applications in environment, agriculture, and so on. However, to our knowledge, a comprehensive review of the microorganism/nanomaterial interface approach has rarely been conducted. The final remarks point the ideal photocatalytic way for the effective prevention/eradication of microorganisms, considering the resistance that the microorganism could develop without the appropriate regulatory aspects for human and ecosystem safety.

A review of electrokinetically enhanced bioremediation technologies for PHs

Since the early 1980's there have been several different strategies designed and applied to the remediation of subsurface environment including physical, chemical and biological approaches. They have had varying degrees of success in remediating contaminants from subsurface soils and groundwater. The objective of this review is to examine the range of technologies for the remediation of contaminants, particularly petroleum hydrocarbons, in subsurfaces with a specific focus on bioremediation and electrokinetic remediation. Further, this review examines the efficiency of remediation carried out by combining bioremediation and electrokinetic remediation. Surfactants, which are slowly becoming the selected chemicals for mobilizing contaminants, are also considered in this review. The current knowledge gaps of these technologies and techniques identified which could lead to development of more efficient ways of utilizing these technologies or development of a completely new technology.

Evaluation of electrokinetics coupled with a reactive barrier of activated carbon loaded with a nanoscale zero-valent iron for selenite removal from contaminated soils

The range between dietary deficient and toxic levels for selenium is quite narrow. In this study, the synergistic effects of electrokinetics (EK) and a permeable reactive barrier (PRB) on the reductive sequestration of Se(IV) oxyanions from spiked soils were investigated in detail. Activated charcoal (AC)-supported Fe(II) and nanoscale zero-valent iron (nZVI) were prepared as the PRB media for use in an electrolyzer. In aqueous equilibrium adsorption tests, the AC-supported nZVI medium had a higher adsorption capacity than that of the other adsorbents. The Se(IV) removal isotherms were well-fitted using the Langmuir model. The Se(IV) removal rates were accurately predicted by both pseudo-first- and pseudo-second-order kinetic models. For the coupled systems, a moderate increase in the number of PRBs and decrease in the PRB thickness in the electrolyzer enhanced the removal and catalytic recovery of Se(IV) from the spiked soil samples. A Se(VI) removal efficiency of approximately 95% and Se(VI) reduction efficiency of 90% were achieved in the optimized electrochemical system. The Se(IV) species were reduced to Se° and FeSe by the AC-supported nZVI regardless of the pH distribution. The experimental results provide guidance for the multichannel recovery of Se from abandoned ore tailings or solid wastes.

Integrated electrokinetic processes for the remediation of phthalate esters in river sediments: A mini-review

Concerning the contamination of phthalate esters (PAEs) in river sediments, this mini-review introduces four recently reported novel "integrated electrokinetic (EK) processes" for the remediation purpose, namely two combined technologies of the EK process and advanced oxidation process (EK-AOP Processes) and two combined technologies of the EK process and biological process (EK-BIO Processes). The following is a comprehensive summary for these remediation processes: (1) the EK process coupled with nano-Fe₃O₄/S₂O₈^2- oxidation process - Test results have shown that nanoscale Fe₃O₄ played a significant role in activating persulfate oxidation. Even a recalcitrant compound like di(2‑ethylhexyl)phthalate (DEHP), its concentration in test sediment was reduced to 1.97 mg kg^-1, far below the regulatory levels set by Taiwan EPA; (2) the EK process integrated with a novel Fenton-like process catalyzed by nanoscale schwertmannite (nano-SHM) - Test results have revealed that simultaneous injection of nano-SHM slurry and H₂O₂ into the anode reservoir and sediment compartment is a good practice. 70-99% in removal efficiency was obtained for various target PAEs; (3) enhanced in situ bioremediation coupled with the EK process for promoting the growth of intrinsic microorganisms by adding H₂O₂ as an oxygen release compound (ORC) - Test results have demonstrated that an intermittent mode of injecting lab-prepared ORC directly into the contaminant zone would be beneficial to the growth of intrinsic microorganisms in test sediment for in situ bioremediation of target PAEs; and (4) coupling of a second-generation ORC (designated 2G-ORC) with the EK-biological process - Test results have proved that 2G-ORC is long-lasting and can be directly utilized as the carbon source and oxygen source for microbial growth resulting in an enhanced biodegradation of PAEs. Except DEHP having a residual concentration of 4 μg kg^-1, all other target PAEs in test sediment were totally removed by this novel combined remediation process.

Improvement of the electro-bioremediation process of a non-polar herbicide-polluted soil by means of surfactant addition

Oxyfluorfen is a non-polar herbicide that may cause severe soil pollution. The present work studies the possible improvement due to surfactant addition in the efficiency of electro-bioremediation of a clay soil polluted which such a non-polar, low-mobility pollutant. Two-week-long batch electro-bioremediation experiments were performed in a bench-scale device. Oxyfluorfen-polluted soil (20 mg kg^-1) was inoculated with an acclimated microbial culture, and several experiments were performed using different surfactant concentrations in the electrode wells (0.0, 2.5, 5.0, 10.0 and 20.0 g L^-1 of SDS, sodium dodecyl sulphate). Experiments were performed under 1.0 V cm^-1 and electrode polarity reversal. It was observed that the electro-osmotic flow (EOF) increased with SDS concentration and that SDS was successfully distributed across the soil, probably improving the oxyfluorfen mobility. Additionally, microbiological activity was fully maintained during the experiments. Electro-bioremediation without SDS removed 14% of the oxyfluorfen, while under 2.5 g L^-1 SDS, the efficiency increased to 22% because of an expected improvement in the contact between the different species in the soil. However, higher SDS concentrations (between 10.0 and 20.0 g L^-1) caused a decrease in the oxyfluorfen removal efficiency, as SDS is an easily biodegradable compound and was preferably used as substrate by the microbial culture instead of oxyfluorfen. Additionally, the use of high concentrations of SDS was clearly inefficient, as high amounts of the surfactant were lost through the EOF, and even low amounts of oxyfluorfen were removed to the electrode wells, which means that ex situ treatment of the polluted water would be needed.

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.

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.

Effect of the polarity reversal frequency in the electrokinetic-biological remediation of oxyfluorfen polluted soil

This work studies the feasibility of the periodic polarity reversal strategy (PRS) in a combined electrokinetic-biological process for the remediation of clayey soil polluted with a herbicide. Five two-weeks duration electrobioremediation batch experiments were performed in a bench scale set-up using spiked clay soil polluted with oxyfluorfen (20 mg kg^-1) under potentiostatic conditions applying an electric field between the electrodes of 1.0 V cm^-1 (20.0 V) and using PRS with five frequencies (f) ranging from 0 to 6 d^-1. Additionally, two complementary reference tests were done: single bioremediation and single electrokinetic. The microbial consortium used was obtained from an oil refinery wastewater treatment plant and acclimated to oxyfluorfen degradation. Main soil conditions (temperature, pH, moisture and conductivity) were correctly controlled using PRS. On the contrary, the electroosmotic flow clearly decreased as f increased. The uniform soil microbial distribution at the end of the experiments indicated that the microbial activity remained in every parts of the soil after two weeks when applying PRS. Despite the adapted microbial culture was capable of degrade 100% of oxyfluorfen in water, the remediation efficiency in soil in a reference test, without the application of electric current, was negligible. However, under the low voltage gradients and polarity reversal, removal efficiencies between 5% and 15% were obtained, and it suggested that oxyfluorfen had difficulties to interact with the microbial culture or nutrients and that PRS promoted transport of species, which caused a positive influence on remediation. An optimal f value was observed between 2 and 3 d^-1.

Coupling electrokinetics with microbial biodegradation enhances the removal of cycloparaffinic hydrocarbons in soils

An innovative approach that couples electrokinetics with microbial degradation to breakdown cycloparaffinic hydrocarbons in soils is described. Soils were spiked with cyclododecane, used as a model pollutant, at approximately 1000mgkg^-1. A mixture of petroleum-utilizing bacteria was added to achieve about 10⁶-10⁷ CFUg^-1. Then, three treatments were applied for 25 days: (1) no electric field, control; (2) a constant voltage gradient of 1.3Vcm^-1 in one direction; and (3) the same electric field, but with periodical switching of polarity. The degradation pathway of cyclododecane was not changed by the electric field, but the dynamic processes were remarkably enhanced, especially when the electric field was periodically switched. After 25 days, 79.9% and 87.0% of the cyclododecane was degraded in tests 2 and 3, respectively; both much higher than the 61.5% degraded in test 1. Analysis of the intermediate products strongly indicated that the competitive advantage of the electric field was the increase in ring-breaking of cyclododecane, resulting in greater concentrations of linear substances that were more susceptible to microbial attack, that is, β-oxidation. The conditions near the cathode were more favorable for the growth and metabolism of microorganisms, which also enhanced β-oxidation of the linear alkanoic acids. Therefore, when the electric field polarity was periodically switched, the functions of both the anode and cathode electrodes were applied across the whole soil cell, further increasing the degradation efficiency.

From oil spills to barley growth - oil-degrading soil bacteria and their promoting effects

Heavy contamination of soils by crude oil is omnipresent in areas of oil recovery and exploitation. Bioremediation by indigenous plants in cooperation with hydrocarbon degrading microorganisms is an economically and ecologically feasible means to reclaim contaminated soils. To study the effects of indigenous soil bacteria capable of utilizing oil hydrocarbons on biomass production of plants growing in oil-contaminated soils eight bacterial strains were isolated from contaminated soils in Kazakhstan and characterized for their abilities to degrade oil components. Four of them, identified as species of Gordonia and Rhodococcus turned out to be effective degraders. They produced a variety of organic acids from oil components, of which 59 were identified and 7 of them are hitherto unknown acidic oil metabolites. One of them, Rhodococcus erythropolis SBUG 2054, utilized more than 140 oil components. Inoculating barley seeds together with different combinations of these bacterial strains restored normal growth of the plants on contaminated soils, demonstrating the power of this approach for bioremediation. Furthermore, we suggest that the plant promoting effect of these bacteria is not only due to the elimination of toxic oil hydrocarbons but possibly also to the accumulation of a variety of organic acids which modulate the barley's rhizosphere environment.

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.

Electrokinetic-Fenton technology for the remediation of hydrocarbons historically polluted sites

The feasibility of the electrokinetic-Fenton technology coupled with surfactants in the treatment of real historically hydrocarbons polluted soils has been studied. The characterisation of these soils from Spain and Romania was performed and identified as diesel and diesel-motor oil spillages, respectively. Moreover, the ageing of the spillages produced by the soil contamination was estimated showing the historical pollution of the sites (around 11 and 20 years for Romanian and Spanish soils, respectively). An ex-situ electrochemical treatment was performed to evaluate the adequacy of surfactants for the degradation of the hydrocarbons present in the soils. It was found an enhancement in the solubilisation and removal of TPHs with percentages increasing from 25.7 to 81.8% by the presence of Tween 80 for Spanish soil and from 15.1% to 71.6% for Triton X100 in Romanian soil. Therefore, the viability of coupling enhanced electrokinetic and Fenton remediation was evaluated through a simulated in-situ treatment at laboratory scale. The results demonstrated that the addition of the selected surfactants improved the solubilisation of the hydrocarbons and influenced the electroosmotic flow with a slight decrease. The efficiency of the treatment increased for both considered soil samples and a significant degradation level of the hydrocarbons compounds was observed. Buffering of pH coupled with the addition of a complexing agent showed to be important in the treatment process, facilitating the conditions for the degradation reactions that take place into the soil matrix. The results demonstrated the effectiveness of the selected techniques for remediation of the investigated soils.

Effect of electric field on the performance of soil electro-bioremediation with a periodic polarity reversal strategy

In this work, it is studied the effect of the electric fields (within the range 0.0-1.5 V cm(-1)) on the performance of electrobioremediation with polarity reversal, using a bench scale plant with diesel-spiked kaolinite with 14-d long tests. Results obtained show that the periodic changes in the polarity of the electric field results in a more efficient treatment as compared with the single electro-bioremediation process, and it does not require the addition of a buffer to keep the pH within a suitable range. The soil heating was not very important and it did not cause a change in the temperature of the soil up to values incompatible with the life of microorganisms. Low values of water transported by the electro-osmosis process were attained with this strategy. After only 14 d of treatment, by using the highest electric field studied in this work (1.5 V cm(-1)), up to 35.40% of the diesel added at the beginning of the test was removed, value much higher than the 10.5% obtained by the single bioremediation technology in the same period.

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.