Enhancement Of Electro-Osmotic Flow During The Electrokinetic Treatment Of A Contaminated Soil

Bibliometric analysis and systematic review on the electrokinetic remediation of contaminated soil and sediment

Electrokinetic remediation (EKR) is a proficient, environmentally friendly separation technology for in-situ removal of contaminants in soil/sediment, distinguished for its ease of implementation and minimal prerequisites compared to other remediation technologies. To comprehensively understand the research focus and progress related to EKR of contaminated soil/sediment, a bibliometric analysis was conducted on 1593 publications retrieved from the Web of Science Core Collection (WOSCC) database. This analysis utilized data mining and knowledge discovery techniques through Bibliometrix, VOSviewer, and CiteSpace software. The results revealed a rising trend in annual publication numbers, with China leading in the number of publications. The primary journals in this field included the Journal of Hazardous Materials, Chemosphere, and Separation and Purification Technology. The primary disciplines contributed to this field included "Environmental Sciences", "Engineering, Environmental", "Engineering, Chemical", and "Electrochemistry". Keyword co-occurrence and burst analysis indicated that current EKR-related research mainly focuses on the remediation of soil/sediments contaminated by heavy metals (HMs) and organic pollutants (OPs). Furthermore, the EKR remediation improvement method emerged as the prevailing and future research hotspots and development directions. Future research could integrate numerical simulations and various methodologies to predict and assess the migration of pollutants and the efficiency of remediation efforts. Additionally, these studies could explore the effects of EKR on the physicochemical properties and microbial diversity of soil/sediment to provide a theoretical foundation for applying EKR in soil/sediment remediation.

Electrokinetic remediation of cadmium-contaminated soil using polarity reversal method: Optimization analysis and mechanism exploration

Electrokinetic remediation (EKR) has been applied for in-situ removal of Cd from contaminated soil, and the EKR enhanced with polarity reversal has achieved a higher Cd removal efficiency. However, the migration and accumulation mechanisms of Cd in the EKR process have not been investigated. In this paper, the cross-impacts of the voltage gradient, citric acid concentration in the electrolyte, and polarity reversal frequency on the removal efficiency by EKR of Cd and the optimization conditions were investigated. The migration and accumulation mechanisms of Cd were explored by analyzing the changes in electrokinetic process parameters, experimental phenomena, and X-ray diffraction (XRD) analysis. The results showed that the maximum removal efficiency of Cd reached 82.26%. The optimal conditions were determined by fitting the RSM model using the BBD design. In the EKR experiment with polarity reversal, Cd accumulated mainly in the middle part of the soil, attributed to the formation of chemical precipitation focusing area caused by soil pH transition, ion-induced potential gradient well trapping effect (IIPGWTE), or soil compaction induced by water loss. In conclusion, the various parameters have cross-impacts on the EKR of Cd-contaminated soil, and efficient in-situ removal of Cd from the contaminated soil can be achieved by adjusting the parameter conditions.

PFOA remediation from kaolinite soil by electrokinetic process coupled with activated carbon/iron coated activated carbon - permeable reactive barrier

This study applied electrokinetic (EK) in situ soil remediation for perfluorooctanoic acid (PFOA) removal from kaolinite soil. The kaolinite soil was spiked with 10 mg/kg PFOA for the EK treatment using Sodium Cholate bio-surfactant coupled with Activated Carbon (AC) or iron-coated Activated Carbon (FeAC) permeable reactive barrier (PRB). The study also evaluated the impact of AC and FeAC PRBs' position on the EK process performance. In the EK with the PRB in the middle section, PFOA removal from kaolinite was 52.35 % in the AC-EK tests and 59.55 % in the FeAC-EK. Experimental results showed the accumulation of PFOA near the cathode region in FeAC PRB tests, hypothesising that Fe from the PRB formed a complex with PFOA ions and transported it to the cathode region. Spent PRBs were regenerated with methanol for PFOA extraction and reuse in the EK experiments. Although FeAC PRB achieved better PFOA removal than AC PRB, the EK tests with regenerated AC-EK and FeAC-EK PRBs achieved 40.37 % and 20.62 % PFOA removal. For EK with FeAC PRB near the anode, PFOA removal was 21.96 %. Overall, using PRB in conjunction with the EK process can further enhance the removal efficiency. This concept could be applied to enhance the removal of various PFAS compounds from contaminated soils by combining a suitable PRB with the EK process. It also emphasizes the feasibility of in-situ soil remediation technologies for forever chemical treatment.

Electrokinetic Remediation of Cu- and Zn-Contaminated Soft Clay with Electrolytes Situated above Soil Surfaces

Electrokinetic remediation (EKR) has shown great potential for the remediation of in situ contaminated soils. For heavy metal-contaminated soft clay with high moisture content and low permeability, an electrokinetic remediation method with electrolytes placed above the ground surface is used to avoid issues such as electrolyte leakage and secondary contamination that may arise from directly injecting electrolytes into the soil. In this context, using this novel experimental device, a set of citric acid (CA)-enhanced EKR tests were conducted to investigate the optimal design parameters for Cu- and Zn-contaminated soft clay. The average removal rates of heavy metals Cu and Zn in these tests were in the range of 27.9-85.5% and 63.9-83.5%, respectively. The results indicate that the Zn removal was efficient. This was determined by the migration intensity of the electro-osmotic flow, particularly the volume reduction of the anolyte. The main factors affecting the Cu removal efficiency in sequence were the effective electric potential of the contaminated soft clay and the electrolyte concentration. Designing experimental parameters based on these parameters will help remove Cu and Zn. Moreover, the shear strength of the contaminated soil was improved; however, the degree of improvement was limited. Low-concentration CA can effectively control the contact resistance between the anode and soil, the contact resistance between the cathode and soil, and the soil resistance by increasing the amount of electrolyte and the contact area between the electrolyte and soil.

Highly alkaline electrokinetic extraction: Characteristics of chromium mobilization, conversion and transport in high alkalinity soil

In site chromium (Cr) contaminated soil characterized by high alkalinity and carbonate content, protons are not effectively targeted for Cr(III) mobilization but rather accelerate the reduction of easily transportable Cr(VI) within the acidification electrokinetic (EK) system. As an alternative, the highly alkaline extraction conditions (HAECs) maintained by anolyte regulation are explored owing to the ability to desorb strong binding Cr(VI) and form anionic Cr(III)-hydroxides (Cr(OH)4-, Cr(OH)52-). The results demonstrate that HAECs were more efficient in mobilizing ions in severe alkalinity and electrical conductivity soil compared to organic acid acidifying extraction conditions (OAECs). Simultaneously, a limited amount of soluble Cr(III) was produced; however, its transportation was hindered and more noticeable in the case of Cr(VI), displaying a distinct retention phase within the intermediate soil chamber. The antagonistic interplay between electromigration and electroosmotic flow was considered the main responsible factor. The conversion intensity of Cr(VI) to Cr(III) was inhibited at HAECs. The promising mobilization and low conversion intensity contributed to total Cr removal. At HAECs, enhanced electromigration and electroosmotic flow combined with a favorable oxidation environment may facilitate in situ delivery of oxidants, offering practical implications for the EK detoxification of high alkalinity site soil contaminated with Cr. The practicability of HAECs is likely to be enhanced when the cost-benefit balance of providing a simultaneous energy supply during site treatment is resolved.

Electroosmotic flow in soft clay and measures to promote movement under direct current electric field

Electroosmosis has been proposed as a technique to reduce moisture and thus increase the stability of soft clay. However, its high energy consumption and uneven reinforcement effect has limited its popularization and application in practical engineering. This paper presents the results of some electrokinetic tests performed on clayey specimens with different electrification time and anode boundary conditions. The results indicate that the timing of the formation of electroosmotic flow (EF) by the water originally contained in different soil cross sections, from the anode to the cathode, varies. The measuring soil cross section nearest the anode first reached the limiting water content of 22%±3% and electroosmosis had to be stopped. Water injection into the anode during electroosmosis enhanced further drainage of other four measuring soil cross sections until the second soil cross section from the anode reached the limiting water content of 30%±2%. Electroosmosis with water injection into the anode technique provides more uniform reinforcement, increasing EF, and environmental protection. The experimental results highlighted the relevant and expected contribution of water injection into the anode on the effectiveness of the electroosmotic treatment as a soft clay improvement technique.

Effect of experimental boundary conditions and treatment-time on the electro-desalination of soils

This study investigates the effect of boundary conditions and treatment-time on the electro-desalination of artificially-contaminated soil. The effect of ion exchange membranes (IEM), calcium chloride (CaCl2), and ethylenediaminetetraacetic acid (EDTA) on the removal of salt (i.e., Na+, Cl-, and Ca2+) and metal (i.e., Co2+ and Fe2+) ions from the soil by electrokinetic (EK) was studied. The outcomes demonstrate that an increase in treatment-time decreases the electroosmosis and ion removal rate, which might be attributed to the formation of acid-base fronts in soil, except in the IEM case. Because a high pH jump and electroosmotic flow (EOF) of water were not observed within the soil specimen due to the IEM, the removal of ions was only by diffusion and electromigration. The collision of acid-base fronts produced a large voltage gradient in a narrow soil region with a reduced electric field (EF) in its remaining parts, causing a decrease in EOF and ion transport by electromigration. The results showed that higher electroosmosis was observed by using CaCl2 and EDTA; thus, the removal rate of Co2+, Na+, and Ca2+ was greater than Cl- due to higher EOF. However, for relatively low EOF, the removal of Cl- exceeded that of Co2+, Na+, and Ca2+, possibly due to a lack of EOF. In addition, the adsorption of Fe2+ in soil increased with treatment-time due to the corrosion of the anode during all EK experiments except in the case of IEM, where an anion exchange membrane (AEM) was introduced at the anode-soil interface.

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.

A new method for assessment of electro-osmotic permeability through the integration of theoretical and experimental ion flux in electrokinetic processes

Electrokinetic (EK) technology is promising for removing heavy metals from contaminated unsaturated soils. It is crucial to accurately determine the unsaturated electro-osmotic permeability for predicting the efficiency of EK treatment, optimizing treatment strategies, and accurately predicting the distribution of contaminant concentrations. However, the current approach of estimating unsaturated electro-osmotic permeability, which involves measuring effective voltage, drainage volume, and performing exponential fitting, fails to address the issue of uneven voltage gradient distribution during EK treatment. Herein, a novel method was presented for estimating the electro-osmotic permeability of unsaturated porous media. This method quantifies the electro-osmotic flow in an unsaturated porous medium by considering the difference in mass-transfer efficiency (MTE) between real (with electro-osmotic flow) and hypothetical cases (without electro-osmotic flow). This difference serves as a metric for estimating the electro-osmotic permeability. Results revealed a linear relationship between the electro-osmotic permeability and the product of volumetric moisture content and tortuosity, with the slope related to the ionic mobility of target ions, hypothetical and actual MTE. To validate this method, hexavalent Cr (Cr(VI)) was selected as the target contaminant and six EK experiments were conducted with varying initial volumetric moisture content. The feasibility of the method was evaluated by fitting the results of these experiments to obtain the specific slope of the porous medium used. Compared to the existing effective voltage-drainage volume-exponential fitting method, the proposed method offers several advantages. First, it effectively addressed the issue of nonuniform voltage distribution during EK treatment in the unsaturated porous medium. Second, it overcame the problem of a nonzero electro-osmotic permeability at zero volumetric moisture content in the exponential empirical formula. Third, the proposed method was based on theoretical derivations instead of relying solely on empirical fitting. Finally, the proposed method does not require a prior estimate of the saturated electro-osmotic permeability of the porous medium.

Applying fulvic acid for sediment metals remediation: Mechanism, factors, and prospect

Fulvic acid (FA) has been shown to play a decisive role in controlling the environmental geochemical behavior of metals. As a green and natural microbial metabolite, FA is widely used in environmental remediation because of its good adsorption complexation and redox ability. This paper introduces the reaction mechanism and properties of FA with metals, and reviews the progress of research on the remediation of metal pollutant by FA through physicochemical remediation and bioremediation. FA can control the biotoxicity and migration ability of some metals, such as Pb, Cr, Hg, Cd, and As, through adsorption complexation and redox reactions. The concentration, molecular weight, and source are the main factors that determine the remediation ability of FA. In addition, the ambient pH, temperature, metal ion concentrations, and competing components in sediment environments have significant effects on the extent and rate of a reaction between metals and FA during the remediation process. Finally, we summarize the challenges that this promising environmental remediation tool may face. The research directions of FA in the field of metals ecological remediation are also prospected. This review can provide new ideas and directions for the research of remediation of metals contaminants in sediments.

A closed-loop electrokinetic system for recovery of PbO2@Fe composite derived from lead-containing sludge

Lead-containing sludge produced from lead-acid battery factory will cause environmental hazards if they are not treated properly. A novel process was developed to recycle lead from sludge back into Fe-doped PbO₂ electrodes and realize sludge reduction in this study. The effects of Fenton conditioning on Pb removal efficiency in electro-kinetic (EK) treatment process and its mechanism as well as electro-dewatering (ED) performance were investigated. It was found that the oxidation of Fenton can promote desorption and release of Pb from the organic binding state, and improve the removal efficiency of Pb during EK process, as well as enhance sludge ED performance. About 63.8 wt% Pb can be removed from sludge during EK process, achieving sludge reduction of 63.5 wt% by ED treatment. The composite PbO₂@Fe electrode recovered from lead-containing sludge showed a high electrocatalytic activity for acid red G (ARG) degradation. The electrode obtained by electrodeposition at 20 mA cm^-2 had the largest exchange current density (3.26 × 10^-5 A cm^-2). In the experiment of dye wastewater electrocatalytic degradation, over 99.5% organic matter was degraded within 80 min.

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.

The Remediation Characteristics of Heavy Metals (Copper and Lead) on Applying Recycled Food Waste Ash and Electrokinetic Remediation Techniques

Most food waste is incinerated and reclaimed in Korea. Due to the development of industry, soil and groundwater pollution are serious. The purpose of this study was to study recycled materials and eco-friendly remediation methods to prevent secondary pollution after remediation. In this study, recycled food waste ash was filled in a permeable reactive barrier (PRB) and used as a heavy metal adsorption material. In situ remediation electrokinetic techniques (EK) and acetic acid were used. Electrokinetic remediation is a technology that can remove various polluted soils and pollutants, and is an economical and highly useful remediation technique. Thereafter, the current density increased constantly over time, and it was confirmed that it increased after electrode exchange and then decreased. Based on this result, the acetic acid was constantly injected and it was reconfirmed through the water content after the end of the experiment. In the case of both heavy metals, the removal efficiency was good after 10 days of operation and 8 days after electrode exchange, but, in the case of lead, it was confirmed that experiments are needed by increasing the operation date before electrode exchange. It was confirmed that the copper removal rate was about 74% to 87%, and the lead removal rate was about 11% to 43%. After the end of the experiment, a low pH was confirmed at x/L = 0.9, and it was also confirmed that there was no precipitation of heavy metals and there was a smooth movement by the enhancer and electrolysis after electrode exchange.

Influence of electrical fields enhanced phytoremediation of multi-metal contaminated soil on soil parameters and plants uptake in different soil sections

The influence of electrical fields on phytoremediation of multi-metal (Cd, Cu, and Zn) naturally contaminated soils has been investigated based on different soil sections. After ryegrass and hybrid penisetum were sowed for 30 d, electrical fields were applied during 30 days with the switching polarity every 30 min and continuing for 16 h d^-1. After electrokinetic (EK) assisted phytoremediation process, soil electrical conductivity (EC) in anode section and available soil potassium (K) in cathode section were obviously elevated. Plants biomass in middle and cathode sections were increased in both plants, especially in middle section the overall biomass of hybrid penisetum increased by 68.8%. The influence of electrical field on the contents of heavy metals in plants was different depending on the species of plants, kind of heavy metals and soil section. For Cd, Cu, and Zn co-contaminated soils, shoot metals accumulation in middle section in both plants were improved at least about 20% (with the exception of Zn in ryegrass). Electrical fields had the most significant effect on copper absorption by ryegrass and shoot Cu accumulation were elevated 32.5% in all the section. The soil EC maybe an important factor that affected electrical fields enhanced plants growth, plant metals concentrations and remediation efficiency.

Investigation of a biosurfactant-enhanced electrokinetic method and its effect on the potentially toxic trace elements in waterways sediments

In this study, the biosurfactant-enhanced electrokinetic method was investigated for the removal of potentially toxic trace elements (As, Ba, Cd, Cr, Cu, Mo, Ni, Pb, Sb, Se and Zn) in waterways sediments. The effect of this method was compared to the removal capacities of deionized water in the same conditions in order to assess its efficiency. After treatment, batch leaching tests have shown that almost toxic elements (As: 81.3%; Ba: 80%; Cr: 97.3%; Cu: 82%; Zn: 94.5%; Mo: 13.8%; Ni: 62.7%; Se: 66.8% and Sb: 9.3%) were less released in waters. On the whole sediment samples, Ba and Cd displayed the highest removal rates (Ba: 71.2% and Cd: 77.5%). The use of biosurfactant enhanced the electrokinetic method by improving the trace elements migration and altering pH and Eh locally generated by the system. Overall, the application of this new approach dredged sediments seems to be promising but needed further investigations for industrial applications.

Coupled with EDDS and approaching anode technique enhanced electrokinetic remediation removal heavy metal from sludge

In this work, the novel technology was used to remove heavy metal from sludge. The coupled with biodegradable ethylenediamine disuccinic acid (EDDS) and approaching anode electrokinetic (AA-EK) technique was used to enhance heavy metal removing from sludge. Electric current, sludge and electrolyte characteristics, heavy metal removal efficiency and residual content distribution, and heavy metal fractions percentage of variation were evaluated during the electrokinetic remediation process. Results demonstrated that the coupled with EDDS and AA-EK technique obtain a predominant heavy metal removal efficiency, and promote electric current increasing during the enhanced electrokinetic remediation process. The catholyte electrical conductivity was higher than the anolyte, and electrical conductivity of near the cathode sludge achieved a higher value than anode sludge during the coupled with EDDS and AA-EK remediation process. AA-EK technique can produce a great number of H⁺, which caused the sludge acidification and pH decrease. Cu, Zn, Cr, Pb, Ni and Mn obtain the highest extraction efficiency after the coupled with EDDS and AA-EK remediation, which were 52.2 ± 2.57%, 56.8 ± 3.62%, 60.4 ± 3.62%, 47.2 ± 2.35%, 53.0 ± 3.48%, 54.2 ± 3.43%, respectively. Also, heavy metal fractions analysis demonstrated that the oxidizable fraction percentage decreased slowly after the coupled with EDDS and AA-EK remediation.

Remediation of heavy metal-contaminated soils by electrokinetic technology: Mechanisms and applicability

Electrokinetic remediation is a widely admitted technology forrectifying heavy metal-contaminated soil. Various technologies have been effectively developed to improve the metal removal efficiency of contaminated soil by electrochemical treatment alone or in combination with other remediation technologies. The working components for electrokinetic system, such as supplying power for electric fields, installing electrodes to generate electric fields, introducing electrolytes and other potential materials as a reactive medium are crucial. This review focuses on the specific functions of the working components in electrokinetic systems and their effects on the efficiency of heavy metal removal using electrochemical process. The advancements in working components were systematically summarized, such as power for electric fields, electrodes, electrolytes and ion exchange membrane, which have various impacts on the effectiveness of electrokinetic remediation. Additionally, this study introduces the application of dominating technologies at present coupled with electrokinetics. Overall, a judicious design and reasonable operation in the application of electrokinetic-coupled remediation should be implemented to enhance the removal process of heavy metals from contaminated soil.

Electrokinetic remediation of Pb near the e-waste dismantle site with Fe(NO3)3 as cathode electrolyte

In this study, Pb-contaminated soil in the e-waste dismantle site was remediated by activated carbon fiber (ACF) enhanced electrokinetic remediation. Experiments were conducted using Fe(NO₃)₃ as catholyte and citric acid-sodium citrate as anolyte with different conditions: pH value of anolyte, voltage and the electrode gap. At the same time, we set up a group of contrast test without ACF to investigate the adsorption performance of ACF for Pb. Results showed that the highest removal rate of Pb after the remediation was 80.53% at 4 cm from the anode when the electrode gap was 31 cm, pH value was 3 and the voltage was 28 V, and the total removal rate increased significantly with the decrease of the pH value of anolyte and the increase of voltage. Characterization of ACF after reaction showed that ACF effectively adsorbed heavy metal Pb, and the adsorption amount was 1.42 mg/g. Sequential extraction analysis revealed that Pb mainly existed in the forms of organic matter bound and residual in the soil after remediation. These forms are relatively stable and low toxicity, indicating that the remediation has significantly reduced the harm of Pb to the environment.

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.

Microbial community responses to soil parameters and their effects on petroleum degradation during bio-electrokinetic remediation

This study evaluated the interactions among total petroleum hydrocarbons (TPH), soil parameters, and microbial communities during the bio-electrokinetic (BIO-EK) remediation process. The study was conducted on a petroleum-contaminated saline-alkali soil inoculated with petroleum-degrading bacteria with a high saline-alkali resistance. The results showed that the degradation of TPH was better explained by second-order kinetics, and the efficacy and sustainability of the BIO-EK were closely related to soil micro-environmental factors and microbial community structures. During a 98-d remediation process, the removal rate of TPH was highest in the first 35 d, and then decreased gradually in the later period, which was concurrent with changes in the soil physicochemical properties (conductivity, inorganic ions, pH, moisture, and temperature) and subsequent shifts in the microbial community structures. According to the redundancy analysis (RDA), TPH, soil temperature, and electric conductivity, as well as SO₄^2-, Cl^-, and K⁺ played a better role in explaining the changes in the microbial community at 0-21 d. However, pH and NO₃^- better explained the changes in the microbial community at 63-98 d. In particular, the dominant genera, Marinobacter and Bacillus, showed a positive correlation with TPH, conductivity, and SO₄^2-, Cl^-, and K⁺, but a negative relationship with pH and NO₃. Rhodococcus was positively correlated with soil temperature. The efficacy and sustainability of the BIO-EK remediation process is likely to be improved by controlling these properties.

Impacts of electrokinetic isolation of phosphorus through pore water drainage on sediment phosphorus storage dynamics

Pore water is a crucial storage medium and a key source of sediment phosphorus. A novel equipment based on electrokinetic geosynthetics (EKGs) was used for isolating phosphorus from eutrophic lake sediments through pore water drainage. Three mutually independent indoor group experiments (A, B, and C) were conducted to investigate the effects of voltage gradient (0.00, 0.25, and 0.50 V/cm) on pore water drainage capacity, phosphorus removal performance, sediment physicochemical properties, and phosphorus storage dynamics. The average reduction in the sediment moisture and total phosphorus content was 2.5%, 4.3%, and 4.6% and 28.15, 75.95, and 112.65 mg/kg after 6 days of treatment for A, B and C, respectively. Efficient pore water drainage through gravity and electroosmotic flow and electromigration of phosphate were the main drivers of sediment-dissolved and mobilized phosphorus separation. A high voltage gradient facilitated the migration of pore water and the phosphorus in it. The maximal effluent total phosphorous (TP) concentration was up to 27.9 times that in the initial pore water samples, and negligible effluent TP was detected when the pore water pH was less than 2.5. The TP concentration was exponentially and linearly related to the pH and electronic conductivity of the electroosmotic flow, respectively. The migration of H⁺ within the sediment matrix promoted the liberation of metals bounded to phosphorus, particularly of Ca-P and Fe-P. Pore water drainage through an EKG resulted in Ex-P separation of up to 87.50% and a 13.84 mg/kg decrease in Ca-P and 125.35 mg/kg accumulation of low mobile Fe-P in the weak acid anode zone.

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.

Enhancement of electrokinetic-bioremediation by ryegrass: Sustainability of electrokinetic effect and improvement of n-hexadecane degradation

Phytoremediation-assisted electrokinetic-bioremediation is a novel technology for soil remediation. We aimed to study the effects of a plant (ryegrass) on electrokinetic-bioremediation in n-hexadecane-contaminated soil. After treatment for 40 days, the n-hexadecane degradation ratio of electrokinetic-bioremediation-ryegrass (EK-Bio-RG) was 4.86% higher than that of electrokinetic-bioremediation (EK-Bio) (p < 0.05), with a maximum constant degradation rate (107.23 ± 4.62 mg kg^-1· d^-1). Owing to the improved electrical conductivity, 73.28% of the initial current was maintained on the 40th day in EK-Bio-RG, which was 1.62 times that in EK-Bio. Furthermore, ryegrass reduced the soil zeta potential, which indicated the alleviation of the soil electric double layer compression and prevention of the aggregation of small soil colloids into larger ones. The fine colloidal structure was conducive to mass transfer in electrokinetic-bioremediation. An analysis of the microbial community showed that the degradation of n-hexadecane was mainly attributable to gram-positive bacteria, and a new microbial community was gradually constructed in the rhizosphere, which still metabolized n-hexadecane. The results indicated that the sustainability of the electrokinetic effect was improved combined with ryegrass, and the harmonious micro-environment in the rhizosphere was constructed which furtherly optimized the EK-Bio technology to remediate organics-polluted soil.

Effect of processing time on removal of harmful emerging salt pollutants from saline-sodic soil during electrochemical remediation

The recent climate change has spawned a new emerging environmental hazard known as soil salinity. The lacks of rainfall and global warming have drastically increased the concentration of harmful emergent salt pollutants (HESPs) to toxic levels for natural and human assets. Electrochemical remediation (ECR) has been successfully used in remediation of contaminated soils. This research aims to investigate the effect of processing time (PT) on the removal of HESPs during ECR. The experiments were operated in a designed laboratory cell, using two different PT of 3 and 5 days with a voltage gradient of 1.5 V/cm. The results show that the increase of PT enhances the removal of monovalent cations, including sodium (Na⁺) and potassium (K⁺), reaching an efficiency of 63 and 83%, respectively. However, calcium (Ca²⁺) and magnesium (Mg²⁺) have exhibited irreversible behavior, where the increase of PT seems hindering their removal, namely near the cathode. Longer periods induce sharp increase in the basic front, which curb their desorption and mobilization. For the anionic salts, the raises of PT lead to better elimination of monovalent anions, with a removal of 92 and 63% for nitrate (NO₃^-) and chloride (Cl^-), respectively. Nevertheless, the effect of PT was less significant on the elimination of sulfate (SO₄^2-), due to their chemical nature. It can be concluded that the removal rate is an intrinsic parameter, strongly related to set of parameters, including the soil pH, chemical nature, ionic valence of the targeted salts and their selectivity on clay particles in clay-water-electrolyte system.

Low permeability zone remediation of trichloroethene via coupling electrokinetic migration with in situ electrochemical hydrodechlorination

To address the challenge of trichloroethene (TCE) remediation in low permeability zone, an inexpensive Cu-Ni bimetallic cathode was proposed in electrokinetic (EK) remediation system to couple electrokinetic migration with in situ electrochemical hydrodechlorination. Aqueous phase TCE was originally added into the anolyte so that breakthrough curves through the low permeability porous soil compartment could be obtained to better understand TCE migration driven by electroosmosis flow using different cathodes. The Cu-Ni cathode resulted in more TCE migration of 7.64 mg compared to that of 5.99 mg with Ni and 4.22 mg with mixed metal oxide (MMO) cathode, suggesting that the Cu-Ni cathode was capable of driving more TCE flux out of the contaminated soil. With the Cu-Ni cathode, 98.4% of TCE flux that reached the cathode was electrochemically reduced on the cathode, which was much higher than that with MMO cathode (77.9%) or Ni cathode (59.6%). TCE mass that was transported by electroosmosis flow increased from 2.04 to 6.68 mg when the voltage gradient increased from 1 to 4 V cm^-1, with the normalized energy consumption increasing from 0.06 to 0.16 kWh kg^-1 per unit water movement, and from 0.54 to 2.55 kWh g^-1 per unit TCE transport. For TCE that did reach the cathode compartment, > 98% degradation maintained at the Cu-Ni cathode with various voltage gradients. The coupled electrokinetic and electrochemical hydrodechlorination technology appears to be a promising strategy for the remediation of low permeability porous media.

Analysis and Optimization of Mn Removal from Contaminated Solid Matrixes by Electrokinetic Remediation

Electrokinetic remediation is a useful technique for the removal of ionic contaminants in soils, sediments, sludges, and other solid porous matrixes. The efficiency of metal removal and the electricity consumption in the electrokinetic treatment of soils largely depend on electric and physicochemical conditions. This study analyzes the electrokinetic treatment of Mn contaminated kaolinite clay specimen and the influence of voltage, current intensity, moisture content, pH, and facilitating agents on metal removal and energy consumption. The objective of this study is to identify the influence of the typical variables used in electrokinetic remediation. The results showed that the operation at constant voltage or constant current intensity were equivalent in terms of metal removal and energy consumption, as long as the electric field intensity was kept low to minimize the consumption in parallel electrochemical reactions, especially the electrolysis of water. The moisture content had a significant influence on the Mn removal. Moisture content higher that 50 percent resulted in very effective Mn removal as compared with kaolinite specimens with lower moisture. The control of pH in the electrolyte solutions and the addition of facilitating agents (organic acids) enhanced the removal of Mn but increased the electric energy cost. Overall, the best conditions for Mn removal involved low to moderate electric potential difference (10 to 30 V), the use of citric acid as the facilitating agent, and the pH control in the cathode at a slightly acid pH. The electrokinetic treatment of a sludge from a water treatment plant contaminated with Mn was effective when pH control on the cathode was used. Mn and various metals (66% of Mn, 30% of Cu, 56% of Zn, 21% Sr, and 21% of Fe) were removed with moderate electricity and acid consumption.

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.

The enhancement of synthesized wastewater on non-uniform electrokinetic remediation of a Cd-spiked natural clayey soil

It is usually very difficult to achieve satisfactory extraction efficiencies in electrokinetic remediation of heavy metal-contaminated soils of high acid/base buffer capacity. Enhancement agent is often required. In this study, synthesized citric acid industrial wastewater (CAIW) is used as the enhancement agent to remediate cadmium-spiked natural clayey soil from Shanghai, China. Four electrokinetic extraction experiments were carried out to evaluate the enhancement effects of CAIW on the remediation of metal-spiked clayed soil of high buffer capacity and the effects of treatment time and initial cadmium concentration on the migration of cadmium in the specimen. The results of electrokinetic experiments indicated that CAIW can efficiently enhance the transport of cadmium in comparison with HNO₃ of the same pH. Cadmium mobilization was enhanced with prolonged treatment time from 104 to 261.2 h, but the average cadmium removal efficiency was not significantly enhanced. A non-uniform cadmium distribution in the specimen was observed after the enhanced electrokinetic experiments due to the localized electrical gradients with an electrical gradient of approximately 1 V/cm and a ratio of the distance between electrodes of the same polarity to the outer diameter of electrode of 2.8 (50:18 mm).

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.

Removal of heavy metals from contaminated soil by electrodialytic remediation enhanced with organic acids

The soil from an industrial area in Algeria was contaminated with Cr (8370 mg kg^-1), Ni (1135 mg kg^-1) and zinc (1200 mg kg^-1). The electrodialytic remediation of this soil was studied using citric acid and EDTA as facilitating agents. 0.1 M citric acid or EDTA was added directly to the soil before it was introduced in an electrodialytic cell in an attempt to enhance the heavy metal solubility in the interstitial fluid. The more acidic pH in the soil when citric acid was used as the facilitating agent was not enough to mobilize and remove the metals from the soil. Only 7.2% of Ni and 6.7% of Zn were removed from the soil in the test with citric acid. The best results were found with EDTA, which was able to solubilize and complex Zn and Ni forming negatively charged complexes that were transported and accumulated in the anolyte. Complete removal was observed for Ni and Zn in the electrodialytic treatment with EDTA. Minor amounts of Cr were removed with both EDTA and citric acid.