Removal of Contaminants from Soils by Electric Fields

Spatiotemporal dynamic temperature variation dominated by ion behaviors during groundwater remediation using direct current

Direct current (DC) electric field has shown promising performance in contaminated site remediation, in which the Joule heating effect plays an important role but has been previously underappreciated. This study focuses on the spatiotemporal characteristics and mechanism of temperature change in heterogeneous porous media with applied DC. The heating process can be divided into four phases: preferential heating of the low permeability zone (LPZ), rapid heating in the middle region, temperature drop and hot zone shift, and reheating. The dynamic ion behaviors with complex interplays among reactions, electrokinetic-driven migration, and mixed convection induced an uneven redistribution of ions and dominated the heating rate and temperature distribution. The concentration of major ions near the pH jump decreased to 1% of the initial value, even though ions were continuously pumped into the heating zone. This ion depletion caused a drop in current, heating rate, and temperature. Here ions cannot be delivered rapidly into the ion-depleted zone by electromigration due to the potential flattening in the surrounding region. The presence of LPZ intensified the nonuniformity of ion redistribution, where a regional focusing of water-soluble ions was observed, and weakened the temperature rebound compared with that using homogeneous sand. These results provide a new perspective on the regulation of DC heating in site remediation.

Insights into the Mechanism of Selective Removal of Heavy Metal Ions by the Pulsed/Direct Current Electrochemical Method

Heavy metal pollution treatment in industrial wastewater is crucial for protecting biological and environmental safety. However, the highly efficient and selective removal of heavy metal ions from multiple cations in wastewater is a significant challenge. This work proposed a pulse electrochemical method with a low-/high-voltage periodic appearance to selectively recover heavy metal ions from complex wastewater. It exhibited a higher recovery efficiency for heavy metal ions (100% for Pb2+ and Cd2+, >98% for Mn2+) than other alkali and alkaline earth metal ions (Na+, Ca2+, and Mg2+ were kept below 3.6, 1.3, and 2.6%, respectively) in the multicomponent solution. The energy consumption was only 34-77% of that of the direct current electrodeposition method. The results of characterization and experiment unveil the mechanism that the low-/high-voltage periodic appearance can significantly suppress the water-splitting reaction and break the mass-transfer limitation between heavy metal ions and electrodes. In addition, the plant study demonstrates the feasibility of treated wastewater for agricultural use, further proving the high sustainability of the method. Therefore, it provides new insights into the selective recovery of heavy metals from industrial wastewater.

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.

The technique of arsenic elimination from contaminated soil with enhanced conditions by electro-kinetic remediation

Remediation of arsenic-contaminated soil by the technique of electro-kinetic using various enhanced conditions is the thrust of this study. Many tests were performed with different operating conditions, such as the electrodes solution (tap water) with a variable pH at (2, 5, and 8). Cyclodextrin (10% hydroxypropyl-beta-cyclodextrin, or HPCD) has been used as an activating agent that helps enhance this process of soil arsenic clean-up. Agricultural wastes (pomegranate peels) were used after treating them as an adsorbent to impede the return of pollutants to the soil as a result of reverse osmosis. The results obtained after the remediation's completion indicated that the best elimination capacity was 72% at a pH of 2. This indicates that the elimination capacity increases with the decreasing pH of the electrodes solution. Mixing the HPCD solution with soil as an enhanced solution increased the solubility and desorption of arsenic from the soil. Then, arsenic-containing HPCD micelles readily moved to the cathode electrode by electro-osmotic flow, and the elimination capacity increased (84%). It was concluded that pomegranate peels are a good adsorbent for arsenic returning to the soil during the treatment process as a result of reverse osmosis. Hence, the application of this material in this research gave a perfect impression of its use as an inexpensive, environmentally friendly adsorbent and an alternative to commercial absorbents.

Sustainable decontamination of heavy metal in wastewater and soil with novel rectangular wave asymmetrical alternative current electrochemistry

A new concept of removal and recovery of heavy metals and simultaneous regeneration and reuse of ethylenediamine-tetraacetic acid (EDTA) in soil washing effluent containing metal-EDTA complexes is proposed, which is used to remediate heavy metal contaminated soil. To achieve this goal, soil washing approach coupled with rectangular wave asymmetrical alternative current electrochemistry (RW-ACE) equipped with amidoxime-functionalized electrodes (Ami-CF) is employed. With high hydrophilicity and strong binding affinity, Ami-CF could specifically compete for heavy metals over EDTA under electric field. RW-ACE system is found successfully to achieve the non-destructive decomplexation of heavy metal-EDTA, and then regenerate EDTA for highly recycling, which saves as high as 98.9 % EDTA consumption compared with conventional washing method. Moreover, more than 90% of heavy metals are recovered and deposited on the electrode with a majority of them existed as zero-valence state as evidenced by XPS. The RW-ACE method is universal for various heavy metals such as Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ in an authentic contaminated soil, and the loss of soil nutrient is very limited. Along with long-term assessment and operation cost estimation, the RW-ACE method is a sustainable remediation approach for the heavy metal polluted wastewater and soils, and easily scaled up for field practice.

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

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

Micro/nanomotor technology: the new era for food safety control

Food poisoning caused by eating contaminated food remains a threat to global public health. Making the situation even worse is the aggravated global environmental pollution, which poses a major threat to the safety of agricultural resources. Food adulteration has been rampant owing to negligent national food safety regulations. The speed at which contaminated food is detected and disposed of determines the extent to which consumers' lives are safeguarded and agricultural economic losses are prevented. Micro/nanomotors offer a high-speed mobile loading platform that substantially increases the chemical reaction rates and, accordingly, exhibit great potential as alternatives to conventional detection and degradation techniques. This review summarizes the propulsion modes applicable to micro/nanomotors in food systems and the advantages of using micro/nanomotors, highlighting examples of their potential use in recent years for the detection and removal of food contaminants. Micro/nanomotors are an emerging technology for food applications that is moving toward mass production, simple preparation, and important functions.

Rapid Cr(VI) reduction structure in chromium contaminated soil: The UV-assisted electrokinetic circulation of background iron

Substantially decreasing the severe hazards connected with the toxic Cr(VI), developing effective reduction remediation strategies may be crucial under favorable economic conditions for the contaminated soil containing Cr(VI) to protect human health. Several typical enhancers (phosphate, fulvic acid, citric acid) were used to test electrokinetic remediation (EKR) coupled with UV radiation-induced photochemical reduction for contaminated soil containing Cr(VI). The added citrate, while improving the Cr(VI) electromigration, worked as the ultimate sacrificial electron donors, with the dissolved soil background Fe(III) as electron shuttle, to Cr(VI) rapid reduction. The dissolved soil background Fe(III) convert into Fe(II) ions through the UV radiation-induced ligand-metal charge transfers reaction, which constituted a novel electrokinetic circulation reduction pathway for the elimination of surface-bound/dissolved Cr(VI) (difficult to electromigration) in the near-anodic soil layers. More than 80% dissolved and surface-bound Cr(VI) was eliminated from the soil. In particular, the dissolved and surface-bound Cr(VI) was enhanced by more than 62.37% removal in near-anodic soil layers compared to conventional citric acid-enhanced EKR and provided no extra cost other than UV radiation. This configuration may be a cost-effective and feasible remediation design in the future for the in-situ Cr(VI) reduction of contaminated sites.

On the interplay between electromigration and electroosmosis during electrokinetic transport in heterogeneous porous media

Electrokinetic techniques represent a valuable approach to enhance solute transport, reactant delivery and contaminant degradation in complex environmental matrices, such as contaminated soil and groundwater, and have a great potential for the remediation of many organic and inorganic pollutants. This study investigates the complex interplay between the key electrokinetic transport mechanisms, electromigration and electroosmosis, in physically heterogeneous porous media and its impact on tracer distribution, reactant mixing and degradation efficiency. We perform experiments in a multidimensional setup, considering different types of heterogeneities, injected tracers and reactants, as well as background electrolyte pore water with different chemical composition and pH. We show that EK transport is significantly affected by the physical heterogeneities, due to the interaction between electrokinetic and hydraulic processes, and by the pore water chemistry that plays a key role on the magnitude and spatial distribution of electroosmotic fluxes. The latter affect the overall transport of charged and non-charged species, including the migration velocity of injected plumes, their spatial patterns, spreading and mixing with the background groundwater, and the extent of degradation and the spatio-temporal evolution of reactive zones in the heterogeneous porous media. Process-based numerical modeling allowed us to interpret the experimental observations and to disentangle the coupled effects of physical, chemical and electrostatic processes in the multidimensional, heterogeneous setups. Besides elucidating the mechanisms controlling electrokinetic transport, the results of this study have also important implications for practical field implementation of EK approaches in intrinsically heterogeneous subsurface systems.

Reversal of Electroosmotic Flow in Charged Nanopores with Multivalent Electrolyte

We study the reversal of electroosmotic flow in charged cylindrical nanopores containing multivalent electrolyte. Dissipative particle dynamics is used to simulate the hydrodynamics of the electroosmotic flow. The electrostatic interactions are treated using 3D Ewald summation, corrected for a pseudo-one-dimensional geometry of the pore. We observe that, for sufficiently large surface charge density, condensation of multivalent counterions leads to the reversal of the pore's surface charge. This results in the reversal of electroosmotic flow. Our simulations show that the Smoluchowski equation is able to quantitatively account for the electroosmotic flow through the nanopore, if the shear plane is shifted from the position of the Stern contact surface.

Electro‐osmotic flow of fractional second‐grade fluid with fractional Cattaneo heat flux through a vertical microchannel

This study investigates the unsteady electro‐osmotic flow (EOF) of a fractional second‐grade fluid through a vertical microchannel with convection heat transfer. The fractional Cattaneo heat flux model will be used to modify the heat equation. The solutions for the velocity and the temperature have been derived by employing the Laplace and finite Fourier sine transforms and their numerical inverses. The results show that at the beginning of the time period, the fractional parameter postpones the movement of the fluid. Furthermore, the results show that at the high values of retardation time (non‐Newtonian case), the required time for the velocity and the flow rate to reach the steady state increases. Moreover, the heat relaxation time reduces the heat transfer until a critical time, and then the effect reverses.

Electroosmotic Flow in Polarizable Charged Cylindrical Nanopores

We present a simulation method to study electroosmotic flow in charged nanopores with dielectric contrast between their interior and the surrounding medium. To perform simulations, we separate the electrostatic energy into the direct Coulomb and the polarization contributions. The polarization part is obtained using periodic Green functions and can be expressed as a sum of fast converging modified Bessel functions. On the other hand, the direct Coulomb part of the electrostatic energy is calculated using fast converging three-dimensional (3D) Ewald summation method, corrected for a pseudo one-dimensional (1D) geometry. The effects of polarization are found to be particularly important for systems with multivalent counterions and narrow nanopores. Depending on the surface charge density, polarization can increase the volumetric flow rate by 200%. For systems with 3:1 electrolyte, we observe that there is a saturation of the volumetric flow rate. In this case, for polarizable pores, the flow rate is 100% higher than for nonpolarizable pores.

Electrokinetic sweeping of colloids at a reactive magnesium oxide interface

Investigating the electrokinetic (EK) response in the vicinity of interfaces has regained interest due to the development of new membrane based processes for energy harvesting or soil depollution. However, the case of reactive interfaces, ubiquitous in these processes, remains scarcely explored. Here we experimentally investigate the EK response of a model interface between an aqueous electrolyte and a bulk MgO crystal surface (100), for different pH. For that purpose, we use a lab-scale non invasive method to monitor the zeta potential of the interface versus time, by confocal fluorescent particle tracking. An unexpected motion of the particles, repelled and then attracted again by the interface is observed. We attributed this motion to the surface reactivity, inducing ion concentration gradients perpendicular to the interface and subsequent diffusiophoresis of the charged particle. Accordingly, we could describe at a semi-quantitative level the particle dynamics by solving numerically the Poisson-Nernst-Planck equations to establish concentration profile in the system and subsequent diffusiophoretic motion. These experiments open the way to the characterization of both the EK response and the reaction rate in the vicinity of reactive interfaces.

Quantitative research on heavy metal removal of flue gas desulfurization-derived wastewater sludge by electrokinetic treatment

Flue gas desulfurization-derived wastewater sludge (FGD-WWS) has been produced increasingly in China and India etc., and its content of heavy metals (HMs) including Cd, Cr, Cu, Hg, Ni and Zn seriously exceeds the limits allowed. Developing the suitable disposal of FGD-WWS is therefore significantly important and necessary. The novel process of electrokinetic treatment combined with chemical pretreatment of HMs in FGD-WWS were proposed here to improve the removal efficiency. Results indicate that the effects of different pretreatment agents (citric acid (CA), ammonia, tetrasodium of N, N-bis (carboxymethyl) glutamic acid (GLDA), and rhamnolipid) on the ET of HMs were different. To investigate the mechanism of combined process, the transformation potential (TP), exchange potential (EP) and removal potential (RP) were calculated. Correlation analysis shows the correlation between TP and RP was higher than that between EP and RP, indicating that the removal efficiency is mainly affected by the fraction transformation of HMs. Electric field, pH and pretreatment agents are main factors causing fraction transformation and affecting TP. Focusing on fraction transformation is an efficient way to improve further the removal efficiency. The work is promisingly valuable for developing the technology of treating FGD-WWS.

Toward a more sustainable mining future with electrokinetic in situ leaching

Metals are currently almost exclusively extracted from their ore via physical excavation. This energy-intensive process dictates that metal mining remains among the foremost CO₂ emitters and mine waste is the single largest waste form by mass. We propose a new approach, electrokinetic in situ leaching (EK-ISL), and demonstrate its applicability for a Cu-bearing sulfidic porphyry ore. In laboratory-scale experiments, Cu recovery was rapid (up to 57 weight % after 94 days) despite low ore hydraulic conductivity (permeability = 6.1 mD; porosity = 10.6%). Multiphysics numerical model simulations confirm the feasibility of EK-ISL at the field scale. This new approach to mining is therefore poised to spearhead a new paradigm of metal recovery from currently inaccessible ore bodies with a markedly reduced environmental footprint.

Electroosmotic Flow Grows with Electrostatic Coupling in Confining Charged Dielectric Surfaces

In this work, the effects of polarization of confining charged planar dielectric surfaces on induced electroosmotic flow are investigated. To this end, we use dissipative particle dynamics to model solvent and ionic particles together with a modified Ewald sum method to model electrostatic interactions and surfaces polarization. A relevant difference between counterions number density profiles, velocity profiles, and volumetric flow rates obtained with and without surface polarization for moderate and high electrostatic coupling parameters is observed. For low coupling parameters, the effect is negligible. An increase of almost 500% in volumetric flow rate for moderate/high electrostatic coupling and surface separation is found when polarizable surfaces are considered. The most important result is that the increase in electrostatic coupling substantially increases the electroosmotic flow in all studied range of separations when the dielectric constant of electrolytes is much higher than the dielectric constant of confining walls. For the higher separation simulated, an increase of around 340% in volumetric flow rate when the electrostatic coupling is increased by a factor of two orders of magnitude is obtained.

Electrokinetic Delivery of Reactants: Pore Water Chemistry Controls Transport, Mixing, and Degradation

Electrokinetics in porous media entails complex transport processes occurring upon the establishment of electric potential gradients, with a wide spectrum of environmental applications ranging from remediation of contaminated sites to biotechnology. The resulting electric forces cause the movement of pore water ions in opposite directions, leading to charge interactions that can affect the distribution of charged species in the domain. Here, we demonstrate that changes in chemical conditions, such as the concentration of a background electrolyte in the pore water of a saturated porous medium, exert a key control on the macroscopic transport of charged tracers and reactants. The difference in concentration between the background electrolyte and an injected solute can limit or enhance the reactant delivery, cause nonintuitive patterns of concentration distribution, and ultimately control mixing and degradation kinetics. With nonreactive and reactive electrokinetic transport experiments combined with process-based modeling, we show that microscopic charge interactions in the pore water play a crucial role on the transport of injected plumes and on the mechanisms and rate of both physical and chemical processes at larger, macroscopic scales. Our results have important implications on electrokinetic transport in porous media and may greatly impact injection and delivery strategies in a wide range of applications, including in situ remediation of soil and groundwater.

Glossary on atmospheric electricity and its effects on biology

There is an increasing interest to study the interactions between atmospheric electrical parameters and living organisms at multiple scales. So far, relatively few studies have been published that focus on possible biological effects of atmospheric electric and magnetic fields. To foster future work in this area of multidisciplinary research, here we present a glossary of relevant terms. Its main purpose is to facilitate the process of learning and communication among the different scientific disciplines working on this topic. While some definitions come from existing sources, other concepts have been re-defined to better reflect the existing and emerging scientific needs of this multidisciplinary and transdisciplinary area of research.

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.

Remediation of Multiply Contaminated Ground via Permeable Reactive Barrier and Electrokinetic Using Recyclable Food Scrap Ash (FSA)

A study of the application of electrokinetic (EK) remediation and Permeable Reactive Barriers (PRB) using recyclable Foods Scrap Ash (FSA) in multiple contaminated soils was carried out. An FSA was chosen as a PRB fill material due to its highly efficient capacity for contaminant removal. Acetic acid and Brij30 were used as enhancers on copper and phenanthrene, respectively, to improve EK remediation performance in removing the heavy metal and organic contaminants. Copper adsorption in PRB was so substantial that the confirmed removal efficiency was 83.86–90.17% and the remaining amount was 105–212 mg. While a high removal efficiency of acetic acid was observed on copper in multiple contamination soils; the removal of phenanthrene was hardly detected and the recovery rate of the contaminant was low during pretreatment. Therefore; an additional study of pretreatment on the phenanthrene-contaminated kaolinite needs to be performed.

Electro-assisted adsorption of heavy metals from aqueous solutions by biochar

Electro-assisted adsorption was investigated for Pb²⁺, Cu²⁺ and Ni²⁺ removal using date seed biochar (DSB-Electro). Compared with pristine biochar, the results showed that DSB-Electro effectively increased the adsorption capacity of Pb²⁺, Cu²⁺ and Ni²⁺ by 21% to 94%. Significant differences were observed between Pb²⁺ and Cu²⁺ adsorption compared with Ni²⁺, which could be explained based on ion polarizing power. Under the same voltage, Ni²⁺ solution shows the highest electric conductivity; thereby more Ni²⁺ is transported to the biochar anode, giving them a greater chance to interact with the surface groups. Electro-assisted adsorption occurred rapidly as around 88% of Pb²⁺ and Ni²⁺ adsorbed within the first 3 h, while 96% of Cu²⁺ occurred within the first hour of contact. Reversing the polarity did not seem to cause significant desorption of the adsorbed ions as the amount released from reversing polarity was less than 38%, indicating that only a small fraction of the ions was held by the electrostatic charge introduced by the current. It was likely that the enhanced charge facilitated other adsorption mechanisms by bringing the ions in contact with the biochar initially via electrostatic force. Electro-assisted adsorption can improve the biochar economic feasibility for metals removal (particularly Ni²⁺) from industrial streams.

Enhanced electrokinetic remediation and simulation of cadmium-contaminated soil by superimposed electric field

The 'focusing' effect has become a limiting factor for the removal of heavy metals from soils by electrokinetic (EK) remediation. A superimposed electric field EK (SE-EK) method is proposed to address this problem. Two sets of fixed electrodes placed at different positions were switched to move the 'focusing' region of Cd to the cathode by controlling the location of the pH jumping front. Moreover, a model was established to simulate and optimize the process of Cd transport in soil under the superimposed electric field. Results showed that, after 35 d of SE-EK remediation, Cd was mainly accumulated in the soil section near the cathode (S5), where the acid and alkaline fronts converged. The removal rate of Cd in the soil sections from S1 to S4 reached 87.60%, which was 6.13 times that in conventional EK remediation. Meanwhile, the energy utilization efficiency in SE-EK was 6.38 times that in conventional EK. The pH changes and Cd distribution during the SE-EK experiment were simulated well, with good agreement between the modeled and experimental data. The removal of Cd in SE-EK remediation could therefore be optimized through simulating the distribution of Cd in five situations with differences in switching time and electrode position. This research provides valuable technical support for effective EK remediation of heavy metal contaminated soil.

Soil heterogeneity and surfactant desorption influence PAH distribution during electroremediation at a tar oil-contaminated site

After a field experiment utilising electroosmosis and non-ionic surfactant Tween 80 as a remediation effort on the removal of polycyclic aromatic hydrocarbons (PAHs) from a long-term asphalt-contaminated soil, the PAH heterogeneity in the soil was yet extensive. This study come as a follow-up to address the following questions: (i) was PAH (re)distribution a consequence of the treatment? and (ii) to what extent does the surfactant affected PAH desorption and subsequent bioavailability? To answer question (i), we selected random soil samples from different locations of the field site before in situ remediation took place, and quantified and characterised soil organic matter by elemental analysis and solid-phase ¹³C nuclear magnetic resonance spectroscopy and PAH concentrations. Finally, batch desorption experiments with selected contaminated soil samples were carried out with and without 1% Tween 80 in the solution phase to address question (ii). Data shows that PAH concentrations were related neither to organic matter content nor to a high aromaticity of the organic matter, which serves as a proxy for the presence of tar oil. Soil heterogeneity is likely to be the cause of PAH heterogeneous distribution, but it is inferred that remediation causes weathering of the tar oil phase, resulting in the release and subsequent transport and sorption of PAH to inherent organic material. The results of the batch desorption experiments demonstrate PAH desorption up to 146 times when surfactant is present. However, Tween 80 does not enable biodegradation, since desorbed PAH molecules are entrapped inside surfactant micelles.

Remediation of heavy metal contaminated soil by asymmetrical alternating current electrochemistry

Soil contamination by heavy metals constitutes an important environmental problem, whereas field applicability of existing remediation technologies has encountered numerous obstacles, such as long operation time, high chemical cost, large energy consumption, secondary pollution, and soil degradation. Here we report the design and demonstration of a remediation method based on a concept of asymmetrical alternating current electrochemistry that achieves high degrees of contaminant removal for different heavy metals (copper, lead, cadmium) at different initial concentrations (from 100 to 10,000 ppm), all reaching corresponding regulation levels for residential scenario after rational treatment time (from 30 min to 6 h). No excessive nutrient loss in treated soil is observed and no secondary toxic product is produced. Long-term experiment and plant assay show the high sustainability of the method and its feasibility for agricultural use.

Soil pollution by heavy metals is a problem of global concern, requiring the development of remediation technologies. Here the authors report a method based on asymmetrical alternating current electrochemistry, which enables recycling of soil washing chemicals and eliminates secondary pollution.

Electrokinetic remediation of antibiotic-polluted soil with different concentrations of tetracyclines

This study investigated the efficacy of electrokinetic remediation of soils polluted with different concentrations of tetracyclines (TCs). Three widely used TCs (oxytetracycline, chlortetracycline, and tetracycline) were selected, and concentrations of 0, 5, 10, 20, and 50 mg/kg (C0, C5, C10, C20, C50) were selected for comparison. Antibiotic-polluted soils with no electric field served as controls. The average removal rates of TCs in different treatments ranged from 25 to 48% after 7-day remediation. The contributing ratios of electrokinetics to TCs removal varied from 22 to 84%. The concentrations of NH₄⁺ increased in soils and electrolytes, which indicated the decomposition of TCs in the electric field. The highest removal amount of TCs was obtained in the C50 treatment, due to efficient reactions of TCs with oxidative radicals generated during the electrolysis. The fluctuant range of pH in the electrolytes was decreased with increasing concentration of TCs, while the soil pH was increased. The removal rate of antibiotic-resistant bacteria (ARB) in the C5 treatment was significantly higher than that in other treatments. The abundance of antibiotic resistance genes (ARGs) increased with the concentrations of TCs in soils. It might result from the induction of increasing selective pressure of antibiotics. Significant removal of ARGs occurred in the C50 treatment (38-60%). In terms of controlling ARB and ARGs, which were more resistant, the electrokinetic technology showed advantageous effects. Above all, electrokinetic technology provides an effective remediation method, especially for TC-polluted soil with a concentration of 20-50 mg/kg.

An extensive review on restoration technologies for mining tailings

Development of mineral resources and the increasing mining waste emissions have created a series of environmental and health-related issues. Nowadays, the ecological restoration of mining tailings has become one of the urgent tasks for mine workers and environmental engineers all over the world. Aim of the present paper is to highlight the previous restoration techniques and the challenges encountered during the restoration of mine tailings. As it is a common practice that, before restoring of tailings, the site should be evaluated carefully. Studies showed that the mine tailings' adverse properties, including excessive heavy metal concentration, acidification, improper pH value, salinization and alkalization, poor physical structure and inadequate nutrition, etc., are the major challenges of their restoration. Generally, four restoration technologies, including physical, chemical, phytoremediation, and bioremediation, are used to restore the mining tailings. The working mechanism, advantages, and disadvantages of these techniques are described in detail. In addition, selection of the suitable restoration techniques can largely be carried out by considering both the economic factors and time required. Furthermore, the ecosystem restoration is perceived to be a more promising technology for mine tailings. Therefore, this extensive review can act as a valuable reference for the researchers involved in mine tailing restoration.

Transient reactive transport model for physico-chemical transformation by electrochemical reactive barriers

A comprehensive model that integrates coupled effects of chemical, physical, and electrochemical processes, is necessary for design, analysis, and implementation of the electro-remediation of groundwater under flow conditions. A coupled system of equations to solve for transport and multiple reactions in an electrochemical reactor is numerically intensive due to highly stiff nature of reaction model formulation. In this study, the focus is to develop an efficient model for reactions associated with the transport and physico-chemical transformation in an electrochemical reactor. The model incorporates effects of transport mechanisms as well as chemical and electrochemical reactions. Model verification is provided for pH profiles under different electrolyte compositions in two sets of reactors; a batch and a flow-through reactor. The model is able to predict the concentration of species during the electrochemical remediation process with a close correlation to experimental data (R² = 0.99 for batch and R² = 0.78 for flow-through reactor.) Imposing polarity reversal to the system will cause fluctuation of pH, however, the trend stays the same as if no polarity were applied. Ultimately, volumetric charge flow is introduced as a unique parameter characterizing the electroremediation reactor for operating purposes.

Improved isolation of cadmium from paddy soil by novel technology based on pore water drainage with graphite-contained electro-kinetic geosynthetics

Novel soil remediation equipment based on electro-kinetic geosynthetics (EKG) was developed for in situ isolation of metals from paddy soil. Two mutually independent field plot experiments A and B (with and without electric current applied) were conducted. After saturation using ferric chloride (FeCl₃) and calcium chloride (CaCl₂), soil water drainage capacity, soil cadmium (Cd) removal performance, energy consumption as well as soil residual of iron (Fe) and chloride (Cl) were assessed. Cadmium dissolved in the soil matrix and resulted in a 100% increase of diethylenetriamine-pentaacetic acid (DTPA) extracted phyto-available Cd. The total soil Cd content reductions were 15.20% and 26.58% for groups A and B, respectively, and electric field applications resulted in a 74.87% increase of soil total Cd removal. The electric energy consumption was only 2.17 kWh/m³ for group B. Drainage by gravity contributed to > 90% of the overall soil dewatering capacity. Compared to conventional electro-kinetic technology, excellent and fast soil water drainage resulted in negligible hydrogen ion (H⁺) and hydroxide ion (OH^-) accumulation at nearby electrode zones, which addressed the challenge of anode corrosion and cathode precipitation of soil metals. External addition of FeCl₃ and CaCl₂ caused soil Fe and Cl residuals and led to 4.33-7.59% and 139-172% acceptable augments in soil total Fe and Cl content, correspondingly, if compared to original untreated soils. Therefore, the novel soil remediation equipment developed based on EKG can be regarded as a promising new in situ technology for thoroughly isolating metals from large-scale paddy soil fields.

Electrokinetic Control of Viscous Fingering

We present a theory of the interfacial stability of two immiscible electrolytes under the coupled action of pressure gradients and electric fields in a Hele-Shaw cell or porous medium. Mathematically, our theory describes a phenomenon of "vector Laplacian growth," in which the interface moves in response to the gradient of a vector-valued potential function through a generalized mobility tensor. Physically, we extend the classical Saffman-Taylor problem to electrolytes by incorporating electrokinetic (EK) phenomena. A surprising prediction is that viscous fingering can be controlled by varying the injection ratio of electric current to flow rate. Beyond a critical injection ratio, stability depends only upon the relative direction of flow and current, regardless of the viscosity ratio. Possible applications include porous materials processing, electrically enhanced oil recovery, and EK remediation of contaminated soils.

Multiscale Model for Electrokinetic Transport in Networks of Pores, Part II: Computational Algorithms and Applications

The first part of this two-article series presented a robust mathematical model for the fast and accurate prediction of electrokinetic phenomena in porous networks with complex topologies. In the second part of this series, we first present a numerical algorithm that can efficiently solve the model equations. We then demonstrate that the resulting framework is capable of capturing a wide range of transport phenomena in microstructures by considering a hierarchy of canonical problems with increasing complexity. The developed framework is validated against direct numerical simulations of deionization shocks in micropore-membrane junctions and concentration polarization in micro- and nanochannel systems. We demonstrate that for thin pores subject to concentration gradients our model consistently captures correct induced osmotic pressure, which is a macroscopic phenomena originally derived from thermodynamic principles but here is naturally predicted through microscopic electrostatic interactions. Moreover, we show that the developed model captures current rectification phenomena in a conical nanopore subject to an axial external electric field. Finally, we provide discussions on examples involving stationary and moving deionization shocks in micropore nanopore T-junctions as well as induced-flow loops when pores of varying sizes are connected in parallel.

In situ construction of low permeable barrier in soil to prevent pollutant migration by applying weak electric field

In order to prevent vertical migration of pollutant in soil matrix, this study firstly proposed to construct an in situ low permeable barrier (LPB) through synchronously transporting calcium and carbonate. After LPB construction, the soil permeability was declined tenfold. Exchangeable calcium (37.3%) and calcium bonding to carbonate (41.7%) respectively alleviated flocculation of microaggregates and cementation of marcoaggregates. Accordingly, smaller particles (<1 mm) aggregated into bigger ones (>2 mm) after electrokinetic remediation. The other soil characters like pH, moisture, and bacterial communities were well preserved after remediation. In addition, the pollutant prevention was divided into two phases as unsaturated phase and saturated phase. In unsaturated phase, phenol, F^-, Cd²⁺, and Ni²⁺ in filtrate were all lower than 0.1 mg, and Cr₂O₄^2--Cr discharged from LPB was 1/5.1 than that from initial soil. In saturated phase, LPB prevented 4.3-12.1 fold pollutant than initial soil. Taken together, proposed method could effectively prevent vertical migration of pollutants, indicating significant values for saving soil remediation cost or avoiding contamination of underground water.