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Multispecies reactive transport modelling of electrokinetic remediation of harbour sediments
Abstract
We implemented a numerical model to simulate transport of multiple species and geochemical reactions occurring during electrokinetic remediation of metal-contaminated porous media. The main phenomena described by the model were: (1) species transport by diffusion, electromigration and electroosmosis, (2) pH-dependent buffering of H⁺, (3) adsorption of metals onto particle surfaces, (4) aqueous speciation, (5) formation and dissolution of solid precipitates. The model was applied to simulate the electrokinetic extraction of heavy metals (Pb, Zn and Ni) from marine harbour sediments, characterized by a heterogeneous solid matrix, high buffering capacity and aged pollution. A good agreement was found between simulations of pH, electroosmotic flow and experimental results. The predicted residual metal concentrations in the sediment were also close to experimental profiles for all of the investigated metals. Some removal overestimation was observed in the regions close to the anode, possibly due to the significant metal content bound to residual fraction.
... + E-mail: koteswarareddy@kluniversity.in 1021-9986/2022/5/1573- 1587 15/$/6.05 Research Article Seabed area Stirred suspension (1300 rpm) 20 18-21 [18] Co Mine tailing Nitric acid, Humic acid, Fulvic acid 20 29-38 [19] Cu Mine tailing Pulse current frequency 10 1.5 [20] Industrial waste Diffuse double-layer processes 5 - [21] Seabed area Stirred suspension (1300rpm) 20 41-67 [18] Municipal sludge Nitric acid, Sodium nitrate 6 83 [22] Ni Seabed area Stirred suspension (1300rpm 20 24-49 [18] Mine tailing Nitric acid, Humic acid, Fulvic acid 20 41-70 [19] Harbour sediment Nitric acid 120 42 [23] Municipal sludge Nitric acid, Sodium nitrate 6 73 [22] Zn Seabed area Stirred suspension (1300 rpm) 20 71-74 [18] Mine tailing Nitric acid, Humic and Fulvic acid 20 25-47 [20] Harbour sediment Nitric acid 120 28 [23] Mn Seabed area Stirred suspension (1300rpm) 20 36-37 [18] Mine tailing Nitric acid, Humic acid, Fulvic acid 20 38-64 [19] *Removal efficiency (%) System (GIS) particularly for groundwater [4,5]. The environmental impact assessment of HM polluted water and soil were estimated in order to mitigate the environmental pollution [6]. ...
... + E-mail: koteswarareddy@kluniversity.in 1021-9986/2022/5/1573- 1587 15/$/6.05 Research Article Seabed area Stirred suspension (1300 rpm) 20 18-21 [18] Co Mine tailing Nitric acid, Humic acid, Fulvic acid 20 29-38 [19] Cu Mine tailing Pulse current frequency 10 1.5 [20] Industrial waste Diffuse double-layer processes 5 - [21] Seabed area Stirred suspension (1300rpm) 20 41-67 [18] Municipal sludge Nitric acid, Sodium nitrate 6 83 [22] Ni Seabed area Stirred suspension (1300rpm 20 24-49 [18] Mine tailing Nitric acid, Humic acid, Fulvic acid 20 41-70 [19] Harbour sediment Nitric acid 120 42 [23] Municipal sludge Nitric acid, Sodium nitrate 6 73 [22] Zn Seabed area Stirred suspension (1300 rpm) 20 71-74 [18] Mine tailing Nitric acid, Humic and Fulvic acid 20 25-47 [20] Harbour sediment Nitric acid 120 28 [23] Mn Seabed area Stirred suspension (1300rpm) 20 36-37 [18] Mine tailing Nitric acid, Humic acid, Fulvic acid 20 38-64 [19] *Removal efficiency (%) System (GIS) particularly for groundwater [4,5]. The environmental impact assessment of HM polluted water and soil were estimated in order to mitigate the environmental pollution [6]. ...
... The removal of copper (Cu) ions about 83% from municipal sludge by the addition of nitric acid and sodium nitrite reagents within 6 days of operation [21,22]. The amount of nickel (Ni) ions, is removed by almost 73% from mine tailing soils in 6 days of treatment under enhanced conditions [18][19][20][21][22][23]. In the same way, the amount of zinc (Zn) and manganese (Mn) ions were removed around 25-47% and 38-64% from mine tailing soil by the addition of nitric acid, humic acid, and fulvic acid as enhancement reagents within 20 days of operation on a laboratory scale [18,19]. ...
The goal of the study is to incorporate the electrokinetic models and estimate the remediation time for maximum removal of Heavy Metals (HMs) from polluted soils. Most of conventional electrokinetic technologies have not considered the electrokinetic models in the removal of HMs from polluted soils. We addressed this problem and incorporated the electrokinetics and applied it to the experimental ElectroKinetic Soil Remediation (EKSR) process particularly, to extract the numerical data between the removal performance of HMs versus remediation time with the help of the MATLAB program. In the experimental study, chelating chemical agents (citric acid and ethylenediaminetetraacetic acid (EDTA)) were used in EKSR process under a constant voltage gradient (2V/cm) for the removal of Chromium (Cr), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn) and Manganese (Mn) ions from granite dump soil. We experimentally investigated that the removal performance of HMs in chelating agents enhanced EKSR was about 6 to7 times more than when unenhanced in 20 days of treatment. Furthermore, we estimated the remediation time of about 52 to 54 days for the complete removal of HMs using electrokinetic models. The study may be useful for the researcher particularly, in soil decontamination studies to overcome the uncertainty in the process optimization and scale up the process to the pilot plant and field level.
... These ionic species transfer into the soil and pore water under the control of electric field and porous fluid flow. EKR performances depend on the pH profile of soil water, as pH profile leads to physicochemical processes beside electro-kinetic ones, for example, precipitation/dissolution of minerals and metals, adsorption/desorption of pollutants and ion equilibrium between the soil and the pore water (López-Vizcaíno et al. 2017;Lopez Vizcaino et al. 2018;Masi et al. 2017b). Additionally, chemical speciation is strongly related to the pH of the pore water, which affects the efficiency of EKR process. ...
... Mathematical modeling methods have been recognized as essential tools for electro-kinetic remediation since they are essential for understanding the coupled mass transfer and porous fluid flow processes under the control of electric field, as well as for the design, optimization and control of EKR systems Paz-García et al. 2016;Sprocati et al. 2019). Several models have been developed for 1-D or 2-D dimensions (2014( , Masi et al. 2017bMiao and Pan 2015;Paz-García et al. 2016;Rubio-nieblas et al. 2014a, b;Sprocati et al. 2019;Vereda-Alonso et al. 2007;Wu et al. 2012b), with most of these studies typically including the coupling of the physical mass transfer processes with geochemical databases to simulate EKR in saturated porous media (Masi et al. 2017a, b;Paz-García et al. 2016;Sprocati et al. 2019;Wu et al. 2012a). In these models, fluid flow, solute transport (including electro-migration and electro-osmosis), columbic interactions between transported species and equilibrium reactions play major roles in the EKR performance (Alvarez et al. 2017;Masi et al. 2017b;Sprocati et al. 2019;Sprocati and Rolle 2020). ...
... Several models have been developed for 1-D or 2-D dimensions (2014( , Masi et al. 2017bMiao and Pan 2015;Paz-García et al. 2016;Rubio-nieblas et al. 2014a, b;Sprocati et al. 2019;Vereda-Alonso et al. 2007;Wu et al. 2012b), with most of these studies typically including the coupling of the physical mass transfer processes with geochemical databases to simulate EKR in saturated porous media (Masi et al. 2017a, b;Paz-García et al. 2016;Sprocati et al. 2019;Wu et al. 2012a). In these models, fluid flow, solute transport (including electro-migration and electro-osmosis), columbic interactions between transported species and equilibrium reactions play major roles in the EKR performance (Alvarez et al. 2017;Masi et al. 2017b;Sprocati et al. 2019;Sprocati and Rolle 2020). Despite the increasing number of numerical simulators for EKR process, a modeling approach capable of coupling fluid flow and mass transfer of H + and OH − is still lacking. ...
Purpose
Soil electro-kinetic remediation (EKR) has received significant attention owing to its environmental sustainability. Water electrolysis at electrode surface changes the pH profile of soil water. The pH profile has a strong impact on EKR performances. The aims of this study were to quantify the mass transfer of H⁺ and OH⁻ and investigate the coupled relationship among H⁺ and OH⁻ mass transfer, electric field and porous fluid flow.
Materials and methods
Herein, multi-dimensional (1D and 2D) models capable of coupling fluid flow and mass transfer were established to study the coupled relationship among H⁺ and OH⁻ mass transfer, electric field and porous fluid flow. The multi-dimensional (1D and 2D) models were validated by lab scale experiments.
Results and discussion
The characteristics of pH front and pH profile was proven to be dominated by electric field, mass transfer and porous fluid flow. The movement of pH front and pH profiles dominates the EKR performance. The conductivity rise and the electric field distribution variations were quantified and proven to be caused by the H⁺ and OH⁻ mass transfer. After a certain EKR time, in the areas near the electrodes where the H⁺ and OH⁻ are generated, the mass transfer flux of H⁺ and OH⁻ is gradually close to its releasing rate, the ionic species H⁺ and OH⁻ stop accumulating and the concentration of both tends to steady state, so does the conductivity.
Conclusions
We demonstrated that the coupled relationship among mass transfer of H⁺ and OH⁻, electric field, and porous fluid flow dominates the movement of pH profiles and the conductivity rise.
... Furthermore, unlike conventional remediation methods, electrokinetic remediation can be used effectively for both ex situ and in situ remediations of low permeability soils [8]. It can also be used in railway soils and residential areas where it could be difficult to excavate [8,9]. ...
... These may include adsorption-desorption, dissolution-precipitation reactions, acid-alkaline reactions, and redox reactions [27,47,49,50]. Due to numerous and complex electrochemical and physico-chemical processes within the electrokinetic system, meticulous understanding of the following is required [9,51]: ...
... The interaction between the electrolytes and the electrodes leads to formation of • OH, as shown in Equation (8), where the electrode is denoted as MO x [76]. In the presence of active electrodes at the anode, the • OH strongly adsorbs to the electrode to form a metal oxide (MOx + 1), as shown in Equation (9). Organic contaminants are either oxidised directly by the electrodes, as shown in Equations (10) and (11), or indirectly by high energy free radicals • OH (Equation (8)), H 2 O 2 (Equation (12)), and O 3 (Equation (13)) to form carbon dioxide [76]. ...
Electrokinetic remediation has, in recent years, shown great potential in remediating polluted environments. The technology can efficiently remove heavy metals, chlorophenols, polychlorinated biphenyls, phenols, trichloroethane, benzene, toluene, ethylbenzene, and xylene (BTEX) compounds and entire petroleum hydrocarbons. Electrokinetic remediation makes use of electrolysis, electroosmosis, electrophoresis, diffusion, and electromigration as the five fundamental processes in achieving decontamination of polluted environments. These five processes depend on pH swings, voltage, electrodes, and electrolytes used in the electrochemical system. To apply this technology at the field scale, it is necessary to pursue the design of effective processes with low environmental impact to meet global sustainability standards. It is, therefore, imperative to understand the roles of the fundamental processes and their interactions in achieving effective and sustainable electrokinetic remediation in order to identify cleaner alternative solutions. This paper presents an overview of different processes involved in electrokinetic remediation with a focus on the effect of pH, electrodes, surfactants, and electrolytes that are applied in the remediation of contaminated soil and how these can be combined with cleaner technologies or alternative additives to achieve sustainable electrokinetic remediation. The electrokinetic phenomenon is described, followed by an evaluation of the impact of pH, surfactants, voltage, electrodes, and electrolytes in achieving effective and sustainable remediation.
... The application of a low-intensity electric field or continuous current mobilizes contaminants and water through the porous medium towards the electrodes due to four main transport mechanisms: diffusion-generated mass flow, electromigration, electroosmosis, and electrophoresis [17]. Other effects such as pH change, electrode reactions, and geochemical reactions may also take place [20]. In addition, water electrolysis occurs in the electrodes generating hydroxides in the cathode and hydrogen ions in the anode, which are transported by the electric field towards the opposite-charge electrodes, determining acid fronts, which are favorable to desorption of contaminants, and alkaline fronts that may produce precipitations, slowing down desorption, moving in opposite directions. ...
... Theoretical descriptions of the transport phenomena on electrical and hydraulic potentials have been proposed by several authors [17,29,30,31,32,33] and adapted to the most diverse experimental conditions [20,[34][35][36][37][38][39]. ...
... The electroosmotic flow (J e ) was evaluated based on the mass balance measuring the volume alterations in electrolyte overflows [20]. The system was filled out with electrolyte and kept under a potential of 11 V for 48 h, without electrolyte pumping, and the overflow of electrolytes during that period of time was observed. ...
Transport phenomena are investigated which are involved in the electrokinetic remediation process used for removing vanadium from deactivated catalysts from oil catalytic cracking that are currently allotted to cement plants and class‐I landfills. Variables such as the concentration of electrolyte, electric potential, and applied electric current were evaluated in order to determine the effects produced by electroosmosis, diffusion, hydraulic gradient, and electromigration on the removal of vanadium from the catalyst. It was observed that migration is the most relevant phenomenon in the remediation tests, and for the best remediation condition, the migratory flow accounted for about 87 % of the vanadium removal.
... This model ignores electrophoresis due to the inhibition of colloid migration by the immobile porous medium. With consideration of diffusion, hydraulic gradient, electromigration, and electroosmosis, the Cd(II) transport driven by EKR can be described as: (Alshawabkeh and Acar, 1992;Masi et al., 2017). ...
... The volume-averaged EOF Q eo,x (m 3 s −1 ) along the x-direction can be expressed as (Masi et al., 2017): ...
Enhanced electrokinetic remediation (EKR) allows the rapid remediation of heavy metal-contaminated clay, but the impacts of this process on soil micro-ecology have rarely been evaluated. In this study, nitric acid, acetic acid, and EDTA were applied for enhancement of EKR and the effects on Cd removal, soil enzyme activity, and soil bacterial communities (SBCs) were determined. Nitric acid and acetic acid allowed 93.2% and 91.8% Cd removal, respectively, and EDTA treatment resulted in 40.4% removal due to the formation of negatively charged EDTA-Cd complexes, resulting in opposing directions of Cd electromigration and electroosmosis flow and slow electromigration rate caused by low voltage drop. Activities of soil beta-glucosidase, acid phosphatase, and urease, were all reduced by enhanced EKR treatment, especially nitric acid treatment, by 46.2%, 58.8% and 57.7%, respectively. The SBCs were analyzed by high-throughput sequencing and revealed significantly increased diversity for acetic acid treatment, no effect for EDTA treatment, and reduced diversity for nitric acid treatment. Compared with nitric acid and EDTA, acetic acid treatment enhanced EKR for higher Cd removal and improved biodiversity.
... Where A is the area of the perpendicular surface to the direction of the flow, E z is the voltage gradient in the direction of the flow (z in this case), k e * is the effective coefficient of electro-osmotic permeability. The k e * is directly related to the dielectric constant ε (F m −1 ), zeta potential ξ (V), and the fluid viscosity δ (N s m −2 ) (Choi and Lui, 1995;Masi et al., 2017b): ...
... CPI value distribution of common ions in soil (Date sources:Kim et al., 2009;Paz-García et al., 2012;López-Vizcaíno et al., 2017;Masi et al., 2017b;Rezaee et al., 2019)). D.Wen, et al. ...
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.
... Reactive transport models implementing surface complexation reactions have also been applied to describe systems impacted by transient surface-groundwater interactions, such as those observed at the Hanford site and affecting uranium transport (Greskowiak et al. 2011;Ma et al. 2014), as well as the river-groundwater exchange important for arsenic transport documented at a field site in Vietnam (Wallis et al. 2020). Furthermore, the model-based description of transport and surface complexation reactions is important in engineering applications such as remediation of contaminated sites and managed aquifer recharge (e.g., Masi et al. 2017;Rathi et al. 2017). ...
... Despite these advantages, the complexity of the processes occurring during EK makes it challenging to isolate the contributions of different transport mechanisms and to estimate the parameters that govern them [17,24,[29][30][31][32]. The accurate identification of these controlling parameters is essential for the effective scale up of EK technologies [30,[33][34][35][36][37][38]. ...
... Processes 2024, 12, 1157 2 of 18 after remediation [22][23][24]. Reactive transport models (RTMs) have been used to simulate the fate and transport of contaminants under both natural and remedial conditions that can be used to assist the design and optimization of the remediation strategies [25,26]. The computational cost is, however, the constraint for applying RTMs in designing and optimizing remediation technologies, especially for a large site with a complex mixture of different contaminants [27][28][29]. ...
A numerical approach assisted by machine learning was developed for screening and optimizing soil remediation strategies. The approach includes a reactive transport model for simulating the remediation cost and effect of applicable remediation technologies and their combinations for a target site. The simulated results were used to establish a relationship between the cost and effect using a machine learning method. The relationship was then used by an optimization method to provide optimal remediation strategies under various constraints and requirements for the target site. The approach was evaluated for a site contaminated with both arsenic and polycyclic aromatic hydrocarbons at a former shipbuilding factory in Guangzhou City, China. An optimal strategy was obtained and successfully implemented at the site, which included the partial excavation of the contaminated soils and natural attenuation of the residual contaminated soils. The advantage of the approach is that it can fully consider the natural attenuation capacity in designing remediation strategies to reduce remediation costs and can provide cost-effective remediation strategies under variable constraints for policymakers. The approach is general and can be applied for screening and optimizing remediation strategies at other remediation sites.
... Research studies have been conducted on the effectiveness of EO in consolidating soft soils, providing a sound understanding of consolidation mechanism for engineering applications (Lo et al., 1991;Micic et al., 2001;Citeau et al., 2011;Zhang et al., 2017;. These studies have explored various facets, including electroosmotic theory (Hu et al., 2010;Wang et al., 2019), pH influences (Hu et al., 2019), chemical grouting (Ou et al., 2015;Zhang et al., 2017;Xue et al., 2018;Du et al., 2022), combined applications with other methods (Deng and Zhou, 2016;Wang et al., 2016;Zhou and Deng, 2019;, and both theoretical and numerical simulations (Matteo et al., 2017;Zhang and Hu, 2022;Zhou et al., 2022). In recent years, research has focused on emerging areas, including the bio-inspired heavy metal immobilization method and the enzyme-induced carbonate precipitation method. ...
This study conducted experimental investigations into the effects of electrode material and potential gradient on the effectiveness of electro-osmotic consolidation (EO) in strengthening soft soils. Seven laboratory tests were conducted on high-water-content marine clayey soils through EO. In these experimental tests, four different types of electrodes made of steel, copper, aluminum, and composite carbon fiber (CCF) were employed in four tests each to evaluate the consolidation effectiveness. Additionally, four tests, one was the comparitive study for different eletrode materials, were carried out to determine the optimal gradient for the EO using CCF electrode. Several critical properties of the tested soils were examined and evaluated in this study, including the effective voltage utilization, potential distribution, water discharge, discharge rate, energy consumption, and soil bearing capacity. The test results indicated that the CCF electrode had superior performance in water discharge, discharge rate, and average soil water content compared to metal electrodes. Furthermore, CCF led to uniform enhancement of soil strength, with treated soil bearing capacities 6.3 to 12 times higher than initial values, and 1.9 to 2.5 times higher than those attained with metal electrodes. Additionally, an effective potential gradient of 1 V/cm was identified for the EO with the CCF electrode, providing a higher discharge rate and a larger soil strength in a uniform distribution. Moreover, the use of CCF electrode significantly reduced corrosion compared to metal electrodes during the consolidation process, further contributing to improved consolidation efficiency. This study offers valuable insights and recommendations for the utilization of CCF in marine clayey soils, effectively addressing the challenges posed by electrode corrosion and high energy consumption in EO applications.
... EK remediation technique is based on the application of a low electric field between two or more electrodes inserted in the soil to mobilize pollutants toward the electrodes through the combined mechanisms of electroosmosis (movement of water towards the cathode), electrophoresis (movement of colloidal particles towards the anode), and electromigration (moving ions toward the opposite pole). In the literature, several studies can be found that applied EK technique for the remediation of soils contaminated with HMs including Cu [6,7]. However, removal rate of using EK alone is relatively low [8]. ...
Heavy metals (HMs) removal from clay soils by conventional soil washing techniques is a challenging process due to the soil low permeability. In this study, removal of copper (Cu) from contaminated clay soil was investigated using electrokinetic (EK) technology coupled with zero-valent iron (ZVI) permeable reactive barrier medium (PRB). Reverse polarity (REK) and pH control (EK-pH) strategies were used to improve the EK performance. According to the result, for a constant electric field (1.0 V.cm-1) applied during 15 days, the pH control strategy (EK-pH) attained a high removal efficiency of 83.7% where the better soil conditions such as stable pH, highest EC (85 mA) and EOF (7.9 mL/d) were observed. However, the best Cu removal efficiency of 84.3% was achieved in the case of polarity reversion (REK) due to the pH stability and equal water movement to the PBR from the two sides. The results of this work suggest a promising technique for effective heavy metal removal from contaminated soil.
... To establish a connection between the FEM software and thermodynamic codes, an interface needs to be developed. Serval interfaces have been created, including the COMSOL-PHREEQC interface (Guo et al. 2018a, b;2021a, b;Masi et al. 2017;Nardi et al. 2014;Rolle et al. 2018;Wissmeier and Barry 2011), the UTCHEM-PHREEQC interface (Korrani et al. 2015), the COMSOL-GEMS interface (Azad et al. 2016) and the OpenGeoSys-GEMS interface (Kosakowski and Watanabe 2014). Figure 4 presents the COMSOL-PHREEQC interface to solve the Fig. 4 The COMSOL-PHREEQC interface based on SNIA to solve chloride reactive transport model in concrete (Guo et al. 2021a, b) chloride reactive transport model in concrete. ...
The study reviews the theoretical models, numerical implementation and practical applications of chloride reactive transport in concrete. Thermodynamic modeling is capable of accurately predicting chloride binding behaviors across the entire concentration range. It also considers the impact of the pH variation in the pore solution. Thus, the reactive transport model, integrating thermodynamic calculations into transport equations, can provide a more comprehensive representation of chloride ingress in concrete. Furthermore, we discuss the effects of water transport and external stresses on chloride reactive transport. In addition to the well-known advection phenomenon, water transport has the ability to alter the effective transport pathway and influence chloride binding reactions. These three influences exhibit typical temporal and spatial characteristics. Capturing the temporal and spatial characteristics in chloride reactive transport model can be achieved by continuously updating the saturation degree and chloride diffusion coefficient at each finite element mesh node. The effect of stress on chloride reactive transport can be categorized into two scenarios based on the response of transport pathway to external loads: (1) high stress levels, which result in the formation of cracks in concrete, and (2) low stress levels, where concrete remains crack-free. Quantitating the influence of stress levels on the transport pathway is crucial for simulating chloride reactive transport.
... where q M adv is the advective velocity and j L dif− dis,i is the individual diffusive-dispersive flux of each i-th species evaluated at the L-th modeling level. These fluxes are defined by the well-known Nernst-Planck equation based on the assumption that the gradient of the electrochemical potential of each species acts as a driving force for the diffusion-dispersive transport mechanism Masi et al., 2017;Tournassat et al., 2020): ...
... Recent electrokinetic modeling has focused on incorporating geochemical effects, including adsorption/desorption, precipitation/dissolution, hydrolysis, surface complexation, ion exchange, and kinetics (Al-Hamdan and Reddy 2008;Mascia et al. 2015;Masi et al. 2017Masi et al. , 2019Rolle et al. 2018;Sprocati et al. 2019). However, most models focus on saturated porous media, while the simulation of ion migration in unsaturated zones remains underexplored. ...
In the study of remediation of heavy metal contaminated sites by electrokinetic method, the nonuniform distribution of voltage gradient may lead to the high concentration of target ions and reduce the remediation effect. However, the relationship between the voltage gradient difference and the mass transfer efficiency between adjacent areas has not been clarified in previous studies. This study aimed to investigate the relationship between voltage heterogeneity and mass transfer efficiency (MTE) of heavy metals during electrokinetic (EK) remediation. Although voltage heterogeneity has been shown to directly affect the electromigration of heavy metal contaminants in soils, the specific effects on MTE in adjacent regions are unclear.
To characterize this relationship, we performed nine bench-scale EK experiments using sandboxes with a total applied voltage of 9 V spiked with Cr(VI) as a model contaminant. The EK test zone was divided into equal-volume contaminated and uncontaminated subzones. The initial voltage percentage in the uncontaminated zone (IVPF) ranged from 23.4 to 98% of the total voltage.
The MTE of total Cr showed a quadratic response to IVPF (R2 > 0.95), with peak efficiency at ~ 54% IVPF. Increasing IVPF enhanced the migration of acid and base fronts. Both contaminated and uncontaminated zones were acidified when IVPF exceeded 90%.
Our results indicate that initial voltage distribution significantly affects the MTE of heavy metals during EK remediation. We highlight the importance of considering initial voltage heterogeneity in the design of EK systems.
... There was no mention of other condition solutions added to the contaminated soil (Table 5). Masi et al. (2017b) studied the SEKR modeling of multispecies reactive heavy metal (Pb, Zn, and Ni) transportation in harbor sediments. In this study, HNO 3 was the added conditioning solution to the catholyte solution (Table 6). ...
This review is a follow-up to our previous review titled “comprehensive review of progress made in soil electrokinetic research during 1993–2020, Part I: process design modifications with brief summaries of main output”. In this review, we have summarized the material additives that are utilized for the enhancement of soil electrokinetic remediation (SEKR) efficiency within the last four years (2017–2020). This review has been divided into six subjects according to determined SEKR categories including removal of inorganic contaminants, removal of organic contaminants, SEKR enhancement using permeable reactive barrier (PRB), SEKR for solidification and soil improvement, SEKR enhancement using an oxidation process, and SEKR enhancement using bioremediation and phytoremediation. Various enhancement materials have been added to improve the primary endeavors of SEKR and the summaries of the output were included in this review. In the SEKR experiments in which inorganic and organic contaminants were removed, the process was evaluated based on the process driving mechanism, particularly electromigration and electro-osmosis. Once the performance of SEKR was inefficient, the SEKR processing was integrated with PRB, oxidation, and bioremediation for significant improvements in SEKR performance. Therefore, the primary aim of our review is to provide a comparison of material additives for the SEKR removal intensification process from 2017 to 2020 to provide a review of the past materials that improved the SEKR performances and to simplify future innovation. During 2017–2020, SEKR studies focused on inorganic and organic contaminants remediation, whereas, few SEKR studies have focused on solidification and soil improvement. The SEKR has been utilized for dewatering, but in this case, the materials added were limited. The insufficient of relevant information on this subject was our primary motivation to write this review.
... (12) (Lorenz 1969;Eykholt and Daniel 1994), which is currently popular and applied directly to simulate the nonlinear EOF by lots of scholars (Kim et al. 2005;Beddiar et al. 2005;Asadollahfardi et al. 2016;Ahmed 2020). Lately, other researchers (Masi et al. 2017;Zhang and Hu 2022) believe that the parameters in this empirical function should be re-calibrated for different types of soil other than the kaolinite. Thus, in the present study, the tested zeta potential of the RF soil in the 0.05 mol/L NaCl solution was also fit by Eq. (12) seen in Fig. 6. ...
Electrokinetic remediation (EKR) is a promising alternative for the contaminated soil with low hydraulic permeability. The nonlinearity of the electroosmotic flow (EOF) is mainly induced by the nonuniform variation of the pH and thus the zeta potential of the soil during the EKR process. The empirical relation between the zeta potential and the pH for kaolinite is currently applied to analyze the nonlinearity of the EOF. A new perspective for theoretical determination of the zeta potential for the variable charge soil is proposed in this study. The prediction model incorporates the pH, the valence and concentration of the electrolyte, and the temperature and permittivity of the solvent surrounding the clay particles. Satisfying agreement between the calculated and measured curves of zeta potential versus pH for three types of variable charge soil was achieved. This model would act as a useful tool to simulate the nonlinearity of the electroosmosis of the variable charge soil and provide guidance and precise control mechanism for maximizing the efficiency of the EOF.
Graphical Abstract
... A second effect is a consumption of permanganate, which is unstable outside the 3.5-12 pH range, hence advancing acidic and alkaline fronts promote permanganate transformation to Mn 2+ and MnO 2 , respectively (Hodges et al., 2013;Reynolds et al., 2008;Roach, 2006), which are inactive species in the remediation process. Additionally, the migration of pH fronts from the electrode reservoirs in the porous medium can induce leaching of contaminants such as heavy metals from the solid matrix (Iannelli et al., 2015;Masi et al., 2017;Reddy et al., 2001). ...
The use of electrokinetics (EK) has great potential to deliver reactants in impervious porous media, thus overcoming some of the challenges in the remediation of contaminants trapped in low-permeability zones. In this work we experimentally investigate electrokinetic transport in heterogeneous porous media consisting of a sandy matrix with a target clay inclusion. We demonstrate the efficient EK-delivery of permanganate in the target clay zone (transport velocity 0.3–0.5 m day⁻¹) and its reactivity with Methylene Blue, a positively charged contaminant trapped within the inclusion. The delivery method was optimized using a KH2PO4/K2HPO4 buffer to attenuate the effect of electrolysis reactions in the electrode chambers, thus mitigating the propagation of pH fronts and preventing the phenomenon of permanganate stalling. The experiments showed that the buffer electrical conductivity greatly impacts the potential gradient in the heterogeneous porous medium with implications on the observed rates of electrokinetic transport (variation up to 40%). The reactive experiments provided direct evidence of the permanganate penetration within the clay and of its capability to degrade the target immobilized contaminant. The experimental results were analyzed using a process-based model, elucidating the governing transport mechanisms and highlighting the effect of different mass transfer processes on conservative and reactive electrokinetic transport.
... As REEs are species that have a high number of coordination (Ma et al., 2017), the size of the molecules that constitute the leaching agents should be considered. As the effective mobility of species depends on the porosity and tortuosity of the specimen (Masi et al., 2017), the mass transport of higher magnitude complexes may be affected, resulting in low REE removal efficiencies using DES-based electrolytes. Fig. 5 shows that the desorption of species decreases with the pH increase for the REEs analyzed. ...
A R T I C L E I N F O Keywords: Cerium Lanthanum Neodymium Light rare earth elements Electromining Selective removal A B S T R A C T Rare earth elements play an important role in our society, as they are used in green energy technologies. However, they are considered critical raw materials. For this reason, there is a concern for obtaining alternative and complementary sources for conventional mining. In light of this view, electric field assisted mining arises as a technique to extract species from soils using green electrolytes to help in the extraction of metals. The aim of this paper was to evaluate the effect of different types of biodegradable electrolytes, including the use of deep eutectic solvents, in the electromining process. Six experiments were conducted applying an electric field of 1.0 V cm − 1 , and all electrolytes were used at a concentration of 0.1 mol L − 1. The results showed that different electrolytes achieved different selectivities. The maximum efficiency using acetic acid resulted in 69.1% of Ce 4+ , citric acid removed 62.3% of La 3+ , and oxalic acid extracted 21.5% of La 3+. The electromining efficiencies using deep eutectic solvents presented minor results. Therefore, considering the biodegradability and selectivity of the organic acids used, electro-mining showed to be a promising eco-friendly alternative for preferential extraction of metal species from soils.
... EK transport processes have received increasing attention for their application for in situ soil and groundwater remediation due to their potential of providing considerable mass transfer rates also in low-permeability porous media where most of the in situ remediation approaches are not applicable or severely challenged. Electrokinetic remediation approaches have been proposed for different purposes, including the mobilization and removal of organic and inorganic contaminants (e.g., Kim et al., 2009;Jung et al., 2015;Lima et al., 2017;López-Vizcaíno et al., 2017a;López-Vizcaíno et al., 2017b;Mao et al., 2019;Masi et al., 2017a;Masi et al., 2017b;Reddy et al., 2003;Ribeiro and Mexia, 1997;Virkutyte et al., 2002), the delivery of charged reactants such as permanganate and persulfate to promote in situ chemical oxidation (e.g., Chowdhury et al., 2017a;Chowdhury et al., 2017b;Fan et al., 2016;Head et al., 2020;Hodges et al., 2013;Reynolds et al., 2008;Wu et al., 2012a;Wu et al., 2012b), and the delivery of amendments (i.e., substrates and microorganisms) to stimulate in situ biodegradation of organic contaminants (e.g., Gill et al., 2014Gill et al., , 2015Hyldegaard et al., 2019;Inglis et al., 2021;Mao et al., 2012;Ottosen et al., 2019;. Most literature studies on electrokinetic transport in porous media have focused on low-permeability matrices, where mass transfer is limited to diffusive processes (Jin et al., 2014;Yang et al., 2017) leading to persistent contamination (Parker et al., 2008). ...
The application of electrokinetic techniques in porous media has great potential to enhance mass transfer rates and, thus, to mobilize contaminants and effectively deliver reactants and amendments. However, the transport mechanisms induced by the application of an external electric field are complex and entail the coupling of physical, chemical and electrostatic processes. In this study we focus on electromigration and we provide experimental evidence of the impact of compound-specific properties, such as the aqueous diffusivity and the valence of charged species, on the macroscopic electrokinetic transport. We performed a series of multidimensional experiments considering the displacement of three different tracer plumes (i.e., permanganate, allura red and new coccine) in different background electrolyte solutions. The outcomes of the experiments clearly show that both the compound-specific diffusivity and the charge of the injected and resident ions impact the transport of the selected color tracer plumes, whose evolution was monitored with image analysis. The investigated experimental scenarios led to distinct plume behavior characterized by different mass distribution, average displacement velocities, longitudinal and lateral plume spreading, shape of the invading and receding fronts, as well as dilution of the injected solutes. A numerical simulator, based on the Nernst-Planck-Poisson equations and on aqueous speciation reactions in the pore water, allowed us to quantitatively interpret the experimental results, to capture the observed patterns of plume evolution, and to illuminate the coupling between the governing physico-chemical mechanisms and the controlling role of small scale compound-specific and electrostatic properties. Finally, the model was also extended to a typical configuration of in situ electrokinetic remediation of contaminated groundwater to show the impact of such mechanisms at larger scale.
... A time-dependent equation for the conservation of mass is used to describe transient the mass transport of species i and can be written as mentioned below (Kim et al., 2005), where R i ( molm −3 s −1 ) represents a volumetric net source or sink of ionic species (i) due to chemical reactions, viz. adsorption/ desorption, precipitation/ dissolution and aqueous reactions (Masi et al., 2017b). ...
Electrokinetic remediation technique is widely applied for the removal of heavy metal from contaminated soil, but the soil buffering capacity and fractionation of heavy metals mainly affect the cost and duration of the treatment. This study aims to treat heavy metal-contaminated sediments by electrokinetic remediation (EKR) technique by using various enhancing agents such as EDTA, [Formula: see text], HCI, [Formula: see text], acetic acid and citric acid for optimizing the cost and treatment duration. The optimum molar concentration of enhancing agent for treatment was estimated by batch experiments to maximize the dissolution of target heavy metals and reduce the dissolution of earth metals (Fe, Al and Ca) to maintain soil health. The EKR experiments were performed up to 15 days with the above enhancing agents to reduce the risk associated with heavy metals and the selection of enhancing agents based on removal efficiency was found to be in an order of EDTA > citric acid > acetic acid > [Formula: see text] > HCl [Formula: see text] [Formula: see text]. Also, a numerical model has been developed by incorporating main electrokinetic transport phenomena (electromigration and electroosmosis) and geochemical processes for the prediction of treatment duration and to scale up the EKR process. The model predicts well with experimental heavy metal removal with a MAPD of [Formula: see text] 2-18 %. The parametric study on electrode distance for full-scale EKR treatment was found in this study as [Formula: see text] 0.5 m.
... Considering four transport processes in the electro-kinetic system, namely, electro-migration, diffusion, electro-osmosis and advective flow under hydraulic gradients (Acar and Alshawabkeh, 1993;Shapiro and Probstein, 1993;Acar et al., 1995;Yeung et al., 1997), several numerical models have been developed for the predication of contaminant ion transport in electro-kinetic extraction and remediation (Choi and Lui, 1995;Alshawabkeh and Acar, 1996;Mattson et al., 2002;Kim et al., 2003Kim et al., , 2004Amrate et al., 2005;Al-Hamdan and Reddy, 2008;Paz-García et al., 2011;Masi et al., 2017). Meanwhile, Yeung and Mitchell (1993) have developed a general coupled flow theory for the electro-kinetic barrier on the basis of the formalism of non-equilibrium thermodynamics for analysis of the physics involved in the transport processes. ...
To prevent contaminant ions from spreading over the adjacent environment, it is of great interest to consider using electro-kinetic barriers to counteract contaminant transport through the combined effects of electro-osmosis and electro-migration. In this study, considering the bottom surface as a Cauchy boundary or a Drichlet boundary condition, two analytical solutions are proposed to predict the contaminant ion transport in the electro-kinetic barrier. Analytical solutions are verified against experimental data and numerical solutions from the previous literature. Subsequently, the proposed analytical solutions are used to investigate the effects of average applied voltage gradient, barrier thickness, diffusion coefficient, retardation factor and electro-osmotic conductivity on the transport of contaminant through the electro-kinetic barrier. The results show that the steady state base contaminant concentration decreases with the increase in average voltage gradient and barrier thickness. Additionally , the logarithm of the final stable base contaminant concentration changes linearly with the average applied voltage gradient and barrier thickness. For the contaminants with a high diffusion coefficient, a larger average voltage gradient should be applied to improve the electro-kinetic effects. Clayey soils with high sorption capacity and relatively high electro-osmotic conductivity are the suggested material choice for constructing compacted soil liners within the electro-kinetic barrier, due to distinct advantages in contaminant migration prevention. Finally, a dimensionless factor is introduced to uniformly describe influences on the base relative concentration, and it can be used as an important parameter for the design of an electro-kinetic barrier.
... Specifically, the application of the electrical field causes the oxidation of water at the surface of the anode, which causes a reduction in pH due to the production of H + ions. Hence, an acid front is electrogenerated at the anode region favoring the dissolution of the heavy metals migrated towards the cathode (Figueroa et al., 2016;Habibul et al., 2016;Masi et al., 2017). At the same time, an increase in the soil pH near the cathode side is derived from the furtherance of OH − created by the reduction of water at the cathode surface (Habibul et al., 2016). ...
Electrokinetic (EK) remediation is a promising technology for soil decontamination, although basic pH in the soil close to cathode has constrained EK effectiveness due to heavy metal precipitation. This study aimed to enhance copper removal from kaolinite soil by integrating EK with compost (C) as recyclable reactive filter media (RFM) for the first time. Compost placed near the cathode served as an adsorbent to bind copper ions while buffering the advancement of the alkaline front in soil. The total copper removal rate increased from 1.03% in EK to 45.65% in EK-100%C under an electric potential of 10 V. Further experiments conducted by using biochar (BC) and compost/biochar (C + BC) mixture RFM at different ratios showed total Cu removal efficiency decreasing as EK-100%C > EK-(10%BC + 90%C) > EK-(20%BC + 80%C) > EK-(30%BC + 70%C) > EK. The application of a constant electric current of 20.00 mA further enhanced copper removal to 84.09% in EK-100%C although did not show significant enhancement in EK-(BC + C). The compost RFM was regenerated by acid extraction and then reused twice, achieving a total removal of 74.11%. The findings demonstrated compost as a promising and reusable RFM for the efficient removal of copper in contaminated soil.
... The first method represents geochemical reactions in the same calculation code as the transport code and uses a retardation factor to model only the sorption process (Baraud et al. 1997;Haran et al. 1997;Malusis and Shackelford 2002;Park et al. 2003;. The second method couples the transport calculation code with a geochemical reaction calculation code such as Phreeqc (Mao et al. 2019;Masi et al. 2017Masi et al. , 2019Sprocati et al. 2019;Wu et al. 2012) or CHEMSPC (Al-Hamdan and Reddy 2008). This second method is complex because it requires the comprehension of all geochemical reactions. ...
This paper presents an experimental and numerical investigation of the electrokinetic propagation of acid or base fronts through clayey soil. The experimental study included measuring the electric current, the mass of water extracted, and the advance of the acid and base fronts in synthetic sediment composed of 25% kaolinite and 75% silt. The results show that the flux rate remains constant over time and that the medium acidifies with time. The numerical model is based on the transport equations and, by considering adsorption and auto-ionization of water, it describes the temporal evolution of pH in the specimen. The experimental evaluation of the adsorption isotherms allows us to calculate the retardation factor, which are 69 and 19.5 for the hydronium and hydroxyl ions, respectively. Given these retardation factors, the results of the numerical model are consistent with experimental results and show how the acid and base fronts evolve in a saturated porous medium. Numerical model results show that electromigration is the major contributor to the migration of ions in the porous medium during electrokinetic treatment.
... Masi et al. [6] developed a two-dimensional reactivetransport model for the cost optimization of electrokinetic treatment of contaminated dredged sediments. The model was applied to an EKR prototype plant [23] treating 150 m 3 of dredged sediments, and the model parameters were either calibrated using a lab-scale study (1.5 10 À3 m 3 of sediment) previously conducted for the same soil. ...
Most of electrokinetic remediation (EKR) reported researchs were performed on small-scale laboratory devices and less field-scale studies are available in literature. Understanding the scaling-up process is essential for the application of the EKR at field scale. This paper presents the results of laboratory experiments performed at two scales (involving 0.4 kg and 40 kg of dredged sediment) to evaluate the potential of EKR process on both organic and inorganic contaminants, and the scaling-up effect. Mixtures of eco-friendly enhancing additives were used: a biodegradable chelating agent (citric acid) combined with a nonionic surfactant (Tween 20) were promising candidates for the simultaneous EK remediation of a multi-contaminated harbor sediment. The values of energy consumption from the large scale tests showed that efficient decrease of pollutant concentrations and sustainable remediation could be achieved with moderate energy costs. Obtained results also indicated that better removals of Cr, PAHs and PCBs were achieved with the large-scale device using less energy and additives. However, the distribution of the pollutants in the specimen after the EK remediation indicated that the electric field did not totally control the migration process and that the interaction with likely heterogeneity and inertial effects reduced the EK effectiveness at the large scale. This scaling-up investigation allows considering EK tests at a larger scale (field installation) with adjusted parameters from a small-scale investigation.
... In the multi-species transport model proposed by Masi, Ceccarini, and Iannelli (2017), along with surface and aqueous speciation and clay mineral dissolution, the pHdependent buffering of H + ions were incorporated. The significance of pH buffering in soil electro-kinetic applications was previously reported in the studies of Sumbarda-Ramos et al. (2010). ...
Electrochemical treatment of soil, ECT is the technique of deploying an external electric potential for introducing stabilizing chemicals into the intended zone of soil improvement. The current flow modifies the basic soil-electrolyte chemistry, reflected mostly in the form of soil pH. In view of this, the manuscript reviews the alterations in clay fabric and ionic-species interactions instigated with the modifications in surface charge due to soil pH alterations. The derived consequences of system chemistry changes are discussed concerning its impact on the migration mechanisms during ECT viz. electro-osmosis and electro-migration. Though ECT is familiar to the research fraternity for decades, the attempts made hitherto for compiling the available literature on ECT are limited. Given this, by depicting the pioneering developments over time, the present study also provides a concise review of the research carried out in the area of ECT by highlighting the impact of electrochemical modifications upon the treatment efficiency.
The hydrophobic, hard-to-naturally-decompose compounds, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and pesticides, are categorized as persistent organic pollutants (POPs). POPs are toxic/hazardous and present serious risks to human health. Electrokinetic (EK) remediation is highly flexible and cost-effective, suitable for both in-situ and ex-situ applications. It effectively targets a wide range of contaminants, including metals and organic compounds, especially in low-permeability and low-hydraulic conductivity soils, where traditional methods are less effective. This technology is easy to install and can be combined with other strategies for enhanced remediation in complex soil environments. This paper underscores EK remediation as a promising method for addressing soil pollution caused by these organic pollutants, especially in low-permeability soil. The present review starts with the classification, toxicity effects, and source of POPs in the environment. Theoretical aspects and fundamentals of EK, including transport mechanisms and principles, are also reviewed. The theoretical underpinnings of effective factors are comprehensively explored, such as surface charge, zeta potential, pHpzc, and numerical modeling of transport fluxes. Moreover, a comprehensive examination is undertaken regarding the operation and design considerations of the EK process, encompassing factors like pH, electrode arrangement, electrolyte, and voltage. Subsequently, it is highlighted that EK has the potential to come in synergistically in contact with other remediation technologies to augment the POPs' degradation. Various enhancement techniques are also explored, including solvent extraction, chemical oxidation, bioremediation, and permeable reactive barriers to combine with EK. Each method is examined in terms of its advantages, limitations, recent developments, and ongoing research. Finally, the potential and challenges associated with enhanced EK methods combined with other techniques for the removal of POPs were reviewed.
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.
Electrolyte is one of the critical factors for the efficient and green implementation of electrokinetic remediation (EK). Modelling of enhanced EK in the complex heavy metal contaminated soils was designed. Enhanced electrokinetic remediation experiments with composite electrolyte (citric acid + calcium chloride) were conducted to investigate their influence on the removal of cadmium (Cd) and lead (Pb), depletion of soil cations, and electric energy consumption. The results indicated that the removal efficiency of lead and cadmium rose from 10.8% and 14% to 15.1% and 22.5%, at the same time, the power consumption increased from 140.4 to 402 Wh since calcium chloride was added to citric acid. With the extension of time, the highest removal of Pb and Cd were 58% and 99.2%, respectively, in the section of S1 where soil pH was 2.5. The dominant cations (detected) in the pore water of the soil were Ca²⁺, Mg²⁺, and Mn²⁺, which was more evident with the extension of time. Lead-cadmium transmission efficiency under the electric field was lowered because the ratio of the heavy metal ions to the total amount of cations was very low, especially with the addition of calcium chloride.
Dissolved organic matter (DOM) in the sediment matrix affects contaminant remediation through consumption of oxidants and binding with contaminants. Yet the change in DOM during remediation processes, particularly during electrokinetic remediation (EKR), remains under-investigated. In this work, we elucidated the fate of sediment DOM in EKR using multiple spectroscopic tools under abiotic and biotic conditions. We found that EKR led to significant electromigration of the alkaline-extractable DOM (AEOM) toward the anode, followed by the transformation of aromatics and the mineralization of polysaccharides. The AEOM remaining in the cathode (largely polysaccharides) were resistant to reductive transformation. There was little difference in the translocation and transformation of AEOM between the abiotic and biotic conditions, indicating the dominance of electrochemical processes when relatively high voltages were applied (1-2 V/cm). The water-extractable organic matter (WEOM), in contrast, showed an increase at both electrodes, which was likely attributable to pH-driven dissociations of humic substances and amino acid-type constituents at the cathode and the anode, respectively. Nitrogen migrated with the AEOM toward the anode, but phosphorus remained immobilized. Understanding the redistribution and transformation of DOM could inform studies on contaminant degradation, carbon and nutrient cycling, and sediment structural changes in EKR.
Electrokinetic remediation (EKR) is a promising remediation technology for low-permeability porous media contaminated with organic pollutants. To reveal the processes of nonlinear electroosmotic flow (EOF) and to elucidate the key factors controlling the efficiency of EKR, a process-based pore-scale numerical model was established. Nernst-Planck-Poisson Equation is coupled with Navier-Stokes Equation to describe the transport of species. 1-pK model is applied to evaluate time-varying zeta potential during EKR process. Totally 33 cases are simulated, which include the settings of 3 background concentrations (Cb) of ions in media, 3 pK values, 3 enhanced factor (d, corresponding to reciprocal of diameter of electrode well) values, and two different initial conditions. It is found that all of Cb, pK and d impact EOF and the removal efficiency of organic species. From this study, media with pK of 3.5 and 8.5 are more suitable for EKR with about 5-10 times faster remove rate than media with pK of 5.5. Nonlinear EOF, including decreasing EOF rate and reverse EOF, is more likely to occur in cases with pK of 5.5 and higher values of Cb and d, which can reduce efficiency of EKR by 2-4 times. Using an electrolyte with ion concentration closer to the original background concentrations of ions in soil can favour better removal efficiency.
Electrokinetic techniques have attracted considerable attention due to their competitive advantage in enhancing transport phenomena and remediation of low permeability soils. However, many types of low-permeability porous media, such as active clays like bentonite, present a double porosity structure that strongly affects mass transfer processes. In this work, we develop a modeling approach to simulate electrokinetic transport in double porosity media. The proposed model is based on the previously released code M4EKR and explicitly considers the transport due to electromigration as well the effects of the electrical diffuse layer in charged porous media. For this purpose, two modeling levels have been established: one associated with the soil macrostructure (bulk water) and the other with the microstructure (diffuse layer). At the latter level, the effect of the electrostatic interactions imposed by the negative charge of the clay particles is modeled with a Donnan approach. The comparison of the results between synthetic cases in single and double porosity systems shows the fundamental role of the microstructural modeling level. High enrichment of cations and, therefore, high cationic fluxes are produced in such microstructural level. These mechanisms significantly modify the extent of cation transport by electromigration and, thus, directly influence the transport of other species determined by the electrostatic interactions.
This work reviews the recent progress in physical and mathematical modelling of electrochemically assisted soil remediation processes and focuses especially on those treatments that aim to remove organic pollution. It introduces the importance of modelling and simulation, not only to understand, but also to manage more efficiently the remediation of polluted sites. Special emphasis is taken in the information generated in the last five to ten years, trying to shed light on how the understanding of these treatments has improved in this period. To do this, two different sections introduce the main challenges in mathematical and physical modelling, merging in a final section, that shows the expected future trends in electrochemically assisted soil remediation modelling. Regarding physical modelling, the size of the installation used in the study and the novelty of the application of the technology have been used as the key inputs for this review. Regarding mathematical modelling, the inputs used to conduct the review have been the typology of numerical code, the processes included in the model and the way in which the electrolyte is considered in the model. Final remarks highlight the importance that modeling will take in the near future to increase the technology readiness level of these treatments and to promote their massive application to the remediation of polluted sites.
Porewater is the primary carrier of sediment nitrogen and a crucial source of overlying water nitrogen; its separation thus is essential for restraining nitrogen release from sediment to overlying water. We developed a novel device using electrokinetic geosynthetics to drain porewater with nitrogen and restrain nitrogen release. A batch experiment lasted 1120 h (about 47 days) was conducted with 20 cm depth of overlying water under three conditions, i.e., undrained at 0 V/cm voltage gradient (control), drained at 0 V/cm, and drained at 0.5 V/cm. Under the pulsed direct current, once porewater drained, overlying water replenished sediment pore space and supplied porewater. Along with porewater drainage, sediment nitrogen concentration was reduced by 11%–30%, decreasing nitrogen release from sediment to overlying water from 83 mg/m² in the first 100 h to −95 mg/m² after about 600 h. Processes such as electroosmosis, electromigration, and redox reaction contributed to the restraint on nitrogen release. This research revealed the potentiality of applying electrokinetic geosynthetics to in-situ restraint on sediment nitrogen release in eutrophic waterbodies such as fishponds.
Contaminants with toxic concentrations in bottom marine sediments represent a serious threat to the environment and human health. Dredging of contaminated sediments with subsequent treatment off-site represents one way to remediate environmentally impacted areas. Among ex situ remediation approaches, stabilization/solidification (S/S) represents a key technology for the production of innovative materials for civil engineering, by treatment and reuse of dredged contaminated sediments. S/S involves the addition of binders and additives to a contaminated material in order to produce more chemically stable constituents and to improve the mechanical behavior of sediment. This chapter explores the main aspects of S/S application to contaminated marine sediments: characterization methods; examples of chemical–physical characterization output; main binders and additives used; examples of S/S application to sediments with only organic and mixed inorganic/organic contamination. Finally, the current lines of research will be outlined.
The effect of the electrokinetic process on the extraction of trapped water was evaluated in the Miduk copper mine’s tailings slurry. The effect of the dewatering process on copper removal was also studied. The initial electroosmotic permeability was determined using a one-dimensional cell. In the next step, the effect of voltage and polarity reversal was evaluated for three scenarios: T1 (2 V/cm), T2 (2 V/cm), and T3 (1 V/cm). In addition, the polarity reversal method was applied to T2 and T3. To compare the amount of water extraction and energy consumption, three indices were considered and calculated: index of dry tone, water extraction, and normal water extraction. Moreover, the effect of electrokinetics on the final moisture content, physicochemical variation, and chemical composition were investigated. The results indicated that the calculated electroosmotic permeability ranged from 1.23 × 10–9 to 1.36 × 10–9 m2/V·S, which is acceptable for electrokinetic experiments. The water extraction experiments showed average flow rates of 1.68, 1.84, and 1.73 mL/h in T1, T2, and T3, respectively. The maximum amount of water extracted in T2 was ≈ 6% more than in T3. However, considering electric energy consumption, the highest efficiency of water extraction was achieved by applying a voltage gradient of 1 V/cm for 24 h. Consequently, polarity reversal affected the water extraction efficiency by reducing energy consumption. Furthermore, moisture reduction due to the dewatering process decreased the pH variation and copper release and transport. Roughly 35% of the extractable copper was removed on the anode side of T2, which was determined to be the most efficient for remediation.
Electrokinetic remediation (EKR) is a remediation method that can effectively remove heavy metals from soil. Traditional EKR uses an external power source to consume electrical energy, in the present study, through the experiment of different electrode exchange frequency to remediate Cr contaminated soil, the effect of polarity exchange electric remediation and the influencing factors of Cr(VI) to Cr(III) conversion are discussed. Five experiments were conducted using DC power supply with conventional EKR (T1), DC power supply with polarity exchange EKR (T2, T3) and solar energy with EKR (T4, T5). The results showed that the T1 experiment had the maximum removal rate of total Cr (42.03 %) and Cr(VI) (85.50 %) but had an adverse effect on soil pH. T2, which exchanged polarity every 24 h, and T3, which exchanged polarity after 84 h, had smaller adverse effects on soil pH, but the removal efficiency was reduced due to repeated migration. The reduction of Cr(VI) to Cr(III) occurred during the EKR processes, the content of Cr(III) increased greatly. Furthermore, few differences were observed between DC power and solar power on the EKR experiments. Therefore, the combination of electrokinetics and solar energy is a feasible method for Cr-contaminated soil.
Pore scale modelling in porous media is a field of research where interfaces such as the fluid – mineral boundary are explicitly represented and avoids volume-averaged properties such as porosity, permeability and reaction rates as required in a continuum scale approach. Setting up, running and extracting data from a pore scale model can be complex involving multiple software packages and coding for each problem. A new implementation of a multicomponent reactive-transport model at pore scale is presented using a java interface combining COMSOL and PhreeqC (iCP). The new model is tested using benchmark problems recently published and by simulating a laboratory experiment. The main advantages of this model are the implementation of the aqueous speciation, easy incorporation of different reaction mechanisms and rate laws, parallelization of geochemical calculations and a user-friendly interface. The iCP results agree with the benchmark and experimental results very well confirming its application to reactive-transport problems with dynamic pore geometry evolution. The importance of aqueous speciation and the chosen rate constants are demonstrated for calcite dissolution. Given the model transfers data between two codes and computes the aqueous speciation for every time step, the model is computationally expensive. Its advantages are the user-friendliness and its accuracy in computing local reaction rates and fluid composition.
In this chapter, the importance of mathematical models to analyze the electrokinetic remediation process is highlighted. Due to the amount of coupled physicochemical, electrochemical, and electrokinetic phenomena that take place in this soil treatment, it is necessary to develop conceptual models and numerical tools that, combined with experimental information, improve the global understanding of these processes.
Pollution of sediments in harbors originates from several sources due to both past and present human and industrial activities. Pollutions often have a complex composition of a wide variety of organic and inorganic pollutants. When remedial actions include dredging, there is a need to manage and possibly treat sediments. Electrokinetic remediation (EKR) offers the advantage of separating pollutants from sediment, thereby increasing the reuse options for sediments (non‐polluted) and pollutants (where desirable). This chapter reviews EKR developments for single pollutants and simultaneous removal of several pollutants. Methods to enhance EKR fall into the categories of physical and chemical treatments, e.g. applying different setups or adding surfactants. The advantages and limitations of each enhancement method in terms of environmental benefits and efficiency are discussed. At the end of the chapter, a short discussion of scale‐up challenges and further development of EKR is presented.
Electrokinetic soil remediation (EKR) is one of the most promising techniques for decontamination of low permeability soils, in which the most classical techniques have been found to be less efficient. However, its practical application on a real scale has been rather limited since the phenomena involved in these processes are very complex. For this reason, it is essential to use numerical models that allow us to know the main trends in the behaviour of soils and natural waters subjected to EKR processes. This study presents the numerical model M4EKR (Multiphysics for ElectroKinetic Remediation). The M4EKR module is a reactive transport model for partially saturated soils that allows reproducing the transport of species due to electroosmosis, electromigration, diffusion and advective flow. The model was completely implemented in COMSOL Multiphysics, a partial differential equation solver. The system of differential and algebraic equations to solve the chemical and transport problem was fully defined by the authors, and they were solved by the M4EKR module in COMSOL (monolithic approach). The scope of the model is illustrated by simulating an EKR process of a natural soil and porewater contaminated with a polar pesticide: 2,4-Dichlorophenoxyacetic acid. For simplicity, the M4EKR version used in this study does not include gas transport, it does not consider the deformability of the soil and it is assumed the processes occur under isothermal conditions.
Electroosmotic permeability is a soil parameter that is fundamental for designing and implementing environmental remediation, dewatering, or soil stabilization processes. It presents a narrow range of variation compared to hydraulic permeability, so that in many occasions no experimental tests are carried out to determine it. Traditionally, the Helmholtz-Smoluchowski equation has been used to estimate electroosmotic permeability, despite having several drawbacks that make it difficult to apply, as it was deduced for large capillaries (unusual in clays) and its dependence on the zeta potential of the soil, difficult to measure and dependent on a wide variety of factors. The present work describes a semi-empirical formulation that allows the estimation of the electroosmotic permeability as a function of the specific surface area and the total void ratio and is based on the Donnan equilibrium and the ion hydration models. The parameters for modelling are calibrated and validated with experimental data, obtaining better results than those given by the Helmholtz-Smoluchowski model. In addition, the expected trends in the behavior of clays that can be found in the literature are obtained in a quantitative and consistent manner for different types of clays with variable water content and pore water salinity.
Electrokinetic (EK) has been used in the removal of metal ions from contaminated soil. This study focused on integrating the EK technique with different reactive filter media (RFM) of activated carbon (AC) and biochar (BC) for the first time without adding chemicals to facilitate the removal of copper ions from the contaminated kaolinite soil. Tests based on EK, EK coupled with AC (EK-AC), and EK combined with BC (EK-BC) were performed under an electric potential of 10 V, and the overall removal efficiency of copper ions decreased as EK-BC > EK-AC > EK. The results show that 27% of copper in the soil was captured by BC, compared with only 10% by AC. Additional EK-BC test performed under a constant current (20 mA) revealed that the acid front swept across the soil, resulting in 70.6–95.0% copper removal from soil sections 4 to 1 close to the anode region with more copper accumulation in section 5. Similar to the EK-BC test under a fixed voltage, 26% of copper in the soil was captured by BC during EK-BC treatment under a constant current although with a higher energy consumption. Moreover, RFM was regenerated by flushing with an acid solution, achieving 99.3% of copper recovery in BC and 78.4% in AC. Although the permeability of AC-RFM was higher than that of BC-RFM, copper contaminant was more easily leached out from the BC-RFM. The findings demonstrated the feasibility of contaminant entrapment in BC-RFM and recovery by acid leaching, with potential for sustainable soil remediation.
The potential of electrokinetic remediation technologies (EKR) for the removal of different contaminants from subsurface porous media has been increasingly recognized. Despite electrokinetic applications have shown promising results, quantitative understanding of such systems is still challenging due to the complex interplay between physical transport processes, electrostatic interactions, and geochemical reactions.
In this study, we perform a model-based analysis of electrokinetic transport in saturated porous media. We investigate the effects of: (i) Coulombic interactions between ions in the system mobilized by electromigration, (ii) reaction kinetics on the overall removal efficiency of a non-charged organic contaminant, and (iii) dimensionality and different electrode configurations. The results show that such effects play a major role on the performance of electrokinetic systems. The simulations illuminate the importance of microscopic processes, such as electrostatic interactions and ion-specific diffusivities, and their non-intuitive macroscopic impact on the delivery of charged amendments and on the efficiency of contaminant removal. The insights of this study are valuable to improve and optimize the design and the operational strategies of electrokinetic remediation systems.
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.
In recent years, electrokinetic (EK) remediation method has been widely considered to remove metal pollutants from contaminated dredged sediments. Chelating agents are used as electrolyte solutions to increase metal mobility. This study aims to investigate heavy metal (HM) (As, Cd, Cr, Cu, Ni, Pb and Zn) mobility by assessing the effect of different chelating agents (ethylenediaminetetraacetic acid (EDTA), ethylenediaminedisuccinic acid (EDDS), nitrilotriacetic acid (NTA) or citric acid (CA)) in enhancing EK remediation efficiency. The results show that, for the same concentration (0.1 mol L−1), EDTA is more suitable to enhance removal of Ni (52.8 %), Pb (60.1 %) and Zn (34.9 %). EDDS provides effectiveness to increase Cu removal efficiency (52 %), while EDTA and EDDS have a similar enhancement removal effect on As EK remediation (30.5∼31.3 %). CA is more suitable to enhance Cd removal (40.2 %). Similar Cr removal efficiency was provided by EK remediation tests (35.6∼43.5 %). In the migration of metal–chelate complexes being directed towards the anode, metals are accumulated in the middle sections of the sediment matrix for the tests performed with EDTA, NTA and CA. But, low accumulation of metal contamination in the sediment was observed in the test using EDDS.
The suitability of electrokinetic remediation for removing heavy metals from dredged marine sediments with high acid buffering capacity was investigated. Laboratory-scale electrokinetic remediation experiments were carried out by applying two different voltage gradients to the sediment (0.5 and 0.8 V/cm) while circulating water or two different chelating agents at the electrode compartments. Tap water, 0.1 M citric acid and 0.1 M ethylenediaminetetraacetic acid (EDTA) solutions were used respectively. The investigated metals were Zn, Pb, V, Ni and Cu. In the unenhanced experiment, the acid front could not propagate due to the high acid buffering capacity of the sediments; the production of OH− ions at the cathode resulted in a high-pH environment causing the precipitation of CaCO3 and metal hydroxides. The use of citric acid prevented the formation of precipitates, but solubilisation and mobilisation of metal species were not sufficiently achieved. Metal removal was relevant when EDTA was used as the conditioning agent, and the electric potential was raised up to 0.8 V/cm. EDTA led to the formation of negatively charged complexes with metals which migrated towards the anode compartment by electromigration. This result shows that metal removal from sediments with high acid buffering capacity may be achieved by enhancing the electrokinetic process by EDTA addition when the acidification of the medium is not economically and/or environmentally sustainable.
In this chapter, we present a model for the reactive-transport of chemical species through partially saturated porous media for electrokinetic remediation processes. A generalized theoretical model is presented, easily adaptable to specific remediation setups, target contaminants and supporting matrices; and we give detailed guidelines for the implementation of tailor-made numerical methods for the computer-aided solution. The model is subdivided into two coupled modules: one for reactive-transport, numerically solved by means of a non-linear finite element method; and another one for chemical equilibrium, solved using an enhanced Newton–Raphson method.
The capabilities of the model are illustrated through an example: the remediation of a lead-contaminated soil in a horizontal column using an acetic acid enhanced technique, which buffers the alkaline front and acts as an extracting agent for lead. Simulation results are presented and compared with experimental results, showing the strong prediction capability of the proposed model.
Understanding the possible pH-buffering processes is of maximum importance for risk assessment and remediation feasibility studies of heavy-metal contaminated soils. This paper presents the results about the effect of the buffering capacity of a polluted soil, rich in carbonates, on the pH and on the leaching evolution of its main contaminant (lead) when a weak acid (acetic acid) or a strong one (nitric acid) are slowly added.
In both cases, the behavior of lead dissolution could be predicted using available (scientifically verified freeware) models assuming equilibrium between the solid and the aqueous phase. However, the experimental results indicate that the dissolution of calcium and magnesium carbonates is kinetically controlled. These kinetic limitations affect the overall behavior, and should be considered to understand also the response of the metals under local equilibrium.
The well-known BCR sequential extraction procedure was used before- and after-treatment, to fractionate the lead concentration in the soil according to its mobility. The BCR results were also in agreement with the predictions of the equilibrium model. This agreement allows new insights about the information that could be derived from the BCR fractionation analysis.
Zero valent iron nanoparticles (nZVI) are considered very promising for the remediation of contaminated soils and groundwaters. However, an important issue related to their limited mobility remains unsolved. Direct current can be used to enhance the nanoparticles transport, based on the same principles of electrokinetic remediation. In this work, a generalized physicochemical model was developed and solved numerically to describe the nZVI transport through porous media under electric field, and with different electrolytes (with different ionic strengths). The model consists of the Nernst–Planck coupled system of equations, which accounts for the mass balance of ionic species in a fluid medium, when both the diffusion and electromigration of the ions are considered. The diffusion and electrophoretic transport of the negatively charged nZVI particles were also considered in the system. The contribution of electroosmotic flow to the overall mass transport was included in the model for all cases. The nZVI effective mobility values in the porous medium are very low (10−7-10−4 cm2 V−1 s−1), due to the counterbalance between the positive electroosmotic flow and the electrophoretic transport of the negatively charged nanoparticles. The higher the nZVI concentration is in the matrix, the higher the aggregation; therefore, low concentration of nZVI suspensions must be used for successful field application.
Recent studies suggest that diffusion may be an important, if not dominant, mechanism of contaminant transport through waste containment barriers. This paper represents the first of two papers pertaining to the measurement of diffusion coefficients of inorganic chemicals diffusing in saturated soil. In this paper, both steady-state and transient equations describing the diffusive transport of inorganic chemicals are presented. Several factors affecting diffusion coefficients are identified. A method for measuring diffusion coefficients for compacted clay soil is described. The definition for the diffusion coefficient for diffusion in soil (known as the effective diffusion coefficient, D*) is shown to vary widely. In general, variations in the definition of D* result from consideration of the different factors that influence diffusion of solutes in soil and the different ways of including the volumetric water content in the governing equations. As a result of the variation in the definition of D*, errors in interpretation and comparison of D* values can result if the appropriate definition for D* is not used.
A numerical model was formulated to simulate cadmium (Cd) transport under an electric field using one-dimensional diffusion-advection equations describing the contaminant transport driven by chemical and electrical gradients in kaolinite clay. The numerical model includedcomplex physicochemical factors affecting the transport phenomena, such as soil pH value, zeta potential, aqueous phase reaction, adsorption, and precipitation. One-dimensional finite-difference computer models successfully predicted meaningful values for soil pH profiles and Cd concentration profiles. To verify the results of the proposed model by comparing them with experimental results, two different types of laboratory electrokinetic tests, unenhanced and enhanced tests, were conducted. The numerical and the experimental results showed good agreement. In addition, those results indicate that soil pH is the most important factor in governing the dissolution and/or desorption of Cd in the soil system under electrical fields. The removal efficiency of Cd in the unenhanced test was low (15.6%) due to a high accumulation in the region near the cathode. On the contrary, the cadmium concentration profile of the enhanced test showed a different pattern, and most of the residual concentrations appeared below the initial level at each local point within the soil cell after processing. The removal efficiency of the enhanced test was much higher (42.7%) than that of the unenhanced test, resulting from the prevention of hydroxide precipitation near the cathode using the acidic catholyte. Consequently, the result implies that the enhancement schemes such as conditioning of catholyte should be required to increase the effectiveness of the electrokinetic technology in removing metal contaminants from soils.
The driving mechanisms for flux occur at interface between the soil particles and solution during electrokinetic soil remediation
and the nature of this interface affects the electrokinetic response of the system. The pH-dependent adsorption of heavy metal
contaminants by kaolin and the sensitivity of kaolin zeta potential to the aqueous phase properties are two important aspects
that complicate the metal movement during electrokinteic soil remediation. This paper addresses these aspects and presents
an electrostatic adsorption model that describes the behavior of kaolin surface for Cr(VI), Cr(III), Ni(II), and Cd(II) under
various chemical conditions. This study showed that the aqueous properties: pH, ionic strength and the presence of the heavy
metals Cr(VI), Cr(III), Ni(II), and Cd(II) in the system affect the zeta potential of kaolin surface. The zeta potential of
kaolin shifts to a more negative value if the system pH increases. However, it shifts to a more positive value if the system
ionic strength or metal concentration increases. It was found that the amount of the heavy metal adsorbed by kaolin has a
pronounced sensitivity to the pH. As a result of the adsorption modeling, the constant capacitance protonation-dissociation
intrinsic constants of kaolin are: pK
+int=-3.8±0.5_{+}^{{\mathop{\rm int}}}=-3.8\pm 0.5
and
pK-int=9.4±0.5pK_{-}^{{\mathop{\rm int}}}=9.4\pm 0.5
. The ion-kaolin surface complexation constants (pK
int^{{\rm int}}
) of Cr(VI), Cr(III), Ni(II), and Cd(II) are: −12.5± 0.5, −5.0± 1.0, 2.6± 0.6, and 3.3± 1.0, respectively.
We propose to develop a mathematical model for the electrokinetic remediation of contaminated soil. We assume that the contaminants are mostly heavy metals, water is in excess, the dissociation-association of water into hydrogen and hydroxyl ions is rapid, and that electroosmosis is insignificant when compared to electromigration as a transport mechanism. Steady-state solutions for the model are derived and results of the numerical simulations are given to show that heavy metals in the soil are removed by this method in the long run.
A generalized model applicable to soils contaminated with multiple species under enhanced boundary conditions during treatment by electric fields is presented. The partial differential equations describing species transport are developed by applying the law of mass conservation to their fluxes. Transport, due to migration, advection and diffusion, of each aqueous component and complex species are combined to produce one partial differential equation that describes transport of the total analytical concentrations of component species which are the primary dependent variables. This transport couples with geochemical reactions such as aqueous equilibrium, sorption, precipitation and dissolution. The enhanced model is used to simulate electrokinetic cleanup of lead and copper contaminants at an Army Firing Range. Acid enhancement is achieved by the use of adipic acid to neutralize the basic front produced for the cathode electrochemical reaction. The model is able to simulate enhanced application of the process by modifying the boundary conditions. The model showed that kinetics of geochemical reactions, such as metals dissolution/leaching and redox reactions, may be significant for realistic prediction of enhanced electrokinetic extraction of metals in real-world applications.
Two types of modeling approaches are illustrated for describing inorganic contaminant adsorption in aqueous environments: (a) the component additivity approach and (b) the generalized composite approach. Each approach is applied to simulate Zn2+ adsorption by a well-characterized sediment collected from an aquifer at Cape Cod, MA. Zn2+ adsorption by the sediment was studied in laboratory batch experiments with a range of pH and Zn(II) concentrations selected to encompass conditions observed in the aquifer. In the generalized composite approach, one- and two-site surface complexation model parameters were calibrated with the experimental data using FITEQL. The pH dependence of Zn2+ adsorption was simulated without explicit representation of electrostatic energy terms. Surface acidity constants and ion pair formation by major electrolyte ions were also not required in the model, thereby minimizing the number of fitted parameters. Predictions of Zn2+ adsorption with the component additivity modeling approach did not simulate the experimental data adequately without manipulation of surface area or site density parameter values. To apply the component additivity approach to environmental sorbents, further research is needed to better characterize the composition of sediment surface coatings. The generalized composite modeling approach requires less information and can be viewed as more practical for application within solute transport models. With only three adjustable parameters, this approach could simulate Zn2+ adsorption over a range of chemical conditions that would cause several orders of magnitude variation in the distribution coefficient (Kd) for Zn2+ within the aquifer.
Electrochemical remediation is a promising remediation technology for soils contaminated with inorganic, organic, and mixed contaminants. A direct-current electric field is imposed on the contaminated soil to extract the contaminants by the combined mechanisms of electroosmosis, electromigration, and/or electrophoresis. The technology is particularly effective in fine-grained soils of low hydraulic conductivity and large specific surface area. However, the effectiveness of the technology may be diminished by sorption of contaminants on soil particle surfaces and various effects induced by the hydrogen ions and hydroxide ions generated at the electrodes. Various enhancement techniques have been developed to tackle these diminishing effects. A comprehensive review of these techniques is given in this paper with a view to providing useful information to researchers and practitioners in this field.
Heavy metal contamination has become a worldwide problem through disturbing the normal functions of rivers and lakes. Sediment, as the largest storage and resources of heavy metal, plays a rather important role in metal transformations. This paper provides a review on the geochemical forms, affecting factors and remediation technologies of heavy metal in sediment. The in situ remediation of sediment aims at increasing the stabilization of some metals such as the mobile and the exchangeable fractions; whereas, the ex situ remediation mainly aims at removing those potentially mobile metals, such as the Mn-oxides and the organic matter (OM) fraction. The pH and OM can directly change metals distribution in sediment; however oxidation-reduction potential (ORP), mainly through changing the pH values, indirectly alters metals distribution. Mainly ascribed to their simple operation mode, low costs and fast remediation effects, in situ remediation technologies, especially being fit for slight pollution sediment, are applied widely. However, for avoiding metal secondary pollution from sediment release, ex situ remediation should be the hot point in future research.
Objective:
This study aimed to describe the "fat brook" (FB) in the popliteal fossa of a cadaver and to evaluate its clinical significance.
Methods:
Ten fresh cadaveric knees underwent magnetic resonance imaging and histologic analyses. In addition, magnetic resonance imaging images from 321 patients (108 men, 213 women; mean age, 49.8 years; age range, 5-92 years) were retrospectively reviewed. Two radiologists independently determined the presence/absence of the FB and internal derangement of the knee.
Results:
The FB was present in all cadaveric specimens without synovial lining or joint communication. In the clinical study, the prevalence of FB was 97.8% (314/321). The FB was associated with effusion (P = 0.001) and tear of the medial meniscus (P = 0.022). There was no significant association between prevalence of FB and age or other structures. Determining FB and internal derangement of the knee had excellent interobserver agreement (concordance correlation coefficient = 0.966, 0.834-1.000).
Conclusions:
The FB might be a part of the superficial layer of superficial fascia and not be mistaken for a fat fracture or Morel-Lavallée lesion.
Background
Electrokinetic remediation of fine-grained soils polluted by heavy metals is affected by the equilibria between dissolved and sorbed ions, pH can also influence the surface equilibria. Geochemical models which describe the behaviour of different surface sites and their interactions with heavy metal ions have to be considered for a good mathematical representation of an electrokinetic process.ResultsCd2+ and Pb2+ sorption isotherms were performed at different pH values using kaolinite as sorbent. Electrokinetic tests were performed with samples of kaolinite spiked with Cd2+ and Pb2+ at 0.7 and 2 V cm−1: the concentration profiles have been experimentally obtained at different treatment times. The pH in the compartments was conditioned by acetate buffer. On the basis of the experimental results, a numerical model was formulated to simulate ion transport under electric field combining the diffusion–advection equations with a two sites geochemical model taking into account for ion exchange and surface complexation of the metal ions, as well as the protonation-deprotonation reactions of the surface sites.Conclusions
The process was effective for cadmium removal, while a strong dependence on pH was observed for lead removal. The model allowed predicting time/space profiles in solid and liquid phase: a good agreement with the experimental data was obtained under all the adopted conditions.
We discuss the use of the commercial finite element software COMSOL Multiphysics® for electrochemical analysis. Practical considerations relevant to finite element modelling are highlighted. A review of contemporary applications of this software is supplied; the subjects concerned reveal the particular suitability of general-purpose finite element methods for non-standard geometries, complex reaction chemistry, hydrodynamic electrochemistry, and rapid verification of standard results. Keywords: Simulation, Finite element, COMSOL, Voltammetry, Electroanalysis
An electrokinetic decontamination process has been modeled to investigate the feasibility of using electrokinetic soil remediation technology to remove 137Cs and 90Sr from the soil. 100 V DC current is applied to a 3-meter-long soil column by electrodes connected to the soil. The prediction results have shown that the efficiency of the electrokinetic treatment depends on the sorption and diffusion parameters. High sorption and slow diffusion will demand long treatment time. If the soil to be treated has similar sorption and diffusion properties as bentonite, and when the soil is flushed with saline water that leads to less sorption, both 137Cs and 90Sr may be cleaned by the electrokinetic process within a few months.
Electrokinetic removal of Cu, Zn, Cd, Cr, Pb and Ni from contaminated dredged material has been carried out on thirteen tests evaluating the effectiveness of different enhancing solutions of different nature (distilled water, citric acid, acetic acid, humic acid and EDTA). Several experimental variables were analyzed (type of enhancing solution, concentration, zeta potential, initial and final pH of the anolyte and catholyte, constants of chelation equilibriums (log β) and precipitation (pKs), ionic mobility and charge density passed) and the main mechanisms of their action were critically analyzed. It was derived that there is not just one enhancing solution that is the best universal one for all the metals in the sediment and that modeling of the decontamination process should incorporate the particularities of the different species. However, as a generalization, it could be said that, apart from the charge density passed, the pH of the cathodic solution, quelation ability and the zeta potential seems to appear as the most relevant factors in the efficiency of the process.
The dominant transport process for removing charged species from soils by electric fields is electromigration. In the case of heavy metals, the polarity and magnitude of the charge depends on the pH. Positive ions are generally stable at low pH and negatively charged complexes dominate at high pH. The transport is further complicated by the strong dependence of solubility on pH, with many heavy metals being virtually insoluble in moderately alkaline conditions. It was found experimentally that under certain conditions, strong pH gradienis can develop in the soil trapping the metals by a process of isoelectric focusing, A numerical model of the transport and electrochemical processes was extended for the first time to incorporate complexation and precipitation reactions and was found to closely reproduce the experimental findings. The model demonstrates the role played by background ions and electroneutrality in governing the distribution of species, and how the concomitant variations in the electric field result in the virtual cessation of the transport process. The model confirmed that the focusing effect can be eliminated and high metal removal efficiencies achieved simply by washing the cathode.
Feasibility and cost‐effectiveness of electrokinetic soil processing in the remediation of contaminated clayey soils has been demonstrated by bench‐scale studies and limited pilot‐scale studies. A review of the fundamentals controlling the process demonstrates that contaminant transport and removal by electrokinetics are dependent on several factors including electrode reactions, pH and surface chemistry of the soil, equilibrium chemistry of the aqueous system, electrochemical properties of the contaminants and hydrological properties of the soil medium. Chemical, hydraulic and electrical fluxes in electrokinetic processing are formalized. A theoretical model describing reactive solute transport is developed in a coupled system of differential/algebraic equations. Considerations for solutions are discussed. An important factor in the removal of contaminants is the transport of the acid front developed by the anode reaction. The basic equation describing the pH distribution resulting from electrode reactions is simplified and solved under specific assumptions pertaining to the boundary conditions. The results reasonably predicted the pH distribution across specimens tested under one‐dimensional conditions.
Electrokinetic remediation is effective for extraction of contaminants, such as heavy metals, from fine grained deposits. However, several challenges have led to limited field implementation in the US over the past two decades. The reasons include lower risk of exposure to adsorbed contaminants through groundwater, cost of treatment, technical challenges in difficult soil, and complex geochemical conditions, and the need for acidification to induce desorption. To address practical implementation, a Reactive Transference Factor can be used to identify conditions that are favorable for electrokinetic remediation. The factor measures the reactive transport rates relative to the electrical conductivity of the soil. Other opportunities are identified for implementation of electric-based methods for remediation beyond heavy metal extraction. These include enhancement of bioremediation, which is favorable for remediation of hot spots or source areas, and the development of reactive electrochemical barriers, particularly for the treatment of contaminated groundwater plumes.
A numerical model has been developed to describe the transport and reaction processes in a porous medium in an electrical field. The model discretizes the one-dimensional porous medium by a number of compartments. The governing equations are formulated by material balance over each compartment. The model describes transport processes of advection, dispersion, ionic migration and electroosmosis. Various reactions such as aqueous complexation, precipitation/dissolution, and electrochemical reactions are treated by kinetic approaches. For fast fluid-phase reactions such as complexation, local equilibrium assumption may be applied in the model, which reduces the number of primary components and hence the number of governing equations. The sorption processes occurring at the surfaces of the porous medium are at the moment treated only by a single-component linear isotherm. Numerical solution to the model gives concentration and electrical potential distributions in the porous medium at different times and gives as well the electrical current history. Modelling results for three sample cases are reported to demonstrate the application of the model. In the first sample case, the removal of copper from sand by an electrical field is simulated. The second case concerns an electrokinetic soil remediation process with cathode rinsing. The third case models the selective filtering function of the ion exchange membranes.
A mathematical model is presented for multicomponent species transport under coupled hydraulic, electric, and chemical potential differences. Mass balance of species and pore fluid together with charge balance across the medium result in a set of differential equations. Sorption, aqueous phase, and precipitation reactions are accounted by a set of algebraic equations. Instantaneous chemical equilibrium conditions are assumed. Transport of H{sup +}, OHâ», Pb{sup 2+}, NOââ», the associated chemical reactions, electric potential, and pore pressure distribution across the electrodes in electrokinetic remediation are modeled. Model predictions of acid transport, lead transport, and pore pressure distribution display very good agreement with the pilot-scale test results validating the formalisms offered for multicomponent transport of reactive species under an electric field. The model also bridges the gap between the electrochemistry and mechanics in electroosmotic consolidation of soils.
Electroosmosis in a copper-contaminated kaolinite was highly sensitive to chemical treatment schemes designed to remove the contamination. Nonuniform profiles of electric field intensity and pH as well as negative pore-water pressure develop during sustained electrokinetic treatment of clays. These nonlinearities and nonuniform pore-water pressures cannot be adequately described by classical analysis. Classical analysis is based on assumptions of a uniform and constant electroosmotic permeability coefficient, for instance. An extended capillary model which includes nonuniform contributions to electroosmosis and pore pressures that vary with space and time, is developed and compared with experimental findings. Subtle changes in initial and boundary conditions of the system chemistry have a very large effect on electroosmosis in soils. For instance, acid addition at the cathode reservoir may cause reversal of the direction of electroosmotic flow. Other species, such as the citrate, may form stable complexes with copper ions, thus reducing the impact of copper on the zeta potential of the clay. The model is used to simulate these effects.
Explica los principios del transporte de especies y remocion de metales en suelos bajo un campo electrico. Presenta los resultados de algunos recientes estudios sobre tratamiento electrocinetico y discute la implicacion de la implementacion de esta tecnica
Electrokinetic remediation technology is one of the developing technologies that offers great promise for the cleanup of soils contaminated with heavy metals. However, the performance of an electrokinetic remediation system depends on the interaction of a complex set of interrelated system variables and parameters. Many of these interactions were addressed in this study by incorporating geochemical reactions into electrokinetic remediation modeling. A one-dimensional transport model was developed to predict the transport and speciation of heavy metals chromium, nickel, and cadmium in soil during electrokinetic remediation as a function of time and space. The model incorporates: 1 pH-dependent adsorption of contaminants to the soil surface; 2 sensitivity of soil surface potential and electroosmotic flow to the pore water properties; and 3 synergistic effects of multiple chemical species on electrokinetic remediation. The model considers that: 1 Electrical potential in the soil is constant with time; 2 surface complexation reactions are applicable in the highly concentrated clay suspensions; 3 the effect of temperature is negligible; and 4 dissolution of soil constituents is negligible. The predicted pH profiles, electroosmotic flow, and transport of chromium, nickel, and cadmium in kaolin soil during electrokinetic remedia-tion were found to reasonably agree with the bench-scale electrokinetic experimental results. The predicted contaminant speciation and distribution aqueous, adsorbed, and precipitated allow for an understanding of the transport processes and chemical reactions that control electrokinetic remediation.
Dredged sediments contaminated by heavy metals and PAHs were subjected to both unenhanced and enhanced electrokinetic remediation under different operating conditions, obtained by varying the applied voltage and the type of conditioning agent used at the electrode compartments in individual experiments. While metals were not appreciably mobilized as a result of the unenhanced process, metal removal was found to be significantly improved when both the anodic and cathodic reservoirs were conditioned with the chelating agent EDTA, with removal yields ranging from 28% to 84% depending on the contaminant concerned. As for the effect on organic contaminants, under the conditions tested the electrokinetic treatment displayed a poor removal capacity towards PAHs, even when a surfactant (Tween 80) was used to promote contaminant mobilization, indicating the need for further investigation on this issue. Further research on organics removal from this type of materials through electrokinetic remediation is thus required. Furthermore, a number of technical and environmental issues will also require a careful evaluation with a view to full-scale implementation of electrokinetic sediment remediation. These include controlling side effects during the treatment (such as anodic precipitation, oxidation of the conditioning agent, and evolution of toxic gases), as well as evaluating the potential ecotoxicological effects of the chemical agents used.
Sediments dewatering is frequently necessary after dredging to remediate and treat contaminants. Methods include draining of the water in lagoons with or without coagulants and flocculants, or using presses or centrifuges. Treatment methods are similar to those used for soil and include pretreatment, physical separation, thermal processes, biological decontamination, stabilization/solidification and washing. However, compared to soil treatment, few remediation techniques have been commercially used for sediments. In this paper, a review of the methods that have been used and an evaluation of developed and developing technologies is made. Sequential extraction technique can be a useful tool for determining metal speciation before and after washing. Solidification/stabilization techniques are successful but significant monitoring is required, since the solidification process can be reversible. In addition, the presence of organics can reduce treatment efficiency. Vitrification is applicable for sediments but expensive. Only if a useful glass product can be sold will this process be economically viable. Thermal processes are only applicable for removal of volatile metals, such as mercury and costs are high. Biological processes are under development and have the potential to be low cost. Since few low cost metal treatment processes for sediments are available, there exists significant demand for further development. Pretreatment may be one of the methods that can reduce costs by reducing the volumes of sediments that need to be treated.
An electromigration transport model for non-reactive ion transport in unsaturated soil was developed and tested against laboratory experiments. This model assumed the electric potential field was constant with respect to time, an assumption valid for highly buffered soil, or when the electrode electrolysis reactions are neutralized. The model also assumed constant moisture contents and temperature with respect to time, and that electroosmotic and hydraulic transport of water through the soil was negligible. A functional relationship between ionic mobility and the electrolyte concentration was estimated using the chemical activity coefficient. Tortuosity was calculated from a mathematical relationship fitted to the electrical conductivity of the bulk pore water and soil moisture data. The functional relationship between ionic mobility, pore-water concentration, and tortuosity as a function of moisture content allowed the model to predict ion transport in heterogeneous unsaturated soils. The model was tested against laboratory measurements assessing anionic electromigration as a function of moisture content. In the test cell, a strip of soil was spiked with red dye No 40 and monitored for a 24-h period while a 10-mA current was maintained between the electrodes. Electromigration velocities predicted by the electromigration transport model were in agreement with laboratory experimental results. Both laboratory-measured and model-predicted dye migration results indicated a maximum transport velocity at moisture contents less than saturation due to competing effects between current density and tortuosity as moisture content decreases.
A numerical analysis was undertaken for enhanced electrokinetic soil processing. To perform chemical conditioning of the electrode reservoirs, the electrokinetic soil process employed a membrane as a barrier between the electrode reservoirs and the contaminated soil. An alkaline solution was purged in the anode reservoir that was bounded by the membrane. A mathematical model was used for demonstration of pH change and phenol removal from a kaolinite soil bed, the prediction of pH variations in both electrode reservoirs, and the determination of an optimized injection time of the anode-purging solution. The time-dependent dispersion coefficient was employed in consideration of the averaging effect of the velocity profile on a one-dimensional transport. The estimation of pH and phenol profiles in the soil bed reasonably agreed with the experimental data. The simulation revealed that the removal efficiency of phenol from the kaolinite soil could be improved by maintaining pH of the anode solution.
Operator-splitting (OS) techniques are very attractive for numerical modelling of reactive transport, but they induce some errors. Considering reactive mass transport with reversible and irreversible reactions governed by a first-order rate law, we develop analytical solutions of the mass balance for the following operator-splitting schemes: standard sequential non-iterative (SNI), Strang-splitting SNI, standard sequential iterative (SI), extrapolating SI, and symmetric SI approaches. From these analytical solutions, the operator-splitting methods are compared with respect to mass balance errors and convergence rates independently of the techniques used for solving each operator. Dimensionless times, NOS, are defined. They control mass balance errors and convergence rates. The following order in terms of decreasing efficiency is proposed: symmetric SI, Strang-splitting SNI, standard SNI, extrapolating SI and standard SI schemes. The symmetric SI scheme does not induce any operator-splitting errors, the Strang-splitting SNI appears to be O(N2OS) accurate, and the other schemes are first-order accurate.
Electrokinetic extraction has been tested for lead removal from Algerian contaminated soils. For this purpose, a chelating reagent (EDTA, 0.1M) has been injected into the catholyte. This procedure is expected to enhance the process by desorption of the contaminant and formation of new mobile species negatively charged. A mathematical model based on Nernst-Planck theory has also been developed to predict the evolution of lead distribution across the soil as a function of time. The results of several experiments carried out on various duration (10, 20 and 40 days) have shown the importance on treatment efficiency of pollution age and contaminant speciation as determined by sequential chemical extraction.
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