Electrokinetic Removal Of Heavy Metals Research Articles

Enhancement of the electrokinetic removal of heavy metals, cations, anions, and other elements from soil by integrating PCPSS and a chelating agent

BackgroundRecently, we have developed a new soil electrokinetic remediation (SEKR) process, hereinafter referred to as the perforated cathode pipe SEKR system (PCPSS), to achieve remediation intensification. Some obstacles, such as the low removal of some heavy metals, particularly Ni, limited our previous PCPCS. MethodsWe aim to improve the removal efficiency of the PCPSS by adding a chelating agent (i.e., ethylenediaminetetraacetic acid (EDTA)). Given the unsatisfactory remediation results obtained in our previous studies, the Taguchi approach (L9OA) was exploited in this work as an experimental design to determine the optimal level of factors that could enhance Ni removal. Significant findingsResults showed that an electric potential of 3 V, addition of 0.5 mol of EDTA, and an initial Ni concentration in the range of 10–20 mg kg−1 could optimize Ni removal. Confirmation experiments conducted on natural contaminated soil showed the following order of removal of heavy metals and other elements: Al (70%) > Co (69%) > Fe (63.5%) > Cu (55%) > Mn (52.8%) > Zn (43.17%) > Cr (41.41%) > Ni (22.26%). Among the studied soil cations, Mg showed the best removal rate, Ba showed moderate removal rate, and Ca showed the lowest removal rate. Notably, the removal rate of V was much higher than that of born (B).

Enhancement of the Electrokinetic Removal of Heavy Metals, Cations, and Anions from Soil by Integrating PCPSS and a Chelating Agent

Enhancement of the Electrokinetic Removal of Heavy Metals, Cations, and Anions from Soil by Integrating PCPSS and a Chelating Agent

Enhanced Electrokinetic Removal of Heavy Metals from a Contaminated Lake Sediment for Ecological Risk Reduction

ABSTRACT In this study, electrokinetic remediation (EKR) was performed to reduce the risk of heavy metal contaminants viz., Cd, Cu, Ni, Pb and Zn associated with different fractions of lake sediments. Firstly, batch experiments were performed to estimate the optimal concentration of enhancing agents such as EDTA, nitric acid and acetic acid for effective dissolution of heavy metals from sediments and to minimize the dissolution of earth metals (Fe and Al) to maintain good soil health. In addition to that, the effect of pH on dissolution of these heavy metals with optimized concentration was studied separately. The results from batch experiments showed that EDTA with the concentration of 0.01 M enhanced the heavy metal dissolution as 38–88% in the pH range 2–12. Nitric acid and acetic acid with the concentration of 0.05 M enhanced the heavy metal dissolution as 18–85% and 15–80%, respectively in the pH range 2–6. For enhancing agents like nitric acid and acetic acid, the increase of pH (above 6) led to the formation of metal hydroxides and carbonates, which reduced the efficiency of heavy metal dissolution. Secondly, EKR experiments were conducted in a reactor with optimal concentration of EDTA (0.01 M), nitric acid and acetic acid (0.05 M) as electrolyte and sediment saturation solution for 7–21 days of treatment time. After 21 days of EKR with 0.01 M of EDTA as enhancing agent, the average removal of heavy metals achieved about 46.4–78.8 % and associated risk with metals viz., Cd, Pb, Zn and Cu reduced from high to low risk and Ni reduced from high to medium risk, according to risk assessment category. Alternatively, the EKR treatment using nitric acid and acetic acid showed the average removal of heavy metals of about 17.2–43.60% and 24.9–57.2 %, respectively, and all heavy metals pose medium risk, except for Cd, which showed low risk to the environment.

Regulating and intervening act of Cr chemical speciation effect on the electrokinetic removal in Cr contaminated soil in arid area

Heavy metals chemical speciation significantly effects the electrokinetic removal of heavy metals contaminated soil, but how to regulate and intervene the chemical speciation for improving the removal efficiency in the electrokinetic remediation of the contaminated soil is still an opened problem. In this study, we used increasing citric acid electrolyte concentration and soil pretreatment as regulating measures and the anolyte circulation and reduction wells as intervening act to explore the detailed transformation and migration of Cr speciation over time during electrokinetic remediation, the effect of the transformation and migration of Cr speciation in arid area contaminated soil on the removal efficiency. Based on the results of sequential extraction, the regulating measures can significantly improve the soil pH and the transformation of Cr speciation in arid areas, but the detailed transformation occurs in the early stage of remediation. The material flows showed that the transformation of other speciation to exchangeable (EXC) was higher than its migration to the electrode chamber and the EXC was not completely removed throughout the experiment. By adding HCl to the soil prior to the electrokinetic remediation, more Cr (III) can migrate to the cathode region. Regulating measure for pretreatment of real contaminated soil in arid area based on high concentration of citric acid and intervening act for the anolyte circulation could significantly reduce the bound to Fe/Mn oxides (FM), but the overall removal efficiency of the real contaminated soil (from 44.86% to 44.90%) was not improved, while the removal efficiency of artificially contaminated soil was higher. Furthermore, the multivariate linear regression analysis of the link between the five fractions and removal efficiency of Cr also shows that their correlation was much higher in artificially contaminated soils than in real contaminated soils. The scale effect would affect the transformation of the speciation, and the smaller the scale of the experiment is, the larger the influence on the migration and transformation of Cr speciation will be. The combination of regulating measures and intervening act provide a valuable reference for the specific transformation and migration of heavy metals speciation and the improvement of removal efficiency during EKR of contaminated soil in arid area.

Research on a closed-loop method that enhances the electrokinetic removal of heavy metals from municipal solid waste incineration fly ashes

The high leachability of heavy metals and toxic organic components has severely discouraged the broader resource recycling of municipal solid waste incineration fly ashes. In this study, a recycling system combining water washing treatment, cationic buffer solution recovery, and electrokinetic remediation was designed based on pH controls and comprehensively explored in terms of strengthening the removal of heavy metals from samples and reducing the risk of environmental leaching of heavy metals in fly ashes. The water washing pretreatment removed a considerable amount of soluble minerals from the fly ash and lowered the initial pH of the electrochemical system to below 10. The dosing of buffer cations decreased the thickness of the diffuse double layer and ameliorated the mobility of the heavy metal species in the pore fluid. Cu was most sensitive to the changes in the operating factors during electrokinetics. The effects of the remediation times and voltage gradients were more significant on heavy metal removal than those of the nitric acid concentration in the electrokinetic optimization system. The leaching toxicities of zinc, lead, copper, and cadmium were reduced by 82.59%, 73.64%, 67.07%, and 93.13%, respectively. Generally, the recovery of the water washing leachate not only enhanced the performance of the electrokinetic remediation for the municipal solid waste incineration fly ash but also avoided downstream disposal of the effluent.

Electrokinetic removal of heavy metals from soil

Removal of heavy metal ions from soils by electrokinetic treatment has several advantages. The extent of removal, however, is both soil specific and ion specific. The conditions to be maintained have to be established based on laboratory studies. With a view to maximize the removal of metal ions the trends of removal of heavy metal ions such as iron, nickel and cadmium form a natural Indian kaolinitic red earth during different conditions maintained in the electrokinetic extraction process are studied. A laboratory electrokinetic extraction apparatus was assembled for this purpose. Attempts are also made to elucidate the mechanism of removal of the metal ions from soil. The composition of the flushing fluid, voltage and duration of extraction are varied. While dilute acetic acid has been used to neutralize the alkalinity that develops at the cathode, EDTA solution has been used to desorb heavy metals from clay surface. Generally the extent of removal was proportional to the osmotic flow. Nickel and Cadmium are more effectively removed than iron. The percentage removal of Ni is generally proportional to the osmotic flow but shows sensitivity to the pH of the system. There is an optimum voltage for removal of metal ions from soil. The removal of iron was negligible under different conditions studied.

The Effectiveness of Electro-Remediation of Aged, Metal-Contaminated Sediment in Relation to Sequential Extraction of Metals

Soil pollution is a universal environmental issue, and the clean-up of contaminated soils can be costly and time consuming. Traditional methods often do not provide an effective solution when it comes to fine-grained and highly impermeable soils and/or immobile contaminants. Electro-remediation, however, is a promising technique. The aim of this study was to relate electrokinetic removal of heavy metals from a contaminated, aged sediment sludge to the metal speciation in the sludge. Metal speciation was determined by a Tessier-based sequential extraction method. Two electro-remediation experiments were conducted in the laboratory in order to investigate removal of metals. The studies were done in a cell designed for a predictive laboratory test of feasibility of electro-remediation of soil. Maximum removal percentages during the experiment using a constant current of 1 mA/cm2 over ∼180 h with a corresponding potential gradient of 0.5 V/cm were larger than 15 % for Cd (21 %), Cu (15 %), and Zn (23 %); Ni and Pb showed removal less than 10 %; Cr demonstrated no removal. Removal of non-contaminant metals Mn and Ca exceeded that of heavy metals. The removal rates were compared to metal speciation in the non-treated sludge, measured by the sequential extraction. It was observed that the electrokinetic removal did not directly relate to the metal distribution over the fractions within the sludge. The aging of metals and other geochemical characteristics of the contaminated sludge affected the removal of heavy metals. Therefore, sequential extraction alone does not provide a predictive tool for the effectiveness of electro-remediation. Additional geochemical information is needed.

Non-Uniform Electrokinetic Removal of Heavy Metals from Contaminated Soil with Permeable Reactive Composite Electrodes

A hexagonal unit cell for non-uniform electrokinetic (EK) remediation was constructed and the electric field distribution of the cell was simulated using MATLAB software. The changes of soil pH and residues of heavy metals in different parts of the cell during the permeable reactive composite electrode electrokinetic (PRCE-EK) remediation were also studied. The results showed that, the changes of soil pH, the removal of heavy metals and the distribution of the non-uniform electric field in the cell were closely related to one another during the PRCE-EK remediation. Simulation of electric field distribution could help to predict the changes of soil pH of different parts of the cell, and then help to optimize the EK remediation process. The results also showed that an acidic migration band, which could prevent premature precipitation of heavy metals to a certain extent and promote EK removal of heavy metals, was formed in the whole hexagonal cell when the cathode used PRCE during the non-uniform EK remediation of cationic metallic ions, Cd2+ and Pb2+. After 480 hours of remediation, the total removal efficiency of Cd and Pb were 89.9% and 69.3%, respectively. Further prolonging remediation and constantly replacing electrodes with new ones could obtain better remediation results.

Enhanced Electrokinetic Removal of Heavy Metals in MSWI Fly Ash Assisted by Cation Exchange Membranes

Since MSWI fly ash enriches heavy metals and many toxic components and these components would potentially leach when the ashes are land filled, it is regulated as hazardous waste in many countries. Electrokinetic removal, an electrochemically assisted extraction method, has recently been adopted as a new method for removal of heavy metals from MSWI fly ashes. But the generation of metal hydroxide precipitation near the cathode decreased conductivity of electrokinetic removal system and limited metal extraction from the sample area. In this study, cation exchange membrane was used in electrokinetic removal experiment to improve this method. After 192 hours of electrokinetic removal, about 15%Cd,20%Pb, 30% Zn, and 10% As was removed from 2.850kg MSWI fly ash. It showed positive effect compared to the trial test. But there still exists some problems, such as low removal efficiency. Further work should be done to solve it.Moreover, the high conductivity in fly ash limits the current , which in turn causes low electroosmotic flow. While electroosmotic flow is the main factor leting the heavy meatals release from fly ash, it would become the focus in further experiment to look for ways increasing the electroosmotic flow in sample cell.

Combining Different Frequencies for Electrical Heating of Saturated and Unsaturated Soil Zones

AbstractIn situ electrical heating of soil was studied applying different frequencies: low‐frequency energy for resistive heating and radio‐frequency energy for dielectric heating. Steep temperature gradients were observed for each heating mode under the condition of the coexistence of saturated and unsaturated soil zones. By combining the two heating modes, this undesired effect can be avoided, thus allowing efficient soil remediation especially when organic phases are accumulated at the capillary fringe. A parallel application of both frequencies was demonstrated as the most suitable method to reduce temperature gradients. By using electronic filters, both electric fields can be established by only one electrode array. This innovative concept is especially applicable for optimizing thermal remediation of light non‐aqueous phase liquid contaminations or realizing thermally‐enhanced electrokinetic removal of heavy metals.

Evaluation of electrokinetic removal of heavy metals from sewage sludge

The presence of heavy metals is one of the main obstacles for agricultural use of million tonnes of dewatered sewage sludge produced in wastewater treatment plants. Electrokinetic (EK) treatment can be applied to remove heavy metals from sludge. The aim of this study was to increase the efficiency of electrokinetic removal of heavy metals from dewatered sewage sludge. EK experiments were carried out with and without pH adjustment in cathode chamber of acidified sewage sludge. The selective sequential extraction (SSE) was used to determine the fractionation of heavy metals in sewage sludge. The mobility of heavy metals in sludge significantly increased after its acidification at pH 2.7 and followed the order: Ni, Zn, Cu, As, Cr, Pb. Removal efficiencies of heavy metals in the experiment with acidified sewage sludge and pH adjustment at cathode chamber at 2.0 were: 95% for Zn, 96% for Cu, 90% for Ni, 68% for Cr, 31% for As and 19% for Pb. The concentrations of Zn, Cu, Ni, Cr and Pb after EK treatment were below the United States Environmental Protection Agency limits for biosolids applied to agricultural land, forest, public contact sites or reclamation sites.

Effects of Electric Current and Potential on the Electrokinetic Removal of Heavy Metals from an Abandoned Mine Tailings

In the removal of heavy metals from the mine deposit using electrokinetic processes, the effects of operation under both constant current and constant potential conditions were estimated. The results of soil pH distributions for DDW-20 V and DDW-100 mA cases after the electrokinetic remediation tests were observed. In the former case, soil pH was not much changed and kept to almost constant value just little higher than initial soil pH of 3.52, except near the cathode, which was about pH 5. While in the latter case, soil pHs of anode and the cathode regions were less than pH 3 and about 6, respectively. The electroosmotic flow to the cathode increased rapidly till 10 hrs and decreased steadily and then maintained to constant rate until the end of operation at constant current condition. Electric potential gradient was continuously increased to as much as 34.375 V/cm. At the steady state, values of the apparent electric conductivity for DDW-20 V and DDW-100 mA were around 40 <TEX>${\mu}\textrm{s}$</TEX>/cm and 30 <TEX>${\mu}\textrm{s}$</TEX>/cm, respectively. In the DDW-100mA test, Cu, Cd, and Zn except Pb showed the tendency of moving toward the cathode. While in the DDW-20 V case, it was observed that Cu, Zn, and Pb except Cd were not moved to any directions. The results of the tests demonstrated that the electrokinetic soil remediation process could be operated better under constant current condition than constant electric potential condition.

Pilot scale study on the ex situ electrokinetic removal of heavy metals from municipal wastewater sludges

In order to remove toxic heavy metals from municipal wastewater sludges, the ex situ electrokinetic technique was studied at pilot scale. This study focused on the feasibility of the electrokinetic removal of heavy metals from sludge and the effectiveness of this technique on the variations of abiotic (physicochemical) and biotic (intracellular and extracellular) speciations of heavy metals using several analytical methods. Even though the sludge used was taken from a municipal wastewater treatment plant, the sludge contained relatively high concentrations of target metal contaminants (Cd=6.8 mg/kg, Cr=115.6 mg/kg, Cu=338.7 mg/kg, and Pb=62.8 mg/kg). The removal efficiencies of heavy metals were significantly dependent on their speciations in the sludge matrices. The electrokinetic removal efficiencies of abiotic heavy metals exceeded 70% for the mobile and weakly bound fractions, such as, the exchangeable and carbonate fractions and were lower than 35% for the strongly bound fractions, such as, the organic/sulfide and residual fractions. In the case of the biotic heavy metals, the removal efficiencies of the extracellular fractions were slightly higher than those of the intracellular fractions.

Evaluation of Electrokinetic Removal of Heavy Metals from Tailing Soils

Electrokinetic remediation was studied for the removal of toxic heavy metals from tailing soils. The study emphasized the dependency of removal efficiencies upon their speciations, as demonstrated by the different extraction methods used, which included sequential extraction, total digestion, and 0.1 N HCl extraction. The tailing soils examined showed different physicochemical characteristics, such as initial pH, particle size distribution, and major mineral constituents, and they contained high concentrations of target metal contaminants in various forms. The electrokinetic removal efficiencies of heavy metals were significantly influenced by their partitioning prior to treatment, and the pHs of the tailing soils. The mobile and weakly bound fractions of heavy metals, such as the exchangeable fraction, were easily removed by electrokinetic treatment (more than 90% removal efficiency), but immobile and strongly bound fractions, such as the organically bound species and residual fractions, were not significantly removed (less than 20% removal efficiencies).

Monitoring of electrokinetic removal of heavy metals in tailing-soils using sequential extraction analysis

This research focused on the monitoring of the electrokinetic removal of heavy metals from tailing-soils, and emphasizes the dependency of removal efficiencies upon their physico-chemical states, as demonstrated by the different extraction methods adopted, which included aqua regia and sequential extraction. The tailing-soils examined contained high concentrations of target metal contaminants (Cd=179 mg kg −1, Cu=207 mg kg −1, Pb=5175 mg kg −1, and Zn=7600 mg kg −1). The removal efficiencies of the different metals were significantly influenced by their speciations, mobilities and affinities (adsorption capacities) in the soil matrix. The removal efficiencies of mobile and weakly bound fractions, such as the exchangeable fraction were more than 90% by electrokinetic treatment, but strongly bound fractions, such as the organically bound species and residual fraction were not significantly removed (less than 30% removal efficiencies). In accordance with the general sequence of mobilities of heavy metals in soils, the removal efficiencies of more mobile heavy metals (Cd, Cu, and Zn) were higher than that of less mobile heavy metal (Pb).

Assessment of electrokinetic removal of heavy metals from soils by sequential extraction analysis

Electrokinetic remediation of metal-contaminated soils is strongly affected by soil-type and chemical species of contaminants. This paper investigates the speciation and extent of migration of heavy metals in soils during electrokinetic remediation. Laboratory electrokinetic experiments were conducted using two diverse soils, kaolin and glacial till, contaminated with chromium as either Cr(III) or Cr(VI). Initial total chromium concentrations were maintained at 1000 mg/kg. In addition, Ni(II) and Cd(II) were used in concentrations of 500 and 250 mg/kg, respectively. The contaminated soils were subjected to a voltage gradient of 1 VDC/cm for over 200 h. The extent of migration of contaminants after the electric potential application was determined. Sequential extractions were performed on the contaminated soils before and after electrokinetic treatment to provide an understanding of the distribution of the contaminants in the soils. The initial speciation of contaminants was found to depend on the soil composition as well as the type and amounts of different contaminants present. When the initial form of chromium was Cr(III), exchangeable and soluble fractions of Cr, Ni, and Cd ranged from 10 to 65% in kaolin; however, these fractions ranged from 0 to 4% in glacial till. When the initial form of chromium was Cr(VI), the exchangeable and soluble fractions of Cr, Ni and Cd ranged from 66 to 80% in kaolin. In glacial till, however, the exchangeable and soluble fraction for Cr was 38% and Ni and Cd fractions were 2 and 10%, respectively. The remainder of the contaminants existed as the complex and precipitate fractions. During electrokinetic remediation, Cr(VI) migrated towards the anode, whereas Cr(III), Ni(II) and Cd(II) migrated towards the cathode. The speciation of contaminants after electrokinetic treatment showed that significant change in exchangeable and soluble fractions occurred. In kaolin, exchangeable and soluble Cr(III), Ni(II), and Cd(II) decreased near the anode and increased near the cathode, whereas exchangeable and soluble Cr(VI) decreased near the cathode and increased near the anode. In glacial till, exchangeable and soluble Cr(III), Ni(II), and Cd(II) were low even before electrokinetic treatment and no significant changes were observed after the electrokinetic treatment. However, significant exchangeable and soluble Cr(VI) that was present in glacial till prior to electrokinetic treatment decreased to non-detectable levels near the cathode and increased significantly near the anode. In both kaolin and glacial till, low migration rates occurred as a result of contaminants existing as immobile complexes and precipitates. The overall contaminant removal efficiency was very low (less than 20%) in all tests.