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

Electrokinetic remediation for the removal of heavy metals in soil: Limitations, solutions and prospection

Electrokinetic remediation (EKR) technology is a promising method to remove heavy metals from low permeability soil, because it is environmentally friendly, efficient and economical, and can realize in-situ remediation. In this paper, the basic principles and related physical and chemical phenomena of EKR are systematically summarized, and three limiting problems of EKR technology are put forward: the weak ability of dissolving metals, focusing effect, and energy consumption. There are many methods to solve these technical problems, but there is a lack of systematic summary of the causes of problems and solutions. Based on various enhanced EKR technologies, this paper summarizes the main ideas to solve the limiting problems. The advantages and disadvantages of each technology are compared, which has guiding significance for the development of new technology in the future. This paper also discusses the dissolution of residual heavy metals, which is rare in other articles. The energy consumption of EKR and the remediation effect are equally important, and both can be used as indicators for evaluating the feasibility of new technologies. This paper reviews the influence of various electric field conditions on power consumption, such as renewable energy supply, new electrode materials and electrode configurations, suitable voltage values and functional electrolytes. In addition, a variety of energy consumption calculation methods are also introduced, which are suitable for ohmic heat loss, energy distribution when there is non-target ion competition, and power consumption of specific ions in various metal ions. Researchers can make selective reference according to their actual situations. This paper also systematically introduces the engineering design and cost calculation of EKR, lists the research progress of some engineering cases and pilot-scale tests, analyzes the reasons why it is difficult to apply EKR technology in large-scale engineering at present, and puts forward the future research direction.

Potentially toxic elements in the environment - a review of sources, sinks, pathways and mitigation measures

Pollution of ecosystems with potentially toxic elements (PTEs) has become a global problem with serious consequences for public health. The PTEs are hazardous to humans owing to their longevity, toxicity, and ability to accumulate in the biotic environment. As most PTEs cannot be degraded microbially or chemically, they can persist in soils for a long time. Besides posing a threat to landsphere, they may be transported to surrounding environmental spheres through movement of water, atmospheric circulation, and biological transmission. This can severely affect the ecological equilibrium. Accumulation of PTEs in soils pose serious health hazards to higher organisms leading to various diseases and disorders and significant relationships exist between the occurrence of PTEs and the toxic effects in humans. In natural soils, PTEs accumulate due to weathering of rocks and ores. Furthermore, locally or regionally significant accumulation of PTEs in soils may occur from industrial goods, pesticides and paints, municipal and industrial waste, fertilizer application, mining activities and atmospheric deposition. In response to the growing need to address PTE contamination, remediation methods have been developed employing mechanical, physico-chemical or biological based technologies. In this review, we discuss sources, sinks, pathways and mitigation measures related to natural and anthropogenic PTEs. We focus on As, Cd, Cr, Hg and Pb which are highly toxic and perform no physiological functions in biota. Further, these are the most widely studied PTEs.

Electrokinetic delivery of permanganate in clay inclusions for targeted contaminant degradation

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.

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

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

Two-compartment membrane electrochemical remediation of heavy metals from an aged electroplating-contaminated soil: A comparative study of anodic and cathodic processes

In this study, two-compartment membrane electrochemical remediation (MER) based on the anode process and the cathode process strategies were compared for treating a multi metal -contaminated soil. Remediation effect, as well as energy consumption and risk evaluation of the two strategies under different current density conditions of electroplating-contaminated soil suspension were performed, the following conclusions were drawn. MERs based on both the anode and cathode processes exhibited a synergetic effect because the DC electric field and extractants dissolved more metals from the soil phase into the liquid phase of the suspension compared to a usual soil washing treatment. The maximum Cr, Cu, and Ni removal efficiencies of MERs based on the anode process were 79.5%, 86.2%, and 85.0%, respectively, compared to 27.5%, 72.5%, and 65.9% based on the cathode process. Risk assessment results showed lower soil environmental risk after MER based on the cathode process than after MER based on the anode process. In this study, MER based on the cathode process as an evolving soil remediation strategy was found to present high simultaneous remediation ability for soil heavy metals and leaching materials, showing its advantages of environmental friendliness and economic effectiveness.

Simultaneous copper migration and removal from soil and water using a three-chamber microbial fuel cell

In this study, we constructed a three-chamber microbial fuel cell (TC-MFC) that avoided the adverse effects of H⁺ diffusion on anode microorganisms in the acidic catholyte and the precipitation of heavy metals in the soil near the cathode side (S4), while also achieving migration of copper from the soil and reduction of Cu²⁺ in the catholyte. The removal efficiency of acid-soluble Cu from the soil near the anode region reached 42.5% after 63 days of operation at an external resistance of 100 Ω and electrode spacing of 10 cm, and Cu²⁺ in the catholyte was completely removed within 21 days. Heavy metal mobility index (M_F) values indicated that the bioavailability and mobility of heavy metals were reduced by the TC-MFC. We found that changing the cathode potential and external circuit current in TC-MFC would affect the type (via XRD) and morphology (via SEM) of cathode deposits and the average removal rate of heavy metals. At the meantime, it should be noted that the interaction between the electric-field-dependent soil heavy metal migration and electron-dependent copper reduction in TC-MFC occurred, which was confirmed to have a relationship with the negative correlation between voltage and current during the test.

Application of enhanced electrokinetic approach to remediate Cr-contaminated soil: Effect of chelating agents and permeable reactive barrier

Enhanced electrokinetic (EK) technique was employed to remediate Cr-contaminated soil using a permeable reactive barrier (PRB) and chelating agents. Synthesized nanomagnetic Fe₃O₄ was used as a reactive material in PRB. Moreover, EDTA and citric acid (CA) were used as chelating agents. Sequential extraction method (SEM) was employed to determine Cr-elimination mechanism during the EK process. The results revealed that EDTA (78% Cr removal) was more effective than CA (54% Cr removal) in eliminating Cr from the contaminated soil during the EK process. The application of PRB in combination with EDTA was able to reduce the Cr removal rate to 70 and 66% by locating PRB in the middle section and near the anode/cathode reservoir, respectively. The use of PRB coupled with EDTA near the anode and cathode led to a more uniform Cr removal from the soil during the EK process. The highest energy consumption was 0.12 KWh during the EK remediation using PRB. Traditional EK remediation could only remove exchangeable and carbonate fractions of Cr. The use of chelating agents led to a significant (more than 90%) increase in Cr removal from the following fractions: exchangeable phase, carbonate phase, and bond to Fe-Mn oxides. In addition to electromigration (EM) mechanism, electroosmotic flow (EOF) played an important role in Cr removal during the EK process, especially when coupled with PRB.

Assessing Chromium Contamination in Red Soil: Monitoring the Migration of Fractions and the Change of Related Microorganisms

The improper stacking of chromium (Cr) slag poses a great threat to the environment and human health. The toxicity of Cr in soil is not only related to its total amount, but also to its fractions. A simulated experiment was conducted in laboratory to assess the environmental risk of Cr fractions migration and distribution in red soil. The results showed the content of acid-soluble and reducible Cr significantly decreased (P < 0.05) in top layer but increased in middle and substratum layers over time. This indicated that acid-soluble and reducible Cr migrated downward with time and the relative mobility of acid-soluble Cr (0.038 mg/kg·d·m) was higher than that of reducible Cr (0.028 mg/kg·d·m). Furthermore, correlation analysis between microbial community and chromium fraction showed the relative abundance of Lysobacter, Flavihumibacter, Flavisolbacter, and Altererythrobacter was significantly (P < 0.05) correlated with acid-soluble and reducible fractions. Thus, these microorganisms might be evaluators to assess the migration of acid-soluble and reducible fractions in red soil. In summary, this study provided a new comprehension on remediation of Cr-contaminated soil by monitoring the migration of acid-soluble and reducible fractions and the changes of related microbial groups.

Opportunities of electrokinetics for the remediation of mining sites in Biga peninsula, Turkey

This study investigated the geological conditions of Biga Peninsula. There are metamorphic rocks, ophiolitic melange, plutonic rocks, subvolcanics, volcanic rocks and volcanoclastics along with marine and terrestrial sediments in the region. This variety of rocks and the associated minerals resulted in many interesting metallic ores and coal for commercial exploitation. The mining exploitations in Biga Peninsula (Turkey) pose an environmental risk due to the release of contaminants, metals and arsenic, to the soil and waterbodies. This study analyzed the potential release of As and metals (Al, Fe, Mn) from a sediment sampled in a mine pond. The extraction column tests proved that those contaminants can be dissolved from the sediment using deionized water as eluent. The electrokinetic treatment of the sediment was able to remove Al and Mn, but the removal of Fe and As was negligible. The fractionation of As and metals in the sediment confirmed that the electrokinetic treatment was able to mobilize the contaminants. Based on the results of this study, it has been hypothesized that the toxic elements could be removed by electrokinetics using facilitating agents, neutralizing the alkaline environment of the cathode and increasing the treatment time.

Stratified chemical and microbial characteristics between anode and cathode after long-term operation of plant microbial fuel cells for remediation of metal contaminated soils

The plant microbial fuel cell (PMFC) is considered as a sustainable technology in which plants, microbes, and electrochemical cells are the major components and have the synergistic effect on electricity generation. Recent study has demonstrated the use of the PMFC system for remediation of hexavalent chromium (Cr(VI)) contaminated soils; however, the electrokinetic effects, fate of Cr and microbial community shift after long-term operation of PMFCs still need to be unveiled. In this study, PMFCs with spiking 50 mg/kg Cr(VI) were operated over 10 months and chemical and microbial characteristics of different locations of PMFC systems were investigated. Distinct chemical and microbial properties for different locations of soil samples were observed within PMFCs. For instance, the pH values of soils around the cathode and anode (cathode and anode soils) in PMFCs with Chinese pennisetum (Chinese pennisetum PMFCs) were 7.03 ± 0.15 and 6.09 ± 0.05 respectively, showing significantly higher pH values of cathode soils than those of anode soils. The electrical conductivity (EC) of cathode and anode soils in Chinese pennisetum PMFCs was 78.00 ± 5.61 and 156.25 ± 7.89 μs/cm respectively, showing significantly lower ECs of cathode soils than those of anode soils. The total Cr of cathode and anode soils in Chinese pennisetum PMFCs was 65.75 ± 3.77 and 84.29 ± 2.87 mg/kg respectively, showing significantly lower total Cr of cathode soils than that of anode soils. The permutational multivariate analysis of variance test of results of 16S rRNA gene high-throughput sequencing revealed that microbial communities in anode and cathode samples had significant difference in compositions. The stratified chemical and microbial characteristics between anode and cathode were primarily driven by the bioelectrochemical processes and electrokinetic effects within PMFCs. The findings in this study help to better understand the underlying effects of operating PMFCs and will be beneficial for future application of PMFCs in the remediation of heavy metal-contaminated soils.

Immobilization of hexavalent chromium in contaminated soil using nano-magnetic MnFe2O4

Nano-magnetic MnFe₂O₄ was prepared and examined to immobilize Cr(VI) in the soil. According to the results of scanning electron microscopy (SEM) and X-ray diffraction (XRD) the formation of nano-magnetic MnFe₂O₄ with the particle size of less than 200 nm was demonstrated. Compared with the untreated soil, the leachability of Cr(VI) was reduced from 70.95% to 4.22% through toxicity characteristic leaching procedure (TCLP) at a dosage of 2 g/L of nanoparticles and 192 h remediation time. At the same condition, the physiologically based extraction test (PBET) human bioaccessibility of chromium was reduced from 86.76% to 4.42%. Moreover, the plant bioavailability of hexavalent chromium (using EDTA) was reduced from 83.72% to 5.53%. According to the sequential extraction procedure (SEP) the loosely bounds Cr (90.28%) was converted to the relatively strong bound (Fe-Mn oxides fraction, 92.09%) revealed the significant decrease in risk of release and availability of chromium after immobilization procedure. Further, results of column experiments of Cr(VI) elution revealed that almost all of the water-soluble chromium was converted to the associated synthesized nanoparticles phase. Overall, the present study proved that nano-magnetic MnFe₂O₄ significantly enhanced the hexavalent chromium immobilization through a decrease in leachability, plant bioavailability, human bioaccessibility, and risk of release.

An overview of field-scale studies on remediation of soil contaminated with heavy metals and metalloids: Technical progress over the last decade

Soil contamination by heavy metals and metalloids has been a major concern to human health and environmental quality. While many remediation technologies have been tested at the bench scale, there have been only limited reports at the field scale. This paper aimed to provide a comprehensive overview on the field applications of various soil remediation technologies performed over the last decade or so. Under the general categories of physical, chemical, and biological approaches, ten remediation techniques were critically reviewed. The technical feasibility and economic effectiveness were evaluated, and the pros and cons were appraised. In addition, attention was placed to the environmental impacts of the remediation practices and long-term stability of the contaminants, which should be taken into account in the establishment of remediation goals and environmental criteria. Moreover, key knowledge gaps and practical challenges are identified.

Electrokinetic remediation of heavy metals from municipal solid waste incineration fly ash pretreated by nitric acid

Municipal solid waste incineration (MSWI) fly ash has a high concentration of heavy metals (HMs) which are hazardous to the environment. Moreover, it has high pH and buffering capacity which hinders the removal of HMs. Another constraining factor is the considerable fraction of HMs which exist in oxidizable and reducible states. The acid pretreatment of MSWI fly ash is a key solution to this problem. Therefore, the current experiment is carried out to evaluate the effect of acid pretreatment of MSWI fly ash and reaction/proposed time on the removal efficiency of HMs through an electrokinetic experiment. The leaching experiment results show that acid pretreatment has increased the desorption/release of heavy metal ions (Pb2+, Cd2+, Cu2+ and Zn2+). It enhances the migration of HM ions in electrolytic cells which get precipitated at the cathode, thereby increasing the removal efficiency of HMs in the electrokinetic experiment. Moreover, it is found that prolonged proposed time (12 d) has significant effect on the removal efficiency of HMs. Finally, it is concluded that acid pretreatment and prolonged proposed time have enhanced the removal electrokinetic remediation of HMs which is carried out via three processes, i.e. desorption (enhanced by acidification), migration and precipitation.

Changes in metal speciation and mobility during electrokinetic treatment of industrial wastes: Implications for remediation and resource recovery

Industrial waste deposits contain substantial quantities of valuable metals and other resources, although often in a recalcitrant form that hinders their recovery. This paper reports an experimental programme on the application of electrokinetic (EK) processing to two different waste materials (a mine tailings deposit and a metallurgical furnace dust), with the aim of exploring the effect of EK on metal speciation and extractability, with a focus on Pb and Zn due to their prevalence in these materials. The speciation of metals within the waste was determined based on a selective sequential extraction (SSE) procedure which was applied to the materials before, during and after the application of the EK treatment. The results demonstrate the generation of an acidic front in the mine tailings, which enhanced the transport of ions associated with the more labile fractions, a behaviour typical of materials characterized by a lower buffering capacity. The application of the EK in the furnace dust showed much less effect due to a very high starting pH (10) with the higher buffering capacity posing an obstacle to transport. It is shown that EK has altered the geochemical speciation of the metals in both materials, typically redistributing them from less available SSE fractions to the more labile fractions. Zn was redistributed with the SSE fractions and mobilised to a greater extent than Pb in both samples. The changes in pH and redox potential arising as a result of the application of an electric field are likely to be the main causes of the changes in speciation of both Zn and Pb. The considerable changes in metal fractionation, including removal from more recalcitrant fractions, suggest that EK may facilitate metal recovery processes. This, combined with its applicability to fine grained materials and heterogeneous environments, demonstrates that the technique may be particularly suited to both remediation of, and in-situ resource recovery from, such materials.

Application of manures to mitigate the harmful effects of electrokinetic remediation of heavy metals on soil microbial properties in polluted soils

Ethylenediaminetetraacetic acid (EDTA) used with electrokinetic (EK) to remediate heavy metal-polluted soils is a toxic chelate for soil microorganisms. Therefore, this study aimed to evaluate the effects of alternative organic chelates to EDTA on improving the microbial properties of a heavy metal-polluted soil subjected to EK. Cow manure extract (CME), poultry manure extract (PME) and EDTA were applied to a lead (Pb) and zinc (Zn)-polluted calcareous soil which were subjected to two electric intensities (1.1 and 3.3 v/cm). Soil carbon pools, microbial activity, microbial abundance (e.g., fungal, actinomycetes and bacterial abundances) and diethylenetriaminepentaacetic acid (DTPA)-extractable Pb and Zn (available forms) were assessed in both cathodic and anodic soils. Applying the EK to soil decreased all the microbial variables in the cathodic and anodic soils in the absence or presence of chelates. Both CME and PME applied with two electric intensities decreased the negative effect of EK on soil microbial variables. The lowest values of soil microbial variables were observed when EK was combined with EDTA. The following order was observed in values of soil microbial variables after treating with EK and chelates: EK + CME or EK + PME > EK > EK + EDTA. The CME and PME could increase the concentrations of available Pb and Zn, although the increase was less than that of EDTA. Overall, despite increasing soil available Pb and Zn, the combination of EK with manures (CME or PME) mitigated the negative effects of using EK on soil microbial properties. This study suggested that the synthetic chelates such as EDTA could be replaced with manures to alleviate the environmental risks of EK application.

Remediation of 137Cs contaminated concrete using electrokinetic phenomena and ionic salt washes in nuclear energy contexts

This work describes the first known the use of electrokinetic treatments and ionic salt washes to remediate concrete contaminated with ¹³⁷Cs. A series of experiments were performed on concrete samples, contaminated with K⁺ and ¹³⁷Cs, using a bespoke migration cell and an applied electric field (60V potential gradient and current limit of 35mA). Additionally, two samples were treated with an ionic salt wash (≤400molm^-3 of KCl) alongside the electrokinetic treatment. The results show that the combined treatment produces removal efficiencies three times higher (>60%) than the electrokinetic treatment alone and that the decontamination efficiency appears to be proportional to the initial degree of contamination. Furthermore, the decontamination efficiencies are equivalent to previous electrokinetic studies that utilised hazardous chemical enhancement agents demonstrating the potential of the technique for use on nuclear licensed site. The results highlight the relationship between the initial contamination concentration within the concrete and achievable removal efficiency of electrokinetic treatment and other treatments. This information would be useful when selecting the most appropriate decontamination techniques for particular contamination scenarios.

Remediation of chromium-contaminated soil by electrokinetics and electrokinetics coupled with CaAl-LDH permeable reaction barrier

The remediation of Cr(VI)-contaminated soil was investigated by electrokinetic (EK) and permeable-reactive-barrier assisted electrokinetic (EK-PRB). The medium of PRB was hydrocalumite (CaAl-LDH). The results showed that removal efficiency of hexavalent chromium (Cr(VI)) in EK-PRB and EK system was 96.49 and 85.50%, respectively. Simultaneously, the removal efficiency of total chromium (TCr) was 69.34 and 40.97% after 120-h treatment. The XRD, FTIR, and XPS analyses indicated that the reactive barrier media of CaAl-LDH successfully captured the chromium. Besides, the migration rate of chromium in EK-PRB was relatively faster than EK, since the media of PRB captured chromium in-time and reduced the influence of chromium accumulation on the migration of chromium. Moreover, the trivalent chromium (Cr(III)) was generated in EK/EK-PRB, and the chromium was stabilized in soil with the chemical speciations of oxidizable and residual fractions. Therefore, the treatment of EK-PRB and EK both increased the removal of chromium and decreased its environmental risks.

The application of homemade Neosinocalamus affinis AC in electrokinetic removal technology on heavy metal removal from the MSWI fly ash

This present paper was focused on the manufacture of activated carbon (AC) and its application in the electrokinetic remediation (EKR) technology on removal of the heavy metals (HMs) from the municipal solid waste incineration fly ash. AC was produced from Neosinocalamus affinis (NF) by chemical activation with H₃PO₄ in N₂ atmosphere, the effects of activation temperatures, soaking time and impregnation ratios on the adsorption capacity of AC on HMs were examined through equilibrium adsorption experiments. The AC produced under the condition of 450 °C of activation temperature, 10 h of soaking time and 1.5 of impregnation ration was applied in the EKR experiment. The addition of AC in the S3-region of the electrolyzer could effectively improve the removal efficiencies of HMs. The technical parameters of voltage gradient, processing time and proportion were further optimized in the coupled experiments, the maximum removal of Cu, Zn, Cd, and Pb was 84.93%, 69.61%, 79.57%, and 78.55% respectively obtained under the optimal operating conditions of 2 V/cm of voltage gradient, 8 d of processing time and 20% of proportion.

Salisbury biochar did not affect the mobility or speciation of lead in kaolin in a short-term laboratory study

Salisbury biochar (produced from British broadleaf hardwood) with two different particle sizes (≤2mm and ≤0.15mm) was applied on a kaolin with three different lead (Pb(2+)) contamination levels (50mg/kg, 300mg/kg and 1000mg/kg) at the dosage of 1% in w/w. The short-term impact of biochar on the mobility and speciation of Pb(2+) in the kaolin was investigated using attenuation periods of 1, 7 and 28 days. The leachability and extractability of Pb(2+) in carbonic acid leaching and EDTA extraction tests as well as the speciation of Pb(2+) in soils were not significantly affected by biochar treatment during all periods. The insignificant effects of biochar on Pb(2+) immobilisation were most likely attributed to the high adsorption capacity of Pb(2+) on the kaolin and biochar failed to competitively adsorb Pb(2+) against kaolin. The kaolin immobilised Pb(2+) primarily through cation exchange, which represents the readily bioavailable fractions of Pb(2+) in soils and may still pose environmental risks. This paper suggests the inefficiency of biochar treament on heavy-metal contaminated clay-rich soils. Therefore a laboratory treatablity study with respect to the soil type may be crucial when large-scale biochar applications in heavy-metal associated soil remediation are evaluated.

Remediation of lead contaminated soil by biochar-supported nano-hydroxyapatite

In this study, a high efficiency and low cost biochar-supported nano-hydroxyapatite (nHAP@BC) material was used in the remediation of lead (Pb)-contaminated soil. The remediation effect of nHAP@BC on Pb-contaminated soil was evaluated through batch experiments. The stability, bioaccessibility of Pb in the soil and the change in soil characteristics are discussed. Furthermore, the effects of the amendments on the growth of cabbage mustard seedlings and the accumulation of Pb were studied. The results showed that the immobilization rates of Pb in the soil were 71.9% and 56.8%, respectively, after a 28 day remediation using 8% nHAP and nHAP@BC materials, and the unit immobilization amount of nHAP@BC was 5.6 times that of nHAP, indicating that nHAP@BC can greatly reduce the cost of remediation of Pb in soil. After the nHAP@BC remediation, the residual fraction Pb increased by 61.4%, which greatly reduced the bioaccessibility of Pb in the soil. Moreover, nHAP@BC could effectively reduce the accumulation of Pb in plants by 31.4%. Overall, nHAP@BC can effectively remediate Pb-contaminated soil and accelerate the recovery of soil fertility.

Electrokinetic removal of radionuclides contained in scintillation liquids absorbed in soil type Phaeozem

Control samples of scintillation liquids - Phaeozem soil mixtures were prepared with different scintillation liquids as the support electrolyte, Install Gel^® XF, (Ultima Gold AB™ and Ultima Gold XR™), to construct the polarization curves, and to select the cell potential with the highest mass transfer to remove ²⁴Na (15 h) and ^99mTc (6 h) as radiotracers from polluted Phaeozem soil. During the electrokinetic treatment (EKT), the removal of radionuclides contained in scintillation liquids absorbed in Phaeozem soil, liquid phase was characterized by Gas Chromatography coupled with a Flame Ionization Detector (GC-FID) and Fourier Transform Infrared Spectrometry (FTIR), solids by FTIR, before and after the application of cell potential. In this sense, the support electrolyte was selected based on the highest current generated (1 mA), as in the case of scintillation liquid 50% Ultima Gold XR™ + 50% Water (1:1), which was used for 6 h in the presence of a mesh and a titanium rod, as anode and cathode, respectively. Finally, the removal percentage accumulated in the liquid phase after the EKT of Phaeozem soil polluted by ^99mTc was 61% close to the anode after 4 h. It was also 61% for ²⁴Na close to cathode after 2 h, and after 4 h it was 71.8%.

In situ immobilization of cadmium in soil by stabilized biochar-supported iron phosphate nanoparticles

The potential for nanoscale phosphate amendments to remediate heavy metal contamination has been widely investigated, but the strong tendency of nanoparticles to form aggregates limits the application of this technique in soil. This study synthesized a composite of biochar-supported iron phosphate nanoparticle (BC@Fe3(PO4)2) stabilized by a sodium carboxymethyl cellulose to improve the stability and mobility of the amendment in soil. The sedimentation test and column test demonstrated that BC@Fe3(PO4)2 exhibited better stability and mobility than iron phosphate nanoparticles. After 28 days of simulated in situ remediation, the immobilization efficiency of Cd was 60.2 %, and the physiological-based extraction test bioaccessibility was reduced by 53.9 %. The results of sequential extraction procedures indicated that the transformation from exchangeable (EX) Cd to organic matter (OM) and residue (RS) was responsible for the decrease in Cd leachability in soil. Accordingly, the pot test indicated that Cd uptake by cabbage mustard was suppressed by 86.8 %. Compared to tests using iron phosphate nanoparticles, the addition of BC@Fe3(PO4)2 to soil could reduce the Fe uptake of cabbage mustard. Overall, this study revealed that BC@Fe3(PO4)2 could provide effective in situ remediation of Cd in soil.

Moisture content-affected electrokinetic remediation of Cr(VI)-contaminated clay by a hydrocalumite barrier

An electrokinetic-permeable reaction barrier (EK-PRB) system was introduced in this study with hydrocalumite as the barrier material. The combined system effectively remediated the Cr(VI)-contaminated clay after a 72-h treatment, and the Cr(VI) removal efficiency increased with the initial soil moisture content. Further evidence was found that the changing soil pH value and current density were highly associated with the initial moisture content, showing its important roles in the Cr(VI) removal process. Additionally, the total Cr removal efficiency was much lower than that of Cr(VI) owing to the partial conversion of Cr(VI) to Cr(III) in the electrokinetic remediation process. Under high soil moisture conditions (40%), the removal efficiency of Cr(VI) and total Cr was 96.6 and 67.3%, respectively. Further analysis also revealed the new mineral phase, chromate hydrocalumite, for Cr fixation in the hydrocalumite barrier, which was significantly affected by the initial soil moisture content. Our results showed that the EK-PRB system with a hydrocalumite barrier is highly promising with great potential for the effective remediation of Cr(VI)-contaminated clay and engineering implementation.

Remediation of Cd(ii)-contaminated soil by three kinds of ferrous phosphate nanoparticles

Three kinds of iron phosphate nanoparticles were synthesized and they could reduce leachability and bioaccessibility of Cd effectively.

The transport behavior of As, Cu, Pb, and Zn during electrokinetic remediation of a contaminated soil using electrolyte conditioning

Electrokinetic remediation (also known as electrokinetics) is a promising technology for removing metals from fine-grained soils. However, few studies have been conducted regarding the transport behavior of multi-metals during electrokinetics. We investigated the transport of As, Cu, Pb, and Zn from soils during electrokinetics, the metal fractionation before and after electrokinetics, the relationships between metal transport and fractionation, and the effects of electrolyte conditioning. The main transport mechanisms of the metals were electroosmosis and electromigration during the first two weeks and electromigration during the following weeks. The direction of electroosmotic flow was from the anode to the cathode, and the metals in the dissolved and reducible-oxides fractions were transported to the anode or cathode by electromigration according to the chemical speciation of the metal ions in the pore water. Moreover, a portion of the metals that were initially in the residual fraction transitioned to the reducible and soluble fractions during electrokinetic treatment. However, this alteration was slow and resulted in decreasing metal removal rates as the electrokinetic treatment progressed. In addition, the use of NaOH, H3PO4, and Na2SO4 as electrolytes resulted in conditions that favored the precipitation of metal hydroxides, phosphates, and sulfates in the soil. These results demonstrated that metal removal was affected by the initial metal fractionation, metal speciation in the pore solution, and the physical-chemical parameters of the electrolytes, such as pH and electrolyte composition. Therefore, the treatment time, use of chemicals, and energy consumption could be reduced by optimizing pretreatment and by choosing appropriate electrolytes for the target metals.

Immobilization and phytotoxicity of chromium in contaminated soil remediated by CMC-stabilized nZVI

The toxic effect of Cr(VI)-contaminated soil remediated by sodium carboxymethyl cellulose stabilized nanoscale zero-valent iron (CMC-stabilized nZVI) was assessed through in vitro toxicity and phytotoxicity tests. In vitro tests showed that 0.09 g L(-1) of Fe(0) nanoparticles (soil-to-solution ratio was 1 g:5 mL) significantly reduced the toxicity characteristic leaching procedure (TCLP) leachability and physiological based extraction test (PBET) bioaccessibility of Cr by 82% and 58%, respectively. Sequential extraction procedures (SEP) revealed that exchangeable (EX) Cr was completely converted to Fe-Mn oxides (OX) and organic matter (OM). Accordingly, phytotoxicity tests indicated that after 72-h remediation, Cr uptakes by edible rape and Chinese cabbage were suppressed by 61% and 36%, respectively. Moreover, no significant increase in Cr uptake was observed for either species after a 1-month static period for the amended soil. Regarding Fe absorption, germination and seedling growth, both plant species were significantly affected by CMC-nZVI-exposed soils. However, similar phytotoxicity tests conducted after 1 month showed an improvement in cultivation for both plants. Overall, this study demonstrated that CMC-nZVI could significantly enhance Cr immobilization, which reduced its leachability, bioavailability and bioaccumulation by plants. From a detoxification perspective, such remediation is technologically feasible and shows great potential in field applications.

Enhanced electrokinetic remediation of lead-contaminated soil by complexing agents and approaching anodes

Optimizing process parameters that affect the remediation time and power consumption can improve the treatment efficiency of the electrokinetic remediation as well as determine the cost of a remediation action. Lab-scale electrokinetic remediation of Pb-contaminated soils was investigated for the effect of complexant ethylenediaminetetraacetic acid (EDTA) and acetic acid and approaching anode on the removal efficiency of Pb. When EDTA was added to the catholyte, EDTA dissolved insoluble Pb in soils to form soluble Pb-EDTA complexes, increasing Pb mobility and accordingly removal efficiency. The removal efficiency was enhanced from 47.8 to 61.5 % when the EDTA concentration was increased from 0.1 to 0.2 M, showing that EDTA played an important role in remediation. And the migration rate of Pb was increased to 72.3 % when both EDTA and acetic acid were used in the catholyte. The "approaching anode electrokinetic remediation" process in the presence of both EDTA and acetic acid had a higher Pb-removal efficiency with an average efficiency of 83.8 %. The efficiency of electrokinetic remediation was closely related to Pb speciation. Exchangeable and carbonate-bounded Pb were likely the forms which could be removed. All results indicate that the approaching anode method in the presence of EDTA and acetic acid is an advisable choice for electrokinetic remediation of Pb-contaminated soil.

Monitoring the effects of chelating agents and electrical fields on active forms of Pb and Zn in contaminated soil

The application of electrical fields and chelating agents is an innovative hybrid technology used for the decontamination of soil polluted by heavy metals. The effects of four center-oriented electrical fields and chelating agents on active fractions of lead and zinc were investigated in this pot experiment. Ethylenediaminetetraacetic acid (EDTA) as a synthetic chelator and cow manure extract (CME) and poultry manure extract (PME) as natural chelators were applied to the pots (2 g kg(-1)) 30 days after the first irrigation. Two weeks later, four center-oriented electrical fields were applied in each pot (in three levels of 0, 10, and 30 V) for 1 h each day for 14 days. The soil near the cathode and anodes was collected and analyzed as cathodic and anodic soil, respectively. Results indicated that the soluble-exchangeable fraction of lead and zinc were decreased in the cathodic soil, while the carbonate-bound fractions were increased. In the anodic soil, however, the opposite result was observed. EDTA enhanced the soluble-exchangeable form of the metals in both anodic and cathodic soils. Furthermore, the amounts of carbonate-bound heavy metals were increased by the application of CME in both soils. The organic-bound fraction of the metals was increased by the application of natural chelators, while electrical fields had no significant impacts on this fraction.

Effects of electrokinetic treatment of contaminated sludge on migration and transformation of Cd, Ni and Zn in various bonding states

This study assesses the effect of electrokinetic processes on the migration and bonding states of various heavy metals in municipal sludge. The transformation and migration are discussed through the examination of sludge characteristics and distribution of Cd, Zn and Ni after electrokinetic treatments. The migration and distribution of the contaminants after the electrokinetic treatments were determined for each sludge sample by sequential extraction. The noticeable changes on the average speciation fractions of Cd, Zn and Ni were observed that oxidizable heavy metals increased and reducible fraction decreased due to the application of voltage. Bivariate correlation analysis indicated that the amounts of different bonding states of Zn and Ni were significantly correlated (P<0.05) with durations and resistance. The oxidizable Zn was negatively correlated with exchangeable and reducible Zn. Moreover, reducible Zn had a close negative relationship with residual Zn. The bonding state of Ni was significantly related to the durations of electrokinetic processes, indicating the existing of mutual transformation between different speciation fractions over time. The analysis also indicated that the exchangeable Cd showed a close negative relationship with reducible Cd (P<0.01), whereas the reducible Cd was negatively related to residual Cd (P<0.05).

Pulse current enhanced electrodialytic soil remediation--comparison of different pulse frequencies

Energy consumption is an important factor influencing the cost of electrodialytic soil remediation (EDR). It has been indicated that the pulse current (in low frequency range) could decrease the energy consumption during EDR. This work is focused on the comparison of energy saving effect at different pulse frequencies. Based on the restoration of equilibrium, the relaxation process of the soil-water system was investigated by chronopotentiometric analysis to find the optimal relaxation time for energy saving. Results showed that the pulse current decreased the energy consumption with different extent depending on the pulse frequency. The experiment with the frequency of 16 cycles per day showed the best restoration of equilibrium and lowest energy consumption. The energy consumption per removed heavy metals was lower in pulse current experiments than constant current and increased with the pulse frequency. It was found that the transportation of cations through the cation exchange membrane was the rate controlling step both in constant and pulse current experiments, thus responsible for the major energy consumption. Substitution of the cation exchange membrane with filter paper resulted in a dramatic decrease in energy consumption, but this change impeded the acidification process and thus the removal of heavy metals decreased significantly.

Enhanced heavy metal immobilization in soil by grinding with addition of nanometallic Ca/CaO dispersion mixture

This study investigated the use of a nanometallic Ca and CaO dispersion mixture for the immobilization of heavy metals (As, Cd, Cr and Pb) in contaminated soil. Simple grinding achieved 85-90% heavy metal immobilization, but it can be enhanced further to 98-100% by addition of a nanometallic Ca/CaO dispersion mixture produced by grinding. Observations using SEM-EDS elemental maps and semi-quantitative analysis showed that the amounts of As, Cd, Cr, and Pb measurable on the soil particle surface decrease after nanometallic Ca/CaO treatment. The leachable heavy metal concentrations were reduced after nanometallic Ca/CaO treatment to concentrations lower than the Japan soil elution standard regulatory threshold: <0.01 mg L(-1) for As, Cd, and Pb; and 0.05 mg L(-1) for Cr. Effects of soil moisture and pH on heavy metal immobilization were not strongly influenced. The most probable mechanisms for the enhancement of heavy metal immobilization capacity with nanometallic Ca/CaO treatment might be due to adsorption and entrapment of heavy metals into newly formed aggregates, thereby prompting aggregation of soil particles and enclosure/binding with Ca/CaO-associated immobile salts. Results suggest that the nanometallic Ca/CaO mixture is suitable for use in immobilization of heavy-metal-contaminated soil under normal moisture conditions.

Near-anode focusing phenomenon caused by the high anolyte concentration in the electrokinetic remediation of chromium(VI)-contaminated soil

The effects of the concentration and the low pH value of anolyte on the electrokinetic remediation (EKR) of chromium-contaminated soil were investigated using chromium-spiked kaolin-gypsum soil. Results of visual observation and X-ray fluorescence analysis show that high anolyte concentrations could cause an ion-induced potential gradient well trapping effect on the soil near the anode, and consequently cause a focusing phenomenon (FP) without chemical precipitation. This FP significantly prolonged the remediation duration and reduced chromium removal. The low pH value of soil aggravated the FP, resulting in a quasi-dead zone near the anode caused by the reduction in soil resistance, rather than the adsorption of chromium (VI) ions. The high anolyte concentration also resulted in high energy consumption. The FP and low pH value collectively decreased the energy efficiency by more than 96%. This kind of FP can be predicted via the online monitoring of the potential gradient profiles of the soil between the electrodes in the EKR.

Electro-migration of heavy metals in an aged electroplating contaminated soil affected by the coexisting hexavalent chromium

Cr(VI) was often reported to oxidize soil organic matter at acidic environments due to its high ORP, probably thus changing cationic metal species bound to soil organic matter, and influencing their electro-migration patterns. However, such an effect on the electro-migration was not confirmed in most previous studies. Therefore, this study applied a fixed voltage direct current field on an aged electroplating contaminated clayed soil, with a special interest in the direct or indirect influence of Cr(VI) on the electro-migration of other coexisting metals. After 353 h electrokinetic process, 81% of Zn, 53% of Ni and 22% of Cu in the original soil were electro-migrated into the electrolyte, and most of the remaining concentrated near the cathode. The Cr(VI) oxidized some soil organic matter along its migration pathway, with a pronounced reaction occurred near the anode at low pHs. The resulting Cr(III) reversed its original movement, and migrated towards the cathode, leading to the occurrence of a second Cr concentration peak in the soil. Metal species analyses showed that the amount of metals bound to soil organic matter significantly decreased, while a substantial increase in the Cr species bound to Fe/Mn (hydro-)oxides was observed, suggesting an enhancement of cationic metal electro-migration by the reduction of Cr(VI) into Cr(III). However, the Cr(VI) may form some stable lead chromate precipitates, and in turn demobilize Pb in the soil, as the results showed a low Pb removal and an increase in its acid-extractable and residual fractions after electrokinetic remediation.