Application of EAPR system on the removal of lead from sandy soil and uptake by Kentucky bluegrass (Poa pratensis L.)

Hybrid and enhanced electrokinetic system for soil remediation from heavy metals and organic matter

The electrokinetic (EK) process has been proposed for soil decontamination from heavy metals and organic matter. The advantages of the EK process include the low operating energy, suitability for fine-grained soil decontamination, and no need for excavation. During the last three decades, enhanced and hybrid EK systems were developed and tested for improving the efficiency of contaminants removal from soils. Chemically enhanced-EK processes exhibited excellent efficiency in removing contaminants by controlling the soil pH or the chemical reaction of contaminants. EK hybrid systems were tested to overcome environmental hurdles or technical drawbacks of decontamination technologies. Hybridization of the EK process with phytoremediation, bioremediation, or reactive filter media (RFM) improved the remediation process performance by capturing contaminants or facilitating biological agents' movement in the soil. Also, EK process coupling with solar energy was proposed to treat off-grid contaminated soils or reduce the EK energy requirements. This study reviews recent advancements in the enhancement and hybrid EK systems for soil remediation and the type of contaminants targeted by the process. The study also covered the impact of operating parameters, imperfect pollution separation, and differences in the physicochemical characteristics and microstructure of soil/sediment on the EK performance. Finally, a comparison between various remediation processes was presented to highlight the pros and cons of these technologies.

Lead dissociation and redistribution properties of actual contaminated farmland soil after long-term EKAPR treatment

Electrokinetic-assisted phytoremediation (EKAPR) is a potential technology much affected by the metal species and accessibility to plant roots. In this study, Pb-contaminated red soil was remediated with Sedum plumbizincicola to investigate the changes in soil pH, available nutrients, dissociation and redistribution of Pb under a long-term periodic reversal direct-current electric field. This approach could effectively activate soil P, K, organic matter (OM) and Pb, without significant soil acidification; the effect was positively correlated with applied voltage. Soil Pb can be continuously dissociated, migrated, and tended to accumulate in the middle region. The maximum Pb removal rate in the anodic section of the EKAPR system was 21.4%, and the aggregation rate in middle regions was 14.4%, higher than the available Pb content of the original soil. The Pb desorption in aqueous solution increased significantly with increasing voltage, irrespective of the solution pH. At a voltage of 20 V, the Pb cumulative desorption content reached 91.1 mg kg-1 (pH = 7), which was 2.7 times than that without electric field (33.2 mg kg-1). Compared to original soil (2.80 mg kg-1) and the control (14.54 mg kg-1), the available Pb in the anode section of EKAPR system (20.66 mg kg-1) increased by 637.9% and 42.1%, respectively. These results indicated that except for the indirect influence of soil pH changes, electrodynamics can directly promote the bioavailability and dissociation of Pb at the soil-water interface. This finding provides a new perspective for further studies on the mechanism of Pb speciation evolution and accumulation changes using EKAPR.

Electro-enhanced phytoremediation system on the removal of trace metal concentration from contaminated water

The combination of electro-enhanced and hydroponic phytoremediation hereinafter referred to as electro-enhanced phytoremediation (EP) system, has been employed for rapid removal of trace metal concentration of lead (II) from contaminated water using Kentucky bluegrass (Poa pratensis L.) as accumulator plant. In this study, for rapid assessment the effectiveness of two-dimensional (2D) electrode configuration in electro-enhanced system was evaluated by agar media for 48h period of time. Furthermore, these configurations were applied to enhance the EP system for 9d period of time. Also, a common agrochemical-urea as chaotropic agent to facilitate the healthy growth of plant in contaminated water was evaluated. The results showed that the accumulation of lead (II) concentration was higher in the plant roots (i.e. high bioaccumulation coefficient (BC) value) than in aerial parts of plant (i.e. low translocation factor (TF) value). Also, the accumulation of lead (II) concentration in plant was higher under the treated urea of EP system. The chlorophyll content, biomass accumulation productivity, and water content (i.e. dry weight-fresh weight (DW/FW) ratio) of plant either under the treated urea or untreated urea with high accumulation of lead (II) concentration revealed that the Kentucky bluegrass has able to hold out the plant stress.

The effects of different electrode materials on seed germination of Solanum nigrum L. and its Cd accumulation in soil

The effects of different electrode on Solanum nigrum L. seed germination were determined. The result showed that germination percentage (GP) of seeds in treatment T2 (titanium electrode) was 26.6% higher than in control (CK, without electric field). High potassium and calcium concentrations were beneficial for seed enzymatic activity in treatment T2, which could partly explain the increase in GP. Cd accumulation (μg/pot) in S. nigrum treated with any electric field was significantly higher (p<0.05) than in CK without electric field. Specifically, Cd accumulation under the treatment T3 (stainless steel electrode) was the highest both in roots and shoots; this accumulation in shoots and roots were 74.7 % and 67.4 % higher for stainless steel than in CK. This increase must have been associated with a higher Cd concentration in plants and did not exert a significant effect on the biomass. In particular, Cd concentrations in roots and shoots under stainless steel treatment were both significantly higher than in CK (p<0.05), which had to be related to the higher available Cd concentration in the soil in the middle region. Furthermore, it could be attributed to altered soil pH and other soil properties. Moreover, none of the biomasses were significantly affected (p<0.05) by different electrode materials compared to CK.

Evaluation of electro-assisted phytoremediation (EAPR) system for heavy metal removal from synthetic leachate using Pistia stratiotes

The highest waste generated in Malaysia is composed of municipal solid waste, which is mainly managed by landfilling. Heavy metals in leachate generated from landfill could have caused hazardous effects to human and environment. EAPR has been increasingly applied to treat soil and wastewater. This technique serves as a potential tool for remediation of real leachate. Metals (Mn, Cd, Fe, Ni, Pb, Zn) uptake by Pistia stratiotes were evaluated via flame atomic adsorption spectrophotometer. Pb and Fe could be the elements that were more efficiently removed by P. stratiotes in the EAPR system. The removal efficiency was 59.86 ± 9.98 and 56.56 ± 18.08% for Pb and Fe, respectively. EAPR significantly reduced the BOD (9.37 ± 2.36 mg/L), color (120.00 ± 5.77 PtCo), and turbidity (25.50 ± 11.96 NTU) of synthetic leachate. An obvious accumulation of heavy metals was observed at roots based on BCF and TF values. BCF values of Pb (18,999.06 ± 8,321.76) and Fe (16,090.81 ± 5,844.36) in the EAPR system were more than 10³, which indicates that P. stratiotes is a hyperaccumulator. Further study on the upregulated genes is needed to comprehend the molecular basis of heavy metal stress tolerance.

An overview of in-situ remediation for nitrate in groundwater

Faced with the increasing nitrate pollution in groundwater, in-situ remediation has been widely studied and applied on field-scale as an efficient, economical and less disturbing remediation technology. In this review, we discussed various in-situ remediation for nitrate in groundwater and elaborate on biostimulation, phytoremediation, electrokinetic remediation, permeable reactive barrier and combined remediation. This review described principles of each in-situ remediation, application, the latest progress, problems and challenges on field-scale. Factors affecting the efficiency of in-situ remediation for nitrate in groundwater are also summarized. Finally, this review presented the prospect of in-situ remediation for nitrate pollution in groundwater. The objective of this review is to examine the state of knowledge on in-situ remediation for nitrate in groundwater and critically evaluate factors which affect the up-scaling of laboratory and bench-scale research to field-scale application. This helps to better understand the control mechanisms of various in-situ remediation for nitrate pollution in groundwater and the design options available for application to the field-scale.

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

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

Enhancement Effect of Humic Acid on Removal of Lead from Soil by Electrokinetic Process

The environmentally friendly electrokinetic process was investigated using model contaminated soil with Pb and humic acid. Although humic acid has a negative charge, it moved toward the cathode side by electroosmotic flow. The removal efficiency of Pb from model contaminated soil was 48.0% under 1 V/cm of the potential gradient after 72 h of operation, while it improved to 72.6% with humic acid. These results indicated that humic acid has high potential for application in the environmentally friendly remediation of soil.

The effects of different electric fields and electrodes on Solanum nigrum L. Cd hyperaccumulation in soil

Electrokinetics is a new attempt of strengthening hyperaccumulator Solanum nigrum L. Cd extraction. The effects of different electric fields and electrodes on S. nigrum accumulating Cd among of four electric field conditions (1 V cm^-1) and two electrodes were determined. The results showed that the AC electric field significantly stimulated (p < 0.05) the growth of S. nigrum, and the biomass increased nearly by 40% compared with the control, while the DC electric field (including the switching polarity) had no significant effect. Electric field significantly increased (p < 0.05) Cd concentration in S. nigrum and the highest one was recorded for the DC electric field with switching polarity. S. nigrum Cd accumulation (ug pot^-1) was the highest under the AC electric field, which was nearly 70% higher compared to the control. The innovation found was that the role of biomass enhancement for S. nigrum accumulating Cd (ug pot^-1) was the first and increased Cd concentration was secondary under AC electrical field, which might be relative with S. nigrum is a weed species. Basically, there was no significant difference between the graphite and stainless steel electrode, but under AC electric field conditions, Cd accumulation of S. nigrum was significantly higher (p < 0.05) than stainless steel electrode, which is worthy of further revelation besides of statistic factor. In general, the accumulation (ug pot^-1) of Cd by S. nigrum was the highest under the AC electric field, which was a very important reference for the electrokinetic conditions to be used in the practice of phytoremediation.

Scaling up the electrokinetic-assisted phytoremediation of atrazine-polluted soils using reversal of electrode polarity: A mesocosm study

Electrokinetic-assisted phytoremediation (EKPR) has been recently proposed for the removal of pesticides from polluted soils. In this work, we report the results from an EKPR experiment that was carried out in a mesocosm mock-up of 0.386 m³ using ryegrass (Lolium perenne L.) and a low permeability soil spiked with atrazine. Plants were initially grown for 35 days; then, the soil was spiked with atrazine at a dose of 2 mg kg^-1 soil. A DC electrical field of 0.6 V cm^-1 was applied 24 h every day, switching polarity daily. Another identical mock-up with the same experimental conditions but without plants was used for comparison purposes. The duration of the EKPR test was 19 days during which some operational parameters were registered (electric current intensity, soil pH and temperature) and soil porewater samples were taken and analysed. Plant tissues and soil samples from the different sections in which the mock-ups were divided, were also collected and analysed at the end of the experiment. 3-D profiles of soil pH, water content and atrazine residues concentration in plants and soil were obtained and discussed. The results of this experiment were compared with others previously reported by us from a similar EKPR pot test. In spite of the difficulties to get an adequate geometric and operational similarity between setups of different scale, the main output parameters of the EKPR process (electric current, specific current charge, overall atrazine removal, specific atrazine removal efficiency, root biomass:soil weight ratio) were discussed. It was shown that, although the processes carried out are essentially the same in both scales, their extent may be quite different; it highlights the limitations of small-scale experiments to predict the results at field conditions.

Interactions between electrokinetics and rhizoremediation on the remediation of crude oil-contaminated soil

An electrokinetics (EK)-enhanced phytoremediation system with ryegrass was constructed to remediate crude oil-polluted soil. The four treatments employed in this study included (1) without EK or ryegrass (CK-NR), (2) EK only (EK-NR), (3) ryegrass only (CK-R), and (4) EK and ryegrass (EK-R). After 30d of ryegrass growth, EK at 1.0 V·cm^-1 with polarity reversal (PR-EK) was supplied for another 30 d. The electric current was recorded during remediation. The pH, electrical conductivity, total petroleum hydrocarbon content (TPH), 16S rDNA, functional genes of AlkB, Nah, and Phe, DGGE, and dehydrogenase activity in soil were measured. The physical-chemical indexes of the plant included the length, dry mass, and chlorophyll contents of the ryegrass. Results showed that EK-R removed 18.53 ± 0.53% of TPH, which was higher than that of other treatments (13.34-14.31%). Meanwhile, the values of 16S rDNA, AlkB, Nah, Phe, and dehydrogenase activity in the bulk soil of EK-R all increased. Further clustering analysis with numbers of genes and DGGE demonstrated that EK-R was similar to the ryegrass rhizosphere soils in both EK-R and CK-R, while the EK treatment of EK-NR was similar to that of CK-NR without EK and ryegrass. These results indicate that the PR-EK treatment used in this experiment successfully enlarged the existing scale of the rhizosphere microorganisms, improved microbial activity and enhanced degradation of TPH.

Comparing storage battery and solar cell in assisting Eucalyptus Globulus to phytoremediate soil polluted by Cd, Pb, and Cu

Metal decontamination and leaching alleviation capacity of Eucalyptus globulus with and without electric field were investigated using ICP-MS. The biomass production of the chosen plant increased from 0.87 kg in planting control without electrokinetic treatment to 1.16 and 1.42 kg in experiments with electric field supplied by storage battery and solar cell, respectively. Under the influence of electric field with a voltage of 6.5 V, significantly more Cd, Pb and Cu were extracted by the species. Precipitation simulation was performed to evaluate the capacity of battery and solar panel to intercept leaching. The total volume of leachate gathered from the control decreased from 1012 mL to 299 and 336 mL in containers treated by storage battery and solar cell, respectively. In addition to reduction of leachate, the leaching mass of Cd, Pb and Cu was decreased significantly by electric fields (both battery and solar cell) treatments. The effect of remediation and environmental risk alleviation by solar cell was comparable with storage battery, at least during the 30-day experimental period. On the basis of the present study, solar cell should be a suitable substitute for conventional power supply to improve metal polluted soil when considering phytoremediation efficiency and energy consumption.

Using solar cell to phytoremediate field-scale metal polluted soil assisted by electric field

Eucalyptus globulus were used to remediate a real scale site endangered by e-waste with electric fields supplied by solar cell and conventional storage battery. The capacity of the species to produce biomass, absorb pollutants and decontaminate metals, as well as the soil moisture of various layers under different treatments was compared. During the 3-month experiment, the output potential of solar cell influenced by weather conditions was less stable (ranging from 0 to 8.3 V) comparing with traditional power supply. Solar cell and storage battery stimulated the growth of the species from 5.92 in control to 7.21 and 7.38 kg per plant, respectively, demonstrating their similar improvement effect. Electric fields of either power source increased the metal concentrations of plant roots and shoots in equal proportions and subsequently greatly promoted the efficiency to decontaminate pollutants. Relative to the control without electric field, solar cell and storage battery treatments reduced the soil moisture of each corresponding layer and consequently, alleviated the leaching risk. At the termination of the experiment, metals tended to distribute in the surface layer under electric field assisted phytoremediation either by solar cell or storage battery. Comparing with conventional battery, solar cell has similar effect on improving remediation and mitigating leaching risk, but is less energy consuming and easier to manage, especially under real scale field. Solar cell treatment was suggested to be a suitable supplementary means to improve phytoremediation efficiency.

Can electrochemistry enhance the removal of organic pollutants by phytoremediation?

An electrokinetic-assisted phytoremediation test using maize (Zea mays L.) was conducted in order to assess the role of the electric field on the enhancement of plant uptake and degradation of the moderate polar pesticide atrazine in spiked soils. Twelve different treatments, including two different initial atrazine soil doses (5 and 10 mg kg^-1) and two different values of the electric field applied (2 and 4 V cm^-1), together with the corresponding control treatments without plants and/or without electric current, were tested. The application of an electric field during a period of 4 h a day and with periodical polarity inversion (each 2 h) did not caused significant changes in soil pH; moreover, maize plants increased the buffering capacity of the soil. The application of an electric field of 2 V cm^-1 led to a slight decrease on maize biomass while the accumulation of atrazine and its main metabolites in plant tissues was significantly enhanced. On the overall, the yield of atrazine removal by electrokinetic-assisted phytoremediation with maize was increased up to 36.5% with respect to the phytoremediation process without electricity. On our knowledge, this work is the first one specifically focused on the removal of organic pollutants from soils by using the combination of phytoremediation and electrokinetic remediation.

A multi-technique phytoremediation approach to purify metals contaminated soil from e-waste recycling site

Multiple techniques for soil decontamination were combined to enhance the phytoremediation efficiency of Eucalyptus globulese and alleviate the corresponding environmental risks. The approach constituted of chelating agent using, electrokinetic remediation, plant hormone foliar application and phytoremediation was designed to remediate multi-metal contaminated soils from a notorious e-waste recycling town. The decontamination ability of E. globulese increased from 1.35, 58.47 and 119.18 mg per plant for Cd, Pb and Cu in planting controls to 7.57, 198.68 and 174.34 mg per plant in individual EDTA treatments, respectively, but simultaneously, 0.9-11.5 times more metals leached from chelator treatments relative to controls. Low (2 V) and moderate (4 V) voltage electric fields provoked the growth of the species while high voltage (10 V) had an opposite effect and metal concentrations of the plants elevated with the increment of voltage. Volumes of the leachate decreased from 1224 to 134 mL with voltage increasing from 0 to 10 V due to electroosmosis and electrolysis. Comparing with individual phytoremediation, foliar cytokinin treatments produced 56% more biomass and intercepted 2.5 times more leachate attributed to the enhanced transpiration rate. The synergistic combination of the individuals resulted in the most biomass production and metal accumulation of the species under the stress condition relative to other methods. Time required for the multi-technique approach to decontaminate Cd, Pb and Cu from soil was 2.1-10.4 times less than individual chelator addition, electric field application or plant hormone utilization. It's especially important that nearly no leachate (60 mL in total) was collected from the multi-technique system. This approach is a suitable method to remediate metal polluted site considering its decontamination efficiency and associated environmental negligible risk.

Efficiency of nanoscale zero-valent iron on the enhanced low molecular weight organic acid removal Pb from contaminated soil

The Pb removal efficiencies from contaminated soils by low molecular weight organic acid (LMWOA) and nanoscale zero-valent iron (nZVI) were investigated through batch soil washing experiments. Results showed that significant promotion on Pb-removal with the mixed solutions of LMWOA and nZVI (p < 0.05). The Pb removal efficiencies reached 64% and 83% for mine and farmland soil by addition of 0.2 M citric acid and 2.0 g L−1 nZVI, respectively. They decreased with increasing pH from 3 to 9. The mixed solutions of LMWOA and nZVI induced Pb(II) releases processes including a rapid desorption within 4 h and a slow desorption in the following hours. The second-order model was the most appropriate for describing the kinetic processes of Pb(II) desorption. The main fractions of Pb removal were exchangeable and reducible. Compared with LMWOA, the loss rates of nitrogen, phosphorus and potassium decreased after washing with the mixed solutions. Our study suggests that combining of LMWOA and nZVI would be a promising alternative approach for remediation Pb-contaminated soils.

Field application of electrokinetic remediation for multi-metal contaminated paddy soil using two-dimensional electrode configuration

In this study, we evaluated the feasibility of in situ electrokinetic remediation for arsenic (As)-, copper (Cu)-, and lead (Pb)-contaminated soil, in a pilot-scale field application with two-dimensional electrode configurations. Square and hexagonal configurations with different electrode spacing, 1 m and 2 m, were investigated under a constant 100 V. A square configuration with electrode spacing of 2 m removed 61.5 % of As, 11.4 % of Cu, and 0.9 % of Pb, respectively, and a hexagonal configuration with the same spacing showed a higher removal efficiency in top (59 % of As, 0-0.5 m) and middle (53 % of As, 0.5-1.0 m) layers, but much lower removal efficiency in the bottom layer (1-1.5 m), which was thought to be due to groundwater flow through periodic rise and fall of tides. Fractionation analysis showed that As bound to Fe-Mn oxyhydroxide was the main form of As removed by the electrokinetic process. The two-dimensional configuration wasted less electrical energy by Joule heating, and required fewer electrode installations, compared to the one-dimensional electrode configuration.

Electrokinetic-enhanced phytoremediation of soils: status and opportunities

Phytoremediation is a sustainable process in which green plants are used for the removal or elimination of contaminants in soils. Both organic and inorganic contaminants can be removed or degraded by growing plants by several mechanisms, namely phytoaccumulation, phytostabilization, phytodegradation, rhizofiltration and rhizodegradation. Phytoremediation has several advantages: it can be applied in situ over large areas, the cost is low, and the soil does not undergo significant damages. However, the restoration of a contaminated site by phytoremediation requires a long treatment time since the remediation depends on the growth and the biological cycles of the plant. It is only applicable for shallow depths within the reach of the roots, and the remediation efficiency largely depends on the physico-chemical properties of the soil and the bioavailability of the contaminants. The combination of phytoremediation and electrokinetics has been proposed in an attempt to avoid, in part, the limitations of phytoremediation. Basically, the coupled phytoremediation-electrokinetic technology consists of the application of a low intensity electric field to the contaminated soil in the vicinity of growing plants. The electric field may enhance the removal of the contaminants by increasing the bioavailability of the contaminants. Variables that affect the coupled technology are: the use of AC or DC current, voltage level and mode of voltage application (continuous or periodic), soil pH evolution, and the addition of facilitating agents to enhance the mobility and bioavailability of the contaminants. Several technical and practical challenges still remain that must be overcome through future research for successful application of this coupled technology at actual field sites.