Sequentially Enhanced Electrokinetic Remediation of Heavy Metals in Low Buffering Clayey Soils

This paper presents the results of an experimental investigation undertaken to evaluate different purging solutions to enhance the removal of multiple heavy metals, particularly chromium, nickel, and cadmium, from a low buffering clay, specifically kaolin, during electrokinetic remediation. Experiments were conducted on kaolin spiked with Cr(VI), Ni(II), and Cd(II) in concentrations of 1,000, 500, and 250 mg/kg, respectively, which simulate typical electroplating waste contamination. A total of five different tests were performed to investigate the effect of different electrode purging solutions on the electrokinetic remedial efficiency. A constant DC voltage gradient of 1 V/cm was applied for all the tests. The removal of heavy metals from the soil using tap water as the purging solution was very low. When 1 M acetic acid was used as the purging solution in the cathode, the removal of chromium, nickel, and cadmium was increased to 20, 19, and 13%, respectively. Using 0.1 M ethylene diamine tetraacetic acid as the purging solution in the cathode, 83% of the initial Cr was removed; however, the nickel and cadmium removal was very low. A sequentially enhanced electrokinetic remediation approach involving the use of water as a purging solution at both the anode and cathode initially, followed by the use of acetic acid as the cathode purging solution and a NaOH alkaline solution as the anode purging solution was tested. This sequential approach resulted in a maximum removal of chromium, nickel, and cadmium of 68–71, 71–73, and 87–94%, respectively. This study shows that the sequential use of appropriate electrode purging solutions, rather than a single electrode purging solution, is necessary to remediate multiple heavy metals in soils using electrokinetics.     

Complicating Factors of Using Ethylenediamine Tetraacetic Acid to Enhance Electrokinetic Remediation of Multiple Heavy Metals in Clayey Soils

Batch and electrokinetic experiments were conducted to investigate the removal of three different heavy metals, chromium(VI), nickel(II), and cadmium(II), from a clayey soil by using ethylenediamine tetraacetic acid (EDTA) as a complexing agent. The batch experiments revealed that high removal of these heavy metals (62–100%) was possible by using either a 0.1?M or 0.2?M EDTA concentration over a wide range of pH conditions (2–10). However, the results of the electrokinetic experiments using EDTA at the cathode showed low heavy metal removal efficiency. Using EDTA at the cathode along with the pH control at the anode with NaOH increased the pH throughout the soil and achieved high (95%) Cr(VI) removal, but the removal of Ni(II) and Cd(II) was limited due to the precipitation of these metals near the cathode. Apparently, the low mobility of EDTA and its migration direction, which opposed electroosmotic flow, prevented EDTA complexation from occurring. Overall, this study found that many complicating factors affect EDTA-enhanced electrokinetic remediation, and further research is necessary to optimize this process to achieve high contaminant removal efficiency.     

Sequential Electrokinetic Remediation of Mixed Contaminants in Low Permeability Soils

The coexistence of heavy metals and polycyclic aromatic hydrocarbons (PAHs) at many of the contaminated sites poses a severe threat to public health and the environment. Very few technologies, such as soil washing/flushing and stabilization/solidification, are available to remediate such sites; however, these technologies are ineffective and expensive to treat contaminants in low permeability clayey soils. Previous studies have shown that electrokinetic remediation has potential to remove heavy metals and organic compounds when they exist individually in clayey soils. In the present study, the feasibility of using surfactants and organic acids sequentially and vice versa during electrokinetic remediation was evaluated for the removal of both heavy metals and PAHs from clayey soils. Kaolin was selected as a model clayey soil and it was spiked with phenanthrene and nickel at concentrations of 500 mg/kg dry each to simulate typical field mixed contamination. Bench-scale electrokinetic experiments were performed with the sequential anode conditioning with: (1) 1 M citric acid followed by 5% Igepal CA-720; (2) 1 M citric acid followed by 5% Tween 80; and (3) 5% Igepal CA-720 followed by 1 M citric acid. A periodic voltage gradient of 2 V/cm (with 5 days on and 2 days off cycles) was applied in all the tests. A removal of about 96% of phenanthrene was observed in the test with 5% Igepal CA-720 followed by 1 M citric acid sequence. Most of the nickel (>90%) migrated from anode to cathode in this test; however, it precipitated in the section very close to the cathode due to the high pH conditions. Conversely, the removal efficiency of nickel was about 96 and 88% in the tests with 1 M citric acid followed by 5% Igepal CA-720 sequence and 1?M citric acid followed by 5% Tween 80 sequence, respectively. However, the migration and removal efficiency of phenanthrene in both of these tests were very low. Overall, it can be concluded that the sequential use of 5% Igepal CA-720 followed by 1 M citric acid may be an effective remedial strategy to remove coexisting heavy metals and PAHs from clayey soils.     

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).     

Iodide-Enhanced Electrokinetic Remediation of Mercury-Contaminated Soils

This study investigates using an iodide-enhanced solution at the cathode during electrokinetic treatment to optimize the removal of mercury from soils. The experimental program consisted of testing two types of clayey soils, kaolin, and glacial till, that were initially spiked with 500 mg/kg of Hg(II). Experiments were conducted on each soil type at two voltage gradients (1.0 or 1.5 VDC/cm) to evaluate the effect of the voltage gradient when employing a 0.1 M KI solution. Additional experiments were performed on each soil type to assess the effect of using a higher iodide concentration (0.5 M KI) when using a 1.5 VDC/cm voltage gradient. The tests conducted on the kaolin soil showed that when the 0.1 M KI concentration was employed with the 1.0 VDC/cm voltage gradient, approximately 97% of the mercury was removed, leaving a residual concentration of 16 mg/kg in the soil after treatment. The tests conducted on glacial till indicated that it was beneficial to use the higher (0.5 M KI) iodide concentration and the higher (1.5 VDC/cm) voltage gradient to enhance mercury removal, because, under these conditions, a maximum of 77% of the mercury was removed from the glacial till, leaving a residual concentration of 116 mg/kg in soil after electrokinetic treatment. Compared to kaolin, the lower mercury removal from the glacial till soil is attributed to the more complicated soil composition, such as the presence of carbonates and organic matter, which caused Hg(II) to adsorb to the soil and/or exist as an immobile chemical species.     

Enhanced Soil Flushing for Simultaneous Removal of PAHs and Heavy Metals from Industrial Contaminated Soil

Polycyclic aromatic hydrocarbons (PAHs) and heavy metals are environmental concerns and must be removed to acceptable levels. This paper evaluates different flushing agents to enhance the remediation of soil contaminated with PAHs and heavy metals at a former manufactured gas plant site. Four flushing column tests at a constant hydraulic gradient of 1.2 were conducted using four different flushing agents, which included deionized water, chelant (0.2?M EDTA), surfactant (5% Igepal CA-720), and cyclodextrin (10% hydroxypropyl-β-cyclodextrin or HPCD). Additional column tests using Igepal and HPCD at a lower hydraulic gradient of 0.2 were conducted to investigate the effects of rate-limited desorption or solubilization of PAHs. The results showed that the EDTA produced the maximum metal removal from the soil compared with deionized water, Igepal, and HPCD under different hydraulic gradient conditions. The 0.2?M EDTA flushing solution removed approximately 25–75% of the toxic heavy metals found in the soil. None of the PAHs were removed from the soil when deionized water and EDTA were the flushing solutions. The PAHs removal efficiencies in the Igepal and HPCD systems decreased as the hydraulic gradient decreased. However, the surfactant-enhanced systems were more efficient in removing PAHs from the soil than the HPCD systems under high- and low-hydraulic gradients. The results also demonstrated that the removal of PAHs in surfactant-enhanced systems depended upon the micelles formation, whereas in the HPCD-enhanced systems, it depended upon the sterioselective diffusion of the PAHs to the nonpolar cavity of the HPCD. Overall, this study showed that the contaminant removal in soil flushing systems depends on the flushing solution affinity and selectivity toward the target contaminant and the existing hydraulic gradient condition.     

Cyclodextrin-Enhanced Electrokinetic Remediation of Soils Contaminated with 2,4-Dinitrotoluene

The removal of 2,4-dinitrotoluene (2,4-DNT), a munitions waste constituent and an industrial intermediate, from contaminated soils was evaluated using enhanced electrokinetic (EK) remediation. Two model soils were spiked with 480?mg of 2,4-DNT/kg of dry soil for the EK experiments. The spiked soils were kaolin, a low-buffering clayey soil, and glacial till, a high-buffering silty soil. The glacial till was obtained from a field site and contained 2.8% organic matter. Deionized (DI) water and cyclodextrin solutions were used as the EK purging solutions. Cyclodextrin was selected as a nonhazardous solubility enhancer for enhancing the desorption and removal of 2,4-DNT from soils in EK remediation. Two aqueous solutions of hydroxypropyl β-cyclodextrin (HPCD) at concentrations of 1 and 2% were selected for kaolin and glacial till, respectively, based on results for batch extraction of 2,4-DNT from the same soils. During the EK experiments, greater current and electro-osmotic flow were observed for HPCD solutions than for DI water. After the completion of the EK experiments, the soils in the EK cell were extruded and the residual 2,4-DNT in the soils was determined. Less 2,4-DNT remained in the kaolin soil (up to 94% transformed) than in the glacial till soil (20% transformed) due to strong retention of 2,4-DNT by the soil organic matter in glacial till. For kaolin, less 2,4-DNT remained in the soil using HPCD solutions than using DI water. For glacial till, comparable levels of 2,4-DNT remained in the soil for both EK solutions. Since no 2,4-DNT was detected in the effluents from the EK cells, the decrease in 2,4-DNT concentration in the kaolin and glacial till soils was attributed to electrochemical transformation of 2,4-DNT to other species.     

Electrokinetic Remediation Modeling Incorporating Geochemical Effects

Electrokinetic remediation technology is one of the developing technologies that offers great promise for the cleanup of soils contaminated with heavy metals. However, the performance of an electrokinetic remediation system depends on the interaction of a complex set of interrelated system variables and parameters. Many of these interactions were addressed in this study by incorporating geochemical reactions into electrokinetic remediation modeling. A one-dimensional transport model was developed to predict the transport and speciation of heavy metals (chromium, nickel, and cadmium) in soil during electrokinetic remediation as a function of time and space. The model incorporates: (1) pH-dependent adsorption of contaminants to the soil surface; (2) sensitivity of soil surface potential and electroosmotic flow to the pore water properties; and (3) synergistic effects of multiple chemical species on electrokinetic remediation. The model considers that: (1) Electrical potential in the soil is constant with time; (2) surface complexation reactions are applicable in the highly concentrated clay suspensions; (3) the effect of temperature is negligible; and (4) dissolution of soil constituents is negligible. The predicted pH profiles, electroosmotic flow, and transport of chromium, nickel, and cadmium in kaolin soil during electrokinetic remediation were found to reasonably agree with the bench-scale electrokinetic experimental results. The predicted contaminant speciation and distribution (aqueous, adsorbed, and precipitated) allow for an understanding of the transport processes and chemical reactions that control electrokinetic remediation.     

Electrokinetic Remediation of Cadmium-Contaminated Clay

Electrokinetic extraction has been demonstrated to be very effective in removing heavy metals from Georgia kaolinite. The relatively high removal efficiency depends on the extremely acidic soil environment generated by the electrokinetic process. However, the efficiency observed in Georgia kaolinite cannot be achieved in soils of high acid/base buffer capacity without enhancement. In this study, the effect of ethylenediaminetetraacetic acid (EDTA) to enhance electrokinetic extraction of cadmium from Milwhite kaolinite was examined. The influence of electro-osmotic flow direction on the migration of cadmium, EDTA, and their complexes were also investigated. It was observed that injection of EDTA from the cathode reservoir by a reverse electro-osmotic flow could mobilize the cadmium in the specimen effectively. A less significant mobilization of cadmium was observed when the electro-osmotic flow was directed toward the cathode. However, accumulation of cadmium near the anode was observed regardless of the electro-osmotic flow direction.     

Fenton-Like Oxidation of Polycyclic Aromatic Hydrocarbons in Soils Using Electrokinetics

An integrated electrochemical oxidation process that utilizes electrokinetics (EK) to deliver the oxidant (5–10% hydrogen peroxide, H2O2) and chelant [40 mM of ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA)] or iron chelate (1.4 mM Fe-EDTA or Fe-DTPA) to oxidize polycyclic aromatic hydrocarbons (PAHs) in soils was investigated. Batch and bench-scale EK experiments were conducted using: (a) kaolin, a low permeability clayey soil, spiked with phenanthrene at 500 mg/kg, and (b) former manufactured gas plant (MGP) soil, a high buffering silty soil, contaminated by a variety of PAHs (1493 mg/kg). Batch experiments showed that chelant solutions dissolve native iron minerals to form soluble Fe-chelates that remain available even at higher pH conditions of soil for the Fenton-like oxidation of the PAHs. In EK experiments, a 5–10% H2O2 solution was delivered from the anode and a chelant solution or iron-chelate was delivered from the cathode. Preflushing of soil with 5% ethanol and ferrous sulfate (1.4 mM) prior to oxidant delivery was also investigated. An electric potential of 2 VDC/cm was applied in all tests to induce electroosmotic flow for 5–8 days for kaolin and 25 days for the MGP field soil. In the absence of any chelating agent, phenanthrene oxidation was catalyzed by native iron present in kaolin soil, and 49.8–82.3% of phenanthrene was oxidized by increasing H2O2 concentration from 5–10%. At 5% H2O2 concentration, phenanthrene oxidation was not increased by using 40 mM EDTA, 40 mM DTPA or 1.4 mM Fe-DTPA, but it increased to 70% using 1.4 mM Fe-EDTA. Maximum phenanthrene oxidation (90.5%) was observed by 5% ethanol preflushing and then treating with 5% H2O2 at the anode and 1.4 mM Fe-EDTA at the cathode. However, preflushing with 1.4 mM ferrous sulfate did not improve phenanthrene oxidation. The results with the MGP field soil indicated that delivery of 5% H2O2 alone resulted in oxidation of 39.8% of total PAHs (especially 2- and 3-ring PAHs). The use of EDTA and Fe-EDTA did not increase PAHs oxidation in this soil. Overall, the results reveal that an optimized in situ combined technology of EK and Fenton-like process has the potential to oxidize PAHs in low permeability and/or high buffering soils.     

Experimental Studies on Heavy Metal Extraction from Contaminated Soil Using Ammonium Citrate as Alkaline Chelate during the Electrokinetic Process

Researchers have performed experimental studies using ammonium citrate (AC) during the electrokinetic (EK) remediation process for the extraction of cadmium (Cd) and copper (Cu) from the contaminated soil. They evaluated the efficiency of ammonium citrate by considering it as a washing solution and a purging solution at the anode electrode compartment. The efficiency of electrokinetic extraction was observed to be significantly influenced by the pH and buffering nature of the soil medium. The experimental studies indicate that the removal of cadmium and copper was 48.9% and 30.0%, respectively, when ammonium citrate was used both washing and purging solution. The solubility of both cadmium (Cd++) and copper (Cu++) in EK-treated soils has also been estimated by sequential extraction studies with deionized (DI) water. The analytical techniques, X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscope (SEM) provide the evidence of migration of cations during treatment of contaminated soil by process of electroosmosis (EO). The SEM images of both cadmium- and copper-contaminated soils show that these soils have a fluffier and more porous structure. This might be caused by the change in surface charges of the clay particles as a result of introduction of heavy metals. The mineralogical compositions of soil are not altered significantly by electrokinetic process.     

Prediction of Heavy Metals Mobility and Bioavailability in Contaminated Soil Using Sequential Extraction and Biosensors

Several chemical and biological methods have been developed in the last decade to evaluate heavy metals mobility and bioavailability in contaminated soils. In this study, two methods, Biomet sensors and chemical sequential extraction [potentially bioavailable assessment sequential extraction (PBASE) method], were used to predict heavy metals bioavailability in the surface and heavy metals mobility in the subsurface of smelter-contaminated soils, respectively. The heavy metals considered (arsenic, copper, iron, lead, and zinc) were those detected in a previous sampling campaign performed in the contaminated area. Biomet biosensor results indicated that 15–25% of Cu and Zn were bioavailable for plants and animals uptake in the soil surface, whereas higher values were obtained for As and Pb (>60%). In the soil subsurface, iron was identified as the less mobile element, followed by As and Pb, since they were mainly present in the nonsoluble fractions of PBASE method. In contrast, Cu and Zn showed similar distribution between the soluble and nonsoluble fractions. Therefore, PBASE and Biomet are useful and complementary methods which supply different information about heavy metals occurrence in contaminated soils: the first method indicates their potential mobility, whereas the second one shows their potential bioavailability for biota.     

Heavy Metals Removal from Acidic and Saline Soil Leachate Using Either Electrochemical Coagulation or Chemical Precipitation

This study compares electrocoagulation and chemical precipitation for heavy metals removal from acidic soil saline leachate (SSL) at the laboratory pilot scale. The electrocoagulation process was evaluated via an electrolytic cell [12 cm (width)×12 cm (length)×19 cm (depth)] using mild steel electrodes (10 cm width×11 cm high), whereas chemical precipitation was evaluated using either calcium hydroxide [Ca(OH)2] or sodium hydroxide (NaOH). By comparison with chemical precipitation at a pH varying between 7 and 8, electrocoagulation was more effective in removing metals from SSL having a relatively low contamination level (124?mg?Pb/L and 38?mg?Zn/L). For SSL enriched with different heavy metals (each concentration of metals was initially adjusted to 100 mg/L) and treated at a pH lower than 8.5, with the exception of Cd, the residual metal concentrations at the end of the experiments were below the acceptable level recommended for effluent discharge in urban sewage works (less than 4 mg/L of each residual metal concentration was recorded) using electrocoagulation, contrary to chemical precipitation using NaOH (more than 15 mg/L of each residual metal concentration was recorded). By comparison, chemical precipitation using Ca(OH)2 was effective in reducing Cr, Cu, Ni, and Zn under the permissive level, but not for Cd and Pb. However, both chemical precipitation processes needed to be operated at higher pH values (around 10.0) to be more effective in reducing metals from SSL and, therefore, required a pH adjustment of the effluent before discharge, whereas electrochemical treatment had a practical advantage of producing an effluent having a pH close to the neutral value and suitable for stream discharge in the receiving water. On the other hand, electrocoagulation was also found to be very efficient for removing Pb from very contaminated solutions (250–2,000 mg?Pb/L). At least 94% of Pb was removed regardless of the initial Pb concentration in the SSL. Electrochemical coagulation involves a total cost varying from 8.67 to 13.00 $/tds, whereas 0.84 to 16.73 $/tds is recorded using chemical precipitation. The cost included only energy consumption, chemicals consumption, and metallic sludge disposal.     

Evaluating an Electrokinetically Driven Extraction Method for Measuring Heavy Metal Soil Contamination

Measuring the heavy metal burden of “old contamination” soils (soils with aged contamination) can be challenging. Many laboratory procedures are currently in use and these can yield a wide range of burden estimates for the same soil. The appropriate extraction method selection depends on the intended use of the information and on compatibility with the procedures used to generate the “reference” data to which results are compared. In this work, results for an extraction based on the electrokinetic mobilization of old contamination heavy metals were compared to the results of established single analyte and sequential extraction methods. Accomplishing extractions electrokinetically offers promise for simplifying processes and for evaluating the electrokinetic remediation potential of old contamination soils. On the brownfield soils tested, electrokinetic extraction identified an average of 82% of the soil’s Cr, Cu, Pb, and Zn burden relative to Environmental Protection Agency Method 3050B extractions. However, results also indicated that neither of these methods were successful at extracting all of the sequestered (residual) fraction of heavy metals and thus underestimated the total contamination burden of the soil.     

EDTA-Enhanced Washing for Remediation of Pb- and/or Zn-Contaminated Soils

The effects of the operating conditions, the initial concentrations of heavy metals in soils, and the competition among heavy metals during ethylenediaminetetraacetic acid (EDTA)-enhanced soil washing were extensively investigated using batch experiments with Pb- and/or Zn-contaminated soils aiming to determine the heavy-metal removal for different types of contaminations and to optimize the process parameters. Pb or Zn removal efficiency was found to be dependent on contact time, pH, concentrations of EDTA, and their initial concentrations in contaminated soils. The experimental results showed that the heavy-metal removal efficiency increased with a higher initial concentration of heavy metals in soils, and the concentrations of heavy metals in the solutions after washing were linearly correlated with their initial concentrations in soils. The study of the competition among heavy metals indicated that when EDTA was present in solution with the concentration less than the stoichiometric requirements, Pb removal efficiency was higher than that of Zn; on the other hand, when EDTA concentration was greater, Pb and Zn removal efficiencies were almost the same.     

Lead Extraction from Contaminated Soil Using Water-Soluble Polymers

The applicability of water-soluble polymers as extractants for the remediation of heavy metal-contaminated soils has been explored using a lead-contaminated Superfund soil as a sample system. Polyethylenimine (PEI) was functionalized with bromo- or chloroacetic acid to give an aminocarboxylate chelating group, which effectively binds lead. The resulting polymer, PEIC, has extraction properties similar to the molecular chelator ethylenediaminetetraacetic acid. A series of studies was done to probe optimum conditions for lead extraction from soils obtained from the Cal-West Superfund site in New Mexico that contained approximately 10,000 ppm of Pb. In batch extraction experiments using polymer functionalized at two different levels, the polymers removed greater than 97% of the lead from the soils. Subsequent experiments demonstrated that the selective extraction of lead could be controlled by varying polymer functionalization levels. Concentration and regeneration of the polymers using ultrafiltration was also demonstrated. Release of lead from the polymer was accomplished by acidification to pH 1 with HCl. Subsequent ultrafiltration allowed recovery of the extractant polymer for reuse.     

湖南重金属污染现状分析及其修复对策

近年来随着湖南工农业的快速发展,矿产资源的开发,湖南重金属污染日趋严重。文章着重调查了湖南重金属污染现状与己造成的影响,对重金属污染来源与原因进行了分析。同时讨论了修复重金属污染的对策和措施,包括物理化学修复、化学修复和生物修复等措施。     

Clean up of Petroleum-Hydrocarbon Contaminated Soils Using Enhanced Bioremediation System: Laboratory Feasibility Study

The objective of this study was to assess the potential of applying enhanced bioremediation on the treatment of petroleum-hydrocarbon contaminated soils. Microcosm experiments were conducted to determine the optimal biodegradation conditions. The control factors included oxygen content, nutrient addition, addition of commercially available mixed microbial inocula, addition of wood chip and rice husk mixtures (volume ratio = 1:1) as bulking agents, and addition of organic amendments (chicken manures). Results indicate that the supplement of microbial inocula or chicken manures could significantly increase the microbial populations in soils, and thus enhance the efficiency of total petroleum hydrocarbon (TPH) removal (initial TPH = 5,500?mg/kg). The highest first-order TPH decay rate and removal ratio were approximately 0.015?day?1 and 85%, respectively, observed in microcosms containing microbial inocula (mass ratio of soil to inocula = 50:1), nutrient, and bulking agent (volume ratio of soil to bulking agent = 10 to 1) during 155 days of incubation. Results indicate that the first-order TPH decay rates of 0.015 and 0.0142?day?1 can be obtained with the addition of microbial inocula and chicken manures, respectively, compared with the decay rate of 0.0069?day?1 under intrinsic conditions. Thus, chicken manures have the potential to be used as substitutes of commercial microbial inocula. The decay rate and removal ratio can be further enhanced to 0.0196?day?1 and 87%, respectively, with frequent soil shaking and air replacement. Results will be useful in designing an ex situ soil bioremediation systems (e.g., biopile and land farming) for practical application.     

Influence of Citrate on Adsorption of Zinc in Soils

Adsorption of zinc to two soils was quantified in the presence of citrate ranging in concentration from 0 to 10 mM/L. For both soils, Zn adsorption increased with increasing citrate up to a concentration of 2mM/L. With increasing citrate concentrations between 2 and 10 mM/L, Zn adsorption increased in one soil (pH 4.8) but decreased in the second (pH 5.8). Adsorption of a Zn-citrate complex is key to the Zn adsorption in both soils. The soil with the higher pH appears to have a significantly lower adsorption capacity for the complex than the soil with pH 4.8. Activity-based adsorption modeling supports these experimental observations.     

Chemical-Enhanced Washing for Remediation of Soils Contaminated with Marine Diesel Fuel in the Presence/Absence of Pb

The effects of the operating conditions, the initial concentrations of marine diesel fuel (MDF) and the coexisting Pb in the soil, and the ethylene diamine tetra acetic salt (EDTA) in solution on MDF removal by sodium dodecyl sulfate (SDS) washing were extensively investigated with the aim of optimizing the process parameters and determining the MDF removal efficiency by SDS under different contamination conditions. The experimental results from batch tests indicated that the majority of MDF was removed by SDS in the first 2?h, and its optimal pH was nearly neutral. Increasing the SDS concentration linearly increased the MDF removal efficiency. At a given SDS concentration, the removal efficiency was dependent on the existing forms of MDF in soils, and the free phase of MDF was found to be more easily removed than the adsorbed phase. MDF removal by SDS was significantly reduced by the coexisting Pb in soils, which likely forms a complexation with SDS and thereby enhances the partitioning of MDF in the soil by the re-adsorption of released MDF onto the hydrophobic tails of the adsorbed SDS. EDTA alone, or with SDS, could remove MDF, but the remaining MDF in the contaminated soil after EDTA washing became more difficult to be removed by SDS. Therefore, the EDTA washing followed by SDS washing is not recommended for MDF removal.