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).     

Multivariate Study on Phenanthrene Sorption in Soils

Batch tests and different statistical tools of data analysis were used to re-evaluate the overall effect of soil characteristics and liquid phase composition on the extent of phenanthrene adsorption in complex soil-water systems. The linear isotherm models was capable of adequately describing the equilibrium data under extremely varying conditions of soil type, environmental conditions (pH, temperature, ionic strength) and amendments (surfactant, oil, dispersing agent, glucose). Consistent with existing mechanistic models, the multivariate approach also identified the organic carbon content (foc) of soil as the key parameter controlling the phenanthrene adsorption constant (Kd) in nonamended systems (Koc was 17,700 mL/g). From studying the effect of the amendments, two interactions (surfactant-pH and surfactant-oil) and two main effects (surfactant and oil) have been detected. An empirical linear model of Kd as a function of foc, pH, oil content of soil, and surfactant dose was developed for the range of conditions studied. The proposed model and modeling approach can be adapted to other types of contaminants or variables for specific natural and engineered systems.     

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.     

Simultaneous Removal of Naphthalene and Sulfur Dioxide using Surfactant

Anionic surfactant was added during absorption to investigate the solubility of vapor phase naphthalene and SO2 in water. Anionic surfactant employed was sodium dodecyl sulfate. Lower than critical micelle concentration (CMC), the apparent solubility and absorption rates of SO2 or naphthalene with or without SO2 were practically identical to those of pure water. However, higher than CMC, equilibrium SO2 or naphthalene apparent solubility increased linearly in proportion to the surfactant concentrations. The solubilization effect of micelles resulted in the increase. Because the micelle solubilization effect was greater than that of the decrease of the mass transfer coefficient, the gas absorption rate increased. When surfactant concentration was 0.1 M, the enrichment factor (EF) value of naphthalene with SO2 was 4.54, which was only half of its value without SO2. When surfactant concentration was 0.2 M, the SO2 EF values increased to 2.24. These empirical findings confirm that to increase the removal efficiency of simultaneous absorption of hydrophobic organic compounds and SO2 via a spray or packed tower, an anionic surfactant can be employed.     

Simultaneous Removal of Cd and Cr(VI) Using Fe-Loaded Zeolite

Current research focuses on the simultaneous removal of Cd and Cr(VI) in water by a newly developed material having both abilities of sorption and electrochemical reduction. The material was derived from the zeolite modified by Fe(II) chloride followed by sodium borohydride reduction. The Fe-loaded zeolite simultaneously removed Cd and Cr(VI) to below the detection limit at a fairly rapid rate within 1?h for Cd and within 20?h for Cr(VI), under the pH ranging from slightly acid to around neutral. At high concentration of coexisting Cr(VI), the removal efficiency of Fe-loaded zeolite for Cd slightly decreased due to surface fouling by Cr(III) hydroxide precipitations. On the contrary, the coexisting Cd was found to increase the removal rate of Cr(VI) by Fe-loaded zeolite. From the test results, the Fe-loaded zeolite was found to be a possible alternative in simultaneous removal of Cd and Cr(VI) in the aqueous phase.     

Removal of Lead from Contaminated Water and Clay Soil Using a Biosurfactant

Lead removal from water and contaminated soils was investigated using biosurfactant, anionic, and nonionic surfactants in continuously stirred batch reactors. Lead-contaminated water up to 100?mg/L and clay soil up to 3,000?mg/kg were used in this investigation. The surfactant concentration up to 10 critical micelle concentration was used. The speciation of lead into the micelles was quantified and the lead removal efficiency depended on the level of contamination, surfactant type, and concentration. Of the surfactants used, biosurfactant (produced from used vegetable oil) had the best removal efficiency (75%) at a lead contamination of 100?mg/L in water at pH of over 12. The Fourier-transformed infrared spectroscopy study showed that the carboxyl group in the biosurfactant was effective in removing the lead from the solution. Langmuir and Freundlich relationships were used to represent the micelle partitioning of lead in the surfactant solutions. Desorption of lead from contaminated kaolinite clay was represented using linear isotherms. The biosurfactant solution had a higher micelle partitioning for the lead from contaminated water and desorbing the lead from the contaminated soil compared to the other chemical surfactants.     

Removal of Copper Ions from Waters Using Surfactant-Enhanced Hybrid PAC/MF Process

This paper deals with the removal of copper ions from aqueous solutions by using surfactant-enhanced powdered activated carbon (PAC)/microfiltration (MF) hybrid process, including the evaluation of process performance and fouling dynamics at various linear alkyl benzene sulfonate (LABS), PAC, and Cu2+ concentrations of feed solution. Although the use of surfactant as an additive material increased the adsorption efficiency in PAC/MF hybrid process, a considerable amount of the flux was lost for surfactant concentration above critical micelle concentration. The process could be employed with a performance of 74.7%, 97.2% and 87?L/m2?h for LABS rejection, Cu2+ rejection and permeate flux at the conditions of 2?g PAC/L, 5?mM LABS, 0.2?mM Cu2+, and 60-min process time. Cu2+ rejection, which increased with increasing of LABS, and PAC amounts decreased with the increase in Cu2+ concentration. It was understood that the increments in LABS, PAC, and Cu2+ concentrations being an indicator for the feed solution quality led to the occurrence of more fouling on the membrane. The analyses of dynamics concerning the fouling behaviors, which were carried out using single and combined pore blocking models, put forward that the cake formation was the main predominant mechanism in the process. It was also determined that the variation of feed contents deduced the presence of rather complex fouling behaviors as a simultaneous function of secondary membrane layer formation and clogging and narrowing of membrane pores by surfactants.     

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.     

Effect of Loading in Soil Slurry-Sequencing Batch Reactors on Biosurfactant Production and Foaming

A soil contaminated with diesel fuel (DF) was treated in 8-L soil slurry sequencing batch reactors with 10-day retention times and different volumetric loadings: 5, 10, and 50% of the reactor volume per cycle. Concentrations of DF, DF-degrading microorganisms, and biosurfactant were measured, with emulsification capacity (EC), foam thickness, and O2 uptake. Foaming coincided with nonzero values of EC, a measure of free (i.e., non-DF-bound) surfactants. Higher surfactant levels increased DF emulsification and foaming and reduced DF stripping. Concentrations of Candida tropicalis, Brevibacterium casei, Flavobacterium aquatile, Pseudomonas aeruginosa, and Pseudomonas fluorescens were determined. Biosurfactant production and DF degradation increased with increased loading. Biosurfactants exceeded the critical micelle concentration early in the cycle but were completely degraded by the cycle’s end. Orders-of-magnitude differences in effluent concentrations of individual species were observed. Culture-based counts of surfactant-producing species (C. tropicalis, P. aeruginosa, P. fluorescens) relative to total counts increased from 21 to 86% as loading increased from 5 to 50%.     

Reinforced Soil Using Cohesive Fill and Electrokinetic Geosynthetics

An electrokinetic geosynthetic (EKG), is a polymeric geosynthetic material, enhanced to conduct electricity, which can be used to transport water in fine-grained soils by electrokinetic means. This paper describes the design, construction details, and analysis of a reinforced soil wall using EKG and wet cohesive fill. In order to establish an initial design layout, a long-term stability analysis of the wall was carried out using the soil’s critical state shear strength parameters. The long-term design was then checked for short-term stability based upon a minimum required undrained shear strength for the clay utilizing four different short-term analytical methods: critical height, Coulomb, discrete theory, and composite theory. The electroosmosis design was then undertaken, based upon the water content—undrained shear strength curve for the fill material ascertained from laboratory testing. Using this curve the difference between the as-placed water content and the water content corresponding to an undrained shear strength of 20?kPa was calculated, giving the volume of water that needed to be removed from each lift of clay fill. Using this volume of water the electroosmosis calculations were undertaken. A simplistic analysis was undertaken using a linear voltage gradient and fixed soil parameters, followed by a more complex analysis using finite difference techniques to establish the voltage gradient. The variation in the value of electro-osmotic permeability ke were estimated using both an empirical model and a graphical interpretation of the actual variation of ke measured in the laboratory. The results of these analyses yielded estimated treatment times and undrained shear strength of the clay.     

Removal of Hexachlorobenzene and Phenanthrene from Clayey Soil by Surfactant- and Ultrasound-Assisted Electrokinetics

Removal of nonpolar contaminants such as most organic compounds are transported primarily by electroosmosis in electrokinetic remediation, thus the process is effective only if the contaminants are soluble in pore fluid. Hydrophobic organic compounds such as hexachlorobenzene (HCB) and phenanthrene (PHE) can adsorb strongly to clayey soil. Therefore, in this study, enhancements were done by adding 2-hydroxylpropyl-β-cyclodextrin surfactant and ultrasonication comparably to assist the electrokinetic treatment in improving the mobility of these hydrophobic compounds. The results show that HCB and PHE were mobilized and removed in both cases. But HCB is more difficult to remove than PHE, because of its highly stable nature and low water-solubility property. Ultrasound-assisted test performed better PHE reduction than surfactant-assisted test, because ultrasound can degrade the contaminant through oxidation by free radicals.     

BTEX Removal from Produced Water Using Surfactant-Modified Zeolite

Produced water (water generated during recovery of petroleum) contains large amounts of various hazardous organic compounds such as benzene, toluene, ethylbenzene, and xylenes (BTEX). With increasing regulations governing disposal of this water, low-cost treatment options are necessary. This study evaluated the effectiveness of surfactant-modified zeolite (SMZ) for removal of BTEX from produced water. The long-term effectiveness of SMZ for BTEX removal was investigated along with changes in sorption properties with long-term use. The results of these investigations show that SMZ completely removes BTEX from produced water up to a compound-specific capacity, and that SMZ can be regenerated via air sparging without loss of sorption capacity. The BTEX mobility in laboratory columns of SMZ was in the order of decreasing water solubility and increasing Kow. The most soluble compound, benzene, began to elute at 8 pore volumes (PV), while the least soluble compounds, ethylbenzene and xylenes, began to elute at 50 PV. After treating 4,500 PVs of water in the column system over 10 sorption/regeneration cycles, no significant reduction in sorption capacity of the SMZ for BTEX was observed. The mean Kds determined in these column experiments ranged from 18.3?L/kg for benzene to 95.0?L/kg for p- and m-xylene. Laboratory columns were upscaled to create a field-scale SMZ treatment system. The field-scale system was tested at a natural gas produced-water treatment facility near Wamsutter, Wyo. We observed even greater sorption of BTEX in the field column than predicted from the laboratory results. In the field column, initial benzene breakthrough occurred at 10 PV and toluene breakthrough began at 15 PV, and no breakthrough of ethylbenzene or xylenes occurred throughout the 80 PV experiment. The field and laboratory results, along with the low price of SMZ (about $460?per?metric?t), suggest that SMZ has a potential role in a cost-effective produced water treatment system.     

Sediment Removal Efficiency of Vortex Chamber Type Sediment Extractor

Experimental results on sediment removal efficiency of vortex chamber type sediment extractors are reported. A geometric configuration of the extractor is identified that is able to remove even the fine sediment (0.055 ? d ? 0.22?mm) from flow with high efficiency. For the purpose of analysis, the experimental data of present study are compiled with the data from other laboratories and the field that were collected previously by other investigators. Since the existing relations are not found to produce satisfactory results, a new relationship is developed for determination of the sediment removal efficiency of the vortex chamber type sediment extractors.     

Enhanced Electrokinetic Remediation of Heavy Metals in Glacial Till Soils Using Different Electrolyte Solutions

Previous electrokinetic remediation studies involving the geochemical characterization of heavy metals in high acid buffering soils, such as glacial till soil, revealed significant hexavalent chromium migration towards the anode. The migration of cationic contaminants, such as nickel and cadmium, towards the cathode was insignificant due to their precipitation under the high pH conditions that result when the soil has a high acid buffering capacity. Therefore the present laboratory study was undertaken to investigate the performance of different electrolyte (or purging) solutions, which were introduced to either dissolve the metal precipitates and/or form soluble metal complexes. Tests were conducted on a glacial till soil that was spiked with Cr(VI), Ni(II), and Cd(II) in concentrations of 1,000, 500, and 250 mg/kg, respectively, under the application of a 1.0 VDC/cm voltage gradient. The electrolyte solutions tested were 0.1M EDTA (ethylenediaminetetraacetic acid), 1.0M acetic acid, 1.0M citric acid, 0.1M NaCl/0.1M EDTA, and 0.05M sulfuric acid/0.5M sulfuric acid. The results showed that 46–82% of the Cr(VI) was removed from the soil, depending on the purging solution used. The highest removal of Ni(II) and Cd(II) was 48 and 26%, respectively, and this removal was achieved using 1.0M acetic acid. Although cationic contaminant removal was low, the use of 0.1M NaCl as an anode purging solution and 0.1M EDTA as a cathode purging solution resulted in significant contaminant migration towards the soil regions adjacent to the electrodes. Compared to low buffering capacity soils, such as kaolin, the removal of heavy metals from the glacial till soil was low, and this was attributed to the more complex composition of glacial till. Overall, this study showed that the selection of the purging solutions for the enhanced removal of heavy metals from soils should be primarily based upon the contaminant characteristics and the soil composition.     

New Approach: Waste Materials as Sorbents for Arsenic Removal from Water

The sorption of inorganic arsenic species (arsenite and arsenate) from aqueous solutions onto steel-mill waste and waste filter sand, under neutral conditions, was investigated in this study. Additionally, the steel-mill waste material was modified in order to minimize its deteriorating impact on the initial water quality and to meet the drinking water standards. The influence of contact time and initial arsenic concentration was investigated using batch system techniques. To evaluate the application for real groundwater treatment, the capacities of the obtained waste materials were further compared to those exhibited by commercial sorbents, which were examined under the same experimental conditions. Kinetic studies revealed that waste slag materials are the most efficient in arsenic removal, reaching equilibrium arsenic sorption capacities in the range 47.6–55.2?μg/g, while waste filter sand exhibited capacities of 25.4–29.8?μg/g (for an initial arsenic concentration Co = 0.5?mg/L). The higher iron content in the slag materials was considered to be responsible for the better removal efficiencies, and the specific arsenic removal efficiency was estimated to be 220?μgAs/gFe. The specific arsenic removal efficiency of the second active substance found in waste filter sand, manganese, was estimated to be 115?μgAs/gMn. Equilibrium studies revealed the occurrence of both chemisorption and physical sorption processes. All the waste materials exhibited higher performances for As(V). The highest maximum sorption capacity was obtained by waste iron slag: 4040?μg/g for As(V). The waste materials reached the arsenic removal capacities of the examined commercial materials, suggesting the feasibility of their application in real groundwater treatment.     

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.     

Laboratory-Scale Flotation Process for Treatment of Soils Contaminated with Both PAH and Lead

A soil decontaminating process has been studied at laboratory scale for the treatment of one soil polluted by both polycyclic aromatic hydrocarbons (PAHs) and lead (Pb). This process first includes attrition and sieving steps to separate the coarse (>2?mm) from the fine (<2?mm) fractions, followed by a flotation step using an amphoteric surfactant in acid and saline conditions for the treatment of the fine contaminated particles. Electrodeposition and chemical precipitation using sodium hydroxide have been compared to ensure a possible reuse of wastewaters without disturbing the efficiency of the process. The performance of the process has been estimated considering soil quality after treatment with respect to the limit regulatory levels for commercial or industrial use in Quebec (Canada). Precipitation of lead hydroxides was efficient after five cycles of wastewaters reuse, while electrodeposition did not maintain efficiency of the flotation step with regard to PAH levels in soil after treatment. The complete process including Pb precipitation ensured the removal of 89±8 and 76±10% of total PAH, respectively, for the coarse (>2?mm) and fine (<2?mm) fractions, while Pb was removed at 88±10 and 65±2%, respectively, for the same fractions of the soil.     

Removal of Inorganic Mercury from Polluted Water Using Structured Nanoparticles

This paper presents a process for the removal of inorganic mercury from aqueous solutions using alumina nanoparticles, which were prepared by the sol-gel method. Different amounts of mercury were added to the particles until a critical concentration was achieved, thus inducing the alumina sol flocculation. Particle growth was monitored during the process using dynamic light scattering. The amount of metal ion adsorbed on the surface of the alumina sols was determined by atomic absorption spectroscopy. Initial mercury concentrations ranging between 50 and 100 ppm decreased to below 1 ppb in a short time.     

Removal of Mercury from Low-Concentration Aqueous Streams Using Chemical Reduction and Air Stripping

Field, laboratory, and engineering data confirmed the efficacy of chemical reduction and air stripping as a low concentration mercury treatment concept for water containing Hg(II). The process consists of dosing the water with low levels of stannous chloride [Sn(II)] to convert the mercury to elemental mercury (Hg0). Hg0 can easily be removed from the water by air stripping or sparging. We studied this concept for groundwater containing initial mercury concentrations of approximately 138 ng/L (0.00069 μmol/L). In undosed samples, sparging removed 0% of the initial mercury. Removal in the treated samples varied by reagent dose. Low reagent doses, with Sn:Hg stoichiometric ratios <1, showed little removal. High reagent doses, with Sn:Hg stoichiometric ratios greater than about 5 to 25, showed relatively complete removal (>94%) and yielded final mercury concentrations <10 ng/L (<0.00005 μmol/L). At intermediate doses, mercury removal was a function of the dose. A kinetic study indicated that addition of the Sn(II) reagent resulted in rapid reduction of Hg(II) to Hg0. When combined with standard supporting engineering techniques (e.g., treating the purge air) as needed, a simple system of chemical reduction and stripping may be useful and cost effective.     

Lead Removal from Acidic Solutions by Sorption on Cocoa Shells: Effect of Some Parameters

The objectives of this work were to evaluate the effects of different parameters (Pb concentration, solution pH, Ca/Mg/Na/K salt concentrations) on Pb uptake by cocoa shells and to study the mechanisms of Pb removal in very acidic conditions. Sorption tests were conducted in shaken flasks with synthetic Pb solutions and 15 g/L cocoa shells. A lead uptake value of 161 mmol/kg was measured during the assay with [Pb]i = 3.66?mmol/L in solution at pHi = 2.0 and T = 22°C. Results show that Pb uptake is very similar (14.5–16.0 mmol/kg) for an initial pH between 2.0 and 4.0, but a moderate decrease (10.8 mmol/kg) occurred when the initial pH=1.5 and the [Pb]i = 0.25?mmol/L. High Ca and Mg concentrations (2.35 mol/L) in solution induced a significant decrease in Pb removal by cocoa shells, whereas high K and Na concentrations did not affect Pb uptake by this sorbent. This suggests that Pb uptake by cocoa shells is controlled by ion-exchange reactions with Ca/Mg ions and protons. Finally, chemical tests have shown that carboxyl and amine functional groups play a dominant role in Pb uptake by cocoa shells.