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.     

ELECTROKINETIC EFFECTS: CARBONIC ANHYDRASE AND CO2 ELECTRODE DYNAMICS

It is well known that the dynamic response rate of the Severinghaus-type CO₂ electrode is improved significantly in some cases by addition of the enzyme carbonic anhydrase to the electrode assembly. Hysteresis in the response rate also is reduced. Experimental data and modelling results indicate that catalysis of the CO₂ hydration reaction in the bulk of the bicarbonate layer (the Nernst film) is not responsible for the improved response behavior, Evidence is presented to show that catalysis in the electrostatic double layer region at the glass electrode surface is a possible explanation. This proposed phenomenon may have widespread implications for the optimal design of analytical devices, commercial processes involving electrochemical phenomena, and may also provide insight into electrobiologi-cal processes.     

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.     

Mechanism of boron tribromide electroreduction

This study was carried out to investigate the electrochemical behavior of boron tribromide in dimethlyformamide. The reduction of the compound was found to follow a CE mechanism. The kinetic parameters and the diffusion coefficient were calculated by the use of ultramicrodisc electrodes and chronoamperometry. The number of electrons transferred was found to be 2 by rotating disc and ultramicro disc electrodes and 3 by coulometry. These results are in good accordance with those obtained from molten boron salts. This study is important in regard to electrochemical boronizing at low temperatures.     

Electrophoretic mobilities of dissolved polyelectrolyte charging agent and suspended non-colloidal titanium during electrophoretic deposition

Coarse (≤20 μm) titanium particles were deposited on low-carbon steel substrates by cathodic electrophoretic deposition (EPD) with ethanol as suspension medium and poly(diallyldimethylammonium chloride) (PDADMAC) as polymeric charging agent. Preliminary data on the electrophoretic mobilities and electrical conductivities on the suspensions of these soft particles as well as the solutions themselves as a function of PDADMAC level were used as the basis for the investigation of the EPD parameters in terms of the deposition yield as a function of five experimental parameters: (a) PDADMAC addition level, (b) solids loading, (c) deposition time, (d) applied voltage, and (e) electrode separation. These data were supported by particle sizing by laser diffraction and deposit surface morphology by scanning electron microscopy (SEM). The preceding data demonstrated that Ti particles of ∼1-12 μm size, electrosterically modified by the PDADMAC charging agent, acted effectively as colloidal particles during EPD. Owing to the non-colloidal nature of the particles and the stabilization of the Ti particles by electrosteric forces, the relevance of the zeta potential is questionable, so the more fundamental parameter of electrophoretic mobility was used. A key finding from the present work is the importance of assessing the electrophoretic mobilities of both the suspensions and solutions since the latter, which normally is overlooked, plays a critical role in the ability to interpret the results meaningfully. Further, algebraic uncoupling of these data plus determination of the deposit yield as a function of charging agent addition allow discrimination between the three main mechanistic stages of the electrokinetics of the process, which are: (1) surface saturation; (2) compression of the diffuse layer, growth of polymer-rich layer, and/or competition between the mobility of Ti and PDADMAC; and (3) little or no decrease in electrophoretic mobility of Ti, establishment of polymer-rich layer, and/or dominance of the mobility of the PDADMAC over that of Ti.     

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.     

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.     

Nuclear decontamination of cementitious materials by electrokinetics: An experimental study

The objective of this paper is to evaluate the efficiency of an electrokinetic method of radionuclide removal from a cement-based material. This work is a two-part process. In the first part, a sample of mortar was uniformly contaminated for use as a reference. In order to ensure a uniform contamination, the ingress of the radioelement (cesium) was controlled by an external electrical field. The second part of this work concerns the removal of cesium from the contaminated mortar samples. This second, decontamination, phase was driven by the same electrical field. No electroosmosis was detected. Both phases were characterized by analyses of cesium and calcium concentrations in cathodic and anodic solutions, and by measurements of cesium content in the samples at the end of each phase. In addition to the electrical current, pH, and conductivity were measured during the experiments. Finally, the efficiency of the electrokinetic method was described in terms of decontamination factor, leading to promising results.     

Experimental study and mathematical model on remediation of Cd spiked kaolinite by electrokinetics

An experimental study on electrokinetic removal of cadmium from kaolinitic clays is presented in this work, which is aimed to investigate the effect of surface reactions on the electrokinetic process. Enhanced electrokinetic tests were performed in which the pH of the compartments was controlled. Cadmium spiked kaolin was adopted in the experimental runs. On the basis of the experimental results, a numerical model was formulated to simulate the cadmium (Cd) transport under an electric field by combining a one-dimensional diffusion-advection model with a geochemical model: the combined model describes the contaminant transport driven by chemical and electrical gradients, as well as the effect of the surface reactions. The geochemical model utilized parameters derived from the literature, and it was validated by experimental data obtained by sorption and titration experiments. Electrokinetic tests were utilized to validate the results of the proposed model. A good prediction of the behaviour of the soil/cadmium ions system under electrical field was obtained: the differences between experimental and model predicted profiles for the species considered were less than 5% in all the examined conditions.     

Feasibility of using electrokinetics and nanomaterials to stabilize and improve collapsible soils

Loess as a subcategory of collapsible soils is a well-known aeolian deposit generally characterized as a highly-porous medium with relatively low natural density and water content and a high percentage of fine-grained particles.Such collapsible soil sustains large stresses under a dry condition with natural water content.However,it can experience high and relatively sudden decreases in its volume once it reaches a certain water content under a certain load and therefore,the natural condition of the soil might not be suitable for construction if the possibility of the exposure of the soil to excessive water exists during the lifetime of the project.This research presents the utilization of an innovative method for stabilization and improvement of Gorgan loessial soil.This method uses electrokinetics and nanomaterials to instigate additives to move through soil pores,as an in situ remedial measure.To assess the acceptability of this measure,the deformability and strength characteristics of the improved collapsible soil are measured and compared with those of the unimproved soil,implementing several unsaturated oedometer tests under constant vertical stress and varying matric suction.The result emphasizes the importance of the matric suction on the behavior of both improved and unimproved soils.The test results indicate that the resistance of the soil was highly dependent on the water content and matric suction of the soil.The oedometer tests on samples improved by 3% lime and 5% nanomaterials show considerable improvement of the collapse potential.Results also reveal that stabilized samples experience notably lower volume decrease under the same applied stresses.