[show abstract][hide abstract] ABSTRACT: Electrokinetic remediation of metal-contaminated soils is strongly affected by soil-type and chemical species of contaminants. This paper investigates the speciation and extent of migration of heavy metals in soils during electrokinetic remediation. Laboratory electrokinetic experiments were conducted using two diverse soils, kaolin and glacial till, contaminated with chromium as either Cr(III) or Cr(VI). Initial total chromium concentrations were maintained at 1000mg/kg. In addition, Ni(II) and Cd(II) were used in concentrations of 500 and 250mg/kg, respectively. The contaminated soils were subjected to a voltage gradient of 1 VDC/cm for over 200h. The extent of migration of contaminants after the electric potential application was determined. Sequential extractions were performed on the contaminated soils before and after electrokinetic treatment to provide an understanding of the distribution of the contaminants in the soils. The initial speciation of contaminants was found to depend on the soil composition as well as the type and amounts of different contaminants present. When the initial form of chromium was Cr(III), exchangeable and soluble fractions of Cr, Ni, and Cd ranged from 10 to 65% in kaolin; however, these fractions ranged from 0 to 4% in glacial till. When the initial form of chromium was Cr(VI), the exchangeable and soluble fractions of Cr, Ni and Cd ranged from 66 to 80% in kaolin. In glacial till, however, the exchangeable and soluble fraction for Cr was 38% and Ni and Cd fractions were 2 and 10%, respectively. The remainder of the contaminants existed as the complex and precipitate fractions. During electrokinetic remediation, Cr(VI) migrated towards the anode, whereas Cr(III), Ni(II) and Cd(II) migrated towards the cathode. The speciation of contaminants after electrokinetic treatment showed that significant change in exchangeable and soluble fractions occurred. In kaolin, exchangeable and soluble Cr(III), Ni(II), and Cd(II) decreased near the anode and increased near the cathode, whereas exchangeable and soluble Cr(VI) decreased near the cathode and increased near the anode. In glacial till, exchangeable and soluble Cr(III), Ni(II), and Cd(II) were low even before electrokinetic treatment and no significant changes were observed after the electrokinetic treatment. However, significant exchangeable and soluble Cr(VI) that was present in glacial till prior to electrokinetic treatment decreased to non-detectable levels near the cathode and increased significantly near the anode. In both kaolin and glacial till, low migration rates occurred as a result of contaminants existing as immobile complexes and precipitates. The overall contaminant removal efficiency was very low (less than 20%) in all tests.
Journal of Hazardous Materials 07/2001; 84(2-3):279-96. · 3.93 Impact Factor
[show abstract][hide abstract] ABSTRACT: Several low cost biomaterials such as baggase, charred rice husk, activated charcoal and eucalyptus bark (EB) were tested for removal of chromium. All the experiments were carried out in batch process with laboratory prepared samples and wastewater obtained from metal finishing section of auto ancillary unit. The adsorbent, which had highest chromium(VI) removal was EB. Influences of chromium concentration, pH, contact time on removal of chromium from effluent was investigated. The adsorption data were fitted well by Freundlich isotherm. The kinetic data were analyzed by using a first order Lagergren kinetic. The Gibbs free energy was obtained for each system and was found to be -1.879 kJ mol(-1) for Cr(VI) and -3.885 kJ mol(-1) for Cr(III) for removal from industrial effluent. The negative value of deltaG0 indicates the feasibility and spontaneous nature of adsorption. The maximum removal of Cr(VI) was observed at pH 2. Adsorption capacity was found to be 45 mg/g of adsorbent, at Cr(VI) concentration in the effluent being 250 mg/l. A waste water sample containing Cr(VI), Cr(III), Mg, and Ca obtained from industrial unit showed satisfactory removal of chromium. The results indicate that eucalyptus bark can be used for the removal of chromium.
[show abstract][hide abstract] ABSTRACT: This paper presents the results of a laboratory investigation performed to evaluate the effects of the initial form of chromium on the electrokinetic remedial efficiency for contaminated clays. Electrokinetic experiments were conducted by contaminating clays with chromium in three different forms: Cr(III) alone, Cr(VI) alone, and a combination of Cr(III) and Cr(VI). The same total chromium concentration of 1000 mg/kg was maintained in all cases. Ni(II) and Cd(II) in concentrations of 500 mg/kg and 250 mg/kg, respectively, were also introduced into the clays as co-contaminants to simulate typical electroplating waste constituents. Two different clays, kaolin, a typical low buffering clay and glacial till, a typical high buffering clay, were tested. All tests were conducted with a constant voltage gradient of 1.0 VDC/cm. The test results showed that chromium migration was highest when it was present in kaolin in the Cr(III) form and in glacial till in the Cr(VI) form. When chromium was present in Cr(III) form, migration occurred towards the cathode due to the existence of Cr(III) as cation and cationic hydroxide complexes. Cr(III) migration was not observed in glacial till because of precipitation that resulted from high pH conditions that existed throughout the glacial till. However, when chromium was present in Cr(VI) form, the migration occurred towards the anode, due to the existence of Cr(VI) as soluble oxyanions. The migration of Cr(VI) was higher in glacial till as compared to kaolin due to alkaline conditions that existed in the glacial till, resulting in negligible Cr(VI) adsorption to soil solids. When chromium was present as a combination of Cr(VI) and Cr(III), Cr(VI) migrated towards the anode, while Cr(III) migrated towards the cathode. For these cases, the total chromium migration was lower than the migration observed when only Cr(III) was present in kaolin or when only Cr(VI) was present in glacial till. No migration was observed for the co-contaminants, Ni(II) and Cd(II), in glacial till due to precipitation as a result of alkaline conditions. In kaolin, however, Ni(II) and Cd(II) migrated towards the cathode. Overall, the test results show that significant removal of contaminants from the soils was not achieved for the processing periods utilized. This study clearly demonstrated that the efficiency of the electrokinetic removal of chromium, nickel and cadmium from the contaminated clays depends on the initial form of chromium as well as the soil chemistry. Enhancement strategies should be investigated in order to enhance contaminant migration and to achieve high removal efficiencies.
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