Removal of hexavalent chromium of contaminated soil by coupling electrokinetic remediation and permeable reactive biobarriers

PURPOSE: In this study, a novel and ecological alternative have been developed to treat soils contaminated with hexavalent chromium coupling two well-known systems: electrokinetic remediation and permeable reactive biobarriers. The electric field promotes the electromigration of the hexavalent chromium oxyanions towards the anode. The biobarriers were placed before the anode electrode, in order to promote the reduction and retention of the chromium migrating in its direction. Thus, this technology provided a global treatment to soil removal without subsequent treatments of the contaminated effluents. METHODS: The electrokinetic system was coupled with two different permeable reactive biobarriers composed by Arthrobacter viscosus bacteria, supported either in activated carbon or zeolite. An electric field of 10 V was applied and two different treatment times of 9 and 18 days were tested. RESULTS: Removal values of 60% and 79% were obtained when electrokinetic treatment was coupled with zeolite and activated carbon biobarriers, respectively, for a test period of 18 day. The reduction of hexavalent chromium to trivalent chromium was around 45% for both systems. CONCLUSIONS: In this work, two types of biobarriers were efficiently coupled to electrokinetic treatment to decontaminate soil with Cr(VI). Furthermore, the viability of the new coupling technology developed (electrokinetic + biobarriers) to treat low-permeability polluted soils was demonstrated.This work was supported by the Spanish Ministry of Science and Innovation (CTQ2008-03059/PPQ), Xunta de Galicia (08MDS034314PR). The authors are grateful to the Spanish Ministry of Science and Innovation for providing financial support for Marta Pazos under the Ramon y Cajal program and the Fundacao para a Ciencia e Tecnologia, Ministerio da Ciencia e Tecnologia, Portugal through the PhD grant of Bruna Fonseca (SFRH/BD/27780/2006)

Comparison of three methods to calibrate TDR for monitoring solute movement in undisturbed soil

Time domain reflectometry (TDR) is rapidly becoming a popular method for measuring solute concentrations in the laboratory as well as in the field. Success or failure of TDR to represent solute resident concentrations depends on the accuracy of the invoked calibration. In this study, we compared three commonly used calibration methods that relate the impedance, Z+ as measured with TDR, to the solute concentration such as the inlet concentration, CO. The comparison was carried out using solute transport data obtained from l-m-long, 0.3-m-diam. undisturbed saturated soil columns. The first method comprised the application of a long enough solute pulse such that the concentration in a soil column became equal to the input concentration. The second method involved numerical integration of the observed response to a tracer pulse input function from which ZO could be obtained. The third method determined & using an independently measured relationship between the impedance and the solute concentration. The three calibration methods gave approximately the same results for the first observation depth at x = 0.05 m. However, the presence of heterogeneous transport processes involving solute diffusion from mobile to immobile water regions predicated the use of excessively long solute pulses in order to equilibrate the entire soil column to the input concentration. The first method hence w a s useful only for the shallower depths. The second method could be applied throughout the soil profile, provided impedance measurements were made for a reasonable time period, especially in the case of nonequilibrium transport. The procedure using an independently measured Z-C relationship underpredicted Z.O in about 50% of the cases, presumably because of the use of repacked soil in the calibration