Concentration fluctuation and dead zone phenomena caused by ion-induced trapping effect in electrokinetic remediation of chromium-contaminated soil

Concentration fluctuation and dead zone phenomena (CFDZP) is ubiquitous in the field, pilot and bench tests of electrokinetic remediation (EKR) of metal contaminated soils and has severely impeded the application of EKR in fields. In this study, a series of deliberately designed experiments with chromium-spiked agar and soil were performed to explore the mechanism of CFDZP caused by ion-induced trapping effect (ITE) which is interpreted as that the lower potential gradient caused by co-existing ions of high concentration in some zones will cause significant velocity drop for the target ions when they migrate into these zones. The accumulation of “trapped” target ions would result in “hot spots” or more severely, “dead zones” in the sites. CFDZP could dramatically increase the electric power consumption and remediation duration to an unpredictable magnitude, rendering the estimation of cost based by bench or pilot tests using homogenized soils meaningless. A comprehensive distribution map of soil conductance and concentrations of target ions in the sites can help to predict and eliminate the potential “hot spots” or “dead zones” in advance. An economical EKR duration could be determined by the moment when “dead zones” occurred.