Near-anode focusing phenomenon caused by the high anolyte concentration in the electrokinetic remediation of chromium(VI)-contaminated soil

Graphical abstract: Mechanism illustration of the focusing band by ion-induced potential gradient well trapping effect (IIPGWTE), Cr(VI) denotes the CrO42−/HCrO4− ions; ION denotes all other ions except Cr(VI) ions. The potential gradient (Ea) and ions' velocity (va) in the soil near the anode was lower than that in other parts (Ec, vc) forming a potential gradient well, because of invasion of anolytes (from the left end). Highlights: ► High anolyte concentration causes focusing phenomena in soil near anode. ► Low pH value aggravates the focusing phenomenon. ► Focusing phenomenon significantly increase the remediation duration and energy consumption. - Abstract: The effects of the concentration and the low pH value of anolyte on the electrokinetic remediation (EKR) of chromium-contaminated soil were investigated using chromium-spiked kaolin-gypsum soil. Results of visual observation and X-ray fluorescence analysis show that high anolyte concentrations could cause an ion-induced potential gradient well trapping effect on the soil near the anode, and consequently cause a focusing phenomenon (FP) without chemical precipitation. This FP significantly prolonged the remediation duration and reduced chromium removal. The low pH value of soil aggravated the FP, resulting in a quasi-dead zone near the anode caused by the reduction in soil resistance, rather than the adsorption of chromium (VI) ions. The high anolyte concentration also resulted in high energy consumption. The FP and low pH value collectively decreased the energy efficiency by more than 96%. This kind of FP can be predicted via the online monitoring of the potential gradient profiles of the soil between the electrodes in the EKR.