Effect of low permeability zone location on remediation of Cr(VI)-contaminated media by electrokinetics combined with a modified-zeolite barrier

Abstract

Research on electrokinetics-permeable reactive barrier (EK-PRB) remediation to date has mainly focused on homogeneous soils or soils with micro-scale heterogeneities. The potential impact of macro-scale physical heterogeneities, such as stratified layers or lenses, on EK-PRB remediation has not received much attention. This study investigates the effect of a low permeability stratum on EK-PRB remediation of hexavalent chromium (Cr(VI)). Sandbox experiments were conducted to treat Cr(VI)-contaminated kaolinite/sand media, consisting of vertically-layered high permeability (HPZ) and low permeability zones (LPZ), where distance between LPZ and anode (DLA) was 3, 9, or 15 cm. Parameters including current, moisture content (MC), pH, and removal of Cr(VI) were evaluated. With 72 h of EK-PRB treatment, tests with larger DLA (15 cm) had greater Cr(VI) migration from contaminated area to modified-zeolite PRB. Cr(VI), Cr(III), and Cr(Total) removal and energy utilization efficiency followed the trend as: DLA-15 > DLA-9 > DLA-3. MC generally decreased from anode towards cathode and pH was alkaline in all the zones for DLA-3 and DLA-15. In DLA-9 (LPZ in the middle), MC increased and pH was alkaline in HPZs near cathode whereas HPZs near anode were very dry (MC < 1%) and acidic (pH < 5.5). Our results show that the location of LPZ relative to electrode locations has a significant influence on Cr(VI) removal efficiency and macro-scale physical heterogeneity is an important factor to be considered during EK-PRB remediation.