Retrieval of trivalent chromium by converting it to its dichromate state from soil using a bipolar membrane electrodialysis system combined with H2O2 oxidation

Abstract

When a bipolar membrane electrodialysis (BMED) system is used to remove Cr(III) from the soil, the existing metal cations easily affect this removal owing to the migration competition of metal cations. To address this problem, herein, a BMED system combined with H2O2 oxidation was used to recover Cr(III) from soil in the form of Na2CrO4. The experimental results show that H2O2 can oxidise Cr(III) to Cr(VI), which can be recovered as Na2CrO4. The concentrations of electrolyte and humic acid in soil, current densities and addition time of H2O2 influenced Cr(III) recovery, current efficiency and specific energy consumption. Under optimal experimental conditions of a current density of 2 mA/cm2 and H2O2 addition time of 2 h after the BMED system was powered on, more than 69 % of Cr(III) could be recovered. When the number of equipped soil compartments increased from one to two and then to three, the current efficiency increased from 9.9 % to 19.8 % and then to 29.1 % and the specific energy consumption of chromium removal decreased from 7.04 × 10−2 to 5.51 × 10−2 and then to 4.72 × 10−2 kW∙h/g, respectively. When the BMED system was used to recover Cr(III) from actual soil, the recovery rate decreased to 55.9 % owing to the low Cr(III) oxidation rate resulting from more residual state Cr(III) being present in actual soil. Following treatment, the remaining Cr(VI) in soil can be reduced to Cr(III) by adding a reductant. Therefore, the BMED system combined with H2O2 oxidation can be regarded as an effective method for recovering Cr(III) from soil.