Novel remediation strategies for Cr(VI)-contaminated groundwater at low temperature: Study on the effects of combining ascorbic acid reduction with biostimulation.

Hexavalent chromium (Cr(VI)) bioreduction at low temperatures is slow to initiate and proceeds at a low rate. To accelerate the initiation of bioreduction and broaden the applicable range of biostimulation, ascorbic acid (AA) combined with molasses was used to conduct Cr(VI) remediation experiments at 8 °C. The results demonstrated that the Cr(VI) reduction efficiency was significantly inhibited by both dissolved oxygen and aquifer media when treated with AA. However, excess-addition of AA (n(AA): n(Cr(VI)) = 2:1) could eliminate the inhibitory effect of non-target reactions. Compared to the molasses-only control, the average removal rates of Cr(VI) in the systems with the additional 38, 86, and 170 mg/L of AA were increased by 0.87, 1.07, and 2.24 mg/(L·d). AA rapidly reduced Cr(VI) within 6 h, and the subsequent bioavailable molasses had the greatest Cr(VI) removal rate (2.47 mg/(L·d)) when the Cr(VI) concentration was decreased to near 40 mg/L. Higher AA additions under combined treatment increased the relative abundance of Bacillus from 26% to 46% and 50%, respectively. Additionally, the expression of genes related to Cr(VI) detoxification was upregulated by AA combined with molasses remediation. The expression of ChrA in AA-molasses-amended systems was 1.52 times (38 mg/L AA) and 1.68 times (86 mg/L AA) that of molasses-amended system, respectively. The relative expression of SOD1 in AA-molasses-amended groups was 1.56 (38 mg/L AA) and 1.84 times (86 mg/L AA) that in molasses-amended system. The results indicate that the combined treatment holds great promising for the remediation of Cr(VI)-contaminated groundwater.