Woodchip-sulfur based mixotrophic biotechnology for hexavalent chromium detoxification in the groundwater

This study investigated groundwater hexavalent chromium (Cr(VI)) decontamination by a biological permeable reactive barrier (bio-PRB), where a woodchip-elemental sulfur [S(0)] based mixotrophic process was established. 89.0 ± 0.27% of Cr(VI) was removed from the synthetic groundwater after 72 h at a concentration of 50 mg/L during the preliminary batch experiment. The impact of geochemical and hydrodynamic conditions Cr(VI) removal was investigated in the bio-PRB over 225 days. Although elevated Cr(VI) (i.e., 75 mg/L), addition of nitrate and short hydraulic retention time reduced the Cr(VI) removal, 87.2 ± 2.09% of Cr(VI) removal was accomplished. Characterization of the solids indicated that the soluble Cr(VI) was converted and immobilized as the insoluble trivalent chromium [Cr(III)]. 16S rRNA gene based sequencing results suggested that micromolecules produced by woodchip cellulose hydrolyzing- and sulfur oxidizing bacteria were further used by functional Cr(VI) removal bacteria (e.g., Geobacteraceae and Pseudomonas). The extracellular protein and humic-like substances in extracellular polymeric substances (EPS) can bind toxic Cr(VI) through carboxyl and hydroxyl groups, and reduce Cr(VI) in an enzymatic manner. The preliminary finding of this study provided a potential way to utilize agricultural waste for in-situ Cr(VI) contaminated-groundwater remediation.