Sulfur-based mixotrophic bio-reduction for efficient removal of chromium (VI) in groundwater

摘要

Organic matter and reduced sulfur compounds commonly coexist in groundwater aquifers and their respective roles in Cr (VI) bio-reduction have been well established, but Cr(VI) bio-reduction under mixotrophic condition, where organics and elemental sulfur simultaneously occur as co-donors of electrons, remains largely unknown. Herein a sulfur-based mixotrophic bio-reduction process is demonstrated to be effective to detoxify Cr(VI), with a removal efficiency of 95.5 +/- 0.74% within 48 h at an initial concentration of 50 mg/L. In addition to direct reduction by heterotrophic Cr(VI) reducers such as Desulfovibrio and Desulfuromonas, volatile fatty acids (VFAs) produced from autotrophic sulfur oxidation served as electron donors for heterotrophic Cr(VI) reducers. Part of VFAs was also assimilated and accumulated as glycogen within cells, which enhanced their Cr(VI) removal capacity. Metabolic pathway analysis suggested that Cr(VI) was reduced to insoluble Cr(III) both extracellularly by cytochrome c and intracellularly by nicotinamide adenine dinucleotide in the presence of upregulated chrA gene. Constituents of extracellular polymeric substances (EPS) also contributed to Cr(VI) reduction enzymatically, through binding of toxic Cr(VI) by carboxyl and hydroxyl groups. Results from this study have important implications for understanding the biogeochemical behavior and environmental remediation of Cr(VI) in groundwater aquifers and sediments/soils. (C) 2019 Elsevier Ltd. All rights reserved.