Bioreduction of hexavalent chromium via Bacillus subtilis SL-44 enhanced by humic acid: An effective strategy for detoxification and immobilization of chromium

As an organic macromolecule, humic acid (HA) has been extensively used as the protectant for bacteria applied in Cr(VI) microbial remediation. However, the effect of the structural properties of HA on the reduction rate of bacteria and the respective contribution of bacteria and HA to soil Cr(VI) management remained uncertain. In this paper, the structural differences between two kinds of humic acid (AL-HA and MA-HA) were explored by means of spectroscopy and electrochemical characterization, and the potential influence of MA-HA on Cr(VI) reduction rate and physiological characteristics of bacteria (Bacillus subtilis, SL-44) also were analyzed. The results showed that the phenolic groups and carboxyl on the surface of HA are firstly complex with Cr(VI) ions, and the fluorescent component with more п-п conjugate structure in HA is the most sensitive species. Compared with single bacteria, the application of SL-44 and MA-HA complex (SL-MA) not only enhanced the reduction of 100 mg/L Cr(VI) to 39.8 % within 72 h and formation rate of intermediate Cr(V), but also reduced the electrochemical impedance. Moreover, the addition of 300 mg/L MA-HA also relieved the Cr(VI) toxicity and decreased the accumulation of glutathione to 94.51 % in bacterial extracellular polymeric substance, furthermore down-regulated the gene expression related to amino acid metabolism and polyhydroxybutyric acid (PHB) hydrolysis in SL-44. Finally, the application of SL-MA also enhanced the stability of chromium in soil and decreased its phytoavailability to 86.09 %, which further reduced chromium enrichment in cabbage organs. These findings provide new insights into Cr(VI) removal, which is also critical for evaluating the application potential of HA for enhancing Cr(VI) bio-reduction.