Effect of the reduction–mineralization synergistic mechanism of Bacillus on the remediation of hexavalent chromium

Abstract

Through the synergistic effect of reduction and mineralization by Bacillus strain T124, hexavalent chromium (Cr(VI)) is reduced to a state with lower toxicity, where stable mineralization occurs, and this results in the reduction of the migration and bioavailability of Cr(VI) in the environment. In this study, a chromium (Cr)–resistant mineralization strain, Bacillus thuringiensis T124, was selected from the soil around an electroplating company, and the mechanism underlying the reduction–mineralization of Cr by this strain was studied. The reduction process mainly occurred on the cell wall membrane. The reducing enzymes were found in each component of the strain; however, a majority of them were internal. The strain could tolerate and reduce the toxicity of Cr toward itself. On fitting the data to enzyme kinetics, the K m was found to be 2925.03 μmol·L −1 , V m was 7.48 mg·(h·L) −1 , and urease activity was 66.4 U·mL −1 . The final mineralization of Cr(VI) by the strain yielded CaCO 3 and Ca 10 Cr 6 O 24 (CO 3 ). The proposed mechanism is that Cr(VI) is reduced by biological reductase, which reduces the toxicity of Cr and indirectly increases urease activity of the strain. Urease can decompose the substrate to form a CaCO 3 precipitate with Ca(II). Cr replaces a part of the lattice of CaCO 3 to mineralize Cr, thereby forming a Ca Cr coprecipitate. These minerals occupy a part of the binding site of the strain. The reduction–mineralization synergistic mechanism thus plays an important role in the immobilization of Cr(VI) on the cell wall of the strain. • Bacillus strain T124 reductase can reduce Cr(VI) to Cr(III), reducing the toxicity of Cr. • The Cr ion was captured during the formation of CaCO 3 and fixed in the lattice. • Cr(VI) can be fixed as Ca 10 Cr 6 O 24 (CO 3 ), with a good crystal form and stable structure. • The reduction-mineralization system has a synergistic effect, which can better repair Cr (VI).