| A comprehensive understanding of the detailed mechanism is expected to have a significant impact in the design of low-cost workable strategies for the treatment and recycling of Cr(VI) industrial wastes. Though the interaction mechanism between Cr(VI)-reducing bacteria and Cr(VI) was extensively studied, however, for a specific bacteria, literatures only contain one or two points. The comprehensive understanding of the Cr(VI) uptake mechanism of a specific bacterial strain is quite absent. In this work, with a Cr(VI)-tolerant strain Bacillus cereus (B. cereus) as an example, complete understanding of Cr(VI) reduction and Cr(III) immobilization mechanism were obtained. For the first time, it was found that the extracellular secretion of bacteria could play an important role during the Cr(VI) reduction and Cr(III) immobilization processes. Protein denaturant experiments suggested no reducing enzyme participated in the reduction process of Cr(VI) by both extracellular secretion and cell debris of B. cereus. X-ray photoelectron spectroscopy (XPS) and electron spin resonance spectroscopy (ESR) analysis revealed that the end product was Cr(III), with Cr(V) as intermediate. Both intracellular and extracellular immobilization of Cr(III) were observed by atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis revealed that the Cr(III) distribution intracellularly and extracellularly was about 31.4% and 68.6%, respectively. With Fourier transform infrared (FT-IR) and extended X-ray absorption fine structure (EXAFS) analysis, we confirmed the amino and carboxyl groups from the bacterial surface as well as the extracellular secretion might involve in the Cr immobilization process, the possible coordination structure could be similar to Cr(Glycine)3·H2O. |
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