Formation of soluble Cr(III) end-products and nanoparticles during Cr(VI) reduction by Bacillus cereus strain XMCr-6

The Bacillus cereus strain XMCr-6 was isolated from chromium-contaminated soils and was identified on the basis of biochemical characteristics and 16S rRNA gene sequence analysis. The bacterium exhibited complete reduction of 100 mg L−1 Cr(VI) within 48 h under aerobic conditions. The reduction of Cr(VI) by XMCr-6 was attributed to cell debris rather than cell free extract, and the reduction mechanism was found to be enzyme-mediated. It was demonstrated that the reduced Cr(III) could bind to cells by coordination with functional groups on the bacterial surfaces while it simultaneously formed soluble Cr(III) end-products through interactions with small organic molecules. Discrete spherical Cr2O3 nanoparticles were observed on the surface of the bacterial cells after Cr(VI) reduction by XMCr-6 in the absence of an appropriate chelating chemical. Our results offer insight into bacterial Cr(VI) reduction processes and will be valuable for understanding reduction mechanisms and Cr(VI) bioremediation.

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► The strain XMCr-6 could reduce sufficient Cr(VI) completely within 48 h.
► The Cr(VI) reduction mechanism by XMCr-6 was enzyme-mediated in the cell debris.
► Small organic molecules compete with bacteria for reduced Cr (III) coordination.
► Role of small organic molecules in determining the types of Cr(III) end-products.
► Reduced Cr(III) could form discrete spherical Cr2O3 nanoparticles.