Cr(III) oxidation coupled with Mn(II) bacterial oxidation in the environment

Cr(III) oxidation to Cr(VI) significantly increases Cr mobility and toxicity and thus its environmental risks. Manganese (Mn) oxides may serve as the potential oxidants of Cr(III) in environment. Natural Mn oxides in the environment are believed to be derived from bacterial oxidation. The objective of this study was to examine the Cr(III) oxidation capacity of biogenic Mn oxide and the role of Mn-oxidizing bacteria in Cr(III) oxidation. Batch experiments were conducted to investigate the capacities of Cr(III) oxidation by chemically synthetic Mn oxides and biogenic Mn oxide. Biogenic Mn oxide was formed by Bacillus sp. WH4, a Mn-oxidizing bacterium isolated from Fe-Mn nodules of a Chinese soil. Various Cr(III) and Mn(II) were added to the growth medium of Bacillus sp. WH4 to evaluate Cr(III) oxidation coupled with Mn(II) bacterial oxidation. The Cr(III) oxidation capacity of biogenic Mn oxide was 0.24 mmol g(-1) and higher than three chemically synthetic Mn oxides. No Mn(III) intermediate was detected during Mn(II) bacterial oxidation. Bacillus sp. WH4 could promote Cr(III) oxidation through oxidizing Mn(II), although it could not oxidize Cr(III) directly. The participation of Mn-oxidizing bacteria makes Cr(III) oxidation more complicated in environment. These findings illustrate the need to consider bacterial activity and the Mn(II) level when predicting the fate of Cr and the potential applications of Mn oxides in the remediation of pollutants in environment.