Chromium reduction by indigenous Bacillus sp. isolate S1: optimal performance, characterization, and pathway

Abstract

Anthropogenic activities associated with the metal industry in Malaysia have led to an overabundance of heavy metal waste, particularly hexavalent chromium, Cr(VI), which is highly hazardous and has accumulated in the surrounding environment. No previous study has given sufficient consideration to Cr(VI) removal, and its management in Malaysia has led to conflicting findings about the presence of bacteria resistant to Cr(VI). Thus, the objectives of this study were to identify the Cr(VI)-resistant bacteria in metal waste, optimize Cr(VI) reduction by Response Surface Methodology (RSM), characterize bacteria through Scanning Electron Microscope (SEM), and propose the Cr(VI) reduction pathway. The BLAST finding of a 16S rDNA sequence analysis revealed an isolated bacterium resistant to 1,000 mg/L of Cr(VI), identified as Bacillus sp., and chosen to optimize Cr(VI) reduction. The Analysis of Variance (ANOVA) of the quadratic model suggested that the model was acceptable, as it still had Prob > F (0.0263), which signifies significance for determining the model's reliability. The optimum conditions for Bacillus sp. S1, resulting in a 100% reduction of Cr(VI) with an initial Cr(VI) concentration of 50 mg/L, was at pH 6 and an exposure period of 3 days. The Cr(VI)-resistant bacterium Bacillus sp. could eliminate Cr(VI) under suitable conditions. SEM examination demonstrated that the reduction of Cr(VI) by Bacillus sp. S1 took place inside the cell. This study provides a scientific reference on the Cr(VI) reduction pathway for research on Cr(VI)-resistant bacteria.