Reduction of Cr(VI) to Cr(III) by thermal Bacillus licheniformis B22 under different temperatures using binary and ternary combinations of organic acids

Abstract

This study presents optimization of process variables for hexavalent chromium reduction using thermotolerant Bacillus licheniformis B22 isolated from Pamukkale (Denizli, Turkey). We examined the effects of binary and ternary combinations of different electron-donating substrates (galacturonic acid, glucuronic acid, and humic acid) at 45 and 50°C. The influence of different pH values (6.0, 7.0, 8.0, 9.0, and 10.0) and initial inoculation rates (2, 4, 6, and 8%) were also enumerated. The strongest stimulatory effect on Cr(VI) reduction was obtained with ternary combination of galacturonic acid, glucuronic acid, and humic acid. At 45 and 50°C, 8% inoculation rate, the reduction in ternary combination is relatively fast, completely reducing 100 mg/l Cr(VI) in 6 h compared with 2% inoculation rate (12 h). This bacterium exhibited a rapid Cr(VI) reduction ability under optimized conditions. Cr(VI) reduction of B. licheniformis B22 increased with an increase in initial inoculation rate, and the optimum initial pH for Cr(VI) reduction was 7.0. In addition, the results suggested that the reduced Cr(III) was not precipitated in the form of Cr(OH)3, and that organic acids significantly enhanced microbial Cr(VI) reduction rates by forming less toxic and highly soluble organo-Cr(III) complexes despite Cr(III) having very low solubility.