Bioremediation of hexavalent chromium-contaminated wastewater by Bacillus thuringiensis and Staphylococcus capitis isolated from tannery sediment

Abstract

The widespread use of hexavalent chromium Cr(VI) in the leather industry causes substantial environmental problems when effluents are left untreated. Therefore, the present work attempts to assess the ability of Bacillus thuringiensis (V45) and Staphylococcus capitis (S21), isolated from tannery industry sediment, to detoxify Cr(VI) by reducing the oxidation state. Initially, the minimum tolerance of chromium by both bacteria was found up to 1000 μg/mL. V45 could tolerate Cr(VI) (520 μg/mL), and S21 could also tolerate hexavalent Cr(VI) (340 μg/mL). Similarly, both bacteria were able to tolerate other metals such as Hg2+ (40 μg/mL), Cu2+ (30 μg/mL), Ni2+ (60 μg/mL), Zn2+ (40 μg/mL), and Pb2+ (30 μg/mL). V45 and S21 could decrease Cr(VI) at a primary concentration of 50 μg/mL up to 86.42% and 97.34%, respectively. In optimization experiments, the best temperature to decrease Cr(VI) was shown to be 35 °C with pH 7 for 96 h. The occurrence of Cu2+ and Na+ slightly increased during the decrease of hexavalent Cr(VI) by V45, while the isolate S21 exhibited the same effects with Cu2+, Mn2+, and Na+. The carboxylate and amino conjugates in the biomass are intricate in the bioreduction of Cr(VI), as confirmed by FTIR spectroscopy. In addition, SEM imagery revealed the accumulation of Cr(VI) around both types of bacterial cells. The occurrence of other elements was evident from SEM-EDS spectroscopy. This study demonstrated the ability of native bacterial populations (V45 and S21) in tannery sediment to reduce Cr(VI) compounds.