In situ bioremediation of chromium (VI) in a simulated ferrochrome slag

Abstract

Stability of chromium in the tailing dumps and stockpiles of chromium concentrate at chrome mining fields is affected by pH, redox potential, and the presence of other metallic species in the waste rocks. It is desirable to keep chromium in the dump site in the trivalent state since the hexavalent state, Cr(VI), is highly mobile and carcinogenicity to aquatic life forms and humans, whereas the trivalent state, Cr(III), precipitates easily and is less toxic than Cr(VI). In this study, microbial remediation with indigenous bacteria was used to reduce Cr(VI) to Cr(III) in presence of Fe(II)/Fe(III) redox couple in simulated ferrochrome slag. The consortium of bacterial culture strains were isolated from sand drying beds and dried sludge from the belt filter press at the Wastewater Treatment Works. Culture achieved 100 % Cr(VI) removal under aerobic condition for 100 mg/L Cr(VI) in 24 h and 100 % Cr(VI) removal under anaerobic condition for 20 mg/L Cr(VI) in 48 h. Increased Cr(VI) reduction occurred in the presence of Fe(II) which confirmed that Fe(II) served as an alternative electron donor for Cr(VI) reduction. Complete reduction of 50 mg/L Cr(VI) was achieved within 12 h using 50 - 100 mg/L Fe(II). Kinetic evaluation in the presence of Fe(III) found that Cr(VI) reduction was noncompetitively inhibited by the presence of Fe(III).