| Abstrakt: |
The aim of the present study was to demonstrate the loss of Cr(VI) from aqueous solutions via microbial Cr(VI) reduction. Aerobic, anaerobic and microaerophilic cultures of Shewanella oneidensis were tested for Cr(VI) reduction under a variety of conditions (i.e., various Cr(VI) concentrations, carbon sources, pH, temperature, bacterial cell densities, presence of cationic metals, sea sand, illite clay, heterologous bacteria commonly found in soil and water or the soluble electron shuttle anthraquinone 2,6 disulphonate [AQDS]). Optimum reduction of 5 mg/l Cr(VI) was observed at: pH 7, at 25–30 °C and in lactate-amended cultures. Anaerobic cells reduced Cr(VI) at an average rate of 28 ng Cr(VI)/min × mg protein versus 13 ng Cr(VI)/min × mg protein for aerobically grown cells. The presence of cationic metals [Co(II), Ni(II)] inhibited the reduction of 50 mg/l Cr(VI) by aerobic S. oneidensis cultures but not anaerobic or microaerophilic cultures. In aerobic cultures, toxic effects due to high levels (50 mg/l) of Cr(VI) and cationic metals were alleviated by the presence of sand and illite clay particles, which sorbed Cr(VI). When 50 mg/l Ni(II) was present, more Cr(VI) was reduced by aerobic S. oneidensis cultures containing either sand or illite clay versus cultures lacking these amendments. Addition of an electron shuttle (AQDS) to the medium also enhanced 50 mg/l Cr(VI) reduction in the presence of 50 mg/l Ni(II). Cr(VI) reduction rates were similar with and without AQDS when 50 mg/l Co(II) was present. Reduction of 50 mg/l Cr(VI) was facilitated by the addition of other bacteria (Acinetobacter calcoaceticus, Pseudomonas putida and Bacillus circulans) in mixed cultures with S. oneidensis.This study demonstrates that aerobic, anaerobic and microaerophilic cultures of S. oneidensis reduce Cr(VI). Bacterial mixtures, addition of AQDS and presence of sand or illite clay may enhance Cr(VI) reduction by this bacterium in the presence of inhibitory agents. |
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