Simultaneous denitrification and hexavalent chromium removal by a newly isolated Stenotrophomonas maltophilia strain W26 under aerobic conditions

Environmental context Industrial development has caused the release of hexavalent chromium and nitrates into the environment. Interactions of hexavalent chromium and nitrates with microorganisms are important both for understanding environmental behaviour and for treatment options. Bacterial removal of both chromium and nitrate was optimised in waters relevant to waste streams and the environment. An isolated strain of the bacterium Stenotrophomonas maltophilia strain W26, is shown to be capable of the simultaneous removal of nitrate and Cr-VI under aerobic conditions. Notably, 10 mg L-1 of Cr-VI and 500 mg L-1 of nitrate were reduced by 92.6 % and 85.2 %, respectively, by strain W26. Results showed that an excellent denitrification efficiency of 96.0 % could be reached at the optimal conditions of a C/N ratio of 10, using a carbon source of trisodium citrate, at pH 7.5, and a nitrate concentration of 500 mg L-1. Strain W26 could also effectively remove high concentrations of Cr-VI (50 mg L-1, 93.2 %) and nitrate (700 mg L-1, 97.4 %). By using the N balance analysis, energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), the denitrification and Cr-VI transformation processes were verified. Cr-VI, immobilised on the cell surface by W26, was reduced to Cr-III, through interaction with of O=C and N-H groups. This study indicates that the isolated strain W26 has the potential to treat wastewater containing combined nitrate and Cr-VI contamination.