Chromium (VI) reduction in aqueous solutions using coal gangue

Coal gangue can be applied to remove chromium (VI) from wastewater because it exhibits excellent reduction ability. Cr(VI) could be almost completely reduced to the lower toxicity form, Cr(III). Coal gangue before and after treatment was characterised by X-ray diffraction, SEM, EDS and FT-IR spectrums. The main active materials in coal gangue were siderite and pyrite. Dynamic experiments with different initial concentrations of Cr(VI) demonstrated that the reduction of Cr(VI) showed zero-order kinetics. The zero-order plots of c vs. t represented ideal linearity and regression coefficients were above 0.938. The treatment capacity calculated from the kinetics model was in good agreement with the experimental data, both of which were greater than 18.5 mg/g. The reduction of Cr(VI) by coal gangue was strongly pH-dependent. In fact, both the decomposition of siderite and pyrite and the reduction of Cr(VI) hardly occurred without the presence of H+. Under the optimum conditions, removal of Cr(VI) by coal gangue was effective and economical. The concentration of hexavalent chromium in the treatment water was lower than 0.05 mg/L, and the total chromium was far below the emission standard of 1.5 mg/L.