High concentration hexavalent chromium removal performance of a sulfidogenic activated carbon-bed bioreactor at moderate temperature

This study intended to assess the performance of a sulfidogenic activated carbon-bed bioreactor (SACB) receiving high concentrations of hexavalent chromium (Cr6+). SACB was tested for 126 days at moderate temperature (20 degrees C) with a synthetic feed carrying ethanol as carbon source while employing a hydraulic retention time of 24 h. Excellent sulfidogenic performance was obtained during both non-Cr6+ added and 100 mg L-1 Cr6+ added periods. Sulfate reduction and carbon oxidation performances initially were 99% and 93%, respectively, and introduction of 100 mg L-1 Cr6+ to the feed at pH 4.0 resulted in only 3% reduction for both performances. During initial testing period, alkalinity generation was 99% of the theoretically expected value, and effluent pH increased up to 8.2 from pH 6.7. During the Cr6+ fed period, average alkalinity concentration was 2316 mgCaCO3 L-1 while pH increased to 8.2 from pH 4.0 with additional acidity consumption during Cr6+ reduction by the biologically generated sulfide. Effluent Cr6+ concentration remained below 0.5 mg L-1 with an average of 0.13 mg L-1, whereas average total chromium concentration in the effluent remained <= 0.2 mg L-1. Batch studies were carried out to better understand stoichiometry and kinetics of Cr6+ reduction with biogenic sulfide.