Comparison of Cr(VI) removal by activated sludge and dissolved organic matter (DOM): importance of UV light

Removal of toxic Cr(VI) by activated sludge and DOM derived from activated sludge was investigated in this study. A rapid increase in TOC concentration from 50.93 to 127.40 mg L-1 is observed during the Cr(VI) removal process by activated sludge in the pH range of 2-9. Removal efficiencies of Cr(VI) by either activated sludge or DOM greatly decreased with the increasing initial pH. Kinetics of Cr(VI) removal by activated sludge indicate that both biosorption and bioreduction are involved in the Cr(VI) removal. Cr(VI) removal by DOM is slow in dark, but it is greatly enhanced when UV light is applied. The first-order constant increases from 0.0033 (in dark) to 0.079 min(-1) (UV illumination) at pH 2.0 and 1068 mg L-1 DOM. The enhancement of Cr(VI) reduction is due to the generation of the reactive intermediates such as O-2(center dot-) and DOM* as DOM absorbed light energy, which plays important roles in the reduction of Cr(VI).