Enhanced biosorption of Cr(VI) from synthetic wastewater using algal-bacterial aerobic granular sludge: Batch experiments, kinetics and mechanisms

Biosorption is considered as a cost-effective and environmentally friendly Cr(VI) removal process from wastewater. This study is the first report on algal-bacterial aerobic granular sludge (AGS) being used as biosorbent for Cr(VI) removal in comparison to conventional bacterial AGS with respect to biosorption performance. Batch tests revealed that Cr(VI) biosorption onto algal-bacterial AGS was highly pH dependent and the maximum Cr(VI) biosorption capacity of 51.0 mg g⁻¹ occurred at pH 2. The Cr(VI) removal by algal-bacterial AGS was found to be accomplished by both biosorption and bioreduction reactions, in which four mechanisms, namely electrostatic interactions, ion exchange, surface complexation and bioreduction were involved. In addition, desorption tests showed that 64–73% of the adsorbed Cr onto the algal-bacterial AGS could be recovered by NaHCO₃, and most of the adsorbed Cr was in the form of Cr(III). Compared to the conventional bacterial AGS, algal-bacterial AGS demonstrated higher biosorption capacity and better granular stability, implying that algal-bacterial AGS can be more potentially utilized as a Cr(VI) removal and recovery biomaterial for the treatment of Cr(VI)-containing wastewater.