Up-flow anaerobic column reactor for sulfate-rich cadmium-bearing wastewater purification: system performance, removal mechanism and microbial community structure.

This study constructed an up-flow anaerobic column reactor fed with synthetic sulfate-rich cadmium (Cd(II))-bearing wastewater, for investigating its Cd(II) removal performance and mechanism. Long-term experiment results manifest that introducing Cd(II) into influent led to an enhanced sulfate removal but did not increase the effluent sulfide concentration, implying the CdS formation. When influent Cd(II) concentration was shifted from 50 to 100 mg/L, the median Cd(II) removal rate was increased from 13.6 to 32.2 mg/(L·d). Batch tests indicate that the uptake and sequestration function of anaerobes merely led to a small portion of Cd(II) removal. A majority of aqueous Cd(II) (86.3%) was eliminated by precipitation reactions. The generated precipitates were found to be dominantly presented in carbonate, Fe-Mn oxide, sulfide bound and residue forms, which account for 92.6-93.9% of total Cd content of sludge obtained at diverse operation phases. The crystallographic CdS (i.e., residue fraction) particles have nano-scale sizes, and the relatively high atomic ratio of S to Cd was likely due to the adsorption/deposition of other sulfides. The dominant sulfate-reducing bacteria (SRB) were recognized as Desulfurella, Desulforhabdus and Desulfovibrio, and the primary competitor with them for substrate utilization were identified to be methanogens.