Biosynthesis of CdSe nanoparticles by anaerobic granular sludge
This study investigated the feasibility of combining bioremediation of selenium (Se) containing wastewater and biorecovery of Se as cadmium selenide nanoparticles (CdSe NPs). A microbial community of anaerobic granular sludge was enriched for 300 days in the presence of Cd(II) and selenite (Se(IV)). Complete Se(IV) (79 mg L−1) reduction in the presence of Cd (30 mg L−1) was observed after the 16th transfer (8 months) with the formation of both elemental selenium (Se(0)) and dissolved selenide (Se(−II)). Cd either was associated with Se(0) or remained in the aqueous phase as free Cd(II) ions and/or CdSe. The absorption and the fluorescence spectra of the aqueous phase showed formation of CdSe NPs. UV-vis and X-ray photoelectron spectroscopy (XPS) confirmed that the CdSe NPs were capped by extracellular polymeric substances (EPS) originating from the anaerobic granular sludge. Raman spectroscopy and XPS analysis further confirmed the presence of CdSe NPs in the aqueous phase, while Cd present in the Se(0) pellet after centrifugation was mainly precipitated as a Se(0)–Cd complex. A CdSe/CdS core/shell structure was found in the sludge, suggesting that Cd(II) ions on the surface of the CdSe core interact with the sulfhydryl (–SH) groups present in the EPS of the UASB granules.