Promotion of Ni(2+) Removal by Masking Toxicity to Sulfate-Reducing Bacteria: Addition of Citrate

The sulfate-reducing bioprocess is a promising technology for the treatment of heavy metal-containing wastewater. This work was conducted to investigate the possibility of promoting heavy metal removal by the addition of citrate to mask Ni(2+) toxicity to sulfate-reducing bacteria (SRB) in batch reactors. SRB growth was completely inhibited in Ni(2+)-containing medium (1 mM) when lactate served as the sole carbon resource, leading to no sulfate reduction and Ni(2+) removal. However, after the addition of citrate, SRB grew well, and sulfate was quickly reduced to sulfide. Simultaneously, the Ni-citrate complex was biodegraded to Ni(2+) and acetate. The NiS precipitate was then formed, and Ni(2+) was completely removed from the solution. It was suggested that the addition of citrate greatly alleviates Ni(2+) toxicity to SRB and improves the removal of Ni(2+), which was confirmed by quantitative real-time PCR targeting dissimilatory sulfite reductase (dsrAB) genes. Analysis of the carbon metabolism indicated that lactate instead of acetate served as the electron donor for sulfate reduction. This study offers a potential approach to increase the removal of heavy metals from wastewater in the single stage SRB-based bioprocess.