Fluidized-bed denitrification of mining water tolerates high nickel concentrations

•Ni impact on denitrification was revealed in two continuous FBRs.•Denitrification tolerated soluble Ni concentrations as high as 500 mg/L.•Ni speciation was investigated by using both modeling and XRD analysis.•Ni3(PO4)2 precipitates were detected by XRD on activated carbon at FBR termination.•The high tolerance of Dechloromonas to Ni successfully maintained denitrification.

This study revealed that fluidized-bed denitrifying cultures tolerated soluble Ni concentrations up to 500 mg/L at 7–8 and 22 °C. From 10 to 40 mg/L of feed Ni, denitrification resulted in complete nitrate and nitrite removal. The concomitant reduction of 30 mg/L of sulfate produced 10 mg/L of sulfide that precipitated nickel, resulting in soluble effluent Ni below 22 mg/L. At this stage, Dechloromonas species were the dominant denitrifying bacteria. From 60 to 500 mg/L of feed Ni, nickel remained in solution due to the inhibition of sulfate reduction. At soluble 60 mg/L of Ni, denitrification was partially inhibited prior to recover after 34 days of enrichment by other Ni-tolerant species (including Delftia, Zoogloea and Azospira) that supported Dechloromonas. Subsequently, the FBR cultures completely removed nitrate even at 500 mg/L of Ni. Visual Minteq speciation model predicted the formation of NiS, NiCO3 and Ni3(PO4)2, whilst only Ni3(PO4)2 was detected by XRD.