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.