BACKGROUNDMetallurgic wastewaters are acidic effluents containing large amounts of nitrate and heavy metals. Citric acid is one of the acids used by this industry and forms stable complexes with metal ions. The aim of this study was to elucidate the chemical aspects driving inhibitory or stimulatory effects of heavy metals on denitrifying processes, based on speciation analysis and monitoring key denitrification intermediates (nitrite and N2O). RESULTSDenitrifying sludge incubations were conducted with iron, chromium and nickel, in single and multi-metal assays, using citrate and ethanol as electron donors. Ferric-citrate complex, [Fe-cit]((aq)), was readily consumed, while complexes of divalent metals, [Fe-cit](-) and [Ni-cit(2)](4-), remained very stable affecting denitrification. Nitrate reduction was affected by Ni2+, while nitrite and N2O accumulation was observed with NiCO3 and Ni(OH)(2) oversaturation. [Cr-cit(2)](4-) resulted in overall denitrification inhibition, while species of [Cr-cit](2-) caused denitrification stimulation. CONCLUSIONFe and Cr inhibited the overall denitrification process, while Ni caused accumulation of intermediates. Synergistic inhibition imposed by multi-metal systems revealed lower inhibitory effects compared with those observed by the sum of individual effects of metals. This study elucidates chemical aspects determining the effects of heavy metals on denitrification, which is relevant to develop efficient biological processes for metallurgic effluents. (c) 2017 Society of Chemical Industry