Effect of copper dosing on sulfide inhibited reduction of nitric and nitrous oxide

The stimulating effect of copper addition on the reduction rate of nitrous oxide (N2O) to dinitrogen (N2) in the presence of sulfide was investigated in batch experiments (pH 7.0; 55 °C). N2O was dosed either directly as a gas to the headspace of the bottles or formed as intermediate during the denitrification of nitrite in Fe(II)EDTA2−-containing medium and nitrate in Fe(II)EDTA2−-free medium. Sulfide was either dosed externally or generated from endogenous sulfur sources during anaerobic incubation of the sludge. In the presence of sulfide (from 15 μM to 1 mM), heterotrophic denitrification using ethanol as electron donor was incomplete, i.e., N2O accumulated instead of N2 or was transiently formed. Copper addition (60 μM) rapidly stimulated the reduction of N2O to N2. Zinc addition (60 μM) did not have a similar strong stimulating effect as observed for copper and the N2O reduction rate was not stimulated at all upon supply of FeCl3 (2 mM). Thus, a copper deficiency for N2O reduction is most likely developed in the presence of sulfide. It is suggested that sulfide induces this deficiency as it readily precipitates as copper sulfide and thus scavenges copper in the medium or that sulfide inactivates the N2OR reductase as it sequesters the copper of this metalloenzyme.