Inhibition of bacterial oxidation of ferrous iron by lead nitrate in sulfate-rich systems

Inhibition of bacterial oxidation of ferrous iron (Fe(II)) by Pb(NO₃)₂ was investigated with a mixed culture of Acidithiobacillus ferrooxidans. The culture was incubated at 30 °C in ferrous-sulfate medium amended with 0–24.2 mM Pb(II) added as Pb(NO₃)₂. Anglesite (PbSO₄) precipitated immediately upon Pb addition and was the only solid phase detected in the abiotic controls. Both anglesite and jarosite (KFe₃(SO₄)₂(OH)₆) were detected in inoculated cultures. Precipitation of anglesite maintained dissolved Pb concentrations at 16.9–17.6 μM regardless of the concentrations of Pb(NO₃)₂ added. Fe(II) oxidation was suppressed by 24.2 mM Pb(NO₃)₂ addition even when anglesite was removed before inoculation. Experiments with 0–48 mM KNO₃ demonstrated that bacterial Fe(II) oxidation decreased as nitrate concentration increased. Therefore, inhibition of Fe(II) oxidation at 24.2 mM Pb(NO₃)₂ addition resulted from nitrate toxicity instead of Pb addition. Geochemical modeling that considered the initial precipitation of anglesite to equilibrium followed by progressive oxidation of Fe(II) and the precipitation of jarosite and an amorphous iron hydroxide phase, without allowing plumbojarosite to precipitate were consistent with the experimental time-series data on Fe(II) oxidation under biotic conditions. Anglesite precipitation in mine tailings and other sulfate-rich systems maintains dissolved Pb concentrations below the toxicity threshold of A. ferrooxidans.