Sulfate-reducing bacteria (e.g. Desulfovibrio) reduce $SO_4^{2-}$ to $H_2S$ under anaerobic conditions: $SO_4^{2-} + 8H^+ + 8e^- \rightarrow H_2S + 2H_2O + 2OH^-$. The generated $H_2S$ reacts with dissolved heavy metals (e.g. $Cd^{2+}$, $Zn^{2+}$, $Pb^{2+}$, $Cu^{2+}$) to form insoluble metal sulfide precipitates: $M^{2+} + H_2S \rightarrow MS \downarrow + 2H^+$. This is exploited in passive bioremediation of acid mine drainage.