Role of biogenic sulfide in attenuating zinc oxide and copper nanoparticle toxicity to acetoclastic methanogenesis

•Biogenic sulfide decreased Zn and Cu toxicity to methanogens.•Inhibition was most strongly correlated with metal soluble ions.•Long-term experiments revealed distinct toxic effects of Zn and Cu. Soluble ions released by zinc oxide (ZnO) and copper (Cu0) nanoparticles (NPs) have been associated with toxicity to methanogens. This study evaluated the role of biogenic sulfide in attenuating ZnO and Cu0 NP toxicity to methanogens. Short- and long-term batch experiments were conducted to explore ZnO and Cu0 NPs toxicity to acetoclastic methanogens in sulfate-containing (0.4mM) and sulfate-free conditions. ZnO and Cu0 were respectively 14 and 7-fold less toxic in sulfate-containing than in sulfate-free assays as indicated by inhibitory constants (Ki). The Ki with respect to residual soluble metal indicated that soluble metal was well correlated with toxicity irrespective of the metal ion source or presence of biogenic sulfide. Long-term assays indicated that ZnO and Cu0 NPs caused different effects on methanogens. ZnO NPs without protection of sulfide caused a chronic effect, whereas Cu0 NPs caused an acute effect and recovered. This study confirms that biogenic sulfide effectively attenuates ZnO and Cu0 NPs toxicity to methanogens by the formation of metal sulfides.