Elemental copper nanoparticle toxicity to different trophic groups involved in anaerobic and anoxic wastewater treatment processes

Elemental copper nanoparticles (Cu⁰ NPs) are potentially inhibitory to the different key microbial trophic groups involved in biological wastewater treatment processes. Cu-based NPs are known to be toxic to methanogens at low concentrations. However, very little is known about the toxic effect of Cu⁰ NPs on other microbial groups involved in either upper trophic levels of anaerobic digestion or anoxic nitrogen removal processes. This study evaluated the toxicity of Cu⁰ NPs to glucose fermentation, syntrophic propionate oxidation and denitrification in shaken batch bioassays with soluble substrates. Batch experiments were also supplemented with CuCl₂ to evaluate the inhibitory impact of soluble Cu(II) ions. Syntrophic propionate oxidation and glucose fermentation were the least and most inhibited processes with inhibition constant (K_i) values of 0.202 and 0.047mM of added Cu⁰ NPs, respectively. Further analyses revealed that the K_i values calculated as a function of the free soluble Cu concentration were <0.003mM for every biological process tested and most of these K_i values were similar in order of magnitude regardless of whether the Cu source was CuCl₂ or Cu⁰ NPs. The results taken as a whole indicate that Cu⁰ NPs are toxic to all the microbial processes studied. Therefore, Cu⁰ NPs can potentially be an important inhibitor of anaerobic wastewater treatment processes that rely on these trophic groups. The evidence suggests that the inhibitory impact of Cu⁰ NPs was mainly due to the release of toxic Cu(II) ions originating from the corrosion and dissolution of Cu⁰ NPs.