Elemental copper nanoparticle toxicity to anaerobic ammonium oxidation and the influence of ethylene diamine-tetra acetic acid (EDTA) on copper toxicity

Soluble ions released by elemental copper nanoparticles (Cu⁰ NP) are toxic to key microorganisms of wastewater treatment processes. However, their toxicity to anaerobic ammonium oxidation (anammox) has not yet been studied. Chelating agents occurring in wastewater may decrease copper ions (Cu²⁺) concentration and consequently, decrease copper toxicity. This study evaluated Cu⁰ NP and CuCl₂ toxicity to anammox and the influence of ethylene diamine-tetra acetic acid (EDTA) on copper toxicity. Bioassays were supplemented with Cu⁰ NP or CuCl₂ with and without EDTA. Anammox activities were used to calculate inhibition constants (K_i). Results showed that Cu⁰ NP are toxic to anammox. K_i constants with respect to added copper were 1.8- and 2.81-fold larger (less toxic) in EDTA-containing assays for Cu⁰ NP and CuCl_2, respectively, compared to EDTA-free assays. Additionally, K_i constants calculated in EDTA-free assays with respect the measured dissolved copper concentration were 0.023 mM Cu⁰ NP and 0.014 mM CuCl₂. The similarity of these K_i constants indicates that Cu⁰ NP toxicity to anammox is caused by the release of Cu²⁺. Finally, severe toxicity caused by 0.315 mM and Cu⁰ NP 0.118 mM CuCl₂ was attenuated by 88–100% when 0.14 mM EDTA was supplied. Toxicity attenuation likely occurred because EDTA complexed Cu²⁺ ions, thus, decreasing their bioavailability. Overall, this study indicates that Cu⁰ NP and CuCl₂ are toxic to anammox, and furthermore, that EDTA attenuates Cu⁰ NP and CuCl₂ toxicity to anammox by complexing Cu²⁺ ions.