Stimulating effect of copper ions and copper nanoparticles on a nitrifying sludge process: microrespirometric and physicochemical assessment

BACKGROUND Copper nanoparticles are used at an accelerated rate in several sectors, generating concern about their impact on human health and the environment. In this study, we characterized the physicochemical behavior and physiological effects of copper ions (Cu(II) ions) and copper nanoparticles (CuNPs) on a nitrifying sludge consortium. RESULTS Both Cu(II) ions and CuNPs exerted a hormetic effect, stimulating nitrifying sludge biomass. Stimulation was quantified by the percentage of enhancement (EOUR), maximum specific oxygen uptake rate (OURmax), and substrate affinity constant (KS). The results showed that, within the range of 0–20 mg L−1, both Cu(II) ions and CuNPs stimulated the nitrifying respiratory process, with maximum stimulation effects of 82.8% and 61.1% at 5 mg L−1, respectively. Both Cu(II) ions and CuNPs only increased the OURmax, but not the KS. CONCLUSION These findings indicate that copper deficiency could limit nitrogen removal in wastewater treatment. Furthermore, physicochemical characterization allowed us to determine that the observed stimulating effect may be caused by different factors, such as the aggregation, speciation, and adsorption of copper species in the nitrifying sludge consortium.