Enhanced stability and dissolution of CuO nanoparticles by extracellular polymeric substances in aqueous environment

Stability of engineered nanoparticles in aquatic environment is an essential parameter to evaluate their fate, bioavailability, and potential toxic effects toward living organisms. As CuO NPs enter the wastewater systems, they will encounter extracellular polymeric substances (EPS) from microbial community before directly interacting with bacterial cells. EPS may play an important role in affecting the stability and the toxicity of CuO NPs in aquatic environment. In this study, the influences of flocculent sludge-derived EPS, as well as model protein (BSA) and natural polysaccharides (alginate) on the dissolution kinetics and colloidal stability of CuO NPs were investigated. Results showed that the presence of NOMs strongly suppressed CuO NPs aggregation, confirmed by DLS, zeta potentials, and TEM analysis. The enhanced stability of CuO NPs in the presence of EPS and alginate were attributed to the electrostatic combined with steric repulsion, while the steric-hindrance effect may be the predominant mechanism retarding nano-CuO aggregation for BSA. Higher degrees of copper release were achieved with the increasing concentrations of NOMs. EPS are more effective than alginate and BSA in releasing copper, probably due to the abundant functional groups and the excellent metal-binding capacity. The ratio of free-Cu²⁺/total dissolved Cu significantly decreased in the presence of EPS, indicating that EPS may affect the speciation and Cu bioavailability in aqueous environment. These results may be important for assessing the fate and transport behaviors of CuO NPs in the environment as well as for setting up usage regulation and treatment strategy.