Tannic acid promotes ion release of copper oxide nanoparticles: Impacts from solution pH change and complexation reactions

The increasing number of applications in which copper oxide nanoparticles (CuO NPs) are used, may lead to potential release of CuO NPs into the environment. However, the impact of natural organic matters on the behavior and fate of CuO NPs in aquatic media is still largely unknown. In this study, the dissolution and aggregation of CuO NPs under the exposure of tannic acid (TA) were monitored over a period of 72 h, with a focus on assessing the contributions of solution pH changes and complexation reactions. Results showed that the total amount of Cu2+ released from CuO NPs increased in the presence of TA especially at the highest TA concentration of 73.5 μmol/L. Although TA was observed to wrap around the CuO NPs, the aggregation of CuO NPs was not strongly influenced by TA and by the solution pH as investigated in this study. The kinetics of Cu2+ release were fitted using the modified pseudo second-order model and the rate of dissolution was assessed to be highest at TA = 14.7 μmol/L. At pH = 4, the increased H+ concentration was responsible for increased Cu2+ release, whereas the complexation reaction between Cu2+ and TA dominated at pH = 7. These findings suggested that the effects of TA on the dissolution of CuO NPs were a combination of solution pH change and complexation reaction, the relative fractions of which also depended on the solution pH. Additionally, the percentage of Cu2+ released from the CuO NPs was found to increase upon decreasing concentrations of CuO NPs. Our work helps to further understand how and to which extent natural organic matters affect the behavior and fate of CuO NPs.