Quantitative investigation of ZnO nanoparticle dissolution in the presence of δ-MnO₂

The widespread use of zinc oxide nanoparticles (ZnO NPs), the second most produced nanomaterial, inevitably leads to their release into the environment. In this study, dissolution and transformation of ZnO NPs in the presence of δ-MnO₂, an abundant and ubiquitous manganese (Mn) oxide mineral, was investigated via a suite of techniques covering bulk to molecular scales. Dissolution kinetics indicated that the presence of δ-MnO₂ significantly affected ZnO NP dissolution rate/trend and equilibrium Zn²⁺ concentration, which were found to be mainly dependent on the concentration and mass ratio of ZnO NPs and δ-MnO₂. Approximately 300 mg ZnO NPs per g δ-MnO₂ was expected for ZnO NP uptake at pH 7.0 via ZnO NP dissolution and surface Zn²⁺ adsorption. X-ray diffraction (XRD), ζ potential, high-resolution transmission electron microscopy (HR-TEM), and Zn K-edge X-ray absorption spectroscopy (XAS) results revealed that when the mole content of ZnO NPs was less than the total adsorption sites of δ-MnO₂ surface, ZnO NPs were completely dissolved and adsorbed on δ-MnO₂ surface in the form of inner-sphere complexes. A fraction of ZnO NPs persisted when the mole ratio of ZnO to δ-MnO₂ further increased. These results suggest that the transformation and fate of ZnO NPs is affected by environment-relevant minerals such as Mn oxides due to their huge capacity of fixing dissolved metal cations at the surface or interlayer structure.