Dissolution and Transformation of ZnO Nano- and Microparticles in Soil Mineral Suspensions

The increasing use of ZnO nanoparticles (NPs) has generated serious concern about their fate, transportation, and toxicity in the environment. The present study focuses on the key geochemical processes controlling the environmental fate of NPs, that is, dissolution and transformation of ZnO NPs in a large time scale with two common soil mineral (gamma-Al2O3 and goethite) suspensions in batch reactors for up to 360 days using synchrotron-radiation powder X-ray diffraction, extended X-ray absorption fine structure spectroscopy, and high-resolution transmission electron microscopy. The aqueous dissolution experiments showed that smaller size ZnO NPs dissolved faster at lower pH. The combined spectroscopic analyses revealed that the interaction between ZnO NPs and gamma-Al2O3 promoted dissolution of ZnO NPs by transforming into the Zn-Al-layered double hydroxide (LDH) precipitate at pH 7.5 within 0.5 h, and the fraction of Zn-Al LDH precipitate increased with the incubation time. At pH 5.5, Zn-Al LDH precipitate was also observed, which was attributed to the locally elevated pH and Zn concentration near the ZnO NP surface during the dissolution. In the presence of goethite, ZnO NPs dissolved less and Zn mainly existed as ZnCO3 at pH 5.5, but at pH 7.5, ZnO NPs barely dissolved and transformed, even with a prolonged incubation time. The findings of this study will facilitate a better understanding of the fate of ZnO NPs in soil mineral suspensions, which can be leveraged for remediation of ZnO NP-polluted soils.