Overlooked Role of Carbonyls of Natural Organic Matter on the Dissolution of Zinc Oxide Nanoparticles

Zinc oxide nanoparticles (ZnO NPs) are harmful because of the release of cytotoxic Zn²⁺ during dissolution. The dissolution process of ZnO NPs is affected by natural organic matter (NOM) in the water environment. In this study, we investigated the role of carbonyl functional groups of NOM isolates in the dissolution of ZnO NPs. Sodium borohydride (NaBH₄) can selectively reduce the carbonyls in NOM. We treated seven types of NOMs with NaBH₄. The experimental results show that the NOM treated by NaBH₄ has a significantly reduced ability to dissolve ZnO NPs. A series of model aromatic ketones and quinones were used to simulate NOM, and these model small-molecule mixtures were selectively reduced by NaBH₄ and sodium dithionite (Na₂S₂O₄). The ability of the treated model small-molecule mixture to dissolve ZnO NPs was reduced. These results further demonstrate that carbonyls play an important role in the dissolution of ZnO NPs. The results of the study allow us to better understand the transfer process of ZnO NPs in NOM-rich aquatic environments.