Effects of polyphosphates and orthophosphate on the dissolution and transformation of ZnO nanoparticles

The fate and toxicity of zinc oxide nanoparticles (ZnO NPs) in nature are affected by solution chemistry such as pH, anions, and natural organic matter (NOM). Inorganic polyphosphates are environmentally ubiquitous phosphorus (P) species that may change the speciation and environmental fate of ZnO NPs. In this study, the interactions of polyphosphates with ZnO NPs and the impacts on ZnO NP dissolution and transformation were investigated and compared with orthophosphate (P₁). The results revealed that pyrophosphate (P₂), tripolyphosphate (P₃), and hexametaphosphate (P₆) enhanced whereas P₁ inhibited the dissolution of ZnO NPs. In addition, P₁, P₂, and P₃ promoted the transformation of ZnO NPs into zinc phosphate (Zn-P) precipitates via interactions with dissolved Zn²⁺. However, P₆-promoted ZnO NP dissolution was through the formation of soluble Zn-P complexes due to the strong capability of P₆ to chelate with Zn²⁺. The transformation of ZnO NPs in the presence of P₃ was affected by reaction time, pH, and P/Zn molar ratio. P₃ first formed inner-sphere surface complexes on ZnO NPs, which gradually transformed into crystalline Zn₂HP₃O₁₀(H₂O)₆ precipitates. This study provided a new perspective for understanding the reactivity of various forms of inorganic phosphate species with ZnO NPs in the natural environment.