Significant zinc release from widely-used commercial lithopone pigments under solar irradiation

Lithopone pigments are an important group of white inorganic pigments, with production exceeding 240,000 tons/year in China. Nevertheless, our understanding of its environmental behavior is still limited. In this work, we reported the fast and extensive release of Zn²⁺ from the commercial lithopone pigment under solar exposure. The lithopone pigment released 40.97% of its total Zn within 24-h simulated sunlight exposure, generating a significant amount of nanoparticles. The commercial lithopone pigment had bandgap energy of 3.63 eV. It can be excited within the solar spectrum (wavelength <342 nm). As suggested by the photocurrent and surface photovoltage results, the pigment generated electrons and holes with good separation and transfer efficiency upon solar irradiation. The pigment also produced hydroxyl radical and superoxide by the oxidation of surface-bound OH^- and H₂O by the holes and direct transfer of the electrons to oxygen molecules. The photogenerated holes can oxidize the pigment, leading to its photo-dissolution and consequently the release of Zn²⁺. The water chemistry, including pH, coexisting anions, and organic carbons, influenced the photo-dissolution kinetics of the pigment by changing the valence band edge, forming stable precipitates, competing for the holes, and screening effect. The release of Zn²⁺ from the commercial lithopone pigment was fast and extensive under natural conditions, with 35.16% Zn released within 6-h solar exposure. Our results highlighted the crucial role of photochemistry in the environmental risk assessments and regulations of commercial lithopone pigments.