The re-precipitation and photo-induced cerium ions release of commercial cerium sulfide pigment in aqueous phase

Cerium sulfide (Ce 2 S 3 ) pigment is an environmentally friendly novel inorganic red-colored metal pigment, representing a potential alternative to traditional heavy metal pigments. In this study, we report concurrent re-precipitation and photo-dissolution of cerium sulfide pigment in a sunlit aquatic phase. Commercial cerium sulfide pigment transforms into its re-precipitation product, CeCO 3 OH, along with the generation of H 2 S gas in its re-precipitation process. Simultaneously, cerium sulfide pigment photo-dissolution results in a significant release of Ce ions. After 4 hours of simulated sunlight irradiation, the particle size of cerium sulfide pigment significantly reduces, and 10.4% of elemental Ce is released into the solution as free Ce ions. The key photo-dissolution mechanism is the oxidation reaction between photogenerated holes and the crystal lattice of the pigment. Cerium sulfide pigment and its re-precipitation product, CeCO 3 OH, can be excited by specific wavelengths in the solar spectrum, mediating the pigment photo-dissolution. UV light shielding can inhibit the cerium sulfide pigment photo-dissolution. Superoxide and hydroxyl radicals are generated during the photoreaction process, in which the hydroxyl radical may oxidize the photo-induced released Ce(III) to Ce(IV) and the re-precipitation-generated H 2 S to SO 4 2- . Our research indicates that commercial cerium sulfide pigment undergoes a series of transformation processes in aquatic environments, potentially posing certain environmental risks.