Phototransformation of microplastic derived dissolved organic matter reduces its adsorption capacity on ferrihydrite: Effects of additive types

Microplastics derived dissolved organic matter (MPDOM) undergoes coupled processes of phototransformation in surface water and adsorption on sediments in hydrological cycle. However, the role those plastic additives play during the interaction between phototransformation and adsorption of MPDOM remains unclear. Herein, MPDOM from polystyrene (PS) without additives, expanded polystyrene (EPS) with phenolic antioxidants, and clear polystyrene (CPS) with S-containing surfactants were selected for exploring the effect of additives-mediated MPDOM phototransformation on its adsorption on ferrihydrite. Carboxyl structure of tannins-like components determined the adsorption capacity of MPDOM. PSDOM was phototransformed into carboxyl-depleted structure under the attack of photogenerated DOM* and O, resulting in a 49.9 %-62.3 % decrease in adsorption on Fh. However, the adsorption of EPSDOM and CPSDOM decreased by only 6.0 %-27.7 % and 3.0 %-16.4 % after irradiation, respectively, attributing to additive-mediated photoprotection and photocombination mechanisms. Specifically, phenolic/quinone moieties of EPSDOM additives can photoprotect carboxyl of intrinsic molecule from phototransformation, while alkyl sulfonates structures of CPSDOM additives can photocombine with its intrinsic molecule to form aromatic CHOS and carboxyl-rich CHO components. The enhanced ecotoxicity of additive phototransformation products and their reduced adsorption capacity on ferrihydrite significantly increased environmental risk of MPDOM. This study provides new insights about the coupling behavior of photochemical transformation and adsorption deposition of MPDOM affecting by additives.