The underestimated environmental risk of tris (2-chloroethyl) phosphate photodegradation in aqueous environment induced by polystyrene microplastics.

Tris(2-choroethyl) phosphate (TCEP) is commonly utilized as a flame retardant and plasticizer, which inevitably coexists with polystyrene microplastics (PS-MPs) in aquatic environments. In this work, the promoting effect of pristine and aged PS-MPs on the photodegradation of TCEP was observed, and the reaction mechanisms and environmental risks of PS-MPs enhancing TCEP photodegradation were clearly revealed. The aged PS-MPs presenting more significant enhancement was attributed to more generation of reactive oxygen species (ROS). The yield of •OH generated in the aged PS-MPs system was 21 times as high as that of pristine PS-MPs. Contributions of •OH, O and O to the degradation of TCEP induced by aged PS-MPs were 59.07 %, 34.98 % and 7.8 %, respectively. Possible degradation products of TCEP were identified, primarily formed through hydroxyl substitution, intramolecular cyclization and carboxylation. Density functional theory calculations demonstrated that •OH was more likely to attack the P atom of TCEP than the C atom. The low molecular derivatives containing oxygen group of PS-MPs reacted with •Cl/•OCHCl released by TCEP to generate interaction products. Toxicity evaluation of interaction products suggested some interaction products were more toxic than TCEP, indicating that these interaction products contributed to the underestimation of transformation risk of TCEP and coexisting PS-MPs. This study provides a novel insight into the fate and the ecological risks associated with the combined contamination of MPs and coexisting plastic additives.