UV aging and soil organic matter co-regulate the adsorption of organophosphate flame retardants on PVC and PS: Kinetics and mechanisms

As used plastic additives, organophosphate flame retardants (OPFRs) widely coexists with microplastics (MPs) in agricultural soil, where MPs could act as carriers, increasing their environmental risk. However, the combined effects of UV aging and soil dissolved organic matter (DOM) on OPFR adsorption to MPs remains unclear. This study investigated the adsorption kinetics and mechanisms of six OPFRs on polyvinyl chloride (PVC) and polystyrene (PS). Results showed that OPFR adsorption positively correlated with their hydrophobicity (logK), dominated by hydrophobic interaction and partition. Kinetic studies indicated chemisorption governed adsorption, following pseudo-second-order kinetics. Upon UV aging, PVC exhibited increased affinity for polar/medium-polar OPFRs (triethyl phosphate, tributyl phosphate, and tris(2-butoxyethyl) phosphate), attributed to hydrophilic microzones formation, while PS preferentially adsorbed non-polar OPFRs (triphenyl phosphate and tris(2-ethylhexyl) phosphate) due to increased surface roughness and abundant binding sites. For soil DOM, it competitively inhibited OPFRs adsorption via hydrogen bonding on PVC but enhanced the OPFRs adsorption on PS through π-π interactions, acting as a molecular bridge. These findings highlight the critical role of polymer type, UV aging, and DOM in regulating OPFRs fate in soil. This will enhance our understanding of the environmental risks caused by the combined pollution resulting from MPs in soil environment.