Experimental Study of OH-Initiated Heterogeneous Oxidation of Organophosphate Flame Retardants: Kinetics, Mechanism, and Toxicity

The environmental risks and health impacts associated with particulate organophosphate flame retardants (OPFRs), which are ubiquitous in the global atmosphere, have not been adequately assessed due to the lack of data on the reaction kinetics, products, and toxicity associated with their atmospheric transformations. Here, the importance of such transformations for OPFRs are explored by investigating the reaction kinetics, degradation chemical mechanisms, and toxicological evolution of two OPFRs (2-ethylhexyl diphenyl phosphate (EHDP) and diphenyl phosphate (DPhP)) coated on (NH₄)₂SO₄ particles upon heterogeneous OH oxidation. The derived reaction rate constants for the heterogeneous loss of EHDP and DPhP are (1.12 ± 0.22) × 10^-12 and (2.33 ± 0.14) × 10^-12 cm³ molecules^-1 s^-1, respectively. Using recently developed real-time particle chemical composition measurements, particulate products from heterogeneous photooxidation and the associated degradation mechanisms for particulate OPFRs are reported for the first time. Subsequent cytotoxicity analysis of the unreacted and oxidized OPFR particles indicated that the overall particle cytotoxicity was reduced by up to 94% with heterogeneous photooxidation, likely due to a significantly lower cytotoxicity associated with the oxidized OPFR products relative to the parent OPFRs. The present work not only provides guidance for future field sampling for the detection of transformation products of OPFRs, but also strongly supports the ongoing risk assessment of these emerging chemicals and most critically, their products.