Mineral- and Base-Catalyzed Hydrolysis of Organophosphate Flame Retardants: Potential Major Fate-Controlling Sink in Soil and Aquatic Environments.

Abstract

The ubiquitous occurrence of organophosphate flame retardants (OPFRs) in aquatic and soil environments poses significant risks to human health and ecosystems. Here, we report on the hydrolysis of six OPFRs and three structural analogues in the absence and presence of metal (hydr)oxide minerals. Eight of the target compounds showed marked degradation in alkaline solutions (pH 9-12) with half-lives ranging from 0.02-170 days. Kinetics follow a second-order rate law with apparent rate constants for base-catalyzed hydrolysis (kB) ranging from 0.69-42 000 M-1 d-1. Although hydrolysis in homogeneous solution at circumneutral pH is exceedingly slow (t1/2 > 2 years, except for tris(2,2,2-trichloroethy) phosphate), rapid degradation is observed in the presence of metal (hydr)oxide minerals, with half-lives reduced to <10 days for most of the target OPFRs in mineral suspensions (15 m2/L mineral surface area loading). LC-qToF-MS analysis of transformation products confirmed ester hydrolysis as the active degradation pathway. Values of kB for individual OPFRs are highly variable and correlate with acid dissociation constants (pKa) of the corresponding alcohol leaving groups. In contrast, kinetic parameters for mineral-catalyzed reactions are much less sensitive to OPFR structure, indicating that other factors like mineral-OPFR interactions are rate controlling. Given the documented recalcitrance of OPFRs to biodegradation and photodegradation, these results suggest that mineral-catalyzed hydrolysis may be a major fate-controlling sink in natural environments.