Unraveling the phototransformation of 2,4,6-tribromophenol: Aqueous mechanisms involving ·OH and the emergence of hydroxylated PBDEs.

Bromophenols (BPs) are ubiquitously present in aquatic environments due to industrial discharge and natural biosynthesis, yet their aquatic photochemical transformation is not fully elucidated. Herein, we investigate the phototransformation of 2,4,6-tribromophenol (TBP), a widely used BP, with a particular focus on debromination pathways under ·OH-involved conditions. Using a combination of mass spectrometric screening, HO labeling, spin-trapping experiments, quantum chemical calculations, and hierarchical clustering of transformation products, we demonstrate that TBP undergoes radical-coupling pathways driven by bromophenoxy radicals besides photohydrolytic and hydroxyl radical-mediated debromination. Elevated ·OH not only accelerate TBP attenuation but also significantly alter transformation pathways. ·OH-promoted radical coupling generates a markedly broader spectrum of dimeric products such as hydroxylated polybrominated diphenyl ethers (OH-PBDEs) and polyhydroxylated dibenzo-p-dioxins (OH-PBDDs). Toxicity prediction highlights that these dimeric products exhibit stronger potential for bioaccumulation than TBP. These findings show broad implications for understanding the phototransformation of BP and structurally related halogenated phenols under ·OH-involved conditions.