A DFT/TDDFT study on the mechanisms of direct and indirect photodegradation of tetrabromobisphenol A in water

Abstract

Tetrabromobisphenol A (TBBPA) is the most widely used commercial brominated flame retardant. However, the mechanisms underlying the photodegradation of TBBPA remain unclear. Here we use density functional theory and time-dependent density functional theory to examine the photodegradation of the two species of TBBPA in water: TBBPA (neutral form) and TBBPA− (anionic form). The study includes direct photodegradation and indirect photodegradation of TBBPA with ·OH and 1O2. The results of the calculations indicate that indirect photodegradation of TBBPA and TBBPA− with ·OH occurs via OH-addition and Br-substitution. All of the OH-addition and Br-substitution pathways are exothermic. Indirect photodegradation of TBBPA and TBBPA− by 1O2 proceeds via H abstraction by 1O2.Ea was higher for H abstraction of TBBPA than H abstraction of TBBPA−. The mechanisms for the direct photodegradation of TBBPA and TBBPA− include debromination, C1C7/C7C13 cleavage, and cyclization. CBr cleavage was observed in the optimized geometries of TBBPA and TBBPA− at the lowest excited triplet state. However, high Ea values and an endothermic nature indicated that C1C7/C7C13 cleavage and cyclization reactions were not the main pathways. OH-adducts, Br-substitution products, H-abstraction (by 1O2) products, and debromination products were the main products of photodegradation of TBBPA. These findings provide useful information for risk assessment and pollution control of brominated flame retardants.