Ozonation of pentabromophenol in aqueous basic medium: Kinetics, pathways, mechanism, dimerization and toxicity assessment.

Ozonation has been identified effective technique to degrade phenolic compounds, and production of intermediate dimers are major threat. In this study, we systematically investigated the degradation of Pentabromophenol (PBP) in an aqueous medium by using two different ozone generators (sources: air and water). We studied various factors that influenced the degradation kinetics of PBP, including the pH (7.0, 8.0, and 9.0), humic acid (HA) and anions (Cl-, SO42-, NO3-, and HCO3-). PBP was efficiently degraded within 5 min (O3 source: water) and 45 min (O3 source: air) at pH 8.0 maintained by phosphate buffer. Reaction kinetics revealed 17 b y-products with five possible pathways, including dimers with their isomers and lower bromophenols. Furthermore, the frontier molecular orbital theory was employed to confirm the proposed ozonation pathways, including the breakage of the CO bond at C5 and C4 positions, and the cleavage of the CC bond at C3 and C6 position. Product P5, P14 (hydroxyl-nonabromophenyl ether) and P15 (dihydroxyl-octabromophenyl ether) were identified with isomers. Ecological Structure Activity Relationships toxicity assessment resulted into the conversion of highly toxic PBP (acute toxicity: LC50 = 0.11 mg L-1 for fish, LC50 = 0.124 mg L-1 for daphnia, and EC50 = 0.118 mg L-1 for green algae) to less harmful products aside from dimers. P14 (acute toxicity: LC50 = 1.04 × 105) found to be more toxic as compare to PBP. From these findings, we concluded that ozonation is an effective and ideal process for PBP degradation.