Rate Constants and Mechanisms for Reactions of Bromine Radicals with Trace Organic Contaminants

Abstract

Bromine radicals can pose great impacts on the photochemical transformation of trace organic contaminants in natural and engineered waters. However, the reaction kinetics and mechanisms involved are barely known. In this work, second-order reaction rate constants with Br^• and Br₂^•- were determined for 70 common trace organic contaminants and for 17 model compounds using laser flash photolysis and steady-state competition kinetics. The <i>k</i>_Br^• values ranged from <10⁸ to (2.86 ± 0.31) × 10¹⁰ M^-1 s^-1 and the <i>k</i>_Br₂^•- values from <10⁵ to (1.18 ± 0.09) × 10⁹ M^-1 s^-1 at pH 7.0. Six quantitative structure-activity relationships were developed, which allow predicting additional unknown <i>k</i>_Br^• and <i>k</i>_Br₂^•- values. Single-electron transfer was shown to be a favored pathway for the reactions of Br^• and Br₂^•- with trace organic contaminants, and this was supported by transient spectroscopy and quantum chemical calculations. This study is essential in advancing the scientific understanding of halogen radical-involved chemistry in contaminant transformation.