The roles of HO•, ClO• and BrO• radicals in caffeine degradation: A theoretical study

Abstract

ClO• and BrO• are newly discovered reactive radicals that contribute to the degradation of micropollutants. However, the research on pollutant degradation by ClO• and BrO• is still lacking. Thus, the mechanism, kinetics, and toxicity of caffeine degradation by HO•, ClO•, and BrO• were computationally studied and compared. Results showed that radical adduct formation (RAF) reaction was dominant for HO•, ClO•, and BrO• initiated reactions of caffeine. The main reaction sites were C5 and C8 of caffeine for HO•, while only the RAF reaction on C8 was prominent for ClO• and BrO•. The initiated reaction rate constants of caffeine by HO•, ClO•, and BrO• were in the order of HO• (5.29 × 109 M−1 s−1) > ClO• (1.40 × 109 M−1 s−1) > BrO• (2.17 × 108 M−1 s−1). The kinetic simulation verified that ClO• played a crucial role in the degradation of caffeine by the UV/chlorine process. In addition to HO-adducts, the subsequent reaction mechanisms of ClO- and BrO-adducts have also been investigated. The formation mechanisms of several important products, namely dimethylparabanic acid (P2), di(N-hydroxymethyl) parabanic acid (P5), 1,3,7-trimethyluric acid (P6), and 8-oxocaffeine (P11), were elucidated. Remarkably, stable chlorinated and brominated intermediates or products were not generated in ClO•- and BrO•-mediated subsequent degradations of caffeine. The assessment of aquatic toxicity and health effects showed that caffeine could penetrate the blood-brain barrier (human), and caffeine and its degradation products were potentially harmful to the aquatic environment.