Chlorine Dioxide-Pollutant Transformation and Formation of Hypochlorous Acid as a Secondary Oxidant

Chlorine dioxide (ClO₂) has been used as a disinfectant in water treatment for a long time, and its use for micropollutant abatement in wastewater has recently been suggested. Surprisingly, a mechanistic understanding of ClO₂ reactions in (waste)water matrices is largely lacking. The present study contributes to this mechanistic understanding by performing a detailed investigation of ClO₂ reactions with organic matter using phenol as a surrogate for reactive phenolic moieties. A concept for indirectly determining HOCl using 2- and 4-bromophenol was developed. The reaction of phenol with ClO₂ formed chlorite (62 ± 4% per ClO₂ consumed) and hypochlorous acid (HOCl) (42 ± 3% per ClO₂ consumed). The addition of ClO₂ to wastewater (5 × 10^-5 M ClO₂) resulted in 40% atenolol and 47% metoprolol transformation. The presence of the selective HOCl scavenger glycine largely diminished their transformation, indicating that atenolol and metoprolol were transformed by a fast reaction with HOCl (e.g., k (atenolol + HOCl) = 3.5 × 10⁴ M^-1 s^-1) that formed in ClO₂ reactions with the wastewater matrix. The formation of HOCl may thus increase the number of transformable micropollutants in ClO₂ applications. However, chlorine related byproducts may also be formed.