Azo compound degradation kinetics and halonitromethane formation kinetics during chlorination

The chlorination of azo compounds can produce halonitromethanes (HNMs), which have attracted increasing concern due to their high genotoxicity. By impacting the speciation of chlorine and azo compounds, pH impacts apparent second-order rate constants of Methyl Orange (MO, 27.5–1.4 × 10³ M⁻¹ s⁻¹), Acid Orange II (AO, 16.7–99.3 M⁻¹ s⁻¹), and Acid Red 1 (AR 1, 3.7–72.5 M⁻¹ s⁻¹) (pH range 6.3–9.0). The two-compartment first-order model successfully described the chloropicrin (TCNM) formation kinetics, suggesting that both fast- and slow-reacting precursors of TCNM are generated from the chlorination of azo compounds. The ratios between fast and slow formation rate constants for MO and AO were 15.6–5.4 × 10², while that of AR 1 was 9.8–19.4 (pH range 6.5–9.0). The fraction of the fast-reacting TCNM precursors decreased with increasing pH for MO and AO; while that for AR 1 decreased when pH increased from 6.5 to 8.0, and then increased when pH increased from 8.0 to 9.0. The impact of pH on TCNM formation was also precursor-specific. The highest molar yields of TCNM predicted from the model in this study were 2.4%, 2.5%, and 1.5% for MO, AO, and AR 1, respectively. The study demonstrates that azo compounds are important HNM precursors, and pose a potential threat to drinking water safety.