Effect of pH on ·OH-induced degradation progress of syringol/syringaldehyde and health effect

Syringol and syringaldehyde are widely present pollutants in atmosphere and wastewater due to lignin pyrolysis and draining of pulp mill effluents. The hydroxylation degradation mechanisms and kinetics and health effect assessment of them under high and low-NO_x regimes in atmosphere and wastewater have been studied theoretically. The effect of pH on reaction mechanisms and rate constants in their ·OH-initiated degradation processes has been fully investigated. Results have suggested that aqueous solution played a positive role in the ·OH-initiated degradation reactions by decreasing the energy barriers of most reactions and changing the reactivity order of initial reactions. For Sy^- and Sya^- (anionic species of syringol and syringaldehyde), most initial reaction routes were more likely to occur than that of HSy and Hsya (neutral species of syringol and syringaldehyde). As the pH increased from 1 to 14, the overall rate constants (at 298 K) of syringol and syringaldehyde with ·OH in wastewater increased from 5.43 × 10¹⁰ to 9.87 × 10¹⁰ M^-1 s^-1 and from 3.70 × 10¹⁰ to 1.14 × 10¹¹ M^-1 s^-1, respectively. In the NO_x-rich environment, 4-nitrosyringol was the most favorable product, while ring-opening oxygenated chemicals were the most favorable products in the NO_x-poor environment. On the whole, the NO_x-poor environment could decrease the toxicities during the hydroxylation processes of syringol and syringaldehyde, which was the opposite in a NO_x-rich environment. ·OH played an important role in the methoxyphenols degradation and its conversion into harmless compounds in the NO_x-poor environment.