Aquatic photochemistry of sulfamethazine: multivariate effects of main water constituents and mechanisms

Abstract

The ubiquity of sulfonamides (SAs) in natural waters requires insight into their environmental fate for ecological risk assessment. Extensive studies focused on the effect of univariate water constituents on the photochemical fate of SAs, yet the multivariate effects of water constituents in environmentally relevant concentrations on SA photodegradation are poorly understood. Here, response surface methodology was employed to explore the integrative effects of main water constituents (dissolved organic matter (DOM), NO₃⁻, HCO₃⁻, Cu²⁺) on the photodegradation of a representative SA (sulfamethazine). Results showed that besides single factors, interaction of factors also significantly impacted the photodegradation. Radical scavenging experiments indicated that triplet-excited DOM (³DOM*) was responsible for the enhancing effect of DOM on the photodegradation. Additionally, DOM may also quench the ³DOM*-mediated oxidation intermediate of sulfamethazine causing the inhibiting effect of DOM–DOM interaction. We also found that HCO₃⁻ was oxidized by triplet-excited sulfamethazine producing CO₃˙⁻, and the high reactivity of CO₃˙⁻ with sulfamethazine (second-order rate constant 2.2 × 10⁸ M⁻¹ s⁻¹) determined by laser flash photolysis revealed the enhancing photodegradation mechanism of HCO₃⁻. This study is among the first attempts to probe the photodegradation of SAs considering the integrative effects of water constituents, which is important in accurate ecological risk assessment of organic pollutants in the aquatic environment.