Photodegradation of sulfapyridine under simulated sunlight irradiation: Kinetics, mechanism and toxicity evolvement

• Sulfapyridine was effectively photodegraded under UV–vis irradiation.
• Factors affecting the photodegradation process were investigated.
• Degradation intermediates were identified and possible pathway was proposed.
• Bioassay test showed reaction products were much more toxic than parent compound.

In this study, the photoinduced degradation of sulfapyridine (SPY) was investigated under simulated light irradiation (λ > 200 nm). The effect of pH and main water constituents including nitrate ion, bicarbonate, dissolved organic matter (DOM) and iron(III) on the photodegradation was explored. SPY was effectively removed in aqueous solution at pH 8 under UV–vis irradiation, with removal efficiency of 100% within 120 min. DOM and iron(III) had retarding influence on the SPY removal, whereas nitrate ion and bicarbonate did not show any obvious effect. Under UV–vis irradiation, the formation of singlet oxygen (1O2) accelerated the SPY photodegradation and the contribution of indirect photolysis due to reaction with 1O2 was up to 42%. The transformation products of SPY were identified by HPLC–MS and the possible photoreaction pathways were proposed. It showed that photoinduced hydrolysis, photo-oxidation via 1O2 and desulfonation were the main degradation ways for SPY decomposition. Toxicity assays by Vibrio fischeri proved that the transformation products were more toxic than the parent compound.