Reevaluation of the Reactivity of Superoxide Radicals with a Sulfonamide Antibiotic, Sulfacetamide: An Experimental and Theoretical Study

Superoxide radical (O-2(center dot-)) is a ubiquitous reactive oxygen species in sunlit natural waters. Here, we selected sulfacetamide (SCT) as a representative sulfonamide antibiotic to investigate its reactivity with O-2(center dot-), as this reaction was reported to be fast with the rate constant k of 7.0 x 10(7) M-1 s(-1). The degradation kinetics of SCT were first studied in the classic McDowell systems (i.e., ultraviolet, acetone, and alcohols) for optimal O-2(center dot-) productivity. Comparison of the apparent first-order degradation rates (k(app)) revealed that 70 mM acetone and 1 M isopropanol were optimal for k(app), but it was only similar to 10% faster than the control. The small degree of enhancement of k(app) led us to suspect its high reported reactivity with O-2(center dot-). Therefore, we reevaluated this reaction in organic and aqueous solutions by a suite of spectroscopic techniques, and all evidence indicated that the measured k values were 6 orders of magnitude smaller than the reported one. The low reactivity was also confirmed by theoretical calculations at the M06-2X level of theory. Our results are of practical and scientific importance for the holistic assessment of the role of O-2(center dot-) in remediation and attenuation of organic contaminants in natural and engineered waters.