Efficient removal of sulfamethoxazole by resin-supported zero-valent iron composites with tunable structure: Performance, mechanisms, and degradation pathways

Abstract

Nanoscale zero-valent iron loaded polymer-based composites (D201-nZVI) are effective materials for the removal of inorganic contaminants from water. However, the removal efficiency of organic contaminants and the role of the distribution of nZVI in the performance of the composites still remains unclear. Herein, four resin-supported nZVI composites with different nZVI distributions (D1, D2, D3, and D4) were prepared and used for sulfamethoxazole (SMX) degradation. The four composites, D1–D4, demonstrated a high efficiency of SMX removal (99.02%, 94.61%, 89.00%, and 86.28%, respectively, at pH 5.0). In addition, the performance of D201-nZVI only dropped by approximately 10% after five cycles, indicating its strong potential for practical application. On the basis of kinetic and electron spin resonance (ESR) spectral analyses, this study showed that the formation of hydroxyl radicals (⋅OH) and superoxide radicals (⋅O2-) is the main mechanism of SMX degradation. Finally, based on six major degradation intermediates of SMX, five possible degradation pathways were proposed, including the coupling of N-centered radicals, demethylation, the isomerization of isoxazole rings, the oxidation of amino groups, and the S–N bond cleavage in the D201-nZVI system. These results are not only important for better understanding the role of Fe distribution in the removal of SMX but are also crucial for the potential application of D201-nZVI composites with a different Fe distribution in many other scenarios.