Selective photo-Fenton degradation of glyphosate using Schiff base metal complexes: Insights into metal-mediated pathways

Developing efficient strategies for glyphosate (PMG) degradation while minimizing the formation of the toxic intermediate, aminomethylphosphonic acid (AMPA), is crucial for environmental remediation. In this study, two Schiff base metal complexes, Fe-SPA and Cu-SPA (SPA: salicylaldehyde o-phenylenediamine), were prepared and evaluated for their catalytic activity in activating H 2 O 2 under visible light irradiation (Vis, λ ≥ 420 nm) to degrade PMG. The Fe-SPA/H 2 O 2 (Vis) system achieved 99.9% degradation of 200 μmol/L PMG within 2 h, while the Cu-SPA/H 2 O 2 (Vis) system achieved complete degradation within 5 h. Interestingly, the Fe-SPA/H 2 O 2 (Vis) system selectively catalyzed PMG degradation via an environmentally friendly pathway, producing glycine as the final product. While in the Cu-SPA/H 2 O 2 (Vis) system, in addition to glycine, initially produced toxic AMPA, which was subsequently mineralized into non-toxic inorganic products. These differences in degradation pathways could be attributed to the preferential adsorption behavior of PMG and H 2 O 2 on the M-SPA complexes. Specifically, Fe-SPA selectively adsorbed the phosphate group of PMG, facilitating the C−P bond cleavage. Conversely, Cu-SPA preferentially adsorbed H 2 O 2 , resulting in a nonselective PMG degradation. Furthermore, hydroxyl radicals (·OH) and photogenerated holes (h⁺) were identified as the dominant reactive species in both systems. Fe-SPA exhibited excellent stability and maintained high catalytic performance across a wide pH range (3–10) and after five cycles of use. These findings highlighted the pivotal role of the central metal in determining pollutant degradation pathways. This study offers a promising strategy for efficient and environmentally safe PMG remediation.