Oxalic acid drove the permutation decomplexation of the Fe-salicylhydroxamic acid complex and the photocatalytic removal of salicylhydroxamic acid

Residual salicylhydroxamic acid (SHA) and Fe3+ in mining sites often form stable, toxic Fe3+-SHA complexes that are hard to remove and ecologically risky. There's an urgent need for eco-friendly, low-energy methods to remove this complex metal–organic ligand co-pollution. Here, a decomplexation of Fe3+-SHA was successfully achieved using the competitive coordination effect of oxalic acid (OA) without external energy. The formed Fe3+-OA complex was used to construct an in situ photo-Fenton system for efficient SHA removal. Due to the lower binding energy of Fe3+-OA (-46.59) compared to Fe3+-SHA (-24.48), OA could release Fe3+ from the Fe3+-SHA complex and rebind with it to form photochemically active iron oxalate complexes (primarily [Fe(C2O4)3]3-). Under the UV light, Fe2+ and a series of reactive oxygen species including CO2•–, O2•–, H2O2 and HO• were generated. Among these radicals, HO• is responsible for SHA degradation. The key role of HO• was further confirmed through the detection of 6 intermediate products using GC–MS and FTIR. Besides metal cation and HCO3–, this system showed minimal sensitivity to anions and good efficacy in actual polluted water samples. In brief, this study provides new insights into the fate of SHA in mine-contaminated sites where Fe3+ and OA are widespread.