Complex formation of water-soluble iron(III)-porphyrin with humic acids and their effects on the catalytic oxidation of pentachlorophenol

The effects of peat humic acids (HAs) on the enhancement of peroxosulfate oxidation of pentachlorophenol (PCP) by iron(III)-tetrakis(p-sulfonatophenyl)porphyrin (Fe(III)-TPPS) were studied mechanistically. Eight types of cyclodextrins (CDs) were used as model compounds for the polysaccharide moieties in the HAs, and their effects were compared with those for the HAs. The results indicate that the presence of hydroxypropyl-β-CD (HP-β-CD), β-CD, HP-γ-CD, γ-CD and HAs were effective in suppressing the self-oxidation of Fe(III)-TPPS and resulted in an enhanced PCP oxidation. The stabilization of the Fe(III)-TPPS catalyst can be attributed to complexation of the catalyst with non-ionic hydrophobic regions of HAs. The conditional formation constants (Kf) for the complexes of Fe(III)-TPPS with CDs and HAs were evaluated by the changes in UV–vis absorption spectra of the Soret band of Fe(III)-TPPS. The log Kf values increased with decreasing rate of self-oxidation of Fe(III)-TPPS, showing that the complexation of Fe(III)-TPPS with CDs and HAs plays an important role in stabilizing Fe(III)-TPPS. 1H NMR studies of D2O solutions of Fe(III)-TPPS, HAs and mixtures thereof showed that the hydrophobic regions of HAs contributed to the binding of the sulfonatophenyl group in Fe(III)-TPPS. In addition, complexes of Fe(III)-TPPS with HAs were isolated by ultrafiltration.

Graphical abstractThe addition of humic acid (HA) was effective in suppressing the self-oxidation of iron(III)-tetrakis(p-sulfonatophenyl)porphyrin (Fe(III)-TPPS) and resulted in an enhanced oxidation of pentachlorophenol in the Fe(III)-TPPS/KHSO5 catalytic system. As shown in Figure, 1H-NMR studies indicate that the stabilization of the Fe(III)-TPPS catalyst can be attributed to complexation between the hydrophobic regions of HAs and the sulfonatophenyl group in Fe(III)-TPPS.Figure optionsDownload full-size imageDownload as PowerPoint slide