Function of humic substances for enhancing catalytic oxidation of chlorinated organic compounds

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The decrease of catalytic activity of an iron(III)-porphyrin catalyst (Fe-Por) can be attributed to self-degradation of Fe-Por. To prevent the self-degradation, we first examined the addition of hydroxypropyl-β-cyclodextrin (HP-β-CD) that is regarded as a model of hydrophobic parts in humic substances (HSs). The self-degradation of Fe-Por was suppressed by forming supramolecular complex between Fe-Por and HP-β-CD, and this resulted in enhancing oxidative degradation of pentachlorophenol (PCP). Second, the effects of HSs on the enhancement of peroxosulfate oxidation of PCP by Fe-Por were studied mechanistically. The addition of HSs was also effective in suppressing the self-degradation of Fe-Por and resulted in an enhanced PCP oxidation. ^1H NMR studies of D_2O solutions of Fe-Por, HSs and mixtures there of showed that the aromatic moieties in HSs contributed to the binding of the sulfonatophenyl groups in Fe-Por. Third, an HS was separated according to molecular size using size exclusion chromatography. Three fractions were obtained and their aromatic carbons were determined by solid-state CPMS ^<13>C NMR. The rate of self-degradation of Fe-Por decreased with increasing the content of aromatic carbon in the fractionated HS. In addition, formation constant of supramolecular complex between Fe-Por and fractionated HS increased with increasing the aromaticity of HS sample. This supports that formation of supramolecular complex is due to interactions between sulfonatophenyl groups in Fe-Por and aromatic moieties in the HS. These results led to a conclusion that function of HS for enhancing peroxosulfate oxidation of PCP by Fe-Por can be attributed to stabilization of the catalyst by forming supramolecular complex.