The effects of peat humic acids (HAs) on the enhancement of peroxosulfate oxidation of pentachlorophenol (PCP) by iron(III)-tetrakis(psulfonatophenyl)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-beta-CD (HP-beta-CD), beta-CD, HP-gamma-CD, gamma-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 (K-f) 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 K-f 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(II)-TPPS. H-1 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. (c) 2005 Elsevier B.V. All rights reserved.