Catalytic decomposition of pentachlorophenol by the iron Fenton system: The dual role of humic acid

Abstract

The effect of Fenton [Fe/H2O2] on the catalytic decomposition of pentachlorophenol (PCP) in the presence and absence of humic acid (HA) was evaluated. Numerous studies have shown that HA can act either as an enhancer or inhibitor of the Fenton reaction. Herein, by combining electron paramagnetic resonance (EPR) spectroscopic and catalytic data, it is shown that HA can act in a dual manner, e.g. either as an enhancer or as an inhibitor for PCP oxidation, depending on the initial Fe/HA ratio. A quantitative limit was established: (i) at low Fe/HA < 1.17 mmol/gr, the presence of HA resulted in a decrease in PCP decomposition while (ii) at high Fe/HA > 1.17 mmol/gr, a significant enhancement was observed. EPR spectroscopy revealed that HA acted (i) as a chelator of Fe and (ii) as a redox agent. The data reveal the presence of two types of Fe-binding sites in HA: (a) strong Fe-binding sites and (b) weak Fe-binding sites where Fe is susceptible to reduction.