Effect of humic acid on chemical oxidation of organic pollutants by iron(II) and H2O2: A dual mechanism

The capacity of Fenton reaction on the 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 is established: (i) at low Fe/HA < 1.17 mmol/g the presence of HA resulted to a decrease of PCP decomposition while (ii) at high Fe/HA > 1.17 mmol/g 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, (b) weak Fe-binding sites susceptible to reduction.

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