Hydroxyl radical involvement in the decomposition of hydrogen peroxide by ferrous and ferric-nitrilotriacetate complexes at neutral pH

Abstract

The relative rates of degradation of three hydroxyl radical probe compounds (atrazine, fenuron and parachlorobenzoic acid (pCBA)) by Fe^III/H₂O₂ (pH = 2.85), Fe^IIINTA/H₂O₂ (neutral pH), Fe^II/O₂, Fe^IINTA/O₂, Fe^II/H₂O₂ and Fe^IINTA/H₂O₂ (neutral pH) have been investigated using the competitive kinetic method. Experiments were carried out in batch and in semi-batch reactors, in the dark, at 25 °C. The data showed that the three probe compounds could be degraded by all the systems studied, and in particular by Fe^IINTA/H₂O₂ and Fe^IIINTA/H₂O₂ at neutral pH. The relative rate constants of degradation of the three probe compounds obtained for all the systems tested were identical and equal to 1.45 ± 0.03 and 0.47 ± 0.02 for k_Atrazine/k_pCBA and k_Fenuron/k_pCBA, respectively. These values as well as the decrease of the rates of degradation of the probe compounds upon the addition of hydroxyl radical scavengers (tert-butanol, bicarbonate ions) suggest that the degradation of atrazine, fenuron and pCBA by Fe^IINTA/O₂, Fe^IINTA/H₂O₂ and Fe^IIINTA/H₂O₂ is initiated by hydroxyl radicals.

Introduction

Advanced Oxidation Processes (AOPs) can be successfully used in the field of wastewater treatment to reduce the chemical oxygen demand and the toxicity of industrial wastewaters, to convert toxic and biorecalcitrant contaminants into biodegradable by-products, to remove colour or to obtain a complete mineralization of organic pollutants (Pera-Titus et al., 2004). Among the AOPs, the Fenton and the Fenton-like oxidation processes (Fe^II/H₂O₂, Fe^III/H₂O₂) have been applied to many industrial wastewaters such as chemical, petrochemical, pharmaceutical and textile effluents (Pignatello et al., 2006, Bautista et al., 2008). These oxidation processes are also popular methods for the remediation of contaminated soils and groundwater (Bergendahl et al., 2003, Watts and Teel, 2005). The main benefits of the Fenton and Fenton-like reactions are the use of environmentally friendly and low cost reagents. However, these oxidation processes have also some limitations. The Fe^II/H₂O₂ and Fe^III/H₂O₂ systems must be operated at low pH values (pH ≈ 3) to prevent the precipitation of ferric oxyhydroxides and to produce hydroxyl radicals. In addition, the efficiency of the Fenton reaction can be markedly decreased in the presence of high concentrations of inorganic salts such as chloride and sulfate ions because of the formation of inactive iron(III)-complexes (De Laat and Le, 2005, De Laat and Le, 2006). To overcome these drawbacks, the use of organic and inorganic iron-chelating agents has been investigated by several authors in order to enhance the efficiency of the homogenous Fenton-like oxidation processes at neutral pH (Sun and Pignatello, 1992, Sun and Pignatello, 1993, Li et al., 2007, Lee and Sedlak, 2009, Rastogi et al., 2009) or of heterogenous systems involving ion bearing minerals (Xue et al., 2009) or granular zero-valent iron (Keenan and Sedlak, 2008, Lee et al., 2008). In their pioneering works, Sun and Pignatello (1992) have tested several classes of iron(III)-chelates for the decomposition of H₂O₂ and of 2,4-dichlorophenoxyacetic at pH 6. Of the 50 compounds tested, nitrilotriacetic acid (NTA) was one of the most active iron(III)-chelate. The iron-NTA/H₂O₂ system was also found to be effective for the degradation of tetrachloroethene in contaminated soils (Howsawkeng et al., 2001, Ndjou’ou et al., 2006). Kim and Kong (2001) showed that the Fe^IIINTA/H₂O₂ system degraded more efficiently 1-hexanol and carbon tetrachloride at pH 9 than at pH 3 and that the degradations of 1-hexanol and carbon tetrachloride are initiated by hydroxyl radical (HO) and by superoxide anion radical (HO₂/O₂⁻), respectively.

The nature of the reactive oxidant species (free and bound HO radicals, high-valent-oxoiron species) generated by the reaction of H₂O₂ with free and complexed Fe(II) and Fe(III) species in aqueous solution remains a controversial issue (Walling et al., 1975, Yamazaki and Piette, 1991, Bossmann et al., 1998, Pignatello et al., 2006). In the absence of chelating agents, it is generally accepted that the reaction of H₂O₂ with Fe²⁺ at low pH yields hydroxyl radicals whereas other oxidants such as the ferryl ion may be produced at circumneutral pH (Gallard et al., 1998, Rivas et al., 2001, Hug and Leupin, 2003, Keenan and Sedlak, 2008, Katsoyiannis et al., 2008). In all these studies, proofs of the formation of one or another oxidizing species are based on EPR spin-trapping techniques, distribution of oxidation by-products, effects of added hydroxyl radical scavengers on reaction rates or on kinetic modeling studies.

In this work, the competitive kinetic method has been applied to the degradation of three hydroxyl radical probes (atrazine, fenuron and parachlorobenzoic acid) in order to highlight the formation of hydroxyl radicals by the Fe^IINTA/H₂O₂ and Fe^IIINTA/H₂O₂ systems at neutral pH. The degradation of probe compounds by the Fe^II/O₂ and Fe^IINTA/O₂ systems has also been investigated because dissolved oxygen readily oxidizes ferrous ion (King et al., 1995, Rose and Waite, 2002) and Fe^IINTA (Harris and Aisen, 1973, Welch et al., 2002) at neutral pH. The effects of the addition of HO radical scavengers (tert-butanol or bicarbonate ions) on the degradation rates of probe compounds and of H₂O₂ will also be examined. However, the effects of experimental parameters on the rate of decomposition of H₂O₂ by Fe^IINTA and Fe^IIINTA will be presented in a next paper and will not be discussed here.