A comparative study of the effects of chloride, sulfate and nitrate ions on the rates of decomposition of H2O2 and organic compounds by Fe(II)/H2O2 and Fe(III)/H2O2

doi:10.1016/j.chemosphere.2003.11.021

Chemosphere

Volume 55, Issue 5, May 2004, Pages 715-723

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Abstract

The effects of chloride, nitrate, perchlorate and sulfate ions on the rates of the decomposition of hydrogen peroxide and the oxidation of organic compounds by the Fenton’s process have been investigated. Experiments were conducted in a batch reactor, in the dark at pH ⩽ 3.0 and at 25 °C. Data obtained from Fe(II)/H₂O₂ experiments with [Fe(II)]₀/[H₂O₂]₀ ⩾ 2 mol mol⁻¹, showed that the rates of reaction between Fe(II) and H₂O₂ followed the order SO₄²⁻ > ClO₄⁻NO₃⁻Cl⁻. For the Fe(III)/H₂O₂ process, identical rates were obtained in the presence of nitrate and perchlorate, whereas the presence of sulfate or chloride markedly decreased the rates of decomposition of H₂O₂ by Fe(III) and the rates of oxidation of atrazine ([atrazine]₀=0.83 μM), 4-nitrophenol ([4-NP]₀=1 mM) and acetic acid ([acetic acid]₀=2 mM). These inhibitory effects have been attributed to a decrease of the rate of generation of hydroxyl radicals resulting from the formation of Fe(III) complexes and the formation of less reactive (SO₄⁻) or much less reactive (Cl₂⁻) inorganic radicals.

Introduction

Advanced oxidation processes (AOPs) which involve the generation of the highly reactive and non selective hydroxyl radical (OH) are of interest for the destruction of organic pollutants in wastewater. Among these AOPs, the Fenton’s reagent (Fe(II)/H₂O₂) and the Fenton-like reagent (Fe(III)/H₂O₂) have been the subject of numerous studies in order to study the mechanism and the kinetics of the reaction (Haber and Weiss, 1934; Barb et al., 1951a, Barb et al., 1951b; Walling, 1975; Sychev and Isaak, 1995; and references therein) or to examine the efficiency of the process for the removal of organic pollutants (Safarzadeh-Amiri et al., 1996).

In a recent study conducted at 25 °C and in NaClO₄/HClO₄ solutions (De Laat and Gallard, 1999; Gallard et al., 1999; Gallard and De Laat, 2000), we validated over a wide range of experimental conditions (0 ⩽ pH⩽ 3, 0 M ⩽ [H₂O₂]₀ ⩽ 1 M, 0 mM ⩽ [Fe(II)]₀ or [Fe(III)]₀ ⩽ 1 mM) a kinetic model which predicts very accurately the experimental rates of decomposition of H₂O₂ and of a model organic compound (atrazine) in dilute aqueous solutions ([atrazine]₀ ⩽ 1 μM). This kinetic model assumes the formation of the hydroxyl radical by the reaction of H₂O₂ with ferrous ion (reaction 1 in Fig. 1) and the regeneration of ferrous ion from Fe(III)-hydroperoxo complexes (reactions 2 and 3 in Fig. 1). The model also includes known propagation and termination reactions involving HO₂/O₂⁻ and OH. This kinetic model was useful in order to better understand the effects of pH, reactant doses and the nature of iron salt (Fe(II) or Fe(III)) on the complex kinetics of the decompositions of H₂O₂ and atrazine by the Fe(II)/H₂O₂ and Fe(III)/H₂O₂ systems.

Our kinetic model has only been validated for reactions conducted with Fe(ClO₄)₂ or Fe(ClO₄)₃ as iron sources, HClO₄ and NaClO₄ for pH (pH ⩽ 3) and ionic strength (I) adjustments and atrazine as the model organic solute. The efficiency of the Fenton’s reagent can also be affected by other parameters such as temperature and concentration of dissolved oxygen, and the nature and concentration of organic compounds (Kang et al., 2002; Lee et al., 2003). Organic solutes and their oxidation by-products (i.e. R and ROO radicals, quinones, hydroxylated derivatives and carboxylic acids) can also have an influence on the efficiency of the Fe(II)/H₂O₂ and Fe(III)/H₂O₂ processes because of the possible reactions with the iron species (Chen and Pignatello, 1997; Gallard and De Laat, 2001; Chen et al., 2002; Kang et al., 2002).

Inorganic anions (Cl⁻, SO₄²⁻, H₂PO₄⁻/HPO₄²⁻, etc.) present in wastewater or added as reagents (FeSO₄, FeCl₃, HCl, H₂SO₄) may also have a significant effect on the overall reaction rates in the Fenton process. The possible effects are (Fig. 1) (i) complexation reactions with Fe(II) or Fe(III) which can affect the distribution and the reactivity of the iron species, (ii) precipitation reactions phosphate which lead to a decrease of the active dissolved iron(III), (iii) scavenging of hydroxyl radicals and formation of less reactive inorganic radicals (Cl⁻, Cl₂⁻ and SO₄⁻) and (iv) oxidation reactions involving these inorganic radicals.

Table 1, Table 2 report the main reactions with chloride and sulfate ions, respectively. The distribution curves in Fig. 2a and b show that dichlorine anion (Cl₂⁻) and sulfate (SO₄⁻) radicals represent the predominant radicals in acidic solutions corresponding to the pH range used for the Fe(II)/H₂O₂ and Fe(III)/H₂O₂ processes. Cl₂⁻ and SO₄⁻ are strong oxidant species. The second-order rate constants for their reactions with most of the organic solutes are within the range of (10³–10⁸) M⁻¹ s⁻¹ for Cl₂⁻ and (10⁶–10⁹) M⁻¹ s⁻¹ for SO₄⁻ (Neta et al., 1988). These values indicate that Cl₂⁻ and SO₄⁻ radicals are less or much less reactive than the OH radical ((10⁷–10¹⁰) M⁻¹ s⁻¹) (Buxton et al., 1988). Furthermore, Cl₂⁻ and SO₄⁻ oxidize Fe(II) and H₂O₂ with rate constants of the same order of magnitude than those with OH.

Usually, the effects of inorganic salts on the overall rates of decomposition of H₂O₂ and organic compounds are ignored. However, a few studies examined the effects of anions on the Fenton’s reaction. Pignatello (1992) showed that the rates of degradation of 2-4-dichlorophenoxyacetic acid by Fe(III)/H₂O₂ at pH 2.7–2.8 followed the order ClO₄⁻ ≈ NO₃⁻ > Cl⁻ ≈ SO₄²⁻, whereas the first-order rate constant for the Fe(III)-catalyzed decomposition of H₂O₂ followed the order ClO₄⁻ ≈ NO₃⁻ > Cl⁻ ≫ SO₄²⁻. Another study showed that the rates of oxidation of 4-chlorophenol by Fe(II)/H₂O₂ ([Fe(II)]₀=40 μM, [H₂O₂]₀=4 mM) at neutral pH (6 < pH < 8.2) were found to decrease in the following order: ClO₄⁻ ≈ NO₃⁻ > SO₄²⁻ > Cl⁻ ≫ HPO₄²⁻ > HCO₃⁻ (Lipczynska-Kochany et al., 1995). Tang and Huang (1996) observed an inhibitory effect of chloride ion on the oxidation of 2,4-dichlorophenol by the Fenton’s reaction in Na₂SO₄ solution. Lu et al. (1997) showed that the rate of oxidation of dichlorvos by Fe(II)/H₂O₂ ([Fe(II)]₀=0.25 mM, [H₂O₂]₀=5 mM, pH=3) decreased in the following sequence: ClO₄⁻ ≈ NO₃⁻ ≫ Cl⁻ ≫ HPO₄²⁻. Kiwi et al. (2000) showed a significant decrease in the rate of decoloration of Orange II by Fe(III)/H₂O₂ upon addition of Cl⁻. They observed the formation of chlorinated organic products and determined the rate constants for the reaction of OH and Cl₂⁻ with Orange II by time-resolved laser kinetic spectroscopy. From a kinetic study of the oxidation of Fe(II) by H₂O₂, conducted in a bicarbonate buffer (pH range 5–8), King and Farlow (2000) reported that the Fe(CO₃) complex was the most kinetically active iron(II) species for the decomposition of H₂O₂ in organic-free water.

Because of the lack of data in literature, this work was undertaken in order to compare under identical conditions, the effects of chloride, sulfate and nitrate on the overall rates of decomposition of H₂O₂ by Fe(II) and Fe(III) and on the rates of oxidation of three organic solutes (atrazine, 4-nitrophenol and acetic acid). Furthermore, several experiments were also conducted in the presence of perchlorate in order to confirm the rates obtained in the presence of nitrate because these two anions do not form complexes with Fe(II) and Fe(III) and are not reactive toward hydroxyl radicals.

Section snippets

Reagents and preparation of solutions

All reagents used in this work were analytical grade and were used without any further purification. Ferric perchlorate and ferrous perchlorate were used as the iron sources. Unstabilized hydrogen peroxide (30% w/w, Fluka) was purchased from Fluka. NaClO₄, NaNO₃, NaCl, Na₂SO₄ and HClO₄ were used to adjust the total concentration of an inorganic anion, the pH and the ionic strength (I). All solutions were prepared in Milli-Q water (Millipore).

Great care was taken to prepare ferrous and ferric

Effects of chloride and sulfate ions on the reaction rates in organic-free water

In the first part of this work, the rates of oxidation of Fe(II) by H₂O₂ (Fe(II)/H₂O₂ process) and the rates of decomposition of H₂O₂ by Fe(III) (Fe(III)/H₂O₂ process) in the presence of ClO₄⁻, NO₃⁻, Cl⁻ or SO₄²⁻ have been compared in the absence of organic solutes.

Conclusions

The present work has shown that the efficiency of the Fe(III)/H₂O₂ oxidation process can be markedly decreased in the presence of chloride and sulfate ions. These effects have been attributed to (i) a decrease of the rate of generation of hydroxyl radicals because the formation of chloro– and sulfato–Fe(III) complexes decreases the rates of generation of Fe(II) and (ii) the formation of inorganic radicals (Cl₂⁻ and SO₄⁻) which are less reactive than OH. This study also confirms that kinetic

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A comparative study of the effects of chloride, sulfate and nitrate ions on the rates of decomposition of H2O2 and organic compounds by Fe(II)/H2O2 and Fe(III)/H2O2

Abstract

Introduction

Section snippets

Reagents and preparation of solutions

Effects of chloride and sulfate ions on the reaction rates in organic-free water

Conclusions

Chemosphere

Water Res.

Chemosphere

Mar. Chem.

Chemosphere

Chemosphere

Chemosphere

Trends Biochem. Sci.

Comput. Environ. Urban Syst.

Talanta

Reactions of ferrous and ferric ions with hydrogen peroxide. Part I––The ferrous ion reaction

Trans. Faraday Soc.

Reactions of ferrous and ferric ions with hydrogen peroxide. Part II––The ferric ion reaction

Trans. Faraday Soc.

Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O−) in aqueous solution

J. Phys. Chem. Ref. Data

Radiolytic transients from p-nitrophenol and their inter- and intramolecular reactions

Adv. Chem. Ser.

Role of quinone intermediates as electron shuttles in Fenton and photoassisted Fenton oxidations of aromatic compounds

Environ. Sci. Technol.

A comparative study of the effects of chloride, sulfate and nitrate ions on the rates of decomposition of H₂O₂ and organic compounds by Fe(II)/H₂O₂ and Fe(III)/H₂O₂

Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O⁻) in aqueous solution