Iron (III) aquacomplexes as effective photocatalysts for the degradation of pesticides in homogeneous aqueous solutions

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Abstract

The degradation of the herbicide asulam (4-amino-benzosulfonyl-methylcarbamate) was studied by excitation of iron (III) aquacomplexes in aqueous solutions at 365 nm as well as by exposition to solar light. Sulfanilamide was also studied as a model molecule. The initial step of asulam disappearance was shown to be due to the formation of hydroxyl radicals generated from the excitation of Fe(OH)2+, the most photoactive iron (III) monomeric species. However, when the iron (III) species was totally photoreduced to iron (II), the degradation of asulam in the presence of oxygen continued to completion. A photoreactivity of iron (II) species and/or iron (II) complexes under our experimental conditions was proposed. The experimental results indicate that the presence of iron (III), iron (II) and molecular oxygen is the condition for achieving the complete mineralization of the solution. The proposed photocatalytic cycle could provide an interesting tool for oxidations in the environment.

Introduction

In the last few decades, the pollutants coming from a variety of human activities, e.g. industrial and agricultural, have received considerable attention because of their toxicity and persistence. They can be found in surface waters and groundwater wells where they have to be removed in order to protect the environment and human health. These reasons led to the demand for the development of new methods of treating contaminated waters. From the viewpoint of energy conversion from solar energy, special emphasis has been given to photochemical methods of decontamination. The application of photochemistry to environmental remediation and treatment has been an active area of research (Helz et al., 1994). When the pollutant does not absorb solar light, photo-induced degradation processes appear to be of considerable interest. Among these, TiO2 (Bellobono et al., 1994, Mattews, 1987, Kiwi et al., 1993, Pruden and Ollis, 1983, Al-Ekabi et al., 1988), UV-H2O2 (Hager, 1992), UV-O3 (Abe and Tanaka, 1996, Abe and Tanaka, 1997) and photo-Fenton (Fe3+/Fe2++H2O2+hν) (Zepp et al., 1992, Sun and Pignatello, 1993, Ruppert et al., 1993, Bossman et al., 1998) appear to represent potential techniques for the degradation of water contaminants. The common point of these methods is the formation of highly oxidative species, namely hydroxyl radicals, which degrade many organic compounds in water with rate constants close to that for a diffusion-controlled process (Buxton et al., 1988).

In our previous studies, we noted the importance of systems such as iron (III) aquacomplexes for the photogeneration of hydroxyl radicals (Brand et al., 1998, Mazellier et al., 1997a, Mazellier et al., 1997b, Brand et al., 2000, Mailhot et al., 1999). The interesting point in such a system, compared to the photo-Fenton process is that no addition of hydrogen peroxide is needed. The excitation of [Fe(OH)(H2O)5]2+, the dominant monomeric species of aqueous ferric ion in acidic solution, is known to yield radical dotOH (Eq. (1)) with a quantum yield of 0.075 at 360 nm (Benkelberg and Warneck, 1995).FeOH2+Fe2++OHwhere Fe(OH)2+ refers to [Fe(OH) (H2O)5]2+.

This electron transfer process has been efficiently used to study the degradation of several organic pollutants in aqueous solution. In most cases, the complete degradation of the pollutant as well as the total mineralization of the solution was observed.

The present communication deals with the total mineralization of asulam (4-amino-benzosulfonyl-methylcarbamate) by excitation of an iron(III) species in aqueous solution and may contribute to the knowledge required to develop decontamination systems. Asulam is mostly used as a main and a selective herbicide for controlling bracken, an invasive weed that causes problems for agriculture (Marrs et al., 1992, Pakeman et al., 1988, Bruff et al., 1995, Pakeman et al., 1997, Marrs and Frost, 1996). In order to treat lands that are not easily accessible by vehicles, asulam is mainly applied by aerial application at levels that varies with seasons (Marrs et al., 1992, Pakeman et al., 1988). Since asulam represents a major pollutant in surface water, its fate in environmental conditions has to be well known. Studies of the degradation of asulam by ozonation as well as by ozonation under UV illumination in the presence of Fe3+ have already been reported (Abe and Tanaka, 1996, Abe and Tanaka, 1997). The complete mineralization was obtained via continuous formation of hydroxyl radicals.

In the present study, we focused our attention on the role of the iron species, iron (III) and iron (II) in the complete mineralization of asulam and also sulfanilamide chosen as its model molecule. A photocatalytic cycle involving iron (III), iron (II) and O2 will be proposed.

Section snippets

Materials

Except when stated, all reagents were of the purest grade commercially available and were used without further purification. The solutions were made up with deionized water (Milli-Q) in equilibrium with air, saturated with oxygen or deaerated by bubbling with nitrogen or argon for 1 h at 22 °C. For prolonged irradiations, the bubbling was maintained all along the experiments. The ionic strength of the solutions was not controlled.

Asulam and sulfanilamide were obtained from Dr

Results

The hydrolysis of iron (III) in dilute aqueous solution has been studied in detail (Faust and Hoigné, 1990). Briefly, in fresh dilute aqueous solution with 1.0×10−4 M<[iron(III)]<3.0×10−4 M corresponding to 3.5>pH>3.3, the predominant monomeric iron (III) species is Fe(OH)2+ (the water molecules were omitted for the sake of clarity). In the dark, the disappearance of Fe(OH)2+ species was observed and the formation of oligomeric species or soluble aggregates was proposed. The percentage of Fe(OH)

Discussion

The induced degradation of asulam by excitation of iron (III) aquacomplexes at 365 nm involved two different stages. The first and rapid part of the kinetics was owing to the excitation of Fe(OH)2+ which constitutes an efficient source of hydroxyl radicals via Eq. (1) (Faust and Hoigné, 1990). This fact is supported by the dependence of asulam disappearance rate on Fe(OH)2+ concentration and also by the inhibition of the degradation in the presence of 2-propanol used as hydroxyl radical

Conclusion

The present work illustrates the efficiency of photochemical degradation and also mineralization of asulam when the process is photo-induced by iron (III). The disappearance of asulam is mainly due to the formation of hydroxyl radicals formed from the excitation of Fe(OH)2+. However, when iron (III) is totally photoreduced, the generated iron (II) is involved, in the presence of oxygen, in a photocatalytic cycle which leads to the formation of iron (III), which can be excited again and

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