Degradation of linuron in aqueous solution by the photo-Fenton reaction

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Abstract

The photodegradation of linuron, which is one of phenylurea herbicides, was carried out in the presence of Fenton reagent. The degradation rate was strongly influenced by the pH and initial concentrations of H2O2 and Fe(II). An initial linuron concentration of 10 mg L−1 was completely degraded after 20 min under the optimum conditions. The decrease of TOC as a result of mineralization of linuron was observed during the photo-Fenton process. The degree of linuron mineralization was about 90% under UV irradiation after 25 h. The formations of chloride, nitrate and ammonium ions as end-products were observed during the photocatalytic system. The decomposition of linuron gave eight kinds of intermediate products. The degradation mechanism of linuron was proposed on the base of the evidence of the identified intermediates. Based on these results, the photo-Fenton reaction could be a useful technology for the treatment of wastewater containing linuron.

Introduction

Herbicides represent the largest group of chemicals used as plant protection agents. One class of herbicides widely used in pre- and post-emergencies is represented by the substituted phenylureas.

The phenylureas are persistent herbicides. At high rates of application they are useful as total weed killers, but at low rates many can be used for selective weed control in a wide range of crops. Linuron, 3-[3,4-(dichlorophenyl)-1-methoxy-1-methylurea], one of the most important commercial ureas, has good contact activity and it may kill emergent weed seedlings [1]. In addition, linuron is a weak competitive androgen receptor antagonist in vitro, induces a positive response in the immature and adult rat Hershberger assay and suppresses androgen-dependent gene expression [2], [3], [4].

Half-life in soil ranges from 38 to 67 days for linuron [5]. Therefore, this compound has been found as contaminants in surface and ground waters [6] and microbial degradation is considered to be the primary mechanism for their dissipation from soil. Many bacterial [7] and fungal isolates [8] that are able to (partially) break down phenylureas have been reported. In general, biological methods commonly require long time for the wastewater containing linuron at high concentration. Therefore, the rapid and simple wastewater treatment of linuron is now required urgently.

A few of treatment techniques for wastewater which contains linuron has been reported by using O3/H2O2 [9], direct photolysis [10] and Fenton [11] procedures. Recent reports indicate that a combination of H2O2 and UV irradiation with Fe(II), so-called the photo-Fenton process, can significantly enhance decomposition of many refractory organic compounds. Until now, the photo-Fenton process has been applied to the degradation of pesticides [12], [13], nitrobenzene and other organics [14], chlorophenols [15], [16], [17], nitrophenols [18], dibutyl-phthalate [19], PCBs [20] and bisphenol A [21]. The acceleration for decomposition of organic compounds is believed to be in order to photolysis of iron aquacomplex, Fe(H2O)5(OH)2+ (represented hereafter by Fe(OH)2+), to provide a new importance source of OHradical dot radicals [22], [23]. Further, the photolysis of Fe(OH)2+ regenerates Fe(II) (Eqs. (1) and (2)), which means that the photo-Fenton reaction would need low Fe(II) concentration compared with the Fenton process.Fe2+ + H2O2  Fe(OH)2+ + OHradical dotFe(OH)2+ +   Fe2+ + OHradical dot

Furthermore, the Fe(OH)2+ can absorb light at wavelengths up to ca. 410 nm, while TiO2 photocatalysis can use photon with wavelength close to 380 nm [24]. Therefore, the photo-Fenton process can be expected to be an efficient method for wastewater treatment and promotes the rate of degradation of various organic pollutants.

In the present study, we have investigated the degradation and mineralization of linuron in water using photo-Fenton process. The many factors, such as pH value and initial concentrations of Fe(II) and H2O2, affected on the degradation were evaluated. The progress of mineralization of linuron was monitored by total organic carbon (TOC) content and ionic chromatography. Furthermore, the photoproducts of linuron during this photocatalytic process have been identified by gas chromatography–mass spectrometry (GC–MS). The degradation pathway was proposed on the basis of intermediates formed.

Section snippets

Reagents

Linuron was purchased from Wako Pure Chemical Industries (Osaka, Japan) and was used as received (HPLC grade >99.0%). Analytical grade hydrogen peroxide solution (30%, w/w) and ferrous sulfate heptahydrate (FeSO4·7H2O) were purchased from Wako Pure Chemical Industries (Osaka, Japan) and Nacalai Tesque (Kyoto, Japan), respectively, and were used as received. All other chemicals and solvents were of the purest grade commercially available and were used without further purification. All aqueous

Effect of variables on the degradation of linuron

The time courses of the UV absorption spectra of linuron solution in the photocatalytic degradation by the photo-Fenton reaction were observed in the range of 0–60 min. The experimental conditions were: [Fe(II)]0 = 1.0 × 10−5 mol L−1, [H2O2]0 = 1.0 × 104 mol L−1, pH 3.0 and light intensity = 2.0 mW cm−2 Other conditions were described in Section 2. The UV spectrum of linuron solution before illumination presented one absorption maximum at 250 nm. The disappearance of 250 nm absorption band was observed with

Conclusions

The photodegradation of linuron in aqueous solution was investigated by the photo-Fenton treatment. The degradation rate was strongly affected by many factors, such as the pH value and the initial concentrations of H2O2 and Fe(II). Linuron achieved complete degradation after 20 min under the optimum conditions.

The disappearance of TOC was observed during the photo-Fenton process and the remaining TOC achieved 0.4 mg L−1 after 25 h (90% mineralization). The formations of chloride, nitrate and

Acknowledgment

This work is a part of projects of Satellite Venture Business Laboratory (SVBL), Mie University.

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