Environmental remediation by an integrated microwave/UV-illumination technique: IV. Non-thermal effects in the microwave-assisted degradation of 2,4-dichlorophenoxyacetic acid in UV-irradiated TiO2/H2O dispersions

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Abstract

After establishing proof of concept in the first three articles of this series, in which we used the rhodamine-B dye to test the methodology, we herein report a study on the photocatalyzed degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous TiO2 dispersions that were subjected to simultaneous irradiation by UV light and microwave (MW) radiation. The characteristics of the process were monitored for the effect of microwave radiation by examining the fate of each substituent group, viz. the carboxylic acid group, the chlorine substituents and the benzene ring of the 2,4-D structure. The degradation dynamics of the microwave-assisted process were examined by decrease of UV absorption, loss of total organic carbon (TOC), formation of Cl ions (dechlorination), and identification of intermediates by electrospray mass spectral techniques. The thermal effect of the microwave radiation to the system was considered by comparing results from MW-generated heat versus externally applied heat. The effect of dissolved oxygen or absence of oxygen on the degradative process was also investigated. The greater efficiency of the MW-assisted process is ascribed to a non-thermal effect of microwave radiation on the break-up of the aromatic ring of 2,4-D (oxidation) but evidently not on the dechlorination process (reduction) for which MW radiation seems to have a negligible influence, if any. Factors involved in this non-thermal component and a mechanism are proposed for the initial stages of the degradation.

Introduction

The agricultural chemical 2,4-dichlorophenoxyacetic acid (2,4-D) is a typical widely used and highly toxic synthetic phytohormone (toxin). Not surprisingly, the United States Environmental Protection Agency has classified 2,4-D as a suspected endocrine disruptor. Many studies have been reported on the photocatalyzed decomposition of chlorinated toxic compounds such as the agrochemicals containing the triazine skeleton (Atrazine) [1], [2], [3], [4], 2,4-D [5], [6], [7], [8], dibromochloropropane [9], [10], [11], hexachlorocyclohexane [11], p,p′-DDT [12], [13], p,p′-DDE [14], 2,4-dichlorophenol [15], [16], [17], [18], [19], kelthane [20], polychlorinated biphenyls (PCB) [21], [22], [23], [24], polychlorinated dibenzo-p-dioxins [22], [25], [26], pentachlorophenol (PCP) [27], [28], [29], [30], [31], and 2,4,5-trichlorophenoxyacetic acid [32]. Complete dechlorination (reduction reaction) of many chlorinated compounds by the photocatalytic degradative route tends to be rather slow toward the mineralization of organic carbon atoms (oxidation reaction) to carbon dioxide. Dechlorination of chlorinated systems is improved greatly when using platinized TiO2 aqueous dispersions [33], albeit the photocatalytic process occurring in these TiO2/Pt dispersions can be rather expensive. Accordingly, treatment of large quantities of chlorinated pollutants is not likely to be economically viable. Alternatives are thus desirable. We have recently reported that the cationic rhodamine-B dye, a typically difficult dye to degrade, can be decomposed efficiently by concomitant irradiation of dye/TiO2 suspensions with UV light and microwave radiation [34], [35], [36], [37], [38]. The latter studies established proof of concept of the methodology.

In this study, we examine the degradation of an aqueous solution of 2,4-D in the presence of TiO2 particles irradiated concomitantly by microwaves and UV light. The strategy of our study was fourfold: (i) assess the generality of the coupled MW/photocatalytic routes by examining the photodegradation of 2,4-D under concomitant illumination with microwave radiation and UV light; (ii) assess the different pathway characteristics of the degradation of 2,4-D illuminated by UV light alone and by integrated UV/MW radiation; (iii) assess the degradative features of photooxidation and photoreduction under air-equilibrated conditions, as well as in oxygen- and nitrogen-purged dispersions; (iv) examine the degradation of 2,4-D using a microwave-powered double quartz cylindrical plasma photoreactor (DQCPP), and finally assess what factors of the microwave radiation influence the photomineralization process.

Section snippets

Chemical reagents and degradation procedures

The photocatalyst was Degussa P-25 TiO2 (BET specific surface area, 53 m2 g−1; particle size, 20–30 nm by TEM; composition, 83% anatase, 17% rutile by X-ray diffraction). High purity grade 2,4-dichlorophenoxyacetic acid (2,4-D) was supplied by Wako Pure Chem. Co.

Microwave irradiation was carried out with a Shikoku Keisoku ZMW-003 system (Fig. 1) manufactured by Shibaura Mechatronics Co. Ltd. It consisted of a microwave generator (frequency, 2.45 GHz; maximal power, 1.5 kW), a three-stub tuner, a

Results and discussion

An aqueous solution of 2,4-dichlorophenoxyacetic acid displays absorption bands at 285, 230 and 204 nm. Calculations of absorption band positions with the ZINDO software for the chemical structure of 2,4-D indicated that the 230 and 285 nm bands are likely those of a substituted benzene ring containing two chlorines and one carboxylic acid group, whereas the 204 nm band is consistent with absorption by a naked benzene ring. Loss of UV absorption features at these three wavelengths and loss of

Acknowledgements

We are grateful to the Japanese Ministry of Education, Culture, Sports, Science and Technology for a Grant-in-aid for Scientific Research (no. 10640569 to HH), and to the Natural Sciences and Engineering Research Council of Canada (no. A5443 to NS) for support of our work. We are also grateful to N. Watanabe, A. Tokunaga and A. Saitou for expert technical assistance.

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