Application of photoactivated periodate to the decolorization of reactive dye: reaction parameters and mechanism

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Abstract

The decolorization of the reactive dye, reactive black 5 (RB5), by the photoactivated periodate (UV/IO4) was investigated under ultraviolet (UV) irradiation at 254 nm, and its possible mechanism based on periodate photochemistry was discussed, in order to elucidate the main oxidants involved in the decolorization of RB5 as the reaction parameters, such as the light intensity, periodate concentration, dissolved oxygen, and solution pH, were varied, with the intermittent employment of an radical dotOH scavenger. The decolorization rate of RB5 increased linearly with increasing light intensity. The decolorization rate was found to increase at low periodate concentrations (0–5 mM), but slightly decrease at higher concentrations (>5 mM). Dissolved oxygen did not have any measurable effect on the decolorization rate of the reactive dye. No pH dependence of the decolorization rate was observed over the pH range of 1.5–10, regardless of the periodate speciation. Based on these results, the main oxidants responsible for the RB5 decolorization with UV 254 were found to be radical dotOH and IO3radical dot (IVIradical dot), after considering the six intermediate species which are reported to be produced as a result of periodate photolysis, namely the radical species (radical dotOH, IO3radical dot, and IO4radical dot) and the non-radical species (O3, IO4, and IO3).

Introduction

The effective decolorization of colored effluents has become an important problem in the treatment of wastewater originating from the textile industry. Conventionally, physical (adsorption, filtration, and flotation), chemical (coagulation, oxidation, reduction, and electrolysis), and biological methods have been employed to treat wastewater containing organic dyes and pigments. However, it is difficult to find a convenient method which can effectively treat all types of dyestuff wastewater. In particular, conventional biological treatment alone is not sufficient to treat modern dyes, due to the large degree of aromaticity and stability of the molecules involved [1], [2]. Accordingly, advanced oxidation processes (AOPs) are under investigation as an alternative means of overcoming the limitations of the conventional techniques [3], [4], [5].

AOPs which generate radical dotOH as a non-selective oxidant, are emerging technologies in the effective degradation of the recalcitrant organic pollutants present in wastewater. The UV/O3, UV/H2O2, UV/TiO2, and hv/Fe(III)/H2O2 processes (photo Fenton process), to mention only a few of the AOPs currently being studied employ ultraviolet irradiation of an aqueous solution, in order to produce radical dotOH or enhance the radical dotOH production rate.

Recently, the photoactivated periodate (UV/IO4) system was proposed as a novel advanced oxidation technology, and which was shown to remove aqueous organic compounds in several studies [6], [7], [8], [9], [10], [11]. Although, the periodate ion (IO4) itself is a well-known oxidant, whose reduction potential was reported to be +1.60 V, oxidation reactions initiated by this ion are known to be selective and significantly slow compared with those involving radical dotOH [12]. Periodate is effectively applicable to the oxidation of the compounds in which the carbonyl group or hydroxyl group is adjacent to a second carbonyl or hydroxyl group, such as α-diols, α-ketols, α-diketones, and α-ketonic aldehydes. Despite the selective reactivities of periodate to substrates with specific functional groups, UV light can greatly enhance the reactivity and reduce the selectivity of the periodate oxidation reaction. This is because periodate produces radical dotOH and other reactive radical and non-radical oxidants, through the photodecomposition which occurs when it absorbs UV light at wavelengths of less than 300 nm. Although the photochemistry of the periodate ion in the aqueous phase has been extensively investigated, and has been relatively well elucidated in many previous studies [13], [14], [15], [16], [17], there are still few reports on the application of photoactivated periodate to water treatment. In particular, no research has been done to investigate the characteristics of the dyestuff decolorization process induced by the photoactivated periodate. Hence, in this study, we attempted to investigate the decolorization of wastewater containing RB5 in the photoactivated periodate system, as the reaction parameters affecting the decolorization of RB5, such as the light intensity, periodate concentration, dissolved oxygen, and solution pH, were varied, with the intermittent employment of an radical dotOH scavenger. The objectives of this study were two-fold: (1) to investigate the effects of the reaction parameters on the reactive black 5 (RB5, Fig. 1) decolorization process and (2) to elucidate the main oxidants responsible for the decolorization of RB5 in the photoactivated periodate system.

Section snippets

Photochemistry of periodate in aqueous solution

The photolysis of periodate and periodic acid in aqueous solution have been extensively studied, and the reactions involved in periodate photolysis have been discussed in a number of studies [13], [14], [15], [16], [17]. In spite of the complicated nature of periodate photolysis, Wagner and Strehlow [15] suggested the simple reaction scheme as shown in Table 1. According to this scheme, periodate can be photodecomposed by two photo-initiated pathways. As shown in Table 1, periodate can be

Reagents

All reagents used were of reagent grade and used without further purification. Sodium periodate (NaIO4), sodium iodate (NaIO3), RB5, perchloric acid (HClO4), sodium hydroxide (NaOH), and tert-butanol (t-BuOH) were purchased from Aldrich (USA). All stock solutions were prepared in distilled and deionized water (Barnstead NANO Pure, USA). RB5 has a molecular weight of 991.82 and its chemical structure is depicted in Fig. 1. However, the RB5 reagent used in this study has an ash content of 55% and

Periodate speciation

The various IVII species (H5IO6, H4IO6, H3IO62−, IO4, H2I2O104−, and H2IO63−) coexist in an aqueous solution of NaIO4. The equilibria among the species are presented in , , , , [6].H5IO6H4IO6+H+,pKa1=1.64H4IO6H3IO62−+H+,pKa2=8.36H3IO62−H2IO63−+H+,pKa3=12.2H4IO6IO4+2H2O,K14=402H3IO62−H2I2O104−+2H2O,K15=141M−1The speciation of the IO4 solution was calculated using the Mathematica 4.0 program (Wolfram Research, USA) on the basis of the above equations and constants. Fig. 2 shows the

Conclusion

This study demonstrated that the UV/IO4 system constituted an effective process for the treatment of the colored wastewater containing the reactive dye, RB5. The principle results obtained by varying the reaction parameters were as follows. Increasing the light intensity linearly increased the RB5 decolorization rate. The RB5 decolorization rate linearly increased with increasing periodate concentration, until IO4 itself became a significant radical dotOH scavenger. The RB5 decolorization rate was not

Acknowledgements

This research was partially supported by the Brain Korea 21 Program of the Ministry of Education.

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