ReviewPhotocatalytic degradation of organic dyes in the presence of nanostructured titanium dioxide: Influence of the chemical structure of dyes
Graphical abstract
This review explains the photocatalytic properties of nanostructured TiO2 and the influence of structure of organic dyes containing different functionalities on their photocatalytic degradation rates.
Introduction
Large amounts of dyes are annually produced and applied in different industries including textile, cosmetic, paper, leather, pharmaceutical and nutrition industries. There are more than 100,000 commercially available dyes with an estimation annual production of over 70,000 tons, 15% of which is lost during the dyeing process [1]. The presence of even trace concentration of dyes in effluent is highly visible and undesirable. It causes some serious problems to aquatic life and human health disorders [2]. These concerns have led to new and/or strict regulations concerning colored wastewater discharge as well as developing more efficient treatment technologies.
Various methods have been suggested to handle the dye removal from water such as biodegradation, coagulation, adsorption, advanced oxidation processes (AOPs) and the membrane process [3], [4], [5], [6], [7], [8]. All these processes have some advantages or disadvantages over the other methods. A balanced approach is therefore needed to look into the worthiness on choosing an appropriate method which can be used to degrade the dye in solution.
Among the new methods of colorful wastewater treatment, AOPs based on the generation of very reactive species such as hydroxyl radicals have been proposed to oxidize quickly and none selectively a broad range of organic pollutants [9], [10], [11]. AOPs have been growing during the last decade since they are able to deal with the problem of dye destruction in aqueous systems. Among AOPs, heterogeneous photocatalysis using TiO2 nanomaterials as photocatalyst appears as the most emerging destructive technology [12], [13], [14], [15], [16], [17]. The key advantage of the photocatalytic process is its inherent destructive nature: it does not involve mass transfer; it can be carried out under ambient conditions (atmospheric oxygen is sufficient as oxidant) and may lead to complete mineralization of organic carbon into CO2. Moreover, nanostructured TiO2 photocatalyst is largely available, inexpensive, non-toxic and shows relatively high chemical stability. Finally, TiO2 photocatalytic process is receiving increasing attention because of its low cost when the sunlight is used as the source of irradiation [2], [10].
The earliest description of photodecomposition of organic compounds and studies of effects of reaction parameters were reported by Kraeutler and Bard [18]. Heterogeneous photocatalysis has attracted constant research since its infancy considering the high number of excellent reviews and books devoted by many researchers [19], [20].
Although photocatalytic degradation has broad generality for destruction of both organic and inorganic compounds, the focus of this review is on organic dyes. There are many studies dealing with the photocatalytic decolorization of specific textile dyes from different chemical categories, and most of them include a detailed examination of working conditions [21], [22], [23], [24], [25], [26], [27]. There are also some reviews concerning the mechanism and fundamentals of photocatalytic degradation of organic dyes [28], [29], [30]. On the contrary and to the best of our knowledge, there is not a review dealing with the influence of the structure of organic dyes on photocatalytic degradation efficiency in the presence of nanostructured titanium dioxide. The present review sheds light on the structure-degradability relation of the heterogeneous photocatalytic degradation of organic dyes and reports the effect of the dye chemical structure on the process efficiency.
Section snippets
Titanium dioxide nanomaterials: structural and photocatalytic properties
Titanium dioxide, C.I. No. 77891, also known as titanium(IV) oxide, CAS No. 13463-67-7 with molecular weight of 79.87 (g mol−1) is the naturally occurring oxide of titanium with the chemical formula TiO2. When used as a pigment, it is called “Titanium White” and “Pigment White 6”. Titanium dioxide is extracted from a variety of naturally occurring ores that contain ilmenite, rutile, anatase and leucoxene, which are mined from deposits throughout the world. Most of titanium dioxide pigment in
Photocatalytic removal of dyes by titanium dioxide nanomaterials
A large amount of scientific articles have reported TiO2-mediated photocatalytic degradation of organic dyes that Table 2, Table 3, Table 4, Table 5, Table 6 summarize the results obtained.
Approximately 50–70% of the dyes available on the market are azo compounds followed by the anthraquinone group [66]. Some azo dyes and their dye precursors have been reported as human carcinogens [67], [68], [69], [70]. Therefore azo dyes are pollutants of high environmental impact and were selected as the
Influence of type of the dye on photocatalytic process efficiency
The chemical structure of the organic dyes has a considerable effect on the reactivity of these dyes on a TiO2-mediated photodegradation system. This effect has been explored by different researchers. Neppolian et al. [74], [111] have compared the photocatalytic degradation of three dyes (i.e. Reactive Yellow 17 (RY17), Reactive Red 2 (RR2) and Reactive Blue 4 (RB4)) in suspended and immobilized UV/TiO2 systems. The energy source was sunlight and UV-C lamp (254 nm) and the photocatalyst type was
Influence of methyl group
To find out the effect of methyl substituent on photocatalytic process efficiency, the decolorization of Acid Orange 7 (AO7) and Acid Orange 8 (AO8) could be a typical example. These two azo dyes have nearly the same structure (see Table 2); but the presence of –CH3 group in AO8 molecular structure can slightly decrease the reactivity of this dye [74].
Khataee et al. [92] have studied the effect of molecular structure of three azo dyes (i.e. Acid Orange 10 (AO10), Acid Orange 12 (AO12) and Acid
Conclusion
TiO2-mediated photocatalytic processes have been widely used for degradation of organic dyes, due to the cost effectiveness and the catalyst inert nature and photostability. Photocatalytic process efficiency depends on different parameters, particularly on the type and surface status of TiO2, the chemical structure of the dyes and the nature of functional groups attached on the dye molecule. This review focuses on the influence of the chemical structure of organic dyes on photocatalytic
Acknowledgement
The authors thank the University of Tabriz, Iran for financial supports.
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