TiO2-mediated photocatalytic degradation of a triphenylmethane dye (gentian violet), in aqueous suspensions
Introduction
Wastewater from the textile industry is highly coloured and of a complex and variable nature [1]. The large amount of dyestuffs used in the dyeing stage of textile manufacturing processes represent an increasing environmental danger due to their refractory nature [2], [3]. A substantial amount of dyestuff is lost during the dyeing process in the textile industry [4], which poses a major problem for the industry as well as a threat to the environment [4], [5], [6], [7], [8], [9]. Decolourization of dye effluents has therefore acquired increasing attention. During the past two decades, photocatalytic process involving TiO2 semiconductor particles under UV light illumination have been shown to be potentially advantageous and useful in the treatment of waste water pollutants. Earlier studies [10], [11], [12], [13] have shown that a wide range of organic substrates can be completely photomineralized in the presence of TiO2 and oxygen.
There are several studies related to the use of semiconductors in the photomineralization of photostable dyes [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. The photocatalysed degradation of various organic systems employing irradiated TiO2 is well-documented in the literature [10]. The initial step in the TiO2 mediated photocatalysed degradation is proposed to involve the generation of (e−/h+) pair leading to the formation of hydroxyl radical and superoxide radical anion [, , ];
It has been suggested that the hydroxyl radicals and superoxide radical anions are the primary oxidizing species in the photocatalytic oxidation processes. These oxidative reactions would result in the bleaching of the dye and the efficiency of the degradation will depend upon the oxygen concentration, which determines the efficiency with which the conduction band electrons are scavenged and the (e−/h+) recombination is prevented. Alternatively, the electron in the conduction band can be picked up by the adsorbed dye molecules, leading to the formation of dye radical anion and subsequent reaction of the radical anion can lead to degradation of the dye.
An example of triphenylmethane dye (gentian violet) is extensively used in textile dyeing, paper printing, as a biological stain and as a dermatological agent [31], [32], Gentian violet is a mutagen, a mitotic poison and clastogen and has been used for many years in veterinary medicine and as an additive to poultry feed to inhibit propagation of mold, intestinal parasites and fungus [33], [34], [35], [36]. Littlefield et al. [37] found that gentian violet is carcinogenic in mice at several different organ sites. The carcinogenic effects of gentian violet in rodents has also been reported [38]. McDonald and Cerniglia [39] reported the reduction of gentian violet to leucogentian violet by human, rat and chicken intestinal microflora under anaerobic conditions. They showed that the major portion of the metabolites was often bound to the cells (up to 87% of the metabolite produced by human microflora). Because of its low cost, its effectiveness as an antifungal agent for commercial poultry feed, and its ready availability, the general public may be exposed to the dye and its metabolites through the consumption of treated poultry products. Therefore, there are both environmental and human health concerns regarding the bio-accumulation of gentian and leucogentian violet.
Biological decolourization of triphenylmethane dyes including gentian violet is widely reported in the literature [40], [41], [42], [43], [44], [45], [46]. There are no reports dealing with the photocatalytic oxidation of the related system by titanium dioxide. With this view we have undertaken a detailed study on the photodegradation of the triphenylmethane dye (gentian violet, 1) sensitized by TiO2 in aqueous solution.
Section snippets
Reagent and chemicals
Gentian violet was obtained from B.D.H., Poqle, England and used as such without any further purification. The water employed in all the studies was double distilled. While the photocatalyst titanium dioxide, P25 (Degussa AG) was used in most of the experiment, other catalyst powders, namely Hombikat UV100 (Sachtleben chemie GmbH) and PC500 (Milenium inorganic chemicals), were used for comparative studies. P25 consists of 75% anatase and 25% rutile with a specific BET-surface area of 50 m2 g−1
Photolysis of TiO2 suspensions containing gentian violet (1)
Fig. 1 shows the degradation and depletion in TOC for irradiation of an aqueous solution of gentian violet (1, 0.18 mM) in the presence of the photocatalyst (P25, 1 g l−1 ) by the “Pyrex” filtered output of a 125 W medium pressure mercury lamp. It was observed that 99 and 85% decomposition and mineralization of the dye takes place, respectively, after 90 min of illumination. Both the mineralization and degradation curves can be fitted reasonably well by an exponential decay curve suggesting
Discussion
The results on the photodegradation of the model compound using different kinds of TiO2 photocatalyst with different bulk and surface properties, i.e. BET-surface, impurities, lattice mismatches or density of hydroxyl groups on the catalyst's surface, indicate that the latter is apparently not responsible for the photocatalytic activity or alternatively just compensate each other. It has been shown earlier that Degussa P25 owes its high photoreactivity due to slow recombination of (e−/h+) pair
Conclusion
TiO2 can efficiently photocatalyse the triphenylmethane dye derivative 1 in the presence of light and oxygen. The observations of these investigations clearly demonstrate the importance of choosing the optimum degradation parameters to obtain high degradation rate, which is essential for any practical application of photocatalytic oxidation processes. The best degradation condition depends strongly on the kind of pollutant. The model compound was found to degrade more rapidly in the presence of
Acknowledgements
Total organic carbon (TOC) analyzer used for the analyses of the samples was a gift equipment from the Alexander von Humbold Foundation, Germany. Financial support by the Department of Science and Technology (DST), Government Of India, New Delhi and The Third World Academy of Sciences Triesty, Italy is gratefully acknowledged.
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