Effects of acidity and inorganic ions on the photocatalytic degradation of different azo dyes

Abstract

Effects of acidity and inorganic ions that are common in industrial effluent on the photocatalytic degradation of azo dyes, Procion Red MX-5B (MX-5B) and Cationic Blue X-GRL (CBX), have been investigated in UV illuminated TiO₂ dispersions. There are significant differences between adsorption and photodegradation of MX-5B and those of CBX to the change of solution pH. The results indicated that CBX photodegradation was favored at the surface of TiO₂, while that of MX-5B occurred in the aqueous phase. At pH 2.4, SO₄²⁻, H₂PO₄⁻, ClO₄⁻ and F⁻ in general increased the decolorization rates of MX-5B and CBX by enhancing the adsorption of the selected dyes on the surface of TiO₂. At pH 10.8, most of the selected anions inhibited the photocatalytic oxidation (PCO) to decolorize and degrade CBX and MX-5B. These results demonstrated that inorganic anions affect the photodegradation of dyes by their adsorption onto the surface of TiO₂ and trapping positive hole (h⁺) and OH. Inorganic cationic ions, such as Cu²⁺ and Ni²⁺ had strong inhibition on the PCO decolorization of MX-5B at pH 10.8. On the contrary, no significant effect was observed at the same concentration of metal cations at pH 2.4.

Introduction

Heterogeneous photocatalytic oxidation (PCO) for wastewater treatment has received much attention by various research groups. Numerous types of catalysts and contaminants have been investigated on laboratory scale [1], [2]. However, very few commercial applications of this technology are available at present. One of the major reasons is the deactivation of the photocatalyst when photocatalysis is utilized to treat real wastewater [3]. Therefore, information on the long-term activity of photocatalysts is needed for the application of photocatalysis. Heterogeneous photocatalysis is a process in which the illumination of an oxide semi-conductor, usually TiO₂ in the form of anatase or rutile, produces photo-excited electrons (e⁻) and positive charged holes (h⁺). The photo-excitation of the semi-conductor particles by UV light changes the energy state of the electrons from the valence band of the solid to the conduction band. The formation of active species was previously discussed in detail [4]. The effects of salty water on the PCO products of liquid 3-octanol and 3-octanone were studied [5]. A number of studies have examined the effect of various inorganic ions on TiO₂ slurry photocatalysts [6], [7] and on fixed-bed photocatalysts where the TiO₂ was attached to a support, e.g. the inside wall of a borosilicate glass spiral [5], [8], and the surface of porous silica gel or fiberglass mesh [9]. There seems to be a general consensus that the effect of organic ions on photocatalysis depends on geometric configuration of the reactor and the structures of the organic compounds to be degraded.

Organic dyes are one of the major groups of pollutants in wastewaters produced from textile and other industrial processes [10]. These wastewaters are highly colored. The more complex environmental problem associated with the textile industry is that organic dyes are resistant to microbial degradation and they can be converted to toxic or carcinogenic compounds [11]. Heterogeneous photocatalysis has been considered as a cost-effective alternative as pre- or post-treatment of the biological treatment process for the purification of dye-containing wastewater [11], [12]. The occurrence of dissolved inorganic ions is rather common in dye-containing industrial wastewaters. They can significantly influence the photocatalytic reactions for the treatment of dyes. A study on the effects of inorganic ions on PCO degradation of dye will therefore be very useful for understanding the mechanism of the photocatalytic decolorization of dyes in TiO₂ dispersions under UV irradiation.

In this paper, we examined the effects of several types of inorganic ions (SO₄²⁻, H₂PO₄⁻, ClO₄⁻, F⁻, Cu²⁺ and Ni²⁺) that are common in synthetic dye-containing effluent on the photodegradation of two selected azo dyes in TiO₂ aqueous dispersions under UV irradiation. These two azo dyes have different adsorption modes on the surface of TiO₂. The photodegradation rates showed different patterns with the changes of pH. In addition, the effects of different inorganic ions on and the mechanisms of photodegradation of these two azo dyes were discussed.