Effect of inorganic anions on Rhodamine B removal under visible light irradiation using Bi₂O₃/Ti rotating disk reactor

Abstract

In this study, Bi₂O₃ with band gap energy of 2.6 eV was coated on a Ti disk and used in a rotating disk reactor to treat 10 mg L⁻¹ Rhodamine B (RB) solution under UV and visible light irradiation. The effects of different inorganic anions (including ${\text{CO}}_3^{2-}$, S²⁻, ${\text{SO}}_4^{2-}$, F⁻ and Cl⁻) were investigated and the results shown that the color removal efficiency could be greatly improved in the presence of Cl⁻. RB adsorption capacity could be enhanced when Cl⁻ was added and RB could further be degraded due to indirect dye self-photosensitization under visible light irradiation. The radicals generated during the reaction were detected by adding radical trapping agents and other experimental conditions like Cl⁻ concentration, rotating speed and pH of the process were investigated and optimized.

Introduction

Photocatalytic (PC) oxidation technology has drawn great attention in wastewater treatment in recent years because the use of stable, non-toxic photocatalysts and the direct utilization of solar light [1], [2], [3]. However there are two main factors that limit the practical application of this technique. One is the development of effective PC reactor to improve the light utilization efficiency for high concentration wastewater treatment, the other is that the traditional TiO₂ photocatalyst can only be excited by ultraviolet radiation that only occupies less than 5% of the solar light due to its wide band gap (3.2 eV).

For the first factor, many new reactors have been developed and most of them are with immobilized photocatalyst because the traditional powder catalyst is hard to separate and recover [4], [5], [6]. Meanwhile the development of thin-film PC reactor is also a great improvement for the application of PC technology because the solution adsorption of irradiation light is also a very serious problem [7]. The rotating disk reactor, first developed by Dionysiou’s group [8], combines a highly effective thin-film PC and a conventional PC process on a single electrode which is coated with photocatalyst, and has been proved to be an efficient PC reactor to treat high concentration pollutants and owns high light utilization efficiency [9], [10]. The upper part of the round disk is coated with a thin-film (μm level) of wastewater and irradiates with light directly during the treatment. Compared with other traditional immobilized PC reactor in which light has to penetrate a thick solution layer before it can irradiate on the surface of the photocatalyst, loss of irradiation power due to absorption by the thin solution film can be negligible. The lower part of the disk electrode is immersed in the wastewater to perform conventional treatment. The electrode keeps rotating during the treatment, through which the thin aqueous film on the upper part of the electrode is continuously refreshed, and the mass transfer is also enhanced in sample solution.

For the second factor, in order to extend its visible-light photocatalytic activity, TiO₂ has been doped with various non-metal and metal atoms (like S, I, Fe, La, and V) [11], [12], [13] or coupled with other catalysts with narrow band gap (like CdS/TiO₂ and Cu₂O/TiO₂) [14], [15]. Meanwhile, other non-TiO₂ photocatalysts with narrow band gap have also been widely investigated to enhance the utilization efficiency of solar light [16], [17].

Bismuth oxide (Bi₂O₃) is an important metal-oxide semiconductor and due to its unique electrical and optical properties it has been widely investigated for various applications such as electronic ceramics, optoelectronic equipments, high temperature superconductors, catalysts, and optical coatings [18], [19], [20]. Meanwhile, the band gap of Bi₂O₃ is 2.58–2.85 eV, and it has also been utilized as visible-light photocatalyst [21]. Many studies have been done to treat dye solutions and other organic compounds using Bi₂O₃ as photocatalyst under UV or visible-light irradiation [22], [23]. However, the immobilized Bi₂O₃ with large grain size has some disadvantages such as small specific surface areas, long migration distances for the photoexcited electron–hole pairs and an increase in the recombination of electron–hole pairs, which adversely affect the photocatalytic activities when Bi₂O₃ is used alone [24], [25]. Meanwhile most of the researches about Bi₂O₃ PC process have been done using distilled water as solvent, the effects of other ions in water were rarely investigated. In this study, Bi₂O₃ was coated on the surface of a Ti disk and used in rotating disk reactor to treat Rhodamine B (RB) dye solution under UV and visible light irradiation. For the dye industries normally use different electrolytic additives (as mordants), the dye is expected to accompany them when they are discharged [26]. The effects of different inorganic anions on RB removal efficiency were investigated and other experimental conditions were also optimized.