Photocatalyzed destruction of organic dyes using microwave/UV/O₃/H₂O₂/TiO₂ oxidation system

Abstract

Photocatalytic decomposition characteristics of three different single-component organic dyes and their mixture using microwave/UV/O₃/H₂O₂/TiO₂ photocatalysts hybrid process system were investigated in this study. The decomposition rates of all organic dyes were higher in single-component experiments than in mixed solution experiments. The decomposition rates of all organic dyes increased with the ozone injection rate. The decomposition rate increased with increasing hydrogen peroxide concentration until it reached a maximum point beyond which it decreased with increasing hydrogen peroxide concentration. The result demonstrates that combination of elemental techniques that have little effect when used alone can make a synergy effect.

Introduction

The treatment of wastewater containing dyes is difficult. Generally, adsorption using activated carbon and biological treatment using microorganisms are used to remove organic pollutants such as dyes contained in waste water. However, these methods do not easily remove the complex aromatic compounds with various substitutions contained in dye wastewater and causes generation of large amount of sludge and solid waste leading to high treatment cost. Oxidation has been widely used to convert toxic non-biodegradable materials into biodegradable forms. Conventional oxidation processes using ozone or hydrogen peroxide (H₂O₂), however, has limits in treating a number of different kinds of pollutants, calling for a more efficient oxidation process. Traditional methods (for example adsorption on activated carbons [1]) only transfer contaminations from one phase to another. The most promising way for removing dyes is photo-catalysis, because this process decomposes the end dyes to water and carbon dioxide [2]. Application of TiO₂ photocatalyst in water treatment has recently been investigated widely [3], [4]. There are still many problems yet to be solved, however, in application of TiO₂ photocatalyst in the treatment of non-biodegradable materials. First, photocatalysis has usually been used in air pollutants treatment because it is suitable for treatment of low-concentration pollutants. Concentrations of water pollutants, however, are much higher than those of air pollutants. Thus, their treatment by photocatalysis is difficult compared to that of air pollutants. Second, polluted water often contains mixture of hydrophilic and hydrophobic materials. Therefore, it is not easy for the pollutants to be adsorbed on the photocatalyst surface. Third, polluted water has high turbidity, hence low transparency, hindering activation of photocatalysts by ultraviolet (UV) rays. Fourth, some materials are not easily degraded by photocatalysis only. Fifth, the amount of oxygen available for photocatalytic oxidation is very low in water compared to in air. Due to these reasons, photocatalytic oxidation of water pollutants has not received large attention thus far. Recently, researches have been conducted actively to improve oxidative degradation performance by adding microwave irradiation as an effort to utilize TiO₂ photocatalyst in water treatment more efficiently [5], [6], [7].

In many photo-decomposition reaction systems, TiO₂ powders are often used as a photocatalyst [8]. However, powder photocatalysts have several problems, such as (1) difficulties in the separation of the catalyst from suspension after the reaction, (2) difficulties in the prevention of aggregation in high concentration suspensions. To avoid such agglomeration, suspension must be diluted. Then the overall reaction rate tends to be slow. On the one hand, these problems can be solved by the use of immobilized (i.e., coated) catalyst particles. However, the coated catalysts are easily detached from the supports. To avoid these problems, TiO₂ thin films have been prepared by the sol–gel method [9], the sputter method [10] and the chemical vapor deposition (CVD) method [11], [12]. Among these, CVD is considered as a promising method to prepare high-quality thin films over large surface area with a well-controlled composition and low defect density.

Microwave energy has been used more and more on synthetic organic chemistry because of its great ability to accelerate reactions and to improve yields and selectivity [13]. Some researchers have been conducted to investigate photocatalytic reactions assisted with microwave irradiation. Kataoka and co-workers found that the photocatalytic oxidation of ethylene proceeded faster (83.9%) in the presence of the microwave irradiation than in the absence [7]. Horihoshi et al. proved by electron spin resonance (ESR) that about 20% more OH radicals were generated by photocatalysis with microwave irradiation than photocatalysis alone [14]. Although microwave effectively accelerates photocatalytic degradation, traditional Hg lamp was not laid in microwave under microwave irradiation. The measures to avoid electrode spoilage would complicate equipment. The problem will be solved if microwave electrodeless lamp substitutes traditional lamp as light source. Moreover, microwave electrodeless lamp has some unique advantages, such as, good photochemical efficiency, long life, low cost, and simple photocatalytic equipment [15]. Horihoshi et al. proved that photocatalysis with electrodeless lamp (a double quartz cylindrical plasma photoreactor) was about 10 times more efficient than the photocatalysis using traditional lamp [16].

In this study, a microwave/UV/O₃/H₂O₂/TiO₂ photocatalyst hybrid process system, in which various techniques that have been used for water treatment are combined, is evaluated to develop an advanced technology to treat non-biodegradable water pollutants efficiently. The objective of this study is to develop a novel advanced oxidation process that overcomes the limitations of existing single-process water treatment methods by adding microwave irradiation to maximize the formation of active intermediate products, e.g., OH radicals, with the aid of UV irradiation by MDEL, photocatalysts, and auxiliary oxidants. In particular, photocatalytic decomposition characteristics of three different single-component organic dyes and their mixture were investigated in this study.