Investigation into photocatalytic decolorisation of CI Reactive Black 5 using titanium dioxide nanopowder Fiona Chai Foong Low, a Ta Yeong Wu, a, * Chee Yang Teh, a Joon Ching Juan b and N Balasubramanian c a Chemical and Sustainable Process Engineering Research Group, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150, Selangor Darul Ehsan, Malaysia Email: wu.ta.yeong@eng.monash.edu.my; tayeong@hotmail.com b School of Science, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150, Selangor Darul Ehsan, Malaysia c Department of Chemical Engineering, A.C. Tech Campus, Anna University–Chennai, Chennai 600025, India Received: 28 September 2010; Accepted: 6 June 2011 The photocatalytic decolorisation of CI Reactive Black 5 using titanium dioxide nanopowder as a catalyst was studied and the results obtained are discussed in terms of its decolorisation efficiency. All experiments were performed using a double-walled quartz immersion well batch reactor in which the slurry form of the reactants was at its natural pH of 5.1. The performance of titanium dioxide nanopowder (size <25 nm; surface area 200–220 m 2 ⁄ g) was compared with that of reference titanium dioxide powder (size ca. 230 nm; surface area 11 m 2 ⁄ g); in both cases, the titanium dioxide samples were anatase. It was found that the photocatalytic decolorisation efficiencies obtained using titanium dioxide nanopowder were higher than those of the reference titanium dioxide powder, with the latter taking approximately 8 min longer to achieve almost complete decolorisation of 10 mg ⁄ l CI Reactive Black 5. The photocatalytic decolorisation rate of CI Reactive Black 5 using both titanium dioxide photocatalysts typically followed a first-order reaction and the decolorisation kinetics were successfully fitted to a simplified Langmuir–Hinshelwood kinetic model. In addition, the effects of light type and intensity, catalyst loading and initial CI Reactive Black 5 concentration were investigated using titanium dioxide nanopowder as the photocatalyst in the decolorisation of the dye. This study shows that the recommended parameters for treating 10 mg ⁄ l CI Reactive Black 5 based on the experimental set-up and operating conditions are an ultraviolet light power of 125 W (39.3 mW ⁄ cm 2 ) and a 0.3-g ⁄ l catalyst loading. Introduction Dyes are extensively used in many industries, especially in textile manufacturing. Currently, it is estimated that there are more than 10 000 commercial dyes used, with an annual production of over 7 · 10 5 tons [1–5]. Among the available types of dyes, azo dyes, which are characterised by an –N=N– unit in their molecular structure, constitute a significant portion of approximately 50–70% [3–8]. Of these dyes, it is estimated that 1–15% is discharged to the environment as industrial effluent during manufacturing and processing operations [3,7,9–11]. Fixation rates of dyes can vary from 60 to 90%, leaving ca. 20% of unfixed dyes in wastewater [4]. The release of these coloured pollutants to our ecosystems not only causes eutrophication and disturbances in aquatic life but is also a source of aesthetic pollution because the presence of even small amounts of dyes (below 1 mg ⁄ l) is clearly visible [1,3,7– 9,12,13]. Furthermore, most of these dyes are toxic and potentially carcinogenic in nature [1,14–17]. With more information available on the consequences of dye pollution, coupled with tighter regulations and increased enforcement of wastewater discharge in many countries, it is vital that dye wastewater be treated before being released as effluent into the environment [7,18]. Most azo dyes are chemically stable because they are synthetic in origin and exhibit a complex molecular structure. Thus, the dyes do not biodegrade easily and are generally resistant to conventional aerobic biodegradation treatments [7,18–21]. Under anaerobic conditions, however, these azo dyes may degrade to give aromatic amines, which are potentially more hazardous and carcinogenic [7,18,20]. Other traditional wastewater treatments, such as direct precipitation, adsorption and membrane filtration, have been shown to have disadvantages as they are typically non-destructive; they simply transfer the pollutant from one phase to another, generating secondary pollution [12,16,17,20,22,23]. This leads to the focus of many studies on advanced oxidation processes (AOPs), characterised by the production of free hydroxyl radicals (OH . ), which can successfully oxidise various organic compounds due to their strong oxidative potential (E 0 = +2.8 V) [18,21,24– 26]. Recent studies have demonstrated that photocatalysis: may lead to the total mineralisation of many organic compounds/degradation of dyes, does not require the use of expensive oxidants, can be performed under ambient conditions and can possibly employ sunlight as the source of irradiation [12,13,21,27,28]. Titanium dioxide (TiO 2 ) is commonly used in the doi: 10.1111/j.1478-4408.2011.00326.x 44 ª 2011 The Authors. Coloration Technology ª 2011 Society of Dyers and Colourists, Color. Technol., 128, 44–50 Coloration Technology Society of Dyers and Colourists