Chemical Engineering Journal 158 (2010) 418–425 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Dual-effects of adsorption and photodegradation of methylene blue by tungsten-loaded titanium dioxide Saepurahman, M.A. Abdullah ∗ , F.K. Chong Department of Chemical Engineering, Universiti Teknologi Petronas, Bandar Seri Iskandar, 31750 Tronoh, Perak D.R., Malaysia article info Article history: Received 3 August 2009 Received in revised form 31 December 2009 Accepted 7 January 2010 Keywords: Photocatalysis Adsorption Titanium dioxide Tungsten loading Environmental remediation abstract Surface modification of titanium dioxide (TiO 2 ) with tungsten was carried out to increase the photo- catalytic degradation of methylene blue (MB). The modified photocatalyst in dark experiment had high affinity towards MB with the amount adsorbed proportional to the tungsten loading and the highest adsorption was at 6.5 mol% tungsten loading. The adsorption isotherm study showed that the adsorption followed Langmuir model with maximum adsorption capacity of 95.9 × 10 -3 mmol g -1 , 8-fold higher than the reported value for unmodified TiO 2 . Under illumination, the modified photocatalyst also enhanced the degradation of MB as compared to the unmodified one. Kinetic studies on the photocatalytic degra- dation of MB using linear and non-linear regression methods suggested that the degradation followed first order kinetics. The photocatalytic activity was greatly affected by the amount of tungsten loading, calcination temperature and calcination duration. The optimum synthesis condition was found at 1 mol% tungsten loading and calcination at 450 ◦ C for 2 h. Using UV filter, the DRUV-Vis analysis confirmed that the enhancement of the photocatalytic activity was not due to the extension of the photoresponse into the visible region. The presence of UV portion rather enhanced the photocatalytic activity. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Photocatalysis is a catalytic reaction under the action of light in the presence of a photocatalyst [1]. TiO 2 photocatalysis is attractive due to the possibility of utilizing solar source as a renewable energy. In addition, TiO 2 is inexpensive where titanium is the world’s sev- enth most abundant metal and ninth most abundant element [2,3]. TiO 2 is non-toxic and has found applications as white pigment and UV absorber in food and paint industries and in cosmetics preparations [3]. TiO 2 is chemically and photo-chemically stable, resistant to corrosion and photo-corrosion, and can be activated by UV light shorter than 388 nm [3–5]. The major drawback in TiO 2 photocatalysis though is its lack of efficiency due to the high rate of recombination of electrons and holes upon photoactivation [5]. Due to its wide bandgap energy, E g , of 3.0–3.2 eV, it can only be activated by UV light, which coincidentally accounts only 3–4% of sunlight spectrum [6]. Although many semiconductors such as CoO, Fe 2 O 3 , MnO, V 2 O 5 and WO 3 have wide bandgap energy (E g 2.0–3.6 eV) and are inefficient as photocatalysts, E g may not be the only factor governing the photocatalytic activity. Other important factors for effective photocatalysis are particle sizes, surface prop- erties, and phase compositions. Efforts have been made to enhance ∗ Corresponding author. Tel.: +60 5 3687636; fax: +60 5 3656176. E-mail address: azmuddin@petronas.com.my (M.A. Abdullah). the efficiency of the process and to modify electronic properties of TiO 2 so that it can utilize higher portion of solar spectrum. TiO 2 has been modified with different group of metals such as alkaline metals [7], earth alkaline metals [8,9], transition metals [10–13], rare earth metals [14,15], and noble metals [16]. Among transition metal dopant, cobalt and tungsten are reportedly the best for pho- todegradation of 4-nitrophenol, benzoic and methanoic acid, while samarium is best for 2-propanol degradation [13,17–19]. Dyes, representing some 9000 colouring agents of which 70% are azo dyes [20], have been of environmental concern due to indis- criminate discharge by textile industries. Discharged dyes do not only affect the water quality, but also affect health as some of these are mutagenic, carcinogenic and teratogenic. Dyes such as ben- zidine, 2-naphtyl amine and its derivatives are known as human bladder carcinogens and are subjected to water regulation and pol- icy [21]. Physical adsorption of discharged dyes by activated carbon is an economically effective process, while adsorption by other adsorbents such as bagasse, woodchips, and peat is gaining atten- tion due to its abundance as agricultural by-products that require no activation. However, all these processes may merely transfer the waste materials from the effluent to the adsorbent. There is a need for post-treatment such as incineration of the wastes adsorbed. Bio- logical methods (aerobic and anaerobic processes) and chemical methods (Fenton’s reagent, ozonation, sodium hypochlorite oxi- dation, and electrochemical oxidation) involve the transformation of the pollutant into a product that does not absorb wavelength in the visible region of the spectrum. Photocatalysis has several 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.01.010