Sandeep Kumar Sharma Haripada Bhunia Pramod Kumar Bajpai Department of Chemical Engineering, Thapar University, Patiala, India Research Article Photocatalytic Decolorization Kinetics and Mineralization of Reactive Black 5 Aqueous Solution by UV/TiO 2 Nanoparticles The photocatalytic decolorization and mineralization of Reactive Black 5 (RB5) dye in presence of TiO 2 Degussa P25 has been studied using artificial light radiation in a shallow pond slurry reactor. The equilibrium adsorption of dye, influence of pH (3–11), catalyst load (0.5–3.0 g/L), and dye concentration (20–100 mg/L) on decolorization kinetics were studied. The effect of area to volume ratio of photoreactor on decoloriza- tion kinetics has been also studied. Mineralization studies were performed at optimized conditions of pH (3) and catalyst load (1.5 g/L). The maximum adsorption (26.5 mg/g) of dye was found to occur at pH 3. The apparent pseudo first order decolorization rate constant (k app ) value followed the order pH 3 > pH 11 > pH 9 > pH 7. As compared to available literature reduction in total organic carbon (TOC) was minimal by the time there was complete decolorization. Initial reduction in TOC was followed by subsequent increasing trend till complete decolorization. Final decreasing trend in TOC was observed only after complete decolorization. Twelve hours of treatment under exper- imental conditions reduced TOC content by 70% only. Discussion of results suggest that photocatalytic treatment of colored effluent under low UV intensity, and low A/V ratio may result in completely decolorized effluent but still having high COD. Keywords: Adsorption; Dye; Shallow pond slurry reactor; TiO 2 Received: October 15, 2011; revised: February 7, 2012; accepted: March 2, 2012 DOI: 10.1002/clen.201100557 1 Introduction Industrial development is pervasively connected with the disposal of a large number of various toxic pollutants that are harmful to the environment, hazardous to human health, and difficult to degrade by natural means. Effluent streams from textile plants, dyeing units, dye, and dye intermediates manufacturing units generate such toxic pollutants. Azo dyes are well known for the toxicity and recalcitrant nature. Azo dyes are characterized by the presence of one or more azo bonds (—N — —N—). More than 15% of the dye remains unfixed at the end of the dying process [1]. This results in huge amount of toxic effluents discharge to water bodies. In some countries, there is a ban over the use of this class of dyes but they continue to be used for the required properties; which are best met by this class of dyes. A number of physical and chemical techniques have been reported for the removal of dye compounds such as adsorption on activated carbon, biodegradation, ozonation, and advanced oxidation pro- cesses (AOPs) such as Fenton and photo-Fenton catalytic reactions [2, 3], H 2 O 2 /UV processes, and TiO 2 photocatalysis [4–8]. Most of the processes, except AOPs, are either ineffective in treating these toxic effluents and/or are merely resulting in transfer of toxicity from effluent to huge amount of solid waste. On the other hand, various AOPs offer the possibility of complete destruction of these toxic compounds without generating solid waste. Among the AOPs, heterogeneous photocatalytic oxidation using TiO 2 has been found to be efficient and potentially advantageous as it leads to complete and fast mineralization of a wide range of compounds to CO 2 , water and inorganic ions [9, 10]. Different authors have studied degradation of Reactive Black 5 (RB5), a well-known diazo dye [11–18]. Kansal et al. [15] have reported photo- catalytic degradation of RB5 by using a immersion well batch reactor with dye in circulation. Low et al. [16] reported that photocatalytic decolorization of RB5 was more energy efficient by UV light rather than by visible light. It has been observed that different photocatalytic studies have been done on different types of photo reactors, with different oper- ating conditions like source of UV, area to volume ratio of reactor, dye purity, etc. [11–17, 19]. However, mainly the parameters under investigations have been adsorption, pH, catalyst loading, dye con- centration. Primarily decolorization studies had been main objec- tive. Some researchers have explored mineralization also [12, 14, 17, 20, 21]. Various parameters determining dye degradation have been found to be interdependent. Also, various UV sources provide differ- ent units of energy per unit volume during photocatalytic treat- ment. Decolorization and mineralization studies during such conditions are not comparable with outdoor solar photocatalytic studies. The photocatalytic studies for such comparison should have UV intensity equal to or below maximum available UV intensity from the sun. Additionally, the surface area to volume ratio of reactor is Correspondence: S. K. Sharma, Department of Chemical Engineering, Thapar University, Patiala 147004, India E-mail: s123sharma@gmail.com Abbreviations: AOPs, advanced oxidation processes; RB5, Reactive Black 5; TOC, total organic carbon Clean – Soil, Air, Water 2012, 00 (0), 1–7 1 ß 2012 WILEY-VCH Verlag GmbH & Co. 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