World Journal of Nano Science and Engineering, 2013, 3, 154-160 Published Online December 2013 (http://www.scirp.org/journal/wjnse) http://dx.doi.org/10.4236/wjnse.2013.34018 Open Access WJNSE Effective Photocatalytic Reduction of Cr(VI) by Carbon Modified (CM)-n-TiO 2 Nanoparticles under Solar Irradiation Yasser A. Shaban 1,2 1 Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, Jeddah, KSA 2 National Institute of Oceanography & Fisheries, Qayet Bay, Alexandria, Egypt Email: yasrsh@yahoo.com Received August 23, 2013; revised September 20, 2013; accepted September 27, 2013 Copyright © 2013 Yasser A. Shaban. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Photocatalytic reduction of toxic Cr(VI) was successfully achieved using carbon modified titanium oxide (CM-n-TiO 2 ) nanoparticles under natural sunlight illumination. Modification of titanium oxide by carbon significantly enhanced the photocatalytic reduction of Cr(VI) under natural sunlight irradiation. The effects of various experimental parameters such as catalyst dose, initial concentration of Cr(VI), and solution pH on the reduction rate of Cr(VI) were investigated. The highest reduction rate of Cr(VI) was obtained at the optimal conditions of pH 5 and 2.0 g·L −1 of CM-n-TiO 2 . Inter- estingly, in the presence of phenol, as a sacrificial electron donor, the rate of Cr(VI) reduction was nearly 1.7 times higher than in its absence. The solar photoreduction of Cr(VI) in aqueous solution using CM-n-TiO 2 obeyed a pseudo-first order kinetics according to the Langmuir-Hinshelwood model. Keywords: Photocatalytic Reduction; Chromium; Titanium Oxide; Carbon Modification 1. Introduction The discharge of toxic heavy metals into aquatic envi- ronment has been known to cause serious pollution prob- lems. Among these pollutants, chromium possesses the most severe environmental concern due to its high toxic- ity, potential carcinogenicity, and high mobility in water [1,2]. The major sources of chromium pollution are elec- trochemical, steel manufacturing industries and leather tanning [3,4]. In aquatic environments, chromium exists in hexavalent (Cr(VI)) and trivalent (Cr(III)) forms, of which hexavalent form is more toxic than the trivalent one, and is known to be human carcinogen [5]. Different techniques have been reported for the treatments of Cr (VI), such as ion exchange [6], membrane separation [7], physical and biological adsorption [8-10], and electroco- agulation [11,12]. However, most of these techniques have several limitations and drawbacks, and they require either high energy or massive use of reducing agents. Semiconductor photocatalysis has attracted consider- able interest as an effective and economical technique for detoxification of polluted waters [13-22]. It can effec- tively reduce highly toxic Cr(VI) into the less harmful Cr(III), which can then be precipitated as Cr(OH) 3 in neutral or alkaline solutions [23]. Titanium dioxide photocatalyst was considered as one of the most practical candidates due to its optical and electronic properties, low cost, high level of photocatalytic activity, chemical stability and non-toxicity. However, its wide band gap (3.0 - 3.2 eV) limits its photoresponse in the ultraviolet region which is only a small fraction (~5%). Therefore, much attention has been devoted to enhancing its cata- lytic efficiency or expanding its applicability under solar irradiation. Recently, carbon modification of n-TiO 2 has been proved to be an effective approach to enhance its catalytic efficiency [21,22,24-26]. Most of the studies on heterogeneous photocatalytic reduction of Cr(VI) using n-TiO 2 were performed under illumination of UV light. Therefore, in this study, visible light active carbon-modified (CM)-n-TiO 2 nanoparticles were prepared via sol-gel method. The photocatalytic performance of CM-n-TiO 2 was examined for the photo- reduction of Cr(VI) in aqueous solution under illumine- tion of natural sunlight. The photocatalytic activity of CM-n-TiO 2 was compared with regular n-TiO 2 . The ef- fects of various experimental parameters such as photo-