WJNSEWorld Journal of Nano Science and Engineering2161-4954Scientific Research Publishing10.4236/wjnse.2013.34018WJNSE-40512ArticlesChemistry&Materials Science Engineering Physics&Mathematics Effective Photocatalytic Reduction of Cr(VI) by Carbon Modified (CM)-n-TiO2 Nanoparticles under Solar Irradiation asserA. Shaban1*Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, Jeddah, KSA; National Institute of Oceanography & Fisheries, Qayet Bay, Alexandria, Egypt* E-mail:yasrsh@yahoo.com051220130304154160August 23, 2013September 20, 2013 September 27, 2013© Copyright 2014 by authors and Scientific Research Publishing Inc. 2014This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

Photocatalytic reduction of toxic Cr(VI) was successfully achieved using carbon modified titanium oxide (CM-n-TiO2) 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-TiO2. Interestingly, 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-TiO2 obeyed a pseudo-first order kinetics according to the Langmuir-Hinshelwood model.

Photocatalytic Reduction; Chromium; Titanium Oxide; Carbon Modification1. Introduction

The discharge of toxic heavy metals into aquatic environment has been known to cause serious pollution problems. Among these pollutants, chromium possesses the most severe environmental concern due to its high toxicity, potential carcinogenicity, and high mobility in water [1,2]. The major sources of chromium pollution are electrochemical, 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 electrocoagulation [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 considerable interest as an effective and economical technique for detoxification of polluted waters [13-22]. It can effectively 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 catalytic efficiency or expanding its applicability under solar irradiation. Recently, carbon modification of n-TiO2 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-TiO2 were performed under illumination of UV light. Therefore, in this study, visible light active carbon-modified (CM)-n-TiO2 nanoparticles were prepared via sol-gel method. The photocatalytic performance of CM-n-TiO2 was examined for the photoreduction of Cr(VI) in aqueous solution under illuminetion of natural sunlight. The photocatalytic activity of CM-n-TiO2 was compared with regular n-TiO2. The effects of various experimental parameters such as photocatalyst loading, Cr(VI) concentration, and pH on the photocatalytic removal rate of Cr(VI) were studied. Additionally, the effect of presence of phenol, as a sacrificial electron donor, on the photocatalytic reduction of Cr(VI) was also investigated.

2. Experimental2.1. Synthesis and Characterization of n-TiO<sub>2</sub> and CM-n-TiO<sub>2</sub> Nanoparticles

Visible light active carbon modified titanium dioxide (CM-n-TiO2) nanoparticals were synthesized by a sol-gel method using titanium butoxide (Ti[O(CH2)3CH3]4, Fluka, 97%), a carbon-containing precursor, as a molecular precursor of TiO2 as well as a carbon source. Regular (unmodified) titanium dioxide (n-TiO2) nanoparticles were synthesized by hydrolysis and oxidation of titanium trichloride (TiCl3 12% in hydrochloric acid (5% - 12%), Sigma-Aldrich) in an aqueous medium. Details on the procedures used for catalysts preparation and characterization can be found elsewhere [22].

2.2. Photocatalytic Experiments

All solar photocatalytic experiments were carried out at the Faculty of Marine sciences, Obhur, Jeddah, KSA, in the daytime between 11:00 am to 15:00 pm, during MayJune, 2013. Experimental set up consisted of a magnetically stirred 500 mL glass photoreactor loaded with the aqueous solution containing different concentrations of Cr(VI) ranging from 1 to 10 ppm, then the synthesized photocatalyst (n-TiO2 or CM-n-TiO2) was added. The photocatalytic reactor was then directly exposed to natural sunlight. The average solar intensity was about 1200 W m−2, measured by Field Scout Light Sensor Reader (Spectrum Technologies, Inc.) equipped with 3670i Silicon Pyranometer Sensor. Prior to analysis, aliquots of treated samples were regularly withdrawn from the reactor and centrifuged immediately to remove the catalyst. The Cr(VI) was determined colorimetrically at 540 nm using a Shimadzu UV-VIS Spectrophotometer (Model PharmaSpec UV-1700) according to the diphenylcarbazide colorimetric method [27].

3. Results and Discussion3.1. Photocatalytic Activity of n-TiO<sub>2</sub> and CM-n-TiO<sub>2</sub>

Figure 1 compares the photoreduction efficiency of 3.0 ppm of Cr(VI) under illumination of real sunlight in the presence of 2.0 g·L−1 of CM-n-TiO2 and n-TiO2, respectively. It is clearly observed that the photocatalytic activity of CM-n-TiO2 is higher than that of n-TiO2 under sunlight irradiation. Complete photoreduction of 3.0 ppm of Cr (VI) was achieved after only 10 min using 2.0 g/L

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