Enhanced Photocatalytic Removal of Methylene Blue From Seawater Under Natural Sunlight Using Carbon-Modified n-TiO 2 Nanoparticles

In this study, photocatalytic removal of methylene blue (MB) from natural seawater was examined using carbon-modified titanium oxide (CM-n-TiO2) nanoparticles under illumination of real sunlight. CM-n-TiO2 nanoparticles exhibited significantly higher photocatalytic degradation efficiency compared to unmodified n-TiO2. Photocatalytic removal studies were carried out at different initial dye concentrations (5-30 μM), catalyst dose (0.5-1.5 gL), and pH (3-9). The highest removal rate of MB was obtained at the optimal conditions of pH 8 and 1.0 gL of CM-n-TiO2. The solar photocatalytic removal of MB from seawater using CM-n-TiO2 obeyed a pseudo-first order kinetics according to the Langmuir-Hinshelwood model.


Introduction
The wastewater from the textile industry is rated as one of the most polluting amongst all industrial sectors, considering the volumes discharged and effluent composition (Bizani, Futianos, Poulios, & Tsiridis, 2005).About 15% of the total amount of dye is lost during the dyeing production process and released in wastewater effluents (Slokar & Le Marechal, 1998).Several classes of dyes are considered as possible carcinogens or mutagens that threaten the entire ecosystem (Flora, Bagnaco, & Zanacchi, 1991).Besides this, the presence of these colored compounds, even in trace quantities, is highly undesirable, as it can block both sunlight penetration and oxygen dissolution, which are essential for aquatic life.Therefore, the contamination of seawater with these compounds is a major environmental concern.
Although seawater is an increasingly important water source, its behavior as a medium for photocatalytic removal of contaminants has not been studied extensively.Most studies dealing with the photodegradation of dyes using n-TiO 2 have been carried out in distilled water under illumination of UV light sources.On the basis of these considerations, visible light active carbon-modified (CM)-n-TiO 2 nanoparticles were prepared by sol-gel method using carbon-containing precursor as a source of both carbon and titanium without using any external source of carbon.The photocatalytic performance of CM-n-TiO 2 was examined for the photocatalytic removal of methylene blue, an intensely colored cationic dye, from natural seawater under sunlight illumination.The photocatalytic activity of CM-n-TiO 2 was compared with regular n-TiO 2 .In succession, the effects of photocatalyst loading, MB concentration, and pH on the photocatalytic removal rate of MB were investigated.

Synthesis and Characterization of n-TiO 2 and CM-n-TiO 2 Nanoparticles
Regular (unmodified) titanium dioxide (n-TiO 2 ) nanoparticles were synthesized by hydrolysis and oxidation of titanium trichloride (TiCl 3 ) in an aqueous medium.Visible light active carbon-modified titanium dioxide (CM-n-TiO 2 ) nanoparticals were synthesized by a sol-gel synthesis using titanium butoxide (Ti[O(CH 2 ) 3 CH 3 ] 4 ), carbon-containing precursor, as a molecular precursor of TiO 2 as well as a carbon source.The preparation and characterization of carbon-modified titanium oxide (CM-n-TiO 2 ) and unmodified n-TiO 2 have been reported in details elsewhere (Shaban et al., 2013).

Photocatalytic Removal Experiments
All solar photocatalytic experiments were carried out at the Faculty of Marine Sciences, Obhur, Jeddah, KSA, in the daytime between 11:00 am and 15:00 pm, during June-July, 2012.Natural seawater samples were collected from Sharm Obhur, Jeddah, KSA.Before spiking with different concentrations of MB, seawater samples were passed through Whatman GFC to remove any solid particles.Experimental set up for photocatalytic degradation consisted of a magnetically stirred 500 mL glass reactor loaded with the seawater solution (400 mL) containing different concentrations of MB ranging from 5 to 30 µM.The synthesized photocatalyst (n-TiO 2 or CM-n-TiO 2 ) was added with continuous stirring for uniform mixing.The photocatalytic reactor was directly exposed to natural sunlight.The average solar intensity was 1200 Wm -2 , measured by Field Scout Light Sensor Reader (Spectrum Technologies, Inc.) equipped with 3670i Silicon Pyranometer Sensor.

Sample Analysis
Aliquots of treated seawater samples were regularly withdrawn from the reactor and centrifuged immediately to remove the catalyst.The samples were analyzed using a Shimadzu UV-VIS Spectrophotometer (Model PharmaSpec UV-1700).The photodegradation efficiency (η) was then calculated from the decrease of absorbance of the dye solution at its maximum absorption wavelength (668 nm) as follows: where C o represents the initial concentration of the dye solution and C t represents the concentration of the dye at solar light irradiation time (t).

Effect of Catalyst Dose
To ensure maximum absorption of photons and to avoid an excess amount of catalyst, the optimum catalyst loading must be determined.The influence of CM-n-TiO 2 dose on photodegradation of MB (20 µM) under natural sunlight is shown in Figure 1a.The photocatalytic removal rate of MB increased with increase in catalyst dose from 0.5 to 1.0 gL -1 .The increase in catalyst amount increases the number of active sites on the photocatalyst surface thus causing an increase in the number of • OH radicals which can take part in decoloration of the dye solution.Further increase in the catalyst loading to 1.5 gL -1 slightly decreases the degradation www.ccsen efficiency.
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