Solar light-induced photodegradation of chrysene in seawater in the presence of carbon-modified n-TiO2 nanoparticles

Yasser A. Shaban

doi:10.1016/j.arabjc.2018.01.007

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Original article

12 (

); 652-663

doi:

10.1016/j.arabjc.2018.01.007

Solar light-induced photodegradation of chrysene in seawater in the presence of carbon-modified n-TiO₂ nanoparticles

Yasser A. Shaban

Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, P.O. Box 80207, Jeddah 21589, Saudi Arabia

Received: 2017-10-27, Accepted: 2018-1-14, Published: 2018-07-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Photocatalytic degradation of chrysene in polluted seawater was successfully achieved under illumination of natural sunlight using carbon modified titanium oxide (C-TiO₂) nanoparticles. The morphological and structural characteristics of the as-synthesized nanoparticles were investigated by X-ray diffraction (XRD), UV–Vis spectra, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS). The characterization results confirmed the successful incorporation of carbon into C-TiO₂ nanoparticles. As a result of C-modification, a significant enhancement of the photocatalytic degradation efficiency was observed for C-TiO₂, compared with pure TiO₂. In order to optimize the operating parameters, the impacts of catalyst loading and pH on the photocatalytic degradation of chrysene were investigated. The best degradation rate was obtained at pH 3 and C-TiO₂ loading of 1.0 g L⁻¹. The photodegradation of chrysene in seawater by using C-TiO₂ was found to follow a pseudo first-order kinetics in terms of the Langmuir-Hinshelwood model.

Keywords

Photocatalysis

C-TiO2

Chrysene

Sunlight

Seawater

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1 Introduction

The release of hazardous polycyclic aromatic hydrocarbons (PAHs) into aquatic environment has received a considerable attention and is considering a serious environmental problem. Due to their toxicity, bioaccumulative and nonbiodegradable nature (IARC, 1987), PAHs have been classified by the US EPA as priority pollutants. Generally, PAHs are generated from natural and anthropogenic sources and are detected in different environmental compartments including soil, air, organisms, and water.

Classical methods for remediation of these pollutants such as high-temperature incineration, landfilling, solvent extraction, and UV radiation are ineffective as they can destroy these contaminants to some extent, leaving sometimes toxic by-products (Kornmüller et al., 1997). Heterogeneous photocatalysis involving semiconductor catalysts under light irradiation has shown potential advantages in water treatment technology (Shaban et al., 2013; Shaban et al., 2016; Xu et al., 2006 ) and various environmental applications (Yu et al., 2016; Zhao et al., 2016). Due to its excellent electronic and optical properties, high photostability, and nontoxicity, titanium oxide (TiO₂) has attracted considerable interest for its applications in elimination of toxic gases (Kavil, et al., 2017; Nguyen and Bai, 2014; Szatmáry et al., 2014; Yang et al., 2017 ), degradation of wide range of organic pollutants (Cojocaru et al., 2017; Govindan et al., 2013; Shaban et al., 2013; Xiong and Hu, 2017; Xu et al., 2006 ) and wastewater treatment (Gebru and Das, 2017; Hu et al., 2017; Oppenlander; 2003; Parsons, 2004 ). However, its wide band gap of 3.0–3.2 eV constrains its absorption of light to the UV region, which impairs its performance. In recent years, many efforts have been devoted to modify its surface and/or electronic properties to broaden its photoresponse for visible light under solar irradiation mainly, by metal ions doping (Al-Azri et al., 2015; Belver et al., 2017; Chen et al., 2015; Garza-Arévalo et al., 2016; Ren and Yang, 2017; Zhang et al., 2017 ), and non-metals doping (Cheng et al., 2016; Fang et al., 2016; Guo et al., 2017; Khan et al., 2002; Pany et al., 2013; Pany et al., 2014; Pany and Parida, 2014; Umebayashi et al., 2002 ). Recent studies have shown that C-modified TiO₂ photocatalyst exhibits an enhanced photocatalytic activity under visible light (Fang et al., 2016; Khan et al., 2002; Liu, et al., 2016; Shaban et al., 2013; Shaban et al., 2016; Xu et al., 2006 ). Irie et al., (2006) reported that the origin of visible-light response of carbon doped TiO₂ is due to the presence of the dopant carbons at the oxygen sites of TiO₂ and the formation of C 2p states above the valence band. Similarly, theoretical confirmation by Lee et al., (2005) and Lu et al., (2012), they reported that the extension of the absorption range of carbon doped TiO₂ to the visible-light region would be attributed to the localization of dopant carbons at the oxygen sites.

Many of the photocatalytic reactions, aimed at photodegradation of PAHs using TiO₂, are limited to the use of artificial light sources. However, the exploitation of real sunlight for the photocatalytic remediation of these pollutants using TiO₂ in seawater is still a rarely discussed topic. Therefore, this study focused on the preparation of carbon-modified (C-TiO₂) nanoparticles, which are capable of absorbing visible-light photons. The photocatalytic activity of the as-prepared nanoparticles was evaluated as their ability to remove chrysene, a model of PAHs, from real polluted seawater under natural sunlight illumination. The effects of physicochemical factors such as photocatalyst loading, solution pH, chrysene concentration on the photocatalytic removal rate were investigated. Finally, the photocatalytic degradation mechanism of chrysene by using C-TiO₂ under illumination of light was also studied.

2 Experimental

2.1

2.1 Catalyst preparation

C-TiO₂ nanoparticles were fabricated via a sonicated sol-gel method. 10 mL of titanium butoxide was added dropwise into an equal volume of C₃H₇OH under continuous ultra-sonication. Carbon modification was performed by gradual addition of 5 mL of 10 µM of glucose solution into the prepared mixture. The solution was subsequently sonicated for further 30 min. The pH was adjusted to be 3.5 by addition of HCl and NaOH. As-prepared TiO₂ gel was subjected to aging at room temperature for 24 h, and then dried overnight at 80 °C. The obtained white solids were ground and finally calcined at 500 °C for 2 h to form C-TiO₂ nanoparticles. Pure TiO₂ was prepared following the same steps for C-TiO₂ except the insertion of glucose, additionally; titanium butoxide was replaced by titanium trichloride, as a carbon-free titanium precursor. TiO₂ P-25 (Degussa, Japan) was used, without any further modification, as a benchmark catalyst for comparison. Detailed characterization for P-25 has been reported in our previous work (Shaban et al., 2016).

2.2

2.2 Catalyst characterization

The crystallinity of the catalysts was determined by using Shimadzu X-ray diffractometer (XRD-6000) equipped with Cu K_α radiation operating at 30 mA and 40 kV, data were collected over the 2θ range of 20–70° at a scan rate of 1.0° min⁻¹. The surface area of the photocatalysts were measured by the Brunauer–Emmett–Teller (S_BET). The measurements were carried out by N₂ adsorption at 77 K using a Quantachrome instrument. Prior to S_BET measurements, samples were degassed at 100 °C for 1 h. A UV/Vis Spectrophotometer (Shimadzu, UV-1700) was used to evaluate the optical properties of the photocatalysts. The chemical structure and functional groups present in the photocatalysts were studied by Fourier transform infrared (FT-IR) spectrometer (Perkin Elmer, UK), samples were scanned within the wave range of 400–4000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a SPECS surface analysis systems operating at a base pressure of 4 x 10⁻¹⁰ mbar. A dual anode non-monochromatic Mg Kα (1253.6 eV) X-ray source was used to irradiate the sample surface with 13.5 kV, 150 W of X-ray power. The surface morphology and the elemental composition of the photocatalysts were examined by using a (SEM, JSM-7600F, JEOL, USA) scanning electron microscopy equipped with an energy dispersive X-ray system (EDS, Oxford, X-Max 50 mm²).

2.3

2.3 Photocatalytic experiments

Samples of clean seawater, collected from Red Sea coast of Obhur, Jeddah, Saudi Arabia, were spiked with various concentrations (5, 10, 20, and 40 ppm) of chrysene. Stock solution were prepared by dissolving a proper amount of chrysene in methanol, before diluting in seawater. Whereas, polluted seawater samples were collected from Al-Arbaeen lagoon, Jeddah, Saudi Arabia. A magnetically stirred 500-mL Pyrex glass reactor loaded with both contaminated samples and the photocatalyst with different dosages (0.25, 0.5, 0.75, 1.0, and 1.5 g L⁻¹) was used to carry out all photodegradation experiments. Prior to illumination with light, the suspensions were kept in the dark for 30 min. Then, the photoreactor was directly placed under natural sunlight. The experiments were carried out on sunny days between 11:00 am to 3:00 pm. A 3670i Silicon Pyranometer Sensor attached to Field Scout Light Sensor Reader (Spectrum Technologies, Inc.) was used to measure the solar intensity. No significant changes in the solar intensity were observed during the experimental period. The average solar intensity was found to be 1200 W m⁻².

To evaluate the stability of C-TiO₂ photocatalyst and the reproducibility of the measurements, five repetitions of the photocatalytic degradation experiment were performed. The photocatalyst nanoparticles were separated from the treated samples by centrifugation for 5 min then washed for several times with deionized water and finally dried at 80 °C before starting a new experiment.

2.4

2.4 Analysis

Samples were collected at regular intervals of irradiation from the photoreactor. The samples were then centrifuged for 5 min to ensure the removal of the catalyst particles. Analysis of chrysene content was performed by using a Shimadzu Spectrofluorometer (Model: RF-5301 PC). The fluorescence of the samples were measured at 310 nm excitation and at 374 nm emission wavelengths. The removal efficiency (E%) was calculated as follows:

(1)

E % = \frac{C_{0} - C_{t}}{C_{0}} \times 100

where C₀ and C_t represent the concentrations of chrysene at zero time and at irradiation time (t), respectively. The intermediates from the photocatalytic degradation of chrysene were analyzed using GC/MS (Shimadzu, 2010) with DB-5MS column (30 m × 0.25 µm). The initial temperature was held at 100 °C for 1 min, and then ramped to 300 °C at rate of 6 °C/min then hold for 3 min. The electron energy of the mass spectrometer was 70 eV. NEST library was used for identification of degraded compounds. Total organic carbon content was measured by Total Organic Carbon (TOC) analyzer (TOC-VCPH: SHIMADZU- Combustion catalytic oxidation/NDIR method, PC-controlled, high-sensitivity model).

3 Results and discussion

3.1

3.1 Catalyst characterization

3.1.1

3.1.1 Crystallinity, particle size and surface area analysis

The crystallinity of the prepared photocatalysts was studied by XRD spectroscopy. The X-ray diffraction patterns of C-TiO₂ and TiO₂ nanoparticles are shown in Fig. 1. The major diffraction peaks are indexed as (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4) and (1 1 6), indicating a typical pattern of anatase phase for both catalysts, and no other crystalline forms were found. These results suggest that the carbon modification does not cause any change of crystalline phase of TiO₂. The diffraction peaks for C were not detected for C-TiO₂, probably due to low carbon content or the high dispersion of carbon over C-TiO₂ surface. The decrease in the intensity and sharpness of the diffraction peaks of C-TiO₂ reveals a smaller particle size compared with the pure TiO₂, which is confirmed by applying the Scherrer formula:

(2)

D = 0.9 λ / (β \cos θ)

where D is the average crystallite size (nm), radiation wavelength (λ) = 1.54056 Å, β is measured in radians and it is equal to the width of the peak at half of the maximum peak intensity, and θ is the diffracted angle at the maximum peak intensity. The calculated average crystal sizes of C-TiO₂ and TiO₂ were found to be 12.3 and 31.8 nm, respectively. The S_BET measurements for the surface area were found to be 105 and 52 m² g⁻¹ for C-TiO₂ and TiO₂, respectively. Notably, carbon modification did not make any growth of C-TiO₂ crystals. Similar results were observed by Zhang et al. (2008a,b) and Chun-Hung et al., (2012). The smaller particle size and the larger surface may result in an enhanced photocatalytic activity for C-TiO₂.

XRD patterns of C-TiO2 and TiO2 nanoparticles.

3.1.2

3.1.2 Surface morphology

The surface morphology of the prepared catalysts was characterized by using SEM, as shown in Fig. 2. It seems from SEM images that the catalyst may be meso porous in nature. The SEM image of C-TiO₂ (Fig. 2b) show that the sample consists of a mass of reasonably large amounts of comparable monodispersed small crystals. The smaller crystals of C-TiO₂ (Fig. 2b) compared to those of TiO₂ (Fig. 2a) is clearly noted, which is in accordance with the average crystal sizes estimated from the XRD patterns. The EDS spectra of both catalysts are displayed in Fig. 3. Well defined peaks for Ti, O and C elements are shown for C-TiO₂, however no carbon was detected for pure TiO₂. The elemental composition of the catalysts determined through the EDS analysis is shown in Table 1. The presence of carbon (7.08 atomic%) in C-TiO₂ nanoparticles evidenced the successful C-modification.

EDS analysis for TiO2 (a) and C-TiO2 (b).

Table 1 Optical properties of C-TiO₂ and TiO₂ nanoparticles.

Catalyst	Crystal phase	Crystalline size (nm)	Bandgap (eV)	Atomic %
Catalyst	Crystal phase	Crystalline size (nm)	Bandgap (eV)	Ti	O	C
C-TiO₂	Anatase	12.3	1.85	30.61	62.31	7.08
TiO₂	Anatase	49.8	2.99	37.33	62.67	00.00

3.1.3

3.1.3 Optical properties

The optical bandgap energy and the light absorption of C-TiO₂ were comparatively evaluated with the pure TiO₂. The UV–Vis spectra of the photocatalysts are shown in Fig. 4a. It is clearly noted that the pure TiO₂ only responds to UV light, whereas the presence of carbon in C-TiO₂ extended its light absorption to the visible light region with broad light absorption up to around 790 nm. The optical bandgap energy was estimated from the plots of transformed Kubelka–Munk function, (αhυ)^1/2, versus photon energy (Kubelka, 1948; Tauc et al., 1966):

(3)

α h ν = A {(h ν - E_{g})}^{1 / 2}

(4)

{(F (R) h ν)}^{n} = A (h ν - E_{g})

where:

(5)

F (R) = {(1 - R)}^{2} / 2 R = α / S

UV–Vis spectra of C-TiO2 and TiO2 nanoparticles (a); the corresponding plot of transformed Kubelka–Munk function (b).

(being α, h, υ, E_g, A, R and S, light absorption coefficient, Planck's constant, frequency of vibration, bandgap, optical constant, diffused reflectance, scattering coefficient, respectively). Significantly, low bandgap energy value of 1.85 eV for C-TiO₂ was observed compared to 2.99 eV for TiO₂ (Fig. 4b). Recent reports have indicated that C can effectively narrow the bandgap of TiO₂ to the values of 1.86 eV (Shaban et al., 2013), 2.35 eV (Fang et al., 2016), and 2.32 eV (Nie and Sohlberg, 2003). Based on the fact that the valence electron number of O is larger than that of C, therefore 2p orbital of C works as an acceptor state. Accordingly, by mixing of the 2p orbital of C with the 2p orbital of O, the bandgap of the C-TiO₂ is reduced (Nakano et al., 2005). Advanced theoretical studies also revealed that carbon doping is responsible for narrowing the bandgap energy of n-TiO₂ (Nie and Sohlberg, 2003; Di Valentin et al., 2005). To incorporate C atoms into TiO₂ lattice, three possible theatrical scenarios were proposed by Di Valentin et al., (2005). Firstly, substitution of a lattice oxygen with a carbon; secondly, the replacement of Ti atoms by C atoms. Lastly, stabilization of carbon at an interstitial position.

3.1.4

3.1.4 FTIR analysis

FTIR analysis was conducted in order to investigate the surface properties of the photocatalysts (Fig. 5). As can be seen for both catalysts, the low FT-IR frequencies at 600–900 cm⁻¹ are assigned to Ti—O—Ti bridge stretching modes and Ti—O bond (Dolat et al. 2012; Zhang et al. 2015). The broad band at of 3000–3500 cm⁻¹ matches to the surface hydroxyl stretching (Zhang et al., 2014a,b). The peak observed at 1680 cm⁻¹ corresponds to the vibrations of OH bonds of surface-adsorbed water molecules on the surfaces of catalyst (Abbasizadeh et al., 2013; Randorn et al., 2004). It is believed that the superficial hydroxyl groups of the photocatalyst contribute to enhancement of its photocatalytic activity (Yu et al., 2007), through the interaction with photogenerated holes, which in turn inhibit the recombination of electron-hole pairs (Ao et al., 2009; Du et al., 2008). Interestingly, a new carbon band at 2900 cm⁻¹ was observed for C-TiO₂ due to C—H stretching_, providing a clear evidence for the successful incorporation of carbon into the structural lattice of the modified catalyst.

3.1.5

3.1.5 XPS analysis

The chemical states of the component elements of the C-TiO₂ catalyst analyzed by X-ray photoelectron spectroscopy (XPS) are shown in Fig. 6. The XPS survey spectra for TiO₂ and C-TiO₂ are shown in Fig. 6a. The Ti 2p spectrum in Fig. 6b demonstrates the spin-orbit split lines of Ti 2p_3/2 and Ti 2p_1/2 at 458.5 and 464.3 eV, respectively, which can be attributed to Ti⁴⁺ oxidation state (Tao et al., 2012). Fig. 6c shows O 1s XPS spectrum, which consists of four peaks 529.8, 530.1, 532.5 and 533.8 eV, corresponding to lattice oxygen of TiO₂ (Ti—O, Ti—O—Ti) and OH⁻ which are resulting mainly from the chemisorbed H₂O or the free hydroxyl group (O—H) on the surface (El-Sheikh et al., 2014; Trevisan et al., 2014; Zhang et al., 2014a,b ). The C 1s XPS spectrum exhibits three peaks at 285.0, 286.8 and 288.8 eV (Fig. 6d), suggesting the existence of C as C—C, C—O/C ⚌ O, and O—C ⚌ O, respectively (Etacheri et al., 2013; Guo et al., 2011; Lei et al., 2015 ).

X-ray photoelectron spectroscopy (XPS) for C-TiO2.

3.2

3.2 Effect of catalyst loading

The effect of C-TiO₂ dose on the photocatalytic degradation of chrysene (5 ppm) in seawater under illumination of sunlight was studied to attain the optimum catalyst loading (Fig. 7). It is clearly seen that the increase in the amount of catalyst from 0.25 to 1.0 g L⁻¹ increases the photocatalytic degradation rate as a result of the elevation of the number of hydroxyl radicals. Further increase in C-TiO₂ loading resulted in a marked decrease in the degradation rate, revealing an optimal catalyst loading of 1.0 g L⁻¹. The reduction of the degradation rate beyond the optimum amount of catalyst loading can be attributed to the agglomeration of catalyst particles, in addition to the increase in the opacity of the suspension, and thus increasing the light shading, consequently less photocatalysts could be activated (Merabet et al., 2009; Wang et al., 2009).

Effect of catalyst dose on the photocatalytic degradation of chrysene in seawater under sunlight.

3.3

3.3 Effect of pH

In heterogeneous photocatalytic systems involving semiconductor oxides, the change of the pH of the medium influences the surface-charge properties and the rate of redox reaction of the semiconductor (Chin et al., 2004; Lu et al., 1993; Zhang et al., 1998 ). The impact of pH on the photodegradation of chrysene (5 ppm) in seawater under illumination of sunlight using C-TiO₂ was studied at four different pH values 3, 5, 8 and 9. As clearly shown in Fig. 8, the rate of the photodegradation process of chrysene, 4-ring PAH, is dependent on the pH values. The highest photocatalytic degradation rate was obtained at pH 3. It is assumed that, the degradation of high molecular weight PAH using TiO₂ is favorable in the presence of H⁺. Whereas in alkaline medium, OH⁻ enhances the degradation of low molecular weight PAHs (Zhang et al., 2008a,b). These results are in a good agreement with those reported by Zhang et al. (2008a,b) and Lehto et al. (2000). Their results showed that the rate constants of high molecular weight PAHs were higher at acidic medium.

Effect of pH on the photocatalytic degradation of chrysene in seawater using 1.0 g L−1 of C-TiO2 under illumination of sunlight.

3.4

3.4 Effect of initial chrysene concentration

The effect of varying the initial concentration of chrysene (2–40 ppm) on its photodegradation rate is shown in Fig. 9. It is clearly demonstrated that the photodegradation rate of chrysene, at the optimized conditions of pH 3 and catalyst loading of 1.0 g L⁻¹, is inversely related to its initial concentration. This observation can be ascribed to the fact that, as the initial chrysene concentration increases, more molecules are adsorbed on the catalyst surface, resulting in a decline of the amount of light photons reaching the active sites of the catalyst, accordingly, a decrease of photodegradation efficiency is observed (Ahmed et al., 2010; Chen et al., 2011; Parida et al., 2006 ).

Effect of initial concentration of chrysene on its photocatalytic degradation at the optimal conditions of pH 3 and 1.0 g L−1C-TiO2 under illumination of sunlight.

3.5

3.5 Chrysene removal by C-TiO₂ and TiO₂

The chrysene (5.0 ppm) removal ability was evaluated at the optimum conditions of pH 3 and 1.0 g L⁻¹ of the photocatalyst under dark and light conditions. To reach the adsorption equilibrium, the photocatalyst was kept in the dark for 1 h. As can be seen in Fig. 10a, both TiO₂ and C-TiO₂ exhibited a negligible degree of chrysene (5.0 ppm) removal through the adsorption process in the dark condition. Whereas, complete degradation of chrysene using 1.0 g L⁻¹ of C-TiO₂ was achieved after only 15 min of solar irradiation. Notably, the irradiation time required to remove the same concentration of chrysene was extended to 30 and 50 min, when P-25 and regular TiO₂ were used.

Removal of chrysene using C-TiO2, TiO2 and P-25 photocatalysts under dark and light conditions (a); Kinetic analysis for the photocatalytic degradation of chrysene at the optimal conditions of pH 3 and 1.0 g L−1 C-TiO2 under illumination of sunlight (b); Cyclic photocatalytic degradation of chrysene under illumination of sunlight at the optimal conditions (c).

3.6

3.6 Kinetic studies

The kinetics of photocatalytic degradation of chrysene was studied using the Langmuir–Hinshelwood (L–H) model. Basically, this model describes the degradation rate (r) of organic compounds in water and its concentration at time t (C) as follows (Bayarri et al., 2005; Kusvuran et al., 2005; Petukhov, 1997 ):

(6)

r = - \frac{dc}{dt} = \frac{k_{r} K_{ad}}{1 + K_{ad} C}

where k_r and K_ad are the rate and the adsorption equilibrium constants, respectively. When the denominator of Eq. (6) is neglected and C_o (the concentration at zero time) is very small, the reaction is essentially a pseudo-first order reaction:

(7)

\ln (\frac{C_{o}}{C}) = k_{r} K_{ad} t = k_{app} t

where k_app is the apparent first-order rate constant. Hence, by plotting ln (C₀/C) against time, the apparent rate constant can be calculated from the slope of the line. Experimental results indicated that the solar photocatalytic degradation of various concentrations chrysene ranging from 2 to 40 ppm in the presence of C-TiO₂ fitted the pseudo-first-order kinetics as confirmed by the obtained linearity of the plots (Fig. 10b).

3.7

3.7 Stability and reusability of C-TiO₂ nanoparticles

To evaluate the stability of C-TiO₂ nanoparticles and to test the reproducibility of the measurements, five repetitions were performed under the same experimental conditions. It is clearly noted that, after five attempts with the use of C-TiO₂, similar results were achieved for the degradation of chrysene (5.0 ppm) under the optimum conditions as shown in Fig. 10c, revealing the stability and the potentiality of C-TiO₂ nanoparticles for continuous reuse.

3.8

3.8 Applicability to real polluted seawater sample

It is interesting to investigate the applicability of the synthesized C-TiO₂ nanoparticles for the removal of chrysene in real polluted seawater samples. The photocatalytic removal of chrysene (0.6 ppm) from polluted seawater was investigated at the natural pH 8 (Fig. 11a) and at the optimal pH 3 (Fig. 11b) using the optimum photocatalyst dosage of 1.0 g L⁻¹ under illumination of solar light. Even with the possible competition with other various types of compounds and pollutants, which are expected to be existed in the polluted seawater samples, the capability of C-TiO₂ for complete removal of chrysene with high efficiency after only 20 min (at pH 3) and 30 min (at pH 8) under natural solar light is clearly demonstrated. Meanwhile, with the use of P-25 this efficiency notably declined by 19% and 12% at pH 8 and pH 3, respectively. With the use of pure TiO₂ the degradation efficiency declined by 23% at pH 8 and 18% at pH 3 was clearly observed compared to those obtained by C-TiO₂. It is worth pointing out that, the apparent rate constant for solar photocatalytic degradation of chrysene in polluted seawater at the optimum pH 3 using C-TiO₂ is 1.89 and 2.32 times as high as that of P-25 and pure TiO₂, respectively (inset of Fig. 11b), which reflects the ability of C-TiO₂ to harvest the maximum solar light photons and hence enhance the degradation rate of chrysene.

Photocatalytic removal of chrysene (0.6 ppm) from polluted seawater at pH 8 (a) and pH 3 (b) using the optimal conditions dosage (1.0 g L−1) of the photocatalyst (C-TiO2, TiO2 and P-25) under illumination of solar light; the insets represent the corresponding kinetic analysis; (c) Removal of TOC using the optimal conditions dosage (1.0 g L−1) of the photocatalyst (C-TiO2, TiO2 and P-25) under illumination of solar light.

Furthermore, to verify the effectiveness of the synthesized C-TiO₂ nanoparticles, the total organic carbon (TOC) was measured during the photocatalytic degradation of chrysene in polluted seawater at the optimum pH 3 using C-TiO_2, P-25 and pure TiO₂ (Fig. 11c). The photocatalytic capability of C-TiO₂ is clearly evidenced through the decline of TOC level from 30.3 mg L⁻¹ to 5.1 mg L⁻¹ after 45 min of sunlight illumination, whereas the remaining amount of TOC increased to 10 mg L⁻¹ and 13.3 mg L⁻¹ with the use of P-25 and pure TiO_2, respectively.

3.9

3.9 Photocatalytic degradation mechanism

Simply, under illumination of light, C-TiO₂ particles absorb light photons whose energy equal or greater than 1.85 eV of the bandgap energy and generate electron/hole pairs. The holes (h⁺) in valence band are subsequently oxidize H₂O molecules or OH⁻ ions to yield hydroxyl radicals (^•OH), having a strong oxidation ability, which can be involved in the oxidation process of chrysene. Fig. 12 shows the possible photocatalytic degradation pathway of chrysene by using C-TiO₂ based on the detection of 2H-1-benzopyran, 3, 4-dihydro-2-phenyl at m/z of 210, benzenemethanol, α-[2-(2-hydroxyphenyl) ethyl] at m/z of 228, 1, 6-heptadiene 2 methyl-6-phenyl at m/z of 186, and 3,5-octadione, 2,7-dimethyl at m/z of 170. The presence of these intermediates suggested that the initial step of the proposed photodegradation mechanism involves the oxidation of chrysene by ^•OH radicals. Subsequently, the intermediates were further oxidized and resulted in ring-opining and breakdown of their aromatic structures, leading to the formation of carbon dioxide.

Schematic illustration of the proposed photocatalytic degradation pathway of chrysene by using C-TiO2.

4 Conclusions

Titanium oxide nanoparticles modified with carbon (C-TiO₂) were prepared via sonicated sol-gel method. The incorporation of carbon into C-TiO₂ nanoparticles was verified by EDS, XPS and FTIR analysis. The results showed a reduction of the optical bandgap of C-TiO₂ from 2.99 eV for TiO₂ to 1.85 eV, which in turn remarkably enhanced its photocatalytic performance towards the solar degradation of chrysene in seawater. The best degradation rate was obtained at pH 3 and C-TiO₂ loading of 1.0 g L⁻¹. The photodegradation of chrysene using C-TiO₂ was found to follow a pseudo first-order kinetics represented by the Langmuir-Hinshelwood model.

Acknowledgment

This work was funded by the Deanship of Scientific Research (DSR) King Abdulaziz University, Jeddah under grant no. (150-386-D1435). The author, therefore, acknowledge with thanks DSR technical and financial support. The author would like to thank Dr. Amr El Maradny for his appreciable help in GC analysis. The author is thankful to Mr. Mosa Alzobidi and Mr. Yasar N.K. for their appreciable help in the experimental analysis.

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Show Sections

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[7] Chen H., Shao Y., Xu Z., Wan H., Wan Y., Zheng S., Zhu D., . Effective catalytic reduction of Cr(VI) over TiO₂ nanotube supported Pd catalysts. Appl. Catal. B. 2011;105:255-262.
[Google Scholar]

[8] Chen W.-T., Chan A., Sun-Waterhouse D., Moriga T., Idriss H., Waterhouse G.I.N., . Ni/TiO₂: A promising low-cost photocatalytic system for solar H₂ production from ethanol–water mixtures. J. Catal.. 2015;326:43-53.
[Google Scholar]

[9] Cheng X., Yu X., Xing Z., Yang L., . Synthesis and characterization of N-doped TiO₂ and its enhanced visible-light photocatalytic activity. Arabian J. Chem.. 2016;9:S1706-S1711.
[Google Scholar]

[10] Chin M.L., Mohamed A.R., Bhatia S., . Performance of photocatalytic reactors using immobilized TiO₂ film for the degradation of phenol and methylene blue dye present in water stream. Chemosphere. 2004;57:547-554.
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[11] Chun-Hung H., Yu-Ming L., I-Kai W., Chun-Mei L., . Photocatalytic activity and characterization of carbon-modified titania for visible light-active photodegradation of nitrogen oxides. Int. J. Photoenergy. 2012;2012:13. Article ID 548647
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[12] Cojocaru B., Andrei V., Tudorache M., Lin F., Cadigan C., Richards R., Parvulescu V.I., . Enhanced photo-degradation of bisphenol pollutants onto gold-modified photocatalysts. Catal. Today. 2017;284:153-159.
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[13] Dolat D., Quici N., Nejman E.K., Morawski A.W., Puma G.L., . One-step, hydrothermal synthesis of nitrogen, carbon co-doped titanium dioxide (N, C – Ti_O2) photocatalysts. Effect of alcohol degree and chain length as carbon dopant precursors on photocatalytic activity and catalyst deactivation. Appl. Catal. B: Environ.. 2012;115–116:81-89.
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[14] Du P., Bueno-Lopez A., Verbaas M., Almeida A.R., Makkee M., Moulijn J.A., Mul G., . The effect of surface OH-population on the photocatalytic activity of rare earth doped P25-TiO₂ in methylene blue degradation. J. Catal.. 2008;260:75-80.
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[16] Etacheri V., Seery M., Hinder S., Michlits G., Pillai S., . A highly efficient TiO₂-XC_x nano-heterojunction photocatalyst for visible-light induced antibacterial applications. ACS Appl. Mater. Interfaces. 2013;5:1663-1672.
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[17] Fang Y., Sun M., Wang Y., Sun S., He J., . Cu₂O decorated carbon-incorporated TiO₂ microspheres with enhanced visible light photocatalytic activity. Mater. Res. Bull.. 2016;74:265-270.
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[18] Garza-Arévalo J.I., García-Montes I., Reyes M.H., Guzmán-Mar J.L., Rodríguez-González V., Reyes L.H., . Fe doped TiO₂ photocatalyst for the removal of As(III) under visible radiation and its potential application on the treatment of As-contaminated groundwater. Mater. Res. Bull.. 2016;73:145-152.
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[19] Gebru K.A., Das C., . Removal of Pb (II) and Cu (II) ions from wastewater using composite electrospun cellulose acetate/titanium oxide (TiO₂) adsorbent. J. Water Process Eng.. 2017;16:1-13.
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[20] Govindan K., Murugesan S., Maruthamuthu P., . Photocatalytic degradation of pentachlorophenol in aqueous solution by visible light sensitive N-F-codoped TiO₂ photocatalyst. Mater. Res. Bull.. 2013;48:1913-1919.
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[21] Guo Q., Zhang Z., Ma X., Jing K., Shen M., Yu N., Tang J., Dionysiou D.D., . Preparation of N, F-codoped TiO₂ nanoparticles by three different methods and comparison of visible-light photocatalytic performances. Sep. Purif. Technol.. 2017;175:305-313.
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[22] Guo X., Mao D., Lu G., Wang S., Wu G., . The influence of La doping on the catalytic behavior of Cu/ZrO₂ for methanol synthesis from CO₂ hydrogenation. J. Mol. Catal. A: Chem.. 2011;345:60-68.
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[37] Merabet S., Bouzaza A., Wolbert D., . Photocatalytic degradation of indole in a circulating upflow reactor by UV/TiO₂ process—Influence of some operating parameters. J. Hazard. Mater.. 2009;166:1244-1249.
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[41] Oppenlander T., . Photochemical Purification of Water and Air. Weinheim, Germany: Wiley-VCH; 2003.

[42] Pany S., Parida K.M., Naik B., . Facile fabrication of mesoporosity driven N-TiO₂@CS nanocomposites with enhanced visible light photocatalytic activity. RSC Adv.. 2013;3:4976-4984.
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