The use of TiO2 immobilized into grape marc-based activated carbon for RB-5 Azo dye photocatalytic degradation

Hanane Belayachi; Benaouda Bestani; Nouredine Benderdouche; Mostefa Belhakem

doi:10.1016/j.arabjc.2015.06.040

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Original article

12 (

); 3018-3027

doi:

10.1016/j.arabjc.2015.06.040

The use of TiO₂ immobilized into grape marc-based activated carbon for RB-5 Azo dye photocatalytic degradation

Hanane Belayachi, Benaouda Bestani^⁎, Nouredine Benderdouche, Mostefa Belhakem

Laboratoire de Structure, Elaboration et Application des Matériaux Moléculaires (SEA2M), Faculté des Sciences et de la Technologie (FST), BP 188, Université Abdelhamid Benbadis, Mostaganem, Algeria

⁎Corresponding author. Tel.: +213 5 52 32 94 07. bestanib@yahoo.fr (Benaouda Bestani)

Received: 2014-11-29, Accepted: 2015-6-30, Published: 2015-12-01

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

Titanium dioxide (TiO₂) in the mineral form of Anatase has been used as a coating agent by impregnation on powdered grape marc-based activated carbon (GMAC) prepared previously by chemical activation. The prepared material (TiO₂/GMAC) acting as a photocatalyst composite was investigated in the removal of reactive black 5 dye (RB5) from aqueous solutions. GMAC and the TiO₂/GMAC were characterized by BET surface area, Scanning electron microscopy, DRX and IRTF. Enhancement of the rate of RB5 dye degradation by the composite photocatalyst GMAC/TiO₂ compared to the GMAC was observed.

Model equations, namely Langmuir, Freundlich and Redlich– Petersen isotherms were used to satisfactorily predict the adsorption of RB5 dye onto the prepared composite. Langmuir–Hinshelwood pseudo first order rate model was applied to the investigation of degradation kinetics. Synergistic effect of the combined material was observed on the surface and inside the pores. COD decrease and, sulfate and nitrate ions release confirm RB5 dye degradation. This novel approach of impregnation process gave the hybrid more homogeneity and stability to better adsorb large molecules such as dyes from industrial effluents.

Keywords

Activated carbon

Anatase

Impregnation

Photocatalysis

RB-5 dye

Regeneration

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1 Introduction

Powdered activated carbon (PAC) obtained from different lignocellulosic wastes is a porous material generally used for wastewater treatment containing persistent organic pollutants compared to other adsorbents (Leary and Westwood, 2011; Aksu, 2005; Dias et al., 2007 ). The fact that these toxic substances are trapped in the pores, does not make this process clean, since it is an ordinary pollution transfer from a liquid to a solid phase. The idea to degrade the molecules adsorbed in the pores of PAC, and consequently regenerating the PAC, is therefore an attractive way to remedy this problem.

In this context, the modification of activated carbons by photo-catalysts nano-structured such as titanium oxide (TiO₂) has been performed by various authors (Torimoto et al., 1997; Ao et al., 2008; Xu et al., 2010; Huang et al., 2011; Cordero et al., 2007 ). The photocatalyst TiO₂ with anatase structure coupled with carbon can be classified into the following three types on the basis of the state of carbon: (1) TiO₂ loading on the surface of PAC using titanium tetraisopropoxide or by the hydrolytic precipitation of tetraisopropyl orthotitanate (Arana et al., 2001); (2) TiO₂-coated PAC through hydrolytic precipitation of TiO₂ from tetrabutylorthotitanate followed by heat treatment (Li et al., 2006); and (3) carbon doping in TiO₂ structure (Tryba et al., 2003, 2004) such as photocatalysts preparation where TiO₂ particles were placed onto the surface of poly(vinyl butyral) through the hydrolysis of tetraisopropyl orthotitanate with carbon precursor and subsequent carbonization to different final temperatures between 700 and 900 °C also prepared photocatalyst TiO₂-mounted PAC through hydrolytic precipitation of TiO₂ from tetraisopropyl orthotitanate, followed by heat treatment at 650–900 °C for 1 h under nitrogen flow (Tryba et al., 2003). These methods are able to degrade strongly organic pollutants in aqueous media. However, they are limited for: (i) the relative high cost of the process; (ii) and the complexity of the modification (synthesis).

In this work, we have chosen to impregnate grape marc-based activated carbon particles with an Anatase TiO₂ semi-conductor by a simple process and appropriate for application in industrial scale. In our humble knowledge, this impregnated hybrid (GMAC + Anatase-TiO₂) has never been studied before for organics removal. To illustrate its high photo-catalytic proprieties and demonstrate the synergistic effects between GMAC and Anatase-TiO₂, some tests are realized on the removal of Reactive Black 5 (RB5) dye from aqueous solution.

2 Experimental

2.1

2.1 Materials

The nanopowdered TiO₂ from Sigma–Aldrich (99.7% on a trace metal basis; 25 nm particle size and surface area of 220 m² g⁻¹), used in this study as received, was mainly Anatase phase. Grape marc as raw material provided from Sidi M’Hamed Benali cavern (Relizane-Algeria) was used as precursor for activated carbon preparation; Absolute ethanol was produced in France (PROLABO). All other chemical reagents used were of analytical grade.

A 10³ mg/L stock solution of the adsorbate was prepared by dissolving the required amount of the Reactive Black (RB 5) dye from sigma–Aldrich in distilled water. Successive dilutions were used to obtain working solutions of the desired concentrations. Fig. 1 shows the chemical structure of the RB5 Azo dye and some important of its characteristics are listed in Table 1.

Table 1 Chemical structure and characteristics of RB-5.

Name	Reactive Black 5 (diazo)
Synonyms	Remazol Black B
Empirical formula	C₂₆H₂₁N₅Na₄O₁₉S₆
Class	Anionic
Molecular weight (g mol⁻¹)	991.82
Dye content (%)	55
λ_max (nm)	595

2.2

2.2 Activated carbon preparation

In a typical experiment, grape marc was washed several times with distilled water and dried at 100 °C overnight, then grinded using Crosschop Vierzen grinder, and sieved to obtain particle diameter lower than 140 μm. Firstly, the resulting material was then impregnated in diluted phosphoric acid solution (40%) and heated at 170 °C for 2 h, filtered off and then dried overnight at 110 °C. Secondly, the chemically activated material was subjected to heat in a muffle furnace at 600 °C for 3 h at heating rate of 10 C min⁻¹. Finally, the obtained material was washed with 0.1 N hydrochloric acid solution, followed by repeated washing with laboratory-produced hot distilled water until the filtrate was free from phosphate ions as shown by conductivity measurements and lead acetate tests. The prepared activated carbon was dried at 100 °C for 24 h, and then ground and sieved to obtain a particle size of <71 μm before adsorption tests. This material was noted as grape marc-based activated carbon (GMAC).

2.3

2.3 Composite photocatalyst preparation

The composite GMAC/TiO₂ was prepared by agitating 1.0 g of GMAC in 10 mL of H₃PO₄ (1 M) for 24 h in which 0.1 g of TiO₂ was added representing 10% ration. The mixture was mechanically stirred at room temperature for 8 h and then filtered by centrifugation. The solid fraction was then washed with distilled water until neutral pH. A closed-loop washing for 2 h using ethanol and drying at 105C were performed on the obtained hybrid. The white layer appearance on GMAC surface means that the composite catalyst preparation is completed.

2.4

2.4 Samples characterizations

The FTIR spectra of the microparticles were determined using the pressed-disk method with potassium bromide on an irprestige-21 Shimadzu FTIR spectrometer. The X-ray diffraction was done using an Empyrean diffractometer. The SEM photomicrograph of the microparticles was taken using a FEI quanta 200 scanning electron microscopy. The surface area and macropore surface area were respectively estimated by BET and t-plot method. The pore size distribution was determined by BJH method. The dye concentration was measured using an UV/visible spectrophotometer Shimadzu. Sulfate and nitrate ions were determined using colorimetric methods and the degradation was estimated by the analysis of the chemical oxygen demand (COD) according to the AFNOR NFT 90-040 and NFT 90-101 standard procedures respectively.

2.5

2.5 Adsorption isotherms

Adsorption isotherm tests were performed by mixing in flasks 50 mL of RB5 dye solutions with known initial concentration ranging from 100 to 1000 mg L⁻¹ separately onto a known amount of GMAC/TiO₂ (2 g/L), GMAC (2 g/L) and Anatase-TiO₂ (1 g/L) via batch mode under magnetic stirring at pH 2 and constant temperature (25 °C) until the predefined equilibrium time is reached. The pH was adjusted using dilute HCl solution. Once the adsorption process has occurred, the resulting solution in each flask was centrifuged at 4000 rpm and the supernatant was analyzed using a UV–Vis 1240 Shimadzu spectrophotometer at wavelength value of 595 nm. Experiments were repeated in triplicate and the average values were reported (±3%.error was obtained). The retained concentrations in the adsorbent phase (q_t) were calculated using the following equation:

(1)

q_{t} = \frac{(C_{0} - C_{t}) \times V}{W_{a}}

where C₀ is the initial dye concentration and C_t the dye concentration (mg L⁻¹) at equilibrium time, V the volume of solution (L) and W_a the adsorbent weight (g).

2.6

2.6 Photocatalysis

Samples photoactivity was tested for RB5 dye degradation from aqueous solutions under UV irradiation using UV lamp (60 W) at 365 nm. Experiments were conducted within a black box plated by mirrors to guarantee both complete irradiation with UV light and isolation in darkness. Measurements of RB5 degradation under UV irradiation without photocatalyst were carried out. Dye decolorization % was determined as follows:

(2)

Dye decolorization rate (%) = (1 - \frac{C_{dye, t}}{C_{dye, 0}}) 100

where C_dye,t and C_dye,₀ are the dye concentrations at reaction time t and t = 0, respectively

3 Results and discussion

3.1

3.1 Characterization

3.1.1

3.1.1 Porosity of the prepared samples

Adsorbents surface area (by BET method) and pore volume (by BJH analysis) are two important characteristics that can be obtained from physisorption isotherms. Fig. 2 shows the adsorption–desorption (2a) isotherms of N₂ at 77 K and pore size distribution (2b) characteristics of grape marc-based activated carbon and the hybrid obtained from BET–BJH analysis indicating that GMAC and GMAC-TiO₂ have a surface areas of 1168 and 914 m² g⁻¹ and total pore volumes of 0.680 and 0.609 cm³ g⁻¹ respectively. These decreases in both surface area and pore volume are mainly due to the Anatase-TiO₂ particles incorporation in the GMAC micropores (5%) and mesopores (22%) volumes indicating the proportionality of the GMAC content of the catalyst in agreement with the one from the surface area measurements as shown in Table 2.

(a) N2 adsorption–desorption isotherms, (b) Pore size distributions of the (▴) hybrid and (●) GMAC.

Table 2 Summary of the BET–BJH analysis.

	A_BET (m² g⁻¹)	A_externe (t-plot) (m² g⁻¹)	V_total (cm³ g⁻¹)	V_meso (cm³ g⁻¹)	V_micro (cm³ g⁻¹)	Mean pore diameter D_p (nm)
GMAC	1168	725	0.710	0.406	0.304	9.9
Hybrid	914	218	0.609	0.320	0.289	9.4
Anatase-TiO₂	117	103	0.309	0.304	0.005	10.1

The same observation was reported by Wang et al. (2009), when studying the immobilization of TiO₂ on active carbon. In other case, the TiO₂ particles can block the activated carbon particles pores, thus reducing its pore fraction (Liu et al., 2007).

In this study, the hybrid material exhibits larger surface area and pore volume to which its adsorption activity is linked, giving then a higher photocatalytic activity.

3.1.2

3.1.2 Fourier transform infrared spectroscopy

Functional groups were determined using FTIR spectra measurements for both GMAC and hybrid using KBr disk. The surface structure transformation of the prepared samples is due to the appearance of new functional groups induced by H₃PO₄ activation on the raw material, and by TiO₂ on the hybrid. Analysis of the measured IR spectra of the GMAC and the Hybrid gives the following information:

The band at 3410 cm⁻¹ associated with the γO—H stretching vibration and indicates the presence of hydroxyl (OH⁻) group and chemisorbed water.
The band at 1724 cm⁻¹ associated with the γC⚌O stretching vibration of the carbonyl group and may indicate the presence of carboxylic acid or ester.
The band at 1564 cm⁻¹ associated with the γC⚌C stretching vibration and may indicate the presence of aromatic ring.
The band at 1020 cm⁻¹ associated with the γC—O stretching vibration and may indicate the presence of phenols, epoxide structures, aromatic ethers, and lactone groups.
The band at 867 cm⁻¹ associated with the γ Ti—O—C vibration and indicates a weak conjugation between Ti–O bonds and PAC.
The band at 500–700 cm⁻¹ associated with the Ti—O stretching vibration, which is caused by the presence of TiO₂ (Bradu et al., 2010; Stylidi et al., 2004; Xu et al., 2011 ).

Also FTIR spectra obtained from hybrid photocatalyst samples collected after 15 and 60 min of irradiation times are shown in Fig. 3.

3.1.3

3.1.3 Scanning Electron Microscopy (SEM)

SEM micrographs of GMAC and hybrid samples are illustrated in Fig. 4. It can be clearly seen that the surface of GMAC is very clean due to the simultaneous activation (chemical pyrolysis at 600 °C). After impregnation process, the major part of fine Anatase particles is deposited inside the GMAC pores and the other covers its surface, making the photocatalytic process favorable, independently of the location of the Anatase-TiO₂ since the latter is light-derived and can receive the UV lights in order to photodegrade the pollutant (Xu et al., 2010).

3.1.4

3.1.4 Crystal structure

In order to investigate the GMAC phase structure changes X-ray diffraction was performed before and after impregnation with Anatase-TiO₂.

Fig. 5 shows the XRD patterns of neat GMAC support, pure Anatase-TiO₂ and hybrid powders. A peak at 26.62 was characteristic of graphite crystallite of GMAC. The characteristic diffraction peaks of Anatase-TiO₂ at 25.49 and 38.10 were clearly observed at values of (2θ) indicating the presence of Anatase-TiO₂ phase in the hybrid (Huang et al., 2011; Barka et al., 2008).

X-ray diffraction patterns of: neat GMAC, Anatase-TiO2 and GMAC deposited with TiO2 films.

3.2

3.2 Adsorption isotherms

Adsorption tests in dark conditions were carried out in order to evaluate the equilibrium constants of the dye adsorption on the three materials such as GMAC, hybrid and Anatase-TiO₂.

Isotherm models namely Langmuir, Freundlich and Redlich–Peterson were used to analyse the adsorption equilibrium data. The Langmuir model is one of the most widely used isotherm equations for modeling the adsorption equilibrium and is valid for monolayer sorption onto a surface with a finite number of binding sites which are homogeneously distributed over the adsorbent surface (Aksu, 2005) and having the same affinity for the adsorption of a single molecular layer in which there is no interaction between adsorbed molecules. The mathematical description of this model is

(3)

\frac{C_{eq}}{q_{e}} = \frac{1}{kb} + \frac{1}{b} C_{eq}

where q_e (mg L⁻¹) is the adsorption density, C_eq (mg L⁻¹) the concentration of adsorbate in solution at equilibrium, b (mg g⁻¹) the adsorption capacity corresponding to complete monolayer coverage, and k (L mg⁻¹) the Langmuir constant related to the energy of adsorption. The linear behavior of C_eq/q_e versus C_eq yields to the Langmuir constants b and k.

The Freundlich isotherm model is an empirical equation to describe heterogeneous systems suggesting that binding sites are not equivalent and/or independent (Bestani et al., 2008). This model is represented by the following form:

(4)

q_{e} = K_{F} \times C_{eq}^{1 / n}

where K_F and n are the Freundlich constants related to the adsorption capacity and adsorption intensity of the sorbent respectively.

The three parameter Redlich–Peterson equation has been proposed to improve the fit by the Langmuir or Freundlich model is given by Eq. (5). These parameters have a linear dependence on concentration in the numerator and an exponential function in the denominator:

(5)

q_{eq} = \frac{K_{RP} C_{eq} M}{1 + {(K_{RP} C_{eq})}^{β}}

where q_eq is the amount adsorbed at equilibrium, C_eq is the equilibrium solution concentration, K_RP is the equilibrium constant, β is the heterogeneity factor, and M is the maximum amount adsorbed.

Related adsorption parameters reported in Table 3, were calculated using nonlinear regression method. The R² coefficient is calculated as an indicator of fitting with the experimental data to the proposed models.

Table 3 Langmuir, Freundlich and Redlich–Peterson parameters for the removal of RB5 by the prepared samples.

	Langmuir-model			Freundlich-model			Redlich–Peterson–model
	b (mg g⁻¹)	K_L (L mg⁻¹)	R²	K_F (mg g⁻¹)	n	R²	K_RP (L g⁻¹)	β	R²
Hybrid	250	0.012	0.98	7.75	65	0.97	34.81	0.89	0.95
GMAC	333	0.04	0.98	7.24	72.77	0.96	79.81	0.79	0.97
Anatase-TiO₂	56	0.45	0.99	4.48	26.42	0.59	0.22	1.06	0.89

Fig. 6 represents the variation of the maximal adsorption capacity versus the concentration of RB5 dye. All three materials show L-type isotherm according to the Giles classification, corresponding then to a monolayer formation of dye on adsorbent surface. As expected, the Langmuir isotherm equation best fits the equilibrium sorption data of RB5 dye with R² values closer to unity compared to the two used models. The maximum adsorption capacities of the three adsorbents were evaluated using Langmuir model. The grape marc based activated carbon yielded the highest uptake of 322 mg g⁻¹ for Reactive Black 5 dye, whereas the hybrid photocatalyst attained 250 mg g⁻¹ while Anatase-TiO₂ gave value of 56 mg g⁻¹.

Adsorption isotherms for RB5 onto GMAC, hybrid and Anatase-TiO2 at pH = 2, solid dose = 2, 2 and 1 g L−1 respectively, and contact time = 100 min.

Lower value of 19.54 mg g⁻¹ was reported in the literature for the RB5 adsorption onto activated carbon felt, 19.54 mg g⁻¹ for RB5 (Donnaperna et al., 2008). Hence, activated carbon prepared from grape marc and used in this study exhibited a high uptake potential toward this pollutant, while the hybrid adsorption capacity is slightly smaller but still remains a good adsorbent with a high adsorption capacity compared to the nontreated grape marc. This can be explained by the viability of the hybrid adsorption surface area and average pore size, which allows adsorption of large molecules.

The Freundlich constants, K_F and n indicate the adsorption capacity and adsorption intensity, respectively. Higher the value of the exponent n, the higher will be the affinity and the heterogeneity of the adsorbent sites (Kurniawan et al., 2012; Srivastava et al., 2006). From Table 3, measured values of K_F showed easy uptake of reactive black 5 with high adsorptive capacity of each sorbent and significant difference in sorption capacities between the three sorbents due to the specific surface area and pore size. The obtained values of n indicated a high absorbability of the dye for all materials.

Redlich–Peterson constant K_RP indicates that the adsorption capacity of all three systems increased with increasing surface area. The exponent β, for both systems GMAC and hybrid was lower than unity except for Anatase-TiO₂ (1063). It should be noted that β normally lying between 0 and 1, indicated a favorable adsorption. Thus, the both isotherm models, Freundlich and Redlich–Peterson lead to the same conclusion (Marouf et al., 2006).

3.3

3.3 Photocatalytic activity

The photocatalytic studies were carried out at the same conditions and at the same parameters such as adsorption pH, ambient temperature and a maximum dose of 2 g L⁻¹ to avoid suspension opacity to UV lights in case of an excess dose >2 g L⁻¹.

3.3.1

3.3.1 Initial dye concentration effect

The percentage removal of RB5 from aqueous solution by hybrid photocatalyst versus UV light irradiation time plot in dark condition after saturation is shown in Fig. 7. The degradation under UV irradiation without photocatalyst was about (5.86%), a negligible value compared to the one obtained using the photocatalyst. As shown in Fig. 7, the increase in the initial dye concentration gradually from 20 to 100 μmol L⁻¹ leads to a decrease in decolorization from 98.93% to 81.77% for a treatment time of 60 min. Removal rate decreases with increasing dye concentration due to increasing amount of dye adsorbed on the surface of the photocatalyst which will inhibit the activity of the OH radicals and prevent irradiation of the media (Karkmaz et al., 2004; Garcia et al., 2007). The same results were found for the decolorization of the azo dye RB5 by Fenton and photo-Fenton oxidation and conclude that increase in dye concentration increases the number of dye molecules and not OH^• radical concentration and so the removal rate decreases (Lucas and Peres, 2006).

Effect of initial RB5 concentration in the presence of 2 g L−1 of hybrid.

3.3.2

3.3.2 Photodegradation kinetics of reactive black 5

The initial dye concentration can affect its degradation rate, since the kinetic rate constant decreases with the concentration (Da and Faria, 2003). The degradation experiments by UV irradiation of Reactive Black 5 in aqueous solutions follow the pseudo-first-order kinetics with respect to the dye concentration in the bulk solution (C) according to the following equation:

(6)

\frac{- dC}{dt} = k_{app} C

Integration of this equation (with the same restriction of C = C₀ at t = 0, with C₀ being the initial concentration in the bulk solution after dark adsorption and t the reaction time) will lead to the expected relation:

(7)

\ln (C / C_{0}) = k_{app} t

where k_app is the apparent pseudo-first-order rate constant. A plot of ln(C/C₀). versus t for all the experiments with different initial bulk concentration of RB5 is shown in Fig. 8. The values of k_app can be obtained directly from the regression analysis of the linear curve in the plot.

Kinetics of RB 5 photodegradation (linear transform ln (C0/C) versus (t) in photocatalytic experiments using hybrid, GMAC and Anatase-TiO2.

The dye photo-oxidation kinetics was analyzed using the Langmuir–Hinshelwood model (L–H) (Aguedach et al., 2005; Matos et al., 2010) given by Eq. (8). Generally, the discoloration rate is represented by the following kinetics equation:

(8)

r = k_{app} C = k_{c} (\frac{K_{c} C}{1 + K_{c} C})

where k_c represents the kinetics constant, K_C represents the adsorption constant and C the dye concentration in the solution. The relationship between k_app and C can be expressed as a linear equation:

(9)

\frac{1}{r} = \frac{1}{k_{app}} = \frac{1}{k_{c} K_{c}} + \frac{C}{k_{c}}

The values of the adsorption equilibrium constant K_c and the kinetic constant k_c were obtained by linear regression of the points calculated by Eq. (9). In Table 4 are shown the kinetic constants for the RB5 according to L–H model.

Table 4 Kinetic constants for the three systems.

Samples	k_app (min⁻¹)	R²	K_C (L mg⁻¹)	k_c (mg L⁻¹ min⁻¹)	R²
Hybrid	0.024	0.99	0.014	2.088	0.99
Anatase-TiO₂	0.018	0.99	0.02	1.04	0.99
GMAC	0.01	0.99	0.016	0.807	0.99

It was evident that the degradation rate on hybrid photocatalyst was faster than that of GMAC and Anatase-TiO₂ as shown in Fig. 8 and the obtained k_app data are shown in Table 4. It can be seen that the hybrid shows the highest k_app value in the investigated three samples.

This reveals that hybrid photocatalyst can enhance the photoactivity under UV light. As discussed above, hybrid photocatalyst can improve the photoactivity, which is in agreement with some previous works (Peralta-Hernandez et al., 2007; Xu et al., 2010; Wang et al., 2009 ).

According to the constants k_c and K_c, we might suppose that the GMAC acts as a physical absorbent absorbs some incident photons but better adsorbs RB5 molecules due to its high surface area. On the other hand, the less active sites of the Anatase oxidize the RB5 molecules and can be degraded more easily (representative potential photocatalyst) (Garcia et al., 2007) and consequently release active surface in favor of other molecules more quickly. Therefore, the hybrid photocatalyst exhibited a much faster photocatalytic rate and better performance making the activated carbon and Anatase-TiO₂ complementary.

3.3.3

3.3.3 Study of absorption spectra

The decolorization of RB5 dye from aqueous solutions (100 μmol/L) is studied by measuring their UV–visible spectra before and after UV lamp treatments. Fig. 9 illustrates the UV–Visible spectra of RB5 dye treated during 45 min. This time was chosen to see clearly the variation of spectra in which the RB5 UV–Vis spectrum is characterized by an important band in the visible region. Due to the presence of azo function, maximum absorbance was obtained at 600 nm. Another band attributed to the benzene cycles substituted by ${SO}_{3}^{-}$ groups is localized in the UV region at 385 nm. Significant diminutions of the absorbance in both the visible and UV spectrum regions were observed due to the photocatalytic treatments in the following order: Hybrid < GMAC < Anatase-TiO₂ from which three different mechanisms of dye removal associated to each material can be mentioned as follows:

Anatase-TiO₂: Adsorption–desorption–degradation on the surface.
GMAC: Adsorption with little degradation due to the photolysis.
Hybrid: Adsorption by GMAC, degradation by TiO₂ and desorption.

UV–Vis spectra of RB5 dye treated during 45 min with different materials.

3.3.4

3.3.4 Chemical oxygen demand measurements (COD)

The degradation was followed by COD determination, which reflects the degree of mineralization. Complete oxidation of RB5 may be written as follows: $C_{26} H_{21} N_{5} {Na}_{4} O_{19} S_{6} + (165 / 2) O_{2} \to 26 {CO}_{2} + 6 {SO}_{4}^{2 -} + (21 / 2) H_{2} O + 4 {Na}^{+} + 5 {NO}_{3}^{-}$

In order to check the accuracy of the analytical method used in this study, the calculated theoretical value of COD (158.4 mg_O2 L⁻¹) was compared with the experimental value (153.6 mg_O2 L⁻¹). The COD values of 100 μmol L⁻¹ RB5 solutions exposed to UV light decrease with increasing exposition time as shown in Fig. 10a, thus, confirming the oxidation of dye molecules. For hybrid photocatalyst, 60 min exposition to UV light was amply sufficient to abate COD from 158.4 to 4.8 mg L⁻¹ of O₂ corresponding to 98% of degradation.

Photocatalytic activity kinetics at 1 g L−1 with different materials: (a) COD, (b) sulfate ions release.

Fig. 10a. shows a slow decreasing phase until 10 min (18.75%), followed by a sharp decrease between 10 and 30 min, when the COD values go from 124.8 to 57.6 mg/L of O₂ corresponding to a 62.5% dye degradation. The overall pollutant concentration is halved within less than 30 min and COD values far below the standard regulation norm (120 mg L⁻¹ of O₂). After 30 min exposition, it is clear that the dye is almost completely degraded.

For Anatase-TiO₂, the kinetics of COD removal depicted in Fig. 10a indicates a decrease of the COD from 153.8 to 115.2 mg L⁻¹ O₂ after 30 min of irradiation corresponding to 25% abatement. Finally, the degradation reaches values of 56.25% and 93.75% after exposition times of 1 h and 3 h respectively. However, 43.75% degradation was obtained using GMAC after irradiation time of 180 min. less degradation of dye molecules adsorption by GMAC was observed in comparison with previous results. On the other hand, pollutant degradation by the hybrid occurs after dye adsorption. Total elimination of RB5 dye from both solution and catalyst surface after 4 h with a mineralization degree >90% was noted by Bradu et al. (2010) using CuO/Al₂O₃ catalyst and taking longer time compared to our photocatalyst.

3.3.5

3.3.5 Mineralization study

Table 5 presents the final concentrations of the mineralized products in solution according to irradiation times and indicates the formation of inorganic species derived from some heteroatoms. Later these are present in azo dyes in the form of Nitrogen (azo groups) and sulfur (sulfur groups). It is clear that dyes containing sulfur atoms under sulfonate form could be mineralized into sulfate through the OH^• radicals (Karkmaz et al., 2004) as follows: $R – {SO}^{3 -} + H_{2} O \to R – H + {HSO}_{4}^{-}$ ${HSO}_{4}^{-} \leftrightarrow {SO4}^{2 -} + H^{+}$

Table 5 Hybrid systems: treatment times effect on mineralized products with final concentration in solution.

Treatment time (min)	[NO₃⁻] (mg L⁻¹)	[SO₄²⁻] (mg L⁻¹)	COD (mg of O₂ L⁻¹)	Degradation rate (%)
0	0.00	0.00	153.60	0.00
5	0.40	2.51	144.00	6.25
10	0.50	10.04	124.80	18.75
15	0.67	12.61	96.00	37.50
30	0.76	18.64	57.60	62.50
45	0.82	28.75	19.20	87.50
60	1.11	30.44	4.80	98.00

Photo-induced activated species should be involved, such as OH^• radicals or photo-induced holes h⁺. The attack by OH^• radicals could generate spontaneously sulfate ions according to ${TiO}_{2} + h ν \to e^{-} + h^{+}$ $H_{2} O + h^{+} \to OH^{•} + H^{+}$

Then, OH^• attack is $R – {SO}_{3}^{-} + OH^{•} \to R^{•} + H^{+} + {SO}_{4}^{2 -}$

The appearance of sulfate ions during the photocatalytic treatment is a consequence of mineralization as shown in Fig. 10b. After 180 min of treatment, the release of sulfate ions was of 30.44, 9.3 and 4.5 mg L⁻¹ respectively for hybrid, Anatase-TiO₂ and GMAC samples.

Using the oxidation reaction (Fig. 11), the limiting value of sulfate concentration calculated from mineralization reaction is 57.6 mg L⁻¹. In this work we found a value of 30.44 mg L⁻¹ (Fig. 11), thus indicating that only 63% of sulfate ions are present in the solution. It should be noticed that a part of the anions formed in the process may still be adsorbed onto the surface of hybrid photocatalyst.

Kinetics of sulfate ions’ appearance for hybrid photocatalyst.

The formation of NO₃⁻ ions could be expected, which is common product of photocatalytic processes. A part of the nitrogen can be released into the atmosphere and part of NO₃⁻ could be adsorbed on the surface of the photocatalyst (Garcia et al., 2007). $Before irradiation : N_{total} = N_{org}$ $After irradiation : N_{total} = {NH}_{4}^{+} + {NO}_{3}^{-}_{(free)} + {NO}_{3}^{-}_{(adsorbed)} + N_{2}$

4 Mechanism of RB5 degradation by the hybrid photocatalysts

Hybrid photocatalyst shows a better performance for RB5 degradation than pure TiO₂. The enhanced photocatalytic activity could be related to a synergistic effect of GMAC adsorption and TiO₂ photocatalysis.

The hybrid photocatalyst should be able to oxidize species in solution by four simultaneous mechanisms: (i) GMAC appears not only as a support for fine TiO₂ particles, but also as a physical absorbent which can absorb some incident photons. On the other hand, another fraction of substrates adsorbed onto GMAC may undergo degradation by attack of reactive oxygen species that are generated on the surface of the Anatase-TiO₂ but migrate onto the carbon surface. As reported (Tatsuma and Kubo, 2005, 2006, 2007; Park and Choi, 2004, 2005), the reactive oxygen species can diffuse over sub-millimeter distances from the surface of Anatase-TiO₂, so some reactive oxygen species could reach the target compound located on the surface of the carbon, where most of the substrate remains adsorbed; (ii) GMAC with very large surface area and well developed pore structure possessed large numbers of active adsorption sites; thus, organic molecules were adsorbed before transferring to the decomposition center of TiO₂ then GMAC concentrated RB5 around the loaded TiO₂ by adsorption with a large amount of molecules. An important step in the photocatalytic process is the adsorption of reacting substances onto the surface of the catalyst (Pecchi et al., 2001). GMAC substrates showed their merits in two aspects; (iii) Direct oxidation on the semiconductor (TiO₂) is by the photo-induced holes which are in the external surface and had much more chance to receive lights. The mechanism of photocatalysis involves the promotion of an electron (e⁻) from the valence band (VB) into the conduction band (CB) of the semiconducting oxide, creating a hole (h⁺) in the VB: TiO₂ + hν → e_cb⁻ + h⁺_vb; (iv) Oxidation of the dyestuff in the solution through successive attacks by the hydroxyl radical is as follows: h⁺_vb + H₂O → OH^• + H⁺. In addition, as RB5 molecules were degraded by GMAC/TiO₂, their amount on the TiO₂ surface decreased, so that the adsorbed RB5 molecules were desorbed from GMAC, since RB5 molecules were liberated in by-products form. Therefore, the adsorption sites were released and GMAC was regenerated. Thus, the dye degradation was achieved through a synergistic mechanism of GMAC adsorption and TiO₂ photocatalysis.

5 Conclusion

Grape marc-based activated carbon-Anatase-TiO₂ photocatalyst was prepared by an original approach of impregnation process using chemical agent. The Anatase-TiO₂ was incorporated in the micropores of GMAC. The hybrid photocatalyst exhibited better photocatalytic performance than Anatase-TiO₂ for Reactive Black (RB5) azo dye degradation from aqueous solution. The pollutant can be degraded both in the mesopores and in the micropores of the GMAC, and then desorbed in the aqueous solution. In this context, the activated carbon can be simultaneously regenerated. The high activity was attributed to the synergistic effect of GMAC adsorbent and Anatase-TiO₂ photocatalyst.

Acknowledgment

The authors thank the National Research Program (PNR) for providing financial support.

References

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Introduction

Experimental

Materials
Activated carbon preparation
Composite photocatalyst preparation
Samples characterizations
Adsorption isotherms
Photocatalysis

Results and discussion

Characterization
Adsorption isotherms
Photocatalytic activity

Mechanism of RB5 degradation by the hybrid photocatalysts

Conclusion

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[1] Aguedach A., Brosillon S., Morvan J., El Kbir L., . Photocatalytic degradation of azo-dyes reactive black 5 and reactive yellow 145 in water over a newly deposited titanium dioxide. Appl. Catal. B: Environ.. 2005;57:55-62.
[Google Scholar]

[2] Aksu Z., . Application of biosorption for the removal of organic pollutants. A review. Process. Biochem.. 2005;40:997-1026.
[Google Scholar]

[3] Ao Y., Xu J., Fu D., Shen X.W., Yuan C., . Low temperature preparation of anatase TiO₂-coated activated carbon. Colloid Surf. A: Physicochem. Eng. Aspects.. 2008;312:125-130.
[Google Scholar]

[4] Arana J., Renden E.T., Rodriguez J.M.D., Melian J.A.H., Diaz D.G., Pena J.P., . Highly concentrated phenolic wastewater treatment by the Photo-Fenten reaction, mechanism study by FTIR-ATR. Chemosphere. 2001;44:1017-1023.
[Google Scholar]

[5] Barka N., Assabbane A., Nouanah N., Albourine A., Ait-Ichou Y., . Degradation photocatalytique de deux colorants séparés et en mélange binaire par TiO₂-supporte. Sci. Technol. A. 2008;27(8):09-16.
[Google Scholar]

[6] Bestani B., Benderdouch N., Benstaali B., Belhakem M., Addou A., . Methylene blue and iodine adsorption onto an activated desert plant. Bioresour. Technol.. 2008;99:8441-8444.
[Google Scholar]

[7] Bradu C., Frunza L., Mihalche N., Avramescu S.M., Neata M., Udreaa I., . Removal of Reactive Black 5 azo dye from aqueous solutions by catalytic oxidation using CuO/Al₂O₃ and NiO/Al₂O₃. Appl. Catal. B: Environ.. 2010;96:548-556.
[Google Scholar]

[8] Cordero T., Chovelon J., Duchamp C., Ferronato C., Matos J., . Surface nano-aggregation and photocatalytic activity of TiO₂ on H-type activated carbons. Appl. Catal. B: Environ.. 2007;73:227-235.
[Google Scholar]

[9] Da S., Faria C.G., . Photochemical and photocatalytic degradation of an azo dye in aqueous solution by UV irradiation. J. Photochem. Photobiol. A: Chem.. 2003;155:133-143.
[Google Scholar]

[10] Dias M., Alvim-Ferraz M.C.M., Almeida M.F., Rivera-Utrilla J., Sanchez-Polo M., . Waste materials for activated carbon preparation and its use in aqueous-phase treatment. A review. J. Environ. Manage.. 2007;85:833-846.
[Google Scholar]

[11] Donnaperna L., Duclaux L., Gadiou R., . Adsorption of Remazol Black B dye on Activated Carbon Felt. India: Applied Science Innovations Pvt. Ltd.; 2008.

[12] Garcia J.C., Oliveira J.L., Silva A.E.C., Oliveira C.C., Nozaki J.N.E., Souza N.E., . Comparative study of the degradation of real textile effluents by photocatalytic reaction involving UV/TiO₂/H₂O₂ and UV/Fe⁺²/H₂O₂ systems. J. Hazard. Mater.. 2007;147:105-110.
[Google Scholar]

[13] Huang D., Miyamoto Y., Ding J., Gu J., Zhu S., Liu Q., Fan T., Guo Q., Zhan D., . A new method to prepare high-surface-area N–TiO₂/activated carbon. Mater. Lett.. 2011;65:326-328.
[Google Scholar]

[14] Karkmaz M., Puzenat E., Guillard C., Herrmann J.M., . Photocatalytic degradation of the alimentary azo dye amaranth Mineralization of the azo group to nitrogen. Appl. Catal. B: Environ.. 2004;51:183-194.
[Google Scholar]

[15] Kurniawan A., Sutiono H., Indraswati N., Ismadji S., . Removal of basic dyes in binary system by adsorption using rarasaponin–bentonite: revisited of extended Langmuir model. Chem. Eng. J.. 2012;189–190:264-274.
[Google Scholar]

[16] Leary L., Westwood A., . Carbonaceous nanomaterials for the enhancement of TiO₂ Photocatalysis. Carbon. 2011;49:741-772.
[Google Scholar]

[17] Li Y., Li X., Li J., Yin J., . Photocatalytic degradation of methyl orange by TiO₂-coated activated carbon and kinetic study. Water Res.. 2006;40:1119-1126.
[Google Scholar]

[18] Liu Y., Yang S., Hong J., Sun C., . Low-temperature preparation and microwave photocatalytic activity study of TiO₂-mounted activated carbon. J. Hazard. Mater.. 2007;142:208-215.
[Google Scholar]

[19] Lucas M.S., Peres J.A., . Decolorization of the azo dye Reactive Black 5 by Fenton and photo-Fenton oxidation. Dyes Pigm.. 2006;71:236-244.
[Google Scholar]

[20] Marouf R., Khelifa N., Marouf-Khelifa K., Schott J., Khelifa A., . Removal of pentachlorophenol from aqueous solutions by dolomitic sorbents. J. Colloid Interf. Sci.. 2006;297:45-53.
[Google Scholar]

[21] Matos J., Garcia A., Zhao L., Titirici M.M., . Solvothermal carbon-doped TiO₂ photocatalyst for the enhanced methylene blue degradation under visible light. Appl. Catal. A.. 2010;390:175-182.
[Google Scholar]

[22] Park J.S., Choi W., . Enhanced remote photocatalytic oxidation on surface-fluorinated TiO₂. Chem. Lett.. 2005;34:1630-1631.
[Google Scholar]

[23] Pecchi G., Reyes P., Sanhueza P., Villasenor J., . Photocatalytic degradation of pentachlorophenol on TiO₂ sol–gel catalysts. Chemosphere. 2001;43:141-146.
[Google Scholar]

[24] Peralta-Hernandez J.M., Manrıquez J., Meas-Vong Y., Rodrıguez F.J., Chapman T.W., Maldonado M.I., Godınez L.A., . Photocatalytic properties of nano-structured TiO₂-carbon films obtained by means of electrophoretic deposition. J. Hazard. Mater.. 2007;147:588-593.
[Google Scholar]

[25] Srivastava C., Mall I.D., Mishra I.M., . Equilibrium modelling of single and binary adsorption of cadmium and nickel onto bagasse fly ash. Chem. Eng. J.. 2006;117:79-91.
[Google Scholar]

[26] Stylidi M., Kondarides D.I., Verykios X.E., . Visible light-induced photocatalytic degradation of Acid Orange 7 in aqueous TiO₂ suspensions. Appl. Catal. B: Environ.. 2004;47:189-201.
[Google Scholar]

[27] Tatsuma T., Kubo W., . Photocatalytic lithography based on photocatalytic remote oxidation. J. Photopolym. Sci. Technol.. 2007;20:83-86.
[Google Scholar]

[28] Torimoto T., Okawa Y., Takeda N., Yoneyama H., . Effect of activated carbon content in TiO₂-loaded activated carbon on photodegradation behaviors of dichloromethane. J. Photochem. Photobiol. A: Chem.. 1997;103:153-157.
[Google Scholar]

[29] Tryba B., Morawski A.W., Tsumura T., Toyoda M., Inagaki M., . Hybridization of adsorptivity with photocatalytic activity—carbon-coated anatase. J. Photochem. Photobiol., A.. 2004;167:127-135.
[Google Scholar]

[30] Tryba B., Tsumura T., Morawski A.W., Inagaki M., . Application of TiO₂-mounted activated carbon to the removal of phenol from water. Appl. Catal. B. 2003;41:427-433.
[Google Scholar]

[31] Wang X., Liu Y., Hu Z., Chen Y., Liu W., Zhao G., . Degradation of methyl orange by composite photocatalysts nano-TiO₂ immobilized on activated carbons of different porosities. J. Hazard. Mater.. 2009;169:1061-1067.
[Google Scholar]

[32] Xu J., Ao Y., Chen M., Fuk D., Yuan C., . Photocatalytic activity of vanadium-doped titania–activated carbon composite film under visible light. Thin Solid Films. 2010;518:4170-4174.
[Google Scholar]

[33] Xu S., Ng J., Zhang X., Bai H., Delai-Sun D., . Adsorption and photocatalytic degradation of Acid Orange 7 over hydrothermally synthesized mesoporous TiO₂ nanotube. Colloids Surf. A.. 2011;379:169-175.
[Google Scholar]