Photocatalytic Disc-Shaped Composite Systems for Removal of Hazardous Dyes in Aqueous Solutions

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Borderless Science Publishing 56

Canadian Chemical Transactions Year 2014 | Volume 2 | Issue 1| Page 56-70

ISSN 2291-6458 (Print), ISSN 2291-6466 (Online)

Research Article

Photocatalytic Disc-Shaped Composite Systems for Removal

of Hazardous Dyes in Aqueous Solutions

Salah A. Hassan1

and Radwa A. El-Salamony2

1Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt

2Department of Process Design and Development, Egyptian Petroleum Research Institute, Nasr City,

Cairo, Egypt

*Corresponding Author, E-mail: [email protected]

Received: November3, 2013 Revised: December 8, 2013 Accepted: December11, 2013 Published: December 12, 2013

Abstract: Rice straw ash (RSA)-based composites, containing titania (TiO2, 50%) or copper

phthalocyanine complex (CuPc, 1.0%) or TiO2-CuPc combination, were prepared in disc- shaped forms.

Mechano-mixing of TiO2 could develop the mesoporous nature of the composites. TiO2 existed as

nanoparticles of average sizes in the range, 16 – 20 nm, homogeneously dispersed on the surface of the

ash matrix. Compression of the solids using polymer binder led to only a small decrease in surface

parameters, pore dimensions and average size of contained silica nanocrystallites. The composites discs

were applied in the adsorption and photo-catalytic degradation (using visible light) of methylene blue

(MB) dye, at ambient temperature and neutral pH. The rate of adsorption of MB on the studied

composites deceased, according to the second order kinetic model, in the order: RSA-TiO2- CuPc > RSA-

TiO2 > RSA > RSA-CuPc. TiO2 or TiO2-CuPc combination enhanced the rate of MB adsorption due to

development of the composite surface characteristics. The rate of visible-photodegradation of MB, in

terms of the first order kinetic model, deceased in the order: RSA ≈ RSA-TiO2 > RSA-CuPc > RSA-

TiO2-CuPc. TiO2, by generated hydroxyl radicals (.OH) and superoxide radicals (

.O2

-) and CuPc

complex, by generated singlet oxygen (1O2) through photosensitization role, could activate the rice straw

ash toward the MB photo -degradation in aqueous solutions. Some retardation in photocatalytic

degradation rate observed in presence of hybrids derived from CuPc and TiO2 was attributed to

probable recombination of most of electrons donated by the bound CuPc with positive holes of tita nia.

Keywords: Rice Straw Ash; Disk Shaped Composites; Photocatalytic Degradation, Methylene Blue

(MB) Dye.

1. INTRODUCTION

Dyes are an abundant class of colored organic compounds that represent an increasing

environmental danger. Most textile dyes are photolytically stable and refractory towards chemical

mailto:[email protected]

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Samples

Crystal sizea

DXRD (nm)

Specific

surface

areab

SBET (m2/g)

Pore

Radiusc

r (nm)

Total pore

Volumec

Vp (cc/g)

RSS

TiO2

RSA 52.4 - 14.49 1.40 0.074

RSA-binder 41.7 - 11.41 1.05 0.020

TiO2 - 23.2 48.18 3.70 0.203

RSA-TiO2(50) 59.8 17.9 22.58 1.90 0.131

RSA-CuPc 52.1 - 9.54 1.17 0.027

RSA-TiO2-CuPc 46.8 23.0 30.90 0.95 0.119

Table 1. Textural and structural properties of original RSA, RSA-binder, TiO2, RSA-TiO2

(50%), RSA-CuPc and RSA-TiO2-CuPc composites

aaccording to XRD analysis. bBET surface area calculated from the linear portion of the BET plot in the relative pressure range of p/p0 = 0.05–0.35. cPore radius and total pore volume estimated using BJH method from the isothermal desorption data.

Table 2. Adsorption kinetic parameters

Catalyst First-order kinetics Pseudo-second-order

kinetics

k1, min-1

R2 k2, g μ g

-1min

-1 R

2

RSA 0.0052 0.9726 2.73E-05 0.999

RSA-TiO2 0.0055 0.9673 7.90E-05 0.9993

RSA-CuPc 0.0033 0.9887 3.54E-06 0.9269

RSA-TiO2- CuPc 0.0058 0.9673 2.91E-04 0.9978

oxidation that renders them resistant towards decolorization by conventional biochemical and

physicochemical methods. Alternative methods, such as activated carbon adsorption and dissolved air

flotation, are not only costly, but result in phase transfer of pollutants. Hence, there is considerable current

interest in developing alternative more cost- effective methods [1].

The development of visible light-driven photocatalysts has been an attractive research field due to

ambitions of water splitting, pollutant destruction, and bacterial disinfection [2]. Most of researches have

been concentrated on anatase (TiO2), which is photostable, nontoxic, cheap, and active. Due to its large

band gap of 3.2 eV, UV light is necessary to generate the electron–hole pairs, thus restricting its

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absorption of solar energy (about 4% of total energy). Therefore, the discovery of a new active and

efficient visible light - driven photocatalyst attracts much attention [3].

The photo-catalyst is usually applied in suspended [4] or in immobilized form [5]. Although

suspended system provides large surface area of the catalyst to precede the reaction with high efficiency,

separation and recycling of the catalyst after the treatment becomes difficult. This problem has been

minimized by using the photo-catalyst in immobilized form. To diversify the range of photo-sensitized

degradation process, a number of products (tiles, glass, ceramic balls, etc.) with photo-catalytic property

have been developed for environmental applications [6].

On the other hand, rice straw (RS), as an agricultural waste produced in significant

quantities on global basis, causes pollution and problems with disposal. When burnt, the rice straw ash

(RSA) is highly pozzolanic and suitable for use in Portland cement replacement. It is insoluble in water,

has good chemical stability, high mechanical strength and possesses a granular structure, making it a good

adsorbent material [7].

The rice straw (RS) was thermochemically modified with citric acid as esterifying agent [8],

where two free carboxyl groups were further loaded with sodium ions to yield potentially biodegradable

cationic sorbent for removal of malachite green dye (MG) from aqueous solution. The MG removal

percentage came up to its maximum value beyond pH 4. Also, potentially biodegradable cationic sorbent

for removal of basic dyes, e.g., basic blue 9 (BB9) and basic green 4 (BG4), was prepared by esterifying

oxalic acid onto RS loaded with sodium ions [9], achieving maximum removal beyond pH 6.

The adsorption potential of low cost bioadsorbent, e.g., rice husk and rice husk ash, was studied

for removal of hazardous methylene blue dye from the waste water [10]. The possibility of using RHA for

removal of Indigo Carmine dye from aqueous solution was also explored [11]. A series of tin and

titanium- incorporated rice husk silica (RHS), synthesized via sol–gel method using cetyltrimethyl

ammonium bromide (CTAB) as structure directing agent, was applied in adsorption of MB. The

adsorption of MB on these catalysts was found to fit the pseudo-second order kinetic model and the

adsorption rate was found to be strongly dependent on the pH of the solution [12].

Generally, it is clear that none of the mentioned researches could approach the design of RHA-

based composites in stable shaped forms, durable and applicable at normal conditions, in adsorption as

well as in photodegradation, with an attempt of regeneration and reuse. For this reason, an approach is

developed, to the first time, in this study to use rice straw ash (RSA) as a base to titania (TiO2) and to

copper phthalocyanine complex (CuPc, Scheme 1) for designing disc–shaped photocatalytic composites.

CuPc was used as a sensitizer in some hybrid composites to enhance the light absorption [13]. Recycled

polymeric foam (P.F-2000), an environmental solid waste, was used for binding. These composites were

applied in the adsorption and the visible light–photodegradation of methylene blue (MB), as a model of

textile dyes creating environmental pollution and toxic problems in industrial waste water.

Scheme 1: Copper Phthalocyanine (CuPc)

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Figure 1. Photographs of RS, RSA, RSA-P.F., RSA-TiO2, RSA-CuPc and

RSA-TiO2-CuPc composite discs

Figure 2. Schematic diagram of the photo-reactor and composite disc

Figure 3. XRD pattern of RSA, RSA-binder and prepared composites

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Canadian Chemical Transactions Year 2014 | Volume 2 | Issue 1| 6-70


2. EXPERIMENTAL

2. 1. Preparation of rice straw ash (RSA)

The collected rice straw (from Kafr El-Zayat region) was thoroughly washed with distilled water

to remove adhering soil and clay and sieved to 250-500 µm. The ash was obtained through controlled

burning in a temperature-programmed oven at 600° C for 2h.

2. 2. Preparation of nano-TiO2

For preparing nanosized TiO2 powder by sol-gel method [14], 25 ml of TiCl4 (15%) was

dissolved in 20 ml distilled water in an ice-water bath. The solution was slowly mixed with 30 ml distilled

water and 20 ml ethanol (dispersing agent). Ammonia was then added drop wise until pH = 9, the solvent

was evaporated at 80° C for 24 h and the formed precipitate was dried at 300o C for 2 h and calcined in

an air stream at 500° C for 4 h.

2. 3. Preparation of RSA-based photocatalytic composites discs

For the first time, the photocatalytic composites, namely, RSA-TiO2, RSA-CuPc and RSA- TiO2-

CuPc, were prepared in disc-shaped forms. A requisite amount of RSA was mixed with TiO2 in a ratio

maintained at 50 % and with CuPc at 1%, then grinded thoroughly in a ball mill to obtain fine powder.

After completing the grinding, the mixture was dried at 110° C and calcined at 250°C for 2h. To prepare

the composite discs, a necessary amount of polymeric foam in chloroform solution was added and the

mixture was shaped using a mold and dried at ambient temperature. To make it in compact form, a

pressure of 1 ton (2000 psi) was applied. To ensure the durability of the obtained disc-shaped composites

of (O.D. = 40 mm and thickness = 2.5 mm, Fig. 1) toward stresses and strains, some essential mechanical

parameters were measured, e.g., the compressive strength and the compressive load (being 9.27 N/mm2

and 11.647 KN, respectively). A universal testing machine, UH Series (SHIMADZU CORP.) was used

for this purpose.

2.4. Characterization techniques

The prepared samples were characterized by transmission electron microscopy (JEOL-2100F)

joined to energy dispersive X-ray spectrometer (EDX). The crystalline structure and the different phases

were investigated via X-ray diffraction analysis (XRD) using Shimadzu XD-1 diffractometer. The phase

identification was made according to the Joint Committee on Powder Diffraction Standards (JCPDS). The

crystallite size, DXRD was calculated according to Scherer equation [15]:

Where K is the Sherrer constant (0.89), λ is the wavelength of the X-ray radiation (0.15418 nm for Cu

Kα), β is the full width at half maximum (FWHM) of the diffraction peak measured at 2θ, and θ is the

diffraction angle. The textural properties were followed up using the N2 adsorption-desorption isotherms

measured at liquid nitrogen temperature (-196°C) using NOVA-2000 gas sorption analyzer

(Quantachrome Corp. system).

2.5. Photocatalytic activity measurements

The methylene blue (MB) azo dye supplied by Aldrich had the following characteristics: C.I.

number 52015; formula weight 319.86; λmax = 665 nm. The photocatalytic efficiencies of the composites

under study were evaluated through the extent of decolorization of MB in aqueous solutions. In a typical

experiment, 150 ml of MB solution (7x10-5

mol dm-3

) was placed in the photoreactor (Fig. 2), and the

prepared composite discs were immersed in the solution while a continuous air stream was bubbled

through the suspension. Prior to irradiation, the disc was immersed into the solution in dark for 1h to

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Figure 4. TEM (from EDX) images of A: RSA, B: Nano TiO2 and C: RSA-TiO2 composite

ensure adsorption/desorption equilibrium. The solution was exposed to visible light at 45° C for 5 h

maintaining the pH at 8.85. A 200W tungsten lamp of visible light (λ > 400 nm) was used as the

irradiation source. In order to monitor the dye concentration in the solution, a 10 ml aliquot was analyzed

using a JENWAY-6505 UV-visible spectrophotometer (at λ= 664 nm for MB), at the appropriate time

intervals.

3. RESULTS AND DISCUSSION

3.1. Structural characteristics (XRD and TEM results)

From the obtained XRD patterns shown in Fig. 3, the original RSA showed amorphous silica

(SiO2) structure (at 2θ between 22o and 25

o) with some small peaks at 2θ = 28.3°, 40.5° and 50.0°,

characteristic probably of nano-crystalline silica (as evidenced from the calculated average crystallite

sizes in Table 1). No detectable change in structure of the contained silica could be detected upon

applying a polymeric binder to RSA, except some damping of characteristic peaks of amorphous and

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Figure 5. N2 absorption-desorption isotherms

crystalline silica. For both pure TiO2 and RSA-TiO2 composite, a single TiO2 (anatase) phase was

observed [JCPDS card 01-071-1168]. In RSA-TiO2 (50 %) system, prepared by mechano-chemical

mixing, titania and silica existed as separate phases. The presence of copper phthalocyanine (CuPc, 1.0

%) was hardly detected by XRD in RSA-CuPc and RSA-TiO2-CuPc composites, due to its low content. In

RSA-TiO2-CuPc system, the damped intensities of both silica and titania peaks might refer to some

interaction of CuPc with the two phases. Similarly, for Pc/Fe-TiO2 nanocomposite of low complex

loading [16], the XRD patterns did not present signals of the Pc that was assigned to its complete

dispersion into TiO2 nanoparticles.

The average crystallite sizes (DXRD) of TiO2 nanoparticles and rice straw silica (RSS),

calculated from XRD patterns [15] were found to be 23.2 and 52.4 nm, respectively, as listed in Table 1.

Addition of polymeric binder seemed to reduce the average crystallite size of RSS to ca. 42 nm. In RSA-

TiO2 (50 wt %) composite, smaller average crystallite size of titania (17.9 nm) was observed, due to its

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Figure 6. Pore size distributions (PSD) curves

Figure 7. Analysis of kinetic results of MB adsorption over RSA, RSA-TiO2, RSA-CuPc and RSA-TiO2-

CuPc composites according to: (A) Pseudo first-order kinetic model and (B) Pseudo second-order kinetic

model.

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surface dispersion, while the average size of the contained RSS was somewhat enlarged (59.8 nm).

Addition of CuPc complex had no effect on the average crystallite size of RSS in the ash, though it

seemed to interact in RSA-TiO2-CuPc composite with RSS, in more extent than with dispersed titania. It

was clear from the TEM images and EDX patterns (Fig. 4), that titania in RSA-TiO2 system has

interacted mainly with the contained silica, whereas carbon has been redistributed in more ordered profile

along the entire surface of the ash. TiO2 existed as nanoparticles of average sizes, 16 – 20 nm,

homogeneously dispersed on the RSA surface, improving thus the surface characteristics.

3.2. Surface characteristics

N2 adsorption–desorption isotherms on prepared disc-shaped composites are depicted in Fig.5.

All the isotherms are of type II, where the adsorption isotherm rises only moderately at intermediate

relative pressures then rises rapidly as the relative pressure approaches unity, being typical for

nanoporous materials. The closed hysteresis loops occurred in the case of RSA, RSA-polymer binder and

RSA-CuPc are of IUPAC-type H3, characteristic of solids consisting of aggregates of particles forming

slit-shaped pores with non-uniform size and shape. For nano-TiO2, RSA-TiO2 and RSA-TiO2-CuPc

composites, the closed hysteresis loops of H2 type define mesoporous materials consisting of

agglomerates with uniform and regular pores (cf., TEM images, Fig. 4) [17, 18]. The obtained surface

parameters including BET-surface area (SBET), mean pore radius (rp) and total pore volume (Vp) are

presented in Table 1. It is clear that the presence of TiO2 has improved the ash surface characteristics.

For more confirmation, the pore size distribution (PSD) curves in Fig. 6 provide unimodal ones for the

original RSA, RSA-polymer binder and RSA-CuPc system, with most probable hydraulic pore radii in

microporous region (rh ≈ 20 Å). However, for the TiO2-containing disc composites, RSA-TiO2, RSA-

TiO2-CuPc as well as TiO2, the observed bimodal PSD curves exhibit a major fraction of wider pores

(rh ≈ 68 Å for composites and 48 Å for titania), together with the narrower fraction of pores

characteristic of pristine ash.

It can thus be concluded that compression of the ash in a disc form using a polymer binder has led

to a small decrease in surface parameters and pore dimensions accompanied with a decrease in the

average size of contained silica nanocrystallites. Addition of CuPc could lead to further decrease in

surface and pore parameters probably by interaction with both silica and titania phases. Mixing with TiO2

seemed to develop the mesoporous nature of the composites, reflecting positively on their adsorption

behavior.

3.3. Kinetics of MB removal over the investigated composites discs

3.3.1. Investigation of the order of reaction in dark

In the case of dark reaction, the amount of MB adsorbed per unit mass (qt, mg g-1

) was calculated

according to equation 1,

(1)

where, m = mass of the composite (g) and V= volume of MB solution (150 mL). The kinetics of

adsorption was investigated due to its importance in waste water treatment processes. The obtained

adsorption results were analyzed according to pseudo first-order kinetic model, based on the solution

concentration,

(2)

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Figure 8. Spectral profiles obtained for visible light-MB photodegradation on RSA and RSA- TiO2

composite discs as a function of time of irradiation. The irradiation times were (top to bottom): 0, 1, 2, 3

and 4h.

Figure 9. (A) Photocatalytic degradation extents of MB under visible irradiation using RSA, RSA-TiO2,

RSA- CuPc and RSA-TiO2- CuPc composites, (B) 1st

order kinetic plots and rate constants (k1) of

photodegradation of MB on the different composites.

And pseudo-second-order kinetic model, based on the sorption equilibrium capacity [19],

(3)

where, k1 (min-1

) and k2 (g μg-1

min-1

) are the respective rate constants and qe is the amount of MB

adsorbed after reaching the equilibrium.

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Figure 10. Photodegradation mechanism of MB by visible light in presence of pure TiO2 (a), CuPc

sensitization (b) and sensitized TiO2 by CuPc

Figure 11. Effect of initial dye concentration on MB removal on RSA and RSA-TiO2 composite discs

under visible light, at pH= 5.85

Figure 12. Reproducibility of the RSA, RSA-TiO2, RSA-CuPc and RSA-TiO2-CuPc composite discs for

25 ppm of MB under visible light, pH= 5.85 and irradiation time of 120 min

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Analysis of kinetic data of MB adsorption over RSA, RSA-TiO2, RSA-CuPc and RSA-TiO2-CuPc

composites according to equations 2 and 3 are illustrated in Fig. 7 and the corresponding adsorption

parameters, derived from the respective slopes and intercepts are summarized in Table 2. The obtained

regression values (R2) show that pseudo second-order kinetics describes the adsorption of MB better than

that of pseudo first -order kinetic model in all cases, except for RSA-CuPc composite, in contrast to

RHA-TiO2 (RHA:TiO2= 75:25 and 50:50) [20]. For RSA-CuPc, the data seem to be better fitting the

pseudo-first order kinetic model. Thus, based on the second order analysis, the rate of adsorption process

of MB dye can be considered to decease in the order: RSA-TiO2-CuPc > RSA-TiO2 > RSA > RSA-CuPc.

In conclusion, while CuPc complex did not change noticeably the adsorption capacity of straw

ash, the presence of TiO2 or TiO2-CuPc combination enhanced markedly the rate of adsorption,

certainly linked with development of mesoporous nature of the composites.

3.3.2. Photodegradation of MB by visible light

Typical time-dependent UV/vis spectra of MB degradation, in visible irradiation, in presence of

RSA and RSA-TiO2 composite discs are depicted in Fig. 8. Prior to irradiation, the MB solution over the

discs was kept in dark for 120 min to attain the adsorption equilibrium. The absorption peaks of MB

solution at λ = 292 nm and 664 nm were decreased with irradiation in presence of the discs with no shift,

i.e., without change in the nature of the degradation products of the MB, up to 4h irradiation.

The photodegradation of MB (25 ppm) dye by using the RSA, RSA-CuPc, RSA-TiO2 (50%), and

RSA-TiO2-CuPc disc-shaped composites is presented as a function of irradiation time in Fig. 9 (A). The

% photodegradation was rapidly increased with the irradiation time up to 60 min, after which the rate of

reaction became almost constant. The overall photodegradation extent decreased, over the composite

systems in the order: RSA-CuPc > RSA- TiO2 > RSA > RSA-TiO2-CuPc. The rate of photo-degradation

process of MB dye was also expressed in terms of the first order rate constant (k1) (equation 2) [21],

derived from the respective slopes (Fig. 9, B) which was deceased according to the used composites in the

order: RSA ≈ RSA-TiO2 > RSA-CuPc > RSA-TiO2-CuPc. It is clear that both TiO2 and CuPc complex

solely can activate the rice straw ash toward the photo-degradation, while TiO2-CuPc combination slows

down the rate of photodegradation of the dye, in spite of its enhancement role in adsorption.

In an attempt to interpret these results, it is worth to discuss the photo-catalytic effects imposed

by our disc-composite systems used in highly desirable visible region. For the well- documented

photocatlytic role of TiO2, the irradiation promotes an electron into the conduction band (CB) of TiO2,

leaving a vacancy in the valence band (VB). The photocatalytic degradation by TiO2 is an oxidative

process brought about by oxidizing radical species, e.g., hydroxyl radicals (.OH) and superoxide radicals

(.O2

-) derived from the reaction of water with electron holes (h

+) and oxygen with electrons (e

-),

respectively (Fig 10, a) [22-24]. On the other hand, phthalocyanines (Pc), being chemically and photo-

physically robust organometallic complexes with strong absorptions in the visible, can act as

efficient sensitizers in the conversion of triplet oxygen (3O2, ground state) into singlet oxygen (

1O2,

excited state) (Fig. 10, b). Singlet oxygen is a highly reactive species that can largely degrade several

organic and biological materials [25, 26]. Due to the chemical inertness of CuPc, it can resist

photodegradation for extended periods of time. Suitable surface modifiers for TiO2, such as CuPc in our

study, is considered as surface bound species acting as electron donor, especially if the energy of its

excited state matches that of the conducting band of TiO2 [27].

In the case of hybrids derived from CuPc and TiO2, as vis-active photocatalyst, the electrons

donated by the bound CuPc to CB of TiO2 promote the generated superoxide radicals (.O2

-) for the dye

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oxidation in solution. However, probable recombination of most of these electrons with TiO2 positive

holes (h+) may lead to marked retardation of the oxidative photodegradation rate (cf., Fig 10, c).

3.3.3. Effect of initial concentrations of MB

The initial concentration of the dye is of practical importance as it is related to the quantity of the

dye adsorbed on the photocatalyst surface. As shown in Fig. 11, photodegradation extent of MB decreases

by increasing its initial concentration (in the range, 5-35 ppm) on RSA and RSA-TiO2 composite systems.

This can be understood, as upon increasing the dye concentration, more of the organic

substance is adsorbed on the surface of the composite, where less number of photons will become

available to reach the catalyst surface and hence less •OH radicals will be produced [21].

3.4. Recycling studies

Fig. 12 represents the reproducibility of using the RSA-TiO2 and RSA-CuPc composite discs for

MB photodegradation in four-cycle experiments. Each experiment was carried out under identical

concentration of 25 ppm of dye, pH of 5.85 and 120 min irradiation. After each experiment, the

concentration of dye was adjusted back to its initial value of 25 ppm. It is clear that, small and gradual

decrease in the activity of discs was observed at the first two cycles. The difference may be due to

accumulation of organic intermediates in the cavities and on the surface of the composites. The RSA-

binder disc seems to be the more resistant, bearable the 4 reuse cycles. The presence of CuPc leads to less

durable discs due to the increased retardation effect on the oxidative photo-degradation rate as mentioned

above.

4. CONCLUSION

The disc–shaped photocatalytic composite systems, explored in this work, included rice straw ash

(RSA) as a base for titania (TiO2, 50 %) or copper phthalocyanine complex (CuPc, 1.0 %) or both of them

in the same weight %. Compression of the ash in a disc form using a polymer binder has led to a small

decrease in surface parameters and pore dimensions, accompanied with a decrease in the average size of

contained silica (RSS) nano-crystallites. In RSA-TiO2 system, TiO2 existed as nano-particles of 16 - 20

nm sizes, homogeneously dispersed in the RSA matrix, interacted mainly with contained silica. Carbon

was redistributed in more ordered profile along the ash entire surface. Mechano-chemical mixing of TiO2

developed the mesoporous nature of the composites. Addition of CuPc complex had no effect on the

average crystallite size of RSS in the ash. The composites were applied in the adsorption and visible light

photo-degradation of methylene blue (MB) at ambient temperature and neutral pH’s. According to the

second order kinetic model, the rate of adsorption process of MB dye on studied composites deceased in

the order: RSA-TiO2-CuPc > RSA-TiO2 > RSA > RSA- CuPc. TiO2 or TiO2-CuPc combination

enhanced markedly the rate of MB adsorption, due to development of mesoporous nature of the

composites. The rate of visible-photodegradation of MB dye, in terms of first order kinetic model, was

deceased according to the used composites in the order: RSA ≈ RSA-TiO2 > RSA-CuPc > RSA-TiO2-

CuPc. It could be concluded that solely TiO2, by generated hydroxyl radicals (.OH) and superoxide

radicals (.O2

-), and CuPc complex, by generated singlet oxygen (

1O2) through photosensitization role,

activated the rice straw ash toward the photo-degradation in aqueous solutions. The retardation observed

in oxidative photo-degradation rate in presence of hybrids derived from CuPc and TiO2 could be

attributed to probable recombination of most of electrons donated by the bound CuPc with titania positive

holes (h+). The oxidative photodegradation rate decreased by increasing the initial concentration of MB in

the range, 5-35 ppm, on RSA and RSA- TiO2 systems, due to more crowded molecules of the adsorbed

dye on the surface, less number of photons became available to reach the catalyst surface and less •OH

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were produced. Reasonable photo-activity was preserved for the RSA, RSA-TiO2 and RSA-CuPc

composite discs up to 4 cycles reuse. The expected future work will be dealing with the effect of

variations in the disc dimensions on its adsorbability and photacatalytic performance in removal of

different textile dyes as well as other organic effluents in the industrial waste water.

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The authors declare no conflict of interest

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Photocatalytic Disc-Shaped Composite Systems for Removal of Hazardous Dyes in Aqueous Solutions

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