# An Experimental Study of Photocatalytic Degradation of Congo Red Using Polymer Nanocomposite Films

Authors: Mashael Nasser Alshabanat; Murefah M. AL-Anazy
Journal: Journal of Chemistry
 (2018)

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## Abstract

Eco-friendly polymer nanocomposite films were synthesized using biodegradable polymers of chitosan and polyvinyl alcohol as polymeric matrices and carbon black nanoparticles as the reinforcement. These films were applied to study their applicability to industrial wastewater purification as a photocatalyst for degradation of Congo red as a target pollutant and to study the effect of the polymeric matrix types of the films on their performance as a semiconductor photocatalyst. Fourier-transform infrared (FT-IR) spectra and X-ray diffraction (XRD) were used to characterize the films. Visible light photocatalytic degradation of Congo red as a pollutant under various operational conditions of pH, dye concentration, contact time, and light intensity was performed. Photocatalytic results revealed that the polymeric substrate type does not play a major role in the photodegradation of the dye, and the best operational conditions were at a pH of 6 and a dye solution concentration of 8 mg/L.


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## Body

## 1. Introduction

Dyes are extensively used in the industrial sector such as the textile industry, rubber, paper, leather, and cosmetics [1]. However, the discharge of dyes could lead to water pollution problems because of their toxicity.Several techniques are used to remove such contaminants. These techniques can be categorized into conventional methods such as adsorption, coagulation, and flocculation; established methods such as ion exchange, membrane separation, and oxidation; and emerging methods such as biodegradation and microbial treatment [2].Currently, the removal process based on photocatalysis is attracting great attention at the applied research level as a possible solution to the environmental pollution problem. The primary mechanism of this process involves the decomposition of organic contaminants into carbon dioxide and water by utilizing light energy. Semiconductor photocatalysts absorb the light energy, thus initiating the oxidation reactions by generating hole (h+) and electron (e−) pairs, which can generate free radicals such as hydroxyl (•OH) radicals [3]. These free radicals work as oxidizers of organic pollutants ([3–5] cited in [3]). Therefore, the photocatalytic oxidation reaction requires a source of light with energy in the visible or UV spectrum, or solar energy. However, to generate the h+ and e− pairs, a semiconductor catalyst must be capable of absorbing light energy equal to or greater than its bandgap energy ([3–5] cited in [3]).Currently, there is a tendency to use semiconductor polymer nanocomposite systems as photocatalysts. Several researchers have published works based on this approach [6–10]. For example, and in previous study, ZnO/PMMA nanocomposite as a photocatalyst had been prepared, and the results demonstrated that the proposed nanocomposite is a promising candidate for water treatment [6]. The photocatalytic activity of TiO2/chitosan (nTC) nanocomposites had also been prepared in another study using hydrothermal process, and the results showed that the best performance of the nanocomposites was with the amount of nTC of 0.25 g and irradiation time of 120 min [7]. A photocatalytic degradation of metronidazole and methylene blue by PVA-assisted Bi2WO6-CdS nanocomposite film had been studied under visible light irradiation by Rajendran et al. [8], and in another work [9], a crosslinked-chitosan/nano-CdS (CS/n-CdS) composite photocatalyst had been prepared and characterized and a photocatalytic activity under visible light irradiation was studied under various conditions of catalyst amount, pH, and concentration of target dye. A review recently published provided comprehensive information on polymer nanocomposites in the field of water purification [2]. According to this review, the features of such materials for water purification is their stability, low cost, and improved processability [2]; in addition, the technique eliminates the need for the recovery of the particles after the treatment process [6].Polymer nanocomposites consist of reinforcement filler nanoparticles embedded and incorporated into the polymeric substrate, thereby providing a combination of great properties of the produced film. The polymeric matrix improves the processability of a nonpolymeric constituent [2]; addition of conductive fillers into an insulating polymeric matrix can provide the film with conductive properties.This study aims to prepare an eco-friendly semiconductor polymer nanocomposite film as a photocatalyst for the degradation of wastewater pollutants and to study the influence of different polymeric matrix types of the nanocomposite thin films on their performance as photocatalysts, considering the effect of different effective factors including the concentration of the dye, pH, light intensity, and contact time. To achieve this goal, two target polymers were used in this study. The first is chitosan because it is biocompatible, is biodegradable, has multiple functional groups, and is nontoxic and cheap [11, 12]. The second is polyvinyl alcohol (PVA) because it is also completely biodegradable and has excellent physical properties and chemical resistance [13].


## 2. Experimental

### 2.1. Materials

Congo red (CR) (Aldrich-Sigma) was used as a model pollutant dye. It has the chemical formula C32H22N6Na2O6S2; i.e., it is a sodium salt of benzidinediazo-bis-1-naphthylamine-4-sulfonic acid [14].Two types of biodegradable polymers were used as previously indicated. They included chitosan with molecular weight of 100,000–300,000 (Acros Organics), PVA with degree of polymerization of 1700–1800, hydrolysis (mole%) of 98-99, volatiles of max 5%, and ash of max 0.7% (Loba Chemie). The reinforcement material of the nanocomposite films is furnace carbon black (Degussa) with a particle size of 95 nm.


### 2.2. Methods

#### 2.2.1. Preparation of Nanocomposite Films

To prepare the polymer nanocomposite film, the nanoparticle carbon materials were spread out over the polymer matrix in a solution form, and the solution was stirred magnetically under a suitable temperature for 3 h. Next, the mixture was cast into a Petri dish and left at ambient temperature to dry, and the solvent was allowed to evaporate completely (Figure1).Figure 1
A photograph of the prepared films.To prepare the polymer solutions, 2 g of chitosan was dissolved in 100 mL of 2% acetic acid and stirred magnetically until a homogenous solution was obtained, while 2 g of PVA was dissolved in 100 mL of distilled water and stirred magnetically at 90°C until a homogenous solution was obtained. The samples were labeled as Chit/C and PVA/C for the chitosan nanocomposites and PVA nanocomposite film photocatalysts, respectively. They were characterized spectroscopically by recording FT-IR spectra using Fourier-transform infrared (FT-IR) spectroscopy 1000, PerkinElmer ranging 4000–400 cm−1, and the X-ray diffraction using PRO X-ray diffractometer made in Holland.


#### 2.2.2. Photodegradation Studies

In general, photodegradation studies were performed via a fixed surface area (1.5 cm × 1.5 cm) of the films, after first placing them in a 250 mL Erlenmeyer flask. Then, 50 mL of the standard dye solution was added to the flask and stirred under dark conditions in a photocatalytic unit for 30 min to ensure an adsorption-desorption equilibrium is attained. A high-pressure mercury lamp (Philips) of 20–120 W emitting visible light irradiation was placed above the dye solution at a fixed distance of 20 cm. The solution was stirred continuously, using a magnetic stirrer (model Thermo Cimarec) at 25°C. The absorption of the samples was measured atλ = 502 nm (using UV-Vis spectrophotometer Jenway-6800, UK) (Figure 2). The degradation yield can be defined as follows [15]:(1)degradationyield%=C0−CeC0×100,where C0 is the initial dye concentration and Ce is the concentration of dye after photodegradation (mg/L).Figure 2
UV-Vis absorption spectrum of CR.To study the effect of pH, a series of solutions with a constant concentration of 8 mg/L of the dye with a pH ranging from 4 to 8 using Hydrolab (WTW multi 340i set, Germany) were used. The solutions were placed in a photocatalytic unit at room temperature (25°C) for a constant contact time of 120 min; the light intensity was 525 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.To study the effect of the dye concentration, and at 25°C, about 50 mL of the solution with different dye concentrations of 2, 4, 6, 8, 10, and 20 mg/L at a fixed pH of 6 was used, and then, the catalyst was added. Before irradiation, the system was magnetically stirred for 30 min under dark conditions to establish the appropriate adsorption-desorption equilibrium between the catalytic surface and the dye. The irradiation time was limited to 120 min with a light intensity of 525 W/m2. The absorbance of the solution before and after the irradiation was measured using the spectrophotometer.To investigate the effect of contact time, a constant concentration of 8 mg/L of the dye with pH adjusted to 6 was placed in the photocatalytic unit at room temperature (25°C). This was done for different contact times ranging from 15 to 180 min with the light intensity set at 525 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.Finally, the effect of light intensity was studied using approximately 50 mL of solution with a fixed dye concentration of 8 mg/L at 25°C and a fixed pH of 6, followed by the addition of a catalyst. Before irradiation, the system was magnetically stirred for 30 min under dark conditions to establish the appropriate adsorption-desorption equilibrium between the catalytic surface and the dye. The irradiation time was limited to 120 min with different light intensity values including 20, 40, 60, and 80 W (175, 350, 525, and 700 W/m2, respectively) using different mercury lamps; each lamp had a power rating of 20 W and emitted a light intensity of 175 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.


## 2.1. Materials

Congo red (CR) (Aldrich-Sigma) was used as a model pollutant dye. It has the chemical formula C32H22N6Na2O6S2; i.e., it is a sodium salt of benzidinediazo-bis-1-naphthylamine-4-sulfonic acid [14].Two types of biodegradable polymers were used as previously indicated. They included chitosan with molecular weight of 100,000–300,000 (Acros Organics), PVA with degree of polymerization of 1700–1800, hydrolysis (mole%) of 98-99, volatiles of max 5%, and ash of max 0.7% (Loba Chemie). The reinforcement material of the nanocomposite films is furnace carbon black (Degussa) with a particle size of 95 nm.


## 2.2. Methods

### 2.2.1. Preparation of Nanocomposite Films

To prepare the polymer nanocomposite film, the nanoparticle carbon materials were spread out over the polymer matrix in a solution form, and the solution was stirred magnetically under a suitable temperature for 3 h. Next, the mixture was cast into a Petri dish and left at ambient temperature to dry, and the solvent was allowed to evaporate completely (Figure1).Figure 1
A photograph of the prepared films.To prepare the polymer solutions, 2 g of chitosan was dissolved in 100 mL of 2% acetic acid and stirred magnetically until a homogenous solution was obtained, while 2 g of PVA was dissolved in 100 mL of distilled water and stirred magnetically at 90°C until a homogenous solution was obtained. The samples were labeled as Chit/C and PVA/C for the chitosan nanocomposites and PVA nanocomposite film photocatalysts, respectively. They were characterized spectroscopically by recording FT-IR spectra using Fourier-transform infrared (FT-IR) spectroscopy 1000, PerkinElmer ranging 4000–400 cm−1, and the X-ray diffraction using PRO X-ray diffractometer made in Holland.


### 2.2.2. Photodegradation Studies

In general, photodegradation studies were performed via a fixed surface area (1.5 cm × 1.5 cm) of the films, after first placing them in a 250 mL Erlenmeyer flask. Then, 50 mL of the standard dye solution was added to the flask and stirred under dark conditions in a photocatalytic unit for 30 min to ensure an adsorption-desorption equilibrium is attained. A high-pressure mercury lamp (Philips) of 20–120 W emitting visible light irradiation was placed above the dye solution at a fixed distance of 20 cm. The solution was stirred continuously, using a magnetic stirrer (model Thermo Cimarec) at 25°C. The absorption of the samples was measured atλ = 502 nm (using UV-Vis spectrophotometer Jenway-6800, UK) (Figure 2). The degradation yield can be defined as follows [15]:(1)degradationyield%=C0−CeC0×100,where C0 is the initial dye concentration and Ce is the concentration of dye after photodegradation (mg/L).Figure 2
UV-Vis absorption spectrum of CR.To study the effect of pH, a series of solutions with a constant concentration of 8 mg/L of the dye with a pH ranging from 4 to 8 using Hydrolab (WTW multi 340i set, Germany) were used. The solutions were placed in a photocatalytic unit at room temperature (25°C) for a constant contact time of 120 min; the light intensity was 525 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.To study the effect of the dye concentration, and at 25°C, about 50 mL of the solution with different dye concentrations of 2, 4, 6, 8, 10, and 20 mg/L at a fixed pH of 6 was used, and then, the catalyst was added. Before irradiation, the system was magnetically stirred for 30 min under dark conditions to establish the appropriate adsorption-desorption equilibrium between the catalytic surface and the dye. The irradiation time was limited to 120 min with a light intensity of 525 W/m2. The absorbance of the solution before and after the irradiation was measured using the spectrophotometer.To investigate the effect of contact time, a constant concentration of 8 mg/L of the dye with pH adjusted to 6 was placed in the photocatalytic unit at room temperature (25°C). This was done for different contact times ranging from 15 to 180 min with the light intensity set at 525 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.Finally, the effect of light intensity was studied using approximately 50 mL of solution with a fixed dye concentration of 8 mg/L at 25°C and a fixed pH of 6, followed by the addition of a catalyst. Before irradiation, the system was magnetically stirred for 30 min under dark conditions to establish the appropriate adsorption-desorption equilibrium between the catalytic surface and the dye. The irradiation time was limited to 120 min with different light intensity values including 20, 40, 60, and 80 W (175, 350, 525, and 700 W/m2, respectively) using different mercury lamps; each lamp had a power rating of 20 W and emitted a light intensity of 175 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.


## 2.2.1. Preparation of Nanocomposite Films

To prepare the polymer nanocomposite film, the nanoparticle carbon materials were spread out over the polymer matrix in a solution form, and the solution was stirred magnetically under a suitable temperature for 3 h. Next, the mixture was cast into a Petri dish and left at ambient temperature to dry, and the solvent was allowed to evaporate completely (Figure1).Figure 1
A photograph of the prepared films.To prepare the polymer solutions, 2 g of chitosan was dissolved in 100 mL of 2% acetic acid and stirred magnetically until a homogenous solution was obtained, while 2 g of PVA was dissolved in 100 mL of distilled water and stirred magnetically at 90°C until a homogenous solution was obtained. The samples were labeled as Chit/C and PVA/C for the chitosan nanocomposites and PVA nanocomposite film photocatalysts, respectively. They were characterized spectroscopically by recording FT-IR spectra using Fourier-transform infrared (FT-IR) spectroscopy 1000, PerkinElmer ranging 4000–400 cm−1, and the X-ray diffraction using PRO X-ray diffractometer made in Holland.


## 2.2.2. Photodegradation Studies

In general, photodegradation studies were performed via a fixed surface area (1.5 cm × 1.5 cm) of the films, after first placing them in a 250 mL Erlenmeyer flask. Then, 50 mL of the standard dye solution was added to the flask and stirred under dark conditions in a photocatalytic unit for 30 min to ensure an adsorption-desorption equilibrium is attained. A high-pressure mercury lamp (Philips) of 20–120 W emitting visible light irradiation was placed above the dye solution at a fixed distance of 20 cm. The solution was stirred continuously, using a magnetic stirrer (model Thermo Cimarec) at 25°C. The absorption of the samples was measured atλ = 502 nm (using UV-Vis spectrophotometer Jenway-6800, UK) (Figure 2). The degradation yield can be defined as follows [15]:(1)degradationyield%=C0−CeC0×100,where C0 is the initial dye concentration and Ce is the concentration of dye after photodegradation (mg/L).Figure 2
UV-Vis absorption spectrum of CR.To study the effect of pH, a series of solutions with a constant concentration of 8 mg/L of the dye with a pH ranging from 4 to 8 using Hydrolab (WTW multi 340i set, Germany) were used. The solutions were placed in a photocatalytic unit at room temperature (25°C) for a constant contact time of 120 min; the light intensity was 525 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.To study the effect of the dye concentration, and at 25°C, about 50 mL of the solution with different dye concentrations of 2, 4, 6, 8, 10, and 20 mg/L at a fixed pH of 6 was used, and then, the catalyst was added. Before irradiation, the system was magnetically stirred for 30 min under dark conditions to establish the appropriate adsorption-desorption equilibrium between the catalytic surface and the dye. The irradiation time was limited to 120 min with a light intensity of 525 W/m2. The absorbance of the solution before and after the irradiation was measured using the spectrophotometer.To investigate the effect of contact time, a constant concentration of 8 mg/L of the dye with pH adjusted to 6 was placed in the photocatalytic unit at room temperature (25°C). This was done for different contact times ranging from 15 to 180 min with the light intensity set at 525 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.Finally, the effect of light intensity was studied using approximately 50 mL of solution with a fixed dye concentration of 8 mg/L at 25°C and a fixed pH of 6, followed by the addition of a catalyst. Before irradiation, the system was magnetically stirred for 30 min under dark conditions to establish the appropriate adsorption-desorption equilibrium between the catalytic surface and the dye. The irradiation time was limited to 120 min with different light intensity values including 20, 40, 60, and 80 W (175, 350, 525, and 700 W/m2, respectively) using different mercury lamps; each lamp had a power rating of 20 W and emitted a light intensity of 175 W/m2. The absorbance of the solution before and after the irradiation was measured using a spectrophotometer.


## 3. Results and Discussion

### 3.1. Characterization

Figure3 shows the FT-IR spectra of the polymers and the prepared nanocomposite films, and Table 1 summarizes their spectral data. The IR characteristics of the original chitosan are 3448, 2925, 2369, 1655, 1405, and 1077 cm−1, assigned to O-H stretching overlapped with N-H stretching, aliphatic C-H stretching, in plane N-H bending, C-O stretching vibration of primary alcoholic group [16, 17], and stretching vibration of hydroxy group ([18, 19] cited in [17]), respectively. These values are shifted to 3425, 2924, 2370, 1546, 1406, and 1069 cm−1 of the Chit/C film, whereas the IR characteristics of the original PVA are 3404, 2926 and 2369, 1656, 1433, 1095, and 670 cm−1 assigned to O-H stretching, CH2 asymmetric and symmetric stretching, C-O stretching, C-H bending, and C-H outside the plan bond, respectively [20], shifted to 3400, 2935, 2370, 1437, 1092, and 669 cm−1, respectively, of the film of PVA/C owing to incorporation of the carbon particles with the polymers.Figure 3
FT-IR spectra of chitosan, PVA, and the prepared films. (a) Chit, (b) Chit/C, (c) PVA, and (d) PVA/C.
(a)
(b)
(c)
(d)Table 1
Spectral data of chitosan, PVA, and the prepared nanocomposite films.
Chitosan
Chit/C
Interpretation
PVA
PVA/C
Interpretation
Position (cm−1)
Position (cm−1)
3448
3425
O-H stretching overlapped with N-H stretching
3404
3400
O-H stretching
2925
2924
Aliphatic C-H stretching
2926
2935
CH2 asymmetric and symmetric stretching
2369
2370
2369
2370
1655
1546
In plane N-H bending
1433
1437
C-H bending
1405
1406
C-O stretching vibration of primary alcoholic group
1095
1092
C-O stretching
1077
1069
Stretching vibration of hydroxy group
670
669
C-H outside the plan bondThe XRD pattern of Chit/C, PVC/C, and the original carbon black is shown in Figure4. The pattern of carbon black shows two broad peaks around 25° and 43° assigned to its amorphous structure [21]. As for the Chit/C film, there are three sharp peaks around 2θ = 10.0°, 12.0°, and 20.0° and three broad peaks around 24°, 32°, and 43°. The sharp beaks at 10.0° and 20.0° are assigned to crystal morphology of chitosan [22], whereas the broad and weak peaks at 24° and 43° are assigned to residual of the carbon black, and the other peaks at 12.0° and 32° are assigned to the formation of nanocomposites. As for the PVA/C, the pattern revealed an amorphous form with three board peaks around 20° and 30°, assigned to the nanocomposite film, and the peak around 40° is assigned to the carbon black, remaining in the nanocomposite. The residual of the carbon black in the XRD pattern of the nanocomposite films of Chit/C and PVA/C suggests that the attraction with the polymer was partial.Figure 4
XRD patterns of carbon black and the prepared films. (a) PVA/C, (b) Chit/C, and (c) carbon black.
(a)
(b)
(c)


### 3.2. Photodegradation Studies

To study the effect of the various parameters on the degradation process, the mechanism of the process should be understood. Overall, the suggested mechanism for the photodegradation process of CR dye is as follows [23, 24].Firstly, the molecules of the CR dye are adsorbed on the photocatalyst surface, which is then irradiated by visible light, leading to the excitation of electrons in the valance band to the conduction band leaving behind positive holes (h+) in the valance band. Subsequently, on the surface of the photocatalyst, the pair of h+ and e− will react with the adsorbed water molecules, dissolved oxygen, and hydroxyl groups of the surface, producing free radicals of hydroxyl and superoxide radicals. The h+ then reacts with water molecules to produce •OH radicals, and the e− reduces the dissolved oxygen to superoxide anion radical O2•−. These photogenerated radicals would degrade the dye molecules, forming intermediate products that completely break into CO2, H2O, and ions of NO3– and NH4+ as expressed in the following equations:The first step (Equation (2)) [23, 24]:(2)Chit./C+hυ⟶Chit./ChVB++eCB−orPVA/C+hυ⟶PVA/ChVB++eCB−The second step (Equations (3) and (4)) [23, 24]:(3)Chit./Ch+orPVA/Ch++H2O⟶H++OH•(4)Chit./Ce−orPVA/Ce−+O2⟶Chit./CorPVA/C+O2•−The third step (Equation (5)) [24]:(5)CRmolecules+OH•+O2•−⟶intermediatecompounds⟶CO2+H2O+NO3−+NH4+In general, the electrical double layer of the solution/solid interface is modified by pH, affecting the adsorption-desorption process, subsequently, releasing the photogenerated pairs e−-h+ on the surface of the photocatalyst ([3, 25] cited in [3]). The absorption process mechanism under the effect of pH could be proposed by electrostatic interaction between the CR molecules and the active sites of the semiconductor surface ([26, 27] cited in [25]), possibly affecting the encounter of the resulting molecules with •OH [26]. Depending on the prevailing forces whether attraction or repulsion, the overall reaction will either improve or restrain ([28] cited in [25]). Thus, pH plays an important role in the photodegradation of the CR dye.Figure5 illustrates the effect of pH on the photocatalytic degradation of CR by both the photocatalyst films. Obviously, the both catalysts mostly follow the same pattern towards the degradation process, wherein the percentage degradation increases with increasing pH until it reaches to 6 then decreases with further increasing in pH. This observation suggests that the strongest electrostatic attraction force between the dye molecules and the surfaces of catalysts is at this value of pH, leading to maximum activity of oxidation, and thereby, high photodegradation efficiency of the CR dye is obtained. The higher adsorption rate obtained at 6 of pH could be explained by the nature of the CR dye itself. The molecule CR dye is a dipolar [1], and it was found that, the color of CR dye solution is changeable from red to dark blue with decreasing pH, suggesting that the ionic character nature of CR dye molecules depend on the medium pH [1]. At the acidic medium, the dye exists as a cationic form and as an anionic form at the basic medium [1]. When the medium of the solution is acidic, the cationic molecules of the CR dye are repulsed with protons ions, resulting in a reduction of the adsorption process. However, the percentage decreases beyond this pH value because more hydroxyl ions are available in the alkaline medium, resulting in repulsion between the negative speciation of the dye molecules and the negatively charged hydroxyl ions.Figure 5
Percentage photodegradation as a function of pH.The mechanism of the adsorption of the CR dye on the Chit/C surface at various pH values could be explained by the electrostatic interaction between the molecules of the dye and Chit/C. In the Chit/C photocatalyst, chitosan contains primary amino groups (–NH2) with the hydroxyl group, making it a highly reactive polysaccharide [7, 9, 29]. In the acidic solution, the reactive –NH2 is easy to protonate to the –NH3+ form, expecting to contribute to the interaction with the ionic CR dye [29]. As mentioned above, CR exists as a cationic form at acidic pH. Therefore, when a solution medium is acidic, the cationic molecules of CR dye are repulsed with the H+ ions and NH3+ ions, resulting in a reduction of the adsorption process on the Chit/C surface. Meanwhile, the adsorption of the CR dye on the PVA/C surface mechanism can be explained as follows.For an acidic medium, the PVA surface is protonated as shown in equation (6) [30], leading to repulsion between the positive charges of the polymer surface and the dye, whereas in an alkaline medium, deprotonation occurs by the release of a free oxygen on the surface of the PVA [30], also leading to electrostatic repulsion as expressed in equation (7) [30]:(6)OH−+H+⟶OH2+OH2++CR+⟶OH2+CR+(7)OH−⟶O−+H+O−+CR+⟶O−CR−Figure6 shows the effect of the initial dye concentrations on the percentage photodegradation by Chit/C and PVA/C catalysts. They almost follow the same pattern. The percentage degradation of the CR dye increases initially with an increase of the concentration until 8 mg/L, because of the availability of the adsorption sites on the photocatalyst surface that enhances the ability of adsorption of the CR molecules. These sites are saturated with time as they get covered with the adsorbed molecules, causing a decline in the percentage of photodegradation with a further increase in the concentration.Figure 6
Percentage photodegradation as a function of initial concentration of CR dye.A similar observation was reported elsewhere [7], which was explained by the increase in the dye concentration that leads to the increase in the dye adsorption on the active sites of the catalyst surface; hence, the competitive adsorption of oxygen on these sites decreases, which leads to a lower formation rate of O2−• and •OH radicals. The study [7] suggested an explanation based on the Beer–Lambert law, wherein the increase in the initial dye concentration decreases the path length of the photons entering the solution, resulting in a lower adsorption for the photon on the catalyst particles, which causes the photodegradation rate to be lower. Another suggested factor proposed by another study [7] is the competition for the photogenerated holes between the adsorbed molecules of water and the CR dye.Figure7 shows the effect of contact time on the photodegradation process for both photocatalysts. Clearly, the behavior of both catalysts seems to be similar. The percentage degradation increases rapidly in the first hour of irradiation exposure because of the enhancement of the excitation process of many electrons from the valence to conduction bands of the semiconductor photocatalysts, generating the pairs e−-h+ in the photocatalyst surface, which are powerful for generating free radical oxidizers. Thereafter, there is a plateau that may result from the consumption of the catalyst.Figure 7
Percentage photodegradation as a function of contact time.Figure8 shows the effect of light intensity on the degradation process. As shown, the patterns of the photodegradation by both the catalysts were initially the same, wherein it linearly increased, after which there was a decrease for the PVA/C photocatalyst, whereas Chit/C continued to increase slightly. The maximum percentage degradation values were 88.4% and 84.2% for Chit/C and PVA/C, respectively.Figure 8
Percentage photodegradation as a function of light intensity.The difference in the behavior and associated values could be attributed to the difference in the photodegradation mechanism. The increase in the photodegradation process with an increase in the light intensity occurred because of the increase in the formation of the pair of e−-h+ and the decrease in the percentage photodegradation for the PVA/C because of recombination of e−-h+.From the above discussion, it is clear that Chit/C and PVA/C mostly behave the same, wherein they both showed a similar trend towards the photodegradation of CR, and the maximum values of percentage photodegradation were very close, as tabulated in Table2, suggesting that the effect of polymeric substrate in our study could be neglected. This is possibly because the adsorbing CR molecules on the photocatalyst surface, which presents a first step in the photodegradation mechanism, occurred through the same bonding forces such as the combination of Van der Waals, hydrogen bonding, and/or of other forces by electrostatic interaction between the CR dye molecules and protonated and deprotonated groups of the polymeric substrate in nanocomposite films. Meanwhile, in other steps of the photocatalysis mechanism, the conductive property of the films is a key factor, wherein the polymeric matrix itself is insolated, and this property is given by the carbon black particles. The presence of carbon black particles lowers the overall resistivity by several orders of magnitude, meaning that the transition of the nanocomposite from insulating to conducting is a function of carbon black concentration, which was fixed in both catalysts. Therefore, in our study, the polymeric substrate, chitosan, and PVA did not play a big role in the photocatalytic degradation of the CR dye.Table 2
Maximum percentage photodegradation (%) of the CR dye by the prepared films with various parameters.
The films
Parameters
pH
Initial concentration
Contact time
Light intensity
Chit/C
85.1
87.6
85.9
88.4
PVA/C
86.4
86
83
84.2


## 3.1. Characterization

Figure3 shows the FT-IR spectra of the polymers and the prepared nanocomposite films, and Table 1 summarizes their spectral data. The IR characteristics of the original chitosan are 3448, 2925, 2369, 1655, 1405, and 1077 cm−1, assigned to O-H stretching overlapped with N-H stretching, aliphatic C-H stretching, in plane N-H bending, C-O stretching vibration of primary alcoholic group [16, 17], and stretching vibration of hydroxy group ([18, 19] cited in [17]), respectively. These values are shifted to 3425, 2924, 2370, 1546, 1406, and 1069 cm−1 of the Chit/C film, whereas the IR characteristics of the original PVA are 3404, 2926 and 2369, 1656, 1433, 1095, and 670 cm−1 assigned to O-H stretching, CH2 asymmetric and symmetric stretching, C-O stretching, C-H bending, and C-H outside the plan bond, respectively [20], shifted to 3400, 2935, 2370, 1437, 1092, and 669 cm−1, respectively, of the film of PVA/C owing to incorporation of the carbon particles with the polymers.Figure 3
FT-IR spectra of chitosan, PVA, and the prepared films. (a) Chit, (b) Chit/C, (c) PVA, and (d) PVA/C.
(a)
(b)
(c)
(d)Table 1
Spectral data of chitosan, PVA, and the prepared nanocomposite films.
Chitosan
Chit/C
Interpretation
PVA
PVA/C
Interpretation
Position (cm−1)
Position (cm−1)
3448
3425
O-H stretching overlapped with N-H stretching
3404
3400
O-H stretching
2925
2924
Aliphatic C-H stretching
2926
2935
CH2 asymmetric and symmetric stretching
2369
2370
2369
2370
1655
1546
In plane N-H bending
1433
1437
C-H bending
1405
1406
C-O stretching vibration of primary alcoholic group
1095
1092
C-O stretching
1077
1069
Stretching vibration of hydroxy group
670
669
C-H outside the plan bondThe XRD pattern of Chit/C, PVC/C, and the original carbon black is shown in Figure4. The pattern of carbon black shows two broad peaks around 25° and 43° assigned to its amorphous structure [21]. As for the Chit/C film, there are three sharp peaks around 2θ = 10.0°, 12.0°, and 20.0° and three broad peaks around 24°, 32°, and 43°. The sharp beaks at 10.0° and 20.0° are assigned to crystal morphology of chitosan [22], whereas the broad and weak peaks at 24° and 43° are assigned to residual of the carbon black, and the other peaks at 12.0° and 32° are assigned to the formation of nanocomposites. As for the PVA/C, the pattern revealed an amorphous form with three board peaks around 20° and 30°, assigned to the nanocomposite film, and the peak around 40° is assigned to the carbon black, remaining in the nanocomposite. The residual of the carbon black in the XRD pattern of the nanocomposite films of Chit/C and PVA/C suggests that the attraction with the polymer was partial.Figure 4
XRD patterns of carbon black and the prepared films. (a) PVA/C, (b) Chit/C, and (c) carbon black.
(a)
(b)
(c)


## 3.2. Photodegradation Studies

To study the effect of the various parameters on the degradation process, the mechanism of the process should be understood. Overall, the suggested mechanism for the photodegradation process of CR dye is as follows [23, 24].Firstly, the molecules of the CR dye are adsorbed on the photocatalyst surface, which is then irradiated by visible light, leading to the excitation of electrons in the valance band to the conduction band leaving behind positive holes (h+) in the valance band. Subsequently, on the surface of the photocatalyst, the pair of h+ and e− will react with the adsorbed water molecules, dissolved oxygen, and hydroxyl groups of the surface, producing free radicals of hydroxyl and superoxide radicals. The h+ then reacts with water molecules to produce •OH radicals, and the e− reduces the dissolved oxygen to superoxide anion radical O2•−. These photogenerated radicals would degrade the dye molecules, forming intermediate products that completely break into CO2, H2O, and ions of NO3– and NH4+ as expressed in the following equations:The first step (Equation (2)) [23, 24]:(2)Chit./C+hυ⟶Chit./ChVB++eCB−orPVA/C+hυ⟶PVA/ChVB++eCB−The second step (Equations (3) and (4)) [23, 24]:(3)Chit./Ch+orPVA/Ch++H2O⟶H++OH•(4)Chit./Ce−orPVA/Ce−+O2⟶Chit./CorPVA/C+O2•−The third step (Equation (5)) [24]:(5)CRmolecules+OH•+O2•−⟶intermediatecompounds⟶CO2+H2O+NO3−+NH4+In general, the electrical double layer of the solution/solid interface is modified by pH, affecting the adsorption-desorption process, subsequently, releasing the photogenerated pairs e−-h+ on the surface of the photocatalyst ([3, 25] cited in [3]). The absorption process mechanism under the effect of pH could be proposed by electrostatic interaction between the CR molecules and the active sites of the semiconductor surface ([26, 27] cited in [25]), possibly affecting the encounter of the resulting molecules with •OH [26]. Depending on the prevailing forces whether attraction or repulsion, the overall reaction will either improve or restrain ([28] cited in [25]). Thus, pH plays an important role in the photodegradation of the CR dye.Figure5 illustrates the effect of pH on the photocatalytic degradation of CR by both the photocatalyst films. Obviously, the both catalysts mostly follow the same pattern towards the degradation process, wherein the percentage degradation increases with increasing pH until it reaches to 6 then decreases with further increasing in pH. This observation suggests that the strongest electrostatic attraction force between the dye molecules and the surfaces of catalysts is at this value of pH, leading to maximum activity of oxidation, and thereby, high photodegradation efficiency of the CR dye is obtained. The higher adsorption rate obtained at 6 of pH could be explained by the nature of the CR dye itself. The molecule CR dye is a dipolar [1], and it was found that, the color of CR dye solution is changeable from red to dark blue with decreasing pH, suggesting that the ionic character nature of CR dye molecules depend on the medium pH [1]. At the acidic medium, the dye exists as a cationic form and as an anionic form at the basic medium [1]. When the medium of the solution is acidic, the cationic molecules of the CR dye are repulsed with protons ions, resulting in a reduction of the adsorption process. However, the percentage decreases beyond this pH value because more hydroxyl ions are available in the alkaline medium, resulting in repulsion between the negative speciation of the dye molecules and the negatively charged hydroxyl ions.Figure 5
Percentage photodegradation as a function of pH.The mechanism of the adsorption of the CR dye on the Chit/C surface at various pH values could be explained by the electrostatic interaction between the molecules of the dye and Chit/C. In the Chit/C photocatalyst, chitosan contains primary amino groups (–NH2) with the hydroxyl group, making it a highly reactive polysaccharide [7, 9, 29]. In the acidic solution, the reactive –NH2 is easy to protonate to the –NH3+ form, expecting to contribute to the interaction with the ionic CR dye [29]. As mentioned above, CR exists as a cationic form at acidic pH. Therefore, when a solution medium is acidic, the cationic molecules of CR dye are repulsed with the H+ ions and NH3+ ions, resulting in a reduction of the adsorption process on the Chit/C surface. Meanwhile, the adsorption of the CR dye on the PVA/C surface mechanism can be explained as follows.For an acidic medium, the PVA surface is protonated as shown in equation (6) [30], leading to repulsion between the positive charges of the polymer surface and the dye, whereas in an alkaline medium, deprotonation occurs by the release of a free oxygen on the surface of the PVA [30], also leading to electrostatic repulsion as expressed in equation (7) [30]:(6)OH−+H+⟶OH2+OH2++CR+⟶OH2+CR+(7)OH−⟶O−+H+O−+CR+⟶O−CR−Figure6 shows the effect of the initial dye concentrations on the percentage photodegradation by Chit/C and PVA/C catalysts. They almost follow the same pattern. The percentage degradation of the CR dye increases initially with an increase of the concentration until 8 mg/L, because of the availability of the adsorption sites on the photocatalyst surface that enhances the ability of adsorption of the CR molecules. These sites are saturated with time as they get covered with the adsorbed molecules, causing a decline in the percentage of photodegradation with a further increase in the concentration.Figure 6
Percentage photodegradation as a function of initial concentration of CR dye.A similar observation was reported elsewhere [7], which was explained by the increase in the dye concentration that leads to the increase in the dye adsorption on the active sites of the catalyst surface; hence, the competitive adsorption of oxygen on these sites decreases, which leads to a lower formation rate of O2−• and •OH radicals. The study [7] suggested an explanation based on the Beer–Lambert law, wherein the increase in the initial dye concentration decreases the path length of the photons entering the solution, resulting in a lower adsorption for the photon on the catalyst particles, which causes the photodegradation rate to be lower. Another suggested factor proposed by another study [7] is the competition for the photogenerated holes between the adsorbed molecules of water and the CR dye.Figure7 shows the effect of contact time on the photodegradation process for both photocatalysts. Clearly, the behavior of both catalysts seems to be similar. The percentage degradation increases rapidly in the first hour of irradiation exposure because of the enhancement of the excitation process of many electrons from the valence to conduction bands of the semiconductor photocatalysts, generating the pairs e−-h+ in the photocatalyst surface, which are powerful for generating free radical oxidizers. Thereafter, there is a plateau that may result from the consumption of the catalyst.Figure 7
Percentage photodegradation as a function of contact time.Figure8 shows the effect of light intensity on the degradation process. As shown, the patterns of the photodegradation by both the catalysts were initially the same, wherein it linearly increased, after which there was a decrease for the PVA/C photocatalyst, whereas Chit/C continued to increase slightly. The maximum percentage degradation values were 88.4% and 84.2% for Chit/C and PVA/C, respectively.Figure 8
Percentage photodegradation as a function of light intensity.The difference in the behavior and associated values could be attributed to the difference in the photodegradation mechanism. The increase in the photodegradation process with an increase in the light intensity occurred because of the increase in the formation of the pair of e−-h+ and the decrease in the percentage photodegradation for the PVA/C because of recombination of e−-h+.From the above discussion, it is clear that Chit/C and PVA/C mostly behave the same, wherein they both showed a similar trend towards the photodegradation of CR, and the maximum values of percentage photodegradation were very close, as tabulated in Table2, suggesting that the effect of polymeric substrate in our study could be neglected. This is possibly because the adsorbing CR molecules on the photocatalyst surface, which presents a first step in the photodegradation mechanism, occurred through the same bonding forces such as the combination of Van der Waals, hydrogen bonding, and/or of other forces by electrostatic interaction between the CR dye molecules and protonated and deprotonated groups of the polymeric substrate in nanocomposite films. Meanwhile, in other steps of the photocatalysis mechanism, the conductive property of the films is a key factor, wherein the polymeric matrix itself is insolated, and this property is given by the carbon black particles. The presence of carbon black particles lowers the overall resistivity by several orders of magnitude, meaning that the transition of the nanocomposite from insulating to conducting is a function of carbon black concentration, which was fixed in both catalysts. Therefore, in our study, the polymeric substrate, chitosan, and PVA did not play a big role in the photocatalytic degradation of the CR dye.Table 2
Maximum percentage photodegradation (%) of the CR dye by the prepared films with various parameters.
The films
Parameters
pH
Initial concentration
Contact time
Light intensity
Chit/C
85.1
87.6
85.9
88.4
PVA/C
86.4
86
83
84.2


## 4. Conclusion

Chitosan/carbon black and polyvinyl alcohol/carbon black nanocomposite films were prepared and characterized using Fourier-transform infrared (FT-IR) spectra and X-ray diffraction (XRD) techniques. Visible light was used as a source of light for the photocatalytic degradation process under various operational conditions of pH, dye concentration, contact time, and light intensity. The findings showed that the role of the polymeric matrix on the photodegradation of the dye was not very impressive. Furthermore, the photodegradation process improves as the light intensity was high and on increasing the contact time. The best operational conditions of the pH and concentration dye solution were at 6 and 8 mg/L, respectively. The results showed that these films could be good materials as photocatalysts and applicable for the photodegradation of dye pollutants, particularly Congo red.



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*Source: 9651850-2018-12-02.xml*