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.
Dyes are extensively used in the industrial sector such as the textile industry, rubber, paper, leather, and cosmetics [
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 [
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 [
Currently, there is a tendency to use semiconductor polymer nanocomposite systems as photocatalysts. Several researchers have published works based on this approach [
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 [
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 [
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 [
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.
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 (Figure
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.
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 at
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.
Figure
FT-IR spectra of chitosan, PVA, and the prepared films. (a) Chit, (b) Chit/C, (c) PVA, and (d) PVA/C.
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 bond |
The XRD pattern of Chit/C, PVC/C, and the original carbon black is shown in Figure
XRD patterns of carbon black and the prepared films. (a) PVA/C, (b) Chit/C, and (c) carbon black.
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 [
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 (
The second step (Equations (
The third step (Equation (
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 ([
Figure
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 [
For an acidic medium, the PVA surface is protonated as shown in equation (
Figure
Percentage photodegradation as a function of initial concentration of CR dye.
A similar observation was reported elsewhere [
Figure
Percentage photodegradation as a function of contact time.
Figure
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 Table
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 |
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.
All the experimental results that we have obtained for this study were presented in this manuscript, and their discussion is supported by the references cited. I have no other data.
The authors declare that there are no conflicts of interest regarding the publication of this article.
The authors would like to thank the Environmental Pollution Chair at Princess Nourah Bint Abdulrahman University for financial support.