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Article

Chitosan/Polyvinyl Alcohol/g-C3N4 Nanocomposite Film: An Efficient Visible Light-Responsive Photocatalyst and Antimicrobial Agent

by
Murugan Sutharsan
1,
Krishnan Senthil Murugan
1,*,
Kanagaraj Narayanan
1 and
Thillai Sivakumar Natarajan
2,3,*
1
PG & Research Department of Chemistry, Sri Paramakalyani College (Affiliated to Manonmaniam Sundaranar University, Tirunelveli), Alwarkurichi 627 412, Tamil Nadu, India
2
Environmental and Clean Energy Research Group, Environmental Science Laboratory, CSIR-Central Leather Research Institute (CSIR-CLRI), Adyar, Chennai 600 020, Tamil Nadu, India
3
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 600 113, Uttar Pradesh, India
*
Authors to whom correspondence should be addressed.
Processes 2025, 13(1), 229; https://doi.org/10.3390/pr13010229
Submission received: 28 November 2024 / Revised: 8 January 2025 / Accepted: 10 January 2025 / Published: 15 January 2025
(This article belongs to the Special Issue Nanomaterials for Environmental Remediation Processes)
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Abstract

:
Biopolymer-based nanocomposite film is an efficient material for addressing the increasing levels of pollutants in the environment and also for the production of antimicrobial packing material due to its good film-forming properties, biodegradability, and minimal environmental impact. In particular, chitosan/polyvinyl alcohol/g-C3N4 (CS/PVA/g-C3N4) nanocomposite films with different weight percentages of PVA were prepared using simple methodologies and characterized using XRD, TGA, FT-IR, DSC, FE-SEM, EDX, and elemental mapping analysis. The XRD and FT-IR results validated the nanocomposite film formation. The FE-SEM images showed the smooth surface of the composite films without any wrinkles; the smoothness of the film increased with increases in the PVA loading, and the surface morphologies of the films were largely unchanged. The EDX and elemental mapping analysis validated the presence and uniform dispersion of g-C3N4 within the nanocomposite film. The photocatalytic activity of the CS/PVA/g-C3N4 composite films was assessed by the degradation of rhodamine B dye (RhB) and acetophenone under direct sunlight irradiation. The CS/PVA/g-C3N4 nanocomposite films exhibited superior degradation efficiency toward the RhB dye and acetophenone compared to the bare polymeric film and the g-C3N4 material. The order of degradation for the RhB dye and acetophenone was CS/PVA (1.0) g-C3N4 (95.34%, 33.33%) > CS/PVA (1.5) g-C3N4 (93.18%, 31.31%) > CS/PVA (0.5) g-C3N4 (93.02%, 29.29%) > CS/PVA (90.69%, 26.26%) > g-C3N4 (87.56%, 24%), respectively. Furthermore, the antimicrobial activity of the nanocomposite films was tested against E. coli, Pseudomonas sps., Klesiella sps., and Enterococcus sps., and the CS/PVA (1.5)/g-C3N4 nanocomposite film offered better antimicrobial properties than the other composite films and bare materials. In conclusion, these biopolymer-based nanocomposites are highly efficient and provide a promising path for the development of various biodegradable polymeric nanocomposites for environmental remediation and antibacterial packing applications.

Graphical Abstract

1. Introduction

Rapid population growth and the expansion of industrial activities have exacerbated serious environmental issues, notably water pollution, leading to widespread challenges in accessing clean water. Majorly, industries like chemical manufacturing, pharmaceuticals, textiles, tanneries, petroleum, etc., are continuing to release substantial quantities of polluted wastewater containing hazardous materials like dyes, drugs, acids, paints, oils, other organics, heavy metals, and volatile organic compounds, which creates significant health and environmental threats [1,2,3]. Various conventional chemical, physical, and biological methods are available for treating the wastewater containing pollutants. Subsequently, various modifications and integrations of these techniques have been performed to improve degradation efficiency. Nevertheless, these methods are inefficient in the complete removal of pollutant molecules from wastewater, which limits their commercial-scale applications [4,5,6].
In addition to inorganic and organic water pollutants, wastewater contains excessive amounts of gram-positive and gram-negative bacteria, which leads to severe health problems and even death in human beings and animals. Moreover, food packaging mainly relies on polyethylene terephthalate, polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, etc.-based films, which are difficult to recycle, and these films offer no antibacterial activity [7]. This has led to the exploration of biodegradable and recyclable biopolymer-based nanocomposite based films with excellent antibacterial properties. Chitosan (CS) is especially actively used for food packaging applications due to its nontoxicity, biodegradability, biocompatibility, excellent film-forming capabilities, bactericidal properties, and high absorbance ability. However, the poor mechanical properties and limited antibacterial activity of pure chitosan films limits their practical use. To address these issues, the coupling of CS with water-soluble polyvinyl alcohol (PVA) has received significant attention, which has helped to improve the mechanical strength and the antimicrobial properties of the resulting composite films [8,9,10]. Nevertheless, the upscaling of the antimicrobial activity of binary CS/PVA composite films is limited, thus necessitating the coupling with other visible light-responsive nanomaterials to produce ternary biopolymer nanocomposite-based antimicrobial agents. The coupling of light-responsive nanomaterials induces the production of reactive oxygen species that help to enhance the antibacterial activity of CS/PVA/nanomaterial-based composite films and also assist in pollutant degradation efficiency [11,12,13].
Similarly, advanced oxidation processes (AOPs) are a group of wastewater treatment techniques that generate highly reactive hydroxyl radicals that efficiently break down persistent contaminants that are resistant to conventional treatment methods, thereby improving overall removal efficacy [14,15]. Semiconductor photocatalysis is an efficient AOP for the treatment of wastewater. Photocatalysts based on various metal oxides (e.g., TiO2, ZnO, WO3, etc.) and sulfides (e.g., CdS, ZnS, etc.) and their composites have been developed for the degradation of pollutants [16,17]. However, these photocatalysts suffer from low photon-utilization efficiency, poor visible-light absorbance (e.g., wide band gaps), and degradation over prolonged time of reaction and stability issues (photo-corrosion). Subsequently, the development of non-metal-based, visible light-responsive semiconductors has increased and shown superior catalytic activity in wastewater treatment [18,19].
Graphitic carbon nitride (g-C3N4) is a non-metal semiconductor with a low band gap (~2.7 eV) that enables effective visible-light absorption. Its unique combination of high stability, nontoxicity, and ease of synthesis makes it a highly promising semiconductor for environmental remediation and renewable energy production under solar light irradiation [20,21,22,23]. In addition, g-C3N4 nanomaterials have shown excellent bacterial inhibition efficiency under visible-light irradiation. Nevertheless, there are some common problems associated with g-C3N4-based systems, such as fast recombination of photo-generated charge carriers, low quantum efficiency, low surface area, and poor adsorption capacity [24]. Therefore, advances in modifying the g-C3N4 structure—specifically, coupling it with polymeric materials (CS, PVA, etc.)—can address these shortcomings and enhance practical applications [25,26]. Specifically, the production of binary composite films by the coupling of g-C3N4 with PVA and CS was performed separately and showed excellent antimicrobial activity, antibiofilm activity, and environmental pollutant degradation efficiency [27,28,29,30]. However, the swelling phenomenon of PVA-based materials has led to poor stability and mechanical strength in aqueous solutions and decreased the activity of g-C3N4/PVA and g-C3N4/CS composite films [31,32]. In addition, the antibacterial activity and pollutant degradation efficiency of binary composites are lacking in practical scale-level implementation. In order to overcome these factors, the development of ternary nanocomposites by the coupling of g-C3N4 with CS/PVA composite biopolymer films has received significant attention in the scientific community [10,12,13,29,33]. The cationic chitosan biopolymer is rich in hydroxyl and amino groups, which has improved the adsorption capacity and the removal efficiency toward environmental pollutants [26,34,35,36,37]. Furthermore, chitosan carries a positive charge under acidic conditions, enabling it to interact with the negatively charged cell membranes of bacteria, and PVA benefits occur in the formation of films that facilitate the controlled release of the antimicrobial agent (chitosan), thereby extending its effectiveness. Also, the coupling of g-C3N4 may further improve the visible-light responses of CS/PVA films, which would enhance the antimicrobial properties by introducing oxidative stress or disrupting microbial membranes [38,39]. Therefore, a ternary g-C3N4/CS/PVA nanocomposite film would be an efficient photocatalyst and antibacterial agent. Nevertheless, the making of ternary nanocomposite films by the coupling of g-C3N4 with chitosan and PVA polymers and their visible-light photocatalytic activity under direct sunlight irradiation and antimicrobial activity against gram-positive and gram-negative bacteria have rarely been investigated in the literature.
Herein, ternary biopolymer CS/PVA/g-C3N4 nanocomposite films with different weight percentages of PVA are synthesized by simple methodologies and their structural and morphological properties are studied using various physiochemical techniques. The CS/PVA/g-C3N4 nanocomposite films showed excellent photocatalytic activity toward RhB dye and acetophenone degradation under direct sunlight irradiation, as well as bacterial inhibition against gram-positive and -negative bacteria. The results conclude that synthesized biopolymer-based ternary nanocomposite films are highly efficient and pave the way for the development of biodegradable polymeric nanocomposite films for environmental remediation and antibacterial applications.

2. Materials and Methods

2.1. Chemicals

Chitosan (CS, 3800–20,000; Dalton’s degree of deacetylation: >75%) and polyvinyl alcohol (hot water-soluble; average molecular weight: 70,000 to 100,000) were purchased from Hi-Media Laboratories Pvt. Ltd., Thane, Maharashtra, India. Melamine (AR, 98%), acetic acid (99.5%), and rhodamine B dye (95%) were purchased from Isochem Laboratories, Tamil Nadu, India. Acetophenone (98%) was purchased from Thermo Fisher Scientific India Pvt. Ltd., Nashik, Maharashtra, India. Double-distilled water was used for the preparation of all experimental solutions.

2.2. Synthesis of g-C3N4

g-C3N4 was synthesized by the direct combustion of melamine, following the procedure mentioned in our earlier report with slight modification [27,40,41]. Briefly, the melamine (5 g) was placed in an alumina crucible, which was covered with a lid and heated at 500 °C for 4 h in a muffle furnace (Scheme 1). After the heating period, the crucible was allowed to cool to room temperature. The resulting yellow product was then ground into a fine powder using a mortar and pestle and stored for subsequent nanocomposite film synthesis.

2.3. Preparation of CS/PVA and CS/PVA/g-C3N4 Composite Films

The schematic representation of the CS/PVA and CS/PVA/g-C3N4 composite film synthesis is shown in Scheme 2. Initially, 0.5 g of chitosan (CS) was dissolved in 20 mL of 1% (vol%) acetic acid solution and stirred at room temperature (approximately overnight) until fully dissolved. Concurrently, polyvinyl alcohol (PVA) was dissolved in hot water at varying volume ratios (10, 20, and 30 mL) and vigorously stirred at room temperature to form a clear solution. The PVA solution was then combined with the CS solution and stirred for 2 h to achieve a homogeneous mixture. To prepare the CS/PVA composite films, the resulting gel-like solution was poured into a dust-free petri dish and dried at 60 °C for 7 h in a vacuum oven. Afterward, the formed films were slowly peeled off from the petri dish using forceps and stored for further characterization and application. The synthesized film was abbreviated as CS/PVA.
The g-C3N4 solution was prepared by dispersing the synthesized g-C3N4 (10 mg) using sonication and adding it into the CS/PVA solution mixture, stirring at room temperature for 2 h to obtain a homogeneous CS/PVA/g-C3N4 solution mixture. The prepared gel-like CS/PVA/g-C3N4 solution was carefully poured into a dust-free petri-dish and dried at 60 °C for 7 h in a hot-air oven (Make: Digiqual, Chennai, India). Similarly, CS/PVA/g-C3N4 nanocomposite films with different PVA loadings were prepared and denoted as CS/PVA (X − 0.5, 1 and 1.5)/g-C3N4. The synthesized composite films were stored in an airtight glass vial that was preserved in a desiccator for photo-degradation reaction [42].

2.4. Characterization

The crystal structure and formation of each composite film was analyzed by X-ray diffraction using a Rich Siefert 3600 diffractometer with Cu K α 1 radiation (λ = 1.5404 Å) in the 2θ range of 5–80°. The morphology, elemental composition, and mapping of the films were assessed using a field emission–scanning electron microscope (FE-SEM, TESCON CLARA, Brno-Kohoutovice, Czech Republic) along with an EDX attachment. The functional group identification of the bare and composite films was analyzed by Fourier transform infrared spectroscopy analysis (Nicolet IS5R FT-IR spectrometer, Brno-Kohoutovice, Czech Republic). The thermal stability of the films was determined using thermogravimetric analysis (TGA) using TGA Q50 (V20.6 Build 31) in a nitrogen environment in the temperature range of 30 to 800 °C with a temperature gradient of 10 °C min−1. Similarly, differential scanning calorimetry (DSC) of the composite materials was performed in a nitrogen atmosphere in the temperature range of 30 °C to 360 °C using TGA Q50 (New Castle, DE, USA, V20.6 Build 31).

2.5. Visible-Light Photocatalytic Activity

The visible-light photocatalytic activities of the synthesized CS/PVA/g-C3N4 composite films with various concentrations of PVA (0.5 wt%, 1 wt%, 1.5 wt%) were evaluated by the degradation of aqueous solution of RhB dye (10 mg/L) and acetophenone (10 mg/L) under direct solar light irradiation. A pictorial representation of the experimental setup for the degradation reaction is shown in Figure 1. The degradation reaction was carried out in the months of April and May from 10.30 a.m. to 4.30 p.m. The RhB dye solution (10 mg/L, 100 mL), acetophenone (10 mg/L, 100 mL), and required amounts of composite films (CS/PVA, CS/PVA (0.5)/g-C3N4, CS/PVA (1)/g-C3N4, CS/PVA (1.5)/g-C3N4; 100 mg) were separated in beakers. To facilitate the adsorption–desorption equilibrium, the reaction mixture was kept in a dark atmosphere for 30 min, followed by irradiation under direct sunlight. The reaction samples were collected using a syringe with a time interval of 1 h, and the concentrations of RhB dye and acetophenone were estimated by determining the absorbances at of 554 nm and 245 nm using a UV–visible spectrophotometer (UV-2600, Shimadzu, Kyoto, Japan). The degradation percentages of the RhB dye and acetophenone were determined using the following formula:
Percentage degradation (%) = {C0 − Ct/C0} × 100
where C0 is the initial concentration and Ct is the final concentration of the pollutants at time t (ppm).

2.6. Antimicrobial Activity

The antibacterial activity of the nanocomposite films was tested against gram-negative bacterial species such as Pseudomonas sps. and Klesiella sps. and gram-positive bacteria such as Enterococcus sps. and E. coli using the inhibition circle method. Mueller–Hinton agar petri dish plates were prepared and sterilized for 20 min. The prepared media were poured onto the plates to allow solidification. After solidification, the bacterial cultures were swabbed over the prepared petri dishes, and the CS/PVA, CS/PVA (0.5)/g-C3N4, CS/PVA (1)/g-C3N4, and CS/PVA (1.5)/g-C3N4 composites were placed on the surfaces of the agar plates. The plates were incubated for 24 h at 37 °C in an incubator. After incubation, the inhibition of the bacterial growth was measured in terms of the diameter of the zone and the antimicrobial efficiency was calculated.

3. Results and Discussion

3.1. XRD Analysis

The XRD patterns of the g-C3N4, CS/PVA, and CS/PVA/g-C3N4 nanocomposite films with different concentrations of PVA are shown in Figure 2. The XRD pattern of g-C3N4 presents two diffraction peaks at the 2θ values of 12.78° and 27.52°, which are indexed to the (100) and (002) crystal planes of the graphite-like structure of g-C3N4. Furthermore, these peaks are associated with the tri-s-triazine unit in g-C3N4 and reflection from the inter-planar stacking of the conjugated aromatic systems and the structural packing of the (002) plane in graphite-like materials [40,43].
For the CS/PVA/g-C3N4 nanocomposite films, the diffraction peak at the 2θ value of 18° is indexed to the PVA [44] and the 2θ values of 10°, 15°, and 20° correspond to the chitosan materials [45,46], which validates the presence of chitosan and PVA in the nanocomposite film. Furthermore, the diffraction peak intensities at 2θ of 15°, 18°, and 20° increased with increases in the PVA concentration and shifted slightly to the higher diffraction angle, which confirmed the successful formation of the composite film. Additionally, the shift in the diffraction-peak position as a function of PVA loading indicates that there might have been a structural change in the crystalline domains in the polymer matrix. Similarly, Thurston et al. observed changes in the crystal structure in the development of g-C3N4/PVA composite films [27]. Also, the diffraction peak (2θ = ~12–13°) corresponding to the g-C3N4 in the composite films was merged with the PVA peak, and another diffraction peak at a 2θ value of 27–28° was not observed in the composite films, which may be due to the low concentration of g-C3N4 [41,47].

3.2. FE-SEM and Elemental Mapping Analysis

The surface morphologies of the synthesized nanocomposite films were analyzed by FE-SEM analysis (Figure 3). Figure 3 reveals the smooth, uniform surface of the films, and the smoothness of the surface increased with increases in the PVA percentage loading. A similar smooth surface of a film was observed by Liu et al. in the synthesis of a CS/PVA/g-C3N4 composite film [10]. The nanocomposite film composition was determined using elemental mapping analysis, and the results are shown in Figure 4 [48]. The results demonstrate that added g-C3N4 has good compatibility with the CS-PVA matrix and was uniformly distributed on the surface of the composite film [10,41]. Also, the synthesized composite film was highly pure, and no other impurities were observed. The FE-SEM and mapping results corroborate the successful synthesis of the CS/PVA/g-C3N4 composite films and match with the XRD results.

3.3. FT-IR Analysis

The functional groups present in the surfaces of the prepared composite (CS/PVA/g-C3N4) films were identified using FTIR spectroscopy, and the results are shown in Figure 5 and Table S1. The characteristic peak at 806 cm−1 corresponds to the characteristic peak of the triazine rings of the g-C3N4. Furthermore, the wave member in the region of 1230–1640 cm−1 are indexed to the C-N bond stretching vibration. The observed aromatic peaks at 1638 cm−1 and 1407 cm−1 are indexed to the C-N and C=N bond vibrations of the g-C3N4 material. For the CS/PVA/g-C3N4 film, the peaks at 3256 cm−1 and 2939 cm−1 correspond to the –OH and –C-H stretching vibrations of the composite films. The broadness of this peak decreased and shifted to lower wavenumbers with increases in PVA concentration. The shift in vibrational frequencies was attributed to the formation of new hydrogen bonding with the PVA polymer matrix and the g-C3N4 particles [10,27]. Also, the peaks at 848 cm−1, 1419 cm−1, and 1090 cm−1 were assigned to the C=C deformations, C-H bending, and C-O-H stretching vibrations of the PVA in the composite film. Moreover, the peaks at 1040 cm−1 and 1670 cm−1 are mapped to the C-O and C=O stretching vibrations of the chitosan materials in the composite film. The FT-IR results validate the successful synthesis of the CS/PVA/g-C3N4 composite films in accordance with the XRD and FE-SEM results.

3.4. Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) was used to measure the temperature (heat absorbed or desorbed) through the physical or chemical alterations of the synthesized CS/PVA/g-C3N4 nanocomposite films, and the results are given in Figure 6. The results demonstrate that the broadness of the curve increased with an increase in the PVA loading. Each sample showed an initial endothermic peak in the range of 98 °C to 123 °C, indexed to the adsorbed water. The second peak, in the range of 215–265 °C, was attributed to the cross-linking of the chitosan and the PVA with the g-C3N4. Specifically, the peak at ~220 °C was assigned to the crystalline polymer fraction of the PVA in the composites [49]. The third exothermic peak, at ~280 °C, could be linked to the decomposition of the amine units of the chitosan from the polymer matrix [50]. It was also observed that the crystallinity of the composite films was decreased compared to the bare CS/PVA film. This may be attributed to the increase in the PVA loading enhancing the polymer–polymer interaction. Also, the hydrogen bonding between the PVA and the chitosan disrupted the crystallinity of the composite films [51].

3.5. Thermogravimetric Analysis

The thermal stability and behavior of the synthesized CS/PV/g-C3N4 nanocomposite films were analyzed by TGA analysis, and the results are shown in Figure 7. The results reveal that three decomposition phases were observed in the TGA analysis of the composite films. The first weight loss, in the temperature range of 100–200 °C, is attributed to the loss of adsorbed water molecules from the surface. The second weight loss, in the temperature range of 200–350 °C, is ascribed to the thermal degradation of the PVA and chitosan polymer structure. The third stage of weight loss, in the temperature range of 350–450 °C, was because of the CS/PVA backbone degradation [49,52,53].

3.6. UV–Visible Spectroscopy Analysis

The UV–visible spectra of the CS/PVA/g-C3N4 nanocomposite films are displayed in Figure 8. It is well-known from the literature that pristine g-C3N4 has a distinctively high absorption peak at 425–450 nm with band gap values of 2.75–2.90 eV. All the samples that showed absorption peaks at 260 nm and 296 nm corresponded to the atomic n- π* and π-π* inter-range shifts [54]. The CS/PVA absorption spectra revealed a distinctive peak with a maximum frequency of 217 nm [55]. The band gap of the composite films was calculated using the Kubelka–Munk function. The band gap of the bare g-C3N4 material was 2.84 eV, and the band gap energy values of the CS/PVA decreased with increases in the PVA loading: from 2.91 eV (CS/PVA) to 2.89, 2.82, and 2.80 eV for the CS/PVA (0.5) g-C3N4, CS/PVA (1.0) g-C3N4, and CS/PVA (1.5) g-C3N4 nanocomposite films, respectively. The decrease in the band gap energy values may be ascribed to the synergistic interaction between the CS/PVA and the g-C3N4 in the CS/PVA/g-C3N4 composite films. The reduction in the band gap was also due to the shift in the electronic stage of the CS/g-C3N4 in the composite film that reduced the charge transfer pathway between the conduction and valence bands of the composite photocatalytic films [32,56].

3.7. Photocatalytic Activity

The catalytic efficiency of the synthesized g-C3N4 and CS/PVA (0, 0.5, 1.0, 1.5) g-C3N4 nanocomposite films were evaluated by the degradation of an aqueous solution of RhB dye and acetophenone pollutants in the presence of direct sunlight irradiation (Figure 9). The UV–visible spectra of the RhB and acetophenone degradation at different time intervals are shown in Figures S1 and S2, and the maximum absorbances at the wavelengths of 553 nm and 246 nm were used for the determination of the degradation percentages at different time intervals [57,58]. In the presence of only light irradiation (photolysis), negligible amounts of RhB dye and acetophenone were degraded, validating that the selected model pollutants are highly stable, which validated the requirement of those catalytic materials for destruction. Figures S1 and S2 reveal that significant decreases in the absorbance intensities of the RhB and acetophenone in the presence of CS/PVA (1)/g-C3N4 composite film were observed compared to the bare CS/PVA film and the g-C3N4 material, suggesting that CS/PVA (1)/g-C3N4 had the optimal ratio favorable for degradation performance. The degradation percentages of the RhB dye and acetophenone were increased with increases in the PVA loading and reached the maximum degradation percentages for CS/PVA (1)/g-C3N4. The CS/PVA (1)/g-C3N4 composite showed 95.34% and 33.33% of the RhB dye and acetophenone degradation after 6 h of sunlight irradiation. The order of degradation for the RhB dye and acetophenone was CS/PVA (1.0) g-C3N4 (95. 34%, 33.33%) > CS/PVA (1.5) g-C3N4 (93.18%, 31.31%) > CS/PVA (0.5) g-C3N4 (93.02%, 29.29%) > CS/PVA (90.69%, 26.26%) > g-C3N4 (87.56%, 24%), respectively. The superior photocatalytic activity of the CS/PVA (1)/g-C3N4 composite film is due to the (i) increased adsorption capacity toward selected model pollutants from the synchronous role of CS and PVA in the CS/PVA/g-C3N4 composite films and (ii) improved visible-light absorption of the CS/PVA/g-C3N4 composite films, which facilitated the enhanced separation rate of the photo-generated charge carriers [26,36,59]. Also, the RhB dye reacted with the film surface through hydrogen bonding, which also helped its adsorption onto the catalyst and enhanced the electron transfer and degradation efficiency [60]. Thus, the enhanced photocatalytic activity of the CS/PVA/g-C3N4 composite films can be attributed to the combined effect of in situ adsorption and photocatalytic degradation.

3.8. Effect of pH

The pH of a solution is regarded as the most important factor in a photocatalytic reaction because of its influence over the photocatalyst surface charge, which impacts the degradation of pollutants. For these reasons, degradation reactions were performed to find the optimal pH values for the degradation of pollutants. Thus, an investigation of pH impact upon RhB dye and acetophenone degradation was carried out in the pH range of between 3 and 9, and the results are shown in Figure 10. The results demonstrated that without changing the pH of the dye solution, the degradation percentage of the RhB dye was 95.3%, whereas the degradation percentage of the RhB dye decreased to 74.4% when the pH increased to 9. Similarly, when the pH decreased to 3 and 5, the degradation percentage was reduced to 77.8, and 73.6%. Under acidic conditions, the film surface was positively charged, and also, the RhB molecules in their cationic form (RhB+), which repelled each other, decreased the adsorption percentage and lowered the degradation percentage. Furthermore, at a much lower pH (pH 3, addition of HCl for pH adjustment), the concentration of Cl− was higher, which may have acted as a scavenger of h+ and •OH radicals and decreased the rate of the degradation process [61]. Under basic conditions, the composite film surface would be negative and the zwitterionic form of the RhB dye would create electrostatic repulsions, resulting in a reduction in the photocatalytic degradation. Also, at a pH of 9, the interaction between OH− ions and •OH radicals would be high, thereby reducing the number of reactive species in the reaction mixture, which would lead to a low RhB degradation percentage [60,62]. For the acetophenone degradation, the degradation percentages at the pH values of 3, 5, 7, and 9 were 55.7, 43.6, 33.3, and 48.9%, respectively. The higher degradation percentage of acetophenone in the acidic medium may have been due to the formation of hydrogen bonding with the –N-H group of the chitosan or the H3O+ group from the reaction medium, which led to enhanced adsorption capacity and improved electron transfer and degradation efficiency. Similarly, the higher concentration of the –OH group in the basic medium may have led to the hydroxylation of the acetophenone, which further facilitated the degradation process under direct sunlight irradiation and yielded a higher removal rate [63,64].

3.9. Regeneration and Reusability of the Catalyst

The CS/PVA (1.0) g-C3N4 composite film stability and reusability were assessed through repeated photocatalytic degradation studies with RhB and acetophenone for up to five cycles. The used composite film was taken out from the reaction mixture, rinsed with distilled water, and reused for the next cycle of degradation experiments, and the results are depicted in Figure 11. The results demonstrate that a slight decrease in the degradation efficiency of the RhB and acetophenone was observed after five cycles of reaction. This decrease in the degradation efficiency may have been due to the active sites of the composite film surface being reduced by the adsorption of pollutants. Nevertheless, after five cycles of experiments, the CS/PVA (1.0) g-C3N4 composite film still showed 89. 72% and 28.12% RhB dye and acetophenone degradation, and photographic images of the film before and after the reaction are shown in Figure S4. Thus, the results conclude that the synthesized composite film is stable and reusable at the current experimental conditions and will pave the way for the development of commercial scale-level biopolymeric composite films.

3.10. Antibacterial Activity

In addition to photocatalytic activity under sunlight irradiation, the antimicrobial properties of the synthesized nanocomposite films against prevalent bacteria, specifically Pseudomonas sps., Klebsiella sps., Enterococcus sps., and E. coli, were investigated by the disc migration methods. The results are shown in Figure 12 and Table 1. The antibacterial activities of the CS/PVA/g-C3N4 (different PVA concentrations: 0, 0.5, 1, 1.5) were observed after 24 h by determining the zone formation in millimeters. The results demonstrated that all the composites showed higher inhibitory effects on E. coli, followed by Pseudomonas sps., Klesiella sps., and Enterococcus sps. [65]. Among the composite films, CS/PVA (1.5)/g-C3N4 showed the highest bacterial growth inhibition rate. The high antimicrobial efficiency of this composite is attributed to (i) the chitosan with a highly charged amino group, which made a natural antibacterial agent that might have interacted with the oppositely charged microbial cell layer, which cross-linked with the PVA to build a gel network structure and contributed to inhibiting bacteria growth, and (ii) the continuous generation of reactive oxygen species (i.e., •OH, O2−•, and H2O2), which reportedly destroys bacterial cell membranes and causes the leakage of intracellular contents, thereby inhibiting bacterial growth [30,66].

4. Conclusions

Chitosan-PVA-g-C3N4 nanocomposite films were successfully synthesized using easy and direct methods, and their structural and morphological properties were evaluated by XRD, FT-IR, DSC, TGA, and FE-SEM with EDX and elemental mapping analysis. The FE-SEM and elemental mapping results revealed the uniform smooth surfaces of the composite films, and the g-C3N4 was uniformly dispersed on the CS/PVA surfaces. The band gap energy of the nanocomposite film decreased with increases in the PVA loading. The band gap energy values are 2.91, 2.89, 2.82 and 2.80 eV for CS/PVA, CS/PVA (0.5) g-C3N4, CS/PVA (1.0) g-C3N4, and CS/PVA (1.5) g-C3N4, respectively. The catalytic activity of the chitosan-PVA-g-C3N4 nanocomposite films was evaluated by the degradation of an aqueous solution of RhB dye and acetophenone in the presence of direct sunlight irradiation. The results revealed that the CS/PVA (1.0) g-C3N4 nanocomposite film showed higher degradation efficiency toward the RhB dye (95. 34%) and acetophenone (33.33%) water pollutants compared to the other PVA percentage-loaded composite films, the bare film, and the g-C3N4 material. The order of degradation for the RhB dye and acetophenone was CS/PVA (1.0) g-C3N4 (95. 34%, 33.33%) > CS/PVA (1.5) g-C3N4 (93.18%, 31.31%) > CS/PVA (0.5) g-C3N4 (93.02%, 29.29%) > CS/PVA (90.69%, 26.26%) > g-C3N4 (87.56%, 24%), respectively. Similarly, the antibacterial activity results demonstrated that the nanocomposite films showed higher antibacterial activity against both gram-negative and gram-positive bacterial species compared to the bare CS/PVA film. Among these, the CS/PVA (1.5)/g-C3N4 composite film showed higher antibacterial activity against the E. coli species. Furthermore, the recyclability test demonstrated that the composite films could be easily recycled for up to five cycles of experiments and reused for the degradation of pollutants and as antibacterial agents. Thus, the chitosan/PVA/g-C3N4 nanocomposite films were efficient photocatalysts against water pollutants and antibacterial agents under the present experimental procedure. These results conclude that the simple and cost-effective method and simple operational nature of the composite films validates that CS/PVA/g-C3N4 composite films offer a promising route for sustainable antibacterial material development and also play a key role in water pollutant treatment technologies. Furthermore, the simple synthesis methodologies provide a new pathway for the upscaling of composite film development at the industrial level and also help to make solar photocatalytic antibacterial packaging films.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13010229/s1, Figures S1 and S2: UV–visible spectra of photocatalytic degradation of RhB dye and acetophenone, Figure S3: Violin plots of the distribution of the degradation ratios of RhB dye and acetophenone, and Figure S4: Photographic images of the CS/PVA (1)/g-C3N4 composite film before and after reaction.

Author Contributions

Conceptualization, M.S. and T.S.N.; data curation, M.S. and K.N.; formal analysis, M.S.; methodology, M.S. and T.S.N.; investigation, M.S.; writing—original draft, M.S., K.S.M. and T.S.N.; writing—review and editing, M.S., K.N. and T.S.N.; funding acquisition, K.S.M. and T.S.N.; supervision, K.S.M. and T.S.N.; project administration, K.S.M. All authors have read and agreed to the published version of the manuscript.

Funding

The author T.S.N. gratefully acknowledges the CSIR-CLRI for financial support through the OLP-2401, OLP-2409, and OLP-2450 projects. T.S.N. thanks MDPI for the full ‘waive-off’ of the APC.

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Acknowledgments

M.S. (Reg. No. 21214012031017) and K.S.M. thank the Sri Paramakalyani College, Alwarkurichi and DST-FIST for the FT-IR UV instrumental facilities at the PG and Research Centre, Department of Chemistry, Sri Paramakalyani College, Alwarkurichi for their financial support. We thank G. Ramanathan, Department of Microbiology, Sri Paramakalyani College for the antimicrobial studies. The author T.S.N. gratefully acknowledges the CSIR-CLRI for the financial support through the OLP-2401, OLP-2409, and OLP-2450 projects. The authors M.S. and T.S.N. thank Shri. K. Patchai Murugan for his valuable support.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Processes 13 00229 sch001
Scheme 1. Schematic diagram of g-C3N4 synthesis.
Scheme 1. Schematic diagram of g-C3N4 synthesis.
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Scheme 2. Schematic diagram of the preparation of CS/PVA/g-C3N4 nanocomposite films.
Scheme 2. Schematic diagram of the preparation of CS/PVA/g-C3N4 nanocomposite films.
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Figure 1. Schematic representation of the photo-degradation setup.
Figure 1. Schematic representation of the photo-degradation setup.
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Figure 2. XRD patterns of g-C3N4, CS/PVA, and CS/PVA (0.5, 1, 1.5%)/g-C3N4 nanocomposite films.
Figure 2. XRD patterns of g-C3N4, CS/PVA, and CS/PVA (0.5, 1, 1.5%)/g-C3N4 nanocomposite films.
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Figure 3. FE-SEM images: (a) CS/PVA, (b) CS/PVA (0.5)/g-C3N4, (c) CS/PVA (1)/g-C3N4, and (d) CS/PVA (1.5)/g-C3N4 nanocomposite films.
Figure 3. FE-SEM images: (a) CS/PVA, (b) CS/PVA (0.5)/g-C3N4, (c) CS/PVA (1)/g-C3N4, and (d) CS/PVA (1.5)/g-C3N4 nanocomposite films.
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Figure 4. Elemental mapping of (a) CS/PVA (0.5)/g-C3N4, (b) CS/PVA (1)/g-C3N4, and (c) CS/PVA (1.5)/g-C3N4 nanocomposite films.
Figure 4. Elemental mapping of (a) CS/PVA (0.5)/g-C3N4, (b) CS/PVA (1)/g-C3N4, and (c) CS/PVA (1.5)/g-C3N4 nanocomposite films.
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Figure 5. FT-IR spectra of CS/PVA and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films.
Figure 5. FT-IR spectra of CS/PVA and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films.
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Figure 6. DSC curves of CS/PVA and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films.
Figure 6. DSC curves of CS/PVA and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films.
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Figure 7. TGA analysis of CS/PVA and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films.
Figure 7. TGA analysis of CS/PVA and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films.
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Figure 8. UV–visible absorbance spectra of CS/PVA and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films.
Figure 8. UV–visible absorbance spectra of CS/PVA and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films.
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Figure 9. Degradation percentages of (A) RhB dye and (B) acetophenone with error bars using g-C3N4, CS/PVA, and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films under direct sunlight irradiation.
Figure 9. Degradation percentages of (A) RhB dye and (B) acetophenone with error bars using g-C3N4, CS/PVA, and CS/PVA (0.5, 1.0, 1.5) g-C3N4 nanocomposite films under direct sunlight irradiation.
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Processes 13 00229 g010
Figure 10. (a) pH effect on degradation of RhB dye and (b) acetophenone using CS/PVA (1.0) g-C3N4 nanocomposite film under direct sunlight irradiation.
Figure 10. (a) pH effect on degradation of RhB dye and (b) acetophenone using CS/PVA (1.0) g-C3N4 nanocomposite film under direct sunlight irradiation.
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Processes 13 00229 g011
Figure 11. Recyclability profiles of CS/PVA (1.0) g-C3N4 composite film for (A) RhB dye and (B) acetophenone degradation under direct sunlight irradiation.
Figure 11. Recyclability profiles of CS/PVA (1.0) g-C3N4 composite film for (A) RhB dye and (B) acetophenone degradation under direct sunlight irradiation.
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Processes 13 00229 g012
Figure 12. Antibacterial activities of the CS/PVA (0, 0.5, 1, 1.5) g-C3N4 nanocomposite films against (a) E. coli, (b) Pseudomonas sps., (c) Klebsiella sps., and (d) Enterococcus sps.
Figure 12. Antibacterial activities of the CS/PVA (0, 0.5, 1, 1.5) g-C3N4 nanocomposite films against (a) E. coli, (b) Pseudomonas sps., (c) Klebsiella sps., and (d) Enterococcus sps.
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Table 1. Antimicrobial activity of CS/PVA (0, 0.5, 1.0, 1.5) g-C3N4 composite films.
Table 1. Antimicrobial activity of CS/PVA (0, 0.5, 1.0, 1.5) g-C3N4 composite films.
Pathogenic BacteriaCS/PVA (mm)CS/PVA (0.5) g-C3N4 (mm)CS/PVA (1) g-C3N4 (mm)CS/PVA (1.5) g-C3N4 (mm)
E. coli-101111.5
Pseudomonas sps.-1010.511
Klebsiella sps.5101111.5
Enterococcus sps.56-10
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MDPI and ACS Style

Sutharsan, M.; Senthil Murugan, K.; Narayanan, K.; Natarajan, T.S. Chitosan/Polyvinyl Alcohol/g-C3N4 Nanocomposite Film: An Efficient Visible Light-Responsive Photocatalyst and Antimicrobial Agent. Processes 2025, 13, 229. https://doi.org/10.3390/pr13010229

AMA Style

Sutharsan M, Senthil Murugan K, Narayanan K, Natarajan TS. Chitosan/Polyvinyl Alcohol/g-C3N4 Nanocomposite Film: An Efficient Visible Light-Responsive Photocatalyst and Antimicrobial Agent. Processes. 2025; 13(1):229. https://doi.org/10.3390/pr13010229

Chicago/Turabian Style

Sutharsan, Murugan, Krishnan Senthil Murugan, Kanagaraj Narayanan, and Thillai Sivakumar Natarajan. 2025. "Chitosan/Polyvinyl Alcohol/g-C3N4 Nanocomposite Film: An Efficient Visible Light-Responsive Photocatalyst and Antimicrobial Agent" Processes 13, no. 1: 229. https://doi.org/10.3390/pr13010229

APA Style

Sutharsan, M., Senthil Murugan, K., Narayanan, K., & Natarajan, T. S. (2025). Chitosan/Polyvinyl Alcohol/g-C3N4 Nanocomposite Film: An Efficient Visible Light-Responsive Photocatalyst and Antimicrobial Agent. Processes, 13(1), 229. https://doi.org/10.3390/pr13010229

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