Rational Engineering of Nanostructured NiS/GO/PVA for Efficient Photocatalytic Degradation of Organic Pollutants

Abstract

A novel nanocomposite film synthesized from an inexpensive and easily accessible polymer such as poly (vinyl alcohol) (PVA), which is coated with nickel sulfide (NiS) and graphene oxide (GO), was obtained from used drinking-water bottles. The produced coated film was examined as a potential photocatalyst film for wastewater treatment promotion in a batch system for the removal of methylene blue (MB) and tetracycline (TC) antibiotics. The experimental results show that the presence of GO significantly increases the photocatalytic efficiency of NiS, and the MB and TC degradation results proved that the incorporation of GO with NiS led to a more than one-and-a-half-fold increase in the removal percentage in comparison with the NiS/PVA-coated film. After 30 min of illumination using GO/NiS/PVA-coated film, the removal efficiency reached 86% for MB and 64% for TC. The photodegradation kinetic rate followed the pseudo-first-order rate. Furthermore, the response surface methodology (RSM) model was utilized to study and optimize several operating parameters. The ideal circumstances to achieve 91% elimination of MB are 12 mg L⁻¹ MB initial concentration, two lamps, and an illumination time of 15 min; however, to achieve 85% TC removal, 11 mg L⁻¹ TC initial concentration, two lamps, and a 45 min illumination time should be used. The fabricated nanocomposite photocatalyst film seems to have promise for use in water purification systems.

1. Introduction

Human life depends essentially on water and oxygen. The water on Earth comes from numerous sources, but only a small number of these are fit for human use. The shortage of drinkable water sources leads to a big problem for human life, while humankind tries to provide a greater quantity of drinkable water sufficient for human daily use. The drinkable water shortage crisis has caused scientists to think of smart water purification techniques [1]. Despite the harmful effects on human health of various industry water waste types, such as dyes, heavy metals, fertilizers, and pharmaceuticals, scientists have hardly tried to develop new methods to free water from industry pollution as the demand for water increases. Methylene blue (MB) from textile factories represents the essential source of water pollution due to its toxicity and carcinogenic effect on human life [2]. The harmful effects of such dyes extend to the breakdown of the aquatic biota and stunt growth due to blocking sunlight transfer into deep water as a result of the heavy dye color, which prevents the photosynthesis process [1]. This means that water purification from dyes is very important not only for human life but also for the life of other marine organisms. In addition, the heavy color of the dyes also causes incomplete bacteria degradation and provides a high water toxicity percentage that anaerobically blocks the sediment process [3].

Furthermore, antibiotics such as tetracycline (TC) represent a serious water waste crisis, coming after dye waste in the list of waste sources. The wide use of antibiotics as medicine to stop bacteria from working makes the research mission of TC removal from water very hard [4,5,6]. Photocatalyst degradation, biological treatment, electrochemical removal, precipitation using chemical methods, reverse osmotic techniques, lime softening, coagulation, and adsorption methods are used for wastewater treatment [7,8,9,10]. The photocatalysis technique is a promising technique among the other mentioned methods that provides a sustainable, simple, eco-friendly, and high-efficiency technique [9,11]. Though this method relies on creating extremely oxidizing •OH radicals in the reaction media, it offers a viable substitute for treating wastewater that contains extremely hazardous organic compounds that are either non-biodegradable or very biodegradable.

From a thorough literature survey, nanocomposites based on graphene oxide showed superiority in visible-light-induced photocatalytic response. For example, Cu/Ni-loaded RGO hybrid nanocomposites and Ni-modified TiO₂/GO nanosheet composites have been employed for hydrogen production with higher yields via visible light photocatalysis [12,13]. Kandhasamy et al. used Ni–Mn-S wrapped with RGO as an ultra-fast photocatalytic for organic dye molecule degradation [14]. Similarly, the efficiency of the photocatalyst could be improved by the incorporation of transition metal sulfides such as MoS₂, NiS, MoS, and CoS, which provide an effective photocatalysis behavior for environmental water waste purification when illuminated with light [15]. Nickel sulfide (NiS) showed amazing photocatalysis effectiveness due to its inexpensive cost, several stoichiometric properties, low toxicity, thermodynamic stability, high electrical conductivity, higher activity to enhance light absorption, and wide photocatalysis applications, as well as being eco-friendly [11,16].

Cutting-edge study findings show that GO-based Ni compounds are a promising class of photocatalysts [17]. Therefore, it would be anticipated that the inclusion of GO along with a non-metal, such as sulfur, and Ni nanoparticles, will demonstrate encouraging photocatalytic activity. Graphene-adorned semiconductor nanoparticles and their photocatalytic uses in environmental cleanup and clean energy production, however, are little documented in the literature. One potential method to address the large-scale separation and recycling of the photocatalyst throughout the wastewater treatment process is to support the catalyst on a polymeric matrix, such as polyvinyl alcohol (PVA).

PVA polymer is cheap, nontoxic, rapidly soluble in water, and degradable among various polymer types, which makes it an excellent green polymer for wastewater treatment [18,19]. Additionally, PVA exhibits a high degree of hydrophilicity because of the presence of OH functional groups, which improve water uptake. However, various studies on heterostructures, combining composite materials for the enhanced photodegradation of various organic contaminants, have been conducted [16,20,21,22,23,24,25,26], and the results revealed that the photocatalytic process benefited from the incorporation of GO, which has a synergistic effect, and hierarchical and porous architectures.

The present work presents a novel photocatalyst composite (GO/NiS/PVA) with high photocatalytic performance for MB and TC. NiS was prepared using a novel technique, where the raw materials (nickel acetate and thiourea) were mixed well and ground to a fine powder using a ball mill to be sure that the mixture could be dissolved well in ethanol. Additionally, GO was fabricated from waste plastic to save the environment by recycling technology and treating wastewater with the materials that were developed. Moreover, the use of (GO/NiS/PVA) photocatalyst film facilitated the separation of the photocatalyst after the treatment process. Additionally, the response surface methodology model (RSM) was obeyed by the prospective mechanism through the (GO/NiS/PVA) film surface and water waste (MB and TC) to examine the ideal values of the most important components through the photocatalytic process.

2. Results and Discussion

2.1. Characterization of Prepared GO, NiS, and Coated Films

The X-ray diffraction (XRD) analysis of NiS fine powder revealed the presence of the hexagonal NiS phase according to the hexagonal NiS reference profile (JCPDS No. 75-0613) [27]. The XRD profile depicted in Figure 1a exhibited a polycrystalline nature, as demonstrated by the presence of peaks at 29.7°, 34.3°, 45.6° and 53° corresponding to (101), (102), (110), and (103) planes, respectively. In contrast, the graphene oxide (GO) spectrum exhibits a broad peak at 26, which is well-matched with the reference data (JCPDS75-2078) [28]. However, the XRD spectrum of GO/NiS/PVA-coated film shows the diffraction peaks corresponding to GO, NiS, and PVA materials.

Fourier transform infrared spectroscopy was used to examine the functional groups of prepared PVA, NiS/PVA, GO/PVA, and GO/NiS/PVA films, as depicted in Figure 1b. The Fourier Transform Infrared (FTIR) spectrum exhibited a broad band at 3465 cm⁻¹ in the FTIR spectrum. In addition to the C-H symmetric stretching vibration at 2159 cm⁻¹, there was also a C-H asymmetric stretching vibration at 2428 cm⁻¹. Nevertheless, the FTIR spectrum of NiS/PVA film showed bands at 3400 cm⁻¹ and 1620 cm⁻¹ that represented the O-H stretching and bending vibrations, respectively. Additionally, the C-O-C group appeared at 1400 cm⁻¹ and 1120 cm⁻¹, representing C-O stretching asymmetric and symmetric bonds. The bending vibration of the bond between Ni-S was documented in the literature using the band at 675 cm⁻¹ for Ni-S [29]. This finding demonstrates a strong concurrence with the data obtained in this study. In the Fourier Transform Infrared (FTIR) spectrum of graphene oxide (GO), a prominent peak at 3446 cm⁻¹ is indicative of O-H stretching. Conversely, the FTIR spectrum exhibits bands at 1636, 1520, and 1100 cm⁻¹, which correspond to C = O, C=C, and O-H bend, respectively, indicating the high oxygen concentration in GO. However, it has been observed that the GO/NiS/PVA spectrum exhibits a combination of functional properties among its constituent components.

The NiS TEM image (Figure 2a) shows the presence of aggregated nanoparticles. The energy-dispersive X-ray spectroscopy (EDS) findings, as demonstrated in the inset of Figure 2a, indicate that the atomic ratios of Ni:S are 48.34:51:66. These ratios closely align with the theoretical values and closely approximate the ideal structure [30]. The average Ni/S ratio agrees and is comparable to those in the literature [31]. However, the TEM image for GO nanoparticles (Figure 2b) shows agglomerated nanosheets. The mixture of graphene oxide (GO) and nickel sulfide (NiS) nanoparticles in a weight ratio of 1:1 is depicted in Figure 2c. Subsequently, the mixture was uniformly dispersed onto the surface of the polyvinyl alcohol (PVA) polymer film. The SEM image of the film reveals a rough surface characterized by numerous irregular aggregates of nanoparticles, which exhibit a microporous structure, as demonstrated in Figure 2d. Furthermore, the microporous structure increases the number of active sites for the adsorption/photocatalysis process.

Table 1. Contact angle values of prepared films.

Nanocomposite Film	Contact Angle
PVA	∼49.5° ± 3
NiS/PVA	∼114° ± 5
GO/PVA	∼31.5° ± 2.5
GO/NiS/PVA	∼81° ± 4.5

displays the contact angle data for PVA, GO/PVA, NiS/PVA, and GO/NiS/PVA-coated films. Given the presence of multiple oxygenated functional groups on the surface of GO, GO nanoparticles exhibit a higher degree of hydrophilicity compared to NiS. This observation suggests that the addition of GO results in an increase in the hydrophilicity of the NiS/PVA film.

The UV–Vis absorbance analysis was conducted, and the results are depicted in Figure 2a. The highest level of absorption was observed between 200 and 400 nm. As a result, the produced coated films may have a strong UV and visible activity. However, Equation (2) illustrates the determination of the optical band gap of films coated with GO/PVA, NiS/PVA, and GO/NIS/PVA through UV–Vis absorbance using Tauc’s equation [32]

(αhν)ⁿ = C × (hν − E_g)

(1)

where α is the absorption coefficient, C is a band tail parameter, h is the Planck constant, ν is the light frequency, E_g is the photocatalyst band gap, and n is a constant equal to 2 for indirect transitions in amorphous materials. The gap value can be estimated by extrapolating the linear component that occurs in the absorption threshold, as demonstrated in Figure 3b, which shows the fluctuation of (αhv)n as a function of (hv). By comparing the values of band gaps for the three coated films, it was discovered that combining the GO with NiS resulted in the reduction of the NiS/PVA from 3.1 eV to 2.5 eV for GO/NiS/PVA heterostructures coated film. This observation suggests that the addition of GO, which functions as a narrow bandgap material, in the production of a film coated with GO/NiS/PVA heterostructures leads to an increase in the excited electron transfer to the GO [33]. This prevents electron-hole recombination and increases the generation of OH radicals since the holes are free to react. Therefore, the investigation demonstrates that the produced GO/NiS/PVA heterostructures coated film has promising photocatalytic activity.

2.2. Adsorption and Photocatalytic Performance of GO/NiS/PVA-Coated Film

To investigate the impact of light on the removal characteristics of GO/PVA, NiS/PVA, and GO/NiS/PVA-coated films, the adsorption batch experiments were conducted under dark conditions. This knowledge is crucial for evaluating the photocatalytic activity of these films in the presence of visible light.

The duration of illumination was measured at different intervals, both in the absence of light and in the presence of light. The experiment involved using 0.02 g of film strips saturated with 50 mL of 20 mg L⁻¹ MB dye or TC antibiotic solutions, with a solution pH of 7. The dark adsorption of MB and TC exhibited a positive correlation with time, as depicted in Figure 4a,b, and subsequently plateaued after 60 min.

The observed phenomenon may be attributed to the rapid gradual increase in the amount of adsorped MB or TC molecules with increasing adsorption time. This can be due to the presence of surface pores and active sites on the adsorbent surface, which facilitate the adsorption process. Subsequently, an equilibrium is achieved due to the saturation of the pore structure and surface with pollutant molecules after approximately 60 min. Furthermore, the findings suggest that the GO/NiS/PVA nanocomposite film exhibits a superior removal rate in comparison to the GO/PVA and NiS/PAV films. This can be attributed to combining GO and NiS nanoparticles on the surface of the PVA film, which increases the active site availability. Consequently, this enhanced availability facilitates more effective reactive interactions with the MB and TC molecules.

The adsorbed amount of MB and TC at time t (qt, mg g⁻¹) was calculated using the equation

q_t = (C₀ − C_t) V/m

(2)

where C_o and C_t are the initial and concentration at time (t), respectively, of MB and TC, V is the volume of solution per liter, and m is the mass of coated film’ strips per gram. Based on a comparative analysis of the removal efficacy of the tested films, the following order was determined: GO/NiS/PVA > GO/PVA > NiS/PVA. These results indicate that the presence of GO nanoparticles with NiS nanoparticles on the film’s surface plays a significant role in enhancing the adsorption of MB and TC. This suggests that the heterostructures present in the coated film contribute to the improved MB absorptivity and TC on the film’s surface [34]. Moreover, it is widely recognized that GO sheets can form various bonds and interactions due to the presence of ionic groups and aromatic sp2 domains. Therefore, the formation of electrostatic interactions between MB or TC molecules and aromatic regions of the GO in the coated film can enhance the physical adsorption of MB or TC molecules [35].

GO/NiS/PVA was selected for further photocatalytic tests due to its superior removal efficiency, which was observed during the adsorption process in the dark control experiments. The data presented in Figure 4c demonstrates an increase in photocatalytic activity under visible light. Following approximately 30 minutes of exposure to light using a film coated with GO/NiS/PVA, which has exhibited remarkable photocatalytic characteristics, the results revealed a substantial improvement in photocatalytic activity, reaching 86% for MB and 64% for TC.

2.3. Photocatalytic Mechanism

The mechanism underlying the photocatalytic degradation of MB and TC molecules on the surface of a film coated with GO/NiS/PVA is depicted in Figure 5. Upon exposure to visible light, NiS in the coated film’s conduction band (CB) or stimulated (MB*) or (TC*) molecules might produce electrons. The produced electrons are transferred to GO, therefore inhibiting the charge recombination processes in NiS and enhancing the heterostructure-coated film’s photocatalytic activity [36]. The generation of reactive oxygen species (^•O₂⁻) occurs through the reaction between generated electrons on graphene oxide (GO) sheets and absorbed surface O₂. This also occurs at NiS surfaces as holes and electrons combine to produce ^•OH radicals. The (^•O₂⁻) and (•OH) radicals act as catalysts for the degradation of MB and TC molecules. In addition to charge separation, pollutants’ adsorption, light absorption, and surface oxygen adsorption can impact the photocatalytic activity of the heterostructured GO/NiS/PVA-coated film.

The results of our experiment and prior scholarly deliberations indicate the photocatalytic degradation mechanism that is described below:

$$\mathrm{NiS} \stackrel{hv}{\to }({{\mathrm{h}}^{+}}_{\mathrm{VB}}+{{\mathrm{e}}^{-} }_{\mathrm{CB}})\mathrm{NiS}$$

(3)

(e⁻ _CB) NiS → GO (FL)(e⁻)

(4)

(e⁻ FL) GO + O₂ → ^•O₂⁻

(5)

H₂O + ^•O₂⁻ → ^•OOH + OH⁻

(6)

^•OOH + H₂O → ^•OH + H₂O₂

(7)

H₂O₂ + (e⁻ _FL) GO → ^•OH + OH⁻

(8)

H₂O₂ + (h⁺ _VB)NiS → ^•OOH + H⁺

(9)

H₂O₂ + ^•OOH → OH + H₂O + O₂

(10)

$$\mathrm{MB}\stackrel{hv}{\to }{\mathrm{MB}}^{\ast }\to {\mathrm{MB}}^{+}+\mathrm{NiS}/\mathrm{GO} ({{\mathrm{e}}^{-} }_{\mathrm{FL}})$$

(11)

^•OH+ ^•O₂⁻ + MB⁺ → Degraded products

(12)

$$\mathrm{TC}\stackrel{hv}{\to }{\mathrm{TC}}^{\ast }\to {\mathrm{TC}}^{+}+\mathrm{NiS}/\mathrm{GO} ({{\mathrm{e}}^{-} }_{\mathrm{FL}})$$

(13)

TC⁺ +NiS/GO (e⁻ _FL)

^•OH+ ^•O₂⁻ + TC⁺ → Degraded products

(14)

2.4. Kinetics of the Photocatalytic Degradation

A detailed kinetic study for the degradation of MB and TC using GO/NiS/PVA heterostructure-coated film was conducted, and the results are displayed in Figure 6. In the kinetic experiment, 20 mg of film strips and MB or TC solutions were utilized at six distinct starting concentrations (5, 10, 15, 20, 25, and 30 mg L⁻¹) in a neutral pH solution. The reaction was continued until it reached each concentration’s equilibrium state. At regular intervals, aliquots of 5 ml were extracted and subjected to analysis using a UV–Vis spectrophotometer.

As shown in Figure 6a,b, the reaction completion time increased as the initial dye concentration increased for MB and TC, respectively. After 30 minutes of visible light irradiation, a 100% degradation of the 5 mg L⁻¹ MB dye molecule was observed. Conversely, 5 mg L⁻¹ TC molecules have undergone complete degradation after 60 min.

The kinetic data were then fitted with the pseudo-first-order rate, and the apparent photodegradation rate constant (K) can be calculated using (Equation (13)) [36]. The corresponding results are plotted in Figure 6c,d. The values of rate constant K are displayed in

Table 2. The values of rate constant and apparent photodegradation rate constant (K).

	Initial Concentration (mg L−1)
	5	10	15	20	25	30
MB	0.094	0.054	0.015	0.012	0.011	0.008
TC	0.093	0.049	0.0048	0.0045	0.0044	0.0044

.

$$ln{\displaystyle \frac{C_o}{C_e}}=K t$$

(15)

2.5. Optimization of the Photocatalytic Process Conditions Using the RSM Model

The optimization of the photocatalytic degradation process for MB and TC using GO/NiS/PVA heterostructure film was examined using the Box–Behnken design analysis. The statistical relationship between variables and response, as measured by the yield of removal, was represented by the following equations, assuming a quadratic dependence:

MB Removal (%) = 50.5 + 27.66A − 7.75B +3.91C + 3.3AB − 5.13AC − 2.3BC + 2.69A² + 7.51B² + 3.19C²

(16)

TC Removal (%) = 54 + 2.63A − 10.5B + 8.13C − 4.5AB + 1.75AC − 2.5BC + 6.87A² + 5.12B² + 0.375C²

(17)

An analysis of variance (ANOVA) was utilized to determine the main effect of each factor and their interaction. The statistical analysis revealed that the observed effects exhibit statistical significance (p < 0.05). These findings suggest that the percentage of MB and TC removal is significantly affected by the variables of light intensity, initial concentration, lumination time, and their combined effect. The contour plots shown in Figure 7 indicate a direct relationship between light intensity and removal percentage. In contrast, the fitted surface plots of MB and TC removal % demonstrate a negative correlation between the initial pollutant concentration and the removal percentage. This relationship is contingent upon the combined influence of the initial pollutant concentration and light intensity. Nevertheless, the combined impact of light intensity and lumination time demonstrates a direct correlation with the elimination percentage. Furthermore, it can be observed from the perturbation plot depicted in Figure 8 that as the initial concentrations (variable coded B) increase, there is a corresponding decrease in the removal %. Conversely, it was determined that the degradation process of TC is regulated by the starting concentration, whereas the degradation of MB is primarily influenced by light intensity.

According to the quadratic model, the optimal conditions for the removal of MB were determined to be as follows: an initial MB concentration of 12 mg L⁻¹, two lamps (the maximum light intensity), and a lumination time of 15 min to achieve 91% elimination. The ideal circumstances for removing TC are as follows: to achieve 85% removal, the starting TC concentration should be 11 mg L⁻¹, two lamps should be used, and the lumination time should be equivalent to 45 min.

2.6. Stability and Reusability of GO/NiS/PVA-Coated Film

The GO/NiS/PVA-coated Film Stability Test Was Performed by Measuring the Amount of GO/NiS Nanoparticles Leached to the Solution. A Total of 500 mg/10 cm² Was the Starting Concentration of GO/NiS on the Film Surface. After Five Days of Leaching, It Was Found That 9.3 mg of GO/NiS Nanoparticles Had Undergone Leaching from the Surface of the PVA Film by the Initial Day, Indicating a Leaching Rate of Approximately 63%. The Results Depicted in Figure 9a Demonstrate That a Substantial Amount of Leaching Occurred, As Demonstrated by the Retention of Approximately 71% of the GO/NiS Nanoparticles on the Surface of the PVA Film After 5 Days.

The examination of the reusability of GO/NiS/PVA-coated film, as shown in Figure 9b, revealed that the process is cost-effective. The GO/NiS/PVA film strips were recovered, cleaned in 0.1 M HCl solution, and used five times under ideal circumstances. The film’s strips were repeatedly cleaned with a 70% (v/v) ethanol aqueous solution until TC and MB were removed from the solution. According to the results, efficiency decreases for both TC and MB, decreasing from 64% to 58% and 86% to 81%, respectively. The fouling of the porous surface, the leaching of GO/NiS nanoparticles from the PVA film’s surface, or both may be responsible for the reduction in TC and MB removal efficiency. Nevertheless, these findings suggest that the film coated with GO/NiS/PVA demonstrated sustained functionality even when repurposed.

3. Materials and Methods

3.1. Preparation of Graphene Oxide (GO)

As reported in our previous work [9], GO was produced by pyrolyzing wasted polyethylene terephthalate (PET) plastic water bottles. The final product was ground into a fine powder in a ball mill operating at 500 rpm for 30 min.

3.2. Preparation of Nickel Sulfide (NiS)

A novel method is employed to effectively prepare nickel sulfide (NiS) by combining nickel acetate (Ni(CH3COO)2.2H2O) (99.9% Universal Fine Chemical UFC, Sanborn, New York, NY, USA) with thiourea (99.9% Carl Roth, Karlsruhe, Germany) in a ball mill at 400 rpm for 10 minutes to produce a fine powder. The fine powder is then dissolved in 100 mL of ethanol using an ultrasonic device for 15 minutes. Subsequently, the NiS was obtained by heating the solution in the oven at 200 °C for 2 hours. Finally, the NiS powder was grinding very well using a ball mill at 400 rpm for 20 min to obtain fine powder.

3.3. Preparation of GO/NiS/PVA-Coated Films

The nanocomposite film preparation was as follows: A clear solution was obtained by dissolving PVA (99% hydrolysis, medium MW, Sig-ma-Aldrich, St. Louis, MO, USA) in water at 90 °C for 2 h with vigorous stirring. This resulted in the preparation of a 10 wt.% PVA solution. A suspension of GO and NiS (1:1 wt.%) in a few drops of water was added to the semisolid PVA film, and the polymeric solution was crosslinked by adding 0.5 mL of glutaraldehyde (GA) (50 wt.% in H₂O, Alfa Aesar, Tewksbury, MA, USA) in 10 mL of acetone and stirring thoroughly for 4 h at 40 °C. The coated film was dried in a vacuum oven at 60 °C for the entire night after the GO and NiS suspension were added to the semisolid PVA film.

3.4. Characterization of Prepared Materials

The prepared GO and NiS powder and GO/NiS/PVA-coated film were subjected to basic characterization, Supplementary Materials, which included the analysis of surface functional groups using Fourier Transform Infrared (Shimadzu FTIR-8400, Osaka, Japan). The morphologies of the nanocomposite films were investigated using SEM (JEOL JSM 6360, Tokyo, Japan) and Transmission Electron Microscope (TEM, TECNAI G20, Amsterdam, The Netherlands) to investigate the morphology of GO and NiS. The optical absorption spectra were analyzed using UV–vis spectroscopy (Shimadzu UV-2600, Tokyo, Japan).

3.5. Adsorption/Photocatalytic Tests

Two beakers were filled with 50 mL of 20 mg L⁻¹ MB solution and another with 20 mg L−1 TC solution to investigate the equilibrium time of the adsorption process for the GO/PVA, NiS/PVA, and GO/NiS/PVA-coated films. Approximately 20 mg of film stripes were added to each beaker and shaking was used for 30 min. Each group of the two beakers was placed in the dark to prevent the effect of sunlight.

A Plexiglas cylindrical reactor measuring 15 cm in diameter and 20 cm in height was used to evaluate the photocatalytic degradation of MB dye and TC antibiotic solutions using various fabricated coated films. The tests were conducted at pH 7 of each tested solution under the illumination of LED visible light. The radiation source on top of the reactor was two 12 W lamps (Bareeq, Egypt) with a light intensity of 1200 lm. Samples were taken at regular intervals after the 50 mL of 20 mg L⁻¹ MB or TC solution was magnetically agitated with 20 mg of film strips and exposed to visible light. After irradiation, 2 mL of the reaction mixture for each tested solution was sampled, and the residual concentration was measured using a UV–Vis spectrophotometer at 665 nm for MB and 357 nm for TC. MB and TC’s photocatalytic degradation was computed using the following formulas:

$$photodegradation \%={\displaystyle \frac{C_0-C_t}{C_0}} \times 100$$

(18)

where C₀ and C are the initial and final concentrations, respectively. All adsorption/photocatalytic degradation experiments were replicated three times.

3.6. Optimization of the Degradation Process

To optimize degradation process conditions, we tried to establish a relationship between factors and responses according to a response surface methodology model (RSM). The selected matrix for the response surface methodology followed the Box–Behnken design [37] with 17 trials. To evaluate the adsorption process performance, three factors were used: A (light intensity, lm); B (initial concentration, mg L⁻¹); and C (lumination time, min.), at three levels of −1, 0, and 1 as illustrated in

Table 3. RSM experimental design coded values for MB and TC.

Symbol	Independent Variables	Coded Levels
		−1	0	1
A	light intensity, lm	0	1	2
B	initial concentration/mg L−1	10	20	30
C	lumination time/min.	15	30	45

for MB and TC contaminants. Data analysis and optimization were carried out using Design-Expert, 13.0.9.0 software from STAT-EASE, INC (Minneapolis, Minnesota, USA). Furthermore, the developed polynomial models were statistically validated using analysis of variance (ANOVA), with the F-test used to check their statistical significance. The coefficient of determination R² was used to check their fitting quality [38,39].

3.7. Stability and Recycling Tests

The stability of the GO/NiS layer on the surface of the PVA film was examined in a shaking water bath. After being placed in a shaking water bath, the GO/NiS/PVA-coated film was immersed in a beaker filled with 250 mL of Milli-Q water. Following a period of five days of rigorous agitation at ambient temperature at a speed of 500 revolutions per minute, a fresh batch of Milli-Q water was introduced into the water bath. Subsequently, the film coated with GO/NiS/PVA was immersed in this water bath for a complete day. Shaking induces hydrodynamic shear stress on the surface of the GO/NiS/PVA-coated film, causing the physically adsorbed GO/NiS layer to leak away. Every 24 h, the GO and NiS nanoparticles that had leached out of the 250 mL of water were collected by evaporating the water, drying the precipitate of the final nanoparticles, and weighing it. Regarding the quantity of GO and NiS nanoparticles adhered to the PVA film, the percentage of GO and NiS nanoparticles leached was computed. The experiment was conducted three times.

To assess the regeneration potential of the GO/NiS/PVA-coated film, 20 mg of the film was placed in two beakers, one containing 25 mL of 50 mg L⁻¹ MB solution and the other containing 25 mL of 50 mg L⁻¹ TC solution. The beakers were then shaken at 100 rpm for 30 min. The film was then separated and sonicated for 30 min with a solution of 100 mL ethanol and 10 mL 0.1M NaOH, then filtered, cleaned, and dried at 60 °C for 4 h before being reused. Next, the filter fluid’s concentration was determined. The previous steps were repeated five times.

4. Conclusions

GO was synthesized by thermal catalytic decomposition of drinking-water bottle waste, while NiS was prepared from nickel acetate and thiourea using a novel method. GO and NiS nanoparticles were deposited on PVA to obtain GO/NiS/PVA heterostructure-coated film. The synthesized GO/NiS/PVA heterostructure-coated film was characterized by FT-IR, XRD, UV–Vis-spectroscopy, and SEM. XRD results revealed that the synthesized film is semicrystalline. The SEM imaging confirms the deposition of GO and NiS nanoparticles on the PVA film’s surface. GO/NiS/PVA heterostructure-coated film was tested for its photocatalytic activity for methylene blue and tetracycline degradation under visible light irradiation. However, this film is a highly active photocatalyst in the neutral pH range, with 86% removal efficiency for MB and 64% for TC after 30 minutes of illumination and 90 minutes of overall removal time. The photodegradation of MB and TC molecules followed a pseudo-first-order kinetic model with a maximum rate constant of 0.094 and 0.093 min⁻¹ for MB and TC, respectively. The synthesized film photocatalyst retains the activities of five reusability cycles with low-efficiency reduction. Therefore, the GO/NiS/PVA heterostructure-coated film would continue to function effectively under visible light irradiation even if reused.