Using periwinkle shell ash (PSA) and polystyrene (PS), a new-fangled PSA@PS-TiO2 photocatalyst was fabricated. The morphological images of all the samples studied using a high-resolution transmission electron microscope (HR-TEM) showed a size distribution of 50–200 nm for all samples. The SEM-EDX showed that the membrane substrate of PS was well dispersed, confirming the presence of anatase/rutile phases of TiO2, and Ti and O2 were the major composites. Given the very rough surface morphology (atomic force microscopy (AFM)) due to PSA, the main crystal phases (XRD) of TiO2 (rutile and anatase), low bandgap (UVDRS), and beneficial functional groups (FTIR-ATR), the 2.5 wt.% of PSA@PS-TiO2 exhibited better photocatalytic efficiency for methyl orange degradation. The photocatalyst, pH, and initial concentration were investigated and the PSA@PS-TiO2 was reused for five cycles with the same efficiency. Regression modeling predicted 98% efficiency and computational modeling showed a nucleophilic initial attack initiated by a nitro group. Therefore, PSA@PS-TiO2 nanocomposite is an industrially promising photocatalyst for treating azo dyes, particularly, methyl orange from an aqueous solution.

  • The new photocatalyst prevented TiO2 aggregation into larger microspheres due to the carbon.

  • TiO2 was distributed within the mesoporous (PSA) and microporous (PS) layers of the membrane substrates.

  • Increased wt.% correlated with higher irregularity and roughness owing to the presence of doped PSA and TiO2 nanoparticles.

  • The bandgap reduces from 3.1 to 2.5 eV when PSA content increased from 0.5 to 5.0 wt.%.

A huge variety of compounds from various sources have been discovered as significant organic contaminants in drinking water, surface water, sewage, and groundwater (Li et al. 2021). The greatest source of worry is their negative impact on man's well-being and the ecosystem. One of the utmost public health concerns in most nations across the world is the discarding of dye wastewater effluents from textile industries, on aquatic water bodies being a key organic pollutant (Yaseen & Scholz 2019). The majority of organic contaminants, including dyes that have been detected in water, come from a variety of sources, including wastewater sources. As previously stated, practically all dyes have a significant environmental impact on the world's population (Hernández-Zamora & Martínez-Jerónimo 2019). Besides all forms of dyes have a major impact on health and exhibit strong hazardous activity on the environment (Ledakowicz & Paździor 2021). The azo dyes make up more than half of the classified dyes in the color index, contain an azo functional group (N = N), and are properly bound to the aromatic ring (Al-Rubaihe & Mhessn 2012). Congo red, phenyl orange, methyl red, and methyl orange (MO), among other azo dyes, have also been classified as well-known dyes. According to the literature, all forms of subsurface and surface water, including municipal and potable water, are contaminated with significant levels of dyes (Sharma & Bhattacharya 2017). The morphological flexibility and traits of marine environments can be altered by dye effluent concentrations as low as 0.05 mg/L. As a result, removal from the wastewater before discharge into the environment is crucial (Amin et al. 2016; Krishna Moorthy et al. 2021).

Many strategies for wastewater treatment have been recognized as effective. Reverse osmosis, membrane filtration, adsorption, activated alumina, activated carbon, and photocatalysis are some of these approaches (Tlili & Alkanhal 2019; Abdel-Fatah 2020). The photocatalytic degradation method is regarded as one of the most advantageous wastewater cleanup processes from a variety of sources (Loeb et al. 2019; Nkwoada et al. 2022a). As a result, a good photocatalyst with cost savings, material stability, enhanced band structure, and a greater photodegradation rate is necessary (Yang & Wang 2018; Wang et al. 2020).

Nanocomposites, on the other hand, are a recent discipline of nanoscience concerned with the structure, application, and synthesis of disperse phase and matrix phase in a multiphase system of composites in which nanoparticles are scattered in the matrix (Azad & Gajanan 2017; Ravichandran et al. 2018). Titanium dioxide (TiO2) is the most promising photocatalyst owing to its non-toxicity, chemical stability, and high photocatalytic activity (Binas et al. 2017). It is known that TiO2 with a bandgap energy of 3.2 eV only degrades contaminants in response to UV radiation (Dette et al. 2014). Albeit, under natural solar irradiation or reduced bandgap, this photocatalyst's activity can be improved with suitable substrate and dopants.

According to a reviewed study, polystyrene (PS) structure collapses at 110–120 °C (Maharana et al. 2007) at elevated temperatures. The collapsed structure melts at 160 °C and vaporizes at temperatures greater than about 275 °C, and complete volatilization occurs around 460–500 °C. (Mehta et al. 1995). Likewise knowing this, researchers are advancing the applications of PS material as a photocatalyst membrane support, particularly for TiO2. A research study (Santos et al. 2018) used TiO2 photocatalyst supported on waste PS to photodegrade erythrosine and brilliant blue FCF (binary mixture) food dyes in an aqueous solution. Their findings showed enormous potential in the treatment of industrial effluents contaminated by food dyes and the use of waste PS for membrane support. Interestingly, degradation kinetics was rapid, and the pseudo-first-order kinetics was the best fit for the experimental data. Then again, films from PS with OH end groups and carbon nanotubes (CNTs) were synthesized by chemical vapor deposition using ferrocene and benzene as precursors (Granados-Martinez et al. 2016). The SEM micrographs showed that CNTs were dispersed inside the composites, like a sandwich structure, demonstrating the affinity of the PS–OH matrix and CNTs. While Raman's analysis proved chemical bonding between multi-walled CNTs and PS–OH. Thus, both research studies suggested that PS can be supported with thermally activated carbons for nanocomposite fabrication, whereas Aziz et al. (2023) also suggested that polymers and polymer composites can be improved with significant enhancement in thermal stability and mechanical properties.

On the contrary, periwinkle shell ash (PSA) was a cheap, readily available, and thermally activated carbon material with photocatalytic properties in the heterogeneous photodegradation of aniline aqueous solution and attained optimum degradation efficiency at a contact time of 100 min (Aisien et al. 2014). In addition, other findings (Onuoha et al. 2017) confirmed that the PSA was suitable for the production of recycled polymer composites. Furthermore, a study worked on activated carbon prepared from PSA for the removal of Remazol brilliant violet-5R dye (RBV) (Bello & Ahmad 2011). The work showed that calcination above 700 °C expanded the carbon material, and created a large surface area and high porosity, leading to a high percentage of RBV-5R dye removal. Also, similar work studied the photodecolorization of industrial wastewater from a beverage company by PSA (Aisien et al. 2013). The total decolorization (98%) was obtained in 60 min of solar irradiation when H2O2 was added to the catalyst periwinkle shell and the surface area was determined to be 400 m2/g. Then, work was conducted on the chemical content of the PSA alongside its suitability for thin-layer chromatography (TLC) (Orji et al. 2017). Different researchers (Offiong & Akpan 2017) corroborated similar findings on X-ray fluorescence: SiO2 (33%), Fe2O3 (5%), CaO (41.2%), and ZnO (3.2%) characterization, which led to its photocatalytic behavior.

Interestingly studies of materials with similar compositions (TiO2/SiO2) (Bellardita et al. 2010) performed brilliant photodegradation of dyes. Moreover, the photocatalytic behavior of CaO has been observed (Fatimah et al. 2018) because the precursors can form photoactive calcium titanate (CaTiO3) for dye degradation. PSA material has good physicochemical properties, photonic stability, and low toxicity and is activated by UV-Vis light (Nkwoada et al. 2021). Unfortunately, PSA has not yet been studied as a coupled membrane substrate or dopant in photocatalysis, to the best of our knowledge (Aisien et al. 2015; Nkwoada et al. 2021), and neither has any work shown high-resolution transmission electron microscope (HR-TEM), scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), atomic force microscopy (AFM), and UV-diffuse reflectance spectroscopy (UVDRS) characterization. This PSA material according to our recent work exhibited a high rate of electron–hole recombination (Nkwoada et al. 2022b), with very stable and gradual photodegradation.

Interestingly, titanium dioxide (TiO2) photocatalyst has an appropriate redox behavior and potential synergy (Dong et al. 2015; Orha et al. 2017; Ge et al. 2021). Although the anatase and rutile phases of TiO2 have been established, the combined phase is the most photo-efficient and effective and highly desired. Besides, the broad bandgap (Eg = 3.2 eV) of TiO2 with augmented activity interacts with UV rays from other sources (Gautam et al. 2016; Xing et al. 2016; Shafique et al. 2022). Interestingly a study confirmed that PSA material in micro-sized and even nano-sized particles are a good candidate as filler or in polymer composites (Abdelmalik & Sadiq 2019). Moreover, recent work showed that PSA offers good nanocomposite attributes in the photocatalytic degradation of dyes (Nkwoada et al. 2022b). Studies have also shown that PSA has not yet been used in the synthesis of the polymer nanocomposite photocatalyst that is being suggested. Similarly, unlike other activated carbons or biomaterials (Chai et al. 2021), PSA is an emerging composite that does not yet have enough literature to support useful scale-up experiments in both the laboratory and the industry. Furthermore, reports on investigations on the photodegradation of MO using the suggested nanocomposite photocatalyst and studies using molecular modeling tools have not been carried out.

To this end, researchers are occupied with synthesizing efficient nanomaterials that will effectively reduce the bandgap (de Araujo Scharnberg et al. 2020). Consequently, the current paper aims to synthesize a stable and cheap novel PSA@PS-TiO2 nanocomposite photocatalyst using a hydrothermal approach that will reduce the bandgap of TiO2, as a competitive clean photocatalyst (Kansal et al. 2013; Yudoyono et al. 2016; Mohammad-Salehi et al. 2019). The study objectives include the physicochemical and material characterization (Zhang et al. 2013; Alkayal & Hussein 2019; Kuldeep et al. 2021), right after the synthesis of nanocomposite with a general formula of PSA@PS-TiO2. In addition, the photoactivity of the photocatalyst is evaluated against MO. Optimization processes were studied such as catalyst loading, initial concentration, pH, and likewise industrial feasibility via reusability test. Final objectives include response surface designs and computational studies (Rajesh et al. 2023; Wazzan & Al-Qurashi 2023).

Materials

Sigma Aldrich supplied titanium tetra isopropoxide (TTIP; 99.9% purity), and double distilled water (DDW) was prepared in the laboratory. Ethanol, xylene, HNO3 (>85% purity), NaOH (>97% purity), MO (CAS 547-58-0, 95% purity), and HCl were procured from Gate Laboratory Chemicals Nigeria, Ltd. All were used as purchased without further purification process. The collected waste PS (thermocol) surface (20 g) was scraped off to remove dust and then cut into tiny particles. The tiny particles are stored in a transparent glass bottle and labeled. The density change was determined before (1.03 g/cm3) and after (1.3 g/cm3) crushing (Das & Mahalingam 2019).

Preparation of PSA

Periwinkle (Typanotonus fuscatus) empty waste shell (100 g) was purchased from a local dealer at Relief market Owerri, Imo State, and stored in a polyethene bag. The empty shells were thoroughly washed several times under continuous agitation in flowing tap water to remove loose debris and particles and then soaked in tap water for 10 days to remove other tough impurities. The shells were sun-dried at an average temperature of 29–32 °C for 5 days to remove moisture and finally dried in the oven at 110 °C for 3 h and they became brittle. The shells were pulverized using a ball mill and sieved through a 150-μm sieve. The shells were calcined (improve hydrophobicity) in a muffle furnace (CARBOLITE 300) at 800 °C for 3 h at 100 °C/10 min temperature program. The obtained PSA was again sieved through a 50-μm sieve to remove larger particles and aggregates and then stored in a tight glass bottle until use (Ugoeze & Chukwu 2015; Abdelmalik & Sadiq 2019).

PSA-doped PS/TiO2 photocatalyst preparation

The fabricated PSA@PS-TiO2 photocatalyst is described in this paper and is shown in Table 1. A mass of 10.0 g of PS is dissolved into 20 mL of xylene in a ceramic crucible (100 mm diameter) and placed in the fume cupboard in the dark for 2 h. Then, 7.6 mL of TTIP is added into a 40/20 mL ethanol/DDW mixture and stirred for 30 min at room temperature (29–31 °C) to obtain a uniform aliquot, and prepared in three different flasks. After stirring, measured amounts of 0.5, 2.5, and 5 wt.% of PSA are slowly added into each flask, respectively, and the mixture is stirred for 1 h at 60 °C by a magnetic stirrer (300 rpm) to form the TTIP/PSA aliquot. To create a homogeneous mixture of TTIP/PSA, 1 mole of 10 mL (HCl) initiator was then added drop by drop and agitated for 1 h at 60 °C. Once the PS mixture was homogenized, the TTIP/PSA suspension was gradually added, and the mixture was agitated for 1 h at 300 rpm at 60 °C. Finally, 48 h was given for settling and slow solvent evaporation. Then, the wet slurry was oven dried at 200 °C for 3 h in the muffle furnace for best photoactivity. Finally, it was removed, washed with water, dried in a vacuum at 80 °C for 4 h, and stored in a dark amber bottle. The procedure was repeated according to Table 1.

Table 1

Composition ratio for nanocomposites doped with PSA

S/NTiO2 7.6 mL ≡ 7 g equivalentPolystyrene (g)Periwinkle ash carbon (g)Weight % equivalent (≡)
7.6 10.0 0.09 0.5 
7.6 10.0 0.44 2.5 
7.6 10.0 0.88 5.0 
S/NTiO2 7.6 mL ≡ 7 g equivalentPolystyrene (g)Periwinkle ash carbon (g)Weight % equivalent (≡)
7.6 10.0 0.09 0.5 
7.6 10.0 0.44 2.5 
7.6 10.0 0.88 5.0 

Characterization techniques

Utilizing a TESCAN VEGA 3 LMH scanning electron microscope, physical–chemical and material characterization operations were completed. EDX spectra were plotted using an X-ray micro-analyzer (Oxford 6587 INCA X-sight) attached to the device at an accelerating voltage of 20 kV. A HR-TEM examined the nanoparticle (NP) and surface morphologies using JOEL-TEM 2100F. The samples were prepared by ultrasonically dissolving the specimens in methanol for 45 min. After the solution had dried on a copper grid covered in carbon, it was fed into the HR-TEM equipment. AFM was used to determine the topology and surface roughness (AFM-Nanoscope Bruker multimode 8). The spectral information about the composition of membranes and the presence of different functional groups on the membrane surface describing the membrane chemical structure of the sample was accomplished using Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR Shimadzu IR-Affinity-IS spectrometer). Using a Philips X'Pert PW 3040 Powder X-ray diffractometer with Cu K radiation (= 0.15406 nm), at a scanning rate of 0.02°/s, with a voltage of 40 kV and 40 mA, and data collected in the range of 10–60 (2°), X-ray diffraction (XRD) analysis was used to examine the phases of the nanocomposite. Shimadzu UV-Visible 1800's kinetic mode used UVDRS to analyze the photocatalyst's optical characteristics to estimate its bandgap performance. The experiment was done at room temperature in order to find absorption bands between 200 and 800 nm.

Photocatalytic experiments

The performance of the nanocomposite photocatalyst PSA@TiO2/PS in photocatalyzing the breakdown of MO was evaluated using a Bosch ZF-1 Ultraviolet Analyzer (lamp wavelength 254/365 nm). The simple instrument consists of a 6 W/220 V/ 50 HZ UV light (Bosch UV lamp) and adopts the ultraviolet lamp tube and a filter. Generally, 0.1 g of the photocatalyst was disseminated in the aqueous solution (total volume = 1 L, concentration = 10 mg/L). The suspension was agitated in the dark for 1 h before photo-irradiation to ensure that the photocatalyst and pollutant were in adsorption–desorption equilibrium. After being exposed to light, 5 mL of the supernatant was periodically removed and subjected to Shimadzu UV-1800 spectrophotometer analysis. The photodegradation of MO was investigated by measuring the absorbance at 522 nm (max). To get the photodegradation efficiency, apply Equation (1):
formula
(1)
where Co and Ci represent the pollutant's initial and final aqueous solution concentrations after irradiation. In photocatalytic processes, the Langmuir–Hinshelwood rate term is widely used to estimate the degradation rates of dyes. The pseudo-first-order model, as described in Equations (2)–(4), has been frequently utilized to investigate the catalytic reaction kinetics for organic pollutants such as MO after irradiation, where k1 is the pseudo-first-order photodegradation reaction rate constant. Plot of the process's UV light irradiation time against ln(Co/C):
formula
(2)
formula
(3)
formula
(4)

The influence of several experimental factors on photocatalytic efficiencies, such as pH, starting dye concentration, and catalyst loading, was also investigated. The intermediates and reaction products from photocatalytic degradation were analyzed using a SHIMADZU GCM-2010 gas chromatography-mass spectrometer (Japan). The stability of the photocatalyst was assessed by monitoring photoactivity over a number of cycles until a loss of efficiency. The catalyst was used for the subsequent run after being filtered, washed three times with ethanol and water, and dried for 4 h at 80 °C. Using 0.5 and 1 mL H2O2, the photocatalytic reaction mechanism was examined to ascertain the active species generated during the photocatalytic process. Regression modeling was carried out using Excel VBA and Origin.9.0 software, while computational experiments were carried out using the Accelrys Inc. 7.0 Material studio.

Morphology of photocatalyst

The PSA@TiO2/PS in Figure 1 depict that the microspheres of the photocatalyst were within 10–20 μm (Figure 1a,d,g). Higher magnification of the photocatalyst (Figure 1b,e,h) showed that they appeared as clusters of outwardly aligned crystallites. According to Zhang et al. (2013), the scan demonstrated that the carbon atoms present in PS and PSA prevented TiO2 from agglomerating and growing into larger microspheres. Also, it was observed that the film structure of PS was well-distributed, and there were uniformly formed sizes of porous aggregates distributed on the membrane substrates (Martins et al. 2017). This ensured that TiO2 was distributed within the created mesoporous (PSA) and microporous (PS) layers of the membrane substrates which increased the capacity to receive and utilize light in the photodegradation of dyes as observed by Altin & Sokmen (2014). The histogram (Figure 1c,f,i) established the presence of (prominent peak) anatase and (inconspicuous peak) rutile phases of TiO2. Other elements were Ca and S atoms from PSA, Cl atoms from PS, and O atoms from TiO2 and SiO2. These according to Nath & Mahato (2022) generated more reactive oxygen species, both on the film surface and inside the thin film, thereby enabling the PS matrix to induce faster mineralization of dyes.
Figure 1

SEM-EDX cross-section of PSA@TiO2/PS photocatalyst at (a–c) 0.5 wt.%, (d–f) 2.5 wt.%, (g–i) 5.0 wt.% at 10–20 μm magnification and the micrograph, histogram, and spectrum.

Figure 1

SEM-EDX cross-section of PSA@TiO2/PS photocatalyst at (a–c) 0.5 wt.%, (d–f) 2.5 wt.%, (g–i) 5.0 wt.% at 10–20 μm magnification and the micrograph, histogram, and spectrum.

Close modal

The spectrum (insets Figure 1c,f,i) showed that (Ti) and (O) emerged as the major composites at 45.4 and 29.0 wt.% in all the synthesized photocatalysts, respectively. The amount of C was higher than Ca which increased as the wt.% varied from 0.5 to 5.0 wt.%, while Ti reduced in 5.0 wt.% composition with the appearance of larger clusters of membrane substrates as observed by Xing et al. (2016). These results showed that PSA@TiO2/PS photocatalyst was of high purity and successfully synthesized.

The HR-TEM images of PSA@TiO2/PS in Figure 2(a), 2(c) and 2(e) depict nanoparticles of 50 nm sizes for 0.5–5.0 wt.% photocatalyst, while Figure 2(b), 2(d) and 2(f) represent NP sizes of 100 nm obtained for 0.5–5.0 wt.% photocatalyst. All the figures depict a homogenous morphology of PSA@TiO2/PS formation from the composites (Alejo-Molina et al. 2016). The dark areas showed the TiO2 catalyst which appears spherical-like and agglomerated within the matrix, while the lighter areas represented the PS/PSA matrix. The TiO2 aggregation occurred to form small clusters owing to nucleation and crystal growth into anatase and rutile phases with gradual loss of water. It confirmed that it collapsed after crystallization and not before crystallization was accomplished because of the uniformity in sizes of 0.5–5.0 wt.% photocatalyst (Wang et al. 2012).
Figure 2

HR-TEM cross-section of TiO2-PS/PSA photocatalyst at (a, b) 0.5 wt.%, (c, d) 2.5 wt.%, (e, f) 5.0 wt.% at 50 and 100 nm NP sizes.

Figure 2

HR-TEM cross-section of TiO2-PS/PSA photocatalyst at (a, b) 0.5 wt.%, (c, d) 2.5 wt.%, (e, f) 5.0 wt.% at 50 and 100 nm NP sizes.

Close modal

According to Granados-Martínez et al. (2017), the synthesis of PS nanocomposite material demonstrated the affinity of the (polystyrene–hydroxyl group) PS–OH matrix and other composites, hence a chemical bonding existed between the PS–OH matrix and other composites. The images further elucidated that the thermal treatment of TiO2 precursor (TTIP) yielded a systematic increase of different NP sizes and likewise the reaction temperature did not inhibit the development of TiO2 anatase and rutile nanoparticles which agreed with the SEM-EDX analysis. The images confirmed that the TiO2 catalyst was immobilized on a PS/PSA matrix and was uniformly distributed to obtain a PSA@TiO2/PS nanocomposite photocatalyst. According to Jaleh et al. (2011), the resolved lattice fringes provided evidence that the TiO2 material was highly crystalline in nature and equally established the successful synthesis of NP structures.

The cross-section of the AFM image of the PSA@TiO2/PS film surface is shown in Figure 3(a)–3(c). Figure 3(a) revealed the flattened topology while Figures 3(b) and 3(c) showed the layered topology. Also as the wt.% increased, the level of irregularity and roughness increased owing to the presence of PSA and TiO2. A similar finding on PVC with ZnO nanoparticles showed a membrane with rough surfaces that improved the ion-trapping ability of photocatalysts and also initiated the absorption of ions by the membrane composites with increased permeability and flux (Zarrinkhameh et al. 2013).
Figure 3

Cross-section of AFM images showing the surface and layer roughness of PSA@PS-TiO2: (a) 0.5 wt.%, (b) 2.5 wt.%, and (c) 5.0 wt.% raster metaphors.

Figure 3

Cross-section of AFM images showing the surface and layer roughness of PSA@PS-TiO2: (a) 0.5 wt.%, (b) 2.5 wt.%, and (c) 5.0 wt.% raster metaphors.

Close modal

Also, when compared with mesoporous carbon and TiO2 from another study, the findings of this study showed a significant surface roughness that differed as wt.% increased, while the surface of TiO2 showed a small degree of homogenous roughness and likewise the mesoporous carbon. AFM results demonstrated the effect of mesoporous PSA on the surface roughness of PSA@TiO2/PS (Hosseini et al. 2016). However, based on the image statistics in Table 2 on average roughness (Ra) the 2.5 wt.% had the highest surface roughness of 4.62 nm at 2.5 wt.%. The raw mean and image data showed the position and depth of image capture. The root mean square (Rq) which represented spatial differences to large deviations concerning roughness was also highest at 2.5 wt.% at 7.13 nm. Hence, 2.5 wt.% had a better surface roughness and topography than other 0.5 and 5.0 wt.% of TiO2-PS/PSA photocatalysts.

Table 2

Determined values of image raster and image roughness in TiO2-PS/PSA

TiO2-PS/PSA (wt.%)Mean image data (ra)Mean average roughness (Rq)Raw meanImage data range, z
0.5 1.575 nm 2.024 nm 12.84 nm 36.37 nm 
2.5 4.621 nm 7.131 nm 318.66 nm 80.84 nm 
5.0 2.690 nm 3.585 nm 597.50 nm 40.22 nm 
TiO2-PS/PSA (wt.%)Mean image data (ra)Mean average roughness (Rq)Raw meanImage data range, z
0.5 1.575 nm 2.024 nm 12.84 nm 36.37 nm 
2.5 4.621 nm 7.131 nm 318.66 nm 80.84 nm 
5.0 2.690 nm 3.585 nm 597.50 nm 40.22 nm 

Structure of photocatalyst

The cross-section of the FTIR-ATR spectrum of PSA@PS-TiO2 shown in Figure 4(a) depicts the core functional groups of all the photocatalysts of 0.5–5.0 wt.% and Figure 4(b) shows all the absorption bands detected in a 2.5 wt.% of the photocatalyst. The spectrum identified the composites that made up the photocatalyst as it depicted several functional groups reflected as peaks. As observed in PSA@PS-TiO2, the weak band at 1,798.2 cm−1 represented the C = O bonds formed from carbonate, while the strong band at 986.3 cm−1 showed the presence of Ca–O bonds as similarly reported by (Abdelmalik & Sadiq 2019). Also, the prominent PS matrix (microspheres) was observed at 3,129.0 cm−1 and also at 1,289.3 cm−1 which represents phenyl C–H stretching and C–C stretching vibrations as similarly observed (Zakia & Yoo 2022). Absorption bands at 3,695.6 cm−1 represented the anatase-TiO2 confirmed by a previous study (Shi et al. 2012), whereas, the 2,801.7 cm−1 peaks are the asymmetric and symmetric tensions of CH2 (Granados-Martínez et al. 2017). Likewise, the absorption bands at 2,500.0 cm−1 suggested carboxylic OH and SH thiols (Olusola & Babayemi 2019). The study also suggested that an increased level of interaction took place as the wt.% increased because there was no band displacement of membrane substrates and TiO2, and all the absorption bands were represented in 0.5, 2.5, and 5.0 wt.%. This implied that PS, PSA and TiO2 co-existed in all TiO2-PS/PSA samples and the sol-gel synthesis (hydrothermal method) avoided the loss of PS through calcination (volatilization). Then again PSA did not mask the presence of TiO2-anatase and the surface of the PSA@PS-TiO2 photocatalyst contained advantageous functional groups for enhancement of photocatalytic activity. The results agreed with findings obtained from SEM-EDX, HR-TEM, and AFM results.
Figure 4

Cross-section of FTIR-ATR spectrum of PSA@PS-TiO2 photocatalyst at (a) showing all the as-prepared photocatalysts of different wt.% and (b) enlarged 2.5 wt.% photocatalyst.

Figure 4

Cross-section of FTIR-ATR spectrum of PSA@PS-TiO2 photocatalyst at (a) showing all the as-prepared photocatalysts of different wt.% and (b) enlarged 2.5 wt.% photocatalyst.

Close modal
The XRD diffractogram of PSA@PS-TiO2 photocatalyst is shown in Figure 5, in which Figure 5(a) shows the positions of the miller indices, and Figure 5(b) shows the phases of the composites for 2θ and wt.% respectively. Typical diffraction peaks of PS (membrane substrate), PSA (dopant), and TiO2-anatase (catalyst) are evidenced in the spectrum which confirmed that they formed the main crystalline phases and miller indices. The XRD diffractogram of PSA@PS-TiO2 showed the appearance of peaks at 2θ for 25.3°, 29.1°, 32.1°, 42.0°, and 67.2° corresponding to the (101), (103), (004), and (116) crystalline planes which also matched JCPDS card 01-072-2496.
Figure 5

XRD diffractogram of PSA@PS-TiO2 photocatalyst: (a) 0.5–5.0 wt.% showing the miller indices/crystalline phases and (b) 0.5–5.0 wt.% of the TiO2 phases and membrane substrate.

Figure 5

XRD diffractogram of PSA@PS-TiO2 photocatalyst: (a) 0.5–5.0 wt.% showing the miller indices/crystalline phases and (b) 0.5–5.0 wt.% of the TiO2 phases and membrane substrate.

Close modal

The notable presence of anatase was at 2θ angle 25.1° for all PSA@PS-TiO2 photocatalysts. The nano TiO2-anatase peaks have been observed in a study conducted by authors (El-Didamony et al. 2020). The presence of PS can be seen at 15° 2θ, similarly observed by (Singh et al. 2014) while the presence of ashed carbon was detected at 60° 2θ by (Martins et al. 2017) and rutile phase 2θ was at 67.2° observed by (Wang et al. 2019). The anatase and rutile phases were effectively immobilized on the PS/PSA substrate. Next, the composition of the TiO2 crystalline phases was resolved from integrated intensities of anatase and rutile peaks are shown in Table 3 using the Spurr-Myers formulae.

Table 3

Calculated crystalline properties of the PSA@PS-TiO2 photocatalyst

Photocatalyst (wt.%)Anatase (w/v%)Rutile (w/v%)Lattice strain (%)FWHM (nm)Grain size (Å)
0.5 57.3 42.7 0.17–0.24 0.11–0.94 94–386 
2.5 62.0 38.0 0.17–0.24 0.11–0.94 94–386 
5.0 63.7 36.3 0.17–0.24 0.11–0.94 94–386 
PhotocatalystCrystal systemd-spacing (Å)Calculated density (g/cm3)Cell volume 106 pm
0.5 Rhombohedral 1.1–4.9 3.90–4.57 81.43–422.51  
2.5 Rhombohedral 1.1–4.9 3.90–4.57 81.43–422.51  
5.0 Rhombohedral 1.1–4.9 3.90–4.57 81.43–422.51  
Photocatalyst (wt.%)Anatase (w/v%)Rutile (w/v%)Lattice strain (%)FWHM (nm)Grain size (Å)
0.5 57.3 42.7 0.17–0.24 0.11–0.94 94–386 
2.5 62.0 38.0 0.17–0.24 0.11–0.94 94–386 
5.0 63.7 36.3 0.17–0.24 0.11–0.94 94–386 
PhotocatalystCrystal systemd-spacing (Å)Calculated density (g/cm3)Cell volume 106 pm
0.5 Rhombohedral 1.1–4.9 3.90–4.57 81.43–422.51  
2.5 Rhombohedral 1.1–4.9 3.90–4.57 81.43–422.51  
5.0 Rhombohedral 1.1–4.9 3.90–4.57 81.43–422.51  

The ratio of anatase and rutile phase w/v% in PSA@PS-TiO2 in Table 3 showed that 5.0 wt.% > 2.5 wt.% > 0.5 wt.%. The findings confirmed that TiO2-anatase was the predominant phase in the photocatalyst and increased as the wt.% of the dopant increased. The main crystal phases of TiO2 were rutile and anatase whose combined photo activities are more effective than a single effect from any of the phases in photocatalytic reactions (Bel Hadjltaief et al. 2015). The Xpert highscore software further calculated and predicted other parameters and information about the crystals (full weight at half maximum, FWHM, d-spacing and grain size) known as domain A and problems or defects from the crystals (cell volume, density, and lattice strains) known as domain B. From the table, it was observed that grain size remained stable within a range <200, while the small values of the FWHM and d-spacing, indicated better crystallinity. Also, the density was small which indicated uniformity of the crystal, whereas the cell volume indicated that identical cells (grain size) are packed in the same geometry and the possible predicted crystal system was rhombohedral.

The UVDRS absorption spectrum of PSA@PS-TiO2 in Figure 6 shows the UV-Visible absorption spectra and the Tauc plots showed the reduced bandgap in the photocatalyst. It was observed that in PSA@PS-TiO2, (a) the 2.5 wt.% was the most photoactive in the UV region while 0.5 wt.% was the least (b) the adsorption band of 5.0 wt.% extended the most into the visible regions while 0.5 wt.% was the least. The results confirmed that the 0.5–5.0 wt.% of PSA@PS-TiO2 photocatalysts were all UV-Vis light photoactive.
Figure 6

Absorption spectra from PSA@PS-TiO2 photocatalyst: (a) graph of UV and visible absorption spectra and (b) plot using the Kubelka–Munk formula for the bandgap calculation.

Figure 6

Absorption spectra from PSA@PS-TiO2 photocatalyst: (a) graph of UV and visible absorption spectra and (b) plot using the Kubelka–Munk formula for the bandgap calculation.

Close modal

The findings, therefore, confirmed that the dopants (PSA) improved the absorption of PSA@PS-TiO2 by extending the wavelength into the visible region due to the contributory effect of the different metal oxides on the surface of PS substrates. This broadening of absorption bands was also attributed to electron excitation from the d3 orbital of the metal ions (Fe2+, Al3+, Si4+ found in PSA) to the conduction band of TiO2 (Jafari & Afshar 2016). Hence, the addition of dopants in the polymer nanocomposites enhanced the optical and physicochemical properties of the polymeric membranes due to the homogeneous dispersion of nano-fillers in the polymer matrix. The bandgaps in Table 4 were determined by the Kubelka–Munk formula (Adnan et al. 2019)

Table 4

The calculated band gap of PSA@PS-TiO2 by the Kubelka–Munk equation

Bandgap (eV)0.5 (wt.%)2.5 (wt.%)5.0 (wt.%)
TiO2-PS/PSA 3.1 2.5 3.0 
Nature of band Indirect Indirect Direct 
Bandgap (eV)0.5 (wt.%)2.5 (wt.%)5.0 (wt.%)
TiO2-PS/PSA 3.1 2.5 3.0 
Nature of band Indirect Indirect Direct 

Table 4 shows that 2.5 wt.% of PSA@PS-TiO2 bandgap was better than 0.5 wt.% greater than 5.0 wt.% of the photocatalyst. Also, the quantum effects allowed both the direct (5.0 wt.%) and indirect (0.5 and 2.5 wt.%) electronic transitions (Martins et al. 2017). The data in Table 3 show that all the bandgaps were lower than TiO2 bandgap 3.2 eV (Jafari & Afshar 2016) and all the composites co-jointly contributed to the reduction of the photocatalyst band gap below 3.2 eV. Also, the lowered bandgap consequently extended the wavelength of all the photocatalysts into the range of photoactivation of TiO2 by both visible and ultraviolet radiation (Huang 2016). Therefore, an improved bandgap enhanced the photodegradation of dye wastewater effluents (Lee et al. 2014). Although it can be possible to characterize the photocatalyst using a BET surface analyzer to further establish that the dopants enhanced the surface area and porosity of the photocatalyst in Figure 6(a), it was outside the scope of the study. This possibility has the benefit of equally boosting incident light harvesting in Figure 6(b). The dopants are also attributed to contributing to the small crystallite size and high crystallinity found in the XRD data. When compared to TiO2 NP formations made of disordered TiO2, this attribute enabled the electrons to traverse the material more freely, boosting conductivity in Figure 6(b). The dopants can then act at donor levels as well as charge-trapping centres that reduce the electron recombination process while driving the photogenerated charges.

Photocatalytic performance

The photocatalytic degradation of MO by PSA@PS-TiO2 was carried out using absorbance measurement via UV-Vis spectroscopy. The initial trials obtained are shown in Figure 7(a) and 7(b). In the preliminary experiment, Figure 7(a) and 7(b), photocatalytic degradation of MO by pristine PSA and pristine PS without the TiO2 photocatalyst was observed for 30 min and then for 300 min under UV-Vis irradiation. The results confirmed minor degradation (<25%) of MO by PS or PSA. It showed that composite materials can partake in the photodegradation of MO but are unable to drive the oxidation process to the decomposition of pollutants.
Figure 7

Degradation efficiency of pristine polystyrene and periwinkle ash carbon: (a) dosage amount and (b) irradiation time.

Figure 7

Degradation efficiency of pristine polystyrene and periwinkle ash carbon: (a) dosage amount and (b) irradiation time.

Close modal

Photocatalytic activities

The effect of the initial pH of a pollutant in an aqueous solution plays a vital role in photocatalysis and influences the rate of degradation. Consequently, the effect of initial solution pH on MO azo dye photocatalytic degradation by PSA@PS-TiO2 experiment was performed within the pH range of 3–11. The acquired results presented in Figure 8(a)–8(c) show a steady increase in degradation efficiency is obtained at the natural pH to be 7.2 and decreased going from 8 to 11. The observed pattern in degradation efficiency with pH is associated with the presence of additional hydroxide ions on the TiO2 surface, driving a higher generation of OH species and consequent improvement of MO dye rate of photodegradation. The decrease in photodegradation was due to increased electrostatic repulsion between the PSA@PS-TiO2 and MO dye. Also, it could be that at acidic and alkaline mediums electrostatic repulsion is minimal which favors agglomeration that hinders absorption and transmission of light, in other words at those pH levels the surfaces were covered in dye molecules which prevented active light penetration. Also, the degradation ratio (C/Co) of PSA@PS-TiO2 occurred from 0.6 showing that PSA@PS-TiO2 gradually degraded MO (Table 5). The data were well-fitted using linear curves, and the correlation coefficients and reaction rate were both positive. Beneficial rate constants (k) indicate that membrane substrates had a positive impact on the pH kinetics. Low activation energy and low-rate constants together allowed for more molecules to interact favorably with the photocatalyst.
Table 5

Values from the effect of pH on photodegradation kinetics of MO over PSA@PS-TiO2

Photocatalyst (wt.%)Initial concentration (mg/L)Rate constant K1 (min−1)Rate of reaction (mg/L.min−1)Correlation coefficient (R2)
0.5 10 0.00583 0.5833 0.9640 
2.5 10 0.00428 0.4308 0.9706 
5.0 10 0.00514 0.3649 0.9931 
Photocatalyst (wt.%)Initial concentration (mg/L)Rate constant K1 (min−1)Rate of reaction (mg/L.min−1)Correlation coefficient (R2)
0.5 10 0.00583 0.5833 0.9640 
2.5 10 0.00428 0.4308 0.9706 
5.0 10 0.00514 0.3649 0.9931 
Figure 8

Photocatalysis of PSA@PS-TiO2: (a) effect of pH at 7.2 on the rate of photodegradation of methyl orange; (b) pH of UV-Visible irradiation and methyl orange photodegradation on the photocatalyst before and after irradiation; and (c) first-order rate kinetic graph of the photodegradation of methyl orange at varied photocatalyst pH loadings (0.5–5.0 wt.%, 300 min, and 10 mg/L concentrations).

Figure 8

Photocatalysis of PSA@PS-TiO2: (a) effect of pH at 7.2 on the rate of photodegradation of methyl orange; (b) pH of UV-Visible irradiation and methyl orange photodegradation on the photocatalyst before and after irradiation; and (c) first-order rate kinetic graph of the photodegradation of methyl orange at varied photocatalyst pH loadings (0.5–5.0 wt.%, 300 min, and 10 mg/L concentrations).

Close modal
The effect of the initial concentration of dye on the photodegradation of MO azo dye was studied. The results are shown in Figure 9(a)–9(c) and Table 6. The dye concentration was obtained via serial dilution from stock solution into 10–100 mg/L. It showed that increasing the dye concentration levels led to a significant lowering of PSA@PS-TiO2 efficiency. This correlated to similar findings where a large amount of UV-Vis light is absorbed by the MO dye instead of the catalyst.
Table 6

Effect of initial concentration on photodegradation kinetics of MO over PSA@PS-TiO2

Photocatalyst (wt.%)Initial concentration (mg/L)Rate constant K1 (min−1)Rate of reaction (mg/L.min−1)Correlation coefficient (R2)
0.5 10 0.091 0.9897 0.9249 
2.5 10 0.073 0.7614 0.9681 
5.0 10 0.072 0.4548 0.9861 
Photocatalyst (wt.%)Initial concentration (mg/L)Rate constant K1 (min−1)Rate of reaction (mg/L.min−1)Correlation coefficient (R2)
0.5 10 0.091 0.9897 0.9249 
2.5 10 0.073 0.7614 0.9681 
5.0 10 0.072 0.4548 0.9861 
Figure 9

Photocatalysis of TiO2-PS/PSA: (a) effect of concentration at 10 mg/L on the effectiveness of methyl orange photodegradation; (b) adsorption and methyl orange photodegradation concentrations on the photocatalyst before and after UV-Visible radiation; and (c) first-order rate kinetic graph of the photodegradation of methyl orange at various photocatalyst loadings (0.5–5.0 wt.%, 300 min, pH 7.2).

Figure 9

Photocatalysis of TiO2-PS/PSA: (a) effect of concentration at 10 mg/L on the effectiveness of methyl orange photodegradation; (b) adsorption and methyl orange photodegradation concentrations on the photocatalyst before and after UV-Visible radiation; and (c) first-order rate kinetic graph of the photodegradation of methyl orange at various photocatalyst loadings (0.5–5.0 wt.%, 300 min, pH 7.2).

Close modal

The effect diminishes the photon flux of the PSA@PS-TiO2 surface and results in a decrease in photodegradation efficiency (Santhosh et al. 2018). Hence, the MO azo dye concentration was fixed at 10 mg/L concentration. The degradation ratio (C/Co) showed that PSA@PS-TiO2 occurred within 0.6 ratios and showed gradual degradation (Table 6). The graphs also had a good fitting of the kinetics and the calculated correlated coefficients (R2). Also, positive rate constants suggested that substrates have a positive effect on the composition and a low activation for the generation of electron holes.

The effect of the photocatalyst loading on the photodegradation of azo dye was similarly examined on catalyst loading of 0.5–2 g/L The data are plotted in Figure 10(a)–10(c). The graph clearly showed that increasing the photocatalyst loading consequently improved the photocatalytic degradation efficiency. This increase in catalyst loading caused a rise in the number of generated hydroxyl radicals in addition, it makes for more efficient utilization of UV-Visible radiation. In this study, 1 g/L catalyst loading was optimum for the photodegradation of dye. It showed that the linear graphs had a good fitting and calculated R2 values described a better fitting in all photocatalysts (Table 7). All the rate constants (k) showed positive values which depict that the k values were all positive and indicated that the membrane substrates had a positive influence on the prepared photocatalyst. Also, the low-rate constants depict low activation that allows more molecules to react in the presence of the photocatalyst. Thus, an increase in wt.% of the composite increased photoactivity in 0.5 and 2.5 wt.% but insignificant in 5.0 wt.%. Also, there was no aggregation effect to reduce the interfacial area between the dye solution and the photocatalyst (Santhosh et al. 2018).
Table 7

Effect of catalyst loading on photodegradation kinetics of MO over PSA@PS-TiO2

Photocatalyst (wt.%)Initial concentration (mg/L)Rate constant K1 (min−1)Rate of reaction (mg/L.min−1)Correlation coefficient (R2)
0.5 10 0.0055 0.3519 0.9903 
2.5 10 0.0056 0.3831 0.9916 
5.0 10 0.0039 0.2168 0.9939 
Photocatalyst (wt.%)Initial concentration (mg/L)Rate constant K1 (min−1)Rate of reaction (mg/L.min−1)Correlation coefficient (R2)
0.5 10 0.0055 0.3519 0.9903 
2.5 10 0.0056 0.3831 0.9916 
5.0 10 0.0039 0.2168 0.9939 
Figure 10

Photocatalysis of PSA@PS-TiO2: (a) effect of catalyst loading at 0.1 g over methyl orange, (b) performances of adsorption and photodegradation before and under UV-Visible light, and (c) first-order rate kinetic graph of the photodegradation of methyl orange at varied photocatalyst loading percentages (pH 7.2, loading, 0.5–5.0 wt.%, time, 300 min, concentrations).

Figure 10

Photocatalysis of PSA@PS-TiO2: (a) effect of catalyst loading at 0.1 g over methyl orange, (b) performances of adsorption and photodegradation before and under UV-Visible light, and (c) first-order rate kinetic graph of the photodegradation of methyl orange at varied photocatalyst loading percentages (pH 7.2, loading, 0.5–5.0 wt.%, time, 300 min, concentrations).

Close modal

Addition of peroxides

To determine the role of reactive species involved in the photocatalytic mechanism of PSA@PS-TiO2 photocatalyst, H2O2 was added into the MO azo dye photodegradation process as a scavenger for hydroxyl radicals (OH). This experiment examined the effects of adding (0.5 and 1.0 mL) H2O2 at an initial MO concentration of (100 mL) 10 mg/L and 2.5 wt.% of photocatalyst at their natural pH for a duration of 300 min. Figure 11 demonstrates how the H2O2 addition accelerated the photodegradation of MO dye. After 30 min they attained 70% degradation for 0.5 mL and 75% for 1.0 mL addition of H2O2. Also, further introduction of 1.0 mL reached maximum degradation at 150 min while 0.5 mL attained maximum at 300 mL. Therefore, moderate addition of H2O2 was advantageous for higher photoactivity.
Figure 11

Effect of H2O2 addition on the photodegradation of the MO dye by the photocatalyst.

Figure 11

Effect of H2O2 addition on the photodegradation of the MO dye by the photocatalyst.

Close modal
Hence, using H2O2 as a photo-oxidant causes faster and more efficient mineralization than just individual photocatalysts by scavenging electrons from the conduction band of the photocatalyst (generated electrons) to generate excess hydroxyl radicals as shown in Equations (5)–(7) (Alkayal & Hussein 2019):
formula
(5)
formula
(6)
formula
(7)

Photodegradation mechanism

The degradation pathway of MO by PSA@PS-TiO2 was carried out to investigate the mechanism. The reaction was performed using UV-Vis light irradiation on the dye solution containing the photocatalyst, which was filtered and injected into GC–MS via electrospray ionization at predetermined time intervals to identify the reaction intermediate. The principal intermediates were proposed using the m/z values obtained from the mass spectra (Table 8). The analysis showed the dyes were stable, and then the introduction of a photocatalyst led to the formation of some aromatic intermediates, though some peaks had higher molecular values than 327 g/mol.

Table 8

Mass spectra of some reaction intermediate in MO azo dye degradation

m/zChemical formulaIUPAC nameChemical structure
276 C12H8N2O2S2 1-(Sulfinylamino)-2-[2-(sulfinylamino)phenyl]benzene  
234 C12H10O34-Phenylbenzenesulfonic acid  
198 C10H15NO3 2-(Diethoxymethyl)-1-oxidopyridin-1-ium  
196 C9H12N2OS S-[4-(Dimethylamino)phenyl] carbamothioate  
153 C8H11NS 2,4-Dimethyl-6-methyl sulfanyl pyridine  
m/zChemical formulaIUPAC nameChemical structure
276 C12H8N2O2S2 1-(Sulfinylamino)-2-[2-(sulfinylamino)phenyl]benzene  
234 C12H10O34-Phenylbenzenesulfonic acid  
198 C10H15NO3 2-(Diethoxymethyl)-1-oxidopyridin-1-ium  
196 C9H12N2OS S-[4-(Dimethylamino)phenyl] carbamothioate  
153 C8H11NS 2,4-Dimethyl-6-methyl sulfanyl pyridine  

The one of (m/z = 320) showed higher stability than the parent peak. The other species are moieties of the parent peak. We also find fragments of m/z = 276 breaking into 212 and 156; 234 fragments breaking into 152, and 198 breaking down to 124 and 78. The presence of hydroxyl groups in the aromatic rings of the molecule indeed suggested that a degradation mechanism is operating. Hence, homolytic bond breaking is a major form of bond breaking and bond formation in MO degradation. As similarly observed by Ghattavi & Nezamzadeh-Ejhieh (2020), the azo group receives the primary attack owing to the appearance of superoxide or OH radical as seen on some radicals. As the attack continued, the radicals are broken down into benzene, phenol-fragments like phenoxy radicals, and sulfuric acid. A further attack by *OH or superoxide molecule on benzene and sulfuric acid will yield carbonic acid, carboxylic acid, maleic acid, and oxalic acid which are then broken down into CO2 and H2O via persisting radical attack. While the phenoxy radical is degraded by the superoxide and converted to maleic acid, ethanediol, ethylene glycol, and by persisting OH radical. These smaller intermediates are then mineralized into CO2 and H2O.

Based on the experimental findings in this research work, we attempt to propose the mechanism of the reaction for the photocatalytic degradation of the dyes with the nanocomposite photocatalyst under UV-Visible light. When nano TiO2 is irradiated by UV-Visible light, as shown in Figure 12 with energy equal to or higher than the bandgap of TiO2 (3.2 eV, λ < 385), it initiated the photocatalytic process due to the electronic excitation of TiO2 that created electrons and electron–hole mechanism. This photoexcitation of TiO2, promoted the transfer of electrons, e from the valence band (VB) to the conductance band (CB) which generates an electronic vacancy or positive holes h+ in the valance band and subsequent generation of electron–hole pairs. Then the generated electrons in the CB are transferred to the conduction band of PSA@PS-TiO2, whereas the electron holes are transferred from the valence band of PS to PSA active surface. TiO2 was immobilized on PS/PSA surfaces, which increased the dyes' surface area and improved the TiO2 catalyst's reaction mechanism, facilitating the adsorption process. According to the UVDRS results, the photocatalyst is optically active in the UV-Vis region (with a bandgap of TiO2 3.2), and the semiconductors present in PSA (SiO2, Fe2O3, and Al2O3) form semiconductor heterojunctions that prolong the carrier lifetime and prevent the photogenerated electrons from recombining too quickly.
Figure 12

Proposed photodegradation model for the MO azo dye over PSA@PS-TiO.

Figure 12

Proposed photodegradation model for the MO azo dye over PSA@PS-TiO.

Close modal

Stability and recyclability

The photocatalyst's potential for use in industrial applications was assessed by its recyclability. In succeeding cycles, the test was carried out under ideal circumstances up until a drop in photoactivity. To completely destroy 10 mg/L of dye solutions at the pH of the photocatalyst, each cycle lasted 360 min. Figure 13, which highlights the effectiveness of the photocatalyst, illustrates the dependability of the catalysts. According to the graph, PSA@PS-TiO2 was usable for five runs before degrading on average by 5 and 10%, respectively, in the sixth and seventh cycles. As a result, the photocatalyst is reliable up to the fifth run before the catalyst degrades. It was proposed that the decrease in degradation percentage was caused by intermediate products lowering the electron holes poisoning the catalyst surface. By slowly recovering the catalyst through filtration, washing, and annealing at 110 °C for 1 h, the stability test was carried out. The diffractograms of the recovered photocatalyst did not significantly differ from those in Figure 5. This suggested that the photocatalysts were reliable in the testing environment and appropriate for commercial use and possible scale-up.
Figure 13

Cyclic curve of the degradation of methyl orange by PSA@PS-TiO2 nanocomposite photocatalyst (concentration = 10 mg/L, pH = 7.2, irradiation time = 300 min).

Figure 13

Cyclic curve of the degradation of methyl orange by PSA@PS-TiO2 nanocomposite photocatalyst (concentration = 10 mg/L, pH = 7.2, irradiation time = 300 min).

Close modal

Process optimization

Using the stated regression model in the previous study (Nkwoada et al. 2022b) process optimization was carried out to maximize the impact of time, concentration, and catalyst loading. The Response Surface Methodology (RSM) rigid approach was not followed in the design of the experiment due to the sensitive and unpredictable nature of the investigation. Instead, a novel combination of Excel VBA and Origin 9.0 version 2 was used to obtain meaningful data and mapping. The Multiple Linear Regression tool in Excel VBA was used to estimate the degradation efficiencies that are depicted in Figure 14 for the effects of starting concentration (X2, 10–20), catalyst quantity (X3, 0.5–5.0), and duration (X1, 30–300) on the photodegradation of MO dyes. The model was then modified in Origin 9.0 software to produce both 3D response surface color maps (DOE), which are shown in Figure 15.
Figure 14

Response surface plots showing ongoing interaction in PS/PSA-TiO2 between two parameters, concentration, and amount (a), time and amount (b), and time and concentration (c) on the methyl orange degradation percentage.

Figure 14

Response surface plots showing ongoing interaction in PS/PSA-TiO2 between two parameters, concentration, and amount (a), time and amount (b), and time and concentration (c) on the methyl orange degradation percentage.

Close modal
Figure 15

Predicted values of the response against the actual values plotted against irradiation time and 18 experimental runs in methyl orange.

Figure 15

Predicted values of the response against the actual values plotted against irradiation time and 18 experimental runs in methyl orange.

Close modal

From the data, the highest degradation efficiency was 92% when the concentration was 10 mg/L and the amount was 5.0 wt.%, and it was 95% when the time was 300 min and the amount was 5.0 wt.%, according to the data and the relationship of the variance. Finally, when the time was 300 min and the concentration was 10 mg/L, 98% efficiency was attained.

The projected data for dye photodegradation efficiency were plotted against the actual data based on the graph in Figure 15. High accuracy and good precision were displayed on the graph. This confirms that this model can be used commercially in the decision-making process. According to our ANOVA calculations, the model was statistically significant at a 95% confidence level with an F-value of MO of 13.63 and a p-value of 4.87 × 10–6.

Computational modeling

Using the Accelrys materials studio 7.0 software the DMol3 tool was employed to build and determine the FUKUI indices which include the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) adsorption energy, rigid adsorption energy and deformation energy, etc. The PSA@PS-TiO2 had high energy (repulsion) which implied that it has fewer interacting particles and consequently low deformation energy to achieve geometry in Figure 16(a). This showed that the adsorption energy played a dominant role at this stage and surpassed the influence of other forces which may affect the mechanism of the reaction. Also, the negative value of adsorbate represented a stronger adsorption process. The negative HOMO and LUMO determinants in dye molecules (Figure 16(b)) showed that they are inbound states. Generally, the smaller HOMO (electron donor) and LUMO (electron acceptor) gap between the nucleophile and electrophile depicts a faster reaction kinetics and the band gaps are appropriate for reaction initiation. In the MO; it was a nucleophilic attack initiated at position number 6 by a nitro group (–N = O–) which was the highest value for all attacks shown in Table 9. Oxidation was likely initiated at the amine nitrogen atom to form an azoxy compound (N = N) which eventually drove the photodegradation of the dye molecule into smaller fragments.
Table 9

Fukui indices for nucleophilic attack in methyl orange. Bolded values represent the highest value and point of attack

AtomMullikenHirshfeld
S (1) 0.010 0.020 
O (2) 0.023 0.026 
O (3) 0.035 0.032 
O (4) 0.038 0.033 
N (5) 0.021 0.038 
N (6) 0.089 0.094 
N (7) 0.073 0.073 
C (8) 0.033 0.040 
C (9) 0.012 0.028 
C (10) 0.016 0.031 
C (11) 0.030 0.043 
C (12) −0.008 0.014 
C (13) 0.029 0.035 
C (14) 0.036 0.037 
C (15) 0.004 0.022 
AtomMullikenHirshfeld
S (1) 0.010 0.020 
O (2) 0.023 0.026 
O (3) 0.035 0.032 
O (4) 0.038 0.033 
N (5) 0.021 0.038 
N (6) 0.089 0.094 
N (7) 0.073 0.073 
C (8) 0.033 0.040 
C (9) 0.012 0.028 
C (10) 0.016 0.031 
C (11) 0.030 0.043 
C (12) −0.008 0.014 
C (13) 0.029 0.035 
C (14) 0.036 0.037 
C (15) 0.004 0.022 
Figure 16

Structural modeling of PSA@PS-TiO2 and molecular orbitals of dye molecules.

Figure 16

Structural modeling of PSA@PS-TiO2 and molecular orbitals of dye molecules.

Close modal

However, it is pertinent to explain certain specific photocatalyst limitations and biases that may be present during the degradation of dye effluent. For instance, scaling up to an industrial level with time and cost–effectiveness was outside the scope of the work, even though recovery and reusability were taken into account in the laboratory. Similarly, it is anticipated that using visible-light photocatalysis in place of UV lamps will result in faster degradation, but it is known that under solar illumination, photocatalysts with apparent quantum yields below 600 nm typically have less than 10% efficiency. Moreover, certain commonly used semiconductors, such as ZnS and ZnO, were first discontinued due to their low stability under irradiation, with only TiO2 being evaluated as a potential photocatalyst for this specific nanocomposite synthesis. Also, the volume of data that had to be condensed to fit into this literature made it impossible to give a detailed comparison of the data reported in different studies. A good implication of the constraints mentioned above is that a disturbance-free environment would be necessary if solar energy were to be used in place of UV lamps. The fact that slow kinetics seen in some parameters were thought to be caused by dopant-induced midgap states that affected the surface redox processes as another consequence. Likewise, the thermodynamic viability of the redox processes and the probable degradation products were impacted by the differing CB and VB potentials of the ZnS and ZnO semiconductors. Therefore, studying the thermodynamic driving forces for the reactions would identify a reaction pathway affecting the semiconductors' selectivity. The study also provided a future direction for a robust photocatalyst design for a range of applications. There is still much effort to be done to make these materials and photocatalysts feasible as well as to employ molecular modeling software to comprehend the degradation mechanism. One of the current challenges of any stable photocatalyst is the prevention of electron recombination and low band gap and commercial viability. But the research findings have demonstrated the efficiency of a low-cost photocatalyst that effectively reduced the band gap, prevented fast electron combination and effectively photodegrade MO dye gaining 5× original reusability value. This nano-sized photocatalyst can be modified to meet the current research need in academia and industry to selectively absorb volatile organic compounds, cause water splitting/production of cheap hydrogen, facilitate bond cleavage of aromatic rings and photodegrade emerging contaminants.

The doping of PSA into PS-TiO2 materials resulted in a significant enhancement in the photocatalytic activity of the obtained nanocomposite photocatalyst. The prepared PSA@PS-TiO2 nanocomposite exhibited a reduced bandgap due to the presence of elements in d3 orbitals found in PSA. The X-ray diffractogram showed the photocatalysts containing both anatase and rutile phases suggesting an improved photoactive material. The UVDRS revealed the new photocatalyst can act as donor levels as well as charge-trapping centres that reduce the electron recombination process while driving the photogenerated charges. This resulted in the improvement of the photoactivity of the PSA@PS-TiO2 nanocomposite over regular photocatalysts. The as-prepared photocatalyst showed high photon utilization efficiency of 95% toward the photodegradation of MO dye. Additionally, the photodegradation reaction followed the Langmuir–Hinshelwood kinetic model, and the rate constants (k) showed positive values for all increases in wt.% increase and suggested showing a positive influence on the membrane substrate while the positive rate of reaction suggested low activation energy allowing more molecules to react favorably with the photocatalyst. The chemical structure retained its efficiency after five cycles demonstrating its promising industrial photocatalyst for the removal of toxic dye pollutants. A series of four-step basic mechanisms of adsorption, photoexcitation, charge diffusion, and oxidation/reduction reaction to photodegrade the dyes to carbon dioxide, water, and other intermediate products was observed.

Much gratitude goes to the Department of Chemistry, Meta-Catalysis Laboratory of the University of Johannesburg, Auckland Park, the Republic of South Africa for the use of their modern research equipment for characterization as well as the central laboratory of Bowen University, Iwo Osun State, Nigeria and Laboratory of Chemistry Department, FUTO, Owerri.

A.N. developed the concept, prepared the methodology, collected data, wrote the original draft, did research and formal analysis, and acquired funds. G.O. prepared the methodology, did formal analysis, and supervised the study. M.O. did project administration, supervised the study, and reviewed the article. E.E.O. conceptualized the study, prepared the methodology, wrote, reviewed, edited, and supervised the article.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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