Green synthesis of Zinc sulphide (ZnS) nanostructures using S. frutescences plant extract for photocatalytic degradation of dyes and antibiotics

Pollutants such as dyes and pharmaceuticals have become a problem in the environment, thus there is a need to find multifunctional materials that are safe and can be used for the removal of various pollutants. In this study, we report on the synthesis of Zinc sulphide (ZnS) nanostructures and their use as photocatalysts for the degradation of dyes and various antibiotics. Fourier transform infrared spectroscopy (FTIR) confirmed the functional groups found in plants and these were linked to the biomolecules identified through Liquid chromatography-mass spectrometry (LCMS). Ultraviolet-visible spectroscopy (UV–vis) and x-ray diffraction (XRD) confirmed the formation of the ZnS nanostructures. Thermal Gravimetric Analysis (TGA) and Brunner Emmet Teller (BET) confirmed the material was thermally stable up until 480 °C and mesoporous in nature, respectively. Scanning electron microscope (SEM) and transmission electron microscope (TEM) showed that the material is spherical in shape and energy dispersive spectroscopy (EDS) further corroborated their formation. From the degradation analysis, 90% of the malachite green (MG) dye could be degraded in 60 min at optimum conditions (pH 6, 25 mg and 10 mg l−1) and the holes were responsible for the degradation. Lastly, when tested against antibiotics, the ZnS material managed to degrade both the sulfisoxazole (SSX) and sulfamethoxazole (SMX). These results showed that the ZnS nanoparticles could be used as a multifunctional material for the degradation of various pollutants.


Introduction
Water scarcity has become a threat to the human kind for some decades. The reports from the World Health Organization estimated that about 1.1 billion of people are unable to access safe and clean water worldwide. This is as a result of industrialization, urbanization, climate change, human activities and agricultural activities [1]. The number of contaminants found in our water streams has increased over the years in particular the dyes and antibiotics [2]. This is due to the over reliance on organic dyes by the textile industry especially for dying cloths, thus causing leaching. Also with the high number of patients being prescribed antibiotics such as sulfisoxazole (SSX) and sulfamethoxazole (SMX) due to various ailments, these drugs tend to be excreted through urine and faeces, causing a high concentration in our water streams [3,4]. These pollutants if they are not attended to and exceed the permissible levels, they can be detrimental to both human and aquatic life. Hence, the use of semiconductor photocatalyst has emerged as a promising way to remove these pollutants from wastewater. Several methods have been used for the removal of organic pollutants such as dyes, but because these dyes consist of the complex structure, most methods such as reverse osmosis, adsorption, electrocoagulation etc have fallen short [5]. These methods either rely on expensive adsorbents, produce secondary pollutants, need multiple steps or require high energy, which may not be feasible when one considers cost factors. Antibiotics are also a concern because they are among the fastest-growing new toxins in our waterways, it is imperative to find methods and materials that are environmentally friendly and can be used to target various pollutants [3].
The photocatalysis method has been extensively used in the last few decades due to its fast oxidation, nontoxicity and ability to degrade pollutants into environmentally viable materials [6]. This method uses light to generate electrons and holes which promote the generation of reactive species that are effective oxidizers [7,8]. The pace in which the oxidation and reduction process is occurring during photocatalytic activity determines its removal efficiency.
Semiconductor Zinc sulphide (ZnS) nanostructures is one of the well-studied material which have been employed as a photocatalyst for removal of organic pollutants like Methyl orange (MO), Methylene blue (MB) and Rhodamine blue (RhB). ZnS is known to have a wide band gap, superior light absorption and photocatalytic properties [4]. Moreover, they also have interesting optical and electronic properties. The semiconductor belongs to the II-VI group and has the band gap ranging from 3.63 to 3.92 eV [2]. It normally crystallizes to hexagonal wurzite and cubic zinc blende structure. The ZnS nanostructures have been synthesized using different conventional methods. Salavati-Niasari et al [5] prepared wurtzite ZnS nanorods via the hydrothermal route, with the diameter ranging from 40 to 200 nm. La Porta et al [6] also synthesized cubic ZnS nanoparticles using microwave synthesis. In other studies, Huo et al [7] employed a solid-state method for the synthesis of their particles. Though these conventional methods used had successfully synthesized these particles, they do suffer from some limitations. Generally, it is known that nanostructures agglomerate fast in the presence of capping agents [8,9]. Furthermore, these methods use toxic chemicals which result in poor control of particle size, shape and reproducibility [4]. For those particular reasons, suitable green synthesis methods have been developed in order to avoid such limitations. The green synthesis methods include the uses of plants, bacteria, fungi and algae for the synthesis of nanostructures. Among all these plants are preferred because of their ease at handling and accessibility [10]. Sutherlandia frutescens is the plant of interest which has been traditionally used as a tonic. It has also been reported that its consists of phytochemical like GABA, canavanine and flavonols. The plant has been used before for the synthesis of ZnO [8] and CdS [11] nanoparticles, so this is evident enough that the plant can be used as a reducing and stabilizing agent for the nanostructures. The green synthesis of spherelike ZnS nanoparticles with an average size of 45 nm have been reported using Corymbia citriodora leaf extract [12]. These nanoparticles were evaluated for the photodegradation of MB dye and they showed excellent performance whereby 96% of the dye was degraded using UV light. Senapati et al [13] used Elaeocarpus floribundus leaf extract to synthesize spherical ZnS nanoparticles with a small particle size of 3-8 nm, which were investigated for their electrical properties.
In light of these, there are not many studies where green derived materials have been used for the degradation of antibiotics. In this study, the ZnS nanostructures were synthesized using Sutherlandia frutescence (S. frutescence) plant extracts. The degradation of MG dye at various conditions is investigated. And lastly, we report on the degradation of antibiotics, SMX and SSX using green ZnS for the first time.

Materials and reagents
Sutherlandia frutescens (S. frutescence) tea was purchased from a local store in Polokwane, RSA. MG, SMX, ZnCl 2 and Na 2 S were all purchased from Protea Labs, South Africa. SSX was given by Dr T Leboho from the Chemistry Department, University of Limpopo, RSA.

Extraction of S. Frutescence plant and synthesis of ZnS nanoparticles.
S.frutescence extract was prepared using the tea infusion method as described by Mahlaule-Glory without any modification [8]. The extract was kept in the refrigerator at 4°C and used for further reaction analysis. For the synthesis of ZnS nanoparticles, in 100 ml of S. frutescens plant extract, an amount 2.5 g of ZnCl 2 and Na 2 S salt were added while stirring, and the solution was heated at 80°C for 1 h. Thereafter, the solution was centrifuged at 2000 rpm for 15 min, washed three times with distilled water and dried in the oven at 80°C. The final powder was crushed and calcined at 500°C for 2 h.

Characterization
The phytochemicals present in S. frutescens extracts were confirmed by a synats G 2 quadrupole time-of-flight (QTOF) mass spectrometer (MS) connected to a water acquity ultra-performance liquid chromatography analysis. The functional groups of the materials were analyzed using FTIR, Perkin Elmer spectrum 100 FT-IR spectrometer in a scan range of 500-4000 cm −1 . The multi-purpose x-ray diffractometer (XRD) D8-Advance from Bruker in a scan range of 15-75 2θ was used to determine the crystallite size, crystallinity and phase identification of the synthesized materials. UV-vis, Beckman Coulter DU 730 UV-vis spectrophotometer was used for optical analysis and to track the degradation of the dye and antibiotic, with wavelengths of 665 and 285 nm, respectively. TGA Q500 V20.13 Build 39 was used for the determination of the thermal stability and weight loss of the material using a Perkin Elmer Pyris instrument. For each run, roughly 10 mg of the sample was used and the temperature of focus was between 35 and 900°C and the reaction taking place at a rate of 10°C min −1 . Tristar II BET equipment manufactured by Micromeritics (USA) was used to determine the surface area of the nanostructures. Energy Dispersive x-ray Diffraction Spectroscopy (EDS) was used to analyse elemental composition and mapping, Transmission Electron Microscopy (TEM), JEOL JEM-2100 electron microscope (Japan) operated at an accelerating voltage of 200 kV and Scanning Electron Microscopy (SEM), Hitachi X 650 Scanning electron microscope was used for morphology.

Photocatalytic
The effect of the nanophotocatalysts was demonstrated by photodegradation of MG under the 300 W lamp light source. Following and modifying the methods reported by Ngoepe et al [14], the effect of pH (4-10), catalysts dosage (25-100 mg) and dye concentration (5-20 mg l −1 ) was investigated. Prior to UV irradiation, the solution containing the nanophotocatalysts was stirred continuously for about 30 min in the dark to reach an adsorptiondesorption equilibrium so that the degradation efficiency can be influenced by photodegradation. After reaching the equilibrium, the system was subjected to UV irradiation using a 300 W light source. The distance between the light source and solution surface was kept constant at about 5 cm throughout the photocatalytic experiments. Briefly, 5-20 mg ml −1 concentration of MG dye pollutant was prepared and 25-100 mg of the dosage was added in a 250 ml solution in a 500 ml beaker. The samples were irradiated for 60 min and analysis for the degree of degradation was taken at 15 min intervals. To calculate the % degradation for each experiment, the formula denoted as equation (1) was used.
here A 0 is the initial absorbance and A t is the final absorbance. The kinetics of photocatalytic reaction of MG was calculated using this formula: Where A 0 is the initial absorbance, A t is the final absorbance, k being the degradation kinetic of the photocatalyst and t is the irradiation time.
Similarly, the pharmaceutical pollutants, SSX and SMX were degraded following the same method as above. However, the operating parameters were different and were recorded as follows: natural pH, 25 mg dosage, 10 mg ml −1 concentration, time duration of 60 min and the wavelength of the light used was 300 nm. The decaying and the degradation of the pollutant was recorded at 285 nm.

Reusability and trapping studies
Reusability studies were conducted for 4 cycles under the same conditions as the actual degradation run. For each run, the sample was filtered after the degradation, washed and dried and reused. Also, the trapping of active species experiments was conducted using propan   Lei et al [9], the glycosides and flavanols compounds were identified to be in highest content. FTIR was done to further confirm the functional groups presence in the plant.

Spectroscopic analysis using FTIR
FTIR analysis was used to determine the functional groups involved in the synthesis of ZnS nanostructures using S. fructescens. and the ones present in the S.frutescens plant extract. Figure 2 shows the FTIR spectra of the S. fructescens plant and the SF-ZnS. The S. fructescens plant extract spectra exhibited vibrational stretching at 3400 cm −1 , 2222 cm −1 , 1017 cm −1 and 1645 cm −1 . The strong broad absorbance peak at 3400 cm −1 is due to hydroxyl group (O-H) shifting, the small and sharp peaks at 1339 cm −1 and 1017 cm −1 are as a result of C-N and C-O stretching which might be due to amino groups [13]. The carboxylic group is responsible for the absorbance peak at 1645 cm −1 which matches the C=O groups. These peaks were also observed and deposited on SF-ZnS. The Zn-S vibrational stretching was observed around 680 cm −1 in the fingerprint region confirming the formation of ZnS nanostructures synthesized using S. fructescens plant. Since several phytochemicals have been identified from the plant extract, and these are used as reducing agents, the mechanism of formation can be proposed. The proposed mechanism for the formation of ZnS nanostructures using S. fructescens plant extract is as follows. Firstly, the phytochemicals such as flavonoids, alkaloids, glycosides and phenolic of S. fructescens plant extract bind and reduce the ZnCl 2 into Zn 2+ ions. Thereafter, the added Na 2 S.9H 2 O forms the S 2− ions which bind to the zinc metal connected to the phytochemicals. Then nucleation and growth process occurs on the nanostructures thus the formation of ZnS nanostructures. The phytochemicals on the S. fructescens plant extract bound to the surface of the ZnS nanostructures prevented the agglomeration of the particles while stabilizing the nanostructures. Further, because the phytochemicals comprise of carbonyl groups that have the capacity to direct the shape, the capping of the ZnS nanostructures also occurs. From the FTIR spectrum, the results reveal that the flavonol, canavanine and Gaba with the functional groups like hydroxyl, amino and carboxylic acids might have been absorbed on the surface of the ZnS nanostructures and this suggests that they played an important role during their formation. To further confirm the formation of this material, XRD analysis was conducted.

Crystallinity and peak identification studies
The phase structure of the synthesized SF-ZnS nanostructures was evaluated using XRD as presented in Where K=0.9, indicates the constant shape factor, λ represents the x-ray wavelength of CuK α (1.5404 Å), β is the peak full width at half maximum (FWHM) of the diffraction peak, θ is the bragg's diffraction angle and D is the average crystallite size of the nanostructures.
The average size of the SF-ZnS nanostructures synthesized was estimated to be around 41.22 nm. The calculation of the size was done based on the intense peak using the FWHM of the XRD peak broadening. Additionally, the lattice spacing was estimated using Bragg's formula; where d is the lattice spacing, θ is the Bragg's angle, and λ is the wavelength of x-ray. Table 2 shows the diffraction data that justifies the formation of wurtzite hexagonal structure of SF-ZnS nanostructures with the lattice parameter of a = 3.777 Å and c = 6.188 Å. Similarly, in the study conducted by Sathishkumar et al [18] using the ZnS nanoparticles synthesissed by tridaxprocumbens plant extracts, they showed that the material had a hexagonal wurtzite structure. Further, their calculated crystalline grain size was found to be 40 nm. ZnS nanoparticles with hexagonal wurzite phase was also synthesized using plant extracts of Tridax procumbens by Kannan et al [19]. The crystalline pattern also showed that there were a few impurity peaks, and these impurities are suspected to be from plant phytochemicals, they were also ideintified in EDS analysis. Their calculated crystalline size by Debye-Scherrer equation was found to be 22.70 nm. Further, the crystalline growth of ZnS was reported to be due to the biomolecules like phenols, alkaloids, flavonoids and saponins. To obtain more information on the structure of the material, morphological analysis was also conducted.

Morphological and elemental analysis
The SF-ZnS nanostructures were characterized using HRSEM and TEM. From figure 4(a), SEM shows that the morphology of the particles were spherical in shape and appear as clusters. Using image J, the particle sizes were measured to be between 70-170 nm as shown on the histogram ( figure 4(b)). TEM analysis also confirmed the formation of large particles of spherical morphologies with some clustering. Bai et al, synthesized ZnS nanoparticles using Rhodobacter sphaeroides [4]. Upon conducting a synthesis time study, increasing the time interval also led to an increased average diameter of 4, 8, 30 and 105 nm respectively. The increment in the average diameter was stated to be due to the nucleation effect which leads to the agglomeration of small particles thus larger particles are also identified in our TEM results ( figure 4(b)). A controlled particle size is one of the most important standards in the area of green derived nanomaterials since it has an influence on the application   of the nanostructures. As observed in the TEM image, the particle size of SF-ZnS nanostructures is uniform and large as a results of plant extract covering the surface of SF-ZnS. When the concentration of phytochemicals which are responsible of mediating the synthesis of SF-ZnS is low the interaction is minimum leading to an increase of particle size.
Energy dispersive x-ray spectroscopy (EDS) also provided information about the elemental composition and purity of SF-ZnS nanostructures. Their elemental analysis was obtained from the selected regions of the SEM image. The spectra indicated the presence of zinc and sulphur which confirms indeed that the synthesised ZnS using plant extract of S. fructescens were formed. The presence of other elemental peaks like C, O, Al, Si, Cl, K and Ca maybe due to surface contamination and absorbed residual surfactants on the nanostructure [20]. Moreover, they might be as a result of unreacted biomolecules from the S. fructescens plant extract. Hussain and co-workers [21] reported that the impurities on the ZnS nanoparticles demonstrated on their EDX spectra were coming from the proteins or enzymes of the A. bisporus extract. Thermal stability test using TGA weight loss were conducted to determine the material's stability.

Thermal stability studies
Thermogravimetric analysis is important in determining the thermal stability of the materials. The TGA and DTA curve of SF-ZnS nanostructures are shown in figures 5(a) and (b). From the TGA curve in figure 5(a), the decomposition of the nanostructures started from 100°C up until 800°C. Initially, the percentage (%) weight loss (1.46%) recorded corresponded to the exothermic peak at 106°C on the DTA curve which was attributed to the loss of moisture from the sample. The peaks at 200°C and 590°C with the percentage weight loss of 1.20% and 7.60% respectively were attributed to the breakdown and oxidation of phytochemicals on the plants (biomolecule on S.frutescens plants). Lastly, a large exothermic peak at 723°C which might be because of the crystallization of SF-ZnS nanostructures or the loss of the remaining residues on the sample and 1.61% of the weight loss was observed. Similarly, the same results were recorded by Senapati et al [22] for the synthesis of ZnS nanoparticles using starch. The BET results were explored in order to have a better understanding of the surface area, pore size and volume of the SF-ZnS nanostructures. Figures 6(a)-(c) illustrate the Nitrogen adsorption-desorption isotherms together with the pore diameter and surface area plot of the SF-ZnS nanostructures. From figure 6(a) clearly shows that the isotherms belongs to the standard of IUPAC recommendation type IV isotherms which exhibit a monolayers then followed by multilayer adsorption on the ZnS nanostructures. The surface area of the synthesized SF-ZnS nanostructures is 6.6973 m 2 g −1 . The Barrett-Joyner-Halenda (BJH) technique was used to calculate the pore volume and pore diameter. The pore volume and pore diameter of the SF-ZnS nanostructures were found to be 0.018494 cm 3 g −1 and 36.88 nm respectively. The plot of pore diameter relative to pore volume confirm the presence of mesoporous structure for SF-ZnS (average pore size between 2-50 nm).

Optical properties of SF-ZnS nanostructures
The absorption spectra of SF-ZnS nanostructures was analysed in the range between 300 to 700 nm. The optical absorption spectrum of SF-ZnS nanostructures displayed the absorption peak at 304 nm as seen in figure 7(a). This suggests that the absorption band was blue shifted due to the formation of SF-ZnS nanostructures. In a study done by Senapati et al [23], the ZnS nanoparticles synthesized using mushroom Pleurotuss ostreatu reported that their absorption peaks were blue shifted because of the electrons stimulation from the valence band to the conduction band. Further, this was also observed in the study where they used glucose carboxymethylcellulose (CMC) for the synthesis of ZnS nanoparticles. They demonstrated that their absorption band edge was observed between 270 and 330 nm, and this was blue shifted [24]. The same trend was also observed in studies investigated by Huo et al , Samanta et al and Lalithadevi et al [25][26][27]. The band gap energy for the nanostructures was estimated using Tauc plot. The calculations were done using the expression αhv = (hv-Eg) n : where, h is for the Planck's constant, α stands for the absorption coefficient, hν is for the photon energy and E g is for the optical band gap. The Tauc plot of (αhv) 2 against photon energy (eV) is shown with the respective UV-vis spectrum for nanostructures. The tangent line depicts the band energy on taucs plot. The direct band gap energy was obtained to be 3.60 eV for SF-ZnS. The obtained band gap energy of the semiconductor SF-ZnS nanostructure was lower than the ZnS bulk. This case was also observed in the study  where Kannan et al [28], used Acalypha indica and Tridax procumbens plant extract for the synthesis ZnS nanoparticles. This was because of an increasing particle size which occurred during the growth period of grains thus resulting in the decrease in defect of band edge of wavelength in the UV region.

Photocatalytic activity of SF-ZnS nanostructures synthesised using S. fructescens plant
The photocatalytic activity of SF-ZnS was also investigated against malachite green (MG) dye and antibiotics, sulfisoxazole (SSX) and sulfamethoxazole (SMX). The effect of pH, catalyst loading and dye concentration were investigated for the degradation of MG.

Studies of the different pH, dosage and concentration values for the photocatalytic activity of malachite green using SF-ZnS nanostructures
The photocatalytic activity of SF-ZnS nanostructures was investigated against the malachite green dye at the interval of 15 min within 60 min and it was analysed from pH 4-10 ( figure 8(a)). The semiconductor ZnS is an amphoteric material, which means that there is change in the surface charge of ZnS with the changes in pH values, thus it is important to conduct the effect of pH [29]. The degradation efficiency of MG was found to be 46, 90, 90 and 74% for pH 4, 6, 8 and 10 respectively. From this analysis, it can be noted that pH 6-8 produced the highest degradation of 90%. Generally, since MG is a cationic dye it favours alkaline medium for its removal. When the pH of the medium was increased there was a promotion of reactive species (OH radicals) which are essential during degradation. Nevertheless, the degradation efficiency of the SF-ZnS nanostructures reached a plateau at pH 6-8 and started to decrease after pH 8. This could be due the fact that maybe the produced hydroxyl ions were competing with the dye molecules during the process of absorption on the surface catalyst. In addition, at lower pH, it is difficult to form the needed hydroxyl groups to form hydroxyl radicals since it is in acidic medium and as such the removal is reduced. The SF-ZnS nanostructures were further investigated for their kinetics as indicated in table 3 for better understanding towards the photodegradation. The rate constants were found to be 0.00869, 0.0096, 0.00747 and 0.0164 min −1 corresponding to pH 4, 6, 8 and 10 respectively. The rate constants revealed that the degradation rate of the dye at pH 6 was multiplied more as compared to other pH values thus there was high efficiency of photodegradation of MG. The kinetic studies conducted on different pH values shows that only one R 2 value was greater than 0.90 which is for pH 4. From these, the kinetics studies for pH 4 were fitted for the pseudo-second order. Though that is the case, still none of them really fitted for the second order kinetics reaction. From the investigation, pH 6 was taken as the optimum value for all the experiments, since it had the highest degradation at the lowest pH and a better degradation rate, compared to the other pHs The study on the effect of catalyst loading was conducted using several loadings from 25, 50, 75 and 100 mg as displayed in figure 6(b). The degradation efficiency decreased as the catalyst loading increased. The results show that a highest degradation percentage was obtained when 25 mg loading was used. The degradation removal was found to be 90, 62, 63 and 51% corresponding to 25, 50, 75 and 100 mg respectively. When an amount of the dosage catalyst is increased, the degradation efficiency is supposed to increase because of more available active sites. However, this study showed that a further increase in loading resulted in a decrease in the removal efficiency. This may be due to the aggregation of the SF-ZnS photocatalyst which led to the decrease in the surface area of the particles [14]. The photocatalytic activity rate constant values corresponding to the dosage loading 25, 50, 75 and 100 mg were 0.02073, 0.00967, 0.01253 and 0.0164 min −1 respectively as summarized in table 4. These rate coefficients follow the order 0.00967, 0.01253, 0.0164 and 0.02073 min −1 . The R 2 obtained from the plotted graphs did not fit the second order reaction because their values were too far from the preferred 0.99 value.
After the pH and dosage investigation of SF-ZnS nanostructures, the photocatalytic activity of the photocatalysts in degrading MG with various concentrations (5 mg l −20 mg l −1 ) was also evaluated. figure 8(c) shows the degradation plot related to the photocatalytic activity of MG on the effect of concentration. The removal capacity of MG dye decreased as the dye concentration increased. This occurs because as the concentration is increasing, the lights ability to be transmitted into the solution is limited. The increase in light absorption of the dye molecules causes a lesser amount of the photons to hit the surface of the photocatalyst [30]. From the results, the removal efficiency of MG dye using SF-ZnS nanostructures were 62%, 90%, 70% and 30% respectively. The kinetic studies done on different initial concentrations are given on table 5. These values shows that at 10 mg l −1 concentration of the dye, the k value was greater than when the concentration of the dye was 5 mg l −1 , 15 mg l −1 and 20 mg l −1 . This indicates that at 10 mg l −1 the degradation rate of malachite green was faster resulting in a higher photocatalytic activity.  6, the ZnS nanostructure synthesised using different plant extracts had a higher percentage degradation (ranging from 55%-99%) over a longer period of reaction time (150-240 min) as compared with the present study of the SF-ZnS. This might be due to different factors such as the morphology, particle size, band gap energy, surface area and the active sites. All these studies achieved the highest degradation efficiency with the minimal dosage photocatalyst. The results in the present study showed that about 90% of the MG dye was degraded in 60 min The plant extract played an important role on their photodegradation efficiency. Through photocatalytic activity, the phytochemicals on the surface of the SF-ZnS nanostructures as seen in FTIR spectrum provide the surface charge to avoid aggregation of the particles hence the nanostructures are stabilized. In addition, these phytochemicals also act as a connection points for establishing phytochemicals-MG molecules complex which promotes the decomposition of MG dye.

Reusability studies
The reusability and stability studies (figure S4 (available online at stacks.iop.org/MRX/9/015001/mmedia)) of SF-ZnS nanostructures were investigated for four cycles using the optimum conditions of 25 mg of the photocatalyst and 10 mg l −1 of the concentration. The results as seen in figure 8 show that the photodegradation of MG using SF-ZnS nanostructures decreased drastically after each cycle, for the successive cycles. Their percentage removal was found to be 90%, 70%, 60% and 57% respectively. This means that as the photocatalyst was used continuously, it was losing its stability hence the excessive decrease in its photocatalytic efficiency.
Since the photocatalyst was in small amounts, the filtration and drying process might have contributed to this too much loss of the photocatalyst before the. It is known that the crucial step in photocatalytic activity is an absorption of dye molecules on the surface of the nanostructures. In the study done by Rajabi et al [34], it was reported that the decrease in the photocatalytic efficiency during the reusability studies was because the available active sites were occupied by the absorbed molecules thus this resulted to decrease in photocatalytic activity of the photocatalysts at following cycles. Similarly, this was the same case on the study done by Wang et al [35].
3.12. Quenching of active species during photocatalytic activity Further investigations were carried out to explore the role of different scavengers. For these quenching studies, scavengers such as isopropanol, silver nitrate, EDTA and p-Benzoquinone for hydroxyl radicals, electrons, holes and superoxide respectively were employed. From the bar graph represented in figure 9(f), the addition of those scavengers exhibited that their effect was able to be suppressed. A decrease in the removal efficiency was noticed following the order as O 2 · , h + , OH · and e · and 40%, 41%, 48% and 82% was degraded. From the results, the superoxide and holes played a crucial role in the photodegradation of MG, then followed by the OH − species. In the study done by Samantha et al [26], the degradation of RhB dye was suppressed by the presence of silver nitrate as compared to the addition of EDTA and TBA. This meant that e − was the major oxidative species responsible for degradation of the dye. To further test this material against other pollutants, their efficiency against antibiotics such as sulfisoxazole (SSX) and sulfamethoxazole (SMX).

Photodegradation of SSX and SMX using SF-ZnS nanostructures
The SF-ZnS nanostructures was investigated for the photodegradation of SSX and SMX pharmaceuticals as presented in figures 10(a)-(c). The absorption spectra for SSX and SMX irradiated in the presence of SF-ZnS nanostructures carried out at various time intervals showed a decrease in the intensity of the peak. This suggest that SSX and SMX have been decomposed by photocatalytic treatment. Upon testing the adsorption -desoption capabilities of the materials before the equilibrium was reached, a substantial amount of the SSX and SMX pollutants, respectively were adsorbed. Upon radiating the light on the solution, a maximum of 98% and 96% of SSX and SMX respectively was decayed in 120 min of reaction time. figure 10(d) suggests that the degradation of SMX followed the second order kinetics with the rate constant of 0.02922 min −1 . The R 2 value was found to be 0.9522 which is a better fit for second order kinetics.
Further, the plot of ( ) In Ao At was also used to get the rate constant of SF-ZnS nanostructures when decomposing SSX and it was found to be 0.0378 min −1 with an R 2 value of 0.87936. This R 2 results for SSX did not indicate correlation with Langmuir-Hinshelwood kinetic model. In comparison with other studies, Hu et al [36] found that TiO 2 was able to degrade 95% of the SMX. Similarly, in another study done by Abellan et al [37], the higher efficiency of SMX (82%) was also obtained when decomposing it using TiO 2 . For the degradation of SSX, Hasan at al [38] reported that the photocatalytic test done using TiO 2 showed higher removal efficiency which was 93% over 1 h under irradiation. In their studies they reported that the OH which were produced on the aqueous-TiO 2 interface were the ones responsible for that higher degradation efficiency. Moreover, the tests were also done to support that the hydroxyl radicals were the active oxidizing agents for SMX. Though there are no studies where they have used SF-ZnS nanostructures for the decomposition of SSX and SMX from literature it was anticipated to get higher removal efficiency while using ZnS.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.