As results of industrys fast expansion, water pollu-tion is currently a severe global issue. Huge amounts of waste from many sectors, including those in tex-tile, plastic, paper, and cosmetics, are dumped into ponds, rivers, and lakes, which gravely harms the ecology and causes water pollution (Alqadami et al., 2016). An estimated 7×105 metric tons of dyes are produced annually, of which 12% are lost during production and other handling steps. By using in-dustrial effluent, around 20% of them harm the environment (Samadder et al., 2020). High-colored effluents from the dyeing industry have low biologi-cal oxygen demand (BOD) and high chemical oxy-gen demand (COD) (Nasr et al., 2006). Disposal of these effluents into the water can be toxic to living organisms (Lee et al., 1999; Kadirvelu et al., 2000). The biological activities are spoilt by the dyes in water bodies. They harm our health since they might induce cancer and mutagenesis. It is crucial to remove dyes from wastewater (Papic et al., 2000; Kar et al., 2021; Sivaraj et al., 2001). 

There are different types of methods to reduce dyes from industrial dye-contaminated wastewater such as precipitation (Zhu et al., 2007), ion exchange, photo-catalytic degradation (Yi et al., 2018; Xu et al., 2015; Yang et al., 2018), biological oxidation (Manenti et al., 2014; Ren et al., 2018), adsorption (Fan et al., 2016; He et al., 2018), membrane filtra-tion (Li et al., 2017), electrochemical function coagulation, chemical oxidation, electro-dialysis, adsorption, a spectro-photometric method, liquid chromatography method, etc. The waste water puri-fication from industries that are polluted with dye by photocatalysis has received a lot of interest recently. Advanced oxidation processes (AOPs), which pro-duce electron-hole pair radicals capable of subse-quent reactions are the technology behind photo-catalytic degradation (Banerjee et al., 2006).

In relation to purified polymer or ordinary com-posites (micro and macro composites), clay or nano clay filled polymer nanocomposites represent a new category of materials that have drawn significant attention for their excellent physical properties, including high dimensional stability, gas barrier per-formance, flame retardancy, and mechanical stren-gth. Nanoclays are employed in nanocomposites owing to their natural abundance and extremely high form factor. Natural or fabricated clays are utilized as fillers in nanoplates. Kaolinite, Silhydrite, Sme-ctite, Fluorohectorite, Zeolite, Sepiolite, Kenyaite, Saponite, Magadiite, Kanemite, Ilerite, and other natural and manufactured nanoclays are often emp-loyed as nanofiller in nanocomposite materials (Liu et al., 2003; Lateef et al., 2016). They often have crystalline shapes and a Nanometric thickness. The growing interest in ZnO as a prospective photo-catalyst is being generated by its low cost and relative safety (Lu et al., 2008; Sakthivel et al., 2003; Hariharan et al., 2006). However, several wur-tzite-type ZnO properties result in several unavoi-dable drawbacks when utilized as photo-catalyst. As for example, ZnO can only demonstrate its photo-catalytic ability when exposed to UV light. Its photo-catalytic effectiveness is decreased as a consequ-ence of quick recombination of photoexcited elect-ron-hole pairs (Liu et al., 2012; Lin et al., 2009; Djurišić et al., 2006). Additionally, the high crystal size and rapid growth rate of ZnO result in a low particular surface area, which prevents the catalyst from exposing enough active sites (Greene et al., 2006; Greene et al., 2003; Xu et al., 2011). Numerous engineering techniques, including band structure engineering, micro/nano engineering, bio-nic engineering, co-catalyst engineering, surface/ interface engineering, and others, have been estab-lished to address these problems (Xu et al., 2011; Zhou et al., 2007; Qin et al., 2011; Li et al., 2015). And it is discovered that this innovative approach and photocatalytic efficiency have been used widely in the domains of degrading organic contaminants (Zhou et al., 2012; Linic et al., 2011; Hu et al., 2010; Yan et al., 2015). Numerous ZnO matrix mor-phologies have been documented thus far, including nanospindles (Christopher et al., 2011; Kuriakose et al., 2014), nanoporous microds (Deng et al., 2012), nanorods (Zheng et al., 2007), nanoparticles, nano fibers (Xie et al., 2010), microspheres (Lai et al., 2010), nanosheets (Qin et al., 2011). There is still plenty of room to increase the photo catalytic effect-iveness because ZnO diameters range widely from tens to hundreds of nanometers. Since ZnO has a smaller size, its larger specified surface area exposes more effective sites.

The current work was aimed to remove the textile dye Eosin yellow from the environment for the benefit of the creatures. In the present study, a uni-que Mymensingh clay and ZnO composite (clay-ZnO) was prepared for the purpose of photocatalytic destruction of EY under UV and visible light.

Materials and Reagents

Mymensingh clay was a kind of gift from the My-mensingh local area. All studies utilized deioni-zed water that was acquired from a Millipore water puri-fication device (Takashima Keiki Co Ltd. Japan). Zinc Acetate, Ammonium carbonate, EY, Na2CO3, NaHCO3, Na2SO4, NaCl, Ca(NO3)2.4H2O, Zn(NO3)2, Al(NO3)3, Ba(NO3)2 (Merck, Germany). All of the chemicals were of analytical grade and were utilized directly.

Preparation of clay-ZnO composite

In a 1000 mL beaker, the growth of the clay-ZnO composite photocatalyst was completed. The beaker was first filled with 500 mL of a 0.25M zinc acetate solution. The beaker then was charged with the necessary amounts of natural sources of clay (obtain-ned from Mymensingh); the clay was added to the prior solution to create a suspension, which was then heated to roughly 60 °C. After adding 250 mL of a 0.5M ammonium carbonate solution drop by drop while agitating the suspension at a constant temp-erature of 60°C, zinc carbonate precipitated on the clay particle surfaces. Under experimental condi-tions, the suspension was aged for 2 hours to achi-eve total precipitation. Produced precipitates were filtered and washed with distilled water to remove contaminants. The precipitates were then dried for two hours in an oven set at 110°C. Finally, the samp-les of the produced composites were kept in a fur-nace for three hours at 600°C for calcinations (Qin et al., 2011; Benkelberg and Warneck, 1995). The final, dry composite was crushed and sieved through sieves with a 140 mesh size before being placed in a desiccator for storage. The following reaction occur-red in this scheme:

Zn(CH3COO)2+(NH4)2CO3→ ZnCO3+2 CH3COONH4

ZnCO3 + Clay → Clay-ZnO

Photocatalytic degradation study

Photocatalytic degradation capacity of Clay-ZnO composite was analyzed using EY as a refer-ence dye. 0.2 g of composite was kept in a beaker and charged into 100mL aqueous EY solution (2×10-5M) under UV light (3.31×10-9 Ein cm-3 s-1). The com-posites were separated by using centrifugal force. The excess concentration of EY was determined by UV-1650 PC, Shimadzu, Japan. The degradation capacity was monitored at discrete time interludes. The quantity of dye present in the clay-ZnO composite as flow served as the basis for calculating the rate of degradation.

 Degradation Ratio (%) =  ((Co-Ce))/Co×100%             (1)

Where, Co and Ce are the initial and equilibrated dye concentrations (mol/L).We also compared the degra-dation capacity in different ratio of clay-ZnO com-posite. 

Characterization

A JEOL JSM-6400LA FESEM was employed to analyze the surface morphologies of ZnO, Mymen-singh clay, and Clay-ZnO composites at a 5 kV acce-lerating voltage. An EDX spectrometer connected to the FESEM was utilized to determine the samples elemental compositions. The infrared spectra were recorded on a SHIMADZU IR Tracer-100 infrared spectrophotometer in the region of 4000-400 cm-1. The phase identification of the composites was cen-sured by an XRD investigation carried out main-taining room temperature in the Ultima IV X-ray diffractometer. A UV-Vis spectrophotometer (UV-1650 PC, Shimadzu, Japan) was employed for the calculation of dye concentration.

Surface Morphology Analysis

The SEM images of clay, commercial ZnO, and composite in various ratios are represented in Fig. 1(a), 1(b), and 1(c) to 1(e) with different magni-fication. The images suggest that the composite contains particles of different sizes. Clay particles appeared in different shapes, and since they collect randomly, the surface is heterogeneous. Contrarily, the particles of commercial ZnO are regularly shaped into various geometrical forms, including cubic, hexagonal, rhombic, etc. The surface is homogenous because of these regular shapes. Particle shapes in composites change based on how much clay and ZnO are included. Comparing composite particles to commercial ZnO and clay shows a different mor-phology. Particle size and shape change along with a rise in clay percentage of the composite. These part-icles aggregated, influencing the composites sur-faces morphology. It should be beneficial for photo-catalytic degradation since it enhances surface area (Waldemer and Tratnyek, 2006).

Fig. 1(a): SEM image of Mymensingh Clay.

Fig. 1(b): SEM image of commercial ZnO.

Fig. 1(c): SEM image of prepared composite (clay: ZnO; 12:88).

Fig. 1(d): SEM image of prepared composite (clay: ZnO; 25:75).

Fig. 1(e): SEM image of prepared composite (clay: ZnO; 50:50).

Fig. 2: EDX spectra of prepared composite (composition, clay to ZinO12:88).

XRD Analysis

The hexagonal phase of ZnO mentioned in JCPDS card No. 36-1451, with a = 0.3249 nm and c = 0.5206 nm, is the source of all the diffraction peaks in the XRD of commercial ZnO shown in Fig. 3(a). There seem to be nine peaks at 2θ = 31.77˚, 34.42˚, 36.25˚, 47.53˚, 56.59˚, 62.85˚, 66.38˚, 67.94˚ and 69.09˚, which are (100), (002), (101), (102), (110), (103), (200), (112), and (201), respectively (Habib et al., 2013). The sharpness of all the peaks indicates that these ZnO particles have such a high degree of crystallinity. The prepared composites XRD, which is seen in Fig. 3(b), contains all of the ZnO peaks, but their positions have indeed been slightly shifted, indicating that clay and ZnO have interacted at the molecular level. A composites shifting crystal struc-ture may influence the lattice parameter and, in effect, the surface morphology. Peaks for clay appear and get more intense as the content of clay in the composite increases, although the position has slightly shifted. Due to nearby peaks influence, several peaks have been altered.

Fig. 3: XRD spectra of (a) ZnO and (b) composite of 50:50 compositions.

FTIR analysis

Fig. 4 presents the FT-IR spectra of commercial ZnO, prepared composites, and clay. It displays wide band characteristics below 1000 cm-1 that might be caused by overlapping Zn-O stretching (Viswanatha et al., 2012; Gao et al., 2003; Zuas and Hamim, 2013). The stretching bands of ZnO at 487.99 cm-1 and 437.84 cm-1 are overlapped, appearing in the bands at 495.71 and 439.77 cm-1. The broad peak between in 3100 and 3600 cm-1 is assigned to the hydroxyl groups fundamental stretching vibration (free or bonded) (Wickramasuriya et al., 2021) which is further verified by the weak band at about 1620.21 cm-1 (Klingenberg and Vannice, 1996). The characteristics clay peak between 1000 and 500 cm-1 was observed. The peaks of the composite spectrum that was observed in clay and ZnO have undergone some modification. Few peaks at around 1100 cm-1of the composites spectrum seem to get more intense as the clay content increases, despite the fact that they are primarily remakes of the clay-specific peaks and have shifted positions. The peaks that indicate ZnO is present in the prepared composite do not appear to have altered significantly. It may be because ZnO predominates over clay in the prepared composites. The vibrations of atmospheric CO2 might be accountable for a faint band at about 2400 cm-1. Fig. 2s composite EDX analysis shows that the main components of the prepared composite were clay and ZnO. No other peaks for impurities were observed in the spectrum of composite. So, the prepared composite samples are in pure state.

Fig. 4: FTIR spectra of clay, ZnO and prepared clay-ZnO composite of different compositions, respectively.

Effect of composition of prepared Clay-ZnO com-posite on photodegradation

The efficient clay-ZnO composition for the com-posite was determined by carrying out illumination experiments using 0.20 g composite of different clay-ZnO ratios, 2.0×10-5 M concentration of EY with the light intensity of 3.31×10-9 Ein cm-3 s-1. The blank experiment was conducted using 0.20 g of pure ZnO with the same experimental condition to figure out the composition effect as illustrated in Fig. 5. Based on the observations, 25:75 Clay-ZnO composite displayed photocatalytic degradation efficiency of 44%. Illumination generates electrons and holes in ZnO and these electrons and holes create secondary species/radicals that cause photo degradation. Now, effective electron and hole sepa-ration elongates the charge carriers lifetime and improves the adsorption of interfacial charge transfer of substrates (Serpone et al., 1995). In the com-posites, electrons, and holes separation are efficient owing to mixing of clay with ZnO as it helps to delay the recombination process of the charges. Therefore, the photocatalytic activity is relied on the quantity of clay and ZnO in the developed composite that helps to separate the charges efficiently, ulti-mately enhancing the production of secondary species/radicals (Habib et al., 2013). Optimization was seen at the composition of clay to ZnO at 25:75 which gives the maximum photo degradation of EY nearly 45%. An increase in the composite com-posi-tion causes ZnO particles to accumulate on the clays surface, and it seems that the ZnO particles are trapping the clay. At this stage, a composite mostly made of ZnO acts exactly like pure ZnO.

Fig. 5: Changes in the percentage of EY solution degradation with changes in the clay-ZnO ratio for the composite at various times under UV irradiation using 2.0×10-5 M EY and light intensity of 3.31×10-9 Ein cm-3 s-1.

Effect of prepared composite amount 

Photo degradation experiment was performed using different composite amount varying from 0.1 to 0.5 g in 100 mL solution keeping EY concentration of 2.0×10-5 M and intensity of UV light of 3.31×10-9 Ein cm-3 s-1 constant as shown in Fig. 6. It is found that photo degradation efficiency increases with the increase of the composite amount from 0.1 g to 0.35 g, gives maximum efficiency at 0.35 g then degra-dation efficiency decreases with the increase of com-posite amount. The catalysts surface has excessive active sites due to an increase in composite quantity, which promotes the initial increase in degradation efficiency (Habib et al., 2013). Whereas, the degra-dation efficiency decreases with the increase of composite amount is because the excessive com-posite amount causes the hindrance and blocking of light penetration to the catalyst surface (Kim and Park, 2006). Additionally, particle aggregation is significant at higher amounts of composite, thereby reduces the number of active sites on the catalyst surface. Therefore, the degradation efficiency reac-hes its maximum at 0.35 g of composite amount then decreases with further increase of the composite amount.  

Fig. 6: Change of the degradation percentage of EY solution with the change of the composite in UV irradiation using 2.0×10-5 M concentration of EY with light intensity of 3.31×10-9 Ein cm-3 s-1.

Effect of primary concentration of EY 

As shown in Fig.7, the photo degradation efficiency was originated as a function of primary concent-ration of EY. The photo degradation efficiency was lowered with increasing EY concentration without any distinct pattern or obeying any mathematical equation. No evidence was found that this photo degradation process goes through chain mechanism, therefore, only a constant amount of oxidizing spe-cies is generated from the constant amount of cata-lyst that oxidizes a certain amount of dye molecules. As a result, a fixed quantity of catalyst at constant light intensity can oxidize only a certain amount of dye molecules at a period and that cannot be altered with increasing the quantity of dye molecule. Since, the photo degradation efficiency (percentage of degradation) is calculated with decrease in dye con-centration with respect to primary dye concentration, that justifies why degradation efficiency decreases with increasing initial dye concentration. Addition-ally, a dye solution with a high concentration can block light from reaching the rest of the solution. Higher concentration hence resulted in decreased photo degradation efficiency.

Fig. 7: Change of the degradation percentage of EY at (a) different time for different concentration (b) change of initial dye concentration in UV irradiation with light intensity of 3.31×10-9 Ein cm-3 s-1.

Effect of discrete light sources

Fig. 8 exhibits the utilization of UV light having an intensity of 3.31×10-9 Ein cm-3 s-1 and visible light to observe the impacts of various light sources. In this instance, the optimum EY concentration was 1.00×10-5 M, and the composite weight was 0.35 g. By introducing the dye to UV radiation, the maxi-mum rate of degradation of EY about 82.2% was observed. The degradation between the light sources for visible light is less significant, at around 63%. It is generally known that when the wavelength of the light source is reduced, an increase in the formation of electron-hole pairs occurs. UV light will therefore be more efficient than visible light for lights of equal intensity.

Fig. 8: Variation of degradation percentage of 1.00x10-5 M EY solution under various light sources.

Effect of light intensity

Presented in Fig. 9 is alteration in the photo degraded percentage of EY with time for various light intensity indicating that the degradation of EY enhanced with increasing light intensity and com-plete degradation occurred within 1 hour at the light intensity of or above 1.21×10-9 Ein cm-3 s-1. The proposed photo degradation of EY mechanism said that the quantity of charged particles (electrons and holes) generated from composite are dependent on the quantity of photons entering into composite surface. Therefore, the generation of OH• radicals and other reactive species that oxidize dye molecule are dependent on number of photons and light intensity. This mechanism explains with experi-mental observation for increased photo degradation efficiency with increasing light intensity. It was noticed that to reach complete dye degradation as the light intensity increases from 1.03×10-9 to 1.36×10-9 Ein cm-3 s-1. Even though, experiments were carried out using light intensity higher than 1.36×10-9 Ein cm-3 s-1 but no experiment was possible to carry out at light intensity below 1.03×10-9 Ein cm-3 s-1 due to laboratory limitations. 

Fig. 9: Change of the percentage of degradation of EY solution with the change of light intensity at different time using 2.0×10-5 M concentration of EY.

Therefore, no real trend that photo degradation of EY follow with light intensity was figured out using data from only three different light intensity experi-ments. Nevertheless, attempts were made to figure out a relationship of light intensity with photo degra-dation efficiency using the data acquired from the light intensity experiments that yielded photo degra-dation below 100 percent.

Effect of cations and anions 

Fig. 10 illustrates the findings of the study on the effects of cation and anion using a composite of 0.35 g and 1.00×10-5 M EY solution. Investigated were the effects of Ca2+, Zn2+, Ba2+, and Al3+ cations on photodegradation. In comparison to other cations, the Zn2+ ion among them appears to accelerate photodegradation of EY. It could be because of its significant reduction potential and effective adsor-ption onto the composites surface. Utilizing sodium salts of HCO3-, CO32-, Cl-, NO3-, and SO42-, the effect of anion was studied. All of these have reduced the degradation percentage. These ions decrease poto degradation by scavenging the most reactive hydro-xide radicals (Islam et al., 2010).

Fig. 10: (a) Effect of cations on Photodegradation process, (b) Photodegradation in presence of different cations at 60 minutes, (c) Effect of anions on Photodegradation process, (d) Photodegradation in presence of different anions at 60 minutes.

In the present study, we here report the effective synthesis of clay-ZnO composite photo-catalysts with varying clay and ZnO compositions ranging from 12:88 to 50:50. The interpretation of the clay-ZnO composites by SEM, EDX, FTIR, and XRD has been beneficial. The material with the composition clay: ZnO had the highest % removal of EY, which includes adsorption and photo catalytic degradation (25:75). When l.0105 M EY was exposed to irradi-ation for 1 hour while contained in 3.5 g/L of com-posite suspension, about 82.21% degradation of EY was seen. The photo catalytic degradation of EY was evident to be adversely affected by the conse-quences of several anions, including HCO3-, CO32-, Cl-, NO3-, and SO42-. On the contrary, the addition of Zn2+ accelerated EY degradation rate. Photodegrad-ation was observed to be at its highest using UV light, but visible light showed less photodegradation. A strong UV radiation causes total degradation. Since all these samples are expected to quickly absorb the organic dyes during the continuous flow of effluents into the river and can be applied as an efficacious photocatalyst substitute to treat waste-water containing dyes, it could be concluded that the typical prepared composites should be very import-ant in the industrialized era. Additionally, this com-posite is simple to make from inexpensive clay, making the procedure economical. The method is environmentally friendly since the photocatalyst is recycled.

The authors are thankful to the Committee for Advanced Studies and Research (CASR), BUET, Bangladesh and Ministry of Science and Techno-logy, Peoples Republic of Bangladesh for funding.

The authors declare that they have no conflict of interests.