Preparation and photocatalysis of ZnO/bentonite based on adsorption and photocatalytic activity

This article focus on the adsorption and photocataltytic study of methylene blue (MB) dye from aqueous solution with sodium carbonate-activated bentonite adsorbent and ZnO/Bentonite photocatalyst. The activated bentonite adsorbent and ZnO/bentonite nanocomposites photocatalysts were prepared and characterized using XRD, FT-IR, SEM, TEM and AAS. The adsorption experiment was conducted using 5% sodium carbonate activated bentonite (AB5) to establish the effect of different parameters. The adsorption isotherm was performed using Langiumer, Freundlich, and Temkin isotherm models. The result showed the Langiumer adsorption isotherm best fit the adsorption study for the experimental data with R2 = 1. The kinetic adsorption of MB dye by activated bentonite was conducted using pseudo-first and pseudo-second-order models. The result revealed that the pseudo-second-order fit experimental data well and the process was chemisorption. The endothermic and spontaneous nature of the adsorption process of MB dye AB5 were carried by studying the thermodynamic parameters of the system. Desorption of MB dye from the spent adsorbent and recyclability of adsorbent was studied by thermo-chemical methods. The desorption capacity of MB dye in the selected solvents were small. The adsorbent developed for this study was recycled and its adsorption capacity decreased with recycling time. But, the photo-degradation studies conducted using the synthesized ZnO/Bentonite@550(1:1) photocatalyst with average particle size of 30.06 nm resulted with 99.54% degradation efficiency of MB.


Introduction
Water is basic for life. Clean water is the key to a healthy society. But in the present scenario, clean water is very limited because of the rapid development of urbanization and industrialization [1]. Chemical contamination of drinking water source is a concern for millions of people and long-term exposure to chemical pollutants can have serious health implications. According to recent reports, many industries discharging about 70% of their wastes to water bodies without any treatments and the world's population is facing the challenge of clean water scarcity [2].
Textiles, foods, leathers and other industries discharge a large number of dye effluents, during the printing and coloring process. Dyes are colorful substances that are used widely as part of the coloring processes in textile, paper, plastics, leather tanning, food, polymer, cosmetics, printing, and dye manufacturing industries, to give color to the supplies such as papers, fabrics, or any other type of colorable materials [3].
These materials have been widely used for thousands of years for several applications and released without additional concern into the water environment and posed a serious risk to ecosystems because of their high carcinogenic and mutagenic potential [4,5]. Based on their structures, dyes are divided into anionic, cationic, and non-ionic dyes [6]. Anionic dyes give a net negative charge when dissolved in an aqueous solution, such as those of sulfonate (-SO 3 -) and carboxyl (-COOH) groups. Methyl orange is an example of anionic dyes, toxic, mutagenic, and carcinogenic, a hard-braking molecular structure, and it is difficult for adsorption by adsorbents [7]. On the other hand, cationic dyes are organic dyes that give a net positive charge when dissolved in an aqueous solution, due to their functional groups such as amine or sulfur.Methylene blue (MB) dye is an example of cationic dyes which is used as a reference to show the cation exchange capacity (CEC) of clay. MB dye is highly soluble in water and it is one of the thiazine colorants. It may cause harmful effects on human health because of its carcinogenicity and toxicity in nature, it leads to severe headaches, chest pain, breathing difficulty, eye irritation, vomiting, increased heart rate, diarrhea, and cyanosis [8]. The presence of MB dye in water bodies significantly endangers the aquatic systems by blocking the direct Sunlight for photosynthesis organisms [9]. The removal of MB dye from the wastewater was investigated by methods like: coagulation, chemical oxidation, membrane separation, electrochemical process, photocatalysis, and adsorption methods. Adsorption method was recognized to be promising and cost-effective, simple and flexible, easy to operate and regenerating adsorbent [10]. The choice adsorbent may depend upon parameters like cost, availability, toxicity, regeneration, and re-usability. Some of the adsorbents reported for the removal of methylene blue from wastewater are activated carbon, chitin, cellulose, agricultural wastes, zeolites, and clays minerals [11]. Bentonite clay is a lowcost adsorbent with high specific surface area (SSA),high removal efficiency of cationic dye, cation exchange capacity (CEC), plasticity, and swelling capacity that swell many times in comparison to its original size [12]. Among different kinds of photocatalyst, ZnO is the most widely used for wastewater treatment due to its high oxidizing propertie. This process requires UV-light radiation to promote the electron from the valance band (VB) to the conduction band (CB) and leave the hole (h + ) in the VB. When the hole meets the H 2 O molecule in the presence of oxygen, it will produce hydroxyl radical (OH • ), and this OH • will break the MB dye molecule into a simple compound [13,14]. But, ZnO haslimitations like instability in acidic media, photo-corrosive nature, spontaneous growth, and aggregation restricts its application in wastewater treatment. To avoid such problems, a suitable material is needed to incorporate ZnO particles, porous inorganic bentonite clay material is a good candidate [15]. Study reports on a green and facile synthesis method for a ZnO-bentonite nanocomposite and its photocatalytic performance in terms of the degradation of organic dyes under solar radiation was found to eliminate more than 95% of dye molecules [16].
Hence, the present study was focused on the removal of methylene blue dye from an aqueous solution using activated bentonite through adsorption process and photocatalytic degradation using ZnO/bentonite nanocomposites synthesized by green method with H. Abyssinica.

Experimental details 2.2.1. Preparation of synthetic methylene blue stock solution
The preparation of 1 g l −1 stock solution of MB dye was performed by dissolving 1 g of commercial-grade MB dye in a 1 l volumetric flask by filling with distilled water up to mark at room temperature. Then, the prepared solution of MB dye was protected from light by aluminum foil due to its light sensitive. Then, the solution was analyzed using the UV-vis(Double Beam Spectrophotometer, SM-1600, India) [17]. The standardization curve was obtained with maximum absorbance at λ max 663 nm using MB standard concentration in the range of 80 to 230 mg l −1 . The solution was measured by using a pH meter of pH 21 Hanna instrument.

Purification of natural bentonite
The collected natural bentonite was dried at room temperature for one week and crushed to particle size 125 μm using a ball mill at 350 rpm. To remove quartz, carbonates, calcites, and iron hydroxide (Fe(OH) 2 ); 100 g of the ground bentonite was dispersed in 1 l distilled water at room temperature by mechanical stirrer for 2 h. The suspension was allowed to stand for 24 h to settle the impurities. The clear supernatant was decanted and the solid sample was separated from the suspension and centrifuged at 3000 rpm for 20 min to obtain the bentonite. This process was repeated four times, in order to guarantee obtaining samples in a pure form. The bentonite clay was dried in an oven at 60°C for 12 h, then ground using ball mill and sieved using a 63 μm mesh sieve, and stored in tightly closed plastic bottles till required for the further experiments [18].
2.2.3. Activation of purified bentonite by sodium carbonate (Na 2 CO 3 ) Activation of purified bentonite was carried out to reduce the residual sludge volume and the oil loss in the sludge with little modification [19]. First 100 g of purified bentonite was dispersed into 1 l boiled distilled water for 1 h. Then, different amounts of Na 2 CO 3 (2.5, 5, 7.5, and 15 g) were added to each suspension of bentonite in the beaker. The suspension was stirred vigorously with a magnetic stirrer and boiled for 1 h. After, 1 h the suspension was cooled down to room temperature and diluted with distilled water in three beakers (1 l) in each beaker added bentonite suspension equally. After that distilled water was added up to 1 l mark and waited for 24 h. Over and over again, the dilution was continuous up to clear supernatant appeared above the settlement. After the clear supernatant decant; the supernatant and the dry solid in the beaker were placed in the oven at 105°C for 12 h.

Preparation of Koso leaf extract and synthesis of ZnO/bentonite nanocomposites by green method
The extraction of the leaf H. Abyssinica (local name Koso) was carried out using 25 g of dried koso leaf and added into 500 ml distilled water and heated for 1 h at 60°C. Then, the solution was cooled and separated the solid from the solution using a centrifuge, and the extract was filtered using filter paper, and kept the filtrate for further use [20].

Fourier transforms infrared (FT-IR)
The structural analyses of purified bentonite, activated bentonite (AB5) after adsorption, after desorption, and ZnO/bentonite with different ratios of ZnO to bentonite were performed by FT-IR transmission spectra using the KBr pellets disc technique. The analysis was carried out on (PerkinElmer, Spectrum 65 FT-IR) in the wavenumber range of 400-4000 cm −1 at a resolution of 4 cm −1 , with number of scans 4.

Scanning electron microscopy (SEM)
Morphological analyses of the activated bentonite before (AB5) and after adsorption (AB-MB) were analyzed using SEM (SEM, JCM-6000). Before scanning, the AB5,and AB-MB were coated with gold using a sputter coater, and subsequently, the SEM images were taken.

2.3.4.
Complete silicate analysis of bentonite 0.1 gram of natural bentonite, and activated bentonite was separately added to 1.5 grams of LiBO 2 and the mixture was heated in a furnace for 45 min at 950°C. After it cooled overnight the mixture was stirred with a magnetic stirrer, washed, and transferred to a round bottom flask by adding lanthanum chloride. The prepared solutions were used for AAS to determine metals: Ca, Al, K, Mg, Fe, Na, etc. The amount of silica was determined by taking 0.1 g of bentonite, 3 ml of hydrochloric acid (HCl), and 2 ml of boric acid (H 3 BO 3 ) directly read by AAS, and the amount of TiO 2 and P 2 O 5 were determined by UV-vis spectroscopic method.

Adsorption experiment
To carry out the methylene blue dye adsorption with bentonite adsorbent, various parameters such as pH of solution, adsorbent dosage, initial concentration of MB, temperature and contact time were optimized. To carried out this experiment, contact times : 30 to 90 min, adsorbent dosage: 0.1 to 1.5 g, an initial concentration:80 to 230 mg L −1 , temperature of solution ranged from 25 to 50°C, and pH of initial solution ranged from 4 to 11 adjusted with the addition of 0.1 M HCl or 0.1 M NaOH and the pH change was measured using a pH meter. The aqueous phase was separated from the solid by filtration using Whatman no 1 filter paper and the filtrate was analyzed using UV-vis spectroscopyat a maximum wavelength of 663 nm, for the determination of the residual concentration of MB dye in the filtrate. Then, the adsorption efficiency (%) and adsorption capacity of MB dye q e (mg/g) was calculated using equations (1) and (2). The adsorbent loaded with MB dye after adsorption was dried in an oven at 70°C for 4 h and used for the desorption study: Where C o is the initial concentration of MB dye (mg/l), C r is the residual concentration of MB dye (mg/l), m is mass of adsorbents (g), V is the volume of MB dye solution taken during adsorption (l) and q e is adsorption capacity (mg/g).

Desorption of methylene blue from exhausted bentonite and reuse
Desorption of MB dye from the used bentonite adsorbent for adsorption of MB dye was performed using thermo-chemicals regeneration methods. The spent adsorbent coated by MB dye was heated at 200°C in a muffle furnace for 1 h. Then, the heated AB-MB at 200C added in to solvents (ethanol, acetone, and distilled water) and heated at 25, 35, and 40°C with stirring on the magnetic stirrer for 2 h. After two hours the concentration of MB dye desorbed in solvents was measured using UV-visible spectrophotometer at wavelength of 663 nm. Then, desorption capacity and desorption efficiency of the MB dye was determined using equations (3) and (4) respectively: Where qe: is desorption capacity, m is that the weight of spent adsorbent (g), V is the volume of the solvent (L), C f is the MB dye concentration in the solvent (mg/l). Desorption efficiency is defined as the amount of dye desorbed per gram of spent adsorbent at equilibrium.

Photocatalysis experiments
The experimental method on ZnO/bentonite nanocomposites for photodegradation of MB dyereported by (Bel et al 2017) [24], with little modification was used for this work. The experiment was carried out in a photocatalytic reactor, containing 400 W Mercury lamps. A cooling water jacket was used to maintain the temperature of the reactor. One gram of ZnO/bentonite nanocomposites (ZB11, ZB12, and ZB13) were added into 50 ml MB dye solution with an initial dye concentration of 80 mg L −1 at pH 8 of the solution. Then, the mixture was placed in a photocatalytic reactor, and adequate amount of sample was withdrawn after 60 min. The suspension was centrifuged and filtered; and the residual MB dye concentration present in the filtrate was determined using spectrophotometer at663 nm. The degradation capacity and degradation efficiency of MB dye were calculated using equation (5).
Where, A 0 is the initial absorbance of dye solution, and A t is the dye solution absorbance at a certain reaction time.   [28]. In the XRD patterns of ZnO-bentonite NCs, the left hand peaks (below 2θ of 30°) are strongly shows the bentonite character and right hand peaks (above 2θ of 30°) are the ZnO character. Hence, the XRD pattern revealed the composites synthesized were composed of ZnO and bentonite phases. The bentonite characteristics peaks intensity are lower than that of activated bentonite, this might be due to the intercalation of ZnO into the interlayers of bentonite, which changed the structure of host materials.

Fourier transform IR (FT−IR)
The functional groups of bentonite and composites of ZnO/bentonite were characterized using FT-IR spectroscopy in the range 4000-400 cm −1 . In the present study, FT-IR analysis was conducted for bentonite samples before adsorption, after adsorption, and after desorption of MB dye as indicated in figure 2(a). Also, the calcined and as synthesized ZnO/bentonite (ZB11) was analyzed using FT-IR. This technique has been significantly used to understanding the structure, bonding, and reactivity of clay minerals.  [29].
FT-IR is sensitive to the structural changes which occur in the clay upon soda activation and the IR data recorded in this studysuggested that both the octahedral and tetrahedral sheets are not affected by soda attack. The intensity of stretching band observed at 3620 cm −1 for pure bentonite (Al-OH-Al along with the Al-Mg-OH stretching vibrations) is the same as activated bentonite by sodium carbonate. Activation of bentonite with Na 2 CO 3 has changed the peaks of the bands associated with the absorbed water at 3435 cm −1 (H-O-H stretching) and decreased the peaks of the bands associated with the absorbed water at 1630 cm −1 (H-O-H bending). Changes in the intensities of these bands are explained by loss of water molecules when the pure bentonite changed to activated bentonite. After desorption of MB dye from the adsorbent, the intensity of the band at 1440 cm −1 disappeared. On the other hand, after soda activation, the intensity of the band at 1020 cm −1 increased due to the formation of three-dimensional networks of amorphous Si-O-Si units. But the intensity of the band at 690 cm −1 for (Al-OH-Si bending) almost disappeared with soda activation, signifying the partial dissolution of aluminum ions present in the octahedral sheet of bentonite. The intensity of the band at 522 cm −1 after desorption of MB dye from the spent adsorbent becomes the band of the bending. The results observed indicate the pattern of the spectra differs for the bentonite coated by methylene blue and purified bentonite, and after desorption of MB at the region 1700-500 cm −1 changes are observed on a certain bands [30]. In this region, interaction can occur between bentonite clay and secondary amines functional group of methylene blue in figure 2.
As indicated in figure 2(b) calcined and uncalcined ZnO/bentonite nanocomposites (ZB11) showed broadband absorption between 3400-3700 cm −1 , representing stretching vibration of O-H group of Zn-OH and Si-OH interacted with water molecules to form a hydrogen bond, but small deformation of Zn-OH and Si-OH formed in calcined ZB11. The spectra of calcined and uncalcined ZnO/bentonite nanocomposite exhibited the intensity band at 2843 and 2920 cm −1 , which indicated the stretching vibration of the C-H bond in CH 2 and

Complete silicate analysis of bentonite
Raw bentonite and 5% Na 2 CO 3 activated bentonite (AB5) were analyzed by complete silicate analysis method. For this characterization natural and activated bentonites having 75 μm sizes were analyzed by LiBO 2 fusion, HCl attack, gravimetric, colorimetric, and AAS analytical methods, and the results are listed in table 2.
Previously major and minor oxide composition of Gewane area bentonite was reported by (Mesfin Tessema, 2012) i.e., SiO 2 (54.2%), Al 2 O 3 (12.65%), Fe 2 O 3 (7.75%), TiO 2 (1.35%), CaO (4.15%), Na 2 O (1.85%), K 2 O (1.35%), MnO (0.1%). As indicated in table 2 the present results revealedthe Gewane area bentonite is Cabentonite. But, when natural bentonite which is Ca-bentonite was activated by Na 2 CO 3 it changes the oxide contents and becomes Na-rich bentonite. For instance, the sodium oxide and calcium oxide present in the natural bentonite before activation were analyzed and the results show the Na 2 O of raw bentonite was lower than AB5 as presented in table 2. The percentage of CaO decreases consistently while the percentage of Na 2 O increases, with the percentage increase in a mass ratio of sodium compounds. A similar result was reported by (Evangeline, 2009) when bentonite treated with Na 2 CO 3 [31]. This indicates the Ca 2+ of the raw bentonite was replaced by Na + during activation. The increase in the percentage of Na 2 O is higher than the percentage reduction of CaO in the clay matrix; here the substitution is divalent ions by mono-valent plus other ions like Mg 2+ also replaced by Na + . The λmax (663 nm) absorbance observed on UV-Visible spectrum obtained for stock solutions indicated in figure 4, confirmed the stock solution is MB dye [16]. As indicated in figure 4 Methylene blue has two absorbance peaks at 612 and 663 nm, π-π * of the benzene ring and a band at low energy around 660-670 nm (moving according to the pH of the solution) and n-π * transitions (n is the free doublet on the nitrogen atom of C = N bond and free doublet of S atom on S = C bond) at 605 nm atom on S = C bond) at 605 nm.

MB dye sorption from aqueous solution with activated bentonites
The activated bentonite adsorbents with different amounts of Na 2 CO 3 (2.5, 5, 7.5, and 15 g) per 100 g of bentonite clay was used to remove methylene blue dye from an aqueous solution. The experimental work was carried out by keeping the parameters constant like AB dosage 0.1 g of (AB2.5, AB5, AB7.5, and AB15), pH = 7, at a temperature of 25°C, initial concentration of MB dye 80 mg L −1 , and mixing speed at 300 rpm for 30 min.
The result from this experiment shows the removal efficiency of MB dye increase from AB2.5 to AB5, and the maximum removal efficiency observed by AB5. However, the removal efficiency of MB dye from aqueous solution decrease after AB5 as indicated in figure 5. A high negative charge on the surface of activated bentonite by (AB5) makes the attraction of opposite charge MB dye highly on its surface. Also, the AB5 has higher Na 2 O contents in the layer than AB2.5, AB7.5, and AB15 as from the previous report [31]. The high amount of Na-in Na-bentonite makes the exchange capacity of MB dye from aqueous solution high. Due to the above reason, the high removal efficiency of MB dye occurred by AB5 when compared to AB2.5, AB7.5, and AB15.

Removal of MB dye from aqueous solution with different adsorbents
The optimization tests for adsorbents were carried out to know the best adsorbent for the removal of MB dye from an aqueous solution. For this experiment, RB, PB, AB5, and ZB13 were used for the removal of MB dye from the aqueous solution. The adsorption test was carried out at constant parameters like adsorbent dosage 0.1 g, pH of solution 7, initial concentration of MB dye 80 mg L −1 , the temperature at 25°C, and mixing speed 300 rpm for 30 min. The results showed the 5% Na 2 CO 3 activated bentonite (AB5) has high removal efficiency for MB dye removal from aqueous solution than RB, PB, and ZB13 as indicated in table 3. This may be due to the presence of more negative surface charge on AB5 adsorbent. The more negative surface charge of Na-bentonite makes the best removal of cationic dye from an aqueous solution. The other reason for AB5 with high removal efficiency was due to the interactions between the negative charge of Si-O and the positive charge of methylene blue ion (MB + ), ionic exchange with the Na-bentonite. The result shows the PB has the lowest removal efficiency as indicated in table 3; this may be during the purification of bentonite claythe organic matter is leached out and feldspar can be partly attacked (or dissolved).  This process causes an increase in surface area, surface acidity and introduces permanent mesoporosity and also removes metal ions from the crystal interlayer, which partially delaminated the clay [33]. The other reason why the purified bentonite has the lowest removal efficiency of MB dye is due to the moisture content of the purified bentonite. The moisture in bentonite makes the pore sizes on the surface of bentonite filled with water.

Effect of adsorbent dosage on adsorption efficiency
The effect of adsorbent dosage (AB5) was carried out by keeping all parameters constant : pH 7, initial dye concentration 80 mg l −1 , contact time 30 min, temperature25°C, mixing speed 300 rpm, and volume of MB dye 50 ml. The resultindicates the removal efficiency of MB dye increases as AB5 dosage increases. This might be due to the presence of more adsorption sites present on the AB5 surface when the adsorbent dosage increases. As shown in figure 6(a) there is a slow increase of adsorption efficiency between 0.1 and 0.2 g , above 0.2 g it increases sharply in the adsorption efficiency but reaches almost a no change state beyond and this removal efficiency isvery small, similar result was reported [33] on organo-modified nano clay [33]. As the adsorbent dosage increase the adsorption capacity of MB dye was decreased as shown in figure 6(b). The negative sign showed for the residual concentration of MB dye after adsorption of MB dye from aqueous solution indicates the excessive adsorbent site due to the high adsorbent dosage in the solution.

Effect of initial concentration of MB dye
The experimental study on the effect of initial concentration on the removal efficiency of MB dye was carried out by keeping all parameters constant: AB5 dosage 0.1 g, contact time30 min, pH 7,temperature25°C, mixing speed of 300 rpm, and volume of solution 50 ml,the initial concentration of MB dye was varied with 50 ml interval (80, 130, 180, and 230) mg/l. The result indicates, as the initial concentration of MB dye increases from 80 mg L −1 to 230 mg/l the removal efficiency of MB dye decreasesfigure 7(a). This implies the removal efficiency is inversely proportional to adsorbent surface loading. The reduction of the removal efficiency of MB dye was due to the increased concentration of the methylene blue dye in the solution for the same number of AB5 sites and on the same surface of the AB5. In other words, the decrease in the removal efficiency of MB dye was attributed to the lack of sufficient surface sites to accommodate much more methylene blue dye concentrations present in the solution. At lower concentrations, all dye molecules present in the solution could interact with binding sites and thus be higher than those at higher concentrations. Thus, the removal efficiency of MB dye decreases as concentration increases, similar observations were reported for natural Saudi Red Clay [34].

Effect of pH
During the adsorption process, the pH of the aqueous solution plays an important role [30]. The effect of pH on the removal of MB dye was carried out by keepingparameterslike temperature = 25°C, AB5 dosage = 0.1 g, initial concentration of MB dye = 80 mg L −1 , and mixing speed 300 rpm for 30 min constant. But the pH of the solutions were varied : 4, 7, 9, and 11 to determine the optimum pH of MB dye solution which will give the best removal efficiency of MB dye. The resultindicates the removal efficiency of MB dye increase slightly with an increase in pH of the solution uptake has been observed up to pH 7 thereafter, slightly the decrease in the removal of MB dye has been observed as shown in figure 8. This might be happened when the surface of bentonite is negatively charged in acidic media; H + ions competed with MB + for activating bentonite sites and lead to minimal adsorption of MB dye onto AB5 at low pH of the solution. The other reason why the removal of MB dye increase with the increase in pH is due to the dissolution of MB dye into the water, the amine group in the MB dye structure enters the aqueous solution makes the dye has an overall positive charge and attracted to the negatively charged AB5 adsorbent. With the increase in pH, the number of H + ions decreased and the adsorbent surface carriedmore negative charge resulting in greater attraction between the adsorbent and the cationicadsorbate [35]. A similar result was reported for pH 7 by the previous researcher [36] on Kaolin.

Effect of temperature
The experimental study on the effect of temperature for the removal of MB dye was carried out by keeping five parameters constant: pH = 7, initial concentration of MB dye = 80 mg L −1 , contact time = 30 min, AB5 dosage = 0.1 g, and mixing speed 300 rpm varying temperature in the range of 25°C-50°C. The result indicates the removal efficiency of MB dye using AB5 was observed to increase with increasing temperature as shown in figure 9. This may be due to the increase in the kinetic energy and entropy of the system resulting in more collisionsbetween the activated bentonite and MB dye, similar result was reported previously on natural Saudi Red clay [37].

Effect of contact time
The effect of contact time on removal efficiency of MB dye from aqueous solution was carried out at constant parameters of pH(pH = 7), initial concentration of MB 80 mg l −1 , AB5 dosage 0.1 g, mixing speed 300 rpm, and temperature 25°C, for variable contact time in the range of 30 min to 90 min. The result indicated the effect of contact time on the removal of MB dye from an aqueous solution is increased as the contact time increased. As

Adsorption isotherms
Adsorption isotherm describes the qualitative information on the nature of the adsorbent surface interaction as well as the specific relationship between the concentration of adsorbate, and its degree of accumulation onto the adsorbent surface at a constant temperature. Adsorption isotherms are critical in optimizing the utilization of adsorbents, and therefore the analysis of the isotherm data using different isotherm models is a crucial step to find the suitable model that can be used for design purpose [39]. Hence, threeisotherms were selected for this study, namely Langmuir, Freundlich, and Temkin isotherms.

Langmuir adsorption isotherm model
The Langmuir isotherm model works based on the assumption of a monolayer, and uniform energies of adsorption.Langmuir constant (K L ) and the maximum adsorption capacity (qmax) are obtained from the slope and the intercept of a linearized Langmuir equation [36]. The Langmuir adsorption is indicated by maximum adsorption capacity (qmax) which represents the saturated monolayer adsorption at equilibrium. The result shown in table 3 indicates the Langmuir adsorption isotherm parameters qmax (76.5 mg g −1 ) and R 2 is 1 which is a strong correlation coefficient. The correlation coefficient was obtained from a graph of Ce/qe versus Ce plotted and linearity was determined from the correlation coefficient (R 2 ) as indicated in figure 11. From the correlation coefficient R 2 of Langmuir, the adsorption isotherm indicates the adsorption study of MB dye on activated bentonite was best fits the Langmuir isotherm model than that of Freundlich and Temkin isotherm models. The Langmuir isotherm study indicates the value of RL decided the shape of the isotherm was favorable due to the value of RL for this study was found in between 0 & 1 that means (0 < RL < 1) [40].

Freundlich adsorption isotherm model
The Freundlich equation is an empirical equation employed to describe adsorbate on the surface of the heterogeneous adsorbent that is characterized by the heterogeneity factor 1/n, describes reversible adsorption, and is not restricted to the formation of the monolayer. The affinity between the adsorbate and adsorbent might be indicated by the heterogeneous factor, 1/n. The isotherm parameters result is presented in table 4. The saturated adsorption capacities at 298˚K for AB5 were 39.90 mg g −1 . The Freundlich parameter, of 1/n, was between 0 & 1 which indicates the favorability of MB dye adsorption on activated bentonite. This means the  value of n is greater than 1 which indicates the bond between the MB dye and bentonite adsorbent was strong. The correlation coefficient R 2 of Freundlich was 0.712 this lower than the Langmuir isotherm, this indicating it does not fit the experimental data.
In figure 12 the line does not touch all the dots. So, this model was not fit for this experimental data. The binding of MB molecule to one site on bentonite might facilitate the adsorption of subsequent molecules towards the active sites.

Temkin adsorption isotherm model
The Temkin adsorption isotherm model shows the adsorption heat of all molecules in the layer is assumed to decrease linearly with coverage of the adsorbent surface due to a decrease in the interactions between the adsorbent and adsorbate.
Temkin isotherm model does not fit the experimental data and its parameters were obtained from a graph of qe versus ln Ce plotted in figure 13. Its linearity was determined from the correlation coefficient (R 2 ). As it was observed in table 5 Temkin adsorption isotherm parameters are KT (320.29), B (6.92), and correlation coefficient R 2 = 0.713 which is a weak correlation coefficient. From this point of view adsorption of MB dye on activated bentonite is not packed with a similar structural arrangement.

Adsorption kinetics models
The adsorption kinetics describes the solute uptake rate, which in turn governs the residence time of the sorption reaction. Pseudo-first-order and pseudo-second-order models were applied to analyze the kinetics study of MB  dye adsorption. It was one of the important characteristics in defining the efficiency of sorption. In the present study, the kinetics study of MB dye removal was carried out to understand the behavior of the prepared low-cost adsorbent activated bentonite. The corresponding data was given in table 5 for pseudo-first-order and pseudosecond-order respectively, which indicates the pseudo-second-order kinetic model fit well for MB adsorption data on activated bentonite than the pseudo-first-order kinetic model.

Pseudo-first-order and pseudo-second-order kinetic models
The pseudo-first-order kinetic model specified that the speed of occupation of adsorption sites is proportional to the number of unoccupied sites. The values of log (qe-qt) were linearly correlated with t. The values of K 1 and qe were calculated from the slopes and intercepts of log (qe-qt) against the t plots and the different parameters of pseudo-first-order kinetics are given in table 5.
The result for the present study shows the pseudo-first-order model with a correlation coefficient (0.36) was not fit the experimental data as indicated in figure 14(a). While the pseudo-second-order model fitted very well with R 2 equal to a unit (R 2 = 1) as shown in figure 14(b). The MB dye adsorption on activated bentonite can be described as the pseudo-second-order model, which indicated that the adsorption of MB dye on activated bentonite can be described as chemical adsorption. The kinetic model for the present study results reveals that the adsorption of MB dye onto activated bentonite could be well explained by a pseudo-second-order kinetic model. This is good agreement with the work reported by (Hu et al 2018) on activated red mud [41]. The results of pseudo-first-order and pseudo-second-order kinetic models are summarized in table 5.

Thermodynamic study
Adsorption processes are influenced by temperature; thus in the present study, the effect of temperature was carried out in the temperature range of 298 K to 323°K ( figure 15). The thermodynamic parameters for the adsorption of MB dye on bentonite adsorbent were calculated to determine the thermodynamic feasibility of the thermal effects of the adsorption; the Gibbs free energy (ΔG°), the entropy ΔS°and the enthalpy (ΔH°) were calculated using equation (4.2).   The result for the present study indicates the negative values of ΔG°show the adsorption process on the activated bentonite (AB5) was spontaneous. The increase in negative value with increasing temperature indicates that the adsorption process on activated bentonite becomes more favorable at higher temperatures as shown in table 4. Also, the ΔG°values are in the range between −30 KJ mol −1 and −20 KJ mol −1 shows the physisorption was likely the mechanism [37]. The standard enthalpy of adsorption ΔH°= 56.55 KJ mol −1 indicates an endothermic process and the positive value of ΔS°reflects the affinity of activated bentonite for MB and suggests an increase in disorder/randomness at the adsorbate-adsorbent interface i.e., some structural changes in dye and activated bentonite which agrees with the observations. 3.8. Recyclability of adsorbent & its composites 3.8.1. Desorption of MB dye from the exhausted adsorbent by thermo-chemicals method The desorption study of MB dye from the exhausted adsorbent is important to investigate the retention ability of AB-MB. Desorption of MB dye from the exhausted adsorbent was carried out by thermochemical method using  different solvents. For this study, the dried adsorbent coated by MB dye after adsorption from aqueous solution was used for the desorption study using distilled water, ethanol, and acetone. The result showed desorption of MB dye from the exhausted adsorbent using distilled water (DW) was better when compared with ethanol and acetone as indicated in figure 16. The desorption of MB from the spent adsorbent by ethanol and acetone was relatively low, and similar result was reported for ethanol and thermo-chemical methods [42]. However, the low desorption efficiency of MB dye from adsorbent might be due to a strong electrostatic attraction of MB with the adsorbent and requires higher energy to get rid of the dye through chemical regeneration method. Strong adsorbent-adsorbate (AB-MB) interaction is the cause for the low desorption results of MB dye adsorbed on activated bentonite occurred during the adsorption process. On the other hand, efficient desorption of MB dye from exhausted adsorbent is the crucial step to allow the adsorbent to be regenerated [36]. After desorption of MB dye from the exhausted adsorbent, the adsorbent was reused for recyclability MB dye from an aqueous solution.

Recyclability of adsorbent after usage
After the adsorption study, the adsorbent was coated with the MB dye. Discharge of the spent adsorbent with MB dye to the environment is environmentally not good because the MB dye on the adsorbent is toxic and carcinogenic. Due to this reason recycling adsorbents and adorbates after usage was one of the most important to reduce the dyes that discharged to the environment or water bodies.
The recyclability of adsorbent for removal of methylene blue dye from aqueous solution was carried out by spent adsorbent after desorption of MB dye in distilled water at 35°C for 2 h. The result of this activities indicates the removal efficiency of MB dye from aqueous solution by spent adsorbent for the first, second, and third cycles were (71.89%), (66.71%), and (59.40%) respectively. The decreasing removal efficiency of MB dye from an aqueous solution of adsorbent might be due to the low desorption efficiency of MB dye from the active sites surfaces of adsorbent (AB5). As shown in figure 17 the removal efficiency of MB dye after recycling of adsorbent for the first cycle has high removal efficiency than the second and third cycles similar results were reported by (Kara et al 2021) on the removal of methylene blue dye from wastewater using periodiated modified nanocellulose [43].

Photodegradation of MB dye by ZnO/bentonite nanocomposites (ZB)
As shown in section 3.8.1 the desorption results of the adsorbent and methylene blue are very low, and the disposal of exhausted adsorbent to the environment will create another problem. However, different research reports revealed the high degradation efficiency of organic contaminates with zinc oxide immobilized on bentonite clay surface [44]. Modified bentonite clay increases the adsorptive capacity of organic contaminant (MB dye) and will improve the photocatalytic rate of nano zinc oxide [45,46]. For this reason, in this work, the photo-degradation test of methylene blue using zinc oxide-bentonites was carried out and the results obtained were discussed below. Photodegradation of MB dye using ZnO/bentonite nanocomposites for the removal of MB from the aqueous solution was carried out using UV light. The result for this study showed ZB11 composite has better degradation efficiency (99.53%), as revealed in figure 18. Figure 18 shows the photodegradation efficiency of MB dye using ZnO/bentonite nanocomposites (ZB11, ZB12, and ZB13). The summary of the result of this study indicates that the ZB13 (1:3) ZnO/bentonite nanocomposites showed better removal efficiency (99.94%) of MB, but ZB13 has lower degradation efficiency   relative to the other tested composites (92.02%). On the other hand, the ZB11 has higher photocatalytic activity for degradation of MB from aqueous solution (99.53%). As indicated in figure 19, this might be due to the homogeneous distribution of more number of ZnO nano particles photocatalyst with average particle size of 30.06 nm in the bentonite matrix as it is observed on its TEM micrographs for ZB11 and less number of ZnO nanoparticles photocatalyst in the bentonite matrix for ZB13. Semiconductors metal oxide like TiO 2 , ZrO 2 , CuO and ZnO NPs with optimum size Immobilized on Bentonite Clay resulted with better photo-catalytic efficiency [13,21,25,47,48]. The mechanism of photocatalysis of contaminants using ZnO photocatalyst in presence of UV irradiation is shown below as reported by different groups.

Conclusions
In the present study the development of Green or environmentally friendly adsorbent; bentonite was collected from Afar regional state, dried, ball-milled, sieved, purified, and activated with sodium carbonate with different percentages. The bentonite activated with 5% Na 2 CO 3 (AB 5 ) has 100% removal efficiency of the targeted organic dye (i.e., methylene blue) from an aqueous solution. The batch adsorption experiments were conducted using AB5 to optimize the variables like temperature; pH, initial concentration, adsorbent dosage, and contact time.
The results of the optimized parameters for this study are pH = 7, contact time = 30 min, initial concentration of dye = 80 mg L −1 , at a temperature of 25 C°, and mixing speed 300 rpm. The adsorption isotherm studies result shows the removal of MB from aqueous solution using activated bentonite by Na 2 CO 3 was best fits the Langiumer adsorption isotherm model with the correlation coefficient R 2 = 1. The kinetic adsorption study was best fits the pseudo-second-order model with the correlation coefficient R 2 = 1. The thermodynamic study conducted in the temperature range of 298°K to 323°K results with the negative values of ΔG°, which indicates the adsorption process on the developed adsorbent(AB5) was spontaneous. The standard enthalpy of adsorption ΔH°= 56.55 KJ mol −1 determined, indicates an endothermic process and the positive value of ΔS°reflects the affinity of activated bentonite for MB dye. Desorption tests of MB from exhausted adsorbent (AB5-MB) after adsorption using the thermo-chemical method were carried out to reuses both the adsorbent and the dye. The highest result of the desorption study of MB dye in distilled water from exhausted adsorbent obtained is about 50%. Because of this low desorption result, the photocatalytic degradation test conducted using ZnO/bentonite nanocomposites synthesized for the MB dye degradation showed better results (99.53%) efficiency.