A reusable mesoporous adsorbent effectively treats hazardous triphenylmethane dye wastewater: RSM-CCD optimization and rapid microwave-assisted regeneration | Scientific Reports

2021-11-24 01:57:43 By : Mr. Henry Chen

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Scientific Reports Volume 11, Article Number: 22751 (2021) Cite this article

In this study, mesoporous calcium aluminate nanostructures (meso-CaAl2O4) were synthesized using a citric acid-assisted sol-gel automatic combustion process as a potential adsorbent to eliminate the toxic triphenylmethane dye in synthetic/real wastewater Malachite green (MG). The surface morphology of Meso-CaAl2O4 is highly porous, with nanometer size and a non-uniform surface. The specific surface area, total pore volume and BJH pore diameter of meso-CaAl2O4 are 148.5 m2 g-1, 1.39 cm3 g-1 and 19 nm, respectively. Meso-CaAl2O4 also exhibits very high heat resistance because it only loses 7.95% of its weight at 800°C, which is mainly related to moisture loss. Based on response surface method (RSM)-central composite design (CCD) technology to obtain the best adsorption conditions. The Langmuir isotherm model is used to fit the adsorption measurement, in which the maximum adsorption capacity of the dye is 587.5 mg g-1. It was found that the data obtained from the adsorption kinetic model corresponded to the pseudo-secondary model. In addition, thermodynamic parameters including enthalpy change (ΔH°), entropy change (ΔS°) and Gibbs free energy change (ΔG°) indicate that the adsorption of MG dye by mesoscopic CaAl2O4 is feasible and endothermic. And it happens spontaneously. In addition, under the microwave irradiation of 900 W for 6 minutes, the micro-CaAl2O4 was regenerated. After 5 microwave regeneration cycles, the MG dye removal rate remained above 90%.

Water pollution is one of the most serious problems facing the world today. Among various pollutants such as pesticides, pharmaceutical raw materials, heavy metals, and microplastics, dyes are highly radioactive, non-biodegradable and carcinogenic1. Many manufacturing industries contain large amounts of dyestuffs in wastewater, including food, plastics, cosmetics and textiles2. Therefore, it is important to remove dyes from manufacturing wastewater before they are discharged into the environment. The textile industry ranks first in terms of dye consumption, and textile wastewater treatment is challenging 3, 4. As a triphenylmethane cationic dye, malachite green (MG, Figure 1) is one of the most widely used synthetic colorants for dyeing Wool in the silk, leather, cotton and textile industries5. MG dye is also used in the aquaculture industry because of its highly effective antibacterial, antifungal and antiparasitic agents. Although many countries have banned MG dye in the production of food fish, it is still used because of its low cost and high efficacy6. However, drinking water contaminated with MG dye is dangerous and carcinogenic because of the presence of nitrogen in its structure and causing serious harm to humans and animals7. Therefore, the effective removal of MG dye residues in water and wastewater is still a severe environmental challenge. Various treatment methods such as physical chemistry, biochemistry and electrochemical processes have been used to remove MG dyes from textile wastewater8. On the other hand, the resistance of MG dye to light and oxidants makes its biological and chemical deposits difficult to remove from wastewater. Adsorption is considered to be an effective method due to its low cost, easy operation, simplicity, flexibility, and insensitivity to toxic pollutants, and has been proven to be a successful alternative to traditional treatment methods9. In recent years, the adsorption capacity of various adsorbents for MG dye has been studied based on experience, such as nanoparticles 10, nanosheets 11, nanocomposites 12, polymer aerogels 13, carbon-based materials 14 and metal-organic frameworks ( ZIF-67) 15.

The chemical structure of MG dye.

So far, various applications of calcium aluminate-containing materials have been reported, including catalysts 16, electronic compounds and ion conductors 17, superhydrophobic cements 18, biological materials in the dental and orthopedics fields 19, bone cements 20, hard tissue repair 21, Nano catalyst 22 and adsorbent used to produce biodiesel to remove Cr(VI)23 aqueous solution. However, a review of the literature shows that there is no record of using monocalcium aluminate (CaAl2O4) to remove MG dye from aqueous media. Therefore, the current study first confirmed the use of CaAl2O4 to adsorb MG dye from contaminated water. In order to obtain CaAl2O4, several synthesis methods have been introduced, such as solid-state reaction 24, sol-gel 25, Pechini method 26, and solution combustion method 27. Sol-gel automatic combustion is an innovative method that combines solution-combustion synthesis and sol-gel process. It is based on the gelation of an aqueous solution composed of organic fuels (such as citric acid and metal salts) and the resulting Burning. The method has the advantages of low precursor cost, simple equipment, low processing cost, high yield, low temperature, and ultrafine particles. In this study, we describe the synthesis of mesoporous CaAl2O4 nanostructures by the sol-gel spontaneous combustion method using citric acid as fuel.

On the other hand, different factors including adsorbent dosage, primary concentration, solution pH, temperature and contact time have been shown to simultaneously affect the efficiency of adsorbent29. Therefore, the optimization process requires a thorough understanding of how these factors interact simultaneously to affect adsorption. Therefore, experimental design is used as a set of useful mathematical techniques to improve design and optimize key parameters. RSM (Response Surface Method) is an effective statistical technique that can simultaneously consider several independent variables that affect the objective function and their interactions. This strategy requires steadily reducing the number of trials and testing multiple regressions to find the conditions that produce the best response to the methodological range under consideration31. Central Composite Design (CCD) is a standard, effective and most commonly used RSM design. In addition, in adsorption technology, it is a severe challenge to regenerate the adsorbent with minimal efficiency loss. Therefore, researchers pay close attention to the use of several regeneration methods to reuse adsorbents. Because the microwave-assisted regeneration technology has a unique molecular-level heating capability and can achieve a fast and uniform thermal reaction, it has recently been extensively studied32.

In this study, citric acid-assisted sol-gel spontaneous combustion was used for the first time to synthesize microscopic CaAl2O4 as an adsorbent, and its structure was identified through various state-of-the-art analytical techniques. In addition, by using the RSM-based CCD, the influence of adsorbent dosage, solution pH, contact time, initial dye concentration, and solution temperature was also evaluated and optimized. The equilibrium adsorption isotherm model and the kinetic model and thermodynamics of the adsorption process are also studied to fit the experimental data. Finally, the reusability and performance of the adsorbent in actual wastewater samples were studied.

The chemicals and equipment used in this study are detailed in the "Electronic Supplementary Information".

In order to synthesize meso-CaAl2O4, the citric acid-assisted sol-gel spontaneous combustion technology is as follows: Dissolve an appropriate amount of Al(NO3)3·9H2O and Ca(NO3)3·4H2O (2:1) in ultrapure water and keep stirring until the A clear solution was obtained at room temperature. The solution is heated at 80 °C for 15 minutes, then the citric acid is dissolved in a minimum amount of water and added to the heated solution. The ratio of citric acid to nitrate is 1:1 (mole). The resulting reaction mixture was heated to 80° C., and ammonium hydroxide solution (0.1 mol L-1) was gradually added to adjust the pH to 7. After stirring for 2 hours, the mixture gelled. Then the gel was heated in an oven at 200°C for 1 hour to obtain a yellowish-white substance, and then heated at 400°C for 30 minutes until the automatic combustion process occurred. Finally, the powder was calcined at 700°C for 1 hour to obtain meso-CaAl2O4.

By using a shaker (digital water bath incubator Drawell Scientific, Shanghai, China), the balance test was performed by the following procedure: In the shaker, 50 mL of CaAl2O4 containing mesoscale mass (2-8 mg) and different triphenylmethane dye solutions were filled . Initial dye concentration (20–100 mg L-1). Other experimental conditions are that the temperature of the solution is 5-45 °C, the pH of the solution is 2.0-10, and the contact time is 5-25 minutes. After conducting the experiment at predetermined time intervals, the adsorbent was separated, and then the MG dye remaining in the solution was measured spectrophotometrically at 617 nm at λmax (Figure 2).

Schematic diagram of Meso-CaAl2O4 and microwave-assisted regeneration and adsorption of MG dye.

Perform CCD to check the influence of 33, 34 contact time, solution pH value, initial dye concentration, mesoscopic CaAl2O4 quality, and solution temperature selected as the independent variable. As shown in Table 1, by performing 32 tests, each factor is checked in five levels (- 2,-1, 0, 1 and 2). Complete the analysis of variance (ANOVA) statistical test to evaluate the coefficient of determination (R2), and also apply the F test to define the significance of the influence of each variable. By applying regression equations, studying the inverse response surface plot and setting the constraints of the variable levels, the best values ​​of the variables are obtained.

The regeneration of the dye-loaded meso-CaAl2O4 was carried out by microwave heating using a 2.45 GHz microwave oven, with a maximum output power of 900 W, at different power levels and exposure times. After the MG dye adsorption process, the dye-carrying meso-CaAl2O4 is separated from the reaction medium and placed in a pure alumina crucible in a microwave oven, and heated by microwave. After finishing the heating process with the digital display temperature controller, the overall temperature of the medium-sized CaAl2O4 is measured by quickly inserting the thermocouple into the sample. Finally, the regenerated meso-CaAl2O4 was washed with ultrapure water to remove the degraded MG dye on the meso-CaAl2O4 surface, and dried in an oven before reuse (Figure 2).

SEM images are used to analyze the surface morphology of meso-CaAl2O4. Figure 3a shows the amorphous shape of meso-CaAl2O4 with a size of approximately 8 μm, which has a highly uneven surface and micron-scale cavities. However, the enlarged image shows that this porous structure is composed of aggregation and adhesion of nearly spherical nanoparticles in the range of 10 to 20 nm. In addition, careful observation revealed that the accumulation of these nanoparticles created cavities below 50 nm in the overall structure of the sample, resulting in the formation of a mesoporous structure (pore size range of 2 to 50 nm). The formation of such pores in the sample leads to a high value of specific surface area in the sample. In the TEM image (Figure 3b), the adhesion of about 10 nm nanoparticles and the resulting holes are clearly visible, which is consistent with the SEM image. In Figure 3c, the EDX spectrum shows the presence of Ca, Al, and O, with no more peaks. It is also obvious that there are no other elements in the synthesized adsorbent, and in fact there are no other impurities.

(a) SEM image, (b) TEM image, and (c) EDX analysis of microscopic CaAl2O4.

The structure and phase purity of the synthesized meso-CaAl2O4 were studied by XRD, as shown in Figure 4a. It can be seen that according to JCPDS Card No. 00-001-0888, it can be proved that meso-CaAl2O4 was successfully synthesized, and no other impurities were found. FT-IR spectroscopy was used to determine the surface functional group type of meso-CaAl2O4, as shown in Figure 4b. The deformation vibration of the water molecule is shown by the stretching vibration of OH at 3437 cm-1, which corresponds to the free hydrogen-bonded hydroxyl group. The nitro group of the precursor causes other small peaks to appear at 1122 cm-1 and 1254 cm-135. It can be seen that the stretching vibration of the MO bond (M=Al, Ca) shows a strong absorption band at 823 cm-1. The stretching vibration of the Ca-O bond is shown at 617 cm-1 and 436 cm-1. The band at 528 cm-1 indicates the presence of aluminum ions, which is confirmed by the stretching vibration of Al-O. The bands at 1727 cm-1 and 2925 cm-1 are related to the presence of citrate ions, which still exist after calcination at 700 °C26. Use N2 adsorption/desorption isotherm, BET specific surface area and BJH pore size distribution analysis to evaluate the surface area and pore structure characteristics of Meso-CaAl2O4. According to the IUPAC classification, the N2 adsorption/desorption isotherm of meso-CaAl2O4 (Figure 4c) is a V-shaped isotherm with a H1-type hysteresis loop, which is mainly related to the mesoporous structure. In addition, the H1 type is generally associated with porous materials, which have a narrow pore size distribution 36. These findings mention the fact that meso-CaAl2O4 has a mesoporous structure, which can also be seen in the SEM image. According to the results of BET, the total pore volume and specific surface area of ​​the adsorbent are 1.39 cm3 g-1 and 148.5 m2 g-1, respectively. In order to further determine the pore size distribution of meso-CaAl2O4, the BJH equation is used. According to the IUPAC classification, pores smaller than 2 nm are called micropores, pores smaller than 2 to 50 nm are called mesopores, and pores larger than 50 nm are called macropores37. Figure 4c (inset) shows the pore size distribution between 10 and 40 nm, which proves the meso-CaAl2O4 mesoporous structure, and most of the pores are around 19 nm.

(a) X-ray diffraction, (b) FT-IR spectroscopy, (c) nitrogen adsorption-desorption isotherm and BJH pore diagram (inset), and (d) microscopic CaAl2O4 TG/DTG curve.

Figure 4d shows the TGA-DTG curve of meso-CaAl2O4 heated from 25°C to 800°C in an air atmosphere (heating rate = 20°C min-1). According to the TGA curve, the total weight loss of the sample is 7.95%, confirming that meso-CaAl2O4 has excellent heat resistance. The main weight loss may be related to the loss of moisture in the sample, which occurs between 25 and 250 °C. There is no significant weight loss after 400 °C, and the sample weight remains unchanged. The DTG analysis at 70.8 °C also detected a major weight loss rate, which confirmed the results obtained from the TGA analysis.

For the quadratic model of MG dye adsorption, the coefficient of variation (CV%) obtained by analysis of variance (Table 2) = 2.544% and F value = 217.0 (probability> F less than 0.0001) means that the model is very significant and the experimental results are reliable Accurate 38. The P value of the lack of fit analysis obtained by the quadratic model is not significant if it is higher than 0.05 (0.2861), which confirms the reliability of the model to predict the adsorption of meso-CaAl2O4 to MG. MG concentration (X2), contact time (X4), adsorbent quality (X3), pH value (X1), temperature (X5), X2X4, X3X4, X2X3, X4X5, X1X2, X12, X42 interaction affects significant items , Due to the value of P (< 0.0001) (Figure 5). Other coefficients are not significant (P> 0.05).

Pareto diagram of MG dye adsorption.

According to the statistical results, the prediction model of dye adsorption can be written as follows:

The values ​​of R2 = 0.998 and Adj-R2 = 0.993 indicate that there is a good correlation between the results collected by the equation. (1) The existence of experimental data indicates that the values ​​are accurate and reliable. In addition, the predicted R2 value means that the model has significant blocking effects. The difference between the value of Adj-R2 (0.993) and predicted R2 (0.955) should be approximately 0.2039, and their difference indicates a problem with the model or data. The sufficient accuracy value of 67.20 also confirms the validity of the model.

It is known that the pH value of the solution is an important factor affecting the adsorption performance. The zero charge point (pHpzc) of meso-CaAl2O4 was evaluated by zeta potential analysis to find its surface charge at different pH values. According to Figure 6a, the adsorbent is neutral, positively charged and negatively charged at pH values ​​= respectively 8.7,> 8.7, <8.7.

(a) The zero charge point of meso-CaAl2O4 (error bars indicate ± standard deviation, n = 3), and (b) a two-dimensional graph showing the effect of MG concentration and solution pH on the R% of MG dye.

In order to better understand the effect of pH, the results of the R% MG prediction model as a function of MG concentration and solution pH are displayed as a 2D contour map (Figure 6b). The corresponding graph shows the R% of MG dye at different pH values. It can be seen that in an alkaline medium, R% is greater than in an acidic medium. By increasing the pH from 2.0 to 10, R% increased from 19.2 to 98.8%, which may be due to the increased electrostatic attraction between MG dye molecules and meso-CaAl2O4 molecules. As shown in Figure 6b, when the initial concentration of MG increases from 20 mg L-1 to 100 mg L-1, the percentage of adsorbed dye drops from 98.6% to 24.9%. This may be due to higher dye concentration requiring more active sites for dye adsorption. Therefore, there is more competition between the MG dye molecule and the binding site of meso-CaAl2O4, and the R% of MG is reduced by 11. Under different initial pH values, the meso-scale photo of CaAl2O4 adsorption of MG dye is shown in Figure 6b.

The two-dimensional contour map evaluated the influence of the influencing parameters (including MG concentration, adsorbent quality, contact time and temperature) on the simultaneous effect of the adsorbent on the removal efficiency of MG dye. The results are shown in Figure 7.

The contour plot shows the effect of (a) adsorbent quality-MG concentration, (b) adsorbent quality-contact time, (c) MG concentration-contact time, and (d) contact time-temperature on the R% of MG dye.

Figure 7a shows the simultaneous effect of adsorbent quality and MG concentration on the adsorption percentage of MG dye. It can be seen that the highest R% of MG appears in the range of simultaneous influence of lower dye concentration and higher adsorbent quality. The adsorption percentage of MG dye increased sharply from 19.6% to 98.2%, while the mass of meso-CaAl2O4 increased. This may be due to the increased surface area and the presence of more adsorption sites. The fact is that increasing the quality of the adsorbent can prevent the adsorbent from being saturated during the process 30, 40. Figure 7b shows the simultaneous effect of adsorbent quality and contact time on adsorption efficiency. The results show that the removal efficiency increases with time and the quality of the adsorbent. The results of the study showed that increasing the contact time from 5 minutes to 25 minutes increased the R% of MG from 46.7% to 98.7%. Within the first 15 minutes of the contact time, the R% reached 89.6%. The large number of available vacancies on the surface of the adsorbent can explain the rapid adsorption of the dye in the initial stage.

The adsorption performance hardly changed after 15 minutes, which may be due to the saturation of the adsorption sites on the microscopic CaAl2O4 surface and the penetration of MG dye molecules into the pores31, 42. The MG concentration and the time of exposure simultaneously affecting the removal efficiency of MG dye are shown in Figure 7c. As shown in the figure, the highest MG dye removal efficiency is obtained at low MG concentration and high contact time. Compared with MG dye molecules at low MG concentration, the adsorbent has a higher rate of active adsorption sites. In addition, increasing the contact time will increase the proximity of the dye molecules to the active adsorption sites of the adsorbent until the adsorbent reaches a saturation level. Figure 7d shows the simultaneous effect of contact time and solution temperature on the removal efficiency of MG dye. The results show that as the contact time and temperature increase, the MG dye adsorption efficiency increases. Therefore, as the temperature increases from 5°C to 45°C, the adsorption efficiency of MG increases significantly from 34.8% to 98.2%. However, at the highest 25°C, 96.0% of the adsorption is achieved, and when the temperature exceeds 25°C, the temperature has no significant effect on the adsorption performance (95%-98.2%).

The optimal value of the dependent variable (R% MG) is determined by using the desirability function in the STATISTICA software. The experimental conditions for maximum MG removal (100%) are solution pH (X1) = 8.0, MG concentration (X2) = 50 mg L-1, meso-CaAl2O4 mass (X3) = 8 mg, contact time (X4) = 15 minutes , The solution temperature (X5) = 25 °C (Figure 8). The estimated value is used for experimental testing. The experimental R% (98.68 ± 2.11%) result is almost the same as the value predicted by the model, indicating that the model is highly reliable.

The best MG adsorption conditions in the model.

The adsorption equilibrium isotherm is essential to convey the behavior of the adsorption process. The following four fitting models were used in this study: Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (DR). In addition, a dimensionless constant called separation factor (RL) is used to measure the properties of adsorption, where RL> 1 indicates unfavorable adsorption, RL = 0 corresponds to irreversible adsorption, and 0 <RL <1 indicates favorable adsorption. Table 3 lists the fitting parameters and the correlation coefficient (R2) value of the adsorption isotherm model at different temperatures. According to the obtained R2 value, the Langmuir model (R2 = 0.999) is more suitable for other models at all temperatures, which indicates that the MG dye molecule is monolayer and uniformly adsorbed to the meso-CaAl2O444 surface, 45. It also revealed that the molecules adsorbed by the MG dye onto the meso-CaAl2O4 surface did not interact.

The results show that as the temperature increases from 5 ºC to 45 ºC, the Langmuir theoretical maximum single-layer adsorption capacity increases from 227.0 to 617.1 mg g-1. However, as the temperature increases from 25°C to 45°C, the increase rate of adsorption capacity is not significant, and the maximum single-layer adsorption capacity obtained at 25°C is 587.5 mg g-1, which is different from the application in recent years. The other several adsorbents are equivalent. Remove the MG dye from the aqueous medium (Table 4). Therefore, the characteristics and performance of the studied adsorbent meso-CaAl2O4 support its use as a potential adsorbent for the removal of cationic pollutants, such as MG, from contaminated water systems. In addition, the RL value (0 <RL <1) confirms the good adsorption of MG dye on meso-CaAl2O447. In addition, the adsorption energy (E) value obtained from the DR isotherm model (below 8 kJ mol-1 at all temperatures) indicates that physical adsorption can be considered effective in the adsorption process.

Using quasi-first-order, quasi-second-order, intra-particle diffusion and Elovich kinetics models, the kinetics of the adsorption of MG dye on the meso-scale CaAl2O4 under different solution pH conditions were studied. The results are shown in Table 5. It can be seen that the pseudo-second-order kinetic model is very suitable for the entire pH range of the solution and shows the best correlation coefficient (R2) value. This means that the adsorption process of MG dye in a wide range of solution pH (2-10) mainly follows chemical adsorption 70. It should be noted that the optimal (R2) value is obtained when the pH of the solution is 8 (R2 = 0.999), which indicates that the adsorption is more suitable at this pH. In addition, the qe value calculated from the pseudo second-order model (qe, calc) is very close to the experimental qe (qe, exp) value, which emphasizes the applicability of the pseudo second-order model.

Vanter Hofs diagram [ln KC vs 1/T(K)] is used to quantify thermodynamic parameters, such as Gibbs free energy change (∆G°), entropy change (∆S°), enthalpy change (∆H°) ) And explained the thermodynamic behavior of MG dye adsorbed to meso-CaAl2O4. The thermodynamic parameters of MG dye adsorbed to meso-CaAl2O4 at different MG dye concentrations are shown in Table 6. It can be seen that the ΔG° value is in the negative range, which means that the spontaneous MG dye is adsorbed on meso-CaAl2O471. In addition, the ΔG° value decreases with increasing temperature, which means that adsorption seems to be more needed at higher temperatures. The obtained positive values ​​of ΔH° (45.68, 47.15 and 11.11 kJ mol-1) indicate that the MG dye adsorption process is endothermic. At MG concentrations of 25 and 50 mg L-1, the significant interaction between MG dye and meso-CaAl2O4 can also be concluded by the high value of ΔH°73. However, the decrease of ΔH° at higher MG concentration (100 mg L-1) indicates that the interaction between meso-CaAl2O4 and MG dye is reduced, which may be due to the rapid saturation of active sites on the outer surface with high concentration of meso-CaAl2O4 And prevent more dye molecules from entering more unoccupied active sites. The values ​​of ΔS° (181.55, 183.6 and 39.05 J mol-1,) are also positive, which indicates that the randomness at the meso-CaAl2O4/solution interface increases by 74 during the MG dye adsorption process. This degree of randomness decreases with the decrease of molecular mobility at higher MG concentrations (100 mg L-1).

MG dye molecules can be adsorbed on meso-CaAl2O4 through different mechanisms such as electrostatic interaction, pore diffusion mechanism, hydrogen bond and chemical bond. One of the important experimental data used to study the adsorption mechanism is the data obtained from the adsorption efficiency under different pH values ​​of the solution. The pH value of the solution affects the dissociation constant (pKa) of the MG dye and the pHpzc of meso-CaAl2O4. As shown in Figure 6a, meso-CaAl2O4 has a positive or negative surface charge under different solution pH conditions. In addition, dye molecules also have cationic or anionic properties at different solution pH values. Therefore, under different solution pH conditions, meso-CaAl2O and MG dye molecules have different charges and can interact through electrostatic interactions. The change of adsorption percentage under different solution pH conditions proves that electrostatic interaction is one of the influencing factors of adsorption. Due to the high surface area and extremely porous nature of meso-CaAl2O4, the existence of a pore diffusion mechanism is also possible. Due to pore diffusion or capillary condensation, MG molecules can be adsorbed by diffusing into the microscopic CaAl2O475 pores.

In addition, by analyzing the surface functional groups of meso-CaAl2O4 before and after the adsorption of MG dye, the possibility of chemical bonds participating in the adsorption process is explained. As shown in the FT-IR spectrum (Figure 9), after the MG dye is adsorbed on meso-CaAl2O4, several characteristic peaks of MG (indicated by different colors) are found in the meso-CaAl2O4 spectrum, while in the blank meso-CaAl2O4 spectrum. This means that adsorption may also occur through chemical bonds. This fact can also be seen from the results of the adsorption kinetic evaluation, where the pseudo-second-order fitting model describes the MG adsorption process. Further research on the FT-IR results showed that the absorption peak corresponding to the oxygen-containing stretching vibration on the mesoporous CaAl2O4 shifted after the adsorption of MG, which indicated that a hydrogen bond was formed between the mesoporous CaAl2O4 and the MG dye molecule during the adsorption process. , UV-Vis spectroscopy results also confirmed the microscopic CaAl2O4 adsorption of MG (Figure 10a). It can be implied that the absorbance of MG decreases at 617 nm after the adsorption process. These results confirmed that MG was successfully adsorbed on meso-CaAl2O4. The photos before and after the MG solution is adsorbed on the adsorbent are shown in Figure 10a.

(a) Meso-CaAl2O4 before MG dye adsorption, (b) MG dye, (c) FT-IR spectrum of meso-CaAl2O4 after MG dye adsorption, and (d) TGA/DTG curve of MG dye.

(a) UV-Vis spectra of MG dye solution before and after adsorption (sorbent mass = 8 mg, V = 50 mL, T = 25 °C, pH 8.0, contact time = 15 min), (b) R% varies The regenerative adsorbent irradiated with MW power, (c) the sample temperature curve measured by the thermocouple at different MW power, and (d) the R% of MG dye on meso-CaAl2O4 under different cycles (error bars indicate ±standard ) Deviation, n = 3).

The practical application of MG dye adsorption research is carried out under optimal conditions (sorbent mass = 8 mg, contact time = 15 minutes, initial MG dye concentration = 50 mg L-1, solution pH value is 8.0, solution temperature is 25 ° C). The average percentage of MG removed from the actual textile wastewater is shown in Table 7. According to the obtained value, the adsorption efficiency of the adsorbent for MG is 72.12%±1.81, indicating the effectiveness of meso-CaAl2O4 in practical applications. The characteristics of textile wastewater before and after treatment are shown in Table 8.

The regeneration of the adsorbent is a key factor in the effectiveness and economy of its application. For this reason, people have been considering the realization of a faster and easier regeneration method. In this regard, the microwave-assisted regeneration of dye-loaded meso-CaAl2O4 has been studied. The microwave power in the microwave heating method plays a vital role in the temperature generated and helps the regeneration of the adsorbent. The temperature change of micro-CaAl2O4 under different microwave powers (300 W, 600 W and 900 W) is shown in Figure 10c. It can be seen that the increase in temperature is achieved by the increase in microwave power. In addition, the adsorption efficiency of regenerated medium-sized CaAl2O4 at different microwave powers is shown in Figure 10b. The results show that the R% of the MG of the meso-CaAl2O4 treated at higher microwave power is greater. This may be due to the higher temperature generated at higher power causing MG to degrade faster from meso-CaAl2O4.

In order to study and confirm the degradation of MG during microwave heating, TGA/DTG was applied. The results in Figure 9d show that MG begins to degrade around 200°C, and at 800°C, it loses about 92.4% of its mass. However, the adsorbent shows very high heat resistance and only loses 7.95% of its weight at 800°C, which mainly corresponds to the aforementioned moisture (Figure 4d). Therefore, the results confirmed the successful regeneration of meso-CaAl2O4 using the microwave-assisted heating method. Finally, the microwave-assisted heating method was used to study the regeneration of dye-loaded meso-CaAl2O4. To do this, put the evaporating dish containing saturated adsorbent in a microwave oven and treat it at 900 W for 6 minutes. After the heating process, the treated Meso-CaAl2O4 was washed 3 times with ultrapure water to completely remove carbon black and other substances. The material left behind by MG degradation. The washed adsorbent is then placed in an oven at 110°C to dry for further use. The recyclability of meso-CaAl2O4 treated by microwave heating is shown in Figure 10d. The results show that the adsorbent has good reusability. After repeated use for 5 times, the adsorption rate of MG is still >90%, indicating that the adsorbent is highly efficient and economical.

The citric acid-assisted sol-gel auto-combustion process is used to successfully synthesize the meso-CaAl2O4 as an adsorbent, which is then used to effectively remove MG (a cationic dye) from synthetic/actual wastewater. The synthesized adsorbents were characterized using different techniques. The obtained specific surface area, BJH pore diameter and total pore volume of meso-CaAl2O4 are 148.46 m2 g-1, 19 nm and 1.39 cm3 g-1, respectively. Optimal adsorption conditions, solution pH (X1) = 8.0, MG concentration (X2) = 50 mg L−1, meso-CaAl2O4 mass (X3) = 8 mg, contact time (X4) = 15 min and solution temperature (X5) = 25 °C, based on RSM-CCD. The Langmuir isotherm fitting model explains well the equilibrium adsorption of MG on the adsorbent, and the maximum single-layer adsorption capacity is 587.5 mg g-1. The pseudo-second-order kinetic model fits the experimental kinetic data well. In addition, the obtained values ​​of thermodynamic parameters (positive values ​​of ΔH° and ΔS°, negative values ​​of ΔG°) indicate that the adsorption process of MG dye is endothermic and spontaneous, and indicates that the randomness of the interface increases during the MG adsorption process. The change of the solution. The medium-sized CaAl2O4 supported by the dye was successfully regenerated by the microwave-assisted heating method, and the adsorption rate of the MG dye was still >90% after repeated use for 5 times. In short, it can be recommended to use meso-CaAl2O4 to effectively remove harmful dyes in wastewater, especially MG dyes.

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Department of Chemistry, Tehran North Campus, Islamic Azad University, Tehran, Iran

Payam Arabkhani & Seyed Nabiollah Hosseini

Iranian Chemical and Chemical Engineering Research Center (CCERCI), P.O. Box 14335-186, Tehran, Iran

Yasuj Medical Plant Research Center, Yasuj, Iran

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PA: Methodology, survey, manuscript writing, verification, data management, writing review and editing. AA conceptualization, project management, data specification, methodology, software, formal analysis, drafting manuscripts, writing reviews and editing, SNH surveys, methodology, software. HJ methodology, survey, data specification, writing review and editing. All authors discussed the results and contributed to the final manuscript.

The author declares no competing interests.

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Arabkhani, P., Javadian, H., Asfaram, A. etc. A reusable mesoporous adsorbent for effective treatment of hazardous triphenylmethane dye wastewater: RSM-CCD optimization and rapid microwave-assisted regeneration. Scientific Report 11, 22751 (2021). https://doi.org/10.1038/s41598-021-02213-2

DOI: https://doi.org/10.1038/s41598-021-02213-2

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