Adsorption of Rhodamine 6G Dye onto Al-MCM-41 and MCM-41 Mesoporous Materials

The present study aimed to evaluate the potential of mesoporous silica MCM-41 and Al-MCM-41 (molar ratio Si/Al = 20) for the removal of Rhodamine 6G (Rh6G) from aqueous solution. The adsorbents were characterised by XRD, FTIR, SEM-EDX, BET, and UV-Raman. Obtained results related to XRD analysis indicated that the MCM-41 structure remained intact after the incorporation of Al species. Kinetic adsorption study showed that the higher removal (88.75 %, 26.62 mg g −1 ) of Rh6G dye was achieved by Al-MCM-41 at 60 min. FTIR analysis showed that hydrogen bonding played the dominant role in the Rh6G removal mechanism, while the pH results showed that electrostatic interaction was also a key factor. This study shows that the prepared mesoporous materials are inexpensive and efficient adsorbents for the removal of cationic dyes.


Introduction
The greatest problem facing our world today is environmental pollution. Water and soil contain different kinds of pollutants, i.e., heavy metal toxic ions, 1,2 antibiotics, 3 dyes, pesticides, 4 herbicides, etc. Rhodamine 6G (Rh6G) is a heterocyclic xanthene ring containing cationic dye having high solubility in aqueous medium, 5 it is the typical dye widely exploited as a colourant in cosmetics, textile, printing, and food manufacturing. 6,7 Rh6G dye is carcinogenic, neurotoxic, and causes chronic toxicity in humans and animals. 5 In addition, it may cause various health problems, such as irritation of eyes, skin, and respiratory tract. 6,8 In this sense, developing an effective method for the removal of Rh6G in wastewater is urgently needed.
Reverse osmosis, ion exchange, precipitation, 6,9 electrocoagulation, 10 photodegradation, 11 photocatalyst, 12,13 electrochemical, 14 and adsorption 5,7 have been used for decolouration of Rh6G dye in aqueous systems. However, among these developed processes, it has been proved that the adsorption method is an effective purification technique due to its low cost, ease of operation, design, and environmentally friendly behaviour. 15,16 In addition, adsorption technology has shown its potential in removing different classes of dyes (ionic and molecular dyes) from industrial wastewater, and for single compound and multicomponent pollution. 17 Notably, among the adsorbents reported in previous research, the adsorption efficiency of Rh6G using these adsorbents is still limited because of their properties, nature and lower performance. In this regard, it is significant to develop a new effective and eco-friendly adsorbent to remove the toxic dye Rh6G from wastewater.
The new kind of nanostructured mesoporous materials of the M41S family, discovered by Mobil Corporation in 1992, has attracted intense interest due to well-ordered mesoporous materials with high surface area, controllable pore diameters (2-10 nm), and uniform structure of mesopores-channels. 18 In particular, MCM-41 as mesoporous material has been widely applied for cationic dyes adsorption in environmental wastewater, and is a promising adsorbent for the purification of organic contaminants. [19][20][21][22][23] Improving MCM-41 adsorption capacity and selectivity has attracted significant attention during the last decade. 24 In this context, the introduction of heteroatom or metal oxide into the MCM-41 structure was suggested, which is considered as one of the most effective approaches to enhance its sorption affinity to dye. [24][25][26] On the other hand, the integration of Al into the MCM-41 structure can lead to the formation of Brönsted acid sites. Thus, the adsorption affinity could be enhanced. 27 Similarly, Dhal et al. 24 evaluated the potential of iron oxide impregnation onto MCM-41 for methylene blue (MB) dye removal, and found a higher dye adsorption efficiency of 194 mg g −1 compared to the pure silica MCM-41, 149.5 mg g −1 . Rashwan et al. 28 prepared aluminosilicate cation exchanger Al-MCM-41 by wet-method to remove the MB from aqueous solution, and the results showed maximum monolayer adsorption ability. However, this method has some limitations, such as low surface area (592 m 2 g −1 ) and ion exchange capacity, which has limited its applications. Zanjanchi et al. 29 studied the spectroscopic data of cationic dye loaded Al-MCM-41 (Si/Al = 5) materials at room temperature using aluminium sulphate source. However, the synthesis of this material at room temperature decreases the hydrothermal and structural stability. 30,31 In another research, Al-MCM-41 was synthesized by hydrothermal method, and showed a high affinity toward yellow dye molecules. 32 Compared to the aforementioned approaches, hydrothermal method is the easiest method to prepare MCM-41 and Al-MCM-41 with high stability and better ordered mesoporous structure. Consequently, the application of this material for the removal of other dyes could warrant research in the future.
Thus, the main objective of this study was to examine the potential of mesoporous material as a low-cost adsorbent for the removal of cationic dye in aqueous solution. To the best of our knowledge, the performance of Al-MCM-41 and MCM-41 for Rh6G adsorption from aqueous solution with detailed mechanism has not been investigated previously.
In this study, mesoporous materials such as MCM-41 and Al-MCM-41 were prepared using hydrothermal method. The aluminium was integrated into the structure of MCM-41 by direct synthesis using aluminium isopropoxide with the molar ratio of Si/Al = 20. Due to the highly toxic Rh6G molecules that pollute water, the prepared mesoporous materials were used to remove this toxic dye from aqueous solution by batch adsorption experiments. The uptake capacity of the prepared adsorbents toward Rh6G under different factors, such as solution pH, contact time, Rh6G concentration, and temperature, was investigated. The non-linear modelling of kinetic and isotherm models were exploited to analyse adsorption kinetic data and test the equilibrium data, respectively. In addition, the thermodynamics of the adsorption was further investigated. Finally, the possible mechanism between Rh6G dye and the surface of mesoporous material was discussed.

Synthesis of MCM-41
MCM-41 mesoporous silica was synthesized using a modified method from Kumar et al. 33 A quantity of 2.4 g of CTAB was dissolved in 120 ml of distilled water and stirred continuously to obtain a homogeneous solution. A volume of 8 ml of aqueous ammonia (30 wt. %) was added into the above solution. After 15 min under stirring, a known amount of silica source (TEOS) was added drop by drop into the mixture to prepare a gel with molar ratio of 1 TEOS: 0.15 CTAB: 1.64 NH 4 OH: 126 H 2 O. The designed homogeneous gel was transferred into an autoclave under hydrothermal treatment (100 °C for 72 h), after which the solution was collected by filtration and washed consecutively with distilled water. Finally, the sample was dried and calcined at 550 °C for 6 h in order to remove the trapped surfactant.

Synthesis of Al-MCM-41
Al-MCM-41 material was prepared according to the direct synthesis process in a molar ratio of Si/Al = 20. In a typical synthesis, a quantity of 2.4 g of CTAB was dissolved in 120 ml of distilled water at 25 °C and stirred continuously to obtain a clear homogeneous solution. A volume of 8 ml of aqueous ammonia (30 wt. %) was added into the solution. After 15 min, 10 ml of TEOS and aluminium isopropoxide (Si/Al = 20 molar ratio) were added into the mixture. The designed homogeneous gel was transferred into an autoclave under hydrothermal treatment (100 °C for 72 h). The solid was then filtered and the template was extracted by calcination at 550 °C for 6 h.

Characterisation
The two MCM-41 and Al-MCM-41 calcined materials were analysed using a PANalytical XPERT-PRO X-ray diffractometer with Cu Kα (λ = 0.154060 nm) radiation, operating at 45 kV and 30 mA. The vibrational properties of pure and aluminium-containing MCM-41 were investigated by both FTIR and UV-Raman spectroscopy. FTIR spectra were recorded using BRUKER ALPHA Platinum ATR in the wavenumber range of 4000-400 cm −1 . UV-Raman spectroscopy measurements were carried out at room temperature using HORIBA Scientific LabRAM HR Evolution RAMAN SPECTROMETER system with a 325 nm excitation laser source and 2400 groove/mm grating. The surface morphology and chemical identity of both samples were characterised by scanning electron microscopy (FEI QUANTA 650) equipped with energy-dispersive X-ray spectroscopy (BRUKER X Flash 6/10). The specific surface area and porosity of the solids were studied by N 2 adsorption-desorption measurement at −196 °C (Quantachrome Instruments, Nova 1000e).
The pH at the point of zero charge (pH PZC ) is one of the most important factors in describing the ionisation behaviour of the adsorbent surface. In this study, 0.15 g of the adsorbent was added to 50 ml of 10 −3 M NaCl solution. The initial pH value (pHi) of NaCl solution was adjusted to 2-11 by the addition of 0.1 M H 2 SO 4 or 0.1 M NaOH. The mixture was agitated for 48 h at room temperature to reach equilibrium, and then the final pH (pHf) was measured. The pH PZC was obtained from the plot of pHi and pHf values.

Adsorption experiment
The standard batch process was used for adsorption experiments. The amount of 50 mg of adsorbent was added to 50 ml of Rh6G solution (30 mg l −1 ) in a dark flask of 100 ml at pH 6.42, the resultant mixture was shaken with an orbital shaker (150 rpm) at room temperature for 5 h. The Rh6G solution was then centrifuged to separate Rh6G from the solid by FRONTIER TM 5706 centrifugation model. The supernatant was analysed by UV/Vis spectrophotometer at the maximum absorbance wavelength of 527 nm. The adsorption capacity (q e , mg g −1 ) and the removal efficiency (R, %) of Rh6G adsorbed on adsorbents at equilibrium were calculated using Eqs. (1) and (2), respectively.
where C 0 and C e are the initial and equilibrium Rh6G concentrations (mg l −1 ), respectively. V (l) is the initial volume of solution, and W (g) is mass of adsorbent.
The effect of variation of different adsorption parameters was investigated as a function of solution pH in the range 2-11, contact time (0-120 min), adsorbent dose (5-100 mg), and initial Rh6G concentration (2-80 mg l −1 ). For thermodynamic studies, 25 mg of the adsorbent was added to 50 ml of Rh6G solution. The mixture was agitated in a thermostatic shaker bath at varying temperatures of 20, 30, 45, and 75 °C for 60 min.

Adsorption isotherm studies
Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models were used to describe the adsorption equilibrium of Rh6G onto both adsorbents. The nonlinear equations of these isotherm models 34 are expressed by Eqs. (3)- (6).
where C e (mg l −1 ) and q e (mg g −1 ) are the Rh6G concentration at equilibrium time and the adsorbent adsorption capacity, respectively. q m (mg g −1 ) is the maximum adsorption capacity, and K L (l mg −1 ) the Langmuir constant. 1/n, K F , and K T are adsorption intensity coefficient, Freundlich constant, and Temkin constant, respectively. RT/b t = B is related to the heat of adsorption. q D-R is the maximum adsorption capacity. E (kJ mol −1 ) is the mean sorption energy, β is the activity coefficient, and ε is the Polanyi potential (kJ mol −1 ), which are expressed by Eqs. (7) and (8), respectively.
where R (8.314 J mol −1 K −1 ) is the gas constant, and T (K) is the absolute temperature.

Kinetic studies
The pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich models were employed to evaluate the adsorption kinetics of Rh6G onto MCM-41 and Al-MCM-41. Nonlinear equations of these kinetic models 34 are expressed by Eqs. (9)-(12): where q e and q t are the adsorption capacities (mg g −1 ) at equilibrium and at time t, respectively. K 1 (min −1 ) is the first-order kinetic rate constant, and K 2 (g mg −1 min −1 ) is the pseudo-second-order kinetic rate constant. K 2 q e 2 = h is the initial adsorption rate (mg g −1 min −1 ). K id (mg g −1 min −1/2 ) and C (mg g −1 ) are the rate constant of intraparticle diffusion and the constant varied directly with the boundary layer thickness, respectively. α (mg g −1 min −1 ) is the initial adsorption rate, and β (g mg −1 ) is the extent of surface coverage.
To study the surface morphology and composition of both mesoporous materials, scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra prevailed, and the corresponding images SEM analysis are presented in Fig. 2 (A-B). In both synthesized samples, the particles have a spherical morphology with particle size in the range of 0.4-0.7 µm. Furthermore, the morphology of the mesoporous material Al-MCM-41 was maintained during the synthesis process. The EDX spectroscopy study of Al-MCM-41, shown in Fig. 2D, suggested the presence of sharp peaks corresponding to Al, Si, C, and O atoms, indicating that the Al atom had incorporated into the surface of MCM-41 framework. Moreover, the peak of C in the spectrum was obtained from the carbon used during the preparation as a support of the sample. Therefore, the incorporation of aluminium into the mesoporous systems was confirmed by the EDX analysis (Fig. 2D).  In order to evaluate the textural properties, pore-size distribution, and surface area of both samples, N 2 adsorption-desorption analysis was performed. The N 2 adsorption-desorption isotherms and BJH pore-size distributions of two Al-MCM-41 and pure MCM-41 samples are given in Fig. 4. The BET surface areas, pore sizes, and pore volumes are also presented in Table 2.
The isotherms of both samples observed are typical for type IV adsorption/desorption pattern, corresponding to ordered mesoporous material. 43 BET surface area of pure silica MCM-41 was 998 m 2 g −1 with an average pore diameter of 3.4 nm. Compared to pure silica MCM-41, after aluminium incorporation, the BET surface area and average pore diameter decreased to 969 m 2 g −1 and 3.0 nm, respectively. Nevertheless, the pore volume of pure MCM-41 (0.38 cm 3 g −1 ) was slightly higher than that of Al-MCM-41 (0.35 cm 3 g −1 ).

Effect of pH
It is well known that the properties of the active sites of adsorbents and dye species during the adsorption reaction are affected by pH solution. 26 The effect of initial pH on equilibrium adsorption capacity of Rh6G using the mesoporous material was examined at varied pH from 2 to 11, and the results are given in Fig. 5A. At pH 2, the removal efficiency of Rh6G was very low, while the removal gradually increased with pH. Thus, lower adsorption of Rh6G at acidic pH is probably due to the presence of H + ions competing with the positively charged dye molecules for adsorption sites. The increase in Rh6G adsorption capacity with increasing pH (Fig. 5A), suggested the presence of more Si−O − due to the deprotonation of silanol groups, which caused a strong electrostatic interaction between the Rh6G dye ion and adsorbent. 44 On the other hand, in alkaline solution at pH 11, the adsorption capacity of the cationic dye adsorbed was decreased. This fact can be explained by the interaction of dye with hydroxyl groups appearing to be the prevalent element. A study evaluated by Mathew et al. 45 showed that all dyes, including metanil yellow (MY), methylene blue (MB) and Rh6G, possess a maximum adsorption capacity on succinamic acid-functionalised MCM-41 (SA-MCM-41) observed at pH = 7, where the effect of pH was not important. As displayed in Fig. 5A, the maximum adsorption capacity for Rh6G was observed at pH 6.42 and 9 for Al-MCM-41 and pure silica MCM-41, respectively, but a slight difference was observed at pH 6.42 and 9 for MCM-41. From these results, other adsorption experiments were carried out with a neutral pH of the dye solution.
Another important factor for interpretation of the adsorption process of Rh6G is the point of zero charge (pH PZC ) of the adsorbent surface. The PZC is pH value at which the net total charge on MCM-41 surface is zero. When pH was lower than the pH PZC (pH < pH PZC ), the global surface charge on MCM-41 was positive, which reduced the removal efficiency of cationic dyes Rh6G. Conversely, at pH > pH PZC , the surface was negatively charged, which was further favourable for the removal efficiency of cationic dyes. In this study, the pH PZC values of Al-MCM-41 and pure silica MCM-41 were estimated to be 4.48 and 6.38, respectively (Fig. 5B). Consequently, these results clearly indicated that, when the pH was greater than pH P-ZC (pH > 4.48), deprotonation of the Al-MCM-41 surface (Si−O − ) occurred, resulting in a strong electrostatic attraction between the positively charged Rh6G with the negative surface of Al-MCM-41.

Effect of adsorbent dose
To study the effect of adsorbent dose of mesoporous silica MCM-41 and Al-MCM-41 on Rh6G adsorption, 8 different dosages between 5 to 100 mg were investigated. According to Fig. 6B, the adsorption efficiency increased linearly with the increase in the amount of adsorbent until it reached maximum adsorption efficiency at 25 mg. In our study, the amount of 25 mg was selected as the optimum adsorbent dose.

Effect of contact time and adsorption kinetic
Contact time is one of the very important factors in the adsorption process. The influence of contact time on Rh6G adsorption of both MCM-41 and Al-MCM-41 adsorbents was studied for different times (0-120 min) at room temperature. The effects of contact time on Rh6G adsorption capacity are given in Fig. 6A. It is clear that the rate of adsorption was very quick, and afterward decreased gradually with time until adsorption reached equilibrium. This result was assigned to the progressive saturation of the adsorption sites of mesoporous material. According to as the optimum contact time to assure that equilibrium is achieved.
Adsorption kinetics is one of the more important features in defining the efficiency of pollutants adsorption process.
To understand most of the mechanism of the adsorption process of Rh6G onto both mesoporous materials, four kinetic models were studied: pseudo-first-order, pseudo-second-order, intraparticle diffusion, and the Elovich models. The calculated parameters of these models by nonlinear regression analysis method, as well as correlation coefficients (R 2 ), are presented in Table 3, and the plots in Fig. 7.
Based on the correlation coefficients (R 2 ) of the kinetic models, the R 2 value of the pseudo-second-order kinetic model was much higher compared to the other models (Table 3), which was 0.999 close to unit for both adsorbents. Besides, the value of calculated adsorption capacity (q e , cal ) and experimental adsorption capacity (q e,exp ) were very close compared to q e,cal for pseudo-first-order kinetic model. Therefore, the pseudo-second-order model was the best-fitted kinetic model with satisfactory correlation coefficients. Similar results were reported for the removal of Rh6G on different adsorbents. 46,47 removal efficiency ⁄ % removal efficiency ⁄ %  Based on the intraparticle diffusion model, if the plot of q t versus t 1/2 is linear and passes through the origin, the intraparticle diffusion is the only rate-limiting step. On the contrary, the film diffusion might be involved in the adsorption system. 48,49 From Fig. 7C, intraparticle diffusion plot did not pass through the origin (C ≠ 0), indicating that the intraparticle diffusion was not the only step controlling the rate of Rh6G dye adsorption. According to this plot, two clear stages were observed. The first step is the rapid external mass transfer, while the second step is a gradual adsorption process.

Adsorption isotherm
In order to interpret the experimental data, four models of equilibrium data, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models were applied. The calculated parameters of these models by non-linear regression analysis method are given in Table 4, and the non-linear plots are presented in Fig. 8. The maximum adsorption capacities of pure MCM-41 and Al-MCM-41 calculated from the Langmuir isotherm model were found to be 96.01 and 187.39 mg g −1 , respectively.   In order to evaluate the feasibility of the adsorption process, the constant separation factor (R L ) was calculated from the following equation: 37 The R L values signify isotherm types: unfavourable (R L > 1), irreversible (R L = 0), favourable (0 < R L < 1), and linear (R L = 1). The calculated R L values by this isotherm were found to be less than 1 and greater than 0, revealing the favourable adsorption nature of dye Rh6G onto both mesoporous materials. On the other hand, based on the concept of Freundlich, the values of n for the Rh6G uptake were found to be 1.17 and 1.19 for Al-MCM-41 and MCM-41, respectively. These values are at the limit of 1 < n < 10, which indicates the better adsorption of the cationic dye Rh6G. 50 The value of E is one of the most imperative parameters to evaluate the adsorption type. Furthermore, the physical or chemical type of adsorption process can be predicted depending on the energy value E, which was obtained from the Dubinin-Radushkevich isotherm model. The value of E should be lower than 8.0 kJ mol −1 , the adsorption process is physical in nature, but when the value of energy (E) is be-

Adsorption mechanism
The possible interactions between Rh6G molecules and mesoporous material are presented in Fig. 9. According to the FTIR results, some significant changes on the adsorbent surface after Rh6G adsorption could be related to the interaction of Rh6G molecules with the mesoporous material surface through hydrogen bonding. Consequently, the amine groups (−NHC 2 H 5 ) or oxygen atom (−C−O−C−) of Rh6G dye molecules could interact with the surface hydroxyl groups (−OH) of mesoporous material through hydrogen bonding, thus playing the dominant interaction mechanism. Hence, the mechanism of Rh6G dye from water onto both Al-MCM-41 and MCM-41 were found to result mainly from the electrostatic interaction and hydrogen bonding. Furthermore, the existence of n-π interaction between Al-MCM-41 and Rh6G dye can probably be involved. The oxygen groups of Al-MCM-41 could interact with the aromatic rings of Rh6G dye through n-π interaction. 40

Thermodynamic study
To study the effect of temperature, Rh6G adsorption studies were performed at different temperatures: 20, 30, 45, and 75 °C, during 60 min (Fig. 6C). The thermodynamic parameters, namely, the distribution coefficient (K d ), change of standard Gibbs free energy (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°), were estimated by Eqs. (14) where T (K) is the absolute temperature, R is the gas constant, ΔS° and ΔH° values are calculated from Van't Hoff's equation (Eq. (16)) as the slope and intercept of ln K d vs.
1/T plot. The calculated values of these parameters are given in Table 5.
ΔG° values (−1.37 to −3.87 kJ mol −1 ) confirm the feasibility and spontaneous nature of Rh6G adsorption on the Al-MCM-41 adsorbent. Furthermore, the decrease of ΔG° negative value with increased temperature indicated that the adsorption of Rh6G was more favourable at lower temperature. In contrast, the positive value of ΔG° suggested the nonspontaneous nature of R6G adsorption onto pure silica MCM-41 adsorbent at 45 and 75 °C, which confirmed that the adsorption was spontaneous at lower temperature.
The negative values of ΔH° shown in Table 5 suggest that the interaction of Rh6G adsorbed by the synthesized adsorbent was exothermic, confirming that the adsorption of Rh6G increased as the temperature decreased. Similar results have been reported for the removal of Rh6G by other adsorbents. 47,52 Consequently, the obtained absolute value of ΔH° was in the range of 17-20 kJ mol −1 , indicating the adsorption process to be physical in nature. 16,53 This result was in good agreement with those obtained by the value of E calculated from the R-D model (Table 4). Besides, the negative ΔS° value indicated the increase in the molecular organisation and the decrease in randomness at the solid-solution interface during the adsorption process. 47,54

Comparison with other adsorbents
The maximum adsorption capacities for Rh6G uptake obtained from the Langmuir model by the materials of the current study and different adsorbents of the brief review are summarised in Table 6. It can be observed from this comparison that MCM-41 and Al-MCM-41 exhibited higher adsorption capacity for cationic Rh6G dye compared with other reported adsorbents. Obviously, Al-MCM-41 showed an excellent adsorption capacity (187.39 mg g −1 ) for cationic dye Rh6G at room temperature. Therefore, it exhibits good removal ability for Rh6G.

Conclusions
In this study, the prepared adsorbents were characterised by chemical and physical methods, and applied for removal of Rh6G from aqueous solution. BET surface area and average BJH pore diameter of pure silica MCM-41 decreased with the incorporation of aluminium species onto the surface of MCM-41. XRD analysis showed that the structure of Al-MCM-41 had been preserved during the incorporation process.
The kinetics of the adsorption process and isotherm data were best described by the pseudo-second-order kinetic (R 2 = 0.99) and Langmuir models. Al-MCM-41 showed a higher adsorption capacity of 187.39 mg g −1 than MCM-41 (96.01 mg g −1 ) and other used adsorbents. Therefore, Al-MCM-41 material proved to be a potent and qualified adsorbent for the separation of Rh6G dye from wastewater. The study of intraparticle diffusion indicated that the adsorption phenomenon was not controlled by the intraparticle diffusion.
The spontaneous and exothermic nature of adsorption was demonstrated by the thermodynamic parameters. Moreover, the physical nature of Rh6G uptake was confirmed by the value of standard enthalpy change (ΔH°) and the mean sorption energy E calculated from the Dubinin-Radushkevich model. The negative values of ΔS° (−45.49 to −60.97 J mol −1 k −1 ) suggested that the randomness at the adsorbent/solution interface decreased during the adsorption of Rh6G. Based on the FTIR analysis, the Rh6G molecules were successfully adsorbed onto the surface of both mesoporous materials, confirming that hydrogen bonding was the dominant interaction mechanism, while the pH results showed that the electrostatic interaction was also a key factor. Consequently, the two mesoporous materials have enormous potential for the adsorption of cationic dyes.