Removal of methylene blue from aqueous solution using a thiol-functionalized ionic liquid-based periodic mesoporous organosilica: Kinetic and thermodynamic studies

Abstract

Integrated wastewater handling and organic dye management are important challenges that scientists are working to solve today. A thiol-functionalized ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH) has been prepared and characterized using diffuse reflectance infrared Fourier transform, scanning electron microscopy, transmission electron microscopy and thermal gravimetric analysis and applied as a robust adsorbent for removal of methylene blue (MB) as an organic dye from an aqueous solution. The optimized conditions were achieved by using 0.04 g of PMO-IL-SH, 10 mg L⁻¹ of MB at 35 min of contact time and pH 8. Fitting the equilibrium data shows the suitability of the Langmuir model with a second-order equation to control the kinetics of the adsorption process with the adsorption capacity of 384.61 mg g^–1. This study may provide insight for remarkable removal of organic dyes to refine water challenges.

Keywords

Ionic liquid adsorbent methylene blue kinetic isotherm

One of the most important environmental concerns today is the presence of toxic dyes in industrial wastewater. Today in industries a large amount of dye is used for the production of food, leather, paper, cosmetics and textiles. Several noxious dyes and their derivatives are not readily biodegradable and have a carcinogenic, toxic or mutagenic influence on mankind and animals. So, a significant area of applied and basic research is the removal of dye pollution from the wastewater of industries.¹ Methylene blue (MB) C₁₆H₁₈ClN₃S (Figure 1) is one of the cationic dyes that is used as material for dyeing wood, cotton and silk. The eradication and elimination of MB dye from the aqueous solution is necessary because it has several detrimental effects, such as diarrhea, cyanosis, tissue necrosis, vomiting, jaundice, quadriplegia, shock and increased heart rate in human beings.² Some methods have been used for the removal of toxic dyes from the wastewater of industries, for example, flocculation, membrane separation, aerobic or anaerobic treatment, coagulation, chemical oxidation and adsorption.^3–11 Nevertheless, adsorption is one of the methods that is most widely used because of its specific properties, such as it being inexpensive, easy to handle and impressive for removal of toxic dyes from the wastewater of industries. One of very important and highly effective adsorbents is ordered nanoporous material. In particular, the modification of nanoporous adsorbents by organic and inorganic functional groups has been developed to increase selectivity to target dyes, apart from the high adsorption capacity of conventional porous materials.^12,13 The functional groups are chosen in such a way that one side of the molecules should contain charges or polar groups that have the ability to preferentially interact with the dye molecules of interest, whereas the other side should be atoms or molecules that have the ability to connect to nanoporous surfaces. The selection of appropriate functional groups is one of the important factors that is necessary to consider in order to maximize the efficiency of the adsorption system. Because the adsorption of dye molecules on adsorbents is mostly due to electrostatic attraction between target dyes and functional groups, compatibility between charges of dye and of functional groups needs to be considered. Moreover, ionic liquids (ILs) are an important class of organic salts with unique characteristics, such as tunable chemical and physical properties, high capacity to dissolve a wide range of both organic and inorganic compounds and high thermal and chemical stability.^14,15 These properties make ILs very interesting candidates for many practical applications, such as catalysis, adsorption, reaction media and energy storage.^16–19 In particular, solid supported ILs are more attractive to chemists due to their advantages of high stability, easy recoverability and reusability in a typical chemical process. Accordingly, more recently, we developed a novel and efficient strategy for the chemical incorporation and/or immobilization of ILs into/onto ordered mesoporous silica.^20–25 In continuation of these studies and according to the importance of removing of toxic dyes for environmental health, a noble thiopropyl containing ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH) material is prepared (Scheme 1) and characterized, and its efficiency is developed in the removal of MB dye from aqueous solution.

Figure 1.
Chemical structure of methylene blue.

Scheme 1.
Preparation of the thiol-functionalized ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH) material.

Experimental details

Instruments and reagents

The MB concentration evaluation was carried out using a Jusco ultraviolet (UV)-visible spectrophotometer model V-530 (Jasco, Japan) at a wavelength of 665 nm, while a pH/ion meter model-686 thermometer Metrohm was used for the measurement of pH (Metrohm, Switzerland). The diffuse reflectance infrared Fourier transform (DRIFT) spectra were determined using a Brucker-Vector 22 (Brucker Analytik, Karlsrhur, Germany). The morphology of the PMO-IL-SH was taken by scanning electron microscopy (SEM) on a KYKY-EM3200 microscope with a secondary electron detector (China) and with transmission electron microscopy (TEM), model Belsorp mini II (BEL, Japan). Thermal gravimetric analysis (TGA) was performed using a NETZSCH STA 409 PC/PG instrument from room temperature to 650°C. All chemicals, including NaOH, HCl and MB with the highest purity, were purchased from Merck (Darmstadt, Germany). The stock of MB solution was prepared by dissolving appropriate amounts of solid dye in double distilled water and the desired concentrations of test solutions were prepared by diluting the stock solution. All other chemicals were used as received and purchased from Merck or Fluka.

Measurements of dye uptake

The dye concentrations in the aqueous solution were estimated quantitatively using the linear regression equations obtained at different MB concentrations. The efficiency of MB removal was determined for 10 mg L⁻¹ of initial MB concentration with 0.04 g of PMO-IL-SH at 35 min of contact time and pH 8. The experiments were also performed in the initial MB concentration range of 10–60 mg L⁻¹ to obtain adsorption isotherms. The MB removal percentage was calculated using the following equation¹⁹
$% MB removal = ((C_{0} - C_{t}) / C_{0}) \times 100$
(1)
where C₀ is the initial dye concentration and C_t (mg L⁻¹) is the concentration of the dye at time t.

The adsorbed MB amount (q_e (mg g⁻¹)) was calculated by the following mass balance relationship
$q_{e} = (C_{0} - C_{e}) V / W$
(2)
where C₀ and C_e (mg L⁻¹) are the initial and equilibrium dye concentrations in aqueous solution, respectively, V (L) is the volume of the solution and W (g) is the mass of the adsorbent.

Preparation of thiol-functionalized ionic liquid-based periodic mesoporous organosilica material

The PMO-IL was prepared according to our previous reported procedure.²⁰ Typically, pluronic P123 (1.9 g) was added to a solution containing potassium chloride (10 g), deionized water (11.9 g) and HCl (2M, 44.3 g) with stirring at 40°C. After clear a homogeneous solution was obtained, a mixture of tetramethoxysilane (20 mmol) and 1,3-bis(trimethoxysilylpropyl) imidazolium chloride (2.22 mmol) was added to the reaction vessel and stirred at the same temperature for 24 h. The reaction solution was then transmitted into a Teflon-lined autoclave and heated at 100°C for 72 h under static conditions. After that, the resulting material was filtered, washed completely with deionized water and dried at room temperature. The removal of the surfactant was accomplished by a Soxhlet apparatus using a mixture of ethanol and concentrated HCl. The final PMO material was dried at 70°C for 12 h and denoted as PMO-IL. In the following, the PMO-IL was modified by 3-mercaptopropyl-trimethoxyslane via a grafting approach. In a typical procedure, PMO-IL (1 g) was added to a flask containing 50 mL of dry toluene with stirring at room temperature. After complete dispersion of PMO-IL, 0.5 mmol of 3-thiopropyl-trimethoxysilane was added and the obtained mixture was refluxed under an argon atmosphere for 24 h. The reaction was then cooled to room temperature and the resulting precipitate was filtered and washed thoroughly with dry toluene to remove unreacted thiopropyl-trimethoxysilane. The final solid was dried at 70°C for 12 h and delivered a white powder denoted as PMO-IL-SH. The elemental analysis showed that the amount of thiopropyl groups on the solid surface is 0.38 mmol g^–1.

Results and discussion

Characterization of PMO-IL-SH

The DRIFT spectrum of PMO-IL-SH is illustrated in Figure 2(a). In this analysis, typical bands could be assigned as follows: a broad signal appeared at 3427 cm⁻¹ that can be assigned to O-H bonds of the material surface (Si-OH).²⁵ The peaks cleared at 1042, 1130 and 928 cm⁻¹ are attributed to asymmetric and symmetric stretching vibration of Si-O-Si bonds.^25,26 The double peaks of S-H were found at 2355 and 2341 cm⁻¹, which are typically very weak due to hydrogen binding effects.^25–28 The absorption peaks of other organic functional groups are observed at 3083 cm⁻¹ (for unsaturated C-H stretching), 2929 and 2885 cm⁻¹ (aliphatic C-H stretching of propyl groups), 1653 cm⁻¹ (C=N stretching of the imidazolium ring),²⁸ 1550 cm⁻¹ (C=C stretching of the imidazolium ring), 1450 cm⁻¹ (C-H deformation vibrations), 772 cm⁻¹ (for C-Si stretching vibrations) and 451 cm⁻¹ (bending vibration of Si-O-Si).^25,26 These data strongly confirm successful incorporation and stability of the IL and thiol functional groups in the material network. The DRIFT spectrum of PMO-IL-SH after adsorption of MB showed the same bands with changes in some areas (Figure 2(b)). As shown, the intensity of the bands around 3000 cm⁻¹ (corresponding to aromatic C-H vibrations) and 1550 cm⁻¹ (corresponding to C=C vibrations of aromatic rings) is increased in the spectrum of the dye-loaded sample, confirming successful adsorption of MB dye onto/into PMO-IL-SH. SEM and TEM analyses were performed to evaluate the morphology and surface area of the synthesized material (Figures 3(a) and (b)). As shown in the figures, these analyzes showed well the presence of regular particles with high pores available on the surface and size distribution of about 40–70 nm for the PMO-IL-SH. These types of particles and pores make the material an efficient candidate for dye sorption and provide suitable binding sites for MB dye molecules. The SEM image of the PMO-IL-SH after adsorption of MB dye was also investigated (Figure 3(c)). The SEM image of MB-PMO-IL-SH demonstrated a different morphology in comparison with its PMO-IL-SH parent. This observation may be attributed to adsorption of the MB molecules onto/into the PMO-IL-SH material. TGA of the PMO-IL-SH was next carried out from room temperature to 650°C to study the thermal stability of the material (Figure 4). This showed a weight loss of 6.13% below 120°C, which is attributed to the elimination of water and alcoholic solvents (EtOH and MeOH). The second weight loss between 120°C and 220°C corresponded to the removal of the surfactant template maintained from the extraction process. The third and main weight loss (around 25%) from 230°C to 650°C is attributed to removal of thiopropyl and IL functional groups.^23,25 This data successfully confirms the successful incorporation and/or immobilization of organic functional moieties into/onto the solid network and verifies the high thermal stability of the material. Figure 5 shows the powder X-ray diffraction (PXRD) pattern of the PMO-IL-SH in the low angle range. This analysis illustrates a peak with high intensity at 2θ of 0.89 and a peak with low intensity at 2θ of 1.41, which were indexed as d100 and d110 reflections, respectively. This is characteristic of materials with highly ordered two-dimensional hexagonal mesostructures, confirming that the material has a highly ordered nanostructure.^23,24

Figure 2.
Diffuse reflectance infrared Fourier transform spectroscopy of ionic liquid-based periodic mesoporous organosilica material (a) before and (b) after the adsorption of methylene blue.

Figure 3.
(a) Scanning electron microscopy (SEM) image of ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH) before the adsorption of methylene blue (MB). (b) Transmission electron microscopy image of PMO-IL-SH before the adsorption of MB and (c) SEM image of PMO-IL-SH after the adsorption of MB.

Figure 4.
Thermal gravimetric analysis of the ionic liquid-based periodic mesoporous organosilica material.

Figure 5.
Powder X-ray diffraction pattern of the ionic liquid-based periodic mesoporous organosilica material.

Effect of pH

MB produces molecular cations in aqueous solution. The adsorption of MB on the adsorbent surface is primarily influenced by the surface charge on the adsorbent.²⁹ Silanol groups on this surface become increasingly deprotonated as the pH of the adsorption system rises, thereby increasing the number of negatively charged adsorbent sites.³⁰ The effect of the initial pH on the MB removal was studied in the pH range of 2–10 with MB concentration of 10 mg L⁻¹ and adsorbent dose of 0.04 g. The experiments were conducted for 35 min of contact time at room temperature and the respective results are presented in Figure 6(a). As shown, the pH value significantly affects the extent of adsorption of MB over the adsorbent and the removal percentage significantly increased at high pH values. As can be seen, the maximum removal of the MB is obtained at a pH of 8.0. On the other hand, the point of zero charge (pH_PZC) of an adsorbent is a very important characteristic that determines the pH at which the adsorbent surface has net electrical neutrality. To find out pH_pzc, experiments were conducted at different pH ranges from 2 to 10 under other optimal conditions. The pH_PZC values of PMO-IL-SH and SiO₂ were determined to be 7.7 and 7.5, respectively. At the solution pH > pH_PZC, the adsorbent surface negatively charged and favors the uptake of cationic dyes due to increased electrostatic force of attraction. At lower pH (pH < pH_PZC), the surface of PMO-IL-SH may get positively charged as a result of being surrounded by H₃O⁺ ions and, thus, the competitive effects of H₃O⁺ ions as well as the electrostatic repulsion between the dye molecules and the positively charged active adsorption sites on the surface of the PMO-IL-SH leads to a decrease in the uptake of dye molecules.³⁰ Another important factor for this adsorption process is H-bonding interactions between N sites of dye and OH and SH sites of adsorbent (Scheme 2). The effective π–π interactions between the imidazolium ring of the sorbent and aromatic rings of MB is an additional method for this successful adsorption (Scheme 2). It is important to note that the aforementioned interactions can occur on both inner and outer surfaces of the mesoporous silica adsorbent.³¹ Therefore, according to the above, all subsequent studies were carried out at pH 8 as the optimum pH.

Figure 6.
Effects of (a) pH, (b) contact time, (c) adsorbent dosage and (d) initial dye concentration on the removal of methylene blue (MB) by ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH) and SiO₂.

Scheme 2.
Proposed hydrogen bonding and π–π interaction for the adsorption of methylene blue on ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH).

Effect of contact time

Contact time studies are helpful in understanding the amount of dye adsorbed at various time intervals by a fixed amount of the adsorbent. Equilibrium time is one of the most important parameters in the design of economical wastewater treatment systems. The variation in percentage MB removal with contact time for 25 mL of MB solution with initial concentrations of 10 mg L^–1, pH 8.0 and 0.04 g L^–1 of PMO-IL-SH is presented in Figure 6(b). From Figure 6(b) it is clear that the adsorption of MB shows two phases: (i) an initial rapid phase where adsorption capacity sharply increased within the first 30 min due to the rapid surface adsorption (external surface adsorption) and (ii) a slower phase whose contribution to the total dye adsorption was relatively small (internal surface adsorption). Adsorption equilibrium can be achieved within 35 min, after which the adsorption of MB was negligible. It may also be observed from Figure 6(b) that more than 90% of MB adsorption takes place within the contact time of 25 min and increases gradually thereafter. The rapid adsorption at the initial contact time was due to the availability of more active surface of the adsorbents, which leads to fast adsorption of the dye from the solution at pH 8.0. The study showed that 98% of MB is removed during 35 min. Accordingly, an equilibrium time of 35 min was selected for further experiments. In addition, the equilibrium time of 35 min, under other optimal conditions, was tested on the SiO₂ adsorbent. The result showed that after 35 min, dye uptake was only 44%, which is much lower than those of the PMO-IL-SH. The higher efficiency of PMO-IL-SH in comparison with SiO₂ may be attributed to the IL nature of PMO-IL-SH and H-bonding and π–π interactions between dye molecules and sites of the adsorbent, which increases the adsorption capacity of the material.

Effect of adsorbent dosage

Adsorbent mass and number of reactive sites are the important controlling factors that limit the magnitude of mass transfer. The monitoring correlation between the amount of adsorbent and removal percentage is required to predict the actual behavior of a real system and also to attain the maximum adsorption capacity of the adsorbent.^32,33 The influence mass of PMO-IL-SH on MB removal reveals a change in removal percentage from 44% to 98% by enhancing the PMO-IL-SH mass from 0.01 to 0.04 g (Figure 6(c)). These expected results are attributed to increases in the number of available adsorption sites and progress in the uptake of the MB. At a lower amount, due to the probable saturation of reactive centers, a significant decrease was seen in removal percentage.

Effect of initial dye concentration

The experimental results for MB removal by PMO-IL-SH at various concentrations of MB (10–40 mg L⁻¹) reveal that the removal percentage and actual amount of adsorbed MB have reverse correlation. Higher MB concentration leads to a decrease in removal percentage. This result confirms the high dependency of the adsorption efficiency on the initial MB concentration (Figure 6(d)). Lower MB concentration due to an abundance of non-occupied surface area and subsequently higher concentration lead to increase in the rate of MB diffusion to adsorbent. At a high concentration of MB, the available sites of adsorbent become fewer and mass transfer depends on the initial dye concentrations.^34–38 The percentage difference of dye removal by PMO-IL-SH and SiO₂ dose is also shown in Figure 6(d). This figure shows that the removal dye capacity of PMO-IL-SH is much better than that of SiO₂.

Adsorption equilibrium study

The Langmuir, Freundlich and Tempkin models were utilized to study isotherm adsorption. A comparison of all three model parameters is presented in Table 1. The Langmuir isotherm is valid for monolayer adsorption of solute from liquid solution without a change in the plane of the surface.³⁹ Based on the linear form of the Langmuir isotherm model, the values of K_a (the Langmuir adsorption constant (L mg⁻¹)) and Q_m (theoretical maximum adsorption capacity (mg g⁻¹)) were obtained from the intercept and slope of the plot of C_e/q_e versus C_e, respectively (Figure 7(a)). The high correlation coefficient with maximum monolayer capacity shows strong positive evidence of the fitness of equilibrium data of adsorption of MB using the Langmuir model (Table 1). Similar results were obtained for various adsorbent-pollutant systems in the literature.⁴⁰ The Freundlich isotherm model is applicable for non-ideal heterogeneous sorption.⁴⁰ The applicability of the Freundlich adsorption isotherm was assessed by plotting ln (q_e) versus ln (C_e) (Table 1). Here, K_F strongly gives useful information on the bonding energy and/or distribution coefficient and represents the quantity of dye adsorbed onto an adsorbent; 1/n is the heterogeneity factor used to characterize the heterogeneity of the system. In general, n > 1 suggests that adsorbate is favorably adsorbed on the adsorbent. The higher the n value, the stronger the adsorption intensity.⁴¹ The values of 1/n (0.16) give an indication of the favorability of adsorption and high tendency of MB for adsorption onto PMO-IL-SH. The heat of the adsorption and the adsorbent–adsorbate interaction were evaluated by using the Temkin isotherm model. In this model, B is the Temkin constant related to heat of the adsorption (J mol⁻¹), T is the absolute temperature (K), R is the universal gas constant (8.314 J mol⁻¹ K⁻¹) and K_T is the equilibrium binding constant (L mg⁻¹). Values of B₁ and K_T were calculated from the plot of q_e against ln C_e.⁴² The value of the correlation coefficient (0.774) of this model is lower than those of the Langmuir model (Table 1). Therefore, the Temkin isotherm represents a worse fit of experimental data than Langmuir isotherms. Comparing the results obtained for both adsorbents, it is observed that the PMO-IL-SH adsorbent has a higher adsorption capacity than the SiO₂ adsorbent (Table 1).

Table 1.
Isotherm parameters for adsorption of methylene blue on ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH) and SiO₂

Isotherm Parameters Adsorbent

PMO-IL-SH SiO₂

Langmuir Q_m (mg g^–1) 384.61 1.268

K_a (L mg^–1) 0.032 8.511

R ² 0.994 0.990

Freundlich 1/n 0.168 0.481

K_F (mg^1–1/ ⁿ L^1/ ⁿ g⁻¹) 8.90 1.832

R ² 0.814 0.970

Tempkin B ₁ 1.60 2.591

K_T (L mg^–1) 346.19 0.963

R ² 0.774 0.977

Figure 7.
(a) The Langmuir plot for the adsorption of methylene blue (MB) on ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH) (adsorbent dose: 0.04 g/25 mL, pH: 8.0, 35 min agitation time at speed of 400 rpm) and (b) Pseudo-second-order kinetics plot for the adsorption of MB on PMO-IL-SH (dye concentration: 10 mg L^–1, adsorbent dose: 0.04 g/25 mL, pH: 8.0, stirrer speed: 400 rpm).

Kinetics evaluation

To evaluate the evolution of the adsorption process, some kinetic models were applied to test the experimental data. The kinetic models, including pseudo-first-order, pseudo-second-order, Elovich and intra-particle diffusion, were used to explain the adsorption mechanism based on the equations and conditions presented in Table 2.⁴²^–⁴⁷ The pseudo-ﬁrst-order model can be expressed as
$\log (q_{e} - q_{t}) = \log q_{e} - \frac{k_{1}}{2.303} t$
(3)

Table 2.
Kinetic parameters for the adsorption of methylene blue on ionic liquid-based periodic mesoporous organosilica (PMO-IL-SH) and SiO₂

Model Parameters Adsorbent

PMO-IL-SH SiO₂

First-order kinetic k₁ (min^–1) 0.15 0.030

q_e_, _calc (mg g^–1) 10.13 3.159

R ² 0.678 0.727

Second-order kinetic k₂ (g mg^–1 min) 0.017 0.007

q_e_, _calc (mg g^–1) 7.20 4.448

R ² 0.992 0.900

Intra-particle diffusion K_id (mg g^–1 min^–1/2) 0.69 0.442

C (mg g^–1) 1.63 –0.263

R ² 0.882 0.965

Elovich β (g mg^–1) 0.63 1.113

α (mg g^–1 min^–1) 2.28 0.355

R ² 0.940 0.910

where q_e is amount of MB adsorbed onto PMO-IL-SH (mg g^–1) at equilibrium time and q_t is at any time, t (min); k₁ is the equilibrium rate constant (min⁻¹). By plotting ln (q_e – q_t) versus time one can obtain the values of k₁ and q_e from the slope and intercept, respectively. The pseudo-second-order kinetics can be expressed as
$\frac{t}{q_{t}} = \frac{t}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}$
(4)
where k₂ is the rate constant for pseudo-second-order kinetics (g mg^–1 min). For dye concentrations of 10 (mg L^–1) and the optimized pH (8.0), it is obvious that the kinetics data ﬁtted very well with the pseudo-second-order model with the correlation coefficient values R² ≈ 1.0, and shows good agreement with experimental data (Figure 7(b)). In the literature, similar kinetic fittings were reported for various pollutant-adsorbent systems.⁴⁷

The Elovich equation is as follows
$q_{t} = β \ln (α β) + \ln (t)$
(5)
where q_t is the sorption capacity at time t (mg g^–1) and α and β are the initial sorption rate (mg g^–1 min) and desorption constant (g mg^–1), respectively. Thus, the constants can be obtained from the slope and the intercept, plotting q_t against ln (t). In a well-agitated batch adsorption system, there is a possibility of intra-particle diffusion of the adsorbate, which can be the rate-limiting step. Therefore, the possibility of intra-particle diffusion resistance affecting the adsorption process was explored by the following equation
$q_{t} = K_{i d} t^{0. 5} + C$
(6)
where K_id is the intra-particle diffusion constant (mg g⁻¹ min^0.5) and C is a constant related to the thickness of the boundary layer (mg g^–1). The values of K_id were calculated from the slopes of q_t versus t^0.5, while C was obtained from its intercept. The kinetics parameters obtained from this study are listed in Table 2, which indicates the non-applicability of this model and makes possible the final outcome that the rate-limiting step is not the intra-particle diffusion process.

Effect of temperature

Thermodynamic considerations of an adsorption process are necessary to evaluate whether the process is spontaneous or not. The Gibb’s free energy change (ΔG⁰) is an indication of spontaneity of a chemical reaction. For significant adsorption to occur, the Gibb’s free energy change of adsorption (ΔG⁰) must be negative. Both enthalpy (ΔH⁰) and entropy (ΔS⁰) factors must be considered in order to determine the Gibb’s free energy of the process.⁴⁸ Thermodynamic parameters for the adsorption of MB over the prepared adsorbent have been determined by using Equations (7) and (8)
$Δ G^{0} = Δ H^{0} - T Δ S^{0}$
(7)

$Δ G^{0} = - R T ln K_{c}$
(8)
where K_c is called the adsorption affinity and is obtained from the q_e/C_e equation, q_e is the amount of dye adsorbed per unit mass of adsorbent (mg g^–1), C_e is the equilibrium concentration (mg L^–1) and T is temperature in Kelvin. The combination of Equations (7) and (8) gives
$log (q_{e} / C_{e}) = (Δ S^{0} / 2.303 R) + (- Δ H^{0} / 2.303 R T)$
(9)

From the plots of ln K_c against 1/T, the values of ΔH⁰ and ΔS⁰ can be estimated from the slope and intercept. The effect of temperature on the adsorption of MB on PMO-IL-SH adsorbent was investigated at 293–333 K (Table 3). The values of ΔG⁰ were negative, indicating that the adsorption of MB on the PMO-IL-SH is feasible and spontaneous. The value of ΔH⁰ was observed to be positive (22.49 kJ mol^–1) for the adsorption of MB, corresponding to an endothermic process. The positive value of ΔS⁰ suggests that the adsorbed MB molecules remain random over the adsorbent surface.⁴⁹

Table 3.
Thermodynamic parameters for the adsorption of methylene blue dye on ionic liquid-based periodic mesoporous organosilica

T (K) ΔG⁰ (kJ mol^–1) ΔH⁰ (kJ mol^–1) ΔS⁰ (J mol^–1 K)

293.15 –6.89 22.49 102.13

303.15 –8.01

308.15 –9.96

313.15 –10.16

323.15 –10.50

333.15 –10.84

Various adsorbents for MB removal

Many MB removal processes using various adsorbents have been reported in the literature^50–58 and their performance for MB removal was compared in term of adsorption capacity, amount of adsorbent and contact time. It can be seen that MB removal by PMO-IL-SH (Table 4) is superior to that previously reported in the literature in terms of higher adsorption capacity (384.61 mg g⁻¹), shorter required time (35 min) and using a smaller amount of adsorbent (0.04 g).

Table 4.
Comparison of adsorption results of previously reported methylene blue (MB) removal with the proposed adsorbent

Dye Adsorbent Q_m (mg g^–1) Reference

MB Graphene oxide/calcium alginate composite 181.80 ^[ ⁵⁰ ^]

Polyaniline nanotubes base/silica composite 10.31 ^[⁵¹^]

Iron terephthalate (MOF-235) 187.00 ^[ ⁵² ^]

Activated carbon 285.70 ^[ ⁵³ ^]

Yolk–shell tubular Fe₂O₃-magnesium silicate 188 ^[ ⁵⁴ ^]

Yolk shell magnetic Fe₃O₄-hierarchical hollow silica nanomaterials 71.45 ^[ ⁵⁵ ^]

Activated carbon cloth 202.15 ^[ ⁵⁶ ^]

Magnetic carbon nanotube/Fe₃O₄/κ-carrageenan nanocomposite 39.66 ^[ ⁵⁷ ^]

Agar/κ-carrageenan composite 205.90 ^[ ⁵⁸ ^]

SiO₂ 1.268 Current study

PMO-IL-SH 384.61 Current study

PMO-IL-SH: ionic liquid-based periodic mesoporous organosilica.

Conclusion

In summary, a thiol-functionalized PMO-IL-SH was prepared and characterized, and its performance for the removal of MB dye from aqueous media was developed. It was observed that the PMO-IL-SH with good porous structure is an efficient adsorbent for the removal of MB. The SEM image of PMO-IL-SH after the adsorption of MB dye probably confirms successful adsorption of MB on the PMO-IL-SH surface. The use of 0.04 g of adsorbent, 35 min of contact time and 10 mg L⁻¹ of MB at pH 8.0 were determined as optimum conditions to achieve maximum removal of 98% for the adsorption process. Comparative study also showed that the efficiency of PMO-IL-SH was much better than that of SiO₂, attributing to the IL nature of the PMO-IL-SH. The achievement of the adsorption process was attributed to the electrostatic interaction, π–π stacking and hydrogen bonding between the sorbent and dye molecules. The isotherm and kinetic studies of the adsorption process were investigated and different models were evaluated for the equilibrium data, which showed the Langmuir isotherm model and pseudo-second-order kinetic are successfully fitted. The maximum adsorption capacity of the material was 384.61 mg g^–1. Also, thermodynamic data showed that the absorption process is spontaneous and endothermic.

Isotherm	Parameters	Adsorbent
Langmuir	Q_m (mg g^–1)	384.61	1.268
	K_a (L mg^–1)	0.032	8.511
	R ²	0.994	0.990
Freundlich	1/n	0.168	0.481
	K_F (mg^1–1/ ⁿ L^1/ ⁿ g⁻¹)	8.90	1.832
	R ²	0.814	0.970
Tempkin	B ₁	1.60	2.591
	K_T (L mg^–1)	346.19	0.963
	R ²	0.774	0.977

Model	Parameters	Adsorbent
First-order kinetic	k₁ (min^–1)	0.15	0.030
	q_e_, _calc (mg g^–1)	10.13	3.159
	R ²	0.678	0.727
Second-order kinetic	k₂ (g mg^–1 min)	0.017	0.007
	q_e_, _calc (mg g^–1)	7.20	4.448
	R ²	0.992	0.900
Intra-particle diffusion	K_id (mg g^–1 min^–1/2)	0.69	0.442
	C (mg g^–1)	1.63	–0.263
	R ²	0.882	0.965
Elovich	β (g mg^–1)	0.63	1.113
	α (mg g^–1 min^–1)	2.28	0.355
	R ²	0.940	0.910

T (K)	ΔG⁰ (kJ mol^–1)	ΔH⁰ (kJ mol^–1)	ΔS⁰ (J mol^–1 K)
293.15	–6.89	22.49	102.13
303.15	–8.01
308.15	–9.96
313.15	–10.16
323.15	–10.50
333.15	–10.84

Dye	Adsorbent	Q_m (mg g^–1)	Reference
MB	Graphene oxide/calcium alginate composite	181.80	^[ ⁵⁰ ^]
Polyaniline nanotubes base/silica composite	10.31	^[⁵¹^]
Iron terephthalate (MOF-235)	187.00	^[ ⁵² ^]
Activated carbon	285.70	^[ ⁵³ ^]
Yolk–shell tubular Fe₂O₃-magnesium silicate	188	^[ ⁵⁴ ^]
Yolk shell magnetic Fe₃O₄-hierarchical hollow silica nanomaterials	71.45	^[ ⁵⁵ ^]
Activated carbon cloth	202.15	^[ ⁵⁶ ^]
Magnetic carbon nanotube/Fe₃O₄/κ-carrageenan nanocomposite	39.66	^[ ⁵⁷ ^]
Agar/κ-carrageenan composite	205.90	^[ ⁵⁸ ^]
SiO₂	1.268	Current study
PMO-IL-SH	384.61	Current study

Footnotes

Acknowledgement

The authors acknowledge Payame Noor University for supporting this work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

ORCID iD

frood Shojaeipoor

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