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Abstract

In the present work, coal fly ash-derived mesoporous silica material (CFA-MS) has been successfully fabricated without employing any extra silica source. The obtained CFA-MS was characterized by Fourier transform infrared spectroscopy, nitrogen adsorption–desorption measurement, powder X-ray diffraction and transmission electron microscopy. Nitrogen adsorption–desorption measurement disclosed that CFA-MS possesses Brunauer–Emmett–Teller-specific surface area of 497 m²·g⁻¹ and pore volume of 0.49 cm³·g⁻¹, respectively. Furthermore, CFA-MS was evaluated for the adsorptive removal of methylene blue from aqueous solution. Several influence parameters on the removal of methylene blue including contact time, pH, initial concentration and temperature were studied in detail. Moreover, Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models were employed for interpretation of the adsorption process, while the pseudo-first-order and pseudo-second-order kinetics equations were applied to investigate the adsorption kinetics. Results in the current work demonstrate that CFA-MS can be used as an efficient adsorbent for methylene blue removal.

Keywords

Coal fly ash mesoporous silica dye adsorption isotherm kinetics

Introduction

Nowadays, water pollution caused by dye contaminants has become a severe global issue and aroused tremendous attention, since it is estimated that more than 100 thousand types of commercial organic dyes and pigments have been produced. Meanwhile, huge amounts of dyes have been discharged into natural environment during the production process, which thus causes serious environmental problems (Yagub et al., 2014). Furthermore, the color of dye-containing water is highly visible even at an extremely low concentration and is thought to be aesthetically unpleasant. Therefore, it is quite urgent to remove the dyestuffs from water, and several methods have been devoted to resolving this issue, including photodegradation, adsorption, biodegradation, coagulation–flocculation and so forth (Cai et al., 2017). Among the various strategies for water purification, physical adsorption has attracted great attention due to its simple treatment procedure and sound effectiveness as well as low cost. Conventionally, activated carbons and zeolites are among the major adsorbents applied for the removal of dyes from aqueous solution. However, their adsorption capacities for dyes are limited due to the microporous nature of activated carbons and zeolites (Joo et al., 2009). Recently, mesoporous silica materials have been widely used for the treatment of contamination in water, such as heavy metal ions, dyes, phenolic compounds, pesticides, etc., which is due to their large specific surface area, high pore volume, excellent stability, low toxicity and transparency (Gibson, 2014; Walcarius and Mercier, 2010). For the preparation of mesoporous silica materials, some commercial silica precursors including sodium metasilicate and tetraethyl orthosilicate have been commonly used as silica sources (Gibson, 2014). However, with the increase in consumption of mesoporous silica materials for large-scale industrial applications, the cost of mesoporous silica materials utilizing commercial silica precursors is relatively high (Chandrasekar and Ahn, 2008; Sari Yilmaz and Karamahmut Mermer, 2016). Therefore, it is highly desirable to realize the production of this type of materials through employing cheaper silica sources.

In view of the silicon content in abundance in many industrial solid wastes, such as fly ash and bottom ash, these raw materials have been utilized for the production of mesoporous silicas (Lee et al., 2017). The strategy possesses the advantage of low cost and environmental friendliness (Misran et al., 2007). During the past decades, these industrial solid waste-derived porous silicate materials have been investigated for the applications in the fields of carbon dioxide capture (Panek et al., 2017; Park et al., 2012), catalysis (Dhokte et al., 2011; Zhang et al., 2013), removal of phosphate (Li et al., 2013) and heavy metal ions (Sun et al., 2014). For example, Prof. Ahn and coworkers (Park et al., 2012) reported the synthesis of mesoporous silicas from bottom ash, and the obtained mesoporous materials were further modified with polyethyleneimine for carbon dioxide capture with high efficiency. Arbad and colleagues (Dhokte et al., 2011) synthesized MCM-41 from fly ash and the obtained material was used as catalyst for the synthesis of β-amino carbonyl compounds through Mannich reaction.

However, only very limited works reported the fly ash-derived mesoporous silica materials for the removal of dye from aqueous solution up to date (Zhou et al., 2015). Very recently, Zhou and coworkers synthesized spherical Al-containing MCM-41 materials from coal fly ash (CFA), and further evaluated their adsorption performance for methylene blue (MB) (Zhou et al., 2015). In spite of the pioneering work, for the practical application in industry, the adsorption capacity of fly ash-derived mesoporous silica materials needs to be further enhanced and the adsorption process should be completed in a shorter time. In the present work, a coal fly ash-derived mesoporous silica material (CFA-MS) has been successfully fabricated by employing a triblock copolymer P123 as structure directing agent under acidic condition, and no commercially available silica source was used during the preparation procedure. CFA-MS was fully characterized by infrared spectroscopy, nitrogen physisorption, powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM). Several influence parameters such as contact time, pH, initial concentration and temperature were studied on the adsorption of MB in detail. Besides, the adsorption process was evaluated using pseudo-first- and pseudo-second-order kinetic models as well as adsorption isotherms including Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models. Results reported in this article demonstrate that CFA-MS can serve as an efficient adsorbent for MB removal with excellent adsorption capacity (323.62 mg·g⁻¹) and relatively rapid adsorption process (90 min).

Experimental section

Materials

The CFA used in this study was obtained from an electric power plant in Shandong, China. Concentrated hydrochloric acid (HCl, 36–38%) was purchased from the Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH) was bought from Beijing Tong Guang Fine Chemicals Company. Pluronic P123 (EO₂₀PO₇₀EO₂₀; M_av = 5800) and MB were from Aldrich and Macklin, respectively. All chemicals were directly used and without further purification.

Extraction of silicon from CFA

The pre-treatment process for CFA was performed for the removal of impurities such as iron and calcium ions. Specifically, the as-received CFA was dried in an oven at 100°C until no weight loss. Then, the dried CFA was mixed with 20% HCl and the mixture was stirred at 80°C for 4 h. The mixture was filtrated and the solid was washed sufficiently with deionized water several times until the filtrate became neutral. Afterwards, the obtained powder was dried in an oven at 100°C.

The alkali fusion method was adopted for the extraction of silica from CFA, which was based on the procedure reported by Kumar et al. (2001) with some slight modification. The CFA powder after pre-treatment was mixed with NaOH at a mass ratio of 1:1.2 and then the mixture was heated in an oven at 550°C for 1 h. The fine milled alkali-fused CFA was added to deionized water with a ratio of 1:4 and then stirred at room temperature for 24 h. Then the sodium silicate-containing supernatant was obtained by centrifugation and filtration of the mixture. The silicon, aluminium and sodium ion concentrations of the supernatant were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) method to be approximately 5097.5, 816 and 45,275 ppm, respectively.

Synthesis of mesoporous adsorbent CFA-MS

In a typical procedure for the synthesis of mesoporous adsorbent, 0.720 g Pluronic P123 was dissolved in 30 g HCl (2 M) at 40°C, and then 30 mL supernatant extracted from CFA was dropped into the solution under stirring at room temperature. Thereafter, 5 g of concentrated HCl and 10 mL of deionized water were added rapidly, and the obtained mixture was continued to be stirred at room temperature for 24 h. Afterwards, the mixture was aged at an oven at 100°C for 72 h. The as-synthesized product was obtained by centrifugation and then washed by deionized water. After being dried at 100°C overnight, the as-synthesized product was calcinated at 550°C for 24 h. After cooling to room temperature, a white powder was obtained and denoted as CFA-MS.

Adsorption of MB

The adsorption behavior of MB onto CFA-MS was investigated by varying several parameters including contact time, pH, initial concentration and temperature. Typically, 10 mL of 200 mg·L⁻¹ MB solution was added into a vial containing CFA-MS fine powder (5 mg), and then the vial with mixture solution was shaken in a shaker at 298 K for 90 min (150 r/min). Afterwards, CFA-MS was separated from MB solution by centrifugation. Then, concentration of the supernatant containing remained MB was analyzed by UV-vis spectrophotometer at 664 nm. To obtain the standard curve between absorbance and MB concentration, MB solution with different concentrations (2, 4, 6, 8 and 10 mg·L⁻¹) was prepared, and then its absorbance was, respectively, measured by UV-vis spectrophotometer at 664 nm (λ_max of MB). The standard curve was drawn according to the Lambert-Beer Law, and the linear regression equation obtained by linear fitting is y = 0.06687x–0.00386, where the correlation coefficient R² = 0.9993.

The adsorption capacity (q_e) and removal efficiency (E) of MB onto CFA-MS were calculated by the following equations $q_{e} = \frac{(c_{0} - c_{e}) V}{m}$ (1) $E = \frac{c_{0} - c_{e}}{c_{0}} \times 100 %$ (2)where c₀, c_e and q_e are initial concentration (mg·L⁻¹), concentration at equilibrium (mg·L⁻¹) and adsorption capacity at equilibrium (mg·g⁻¹) for MB, respectively, m represents the mass of CFA-MS (g) and V stands for the volume of MB solution (L).

Measurements

The chemical composition of CFA was determined by X-ray fluorescence (XRF) method (PANalytical B.V.), while the Si, Al and Na concentrations of CFA supernatant were detected by ICP-OES method on a PerkinElmer Optima 8300 Spectrometer. The PXRD patterns for CFA and mesoporous adsorbent were recorded on an X’ Pert Pro X-ray diffractometer using Cu-K_α radiation with a wavelength of 1.54184 Å. Nitrogen adsorption–desorption measurement was carried out on a Micromeritics TriStar II 3020 surface area and porosity analyzer (Micromeritics Instrument Corporation, USA) at 77 K. Before the measurement, the synthesized adsorbent was degassed under vacuum at 120°C for 12 h. The specific surface area was calculated through Brunauer–Emmett–Teller (BET) method, whereas the pore size distribution was determined using the Barrett–Joyner–Halenda (BJH) method. Fourier transform infrared (FT-IR) spectrum was collected on a PerkinElmer Spectrum Spectroscopy using KBr as standard reference in the range of 400–4000 cm⁻¹. TEM measurement was performed using a JEM 1200EX microscope (JEOL, Japan) operated at an accelerating voltage of 120 kV. The concentrations of the MB solution were analyzed using a UV-5100 spectrophotometer (Shanghai Metash Instrument Co., Ltd, China).

Results and discussion

Characterization of CFA and CFA-MS

The chemical composition of as-received CFA was disclosed by XRF, which establishes that the major components in oxide form are SiO₂ (50.76%), Al₂O₃ (36.04%), CaO (3.64%) and Fe₂O₃ (3.31%). Table 1 summarizes the chemical composition in oxide form. As shown in Figure 1(a), the room temperature PXRD pattern of as-received CFA reveals that the major crystalline phases are mullite, quartz and aluminosilicate glass, which is in accordance with the results from XRF measurement (Kumar et al., 2001; Misran et al., 2007; Panek et al., 2017). After the alkali fusion treatment, the CFA predominantly consists of sodium silicate phase, suggesting that the crystalline silica had been successfully transformed into soluble sodium silicate phase (Figure 1(b)) (Kumar et al., 2001; Misran et al., 2007). Therefore, the alkali fusion method can effectively extract available silica species in CFA and convert them into soluble sodium silicate, thereby facilitating the fabrication of mesoporous silica material.

Table 1.

Chemical composition of as-received CFA used in the present contribution.

No.	Compound	Content (wt%)	CFA (mol/100 g)
1	SiO₂	50.76	0.845
2	Al₂O₃	36.04	0.353
3	CaO	3.64	0.065
4	Fe₂O₃	3.31	0.021
5	TiO₂	1.79	0.022
6	K₂O	1.14	0.012
7	SO₃	1.39	0.017
8	CdO	0.54	0.004
9	Na₂O	0.53	0.009
10	MgO	0.43	0.011

Figure 1.

Powder X-ray diffraction (PXRD) patterns of as-received CFA (a) and alkali-fused CFA (b). M, Q, S and F represent mullite, quartz, sodium silicate and faujasite, respectively.

FT-IR spectroscopy was used to investigate the chemical components of the synthesized CFA-MS. As shown in Figure 2(a), the absorption peaks emerged at 1069, 801 and 456 cm⁻¹ corresponds to the asymmetric stretching, symmetric stretching and in-plane bending vibrations of Si–O–Si bond, respectively (Yuan et al., 2018). In addition, a weak peak located at 1630 cm⁻¹ is due to the physically adsorbed water molecules. Therefore, these absorption bands demonstrate the successful formation of silica framework.

Figure 2.

Fourier transform infrared (FT-IR) spectrum (a) and nitrogen adsorption–desorption isotherms (b) of the obtained CFA-MS.

Nitrogen physisorption measurement was also performed to investigate the porosity of the obtained adsorbent CFA-MS. As displayed in Figure 2(b), CFA-MS possesses a type IV isotherm on the basis of IUPAC classification, indicative of the presence of characteristic mesoporous structure (Thommes et al., 2015). Meanwhile, the isotherm exhibits a large H2-type hysteresis loop in the p/p₀ range of ca. 0.45–1.0, which is typical for materials with ink-bottle pores interconnected with narrow necks by either pore blocking or cavitation (Thommes et al., 2015). Furthermore, the BET-specific surface area and pore volume were calculated to be ca. 497 m²·g⁻¹ and 0.49 cm³·g⁻¹, respectively. These values are quite close to that of the SBA-15 material prepared from CFA (Kumar et al., 2001). In addition, its pore size distribution is relatively broad with pore diameter centered at 3.9 nm, whereas its average pore diameter was measured to be 4.9 nm as evaluated from the nitrogen desorption branch of the isotherm by the BJH method. Moreover, the BET-specific surface area and pore volume of CFA were measured to be ca. 9.82 × 10⁻³ m²·g⁻¹ and 4.84 × 10⁻⁴ cm³·g⁻¹, respectively. In comparison to its derived mesoporous material CFA-MS, these values are quite small. Therefore, the results indicate the nonporous nature of CFA and the successful fabrication of mesoporous material CFA-MS in this work.

However, the low-angle PXRD pattern of the obtained material CFA-MS (not shown here) does not show any obvious diffraction peak at the two-theta angle from 0.5 to 10°, indicative of the absence of uniform mesoporous structure. This is in accordance with the previous reports by Chandrasekar and Ahn (2008) and Kumar et al. (2001), because additional sodium metasilicate is usually introduced to improve the order of the mesostructure, while no additional silica form except the supernatant extracted from CFA was used in the present work (Chandrasekar et al., 2008; Sun et al., 2014). In spite of the absence of ordered mesoporous structure, CFA-MS displays enhanced adsorption properties on cationic dye such as MB, which will be discussed in the following section of this article.

The morphology of CFA-MS was observed by TEM. As depicted in Figure 3, the obtained material CFA-MS exhibits irregular shape and worm-like mesostructure, which is consistent with results from aforementioned nitrogen adsorption–desorption measurement and low-angle PXRD.

Figure 3.

TEM images of the obtained CFA-MS showing its irregular shape (a) and worm-like mesopores (b).

Effects of factors on adsorption

The rapid removal of organic dye from aqueous solution is crucially important to an adsorbent. To investigate the effect of contact time on the adsorption of MB, 5 mg of synthesized adsorbent CFA-MS was added into 10 mL of 200 mg·L⁻¹ MB solution at room temperature. As displayed in Figure 4(a), it was observed that the initial adsorption rate increases quickly with increasing contact time, which can be attributed to the large amount of available adsorption sites. When contact time reaches 90 min, the uptake amount is up to the maximum of 316.8 mg·g⁻¹. After the further increase in contact time, the adsorption capacity remains almost constant.

Figure 4.

Effects of contact time (a), pH (b), initial concentration and temperature (c and d) on the adsorption of methylene blue (MB) from aqueous solution onto CFA-MS.

Furthermore, the effect of pH on the adsorption of MB from aqueous solution over obtained adsorbent CFA-MS was also investigated at 298 K. As shown in Figure 4(b), the adsorption amount increases gradually with varying the pH value from 3 to 10, which can be interpreted by the electrostatic interaction between negatively charged silica and positively charged MB molecule. Therefore, the adsorption of MB onto CFA-MS prefers an alkaline condition and the optimal initial pH value is 10. Results shown here are in accordance with the previous reports (Anbia and Hariri, 2010; Han et al., 2009; Zhou et al., 2015).

The effects of initial concentration and temperature for the removal of MB by CFA-MS were also investigated via varying the initial concentration in the range of 25–200 mg·L⁻¹ and temperature at 298, 308 and 318 K, respectively. As depicted in Figure 4(c) and (d), for all the three different temperatures, the adsorption capacity is enhanced with the increase in initial concentration, while the removal efficiency generally decreases when increasing the initial concentration. With regard to the effect of temperature, it is well known that evaluating the temperature is beneficial to the diffusion of the adsorbate molecules from liquid to solid phases because of the decrease in viscosity of dye solution (Doğan et al., 2004). However, both the adsorption capacity and removal efficiency of MB decrease in general with evaluating the temperature in the present work. This can be interpreted by the fact that the increase in temperature can enhance the mobility of adsorbed MB molecules, and thus gives rise to the serious desorption of MB molecules (Alkan et al., 2000; Gao et al., 2014). This trend indicates that the adsorption of MB onto CFA-MS underwent a physisorption process, where the interactions can be attributed to weaker van der Waals and dipole forces, which are usually associated with low heat of adsorption (Alkan et al., 2000).

Adsorption isotherms

For interpretation of the adsorption of MB onto the adsorbent surface of CFA-MS, four different isotherm models including Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models were investigated at 298 K in detail. The Langmuir isotherm (Langmuir, 1916, 1917, 1918) is the most common and still useful model to describe the adsorption of dye onto the surface of adsorbent, which is on the basis of assumptions that the adsorbed molecules are arranged in a monolayer and the adsorbent surface can be viewed as homogeneous. The linear form of Langmuir isotherm can be written as equation (3) $\frac{c_{e}}{q_{e}} = \frac{1}{k_{L} q_{m}} + \frac{c_{e}}{q_{m}}$ (3)where k_L is the Langmuir constant (L·mg⁻¹) related to energy of adsorption, c_e is the equilibrium concentration of MB solution (mg·L⁻¹), and q_e and q_m represent the adsorption capacities (mg·g⁻¹) at equilibrium and the maximum adsorption capacities (mg·g⁻¹) of MB onto adsorbent, respectively.

Conversely, the Freundlich isotherm model (Freundlich, 1906, 1931) assumes that the adsorbent surface is heterogeneous and the adsorbate prefers to occupy the stronger binding sites. Its linear form can be expressed as equation (4) $\log q_{e} = \frac{1}{n} \log c_{e} + \log k_{F}$ (4)where k_F is the Freundlich adsorption constant ((mg·g⁻¹)/(mg·L⁻¹)^1/ ⁿ ) related to adsorption capacity and 1/n represents the adsorption intensity and surface heterogeneity.

Temkin isotherm model (Kim et al., 2004; Temkin and Pyzhev, 1940) takes into account the interactions between adsorbent and adsorbate and can be linearized as equation (5) $q_{e} = \frac{RT}{b_{T}} \ln c_{e} + \frac{RT}{b_{T}} \ln k_{T}$ (5)where k_T means the Temkin isotherm constant (L·g⁻¹), b_T is a constant related to the heat of adsorption, R is universal gas constant (8.314 J·mol⁻¹·K⁻¹) and T is absolute temperature in Kelvin.

The linear isotherms of MB adsorption onto obtained CFA-MS are plotted in Figure 5, and the relevant parameters are listed in Table 2. Based on the linear regression coefficient R² values, the adsorption equilibrium data can be best presented by Langmuir isotherm model rather than Freundlich or Temkin isotherm models. This suggests that the adsorption of MB occurs at specific homogeneous sites and follows a monolayer adsorption process. In addition, the separation factor R_L defined from the Langmuir model in equation (6) can be employed to evaluate the feasibility of adsorption (Weber and Chakraborti, 1974). $R_{L} = \frac{1}{1 + k_{L} c_{0}}$ (6)

Figure 5.

The Langmuir (a), Freundlich (b), Temkin (c) and Dubinin-Radushkevich (d) adsorption isotherms fitted to the adsorption equilibrium data in linear forms.

Table 2.

Linear isotherm parameters for MB adsorption on the obtained CFA-MS at 298 K.

Isotherms	Parameters
Langmuir	q_m (mg·g^–1)	323.62
	k_L (L·mg^–1)	0.29
	R ²	0.980
Freundlich	k_F ((mg·g^–1)/(mg·L^–1)^1/ ⁿ )	91.22
	1/n	0.35
	R ²	0.798
Temkin	b _T	45.10
	k_T (L·g^–1)	6.08
	R ²	0.908
Dubinin-Radushkevich	q_m^D-R (mg·g^–1)	240.07
	β	2.73 × 10^–7
	R ²	0.848

The calculated R_L value in this study is 0.017, indicative of a favorable adsorption process (Heidari et al., 2009). From the Langmuir model, the theoretical maximum adsorption capacity is calculated to be 323.62 mg·g⁻¹, which is in accordance with the experimentally obtained data. It is worth mentioning that this value is much higher than most of the previously reported adsorption capacity by applying mesoporous silica materials (Yagub et al., 2014) and other types of adsorbents as summarized in Table 3. Furthermore, in light of the previously published literature, this result represents the highest adsorption capacity for MB with comparison to the fly ash-derived porous materials (Zhou et al., 2015). For example, Zhou et al. (2015) reported a fly ash-derived Al-MCM-41 material with maximum adsorption capacity of MB of 277.78 mg·g⁻¹ very recently.

Table 3.

Adsorption capacities q_m (mg·g⁻¹) for methylene blue (MB) by various adsorbents.

No.	Adsorbents	Adsorption capacity (mg·g^–1)	References
1	Activated carbon of coir	15.59	Sharma and Upadhyay (2009)
2	CaHAp-Alg	77.3	Guesmi et al. (2018)
3	Raw KT38 kaolin	52.76	Mouni et al. (2018)
4	MGC-4	65.8	Wang et al. (2014)
5	ZFA/HZ	45.09	Lin et al. (2016)
6	Ti-SBA-15	208.3	Qiang et al. (2019)
7	Al-MCM-41	277.78	Zhou et al. (2015)
8	CFA-MS	323.62	This work

Moreover, the Dubinin-Radushkevich (D-R) model (Dubinin and Radushkevich, 1947) was also applied to calculate the apparent free energy of adsorption, and this model is usually used to distinguish the adsorption types, i.e. physical or chemical adsorption. The D-R isotherm can be computed by equation (7) as follows $\ln q_{e} = \ln q_{m}^{D‐R} - β ε^{2}$ (7) $ε = RT \ln (1 + \frac{1}{c_{e}})$ (8) $E = \frac{1}{\sqrt{2 β}}$ (9)where q_m^D-R represents the D-R adsorption capacity (mg·g⁻¹), ε is Polanyi potential, β is D-R constant and E is the mean free energy of adsorption (Hobson, 1969). As for adsorption of MB onto synthesized adsorbent CFA-MS, the calculated E value is 1.35 kJ·mol⁻¹ (below 8 kJ·mol⁻¹), thereby indicating that the adsorption underwent a physical process (Hobson, 1969).

Adsorption kinetics

Several kinetics models can be employed to evaluate the rate and mechanism of mass transfer of MB from liquid phase to the surface of CFA-MS. In the present study, the pseudo-first-order and pseudo-second-order kinetics models were adopted to understand the adsorption mechanism of MB to the obtained mesoporous adsorbent. The pseudo-first-order model was described by Lagergren (1898) in a linear form as displayed in equation (10), and the linear form of pseudo-second-order model was summarized by Ho and McKay (1999) as written in equation (11) $\ln (q_{e} - q_{t}) = \ln q_{e} - k_{1} t$ (10) $\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}$ (11)where q_e and q_t represent the adsorption capacities (mg·g⁻¹) of MB at equilibrium and at time t, respectively, and k₁ (min⁻¹) and k₂ (g·mg⁻¹·min⁻¹) are the corresponding rate constants. The linear curves plotted based on pseudo-first-order and pseudo-second order-kinetics models are given in Figure 6, and the values of q_e, k₁, k₂ and R² are summarized in Table 4. According to Table 4, it can be seen that the calculated adsorption capacity from pseudo-second-order model (316.5 mg·g⁻¹) is very closer to experimental data (316.8 mg·g⁻¹) than that of the pseudo-first-order model (0.789 mg·g⁻¹). Furthermore, the comparison of two correlation coefficients (R²) also demonstrates that the adsorption process of MB from aqueous solution fits to the pseudo-second-order model better.

Figure 6.

Pseudo-first-order and pseudo-second-order kinetics linear plots for adsorption of MB onto CFA-MS at 298 K.

Table 4.

Coefficients of pseudo-first-order and pseudo-second-order adsorption kinetics models.

	Parameters
Pseudo-first order	q_e,cal (mg·g^–1)	0.789
	k₁ (min^–1)	0.015
	R ²	0.615
Pseudo-second order	q_e,cal (mg·g^–1)	316.5
	k₂ (g·mg^–1·min^–1)	0.114
	R ²	1.000

Adsorption mechanism

Several probable adsorption mechanisms, including electrostatic interaction, hydrogen bonding, ion exchange, coordination, acid-base interaction, have been proposed to understand the interactions between solid adsorbent and dye molecules in aqueous solution. In the present research, as a cationic organic molecule, MB was adsorbed onto the surface of synthesized mesoporous silica mainly through electrostatic interaction. This can be explained by the presence of a large amount of silanol groups Si–OH on the silica surface. The adsorption capacity increases with increasing the pH of the MB solution, suggesting that more deprotonated silanol groups exist in the form of Si–O^–, which possess stronger electrostatic interaction with cationic dye MB (Zhou et al., 2015). In addition, the silanol group on the surface of CFB-MS can also interact with the amine group of MB molecule via hydrogen bonding (Wang et al., 2016). Therefore, the adsorption mechanisms can be interpreted by electrostatic interaction and hydrogen bonding (Figure 7).

Figure 7.

The possible adsorption mechanisms of MB onto CFA-MS.

Conclusions

A CFA-MS has been successfully synthesized without employing any commercially available silica source and then characterized through TEM, FT-IR spectroscopy, PXRD and nitrogen physisorption. CFA-MS was utilized for the removal of MB from aqueous solution. The adsorption capacity increased with contact time and the optimum pH was found to be 10. Furthermore, the adsorption amount decreased with increasing the temperature, indicative of exothermic adsorption behavior. In addition, the adsorption equilibrium process can be best described as Langmuir isotherm model, while adsorption kinetics was found to follow the pseudo-second-order equation rather than pseudo-first-order equation. The experimental maximum uptake amount was 316.8 mg·g⁻¹, which is close to the theoretical maximum dye adsorption capacity of 323.62 mg·g⁻¹. This value is much higher than the previously reported counterparts. The present research provides a promising approach for adsorptive removal of MB by mesoporous silica materials prepared from inexpensive industrial solid waste CFA.

Footnotes

Acknowledgement

The authors are thankful to China University of Mining & Technology at Beijing for supporting our 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) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: The work was supported by the Fundamental Research Funds for the Central Universities (2017QH01) and the National Training Program of Innovation and Entrepreneurship for Undergraduates (C201803841).

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Adsorptive removal of methylene blue from aqueous solution using coal fly ash-derived mesoporous silica material

Abstract

Keywords

Introduction

Experimental section

Materials

Extraction of silicon from CFA

Synthesis of mesoporous adsorbent CFA-MS

Adsorption of MB

Measurements

Results and discussion

Characterization of CFA and CFA-MS

Effects of factors on adsorption

Adsorption isotherms

Adsorption kinetics

Adsorption mechanism

Conclusions

Footnotes

Acknowledgement

Declaration of Conflicting Interests

Funding

References