Fabrication of highly selective molecularly imprinted membranes for the selective adsorption of methyl salicylate from salicylic acid

Abstract

Herein, highly selective molecularly imprinted membranes (MIM) for methyl salicylate (MS) are synthesized with 4-vinylpyridine (4-VP), acrylamide (AM) or methacrylic acid (MAA) as the functional monomer based on the Al₂O₃ microporous ceramic membrane. Fast kinetic equilibrium for the rebinding of MS was found on all the imprinted membranes. Compared with AM-MIM and MAA-MIM, 4-VP-MIM presents a stronger adsorption capacity and higher permeation selectivity for MS due to the formation of ionic bonds between MS and 4-VP. SEM analysis shows that the surface morphology of the membranes is strongly influenced by the concentration of vinyltrimethoxysilane. Compared with 4-VP-MIM (1, 2 or 4), 4-VP-MIM3 possesses higher kinetic equilibrium adsorption, binding capacity and better selectivity for MS. It was found that the pseudo-second-order kinetic model is suited to describe the kinetic of 4-VP-MIM3, as determined by multiple regression analysis. The adsorption isotherm analysis exhibits that 4-VP-MIM3 has the maximum adsorption capacity for MS. Moreover, the selectivity experiment shows that the selectivity coefficients of 4-VP-MIM3 for MS relative to salicylic acid (SA) and phenol are 2.4185 and 2.2277, respectively, which are close to the predicted selectivity coefficient values.

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1. Introduction

Salicylic acid (SA) is a very important intermediate and is widely used in the manufacturing of pharmaceuticals.^1–3 Moreover, it is frequently applied in cosmetics to get rid of horniness, trivial wrinkles and to shrink pores.^4,5 However, methyl salicylate (MS), which is an analogue of SA, is usually produced along with the more desirable product, SA, and is toxic and harmful to public health.^6,7 Consequently, the separation of MS is important for ensuring the quality of SA and it is of great necessity to develop efficient methods for the selective separation of MS from SA.

Molecular imprinting is a technique that allows specific recognition sites for target molecules to be found in synthetic polymers through the use of a template.^8,9 Due to their very high selectivity and robustness, molecularly imprinted polymers (MIPs)^10–13 are being increasingly used as specific recognition materials for analytical separation. However, the drawbacks of MIPs are obvious: long preparation time, mechanical deformation of their binding sites during grinding, time-consuming and high material loss in the sieving procedure.

The surface imprinting technique, which combines molecular imprinting and surface modification, builds the recognition system on support materials, and could be an alternative method to prepare the favorable MIPs.^14–16 Most studies with imprinted polymers have been carried out using the surface imprinting method, and focus on selective binding, transport, and separation.^17,18 Although, the above methods can promote the selective recognition of imprint molecules and speed of the imprint molecules for the imprint sites, they are not recycled because of the poor mechanical strength, fouling resistance ability and thermostability of the membrane matrix. Therefore, it is urgent to develop a new method to fabricate imprinted support materials with high mechanical strength.

Inorganic ceramic membranes can be an ideal support for imprinting because they can be used under severe conditions, owing to their chemical and thermal stabilities, mechanical strength, and filtration performance.^19,20 In the present work, an imprinted membrane for MS is prepared using the Al₂O₃ ceramic membrane as the membrane matrix, vinyltrimethoxysilane (VMS) as the crosslinker, and 4-vinyl pyridine (4-VP), acrylamide (AM), and methacrylic acid (MAA) as monomers. This approach allows the use of simple procedures and mild reaction temperatures. Additionally, the MIMs adsorption properties and capacity towards MS are evaluated and investigated.

In this work, we develop a surface imprinting inorganic ceramic membrane for the target species template, molecular MS. The excellent adsorption capacity of the 4-VP functional monomer is much higher than that of non-imprinting membranes. Their excellent selectivity and membrane flux also make the MIMs a good candidate for MS separation and purification.

2. Experiments

2.1. Materials

Methyl salicylate (MS), salicylic acid (SA), phenol, 4-vinylpyridine (4-VP), acrylamide (AM), methacrylic acid (MAA), vinyltrimethoxysilane (VMS) and ethylene glycol dimethacrylate (EDMA) were all obtained from Aladdin Reagent. Acetonitrile, methanol and ethanol were all supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Al₂O₃ microporous ceramic membranes with a nominal pore size of d_p = 2 μm and thickness of 2 mm, were purchased from Hefei Great Wall Xinyuan Film Technology Co., Ltd. All materials were used as received. Ultra pure water was used for the preparation of all aqueous solutions and preservation processes.

2.2. Cleaning/hydroxylating microporous alumina surfaces

The surfaces of the Al₂O₃ microporous ceramic membranes were first boiled in 30% hydrogen peroxide for 30 min to introduce –OH groups on the surface for further modification. Then they were boiled in deionized water for 15 min to clean the surface and dried under nitrogen.

2.3. Synthesis of vinylated Al₂O₃ microporous ceramic membranes

Before the synthesis of the MIMs, the Al₂O₃ microporous ceramic membranes were rinsed with 95% ethanol for 1 h at 30 °C to remove impurities. Moderate VMS was dissolved in ethanol and the pH was adjusted to 3.5 with glacial acetic acid. Then, the solution was stirred for 2 h at room temperature in order to hydrolyze VMS completely. Then, the rinsed Al₂O₃ microporous ceramic membranes were added to the solution and the mixture was stirred at room temperature for 2 h. After that, the ceramic membranes were dried at 120 °C for 2 h. Then the membrane was rinsed with ethanol for 24 h and with DI water for another 24 h. Finally, the vinylated ceramic membranes were dried to constant weight under vacuum at 40 °C and stored in a desiccator.

2.4. Synthesis of Al₂O₃ ceramic membrane-based imprinted and non-imprinted membranes

The template MS (1 mmol) and the functional monomer (4-VP, AM, MAA) (4 mmol), as summarized in Table 1, were dissolved in acetonitrile (15 mL) in a round-bottom flask (50 mL). After shaking for 6 h, the vinylated microporous Al₂O₃ microporous membranes were immersed into the above solution. Then, the crosslinking agent EDMA (20 mmol) and the initiator AIBN (34 mg) were added. The mixture was fully stirred for 4 h at 2 °C and this was followed by thorough purging with nitrogen for 10 min, then the mixture was then sealed in vacuo. Thereafter, polymerization was started by thermal initiation in a water bath at 60 °C for 24 h. Finally, the imprinted membranes (MIMs) were extracted with the mixed solvents of methanol/acetic acid (9 : 1, v/v) in a Soxhlet apparatus to remove the template. The non-imprinted membranes (NIMs) were fabricated in the presence of the template MS using the same method (Fig. 1).

Table 1 Membranes obtained by the polymerization process with different polymerization solutions

Membrane	Composition of polymerization solutions
	MS (mmol)	4-VP (mmol)	MAA (mmol)	AM (mmol)	VMS (mL)	EDMA (mmol)
MAA-MIM	1	0	4	0	2	20
MAA-NIM	0	0	4	0	2	20
AM-MIM	1	0	0	4	2	20
AM-NIM	0	0	0	4	2	20
4-VP-MIM1	1	4	0	0	2	20
4-VP-NIM1	0	0	0	0	2	20
4-VP-MIM2	1	4	0	0	1	20
4-VP-NIM2	0	0	0	0	1	20
4-VP-MIM3	1	4	0	0	1.5	20
4-VP-NIM3	0	0	0	0	1.5	20
4-VP-MIM4	1	4	0	0	2.5	20
4-VP-NIM4	0	0	0	0	2.5	20

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Fig. 1 Schematic of the preparation of the Al₂O₃ microporous ceramic composite imprinted membrane (4-VP as the functional monomer).

2.5. Characterization

Micrographs of the pure Al₂O₃ microporous ceramic membrane and its composite membranes were observed by scanning electron microscopy (SEM, S-4800). Before they were photographed, all the prepared membranes were cleaned with deionized water and dried in vacuum then fractured. The dried membrane samples were gold sputtered to produce electric conductivity, and were observed with the microscope.

The pure Al₂O₃ membrane and MIMs were characterized by Raman spectra (DXR) with a WITEC Spectra Pro 2300I spectrometer equipped with an Ar-ion laser which provided a laser beam wavelength of 532 nm.

2.6. Membrane flux experiments

The membrane was fitted on an ultrafiltration cell (UF-8010, Amicon) with the effective membrane area of 4.9 cm². An aqueous solution containing 100 mg L⁻¹ MS was prepared as the feed solution to measure the membrane flux. The feed solution permeated through the different membranes under the steady operation pressure 0.15 MPa and the flux of the different membranes could be calculated using eqn (1):where, J is the flux of the membrane (mL cm⁻² min⁻¹), V is the volume of the permeate solution (mL), and t and s are the operation time (min) and effective area of the membrane (cm²), respectively.

2.7. Batch mode binding experiments

Adsorption kinetics experiments were carried out to determine the adsorption equilibrium time and rate-limiting factor of the prepared composite membranes for the template MS. The prepared membranes were placed in 50 mL Erlenmeyer flasks containing 20 mL of MS methanol–water solution (100 mg L⁻¹). These mixtures were shaken on a constant temperature shaker. At different points of time, the concentration of MS in the solutions were obtained and determined via UV spectrophotometry. The binding amounts (q, mg g⁻¹) were determined using eqn (2), and the time of adsorption equilibrium was obtained.where, C₀ and C_t (mg L⁻¹) are the feed concentration at the initial time and the sampling time, respectively, and V and W are the volume of the solution (mL) and weight of the prepared membrane (g), respectively.

Batch binding experiments were performed in 50 mL Erlenmeyer flasks containing the prepared membrane and 20 mL MS methanol–water solution with different concentrations ranging from 25 mg L⁻¹ to 300 mg L⁻¹. These Erlenmeyer flasks were placed in a 25 °C water bath for 3.0 h. After reaching adsorption equilibrium, the residual concentrations of MS in the solution were determined via UV spectrophotometry. Then the equilibrium binding amounts (q_e, mg g⁻¹) were calculated using eqn (3):

where, C₀ and C_t (mg L⁻¹) are the feed concentration at the initial time and the saturated binding time, respectively, and V (mL) and W (g) are the volume of the solution and the weight of the prepared membrane, respectively.

2.8. Selective recognition experiments

To investigate the selective properties of MS, both SA and phenol were selected as similar compounds. A piece of membrane was added to a 50 mL Erlenmeyer flask that contained 20 mL of the coexisting compound solution with 100 mg L⁻¹ of MS and the contrast substance (SA and phenol). After adsorption, the concentration of substrate (MS, SA and phenol) in the solution was determined via UV spectrophotometry and the binding amounts of the membrane for MS and the analogues were calculated according to the procedure of the batch binding experiments.

The distribution coefficient (K_d) and selectivity coefficient (α) of SA and phenol with respect to MS were obtained according to eqn (4) and (5):

where, K_d (mL g⁻¹) is the distribution coefficient, and q_e (mg g⁻¹) and C_e (mg L⁻¹) are the equilibrium binding amount and the equilibrium concentration of adsorbate in solution, respectively.

The selectivity coefficient (α) was obtained according to the following equation:

where, i and j represent the template and the competitive analogue, respectively.

3. Results and discussion

3.1. Raman spectroscopy

To ascertain the successful preparation of the functional polymers, Raman spectra (Fig. 2) were obtained for 4-VP-MIM, AM-MIM and MAA-MIM. The observed features at around 1220 cm⁻¹ result from the C–Si vibrations of VMS for the three imprinted membranes. The strong peaks between 2800 and 3000 cm⁻¹ could be ascribed to the stretching vibrations of C–H of MIM (4-VP-MIM, AM-MIM and MAA-MIM). The typical Raman peak at around 1599 cm⁻¹ indicates the C=N stretching vibrations of 4-VP for 4-VP-MIM. Compared with 4-VP-MIM, a characteristic feature of AM-MIM and MAA-MIM is the C=O band from –CONH₂ in AM and –COOH in MAA at around 1689 cm⁻¹. These results suggest three types of MS imprinted membranes were successfully prepared with 4-VP, AM and MAA, each, as functional monomers.

Fig. 2 Raman spectra of 4-VP-MIM2, AM-MIM and MAA-MIM.

3.2. Scanning electron microscopy analysis

SEM images were taken to determine the surface morphology of the various prepared MIMs. Fig. 3 depicts the SEM surface photographs of the membranes. As seen in Fig. 3a, the pure Al₂O₃ ceramic membrane exhibits a more porous and smooth structure. In Fig. 3b–d, the surfaces of the MIMs are much rougher than that of the pure Al₂O₃ membrane. This is due to the formation of a thick imprinted layer after the polymerization procedure on the three imprinted membrane surfaces. Additionally, there were fewer differences between 4-VP-MIM, AM-MIM and MAA-MIM, which indicate that the selectivity of the MIMs with different functional monomers is not entirely due to their difference in morphology, but to their imprinting ability. In Fig. 3e, the formation of a thin polymer layer is observed from the surface morphology of 4-VP-NIM2. These results suggest that the reaction between the pure Al₂O₃ ceramic membrane and polymerization solution is indeed reliable.

Fig. 3 SEM images of surface views of (a) pure Al₂O₃ membrane, (b) 4-VP-MIM2, (c) AM-MIM, (d) MAA-MIM and (e) 4-VP-NIM2.

Fig. 4 presents the significant changes of surface morphology of 4-VP-MIMs, which were prepared with polymerization solutions with different amount of VMS (shown in Table 1). With an increase in the content of VMS, the imprinted membrane covered with imprinting copolymer seemed to be thicker and the pores on the surface became distinctly smaller. However, obvious clavoid substances on the surface of 4-VP-MIM4 were observed for the polymerization solution containing the highest amount of VMS. This clearly shows that excessive amounts of VMS may affect the morphology of the imprinted membrane.

Fig. 4 SEM images of surface views of (a) 4-VP-MIM1, (b) 4-VP-MIM2, (c) 4-VP-MIM3 and (d) 4-VP-MIM4.

Fig. 5 shows the cross-sectional morphology of the pure Al₂O₃ ceramic membrane and 4-VP-MIM2. There are great differences between the morphology of the Al₂O₃ ceramic membrane and 4-VP-MIM2. Compared with the Al₂O₃ ceramic membrane, the cross sections of 4-VP-MIM2 is covered by a thin imprinted layer in the inner pores of the Al₂O₃ ceramic membrane after imprinting polymerization. These results suggest that the imprinted polymer is formed not only on the surface, but also in the inner micropores of the Al₂O₃ ceramic membrane.

Fig. 5 SEM images of cross-sectional views of (a) pure Al₂O₃ membrane and (b) 4-VP-MIM2.

3.3. Membrane flux experiments

To determine the hydrophilicity and antifouling performance of the membranes, membrane flux experiments were performed at the trans-membrane pressure of 0.15 MPa, and the results are presented in Fig. 6.

Fig. 6 Membrane flux of (a) pure Al₂O₃ membrane and 4-VP-MIM2/NIM2; (b) pure Al₂O₃ membrane and AM-MIM/NIM; (c) pure Al₂O₃ membrane and MAA-MIM/NIM; and (d) 4-VP-MIM1, 4-VP-MIM2, 4-VP-MIM3 and 4-VP-MIM4.

As shown in Fig. 6a, the pure Al₂O₃ microporous ceramic membrane has the highest membrane flux, which could be ascribed to the microporous structure and more pores of the pure Al₂O₃ microporous ceramic membrane. Compared with the flux of the pure Al₂O₃ microporous ceramic membrane, 4-VP-MIM2 and 4-VP-NIM2 present lower flux values due to the imprinted polymer in the membrane structure. Furthermore, the flux values were high at beginning and gradually declined with an increase in operation time, which could be ascribed to the pore structure and binding sites in the membranes, similarly to the results discussed in the SEM analysis. We suspect that the decrease in flux values is attributed to jammed inner pores and MS adsorption. In addition, 4-VP-MIM2 exhibited a higher membrane flux than that of 4-VP-NIM2 at similar pressure. This result suggests that 4-VP-MIM2 is beneficial for the mass transport of the MS-contained aqueous solution, which facilitates the recognition capability between the membrane and MS molecules.

The effect of various functional monomers (4-VP, AM and MAA) on the Al₂O₃ microporous ceramic membrane brought changes to the membrane flux (see Fig. 6a–c). As expected, the flux profiles of 4-VP-MIM2/NIM2, AM-MIM/NIM and MAA-MIM/NIM remarkably decreased with the formation of the imprinted layer. This is mainly due to the interaction between the imprinted layer and MS-contained aqueous solution, which leads to the difference in membrane flux. Obviously, the imprinted membrane with 4-VP is beneficial to improve the membrane flux.

Fig. 6d presents the membrane flux profiles of the surface of 4-VP-MIMs, which were prepared with pre-polymerization solutions with different amount of VMS. As expected, the flux profiles of 4-VP-MIM1/NIM1, 4-VP-MIM2/NIM2, 4-VP-MIM3/NIM3 and 4-VP-MIM4/NIM4 remarkably decreased with an increase in the concentration of VMS. This is mainly due to the formation of large number of imprinted sites, which increase the interaction between water molecules and the modified membranes. Consequently, VMS could easily enhance the permeate rate between the membrane and permeants, and thereby lead to high flux of the modified Al₂O₃ microporous ceramic composite membranes.

3.4. Batch adsorption kinetics

Among the various plots employed to analyze the effect of additives of the imprinted membrane on the adsorption process, herein, the adsorption rate was studied. Fig. 7 shows the adsorption kinetic curves of MS on MIMs (4-VP-MIM2, AM-MIM and MAA-MIM) and NIMs (4-VP-NIM2, AM-NIM and MAA-NIM) from methanol aqueous solution containing 100 mg L⁻¹ MS at different sampling times. As shown in Fig. 7, the adsorption equilibrium time of MIM and NIM were 14 and 25 min, respectively. It was reasonable to assume that a large number of affinity binding sites exist in the imprinted membranes, thus the template MS easily entered into the sites and interacted with the functional groups. Moreover, the adsorption rates of MS on MIM present a gradual decrease in the adsorption process, which could be ascribed to the diffusion resistance. Additionally, the data show that the 4-VP-MIM2 has a higher equilibrium adsorption capacity than the other membranes. This might be due to the appreciable imprinted sites which facilitated the MS binding with recognition sites. These results suggest that the composite membrane with the functional monomer 4-VP is more suited to improve the adsorption of MS.

Fig. 7 Comparison of the adsorption kinetics of different membranes: (a) 4-VP-MIM2 and NIM2, (b) AM-MIM and NIM and (c) MAA-MIM and NIM.

The content of VMS on the imprinted membrane is expected to strongly affect the adsorption of MS. Fig. 8 shows that the adsorption capacity of 4-VP-MIM3 toward MS is much higher than that of 4-VP-MIM1, 4-VP-MIM2 and 4-VP-MIM4. This result suggests that 4-VP-MIM3 is the optimal imprinted membrane in the present work.

Fig. 8 Effect of the content of vinyltrimethoxysilane on the adsorption kinetics on 4-VP-MIMs: (a) 4-VP-MIM1, (b) 4-VP-MIM2, (c) 4-VP-MIM3, and (d) 4-VP-MIM4.

To investigate the adsorption rate and rate-controlling mechanism of the adsorption processes, the adsorption kinetic data of 4-VP-MIM3 and 4-VP-NIM3 were analyzed by the typical kinetic models pseudo-first-order (eqn (6)) and pseudo-second-order rate equations (eqn (7)):²¹

q_t = q_e − q_ee^−k₁t (6)

where, q_e and q_t (mg g⁻¹) are the adsorption capacity at equilibrium and any time t (min), respectively, and k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) are the pseudo-first-order and pseudo-second-order rate constant of adsorption, respectively.

In this study, the adsorption kinetic constants and linear regression values are listed in Table 2. The correlation coefficient (R²) was used to judge the applicability of the kinetic models. As shown in Table 2, the correlation coefficient of the pseudo-second-order model for 4-VP-MIM3 (R² = 0.9907) is higher than that of the pseudo-first-order model (R² = 0.7642). This reveals that the adsorption kinetic process perfectly fits with the pseudo-second-order kinetic model and the results for MS binding are presented in Fig. 9. The pseudo-second-order model exhibits favorable agreement between the theoretical q_e values (q_e,cal) and experimental q_e values (q_e,exp), whereas the opposite result was found for the pseudo-first-order model. Therefore, these results suggest that the pseudo-second-order kinetic model is better to predict the adsorption behavior of SA on 4-VP-MIM3.

Table 2 Kinetics constants for the pseudo-first-order and pseudo-second-order equations

Adsorbents	Pseudo-first-order model				Pseudo-second-order model
	qe,exp (mg g^−1)	qe,cal (mg g^−1)	k1 (min^−1)	R^2	qe,cal (mg g^−1)	k2 (g mg^−1 min^−1)	R^2
4-VP-MIM3	0.1351	0.1323	0.8751	0.7642	0.1347	16.5491	0.9907
4-VP-NIM3	0.0667	0.0644	0.4655	0.8063	0.0669	12.5662	0.9931

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Fig. 9 Non-linear regression of the kinetic models for 4-VP-MIM3 and 4-VP-NIM3.

3.5. Adsorption isotherm

To evaluate the adsorption performance of the MIM (4-VP-MIM2, AM-MIM and MAA-MIM) and NIM (4-VP-NIM2, AM-NIM and MAA-NIM) for MS, adsorption isotherm experiments were carried out at room temperature (Fig. 10). As shown in Fig. 10, the equilibrium adsorption capacities increased gradually with an increase in the MS initial concentration, and ultimately reached an equilibrium value due to the saturation of the static adsorption behavior. It can also be observed that the adsorption capacity of MIM is higher than that of NIM, which might be ascribed to the great number of chemical imprinting sites in MIM, hence MS molecules more easily interact with MIM. The NIM showed non-selective physical adsorption toward MS molecules. Furthermore, as seen in Fig. 11, the adsorption capacity of 4-VP-MIM3 towards MS is much higher than that of 4-VP-MIM1 and 4-VP-MIM2, and the adsorption capacity of MS on 4-VP-MIM4 is close to that on 4-VP-MIM3. This might be ascribed to the fact that excessive VMS on the surface limits the effect of the specific adsorption of MS. These results suggest that 4-VP-MIM3 is the optimal imprinted membrane in the present work.

Fig. 10 Comparison of the adsorption isotherms of the different membranes: (a) 4-VP-MIM2 and 4-VP-NIM2, (b) AM-MIM and AM-NIM, and (c) MAA-MIM and MAA-NIM.

Fig. 11 Effect of the content of vinyltrimethoxysilane on MS adsorption on the imprinted membranes, 4-VP-MIM1, 4-VP-MIM2, 4-VP-MIM3/NIM3 and 4-VP-MIM4/NIM4.

In this study, the equilibrium data for MS onto 4-VP-MIM3 and 4-VP-NIM3 were fitted by two classical isotherm models, specifically the Langmuir model (eqn (8))²² and Freundlich model (eqn (9)).²³

q_e = K_FC_e^1/n (9)

where, C_e (mg L⁻¹) represents the equilibrium concentration of MS, and q_e (mg g⁻¹) and q_m (mg g⁻¹) represent the equilibrium amount and the maximum adsorption capacity of MS, respectively. K_L is the Langmuir constant related to the affinity of the adsorption sites. K_F and 1/n are the Freundlich constants related to the capacity and intensity of the adsorption, respectively.

A comparison of the isotherm models for MS adsorption onto 4-VP-MIM3 and 4-VP-NIM3 with nonlinear regression is shown in Fig. 12 and the adsorption isotherm constants are given in Table 3. The correlation coefficient (R²) was used to judge the applicability of the isotherm models. As shown in Table 3, the experimental data were well fitted by the two models (Langmuir and Freundlich models) with R² values of 0.9969 and 0.9669; and 0.9950 and 0.9896 for 4-VP-MIM3 and 4-VP-NIM3, respectively. Apparently, the correlation coefficient is higher with the Langmuir model than with the Freundlich model, which indicates that the Langmuir model provides better fitting for 4-VP-MIM3 and 4-VP-NIM3, separately. The calculated maximum adsorption capacities of 4-VP-MIM3 and 4-VP-NIM3 were 0.3247 mg g⁻¹ and 0.1922 mg g⁻¹, respectively.

Fig. 12 Adsorption isotherms of MS on 4-VP-MIM3 and 4-VP-NIM3 with fitting to the Langmuir model and the Freundlich model.

Table 3 Adsorption isotherm constants for 4-VP-MIM3 and 4-VP-NIM3 at 25 °C

Adsorption isotherm models	Parameters	MIM	NIM
Langmuir model	R^2	0.9969	0.9950
	KL (L mg^−1)	0.0085	0.0055
	qm,cal (mg g^−1)	0.3247	0.1922
Freundlich model	R^2	0.9669	0.9896
	KF (mg g^−1)	0.0139	0.0045
	n	1.9817	1.7176

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3.6. Selectivity binding

The selective adsorption ability of prepared membranes was evaluated towards the competitive species MS, SA and phenol. The selective binding experiments for the similar compounds on different membranes were all carried out under the same experimental conditions.

The results of the distribution coefficient (K_d) and selectivity coefficient of the sorbent (α) are summarized in Table 4. The experimental data show a good separation effect by AM-MIM, MAA-MIM and 4-VP-MIM (1, 2, 3 and 4) with α values for MS relative to SA and phenol: 2.0961, 2.0245, 1.8699, 2.1908, 2.4185 and 2.2559; and 1.9286, 1.8041, 1.7055, 2.1138, 2.2277 and 2.1510, respectively. Clearly, the α values for 4-VP-MIM (2, 3 and 4) were higher than that for AM-MIM and MAA-MIM, which indicate that higher adsorption selectivity occurred on 4-VP-MIM among the MIM. This reveals that the functional monomer may play an important role in the adsorption selectivity of MS. Moreover, as shown in Table 4, the α values of 4-VP-NIM3 for MS relative to SA and phenol were 1.0527 and 1.0645, respectively, which indicate that 4-VP-NIM3 is almost non-selective for MS. Obviously, the above fact fully displays that MIM has stronger selective recognition ability than NIM, whereas the separation factor for SA is higher than that for phenol.

Table 4 Parameters of the batch adsorption selectivity of the prepared membranes

Membrane	MS		α	SA
	qe,MS (mg g^−1)	Kd(MS) (mL g^−1)		qe,SA (mg g^−1)	Kd(SA) (mL g^−1)
MAA-MIM	0.0846	0.9213	2.0961	0.0423	0.4395
AM-MIM	0.0764	0.8263	2.0245	0.0394	0.4081
4-VP-MIM1	0.0841	0.9193	1.8699	0.0470	0.4916
4-VP-MIM2	0.1084	1.2161	2.1908	0.0528	0.5551
4-VP-MIM3	0.1319	1.5218	2.4185	0.0594	0.6292
4-VP-NIM3	0.0556	0.5864	1.0527	0.0530	0.5571
4-VP-MIM4	0.1441	1.6874	2.2559	0.0698	0.7480

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Membrane	MS		α	Phenol
	qe,MS (mg g^−1)	Kd(MS) (mL g^−1)		qe,phenol (mg g^−1)	Kd(phenol) (mL g^−1)
MAA-MIM	0.0839	0.9142	1.9286	0.0453	0.4740
AM-MIM	0.0758	0.8194	1.8041	0.0435	0.4542
4-VP-MIM1	0.0778	0.8438	1.7055	0.0472	0.4947
4-VP-MIM2	0.0972	1.0743	2.1138	0.0485	0.5082
4-VP-MIM3	0.1237	1.4107	2.2277	0.0596	0.6332
4-VP-NIM3	0.0554	0.5837	1.0645	0.0521	0.5484
4-VP-MIM4	0.1501	1.7502	2.1510	0.0756	0.8137

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Furthermore, the α values of 4-VP-MIM4 for MS relative to SA and phenol (2.2559 and 2.1510) were lower than that of 4-VP-MIM3 (2.4185 and 2.2277), however the highest adsorption capacity for the substrate (MS, SA and phenol) was obtained over 4-VP-MIM4. The excessive VMS on 4-VP-MIM4 may limit the imprinting effect for MS with the increase of jammed pores. This suggests that the MIM with a high adsorption capacity may not favor an increase in adsorption selectivity. Considering the adsorption capacity and adsorption selectivity on the MIMs, the 4-VP-MIM3 was the optimal imprinted membrane in our experiment.

3.7. Regeneration

To evaluate the stability and regeneration of 4-VP-MIM3, a regeneration experiment was performed at the MS concentration of 100 mg L⁻¹. After adsorption of MS onto 4-VP-MIM3, the MS-adsorbed 4-VP-MIM3 was regenerated using a methanol/acetic acid (9 : 1, v/v) mixed solvent and deionized water. The adsorption capacity of the 4-VP-MIM3 adsorbent for MS with six consecutive adsorption–regeneration cycles is shown in Fig. 13. It was clearly seen that the adsorption capacity of MS on 4-VP-MIM3 decreased gradually for the first three times, and then the adsorption capacity maintained its recovery rate at the almost constant value of 90.62%. It is reasonable to assume that 4-VP-MIM3 could be reused at least five times without significant decrease in its adsorption capacity.

Fig. 13 Adsorption–desorption cycles for 4-VP-MIM3.

4. Conclusions

In this work, a series of Al₂O₃ microporous ceramic composite imprinted membranes for template MS were synthesized by the surface imprinting technique. Their characteristics, membrane flux, kinetics, adsorption capacities, selectivity and regeneration were studied in detail. Morphological analysis and membrane flux showed that the surface morphology and hydrophilicity were strongly affected by the functional monomers and the amount of vinyltrimethoxysilane in the Al₂O₃ membrane. The adsorption kinetic analysis suggests that 4-VP-MIM3 is the optimal imprinted membrane, which possessed high kinetic equilibrium adsorption capacity for the binding of MS. The pseudo-second-order kinetic model was able to predict the adsorption behavior of MS on 4-VP-MIM3. Moreover, adsorption isotherm analysis showed that 4-VP-MIM3 has a high binding capacity for MS. The selectivity binding results reveal that 4-VP-MIM3 has excellent selective recognition ability for MS with respect to competitive analogues. The regeneration experiment exhibits that 4-VP-MIM3 could be reused for at least six adsorption–desorption cycles without a significant decrease in its binding capacity. Further studies will be performed to extend this research to separate MS from complex matrices.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21406085, U1507118, U1407123 and 21576111), Ph.D. Programs Foundation of Ministry of Education of China (No. 20133227110022), Natural Science Foundation of Jiangsu Province (BK20140580, BK20140534 and BK20151350) and the China Postdoctoral Science Foundation (2014M561588).

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