Synthesis of Novel Composite Membranes Based on Molecularly

Article pubs.acs.org/IECR

Synthesis of Novel Composite Membranes Based on Molecularly Imprinted Polymers for Removal of Triazine Herbicides from Water Chrysoula Gkementzoglou,† Olympia Kotrotsiou,‡ and Costas Kiparissides*,†,‡,§ †

Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box 472, Thessaloniki, 54124 Greece Chemical Process and Energy Resources Institute, CERTH, Thessaloniki, 57001 Greece § Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates ‡

ABSTRACT: Novel molecularly imprinted polymer (MIP) composite membranes for the selective removal of triazine herbicides from polluted water sources were synthesized using two different approaches. According to the first method, sandwich-type composite membranes were prepared that consisted of a middle packed layer of MIP nanoparticles (NPs) confined between two microfiltration membranes. The highly selective MIP NPs were synthesized in the presence of atrazine, acting as template molecule, via the mini-emulsion polymerization method. In the second approach, MIP thin films, formed via an in situ polymerization method in the presence of the template molecule (desmetryn), were deposited on the top surface of ceramic support membranes using 2,2′-azobis (N,N′-dimethylene) isobutyramidine as initiator. The rebinding capacity of the synthesized MIP-ceramic composite membranes toward the template molecules was initially tested in batch-wise guest binding experiments. Subsequently, the synthesized composite membranes were tested in continuous dead-end filtration experiments to assess their binding efficiency, specificity, and their ability to adsorb the template molecules from water samples, at very low concentrations (i.e., down to 1 ppb). A series of experiments were also carried out to assess the binding capacity of the regenerated composite membranes and their long-term performance. The present results clearly demonstrate that the synthesized MIP composite membranes can remove the triazine herbicides of atrazine and desmetryn from water samples at very low concentrations (i.e., down to 1 ppb). Finally, it was found that the MIP composite membranes could be regenerated and reused without loss of their binding capacity and “memory effect”, which underlines their outstanding stability and reusability features in a continuous filtration process.

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INTRODUCTION Pesticides are potential pollutants subjected to strict regulations worldwide. European regulations (i.e., council directive 98/83/ EC) require that the allowable concentration of a single pollutant or of all regulated pesticides in water intended for human consumption is less than 0.1 and 0.5 μg/L, respectively. Two of the most commonly used herbicides in Europe for the control of broadleaf and grassy weeds are atrazine and desmetryn. However, their prolonged use can increase the risk of their retention in crops and soils from which they can in turn pass to surface and ground waters caused by washing and leaching processes. For the removal of these pollutants from water sources, several water treatment methods (i.e., ozonation, UV radiation, activated carbon adsorption, etc.) have been applied. However, these methods are normally nonselective and result in a secondary pollution stream.1−4 A water treatment process that has attracted a great interest over the past four decades is the membrane technology because of its favorable process characteristics (e.g., efficiency, scalability, space/energy/cost saving benefits, and adaptability). Indeed, membrane-based processes offer an unmatched performance with respect to concentration, purification, and separation efficiency of mixed pollutants at levels that other competitive technologies can hardly reach. However, many technical challenges and commercially significant separation problems remain to be solved since the currently available membrane technologies do not allow the selective removal of unwanted water-soluble compounds from complex pollutant © XXXX American Chemical Society

mixtures. Therefore, the development of novel functional materials exhibiting high molecular selectivity to specific organic pollutants is highly desirable.5 One of the most promising technologies for the development of highly selective functional materials is that based on molecularly imprinted polymers.6,7 In molecular imprinting, a functional monomer and a cross-linker are copolymerized, via a free-radical polymerization mechanism, in the presence of a target molecule (i.e., the imprint molecule) that acts as a molecular template. The functional monomer molecules initially form a complex with the imprint molecules via covalent or noncovalent molecular bonding. This is followed by the polymerization of the functional monomer with a bifunctional or trifunctional cross-linker. The resulting highly cross-linked polymeric structure keeps the template molecules in position through covalent or noncovalent interactions between the monomer and template functional groups. Subsequent removal of the imprint molecules reveals the specific binding sites that are complementary in size and shape to the template molecule.8−12 Molecularly imprinted polymers exhibit high Special Issue: Recent Advances in Nanotechnology-based Water Purification Methods Received: February 12, 2013 Revised: April 30, 2013 Accepted: May 1, 2013

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dx.doi.org/10.1021/ie400479c | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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ent morphologies were employed. The type of MIP-ceramic composite membranes offers a number of specific advantages including its low shrinking and swelling characteristics derived from the high thermal and structural stability of ceramic membranes. This largely improves the separation efficiency of the deposited MIP layer in comparison to that of common polymeric membranes. In particular, MIP and NIP thin films, formed via an in situ polymerization method, were deposited on the top surface of ceramic support membranes.26−28 A series of experiments were carried out to ensure the successful grafting of the MIP films onto the ceramic substrate. Initially, the binding efficiency of the synthesized MIP-ceramic composite membranes toward the template molecule (i.e., desmetryn) was tested in batch-wise guest binding experiments. Subsequently, the synthesized MIP-ceramic composite membranes were tested in continuous dead-end filtration experiments to assess their binding efficiency and their ability to adsorb desmetryn from water samples, at very low concentrations (i.e., down to 1 ppb). Finally, a series of experiments were carried out to assess the binding capacity of the regenerated composite membranes and their long-term performance.

physical and chemical resistance against external degrading factors. Thus, they are remarkably stable against mechanical stresses, high temperatures and pressures, and also in a wide range of solvents.13,14 Furthermore, these polymers can be repeatedly used without loss of their “memory effect”. In general, molecularly imprinted polymers (MIPs) are synthesized via free-radical bulk polymerization as porous monoliths. Subsequently, irregular particles in the size range of 5−100 μm are formed via grinding and sieving of the bulk MIPs. Although this method allows the easy production of particulate MIPs, it only yields moderate amounts of irregular size and shape MIP particles (i.e., less than 50%). Thus, alternative polymerization techniques need to be developed for the synthesis of well-defined MIP nano- and microparticles. In contrast to bulk monoliths, MIP nanoparticles (NPs) exhibit a high surface-to-volume ratio that significantly increases the number of accessible specific sites, thus improving the binding capacity of MIPs. This product format largely facilitates the design of NP-based hybrid membranes for novel separation processes. Thus, recent research efforts have been focused on the synthesis of regularly shaped molecularly imprinted polymer particles (i.e., particularly in the nanoscale range) because of easier and better control of the MIPs morphological and specific binding properties.15−17 A commonly employed method for the production of MIP NPs in high yields is the mini-emulsion polymerization. According to this method, the polymerization is carried out in the monomer nanodroplets that are dispersed in the aqueous phase. At the end of polymerization, solid MIP NPs in the size range of 50−500 nm are obtained. In mini-emulsion polymerization, the initial droplet size, and consequently the final particle diameter, can be controlled by a judicious selection of the types and concentrations of surfactant and cosurfactant, the volume fraction of the dispersed organic phase, the densities and viscosities of the organic and aqueous phases, the temperature, and the quality of agitation.18−23 In the present work, highly selective particulate MIPs were incorporated in organic filtration membranes to produce novel composite constructs for the removal of synthetic organic compounds (SOCs) (i.e., pesticides, pharmaceutically active compounds, etc.) from water sources.24,25 In particular, MIP NPs with well-defined morphological, physical, and chemical affinity properties were synthesized via the mini-emulsion polymerization method. Batch-wise guest binding experiments were carried out to determine the rebinding properties of the synthesized MIP NPs toward the selected template molecules. It was shown that MIP NPs exhibited a high selectivity and specificity toward the selected template molecule (e.g., atrazine). Subsequently, composite sandwich-type membranes were prepared via the confinement of MIP NPs (i.e., as a filter cake) between two microfiltration membranes. The efficiency of the synthesized composite flat-type filters was accordingly assessed with respect to their flow and binding characteristics in dead-end filtration experiments. Competitive binding experiments were also carried out in the presence of simazine and propazine pollutants to further assess the selectivity of the sandwich-type composite membranes. Additionally, MIP-based composite membranes, consisting of a thin MIP film, acting as a highly selective layer, deposited on a high permeability alumina support membrane, were prepared. To control the final pore size (i.e., permeability) and the specific surface area (i.e., binding capacity) of the synthesized composite membrane, different ceramic substrates with differ-

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SYNTHESIS AND CHARACTERIZATION OF MOLECULARLY IMPRINTED POLYMERS Materials. Methacrylic acid (MAA), ethylene glycol dimethacrylate (EGDMA), acetonitrile, methanol, and water (HPLC grade) were purchased from Merck. Chloroform (CHCl3), dichloromethane (CH2Cl2), atrazine, and desmetryn were obtained from Riedel de-Haën, and acetic acid was from Carlo Erba. 2,2′-azobis (N,N′-dimethylene) isobutyramidine (ADIA) was purchased from Wako Chemicals GmbH (Neuss, Germany). Trimethylopropane trimethacrylate (TRIM), tetrachloroethylene, sodium dodecyl sulfate (SDS), and 2,2′-azobis(2-isobutyronitrile) (AIBN) were purchased from Aldrich. Hexadecane was purchased from Fluka. Alumina-based ceramic disks of 25 mm (i.e., Al2O3) and 44 mm (i.e., TiO2 modified alumina) in diameter and a thickness of 0.8 mm were provided by Keranor AS. Synthesis and Characterization of MIP NPs. Initially, the oil phase (comprising the functional monomer, the crosslinker, the cosurfactant, the porogen medium, and the initiator) was dispersed in the continuous aqueous phase, containing the dissolved SDS. For the synthesis of particulate MIPs, the organic phase contained a specified concentration of the template molecule as well. The two phases were vigorously stirred for 1 h, at room temperature, followed by sonication for 2 min with the aid of a dispersing unit (operated at 90% of its 650Watt power in 50% pulse mode). Polymerization of the dispersed organic nanodroplets was carried out at 60 °C for 24 h in a laboratory scale, water-jacketed glass reactor of 100 mL working volume, equipped with a six-blade impeller and an overhead condenser. The polymerization temperature was controlled within ±0.05 °C of the set-point value with the aid of a constant temperature bath (Julabo F32). In Table 1, the selected experimental conditions for the production of NIP and MIP NPs are reported. The molar ratio of the cross-linker to the functional monomer was equal to 5:1 for the EGDMA cross-linker and 3.3:1 for the TRIM experiments. Thus, the total concentration of double bonds in both cases was kept the same. In particular, the aqueous phase consisted of 50 mL of water and the specified SDS concentration (1.2 wt % of SDS based on monomers). The oil B



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Subsequently, the reaction bottles were sealed and placed in a thermo stated oven for polymerization at 80 °C.29 A series of experiments were carried out to investigate the effect of polymerization time on the grafted polymer film mass on the ceramic filter. After polymerization, the grafted polymer weight was determined by gravimetry (i.e., by measuring the weight of the ceramic membrane before and after polymerization). It was found that after 2 h of polymerization the mass of grafted polymer did not change significantly. Thus, all subsequent polymerizations for the synthesis of various grafted NIP (nonimprinted) and MIP (imprinted) polymer films on the ceramic filters were carried out for 2 h. Two different values of the molar ratio of the template to the functional monomer were employed (namely, 1/5 and 1/10), while the molar ratio of the functional monomer to the cross-linker was kept the same (i.e., 1/5) in all the experiments (see Table 2). Following

Table 1. Selected Experimental Conditions for the Synthesis of MIP and NIP NPs experiment

template (mass)

monomer (mass/volume)

MIP 1

289 mg

IA, 696 mg

NIP 1 MIP 2

IA, 696 mg 266 mg

NIP 2 MIP 3

MAA, 0.42 mL MAA, 0.42 mL

227 mg

MAA, 0.47 mL

NIP 3

MAA, 0.47 mL

NIP 4

MAA, 0.43 mL

MIP 5

159 mg

NIP 5 MIP 6 NIP 6


159 mg


cross-linker (volume) EGDMA, 5 mL EGDMA, 5 mL TRIM, 5.26 mL TRIM, 5.26 mL EGDMA, 5.25 mL EGDMA, 5.25 mL EGDMA, 4.75 mL EGDMA, 3.7 mL EGDMA, 3.7 mL EGDMA, 3.7 mL EGDMA, 3.7 mL

porogen (volume)

chloroform, 0.6 mL chloroform, 1.8 mL chloroform, 1.8 mL tetrachloroethylene, 1.8 mL tetrachloroethylene, 1.8 mL

Table 2. Selected Experimental Conditions for the Synthesis of MIP and NIP Ceramic-Based Composite Membranes

phase in addition to the selected amounts of monomer, crosslinker, and porogen contained a specified concentration of hexadecane (i.e., 4 wt % hexadecane based on monomers). The initiator concentration in the oil phase was 1.75 wt % based on monomers. After polymerization, the produced polymer particles were placed in fresh water for 1 h (i.e., for five consecutive times) to remove the SDS from the NPs. Subsequently, the template molecules were extracted from the MIP NPs by a successive series of washing cycles of NPs with a methanol-acetic acid solution (9:1 v/v). The template removal was monitored via UV spectroscopy. It was found that at the end of the third washing cycle more than 80% of the initially loaded amount of the template molecule had been removed. For the removal of the residual quantity of template molecule, 3 to 5 additional washing cycles were required. Finally, the washed NPs were conditioned in methanol and collected by ultracentrifugation (at 20 000 rpm for 15 min). Nonimprinted polymeric (NIP) NPs were also prepared under the same polymerization conditions in the absence of the template molecule. The surface morphology of the NPs was determined with the aid of a JSM-6300 scanning electron microscope. The pore size distribution and the specific surface area of the synthesized NPs were measured by Brunauer−Emmett−Teller (BET) analysis, using a Quantachrome Autosorb Automated Gas Sorption apparatus. In Situ Synthesis and Characterization of MIP and NIP Thin Films on Ceramic Substrates. First, the ceramic filters were subjected to a surface functionalization treatment for the incorporation of the azo-initiator onto the ceramic substrate surface. More specifically, the ceramic filters were initially immersed in a solution, containing 0.175 g of ADIA in 10 mL of CHCl3 for 2 h. The surface-modified ceramic filters were dried and then placed in reaction bottles containing a solution of the functional monomer and the cross-linker (i.e., 0.34 mL of MAA, 3.8 mL of EGDMA, and 5.6 mL of dichloromethane). The bottles’ content was purged with N2 for 5 min.

experiment

ceramic disk diameter, mm

template/functional monomer/crosslinker ratio

MIP 1 MIP 2 NIP 1,2 MIP 3 NIP 3

25 25 25 44 44

1/10/50 2/10/50 0/10/50 1/10/50 0/10/50

polymerization, the template molecule was removed from the grafted polymer layer by means of successive washing cycles with methanol. The removal of the template molecule was monitored via HPLC (Agilent 1200). After each washing cycle, the weight of the composite membrane was measured. The surface morphology of the membranes was examined with the aid of a JSM-6300 scanning electron microscope.

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ASSESSMENT OF ADSORPTION CAPACITY AND SPECIFICITY OF COMPOSITE MIP MEMBRANES Testing of MIP/NIP Nanoparticles and NP-Based Sandwich-Type Membranes. To evaluate the adsorption capacity of the produced NIP and MIP NPs, equilibrium batchwise guest binding experiments were conducted. Initially, predetermined weights of MIP or NIP NPs (i.e., 0.15 g) were placed in a 5 mL acetonitrile solution of atrazine (i.e., of 216 ppm or 1 mM initial concentration) for 24 h, for equilibration under relatively mild agitation conditions (with the aid of a magnetic stirrer at around 30 rpm). Moreover, additional batchwise guest binding experiments were carried out in both acetonitrile/water (9/1 v/v) and pure water solutions of atrazine (using a 5 ppm initial concentration). After equilibration, the MIP/NIP NPs were separated from the solution using a PVDF filter with a 0.25 μm pore size. Subsequently, the residual free analyte concentration in the atrazine solution was measured with the aid of HPLC (Agilent 1200). The quantification of the analyte was realized at 260 nm. Accordingly, the bound atrazine concentration was calculated from the difference of the initial atrazine minus the final free analyte concentration. In order to identify any potential errors caused by nonspecific binding of analyte onto the PVDF membranes, the initial analyte solutions in the absence of NPs were also filtrated using the same PVDF filters. HPLC measurements showed that the analyte concentration in the C



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densities, viscosities and interfacial tension) of the continuous and dispersed phases. In Figure 1a,b, SEM images of the synthesized MIP and NIP NPs (i.e., MAA-EGDMA) are depicted. As can be seen, in both

permeate was almost identical to the initial (less than 1% difference). Continuous dead-end filtration experiments were also performed by passing a 20 mL atrazine solution of 1 ppm initial atrazine concentration through a sandwich-type membrane. The sandwich-type composite membrane was made of a polyamide support disc (N66 Pall membrane with a diameter of 44 mm and a pore size of 0.1 μm) coated with the molecularly imprinted P(MAA/EGDMA) NPs. To avoid any loss of NPs during the filtration experiment, the NP-coated support membrane was covered by a second identical polyamide membrane disc. The deposition of the MIP or NIP NPs onto the support membrane was affected by the forced flow (under a constant N2 pressure) of a suspension of 0.05 g of NPs in 50 mL of distilled water through the support membrane, with the aid of an ultrafiltration cell (Amicon 8050, Millipore, Germany). After drying, the NP-coated membrane was covered by the top polyamide membrane. Testing of the Composite MIP- and NIP-Coated Ceramic Membranes. The adsorption capacity of the in situ prepared ceramic-based MIP and NIP composite membranes was initially tested in batch-wise guest binding experiments. In particular, the composite ceramic disks (i.e., MIP1, MIP2, and NIP 1,2) were placed in a 20 mL aqueous solution of desmetryn, at 1 ppm initial concentration, for 24 h under mild agitation conditions (using an Edmund Bühler Tilt Mixer Shaker WS-10 at level 20/min). Continuous dead-end filtration experiments were also performed with the MIP and NIP coated ceramic membranes (i.e., MIP3 and NIP3 composite ceramic filters of 44 mm diameter), using the same filtration apparatus employed in the sandwich-type membranes (Amicon 8050, Millipore, Germany). A water sample (i.e., up to 550 mL) containing the desmetryn pollutant was then forced through the composite membrane by applying a constant positive N2 pressure to the water-holding reservoir. Filtration experiments were carried out at different desmetryn concentrations in aqueous solutions (i.e., of 500, 100, 10, and 1 ppb). Measurements of desmetryn concentration were realized with the aid of HPLC-MS (Agilent 6100) apparatus. The amount of bound analyte was calculated by the difference of the initial desmetryn concentration minus the final free analyte concentration in the permeate stream at the end of the filtration experiment.

Figure 1. SEM images of MIP (a) and NIP (b) NPs prepared with MAA as functional monomer and EGDMA as cross-linker. Panels c and d depict MIP NPs produced with IA as functional monomer and EGDMA as cross-linker and MIP NPs prepared with MAA as functional monomer and TRIM as cross-linker, respectively.

cases (i.e, of MIP and NIP NPs), the size of the NPs is almost identical. It is important to point out that when the MAA functional monomer was replaced by the IA (see Figure 1c) or when the EGDMA cross-linker was replaced by TRIM (see Figure 1d), the final size of NPs was not significantly affected. To determine the adsorption capacity of the synthesized NIP and MIP NPs, batch-wise guest binding experiments were carried out following the experimental procedure described previously. In Figure 2, the rebinding capacity of MIP NPs

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RESULTS AND DISCUSSION Effect of MIP Synthesis Parameters on the Selectivity and Specificity of NP-Based Composite Membranes. In the mini-emulsion polymerization, the organic phase (i.e., containing the functional monomer, the cross-linker, the template molecule, the cosurfactant, and the chemical initiator) is dispersed, with the aid of a highly energy-intensive emulsification process (i.e., sonication), in the continuous aqueous phase containing the surfactant. Subsequently, the temperature is raised to the desired one (i.e., 60 °C) so that the free-radical copolymerization can be initiated. As the polymerization proceeds, the dispersed liquid nanodroplets are gradually transformed to sticky liquid−solid and finally to rigid, spherical polymer particles in the size range of 50−500 nm. One of the main advantages of the mini-emulsion polymerization is the control of the droplet/particle size distribution (DSD/PSD) via the proper selection of the type and concentration of the surface-active agents, the energy input into the emulsification process, and the physical properties (i.e.,

Figure 2. Measurement of the rebinding capacity of NIP and MIP NPs. Batch-wise binding experiments were carried out at a 216 ppm (i.e., 1 mM) initial atrazine concentration in acetonitrile.

(produced under different polymerization conditions, see Table 1) toward atrazine is shown in comparison to that of the NIP NPs. In particular, the effect of the functional monomer (i.e., MAA and IA) on the rebinding capacity of the MIP NPs toward atrazine was experimentally examined, following previous molecular modeling studies that showed that the two selected functional monomers exhibited a good affinity toward atrazine. Note that the two selected functional monomers have very similar solubility values (i.e., 83.3 g/L for IA and 89 g/L for MAA in water).30−32 D



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polymerization recipe, the porosity of the NIP and MIP NPs was significantly increased. The effect of the porogen type as well as its concentration on the specific surface area of the NIP and MIP NPs is shown in Table 3. It is evident that as the porogen concentration increases (i.e., from 10 to 30%) the NPs become more porous. On the other hand, when a porogen with a higher boiling point (e.g., tetra-chroroethylene) was used, the specific surface area of the NPs as well as their total pore volume was increased. This can be explained by the fact that, when the boiling point of the porogen increases, its evaporation rate decreases, resulting in an increase of the polymer-free particle volume. This significantly increases the accessibility of the specific binding sites that, consequently, enhances the rebinding capacity of the MIP NPs as was observed in the case of tetra-chroroethylene.31,32,38 However, in the presence of chloroform, no significant improvement in the rebinding capacity and specificity of the MIP NPs was observed (see Figure 2). This was explained by the fact that, in the case of chloroform, the particle morphology and, thus, its internal porous structure was not sufficiently stable as was evidenced by the measurement of the particle specific surface area before and after washing of the NPs for the removal of the residual template molecule. More specifically, it was found that, after a thorough washing of the NPs, the specific surface area as measured by BET analysis decreased from about 100m2/g (i.e., before washing) to about 40 m2/g (i.e., after washing). The measured large decrease in the particle specific surface area, caused by the partial collapse of the internal porous structure after particle washing, lowered the rebinding capacity and specificity of NPs. The rebinding properties of the NIP and MIP (MAA/ EGDMA) NPs, prepared in the presence of or without a porogen (e.g., tetra-chloroethylene), were assessed using an atrazine aqueous solution (see Figure 3). For the batch-wise

It was found that the overall binding capacity of IA-based MIP NPs was lower than that of MAA-based MIP NPs and that the rebinding properties of the IA-based MIP NPs were nonspecific, since the respective IA-based NIP NPs adsorbed more atrazine than the imprinted ones, even when the difference in the respective values of the specific surface area of MIP and NIP NPs was taken into account (i.e., NIP 1 and MIP 1 in Table 3). Table 3. Specific Surface Area and Total Pore Volume of the Synthesized MIP/NIP Particles experiment

specific surface area (m2/g)

total pore volume (cm3/g)

MIP 1 NIP 1 MIP 2 NIP 2 NIP 3 MIP 4 NIP 4 MIP 5 NIP 5 MIP 6 NIP 6

49.8 72.9 27.3 29.5 48.6 78.7 79.8 94.6 106.9 178.5 182.1

0.2484 0.4596 0.0996 0.0993 0.2413 0.4874 0.4290 0.4872 0.5352 0.6857 0.7228

This can be explained by the fact that in the MAA-based MIPs two molecules of MAA are associated with an atrazine molecule to form a complex, while in the case of IA-based MIPs, only one molecule of IA can be associated with an atrazine molecule. More specifically, in the case of MAA, the carboxyl groups (with a pKa value of 4.66 at 20 °C) of two monomer molecules are associated via noncovalent interactions (i.e., ionic interactions, hydrogen bonds, etc.) with the secondary amino groups of atrazine (i.e., isopropylamine NH groups) or with the triazine’s ring nitrogen. On the other hand, in the case of IA monomer, the active site is formed via the association of the two carboxyl groups of one IA molecule (with pKa values 3.85 and 5.45 at 25 °C) with one template molecule.33−35 As a result, the formed sites are not sufficiently specific, due to the random distribution of the excessive active groups of the IA molecules that gives rise to high-levels of nonspecific interactions. Subsequently, the effect of the cross-linker on the rebinding capacity of MIPs was examined. It was found that the MIP NPs, produced with the trifunctional cross-linker TRIM, exhibited a lower rebinding capacity toward the template molecule due to the significant decrease in the overall specific surface area of the NPs. More specifically, it was found that, in the presence of the trifunctional cross-linker TRIM, the MIP and NIP NPs had a significantly lower specific surface area and a lower total pore volume (see NIP2 and MIP2 in Table 3). On the other hand, NPs prepared in the presence of the bifunctional cross-linker EGDMA had a higher specific surface area and a higher total pore volume (see NIP3 in Table 3). Thus, in the latter case, the number of the accessible surface binding sites was significantly higher, resulting in an increase of the binding capacity of EGDMA-based NPs.36 Another parameter that was found to be directly related to the binding-rebinding properties of MIPs was the presence of a porogen solvent in the oil phase.37 More specifically, when chloroform (i.e., 10% CHCl3 in NIP4 and MIP4 and 30% CHCl3 in NIP5 and MIP5) or tetra-chloroethylene (i.e., 30% C2Cl4 in NIP6 and MIP6) was used as a porogen in the

Figure 3. Measurement of the rebinding capacity of NIP and MIP NPs. Batch-wise binding experiments were carried out using a 5 ppm atrazine aqueous solution.

guest binding experiments, 10 mg of NIP or MIP NPs were equilibrated for 24 h in 20 mL of a 5 ppm aqueous atrazine solution. It was found that when the aqueous atrazine solution contained 10% v/v acetonitrile the rebinding capacity of both NIP and MIP NPs was higher than that measured in pure acetonitrile atrazine solutions (see Figure 2). In the case of pure aqueous atrazine solution, the rebinding capacity of MIP NPs became even higher due to the strong hydrophobic interactions. However, it was found that, when the aqueous volume fraction in the atrazine solution increased from 90% to 100%, the specificity of NPs (i.e., the ratio of the binding E



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MIP/NIP NPs prepared in the presence of tetra-chloroethylene. Additionally, the binding specificity, expressed by the ratio of the binding capacity of MIPs over that of NIPs, was found to be higher for the case of NPs prepared in the absence of a solvent (i.e., 1.5 versus 1.1). This can be attributed to the lower permeability of the composite membranes prepared with NPs produced in the absence of porogen. The lower membrane permeability (caused by the higher packing density of NPs produced in the absence of porogen) resulted in a higher contact time of the atrazine solution with MIP NPs that improved the overall adsorption efficiency of the composite membrane. In fact, it was observed that the total permeation time of 20 mL of atrazine solution through the composite membrane made of NPs prepared in the absence of porogen was 25% higher than that of NPs prepared in the presence of porogen. Finally, the template analogues, simazine and propazine, with similar hydrophobic (log Pow, atrazine: 2.21− 2.75; log Pow, simazine: 1.51−2.26; log Pow, propazine: 2.93−3.02)44 and basic (pKa around 1.7 at 20 °C)45 characteristics to atrazine were tested in rebinding experiments for comparison. It was found that the MIP NPs could also adsorb the above template analogues (see Figure 5). However, in the latter case, the binding capacity of NIP NPs was higher than that of MIP NPs due to the nonselective binding of the template analogues and the fact that the driving force for the analogues binding was primarily dependent on hydrophobic interactions. Effect of Polymerization Conditions on the Specificity of MIP-Ceramic Composite Membranes. The synthesis of MIP films on the functionalized ceramic substrate was carried out using MAA as functional monomer and EGDMA as crosslinker in the presence of desmetryn, acting as the template molecule. Desmetryn was chosen due to its stronger basic nature (pKa = 3.9) in comparison to atrazine (pKa = 1.7). Thus, desmetryn could exhibit higher monomer-template ionic interactions at the specific polymerization conditions (i.e., in the presence of a strongly basic amidine-containing initiator).46,47 As discussed before, prior to the in situ copolymerization of MAA with EGDMA for the formation of a MIP film, the ceramic substrate was functionalized with ADIA initiator that was noncovalently attached to the surface of the ceramic support membrane. This strongly basic amidine-containing initiator was physically adsorbed onto the ceramic support membrane with simultaneous abstraction of hydrogen cations from the ceramic support membrane. More specifically, initiators of this type can be adsorbed onto anionic substrates (e.g., Al2O3 and TiO2) due to their cationic nature over a broad range of pH values. Subsequent homolysis of the adsorbed initiator leads to the formation of two cationic radicals that exhibit a limited solubility in organic solvents. This, theoretically, prevents their desorption from an anionic surface when the functionalized surface is immersed into a monomer(s) mixture for polymer grafting.47 Different polymerization times (e.g., from 15 to 2 h) were tested to find out the optimal time that results in a stable polymer grafted film onto the ceramic substrate. The degree of polymer grafting onto the ceramic substrate was determined gravimetrically. It was found that after 2 h of polymerization, the weight of the grafted polymer film did not vary significantly with time. Thus, in all subsequent in situ experiments, the polymerization time was set to 2 h. A series of MIP- and NIP-ceramic composite membranes were produced under different polymerization conditions. The synthesized MIP-ceramic composite membranes were washed

capacity of MIPs over that of NIPs) toward atrazine slightly decreased due to the nonspecific hydrophobic interactions.39−42 Following the above batch experiments, 20 mL of an atrazine aqueous solution at 1 ppm initial concentration was run through a sandwich-type membrane.43 The composite membrane consisted of a top and bottom polyamide membrane discs (N66, Pall) and a middle-layer of NIP or MIP P(MAA/ EGDMA) NPs. In Figure 4, the measured decrease in the water

Figure 4. Time variation of the water flow rate through the deposited layer of MIP/NIP NPs onto the bottom polyamide disk of the sandwich-type composite membrane.

flow rate through the deposited NPs layer is depicted during NP deposition process. It is evident that as the thickness and packing density of the deposited NPs increase, the flow rate through the deposited NP layer decreases. As can be seen, MIP and NIP NPs prepared under different synthesis conditions exhibit different flow rate profiles with respect to time. This can be explained by the fact that when tetra-chloroethylene was used as porogen in the polymerization recipe the mean particle diameter (i.e., D3,2) was slightly increased to ∼130 nm in comparison to the size of NPs prepared in the absence of tetrachloroethylene (i.e., ∼110 nm). As a result, the flow rate was increased since the pressure drop is inversely proportional to the size of the deposited NPs. This has a direct impact on the binding capacity of the sandwich-type composite membranes. In fact, it was found (see Figure 5) that the binding capacity of the composite membranes was higher for MIP/NIP NPs prepared in the absence of prorogen (i.e., tetra-chloroethylene) than that of

Figure 5. Measurement of the rebinding capacity and specificity of NIP and MIP NPs prepared in the presence or absence of porogen. F



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composite membranes retain their specificity, after the regeneration process, which proves their good reusability and stability characteristics. In the continuous dead-end filtration experiments, the aqueous solution, containing the undesired pollutant, was forced through the polymer-ceramic membrane (of 44 mm in diameter) by applying a constant nitrogen pressure (around 1 bar) on the solution feeding tank. The flow rate of the desmetryn solution through the different NIP- and MIPceramic membrane was monitored so the flux characteristics of the different grafted polymer films could be assessed. It was found that, in all filtration experiments, the flow rate exhibited a decrease with time. In particular, the flow rate decreased from an initial value of approximately 4 mL/min to a value of 0.5−1 mL/min at the end of the filtration experiment, due to changes in the polymer membrane’s hydrophobicity and permeability caused by the adsorption of desmetryn molecules.48,49 Additional binding experiments to assess the specificity and adsorption properties of MIP-ceramic composite membranes were carried out using aqueous solutions of desmetryn at very low concentrations (i.e., down to 1 ppb). Initially, 50 mL of an aqueous solution of desmetryn at 500 ppb concentration was passed through the composite membrane. As can be seen from the results of Figure 8, the MIP-ceramic composite membranes

thoroughly with methanol for the removal of the template molecule. During the template removal process, the weight of the grafted polymer was monitored gravimetrically. It was found that, during the polymer film washing process, a total reduction of up to 35 wt % in the initially grafted polymer mass was measured. In Figure 6, typical SEM images show the

Figure 6. SEM images of the ceramic substrate: (a) before and (b) after in situ polymerization.

surface morphology of the ceramic membrane before and after polymerization. It is evident that, after polymerization, the ceramic substrate (see Figure 6a) is uniformly covered by a grafted polymer film (see Figure 6b). To assess the binding capacity of the synthesized composite membranes, both batch-wise and continuous filtration experiments were realized. For the batch-wise guest binding experiments, the ceramic-polymer composite membranes (of 25 mm in diameter) were placed in 20 mL of aqueous solution of desmetryn at 1 ppm initial concentration for 24 h, under mild agitation conditions in an Edmund Bühler Tilt Mixer Shaker WS-10 at level 20/min. As shown in Figure 7, the MIP-

Figure 8. Measurement of the rebinding capacity of the NIP- and MIP-ceramic composite membranes.

adsorbed a higher amount of desmetryn than the NIP-based composite membranes. In addition, the specificity of the MIPceramic membranes, expressed by the ratio of the binding capacity of MIP-ceramic membranes over the capacity of NIPceramic membranes, was found to be equal to 1.9−2, a clear indication that the MIP-ceramic composite membranes had a higher binding capacity than the NIP-ceramic ones. Subsequently, the rebinding efficiency of the composite membranes was evaluated by passing a larger quantity (e.g., 350−550 mL) of an aqueous solution of desmetryn of very low concentration (i.e., 1−100 ppb). From the results of Figure 9, it can be seen that the MIP-ceramic composite membranes have a higher adsorbing efficiency in removing desmetryn from polluted water than that exhibited by the NIP-ceramic membranes. In fact, the MIP-ceramic membranes adsorb nearly 1.5−2 times the pollutant mass adsorbed by the respective NIPceramic composite membranes. In addition to the rebinding tests carried out with the NIPand MIP-ceramic composite membranes, a series of tests were carried using the original ceramic support membranes. As can

Figure 7. Measurement of the rebinding capacity (%) of the NIP- and MIP-ceramic composite membranes. (a) Initial composite membranes. (b) Regenerated composite membranes.

ceramic composite membranes exhibit a higher adsorption capacity than that of NIP-ceramic membranes that demonstrates their higher specificity toward the template molecule in aqueous solutions of the desmetryn pollutant. In the same figure, the rebinding capacity of the regenerated MIP-ceramic membranes is depicted. It is clear that the rebinding capacity of the regenerated MIP-ceramic composite membranes is similar to that measured for the newly synthesized composite membranes. For the regeneration of the MIP-ceramic membranes, the pollutant-loaded membranes were first subjected to a series of washing cycles with 10 mL of methanol-acetic acid solution (5% v/v). Subsequently, the ceramic composite membranes were conditioned in methanol and then dried. As shown in Figure 7, the MIP-ceramic G



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polymeric substrates in the form of sandwich-type composite membrane or by grafting a thin polymer film on ceramic support via an in situ polymerization method. The composite membranes were tested both in batch-wise guest binding experiments and in a dead-end continuous filtration process in order to assess their rebinding ability to remove the target template molecules from polluted water samples. The MIPbased composite membranes exhibited a higher binding capacity than that of the NIP-based membranes and had a superior specificity toward the template molecules. Moreover, they could remove the undesired pollutants from water samples at very low concentrations (i.e., down to 1 ppb). Finally, it was shown that the MIP composite membranes could be regenerated and reused without loss of their binding capacity and “memory effect”, thus clearly demonstrating their outstanding stability in a continuous filtration process.

Figure 9. Measurement of the rebinding capacity of the NIP- and MIP-ceramic composite membranes and untreated ceramic support.

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be seen from the results of Figure 9, the untreated ceramic membranes adsorbed a significantly lower amount of desmetryn than that measured for the NIP and MIP grafted composite membranes. Finally, in Figure 10, the rebinding capacity of the

AUTHOR INFORMATION

Corresponding Author

*Tel: + 302310 99 6211. Fax: +2310 99 6198. E-mail: [email protected]. Funding

This research has been cofinanced by the European Union (European Social Fund − ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) - Research Funding Program: Heracleitus II. (Investing in knowledge society through the European Social Fund) and b) the FP7 Project “Water Treatment by Molecularly Imprinted Materials − WATERMIM” (CP-FP 226524). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the European Commission for the financial support of the present work under the FP7 Project CP-FP 226524 .

Figure 10. Measurement of the rebinding capacity of the new and regenerated MIP-ceramic composite membranes.

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regenerated MIP-ceramic composite membranes is depicted. It is clear that the rebinding capacity of the regenerated MIPceramic composite membranes is similar to that measured for the freshly synthesized composite membranes. For the regeneration of the MIP-ceramic membranes, the pollutantloaded membranes were first washed with 50 mL of methanolacetic acid (5% v/v) solution. Subsequently, the ceramic composite membranes were washed with 50 mL of methanolacetic acid (20% v/v) and then conditioned with 50 mL of methanol and finally with 50 mL of water. The filters were then dried and weighed. In Figure 10, the rebinding capacity of the initial and regenerated MIP-ceramic composite membranes is shown for two different desmetryn concentrations (i.e., 100 and 500 ppb). As can be seen, the rebinding capacity of both new and regenerated MIP-ceramic composite membranes is similar for the same loading of 5 μg desmetryn (i.e., 10 mL of a desmetryn solution at 500 ppb or 50 mL of a desmetryn solution at 100 ppb).

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ABBREVIATIONS AIBN = 2,2′-azobis-(2-isobutyronitrile) ADIA = 2,2′-azobis (N,N′-dimethylene)isobutyramidine BET = Brunauer−Emmett−Teller CHCl3 = chloroform CH2Cl2 = dichloromethane DSD = droplet size distribution EGDMA = ethylene glycol dimethacrylate MAA = methacrylic acid MIP = molecularly imprinted polymer NPs = nanoparticles NIP = nonimprinted polymer PSD = particle size distribution SEM = scanning electron microscopy TRIM = trimethylopropane trimethacrylate SDS = sodium dodecyl sulfate SOCs = synthetic organic compounds REFERENCES

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CONCLUSIONS MIP composite membranes that can specifically adsorb the target molecules (i.e., atrazine and desmetryn) were synthesized using either preformed MIP NPs supported between two H



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Synthesis of Novel Composite Membranes Based on Molecularly

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