Molecular imprinted polymer to remove tetracycline from aqueous solutions

Microporous and Mesoporous Materials 203 (2015) 32–40

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Molecular imprinted polymer to remove tetracycline from aqueous solutions M. Sánchez-Polo ⇑, I. Velo-Gala, Jesús J. López-Peñalver, J. Rivera-Utrilla Department of Inorganic Chemistry, Faculty of Science, University of Granada, 18071 Granada, Spain

a r t i c l e

i n f o

Article history: Received 23 May 2014 Received in revised form 6 October 2014 Accepted 9 October 2014 Available online 18 October 2014 Keywords: Molecularly imprinted polymer Tetracycline Selective adsorption Water treatment

a b s t r a c t The objective of this study was to synthesize a molecularly imprinted polymer (MIP) for tetracycline (TC), prepared in the form of well-defined polymer microspheres, and its corresponding non-imprinted polymer (NIP). TC adsorption on these polymers was investigated, determining: (i) the interactions responsible for the adsorption, (ii) the chemical and textural characteristics of the polymers, and (iii) the role of the solvent in the process. The two polymers were characterized texturally by different techniques: (i) the surface area was larger for the MIP than for the NIP, whereas the micropore volume was considerably lower for the MIP; (ii) XPS analysis showed very high and similar surface oxygen percentages in both materials. The adsorbent properties of the materials were assessed by determining the TC adsorption isotherms: the Freundlich model indicates the high homogeneity of the MIP and Shimizu model shows the total number of binding sites was 8.7 lmol g1 for the MIP vs. 0.17 lmol g1 for the NIP. Analysis of the influence of pH and ionic strength revealed that adsorbent–adsorbate electrostatic interactions play a key role in TC adsorption on the MIP. The adsorption capacity of the MIP was also found to decrease with temperature, showing it to be an exothermic process. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Pharmaceuticals and their metabolites are characterized by high persistence and low biodegradability, and they are increasingly detected in wastewater treatment plant (WWTP) effluents, surface waters, ground waters, and drinking waters [1–3]. These compounds are of major concern because of their potential impact on human health and the environment, even at low concentration levels [4–8]. Tetracycline (TC) is very widely administered and is one of the most frequently detected pharmaceutical residues in water bodies to date. It has been found that biodegradation and chlorination processes do not achieve complete TC mineralization [9–12], and it has been detected at concentrations in the lg L1 range in all types of waters [1,13–16]. The inability of municipal WWTPs to degrade this compound is well documented [17–19]. Ozonation, UV radiation, and activated carbon adsorption might potentially improve the effectiveness of TC removal in these plants. The implementation of ozonation and photodegradation is only partially effective, due to the possibility of generating toxicologically relevant oxidation by-products. Thus, although advanced oxidation processes can effectively remove ⇑ Corresponding author. Tel.: +34 95824288; fax: +34 958243856. E-mail address: [email protected] (M. Sánchez-Polo). http://dx.doi.org/10.1016/j.micromeso.2014.10.022 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

numerous pollutants [20], they are limited in the competitive removal of interfering organics, and the toxicity of treated water arising from a mixture of stable transformation products may exacerbate their environmental and health impact [2]. Adsorption on activated carbon is only effective for hydrophobic contaminants [10] and is considerably impaired by the presence of interfering substances such as humic acid or surfactants [21,22]. Moreover, these water treatment methods all have a relatively low selectivity, explaining the major interest in developing more selective, simple and reliable wastewater treatment processes. Selective uptake using molecularly imprinted polymers (MIPs) appears to be a promising approach for the removal of trace pollutants in water. The synthesis of a polymer in the presence of a template molecule and the subsequent removal of the template furnish a robust material with ‘‘memory’’ sites, with the ability to selectively rebind the original template from a mixture. MIPs are synthesized with tailor-made binding sites for a template, with which they strongly interact [23,24] and they have been utilized to selectively remove templates from more complex environmental matrices. A MIP developed by Yao et al. [25] using computerassisted design demonstrated high potential to eliminate aniline from contaminated water. The molecular imprinting technique may also remove trace phenolic estrogens from different waters [26] and proved effective to remove 2,4-dichlorophenol from water

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[27]. In addition, these polymers may be useful for complex separation in analytical chemistry [28–33] especially when the target product and other compounds have similar properties. With this background, the objectives of this study were: to obtain a MIP specifically designed for the effective and selective adsorption of TC; to characterize this material texturally and chemically, identifying the main factors involved in the adsorption process; to determine the influence of key operational variables on the adsorption process, identifying the main interactions involved; and to establish the optimal conditions for TC adsorption on this MIP.

adsorption isotherms were used to determine the surface area (SBET), micropore and ultramicropore volume (W0(N2) and W0(CO2), respectively), and mean micropore width (L0). The surfaces of the materials were chemically characterized by determining the point of zero charge, pHPZC, and by X-ray photoelectron spectroscopy (XPS), obtaining data on the percentages of surface oxygen and carbon. Further details of the procedures employed have been reported elsewhere [34–36]. 2.4. Measurement of TC concentration TC concentration was measured by fluorimetry using a Cary series 50 spectrophotometer, considering two TC adsorption maxima (kmax 1 = 270 nm and kmax 2 = 370 nm).

2. Experimental 2.1. Materials

2.5. Analysis of the retention capacity and selectivity of the materials The study used: Tetracycline (TC); acetonitrile (ACN); methanol (MeOH); 2,20 -azobis(isobutyronitrile) (AIBN) initiator; methacrylic acid (MAA), as functional monomer; and ethyleneglycoldimethacrylate (EDMA), as cross-linker. All were supplied by Sigma–Aldrich. 2.2. MIP synthesis by precipitation polymerization The precipitation polymerization method was used preparing the MIP by dissolving TC (45 mg L1), EDMA, MAA and AIBN in a mixture of methanol and acetonitrile in a 250 mL polypropylene bottle. The mixture was degassed with oxygen-free nitrogen for 10 min while cooling in an ice bath, sealed under nitrogen atmosphere, and left to polymerize on a low-profile roller (Stovall, Greensboro, NC) housed within a temperature-controlled incubator (Stuart Scientific, Surry, ÇUK). The reaction temperature was raised from 25 °C to 53 °C over 2 h and then kept at 53 °C for a further 24 h. After polymerization, the microspheres were separated from the reaction medium by vacuum filtration on a nylon membrane filter, Soxhlet extraction was carried out with methanol for 24 h to remove the TC and unreacted monomers, and the microspheres were then vacuum-dried overnight at 40 °C. The corresponding non-imprinted polymer (NIP) was prepared in the same manner but without the template. The molar monomer composition was 1:7:26 (template:functional monomer:cross-linker) for the precipitation polymerization synthesis of the MIP for TC. Table 1 gives the doses of the reagents used to prepare the MIP and corresponding NIP, NIP and MIP particle size was between 0.05 and 0.10 lm.

The retention capacity and selectivity of the materials was assessed by determining the corresponding TC adsorption isotherms, using 30 mg of material in 3.5 mL TC solution (1– 100 mM) in a medium of ACN/H2O (70/30 v/v). After six hours of contact, the solution was centrifuged and the TC concentration was determined by fluorimetry at a wavelength of 370 nm. The time to equilibrium (6 h) was based on the adsorption kinetics of TC. 2.6. Analysis of influence of the medium polarity on TC adsorption on the MIP TC adsorption on the MIP was compared among media with different polarities, placing 7.5 mg L1 TC with 0.1 g MIP in a total volume of 3.5 mL solution with one of four different ACN/H2O ratios (100/0, 70/30, 50/50 and 0/100). Experiments were conducted with contact times of 30 and 120 min. 2.7. Analysis of influence of temperature on TC adsorption on the MIP The influence of temperature on the adsorption process was determined by placing 0.1 g of the MIP in contact with a solution of 7.5 mg L1 TC in a total volume of 3.5 mL ACN/H2O solution (70/30 v/v) at temperatures of 15 °C, 25 °C and 35 °C. The contact time was six hours. 2.8. Analysis of influence of solution pH on TC adsorption on the MIP

2.3. Textural and chemical characterization of the MIP and NIP Materials were morphologically analyzed by transmission electron microscopy (TEM), using a LIBRA 120 PLUS SMT transmission electron microscope (Carl Zeiss tec.), and by atomic force microscopy (AFM), using a Nanoscope IIIa Multimode apparatus (Veeco Instruments). Polymers were texturally characterized by determining the adsorption isotherms of the gases (N2 at 77 K, and CO2 at 273 K) using AUTOSORB-1C volumetric equipment (Quantachrome Instruments). N2 and CO2 isotherms were obtained by introducing 0.1 g of each polymer into a glass tube and degasifying for eight hours in a dynamic vacuum of 106 mbar. The N2 and CO2

The influence of solution pH on the adsorption process was determined by placing 0.1 g MIP in contact with a solution of 7.5 mg L1 TC in a total volume of 3.5 mL solution of ACN/H2O (70/30 v/v) at different pH values (1, 6, 9 and 12). The contact time was six hours. 2.9. Analysis of influence of ionic strength on TC adsorption on the MIP The influence of ionic strength on the adsorption process was determined by placing 0.1 g MIP in contact with a solution of 7.5 mg L1 TC in a total volume of 3.5 mL solution of ACN/H2O (70/30v/v) and conducting experiments with different NaCl

Table 1 Dose of reagents used in polymer synthesis. Polymer

TC (mg)

XACN

XMeOH

AIBN (mg)

VACN (mL)

VMeOH (mL)

VTOTAL (mL)

MAA (lL)

EDMA (mL)

MIP NIP

45 0

0.67 0.67

0.33 0.33

10 10

11.5 11.5

4.5 4.5

16 16

60 60

0.54 0.54

X = mole ratio ACN:MeOH.

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concentrations (0.1 M, 0.01 M and 0.001 M). The contact time was six hours.

TC over a given concentration range. We used 30 mg of material in 3.5 mL solution at different TC concentrations (1–100 mg L1) in ACN/H2O (70/30 v/v). TC concentration was determined after six hours of contact, calculating the amount of adsorbed TC by subtracting the free concentration after equilibrium from the total. The experimental binding data were modeled with the Freundlich isotherm (FI) equation (Eq. A.1). The FI model was recently found to be broadly applicable to noncovalent MIPs [37].

2.10. Analysis of the binding properties of the materials The binding properties of the MIP and NIP were determined by analyzing the TC adsorption isotherms, which indicate the relationship between the equilibrium concentration of bound and free

Table 2 Textural and chemical characteristics of the polymers. Polymer

SBET (m2/g)

W0(N2) (cm3/g)

W0(CO2) (cm3/g)

L0(N2) (nm)

L0(CO2) (nm)

W0(N2)/W0(CO2)

IEP (mV)

pHPZC

Oxygen (%)

Carbon (%)

MIP NIP

41 7

1.37 2.92

0.71 0.59

1.99 2.24

0.79 0.78

2.60 4.95

39.7 39.5

3.1 3.1

24.62 24.23

75.38 75.77

SBET = Surface area determined by N2 adsorption isotherms at 77 K. W0(N2) = Micropore volume determined by adsorption isotherms of N2 at 273 K. W0(CO2) = Ultramicropore volume determined by CO2 adsorption isotherms at 273 K. L0(N2) = Mean micropore diameter. L0(CO2) = Mean ultramicropore diameter. IEP = Isoelectric point for pH = 6, NaCl 0.02 M. pHPZC = pH of point of zero charge.

Fig. 1. Images of polymers (MIP and NIP) by TEM.

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Binding sites (Nt) and the global mean affinity constant (K0) is calculated by Langmuir–Freundlich adsorption isotherm model in homogeneous systems [38–41] (Eqs. A.2 and A.3). Shimizu et al. [38,42,43] proposed an analytical expression to calculate the affinity distribution for MIPs with a better fit to a Freundlich isotherm (Eq. A.4). It is also possible to calculate the number of binding sites (Nkminkmax) (Eq. A.5) and the weighted average affinity (Kkminkmax) (Eq. A.6), where a and m are equivalent to Freundlich parameters [42].

3. Results and discussion 3.1. Chemical and textural characterization of the synthesized polymers Table 2 shows the results of the textural and chemical characterization of the MIP and NIP. No major differences were observed in their textural properties, with both having a very low SBET value (<50 m2/g) if we compare it with other materials such as activated carbons. The value was higher for the MIP (41 m2 g1) than for the NIP (7 m2 g1) due to the larger ultramicropore volume, W0(CO2), of the MIP and the smaller size of the pores accessible to N2. The W0(N2)/W0(CO2) ratio was higher for the NIP than for the MIP, indicating that the microporosity of the NIP is more heterogeneous [44,45]. Figs. 1 and 2 depict the morphological analysis of the two polymers by TEM and AFM. The micrographs obtained are similar for both materials, confirming the absence of major differences in their textural properties. With regard to their surface chemical characterization, XPS results showed that the materials have the same surface carbon (C ffi75%) and oxygen (O ffi25%) percentages and the same pHPZC (3.1), while the surface charge for pH = 6 is close to 39 mV. Hence, no appreciable differences can be detected between these

Fig. 3. Adsorption isotherms of TC on the MIP (solid line) and NIP (dashed line).

materials using any of the techniques commonly used for the chemical characterization of solids.

3.2. Adsorption of TC on the MIP and NIP Fig. 3 depicts the TC adsorption isotherms on the MIP and NIP, showing the adsorption capacity to be much higher for the MIP, regardless of the equilibrium concentration considered. This demonstrates the presence of specific adsorbent–adsorbate interactions attributable to the porosity of the MIP. The results in Fig. 3 yielded the values of the parameters in the Freundlich and Shimizu equations (Table 3). The value of ‘‘m’’ was close to 1 for the MIP, indicating the low heterogeneity of its porosity, but was considerably above unity for the NIP, verifying the much more heterogeneous porosity of this material.

Fig. 2. Images of polymers (MIP and NIP) by AFM.

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Table 3 Parameters obtained for the Freundlich, Shimizu, and Langmuir–Freundlich models in medium of ACN/H2O (70/30 v/v). Polymer

m

a (L g1)

Nkminkmax (lmol g1)

Kkminkmax  105 (g1)

Nt (lmol g1)

K0  104 (g1)

R2

MIP NIP

0.954 1.663

3378.41 843.65

0.020 –

262.797 5.555

8.772 0.175

2.902 8.018

0.999 0.934

m, heterogeneity index according to the Freundlich model; a, equilibrium constant or adsorption capacity according to the Freundlich model; Nkminkmax, binding sites in the affinity distribution area according to the Shimizu model; Kkminkmax, affinity in the affinity distribution area according to the Shimizu model; Nt, total number of binding sites according to the Langmuir–Freundlich model; K0, mean affinity index according to the Langmuir–Freundlich model.

As expected, the value of m was lower for the MIP than for the NIP, indicating a higher percentage of high-affinity binding sites (Table 3). We highlight the values obtained for the Shimizu model parameters, showing that the Nkminkmax was higher in the MIP than in the NIP and that the Kkminkmax value was more than 47fold higher in the MIP. Hence, the number of sites with appropriate geometry and functionality to adsorb TC is much higher in the MIP, indicating the success of the imprinting process. The affinity distributions of the MIP, based on the Freundlich model and calculated by Eq. (4), were plotted as N(K) vs. Log(K). The affinity distribution in N(K) vs. log(K) format was an exponentially decreasing function for the TC interactions (Fig. 4). The exponentially tailing part corresponds to the lower concentration portion of the isotherm, where high-affinity binding sites are preferentially occupied. In this region, the polymer is at low loadings, far from saturation. For the majority of non-covalent MIPs, this is typically the subset of sites that is most frequently measured and utilized in applications [46–48]. This is because it is very difficult to reach saturation in most non-covalently imprinted polymers due to their heterogeneity. The affinity distribution function tends towards zero for the highest affinity constant and tends towards infinity for the lowest affinity constant. The MIP had a greater TC retention capacity, as observed in the adsorption isotherms for both adsorbents in Fig. 3. The difference in adsorption is attributable to specific interactions attributable to the ‘‘TC footprint’’ on the MIP. In contrast to adsorption on the NIP, the isotherm for the MIP reaches a plateau at very low TC concentrations due to its textural homogeneity. At low initial adsorbate concentrations, TC adsorption is exclusively due to the specific TC molecule-shaped porosity of the MIP. However, as saturation of the polymers approaches, non-specific adsorbate– adsorbent interactions become more important, and these are the only interactions operating in the case of the NIP. These findings are in agreement with the occupation of the high-affinity binding sites of the polymer with TC at low adsorbate concentrations.

Fig. 4. Affinity distribution of the MIP based on the Freundlich model. ACN:H2O (70:30); pH = 6.5.

3.3. Influence of operational variables on TC adsorption on the MIP The main interactions involved in TC adsorption on the MIP were explored by studying the influence of operational variables on the adsorption process: (i) polarity of the solvent, (ii) solution pH, (iii) temperature, and (iv) presence of electrolytes (NaCl).

3.3.1. Influence of solvent polarity Fig. 5 depicts the TC percentages adsorbed on the MIP after contact times of 30 min and 2 h in media with different proportions of ACN:H2O. It can be observed that the TC adsorption was very rapid on the MIP, because a similar amount was adsorbed during both time periods. TC adsorption was most favored in a medium of 100% ACN or 100% H2O, with the latter obtaining the highest percentage removal values. In the experiments in mixtures of the two solvents, adsorption on the MIP even reached null values.

Fig. 5. TC adsorption on the MIP as a function of medium composition after 30 min (light gray) or 120 min (dark gray) of adsorption time [TC]0 = 7.5 mg L1. pH = 6.5.

Table 4 shows the results obtained after applying the Freundlich and Shimizu models to the TC adsorption isotherms on the MIP as a function of the ACN:H2O ratio of the medium (Fig. 6). According to the results in Table 4, the MIP has a high selectivity for TC, involving both specific adsorption (due to the active adsorption centers in pores [‘‘footprints’’]) and non-specific adsorption, due to electrostatic attraction of covalent bounding between the MIP and TC. The results in Fig. 6 and Table 4 indicate that the specific interactions have a greater effect on the adsorption, given that they are highly influenced by the medium polarity.

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M. Sánchez-Polo et al. / Microporous and Mesoporous Materials 203 (2015) 32–40 Table 4 Parameters derived from applying the Freundlich and Shimizu models to the TC adsorption isotherms on the MIP as a function of the ACN:H2O ratio of the medium. ACN:H2O

m

a (L g1)

Nkminkmax (lmol g1)

Kkminkmax  105 (g1)

Nt (lmol g1)

K0  104 (g1)

R2

100:0 70:30 50:50 0:100

0.787 0.954 1.134 0.767

810.24 3378.41 1136.38 6676.63

1.153 0.020 9.946 0.151

100.755 262.797 6.755 122.665

2.010 8.772 0.224 17.405

16.280 2.902 8.120 1.076

0.878 0.999 0.995 0.979

TC adsorption on the MIP was virtually null when the ACN/H2O ratio was 50:50, because TC solubility is very high in this situation, and the TC molecules are strongly solvated in solution. The mean affinity index (k0) and total number of binding sites (Nt) were obtained (Table 4). The higher Nt value observed in 100% H2O may be due to the greater affinity of TC for the polymer than for the water. [49,50].

Fig. 6. Influence of medium polarity on TC adsorption isotherms on the MIP. ACN:H2O: N, 100:0; d, 70:30; j, 50:50; ⁄, 0:100; , Langmuir model. [TC]0 = 7.5 mg L1. pH = 6.5.

3.3.2. Effect of solution pH The medium pH is a significant factor in adsorption because of its influence on the properties of the MIP surface and on the speciation of the target compound. Skudar et al. [51] found that the discriminating ability of the polymer was lost when the pH of water exceeded the apparent pKa of the polymer. Another study showed that an electrostatic retention model could account for the retention of target compounds when the pH ranged between 2 and 9 [52]. Furthermore, an increase in the adsorption of basic compounds to methacrylic acid-type polymers has been reported with higher pH [53] and an increased adsorption of acidic compounds to vinylic-type polymers with lower pH [54]. It is therefore necessary

Fig. 7. (A) Chemical structure of tetracyclines; (B) speciation diagram of TC as a function of solution pH.

Fig. 8. (A) Adsorption isotherms of TC on the MIP as a function of pH; (B) Shimizu model for different pH values. j, pH 1; N, pH 6; d, pH 9; x, pH 12. 30 mg of MIP, [TC]0 = 1–100 mg L1, ACN/H2O = 70/30 v/v, six hours of contact time.

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Table 5 Parameters derived from applying the Freundlich and Shimizu models to TC adsorption isotherms on the MIP as a function of the solution pH. pH

m

a (L g1)

Nkminkmax (lmol g1)

Kkminkmax  104 (g1)

Nt (lmol g1)

K0  105 (g1)

R2

1 6 9 12

0.649 0.767 0.795 –

7950.609 6676.625 13081.526 –

0.485 0.151 0.135 –

2.969 12.267 2.102 –

3.805 17.405 6.147 –

6.356 1.076 4.845 –

0.995 0.979 0.997 –

Fig. 9. TC adsorption isotherms on the MIP as a function of solution temperature at pH = 6.5. Temperature: d, 15 °C; h, 25 °C; ⁄, 35 °C. 30 mg of MIP, [TC]0 = 1–100 mg L1, ACN/H2O = 70/30 v/v, six hours of contact time.

to study the effect of the solution pH on each adsorption system. Fig. 7 depicts the speciation diagram of TC as a function of pH, which was required for investigation of the present system. It is of interest to analyze TC affinity parameters as a function of the medium pH, which affects the ionic form of the TC molecule (Fig. 7B). Fig. 8 depicts the TC adsorption isotherms on the MIP (A) and the Shimizu model results for pH values of 1, 6, 9, and 12 (B). Fig. 8 shows that the adsorption of TC on the MIP was virtually null at a pH of 12, explained by the ionized form of the TC molecules at this pH, generating repulsive electrostatic interactions between the negatively-charged TC molecules and the negatively charged MIP surface (pHPZC = 3.1). In the range of pH values between 1 and 9, the lowest adsorption takes place at pH 1, when the MIP is positively charged and therefore repels the positivelycharged TC molecules (Fig. 7B) from its active sites. The highest TC adsorption was observed at pH = 6, due to electrostatic attraction between the TC molecules and the negatively-charged MIP (Table 5 and Fig. 8). This type of interaction could also be expected with the NIP, which has the same pHPZC value as that of the MIP. However, the results of this study indicate that TC adsorption on the MIP is favored not only by electrostatic interactions but also by the porosity of the material, characterized by specific active adsorption centers (TC footprints). Fig. 8B depicts the distribution of the affinity of the MIP with TC, which corresponds to a decreasing exponential function, confirming the specificity of TC adsorption regardless of the solution pH.

Fig. 10. Adsorption isotherms of TC on the MIP as a function of the ionic strength of the medium. [NaCl]: d, 0.1 M; h, 0.01 M; ⁄, 0.001 M. 30 mg of MIP, [TC]0 = 1–100 mg L1, ACN/H2O = 70/30 v/v, six hours of contact time.

The exponentially tailing portion of the exponential curve corresponds to lower TC concentrations and therefore represents the high-affinity binding sites. In this portion, adsorption on the MIP has a higher affinity at medium pH values 6 6; however, the affinity of the MIP for TC decreases with higher pH value until it becomes null at pH = 12. According to the results presented in Section 3.3.1 and this section, some experiments were carried out to check the reusability of the MIP. Thus, exhausted TC-MIP was added to 50 mL solution at pH = 12, detecting that 80% of the adsorption capacity was recovered. 3.3.3. Influence of temperature Fig. 9 depicts the results obtained for the influence of solution temperature on TC adsorption on the MIP. At lower TC concentrations, adsorption was favored at lower solution temperature; nevertheless, adsorption was favored at higher temperature for higher TC concentrations. Results obtained with the Langmuir–Freundlich model confirm a larger number of total binding sites (Nt) at lower solution temperatures (Table 6), explained by the reduction in kinetic energy at lower temperatures and the consequent predominance of adsorption by specific or high-affinity interactions, which characterizes the adsorption process at low initial TC concentrations. At higher solution temperatures, the increased kinetic energy of the molecules means that non-specific interactions are predominant in the adsorption process.

Table 6 Parameters obtained by applying the Freundlich and Shimizu models to the adsorption isotherms of TC on the MIP as a function of solution temperature. Temperature (°C)

m

a (L g1)

Nkminkmax (lmol g1 )

Kkminkmax  104 (g1)

Nt (lmol g1)

K0 106 (g1)

R2

15 25 35

0.617 1.089 1.150

36885.615 7627.195 4437.914

0.035 0.070 0.893

7.933 10.004 1.445

8.688 4.157 2.553

4.889 144.89 246.22

0.806 0.957 0.951

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M. Sánchez-Polo et al. / Microporous and Mesoporous Materials 203 (2015) 32–40 Table 7 Parameters derived from applying the Freundlich and Shimizu models to adsorption isotherms of TC on the MIP as a function of the ionic strength of the medium. NaCl (M)

m

a (L g1)

Nkminkmax (lmol g1)

Kkminkmax  104 (g1)

Nt (lmol g1)

K0  105 (g1)

R2

0.1 0.01 0.001

0.588 1.486 1.564

21028.2 4010.73 5696.26

0.179 0.977 4.765

3.116 8.062 1.849

10.087 2.639 3.884

0.978 30.133 24.578

0.99 0.99 0.98

The above results were corroborated by the enthalpy values obtained for TC adsorption on the MIP, calculated as a function of the initial adsorbate concentration [55]; they were 1.073  102 J, 3.615  104 J, 3.192  104 J and 4.623  104 J for initial TC concentrations of 1, 3, 7 and 10 mg L1, respectively. Hence, the process is more exothermal at lower TC initial concentrations, because the absolute value of DH is higher and the adsorbate–adsorbent bond is therefore stronger. We can affirm that the binding strength is greater when specific interactions are predominant, i.e., at low initial TC concentrations. 3.3.4. Influence of the ionic strength of the medium The ionic strength of the solution can affect, to a greater or lesser extent, the adsorption process in aqueous phase. Thus, according to Radovic et al. [56], the presence of solution electrolytes can modify the strength of adsorbate–adsorbent electrostatic interactions. These interactions, either attractive or repulsive, can increase or decrease, by varying the ionic strength of the solution. Fig. 10 and Table 7 show the results of TC adsorption on the MIP in the presence of different NaCl concentrations (0.1 M, 0.01 M, 0.001 M). The adsorption isotherms (Fig. 10) reveal that an increase in the ionic strength of the medium produces a decrease in the maximum TC adsorption on the MIP, which is reached at high TC concentrations, indicating attractive electrostatic interactions between the TC molecules and the MIP. Table 7 exhibits that the mean affinity index value is considerably lower with greater ionic strength of the solution, which may be due to a screening effect of the polymer surface caused by the added salt, given the localization of the electrolytes between the MIP surface and TC molecules. However, the total number of binding sites of the TC molecules on the MIP is much higher at increased salt concentrations, which may be attributable to the lower TC solubility in these conditions. Therefore, the general attraction of the adsorbate to the polymer is stronger, increasing the number of total binding sites, as evidenced by the greater adsorption at low TC concentrations when 0.1 M NaCl was present in the solution (Fig. 10). 4. Conclusions The MIP for TC and its corresponding NIP both have small surface areas. The porosity of the MIP is more homogeneous than that of the NIP. Both polymers do not differ in carbon or oxygen percentages, pHPZC or surface charge at pH 6. The TC adsorption isotherms on the MIP and NIP show that the adsorption capacity of the MIP is markedly higher, due to the specificity of its porosity, characterized by the TC adsorption memory. Adsorption isotherm analysis with the Freundlich, Freundlich– Langmuir and Shimizu models corroborated the homogeneity of the MIP porosity and the larger number of pores with appropriate functionality and geometry in comparison to the NIP. Additionally, the high-affinity binding sites for TC of the MIP are occupied at low TC concentrations. The polarity of the medium affects the capacity of the MIP for TC adsorption, indicating that specific interactions with active centers in the MIP ‘‘footprints’’ have the greatest impact on the adsorption, given that they are highly influenced by the solution

polarity. In other words, the MIP footprints demonstrate a memory effect for TC. The pH of the medium has a major influence on the adsorption by determining the ionic species of TC and the surface charge of the MIP, indicating that electrostatic interactions play an important role in the adsorption of TC on MIPs. At low TC concentrations, the adsorption is favored at lower solution temperatures due to the consequent reduction in kinetic energy, producing a predominance of adsorption by specific or high-affinity interactions. At higher TC initial concentrations, however, the adsorbent–adsorbate bond is favored at higher solution temperatures due to the increased kinetic energy of the molecules, leading to a predominance of adsorption through non-specific interactions. An increase in the ionic strength of the medium produces a reduction in the maximum TC adsorption capacity of the MIP. This indicates the presence of attractive interactions between TC molecules and the MIP, which are reduced by a higher concentration of added salt due to a screening effect between the MIP surface and the TC molecules. Acknowledgements The authors are grateful for the financial support provided by the Junta de Andalucía (RNM7522) and the Spanish Ministry of Science and Innovation (CTQ2011-29035-C02-02). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso. 2014.10.022. References [1] H. Thomas, Toxicol. Lett. 131 (2002) 5–17. [2] S. Mompelat, B. Le Bot, O. Thomas, Environ. Int. 35 (2009) 803–814. [3] L.-J. Zhou, G.-G. Ying, J.-L. Zhao, J.-F. Yang, L. Wang, B. Yang, S. Liu, Environ. Pollut. 159 (2010) 1877–1885. [4] P. Gao, D. Mao, Y. Luo, L. Wang, B. Xu, L. Xu, Water Res. 46 (2012) 2355–2364. [5] K. Kümmerer, J. Antimicrob. Chemother. 54 (2004) 311–320. [6] T. Schwartz, W. Kohnen, B. Jansen, U. Obst, FEMS Microbiol. Ecol. 43 (2003) 325–335. [7] S.R. Kim, L. Nonaka, S. Suzuki, FEMS Microbiol. Lett. 237 (2004) 147–156. [8] K. Kümmerer, J. Antimicrob. Chemother. 52 (2003) 5–7. [9] A.L. Batt, S. Kim, D.S. Aga, Chemosphere 68 (2007) 428–435. [10] T.A. Ternes, M. Meisenheimer, D. McDowell, F. Sacher, H.J. Brauch, B. HaistGulde, G. Preuss, U. Wilme, N. Zulei-Seibert, Environ. Sci. Technol. 36 (2002) 3855–3863. [11] A. Gulkowska, H.W. Leung, M.K. So, S. Taniyasu, N. Yamashita, L.W.Y. Yeung, B.J. Richardson, A.P. Lei, J.P. Giesy, P.K.S. Lam, Water Res. 42 (2008) 395–403. [12] P. Sukul, M. Spiteller, Rev. Environ. Contam. Toxicol. (2007) 131–162. [13] K. Klaus, Chemosphere 75 (2009) 417–434. [14] K. Klaus, Chemosphere 75 (2009) 435–441. [15] A.K. Sarmah, M.T. Meyer, A.B.A. Boxall, Chemosphere 65 (2006) 725–759. [16] S. Webb, T. Ternes, M. Gibert, K. Olejniczak, Toxicol. Lett. 142 (2003) 157–167. [17] B. Halling-Sørensen, S. Nors, Chemosphere 36 (1998) 357–393. [18] P. Gao, M. Munir, I. Xagoraraki, Sci. Total Environ. 421–422 (2012) 173–183. [19] B. Li, T. Zhang, Chemosphere 83 (2011) 1284–1289. [20] T.A. Ternes, J. Stüber, N. Herrmann, D. McDowell, A. Ried, M. Kampmann, B. Teiser, Water Res. 37 (2003) 1976–1982. [21] T. Fukuhara, S. Iwasaki, M. Kawashima, O. Shinohara, I. Abe, Water Res. 40 (2006) 241–248.

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