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Novel Silica Sorbents Surface-Functionalized with a Salicylhydroxamic Acid-Based Ion-Imprinting Polymer for the Selective Removal of Pb(II) from Aqueous Solution Ruixin Wang,* Xiaohui Shi, Hongjing Wang, and Caiping Lei School of Chemical and Environmental Engineering, North University of China, Taiyuan 030051, P. R. China S Supporting Information *

ABSTRACT: A new Pb(II)-imprinted silica sorbent IIPSHA/SiO2 has been successfully prepared using the surface imprinting technique. The bidentate and rigid salicylhydroxamic acid was used as the chelating ligand, and a nucleophilic ring-opening reaction of glycidyl with phenolic hydroxyl group was employed as the cross-linking chemistry. The resulting ion-imprinted silica shows enhanced adsorption and more importantly remarkable selectivity toward Pb(II) and excellent reusability in both batch and dynamic adsorption experiments, making it an appealing material for Pb(II) removal from aqueous medium. The spontaneous adsorption process was found to be entropy driven and endothermic. The positive entropy change associated with the adsorption process is unusually large, indicating that the binding pockets after template ion removal are rather rigid which is believed to be responsible for the enhanced selectivity.

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INTRODUCTION Heavy metals, being nonbiodegradable, once released into the environment can remain present and accumulate and thus pose significant health risk to human beings.1,2 Lead (Pb) is one of the most toxic heavy metals found in water and food. Lead poisoning even at very low concentrations can cause serious health problems such as damage to the liver, kidney, brain, and the nervous system.3 It is thus highly desirable that separation techniques which can efficiently remove Pb(II) from industrial effluents can be developed. Compared to other physicochemical methods such as chemical precipitation, electrochemical treatment, membrane filtration, ion exchange, coagulationflocculation, and so on, adsorption is the most popular method for heavy metal removal because of its simplicity, reusability, high efficiency, low cost, easy handling, and the availability of a variety of solid adsorbents (for example, silica gels, zeolites, hydroxyapatite, as well as natural and synthetic polymers).4−6 In recent years, solid adsorbents containing chelating agents, such as polymer resins or inorganic particle−polymer matrix with chelating groups (e.g., Schiff base, amidoxime, 8hydroxyquinoline, and hydroxide acid) have drawn increasing attention due to their high adsorption capacities and excellent mechanical properties.7−9 These adsorbents, however, often lack selectivity and thus cannot be used to target specific toxic heavy metal ions. In this regard, ion imprinted polymers (IIPs) are more appealing.10,11 Similar to molecular imprinted polymers (MIPs), IIPs are a class of well-tailored functional materials, in which specific cavities that match the target ion (namely the template ion) in shape, size, and functional groups are distributed, rendering high binding affinity and specificity to the target ion.12−14 In view of the deep embedding of the binding sites by matrix © XXXX American Chemical Society

formation in bulk during the preparation procedures of IIPs, which makes those sites poorly accessible to target ions, surface imprinting polymerization has been developed. Indeed a number of surface-imprinted IIPs as selective solid sorbents for the removal of Pb(II) ions have recently been reported.15−19 Most of those IIPs are based on amine ligands. While the dative bond between amine N and Pb(II) is strong, organic amines often lack chemical stability as they are prone to oxidation. It is therefore worthwhile exploring IIPs containing more chemically stable multidentate ligands. Hydroxamic acids are well-known metal ion chelating agents. As hard bases, hydroxamic acids can bind to almost all hard acids and borderline metal ions to form stable five-membered ring complexes.20,21 In this contribution, a new Pb(II)imprinted silica sorbent containing hydroxamic acids as the ion-chelating ligands was successfully prepared using the surface imprinting technique developed in our laboratory.22,23 The resulting IIPs showed attractive binding affinity and specificity to Pb(II), indicating its promising potential as new adsorbents for the selective Pb(II) removal from aqueous solution.

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. Silica gel was obtained from Ocean Chemical Co. Ltd. ((120−160) mesh, about 125 μm in diameter, pore size: 6 nm, pore volume: 1.0 mL·g−1, surface area: 350 m2·g−1. Qingdao, China). Silica surface-grafted Received: December 22, 2014 Accepted: March 4, 2015

A

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with poly(2-hydroxyethyl methacrylate) (PHEMA) (PHEMA/ SiO2) was prepared following literature procedures.24 Salicylhydroxamic acid (SHA) was obtained from Tianfeng Fine Chemical Co. (Henan, China). Ethylene glycol diglycidyl ether (EGDE) was purchased from Wuxi Wanrong Material Co., Ltd. (Jiangsu, China, CR grade). Anhydrous tin tetrachloride (SnCl4) was obtained from Yuanli Chemical Ltd. (Tianjin, China). 1,4-Dichloromethoxybutane (BCMB) was synthesized according to previously reported procedures.25 All the other reagents used were of analytical grade and purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Fourier transform infrared (FT-IR) spectra were recorded using a PerkinElmer1700 infrared spectrometer (PerkinElmer Company, American) in KBr matrix. The adsorption experiments were performed using a SHA-C water-bathing constant temperature vibrator (Eltong Electronics Ltd. of Jiangsu, China). A PHS-3C pH meter (Precision Scientific Apparatus Inc. of Shanghai, China) was used for the pH adjustment at ambient temperature. An STA449 Thermo Gravimetric Analyzer (TGA, Netzsch Company, Germany) was used for the thermal stability analysis. 2.2. Preparation and Characterization of IIP Silica Sorbent. First, 5-chloromethyl-salicylhydroxamic acid (CMSHA) was obtained using BCMB as the chloromethylating reagent. Then using the pending hydroxyl groups on PHEMA, SHA was attached to PHEMA/SiO2 by a nucleophilic substitution reaction of CMSHA with PHEMA, resulting in the chelating sorbent SHA-PHEMA/SiO2. The chemical structure of SHA-PHEMA/SiO2 (see Scheme S1, Supporting Information (SI)) was characterized by IR spectrum. Finally, The Pb(II) ion imprinted silica sorbent IIP-SHA/SiO2 was prepared as follows. A total of 0.1 g of SHA-PHEMA/SiO2 particles was added into a 50 mL Pb(NO3)2 solution with a Pb(II) concentration of 1000 mg·L−1 under pH 6. Once the SHA-Pb(II) binding reaches equilibrium, the Pb(II) chelated SHA-PHEMA/SiO2 particles were filtrated, washed repeatedly with water, and dried under vacuum. Subsequently, 1 g of the above Pb(II) chelated SHA-PHEMA/SiO2 was added into a mixed solvent of ethanol and water (V:V = 1:1), followed by the addition of 0.09 mL of cross-linker EGDE. The mixture was stirred at 318 K for 10 h to ensure the completion of the cross-linking reaction. The resulting functionalized silica particles were collected by filtration and then washed with 0.1 M hydrochloric acid solution to remove the template ion (Pb(II)) until Pb(II) could not be detected by EDTA complexometric titration. The final ion imprinted silica IIP-SHA/SiO2 was collected and dried under vacuum. The chemical structure of IIP-SHA/SiO2 was characterized by IR spectrum. Its thermal stabilization was tested by TGA with a heating rate of 10 °C·min−1 in air. 2.3. Static Adsorption Experiments of IIP-SHA/SiO2 toward Pb(II). 2.3.1. Kinetic Adsorption Experiments. To a set of conical flasks was each added 30 mL (V, L) of a Pb(II) solution with an initial Pb(II) concentration of 600 mg·kg−1 (C0, mg·kg−1). The pH value of all solutions was adjusted to 5. To each of those solutions was then added 30 mg (m, g) of IIPSHA/SiO2 at 293 K. The resulting mixtures were stirred for different amounts of time. The concentrations (Ct, mg·L−1) of Pb(II) in the supernatants at different times (t) were determined by EDTA complexometric titration. The corresponding adsorption amounts (Q, mg·g−1) were calculated according to eq 1, and the adsorption kinetic curve (Q versus t) was plotted.

Q=

V (C 0 − C t ) m

(1)

2.3.2. Isothermal Adsorption Experiments. Multiple sets of conical flasks, each containing 30 mL of a Pb(II) solution with a different initial concentration (C0 = 200, 300, 400, 500, 600, 800, and 1000 mg·kg−1) were prepared. Each set of solutions was adjusted to a specific pH (from 2.5 to 6.5). To each of those solutions was then added 0.03 g of IIP-SHA/SiO2 (or SHA-PHEMA/SiO2). The resulting solutions were shaken in a shaker at different temperatures ((283 to 313) K) for 2 h. After the adsorption reached equilibrium, the equilibrium Pb(II) concentration (Ce, mg·kg−1) in the supernatant was determined by EDTA complexometric titration, and the equilibrium adsorption capacity (Qe, mg·g−1) was calculated according to eq 2. The adsorption isotherm was then plotted.

Qe =

V (C0 − Ce) m

(2)

where V (L) is the volume of the metal ion solutions and m (g) is the weight of the IIP-SHA/SiO2. 2.3.3. Selectivity Experiments. To gauge the selectivity of IIP-SHA/SiO2 for Pb(II) with respect to Cu(II) and Cd(II), 0.03 g of IIP-SHA/SiO2 (or SHA-PHEMA/SiO2) was added into binary metal ion aqueous solution of Pb(II)/Cu(II) and Pb(II)/Cd(II) containing 600 mg·kg−1 for each metal ion at pH 6, respectively. After adsorption reached equilibrium, the concentrations of metal ions in supernatants were determined by EDTA under the masking agent H2SO4 respectively. The adsorption capacity of metal ions (Q, mg·g−1) can be calculated from equilibrium data according to eq 1. 2.4. Reusability Experiments. To evaluate whether the Pb-adsorbed sorbent can be recycled and reused, desorption experiments of Pb(II)-adsorbed IIP-SHA/SiO2 were performed using 0.1 mol·kg−1 hydrochloric acid solution at ambient temperature for 5 h. It was found that the Pb(II) ion could be completely desorbed from IIP-SHA/SiO2 using 0.1 mol·kg−1 hydrochloric acid solution as the stripping agent. The adsorption−desorption experiments for Pb(II) ions were repeated 9 times consecutively using the same IIP-SHA/SiO2 to confirm its reusability. 2.5. Dynamic Adsorption and Elution Experiments. A certain amount of IIP-SHA/SiO2 particles were packed in a glass column with an inside diameter of 8 mm and 2 mL of bed volume (BV). At 293 K, a Pb(II) or a Cu(II) solution with a concentration of 100 mg·kg−1 and a pH of 5.5 was allowed to flow through the column at a rate of four BVs per hour (4 BV· h−1) in a countercurrent manner. The effluents with two bed volume interval were collected and the concentrations of Pb(II) or Cu(II) ions were determined by EDTA complexometric titration. Using the above column filled with IIP-SHA/SiO2 particles saturated with adsorbed Pb(II) ions, elution experiment was carried out using 0.1 mol·kg−1 hydrochloric acid solution as the eluent with a flow rate of 1 BV·h−1. The eluent with one BV interval was collected, and the concentration of Pb(II) ion was determined.

3. RESULTS AND DISCUSSION 3.1. Preparation and Structure Characterization of Surface Ion-Imprinted Silica IIP-SHA/SiO2. The Pbchelated SHA-PHEMA/SiO2 was cross-linked with EGDE after adsorping Pb(II) where each Pb(II) chelates with two B

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Scheme 1. Preparation Procedures of Pb(II)-Imprinted Sorbent IIP-SHA/SiO2

substituted phenyl rings. Careful comparison of the two spectra does show some difference. In particular, the clear absorption bands at (1398 and 668) cm−1 observed in the IR spectrum of SHA-PHEMA/SiO2 were either significantly weakened or completely missing in the spectrum of IIP-SHA/SiO2. These two bands correspond to the in-plane and out-of-plane bending vibrations, respectively, of the phenolic hydroxyl group in SHA. The disappearance or significant decrease in intensity of these bands after ion-imprinting indicates that the phenolic hydroxyl groups have nearly completely reacted with EGDE. The thermal stability of IIP-SHA/SiO2 was studied by thermogravimetric analysis (TGA), and the result is shown in Figure 2. At a heating rate of 10 °C·min−1 in air, a slight weight loss (∼ 3 %) was observed below 100 °C, which can be attributed to the loss of adsorbed water. Sharp and significant weight loss occurred in the temperature range of (245−400)

SHA units. The cross-linking reaction occurs between the phenolic hydroxyl group of SHA and the glycidyl groups of EGDE. Subsequent removal of Pb(II) ions from the crosslinked silica gave the resulting ion-imprinted silica (IIP-SHA/ SiO2) with cavities fitting the template ion in size, shape, and chelating environment. The stepwise preparation process from SHA-PHEMA/SiO2 to IIP-SHA/SiO2 was schematically shown in Scheme 1. The FTIR spectra of IIP-SHA/SiO2 and SHA-PHEMA/SiO2 are shown in Figure 1. Both spectra show clear absorption bands at (1732 and 1635) cm−1 which were attributed to the ester carbonyl (in HEMA) and N-hydroxyl carbonyl (in SHA) groups, respectively. Both spectra also show bands at (1559 and 1535) cm−1. These bands are characteristic absorption bands of

Figure 1. IR spectra of chelating sorbent SHA-PHEMA/SiO2 and Pb(II)-imprinted sorbent IIP-SHA/SiO2.

Figure 2. TGA thermogram of IIP-SHA/SiO2. C

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°C, which was due to the decomposition of imprinted polymers. Beyond 400 °C, there was only slow and small weight loss. These results indicate that IIP-SHA/SiO2 has thermal stability up to 245 °C. 3.2. Adsorption Kinetics. The adsorption amount (Q) versus time for IIP-SHA/SiO2 at pH 5 is shown in Figure 3. It

Figure 4. Influence of pH values on the adsorption capacity of IIPSHA/SiO2 toward Pb(II) (initial Pb(II) concentraion: C0 = 600 mg· kg−1; sorption temperature = 293 K; sorption time = 2 h).

which the IIP-SHA/SiO2 exhibits the highest adsorption capacity is around 6. 3.3.2. Adsorption Isotherms. Figure 5 shows the adsorption amount at equilibrium under varied initial ion concentrations for the three ions, and the data are presented in Table 1. The experiments were carried out at 303 K with the pH of all solutions set at 6. The equilibrium adsorption amount at each initial ion concentration was measured after mixing for 2 h which is significantly longer than the time required for adsorption to reach equilibrium. Figure 5 shows clearly that there are significant differences in metal ion adsorption between ion imprinted and nonion-imprinted silica. Specifically, nonion-imprinted silica shows little selectivity while ionimprinted silica exhibits high selectivity toward the template ion (Pb(II)). Without ion imprinting, the maximum adsorption capacity of SHA-PHEMA/SiO2 for Cu(II), Pb(II), and Cd(II) ions was 60.14, 50.58, and 39.58 mg·g−1, respectively. Relative to that of Pb(II), the adsorption amounts of Cu(II) and Cd(II) by SHA-PHEMA/SiO2 differ by only around 20 %, indicating very little ion selectivity. Note that the adsorption amount of Pb(II) onto SHA-PHEMA/SiO2 was between those of Cu(II) and Cd(II). The increased adsorption from Cd(II) to Pb(II) and to Cu(II) reflects the stability order of their metal-SHA chelated complexes. After ion imprinting, the maximum adsorption amount of IIP-SHA/SiO2 for Cu(II) and Cd(II) ions dropped significantly from (60.14 to 15.04) and (39.58 to 11.82) mg·g−1, respectively. The significant drop in adsorption indicates that the binding pockets left in the ion-imprinted silica no longer fit the Cu(II) and Cd(II) ions. The maximum adsorption capacity for Pb(II) after ion imprinting, on the other hand, was even moderately increased from (50.58 to 56.2) mg· g‑1. The adsorption capacity of IIP-SHA/SiO2 for Pb(II) is now 3.7 times that of Cu(II) and 4.8 times that of Cd(II), showing remarkably high selectivity toward the template ion. These results indicate that the cross-linking chemistry employed generates ion-imprinted silica with high fidelity. The binding pockets generated fit Pb(II) just right in size, shape, and spatial arrangement. Although the ionic radii of both Cu(II) (72 pm) and Cd(II) (97 pm) were smaller than that of Pb(II) (120 pm), the pockets appear to be too large for them to form stable chelated complexes. It is interesting to note that while the SHACu(II) complexes are significantly more stable than that of SHA-Cd(II) complexes, their adsorption capacity on Pb(II) ion imprinted silica are nearly the same. This is presumably due to the fact that the size of Cd(II) ion is closer to that of Pb(II), making the binding pockets suit Cd(II) more than Cu(II).

Figure 3. Adsorption capacity Q of IIP-SHA/SiO2 for Pb(II) versus mixing time (initial Pb(II) concentration C0 = 600 mg·kg−1, pH 5, Sorption temperature = 293 K).

shows clearly that the amount of Pb(II) adsorbed increases rather fast in the first 30 min and continues to rise in the next 20 min or so but with a much slower rate and eventually reaches equilibrium at around 60 min. So the adsorption rate is quite fast, which is likely due to the strong chelation between SHA and Pb(II) and may also indicate a small diffusion barrier for Pb(II) due to the surface imprinting technique. 3.3. Static Binding Characteristics of IIP-SHA/SiO2 toward Pb(II) Ion. 3.3.1. Effect of pH on the Adsorption of Pb(II) by IIP-SHA/SiO2. SHA is a weak diprotic acid with pKa of ∼ 7.4 (hydroxamic acid proton) and 9.8 (phenolic proton). It is also a base with three protonation sites: carbonyl oxygen, N-hydroxy oxygen, and phenolic oxygen. Thus, SHA can adopt different structural forms depending on the solution pH. In highly acidic conditions, some of the base sites can be protonated. Protonation on any one of those sites effectively removes its binding ability with metal ions, making SHA a monodentate ligand or even a nonligand. From a reaction equilibrium perspective, chelation of SHA with Pb(II) (or other metal ions) releases one proton from each SHA. Increasing proton concentration therefore shifts the equilibrium toward complex dissociation. In other words, SHA-metal binding decreases as the pH is lowered. In more base conditions (pH > 6.5), the hydroxide ion competes effectively for these heavy metal ions, forming metal hydroxide precipitates. Indeed, SHA is generally found to form metal complexes with Pb(II), Cu(II), Fe(III), Cd(II), etc. in the pH range of 3−6.5. Our pH dependent studies revealed similar trends. As shown in Figure 4, the adsorption capacity of IIP-SHA/SiO2 toward Pb(II) increases with increasing solution pH in the pH range of 1−6. As a matter of fact, at pH 1, no detectable adsorption of Pb(II) on IIP-SHA/SiO2 was observed, indicating that the 0.1 M hydrochloric acid solution could be used as the stripping agent to desorb Pb(II) ions from IIP-SHA/SiO2. At pH above 6, the adsorption capacity drops which, as previously mentioned, is due to the hydrolysis of Pb(II) ions (the first order hydrolysis constant of Pb(II) ion is K1 = 6.3·10−8) that forms Pb(OH)2 precipitates. These results indicate that the optimized pH at D

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Figure 5. Adsorption isotherms of SHA-PHEMA/SiO2 (a) and IIP-SHA/SiO2 (b) toward Pb(II), Cu(II), and Cd(II) (sorption temperature = 303 K; sorption time = 2 h; pH 6).

Table 1. Isotherm Data for Different Metals Ce/mg·kg−1 sorbents

metals

nonimprinted

Pb(II)

Cu(II)

Cd(II)

imprinted

Pb(II)

Cu (II)

Cd (II)

average values/mg·kg

Qe/mg·g−1

−1

165.80 260.94 357.43 454.37 552.30 750.50 949.42 160.57 254.62 349.90 446.06 543.46 740.73 939.86 175.42 270.83 367.36 465.01 563.23 761.26 960.42 162.08 256.62 352.74 449.35 547.02 745.06 943.80 190.14 288.65 387.48 486.52 585.86 785.18 984.97 192.66 291.29 390.26 489.56 589.02 788.44 988.18

E

standard errors

average values/mg·kg−1

standard errors

0.76 0.87 0.87 0.90 0.90 0.89 0.87 0.81 0.89 0.86 0.89 0.93 0.88 0.89 0.71 0.74 0.75 0.78 0.81 0.86 0.88 0.84 0.89 0.89 0.88 0.87 0.90 0.92 0.97 0.93 0.82 0.77 0.81 0.85 0.86 0.81 0.93 0.96 1.03 1.04 1.04 1.07

34.20 39.06 42.57 45.63 47.70 49.50 50.58 39.43 45.38 50.10 53.94 56.54 59.27 60.14 24.58 29.17 32.64 34.99 36.77 38.74 39.58 37.92 43.38 47.26 50.65 52.98 54.94 56.20 9.86 11.35 12.52 13.49 14.14 14.82 15.04 7.34 8.71 9.74 10.44 10.98 11.56 11.82

0.76 0.87 0.87 0.90 0.90 0.89 0.87 0.81 0.89 0.86 0.89 0.93 0.88 0.89 0.71 0.74 0.75 0.78 0.81 0.86 0.88 0.84 0.89 0.89 0.88 0.87 0.90 0.92 0.97 0.93 0.82 0.77 0.81 0.85 0.86 0.81 0.93 0.96 1.03 1.04 1.04 1.07

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Figure 6. Adsorption isotherms of IIP-SHA/SiO2 toward Pb(II) at different temperatures (sorption time = 2 h; pH 6).

Table 2. Isotherm Data for Pd(II) Ions at Different Temperatures Ce/mg·kg−1 T/K 313

303

293

283

−1

average values/mg·kg 156.53 250.21 346.34 443.25 540.46 738.12 937.37 162.08 256.62 352.74 449.35 547.02 745.06 943.80 168.23 263.42 359.65 456.51 554.43 752.24 951.51 173.82 268.91 365.24 462.72 560.84 758.73 957.82

Qe/mg·g−1 standard errors 0.89 0.90 0.88 0.93 0.88 0.86 0.87 0.84 0.89 0.89 0.87 0.87 0.90 0.92 0.76 0.81 0.89 0.89 0.90 0.90 0.92 0.71 0.77 0.77 0.83 0.87 0.91 0.90

43.47 49.79 53.66 56.75 59.54 61.88 62.63 37.92 43.38 47.26 50.65 52.98 54.94 56.2 31.77 36.58 40.35 43.49 45.57 47.76 48.49 26.18 31.09 34.76 37.28 39.16 41.27 42.18

standard errors 0.89 0.90 0.88 0.93 0.88 0.86 0.87 0.84 0.89 0.89 0.87 0.87 0.90 0.92 0.76 0.81 0.89 0.89 0.90 0.90 0.92 0.71 0.77 0.77 0.83 0.87 0.91 0.90

where Ce (mg·kg−1) is the equilibrium ion concentration in solution, Qe and Qm, (mg·g−1) are the equilibrium adsorption amount and the maximum adsorption capacity, respectively, and b is the Langmuir constant. Using the data shown in Figure 6a, Ce/Qe versus Ce was plotted and the data were fitted and shown in Figure 6b, the isotherm data at all four temperatures show rather nice linear correlations between Ce/Qe and Ce with correlation coefficients higher than 0.99 (see Table S1, SI). The Qm’s obtained by curve fitting closely match those obtained experimentally. These results indicate that the adsorption behavior of IIP-SHA/SiO2 conforms to the Langmuir model and is a typical monomolecular layer adsorption.

The effect of temperature on Pd(II) adsorption of ionimprinted silica was also studied. The adsorption isotherms of IIP-SHA/SiO2 toward Pb(II) at (283, 293, 303, and 313) K are shown in Figure 6a, and the data are presented in Table 2. As temperature increases, the equilibrium adsorption capacity increases. At 313 K, the maximum adsorption capacity reached up to 62.7 mg·g−1 which is among some of the highest adsorption capacities for Pb(II) ever reported in the literature.12,16,17,26,27 The temperature dependence indicates that the Pb(II) adsorption process is endothermic. The isotherm data were analyzed using the Langmuir adsorption model as expressed in eq 3. Ce C 1 = e + Qe Qm bQ m

average values/mg·kg

−1

(3) F

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Kd = Q e/Ce

Using the obtained b values, adsorption equilibrium constants K at different temperatures were calculated following eq 4.28

b = (K − 1)M /ρ

Selectivity coefficient (k) of IIP-SHA/SiO2 for Pb(II) with respect to the competitor ion (B) was obtained by eq 8.

(4)

k = Kd(Pb2 +)/Kd(B)

where M and ρ are the mole mass and density of the solvent (water), respectively. With K available, the associated Gibbs free energy change ΔG is obtained using eq 5. where R is the universal gas constant and T is the temperature in Kelvin. The adsorption enthalpy and entropy changes (ΔH and ΔS) were then extracted using eq 6. By fitting ln K versus 1/T (see Figure.S1, SI), ΔH and ΔS were obtained. All thermodynamics parameters were summarized in Table 3. ln K = −

ΔH 1 + ΔS /R R T

k′ = kIIP/kNIP

Table 3. Thermodynamic Parameters Associated with the Adsorption Process of Pb(II) onto IIP-SHA/SiO2 ΔG/kJ·mol−1

ΔH/kJ·mol−1

ΔS/J·mol−1·K−1

278.15 288.15 298.15 303.15

−13.68 −14.6 −15.55 −16.45

12.57 12.57 12.57 12.57

92.75 92.74 92.8 92.73

(9)

The obtained distribution coefficients Kd, selectivity coefficients k, and relative selectivity coefficients k′ are listed in Table 4. For the ion-imprinted silica (IIP-SHA/SiO2), the selectivity coefficients of Pb(II) over Cu(II) and Pb(II) over Cd (II) are 4.3 and 5.3, respectively, indicating again very high selectivity. For the nonion-imprinted silica (SHA-PHEMA/SiO2), however, the corresponding selectivity coefficients are only 0.57 and 1.18, respectively, indicating no selectivity at all for Pb(II). The ion-imprinting effect on the selectivity is further reflected by the relative selectivity coefficients. While ion-imprinting is shown to dramatically enhance the selectivity toward the template ion, the relative selectivity coefficient of Pb(II) over Cu(II) (7.54) is higher than that of Pb(II) over Cd(II) (4.49), presumably due to the larger ionic size difference between Pb(II) and Cu(II). 3.3.4. Effect of Imprinting Conditions on the Adsorption Selectivity of IIP-SHA/SiO2 for Pb(II). To evaluate how the cross-linking process affects the adsorption property of the resulting ion-imprinted silica, a series of IIP-SHA/SiO 2 sorbents were prepared by varying the cross-linking temperature and/or the amount of cross-linking agent EDGE. Their adsorption capacity and selectivity for Pb(II) were measured and systematically compared. 3.3.4.1. Amount of EGDE. Figure 7 shows the change of the selectivity coefficients (K) of ion-imprinted silica (toward Pb2+) prepared using different amount of cross-linking agent. It is clearly shown that the selectivity of IIP-SHA/SiO2 for Pb(II) was the highest when the mole ratio of SHA to EGDE was 1:0.5. This is, interestingly, the stoichiometric ratio at which the cross-linking between phenolic hydroxyl groups on SHA and glycidyl groups on EGDE can be most complete. The more deviated the SHA/EGDE ratio from the stoichiometric ratio is, the less complete the cross-linking reaction as the extent of cross-linking is related to the stoichiometric imbalance. Thus, too high or too low amount of EGDE will compromise the cross-linking process. 3.3.4.2. Temperature. The temperature effect on the crosslinking process was also explored. Figure 8 shows the selectivity coefficient of IIP-SHA/SiO2 (for Pb(II)) prepared at different cross-linking temperatures. As the cross-linking temperature increases from (298 to 318) K, the selectivity coefficient of the

(6)

T/K

(8)

The ratio of kIIP over kNIP, which is called the relative selectivity coefficient, was used to gauge the extent of enhancement in adsorption selectivity of ion imprinted silica (IIP-SHA/SiO2) over the nonion-imprinted material SHAPHEMA/SiO2.

(5)

ΔG = −RT ln K

(7)

As shown in Table 3, ΔG’s at all four temperatures are negative, suggesting that the adsorption process of IIP-SHA/ SiO2 for Pb(II) ion at these temperatures is spontaneous. The positive enthalpy changes indicate again that the adsorption process is endothermic. Significant increase in entropy is observed for the adsorption process. The large positive ΔS indicates that the binding pocket is rather rigid and the chelating groups have limited freedom. This explains why these binding pockets do not fit smaller Cu(II) and Cd(II) ions. Entropy is increased after adsorption since four Pb(II)-binding water molecules are released as free molecules. It is worth noting that the adsorption ΔS of our ion-imprinted silica is significantly higher than those of other reported similar IIP sorbents,19 indicating our IIP exhibits much more rigid binding pockets and thus better template fidelity. The rigidity of our system is likely due to the proximity of the cross-linking phenolic hydroxyl groups to the binding sites. 3.3.3. Adsorption Selectivity. To further evaluate the adsorption selectivity, competitive adsorption experiments were carried out using Pb(II)/Cu(II) and Pb(II)/Cd(II) binary ionic mixtures. Distribution coefficients (Kd) of Pb(II), Cu(II), and Cd(II) were calculated by eq 7.

Table 4. Distribution Coefficients, Selectivity Coefficients, and Relative Selectivity Coefficients of Functionalized Silica with or without Ion-Imprinting Kd/kg·g−1

Kd/kg·g−1

adsorbent

Pb(II)

Cu(II)

k

k′

Pb(II)

Cd(II)

k

k′

IIP-SHA/SiO2 SHA-PHEMA/SiO2

0.344 0.217

0.08 0.38

4.3 0.57

7.54

0.344 0.22

0.065 0.186

5.3 1.18

4.49

G

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Figure 7. Influence of the amount of cross-linking agent on the adsorption selectivity of IIP-SHA/SiO2 for Pb(II) with respect to Cu(II) (cross-linking temperature: 318 K; cross-linking time: 10 h; initial ion concentraion: C0 = 100 mg·kg−1; pH 6; sorption time = 2 h; sorption temperature = 293 K).

Figure 9. Adsorption capacity of IIP-SHA/SiO 2 after each adsorption−desorption cycle (initial Pb(II) concentration: C0 = 600 mg·kg−1; sorption temperature = 293 K; sorption time = 2 h; pH 6).

adsorption capacity during the first three cycles, no further drop is observed in the following cycles. The ion-imprinted silica after 9 cycles still possesses over 91 % of its initial Pb(II) adsorption capacity, indicating its excellent reusability. 3.4. Column Adsorption Characteristics of IIP-SHA/ SiO2 for the Pb(II) Ion. 3.4.1. Dynamic Adsorption Curves. To explore the potential of the ion-imprinted silica as a stationary phase for Pb(II) removal through simple solution filtration, a column packed with IIP-SHA/SiO2 or SHAPHEMA/SiO2 was used for dynamic adsorption studies for both Pb(II) and Cu(II), and the results are shown in Figure 10. For the chelating silica without ion-imprinting (SHA-PHEMA/ SiO2), the leaking of Pb(II) and Cu(II) ions occurred at (130 and 166) BV, respectively. For the ion-imprinted silica, however, the leaking bed volume for Pb(II) and Cu(II) are (176 and 38) BV, respectively. Please note that the leaking volume corresponds directly to the adsorption capacity and the ratio of the leaking volume for different ions reflects the selectivity. The significant drop in leaking bed volume for Cu(II) after ion-imprinting indicates a sharp drop in its adsorption capacity, showing once again the mismatch of the binding pocket left by Pb(II) ion-imprinting with Cu(II). The leaking bed volume for Pb(II), on the other hand, is increased by 30 % after ion-imprinting, indicating the enhanced binding affinity to the template ion after ion-imprinting. The ratio of the leaking volume of Pb(II) over that of Cu(II) is 0.78 and 4.6 for SHA-PHEMA/SiO2 and IIP-SHA/SiO2, respectively, revealing clearly that the chelating silica without ion imprinting does not exhibit any selectivity toward Pb(II) but the ion-imprinted one shows very high selectivity toward the template ion, which is consistent with the batch experiments. The leaking adsorption amount and the saturated adsorption amount of Pb(II) ion on IIP-SHA/SiO2 were calculated to be (29.11 and 31.42) mg·g−1, respectively, whereas they were only about (5.89 and 8.31) mg· g−1 for Cu(II) ion, respectively. These results confirmed again that IIP-SHA/SiO2 has high affinity and high selectivity toward the Pb(II) ion. 3.4.2. Elution Curve. The desorption activities of Pb(II) adsorbed in the IIP-SHA/SiO2 stationary phase using 0.1 mol· kg−1 HCl as the eluent was studied. The eluent upstream passed through the column was collected and the Pb(II) concentration in the solution was measured. As shown in Figure 11, the desorption of Pb(II) occurs as soon as the HCl solution comes in contact with the sorbent as Pb(II) was detected in the first eluting bed volume. The percentage of

Figure 8. Influence of the cross-linkg temperature on the adsorption selectivity of IIP-SHA/SiO2 for Pb(II) over Cu(II) (cross-linking time: 10 h; ratio of SHA over EDGE = 1:0.5; initial metal ion concentration: C0 = 100 mg·kg−1; pH 6 ; sorption time = 2 h ; sorption temperature = 293 K).

resulting IIP-SHA/SiO2 for Pb(II) increases significantly. The reaction between a phenolic hydroxyl group and a glycidyl group is an acid-catalyzed nucleophilic ring opening reaction. The epoxide ring-opening reaction is often associated with slight increase in entropy and therefore a higher temperature may increase the extent of cross-linking reaction. Furthermore, a higher temperature can help expand the PHEMA matrix and increase the diffusion rate of EGDE, leading to more thorough cross-linking. When the temperature is further increased beyond 318 K, the selectivity coefficient of the resulting ionimprinted silica shows no further increase but instead drops slightly. This is likely due to the fact that the template ion removal is carried out at room temperature. A large temperature difference between the matrix setting process (cross-linking) and the template ion removal process may lead to matrix relaxation which distorts the binding pockets and thus decrease the ion-imprinting fidelity. 3.3.5. Desorption and Reusability. To explore the reusability of the ion-imprinted silica for repeated Pb(II) adsorption, Pb(II)-saturated IIP-SHA/SiO2 was stirred in a 0.1 M HCl solution. It was shown that after 5 h, 98.7 % of the adsorbed Pb(II) was removed. The recycled silica was subjected to adsorption studies again. The above adsorption−desorption cycle was repeated 9 times using the same IIP-SHA/SiO2. The adsorption capacity of the recycled ion-imprinted silica after each cycle is shown in Figure 9. Other than a slight decrease in H

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Figure 10. Ion concentrations of the eluent at different bed volumes from an IIP-SHA/SiO2 loaded column.

Funding

The authors gratefully acknowledge the National Young Scientists Fund of China (No. 21307116), Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province, and the Natural Science Foundation of the Shanxi Province of China (No. 20140110175) for financial support of this work. Notes

The authors declare no competing financial interest.

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Figure 11. Pb(II) ion concentration of the eluent at different bed volumes from the column loaded with Pb(II)-saturated IIP-SHA/SiO2 using 0.1 mol·kg−1 HCl as the eluent.

Pb(II) removed after 9 and 13 bed volumes was 96 % and 99.5 %, respectively, indicating that nearly all adsorbed Pb(II) can be conveniently removed. The elution curve was cuspate and without tailing, suggesting good desorption performance. These results indicate again that the Pb(II) imprinted sorbent has excellent reusability.

4. CONCLUSION A new Pb(II)-imprinted silica sorbent IIP-SHA/SiO2 has been successfully prepared using the surface imprinting technique. The bidentate and rigid SHA was used as the chelating ligand and a nucleophilic ring opening reaction of glycidyl with phenolic hydroxyl group was employed as the cross-linking chemistry. The resulting ion-imprinted silica shows enhanced adsorption and more importantly remarkable selectivity toward Pb(II) and excellent reusability, making it an appealing material for Pb(II) removal from aqueous medium. The spontaneous adsorption process was found to be entropy driven and endothermic. The positive entropy change associated with the adsorption process is unusually large, indicating that the binding pockets after template ion removal are rather rigid which is believed to be responsible for the enhanced selectivity.

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ASSOCIATED CONTENT

S Supporting Information *

Additional material as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

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*E-mail: [email protected]. I

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