Poly(acrylic acid) Membrane for

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Cu(II)-Imprinted Poly(vinyl alcohol)/Poly(acrylic acid) Membrane for Greater Enhancement in Sequestration of Copper Ion in the Presence of Competitive Heavy Metal Ions: Material Development, Process Demonstration, and Study of Mechanisms Jinsong He and J. Paul Chen* Department of Civil and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore, Singapore 119260 ABSTRACT: It is a great challenge that a certain type of heavy metals such as copper is separated in the presence of competing metals. In this study, a novel Cu(II)-imprinted poly(vinyl alcohol)/poly(acrylic acid) membrane (Cu-IM) was prepared by semiinterpenetrating polymer network technique for selective copper removal from aqueous solution. The morphology, surface chemistry, stability, and copper adsorption performance of the Cu-IM were investigated. The Cu-IM exhibited a superior chemical stability in different severe environments. The batch adsorption studies showed that the adsorption of copper was highly pH-dependent, and the maximum adsorption reached 1.284 mmol/g at pH 5.0. The adsorption process was well described by the Langmuir isotherm model and the intraparticle pore diffusion model. Compared with nonimprinted PVA/PAA membrane, the Cu-IM exhibited a higher selectivity for copper, with relative selectivity coefficient of 7.78 for Cu2+/Zn2+. Besides, the Cu-IM possessed a high reusability for copper uptake and could maintain 96.25% of the original capacity for copper after 8 repeated cycles. In addition, the filtration studies indicated the Cu-IM could remove copper efficiently with the total treatment volume of 1370 bed volume to meet the EPA standard when the initial copper concentration was as high as 0.348 mM. The copper removal efficiency in the copper/zinc binary mixed solution can still retain 95.69% of that in single copper solution. Finally, the FTIR and XPS studies revealed that the carboxyl and hydroxyl groups played key roles in the copper uptake.

1. INTRODUCTION Heavy metal contamination in waters due to various industrial activities has become a global issue of greatest concern due to the high toxicity, the nonbiodegradability, and serious damage to the aqueous environment. Copper is one of the most commonly existing heavy metals in industrial effluents. The excessive intake of copper can cause a series of adverse effects on humans, such as gastrointestinal disturbance, headaches, stomach aches, dizziness, vomiting, and liver and kidney damage. Thus, strict regulations on copper have been enforced to protect nature water environments.1,2 Such conventional technologies as precipitation, adsorption, ion-exchange, and coagulation are gradually superseded by membrane technology,3−6 which would become more efficient in separation of heavy metals. The adsorptive membrane has received great attention due to the cost-effectiveness.3,4,7−9 These membranes can be prepared by different approaches, such as surface modification,9 polymer blending or coating,3,8 inorganic particles/polymer compositing,5,6 or full/semi-interpenetrating polymer networks (IPNs/s-IPNs).7,10−12 The membrane by the IPNs/s-IPNs becomes increasingly favored for environmental applications because the metal adsorption would take place in the bulk of the membrane rather than only on the membrane surface.7,11 More importantly, this type of membrane can be reused for multiple times owing to its excellent chemical stability and durability.13 However, IPNs/s-IPNs membranes are generally applicable for nonpreferential separation of the heavy metals. The main drawback is the inherent lack of selectivity of species with © 2014 American Chemical Society

similar molecular weights or volumes. Hence, there is a room for further improvement by developing a new type of membrane that has better properties in selective separation and adsorption. The IPNs/s-IPNs membrane could be improved by ionimprinted technology. The ion-imprinted technology was reportedly combined with the IPNs to form an ion-imprinted gel for selective solid-phase extraction of copper ions.14,15 Thus, it would be interesting to find out whether such a combination can be applied in the membrane fabrication. This technology can provide a spatial recognition system in IPN/s-IPNs membranes, which can allow the membrane to achieve the specific removal of certain type of heavy metals.1,16−20 In addition, extra small molecule(s) (e.g., 4-vinylpyridine) as second complexing agent(s) can be added to further enhance the adsorption selectivity.14,21 In our preparation, the imprinted ions could simultaneously complex with the functional groups of both complexing and matrix polymers to achieve the same goal; as a result, it is unnecessary to add the said small molecules in the preparation. In the present study, we aimed to synthesize a novel ionimprinted membrane by combining ion-imprinted technology with s-IPNs. Poly(vinyl alcohol) (PVA) was employed as a membrane matrix due to its good film-forming capacity, Received: Revised: Accepted: Published: 20223

August 18, 2014 November 27, 2014 November 30, 2014 December 1, 2014 dx.doi.org/10.1021/ie5032875 | Ind. Eng. Chem. Res. 2014, 53, 20223−20233

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic diagram of preparation of Cu(II)-imprinted membrane by the s-IPN technique: (a) preparation process of Cu-IM and (b) cross-linking reaction of PVA.

abundance in −OH groups, and nondegradable stability, and poly(acrylic acid) (PAA), that has shown high affinity toward heavy metals due to the abundant −COOH groups, was chosen as the complexing polymer. The Cu(II)-imprinted PVA/PAA membrane was developed by a semi-interpenetrating polymer network technique for the selective copper removal from aqueous solutions. The surface chemistry and chemical stability of the membrane were first investigated. A series of studies on batch and dynamic adsorption was conducted to evaluate the performance of copper removal in the presence of competing heavy metal. Finally, the mechanism for the removal of copper was studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis.

diameter of 9 cm. The water as solvent would be evaporated at room temperature to form solid membrane. The obtained Cu(II)-imprinted membrane (Cu-IM) was subsequently immersed into 0.1 M HNO3 to remove the template and washed by the DI water. Finally, the obtained CuIM was dried at room temperature for the subsequent study. A schematic diagram for the fabrication process of the Cu-IM is demonstrated in Figure 1. Nonimprinted PVA/PAA membrane (NIM) was also prepared by the same procedures except that Cu(II) was absent. It was used as a control for comparison of the membrane performance between the Cu-IM and NIM. 2.3. Membrane Characterization. The morphology of the membrane was investigated by a field emission scanning electron microscopy (SEM) (JEOL JSM-6701F, Japan). The surface morphologies of the Cu-IM with/without Cu2+ template were studied by the SEM. To investigate the surface chemistry of the membranes (i.e., un-cross-linked PVA/PAA polymer, and Cu-IM before and after copper adsorption), Fourier Transform Infrared spectroscopy were measured by a Shimadzu FTIR spectrometer equipped with attenuated total reflection (ATR) objective. The spectra were collected within the wavenumber range from 650 to 4000 cm−1. All measurements were carried out at room temperature. The chemical stability of Cu-IM is an important parameter for membranes in practical applications. To evaluate its stability, the Cu-IM was soaked in different severe environments, i.e. 1-M HCl solution, 1-M NaOH solution, and ethanol for 48 h, respectively. Weight loss of the membrane in different media was calculated as follows

2. MATERIALS AND METHODS 2.1. Materials. All the chemicals used in this study were of analytical grade. Poly(vinlyalcohol) (PVA) with the molecular weight of 124,000−186 000 Da, poly(acrylic acid) (PAA) (Mw = 250 000 Da) in 35 wt % aqueous solution, cupric nitrate, glutaraldehyde (GLA) in 50 wt % aqueous solution, nitric acid, and all other chemicals used in this study were purchased from Sigma−Aldrich (Singapore). 2.2. Preparation of Cu-Imprinted Membrane. PVA as membrane matrix was dissolved in deionized (DI) water (10 g/ L) under intensely stirring at 90 °C; PAA solution (10 g/L) as complexing polymer was then mixed with the PVA solution and stirred together at the mass ratio of 4:6. The 15-mM cupric nitrate solution as the precursor of Cu(II)-template was slowly added into the PVA/PAA solution to obtain the Cu(II)-organic polymer mixed solution. The copper would complex with carboxyl groups of PAA or hydroxyl groups of PVA to form the metal−organic complexes. After that, the resulting mixed solution was reacted with a desired amount of 5 wt % glutaraldehyde (GLA) at pH about 4.5 at room temperature for 1 h to form cross-linked PVA. The chains of the PAA were entrapped into the cross-linked PVA to form the semiinterpenetrating polymer network (s-IPN). 40 mL of the obtained solution was poured into a plastic Petri dish with a

Weight loss (%) =

W′ − W × 100% W′

(1)

where W′ and W are the weight of the dried membrane before and after treatment, respectively. 2.4. Batch Adsorption Experiments. A stock copper solution with concentration of 15-mM was prepared by dissolving cupric nitrate in the DI water. The stock solution 20224

dx.doi.org/10.1021/ie5032875 | Ind. Eng. Chem. Res. 2014, 53, 20223−20233


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Figure 2. SEM images of the membrane surfaces of (a) Cu-IM with template and (b) Cu-IM without template.

aforementioned. Finally, the concentration of copper and zinc in the solutions was measured by the ICP-OES. The selectivity coefficient and relative selective coefficient were determined for copper versus zinc. The relative selectivity coefficient k′ was calculated by

was diluted with the DI water to prepare copper solutions of predetermined concentrations for the adsorption experiments. In the pH and ionic strength effect experiments, 50 mL copper solutions with different pH value and background ionic strengths were prepared. The pH of the copper solution was adjusted by 0.1 M HNO3 or NaOH. Sodium nitrate of 0, 0.005, or 0.05 M was used as ionic strength background. 25-mg CuIM was added into the copper solution. The mixture solutions were shaken on a rotary shaker at room temperature for 24 h. The constant solution pH was controlled throughout the experiment. The samples were finally collected, and the copper concentration was measured by an inductively coupled plasma emission spectrometer (ICP-OES; Thermo iCAP 6300). The copper adsorption capacity was calculated by the following equation q=

k′ =

(3)

where KCu−IM and KNIM are the selective coefficients for Cu-IM and NIM, respectively. The selectivity coefficient KCu/Zn for CuIM or NIM can be obtained by the following equation KCu / Zn =

Kd(Cu2 +) Kd(Zn2 +)

(4)

where Kd is the distribution coefficient (mL/g) calculated as follows: q Kd = e Ce (5)

V (Co − Cf ) W

KCu − IM KNIM

(2)

where q is the metal adsorption capacity, V is the solution volume, W is the amount of sorbent, and Co and Cf are the initial and equilibrium metal concentrations, respectively. In the adsorption kinetics experiments, 0.5-g Cu-IM or NIM was added to a 1-L copper solution with pH controlled at 5.0 ± 0.1. The samples were collected at different time intervals; the copper concentrations were analyzed by the ICP-OES. In the adsorption isotherm experiment, copper solutions with different concentrations ranging from 0.1 to 1.5 mM were prepared. The same procedure was followed as that for the pH effect experiment, except that the pH was controlled at a constant value of 5.0 ± 0.1. In our preliminary study, we found that the H+ ions were released in the solution during the copper uptake, revealing that the H+ might be associated with the copper uptake due to the ion-exchange. To find out the role of H+ ions in the copper uptake, another set of adsorption isotherm experiments was carried out following the above procedure except that the pH was not controlled during the adsorption process. The released amount of H+ ions was determined from the difference between the initial and final pH of the solutions. The copper concentration in the solutions before and after adsorption was measured by the ICP-OES. To evaluate the selectivity of the Cu-IM for copper adsorption, the competitive adsorption of zinc ions with respect to copper was studied. 50-mg Cu-IM or NIM was added to 100 mL of copper/zinc binary solutions, with the initial concentration of single species of 1.0 mM at different pH value. The other procedures were the same as the

To investigate the reusability of the Cu-IM, the regeneration experiments were conducted with eight cycles of adsorption− desorption. In each cycle, the Cu-IM was placed in a 100 mL copper solution with an initial concentration of 1.56 mM at pH 5.0 ± 0.1; the solution with the Cu-IM was shaken for 24 h. Thereafter, the desorption was performed by using 0.1 M HNO3 solution, by which the copper desorption efficiency of 99.5% was found according to our preliminary study. The regenerated Cu-IM was washed and dried for its reuse in the next cycle. The adsorption capacity of copper in each cycle was calculated to evaluate the reusability of the sorbent. 2.5. Filtration Experiments. Filtration experiments for copper removal were conducted by a cylindrical stirred ultrafiltration cell with an effective diameter of 4 cm and a total volume of 50 mL (Model 8050, Millipore) at a pressure of 1 bar and at room temperature. The effective area and thickness of Cu-IM were 12.56 cm2 and 0.035 mm, respectively; the effective bed volume (BV) of 0.044 mL can be obtained. The membrane was placed on the holder of ultrafiltration cell, which was sealed with an O-ring. The filtration was operated under dead-end mode. The flow rate of the filtration was around 0.25 mL/min. The copper concentration in the feed solution initially was set at 0.174 mM and 0.348 mM, respectively. To further evaluate the selectivity of the membrane, the filtration experiment was carried out with copper/zinc binary mixed feed solution. The initial concentration of both species 20225



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groups as shown in Figure 3a and 3b, suggesting the changes in the carboxyl groups in the preparation of Cu-IM. In addition, it is notable that a new peak occurs in the CuIM, compared with the non-cross-linked PVA/PAA membrane. A new absorption peak at 1176 cm−1 present in the Cu-IM can be due to −C−O stretching of ether groups in the cross-linked PVA.11 This result indicates the formation of the polymer networks. The stability of the Cu-IM was investigated in different severe conditions. Weight loss results show that no mass loss of Cu-IM in all media is observed (data not shown), indicating that no organic leachate was found in severe environments. The superior chemical stability of the Cu-IM is basically due to the semi-interpenetrating polymer network structure: the chains of PAA were firmly entrapped in the cross-linked PVA polymer. Furthermore, the excellent chemical stability of Cu-IM in the acid environment will provide the possibility of good reusability. 3.2. Batch Adsorption Study. 3.2.1. Effect of pH and Ionic Strength. The pH of the solution may play an important role in the distribution of copper species and the surface properties of the Cu-IM, which chemically affects the interaction between copper and Cu-IM. In order to evaluate the effect of pH on the adsorption of copper onto the Cu-IM, adsorption experiments were conducted at pH from 1.5 to 5.5, at which no precipitation of copper was observed. The results shown in Figure 4 demonstrate that the adsorption of copper onto the Cu-IM is highly pH-dependent.

was set at 0.348 M. The pH of all feed solution was set at 5.0 according to the pH effect study. The permeate was collected for the measurement of copper concentrations. 2.6. X-ray Photoelectron Spectroscopic Analysis. The chemical analysis on virgin and Cu-loaded Cu-I-sIPN was studied by the X-ray photoelectron spectroscopy (XPS) (Kratos XPS System-AXIS His-165 Ultra, Shimadzu, Japan), with a monochromatic Al Kα X-ray source (1486.6 eV). The high resolution scans were conducted according to the peak being examined with a pass energy of 40 eV and step size of 0.05 eV. To compensate for the charging effects, all spectra were calibrated with graphitic carbon as the reference at a binding energy (BE) of 284.6 eV. The XPS results were collected in binding energy forms and fit using a nonlinear least-squares curve fitting program (XPSPEAK41 soft-ware).

3. RESULTS AND DISCUSSION 3.1. Characterization of Cu-IM. The SEM images of CuIM with/without copper template are shown in Figure 2. It can be observed that the surface morphologies of membranes are different before and after extraction of copper. After extraction, the Cu-IM possesses a rougher surface in comparison with the Cu-IM with template. The rough surface may enhance the adsorption kinetics since it can promote the mass transfer rate of the metal ions to the membrane surface.22 The FTIR spectra of non-cross-linked PVA/PAA membrane and fresh Cu-IM are shown in Figure 3a-b. The broad and

Figure 3. FTIR spectra of membranes: (a) PVA/PAA membrane; (b) pure Cu-IM; and (c) Cu-loaded Cu-IM.

Figure 4. Effect of equilibrium pH and ionic strength on the adsorption of copper onto Cu-IM (sorbent dose = 0.5 g/L, initial concentration = 0.28 mM, T = 20 ± 1 °C).

strong band at 3000−3600 cm−1can be due to the stretching vibrations of −O−H in the alcoholic and carboxyl groups. The band shifts from 3280 cm−1 to a higher frequency at 3358 cm−1 after cross-linking, indicating the involvement of −O−H groups in the ion-imprinted preparation process. The adsorption band at about 2910 cm−1 can be contributed to the stretching vibration of the −C−H. The peaks at 1089 cm−1 (a) and 1099 cm−1 (b) are assigned to the −C−O stretching of alcoholic groups in PVA, and the change further confirms the interaction of alcoholic groups and copper in the Cu-IM. Furthermore, there are two clear band shifts and intensity decrease of both −CO (1705 cm−1 (a) versus 1715 cm−1 (b)) and −C−O (1236 cm−1 (a) versus 1248 cm−1 (b)) stretching of carboxyl

The uptake of copper is extremely low at strong acidic solution with a pH < 3. It increases significantly with the increasing equilibrium solution pH > 3 and reaches a plateau at pH > 4.5. The pH-dependent adsorption behavior is mainly associated with the surface properties of the Cu-IM. It is reported that the polymer PAA has a pKa value between 4 and 6.11 At 1.5 < pH < 4, the functional groups of PAA (carboxylate group) can be easily protonated, leading to an electrostatic repulsion of copper ions. The competition between H+ and copper for the adsorption sites dramatically decreases the adsorption of copper. As pH becomes higher, the degree of ionization of PAA increases, and a large number of carboxylate groups become available for copper ions binding, which results in the 20226



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copper is established after 4 and 12 h for the Cu-IM and the NIM, respectively. This result indicates that the Cu-IM displays a faster adsorption rate and shorter equilibrium time for copper than NIM. The faster kinetics of Cu-IM could be due to high affinity of imprinted cavities of Cu-IM toward copper, which allows the copper ions easily access to the binding sites on the Cu-IM,24,26 in comparison with that of NIM. To better understand the adsorption kinetics of copper on the membranes, the adsorption kinetic data were further described using the pseudo-first-order model and the pseudosecond-order model as well as the intraparticle diffusion model. 3.2.2.1. Pseudo-First/Second-Order Model. The mathematical equations of the pseudo-first-order rate model and the pseudo-second-order rate model are expressed as follows

enhancement of the adsorption of copper. The influence of pH on the copper adsorption is consistent with other reported studies.23,24 Based on this result, pH 5.0 was set in the subsequent adsorption and filtration studies. The effect of ionic strength on the adsorption was studied. As shown in Figure 4, the adsorption capacity of copper decreases slightly with increasing ionic strength from 0 to 0.05 M. This indicates that the ionic strength has little effect on the adsorption. The result suggests that the adsorption of copper onto the Cu-IM may be controlled by inner-sphere complex adsorption mechanism.25 3.2.2. Adsorption Kinetics. The experimental data of the adsorption kinetic studies of copper onto the Cu-IM and NIM together with modeling results are shown in Figure 5. The

qt = qe(1 − e−K1t )

qt =

(6)

K 2qe2t 1 + K 2qet

(7)

−1

where K1 (h ) and K2 ((g/mmol)/h) are the first- and secondorder rate constants, respectively, and qe (mmol/g) and qt (mmol/g) are the amounts of the adsorbate adsorbed at equilibrium and at any time, respectively. The values of qe and K1 can be obtained from the nonlinear curve fitting of experimental data qt versus t. As shown in Figure 5a, the experiment data are better fit with the pseudo-second-order model than the pseudo-first-order model in both situations. The r2 value for the pseudo-secondorder model is a little higher than that for the pseudo-first-order model as presented in Table 1, indicating that the pseudosecond-order model performs better in the description of the adsorption process. 3.2.2.2. Intraparticle Diffusion Model. In this study, the intraparticle pore diffusion model based on Fick’s Law was employed to describe the adsorption kinetics due to the porous structure of the membrane. The mathematical equation is described as follows:27

( ∂c ) −

2 Dp ∂ r ∂r ∂C εp = 2 ∂t ∂r r

ρp ∂q m ∂t

,

0 ≤ r ≤ R,

t>0 (8)

The initial and boundary conditions are as follows

c = 0, ∂c = 0, ∂r Figure 5. Adsorption kinetics for copper removal by Cu-IM and NIM: (a) pseudo-first-order and pseudo-second-order kinetic models and (b) pore diffusion model. Conditions: pH = 5.0 ± 0.1, sorbent dose = 0.5 g/L, initial concentration = 0.32 mM, T = 20 ± 1 °C.

Dp

t=0

(9)

r=0

(10)

∂c = k f (C − c), ∂r

r=R

(11)

where C and q are the concentration of copper ions in bulk and in solid phase, respectively. c is the local concentration within the pores of the membrane, in equilibrium with the corresponding concentration in the solid phase q* according

adsorption of copper quickly takes place within the first 2 h, followed by a relatively slower process. The adsorption of

Table 1. Adsorption Kinetic Constants Obtained by Different Models pseudo-first-order model membranes Cu-IM NIM

qe (mmol/g) 0.603 0.419

K1 (1/h) 1.557 1.258

pseudo-second-order model r

2

0.980 0.776

qe (mmol/g) 0.649 0.451 20227

K2 ((g/mmol)/h) 3.454 4.178

intraparticle pore diffusion model r

2

0.989 0.908

kf (m/s) −6

5.2 × 10 3.4 × 10−6

Dp (m2/s) 1.0 × 10−9 5.0 × 10−12



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to the Langmuir isotherm equation. m is the concentration of the membrane. Dp is the pore diffusion coefficient within the adsorbent, ρp is the membrane density, R is the effective radius of the membrane pores, r is the radius distance measured from the adsorptive site, kf is the external mass transfer coefficient, and t is the time. It can be found in Figure 5b that the modeling can well describe the experimental data for both Cu-IM and NIM. This result indicates that the copper adsorption onto Cu-IM and NIM can be controlled by the intraparticle pore diffusion mechanism. More importantly, the values of kf and Dp summarized in Table 1 for Cu-IM are 5.2 × 10−6 m/s and 1.0 × 10−9 m2/s, respectively, which are higher than those for NIM. 3.2.3. Adsorption Isotherm. Adsorption isotherm experiments of copper on the Cu-IM and NIM were conducted at optimal pH of 5.0 ± 0.1. The experimental data and fitting curves with the isotherm models are presented in Figure 6a. Both Langmuir and Freundlich models are employed to describe the adsorption isotherms, with the equations expressed as follows

qe =

qmaxbCe 1 + bCe

qe = k f Ce1/ n

(12) (13)

where qmax is the total content of adsorption site (i.e., maximum adsorption capacity) (mmol/g), C e is the equilibrium concentration of adsorbate (mmol/L), qe is the adsorption capacity (mmol/g), b is the adsorption reaction constant (L/ mmol), and kf and n are the empirical constants. The constants of both models are summarized in Table 2. According to the correlation coefficient (r2), the Langmuir model performs much better in description of the adsorption isotherm data with a higher r2 than the Freundlich model. The nonlinear chi-square test (χ2) was used to further validate the appropriate isotherm model for the sorption system. The calculation of the χ2 value can be found in our previous study.28 The obtained results from the χ2 test in Table 2 are consistent with the above findings. Based on the Langmuir model, the maximum adsorption capacity of copper is 1.284 mmol/g for Cu-IM and 1.268 mmol/g for NIM, respectively. The experimental data shown in Figure 6a demonstrates the uptake of copper by Cu-IM is higher than that by NIM at the same copper equilibrium concentration, especially at a lower equilibrium concentration. In addition, the value of b for the Cu-IM is much higher than that for the NIM, indicating that Cu-IM possesses a higher affinity toward copper than NIM. The above results are mainly due to the formation of Cu(II)template on the Cu-IM. Cu-IM creates a recognition system of binding sites for copper: due to the imprinted process initially, the template-defined structures of carboxyl/hydroxyl groups and copper ions were formed by complexation during the membrane preparation process, and after the extraction of the copper, the Cu-IM retained recognition cavities to provide a spatial configuration for copper. Each copper ion can exactly occupy one cavity and complexes with four ligands inside the cavity. However, the carboxyl/hydroxyl groups in NIM were randomly distributed because of the absence of the imprinted process, leading to less available adsorptive sites for spatial configurations for copper complexations. It could be anticipated that the imprinted process can enhance the adsorption affinity toward the target ions. 3.2.4. Selectivity in Competitive Adsorption. The selective adsorption of Cu-IM and NIM in the copper/zinc binary solutions was studied at different pHs. As shown in Figure 7, the adsorption of copper and zinc on both Cu-IM and NIM increases as the PH goes up. At pH 5.0, the copper adsorption on the Cu-IM is not obviously reduced in the presence of zinc, which is around 92% of the maximum copper adsorption capacity. In addition, the Cu-IM shows a higher selectivity toward copper with respect to zinc in comparison with NIM, indicating the existence of a large amount of binding sites with specific affinity toward copper in Cu-IM due to the imprinted process. The selectivity coefficient and relative selectivity coefficient were calculated as shown in Table 3. The results demonstrate that the selectivity value for Cu-IM is 7.78 times greater than that of NIM for Cu2+/Zn2+ at optimal pH 5.0, further confirming the high selectivity for copper. It can be concluded that the integration of ion-imprinted and s-IPN technologies can be provided as an alternative method for the selective removal of the target ions from water, which is in great agreement with our recent report.29

Figure 6. (a) Adsorption isotherm of copper on the Cu-IM and NIM and (b) relationship between the copper uptake and release of H+ ions on Cu-IM. Conditions: membrane dosage = 0.5 g/L, T = 20 ± 1 °C. 20228



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Table 2. Langmuir and Freundlich Isotherm Constants for Adsorption of Copper on the Membranes Langmuir isotherm

Freundlich isotherm

membrane

qmax (mmol/g)

b(L/mmol)

r2

x2

kf

n

r2

x2

Cu-IM NIM

1.284 1.268

155.537 11.770

0.893 0.984

0.021 0.002

1.550 1.376

5.264 2.896

0.745 0.941

0.745 0.008

3.2.5. Reusability Performance. From the standpoint of practical application, materials with high reusability are more favored due to the economic feasibility. Several cycles of adsorption−desorption experiments were conducted to evaluate the reusability. The resulting adsorption capacity for the Cu-IM in 8 cycles is shown in Figure 8. It can be found that the

Figure 7. Selective adsorption of copper and zinc on the Cu-IM and NIM. Conditions: membrane dosage = 0.5 g/L, initial concentration of copper/zinc = 1.0 mM, T = 20 ± 1 °C.

Table 3. Selective Adsorption Performance of Cu-IM and NIM Kd (mL/g)

KCu/Zn

metal ions

pH

Cu-IM

NIM

Cu-IM

NIM

k′Zn

Cu2+ Zn2+

5.0 5.0

2855 81

1295 286

35.25

4.53

7.78

Figure 8. Performance of reusability of the Cu-IM. Conditions: pH = 5.0 ± 0.1, membrane dosage = 0.5 g/L, initial concentration = 1.56 mM, T = 20 ± 1 °C.

adsorption capacity nearly maintains the same level in the eight cycles, i.e. the adsorption capacity in Cycle 8 can still reach up to 96.25% of that of Cycle 1. This finding suggests that the CuIM is chemical stable and effective for copper uptake for multiple cycles. 3.2.6. Filtration Study. The filtration experiments for copper removal in single-component copper solution and copper/zinc binary mixed solution were conducted, respectively. As shown in Figure 9, with the initial concentration of 0.174 mM, initially the copper concentration of the permeate is extremely low until a sharp breakthrough occurs at a volume of 1710 BV (break point was set at 0.02 mM (1.3 mg/L, i.e. the maximum copper concentration level in drinking water regulated by EPA)). This result indicates the membrane has an excellent performance for copper removal. As the initial concentration increases to 0.348 mM, the breakthrough takes place after the production of a total volume of 1370 BV. The corresponding total volume of permeate decreased as compared to the volumes from the filtration of low copper initial concentration. The decrease in volumes of permeate produced is attributed to the larger amounts of copper in influent. A higher copper concentration causes a higher concentration gradient that provides a stronger driving force to enhance adsorption rate. Therefore, the breakthrough point occurs much faster than that from filtration with low initial copper concentration. For the filtration of copper/zinc binary mixed solution, the breakthrough occurs at a permeate volume of 1311 BV, which is 95.69% of the treated volume of the above single copper

The performance characteristics of different Cu-imprinted adsorbents are summarized in Table 4. It can be concluded that the adsorption capacity of our membrane is much higher than that of many other adsorbents and that the selectivity for CuIM is comparable to other reported adsorbents. Table 4. Comparison of the Sorbent Characteristics of Various Cu-Imprinted Sorbents

imprinted sorbent

pH

adsorption capacity (mmol/g)

Cu-MAH microbead Cu-CS gel Cu-VP/ DBDA15C4 polymer Cu-magnetic ionimprinted polymer Cu-PVA/SA film Cu-MTMAAm polymer Cu-chitosan/ sargassum sp. pellet Cu-PVA/PAA membrane

7.0

0.75

7.4

30

7.0 7.0

0.74 0.07

5.8 16.1

31 32

7.0

0.93

12.36

26

6.0 6.0

1.24 0.08

9.1

1 22

5.0

1.08

5.0

1.284

relative selectivity coefficient (k′Zn)

ref

23 7.78

present study 20229



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the alcoholic groups in PVA may be involved in the uptake of copper. 3.3.2. X-ray Photoelectron Spectroscopy Study. The XPS wide scan spectra of the virgin and Cu-loaded Cu-IM are shown in Figure 10. The wide scan XPS spectra shows that the main

Figure 9. Breakthrough curves for copper from Cu-IM with different initial concentration. Conditions: bed volume = 0.044 mL, T = 20 ± 1 °C.

solution (Co = 0.348 mM). This result implies that the presence of zinc in the binary mixed solution does not hinder the removal of copper by Cu-IM obviously. It is also indicative that the Cu-IM exhibits high selectivity toward the target copper ions. 3.3. Adsorption Mechanism Studies. 3.3.1. Fourier Transform Infrared Spectroscopy Study. The FTIR spectra of the virgin and Cu-loaded Cu-IM are shown in Figure 3b-c. A sharp adsorption peak at 1715 cm−1 and a weak peak at 1248 cm−1 observed in the virgin membrane correspond to the stretching vibrations of the carbonyl double bond (υCO) and the carbon−oxygen single bond (υC−O) in carboxyl groups, respectively.33 Compared with the FTIR spectrum of virgin membrane, both above adsorption peaks show obvious differences after copper adsorption. Both the carbonyl double bond and the carbon−oxygen single bond bands shift to different frequency. The carbonyl double bond stretching band exhibits a clear shift to a lower frequency at 1618 cm−1, and the carbon−oxygen single bond band shifts to a lower frequency at 1242 cm−1, corresponding to the complexation of copper to CO and C−O bonds. This result indicates a typical carboxylic adsorption. The differences of frequencies of both CO and C−O bond stretching (Δ = υCO − υC−O) in virgin and Cu-loaded membrane are 467 and 376, respectively. It is reported that the difference between CO and C−O bond stretching is associated with the relative symmetry of these two carbon− oxygen bonds and reflects the nature of the carboxyl group binding status.34 The virgin film has a larger Δ value than the Cu-loaded membrane. The change in Δ in the presence of copper clearly reveals the involvement of carboxyl groups forming complexes with copper ions. Another notable change is that the intensity of the carbonyl double bond (CO) band at 1618 cm−1 after the adsorption becomes lower than that for the CO band at 1715 cm−1 before adsorption. This result further reveals that carboxyl groups are involved in the metal binding, which is consistent with other reported studies.2,34 The carboxyl groups are active functional groups for copper ions uptake through the formation of the Cu2+−COO− complex. In addition, the peaks attributed to the C−O bond of alcoholic groups also shows a shift (1099 cm−1 before adsorption versus 1107 cm−1 after adsorption), indicating that

Figure 10. Wide scan XPS spectra of virgin and Cu-loaded Cu-IM.

elements in the virgin membrane are C and O, whereas the main elements detected in the Cu-loaded membrane are C, O, and Cu (i.e., Cu 2p and Cu KL), confirming the uptake of copper on the Cu-IM The high resolution C 1s and O 1s spectra of the virgin and Cu-loaded Cu-IM are shown in Figure 11. The C 1s spectrum of virgin Cu-IM comprises four peaks with binding energies of 284.69, 285.82, 286.40, and 288.47 ev, identified via deconvolution. These peaks can be assigned to C−C, C−O− H, C−O−C, and CO, and the last three can be attributed to alcohol, ether, and carboxyl groups, representing the different chemical status of carbon atoms in the membranes.23,35 Among them, alcoholic and ether carbons come from the cross-linked PVA, whereas carboxyl carbons are from the PAA polymer. After the copper adsorption, the peak of the CO bond shows a clear shift to a higher binding energy at 288.87 eV. This result reveals that the carboxyl groups are associated with the uptake of copper, in which the oxygen donates electrons to copper ions and the electron density at the adjacent carbon atoms in CO decrease. Thus, the binding energy of the above carbons shifts up. Additionally, the peak of the C−O−H bond also exhibits a small shift to a higher frequency after adsorption, indicating the involvement of alcohol groups in the uptake of copper. The O 1s spectrum of the virgin membrane can be decomposed into two component peaks, which come from different functional groups and overlap on each other as shown in Figure 11c. The peaks at binding energies of 531.41 and 532.21 eV can be assigned to the CO (carboxyl groups) and C−O or O−H (alcohol or ether groups), respectively.35,36 20230



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Figure 11. High resolution XPS spectra of Cu-IM: (a) C 1s; (b) C 1s after adsorption; (c) O 1s; and (d) O 1s after adsorption.

Figure 12. Schematic diagram of mechanism for adsorption and desorption of copper by Cu-IM.

addition, the H+ ions from carboxyl groups were found to be released in the solution during the copper uptake in our preliminary study, suggesting the ion-exchange mechanism between H+ and copper ions for copper uptake. To find out the role of H+ ions in the copper uptake, the relationship between the uptake of copper and release of H+ in the adsorption process was investigated. The result present in Figure 6b demonstrates that there is a clear linear relationship between the uptake of copper and the release of H+ ions. This result indicates that an ion-exchange process is involved in the adsorption process, further revealing that the uptake of one copper leads to the release of two H+ ions.

Compared with the spectrum of the virgin membrane, a new peak at 531.39 eV present after copper adsorption can be due to the binding of copper onto the O atoms as shown in Figure 11d. In addition, it is also observed that the binding energy of both CO and C−O peaks shifts to the higher energy at 531.91 and 532.51 eV, respectively, which could be due to the decrease of the electron density of the O atoms bound with copper. From these results, it can be concluded that both carboxyl and alcohol groups are associated with the uptake of copper. 3.3.3. The Effect of Ion-Exchange. As discussed in the above section, both the carboxyl group of PAA and the alcohol group of PVA are found to be coordinated with copper ions. In 20231



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(3) Ç ifci, C.; Kaya, A. Preparation of poly(vinyl alcohol)/cellulose composite membranes for metal removal from aqueous solutions. Desalination 2010, 253, 175−179. (4) Han, K. N.; Yu, B. Y.; Kwak, S.-Y. Hyperbranched poly(amidoamine)/polysulfone composite membranes for Cd(II) removal from water. J. Membr. Sci. 2012, 396, 83−91. (5) He, J.; Matsuura, T.; Chen, J. P. A novel Zr-based nanoparticleembedded PSF blend hollow fiber membrane for treatment of arsenate contaminated water: Material development, adsorption and filtration studies, and characterization. J. Membr. Sci. 2014, 452, 433−445. (6) Zheng, Y.-M.; Zou, S.-W.; Nanayakkara, K. G. N.; Matsuura, T.; Chen, J. P. Adsorptive removal of arsenic from aqueous solution by a PVDF/zirconia blend flat sheet membrane. J. Membr. Sci. 2011, 374, 1−11. (7) Bessbousse, H.; Verchère, J.-F.; Lebrun, L. Characterisation of metal-complexing membranes prepared by the semi-interpenetrating polymer networks technique. Application to the removal of heavy metal ions from aqueous solutions. Chem. Eng. J. 2012, 187, 16−28. (8) Irani, M.; Keshtkar, A. R.; Mousavian, M. A. Removal of Cd(II) and Ni(II) from aqueous solution by PVA/TEOS/TMPTMS hybrid membrane. Chem. Eng. J. 2011, 175, 251−259. (9) Salehi, E.; Madaeni, S. S.; Heidary, F. Dynamic adsorption of Ni(II) and Cd(II) ions from water using 8-hydroxyquinoline ligand immobilized PVDF membrane: Isotherms, thermodynamics and kinetics. Sep. Purif. Technol. 2012, 94, 1−8. (10) Bessbousse, H.; Verchère, J. F.; Lebrun, L. Increase in permeate flux by porosity enhancement of a sorptive UF membrane designed for the removal of mercury(II). J. Membr. Sci. 2010, 364, 167−176. (11) Lebrun, L.; Vallée, F.; Alexandre, B.; Nguyen, Q. T. Preparation of chelating membranes to remove metal cations from aqueous solutions. Desalination 2007, 207, 9−23. (12) Bessbousse, H.; Rhlalou, T.; Verchère, J. F.; Lebrun, L. Removal of heavy metal ions from aqueous solutions by filtration with a novel complexing membrane containing poly(ethyleneimine) in a poly(vinyl alcohol) matrix. J. Membr. Sci. 2008, 307, 249−259. (13) Lebrun, L.; Follain, N.; Metayer, M. Elaboration of a new anionexchange membrane with semi-interpenetrating polymer networks and characterisation. Electrochim. Acta 2004, 50, 985−993. (14) Wang, S.; Zhang, R. Selective Solid-Phase Extraction of Trace Copper Ions in Aqueous Solution with a Cu(II)-Imprinted Interpenetrating Polymer Network Gel Prepared by Ionic Imprinted Polymer (IIP) Technique. Microchim. Acta 2006, 154, 73−80. (15) Wang, J.; Liu, F. Enhanced and selective adsorption of heavy metal ions on ion-imprinted simultaneous interpenetrating network hydrogels. Des. Monomers Polym. 2013, 17, 19−25. (16) Vatanpour, V.; Madaeni, S. S.; Zinadini, S.; Rajabi, H. R. Development of ion imprinted technique for designing nickel ion selective membrane. J. Membr. Sci. 2011, 373, 36−42. (17) Li, Z.-C.; Fan, H.-T.; Zhang, Y.; Chen, M.-X.; Yu, Z.-Y.; Cao, X.Q.; Sun, T. Cd(II)-imprinted polymer sorbents prepared by combination of surface imprinting technique with hydrothermal assisted sol−gel process for selective removal of cadmium(II) from aqueous solution. Chem. Eng. J. 2011, 171, 703−710. (18) Chen, J. H.; Li, G. P.; Liu, Q. L.; Ni, J. C.; Wu, W. B.; Lin, J. M. Cr(III) ionic imprinted polyvinyl alcohol/sodium alginate (PVA/SA) porous composite membranes for selective adsorption of Cr(III) ions. Chem. Eng. J. 2010, 165, 465−473. (19) Du, X.; Zhang, H.; Hao, X.; Guan, G.; Abudula, A. Facile Preparation of Ion-Imprinted Composite Film for Selective Electrochemical Removal of Nickel(II) Ions. ACS Appl. Mater. Interfaces 2014, 6, 9543−9549. (20) Wang, J.; Li, X. Ion-Imprinted Composite Hydrogels with Excellent Mechanical Strength for Selective and Fast Removal of Cu2+. Ind. Eng. Chem. Res. 2012, 52, 572−577. (21) Branger, C.; Meouche, W.; Margaillan, A. Recent advances on ion-imprinted polymers. React. Funct. Polym. 2013, 73, 859−875. (22) Yılmaz, V.; Hazer, O.; Kartal, Ş. Synthesis, characterization and application of a novel ion-imprinted polymer for selective solid phase

A stoichiometric equation for the copper uptake due to the ion-exchange process is therefore proposed as follows 2RCOOH + Cu 2 + = (RCOO)2 Cu + 2H +

(14)

where X̅ and X represent the concentration in the solid phase and solution, respectively. Thus, we can obtain the following equation: 2Δ[Cu 2 +] = Δ[H +]

(15)

The data in Figure 6b demonstrates that the process follows the ion exchange reaction given in eq 14. From the spectroscopic analysis and experimental data, the mechanism for copper adsorption on the Cu-IM was deducted. The copper ions are bound to the membrane by exchanging the H+ ions from carboxyl groups as shown in Figure 12. The bounded copper may form tetradentate complexes onto the Cu-IM.

4. CONCLUSIONS An innovative Cu(II)-imprinted PVA/PAA membrane was prepared for the selective copper removal by a semiinterpenetrating polymer network technique. Chains of the PAA polymer complexed with the Cu(II)-template were entrapped in the cross-linked PVA membrane matrix. The Cu-IM showed good chemical stability in various severe environments. The batch adsorption experiments revealed that the Cu-IM exhibited high adsorption capacity and good reusability for copper. The Cu-IM showed a much higher selectivity toward copper ions versus zinc than NIM. In addition, the membrane was confirmed to be effective and selective for the removal of copper in the presence of zinc under the continuous filtration mode. Finally, the FTIR and XPS analyses indicated that the adsorption of copper was mainly associated with the carboxyl groups of PAA and hydroxyl groups of PVA.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.S.H. would like to thank the National University of Singapore (NUS) for providing the NUS President Scholarship during his Ph.D. study. The authors would like to acknowledge the Agency for Science, Technology and Research (A*Star) of Singapore under Grant No. 092 101 0059 and the National Research Foundation of Singapore under Grant No. NRF2011NRF-POC001-028 for providing support for the research work.

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REFERENCES

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