Nanostructured Membranes from Triblock Polymer Precursors as High

DOI: 10.1021/acs.langmuir.5b01605. Publication Date (Web): September 21, 2015. Copyright © 2015 American Chemical Society. *(W.A.P.) E-mail: ...
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Nanostructured Membranes from Triblock Polymer Precursors as High Capacity Copper Adsorbents Jacob L. Weidman,† Ryan A. Mulvenna,‡ Bryan W. Boudouris,‡ and William A. Phillip*,† †

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States

‡

S Supporting Information *

ABSTRACT: Membrane adsorbers are a proposed alternative to packed beds for chromatographic separations. To date, membrane adsorbers have suffered from low binding capacities and/or complex processing methodologies. In this work, a polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI−PS−PDMA) triblock polymer is cast into an asymmetric membrane that possesses a high density of nanopores (d ∼ 38 nm) at the upper surface of the membrane. Exposing the membrane to a 6 M aqueous hydrochloric acid solution converts the PDMA brushes that line the pore walls to poly(acrylic acid) (PAA) brushes, which are capable of binding metal ions (e.g., copper ions). Using mass transport tests and static binding experiments, the saturation capacity of the PI−PS−PAA membrane was determined to be 4.1 ± 0.3 mmol Cu2+ g−1. This experimental value is consistent with the theoretical binding capacity of the membranes, which is based on the initial PDMA content of the triblock polymer precursor and assumes a 1:1 stoichiometry for the binding interaction. The uniformly sized nanoscale pores provide a short diffusion length to the binding sites, resulting in a sharp breakthrough curve. Furthermore, the membrane is selective for copper ions over nickel ions, which permeate through the membrane over 10 times more rapidly than copper during the loading stage. This selectivity is present despite the fact that the sizes of these two ions are nearly identical and speaks to the chemical selectivity of the triblock polymer-based membrane. Furthermore, addition of a pH 1 solution releases the bound copper rapidly, allowing the membrane to be regenerated and reused with a negligible loss in binding capacity. Because of the high binding capacities, facile processing method implemented, and ability to tailor further the polymer brushes lining the pore walls using straightforward coupling reactions, these membrane adsorbers based on block polymer precursors have potential as a separation media that can be designed to a variety of specific applications.

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INTRODUCTION The separation of metal ions from aqueous solutions is relevant to numerous fields of science and technology. For example, the effective and efficient capture of metal ions from wastewater and process water sources is important for safeguarding public health and the environment.1 Also, these separations are essential to the development of sustainable processes for the recovery of natural resources (e.g., rare earth elements).2,3 As such, several techniques exist to aid in this class of separations.4,5 Reverse osmosis (RO), which makes use of a size-selective membrane, can concentrate metal ions in an aqueous solution by allowing the selective permeation of water but cannot separate the dissolved ions from one another.6−8 Nanofiltration membranes, which possess slightly larger free volume elements than RO membranes, allow water and monovalent ions to permeate while retaining most multivalent ions. However, they demonstrate modest selectivity for the separation of ions with the same valence number.9−12 Alternatively, electrodialysis can separate monovalent and multivalent ions selectively, but its utility for separating the components of dilute solutions may be limited.13−15 In fact, none of the techniques listed above are capable of separating © 2015 American Chemical Society

ions that have similar sizes and similar valence number values from one another selectively. For these reasons, much of the research focused on the separation of metal ions from one another makes use of chemical affinity between the dissolved ions and a substrate to effect a separation. In particular, the specific and selective binding of metal ions to polymeric moieties has been shown to be an effective means of separating ions in adsorption processes.1−5 Typically, adsorptive separation processes that make use of chemically selective substrates are operated using packed beds filled with microporous beads in order to maximize the active surface area to volume ratio and to yield high capacities. However, the convective flow, which drives the solution through the packed beds, occurs around the microporous beads and bypasses the pores within the beads. Therefore, these processes rely on a diffusive transport mechanism to bring the dissolved solutes from the exterior surface of the bead to the active sites within the beads, which can be distances of ≥100 Received: May 4, 2015 Revised: September 15, 2015 Published: September 21, 2015 11113

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Langmuir μm for standard adsorbents.16 This distance necessitates long residence times to ensure that the active sites saturate with solute; in turn, this decreases the overall packed bed throughput. As such, the size of the equipment required for a packed-bed process must be increased, which increases capital and operating expenses. Furthermore, the possibility of backflow and the presence of dead volumes within the bed also can limit the effectiveness of the separation, and additional energy may be required in these systems due to the large pressure drops needed to drive flow through a packed bed.17,18 To address these issues, much work has used membranes as adsorbents in place of microporous beads. Membrane separations typically can be operated with lower pressure drops compared to packed beds and require a smaller capital footprint.17−19 Additionally, because solution flows through the substrate containing the active sites, the capture relies on very short diffusion lengths (i.e., on the scale of nanometers rather than micrometers). This shortens the time required for the solutes to bind with active sites and increases the overall throughput capabilities of the process.17,18 Currently, membranes that possess these advantages can be produced in several ways. These include (1) layer-by-layer methods, in which polyelectrolyte groups are deposited within pores to provide a binding substrate;15,20−24 (2) molecular ion imprinting, in which a polymer is doped with ions during membrane fabrication in order to create an ion-selective framework within the membrane;25,26 (3) grafting-from methods, in which the polymerization of polymer brushes capable of binding metal ions is initiated from a membrane surface;27−29 (4) grafting-to methods, in which polymers are synthesized in solution and subsequently attached covalently to the membrane surface;30,31 and (5) hydrogel fabrication, in which a cross-linked polymer acts as a solvent-responsive substrate containing ion-binding sites.32 Though these fabrication techniques make use of the mass transfer advantages adsorption membranes provide, the materials tend to have relatively low total binding capacities compared to the microporous beads.33,34 Recently, layer-bylayer methods have been used to generate high capacity membrane adsorbers, but the extended processing routes required for membrane fabrication may make them less favorable for large-scale membrane production.15,20−24 Membrane adsorbers fabricated from self-assembled block polymer precursors could address the low capacities and complicated processing routes associated with many existing membrane adsorbers.35 Previously, asymmetric membranes with an active layer possessing a high density of consistently sized pores and an underlying gutter layer, which provides mechanical reinforcement but contributes a negligible resistance to flow, were fabricated from self-assembled block polymers using the self-assembly and nonsolvent induced phase separation (SNIPS) casting method.36−38 The result is a robust membrane with a high permeability that is capable of filtering solutes from solution selectively.39 Additionally, the pore size of these membranes is tunable both before membrane fabrication, by adjusting the parameters of the block polymer synthesis, and after fabrication, by changing the state of the surrounding solution or by selective functionalization of the pore-lining block.39−41 Using these methods, membranes with pore diameters that range from ∼2 nm39 to 60 nm42 have been fabricated. These small, uniformly sized pores should result in short diffusion lengths that could reduce mass transfer limitations in adsorption processes.17 Furthermore, the repeat units of the pore-lining block provide a high density of active

site precursors that are already built into the cast membrane.39 By judiciously choosing the pore-lining block, the chemistry of the pores can be tailored in the solid state to produce membranes with new properties tailored to meet the needs of specific applications.39,43 Here, we fabricate high-capacity membrane adsorbers in a straightforward manner by using a polyisoprene-b-polystyreneb-poly(N,N-dimethylacrylamide) (PI−PS−PDMA) triblock polymer as a precursor for the formation of a copper-selective adsorber. The PI−PS−PDMA triblock polymer is fabricated into a functional membrane using the SNIPS process. We demonstrated previously that PDMA brushes line the pores of the resulting membrane.39 Notably, the conversion of the PDMA brushes to a poly(acrylic acid) (PAA) moiety41,43 results in pore walls that can adsorb copper ions reversibly.44,45 As such, these nanoporous membranes fabricated from selfassembled block polymer precursors possess a relatively high copper binding capacity of 4.1 ± 0.3 mmol g−1 membrane. Furthermore, the PAA can be further functionalized to a wide variety of new chemistries through straightforward reaction mechanisms, creating a platform for fabricating high-capacity adsorbers tailored for the separation of specific similarly sized and similarly charged solute molecules.43 When these properties are combined with their high-throughput manufacturing methodologies and facile binding reversibility, these membrane adsorption systems are readily compatible with large-scale applications.

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EXPERIMENTAL SECTION

Polymer Synthesis. The polyisoprene-b-polystyrene-b-poly(N,Ndimethylacrylamide) (PI−PS−PDMA) triblock polymer used in this work was synthesized utilizing a reversible addition−fragmentation chain transfer (RAFT) polymerization mechanism.46−48 All chemicals were purchased from Sigma-Aldrich unless otherwise noted. Membrane Fabrication and Functionalization. Membranes were fabricated using the self-assembly and nonsolvent induced phase separation (SNIPS) method described in detail previously.36−38 Briefly, the PI−PS−PDMA triblock polymer was dissolved in a 70− 30 wt % mixture of dioxane−tetrahydrofuran to produce a casting solution with a concentration of 15 wt % polymer. After drawing the solution into a thin film using a doctors blade, solvent was allowed to evaporate for 45 s before plunging the cast film into a nonsolvent water bath. The conversion of the PDMA block, which lines the pore walls of the membranes, to a poly(acrylic acid) (PAA) block follows closely the process described in previous work.39,43 The membrane was placed in a 6 M hydrochloric acid solution, and the solution was heated to 75 °C and left unstirred for 64 h. After the prescribed reaction time, the hot plate supplying heat to the bath was turned off, and the acidic solution cooled to room temperature slowly. Once the solution was at room temperature, the membrane was removed, rinsed thoroughly with DI water, and stored in a Petri dish containing DI water. Transport Experiments. All transport tests were conducted using an Amicon 8010 stirred cell (EMD Millipore Corporation, Darmstadt, Germany). A 1 in. (2.54 cm) diameter circular hole punch was used to remove samples of triblock polymer membranes, which were placed in the bottom of the sample holder of the stirred cell. After assembly, the cell was filled with 10 mL of a solution, and a predetermined pressure [up to 1400 kPa (20 psi)] was applied to the cell using nitrogen gas (5.0 Ultra High Purity, Airgas, Radnor, PA). The pressure was monitored using a regulator (Victor Professional Single Stage EDGE ESS4, Victor Technologies, Chesterfield, MO). The solution that permeated through the membrane was collected in a scintillation vial that rested atop a balance. Copper adsorption tests were performed using aqueous solutions made with cupric chloride (CuCl2) salt. Experiments were also conducted with aqueous solutions containing 11114

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Langmuir magnesium chloride (MgCl2) and nickel chloride (NiCl2) salts to determine the effects of cations that do not bind strongly to PAA on membrane performance.49 More detailed descriptions of all the experimental protocols are provided in the Supporting Information.

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RESULTS AND DISCUSSION The polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI−PS−PDMA) triblock polymer utilized in this work was synthesized in three sequential steps using a RAFT-mediated polymerization mechanism. Polyisoprene (11.2 kDa) was synthesized first, followed by the addition of polystyrene (20.1 kDa), and finally poly(N,N-dimethylacrylamide) (17.3 kDa) was added to form the triblock polymer. The dispersity of the PI−PS−PDMA polymer, as determined by SEC against polystyrene standards, was 1.4. The triblock polymer was designed such that each block serves a specific purpose when a membrane is fabricated from this material. The hydrophobic PS block forms the matrix of the membrane; the PI block increases the toughness of the membrane relative to the diblock analogue, which makes the membranes easier to handle;41 and based on its unfavorable interactions with the two nonpolar blocks, the PDMA block directs the formation of the self-assembled pores.39 The PDMA block also causes the pore walls to be lined with moieties that can be functionalized in the solid-state using straightforward approaches, which allows new chemistries, designed for specific, directed applications, to be incorporated into the membrane. That is in this work, conversion of the pore walls to poly(acrylic acid) (PAA) produced a membrane that is capable of capturing copper ions selectively. Membranes were fabricated from the PI−PS−PDMA block polymer via the SNIPS method. A solution of 15 wt % polymer in a mixed solvent of dioxane−tetrahydrofuran (70−30 by weight) was spread evenly on a glass substrate using a doctor blade. Solvent was allowed to evaporate from the film for 45 s. During this short evaporation time, the polymer concentration at the air−film interface increased, which resulted in the selfassembly of the block polymer into distinct nanoscale domains. Subsequently, the film was plunged into a nonsolvent bath to precipitate the polymer, which kinetically trapped the selfassembled structure of the block polymer in the region near the air−film interface. The SEM micrographs of the top surface of the PI−PS−PDMA membranes (Figure 1a) reveal a welldefined pore size formed by the PDMA domains, with an average diameter of 38 nm and a standard deviation of 13 nm. The cross-sectional SEM images (Figure 1c,d) show that the self-assembled active layer of the membrane spans 5−8 μm, below which the nanoscale pores open up into larger macrovoids in a gutter layer that spans the remainder of the membrane. These large voids occurred because the solvent concentration remained high in this region during the short evaporation time. This resulted in a fast exchange of solvent and nonsolvent, which precipitates the polymer with large openings in the gutter layer.50,51 The PDMA block was converted to PAA by placing the membrane into a 6 M hydrochloric acid bath heated to 75 °C for 64 h; following this, the heat to the system was turned off, and the acid bath was allowed to cool for several hours to room temperature. The reaction protocol is similar to that described in prior literature.39−41 However, in this study, the reaction was carried out at a lower temperature due to major surface deformations that occurred when the reaction was executed at 85 °C. At a reaction temperature of 75 °C, a porous surface is

Figure 1. SEM micrographs of the top surface of the membrane for the (a) parent PI−PS−PDMA membrane prior to reaction and (b) the PI−PS−PAA after reaction with an aqueous 6 M hydrochloric acid solution. The parent PI−PS−PDMA membrane has a high density of consistently sized pores with an average diameter of 38 nm. After reaction to the PI−PS−PAA membrane, some structural rearrangement occurs. This limits the density of pores but keeps the average size of the pores at ∼45 nm. (c) Cross-sectional SEM micrograph of the membrane after conversion to the PAA functionality. A 6−8 μm thick active layer at the top of the membrane is supported by a spongy gutter layer underneath. The pores open into larger fingerlike macrovoids farther away from the membrane surface, which means that the structure provides a 45 μm thick gutter layer that self-supports the membrane. (d) A high-magnification cross-sectional SEM image at the upper surface interface of the membrane shows the nanostructure of the active layer. Despite a lack of alignment of the assembled pores, this layer has a bicontinuous structure of pore and polymer. This structure allows for permeation of solution through the narrow openings.

still visible in the SEM micrographs, with a pore size of 45 nm (Figure 1b). However, the density of pores has decreased due to the structural rearrangement that still occurred, even at this lower temperature. One likely explanation for the change to the surface is that the glass transition temperature of the majority material, polystyrene (Tg(PS) ∼ 100 °C), is near the reaction temperature. Therefore, there is increased mobility of the matrix phase at this elevated temperature, which could promote pore collapse. Despite this lowered reaction temperature, the conversion of PDMA to PAA goes nearly to completion, as evidenced by the FTIR spectra in Figure S2, which shows the disappearance of the amide peak near ∼1600 cm−1 and the appearance of the carboxylic acid peak near ∼1700 cm−1. Complete conversion at this lower reaction temperature required 64 h due to the lowered reaction rate relative to previous studies.39 Stimuli-Responsive Hydraulic Permeability of PAAFunctionalized Membranes. Conversion of the pore walls from PDMA to PAA affects the transport properties of the membrane. The hydraulic permeability of the membranes was measured in a 10 mL stirred cell using an applied pressure to drive the flow of solution through the membrane. The parent 11115

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to swell toward the pore center, which shrinks the pore diameter and decreases the permeability.39 When the solution pH was well below the bulk pKa of poly(acrylic acid), the permeability began to increase sharply between pH 3.6 and 3.0. This suggests that the PAA chains were being protonated under the acidic conditions, which decreased the swelling of the PAA chains and increased the average pore size. Below a pH of 3, the slight decrease in permeability observed is within the error of the measurements and, thus, cannot be attributed to a change in the pore diameter. The concentration of copper dissolved in the feed solution affected the observed permeability of the PI−PS−PAA membrane. During experiments where a copper chloride solution permeated the membrane, the volumetric flux of solution through the membrane increased monotonically until leveling at a steady state. In Figure 2b, the steady-state permeabilities are plotted as a function of the copper concentration in the feed solution. Solutions containing no copper (i.e., DI water) possessed a permeability of 1 L m−2 h−1 bar−1. The permeability increased sharply as the retentate concentration of copper increases from 2 to 4 to 6 mM CuCl2 and then saturated, reaching a value of 6.8 L m−2 h−1 bar−1 for a 43 mM CuCl2 retentate solution. The increase in permeability was likely the result of the positively charged copper ions interacting with the negatively charged PAA brushes that line the pore walls. The repulsive intrachain interactions between repeat units play a large role in the swelling of the PAA brushes that line the pore walls, and interference with these interactions would cause the PAA brushes to return to less-extended chain conformations; this would directly increase the pore diameter and therefore the hydraulic permeability. The copper ions may interfere either by screening the negative charges between nearby repeat units or by interacting with negatively charged oxygen atoms and eliminating the electrostatic repulsion completely. 53−55 A similar effect was observed during permeability measurements using solutions of magnesium chloride at varied concentrations. This suggests that the increase in pore size with increasing copper concentration is due to the screening of electrostatic repulsion between PAA repeat units by the cations rather than an effect of complexation on the conformation of the PAA chains. It is noted that the concentration where the permeability curve in Figure 2b begins to saturate correlates well with the point in Figure 3a where the binding capacity starts to level after a sharp rise. The permeability does not increase indefinitely with CuCl 2 concentration because the PI−PS matrix forms a rigid boundary, which limits the maximum pore diameter and, therefore, permeability. Comparison of Experimental Ideal Capacities of PAAFunctionalized Membranes. In many deprotonated carboxylic acid systems, including PAA, electrons from the oxygen can interact with copper cations in order to form polymer−metal complexes.55 To demonstrate this effect in the current system, copper capture experiments were performed using a PI−PS− PAA membrane after it had been rinsed with DI water. Then, a solution of CuCl2 in DI water (pH 4.0−5.0) was added to the 10 mL stirred cell, and a low pressure (i.e., 1 psi ≤ ΔP ≤ 6 psi) was applied. The applied pressure was modified based on the differences in hydraulic permeability, which depends on the bulk concentration of copper (Figure 2b). The solution was allowed to permeate through the membrane in order to afford the copper ions access to the PAA binding sites throughout the membrane. After half of the feed solution had permeated

PI−PS−PDMA membrane was tested at room temperature using DI water (pH 5.5), an acidic solution (pH 1), and a basic solution (pH 13) as the feed solutions (Figure 2a). The

Figure 2. (a) Hydraulic permeability of the PI−PS−PDMA and PI− PS−PAA membranes as a function of solution pH. The permeability of the PI−PS−PDMA membrane is not a function of pH and has a permeability value of ∼10 L m−2 h−1 bar−1. The permeability of the PI−PS−PAA membrane depends on solution pH. For solutions above a pH of 3.4, the permeability is low. For solutions below a pH of 3.4, the permeability increases sharply. Error bars indicate propagated uncertainty in the measurements from a single experiment. (b) Steadystate hydraulic permeability of PI−PS−PAA membranes as a function of copper concentration for solutions of CuCl2 and MgCl2 at a pH between 4 and 5. The permeability of the PAA-functionalized membrane increases when the cation concentration is increased until it plateaus at a value between 6 and 7 L m−2 h−1 bar−1 for both salts. Error bars indicate one standard deviation in measurements from multiple experiments conducted at these concentrations.

permeability remained constant at ∼10 L m−2 h−1 bar−1 through the three experiments, suggesting that the pH of the solution has negligible effect on the permeability of the PI− PS−PDMA membrane. The permeability of the functionalized PI−PS−PAA membrane was also measured at varied pH values. When the membrane was exposed to a pH 13 solution, the permeability of the membrane was as low as 0.8 L m−2 h−1 bar−1. The permeability remained at this low value for solutions with pH values above the bulk pKa of the poly(acrylic acid) (6.5).52 This result has been explained previously by the ionization of the poly(acrylic acid) repeat units, which are largely deprotonated above their pKa. The electrostatic repulsion between the like negative charges and the better solvation of the deprotonated PAA by water caused the chains 11116

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Therefore, the copper binding capacity was evaluated using these permeation tests and prolonged soaking experiments. These data are plotted as a function of copper concentration in solution (Figure 3a). The vertical axis represents the binding capacity and the horizontal axis represents the final retentate concentration at equilibrium. The experimental data for the two methods of determining capacity demonstrate strong agreement. For both data sets, the concentration of bound copper increased sharply as the concentration of free copper increase from 0 to 10 mM; the concentration of bound copper increases less rapidly for concentrations of dissolved copper above 20 mM and begins to saturate around 70 mM. The highest capacity determined from transport tests, 4.3 mmol Cu2+ g−1 membrane, slightly exceeded the capacity calculated using the PDMA content of the triblock polymer material, 3.98 mmol Cu2+ g−1 membrane, which is represented by the horizontal dashed line in Figure 3a. This theoretical value was calculated by assuming complete conversion of PDMA to PAA and that each PAA repeat unit binds one copper ion. More detailed calculations for the determination of theoretical and experimental binding capacities can be found in the Supporting Information. The strong agreement between the experimental and calculated saturation capacities suggests that a one-to-one ratio of PAA repeat unit to copper ion is a more accurate reflection of the binding stoichiometry than the more commonly observed ratio of two carboxylic acid moieties to one copper ion. This may be driven by the high concentration of dissolved copper ions at saturation. Prior studies have demonstrated that at high concentrations of copper ions a single carboxylic acid group interacting with a single copper ion is a viable binding mechanism because the anions in solution can help to stabilize the complex.56,57 The agreement between the experimental and theoretical saturation capacities suggests that the copper makes use of all of the available binding sites throughout the entire membrane. This allows us to estimate the binding capacity of the membrane accurately based on the PDMA weight fraction incorporated into the triblock polymer precursor. Or, perhaps more importantly, a membrane with a specific binding capacity can be designed systematically by selecting the triblock polymer compositions wisely. Additionally, the binding capacity can be controlled by the conversion to PAA during the acid reaction step, as partial conversions are easily achieved by stopping the reaction before a full 64 h has elapsed. This control over active site density during the precursor synthesis and membrane functionalization stages provide block polymer materials with advantage over other adsorbents, where capacities are often only determined experimentally after synthesis. In order to quantify the maximum experimental capacity in a systematic manner, the experimental data were fit to a Langmuir isotherm model (eq 1).

Figure 3. (a) Copper uptake capacity as a function of CuCl2 concentration in solution at pH between 4 and 5. The copper capacities were determined by analyzing the concentrations of the permeate and retentate solutions during breakthrough experiments. These data are marked by red diamonds. Copper capacity was also determined by the amount of copper released after exposing a PAAfunctionalized membrane to a pH 1 solution. These data are shown by blue squares. The black dotted line at 3.98 mmol g−1 represents the theoretical copper capacity calculated using the moles of poly(acrylic acid) (PAA) groups in the block polymer precursor. (b) A comparison of the experimental capacity to a Langmuir isotherm, with a solid black line shown as a best fit curve of the data. The inverse of the value of the slope suggests a saturation capacity of 4.1 ± 0.3 mmol g−1 for this section of membrane. Error bars indicate propagated uncertainty in the measurements from a single experiment.

through the membrane, the applied pressure was decreased to zero, and the retentate solution was left in the cell overnight to ensure that the system reached equilibrium. The concentrations of copper in the permeate and retentate samples were analyzed using an ultraviolet−visible (UV−vis) spectrophotometer. These data, in conjunction with simple materials balances, were used to determine the amount of copper that had been removed from solution, which corresponds to the quantity of copper bound by the PAA-functionalized membranes. These data are represented by the red diamonds in Figure 3a. After allowing the membrane and solution to approach equilibrium overnight, the retentate solution was removed, and 4 mL of a pH 1 hydrochloric acid solution was added to the cell to release the bound copper. An applied pressure was used to drive half of the pH 1 solution through the membrane, and then the cell was left stirring for an additional hour to ensure all of the bound copper was released from the membrane. The resulting concentration was used to determine the amount of copper ions released from the PAA-functionalized membrane. These data are shown by the blue squares in Figure 3a.

q=

QKc 1 + Kc

(1)

In this model, the equilibrium binding capacity (q) is a function of the solute concentration in the solution (c), the maximum binding capacity (Q), and the equilibrium constant (K) for the bound and unbound states. If both sides of eq 1 are inverted and multiplied by the concentration of solute in solution, eq 2 is obtained.

c 1 c = + q QK Q 11117

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Langmuir As such, Q can be extracted readily from a plot of cq−1 vs c (Figure 3b). The good agreement of the best-fit line (black line, Figure 3b) suggests that the Langmuir isotherm is a suitable model. The maximum binding capacity, calculated using the inverse of the slope, is determined to be 4.1 ± 0.3 mmol g−1 membrane. Based on the Langmuir linearization of the capacity for the 2.54 cm diameter circular membrane with an average thickness of 55 μm, the maximum binding capacity is 4.1 ± 0.3 mmol g−1 membrane (2.6 mmol cm−3 membrane). Based on the assumption that the majority of the membrane porosity exists in the gutter layer, it is estimated that about 25% of the active sites are located within the selective layer of the membrane. See Supporting Information for more detailed calculations. The total value for the membrane copper binding capacity is slightly higher than several of the highest copper-binding resins (2.2 mmol g−1 iminodiactetate) and beads (3.8 mmol g−1 potato starch-g-PAN), which are the most common forms of adsorbents used in commercial applications.33,34 The PI−PS− PAA material also is on the same order of magnitude as and outperforms other membranes designed for copper capture, including a layer-by-layer poly(allylamine)/poly(N,Ndicarboxymethylallylamine) (PAH/PDCMAA) membrane and an N-6-(t-dodecylamido)-2-pyridinecarboxylic acid membrane with capacities of 2.5 mmol cm−3 membrane22 and 0.4 mmol g−1 membrane,26 respectively. The block polymer membrane also exceeds the capacity of a grafted-from PAA composite with a binding capacity of 0.6 mmol g−1.49 In addition to providing the membrane with facile processing steps for fabrication and functionalization, using the triblock polymer material as a precursor provides the membrane with a high density of built-in active sites for ion adsorption. Block Polymer Membrane Adsorbers Demonstrate a Sharp Breakthrough Curve. The maximum copper capacity of the PI−PS−PAA triblock polymer membrane is comparable with other state-of-the-art resins. Additionally, these membranes may be an attractive option over existing resins because of the reduced mass transfer limitations associated with membranes. For resins, convective flow occurs around the outside of the beads, and diffusion is the only mechanism available to move the solute from the exterior of the beads to the interior of the beads where the active sites reside. The long diffusion lengths needed to access the active sites within the beads (∼100−500 μm) result in long residence times or lessthan-ideal fraction of binding sites used during operation. During metal separations using the triblock polymer membrane, the ions flow with the solution through the small pores lined with active binding sites, where a short diffusion length to the pore walls results in a highly efficient capture rate. The effectiveness of this capture can be demonstrated by a breakthrough experiment performed with 10 mL of an 8 mM CuCl2 solution at an applied pressure of 1 psi (Figure 4). In the first 1 mL of solution permeating through the membrane, a large portion of the copper is captured, as there is a high density of binding sites available. The lower concentration of ions that permeates through the pores in the active layer enter the gutter layer, which also contains a high density of exposed PAA groups. Because of the high porosity of this layer, the superficial velocity of the solution decreases significantly, resulting in a long residence time that allows for the capture of most of the ions. Therefore, the permeate concentration in the first 1 mL remains around zero. As a larger fraction of the sites become occupied, more copper ions begin

Figure 4. Breakthrough curve showing instantaneous permeate concentration as a function of permeate volume for a PAAfunctionalized membrane using an 8 mM CuCl2 feed solution at pH 5.5. An applied pressure of 1 psi pressure was used. 10 mL of feed solution was introduced into the stirred cell at the start of the experiment, and 0.3 mL samples of the permeate were collected as the experiment progressed in time. Error bars indicate propagated uncertainty in the measurements from a single experiment.

to permeate the entire membrane without being captured, leading to the concentration rise between 1 and 2 mL of permeate. After breakthrough, the permeate concentration quickly approaches the steady-state value of 4.2 mM CuCl2. This value differs from the initial feed concentration of 8 mM due to the operation of the cell as a dead-end filtration. In this mode of operation, the upstream solution is not continually refreshed with 8 mM CuCl2 feed solution, which allows the concentration to change with time. In the experiment presented here, because copper ions are transported from the membrane surface to the active sites within the membrane by convection of solution through the membrane as well as by diffusion through the membrane, the final concentration of copper in the retentate is lower than the concentration of copper in the feed solution. As such, the retentate concentration decreases as copper is captured until the concentration of copper ions in solution approaches equilibrium with the copper ions bound to the PAA brushes. Operation at a higher flux would likely result in a broader breakthrough curve that begins at a lower permeated volume. Nevertheless, even at a relatively low velocity, the sharp breakthrough shown in Figure 4 is indicative of a low dispersity in pore size and an efficient separation. A sharp breakthrough, like the one shown in Figure 4, highlights the ability of the membrane to limit permeate concentrations to near zero during loading and to make use of binding sites effectively throughout the membrane. This uniform binding is quantified by the length of unused bed, represented as S′ and defined in eq 3. ⎛ W ⎞ S′ = S⎜ 1 − b ⎟ We ⎠ ⎝

(3)

Here, S represents the thickness of the membrane, Wb is the point of breakthrough, or the point at which the permeate has reached 50% of the equilibrium value, and We is the point where the permeate has approached equilibrium. The values for Wb and We may be reported as a time, a volume of permeate, or the weight of adsorbent used. In the breakthrough experiment performed on this membrane, the 50% mark occurs around 2.0 mL, equivalent to over 33 bed volumes (assumed to be 28 μL based on the membrane thickness), with equilibrium being 11118

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Langmuir approached at 2.5 mL. This results in an S′ of ∼11 μm, compared to a 55 μm thick membrane, or the fraction of unused membrane is 20%. This means that when significant breakthrough of solutes has occurred, 80% of the active sites throughout the membrane have been used to bind copper ions. A more rigorously defined breakthrough point commonly used in membrane separations puts the breakthrough point where the concentration exceeds 10% of the equilibrium value.58 Under this definition, the breakthrough occurred in the membrane after a permeate volume of 1.2 mL, resulting in the fraction of used membrane around 50%. The most effective separations require a small length of unused bed which allows the membrane can be cycled efficiently during continuous separation processes.59,60 The relatively sharp breakthrough curve displayed in Figure 4 is a result of the characteristic diffusion time to active sites being short compared to the residence time for flow through the membrane. To determine how the separation would be affected by the velocity of the solution through the membrane, the membrane was challenged with 10 mL of an 8 mM solution of CuCl2. An applied pressure that varied from 0.3 to 10 psi was used to drive flow. Samples of the permeating solution were collected every 1 mL until a total of 5 mL had permeated through the membrane. The first sample was an exception, and the first 2 mL was collected. This was based on the assumption that the binding sites within the membrane would not be saturated within the first 2 mL of permeate, and the efficiency of capture would not change at this point in the experiment. In Figure 5, the cumulative fraction of copper ion capture is shown as a function of the steady-state flux. At all fluxes, as the

Additionally, the presence of dispersity in the pore sizes that results from structural rearrangement would result in increased flow through the larger pores, making for less efficient separations. On the other hand, at the lowest flux of 0.033 bed volumes min−1, the capture efficiency was found to be less than at 0.08 bed volumes min−1. This may have occurred as the rate of copper diffusion through the membrane became comparable to the rate of convective transport at this flux. PAA-Functionalized Membrane Captures Copper Reversibly. The binding of copper ions to PAA is pHdependent and reversible.22,45,61 Breakthrough experiments were conducted with copper solutions that had a pH less than 8, to avoid the precipitation of the hydroxide salt (Cu(OH)2), but greater than the pKa of the PAA chains because the PAA moieties must be deprotonated to bind copper ions.61 For this reason, copper capture experiments were run with feed solutions of copper ions dissolved in DI water, with a resulting pH between 4 and 5. A membrane at the start of an experiment is shown in the left panel of Figure 6a.

Figure 6. (a) Photographs of the PI−PS−PAA membrane before (left) and after (center) a copper capture experiment. The copper adsorbs through the membrane, giving it a blue-green color. After washing the membrane with an acidic solution (right), the membrane returns to its original off-white color. (b) Increasingly more acidic solutions (moving left to right) were added to the membrane loaded with copper, and the amount of copper released into solution was determined. At each pH, 4 mL of acid was added, and after stirring for 10 min, the solution permeated through the membrane, until 2 mL was collected downstream. The copper concentration in the upstream side of the membrane shows the ions released as the red circles, while the black squares show the ions collected in the permeated solution. The combined total is shown by the blue triangles. (c) Saturation capacity over the course of multiple adsorption experiments at 8 mM CuCl2 feed concentration. The capacity of the first experiment was determined to be 32 μmol, and the concentration of the acid wash solutions following the copper loading was used to compare to this initial value. The values all remain over 80% of the original loading capacity.

Figure 5. Percentage of copper ions captured is compared to the flux of the permeating solution 8 mM CuCl2 solution. The applied pressure was adjusted between 0.3 and 10 psi to change the flux for each experiment. Permeate samples were collected at 1 mL intervals. Each datum shows the cumulative amount of copper captured from the beginning of the experiment at 0 mL permeate collected. The percentage of capture decreases as more volume permeates the membrane, dropping just below 50% at the highest flux and largest volume. At a low flux of 0.08 bed volumes per minute, the rejection begins as high as 91%, but the rejection decreases at even lower fluxes.

volume eluted increased, the cumulative fraction of copper captured decreased. This occurred because the copper ions bind to the available PAA binding sites early in the experiment, leaving fewer free at higher permeated volumes, and therefore there is less chance for the ions to be bound. The percent of copper captured showed a noticeable dependence on flux, where the capture was less efficient at higher fluxes; this is likely due to the decrease in residence time inside the pores.

Because no copper is bound to the membrane initially, the membrane appears off-white in color. After loading with copper during an experiment, the membrane turns a blue-green color, as shown by the center image of Figure 6a. In this case, the membrane was exposed to an 8 mM CuCl2 solution overnight. After loading with copper ions, 4 mL of a hydrochloric acid solution (pH = 1) were added to the cell and stirred to release the bound copper and regenerate the membrane. Within 11119

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and dissolved solutes rather than size-based filtration (like most membranes), the PI−PS−PAA membrane can selectively permeate ions with a similar size as copper, as long as they do not bind to the PAA repeat units. For example, nickel ions have a hydrodynamic radius of 4.04 Å, which is similar to the hydrodynamic radius of copper ions (4.19 Å), but the binding affinity of copper to PAA is an order of magnitude greater than that of nickel.62,63 In order to test the selectivity of the PI−PS− PAA membrane for copper over nickel, a mixture containing 5 mL of a 10 mM NiCl2 solution and 5 mL of a 10 mM CuCl2 solution was added to the cell, leaving a resulting mixture containing 5 mM of each cation. A pressure of 1 psi was applied to drive the solution through the membrane. The permeate was collected in 0.5 mL intervals. After a total of 6 mL of the permeate were collected, the pressure was released and the solution was left in the cell overnight. The following day, the solution was removed, and 4 mL of acidic solution (pH = 1) were added and left for 30 min before collecting a sample. All the samples were analyzed using UV−vis spectrometry to determine the contents of copper and nickel. Absorption readings at λ = 930 nm and λ = 388 nm were used to determine the ion concentrations for the solutions for copper and nickel, respectively. Because copper gives a small absorption reading at 388 nm, UV−vis measurements were made of solutions containing 0 to 5 mM nickel and copper ions in order to determine the correction for the nickel readings at this wavelength based on the observed copper concentration. The permeate concentrations and volumetric flux of solution measured throughout the experiment were used to calculate the average molar flux of both ions over each time interval, as shown by the data in Figure 7. The flux of copper ions (blue

seconds, a change in the color of the membrane surface from blue-green to off-white was observed. The acidic solution was passed through the membrane to ensure that it reached the active and gutter layers of the membrane, and the membrane was left in the solution for a minimum of 30 min to allow adequate time for the desorption of the bound copper. The right photograph in Figure 6a demonstrates that the membrane surface returns to its initial off-white color after regeneration. Following regeneration at pH = 1, a basic solution (pH = 13) was added to the cell to deprotonate the PAA moieties in preparation for another copper capture experiment. In order to determine the relationship between the release of copper ions and the pH of the wash solution, increasingly more acidic solutions were added to a copper-laden membrane after it had soaked in a 100 mM CuCl2 solution overnight. After removing the residual copper retentate solution from the test cell, 4 mL of DI water (pH = 5.5) were added to the cell and stirred, with a low pressure applied to the cell. After 2 mL of the solution had permeated through the membrane and the membrane had been exposed to the wash solution for at least 30 min, the concentration of copper in the retentate and permeate solutions was analyzed. This process was repeated with increasingly more acidic solutions until all the copper ions had been released. The percentage of bound copper released at each pH is shown in Figure 6b. The addition of DI water releases over half of the initially bound copper because the equilibrium between bound and unbound copper, which is quantified by the isotherm in Figure 3a, requires a release of ions into the initially copper-free solution. Following this, the slightly more acidic solutions release small amounts (∼10% total capacity) of additional copper through pH = 3. Below this pH, significant amounts of copper are released as pH decreases, until the remaining bound copper is released completely at pH = 1. A secondary wash at this lowest pH yielded no additional copper release from the membrane. At this low pH, the equilibrium shift likely occurs because the majority of the active sites on the PAA chains are reprotonated. Figure 6b also demonstrates that the concentration of released copper measured in the permeate solutions is greater than that in the retentate. This confirms that the binding is not only occurring on the surface of the membrane but is also occurring within the pores where the majority of the active sites are located. The release of copper from the membrane by simple acid washing is useful, but only if the membrane can capture large quantities of copper upon repeated uses. For this reason, repeated binding and regeneration experiments were performed using an 8 mM CuCl2 feed solution. In these experiments, a membrane was left in a copper solution overnight to ensure that it reached equilibrium. The following day, the cell was emptied, and a pH 1 solution was added to the cell in order to release the bound copper. This procedure was repeated five times, and the amounts of released copper for each trial are shown in Figure 6c. Across all these experiments, the binding capacities did not deviate more than 12% from the average, which suggests that the binding sites are almost entirely regenerated. This is an important property for potential applications of these membranes, where a consistent loading capacity and a low rate of membrane replacement are desired. PAA-Functionalized Membrane Captures Copper Selectively. Because membrane adsorbers separate compounds based on reversible binding between the membrane

Figure 7. An equimolar mixture containing 5 mM CuCl2 and 5 mM NiCl2 was permeated through the PI−PS−PAA membrane at a pressure of 1 psi, with permeate samples collected every 0.5 mL. The concentrations of the ions in solution were determined by UV−vis at λ = 930 nm for copper ions and λ = 388 nm for nickel ions. The copper flux begins below 3 mol m−2 h−1 for the first milliliter before increasing to a steady state of 17 mol m−2 h−1. Error bars indicate propagated uncertainty in the measurements from a single experiment.

squares) and nickel ions (red circles) are plotted as a function of volume of permeate eluted up to 6 mL. The flux of nickel ions remains around the average value of 21 mol m−2 h−1 at all elution volumes. This suggests that there is no significant capture of nickel ions occurring even while a large number of PAA repeat units are available for binding. Meanwhile, the copper ion flux is very low within the first 3 mL of permeation, suggesting that most of the copper ions entering the membrane are bound by the PAA repeat units. As more active sites are 11120

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occupied, the flux of copper increases until leveling near 17 mol m−2 h−1. It was observed that the steady-state flux of the copper ions was lower than that of the nickel ions. This is caused by the reduced concentration of copper ions in the retentate solution, which resulted from operation in dead-end mode. The copper concentration in the retentate after the experiment was 4.3 mM. The concentration of nickel ions in the retentate, on the other hand, was within 5% of the concentration of nickel ions in the initial feed solution, suggesting that the binding of nickel ions to the active sites is negligible. Finally, the amount of copper ions released after the acid wash step was 19 μmol, which was similar to the amount adsorbed during single ion experiments (21 μmol, Figure 3a). The amount of nickel released was significantly lower, but nonzero, around 2.4 μmol. This may indicate that nickel ions can bind to PAA repeat units, though not as favorably as copper, or the ions may have been retained within the pores due to electrostatic interactions.57 Before the breakthrough of copper ions from the retentate solution, the membranes successfully retain the copper ions while allowing the nickel ions to pass. The values of the instantaneous selectivity, which are shown in Figure S3, are determined by dividing the nickel ion flux by the copper ion flux. Within the first 3 mL of permeate the value of the selectivity remains above 10, before beginning to decrease toward roughly 1.3, where it appears to remain after saturation of the active sites. The average value of the selectivity based on the ion fluxes for eluted volumes from 0 to 3 mL is 10.5, which is comparable to other membrane selectivities in the field currently.26,64,65

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01605. Detailed description of the experimental protocols implemented, calculation details for the determination of the experimental and theoretical copper binding capacities, UV−vis absorption spectra for nickel ion and copper ion in water solutions, a sample UV−vis calibration curve used for copper concentration analysis, FTIR of a PI−PS−PDMA membrane and a PI−PS−PAA membrane illustrating conversion of functional groups, and a plot displaying calculated selectivities of nickel/ copper permeability from data shown in Figure 7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(W.A.P.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Portions of this work were made possible with support from the Army Research Office (ARO) through the Polymer Chemistry Program (Award W911NF-14-1-0229, Program Manager: Dr. Dawanne Poree) and the National Science Foundation (NSF) through the Nanomanufacturing Program (Award 1436255, Program Manager: Dr. Khershed Cooper), and we appreciatively acknowledge this support. B.W.B. thankfully acknowledges support from the Ralph W. and Grace M. Showalter Research Trust Award at Purdue University. W.A.P. gratefully acknowledges support from the 3M non-Tenured Faculty Award. We thank the Notre Dame Integrated Imaging Facility (NDIIF) and the Center for Environmental Science and Technology (CEST) at Notre Dame; portions of this research were performed with instruments at these facilities.

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CONCLUSIONS The block polymer membranes show strong potential as a platform for future membrane adsorption devices. Using the SNIPS casting process, which is consistent with roll-to-roll processing methods, to fabricate membranes from a PI−PS− PDMA triblock polymer material yielded a robust membrane with pores lined by PDMA moieties. Though the permeability of the membrane is low compared to other membrane adsorbers, the SNIPS method of casting these membranes involves a number of parameters that can be easily tuned in future studies with this material to produce a membrane structure with optimal nanostructure and porosity to achieve superior membrane properties (e.g., an increased membrane permeability). Conversion of the PDMA block to the PAA functionality, through a straightforward reaction, resulted in a membrane that was responsive to the concentration of copper ion in solution and was capable of binding copper ions in a high capacity. The experimentally determined capacity (4.1 ± 0.3 mmol Cu2+ g−1 membrane) was comparable to that of currently used resins. Furthermore, it was demonstrated that the binding of copper was reversible and selective. The PI−PS−PDMA block polymer precursor provided a high density of built-in binding moieties and also resulted in a self-assembled active layer with consistently sized pores. This made for short diffusive distances, which allowed up to 80% of the binding sites to be utilized before the breakthrough of copper ions into permeate solution occurred. As such, these results demonstrate the future ability of these types of membranes to have tailored functional groups such that specific adsorption of metal ions or other small molecules of interest can occur in a selective and straightforward manner.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b01605 Langmuir 2015, 31, 11113−11123

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DOI: 10.1021/acs.langmuir.5b01605 Langmuir 2015, 31, 11113−11123