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Nanoporous Block Polymer Thin Films Functionalized with Bio-Inspired Ligands for the Efficient Capture of Heavy Metal Ions from Water Jacob Logan Weidman, Ryan A Mulvenna, Bryan W. Boudouris, and William A. Phillip ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 22, 2017

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ACS Applied Materials & Interfaces

Nanoporous Block Polymer Thin Films Functionalized with Bio-Inspired Ligands for the Efficient Capture of Heavy Metal Ions from Water 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, IN 46556-5637, United States ‡

Charles D. Davidson School of Chemical Engineering and ∆ Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States * To whom correspondence should be addressed: [email protected]

Abstract Heavy metal contamination of water supplies poses a serious threat to public health, prompting the development of novel and sustainable treatment technologies. One promising approach is to molecularly engineer the chemical affinity of a material for the targeted removal of specific molecules from solution. In this work, nanoporous polymer thin films generated from tailor-made block polymers were functionalized with the bio-inspired moieties glutathione and cysteamine for the removal of heavy metal ions, including lead and cadmium, from aqueous solutions. In a single equilibrium stage, the films achieved removal rates of the ions in excess of 95%, which was consistent with predictions based on the engineered material properties. In a flow-through configuration, the thin films achieved an even greater removal rate of the metal ions. Furthermore, in mixed ion solutions the capacity of the thin films, and corresponding removal rates, did not demonstrate any reduction due to competitive adsorption effects. After such experiments, the material was repeatedly regenerated quickly with no observed loss in capacity. Thus these membranes provide a sustainable platform for the efficient purification of lead- and cadmiumcontaminated water sources to safe levels. Moreover, their straightforward chemical modifications suggest that they could be engineered to treat sources containing other recalcitrant environmental contaminants as well. Keywords. block polymer self-assembly, heavy metals, water treatment, membrane adsorbers, glutathione, polyisoprene, polystyrene, poly[N-(2-mercaptoethyl)acrylamide]

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Introduction Water is a critical resource for modern society, and a number of high-profile academic and industrial efforts have focused on the preservation and sustainable utilization of this precious resource.1–6 However, recent events have shown that water sources can easily become highly contaminated with pollutants, including toxic heavy metals that pose a clear threat to communities throughout the world.5,7,8 For example, high levels of lead (II) cations can hinder brain development in children who drink from contaminated sources and cause organ damage to people of all ages.7,9 Similarly, water supplies contaminated with cadmium (II) cations cause the weakening of bones and can damage critical organs (e.g., the kidney and liver).8,10,11 These deleterious effects are not unique to the human race, and, in many organisms, there exists a naturally-evolved mechanism of heavy metal ion capture to counter accumulation of these toxins.9,12–15 For example, the peptide glutathione, which is found in many plants and animals can complex with the metal ions, immobilizing and removing the ions from transport pathways within the organism.9,13 Many systems designed to treat water systems draw inspiration from this natural detoxification mechanism by implementing adsorptive groups on material surfaces. And, these adsorbers have displayed high capacities in heavy metal ion uptake, suggesting promise for their applicability to purification in processes utilizing packed bed configurations. However, in the removal of dilute contaminants, packed bed processes are hampered by several deficiencies. The beads used to pack the bed are greater than 0.1 mm in size, which results in long diffusive distances in order to reach binding moieties on the interior of the beads. In turn, this requires long residence times for effective processing, leading to decreased throughput of the treated solution. Additionally, the packed bed systems face issues including channeling, dead zones, and the possibility of backflow.16–18 In order to improve on packed bed separations, porous 2 ACS Paragon Plus Environment

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membranes have been implemented as a simple but effective platform for metal ion capture.17–19 The nanoporous channels severely shorten the diffusive distances, and, therefore, the residence times required for adsorption. Additionally, a low pressure drop across the membrane allows for a high throughput of solution, and the implicit staging of the membrane results in a compounded equilibrium effect, which can reduce concentrations to trace levels through a single membrane. However, effective utilization of membranes in an adsorptive separation critically requires that a high density of binding sites line the membrane surface. In this regard, block polymer membranes are a highly promising material platform as they have a high density of built-in functional groups.20–26 For this reason, a polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PIPS-PDMA) was designed as a precursor material, in which the poly(N,N-dimethylacrylamide) (PDMA) block is easily hydrolyzed to poly(acrylic acid) (PAA) after casting the block polymer into a nanoporous thin film.27–29 These PAA pore walls are then functionalized to a variety of different chemistries, which are tailored to the specific process needs of directed applications, using simple and easily adjusted reaction conditions.30 Thus, this material lends itself to the ready attachment of biochemical functionalities to the nanopore walls in an extremely high density for the high-throughput, low pressure, and high-efficiency chemical-specific separation of toxic heavy metals, including lead and cadmium salts, from aqueous solutions. Here, we design and implement these straightforward coupling chemistries to attach extraordinarily high densities of bio-inspired heavy metal binding ligands within the pores of nanostructured block polymer thin films. In flow-through experiments, we demonstrate the nearly complete removal of lead ions from highly contaminated sources and sources at environmentallyrelevant concentrations (e.g., levels observed in contaminated residential piping due to municipal water supply changes) because of our ability to tune the large number of biochemical interactions 3 ACS Paragon Plus Environment


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within the membrane pores. Due to the versatility of the peptide-like binding moieties, the presence of multiple heavy metals competing for binding sites was found to have a negligible effect on the ability of the thin films to remove ions at low concentrations. Additionally, the material is easily regenerated after adsorption of lead and cadmium ions with no noticeable decrease in uptake in subsequent cycles. Thus, this study displays the ability of this material to operate as an effective separator for the lead and cadmium ions for which it was designed. Furthermore, this technology presents itself as a platform application in a variety of new separations, as the need for these separations emerge, based on the tailorability of the biochemical functional groups that line these nanoporous materials.

Experimental Methods Polymer Synthesis and Thin Film Fabrication The polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-PS-PDMA) triblock polymer utilized in this work was synthesized using the reversible addition-fragmentation chaintransfer (RAFT) polymerization mechanism, as described in full previously.31,32 The polymer was characterized, at the conclusion of each polymerization step, using 1H NMR spectroscopy to measure the molecular weight and using size exclusion chromatography (SEC) to ensure a sufficiently low dispersity.31,32 Subsequently, nanoporous thin films were fabricated following the self-assembly and non-solvent induced phase separation (SNIPS) method.27,28 That is, the PI-PSPDMA block polymer material was dissolved to form a 15% (by weight) solution in a solvent mixture of 70/30 (w/w) dioxane and tetrahydrofuran. After spreading the resulting solution into a thin film using a doctor blade with a gate height set to 254 µm, solvent was allowed to evaporate for 45 s before plunging the thin film into a DI water bath to fix its nanostructure in place.28 The thin films were then stored in a Petri dish containing water until further use. 4 ACS Paragon Plus Environment

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Hydrolysis and Functionalization The PDMA moieties lining the pore walls of the nanoporous PI-PS-PDMA thin films were converted, through acid-catalyzed hydrolysis, to poly(acrylic acid) (PAA) groups, as described in previous work.20,28 The PI-PS-PDMA films were removed from storage in Petri dishes and placed between two PAN 400 membranes. These membranes were then placed between two glass slides in order to prevent curling of the thin films during reaction. The glass slides were placed into 200 mL of a 6 M hydrochloric acid solution at 75 °C for 64 h, then the solution was allowed to cool to room temperature. No precautions to prevent the cross-linking of polyisoprene are taken, as this increases the toughness of the resulting membrane.27 The resulting PI-PS-PAA thin film was then removed from the acid bath and placed into storage in a Petri dish containing water for later use (e.g., analysis by FTIR and SEM or further functionalization). In order to functionalize the thin films to heavy metal adsorbers, the nanoporous PI-PSPAA thin films were reacted via 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) coupling reactions.33 The thin films were placed into 10 mL solutions of 0.20 M 1-ethyl-(3dimethylaminopropyl)-3-ethylcarbodiimide

hydrochloride

(EDC/HCl)

and

0.20

M

hydroxybenzotriazole (HOBt). The presence of HOBt promotes the forward reaction toward the desired amide functionality while limiting the reverse reaction to the initial acrylic acid chemistry. The functional groups were also dissolved into the solution at a concentration of 0.25 M of either glutathione or cysteamine, depending on the desired chemistry. The thin films were covered and left to react in solution for 3 days at room temperature; then the films were removed from the solution and rinsed. Upon functionalization with glutathione, the membranes became polyisoprene-b-polystyrene-b-poly{N-[1-carboxy-3-({[1R]-1-[(carboxymethyl)carbamoyl]-2sulfanylethyl}carbamoyl)propyl]acrylamide} (PI-PS-PAG). Functionalization with cysteamine



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produced

polyisoprene-b-polystyrene-b-poly[N-(2-mercaptoethyl)acrylamide]

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(PI-PS-PASH)

membranes. Thin films were stored in Petri dishes containing water prior to experimentation. Characterization using Fourier Transform Infrared (FTIR) and Scanning Electron Microscopy (SEM) For FTIR analysis, 1 cm × 1 cm sections of the thin films were cut with a razor blade, then dried under vacuum to remove residual water, which can produce overlapping signals in the FTIR spectral range of interest. After at least 30 minutes of drying, the films were scanned using an FTIR (Tensor 27-Platinum ATR, Bruker Optics Inc., Bilerica, MA) over a range from 600 cm-1 ≤ 𝜈 ≤ 4000 cm-1. SEM imaging of the thin film surfaces was also performed after hydrolysis and after functionalization reactions, following a method described previously.20,28 1 cm × 1 cm sections were cut by a razor blade and air-dried before being attached to conductive carbon tape on aluminum SEM stubs (Ted Pella Inc., Redding, CA). After coating the samples with 1.5 nm of iridium, the samples were analyzed in a Magellan 400 Field Emission Scanning Electron Microscope at a working distance of 3 mm and an accelerating voltage of 5kV. Static Cation Adsorption Experiments Equilibrium adsorption experiments were performed using small pieces of functionalized thin films with areas ranging from 0.6-2.4 cm2. These pieces of nanoporous block polymer thin films were placed in 1-2 mL of solution contained in scintillation vials. The solutions contained dissolved metal salts at concentrations between 1-40 mM or between 10-40 ppm. The salts were purchased from Sigma-Aldrich (used as received), and included lead (II) nitrate, cadmium (II) chloride, gold (III) chloride, and silver (I) nitrate. The thin films were left submerged in the solutions for at least 8 hours, before being dipped in a DI water bath to remove residual solution


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on the surface, and then transferred into separate scintillation vials containing 1-2 mL of hydrochloric acid diluted to pH 1 using DI water. The addition of the thin films to the acidic solution resulted in the release of the bound metal cations (with the exception of the covalentlybonded gold ions), and these solutions were stored along with the feed solutions to be analyzed later for their cation concentrations. Selectivity experiments were performed in a similar manner at either high concentrations (10 or 20 mM of each cation) or low concentrations (10 or 20 ppm of each cation) for solutions containing lead (II) nitrate and cadmium (II) chloride. Two additional experiments were carried out at 10 ppm lead (II) nitrate with 10 mM of either potassium chloride or calcium chloride in order to test the effect of competing salts on heavy metal ion removal. Stirred Cell Experiments Thin films were tested for their ability to perform separations in a flow-through configuration using an Amicon 8010 Stirred Cell (EMD Millipore Corporation, Darmstadt, Germany). A 1-inch hole punch was used to separate a circular section of thin film to be tested in the cell from the larger sheet that was cast from solution. The thin film was loaded onto a nonwoven membrane in the base of the cell, and a rubber O-ring was placed on the thin film to seal the cell when the shell was attached. 10 mL of a solution containing 50 ppm lead (II) nitrate was added into the cell, which was capped and pressurized with nitrogen (5.0 Ultra High Purity, Airgas, Radnor, PA) to 3 bar using a regulator (Victor Professional Single Stage EDGE ESS4, Victor Technologies, Chesterfield, MO). The cell contained a magnetic stir bar, which was set to 400 rpm in order to ensure thorough mixing of the retentate during the experiment. The pressure in the cell induced permeation of the solution through the thin film to tubing on the downstream side which connected to scintillation vials. These vials were used to collect solution at 1 mL intervals for a total of 10 collected samples. After the experiment, 2 mL of a pH 1 dilute hydrochloric acid solution were added to the cell and at least 1 mL permeated through the thin film 7 ACS Paragon Plus Environment


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to release bound ions into the solution. The permeate and acid wash solutions were stored for later analysis of cation concentrations. A similar experiment to test the separation of a non-binding divalent cation was carried out using a 50 ppm magnesium chloride solution. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) The samples collected in both the static adsorption experiments and stirred cell experiments were analyzed using ICP-OES. Samples were prepared by adding aliquots of 0.5 mL to 6 mL of a 5% nitric acid solution (by weight) to Falcon tubes. Samples for a calibration curve with 5-6 points (including a blank) were prepared and analyzed along with the samples. A Perkin Elmer Optima 8000 ICP-OES was used for analysis with 4 replicates measured for each sample. A sample calibration curve for lead ions at a wavelength of 220 nm is displayed in Figure S3. The average values of the retentate concentrations and acid wash concentrations were used to determine the concentrations of the samples in the static adsorption experiments, and the reported permeate concentrations were reported for the stirred cell experiment results.

Results and Discussion Functionalization of Pore Walls with Bio-inspired Ligand for Lead and Cadmium Capture The PI-PS-PDMA triblock polymer thin films, which served as structural precursors to the thin film adsorbers developed in this study, were converted to polyisoprene-b-polystyrene-bpoly(acrylic acid) (PI-PS-PAA) thin films via acid-catalyzed hydrolysis. A more detailed description of thin film fabrication and conversion processes can be found in the Experimental Methods section. The PI-PS-PAA films contain a high density of nanoscale pores that are lined by the reactive PAA moiety, which allows the thin films to be modified with new functional groups through simple reactions, and these functional groups can be tailored for specific applications. For instance, in the current study, the naturally-occurring peptide glutathione was chosen as a desired


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functional group because it is a powerful antioxidant that captures heavy metal ions in many living systems,34 and we hypothesized that this property would make it a useful agent in the removal of toxic heavy metal ions from aqueous solutions. The functionalization of the PAA moieties with glutathione occurred using ethylcarbodiimide (EDC) coupling reactions (Figure 1a).33 Fourier Transform Infrared (FTIR) spectra of the PI-PS-PAA and polyisoprene-b-polystyrene-b-poly{N[1-carboxy-3-({[1R]-1-[(carboxymethyl)carbamoyl]-2sulfanylethyl}carbamoyl)propyl]acrylamide} (PI-PS-PAG) thin films confirmed the successful addition of glutathione (Figure 1b). Before the reaction, the PI-PS-PAA spectrum (black trace) displays a prominent peak around 1720 cm-1, indicating the presence of the carboxylic acid group associated with PAA.29 After the reaction, the PI-PS-PAG spectrum (green trace) displays a new peak around 1640 cm-1, indicative of the formation of an amide bond between the glutathione molecule and the PAA repeat units that line the pore wall.29 Peaks remain around 1720 cm-1 because the glutathione molecules also contain carboxylic acid functional groups. The PI moieties crosslink during the conversion of PDMA to PAA, which improves the mechanical stability of the nanoporous block polymer-based thin film, but makes quantifying the conversion of the PAA to the bio-inspired PAG groups difficult because the resulting thin film is insoluble. However, the performance of the PI-PS-PAG thin film in heavy metal capture experiments confirms the successful coupling of glutathione to PAA and demonstrates the utility of the bio-inspired glutathione moiety as a means of removing heavy metal ions from aqueous solutions (Figure 1c). In these experiments, the ability of the thin films to reversibly adsorb heavy metal ions that are considered significant human health threats, namely, cadmium (II) and lead (II), was quantified. These experiments were performed at ion concentrations that ranged from 10 to 40 ppm (0.05 mM to 0.4 mM), which is much higher than most polluted water sources that 9 ACS Paragon Plus Environment


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typically contain concentrations on the order of 10-100 ppb8,35 and, as such, serve as an extreme case of how effectively these separations materials can function. Inductively coupled plasma optically emitting spectroscopy (ICP-OES) was used to quantify the removal of Pb2+ and Cd2+ ions. The results of these measurements are presented in Figure 1c. The red, blue, and magenta bars correspond to initial metal ion concentrations of 10, 20, and 40 ppm, respectively. As anticipated by theory (details of the theoretical calculations are provided in the Supporting Information), there is not a strong dependence of the ion removal on the value of the initial concentration in this low concentration limit. The time dependence of the solute uptake was also quantified through a batch adsorption experiment with a functionalized membrane in a 40 ppm lead (II) nitrate solution. The concentration of lead in solution is displayed as a function of time in Figure S4. Over half of the adsorption occurs within 10 minutes of the experiment, and the system approaches equilibrium within 60 to 90 minutes. The value of the rate constant determined from this experiment is similar in magnitude to that predicted using a thin film theory for mass transfer, suggesting that the uptake of solute is mass transfer limited not reaction limited.


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Figure 1. Coupling glutathione to PI-PS-PAA membranes. (a) Reaction scheme of the conversion of the pore wall lining PAA groups to the bio-inspired PAG groups. (b) FTIR analysis of the membrane before (black) and after (green) reaction with glutathione. The appearance of a peak at 1640 cm-1 suggests successful coupling of the peptide. Spectra of the entire scanned range can be found in Figure S2. (c) Membranes functionalized by glutathione were soaked in either lead or cadmium solutions, with a packing ratio of 20 g L-1. The concentrations of the metal ions in solution were 10 ppm (red bars), 20 ppm (blue bars), or 40 ppm (magenta bars). The white values shown on each group of bars list the average ion removal values over the three concentrations, as determined by ICP analysis. The average heavy metal removal value for both of the metal ions fell between 93% and 97%. Thus, these versatile nanoporous block polymer thin films are able to reduce the concentration of these heavy metal ions in water by a factor of 20 through a single equilibrium adsorption stage. Under operations with multiple equilibrium stages, this separation would be compounded multiplicatively resulting in concentrations that are an order of magnitude lower, making this materials platform suitable for processes that seek to efficiently produce safe, potable water from highly contaminated sources in a low-cost and high-throughput manner. Functionalization with a Simplified Moiety Maintains High Uptake of Heavy Metal Ions 11 ACS Paragon Plus Environment


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The thiol moiety is the critical chemistry required for the ability of glutathione to capture heavy metal ions effectively.12,13 Therefore, we hypothesized that more effective metal capture (on a per mass basis) could be realized by eschewing the extraneous functional groups within glutathione in order to more efficiently incorporate the thiol moiety. This hypothesis was evaluated through the introduction of cysteamine to the pores of the thin film as shown by the magenta structure in Figure 2a. Cysteamine consists of the binding thiol moiety at one end and a primary amine, which enables direct coupling to the PAA repeat units using the straightforward carbodiimide coupling reaction, at the other end. The PI-PS-PAA and polyisoprene-b-polystyreneb-poly[N-(2-mercaptoethyl)acrylamide)] (PI-PS-PASH) spectra before and after reaction with cysteamine, respectively, are displayed in Figure 2b. As can be seen in the spectra, the 1720 cm-1 carboxylic acid peak of the PI-PS-PAA film, shown in the black spectrum, decreases in intensity while the amide peak at 1640 cm-1 grows in intensity as shown in the spectrum of PI-PS-PASH, represented in magenta. Combined, these observations confirm successful introduction of cysteamine within the nanopores of the PI-PS-PASH thin film. The metal binding isotherms of the PI-PS-PASH thin films were established using solutions at higher ion concentrations in order to fully elucidate the salient features of the binding isotherms displayed in Figure 2c. In addition to solutions containing cadmium (II) and lead (II) ions, this set of experiments also involved solutions containing gold (III) and silver (I) ions. The gold (III) ions bonded irreversibly with the thiol groups,13,36 and the gold uptake values were determined solely from the retentate solutions, while the concentration of ions in the acid wash solution was also used to quantify the uptake of the other three metal ions. The silver ions were adsorbed the least in these experiments, with the maximum uptake occurring near a value of 0.7


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mmol g-1, and the gold ions were the most highly adsorbed with its maximum uptake value of 2.8 mmol g-1.

Figure 2. Coupling cysteamine to PI-PS-PAA membranes. (a) Reaction scheme showing the conversion of the pore wall lining PAA groups to bio-inspired PASH groups. (b) Comparison of FTIR spectra of the membrane before (black) and after (magenta) the conversion reaction. The decrease of the carboxylic acid peak (1720 cm-1) and rise of the amide peak (1630 cm-1) suggest functionalization by the cysteamine molecule. (c) Uptake capacity calculated from metal adsorption experiments, as measured by ICP-OES, as a function of metal ion concentration. The ions captured by the membrane include gold (III) (yellow arrows), cadmium (II) (black squares), lead (II) (red circles), and silver (I) (gray arrows). The uptake capacity values were calculated based on both the depletion of ions from the retentate and the concentration of ions in the acid wash solution. The dashed lines represent best fit Langmuir isotherms for each metals ion. The gold ion uptake suggests a density of at least 2.8 mmol g-1 of thiol groups based on a 1:1 binding ratio,36 which suggests at least 70% of the PAA moieties were converted to PASH; these numbers are consistent with the FTIR spectroscopy data collected for this material. The adsorption 13 ACS Paragon Plus Environment


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isotherms for cadmium (II) and lead (II) rise sharply as a function of ion concentration at concentration values less than 5 mM (1000 ppm) before approaching maxima of 1.55 and 1.35 mmol g-1 nanoporous block polymer thin films, respectively, at high ion concentrations. These saturation capacities compete with the highest reported capacities for state-of-the-art lead and cadmium adsorbers.37–39 Material Characterization Enables Accurate Prediction of Thin Film Performance at Equilibrium The isotherms plotted in Figure 2c display a shape typical of a Langmuir isotherm, whose behavior is governed by the following equation. 𝑞=

𝑄𝐾𝑐 1 + 𝐾𝑐

(1)

In equation 1, q represents the moles of ions adsorbed per mass of thin film, Q is the maximum binding capacity, c is the concentration of the metal ions in solution, and K is an equilibrium constant characteristic of the reversible binding reaction. The dashed lines shown in the color matching the data markers in Figure 2c were generated by using values of Q and K determined through linear regression of the linearized Langmuir isotherm (Figure S5). For cadmium and lead ions, the resulting Q values were 1.59 and 1.38 mmol g-1, respectively, in good agreement with reported literature values for the maximum binding capacity.13,36,37 The measured capacities also surpass those of activated alumina which is the most commonly used cadmium and lead adsorbent and has capacities of 0.31 and 0.41 mmol g-1 for cadmium and lead, respectively.40 The equilibrium K values were 0.61 and 0.87 L mmol-1, respectively, are lower than the values typically reported in the literature. However, none of the previously reported values for capacity or equilibrium constant are for adsorption within nanopores or for such a high density of binding moieties, which


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could significantly change their interactions at equilibria and, thus, the values of equilibrium parameters.37,41 The metal removal capabilities of the block polymer thin films at low ion concentrations are determined by these values of K and Q used to fit the Langmuir isotherm. Specifically, when the value of Kc is much less than 1, the percent removal of ions from the solution, R(%), can be resolved in terms of the Langmuir constants, K and Q, and the ratio of the mass of the thin film to the volume of solution treated, m/V̅ , as shown in Equation 2. The derivation of this result is described in the Supporting Information. 𝑚𝑄𝐾 𝑉 𝑅 % = ×100% 𝑚𝑄𝐾 1+ 𝑉

(2)

Note that in this regime the removal of ions is independent of the bulk concentration, provided Kc

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