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Novel Nanofiltration Membranes Consisting of a Sulfonated Pentablock Copolymer Rejection Layer for Heavy Metal Removal Zhiwei Thong,† Gang Han,† Yue Cui,† Jie Gao,† Tai-Shung Chung,*,† Sui Yung Chan,‡ and Shawn Wei§ †

Department of Chemical & Biomolecular Engineering and ‡Department of Pharmacy, National University of Singapore, Singapore 119260 § Kraton Polymers, LLC, 15710 John F. Kennedy Boulevard, Suite 300, Houston, Texas 77032, United States S Supporting Information *

ABSTRACT: Facing stringent regulations on wastewater discharge containing heavy metal ions, various industries are demanding more efficient and effective treatment methods. Among the methods available, nanofiltration (NF) is a feasible and promising option. However, the development of new membrane materials is constantly required for the advancement of this technology. This is a report of the first attempt to develop a composite NF membrane comprising a molecularly designed pentablock copolymer selective layer for the removal of heavy metal ions. The resultant NF membrane has a mean effective pore diameter of 0.50 nm, a molecular weight cutoff of 255 Da, and a reasonably high pure water permeability (A) of 2.4 LMH/bar. The newly developed NF membrane can effectively remove heavy metal cations such as Pb2+, Cd2+, Zn2+, and Ni2+ with a rejection of >98.0%. On the other hand, the membrane also shows reasonably high rejections toward anions such as HAsO42− (99.9%) and HCrO4− (92.3%). This performance can be attributed to (1) the pentablock copolymer’s unique ability to form a continuous water transport passageway with a defined pore size and (2) the incorporation of polyethylenimine as a gutter layer between the selective layer and the substrate. To the best of our knowledge, this is the first reported NF membrane comprising this pentablock copolymer as the selective material. The promising preliminary results achieved in this study provide a useful platform for the development of new NF membranes for heavy metal removal. membrane fouling.4,7 In contrast, NF processes are usually able to alleviate these issues by operating at a lower pressure while maintaining a high permeation flux and reasonable rejection. Various dissolved species can be effectively rejected by NF membranes via the size and the Donnan exclusion (electrostatic repulsion) mechanisms.1,3,5,7−11 Hence, NF has garnered renewed interest and may be a suitable treatment of heavy metal wastewater.12 Currently, most NF membranes are thin-film composites (TFC) with an ultrathin polymeric film deposited on the surface of an asymmetric porous substrate.13 The porous substrate of the TFC membrane provides the necessary mechanical strength for the membrane to withstand the high operating pressure, while the ultrathin polymeric film achieves the rejection of various dissolved species. Therefore, this asymmetric configuration allows the use of complementary materials and methods for modifying each layer separately and optimizing the overall membrane performance.14 However, it

1. INTRODUCTION Heavy metal pollution has become a major problem in numerous countries because of the indiscriminate discharge of untreated wastewater. The situation is expected to be further aggravated in the near future by rapid population growth and economic development. As heavy metals are toxic and nonbiodegradable, their bioaccumulation in the environment further compounds this problem. As such, there is an urgent need to address the current situation to prevent further damage to the environment and human health. Thus, to achieve effective removal of heavy metals and recycling of wastewater, various treatment technologies such as ion exchange, chemical precipitation, adsorption, coagulation and flocculation, flotation, electrochemical methods, and membrane filtration have been adopted.1−4 In particular, membrane filtration technology has been studied extensively2−4 because it can provide (1) high removal efficiency, (2) easy operation and fabrication, (3) space saving, and (4) no phase change.4,5 Among the membrane processes used, reverse osmosis (RO) and nanofiltration (NF) are common means of heavy metal removal.6 Although RO rejects a wide variety of dissolved species, it suffers from the drawbacks of a high operating pressure, a high level of energy consumption, and severe © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13880

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before being fractured in liquid nitrogen. The fractured samples were then subjected to platinum sputtering using a JEOL JFC1100E ion sputtering device before being mounted onto the field emission scanning electron microscope for examination. 2.3.2. Membrane Microstructure. Doppler broadening energy spectroscopy (DBES), conducted using position annihilation spectroscopy (PAS) coupled with a variable monoenergy slow positron beam, was used to examine the membrane microstructure as a function of depth. For each sample, the results of DBES consisted of 29 distinct spectra that were obtained at different incident energies within the range of 0.1−27 keV. The spectra were recorded with an HP Ge detector (EG & G Ortec) at a counting rate of ∼8000 counts/s, and a total of 1.0 million counts were recorded for each spectrum. In this study, the depth profiles of the membranes were characterized by the S parameter of DBES, which refers to the ratio of the integrated counts between 510.3 and 511.7 keV. Given that the S parameter is related to the relative value of the low-momentum section of the positron−electron annihilation radiation, it reflects the changes in the chemistry and free volume of the membranes. Hence, both the chemistry and free volume change of the membrane, as a function of depth, can be determined using the S parameter. More information regarding the use of PAS can be found in other publications.24,25 2.3.3. Membrane Surface Charge. The surface charge of the membranes as a function of pH was determined by a SurPASS electrokinetic analyzer (Anton Paar GmbH). It should be noted that during the analysis, a 0.01 M NaCl solution was allowed to circulate through the measuring cell with its pH being adjusted via autotitrations with 0.1 M NaOH and 0.1 M HCl. Afterward, on the basis of the relationship between surface charge and pH, the isoelectric points of different membranes were determined. 2.3.4. Pure Water Permeability (PWP) and Pore Size Distribution. The pure water permeability (PWP, LMH/bar) of the membranes was determined using dead-end filtration cells at room temperature. In summary, the PWP of the membranes was evaluated using a transmembrane pressure, ΔP, of 10.0 bar and calculated using eq 1:

should be noted that the selective layer of the current state-ofthe-art NF membranes predominantly consists of cross-linked networks of aromatic polyamide prepared by interfacial polymerization.15 As few materials are suitable for the formation of the rejection layer of composite membranes, we endeavor to develop novel membrane materials to produce a more efficient and stable NF membrane for various applications. The molecularly designed Nexar copolymer consisting of pentablock copolymer poly(tert-butylstyrene-b-hydrogenated isoprene-b-sulfonated styrene-b-hydrogenated isoprene-b-tertbutylstyrene) (tBS-HI-SS-HI-tBS) has several advantageous characteristics due to its unique structure. First, it can achieve a high degree of sulfonation without a significant loss of mechanical stability.16−20 Second, the degree of sulfonation of the copolymer can be easily tailored for various applications. Third, the merging of the middle sulfonated styrene block to form a continuous water transport passageway during formation of the film serves to further enhance the water permeability of the membrane.18−20 Consequently, the purpose of this study is to develop an effective composite NF membrane that consists of the sulfonated pentablock copolymer as the thin selective layer and a polyethylenimine (PEI)-modified Matrimid as the substrate. Via the effective control of the membrane preparation process, the composite membranes were designed to possess high water permeability and ion rejections. In addition, the heavy metal removal performance of these new NF membranes was evaluated using a variety of heavy metal solutions, and encouraging results were obtained. The positive results of this study may provide a useful platform for the fabrication of new NF membranes.

2. MATERIALS AND METHODS 2.1. Fabrication of the Matrimid Membrane Substrate. The Matrimid flat sheet membrane substrate was fabricated via a non-solvent-induced phase inversion method.21−23 The detailed procedures for the fabrication are given in the Supporting Information. 2.2. Fabrication of the Composite NF Membrane. The Matrimid membrane substrate was first cross-linked by a PEI aqueous solution (3.0 wt %) at 70 °C for 1 h. Next, the modified substrate was rinsed with deionized water to remove the unreacted PEI. For better illustration of the cross-linking reaction, the chemical structures of Matrimid 5218, PEI, and a possible product of the reaction are shown in Figure S-1 of the Supporting Information. The copolymer selective layer was then formed on the modified substrate by depositing a copolymer solution with a required concentration onto the top layer for 1 min at room temperature (23 °C). Prior to the deposition of the selective layer, the excess water on the surface of the membrane substrate was removed with filter paper. Copolymer concentrations of 0.5, 1, 2, 3, 4, and 5 wt % were used in this study. The resultant NF membranes were then allowed to stabilize in air for 5 min at room temperature (23 °C) before being immersed in ethanol for 1 h to facilitate the removal of nhexane. Finally, the membrane was soaked in deionized water to remove ethanol. 2.3. Characterizations. 2.3.1. Membrane Morphology. Field emission scanning electron microscopy (FESEM) (JEOL JSM 6700) was utilized to examine the cross section and surface morphology of the membrane. During sample preparation, the membrane was first dried with a freeze-dryer

PWP =

Q AΔP

(1)

where Q (liters per hour) is the water flux at the permeate side, ΔP (bars) is the applied transmembrane pressure, and A (square meters) is the effective filtration area of the membrane. The membranes were stabilized for 1 h before any measurements were taken. To ensure the reproducibility, more than three membrane samples were prepared for each experimental condition, and the average results are reported in this study. The mean effective pore size and pore size distribution of each membrane were determined using the solute rejection experiments.26 Various feed solutions consisting of 200 ppm organic solutes were used in this study. Depending on the pore size distributions of the membranes, different organic solutes could be used. Both PEG and PEO, which have larger Stoke diameters, were employed in determining the pore size distributions of the as-prepared and modified Matrimid membrane substrates. In contrast, smaller organic solutes were used for the composite NF membranes.26,27 Table S-1 of the Supporting Information summarizes the molecular weights (MWs) and Stoke diameters (ds) of the smaller organic solutes. The Stokes diameters of the PEG and PEO solutes were calculated by the following equations. For PEG 13881

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Article

(2)

For PEO ds = 20.88 × 10−12 × M 0.587 (MW ≥ 100000)

(3)

During testing, a transmembrane pressure of 10 bar was used to induce the permeation flow across the membrane. Prestabilization was conducted before any measurement to minimize any dilution effect. A total organic carbon analyzer (TOC ASI5000A, Shimadzu) was used to determine the solute concentrations in both the feed and permeate. The effective rejection coefficient R (percent) for each organic solute was obtained using eq 4: ⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝

(4) Figure 1. Plots of ζ potential vs pH of the membranes. Experiments were conducted with 0.01 M NaCl. The composite membrane was obtained using a deposition concentration of 3 wt % and a deposition time of 1 min.

where Cf and Cp are the solute concentrations of in the feed and permeate, respectively. Subsequently, the solute rejection R was plotted against the Stokes diameter on a log-normal probability paper to yield a straight line. The membrane molecular weight cutoff (MWCO), mean effective pore diameter (μp), and geometric standard deviation (σp) were determined from the straight line. The MWCO is defined as the molecular weight of the solute at which the rejection is 90%. μp is the pore diameter at which R = 50%. σp is the ratio of the pore diameters when R = 84.13 and 50%. Afterward, the pore size distribution of the membrane was evaluated using eq 5: dR(d p) dd p

=

⎡ (ln d − ln μ )2 ⎤ p 1 p ⎥ exp⎢ − ⎢⎣ d p ln σp 2π 2(ln σp)2 ⎥⎦

However, the deposition of the pentablock copolymer directly onto the Matrimid substrate is not advisible for a number of reasons. It can be observed from Figure 1 that the ζ potential of the Matrimid substrate decreases as the pH increases. The isoelectric point of the substrate is around pH 3.9, and the substrate is increasingly negatively charged above this point. Given that the pentablock copolymer may form a negatively charged dense film between pH 3 and 10,20 the adhesion between the substrate and the selective layer may be less than ideal because of electrostatic repulsion. The long-term stability of the membrane is questionable. In addition, as seen in Table 1, although the Matrimid substrate has a high PWP of 478 LMH/bar, it has a relatively large mean effective pore diameter of 17.3 nm. This may not be ideal for the deposition of the active layer as severe solution intrusion may occur.28 3.1.2. Modification of the Matrimid Substrate. Consequently, the thermal cross-linking of the Matrimid substrate with PEI is introduced. As illustrated in Figure S-1 of the Supporting Information, during the cross-linking reaction, the highly branched PEI monomers are grafted onto the Matrimid substrate via the formation of amide functional groups. The protonation of these amine groups confers an overall positive charge to the substrate surface for a wide pH range.27,29−33 Hence, it can be observed from Figure 1 that the modified substrate has an isoelectric point of pH ∼10.7 and is highly positively charged at pH 98%) of various heavy metal ions except for HCrO4− (92.3%). The high membrane rejections of cations can be attributed to the small mean effective pore diameter of the composite membrane (0.50 nm), which is significantly smaller than the hydrated diameters of these cations. In addition, because the pH values of these salt solutions are significantly below the isoelectric point of the membrane (i.e., pH 9.2), the membrane is highly positively charged under these conditions (Figure 1). As a result, the Donnan exclusion mechanism plays an additional role in effectively rejecting these cations. Consequently, with the combination of both Donnan and 13885

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note that the rejection of chromate ions increases significantly from 78.0 to 92.3% when the pH of the solution increases from 3.06 to 4.81. Because the proportion of the monovalent anions of HCrO4− remains constant within the entire pH range, the increase in rejection can be attributed to the significant decrease in the surface charge density of the membrane (Figure 1). In other words, the membrane is less positively charged and electrostatic attraction between the membrane and the chromate ions is weakened. As the pH increases from 4.81 to 10.34, the rejection of chromate ions also increases gradually from 92.3 to 98.3%, although CrO42− has a hydrated diameter that is smaller than that of HCrO4−. Similarly, this is likely because the composite membrane becomes less positively charged as the pH increases and eventually becomes negatively charged beyond the isoelectric point. Thus, we can conclude that the solution pH has a significant effect on chromate rejection because the pH determines the surface charge characteristics of the membrane. With the combination of both Donnan and size exclusion at pH 10.34, the composite membrane can also effectively reject the chromate ions. Table 3 summarizes the separation performance of different commercial and various lab-made NF membranes reported in recent years. The commercially available membranes have a comparable PWP but lower rejections of the various heavy metals. In contrast, the previously lab-made membranes generally possess higher heavy metal rejections by sacrificing the PWP that, in turn, reflects the growing trend of having little or no contaminants in the discharge. Compared with the previously lab-made membranes, the newly developed composite NF membrane in this study shows promising rejections (>98%) against various heavy metal ions with a reasonable PWP of 2.4 LMH/bar. It is believed that the newly developed composite membrane may be highly competitive for the removal of heavy metal ions.

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ASSOCIATED CONTENT

S Supporting Information *

Materials, procedures for fabrication of the Matrimid membrane substrate, chemical structures of the different compounds, FESEM images of the Matrimid membrane substrate, and characteristics of small neutral solutes used in the determination of the pore size distribution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +65-65166645. Fax: +65-67791936. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the GlaxoSmithKline-Economic Development Board (GSK-EDB) Trust Fund for the project entitled “New membrane development to facilitate solvent recovery and pharmaceutical separation in pharmaceutical synthese” supported by Grant R-706-000-019-592 and Kraton Polymers, LLC, for the project entitled “Evaluation of Nexar Polymers for Water Filtration Applications” supported by Grant R-279-000396-597. Thanks are also due to Dr. C. L. Willis for his kind support and help. 13886

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