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Surface Modification of Silica- and Cellulose-Based Microfiltration Membranes with Functional Polyamino Acids for Heavy Metal Sorption S. M. C. Ritchie,† L. G. Bachas,‡ T. Olin,§ S. K. Sikdar,| and D. Bhattacharyya*,† Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky, 40506, Department of Chemistry, University of Kentucky, U.S. Army Engr. CEWES-CT, Vicksburg, Mississippi 39180-6199, and U.S. EPA, Cincinnati, Ohio 45268 Received October 15, 1998. In Final Form: May 5, 1999 Functionalized membranes represent a field with multiple applications. Examination of specific metalmacromolecule interactions on these surfaces presents an excellent method for characterization of these materials. These interactions may also be exploited for heavy metal sorption from drinking and industrial water sources. Various low-capacity, silica-based ion-exchange and chelating sorbents (about 0.5 mmol of metal/g of resin) are available for treatment of such waters. Cellulosic membrane-based sorbents, functionalized with polyamino acids, present an excellent approach for high-capacity (3-14 mmol of metal/g of sorbent) metal sorption. Silica-based membrane sorbents possess metal sorption capacities approaching those of cellulosic-based membranes, with the added benefits of excellent acid and solvent resistance. Metal sorption capacities of silica-based membrane sorbents with various polyamino acids range from 0.6 mmol to 1.4 mmol of metal/g of sorbent. Ion exchange, chelation, and electrostatic interactions form the basis of metal sorption. Electrostatic interactions are greatly magnified in membrane-based sorbents, and are partly responsible for their high capacities. Regeneration of these sorbents has also been shown, including the possibility for selective desorption of metals.
1. Introduction The behavior of various materials functionalized with polypeptides and other molecules is a topic of interest because of its applications in affinity separations, biosensors, and other uses involving site-specific interactions.1 An example of the latter involves the removal of heavy metals from aqueous solutions.2-5 Although there are several conventional methods of removing these metals, such as precipitation and reverse osmosis, these techniques have limitations, such as metal solubility limits and high-pressure operation, respectively. Recognitionbased ion-exchange and affinity separations, such as membrane-based sorbents, represent low-pressure alternatives that work well even at low concentrations. These sorbents are made of a variety of materials containing many different functional groups.4,6-15 The advantage of * Corresponding author. Phone: 606-257-2794; fax: 606-3231929; e-mail:
[email protected]. † Department of Chemical and Materials Engineering. ‡ Department of Chemistry. § U.S. Army Engr. | U.S. EPA. (1) Xiao, S.; Textor, M.; Spencer, N. D.; Sigrist, H. Langmuir 1998, 14, 5507. (2) Konishi, Y.; Shimaoka, J.; Asai, S. React. Func. Polym. 1998, 36, 197. (3) Reichert, J.; Binner, J. G. P. J. Mater. Sci. 1996, 31, 1231. (4) Bonn, G.; Reiffenstuhl, S.; Jandik, P. J. Chromatogr. 1990, 499, 669. (5) Bhattacharyya, D.; Hestekin, J. A.; Brushaber, P.; Cullen, L.; Bachas, L. G.; Sikdar, S. K. J. Membr. Sci. 1998, 141, 121. (6) Dias Filho, N. L.; Gushikem, Y. Sep. Sci. Technol. 1997, 32, 2535. (7) Anspach, F. B. J. Chromatogr. 1994, 676, 249. (8) Lessi, P.; Dias Filho, N. L.; Moreira, J. C.; Campos, J. T. S. Anal. Chim. Acta 1996, 327, 183. (9) Leyden, D. E.; Luttrell, G. H. Anal. Chem. 1975, 47, 1612. (10) Lauth, M.; Gramain, P. React. Polym. 1986, 4, 257. (11) Gennaro, M. C.; Mentasti, E.; Sarzanini, C. Polyhedron 1986, 5, 1013. (12) Northcott, S. E.; Leyden, D. E. Anal. Chim. Acta 1981, 126, 117.
affinity separations is that they may be tailored for the desired selectivity and capacity. Typically, affinity separations are performed in packed beds of porous beads that contain some type of active ionexchange, chelation, or affinity groups. Some examples of these groups include amines, carboxylic acids, sulfonic acids, and quaternary ammonium groups.4,7,9,12,14,15 To obtain better selectivity, these groups are often organized in more complex arrangements (e.g., crown ethers) to impart size constraints, rigidity, and specificity to the affinity groups.6,8,10,11,13,15 The support for these groups is often silica-based, because silica provides chemical resistance and is mechanically robust.6,16,17 Beauvais and Alexandratos have provided an excellent overview on selective complexation of metal ions.15 Conventional microfiltration (MF) membranes are typically used for the passive separation of particles from solution. However, MF membranes can be used as active, high-capacity metal ion exchangers if long-chain polymers (with multiple functional groups) are incorporated on the pores.5 The advantages of these functionalized membranebased sorbents include less surface area dependence for attainment of high capacity and rapid sorption kinetics. The objectives of this research are to establish derivatization procedures for composite polymer-silica MF membranes, to determine selected heavy metal sorption (and regeneration) efficiencies by three types of polyamino acids, and to understand the specific metal-polyamino (13) Ion Exchange Technology; Sengupta, A. K., Ed.; Technomic Publ. Co.: Pennsylvania, 1997. (14) Kallury, K. M. R.; Macdonald, P. M.; Thompson, M. Langmuir 1994, 10, 492. (15) Beauvais, R. A.; Alexandratos, S. D. React. Func. Polym. 1998, 36, 113. (16) Biernat, J. F.; Konieczka, P.; Tarbet, B. J.; Bradshaw, J. S.; Izatt, R. M. Sep. Purif. Methods 1994, 23, 77. (17) Ohhira, M.; Ohmura, F.; Hanai, T. J. Liq. Chromatogr. 1989, 12, 1065.
10.1021/la9814438 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/01/1999
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acid interactions that contribute to the observed metal sorption behavior. Results of several derivatized cellulosic membranes have been included for comparison. Silicabased MF membranes were chosen (compared with cellulosic membranes) for increased internal surface area and better acid and solvent stability.17 2. Background and Theory Charged macromolecules that are bound to a surface will behave differently from those in homogeneous solution. The changes in their behavior will depend on the support material, and the orientation, structure, degree of polymerization, and packing density of the macromolecules on the surface. Our original work on functionalized cellulosic membranes5 and the work contained herein on silica-composite membranes examine how metals interact with polyamino acids that are bound to MF membranes. Characteristics of these membranes, polyamino acids, and metal sorption mechanisms are described below. 2.1. Silica-Based Supports. Silica-based support structures are widely used in the field of selective separations because of their well-known acid and solvent stability, as well as their excellent mechanical properties.16,17 Silica-composite membranes can easily withstand long-term immersion in 30 wt % sulfuric acid and are resistant to organic solvents. Silica is also mechanically strong, and does not have the shrink/swell characteristics that affect organic resins such as sepharose and agarose. The surface chemistry of silica also makes it an excellent candidate for modification. One means of silica surface modification is the attachment of silanes. Silanes are coupling agents that typically contain chloride or alkoxy groups at one end, and an organic substituent at the opposite end. The purpose of these coupling agents is the incorporation of different functionalities to silica.18 Modification of silica-based supports has been ongoing since the 1950s and is well understood. Several excellent reviews of the modification of silica surfaces have also been published.16,19 The silica surface may be modified to accommodate virtually any reactive group. Some examples include vinyl groups (for in situ polymerization),20 amine groups (enzyme attachment),14 and glycidoxy groups (IDA chelating agent immobilization).15 In this work, polyamino acids have been coupled to polyethylene-silica composite membranes via the terminal primary amine group of the amino acid. Though many different modes of attachment chemistry may be used, we chose to use a glycidoxymediated one. 2.2. Polyamino Acids. Previous work on cellulosic supports5 used poly-L-glutamic acid (PLGA) as the source of polyamino acid functional groups for heavy metal sorption. In this work, polyaspartic acids (L-form and R,βform) have been used, in addition to PLGA, to provide multiple functional groups for sorption. These polyamino acids are similar in that both have carboxylic acid side groups. When ionized, these side groups form the ligands for heavy metal sorption. Both poly-L-aspartic acid (PLAA) and poly-(R,β)-DLaspartic acid (PDAA) have been used in this research. Schematics of these polyamino acids are shown in Figure 1. The primary difference between these two polyamino acids is the composition of the polymer backbone (i.e., additional methyl groups between sequential peptide (18) Composite Materials: Interfaces in Polymer Matrix Composites; Plueddemann, E. P., Ed.; Academic Press: New York, 1974. (19) Zisman, W. A. Ind. Eng. Chem. Prod. Res. Dev. 1969, 8, 98. (20) Chaimberg, M.; Cohen, Y. Ind. Eng. Chem. Res. 1991, 30, 2534.
Figure 1. Schematics of (a) PDAA, (b) PLAA, and (c) PLGA.
bonds). One method of commercial aspartic acid polymerization is performed through the formation of polysuccinimide.21 This intermediate is also produced by thermal polymerization from maleic anhydride and ammonia. Subsequent alkaline hydrolysis of the succinimide rings leads to formation of PDAA, as it contains a combination of R- and β-peptide bonds. In the case of PLAA, the polymer only contains R-peptide bonds. For the most part PLAA and PDAA perform similarly, though PDAA does have less capacity to form a helix, as will be discussed later. Polyaspartic acid has several unique properties that make it ideal for metal separations. First, as shown in Figure 1, polyaspartic acid has a single terminal amine group. Because attachment to the silica-based membrane support proceeds via this amine, single-point attachment of the polyaspartic acid is achievable. The significance of this mode of attachment is that all of the exchange groups are available in the pore space. Capacity of the membrane sorbent for metal may then be enhanced simply by increasing the molecular weight of the attached polymer. The extent of attachment will of course depend on the support membrane material pore size range and distribution. In addition to polymer size, ionization plays an important role. First, above their pKa, the carboxylic acid (COOH) side groups of the polymer are predominantly ionized, creating one site for metal binding on each repeat unit. The degree of protonation (x) of the polymer chains is given by eq 1,22
x)
K exp(-eoΨ(z)/kT)[H+] 1 + K exp(-eoΨ(z)/kT)[H+]
(1)
where K is the binding constant, eo is the elementary charge, Ψ is the electrostatic charge field, z is the radial distance from the chain, k is the Boltzmann constant, T is temperature, and [H+] is the hydrogen ion concentration. This degree of protonation controls the line charge density encountered in counterion condensation, and will be discussed later. The nitrogen of the peptide bond also creates interesting possibilities for metal sorption. As reported in the literature,23 copper may form complexes with the nitrogen of the backbone amide linkages, thereby increasing the capacity of the sorbent without occupying ionized COOH sites. (21) Wheeler, A. P.; Koskan, L. P. Mater. Res. Soc. Symp. Proc. 1993, 292, 277. (22) Nilsson, S.; Zhang, W. Macromolecules 1990, 23, 5234. (23) Hikichi, K.; Tanaka, H.; Konno, A. Polym. J. 1990, 22, 103.
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Figure 2. Metal sorption mechanisms for cation binding.
Finally, as alluded to earlier, PLAA, and PDAA to some extent, may form helical species.24-26 The extent of helix formation has been described by the Zimm-Bragg model27 in eq 2,
f)
(
)
1 s-1 1+ 2 [(s - 1)2 + 4sσZB]1/2
(2)
where f is the fraction of helix (in helical conformation), s is the propagation parameter, and σZB is the initiation parameter. The propagation parameter is the key to helix formation, and is a function of the electrostatic (∆µel), binding (∆µbinding), and nonelectrostatic (∆µnonel) chemical potential contributions.22 The functionality of s is shown in eq 3.22
s ) exp
(
)
∆µel + ∆µbinding + ∆µnonel -kT
(3)
Electrostatic and binding contributions are positive, and the nonelectrostatic contribution is negative. Hence, in the absence of electrostatic interactions and with complete protonation (x ) 1), s will increase, and the polyamino acid will be in the helix formation. Increases in ∆µel and ∆µbinding, by ionization of the chain, will counterbalance ∆µnonel, and s will approach 0, the coil limit (i.e., fully stretched conformation). Ionic strength and metal cations will also affect helix formation.25 The helix is formed by hydrogen bonding between the amide and carbonyl groups of the polymer backbone. PDAA generally has less helix-forming ability because the interdispersion of methyl groups along the backbone increases the distance over which the hydrogen bonds must form, thereby creating weaker forces to counteract repulsion among the partially ionized side groups. The consequence for eq 3 is a decrease in the magnitude of ∆µnonel, and hence electrostatic interactions may have a greater effect. The effects of helix formation are two-fold. First, helix formation causes axial compression of the polyaspartic acid.22 Compression is due to a relaxation of the repulsion forces between ionized side group COOHs. Though there is some loss of capacity due to fewer ionized groups, closer (24) Pivcova, H.; Saudek, V. Polymer 1985, 26, 667. (25) Kurotu, T. Inorg. Chim. Acta 1992, 191, 141. (26) Saudek, V.; Stokrova, S.; Schmidt, P. Biopolymers 1982, 21, 2195. (27) Zimm, B. H.; Bragg, J. K. J. Chem. Phys. 1959, 31, 526.
residence of the remaining ionized groups magnifies their electrostatic field,27-29 increasing the overall effectiveness of the sorbent. The dramatic effect of the electrostatic field will be described later. This action also causes radial chain expansion,22 allowing formation of a void near the center of the helix. This void is capable of sequestering cations because of the surrounding negative electrostatic field.30 2.3. Membranes as Supports. MF membranes are an ideal support structure for these sorbents. First, the pore size of MF membranes is sufficiently large (0.1-0.5 µm) to accommodate these large polyamino acids (6000-36 000 MW). The polymer chains have adequate access to active groups on the surface and may still dangle freely in the pore space. The open structure of MF membranes also allows for convective flow at low pressure. Finally, attachment of the polyamino acid in a pore allows for dense packing of the attached polyamino acids. This enhances their combined electric charge field in a way not possible on a flat surface. 2.4. Metal Sorption Mechanisms. Figure 2 shows the three primary mechanisms for metal sorption. These are ion exchange, chelation, and electrostatic (in condensation zone) binding. The first two mechanisms occur in conventional ion-exchange and chelation resins. The last mechanism is a function of the polymeric nature of the ligands, and sorption by this technique is referred to as counterion condensation. This final mechanism is partly responsible for the large binding capacities observed with membrane-based sorbents. Au et al.31 have reported the importance of both electrostatic and specific interactions between weak polyelectrolytes and hydrolyzed metal species. 2.4.1. Ion Exchange. Ion exchange is governed by electroneutrality. As shown in eq 4,
2L- + Me2+ S MeL2
(4)
a divalent metal cation will interact with two monovalent ligands to form a stable complex. The molar sorption ratio of divalent metals (moles of metal per mole of COOH) for (28) Katchalsky, A.; Kunzle, O.; Kuhn, W. J. Polym. Sci. 1950, 5, 283. (29) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 1619. (30) Kono, N.; Ikegami, A. Biopolymers 1966, 4, 823. (31) Au, K.; Yang, S.; O’Melia, C. R. Environ. Sci. Technol. 1998, 32, 2900.
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Figure 3. Superposition of electric charge fields from closely packed attached polyamino acid chains (inset for isolated chain).
ion exchange will be 0.5 to ensure electroneutrality. This presents a hindrance to the development of high-capacity ion-exchange resins. 2.4.2. Chelation. Chelation is a metal sorption mechanism based on complex formation. For example, some of the reactions are:
L- + Me2+ S (MeL)+
(5a)
L- + (MeL)+ S MeL2
(5b)
L- + (MeOH)+ S (MeOH)(L)
(5c)
As shown in Figure 2, a stable complex may be formed between a divalent metal and a single chelation group. Complexes may also be formed with nonionic ligands such as amide linkages, where the lone pair of electrons on the nitrogen influences attraction. Typical values of the stability constants for various chelates and metals have been reported in the literature.32 The maximum ratio of divalent metal to exchange group (or ligand) is then 1 mol/mol, again placing a hindrance on the capacity of chelating resins. 2.4.3. Counterion Condensation. Counterion condensation is based on the entrapment of ions in a “solvent sheath” that surrounds polyelectrolytes in solution.33 When the distance between charged groups on the polyelectrolyte becomes less than a critical value, the so-called Bjerrum length, counterion condensation proceeds.34 The Bjerrum length, B (0.71 nm), is given by eq 6,
B)
eo2 4kTπ0
(6)
where is the dielectric constant of water, and 0 is the vacuum permittivity. The distance between charged groups of the polymer chain, b, is given by eq 7,35
b ) L/P
(7)
where L is the contour length of the polyamino acid and (32) Hancock, R. D.; Martell, A. E. Chem. Rev. 1989, 89, 1875. (33) Marinsky, J. A.; Reddy, M. M. J. Phys. Chem. 1991, 95, 10208. (34) Manning, G. S. J. Chem. Phys. 1969, 51, 924. (35) Manning, G. S. J. Chem. Phys. 1969, 51, 934.
P is the number of charged groups. When the charge density parameter, ξ,
ξ ) B/b
(8)
is >1, condensation proceeds. When the counterions are divalent or higher, the Bjerrum length is multiplied by the valency.33 The electrostatic field created by the polyelectrolyte drives counterion condensation. When attached in a pore, charged polyamino acids will attain a stretched configuration. This is due to repulsion among the charged groups. Coulomb’s law for a line charge describes the electrostatic field created by these groups,
Ψ(z) )
λL 2π0zx(4z2 + L2)
(9)
where λ is the linear charge density. The linear charge density will be affected by the degree of protonation according to eq 10,22
λ ) λ0(1 - x)
(10)
where λ0 is the linear charge density for a fully charged (x ) 0) chain. For a single chain, the electric charge field profile will be similar to the inset of Figure 3. Electric charge fields may be superimposed, and hence when the attached chains are tightly packed, their cumulative electric field will resemble Figure 3. Notice that the electric field magnitude is enhanced, and does not return to zero between chains. When Ψ is increased, such as during close packing of the chains, the concentration of counterions [C(z)] near the chain increases. This has been shown by Oosawa36 in eq 11,
C(z) ) A exp
(
)
-0Ψ(z) kT
(11)
where A is a constant. This mechanism is known as counterion condensation,36 because the number of charged species does not increase, but the number of sorbed counterions increases. (36) Oosawa, F. Polyelectrolytes; Marcel Dekker: New York, 1971.
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Figure 4. Schematic of polyamino acid chains attached to (a) a flat surface, (b) in a 0.1 µm membrane pore, and (c) in a 0.6 µm membrane pore. Table 1. Physical Characteristics of Silica and Cellulosic-Based MF Membranes Used in This Work PEb-silica pore size (µm) thickness (µm) internal surface area (m2/g) dry massa(mg)
0.1 200 80 164
CA-compc
CAd
0.6 0.2 100 (with backing) 100 35 10 11 (active layer) 25
a 13.2 cm2 cross-sectional area. b Polyethylene. cCellulose acetate composite (polyolefin backing). d Pure cellulose acetate.
The effect of the electrostatic potential should be magnified as the pore diameter decreases. Eq 12 gives the relation between the relative dissociation constant, β, the apparent volume concentration of the macromolecule, φ, and the charge density parameter.36
ln
(1 -β β) ) ln(1 -φ φ) + βξ ln(φ1)
(12)
The relative dissociation constant relates the number of territorially bound counterions (such as Me2+) to charged groups on the polyelectrolyte. The parameter β decreases as the number of territorially bound counterions increases. Therefore, as shown in Figure 4, as the pore size decreases, φ will increase. Eq 12 shows that β will consequently decrease, and the number of territorially bound counterions will increase. These counterions are not completely free, however, and reside along the polyamino acid chains, as well as inside the concentrated regime created by the bound chains.33 3. Experimental Section 3.1. Materials. The silica-based membranes used in this work are industrially supplied MF membranes. These materials are manufactured by extrusion of a mixture of oil, submicron-sized silica particles, and polyethylene resin. The final membrane material, composed of about 70% silica particles in polyethylene, results from subsequent solvent extraction of oil from the matrix. The gaps between the adjacent silica aggregate particles constitute the membrane pores and the convective flow path. The characteristics of the cellulosic membrane materials were reported in our previous publication.5 Physical characteristics of these membranes are shown in Table 1. The silane used, 3-glycidoxypropyltrimethoxysilane (GOPS), was supplied by Aldrich. The polyamino acids used in this work varied in molar mass from 6000 to 36000 g/mol and were supplied by Sigma. In addition, a low-molar-mass poly-(R,β)-DL-aspartic acid (2500 g/mol) was supplied by Bayer Corporation. All Pb and Cd feed
solutions were made in the lab from reagent-grade nitrate salts to concentrations of 1000 mg/L. All metal solutions were prepared with deionized ultrafiltered water from Fisher Scientific. 3.2. Membrane Functionalization and Operation. The preparation of the membrane-based sorbents consisted of two steps: aldehyde/epoxide derivatization of the membrane support, and polyamino acid attachment (functionalization). The method of derivatization depends on the membrane support used. In the case of cellulosic membranes, derivatization involved hydrolysis followed by periodate oxidation to form aldehyde groups.5 On the other hand, silica membranes required pretreatment before derivatization to remove a thin coating of oil (present in the raw membrane) on the surface. The pretreatment was accomplished by permeation of hexane followed by acetone rinse. The derivatization involved permeation of a 5% solution (v/v) of silane (GOPS) in o-xylene, toluene, or hexane through the membrane under convective flow at 25-60 °C for 2 h. Figure 5 shows the reaction scheme in which the silanol groups on the silica support were reacted with methoxy groups on GOPS. This reaction results in epoxide group formation on the pore surface. The membrane thus modified was then permeated with acetone to remove residual silane. Polyamino acid attachment was performed with 100 mL of a 100 mg/L aqueous solution at pH 9.2-9.5. Polyamino acid functionalization involved the reaction of the terminal amine group with the aldehyde/epoxide groups on the membrane pores. In the case of cellulosic membranes, reaction of the amine with the aldehyde forms an imine double bond. The double bond was subsequently reduced to a stable single bond (to enhance acid resistance) with sodium borohydride.5 As shown in Figure 5, for silica membranes, reaction of the amine with the epoxide group forms a single bond. Hence, no reduction of the bond is required. Typical water fluxes were 60 × 10-6 m3/m2 s at 0.2 bar for functionalized cellulosic membranes and 18 × 10-6 m3/m2 s at 0.7 bar for functionalized silica membranes. Polyamino acid attachment was followed by permeation of water at pH 3 to convert the attached polyamino acid to the H-form. Metal sorption experiments were conducted with feed solutions of 1000 mg/L of Pb2+, Cu2+, and Cd2+. The feed pH for each solution was 5.5 for Pb and Cd, and 5.0 for Cu. Typical experiments involved the permeation of 100 mL of the feed solution for 1-2 h. The permeate was recycled several times to allow the sorbent to reach its equilibrium capacity. The extent of metal sorption was established by permeate sample analysis. The experimental setup used for the silica membrane experiments is shown in Figure 6. Convective flow was obtained with a variable speed pump through the membrane in dead-end operation. The crosssectional area of all of the membranes studied in this work is 13.2 cm2. The temperature of the feed was controlled using an electric heating jacket around a water bath. 3.3. Analysis. The internal surface area of our membranes was determined by N2 adsorption at 77 K with a Micromeritics
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Figure 5. Preparation of silica-composite membranes by silane derivatization followed by polyamino acid functionalization.
Figure 6. Experimental setup for convective flow treatment of silica-based MF membranes. Table 2. Metal Sorption Results for Silica-based MF Membrane Sorbents and Comparison with Other Silica-Based Ion-Exchange/Chelation Sorbents capacity configuration membrane membrane membrane membrane gel granular gel controlled-pore glass a
metal
functional group
g/g
meq/g
ref
Pb Pb Cd Pb Cu Ba Co Ua
PLGA PLAA PLAA PLGA diamine crown ether nitrosonaphthol diamine
0.3 ( 0.1 0.12 ( 0.01 0.08 0.26 0.03 0.02 0.03 0.09
2.8 ( 1 1.2 ( 0.1 1.4 2.5 0.94 0.32 1.0 0.70
this work " " 5 9 10 11 12
As UO22+.
ASAP 2000 pore volume analyzer. The extent of polyamino attachment was determined by homogeneous reaction of the functionalization permeate (containing unreacted polymer) with Cd2+, followed by ultrafiltration with an Amicon 3000 MW cutoff membrane. Analysis of the permeate for Cd2+ concentration allowed determination of polyamino acid attachment. All metal solution concentrations were determined with a Varian AA575 series atomic absorption spectrophotometer at 217.7 nm for Pb, 229.5 nm for Cd, and 325.3 nm for Cu. Metal analysis error was < 5%.
4. Results and Discussion Equilibrium metal sorption capacities for various silicabased sorbents are compared in Table 2. The metal sorption capacities for membrane-based sorbents containing polyamino acids, including our previous preliminary work on poly(vinyl chloride)-silica membranes (Whatman, Inc.),5 were found to be consistently higher than conventional ion-exchange and chelation resins. The
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Figure 7. SEM micrographs of silica-based MF membrane (a) untreated and (b) after epoxide derivatization.
Figure 8. Organic flux (o-xylene) of silylated silica-based MF membrane and pre- and postsilanization water fluxes.
explanation for these differences can be found in how the metal ions interact with the sorbents. The understanding of these interactions can be explored by characterization of the membrane at various stages. Examination of the raw membrane internal surface area and morphology is important because this affects the packing density of the attached polyamino acid chains. The solvent and water permeabilities yield valuable information on surface and pore size modifications. Polyamino acid functionalization is critical because this is the source of the groups with which the metal ions interact. Interactions between metal ions and the attached polyamino acids are the most critical aspect, and lend the most insight into other uses for these and other similar materials. Finally, material regeneration will be examined for further characterization of metalpolyamino acid interactions. The formation of helices at acidic pH and by heavy metals plays an important role in both sorption and regeneration.5 4.1. Membrane Stability. Inorganic membranes are known to have exceptional chemical resistance and mechanical strength. The silica-based membrane used in this work is a composite material, composed of both inorganic (silica) and organic (polyethylene) material. These types of materials are used in battery separators and thus have strong acid resistance. Studies were also performed to determine the chemical resistance of the composite membranes to organic media. Figure 7 shows scanning electron microscope images of the membrane
both before and after functionalization. Besides the silane coating layer on the membrane, there is insignificant alteration of the membrane integrity. Membrane fluxes were measured before and after functionalization. Because the membrane is an MF membrane, the water flux of the untreated membrane is very high (60 × 10-6 m3/m2 s at 0.15 bar). This corresponds to a water permeability (flux/∆P) of 4 × 10-4 m3/m2 s bar. Figure 8 shows the flux of o-xylene through the membrane as a function of pressure. There is a linear relation between the flux and the pressure. However, the solvent permeability of 0.6 × 10-4 is considerably less than that observed with water, indicating deposition of a silane layer on the membrane. The water permeability after silane attachment, 0.3 × 10-4, is even lower because of some crosslinking of the attached silane. When silane attachment takes place in anhydrous organic solvent, methoxy groups are only hydrolyzed by surface water on the silica.37 In this case, because the membrane is prerinsed with hexane and acetone, what little water that remains on the surface is used to hydrolyze methoxy groups before reaction with the silanols. Lack of water may cause incomplete hydrolysis. When the water concentration is high, such as during attachment of the polyaspartic acid, hydrolysis is complete. The remaining (37) Opila, R. L.; Legrange, J. D.; Markham, J. L.; Heyer, G.; Schroeder, C. M. J. Adhes. Sci. Technol. 1997, 11, 1.
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Figure 9. Nitrogen (77 K) adsorption isotherm for silica-based MF membrane (untreated).
Figure 10. Effect of internal surface area on the extent of derivatization (lower points cellulosic MF membranes, upper point silica-composite membrane).
hydroxyl groups are then free to react with each other, cross-linking the adsorbed layer.14,38,39 4.2. Internal Surface Area. Nitrogen adsorption at 77 K was used to measure the internal surface area of the membrane. The adsorption isotherm for the membrane is shown in Figure 9. The shape of the curve is indicative of a type II BET (Brunauer, Emmett, and Teller) adsorption isotherm, that is, for a mesoporous (2-50 nm) solid without micropores (
