Environ. Sci. Technol. 2001, 35, 3252-3258
Polycysteine and Other Polyamino Acid Functionalized Microfiltration Membranes for Heavy Metal Capture STEPHEN M. C. RITCHIE,† KYLE E. KISSICK,† LEONIDAS G. BACHAS,‡ SUBHAS K. SIKDAR,§ CHETAN PARIKH,# AND D I B A K A R B H A T T A C H A R Y Y A * ,† Department of Chemical and Materials Engineering and Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0046, U.S. EPA, Cincinnati, Ohio 45268, and Daramic, Inc., Owensboro, Kentucky 42303
Polycysteine and other polyamino acid functionalized microfiltration membrane sorbents work exceptionally well for the removal and recovery of toxic heavy metals from aqueous streams. These are high capacity sorbents (0.33.7 mg/cm2) with excellent accessibility and selectivity for heavy metals, such as Hg(II), Pb(II), and Cd(II) over nontoxic components such as calcium. Polycysteine functionalized membranes work particularly well for metals such as Hg(II) and Cd(II), even in high total dissolved solids containing streams. Parameters such as permeate flow rate, feed metal concentration, and counterion (for Hg(II)) have also been found to influence sorbent behavior. For multicomponent systems, polyglutamic acid functionalized membranes have been found to selectively sorb Pb(II) versus Cd(II). Selective sorption of Cr(III) has also been observed with actual waste streams containing several heavy metals, hardness, and high sodium (2000 mg/L). The high capacity, site accessibility, and ease of regeneration of these membrane-based sorbents make them ideal for environmental separations when volume reduction or selective recovery is required.
Introduction Aqueous solutions containing toxic heavy metals, such as Hg(II), Pb(II), and Cd(II), are a major concern for environmental separations. Although many techniques exist for the separation of these components, they are often not satisfactory. Factors such as high total dissolved solids (TDS) and competitive sorption of nontoxic species often make traditional ion-exchange less selective. Mass transfer properties of these materials are also often less than desirable, as a high internal surface area is required, and hence the primary mode of transport is diffusion. An alternative to these sorbents would be a high capacity, regenerable membrane-based material with excellent accessibility. * Corresponding author phone: (859)257-2794; fax: (859)323-1929; e-mail:
[email protected]. † Department of Chemical and Materials Engineering, University of Kentucky. ‡ Department of Chemistry, University of Kentucky. § U.S. EPA. # Daramic, Inc. 3252
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Traditional ion-exchange sorbents are typically high surface area materials with some surface functionality. However, since these functionalities are generally COO-, SO3-, NH3+, or NR3+, sorption is governed by electrostatic interactions. Hence, their selectivity is not very great, and increases in ionic strength can mask their charges, decreasing their ability to sorb ions. This situation has led to the development of chelation exchange materials. In general, these materials provide additional metal complex stability by providing multiple binding sites. The resulting multidentate complexes are inherently more stable and can more easily resist regeneration by ionic solutions (1). Other chelating groups provide additional selectivity by addressing a more fundamental aspect of the metal-exchange group interaction. Particularly important in this category is the thiol group (2). Because of the large size and polarizability of sulfur, it does not coordinate well with small ions. Rather, only large, poorly solvated, soft ions, such as Ag(I) and Hg(II), form strongly stable complexes with thiols (3). Since Ca2+, which is a major contributor to solution TDS in the environment, is a hard metal ion, it does not coordinate well with thiols and hence does not compete for sorption sites (4). Amino acids also are an interesting group of chelating species. In all cases, a primary amine and a carboxylic acid are available for metal binding. In addition, some amino acids have side groups that can be used for immobilization on a surface or for additional metal binding stability. For example, the thiol group of cysteine has been used for coupling to a gold surface, while the amine and carboxylic acid are utilized for subsequent copper chelation (5). Homopolymers of amino acids may also be used effectively for metal sorption. It should be noted that polymerization consumes an amine and a carboxylic acid to make a peptide bond. Hence, useful polyamino acids will only be those where the side group can coordinate with metals. In homogeneous solution, polyamino acids have been used effectively for metal sorption (6). These have also been immobilized on a variety of particles (7-9) and membranes for metal capture and recovery (10, 11). A schematic of the membrane-based sorbents containing multiple COO-, NH3+, and SH groups is shown in Figure 1a. The polyamino acids are immobilized on the pore walls by single point attachment, such that the reactive groups are available in the pore space. The number of functional groups can be adjusted by the appropriate molecular weight choice of the polyligands. Cellulose- or silica-based membrane materials are ideal for immobilization of these structures, as modification of their surfaces (creation of sites for single point attachment) is easily accomplished. The chemistry used for functionalization of both silica and cellulosic materials is shown in Figure 1b. Notice that the cellulosic materials are stabilized by reduction of the Schiff-base formed double bond by reduction with sodium borohydride. Since a single bond is formed in the silica reaction scheme, no reduction step is required. The siloxane bond is extremely stable, only becoming susceptible to hydrolysis above pH 8 (12). The focus of this paper will be polycysteine and other polyamino acids functionalized on microfiltration membrane supports. The research objectives of this project include determination of polycysteine functionalization and deprotection efficiency, measurement of metal sorption capacities, and examination of process parameters, such as flow rate, concentration, and composition. These are critical for prediction of sorbent performance, particularly for environmentally relevant systems, such as treatment of Hg(II) containing aqueous streams. Multiple component stream treatment with polyglutamic acid 10.1021/es010617w CCC: $20.00
2001 American Chemical Society Published on Web 07/03/2001
TABLE 1. Characteristics of Nonfunctionalized Membranes Used in This Worka PES-I pore size (µm) thickness (µm) internal surface area (m2/g) silica particle size (nm) pure water permeability (cm3/cm2 s bar) dry mass (mg/cm2)
0.1 200 80
PES-II 0.1 200-1,200 150
PCM 0.1 230 120
250 25 -23 × 10-4 4-20 × 10-4 11 × 10-4 12.4
37.9
11.4
a
PES-I: polyethylene-silica (precipitated) membrane, PES-II: polyethylene-silica (fumed) membrane, and PCM: pure cellulose membrane mat.
FIGURE 1. a: Schematic of membrane-based sorbents containing multiple functional groups, such as (a) COO- (polyglutamic acid, polyaspartic acid), (b) NH3+ (polyarginine, polylysine), and (c) polycysteine. b: Functionalization chemistry for silica and cellulosic materials. functionalized MF membranes, including actual waste streams containing toxic metals and nontoxic components, will also be examined.
Experimental Section Membrane Preparation. The nonfunctionalized polyethylenesilica membranes (PES) were obtained from Daramic, Inc. (Owensboro, KY). These membranes are produced by first compounding the polymer, silica, and a plasticizer using a twin-screw extruder. The plasticizer acts as a lubricant during the processing and fills the pores in the membrane. Two types of silica are used in the manufacture of these membranes: precipitated silica (250 nm porous aggregates) in PES-I and fumed silica (25 nm nonporous particles) in PES-II. The molten compound is calendared and cooled before winding as a continuous sheet. The sheet is next passed through a solvent bath to remove most of the plasticizer. Finally, the sheet is heated in an oven to vaporize the solvent and form a porous structure. The pore sizes may range from less than 0.1 to 1 µm and are interconnected and continuous. Residual plasticizer is extracted from the PES membranes by sequential permeation of toluene, followed by acetone. The membrane thickness can be varied from 200 to 1200 µm, and the material is composed of around 70% silica particles (by weight). The balance of the material is polyethylene that binds the matrix together. Epoxy-functionality is imparted upon the membrane by convective permeation (at ∼1.5 bar) of a solution of 3-glycidoxypropyltrimethoxysilane (GOPS) in anhydrous o-xylene (10). Residual silane is removed by convective permeation of isopropyl alcohol (IPA) through the membrane. The pure cellulose membranes (PCM) are made by dispersion of a microcrystalline (∼30 nm diameter) cellulose in water. The resulting mixture is ozonated to form aldehyde groups on the fiber surfaces. After partial dewatering, the mixture is redispersed in IPA and placed in a large evaporating
dish. The IPA is evaporated over an extended period of time, during which time a membrane mat is formed. The characteristics of these membranes and the PES membranes are shown in Table 1. Because of the higher internal surface area of the PES-II membranes (area that can be utilized for polyamino acid functionalization), most tests were performed with those membranes. Other tests were performed on the pure cellulose membranes. Polyamino Acid Functionalization. Polyaspartic acid and polyglutamic acid functionalization procedures for both silica- and cellulose-based membranes have been published elsewhere (10). Polycysteine (supplied as poly-S-CBZ-Lcysteine, Sigma-Aldrich, St. Louis, MO) was immobilized in the protected state. This was done to prevent cross-linking of the polymer. The poly-S-CBZ-L-cysteine (PLC-CBZ) was first solubilized in pyridine. The solution was then convectively permeated through the membrane in a single pass. Immobilized PLC-CBZ was then deprotected by permeation of a 1-2 g/L solution of NaBH4 in 5 × 10-2 M sodium phosphate buffer. Residual reducing agent was removed by convective rinsing of the membrane with water. Metal Sorption Studies. Metal sorption was conducted in all cases (unless stated otherwise) with nitrate salts of Hg(II), Cd(II), and Pb(II) (Fisher Scientific, Fair Lawn, NJ). All runs were conducted under convective flow operation at a pressure of 1-2 bar. Solution pH was varied for each metal to avoid precipitation and was in the range of 5-6. Concentrations ranged between 5 and 150 mg/L. Additional runs were made with metal solutions containing high TDS concentrations of 4000 mg/L NaNO3. Mixtures of Cd(II) and Pb(II) were examined as well as an actual waste stream, containing Cr(III), Pb(II), Cu(II), Cd(II), and TDS. Analysis. Atomic absorption (Varian FS 200) was used for analysis of all metal solutions. Hg(II), Pb(II), Cd(II), Cr(III), Cu(II), Ca, and Na were analyzed at 253.7, 217.0, 228.8, 357.9, 324.7, 422.7, and 589 nm, respectively. Metal analysis error was < 5%. It should be noted that metal sorption values were determined from material balances performed on the permeated metal solutions both before and after interaction with the functionalized membrane. Scanning electron micrographs were taken of the PES-II and PCM membranes with a Hitachi S900 Field-Emission SEM. Samples were mounted on copper substrates and coated with gold.
Results and Discussion This paper primarily not only deals in the use of polycysteine for single metal capture but also evaluates the use of polyglutamic acid ligands for multicomponent systems. Of particular interest is Hg(II), as it interacts very strongly with thiol groups. Recent work by Holcombe and others on polycysteine functionalized controlled-pore glass has focused on immobilization after deprotection of the thiol side group (8, 9). This technique has inherent difficulties, as the goal of VOL. 35, NO. 15, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Microfiltration membrane supports used for functionalization of polyamino acids including (a) polyethylene-silica-II and (b) pure cellulose membranes. immobilization is the creation of a monolayer of the polyamino acid on the available surface area. When deprotection is performed before immobilization, polymer may be cross-linked by condensation of the thiol groups. Also, if the thiol groups are charged, chain-chain repulsion will result in more diffuse surface coverage. Hence, the polycysteine was immobilized in the protected form (PLC-CBZ). In our case, convective flow was always used, meaning that solutes were transported in the bulk flow of liquid through the membrane. Hence, diffusional mass transfer limitations are minimized, and preparation time of the material as well as stream treatment can be achieved rapidly compared to similar experiments on small, microporous particles in a column. This can have dramatic effects on sorption kinetics and regeneration chemistry, as will be shown later. Role of Membrane-Based Materials. Microfiltration membranes are effective support structures for immobilization of polyamino acids for metal sorption. The characteristics of membranes used in this work are given in Table 1. The pore size of MF membranes is typically in the range of 0.110 µm, which allows for both adequate surface areas (10150 m2/g) and space for polymer immobilization. The open structure also permits high throughput and convective flow of solutions through the membrane at very low pressure. Since transport is by convection, diffusion resistances are minimized. The structures of PES and PCM membranes used in this work are shown in Figure 2. In the case of PES membranes, the silica provides excellent surface area and can be easily modified with a variety of functional silanes. In this work, a glycidyl functional group was reacted with a primary amine on the polyamino acid resulting in a stable single bond. PCM membranes have excellent accessibility (all surface area on outside of fibers) and can be functionalized (aldehyde groups) by simple oxidation. For both membranes, single point attachment of polyamino acids is achieved, and hence side groups used for metal sorption are available in the flow path, away from the surface. Polycysteine Deprotection Efficiency. Removal of the protecting carbobenzoxy (CBZ) group is often achieved by reduction in liquid ammonia with sodium (13). In this work, however, borohydride reduction was chosen for deprotection. Because borohydride is a mild reducing agent, it was necessary to verify that the CBZ was removed. Neutral sulfur donors are known to interact with Cd(II) and Hg(II); however, due to steric hindrances, the CBZ-protected sulfur should interact with metals less than thiol groups. In addition, the polarizability of the CBZ-protected sulfur should also be diminished, making metal coordination less favorable. Figure 3 shows Cd(II) sorption on polycysteine functionalized cellulose membrane before and after deprotection. Notice that Cd(II) breaks through very quickly for membrane 3254
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FIGURE 3. Verification of CBZ removal efficacy by Cd(II) sorption before and after deprotection of polycysteine. functionalized with PLC-CBZ, indicating only marginal interaction of the metal with the protected sulfur. Conversely, the deprotected PLC membrane (with SH groups) effectively removes Cd(II), even after permeation of 400 bed volumes (membrane sorbent still not saturated). Borohydride reduction of the immobilized polycysteine is hence effective. The partial breakthrough behavior of the unsaturated membrane sorbent will be discussed later. It should be noted that the volume flow has been plotted in terms of bed volumes, where one bed volume is defined as the total volume of the membrane. This has been done to facilitate comparison to ion exchange columns, where breakthrough is normally plotted versus empty bed volumes, thus normalizing the data for different size columns. Polycysteine Functionalization Efficiency. Once the deprotection was verified, it was critical to quantify the extent of immobilization of PLC. Mercier and Pinnavaia (14, 15) showed that metal sorption was directly related to the amount of thiol groups available in the sorbent. Hg(II) and Cd(II) interact with thiol groups quantitatively in a ratio of 1 mol metal:1 mol thiol. Hence, if the thiol content is known, subsequent metal sorption capacity can be predicted. Total sulfur analysis was performed on the membranes used in these studies. Although the analysis confirmed the presence of sulfur in the sorbent materials, high material background mass (for both cellulose- and silica-based membranes) precluded quantification. Likewise, fluorescence studies with SBF-Cl (ammonium 4-chloro-7 sulfobenzofurazan) also proved inconclusive, most probably due to steric hindrance. Hence, equilibrium Cd(II) sorption studies were used to determine the accessible functionalized polyamino acid. Cadmium was used since it forms insignificant hydrolysis species at pH 6 and the complex species are well defined. These studies showed up to 20 mg of immobilized polycysteine (as PLC-CBZ @ 10 000 MW) per 13.2 cm2 membrane external area. This corresponds to 0.02 PLC chain/nm2 for PES-II, and 0.07 PLC chains/nm2 for PCM membranes. As observed previously for polyaspartic acid functionalized membranes, this is within an order of magnitude of peptide immobilization studies on titania (10). It should be noted that the method of polycysteine immobilization has not been optimized, and available surface area and extent of derivatization (moles of available aldehyde or epoxide) may allow for increased polyamino acid functionalization. Metal Sorption Capacities. As mentioned in the Introduction, a high capacity sorbent is highly desirable, as onetime use for very toxic materials (Hg(II), Pb(II)) becomes a possibility. For these materials, the metal sorption capacity was determined as the mass of metal sorbed per external area of membrane. For example, Bhattacharyya et al. (11) gave calculations showing the use of polyglutamic acid functionalized membranes (2 mg Pb/cm2 membrane) for the treatment of 40 mg/L Pb(II) solution. In that case, six
TABLE 2. Sorption Capacities of Cd(II), Hg(II), and Pb(II) on Polyamino Acid Functionalized Silica- and Cellulose-Based Membranesa
a
metal
amt sorbed (mg/cm2 external membrane area)
membrane
polyamino acid
pH
feed concn (mg/L)
equilibrium
Pb Pb Cd Cd Cd Hg Hg
3.7 ( 1.4 1.5 ( 0.12 1.0 ( 0.03 0.7 ( 0.01 0.6 ( 0.01 0.3 ( 0.01 0.4 ( 0.01
PES-I PES-I PES-I pure cellulose PES-II pure cellulose PES-II
high MW PLGA high MW PLAA high MW PLAA PLC PLC PLC PLC
5.5 5.5 5.6 5.9 5.9 6.0 5.7
1000 1000 1000 160 80 75 75
yes yes yes no yes yes no
PLGA: poly-L-glutamic acid, PLAA: poly-L-aspartic acid, and PLC: poly-L-cysteine.
FIGURE 4. Effect of concentration on contaminant bypass (pure cellulose membrane) at high flux (18.9 × 10-4 cm3/cm2 s) where Cf is the metal feed concentration and Cp is the metal permeate concentration. 55-gallon barrels of waste could be treated by one 300 ft2 membrane module, resulting in a volume reduction of 380fold, which may be further enhanced by regeneration of the module. The advantage of using high capacity sorbents for the remediation of toxic metal containing streams is thus apparent. With cellulosic membranes we have obtained metal sorption capacities as high as 1.4 g metal/g membrane (16). Polyamino acids have been found to be particularly useful for metal sorption. In addition to electrostatic interactions, hydrogen bonding along the polymer chains permits the formation of secondary structures, such as R-helices and β-sheets. Helix formation has been found to be useful for retention of Pb(II) over Cd(II) (11). In addition, the large electrostatic field caused by close residence of the charged groups allows for counterion condensation, whereby nonspecific interactions increase overall metal sorption (17). Equilibrium metal sorption capacities for several polyamino acid functionalized membranes are given in Table 2. Note that the values range from 0.3 to 3.7 mg metal/cm2 external membrane area. Equilibrium values indicate saturation of the sorbent at the stated feed concentrations. Recall that a capacity of 2 mg Pb/cm2 external area was sufficient for 380-fold volume reduction of a 40 mg/L Pb(II) wastestream. Hence, even nonoptimized Hg(II) sorbents (with polycysteine) should yield at least 40-fold volume reduction. The capacity is a function of the amount of immobilized polyamino acid, the accessibility of the polyamino acids, the stability of the metal complexes formed with the polyamino acids, the feed metal concentration (for ionic polyamino acids), the molecular weight of the polyamino acid, and the membrane thickness. Effect of Flow and Concentration on Metal Sorption. Permeation through the membrane was always performed under convective flow. This was done to minimize diffusional mass-transfer resistance. However, there was still an effect of metal feed concentration (Cf) and flow rate on metal
FIGURE 5. Cumulative Hg(II) (from Hg(NO3)2 at 70 mg/L Hg) sorption for polycysteine (PLC) functionalized pure cellulose (200 µm) membrane at pH 6. sorption. Figure 4 shows how at constant flow, there is incomplete sorption of metal supplied to the membrane. Hence, the metal permeate concentration (Cp) is not equal to zero. At each concentration, only a fraction of the feed metal was sorbed. This is due to feed metal concentration and low residence times for the sorption to occur. Since flow is in a pore, and the polyamino acids are immobilized on the walls of the pore, metal that passes down the center of the pore will not see the sorption groups. This effect is somewhat mitigated due to the tortuosity of the membrane pores, though it was still present at the concentrations tested at 18.9 × 10-4 cm3/cm2 s. Notice that in Figure 4, there is a relation between Cp and Cf. That is, when the concentration is low enough (∼30 mg/L Cd(II)), even at a residence time of only 12 s, there will be nearly 100% sorption of the fed metal in one pass. For higher metal concentrations, multilayered or multiple-pass membranes will overcome breakthrough limitations. The PLC functionalized membranes displayed constant mass sorption behavior that was independent of the metal feed concentration. This behavior is indicative of irreversible chemical reaction (with SH groups) between the metals and the immobilized PLC at the sorption conditions (pH 5-6). It should be noted that at very low pH the reaction is reversible, and metals can be regenerated (8). A representative sorption curve is shown in Figure 5. Notice how after an initial rapid sorption rate, the slope (sorption rate) becomes constant. This is an unusual behavior, as one would predict a constantly changing rate of sorption until most of the sites are occupied. Then, the last 5% of sites would only be fully utilized over a long period of time. In this case, the sorption rate was found to be constant until the sorbent was saturated. The implication of this behavior is that when a sufficiently low concentration stream is permeated through the sorbent, a very sharp breakthrough should occur, which is ideal for application purposes. It should be noted that the maximum Hg(II) sorption is a function of PLC loading. VOL. 35, NO. 15, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Effect of Hg(II) (feed concentrated ) 100 mg/L) counterion on sorption by polycysteine functionalized PES-II membrane (pH of solutions were 5.2 of HgCl20 and 5.7 for Hg(NO3)2). The unusual sorption behavior (constant mass sorption) is most likely due to the structure of the immobilized PLC. PLC was initially functionalized in the CBZ-protected state to prevent cross-linking of the sulfhydryl groups. Once immobilized, the CBZ was removed, and cross-linking may occur. Since the chains are immobilized, it is unlikely that cross-linking occurs by chain-chain interaction. Intramolecular interactions may lead to folding of PLC, particularly when the chains are fully protonated (8). Dipole-dipole interactions with the surface may also stabilize the folded structure, limiting accessibility of sulfhydryl groups, and hence regulating metal sorption. During sorption of divalent metals, complex formation with the PLC produces positively charged sites and repulsion along the polymer chain. The immobilized PLC could then progressively unfold, revealing hidden sulfhydryl groups for metal sorption. Interactions with divalent cations may also disrupt dipole-dipole interactions, reducing PLC interaction with the surface. Since the chains are attached in a microfiltration membrane, the diffusional limitations that normally cause the long periods for complete site utilization would be eliminated. Hence, once the chains became uncoiled, all of the groups would be complexed with metal, and the sorbent would be saturated, as shown in Figure 5. This is in contrast to polyaspartic acid and polyglutamic acid, where the side chains are ionized during metal sorption (fully stretched chains) and are hence always accessible. Role of Mercury Counterion. The metal counterion may also affect interactions with polycysteine. It is well-known in the literature that chloride forms soluble species (log K of HgCl20 ) 14) with Hg(II) (18). These include HgCl20, which is highly soluble, such that mercury-sorbed iminodiacetic acid (IDA) exchangers are readily regenerated with sodium chloride (19). Since sulfhydryl groups form covalent complexes with soft metals, it was hypothesized that chloride complexes would be broken by the PLC functionalized membrane. However, initial experiments with HgCl20 only yielded moderate sorption, as shown in Figure 6. When the counterion was replaced with nitrate, Hg(II) sorption was increased by 7-fold. Dissociation, therefore, plays a critical role in the sorption of mercury species for PLC functionalized materials. For environmental systems, the role of hydrolysis species, particularly for Hg(II), must also be considered. Figure 7a clearly shows that at the pH utilized in these studies, significant Hg(OH)20 is present. In Figure 7b, the presence of chloride greatly changes the predominant Hg(II) species. A preference for complexation with chloride is demonstrated in these figures, thus providing further evidence for the observed behavior. Additional insight is gathered by examining these two systems in the presence of cysteine. Notice 3256
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FIGURE 7. Hg(II) species distribution curves for (a) hydrolysis, (b) hydrolysis with chloride present, (c) cysteine monomer with hydrolysis, and (d) cysteine monomer with hydrolysis and chloride ([Hg]T ) 3.75 × 10-4 M, [Cl]T ) 7.5 × 10-4 M, [Cys]T ) 3.5 × 10-4 M). in Figure 7c that from pH 5-7, a steady 5% of the Hg(II) is complexed with cysteine. However, when chloride is added (Figure 7d) the complexation drops by up to an order of magnitude, similar to what was observed with polycysteine functionalized membranes. All of the curves in Figure 7 were calculated using stability constants from Martell et al. (20), at the same concentrations as used in these studies. It should be noted that these calculations are for the monomer cysteine and are only given as an analogy for the system utilized in this study. In addition, since the reactivity of the membranebound cysteine will be different than that in solution, these diagrams (homogeneous phase) are only intended to give an idea of the species distributions for immobilized case. On the other hand, PLC provides enhanced chelation because of adjacent thiol groups, and thus Hg(II)-PLC species will lower the Hg(OH)20 concentration shown in Figure 7c. TDS Effects on Metal Sorption with PLC. Environmentally relevant systems always contain nontoxic compounds, such as metals related to hardness or sodium. In general, the concentration of these species is in great excess (e.g., Ca2+ > 20 mg/L, TDS 300-2000 mg/L) to toxic compounds. Hence, selectivity for the toxic species in the presence of these components is necessary. Thiols are well-known to be selective for transition metals, even in the presence of high TDS (2). Howard et al. (9) observed excellent recovery of Cd(II), Pb(II), and Cu(II) from synthetic seawater using PLCfunctionalized controlled pore glass. In our work, PLCfunctionalized MF sorbents were also found to work extremely well in this regard, with no difference in metal sorption (with feed metal 75-150 mg/L Cd(II)), even in the presence of 4000 mg/L NaNO3. Selective Metal Sorption in Multicomponent Metal Systems. Selectivity among different heavy metals is critical, particularly when there are differences in toxicity or value. The selectivity of metal ions for a given functionality will depend on many factors, including metal acidity, size, hydration, etc. (4). Table 3 gives some metal complex stabilities for various amino acids. It should be noted that the stability constant (K ) [ML]/([M][L])) for Cd(II) with sulfamic acid (H2NSO3H) is 0.85. This value is given to contrast amino acids with conventional strong acid ion exchangers
TABLE 3. Metal Complex Stability Constants (log K) of Various Metals with Amino Acidsa Ca2+ Cd2+ Cu2+ Ni2+ Pb2+ Hg2+
aspartic acid
glutamic acid
cysteine
1.6 4.4 8.9 7.2 5.9d NAb
1.4 3.8 8.3 5.6 4.4e NAb
NAb 10.1c NAb 9.7 12.2 14.2
a Data taken from ref 20. All stability constants at 25 °C and 0.1 M ionic strength (µ) except where noted. b Not applicable, NA. c 37 °C and µ ) 0.15 M. d µ ) 1.0 M. e µ ) 0.5 M.
FIGURE 9. Cr(III) removal from a multiple metal containing stream (actual wastestream at pH 5) by staged polyglutamic acid functionalized microfiltration membrane sorbent operation.
FIGURE 8. Pb(II)/Cd(II) breakthrough curve showing selective Pb(II) sorption on polyglutamic acid functionalized polyethylene-silica composite membrane. which are generally functionalized with sulfonic acid groups. There are also differences in metal complex stabilities for various amino acids. For example, aspartic and glutamic acids have similar structures and therefore would be expected to have similar stability constants for metals. However, cysteine, which contains a thiol group, has much different stabilities for the same metals. Again, this is because the thiol is a soft group and hence interacts very strongly with soft metal ions, such as Cd(II) and Hg(II). Since Ca2+, Mg2+, and Na+ (major cationic components of TDS) are hard, they have little affinity for thiol groups (4). The stabilities are on a logarithmic scale, however, and thus, even for glutamic and aspartic acid functional groups, there is great latitude for selective sorption of one toxic compound over another. An example of this is shown in Figure 8 for competitive sorption of Cd(II) and Pb(II). In this case, the functionalized polyamino acid is polyglutamic acid. Hence, the side groups are all ionized, and all sites are immediately available for sorption. Initially, all of the Cd(II) and Pb(II) are sorbed. When excess sites are available, all of the ions may be sorbed. However, when free sites become limited, the selectivity of the polyglutamic acid for Pb(II) versus Cd(II) takes precedence, and Pb(II) sorption dominates. Hancock and Martell have reported that the selectivity of negatively charged oxygen donors, such as those of the polyglutamic acid side group carboxylic acids, is a function of metal ion acidity (4). Since the acidity of Pb(II) is greater than Cd(II) (also represented in Table 3), Pb(II) should displace Cd(II) on the sorbent. This was observed by a spike in permeate Cd(II) concentration. Pb(II) continues to sorb until the membrane is saturated, when it breaks through to its feed concentration. Konishi et al. (21) have shown similar results for Co2+/Cu2+ systems using iminodiacetic acid (IDA) functionalized polyethylene MF membranes. Previously, selectivity between Pb(II) and Cd(II) has been observed during regeneration of these materials, as hydrated Pb(II) is retained by the polyamino acid helix, and Cd(II) (which is not highly hydrated) is easily regenerated (11).
Studies of polyglutamic acid functionalized (with only 0.15 mg PLGA loading/cm2) materials have also been performed with actual wastewater. In this case, the wastewater feed contained a wide range of metal ions. These included toxic species, such as Cr(III), to nontoxic materials such as Ca2+ (17.2 mg/L) and Na+ (2000 mg/L). Because there are so many ions in solution competing for sorption sites, selectivity in the sorbent is essential. Figure 9 shows a staged removal of Cr(III) from the solution. Notice how 65-70% of the Cr(III) is removed in a single pass. This corresponds to a residence time of around 9 s. Passage of the permeate solution through a second stage improved the overall removal percentage to almost 90%. It should be noted that the membrane thickness was only 254 µm, and Cr(III) was selectively removed. Single-pass removal could easily be increased by adjusting PLGA loading. In this case, selectivity of Cr(III) was observed because of its large valency, and the metal ion acidity of Cr(III) was the largest of the contaminants. Further, calculation of the Cr(III) hydrolysis species indicates greater than 80% availability as 3+ valency or higher species (stability constants from ref 20). At the pH used in this experiment, 5.0, the predominant species (83%) is Cr3(OH)45+ (11% CrOH2+, 3% Cr(OH)2+, 1.5% Cr3+), and thus strong interactions with the sorbent would be expected. Other metal ions could be removed in successive stages. Hence, toxic metal removal and the discharge of only a TDS-containing stream can be accomplished. Regeneration. Although the need for regeneration of these materials is somewhat mitigated by their high sorption capacity, it is still desirable. Because of the open structure of the membrane, and because all of the exchange groups are located in the flow pathways, regeneration kinetics have been found to be very rapid. With only a single pass of a moderate strength salt or acid solution, nearly 100% regeneration of Cd(II) has been observed from polyglutamic acid functionalized membranes (11). Regeneration of thiolcontaining sorbents has also been explored. Holcombe and others have performed regeneration of thiol-containing materials by high strength acid treatment with 1 M HNO3 (8). Regeneration efficiencies were found to range from 65 to 100%. Regeneration of our membrane-based thiol sorbents was also found to be effective. However, due to the large equilibrium constants for Cd(II) and Hg(II), only high strength (1 M) acid solutions were found to be effective. Regeneration at pH 3 allowed recovery of only 25% of the adsorbed Cd(II). However, 100% of the sorbed Cd(II) was recovered with 1 M HNO3. This was in contrast to previously observed regeneration studies for polyglutamic acid functionalized materials, where pH 3 water was sufficient for metal regeneration. VOL. 35, NO. 15, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Acknowledgments The authors would like to recognize Daramic, Inc., and the U.S. EPA for partial support of the metal sorbent work and the NSF-IGERT program.
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Received for review February 6, 2001. Revised manuscript received May 23, 2001. Accepted May 24, 2001. ES010617W
