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SEPARATIONS Poly(amino acid)-Functionalized Cellulosic Membranes: Metal Sorption Mechanisms and Results J. A. Hestekin,† L. G. Bachas,‡ and D. Bhattacharyya*,† Departments of Chemical and Materials Engineering and of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0046
Passive membranes have been used for separations ranging from seawater desalination via reverse osmosis to the separation of particles with microfiltration membranes. However, active membranes, achieved by immobilization of macromolecules containing multiple functional sites to microfiltration membranes, allow for more selective separations. We have designed a novel membrane system consisting of cellulose-based microfiltration membranes functionalized with poly(amino acid)s (2500-10 000 MW). Because of the high carboxyl content of the poly(amino acid)s, these membranes have been shown to be extremely useful in the separation of heavy metals from aqueous solutions. The advantages of the membrane system, including high capacity and rapid sorption, have been demonstrated in this paper. A model has also been presented relating the effect of the pore size, poly(amino acid) attachment density, pH, and metal type to the initial metal sorption rate. It should be noted that, in contrast to homogeneous systems, the molar sorption capacities of the functional carboxyl sites are significantly enhanced in the membrane pores because of counterion condensation that results partly from the extremely high charge densities in the membrane pores. This phenomenon must also be incorporated in a kinetic model for the prediction of sorption behavior. Introduction Membrane processes are well-developed technologies for separations ranging from solids filtration to drinking water production from seawater.1-3 Traditional membranes are based on selective water transport, and thus consideration of specific exclusions of solutes (e.g., Pb2+ from Ca2+) has not been given. Recent work on new selective solute separations has included the coating or “tethering” of a monomeric or polymeric material to a membrane surface. This allows for tuning of material separation characteristics, while the bulk polymer serves as the membrane support. Significant research has also taken place in membrane functionalization for enzyme bioreactors,4-7 adjustable flux,8 and reduction of membrane fouling.9,10 Functionalized membranes have also been used for the sorption of heavy metals from solution. For instance, one research group11-13 has performed work in this area by attaching monomericto-trimeric chelation groups by plasma-induced radiation grafting. These membrane sorbents have been shown to have a capacity as high as 2.4 mequiv/g14 and have better overall mass-transfer characteristics than ion-exchange columns. Our work has focused on the use of multifunctional ligands for metal capture using a single-point attachment (e.g., amino group) via an aldehyde or an epoxide on a membrane surface. The membranes that we have * Corresponding author. Phone: 859-257-2794. Fax: 859323-1929. E-mail:
[email protected]. † Department of Chemical and Materials Engineering. ‡ Department of Chemistry.
used include pure cellulose (PC), cellulose acetate (CA), modified polysulfone, and polyethylene-silica composites. These materials can only be utilized for poly(amino acid) attachment after derivatization to form aldehyde groups or epoxide groups. Poly(amino acid)s (PAMs), such as poly(L-glutamic acid) (PLGA), poly(aspartic acid) (PAA), and polyarginine, have been immobilized on these membranes.15-18 Depending on the poly(amino acid) type, MW, and loading, metal sorption capacities as high as 27 mequiv/g can be obtained.15,18 For biomolecule (e.g., active enzymes) attachment and biocatalysis, the role of membrane material (hydrophobic vs hydrophilic) and the need for site-directed attachment have been reported in the literature.5-7 In the case of metal-poly(amino acid) interaction, support types (such as fiberous mat vs conventional MF membranes), the ease of derivatization (such as aldehydes and epoxides), and accessibility are quite important. Cellulosic materials have been the focus of this work because both derivatization and poly(amino acid) functionalization can be performed under aqueous conditions. In addition, these materials provide excellent accessibility of metal chelating sites. The objectives of this paper include (1) analysis of the difference between metal chelation in the solution phase and that of attached PAMs, (2) examination of the effect of membrane type on metal chelation, (3) determination of the role of available sites on metal chelation, and (4) quantification of the metal sorption behavior with membrane-bound PAMs under continuous flow conditions. This paper deals only with the use of cellulosic
10.1021/ie000572s CCC: $20.00 © 2001 American Chemical Society Published on Web 05/08/2001
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materials as membrane supports with attached PAA and poly(glutamic acid). Theory and Background To characterize the effects of membrane structure on chelation, several factors must be examined. These include the density and amount of reactive aldehydes available on the membrane support, the metal solution concentration, and charge and pore size effects on the rates of sorption. Because PAMs all contain a single terminal amine group, aldehyde groups on the support provide an excellent means of attachment. Thus, a brief discussion of aldehyde formation on cellulosic materials is useful. Aldehyde Formation. The oxidation chemistry of cellulose is very well-known in the literature.19 The formation of aldehyde groups takes place by the ring opening of the cellulose polymer during oxidation, thus forming two aldehydes per ring. Many oxidizing agents may be utilized for this reaction, though we have focused on sodium periodate and ozone. The reactions from cellulose to cellulose containing aldehyde groups can be defined as O3, NaIO4
(C6O5H11)n 98 O3, NaIO4
(C6O5H9)n 98 cellulose fragments, carboxyl formation (1) This reaction sequence, leading to chain scission, is only thought to be significant at high oxidation times.20 Thus, it is important to control the reaction time in order to prevent irreversible chain degradation. It is important to note the differences between CA, cotton cellulose (CC), and microcrystalline (e.g., bacterial) PC. The primary difference is that, because of the microcrystalline nature of PC, it has a much higher accessibility of hydroxyl groups than CC.21 For this reason, oxidation of CA, which requires prehydrolysis with hydroxide.15 is expected to yield less aldehyde than either pure or CC. A second difference is that PC is known to have a small fibular size, which is part of the reason that it cannot support as high of a surface density of aldehyde groups.21 Again, this should lead to more aldehyde groups as well as a more stable membrane structure upon oxidation. Materials containing the aldehyde groups can easily be functionalized with various PAMs via Schiff base reactions. Because the PAMs provide the sites for subsequent metal capture, the interactions between heavy metals and PAMs (bound and unbound) are discussed below. Aqueous Metal Chelation. Metal chelation with PAMs in the solution phase is dependent on both the charge of the PAMs and the charge of the metal species. The charge of the PAMs has been extensively studied,15,22-24 where researchers have shown that polyelectrolytes go through a broad transition around the pKa. Thus, the degree of protonation in the presence of a polyelectrolyte is written as
X)
K0′ exp(-ψ/kT)[H+] 1 + K0′ exp(-ψ/kT)[H+]
(2)
where ψ is the electrostatic field term and K0′ is the intrinsic binding constant. Notice that the existence of
an electrostatic field around the molecule is responsible for the broad transition. This can be compared to a monomeric amino acid (e.g., glutamic acid) transition of
X)
K0′ exp[H+] 1 + K0′[H+]
(3)
From eqs 2 and 3, it can be observed that a PAM has a broader pH range where it is partially charged than a single charged electrolyte. In addition, a shift in the pKa curve is expected because of this effect. For the metals sorbed on the functionalized membrane, one must have an understanding of the speciation as well. The two metals studied in this work, Pb and Cd, have many hydrolysis states in aqueous solution,25,26 although Pb undergoes polymeric species formation while Cd does not. For instance, the major Cd species is Cd2+ for pH < 6.5. However, with Pb in the same pH range, significant hydrolysis species are formed and are detailed in the equations below. The exact metal species concentration calculation will require the incorporation of hydrolysis reactions.
Pb2+ + OH- S Pb(OH)+
(4)
Pb2+ + 2OH- S Pb(OH)2
(5)
3Pb2+ + 5OH- S [Pb3(OH)5]+
(6)
4Pb2+ + 4OH- S [Pb4(OH)4]4+
(7)
Because the concentrations of these hydrolysis species are significant even at a pH as low as 5.0, they must be considered when looking at the theoretical molar ratio (moles of metal per moles of COOH). The accurate prediction of metal speciation with PAM should allow for the calculation of the ratio metal/COOgroup for the solution phase. The interactions of metals with polyelectrolytes in solution have been studied extensively for polymerenhanced ultrafiltration (PEUF).27-29 PEUF is the solution-phase interaction of metal ions with polymers and subsequent separation with ultrafiltration membranes. A significant disadvantage of this technique is that the metal concentration must be kept low compared to the polymer concentration, such that precipitation does not occur. This would result in extremely slow kinetics of the overall process.30 When the PAMs are attached to a surface, the interactions may be quite different from those in the solution. For microfiltration membranes, because of the confined pore space, the attached PAMs will provide a significant charge field density. A pictorial representation of the differences between the attached and solution PAM interactions with metals is illustrated in Figure 1. Notice that, in addition to multiple mechanisms of sorption for the attached chains, the PAMs are extended away from the surface. However, in solution, the PAMs may reorient and bind with the divalent metal in the ratio of 0.5 (one M2+/two COOH), simply by ion exchange. This results in different molar ratios for solution and membrane-attached PAMs. The differences in molar ratio may be magnified because of counterion condensation. This phenomenon occurs when the electrostatic field nonspecifically binds to small, mono- to
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Figure 2. Schematic of membrane pore dimensions as used in initial breakthrough behavior modeling.
Figure 1. Schematic of metal interaction mechanisms of (a) membrane-bound and (b) homogeneous solution PAMs.
trivalent metal ions.16,31-36 A detailed theory for these interactions has been reported.31 The local concentration of metal species around the polymer as a result of counterion condensation of a monovalent metal ion is proportional to the distance between charged repeat units on a polyelectrolyte (b).
C1loc ∝ (ξb3)-1
(8)
For poly(glutamic acid), this length was found to be 0.35 nm for the random coil state and 0.15 nm for the helical state.24 Because PAA has a similar polymer backbone, this length between charges can be assumed to be nearly the same. The structural charge parameter, ξ, at 25 °C is approximately 7.1/b. It is interesting to note that, in the case of monovalent metal ions, the local concentration near the polyelectrolyte is independent of the solution concentration. When a divalent metal ion interacts with polyelectrolytes, the solution to eq 8 cannot be found directly but rather can be related using31
KN ) [exp(zCAT - 1)](C1loc/C1)zCAT
(9)
where KN is the equilibrium binding constant. Although the value of KN must be determined experimentally, this does show a dependence of the equilibrium metal ion binding on the overall sorption capacity. To understand breakthrough curves involved in continuous operation, the conventional convection-diffusion equation has to include a potential term to describe metal-PAM interactions. The extent of sorption of metals on functionalized membranes will be a function of the charge and size of the metal ions and counterions,
the amount of PAM attached, the pH, the pore size, and the permeate flux. Extension to Continuous Flow Systems. Figure 2 shows a schematic of a pore containing attached PAM chains. These PAM-functionalized membranes are sorbents containing a limited number of sorption sites, and hence the extent of metal pickup is inherently time dependent. However, at very short times, the metal sorption rate is clearly a constant, and thus initial metal sorption rate data from low metal concentration solutions may be utilized in a time-independent model. Because PAM is only completely in the random-coil conformation at the start of the run, establishment of the transport behavior for a well-defined system is best accomplished at the beginning of a run. It is quite wellknown that interaction of divalent metals with PAM leads to helix formations,22 which will influence the driving force of separation. Hence, it should be noted that all experiments were conducted at a total metal sorption of 5 is sufficient for neglecting axial diffusion.38 Second, simulations were performed where the amount of metal sorbed inside the pore ranged from 0 to 5% of the total metal sorption capacity. In this
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range, the metal ion concentration leaving the membrane pore varied by less than 1%. Thus, ∂C1/∂t ) 0 for the initial transport behavior studies, though it would have to be included in the analysis of breakthrough behavior. Because it is difficult to solve for the actual electric field inside the membrane pore, representing the mobility due to charge interaction as a “modified diffusion” was the approach taken. To make this conjecture about the charge of the PAMs, we must assume that the overall charge field in the pore is relatively constant. Because we are dealing with a low concentration range and low sorption times, where the overall charge of the PAMs does not change significantly, anticipating little change in the potential field is a reasonable approach. Further, it is known that the overall potential in a charged system is a function of the counterion concentration, in this case the metal ion concentration. Thus, we are left with the simplified form of the transport equation
U
∂C1 ∂2 - (1 - X)D 2(C1) ∂z ∂r (1 - X)(D + µ) ∂ (C ) ) 0 (11) r ∂r 1
Notice that if ionic attraction dominates, eq 11 becomes a first-order partial differential equation. In addition, for linear concentration profiles, the second derivative in eq 11 would be zero. However, if diffusion dominates, eq 11 must be solved numerically. The mobility due to ionic attraction, µ, has been shown to be proportional to the charge of the ion and counterion39 and inversely proportional to the radius of the ion squared. The latter is due to dispersion of the charge on the surface of the ion. Using this relationship, an empirical relationship for the electrical mobility can be written as
( )( )
µ)a
zcat
rcation2
ranion2 zan
(12)
Table 1. Cellulosic Sorption Materials: Physical Properties matrix type
pore size (µm)
fiber diameter (µm)
surface area (m2/g)
CA membrane PC membrane CC fibers
0.5 0.1 b
a 0.1-0.2 10
35 120 ≈1c
a Membrane made by phase inversion, and therefore it has no fibrous structure. b Not in membrane form. c Estimated based on the fiber diameter because the surface area was too low for accurate prediction with BET.
ity). Finally, although the formation of hydrolysis species is expected to affect the rate of transport, the primary metal used in this study was Cd2+, which forms little or no hydrolysis species in this concentration (
