Surface Effects on Cation Transport across Porous Alumina

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Surface Effects on Cation Transport across Porous Alumina Membranes Elizabeth A. Bluhm,† Eve Bauer,† Rebecca M. Chamberlin,† Kent D. Abney,*,† Jennifer S. Young,‡ and Gordon D. Jarvinen‡ Chemical Science & Technology Division, Los Alamos National Laboratory, MS J514, Los Alamos, New Mexico 87545, and Nuclear Materials Technology Division, Los Alamos National Laboratory, MS E510, Los Alamos, New Mexico 87545 Received March 2, 1999. In Final Form: July 20, 1999 Transport behavior of monovalent and divalent solutes across mesoporous Anopore γ-alumina membranes was investigated as a function of pore diameter, pH, ionic strength, and nature of the salt or complexing species in solution. Radiotracers 137Cs, 85Sr, 22Na, and 45Ca were present in the feed solutions at very low concentrations, ranging from 10-9 to 10-12 M and total salt concentrations from 0.1 to 10-4 M. The divalent cations Ca2+ and Sr2+ exhibit lower diffusion rates (3-7 times slower) than the monovalent cations Cs+ and Na+ for membranes with 20 nm diameter pores. Differences between monovalent and divalent cation diffusion rates for the membranes can be explained in terms of a Donnan exclusion effect from the positively charged alumina surface. The rate of Sr2+ transport across the 20 nm alumina membranes was greatly increased by raising the pH (reducing the membrane surface charge) from 5 to 8 for both the feed and receive sides. Increased ionic strengths and the addition of complexing agents or specific salt solutions also facilitated divalent ion transport. Diffusion coefficients for divalent cations increased 3-fold for the 100 nm pore diameter membranes.

Introduction Chemically robust, selective membranes for the separation of radioactive and hazardous metal cations from contaminated water are an underdeveloped alternative to classical ion exchange and solvent extraction methods. Suitable membrane materials containing well-ordered, homogeneous pore structures have recently become available. For example, track-etched polycarbonate membranes can be chemically modified to cleanly separate hydrophobic and hydrophilic molecules in aqueous solution.1 As an alternative support material, commercially available Anopore anodized γ-aluminum oxide membranes contain large (ca. 20-100 nm) pores in a rigid, honeycomb structure.2 The uniformity of pore diameters and path lengths in the Anopore membranes eliminates many difficulties in physical characterization3 and theoretical modeling4 of transport. γ-Alumina is also thermally robust and can be coated with a thin layer of gold or silicon oxide, then chemically modified with molecular recognition sites to enhance the specificity of the chemical separations. While our ultimate goal is to prepare membrane structures with very selective molecular or ionic recognition properties, the charged surface of γ-alumina itself is expected to influence the transport of cations through the Anodisc membranes. Transport of uncharged solutes through mesoporous γ-alumina has been studied,5 but electrostatic interactions between cationic solutes and * Corresponding author. Mailstop: MS J514. Tel.: 505-665-3894. E-mail: [email protected]. † Chemical Science. ‡ Nuclear Materials. (1) Hulteen, J. C.; Jirage, K. B.; Martin, C. R. J. Am. Chem. Soc. 1998, 120, 6603-6604. (2) Furneaux, R. C.; Rigby, W. R.; Davidson, A. P. Nature 1989, 337, 147-149. Li, F.; Zhang, L.; Metzger, R. M. Chem. Mater. 1998, 10, 2470-2480. (3) Hernandez, A.; Calvo, J. I.; Pradanos, P.; Palacio, L.; Rodriguez, M. L.; Saja, J. A. J. Membr. Sci. 1997, 135, 89-97. (4) Sugawara, S.; Sakurai, I. Konno, M.; Saito, S. J. Chem Eng. Jpn. 1989, 19, 477-480.

charged membrane surfaces are expected to be competitive with diffusion in determining transport rates.6 Since radioactive contaminants such as 137Cs and 90Sr may pose significant human health hazards even when present at parts-per-billion levels, we are especially interested in cation transport at very low concentrations. In this paper, we present preliminary transport data for unmodified γ-alumina membranes and propose a correlation between cation transport and membrane surface interactions. Experimental Section Materials. A. Membranes. Alumina membranes (Anopore, Whatman Corporation) used in this project were 25 mm in diameter with 20 or 100 nm pore diameters and porosities of 25-30% and 40%,7 respectively. Each 20 nm membrane consists of a 100 nm base support with pore shapes that taper down to one or more openings that average approximately 20 nm in diameter. B. Reagents. Reagent-grade chemicals were obtained from commercial sources and used without further purification. 137Cs (5 mCi in 0.1 M HCl, 0.1 mg/mL CsCl) and 45Ca (2 mCi/mL, 0.1 mg/mL CaCl2 carrier in 0.1 M HCl) radiotracer solutions were purchased from Amersham Life Sciences (Arlington Heights, IL). 85Sr (∼3 mCi in dilute HCl, carrier free) and 22Na (∼0.9 mCi, in NaCl) solutions were obtained internally from the Isotope Production and Distribution Program (Los Alamos National Laboratory, Los Alamos, NM). All radiotracer solutions were diluted with distilled-deionized water to obtain working stock solutions with concentrations ranging from 10-9 to 10-12 M. (5) Dalvie, S. K.; Baltus, R. E. J. Membr. Sci. 1992, 71, 247-255. Miller, J. R.; Koros, W. J. Ind. Eng. Chem. Res. 1994, 33, 934-941. Sarrade, S.; Rios, G. M.; Carles, M. J. Membr. Sci. 1994, 97, 155-166. (6) Cot, L.; Larbot, A. Bull. Korean Chem. Soc. 1997, 18, 1028-1031. Itaya, K.; Sugawara, S.; Arai, K.; Saito, S. J. Chem. Eng. Jpn. 1984, 17, 514-520. Peeters, J. M. M.; Boom, J. P.; Mulder, M. H. V.; Strathman, H. J. Membr. Sci. 1998, 145, 199-209. Baticle, P.; Kiefer, C.; Lakhchaf, N.; Larbot, A.; Leclerc, O.; Persin, M.; Sarrazin, J. J. Membr. Sci. 1997, 135, 1-8. Alami-Younssi, S.; Larbot, A. Persin, M.; Sarrazin, J.; Cot, L. J. Membr. Sci. 1995, 102, 123-129. Alami-Younssi, S.; Larbot, A.; Persin, M.; Sarrazin, J.; Cot, L. J. Membr. Sci. 1994, 91, 87-95. (7) Information obtained directly from the Whatman Corporation.

10.1021/la9902441 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/13/1999

Cation Transport across Porous Alumina Membranes

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C. Equipment and Instrumentation. Transport experiments were conducted in Teflon flowcells that resembled a traditional electrochemical H-cell reaction apparatus. Each unit contained 250-mL capacity feed and receive vessels that were connected with Viton O-ring sealed washers. For each experiment, a new membrane was centered between the washers and tightly secured with a C-clamp. The 20 nm membranes were always placed between the two containers so that the 20 nm side of the membrane faced the feed solution. After securing the apparatus, the flowcells were leak-tested with water prior to each study. 85Sr, 137Cs, and 22Na isotope activities were counted on a Packard model 5000 NaI gamma counter. 45Ca activities were measured on a Wallac 1414 liquid scintillation counter. Methods. A. Radiotracer Transport Measurements. Typical transport measurements for each radiotracer consisted of filling both the feed and receive containers with approximately 100 mL distilled-deionized water. Feed and receive pHs were adjusted with dilute KOH. A predetermined volume of radiotracer stock solution was added into the feed side and the entire vessel was then placed on a multi-stirrer set at 300 rpm. Sample aliquots (0.5 mL) were removed from both the feed and receive sides at specific time intervals and diluted with 4.5 mL of water (or Ultima Gold scintillation cocktail, Packard) in a 20 mL plastic liquid scintillation vial. Radioactivity (in units of counts/min/mL) for each sample was subsequently measured by either liquid scintillation β counting or γ counting methods. Transport data were plotted as activity ratios (receive radioactivity/feed radioactivity) versus time. B. Diffusion Constant Calculations. Diffusion constants for the ionic species can be extracted from the ratio of receive to feed radioactivity (Ar/Af) versus time data. For comparison, a theoretical diffusion coefficient for the cation-anion pair in water, D, was calculated from the diffusion coefficients of the ionic species 1 and 2 as follows8

D)

D1D2(z12c1 + z22c2) D1z12c1 + D2z22c2

(1)

where z is the ionic charge and c is the concentration for ion 1 or 2. To determine the effective diffusion coefficients from the transport experiments, the activities (in units of counts/min/ mL) Ar and Af can be calculated as a function of time from the flux across the membrane

dAr F ) dt Vr

(2)

dAf -F ) dt Vf

(3)

where F is the flux and Vr and Vf are the volume of the receive and feed side, respectively. In this case, Vr and Vf are the same, thus

dAr dAf F )) dt dt Vr

(4)

The flux through the membrane was calculated using the following equation

dA dx

F ) -SD

(5)

where S is the surface area of the membrane available for transport, D is the effective diffusion coefficient, and dA/dx is the concentration gradient across the membrane.8 If the membrane can be modeled as a series of parallel pipes, then the area of the membrane available for transport is the product of the membrane area that is exposed to the feed and the porosity of the membrane. The concentration gradient was calculated as shown in eq 6 where (8) Cussler, E. L. Diffusion: Mass Transfer in Fluid Systems, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1997.

l is the thickness of the membrane.

dA Ar - Af ) dx l

(6)

A least-squares fit of the data using the model represented by eqs 2 through 6 was used to determine the effective diffusion coefficients.

Results and Discussion A. Radiotracer Transport Measurements. Alumina is an amphoteric material whose ion exchange behavior depends on solution pH.9 For example, below pH 8 or the point-of-zero-charge (pzc), γ-alumina is protonated and intrinsically behaves as an anion exchange material (eq 7).9

Al-OH + H+ f Al-OH2+

(7)

Above pH 8, the basic form of alumina shows cation exchange behavior because the alumina now has an overall negatively charged surface

Al-OH + OH- f Al-O- + H2O

(8)

A 85Sr transport experiment across the 20 nm alumina membrane was performed with the pH of the feed and receive solutions at approximately 9 (pH>pzc). As time elapsed, we observed a drastic decrease in 85Sr concentrations for both the feed and receive solutions. Evidently, the alumina surface was binding Sr2+ cations at the negative AlO- sites (eq 8). To confirm this observation, we performed a batch distribution coefficient (Kd) test with 85Sr and a 20 nm alumina membrane at pH∼9.10 The alumina membrane removed approximately 95% of the 85Sr from solution with a resultant K value of 2,300 mL/ d g. Additional 85Sr transport studies were conducted as a function of pH in the feed and receive solutions, while the pH was maintained well below 9 to minimize 85Sr adsorption to the membrane surface. Feed and receive side activities were summed to confirm that no sorption was occurring. Three separate experiments were performed with approximate feed and receive pHs of 5, 7, and 8 (Figure 1). Raising the pH by three units (pH 5 to 8) increased the 85Sr diffusion constant by an order of magnitude. An explanation of this result can be found by examining the effect of an electric double layer on the transport of ionic species. Figure 2 represents a general diagram of the electric potential as a function of distance from the alumina membrane surface.11 There are two important regions to note in this schematic. First, there is an electric double layer that resides closest to the alumina surface, which contains mobile anions and is often the termed surface of shear or the diffusive region. A second region exists further away from the surface where the potential decreases exponentially with distance as described by eq 9

Ψ ) Ψ0 e(-κx)

(9)

(9) Clearfield, A. Inorganic Ion Exchange Materials; CRC Press: Boca Raton, FL, 1982. (10) Distribution coefficents were calculated using the following equation: KR ) [(Co - Cf)/Cf](v/m), where Co and Cf are the original and final equilibrium concentrations of the metal ions in solution, respectively, V is the total volume of stock solution, and m represents the mass of membrane. (11) Mulder, M. Basic Principles of Membrane Technology, 2nd ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1998.

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Bluhm et al. Table 1. Experimental and Theoretical Diffusion Constants (cm2/s) for 20 and 100 nm Alumina Membranes experimentala membrane

22Na

137Cs

45Ca

85Sr

20 nm Al2O3 100 nm Al2O3

1.1 × 10-6 1.2 × 10-6

2.3 × 10-6 2.4 × 10-6

1.1 × 10-7 3.7 × 10-7

1.1 × 10-7 3.7 × 10-7

theoretical NaCl 1.6 ×

10-5

CsCl 2.0 ×

10-5

CaCl2 1.3 ×

10-5

SrCl2 1.4 × 10-5

a pH 5 (both feed and rec. for 22Na, 85Sr, and 45Ca); pHs 4.5 (rec.) and 5 (feed) for (137Cs).

Figure 1. 85Sr transport through 20 nm alumina membranes at three different feed and receive pHs (5, 7, and 8).

Figure 3. 85Sr, 45Ca, 22Na, and 137Cs transport through 20 nm alumina membranes (pH 5).

Figure 2. Schematic diagram of an alumina surface and the electrical potential as a function of ionic strength and distance. Reprinted with kind permission from Mulder, M. Basic Principles of Membrane Technology, 2nd ed.; Kluwer Academic Publishers, Dordecht, The Netherlands, 1998; pp 188 and 268, Figures IV-30, V-31. Copyright 1998 Kluwer Academic Publishers.

where Ψ is the electric potential, x is an arbitrary distance along the potential curve, and κ represents the Debye length. At a distance of κ-1, the potential has decreased to a value of 1/e ) 0.37, which is the “thickness” of the electric double layer. The double-layer thickness is not constant but decreases with ionic strength and increases with surface charge.11 Alumina surface charge decreases with increasing pH; therefore, 85Sr transport is facilitated by increasing the solution pH, resulting in a decrease of the electric double layer at the alumina surface. Models by Gouy and Stern12 have shown that alumina membranes with 50 nm pores, (12) Schmid, G. J. Membr. Sci. 1998, 150, 151-157 and references therein.

when placed in dilute electrolyte solutions, have an electric double layer thickness on the order of several hundred angstroms. Their findings lend credence to our conclusion that the electric double layer or Debye length is an important factor that affects transport in the 20 nm alumina membranes. An increased Debye length may hinder transport by repelling cations from the membrane pore openings, and/or by restricting the effective pore diameter through which the cations must diffuse. Table 1 contains experimental and theoretical diffusion constants (assuming free ion diffusion) for 85Sr, 137Cs, 22Na, and 45Ca transport through 20 and 100 nm γ-alumina membranes at low ionic strengths. In both cases, the Ca2+ and Sr2+ cations exhibit smaller diffusion coefficients than the monovalent cations, Cs+ and Na+. Moreover, as noted in Figure 3 and Table 1, monovalent cations have experimental diffusion constants on the order of 3-7 times that of divalent cations. Experimental diffusion constants are indistinguishable between different divalent cations thus indicating a lack of size-exclusion or hydration effects influencing the transport behavior. Further evidence that surface charge plays a significant role in the transport and selectivity between monovalent and divalent cations is the effect of pore diameter on the divalent cation diffusion constants. The 85Sr and 45Ca diffusion coefficients using the 100 nm alumina membranes are approximately three times greater than for the 20 nm membranes, whereas the diffusion coefficients for 22Na and 137Cs are the same. Consequently, the pore

Cation Transport across Porous Alumina Membranes Table 2. 85Sr Diffusion Constants (cm2/s) for the 20 nm Alumina Membranes with 10-4 M Complexants in Feed Solutionsa

Langmuir, Vol. 15, No. 25, 1999 8671 Table 3. 85Sr Diffusion Constants (cm2/s) for the 20 nm Alumina Membranes with Various Concentrations and Types of Magnesium Salt Solutions

complexants

log K (0.1 M)

diffusion constants (cm2/s)

salt solutions

MgSO4

Mg(NO3)2

Na2EDTA oxalic acidb IDA Na acetatec 18-C-6 15-C-5 85Sr

8.68 2.54 2.23 0.49 2.76 1.95 s

2.3 × 10-6 1.8 × 10-6 7.9 × 10-7 4.8 × 10-7 1.6 × 10-7 2.6 × 10-7 1.1 × 10-7

0.0001 M 0.01 M 0.1 M

2.0 × 10-6 2.8 × 10-6 2.8 × 10-6

1.7 × 10-6 2.9 × 10-6 s

size has a strong influence on the diffusion rates for the divalent cations through the membrane and attests to an electrostatic interaction between the pore wall and cations. An explanation for this phenomenon is that a potential difference exists near the membrane surface and within the pores. Pores within the membrane contain an excess of negative ions compared to the external bulk solution. The resulting concentration gradient causes negative ions to diffuse away from the membrane into the bulk solution. Even though small quantities of ions are involved in this diffusion process, the charge transfer is sufficient to create a potential difference near the membrane surface and within the pores. Consequently, the potential difference draws anions back toward the membrane surface and cations back into the bulk solution, resulting in an equilibrium between electric potential and concentration gradients. This “Donnan potential” is an important factor in electrolyte exclusion from alumina membranes as well as ion exchangers. According to classical electrostatics13 and ion exchange phenomena,14 divalent cations, e.g., Sr2+ and Ca2+, experience a stronger repulsion from the positively charged alumina surface than monovalent cations. Conversely, anions would have little difficulty diffusing through the membrane pores. To further support the Donnan exclusion model, theoretical diffusion constants were calculated based on free ion diffusion in water, with no surface-ion attractions or repulsions (eq 1). The theoretical diffusion coefficients are 1-2 orders of magnitude larger than the experimentally determined diffusion coefficients. Therefore, we conclude that surface interactions play a significant role, especially in divalent cation diffusion and to a lesser extent with monovalent cations. Future work will include a more detailed model that fully defines the relationship between electrostatics and ion diffusion through the alumina membranes. B. Complexants. To reduce ion-surface repulsion and thus expedite 85Sr transport, various complexants were added to the feed solution. We specifically examined 85Sr transport through 20 nm alumina membranes with various complexants at 10-4 M concentrations in the feed solution. Representative complexants included disodium EDTA (Na2EDTA), iminodiacetic acid (IDA), sodium acetate (NaAc), 15-crown-5 (15-C-5), 18-crown-6 (18-C6), and oxalic acid. Table 2 shows the resultant 85Sr diffusion constants for all six complexants. Excluding the crown complexes, a positive correlation was observed between diffusion

constants and metal-ligand binding constants (reported as log K) for 85Sr. For example, EDTA has the highest log K (and largest diffusion rate), followed by oxalic acid, iminodiacetic acid, and acetate (with the lowest affinity and slowest transport). Furthermore, the EDTA, IDA, and oxalate complexes of strontium should have a negative or zero charge at pH 5 and, accordingly, reduced electrostatic repulsion from the positively charged alumina surface. Previous research has shown that 18-C-6, and to a lesser extent 15-C-5, exhibit binding capabilities for Sr2+, resulting in positively charged metal-ligand complexes.15 Comparing diffusion constants in Table 2, it is apparent that the 18-C-6-Sr complex has a similar transport rate to 85Sr in water alone, but surprisingly, the smaller 15C-5 complex revealed a slightly higher transport rate through the 20 nm alumina membrane. Steric effects of the bulky, yet larger 18-C-6-Sr complex may contribute to the slower transport rate. However, size exclusion does not appear to greatly influence the rate as long as charge repulsion dominates the diffusion process across the membrane. One other important observation should be noted. If acetate has little binding affinity for Sr2+, then according to Table 2, 85Sr transport with acetate in solution should have a diffusion coefficient similar to that of the crown complexes or Sr2+ alone. In fact, when sodium acetate was present in the feed solution, the rate of 85Sr transport through the 20 nm membrane increased. Based on this observation, an alternative explanation for increased 85Sr transport is required. Possible explanations are the effect of other cations, such as sodium, or an increase in ionic strength. C. Ionic Strength Effects on Ion Transport. To further test the Donnan exclusion model, we examined 85Sr transport through 20 nm membranes with either 0.0001, 0.01, and 0.1 M MgSO4 or Mg(NO3)2 solutions. The effect of Donnan exclusion should diminish as the ionic strength increases. Additionally, according to Donnan theory,11,14 the potential difference at a positively charged membrane surface will decrease with a larger counterion charge. Sulfate and nitrate anions represent the higher and lower valency counterions. The 85Sr diffusion constants shown in Table 3 show a significant increase as the solution concentration increases from 0.0001 to 0.01 M for both salt solutions tested. When the ionic strength decreased to 0.0001 M, the separation between 85Sr diffusion coefficients in the two salt solutions became more apparent. As expected from Donnan theory, the 85Sr rate increased slightly from 1.7 × 10-6 cm2/s in nitrate to 2.0 × 10-6 cm2/s in sulfate due to the higher valency of the sulfate anion. D. Multicomponent Transport Experiments. As mentioned previously, sodium acetate enhanced 85Sr transport more than expected based upon its binding affinity for acetate and the presence of 10-4 M Na+ cations in solution. Additional experiments were performed to examine ionic strength effects on 85Sr or 45Ca transport

(13) Petrucci, S. Ionic Interactions: From Dilute Solutions to Fused Salts; Academic Press: New York, 1971; Vol. 1. (14) Helfferich, F. Ion Exchange; Dover Pubs.: New York, 1995.

(15) Izatt, R. M.; Terry, R. E.; Haymore, B. L.; Hansen, L. D.; Dalley, N. K.; Avondet, A. G.; Christensen, J. J. J. Am. Chem. Soc. 1976, 98, 7620-7626.

a Initial [85Sr] was approximately 4 × 10-12 M. Both the feed and receive solutions were pH 5. Log K values reported from the NIST Standard Reference Database 46-Critically Selected Stability Constants of Metal Complexes, Version 3.0, 1997. b The solubility product of SrC2O4 was not exceeded (pKsp ) 7.5). c log K value reported for acetic acid.

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Table 4. 85Sr and 45Ca Diffusion Constants (cm2/s) with 10-4 M Alkali Metal Salts or 137Cs in the Feed Solutionsa

Table 5.

85Sr Diffusion Constants (cm2/s) with the Feed Solution (20 nm Alumina)a

diffusion constants alkali metal salt

45Ca

85Sr

LiCl NaNO3 KNO3 RbNO3 CsNO3 trace levels 137Csb no added cations

1.6 × 10-6 1.1 × 10-6 9.2 × 10-7 1.1 × 10-7 1.4 × 10-6 2.3 × 10-6 1.6 × 10-7

2.1 × 10-7 7.9 × 10-7 2.2 × 10-7 2.2 × 10-7 2.4 × 10-6 2.2 × 10-6 1.6 × 10-7

a Receive solution is water (20 nm alumina membranes). b Trace levels refer to 137Cs concentrations around 10-9 M. Initial [85Sr] ≈ 4 × 10-12 M and [45Ca] ≈ 1.4 × 10-10. Error is (10%.

Figure 4. 85Sr transport through 20 nm alumina membranes with 10-4 M alkali metal salts (MNO3 where M ) Na+, K+, Rb+, Cs+, or MCl where M ) Li+) and 137Cs. 85

45

properties. Diffusion of Sr and Ca was measured through the 20 nm alumina membranes with macroscopic quantities of various alkali salts in the feed solution. Diffusion coefficients for 45Ca and 85Sr are given in Table 4 and a representative plot for 85Sr transport is presented in Figure 4. The data in Table 4 indicate that cesium nitrate has the largest effect in facilitating 85Sr transfer, followed by Na+, and to a lesser extent, Li+, K+, and Rb+. However, concentration gradients in the feed and receive solutions also contribute to an increase in 85Sr diffusion constants. To confirm this observation, 10-4 M CsNO3 was added to both the feed and receive vessels. The Sr2+ diffusion constant decreased from 2.4 × 10-6 cm2/s (10-4 M CsNO3 only in feed) to 1.0 × 10-6 cm2/s (10-4 MCsNO3 in feed and receive), but was still significantly larger than 85Sr in water alone, 1.1 × 10-7 cm2/s. However, 45Ca transport is most effectively promoted by Li+ salts, followed by Cs+, Na+, Rb+, and finally K+. Thus, there appears to be no correlation between 85Sr or 45Ca transport and the size or hydration energies of the competing ions, nor is the same pattern observed for the two transported cations. An analogous 85Sr facilitation effect was observed when a trace 137Cs quantity was added to the feed, as opposed to macroscopic concentrations of CsNO3. These experiments were repeated twice and found to be reproducible to within 10%. This result is surprising considering that Sr2+ and Cs+ cations are present in 10-9-10-12 M

85Sr 85Sr

+ 137Cs

137Cs

in

0.1 M LiCl

0.1 M NaNO3

H2O

2.4 × 10-6 3.2 × 10-6

2.6 × 10-6 3.7 × 10-6

1.1 × 10-7 2.2 × 10-6

a

Solutions of 0.1 M NaNO3 or LiCl present in both feed and receive vessels.

concentrations and the 20 nm alumina pores are large in comparison to hydrated ionic radii. An identical experiment was repeated with 100 nm alumina membranes, yielding a Sr2+ diffusion constant of 2.1 × 10-6 cm2/s, compared to 7.3 × 10-7 cm2/s (in water with no 137Cs). Tracer-level 137Cs similarly facilitated the transport of calcium ions. Table 5 presents data for the simultaneous transport of 137Cs and 85Sr through a 20 nm alumina membrane with 0.1 M NaNO3 or LiCl in both the feed and receive containers. These experiments were performed to isolate the contribution from ionic strength and alkali metal effects. At high ionic strengths, 85Sr diffusion coefficients in either LiCl or NaNO3 are identical within experimental error and at least 1 order of magnitude greater than 85Sr in water alone. For example, 85Sr diffusion constants in 0.1 M NaNO3 and water were 2.6 × 10-6 and 1.1 × 10-7 cm2/s, respectively. Increasing the ionic strength contracts the electrical double layer extending from the alumina surface which results in faster 85Sr transport through the membrane. 85Sr transport rates also increased when 137Cs was present in the feed (Table 5). Thus, 85Sr facilitation by 137Cs is a real effect and not a manifestation of electrical interactions between the ionic atmospheres of Sr2+ and Cs+. At this point, we are investigating the relation between enhanced Sr2+ transport and Cs+ sorption onto the alumina surface as one plausible explanation. Some irreversible sorption of Cs+ onto the membrane surface may change the electrostatic environment within the pores which ultimately increases the Sr2+ diffusion coefficient. Conclusions In this paper, we present preliminary transport data for unmodified mesoporous alumina membranes and propose a correlation between cation selectivity and membrane structure. The results presented herein also provide a baseline for future work in membrane surface modifications and their subsequent characterization via metal ion transport studies. At low ionic strengths, alumina surface chemistry and large Debye lengths dominate Sr2+, Cs+, Ca2+, and Na+ transport. Observed effects are greatest for 20 nm pores at e 10-4 M salt solutions. These results are in accordance with theoretical models by Stern et al. which show that membranes with 50 nm pores, when placed in dilute electrolyte solutions, have electric double layer thicknesses on the order of several hundred Å.12 Cesium cations, both at macroscopic and trace quantities, are able to facilitate divalent cation transport rates. As ionic strength increases, Cs cations continue to facilitate other cations, specifically 85Sr, but not to the same extent as in pure water solutions. Detailed theoretical calculations are currently underway to fundamentally understand and quantify this observed behavior. Acknowledgment. Funding was provided by the LANL Laboratory Directed Research and Development Program. Los Alamos National Laboratory is operated by theUniversityofCaliforniaunderContractW-7405-ENG-36. LA9902441