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An Investigation of Strong Sodium Retention Mechanisms in Nanopore Environments Using Nuclear Magnetic Resonance Spectroscopy Daniel R. Ferreira* and Cristian P. Schulthess Department of Plant Science and Landscape Architecture, University of Connecticut, 1376 Storrs Road, Storrs, Connecticut 06269-4067, United States
Marcus V. Giotto Institute of Materials Science, University of Connecticut, 97 North Eagleville Road Storrs, Connecticut 06269-3136, United States ABSTRACT: Recent experimental research into the adsorption of various cations on zeolite minerals has shown that nanopore channels of approximately 0.5 nm or less can create an effect whereby the adsorption of ions, especially those that are weakly hydrated, can be significantly enhanced. This enhanced adsorption occurs due to the removal of hydrating water molecules which in turn is caused by the nanopore channel’s small size. A new adsorption model, called the nanopore inner-sphere enhancement (NISE) effect, has been proposed that explains this unusual adsorption mechanism. To further validate this model a series of nuclear magnetic resonance (NMR) spectroscopy studies is presented here. NMR spectra were gathered for Na adsorbed on three zeolite minerals of similar chemical composition but differing nanoporosities: zeolite Y with a limiting dimension of 0.76 nm, ZSM-5 with a limiting dimension of 0.51 nm, and mordenite with a limiting dimension of 0.26 nm. The NMR experiments validated the predictions of the NISE model whereby Na adsorbed via outer-sphere on zeolite Y, inner-sphere on ZSM-5, and a combination of both mechanisms on mordenite. The strong Na adsorption observed in these nanoporous minerals conflicts with sodium’s general designation as a weak electrolyte.
’ INTRODUCTION Recent experimental data on the adsorption of Na, K, Ca, and Ni on zeolite minerals have shown that small nanopores can create unusual adsorption mechanisms that are counter to our traditional understanding of cation adsorption.1,2 These studies showed that monovalent and divalent cations adsorbed weakly, and in similar amounts, on zeolite Y with nanopores of 0.74 nm diameter. The fact that a monovalent cation adsorbed nearly as strongly as a divalent cation is not generally expected under traditional adsorption theories. There are exceptions to this, such as the selective retention of monovalent ions on certain clay minerals (e.g., K+ or Cs+ on illite).35 It should be noted that working with collapsible interlayers confounds the interpretation of what is driving the dehydration of these interlayer cations. The adsorption experiments performed by Schulthess et al.1 and Ferreira and Schulthess2 are different in that they used zeolite minerals with fixed nanopore dimensions. This allowed them to study the adsorption of cations at specific nanopore dimensions and compare the retention mechanisms as a function of nanopore size. Monovalent cations adsorbed much more strongly than divalent cations on zeolites containing nanopores in the 0.350.53 nm size range (ZSM-5 and ferrierite). This was a very curious result, as under most conditions, monovalent cations are not expected to be able to out compete divalent cations in ion exchange reactions at r 2011 American Chemical Society
equimolar concentrations. On mordenite, with a limiting nanopore diameter of 0.26 nm, monovalent and divalent cations all adsorbed equally strongly. These experimental data indicate that the monovalent cations had a dramatic increase in their preferential selectivity in the intermediate pore sizes, while the divalent cations only displayed this enhanced selectivity in the smallest nanopores studied. Traditional adsorption theories are not capable of explaining such an occurrence because they assume that under most conditions higher valence states will generally lead to stronger retention at equimolar concentrations. An ion’s retention on a mineral surface is a balance between its attraction to the surface relative to its affinity for the bulk liquid. At equimolar concentrations, higher valence ions are customarily observed to easily out compete lower valence ions for adsorption sites. This observation leads to the logical assumption that an ion with a greater charge will always have a stronger interaction with an oppositely charged surface than an ion with a weaker charge. However, these traditional theories fail to explain the unusual selectivity inside nanopores observed by Schulthess et al.1 and Ferreira and Schulthess.2 Received: September 29, 2011 Accepted: November 18, 2011 Revised: November 16, 2011 Published: November 18, 2011 300
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Environmental Science & Technology Schulthess et al.1 proposed a new model to explain cation adsorption inside small nanopores. This model, called the nanopore inner-sphere enhancement (NISE) model, predicts that when a cation is attracted to a mineral surface inside a nanopore channel which is smaller than its hydrated diameter, that cation may shed water molecules from its hydration sphere in order to fit inside the nanopore channel and adsorb. Divalent cations are more strongly hydrated than monovalent cations of similar size. In addition, smaller ions are more strongly hydrated than larger ions of similar charge.6 The weaker the bonds are between a cation and its hydration sphere, the more likely it is that the cation will dehydrate and adsorb via an inner-sphere mechanism. The NISE model incorporates hydration energies in describing the selectivity of monovalent cations over divalent cations in ion exchange reactions inside certain nanopores. This new model of cation retention inside small nanopores has potential implications for many environmental processes. Sodium is usually weakly retained in soils, but is strongly retained inside the small nanopores of certain zeolites. This explains the usefulness of zeolites in permeable reactive barriers to increase the retardation of chemically similar monovalent contaminants, which are usually highly mobile in groundwater.7 Molecular dynamics and Monte Carlo simulations have studied the behavior of ions in confining spaces, such as the nanopore channels of zeolite minerals. These simulations range from the behavior of gaseous molecules in zeolites810 to the adsorption of organic chains1113 or even the adsorption of water molecules themselves.1416 There appears to be some disagreement as to whether hydrated ions, when presented with a confining nanopore channel narrower than its hydrated diameter, will be able to enter the channel or not. While some simulations predicted that breaking an ion’s hydration shell presents too high of an energy barrier to overcome,17 other simulations have shown that ions are able to overcome the energy barrier and adsorb inside such channels.14,16,18 These simulations disagreed in describing an ion’s behavior inside nanopore channels. Furthermore, these simulations focused on individual ions under specific conditions, and did not address competition, which is critical for our understanding of selectivity in these environments. Confirmation of the NISE model using more direct experimental measurements is needed to address the differing opinions offered by molecular dynamics and Monte Carlo simulations. Nuclear magnetic resonance (NMR) spectroscopy was chosen to validate the predictions of the NISE model because it can be used to gather data on the nature of an ion’s local chemical environment and how it is bonded to neighboring molecules. Since the atom’s chemical environment and the strength of its bond to the mineral surface will change depending on how it is adsorbed, the NMR spectra can be used to identify whether a particular cation is adsorbed to a mineral’s surface through an inner-sphere or outer-sphere mechanism.19 By comparing the NMR spectra of Na ions adsorbed on different zeolite minerals, the accuracy of the NISE model to predict whether the ions are adsorbed via an inner-sphere or outer-sphere mechanism can be validated. This complements the experimental adsorption data that has already been collected.1,2 This study focused on using NMR spectroscopy to analyze the difference in the adsorption mechanism of Na on zeolite Y, ZSM5, and mordenite. Specifically, the chemical shift of NMR spectra was used to identify whether Na atoms were adsorbing via innersphere or outer-sphere mechanisms as a function of nanopore channel diameter. The ability of NMR to detect nuclear precession
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Table 1. Zeolite Characteristics. Pore Dimensions and Zeolite Codes from the Database of Zeolite Structures (www.izastructure.org) dimensions, nm pore 1
pore 2
SiO2:Al2O3
zeolite name
zeolite code
ratio
zeolite Y
FAU
0.74 0.74
N/A
80:1
ZSM-5
MFI
0.51 0.55
0.53 0.56
80:1
mordenite
MOR
0.70 0.65
0.26 0.57
90:1
is dependent on the type of spin an atom’s nucleus possesses and the frequency at which the atom precesses. Ferreira and Schulthess2 studied Na, K, and Ca adsorption on these zeolites and their Na adsorption data will be compared to the NMR spectroscopy results presented in this study. However, neither Ca nor K were suitable for measurement because their precession frequencies are too low to be detectable by the NMR instrument used in this study.
’ MATERIALS AND METHODS The zeolites used in this study were purchased from Zeolyst International (Conshohocken, PA). Mordenite and zeolite Y were purchased as hydrogen-forms, meaning that their surfaces had only hydrogen ions adsorbed. ZSM-5 was purchased as an ammonium-form, and was converted to a hydrogen form through the volatilization of NH4 to NH3 gas, leaving only H+ ions behind. This process is described in greater detail in Schulthess et al.1 These zeolites were chosen because they have similar chemical compositions, but differing limiting nanopore dimensions. The limiting dimension is the smallest nanopore channel size for each zeolite: 0.74 nm for zeolite Y, 0.51 nm for ZSM-5, and 0.26 nm for mordenite. Table 1 lists all nanopore channel dimensions for each zeolite used in this study, as well as their SiO2:Al2O3 ratios, which are very similar. Samples were prepared by adding 1.166 g of zeolite Y, 0.607 g of ZSM-5, or 0.900 g of mordenite to a centrifuge tube, along with double deionized water, HCl, and NaOH for pH adjustment. These sample amounts were chosen to mimic the sample preparation methods used in Ferreira and Schulthess2 and thus allow for better comparison with their adsorption data. The pH adjustment was performed in such a way that the total Na concentration was kept constant. A fixed quantity of 7 mL of 0.1 M NaOH was added to each tube, and 1.21 M HCl was then used to adjust the pH downward toward the target of pH 7. The zeolite Y sample was pH 7.07, the ZSM-5 sample was pH 7.10, and the mordenite sample was pH 7.11. Two slurries had no Na added in order to analyze the impact of the native Na and had their pH adjusted with Ca(OH)2 instead. The zeolite Y sample with no Na added was pH 6.97 and the ZSM-5 sample with no Na added was pH 6.94. An additional slurry of ZSM-5 was prepared and rinsed three times with 35 mL of 0.5 M KCl followed by a water wash to remove any surface adsorbed Na from the mineral. Each centrifuge tube contained a total liquid volume of 35 mL. The centrifuge tubes were placed on a hematology mixer to equilibrate for 18 to 20 h, followed by centrifugation for 10 min at 7800g to separate the solid and liquid phases. The liquid phase was decanted and the samples were then placed in an oven at 60 °C until they reached a water content of 150% ( 5% by weight. The moisture content was determined gravimetrically as 301
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a ratio of the mass of solid to the mass of the liquid remaining in the sample after heating. At 150% water content, the samples were concentrated slurries, not pastes. The solid-state 23Na magic-angle spin nuclear magnetic resonance (MAS NMR) spectra were acquired with a Bruker DMX 300 NMR spectrometer operating at field strength of 7.05 T and a resonance frequency of 79.38 MHz for 23Na (300 MHz for 1H). Two pulse phase modulation (TPPM) proton decoupling was used with a double resonance 4 mm MAS probe. The magic angle was set using the 79Br resonance of KBr. Approximately 100 mg of each sample was packed in a Zirconia rotor with Kel-F caps and spun at 3000 rpm at room temperature. The short selective pulse width for 23Na was 1.6 μs (π/6) with a delay time of 2 s applied to allow thermal equilibrium. There were 100 000 scans performed yielding 8192 data points and 50 kHz of spectral width. A Lorentzian line broadening of 20 Hz was applied to the data. A 75 kHz proton decoupling field strength was used during the acquisition of the free induction decay (FID). Chemical shifts were given with respect to a 0.1 M NaCl solution as the secondary reference.
can be overcome, especially in situations where the channel wall is charged and the ion has a strong affinity for the surface. The NISE model agrees with this assertion, and states that strong inner-sphere adsorption occurs when the energy of adsorption is greater than the energy of hydration.1 Otherwise, adsorption is via the weaker outer-sphere mechanism. Other simulations have also shown that the retention of organics and ions in nanopores increases as the nanopores become smaller.12,14 Wang et al.14 argued that the activity of water molecules inside nanopore channels of approximately 2 nm diameter is significantly reduced, especially along the channel walls where hydrogen bonding of water molecules is disrupted and solvent density is decreased. This resulted in an increased tendency of ions to form inner-sphere complexes inside nanopore channels.14 This was especially probable when the ion was large in relation to the diameter of the nanopore channel, as argued by Denayer et al.12 for nanopores with channel diameters from 0.54 to 0.76 nm and by Vaitheeswaran et al.16 for nanopores with channel diameters as small as 0.3 nm. Monte Carlo simulations of nanopore channels between 0.125 and 0.975 nm have also been used to compare the retention of Na and K inside nanopore channels. Carillo-Tripp et al.20 showed that K was strongly retained due to its larger ionic diameter inside nanopore channels smaller than 0.65 nm. While the results of the simulations described above agree with the NISE model, it should be noted that these are mathematical approximations and that direct measurements supporting the NISE model, other than the adsorption data presented by Schulthess et al.1 and Ferreira and Schulthess,2 are lacking. Figure 1 shows a stackplot of the NMR spectra for Na on ZSM-5, mordenite, and zeolite Y. The inset shows an overlay of the three peaks to scale so that their peak widths and peak heights can be compared. An aqueous 0.1 M NaCl solution was used as a reference and set to 0 ppm for all three experiments. The samples were prepared as slurries with 150% ( 5% water content to ensure that a solid-aqueous phase equilibrium could be established. Any strong adsorption observed in the NMR spectra is therefore not due to a drying effect. Accordingly, the NMR spectra should display peaks representing aqueous Na (labeled AQ) and separate peaks representing outer-sphere (labeled OS) or inner-sphere (labeled IS) adsorbed Na. The zeolite Y spectrum shows a peak labeled AQ-1 at 0.817 ppm that is very close to the 0 ppm reference point. This indicates that the chemical environment of those Na atoms is very similar to that of the liquid reference and represents the aqueous Na atoms in the liquid phase of the slurry.19 The slight chemical shift from 0 ppm is due to the fact that these ions are not in a purely aqueous solution, and therefore their chemical environment is slightly different from that of the reference. The peak AQ-1 is also the largest of the aqueous Na peaks. Since zeolite Y has a significantly lower amount of Na adsorption than ZSM-5 or mordenite, and the amount of Na added to each sample is fixed, it therefore has the greatest amount of Na remaining in solution. This explains why it has the largest aqueous Na peak among the three zeolites studied. The positive chemical shift of peak OS-1 at 8.463 ppm on zeolite Y indicates that the adsorbed Na is unshielded relative to the aqueous NaCl standard. The electron cloud around the outer-sphere adsorbed Na inside the nanopore is more diffuse than the electron cloud around the aqueous Na represented by the OS-1 peak due to a region of decreased solvent density inside the nanopore.16 This leads to an increased bond length between
’ RESULTS AND DISCUSSION The NISE theory predicts that the more weakly hydrated an ion is, the more easily it can detach from its hydrating water molecules and the more readily it will adsorb in narrow nanopore channels where the dehydrated ion is stabilized.1,2 Monvalent ions have a lower density of charge than multivalent ions and tend to form weaker bonds with the water molecules in their hydration spheres.3 Accordingly, monovalent ions have a significant advantage in ion exchange reactions over divalent ions inside some nanopore channels. Traditional adsorption models of external planar sites or noncollapsible environments focus only on adsorption strength and not on hydration strength. Therefore, they do not correctly predict adsorption in regions where dehydration is stabilized. On mordenite, with a limiting nanopore dimension of 0.26 nm, even divalent cations showed evidence that they could dehydrate and adsorb strongly.1,2 Nevertheless, even inside these smallest nanopores the divalent Ca ion adsorbed on a par with Na and K. It did not out compete them. Since the NISE model takes into account an ion’s energy of hydration in ion exchange reactions, it readily explains the counterintuitive adsorption mechanisms observed by Schulthess et al.1 and Ferreira and Schulthess.2 Abrioux et al.17 compared the results of molecular dynamics and Monte Carlo simulations of Na adsorption on two faujasite zeolites and noted that the molecular dynamics models showed no transfer of the Na ions between the supercages and the sodalite cages in the mineral. Since the hexagonal window between the cages had a diameter of ∼0.5 nm and the hydrated Na ion had a diameter of ∼0.7 nm, they hypothesized that the hydrated ion was unable to pass between the cages due to the high energy barrier associated with the removal of the Na ion’s solvating water molecules. As a result, they concluded that it would be a rare event for an ion to be able to enter a confined space with a diameter less than the hydrated diameter of the ion. This conclusion is in direct contradiction to the NISE model. On the other hand, Zwolak et al.18 used molecular dynamics to show that the hydrophobic pore walls of nanopore channels can in fact break the hydration layers of charged ions. They concluded that although there is an energy barrier to strip away the water molecules in an ion’s hydration sphere, this energy barrier 302
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Figure 1. Stackplot of solid-state 23Na NMR spectra for adsorbed Na on ZSM-5 (A), mordenite (B), and zeolite Y (C). The peaks are shown overlapped in the inset to allow for comparison of unaltered peak widths and heights. Water content was 150% ( 5%. Peaks labeled AQ represent aqueous Na, peaks labed IS represent inner-sphere adsorbing Na, and peaks labeled OS represent outer-sphere adsorbing Na.
peak width for Na ions which are more strongly hydrated.19 Similarly, the OS-1 peak for zeolite Y is narrower than the IS-1 peak for ZSM-5, indicating that the adsorbed Na on zeolite Y is more strongly hydrated than the adsorbed Na on ZSM-5. This reinforces the conclusion that Na is adsorbed via an outer-sphere mechanism on zeolite Y and an inner-sphere mechanism on ZSM-5, since the outer-sphere Na would be the more strongly hydrated of the two. An NMR spectrum was also generated for adsorbed Na on the zeolite mordenite (Figure 1). Mordenite is different from zeolite Y and ZSM-5 in that it contains nanopore channels of two different size classes, large and small (Table 1). Given the fact that these channels are interconnected (Figure 2), this creates two possible environments for the Na atoms to occupy. Ferreira and Schulthess2 showed that Na adsorption on mordenite was 25% lower than on ZSM-5. This was attributed to the fact that only one pore channel was small enough to create the NISE effect. Mordenite’s larger pore channel is similar in size to the pores of zeolite Y, and would, in all probability, not be capable of causing Na to adsorb via an inner-sphere mechanism. The authors presumed that the lower Na adsorption on mordenite as compared to ZSM-5 was due to the fact that Na was likely adsorbing via an inner-sphere mechanism in mordenite’s smaller pores and via an outer-sphere mechanism in the larger pores. The AQ-3 peak, located at 0.021 ppm, represents aqueous Na in solution as described for the AQ-1 and AQ-2 peaks. When the spectrum for mordenite is compared to the spectra from zeolite Y and ZSM-5 in Figure 1, it first appears that Na on mordenite is adsorbing only via an inner-sphere mechanism since peak IS-2 is broad and has a negative chemical shift. There is no obvious indication of any outer-sphere Na in the nanopores, which would be represented by a narrower peak with a positive chemical shift. The adsorbed Na peak height on mordenite is smaller than that of ZSM-5, indicating that there is less Na adsorbed on mordenite than on ZSM-5. The decrease in Na adsorption indicated by the NMR data agrees with the data presented by Ferreira and Schulthess.2
Na ions inside the nanopore channel and the water molecules in their hydration spheres. Finally, this increased bond length decreases the electron cloud density, resulting in a positive chemical shift.21,22 Consequently, OS-1 represents Na adsorbing via an outer-sphere mechanism on zeolite Y. There are also two peaks in the spectrum for ZSM-5. The AQ-2 peak at 0.248 ppm, which is also quite close to the 0 ppm reference, represents aqueous Na in the slurry for the same reasons given above for zeolite Y. The adsorbed Na peak, however, is different on ZSM-5 than on zeolite Y. While the adsorbed Na on zeolite Y had a positive chemical shift, the adsorbed Na on ZSM-5 shows a negative chemical shift. The negative chemical shift of 6.860 ppm means that the electron cloud around the Na adsorbed on ZSM-5 is denser than the aqueous reference. The increased electron cloud density for the Na adsorbed on ZSM-5 is due to its close proximity to the surface of the nanopore channel wall, resulting in the sharing of an electron with the electronegative surface group. Negative chemical shifts are typical of innersphere adsorbed ions.19 Kim and Kirkpatrick19 observed that water washing clay samples treated with Na in order to remove the most weakly held Na resulted in a more negative chemical shift of the remaining adsorbed Na. This demonstrated that the more shielded Na peaks were representative of the most strongly adsorbed Na ions that remained after the washing. Consequently, the location of the IS-1 peak indicates that the adsorbed Na on ZSM-5 is retained via an inner-sphere mechanism. Another important difference between the adsorbed Na peak on zeolite Y and the adsorbed Na peak on ZSM-5 is the peak width. Kim and Kirkpatrick19 observed that as they increased the relative humidity of their samples, and thus presumably the hydration of the Na atoms therein, the width of the Na peaks decreased. They attributed this to increased motional averaging because the mobility of the Na atoms was enhanced as the relative humidity of the samples was increased. The more soluble Na ions gained a higher degree of freedom of movement, resulting in more Na atoms with frequencies closer to the average. This, in turn, produced a decreased 303
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Figure 3. Solid state 23Na NMR spectra of adsorbed Na (top) and native Na (bottom) on zeolite Y at 150% ( 5% water content.
both environments during the time of the NMR scan, the observed peak would be a weighted average of the adsorption strength of the smaller pores and the larger pores. Since mordenite’s large pore is similar in size to zeolite Y’s pore channel and its small pore is similar in size to ZSM-5’s pore channel, we can use their respective chemical shifts as an estimate to represent the peak location of a typical inner-sphere adsorbed and outer-sphere adsorbed Na atom on mordenite. This averaging can be described by the following equation:
Figure 2. A three-dimensional structural rendering of mordenite’s channel system created using Jmol (version 11.2.7, www.jmol.org), an open-source Java viewer for chemical structures in 3D.
Given the similarity in size between mordenite’s larger pore channel and the pore channel of zeolite Y, it seems that there should be some Na adsorbing on mordenite via an outer-sphere mechanism. There is, however, an important difference between the pore channel system of zeolite Y and that of mordenite. Zeolite Y contains only one size pore channel, wherein the outersphere adsorbed Na are held. Ions in these channels can move freely from one site to another, but their chemical environment remains unchanged. Sodium ions in the zeolite Y sample will either be in the aqueous phase or adsorbed in its single nanopore channel. Mordenite, on the other hand, has a series of large and small pore channels that are interconnected with each other (Figure 2). Figure 2 is a top down view of mordenite’s nanopores with the top half of the cylindrical pore channels removed. The interconnection of the smaller pore channels with the large pore channel is clearly visible. Adsorbed Na on mordenite can move easily between two different chemical environments: the large pore channels and the interconnected small pore channels. The typical NMR time-scale for each Na observation is on the order of a few milliseconds.23 The motion of atoms between adsorption sites, however, likely happens on the scale of nanoseconds.24 It is a logical conclusion, therefore, that the NMR acquisition timescale for adsorbed Na on mordenite will represent a motional average of the Na in both environments. The Na atoms will move between the large and small pore environments many times during the course of each NMR observation. The freedom of movement of the Na atom, which impacts the peak location, would be greater whenever the Na atom was further away from the mineral surface in the larger pore. Conversely, the freedom of movement would be more restricted when the Na atom was closer to the mineral surface in the smaller pore. Based on these assumptions and the Na atom occupying
δo ¼
ðδis t is þ δos t os Þ t tot
ð1Þ
where δo = observed chemical shift on mordenite, δis = 6.860 ppm = inner-sphere chemical shift based on ZSM-5 data (IS-1), δos = 8.463 ppm = outer-sphere chemical shift based on zeolite Y data (OS-1), tis = time cation spends as an inner-sphere complex, tos = time cation spends as an outer-sphere complex, and ttot = tis + tos. We can now solve the equation above to determine the ratio of time spent in inner-sphere complexes versus the time spent in outer-sphere complexes necessary to create the chemical shift of 5.350 ppm observed for the IS-2 peak for mordenite. Solving eq (1) yields a tis/ttot ratio of 0.901 and a tos/ttot ratio of 0.099. These ratios are close to the change in the experimentally determined Na adsorption values between completely outersphere Na on zeolite Y and completely inner-sphere Na on ZSM5 presented by Ferreira and Schulthess.2 Those calculations based on Na adsorption on zeolite Y (0.15 μmol m2) and Na adsorption on ZSM-5 (0.78 μmol m2) yield a ratio for innersphere and outer-sphere adsorption strength of 0.84 and 0.16 respectively. The inset in Figure 1 shows the overlapped spectra for Na on the three zeolites. In this way the peak heights and widths for the raw data can be compared. The area under IS-1, IS-2, and OS-1 reflect the quantity of Na adsorbed on each zeolite. It is clear that ZSM-5 had the greatest Na adsorption, whereas zeolite Y had the lowest Na adsorption. Mordenite contains a quantity of adsorbed Na in between ZSM-5 and zeolite Y due to the fact that it has both large and small nanopore channels. These results are consistent with the experimental adsorption data obtained by Ferreira and Schulthess,2 who recorded Na adsorption as 0.78 μmol m2 304
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despite the high energy barrier of dehydration. The ability for monovalent ions to dehydrate and adsorb strongly should be included in future molecular dynamics simulations.
’ ENVIRONMENTAL IMPLICATIONS The validation of inner-sphere adsorption of Na by NMR spectroscopy shows that monovalent ions in the environment can adsorb strongly in the presence of nanoporous minerals. Sodium is often described as an indifferent, inert, or weakly adsorbing electrolye. Our NMR results, however, impact our understanding of this ion’s selectivity in soils. Adsorption models should now, therefore, not only consider the chemical properties of the adsorbent and adsorbate, but also the physical properties of the adsorption environment. ’ AUTHOR INFORMATION Figure 4. Solid state 23Na NMR spectra of adsorbed Na (top) and native Na (bottom) on ZSM-5 at 150% ( 5% water content.
Corresponding Author
on ZSM-5, 0.65 μmol m2 on mordenite, and 0.15 μmol m2 on zeolite Y. The three zeolites have native Na present, accounting for 0.05% of the mineral structure. In order to show that the Na peaks detected during the NMR experiments were due to adsorbed Na and not due to the native Na present in the material, NMR spectra were obtained of zeolite Y and ZSM-5 without any additional Na added. Figures 3 and 4 show spectra of the native Na overlaid underneath the spectra for the samples to which Na was added so as to compare their magnitudes. These overlays with no Na added clearly demonstrate that any contribution of the native Na to the peaks observed in Figures 3 and 4 is negligible. Furthermore, an NMR spectrum was also obtained for a sample of ZSM-5 washed three times with 0.5 M KCl to remove any surface adsorbed Na (data not shown). This spectrum showed no adsorbed Na peak, indicating that the native Na impurities reported by the manufacturer are on the exchange complex and not inside the mineral matrix. The spectra observed in Figure 1 are due to Na ions adsorbing via an inner-sphere mechanism on ZSM-5 and mordenite despite the fact that the samples had a high water content. Sodium cations, which are customarily weakly held on external surface sites, are clearly shown to be dehydrated and adsorbed strongly. The chemical shift of the peak IS-2 on mordenite (Figure 1) represented a motional average of Na ions in two different environments. The chemical shift of this peak was used to calculate the weighted average of both inner-sphere and outersphere adsorption mechanisms, which agrees with equilibrium adsorption data.2 The NISE model predicted, based on experimental adsorption data, that Na would adsorb via an outer-sphere mechanism on zeolite Y and via an inner-sphere mechanism on ZSM-5. The NMR data, which represent a more direct investigation into the cation’s relationships with the mineral surface and hydrating water molecules, agreed with these predictions and further validated the NISE model. The results of conflicting molecular dynamics studies concerning how hydrated cations behave inside these confining nanopore channels were compared. The results of this study provide experimental data that support the idea that hydrated ions can dehydrate in order to enter confining nanopore environments
’ ACKNOWLEDGMENT This work was financially supported by the project “Ion exchange processes in nanopores, clay interlayers and sodic soils” (USDA-Hatch no. CONS00864), funded by the United States Department of Agriculture.
*E-mail:
[email protected].
’ REFERENCES (1) Schulthess, C. P.; Taylor, R. W.; Ferreira, D. R. The nanopore inner sphere enhancement effect on cation adsorption: Sodium and nickel. Soil Sci. Soc. Am. J. 2011, 75, 378–388. (2) Ferreira, D. R.; Schulthess, C. P. The nanopore inner sphere enhancement effect on cation adsorption: Sodium, potassium, and calcium. Soil Sci. Soc. Am. J. 2011, 75, 389–396. (3) Sawhney, B. L. Selective sorption and fixation of cations by clay minerals: A review. Clays Clay Miner. 1972, 20, 93–100. (4) Brouwer, E.; Baeyens, B.; Maes, A.; Cremers, A. Cesium and rubidium ion equilibria in illite clay. J. Phys. Chem. 1983, 87, 1213–1219. (5) Comans, R. N. J.; Haller, M.; De Preter, P. Sorption of cesium on illite: Non-equilibrium behavior and reversibility. Geochim. Cosmochim. Acta 1990, 55, 433–440. (6) Collins, K. D. Charge density-dependent strength of hydration and biological structure. Biophys. J. 1997, 72, 65–76. (7) Robinson, S. M.; Kent, T. E.; Arnold, W. D.; Parrott Jr., J. R. The Development of a Zeolite System for Upgrade of the Process Waste Treatment Plant, ORNL/TM-12063; Oak Ridge National Laboratory, Oak Ridge, TN, 1993. (8) Bezus, A. G.; Kiselev, A. V.; Lopatkin, A. A.; Du, P. Q. Molecular statistical calculation of the thermodynamic adsorption characteristics of zeolites using the atomatom approximation. Part 1.—Adsorption of methane by zeolite NaX. J. Chem. Soc., Faraday Trans. 2 1978, 74, 367–379. (9) Smit, B. Simulating the adsorption isotherms of methane, ethane, and propane in the zeolite silicalite. J. Phys. Chem. 1995, 99, 5597–5603. (10) Leyssale, J. M.; Papadopoulos, G. K.; Theodorou, D. N. Sorption thermodynamics of CO2, CH4, and their mixtures in the ITQ-1 zeolite as revealed by molecular simulations. J. Phys. Chem. B 2006, 110, 22742–22753. (11) Ndjaka, J. M.; Zwanenburg, G.; Smit, B.; Schenk, M. Molecular simulations of adsorption isotherms of small alkanes in FER-, TON-, MTW- and DON-type zeolites. Microporous Mesoporous Mater. 1994, 68, 37–43. (12) Denayer, J. F. M.; Van der Beken, S.; De Meyer, K. M. A.; Martens, J. A.; Baron, G. V. Chromatographic liquid phase separation of 305
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