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Nanopores in minerals can have a significant impact on cation adsorption, known as the nanopore inner‐sphere enhancement (NISE) effect. Four zeolites representing three size classes of nanoporosity were used in this study to further elucidate the NISE phenomenon and describe its effects on the adsorption of Na, K, and Ca. Sodium and K ions have the same charge, but different ionic diameters. Sodium and Ca ions have similar ionic diameters, but different charges. Adsorption envelopes were created for these three cations on the four zeolites: zeolite Y, ZSM‐5, ferrierite, and mordenite. Comparisons of the magnitudes of the adsorption of Na, K, and Ca on the four zeolites indicated that all cations adsorbed weakly on zeolite Y (large nanopores), Na and K adsorbed strongly on ZSM‐5 and ferrierite (medium nanopores) while Ca adsorbed weakly, and all three cations adsorbed strongly on mordenite (small nanopores). It is noteworthy that in the medium nanopores, the two monovalent cations Na and K adsorbed more strongly than Ca, a divalent cation. The NISE effect is responsible for the changes in the relative strength of each cation's adsorption. This is an unusual adsorption mechanism and is counter to the traditional understanding of ion exchange reactions.

This study compared the adsorption behavior of Na+ and Ni2+ by zeolite minerals that differed only in the physical size of the nanopores in their interstitial regions. Any differences observed in the adsorption of these ions were interpreted in terms of how the physical variations of their nanopores might be involved. The adsorption strength of Ni2+ was weak with zeolite Y and ZSM‐5 but strong with mordenite. The adsorption strength of Na+ was weak with zeolite Y but strong with ZSM‐5 and mordenite. Even when Na+ adsorbed weakly, its adsorption strength was comparable to that of Ni2+ Similarly, when both adsorbed strongly, they both adsorbed with nearly the same strength. The adsorption reaction is a three‐way competition between H+, Na+, and Ni2+ for the adsorption sites. We propose a new theory to explain the increase in adsorption by the ZSM‐5 zeolite for Na+ and by mordenite for Na+ and Ni2+ The new theory states that a dehydrated inner sphere adsorbing ion will be stable if either the ion's attraction to the surface site is stronger than toward the bulk water or the ion's attraction to the bulk water is offset by hindering the presence of water in the region. This occurs when the nanopore diameters are larger than the ionic diameter of the ion but much smaller than the hydrated diameter of the ion, which occurs with interstitial cavities that are not greater than approximately 0.3 to 0.5 nm in size in at least one dimension.

Soil Chemistry S chulthess and Taylor (2007) proposed a new theory of cation adsorption to describe the adsorption of Na and Ni within zeolite minerals. This theory, called the nanopore inner sphere enhancement (NISE) theory, explains the unusual adsorption selectivity patterns observed for Na and Ni for Na, K, and Ca (Ferreira and. The NISE theory states that ions can dehydrate to fit into confining nanopore channels that are smaller than their hydrated diameters. In such a case, ions with lower hydration energies are more easily stabilized in their dehydrated states on adsorption than ions with higher hydration energies. Accordingly, in certain nanopore channels, monovalent ions are able to outcompete divalent ions because monovalent ions tend to have lower hydration energies than divalent ones (Collins, 1997). While hydration energy generally correlates well with the dehydration potential needed by the NISE theory, there are cases (such as Cu) where ions with high hydration energies can also dehydrate easily (Ferreira et al., 2012b).On zeolite Y, which contained the largest nanopore channels (0.74-nm limiting diameter), Ferreira and Schulthess (2011) observed that all three cations (Na, K, and Ca) adsorbed weakly and in similar amounts. On mordenite, with the smallest nanopore channels (0.26-nm limiting diameter), all three cations adsorbed strongly and in similar amounts. It is important to point out that the monovalent cations competed equally with a divalent cation at equimolar concentrations in these ion exchange reactions, which is very unusual. The most interesting results, however, The nanopore inner sphere enhancement (NISE) theory provides a new theoretical model of cation adsorption within confining nanopore channels. Inside nanopore channels, hydrated ions can dehydrate and more easily adsorb via an inner sphere mechanism. Adsorption data showed that in certain nanopores, weakly hydrated monovalent cations adsorbed more strongly than divalent cations, which tend to be strongly hydrated. Flow adsorption calorimetry is a valuable tool for directly measuring the heats of the ion exchange process and was used to measure the heats of Na and Ca exchange on three zeolite minerals: zeolite Y, mordenite, and ZSM-5. The data collected showed equal and reversible exchange reactions on mordenite but a strong endothermic Na adsorption and weak exothermic Ca adsorption on ZSM-5. On zeolite Y, the calorimetric signal was below the instrument detection limit of 5 to 7.5 mV. These differences coincide with the adsorption mechanisms and relative competitiveness predicted by the NISE theory for these two ions on the three zeolites studied. These data elucidate an exchange reaction where Ca is outcompeted by Na, which is often considered to be a weak background electrolyte.

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.

The adsorption mechanisms of divalent cations in zeolite nanopore channels can vary as a function of their pore dimensions. The nanopore inner-sphere enhancement (NISE) theory predicts that ions may dehydrate inside small nanopore channels in order to adsorb more closely to the mineral surface if the nanopore channel is sufficiently small. The results of an electron paramagnetic resonance (EPR) spectroscopy study of Mn and Cu adsorption on the zeolite minerals zeolite Y (large nanopores), ZSM-5 (intermediate nanopores), and mordenite (small nanopores) are presented. The Cu and Mn cations both adsorbed via an outer-sphere mechanism on zeolite Y based on the similarity between the adsorbed spectra and the aqueous spectra. Conversely, Mn and Cu adsorbed via an inner-sphere mechanism on mordenite based on spectrum asymmetry and peak broadening of the adsorbed spectra. However, Mn adsorbed via an outer-sphere mechanism on ZSM-5, whereas Cu adsorbed on ZSM-5 shows a high degree of surface interaction that indicates that it is adsorbed closer to the mineral surface. Evidence of dehydration and immobility was more readily evident in the spectrum of mordenite than in that of ZSM-5, indicating that Cu was not as close to the surface on ZSM-5 as it was when adsorbed on mordenite. Divalent Mn cations are strongly hydrated and are held strongly only in zeolites with small nanopore channels. Divalent Cu cations are also strongly hydrated, but can dehydrate more easily, presumably due to the Jahn-Teller effect, and are held strongly in zeolites with medium-sized nanopore channels or smaller.

Abstract:The goal of an attendance policy is to improve the academic success of students. However, current literature does not provide clear conclusions regarding whether enforcing an attendance policy actually improves student performance. This study examined student and faculty perceptions regarding the utility of attendance policies in undergraduate courses at a polytechnic university. Anonymous surveys were completed by 89 faculty members and 455 students from five schools (Engineering, Engineering Technology and Management, Computer and Software Engineering, Architecture, and Arts and Sciences) on a single campus. Comparisons between the perceptions of students and faculty members are presented, as are comparisons between the perceptions of lower-level and upper-level students. Variations in perceptions based on major are also highlighted. Finally, trends in perceptions regarding attendance policies in lower-level versus upper-level undergraduate courses are revealed.Students, regardless of major, class standing, or course level, reported attending more classes in courses that had attendance policies. The most significant impact of an attendance policy on class attendance was observed at the freshman level. While 84% of freshmen reported attending at least 90% of the classes in a course with an attendance policy, only 67% reported attending at that rate in a course without one. Qualitative data containing students' attitudes towards attendance policies are also analyzed and discussed.Even though class attendance appeared to have improved as a result of attendance policies, students' perceptions about these policies varied significantly. Overall, the majority of students (51%) believed that, for a course with an attendance policy, the policy positively affected final grades. For a course without an attendance policy, the majority (57%) felt that the lack of a policy had no impact on final grades. Faculty members' perceptions about attendance policies likewise varied. Overall, 61% of the faculty members surveyed reported having an attendance policy in one or more of their courses. The majority of faculty members believed that an attendance policy led to improvements in students' grades in lower-level courses, but not necessarily in upper-level courses. Further data regarding these perceptions is also discussed in this paper. Collectively, this study will help instructors make better-informed decisions about the use of attendance policies in their courses and give them insight into students' attitudes towards attendance policies.

Many studies have shown that the adsorption of ions like K+ and Cs+ on 2:1 clay minerals can prompt the collapse of their interlayers and render the adsorbing ions nonexchangeable. This study sought to better understand this unique adsorption mechanism through the generation of an adsorption envelope for 133Cs adsorption on vermiculite and the exploration of the kinetics of interlayer collapse. The collapse of the vermiculite interlayer was confirmed via X‐ray diffraction (XRD), and the timing of interlayer collapse was determined by placing Cs+ in competition with K+ at different time intervals. The adsorption envelope for Cs+ on vermiculite showed that although H+ competition does affect the adsorption of Cs+ on vermiculite, the effect of this competition is quite limited, even at very low pH values. This hypothesis is supported by the fact that XRD demonstrated a significant decrease in interlayer dimension after Cs+ adsorption. Finally, kinetics experiments showed that the irreversible adsorption of K+ and the collapse of the interlayer may take place on a much longer time scale than previously considered. Core Ideas Cesium adsorbs extremely strongly on vermiculite, even at very low pH. Cesium adsorption collapses vermiculite's interlayer by >63% to ∼1.1 Å. Attempts to quantify vermiculite interlayer collapse by K+ gave conflicting results.