Vermiculite adsorbed water

As an example, infrared spectroscopy has shown that the lowest stable hydration state for a Li-hectorite has a structure in which the lithium cation is partially keyed into the ditrigonal hole of the hectorite and has 3 water molecules coordinating the exposed part of the cation in a triangular arrangement (17), as proposed in the model of Mamy (J2.) The water molecules exhibit two kinds of motion a slow rotation of the whole hydration sphere about an axis through the triangle of the water molecules, and a faster rotation of each water molecule about its own C axis ( l8). A similar structure for adsorbed water at low water contents has been observed for Cu-hectorite, Ca-bentonite, and Ca-vermiculite (17). 

In view of the problems associated with the expanding 2 1 clays, the smectites and vermiculites, it seemed desirable to use a different clay mineral system, one in which the interactions of surface adsorbed water are more easily studied. An obvious candidate is the hydrated form of halloysite, but studies of this mineral have shown that halloysites also suffer from an equally intractable set of difficulties (JO.). These are principally the poor crystallinity, the necessity to maintain the clay in liquid water in order to prevent loss of the surface adsorbed (intercalated) water, and the highly variable morphology of the crystallites. It seemed to us preferable to start with a chemically pure, well-crystallized, and well-known clay mineral (kaolinite) and to increase the normally small surface area by inserting water molecules between the layers through chemical treatment. Thus, the water would be in contact with both surfaces of every clay layer in the crystallites resulting in an effective surface area for water adsorption of approximately 1000 tor g. The synthetic kaolinite hydrates that resulted from this work are nearly ideal materials for studies of water adsorbed on silicate surfaces. 

The spatial extent of the adsorbed water layer on vermiculite group minerals has been estimated on the basis of thermodynamic properties and self-diffusion coefficients for water on Li- and Na-vermiculite. The... 

The definition of adsorbed water adopted in Sec. 2.3 requires an arrangement of water molecules that differs significantly from that in an appropriate reference aqueous phase. For water on the surfaces of kaolinite group minerals the reference phase is bulk liquid water, whereas for water on vermiculite and smectite surfaces the reference phase is an aqueous solution because of the presence of exchangeable cations on the 2 1 layer silicates. On the ba,si.s of this definition, the consensus developed in Sec. 2.3 is that the spatial extent of adsorbed water on a phyllosilicate... 

Another important chemical property of adsorbed water on vermiculite and smectite surfaces is its Br nsted acidity. This property should refer principally to the acidity of the solvated exchangeable cations, as described by the reaction... 

Malamis, S., Katsou, E., Stylianou, M., Haralambous, K.J. Loizidou, M. (2010) Copper removal from sludge permeate with ultrafiltration membranes using zeolite, bentonite and vermiculite as adsorbents. Water Science and Technology, 61, 581-589. 

Experimentally, the conditions for the study of the infrared spectra of adsorbed water are particularly favorable in expanding lattice clays, /.e., montmorillonites, vermiculites, and halloysite because of the relatively large amount of water present with respect to the amount of solid. 

Machado, L.C.R., Lima, F.W.J., Paniago, R., Ardisson, J.D., Sapag, K. and Lago, R.M. (2006) Polymer coated vermiculite-iron composites novel floatable magnetic adsorbents for water spilled contaminants. Applied Clay Science,... 

Our approach has been to study a very simple clay-water system in which the majority of the water present is adsorbed on the clay surfaces. By appropriate chemical treatment, the clay mineral kao-linite will expand and incorporate water molecules between the layers, yielding an effective surface area of approximately 1000 m2 g . Synthetic kaolinite hydrates have several advantages compared to the expanding clays, the smectites and vermiculites they have very few impurity ions in their structure, few, if any, interlayer cations, the structure of the surfaces is reasonably well known, and the majority of the water present is directly adsorbed on the kaolinite surfaces. 

This fact may explain the superiority of montmorillonite over vermiculite as an adsorbent for organocations (3, 4). Complicating this description, however, is the fact that a sample of any particular layer silicate can have layer charge properties which vary widely from one platelet to another (j>). By measuring the c-axis spacings, cation exchange capacity, water retention, and other properties of layer silicates, one obtains the "average" behavior of the mineral surfaces. 

There ate many environmental applications of adsorption in practice and many others are being developed (Noble and Terry, 2004). Activated carbons and clays are frequently used for the removal of organic contaminants, such as phenol and aniline, both of which are prevalent in industry wastewaters and are known to have a significant negative impact on marine life and human health (IRIS, 1998 Dabrowski et al., 2005). Moreover, the adsorption on inexpensive and efficient solid supports has been considered a simple and economical viable method for the removal of dyes from water and wastewater (Forgacsa et al., 2004). Activated carbon, clays, coal, vermiculite, and other adsorbents have been used for this purpose. Specifically, adsorption can be employed in (Noble and Terry, 2004 Dabrowski, 2001) ... 

Adsorption of aniline on minerals, such as kaolinite, montmorillonite and vermiculite as well as on a-alumina and iron powders, has been studied in order to better understand the interaction of the adsorbate with natural adsorbents154-157. However, the results, similarly to those obtained on a smooth polycrystalline platinum electrode158, are not useful for the removal of aniline from waste water. The commonly used activated charcoal is also deficient owing to slow kinetics of the removal of dissolved pollutants. In this situation, activated carbon fibers, woven as a C-cloth, seem to be the ideal adsorbents for removal of aniline from waste streams. The reports of Niu and Conway159,160 explained the adsorptive and electrosorptive behavior of aniline on C-cloth electrodes and suggested a methodology for clean-up of industrial waste waters. 

In the case of vermiculite, the short-chain organic cations do not penetrate into the interlayer space, and are adsorbed only on the external surface of the mineral. This is clearly seen from the analytical measurements, showing that only 0.06 meq/g of Na" " ions is substituted when Na-vermiculite is treated with tetramethylammonium salt solution, the total exchange capacity being equal to 1.5 meq/g. Therefore, the values of specific surface area obtained from water and hexane adsorption isotherms for the initial and [(CH3)4N] modified vermiculite are virtually the same (Table 3). Such a pronounced difference in the... 

Walker (1956,1957) and van Olphen (1963) discussed the conditions of temperature and humidity at which partly-hydrated Mg-vermiculites exist in air. Walker interpreted differential thermal analyses (DTA) charts as indicating that interlayer water not associated with the hydration of adsorbed Mg ions is released at a lower temperature than is that around the Mg. Most of both of these types of interlayer water is released by 275°C. Vermiculite held under 63 MPa (10,000 p.s.i.) water vapor pressure shows its first dehydration at 550°C (Roy and Romo, 1957). 

Based on the 20 values for the 100 diffraction peak, the interlayer distance for the hydrated and anhydrous samples can be calculated as 1.34 and 0.95 nm, respectively. The observed decrease of the interlayer distance for the expanded sample can be associated with the release of water molecules observed under heating [3]. On the other hand, the hydrophobized matrix exhibits the same XRD pattern of the expanded one. Based on this fact, it can be concluded that the hydrophobizant molecules are adsorbed on the surface of the vermiculite grains and not into the interlayer space, that is, there is not the formation of an vermiculite—hydrophobizant intercalation compound. 

Since water molecules can adsorb on the clay surface, water can be considered as a modifier. To investigate the hydration process of vermiculite, a calorimetric study was performed with the main results as shovm in this section. The vermicuhte sample used was the same as described in the previous section. 

Magnetic resonance (ESR and NMR) studies have provided additional details about the orientational motion of the water molecules adsorbed by vermiculites. ESR spectra of Cu-vermiculite and NMR spectra of Mg- and Na-vermiculite indicate clearly that the primary solvation shells of the cations on the two-layer hydrate are octahedral complexes with a preferred orientation relative to the siloxane surface. For Cu-vermiculite, the symmetry axis through the solvation complex, Cu(H20)6 , makes an angle of about 45 with the siloxane surface on Na-vermiculite the axis through Na(H20)6 makes an angle of 65°. The value of Tc, the correlation time for the rotation of Na(H20)6 around its symmetry axis, is about 10 s at 298 K. This value is four orders of magnitude larger than Tc for a solvation complex around a monovalent cation in aqueous solution. Not quite as disparate are T2 for Na(H20)6, equal to 100 ps at 298 K, and t2 for a monovalent solvation complex in dilute aqueous solution, equal to about 5 ps at the same temperature. These data show that the siloxane surface retards the orientational motion of the water molecules. 

SMECTITE GROUP MINERALS. The structuTe of Water adsorbed by smectite group minerals has been studied extensively in both its static (D structure) and dynamic aspects. As with water molecules on vermiculite, the behavior of water on smectite surfaces is conditioned sensitively on the type of exchangeable cation and on the location of isomorphic cation substitutions in the layer structure. In many respects, a discussion of the configuration of water molecules hydrating smectites is parallel to that for vermiculite. 

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