Vermiculite exchangeable cations

Neutron-scattering and dielectric relaxation studies [23] both indicate that the water molecules solvating monovalent exchangeable cations on montmorillonite are a little less mobile, in respect to translational and reorientational motion, than are water molecules in the bulk liquid. For example, as with vermiculite, neutron-scattering data show that no water molecule is stationary on the neutron-scattering time scale. In the one-layer hydrate of Li-montmorillonite, the residence time of a water molecules is about six times longer than in the bulk liquid, with a diffusive jump distance of about 0.35 nm, and a water molecules reorients its dipole axis about half... 

Exx[Al2-xMgx] Si4 Oio(OH)2> where Ex stands for exchangeable cations, the [ ] symbols signify that Al and Mg2+ an octahedral layer, and the symbols signify that S 4+ is in a tetrahedral coordination. Mg2+ ions can also substitute for A 3+ in the clay mineral chlorite, and is present in octahedral layers of the clay mineral vermiculite.71... 

Aluminosilicates such as montmorillonite, kaolinite, illite, and vermiculite are solids that have structures readily accessible to counter ions. The excess negative charge resulting from isomorphic substitution of Al for Si is primarily distributed over the three adjacent surface O atoms of the layer, where it is electrically balanced by mobile, exchangeable cations- Thermodynamically, ion exchange can be interpreted in terms of the interlayer electrostatic interaction between surface charges and hydrated cations in accordance with the classical Eisenman theory (Eisenman, 1983). A comprehensive description has recently been given by Maes and Cremers (1986). 

Interstratified or intermixed layer silicates, vermiculite (M+(Mg,Fe)s(Si4-n, Aln)0iq(0H)2) and the hydrous (slightly weathered) micas. The M+ represents an exchangeable cation. Whether the minerals of this category are inherited from the parent material or are secondary products derived from inherited minerals is uncertain in many cases. 

Chemical Properties. An important chemical property of clays, which directly affects fines migration is the cation exchange capacity (CEC) (6-9). CEC is a measure of the capacity of a clay to exchange cations. It is usually reported in units of milliequivalents per 100 g of clay (meq/100 g). The CEC depends on the concentration of exchangeable cations in the diffuse Gouy-Chapman layer (see later). This concentration depends on the total particle charge, which may vary with pH. Unless stated otherwise, the reported values of CEC are measured at neutral pH. CEC values (meq/lOOg) of common clay minerals are as follows smectites, 80-150 vermiculites, 120-200 illites, 10-40 kaolinite, 1-10 and chlorite, <10 (10). 

The smectite and vermiculite clays are particularly interesting from a catalytic standpoint since their lamellae are typically held together by hydrated cations that can be exchanged for other cations. The idea of exchanging cations in clays... 

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. 

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... 

Direct coordination to the exchangeable cations has been recognized in complexes of smectites and vermiculites with a wide variety of organic compounds, such as alcohols, amines, nitriles, urea and amides, and pyridine and pesticide molecules. Complexes could be formed with alkaline and alkaline earth cations or with transition metal ions, and excellent reviews have been published on this matter (3-5,7,8). 

FIGURE 7 Proposed interlayer arrangements of exchangeable cations and associated macrocyclic ligands in complexes with montmorillonite and vermiculite. = radius of the macrocyclic cavity r, = radius of cations Mi = interlayer distance. 

Smectite clay - Like mica, smectite cl (commonly called bentonite) has either a pyrophyllite or talc structure. Montmorillonite, a common high-aliunimun smectite, can be characterized by die pyrophyllite crystal structure with a small amount of octahedral Al replaced by Mg. The resulting charge imbalance is compensated by exchangeable cations, usually Na or Ca, between the laminae. In addition to diese counterions, oriented water, similar to that in vermiculite, occupies the interlaminar space. When Ca is the exchangeable cation, there are two water l ers, as in vermiculite when Na is the counterion, there is usually just one water layer. Figure 18 shows the montmorillonite structure. 

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