Smectites, cation exchange properties

Smectites are stmcturaUy similar to pyrophylUte [12269-78-2] or talc [14807-96-6], but differ by substitutions mainly in the octahedral layers. Some substitution may occur for Si in the tetrahedral layer, and by F for OH in the stmcture. Deficit charges in smectite are compensated by cations (usually Na, Ca, K) sorbed between the three-layer (two tetrahedral and one octahedral, hence 2 1) clay mineral sandwiches. These are held relatively loosely, although stoichiometricaUy, and give rise to the significant cation exchange properties of the smectite. Representative analyses of smectite minerals are given in Table 3. The deterrnination of a complete set of optical constants of the smectite group is usually not possible because the individual crystals are too small. Representative optical measurements may, however, be found in the Uterature (42,107). 

Cations exchanged into the interlayers of expandable clays (smectites) are comparatively easy to study with NMR methods because the cations become major components of the phase and their concentrations are often several wt %. In addition to Cs Li, Na, K, and Cd have been studied by NMR. We have chosen to investigate Cs because it is a significant component of nuclear waste, because it provides an end-member case as the least electronegative cation, and because it has desirable nuclear properties (100% abundance, relatively high frequency, 65.5 MHz at H = 11.7 T, and small quadrupole moment)... 

Smectite clays have three important properties related to catalytic activity, intercalation, swelling and cation exchange capacity. When smectites are immersed in water, both intercalation of water molecules and swelling occur. The suspended clay can also freely exchange its interlayer cations for other cations in solution. Interlayer water molecules are dissociated producing protons and exhibit Bronsted acidity (Reaction l).8... 

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 cation exchange ability of smectites is isutilized to incorporate [Ru(bpy)3] cations. The luminescence probe studies have been carried out mainly in colloidal clay systems in which an aqueous solution of [Ru(bpy)3] is mixed with an aqueous suspension of smectites. In some cases, a species that quenches the excited state of [Ru(bpy)3] is added (80,82). The concentrations of hosts, [Ru(bpy)3], and quenchers in the suspensions have been varied in order to see the dynamics of the reactions. The photoprocesses of the [Ru(bpy)3] -smectite systems have revealed that the steady-state and time-re-solved luminescence properties of [Ru(bpy)3] + are not simple, and various interpretations of the unusual properties have been proposed (80,83,91). 

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