Illite exchange

Figure 6. MAS NMR spectra of illite exchanged in 0.1 M NaCl solutions at 25°c. Spectra collected at = 11.7 T, room temperature, and room humidity ca. 35% RH).

Figure 3. CS MAS NMR spectra of Cs- exchanged (A) silica gel, (B) illite, (C) kaolinite, and (D) boehmite collected at H = 11.7 T, room temperature and the indicated relative humidities. The peaks marked by are spinning sidebands. After reference 27.

Figure 5. Cs MAS NMR spectra of (A) illite and (B) kaolinite Cs-exchanged at the indicated CsCl solution concentrations and collected at room humidity (ca. 35% RH) and H = 11.7 T. After reference 27.

Fig. 4.18 shows the result of Cd2+ adsorption on illite in presence of Ca2+ (Comans, 1987). The data are fitted by Freundlich isotherms after an equilibration time of 54 days. It was shown in the experiments leading to these isotherms that adsorption approaches equilibrium faster than desorption. Comans has also used 109Cd to assess the isotope exchange he showed that at equilibrium (7-8 weeks equilibration time) the isotopic exchangeabilities are approximately 100 % i.e., all adsorbed Cd2+ is apparently in kinetic equilibrium with the solution. The available data do not allow a definite conclusion on the specific sorption mechanism. 

In a similar study (Comans et al., 1990), the reversibility of Cs+ sorption on illite was studied by examining the hysteresis between adsorption and desorption isotherms and the isotopic exchangeability of sorbed Cs+. Apparent reversibility was found to be influenced by slow sorption kinetics and by the nature of the competing cation. Cs+ migrates slowly to energetically favorable interlayer sites from which it is not easily released. 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

The data are "normalized" with regard to the ion exchange capacity C of the sorbents. The sorption curves of the illite and of the < 40-pm chlorite are strongly non-linear, whereas that of the montmorillonite approaches linearity. 

The Adsorption of Cs+on Clays - an ion with a simple solution chemistry (no hydrolysis, no complex formation) - can be remarkably complex. Grutter et al. (1990) have studied adsorption and desorption of Cs+ on glaciofluvial deposits and have shown that the isotherms for sorption and exchange on these materials are nonlinear. Part of this non-linearity can be accounted for by the collaps of the c-spacing of certain clays (vermiculite, chlorite). As illustrated in Fig. 4.23 the Cs+ sorption on illite and chlorite is characterized by non-linearity. 

Rather small selectivity differences are observed for homovalent-and heterovalent exchanges involving alkali, alkaline earth, bivalent transition metal ions, aluminium and rare earth cations, as is amply evidenced from the extensive compilation by Bruggenwert and Kamphorst (16). This compilation includes various clay minerals illite, montmorillonite, vermiculite and kaolinlte. 

Highly selective exchange in illite and modified montmorillonites... 

Notwithstanding the exceedingly high selectivities, the process occurring at trace loading is not an irreversible fixation but a reversible ion exchange reaction, as can be deduced from the internal consistency of +the trace Cs and trace Rb adsorption equilibria in Na - and K -illite shown in fig. 6. 

Extremely high selectivities are frequently interpreted as "ion fixation", which suggests an irreversible phenomenon. This is the case for exchanges of Cs, Rb and K in illite clay minerals (95-96) as well as for Cu(NHj) exchange in fluorhectorite (66). However, reversibility was verified from the Hess law for adsorption of Cs, Rb and K on the high affinity sites in illite (91) and modified montmorillonites (101) as well as for the exchange of transition metal complexes (29, 75). 

Figure 2. Plot of XRD peak positions (CuK radiation ethylene glycol-solvated samples) for Kinney smectite treated with 0.05 N Na + K exchange solutions. Experimental points are labeled with percentages of K in solution. The graph, used to determine percentage illite layers and glycol-spacing for illite/smectites having crystallite thickness of 1-14 layers, is from (42).

Table IV. Percentage of Illite Layers and Interlayer Chemistry of K-Smectites Subjected to Wetting and Drying Cycles in Water at 60°C, and Then Exchanged with 0.1 N SrCl2...

Reference Number Sample Number of WD Cycles Number of Sr Exchanges Meq per 100 g Oxide Illite Layers (Percent)... 

N NaCl exchanges used to study diagenetic I/S (44). Thus, the illite layers remaining in the WD clays after three Sr-exchanges may be of comparable stability to those formed by burial diagenesis. 

Figure 5. Percentage illite layers versus layer charge for K-smectites subjected to 100 WD cycles in water at 60°C and 1 Sr-exchange. Numbers in parentheses refer to percentage of octahedral charge. Best fit line is for montmorillonites having 69% or more octahedral charge. Data from Tables III and IV.

Figure 6. Stability of illite layers formed by WD mechanism percentage of change in percentage illite layers between 1 and 3 Sr-exchanges is plotted against Of, the angle of tetrahedral rotation. Data from Table V.

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