ABSTRACT
High cation-exchange capacity (CEC) muscovite mica with a homoionic surface was prepared by replacing the Li(+) surface ions of partially delaminated Li-mica with K(+). The CEC of this K-mica was determined by exchanging its surface cations with Cs(+), NH(+)(4), methylene blue (MB(+)), and copper triethylenetetramine [Cu(trien)(2+)]. The kinetics of these exchange reactions were studied and showed large differences depending on their relative affinities to mica. The NH(4)(+)/K(+) exchange was slow, while the Cs(+) and Cu(trien)(2+)/K(+) exchange was fast. The MB(+)/K(+) exchange was quite slow and was not completed even after 99 h. Insufficient reaction time is one of the main reasons for the contradictory results reported in the literature for the CEC of aluminosilicates obtained by different methods. The CEC of mica can be photometrically measured by exchanging its surface cations with Cu(trien)(2+). Copyright 2000 Academic Press.
ABSTRACT
Model muscovite with high cation exchange capacity was prepared, the surface of which was saturated with a single species of alkali metal ions. The kinetics and thermodynamics of the exchange reaction of the 1-dodecylpyridinium ion (NDP+) with alkali metal ions onto that substrate were studied. The exchange rate depended on the type of alkali metal ions present on the surface because of their different affinities to mica. However, in all cases the reaction was fast at the beginning and about 50% of the ions were exchanged within one hour; then the reaction rate decreased and equilibrium was only reached after several hours. This was attributed to a rate-determining rearrangement step in which the alkyl chains rearrange to adopt a dense packing. The reactivity of the alkali metal ions was in the order Li+ > Na+ > K+ > Rb+, Cs+, and in the case of K+, Rb+, and Cs+, equilibrium was only reached after 72 h. The lithium and sodium ions were exchanged almost quantitatively until a saturation value was nearly reached, while the K+, Rb+, and Cs+ exchange isotherms were less steep. The equilibrium constants (K) as well as the ion exchange capacity (S) were calculated by least-squares fits. Since K is infinite for quantitative exchange and decreases asymptotically upon deviation from this ideal behavior, the high K values (>10) of the NDP+/Li+ and Na+ exchange cannot be accurately determined. K ranges between 1 and 3 for the NDP+/K+, Rb+, and Cs+ exchange. The affinity of NDP+ to muscovite was similar or slightly higher than that of K+, Rb+, and Cs+, but was much higher than that of Li+ and Na+. The presence of oxonium ions in water did not strongly influence the exchange reaction on delaminated mica, as in the case of mica sheets, due to its high CEC. Copyright 1999 Academic Press.
ABSTRACT
The lithium surface ions of a muscovite, which was partially delaminated with a hot saturated lithium nitrate solution, exchange readily with sodium, potassium, rubidium, and cesium ions. The remaining potassium ions in the interlayers of the muscovite do not exchange at ambient conditions. The surface ion exchange is quite fast but can be followed by measuring the change in conductivity of the reaction mixture. In dilute systems, an initial drop in conductivity due to the exchange of the alkali metal ions with Li+ was observed, which was followed by a slow increase over a long period of time. That increase in conductivity is attributed to the formation of alkali bicarbonates due to the CO2 omnipresent in water. The surface Li+ was exchanged almost quantitatively by K+, Rb+, or Cs+ until a saturation value was nearly reached, while the Li+/Na+ exchange was less quantitative. The equilibrium constants (K) of these reactions as well as the ion exchange capacity were calculated by nonlinear least-squares fits. For the Na+/Li+ exchange K was found to be 4, while those of the K+, Rb+, and Cs+ exchange were too high for an accurate determination. The affinity of the alkali metal ions to muscovite decreased in the order K+, Rb+, Cs+ > Na+ > Li+. Copyright 1999 Academic Press.
