Hydronium-promoted equilibrium mechanism for the alkali metal cation exchange reaction in Na-4-mica

A general stoichiometric cation exchange mechanism does not fully explain the unique exchange behaviors of Na-4-mica, the most highly charged swelling 2:1 phyllosilicate. Herein, we present for the first time a novel model of hydronium-promoted equilibrium for the cation exchange reaction of highly charged swelling 2:1 phyllosilicates with alkali metal cations. The exchange reaction of Na-4-mica with alkali metal cations revealed that adsorbed cations undergo further exchange with hydronium ions to result in the unprecedented back-release and position rearrangement of adsorbed cations. During the exchange reaction, hydronium ions play a key role as not competitive but stabilizing cation. Stabilization is accomplished through replacing half of the adsorbed cations and consequently relocating their position. Therefore, this hydronium-promoted equilibrium mechanism would provide a novel intellectual framework not only for understanding cation exchange reactions of various highly charged phyllosilicates but also for exploring their potential for new applications.