Behavior of Crystalline Water During Cation Adsorption and Desorption in Layered Zirconium Phosphate

This study explores the behavior of crystalline water during the adsorption of Na⁺, K⁺, and NH₄ ⁺ ions onto layered zirconium phosphate (ZrP) using powder X-ray diffraction (XRD) and thermogravimetric analysis (TG). Cation uptake proceeded via two discrete adsorption steps, each accompanied by distinct structural transformations and water-related phenomena. For Na⁺, water molecules were incorporated into the interlayer region during the second adsorption stage. In contrast, adsorption of K⁺ and NH₄ ⁺ involved the release of crystalline water during the first stage, followed by water incorporation in the second stage. Notably, ZrP exhibited no measurable dissolution under the analytical conditions employed even under strongly alkaline conditions (pH ≈ 12). Furthermore, acid-induced desorption followed by readsorption confirmed that the layered structure of ZrP remained intact and fully regenerable. These findings demonstrate that ion-dependent migration of crystalline water plays a critical role in the adsorption mechanism and indicate that the dynamic behavior of interlayer water must be considered in the rational design of layered inorganic materials.