Discovery of Unprecedented Ion–exchange Behavior of Nanometric Scheelite Prepared from Single‐Source Precursors

Abstract

AbstractWater‐soluble precursors for nanometric calcium tungstate (CaWO4) were prepared from calcium 2‐hydroxycarboxylate salts. The precursors decompose as low as 425 °C to yield nanocrystalline scheelite. Supported CaWO4 in a convenient form for water treatment processes was prepared by impregnation of high surface area catalyst support pellets with an aqueous solution of a single‐source precursor followed by drying and calcining. Both the powder and supported nanocrystalline forms of CaWO4 underwent ion‐exchange reaction with aqueous lead ions whereby Ca2+ was released and Pb2+ was adsorbed to produce crystalline stolzite (PbWO4). Electron microscopy demonstrated that little morphological change of the particles occurred upon reaction with excess Pb2+. The process appears to occur via an ion‐exchange route. The nonlinear pseudo first‐order kinetic model gave the best fit for both the powder and supported forms of CaWO4, allowing the kinetic results from the two materials to be directly compared to each other. Supporting of CaWO4 improves the kinetics at higher CaWO4:Pb2+ ratios while at lower ratios diffusion of the Pb2+ ions into the porous pellets likely becomes the controlling factor and the powder form uptakes Pb2+ faster. The molar capacity of the material for Pb2+ uptake improved from 70 % to 100 % upon supporting the CaWO4. The novel ion‐exchange materials were demonstrated to be useful for the purification of industrial wastewaters and juice.