Aluminum phosphate dispersed on a cellulose acetate fiber surface Preparation, characterization and application for Li+, Na+ and K+ separation

Cellulose acetate fibers with supported highly dispersed aluminum phosphate were prepared by reacting aluminum-containing cellulose acetate (Al2O3=3.5 wt.%; 1.1 mmol g-1 aluminum atom per gram of the material) with phosphoric acid. Solid-state NMR spectra (CPMAS 31P NMR) data indicated that HPO42- is the species present on the fiber surface. The specific concentration of acidic centers, determined by ammonia gas adsorption, is 0.50 mmol g-1. The ion exchange capacities for Li+, Na+ and K+ ions were determined from ion exchange isotherms at 298 K and showed the following values (in mmol g-1): Li+=0.03, Na+=0.44 and K+=0.50. The H+/Li+ exchange corresponds to the model of the ideal ion exchange with a small value of the corresponding equilibrium constant K=1.1x10-2. Due to the strong cooperative effect, the H+/Na+ and H+/K+ ion exchange is non-ideal. These ion exchange equilibria were treated with the use of models of fixed bi- or tridentate centers, which consider the surface of the sorbent as an assemblage of polyfunctional sorption centers. Both the observed ion exchange capacities with respect to the alkaline metal ions and the equilibrium constants were discussed by taking into consideration the sequence of the ionic hydration radii for Li+, Na+ and K+. The matrix affinity order for the ions decreases as the hydration radii of the cations increase, i.e. Li+>Na+>K+. The high values of the separation factors SNa+/Li+ and SK+/Li+ (up to several hundred) provide quantitative separation of Na+ and K+ from Li+ from a mixture containing these three ions