Ion-exchange properties of NASICON-type phosphates with the frameworks [Ti2(PO4)3] and [Ti1.7Al0.3(PO4)3]

Ion-exchange properties in aqueous A+ solution (A+= Li+, Na+, K+, Ag+, Rb+, NH4+) are investigated for the title materials at room temperature. In LiTi2(PO4)3 and Al-containing Li1.3Ti1.7Al0.3(PO4)3, Li+ is exchanged rapidly and selectively by Na+ and Ag +. The exchange rate of Li1.3Ti1.7Al0.3(PO4)3 is faster than that of LiTi2(PO4)3; this is probably related to a larger diffusion coefficient of Li+ in the former due to its high Li+ conductivity. Large ions such as K+ are not readily accepted by the anion frameworks. Accordingly, Li1.3Ti1.7Al0.3(PO4)3 has excellent Na+ selectivity with fast exchange rate to extract even a very small amount of Na+ in reagent-grade KOH. The Na+-exchanged product of Li1.3Ti1.7Al0.3(PO4)3 can exchange Na+ for Ag+ selectively. The reverse exchange also occurs easily. This material can be used to recover Ag+ in waste solutions.