Behavior of Alkali Metal and Ammonium Salts in Nanofiltration of Their Aqueous Solutions

Abstract

The behavior of salts (chlorides, nitrates, sulfates, phosphates, borates, ferri- and ferrocyanides) of various alkali metals (Li, Na, K, Cs) and ammonium in the course of nanofiltration (NF) of their aqueous solutions was studied. The NF polymer membrane at high salt concentrations (>30 g dm⁻³) retains alkali metal and ammonium ions to a low extent (1–5%). At lower salt concentrations, the retaining ability of the NF membrane increases. In nanofiltration of solutions of Na and K salts with anions of different charge, the retaining ability of the NF membrane increases with an increase in the anion charge. The pH dependence of the ability of the NF membrane to retain sodium nitrate was studied. This parameter was constant at pH in the ranges 2.0–5.5 and 7.5–13.0, but at pH in the range 5.5–7.5 the retaining ability decreased approximately by half. Experiments on retention of sodium borates of various compositions showed that the solutions containing bulky polynuclear anions consisting of 4–5 boron atoms were retained by the NF membrane with 45–50% efficiency, whereas neutral boric acid molecules were not noticeably retained by the membrane. Thus, nanofiltration can be used for selective separation of inactive alkali metal salts from radioactive components of liquid radioactive waste.