Selective sodium removal from aqueous waste streams with NaSICON ceramics

Recent developments in the synthesis and application of the sodium ion conducting polycrystalline Nasicon ceramics allow for selective removal of sodium from aqueous wastes at ambient temperatures by electrochemical salt splitting. In the presence of an applied electric field, sodium ions are transported through the Nasicon structure. The size and electroneutrality constraints allow for selective transport of sodium ions, and exclude other monovalent, divalent and trivalent ions present in the impure reactants from migrating through the membrane. The sodium transport efficiency for generating pure NaOH from nitrate and sulfate industrial wastes is greater than 90%. These ceramic membranes provide the added benefit of very low parasitic losses due to absence of fouling by precipitants. Electro-osmotic transport of H2O through the membrane which is common to polymeric membrane technology is also not observed. While the initial electrochemical evaluation of the ceramic membranes showed high sodium selectivity over other metal cations, the need for improvements in sodium conductivity, long term stability, and durability in strong acid was identified. A new series of Nasicon compositions have shown considerable improvements in properties and exhibit the potential for large-scale, industrial applications. (C) 1999 Elsevier Science B.V.