Modified bipolar membrane for electrodialysis processing of highly concentrated sodium nitrate and boric acid solution

We studied the characteristics of two membranes: the industrial bipolar membrane MB-3, consisting of two layers, i.e., an anion exchanger with quaternary ammonium groups and a cation exchanger with phosphonate groups, and the membrane MB-3M, modified on the cation exchange side with a 100 μm thick layer of perfluorosulfonic acid. The analysis of the selectivity of the initial and modified membranes in a wide range of current densities (0.25–4 A/dm2) showed that the application of a perfluorosulfonic acid layer leads to a decrease in the transport of nitrate anions by 18–40 times, depending on the current density, compared to the initial industrial membrane MB-3. Experiments on electrodialysis processing of a mixed 0.5 M NaNO3 and 0.75 M H3BO3 solution showed that the use of a bipolar membrane modified with a perfluorosulfonic acid, at a sodium nitrate processing degree of 75%, allows obtaining a 1.01 M NaOH solution with a low impurity content (0.08 M sodium nitrate and 0.09 M boric acid) and 1.17 M HNO3 (sodium nitrate content 0.09 M, boric acid 0.09 M). When using the initial MB-3 membrane under the same conditions, 0.77 M NaOH with a high content of impurities (0.25 M sodium nitrate and 0.08 M boric acid) and 1.03 M HNO3 (containing 0.09 M sodium nitrate and 0.09 M boric acid) are obtained. The energy consumption for processing is comparable to that for the initial bipolar membrane MB-3 and amounts to 0.15 kW·h/mol.