Ion Transport Behavior in Bipolar Membrane Electrodialysis: Role of Anions

In this study, the conversion of different Na+-containing inorganic salts (NaF, NaCl, NaNO3, Na2SO4, Na2CO3, and Na3PO4) into their corresponding acids and bases using a bipolar membrane electrodialysis (BMED) process was investigated. The mechanisms governing the transport of various anions (i.e., fluoride, chloride, sulfate, carbonate, phosphate, and nitrate) and one cation (i.e., sodium) through anion- and cation-exchange membranes in the BMED process were explored. The results of ion transport with different ion solutions reveal that intrinsic ion properties, such as symmetry, ion size, hydration number, Gibbs energy, and dehydration rate have a great influence on the ion transport mechanism and acid base generation. The anion transport presented a trend of F– > Cl– > SO42– > NO3– > PO43– > CO32– by producing acids with the concentration order of HF > HCl > H2SO4 > HNO3 > H3PO4 > H2CO3. This trend was consistent with the current efficiency in the acid compartment. The F– ions have a strong hydration shell and low affinity to the polymer matrix, which is beneficial for migrating through the anion-exchange membranes and favoring the acid production performance. The PO43– ions have the largest Stokes radius (4.9 Å) and the highest valency among the various investigated anions, leading to a low acid generation performance. The emission of gaseous carbon dioxide caused the CO32– ions to have the poorest acid generation performance.