The mechanism of boric acid transport during an electrodialytic desalination process

Abstract

► Boric acid transport model across both: the cation- and anion-exchange membrane (in separate) have been examined. ► Both: diffusive and convective H 3 BO 3 transport mechanisms were found to contribute. ► Diffusion coefficient across cation-exchange membrane was found to depend upon the type of the cation and the membrane. ► Diffusion coefficient across anion-exchange membrane depended upon the membrane only. ► The extend of convective transport dependent upon the type of the ion. The applicability of an extended Nernst–Planck equation-based model to predict the rate of boric acid transport during electodialytic (ED) desalination has been examined. Diffusive and convective mechanisms for boric acid transport were found to determine the boric acid transport rate. The effects of ED operating conditions on the diffusion coefficient ( D m ), mass transport coefficient ( k m ) and convective drag transport coefficient ( w m ) were investigated for cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs) separately. In the case of CEMs, it was found that D m and k m were strongly dependent on migrating cations present and increased with different cations in the following order: Mg 2+ < Ca 2+ < Na + < K + , while w m remained constant. This diffusion coefficient series conforms to existing literature data with respect to the transport rates of boric acid across CEMs. In the case of AEMs, D m and k m values were not significantly different with respect to the types of anions present, while w m coefficients decreased with different anions in the following order: SO 4 2− ≫ NO 3 − ≈ Cl − . It was also found that, regardless of ion type, all three transport coefficients were lower by a factor of approximately 10 across CEMs than across AEMs. It was then concluded that, during the course of electrodialysis, boric acid is mainly transported across anion-exchange membranes. Moreover, the diffusion coefficient was found to increase with respect to the type of membrane used, in the following orders: AMV (Selemion, Japan) < AM(H) (Ralex, Czech Rep.) < AMX (Neosepta, Japan) < PC–SA (PCA, Germany) in the case of AEMs and CMV (Selemion, Japan) < CMX (Neosepta, Japan) < CM(H) (Ralex, Czech Rep.) < PC–SA (PCA, Germany) in the case of CEMs. The convective transport coefficient, w m , was found to be affected by the types of cations or anions transported with boric acid rather than the types of membranes used. Finally, a good agreement between the concentrations of boron in the concentrate and electrode rinse solutions during electrodialytic desalination predicted by the model and the concentrations observed experimentally proved that the proposed model is adequate.