Understanding the separation of anion mixtures by TiO2 membranes: Numerical investigation and effect of alkaline treatment on physicochemical properties

Mechanisms governing the rejection of ions from multi-ionic solutions, and especially the selectivity between various anions are not fully understood at the present time. In this study, it is proposed, in a first part, to investigate the evolution of electric and dielectric exclusion mechanisms when anions are mixed in a solution. For this purpose, the volumetric membrane charge X-d and the dielectric constant of the solution confined within pores epsilon(p) are numerically assessed from rejection curves with a classic transport model. This study highlights that the rejection of the various anions is differently governed by electric and dielectric mechanisms but their contribution to mixture separation seems to be proportionally impacted by the solutions mixed. It was for instance demonstrated that the couple (epsilon(p), X-d) obtained with the ternary salt mixture (NaF-NaI-Na2SO4) corresponds to the barycenter of the triangle made by the couples of the three corresponding binary salt mixtures (i.e. NaF-NaI, NaF-Na2SO4 and NaI-Na2SO4). In a second part, the influence of a mild alkaline treatment, consisting in the filtration of a sodium carbonate solution, is investigated from the variation in membrane charge and dielectric constant assessed from ion mixtures. It is shown that the alkaline treatment mostly diminishes dielectric exclusion through a notable increase in dielectric constant inside pores when solution contains fluoride ion. Membrane charge estimated with solutions containing fluoride ion was found to be slightly impacted, but differently depending on the other anions in solution. Finally, exclusion mechanisms are found to be weak and unaffected by the treatment when solution does not contain fluoride ion.