Recovery of metal oxoanions from basic solutions using cooperative sorption - Separation of Na2MoO4 and NaOH

Chromatographic separation of molybdate oxoanions and NaOH was studied using cooperative sorption as a method to recover both Mo and the base as concentrated solutions. Commercial non-ionic microporous adsorbents composed of hyper-crosslinked polystyrene and cross-linked cellulose were investigated at 22 and 60 °C. For comparison, some separation tests were made also with ion exchange resins. Separation was modeled using rigorous thermodynamic treatment and an explicit sorption model. Good separation in batch column was obtained with the non-ionic cellulose adsorbent, while the polystyrene adsorbent and the ion exchangers give only moderate fractionation. Typical cooperative behavior was found in both cases but the sorption mechanisms are different. In the polystyrene adsorbent, uptake of the electrolytes separately and as binary mixtures can be explained in terms of electrostatic and steric exclusion of the constituent ions and no attractive interactions appear to be present. On the contrary, no steric exclusion operates in the cellulose adsorbent and separation is based on attractive interactions between NaOH and the weakly acidic hydroxyl groups of the polysaccharide chains. At the conditions used in this study, separation was not affected by molybdate polymerization taking place at low NaOH concentrations.