Treatment of molybdate solutions by electrodialysis: The effect of pH and current density on ions transport behavior

• Electrodialysis is able to treat molybdate solution in the ohmic current density range.
• Chronopotentiometry gives information about molybdate/anion-exchange membrane system.
• Polynuclear anions are formed under certain solution pH and current density conditions.
• Polynuclear anions formation leads to a membrane poisoning phenomenon.
• A Na2MoO4 pH 4.8 solution shows a non-practicable condition to ED desalting purpose.

Molybdate conversion coatings are seen as a viable alternative to produce chromate-free coatings. By the coating process, molybdate containing wastewater are generated and electrodialysis could be applied for salts recovery and process water reuse. In fact, the electrodialysis efficiency for these purposes is associated to the ions transport properties through the ion-exchange membranes. In this work the transport behavior of molybdate ions through a HDX200 heterogeneous anion-exchange membrane was studied using the electrochemical characterization technique of chronopotentiometry. The study was carried out with 0.005 M Na2MoO4 solutions at different pH values: 4.8, 6.5 and 12.0. Differences in the chronopotentiometric curves shape as well as in the limiting current density and ohmic electrical resistance values were observed and explained with basis on the molybdenum speciation diagram. The obtained results revealed that, according to the solution pH and current density conditions, polynuclear anions can be formed leading to a membrane-poisoning phenomenon, exposing limitations and suitable conditions of using the electrodialysis in the treatment of molybdate wastewater.