Formation of Two Concentration Regions within the Isotherms of Mo(VI) Adsorption on TiO2 by Cause of Oligomerization Processes

Abstract

Modeling of molybdenum adsorption on titanium dioxide can be impeded by oligomerization processes and mutual transformations of molybdenum chemical forms. The scientific community mostly accepts a viewpoint that molybdenum is adsorbed in form of an anion with a tendency towards formation of polymeric forms on the surface, which leads to an emergence of two regions of equilibrium sorbate concentrations on the isotherms at neutral and slightly acidic pH values leastways. In present study, the experiment demonstrated that this approach can be applied to acidic environments having a pH value less than 2, that is a case of neutral and positively charged forms of molybdenum existence in solution. The considered pH region corresponds to several industrially significant processes (e.g., treatment of irradiated solution targets for the production of ⁹⁹Mo and technological procedures of metallurgical production). The mean charge of the adsorbed species was estimated through measuring the electrokinetic potential by means of electrophoretic light scattering method and resulted to be –0.1e as accounted on the slip plane of the electrical double layer after all the screening. The obtained value indicates a predominance of anionic forms in the sorbate. It can be seen that the transition between two regions of adsorption isotherms occurs when the equilibrium sorbate concentration comes down to 10⁻⁴ mol/L by order of magnitude, which corresponds to the conventional lower limit for polymeric molybdenum ions to be observed in an aqueous solution. The total monolayer capacity of the adsorbent in the region of oligomeric forms adsorption (~2 mmol/g) is almost 3 times higher than in the region of monomeric adsorption (~0.7 mmol/g).