Tungsten Adsorption on to Alumina from Ammonium Meta- and Para-Tungstate Solutions — UV-Visible Spectra of the Solutions

The species in solutions used during tungsten adsorption on to alumina from ammonium meta- and para-tungstate solutions have been studied by UV-visible spectroscopy. For metatungstate, the spectra of the impregnant solution placed in contact with alumina and of the solutions corresponding to the dissolution pH and pH- were measured as a function of time. In all the cases, the intensity of the band corresponding to metatungstate decreased with time.

The spectra of paratungstate B solutions placed in contact with alumina as a function of time were very different from those of paratungstate solutions maintained at the dissolution pH and at pH = 8.5. However, these latter spectra were similar to those obtained for metatungstate solution placed in contact with alumina.

The decrease was more rapid for the solution which had been in contact with alumina relative to that to which ammonia had been added (pH = 9). In both cases, the variation in intensity with time was greater than that recorded for the metatungstate solution maintained at the dissolution pH. The observed fraction of metatungstate transformed depended not only on the solution pH but also on the presence of an additional effect due to the interaction of metatungstate ions and alumina. Only the monomeric species WO^2-₄ was present in a solution which had been placed in contact with alumina for 24 h.

During impregnation of an alumina support with paratungstate B and ammonium metatungstate solution, the species adsorbed is the monomer while the metatungstate fraction which does not interact with alumina is slowly transformed to the WO^2-₄ species.