Polymerization and Transformation of Tungsten(VI) Species in Acidic Aqueous Solution by Electrospray Ionization Time-of-Flight Mass Spectrometry

Deep understanding of the polymerization and transformation of tungstate species in aqueous solution is of great significance for hydrometallurgical and chemical separation processes, such as solvent extraction, membrane separation and crystallization. In order to study the polymerization and transformation of tungstate species, the semi-quantitative characterization method of electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) was developed. In this paper, the tungsten speciation in simulative leaching solution was investigated in detail between pH 2.5 and 7 under various conditions (e.g. acid species, initial concentration, pH), and the results indicated that HCl was more conducive than HNO3 for the polymerization of tungsten at the same pH, and HAc accelerated the polymerization of tungsten species, especially in acid solution at lower pH; the morphological changes of each tungsten species tended to be consistent with the increase of tungsten concentration; pH strongly influenced the distribution of tungsten species. Then, the polymerization and transformation laws of tungstate species are summarized. W-1 (monomers) are dominant species in weak acid and neutral solution, and aggregate into oligomeric species (e.g. W-2, W-4, W-6). Transformations also exist among W-2, W-4 and W-6. The polymerization process from W-4 and W-6 species to higher poly-tungstate (W10-12) was observed with the consumption of H+. This work aims to understand the polymerization and transformation of tungsten speciation using ESI-TOF-MS, which should provide useful information for the understanding and optimizing of industrial production and environmental processes, building a bridge between micro tungsten speciation and macro industrial applications.