Interactions of W(VI) and Mo(VI) Oxyanions with Metal Cations in Natural Waters

The association of tungsten(VI) and molybdenum(VI) oxyanions with metal cations was investigated under conditions simulating those prevailing in most natural waters. Potentiometric titrations were carried out for the systems containing tungsten(VI) or molybdenum(VI) anions and metal cations. The selection includes the major metal cations and some other ions of high environmental relevance. At neutral and basic pH values, in the presence of the most abundant metal cations, ionic pair species such as [Ca(WO4)] or [Ca(MoO4)] are formed to high extents. For the acidic pH range, polyoxoanion associations with cations are also found but are only relevant for tungsten(VI) in the conditions of the natural aquatic systems. The present results provide the basis for studying W(VI) and Mo(VI) speciation in natural aquatic systems, on which the environmental fate, bioavailability and toxicity of the elements depend.