Evaluation of the Adsorption of Mono- and Polytungstates onto Different Types of Clay Minerals and Pahokee Peat

Detailed knowledge about the fate and transport of tungsten in soils is critical to understanding and effectively addressing tungsten behavior in the environment. Recent studies have shown that tungsten anions may polymerize (depending upon concentration, pH, and aquatic geochemistry) in aquatic and soil systems. However, to date, of all soluble tungstate species only monotungstates have been scrutinized to a fair extent in adsorption studies. There is a lack of information evaluating adsorption mechanisms of mono- and polytungstates onto clay minerals. The objective of this work is to investigate the adsorption behavior of monotungstates (sodium tungstate, Na₂WO₄) and polytungstates (sodium metatungstate, 3Na₂WO₄·9WO₃) onto different types of clay minerals (montmorillonite, kaolinite, illite) and an organic adsorbent (Pahokee peat). Batch equilibrium experiments as a function of concentration (adsorption isotherms) and pH (adsorption envelopes) were performed to provide information about mono- and polytungstate adsorption onto clays and Pahokee peat. Adsorption equilibrium data for mono- and polytungstates onto different types of clay minerals and Pahokee peat were modeled with Freundlich and Langmuir isotherms. The adsorption affinity of clays and Pahokee peat for monotungstates follows the order: Pahokee peat>kaolinite>montmorillonite>illite; for polytungstates, the order is as follows: kaolinite>Pahokee peat>montmorillonite>illite. Results of this study suggest that the charges of the clay mineral surface, tungsten species, and solution pH are the main factors controlling tungsten adsorption. Moreover, polymeric tungsten species (i.e., metatungstate) appear to be more mobile in the environment than monomeric tungstate.