CHEMICAL SPECIATION CONTROLS ON ARSENIC AND TUNGSTEN IN WATER RESOURCES (Invited Presentation)

Chemical speciation refers to the form that trace elements occur in natural waters, and includes the free hydrated ion, aqueous complexes with inorganic and organic ligands, different valence states, and association with colloids, among others. Chemical speciation controls the mobility, effective solubility, reactivity, and toxicity of trace elements in the environment. For example, trace element toxicity and bioavailability are commonly related to the concentrations of the free hydrated ion, and different valence states of the same trace element can exhibit vastly different toxicities and mobility in the environment. An example is the uncharged and environmentally mobile arsenite oxyanion, H₃As^IIIO₃⁰, which is estimated to be 60-fold more toxic than the “particle reactive” arsenate oxyanion, which occurs as H₂As^VO₄^-or HAs^VO₄^2-in most natural waters. Reactive transport modeling of high As groundwater from the Chicot aquifer in southern Louisiana demonstrates that the arsenite oxyanion is more mobile than the arsenate oxyanion, the mobility of which strongly depends on the amount and type of Fe(III) oxide/oxyhydroxide present in the aquifer. The chemical speciation of tungsten (W) in the environment has until recently been poorly studied chiefly because W was thought to be non-toxic and of low environmental mobility. Nevertheless, the environmental mobility of W increases with increasing pH and decreasing contents of Fe(III) oxide/oxyhydroxide minerals in aquifer sediments. Moreover, W is also mobile anaerobic and sulfidic waters as the tungstate oxyanion (W^VIO₄^2-) is thiolated to various thiotungstate anions. Conversion of tungstate to tetrathiotungstate (W^VIS₄^2-) requires relatively high dissolved sulfide concentrations (i.e., ΣS^-II≥ 60 μmol kg^-1) and is kinetically “sluggish”. Investigation of sulfidic porewaters from southern Louisiana reveals that W is more stable in solution in sulfidic waters than its “chemical twin” molybdenum (Mo).