Sorption of Arsenate, Selenate, and Molybdate on the Barite (001) Surface

Interactions of ions with ionicallybonded minerals such as barite(BaSO4) influence the fate and transport of the ions, whilethe factors that control the sorption of toxic oxyanions on bariteremain elusive. In this study, the sorption of arsenate, selenate,and molybdate on the barite (001) surface was examined at pH & SIM;5using in situ crystal truncation rod analysis, resonant anomalousX-ray reflectivity, and atomic force microscopy. The results showthat arsenate and selenate mainly incorporate into the top monolayerof barite, while molybdate primarily adsorbs above the surface. Thesorption coverage of arsenate is greater (by & SIM;100%) than thatof selenate but similar to that of molybdate. The different incorporationcoverages between arsenate and selenate can be explained by theirdifferent protonation states at pH 5. The incorporated arsenate maybe stabilized by hydrogen bonds between arsenate and oxygen atomsof neighboring sulfate compared to selenate, which exists predominantlyin the deprotonated state. The adsorption of molybdate above the surfaceprobably stems from a surface-induced oligomerization, as the anionand the oligomer may be too large for incorporation. Our observationof these different sorption mechanisms demonstrates how the physicochemicalproperties of the anions control the selective uptake of the toxicmetals on the dominant surface of the ionically bonded mineral barite.