SYNERGISTIC ENHANCEMENT OF LEAD AND SELENATE UPTAKE BY BARITE†

Selenate is the most common selenium (Se) species in an aerobic environment and hence the interactions of selenate with minerals control the bioavailability of Se in the aerobic environment. However, the effects of metal cations on selenate sorption on mineral surfaces are poorly understood. In this work, the sorption of selenate (3 – 100 μM Na₂SeO₄) on the barite (001) surface in the absence or presence of lead (100 μM Pb(NO₃)₂) was studied at pH ~5 using in-situ specular X-ray reflectivity (XR), in-situ atomic force microscopy (AFM), and classical molecular dynamics (CMD) simulations. XR results show that the sorption of selenate on the barite surface is negligible in the absence of Pb while it is substantially enhanced in the presence of Pb. Both ions sorb in essentially the same modes, i.e., primarily incorporated in the top monolayer of the surface with smaller amounts adsorbed above the surface. The synergistic incorporation was also tested with CMD simulations, which show that the co-incorporation of selenate with Pb is the lowest energy state. Combining AFM observations, the sorption mechanisms were identified as lattice incorporation, which dominates at lower coverages, and two-dimensional thin film growth of Se- and Pb-containing phases, which dominates at higher coverages. We also observed a systematic increase in the sorption affinity of lead with increasing selenate sorption coverages. This work provides a fundamental understanding of the role of barite in affecting the transport of essential nutrients and toxic elements in natural and engineered settings.

^†This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Geosciences Program under Contract DE-AC02-06CH11357 to UChicago Argonne, LLC as operator of Argonne National Laboratory.