Selenate, the most common selenium species in aerobic environment, usually adsorbs on mineral surfaces as outer-sphere complexes at circumneutral pH. However, the effect of metal cations on the sorption behavior of selenate on mineral surfaces is poorly understood. In this work, the sorption of selenate (from 3 – 100 μM Na2SeO4) at the barite (001)-water interface in the absence or presence of lead (in 0 or 100 μM Pb(NO3)2) was studied at pH ~5 using in situ specular X-ray reflectivity (XR) 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, suggesting cooperative sorption. Both ions sorb in essentially the same sorption modes, i.e., primarily incorporated in the top unit-cell layer of the surface with smaller amounts adsorbed above the surface, consistent with the interpretation. This idea was also tested with CMD simulations, which show that the co-incorporation of selenate with Pb is the lowest energy state. This presumed co-sorption is likely accomplished by a coupled exchange of lead and selenate for neighboring barium and sulfate, respectively. This work provides fundamental understanding of the role of barite in affecting the transport of these essential nutrients and toxic ions in natural and engineered settings.

Enhanced selenate uptake by lead incorporation at the barite (001)–water interface

ACS Spring 2022

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