Lead and selenite adsorption at water-goethite interfaces from first principles

The complexation of toxic and/or radioactive ions on to mineral surfaces is an important topic in geochemistry. We apply periodic-boundary-conditions density functional theory (DFT) molecular dynamics simulations to examine the coordination of Pb(II), SeO32-, and their contact ion pairs to goethite (101) and (210) surfaces. The multitude of Pb(II) adsorption sites and possibility of Pb(II)-induced FeOH deprotonation make this a complex problem. At surface sites where Pb(II) is coordinated to three FeO and/or FeOH groups, and with judicious choices of FeOH surface group protonation states, the predicted Fe-Pb distances are in good agreement with EXAFS measurements. Trajectories where Pb(II) is in part coordinated to only two surface Fe-O groups exhibit larger fluctuations in Pb-O distances. Pb(II)/SeO32- contact ion pairs are at least metastable on goethite (2 1 0) surfaces if the SeO32- has a monodentate Se-O-Fe bond. Our DFT-based molecular dynamics calculations are a prerequisite for calculations of finite temperature equilibrium binding constants of Pb(II) and Pb(II)/SeO32- ion pairs to goethite adsorption sites.