The persistence of lead (Pb) in contaminated topsoil is ranked as one of the most serious environmental issues in the U.S. and other countries. Adsorption of Pb at the aqueous interface of nanoscale metal oxide and metal (oxy)hydroxide particles is perhaps the most significant process responsible for controlling contaminant sequestration and mobility, but the process is poorly understood at the molecular level. Experimental studies of absorption of Pb onto bulk minerals have indicated significant differences in reactivity, but the molecular basis for these differences has remained elusive due to the challenges of observing and modeling the complex chemistry that exists at the water-oxide interface. In this work, we present a detailed ab initio theoretical investigation aimed at understanding the fundamental physical and chemical characteristics of Pb adsorption onto the (0001) surface of two common minerals, R-Al 2 O 3 and R-Fe 2 O 3 . The results of our periodic density functional theory (DFT) calculations show that the adsorption energy of Pb(II) on hematite is more than four times the value on isostructural alumina with the same fully hydroxylated surface stoichiometry due to bonding interactions enabled by the partially occupied Fe d-band. Site preference for Pb(II) adsorption on alumina is shown to depend strongly on the cost to disrupt highly stable hydrogen bonding networks on the hydrated surface, but is less of a factor for the stronger Pb-hematite interaction. † Part of the special section "Physical Chemistry of Environmental Interfaces".

We report an ab initio thermodynamic analysis of the ␣-Al 2 O 3 ͑1102͒ surface aimed at understanding the experimentally observed terminations over a range of surface preparation conditions as well as a stoichiometric model for the ͑2 ϫ 1͒ surface reconstruction observed after high-temperature annealing. As temperature is increased under both ultrahigh vacuum and ambient hydrated conditions, the predicted minimum-energy structural model goes through the same series of changes: from the hydroxylated "missing-Al" surface model ͑or half-layer model in which the topmost Al site of the stoichiometric surface has zero occupancy͒, to the hydroxylated stoichiometric model, to another hydroxylated missing-Al surface model with tetrahedral coordinated surface Al, and finally to the clean ͑1 ϫ 1͒ stoichiometric model. These results are in agreement with observations of both missing-Al and bulklike stoichiometries under wet conditions and in agreement with similar trends reported for isostructural hematite. However, we observe that the models with excess oxygen have a relatively higher surface-free energy and distinct surface relaxations in the case of alumina as compared to hematite. At very high temperatures where oxygen defects are generated, we find that a stoichiometric, charge-neutral ͑2 ϫ 1͒ structure becomes the most thermodynamically stable. This is consistent with the observation of a ͑2 ϫ 1͒ electron diffraction pattern when the surface is annealed at 2000 K while a ͑1 ϫ 1͒ pattern persists at lower annealing temperatures. A general rule that emerges from our modeling results is that while the full phase space of hydrated and defective surfaces is expansive, model stoichiometries that can be made charge neutral through either hydration or defects offer the greatest thermodynamic stability. However, the unique trends in structure and relative energies of alumina surface stoichiometries as compared to hematite can be understood based on the difference in electronic structure of the substrate.

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Microplastic waste is a worldwide problem, heavily afflicting marine and freshwater environments; the loading of this pollution in water, sediment and living organisms continues to escalate.