Dynamic Surface\nIncorporation of Pb<sup>2+</sup> Ions\nat the Actively Dissolving Calcite (104) Surface

The reaction of dissolved Pb<sup>2+</sup> with calcite\nsurfaces\nat near-equilibrium conditions involves adsorption of Pb<sup>2+</sup> and precipitation of secondary heteroepitaxial Pb-carbonate minerals.\nA more complex behavior is observed under far-from-equilibrium conditions,\nincluding strong inhibition of calcite dissolution, development of\nmicrotopography, and near-surface incorporation of multiple monolayers\n(ML) of Pb<sup>2+</sup> without precipitation of secondary phases\n[where 1 ML ≡ 1 Ca/20.2 Å<sup>2</sup>, the crystallographic\nsite density of the calcite (104) lattice plane]. However, the mechanistic\ncontrols governing far-from-equilibrium reactivity are not well understood.\nHere, we observe the interfacial incorporation of dissolved Pb<sup>2+</sup> during the dissolution of calcite (104) surfaces at pH ∼\n3.7 in a flow-through reaction cell, revealing the formation of a\n∼1 nm thick Pb-rich calcite layer with a total Pb coverage\nof ∼1.4 ML. These observations of the sorbed Pb distribution\nused resonant anomalous X-ray reflectivity, X-ray fluorescence, and\nnanoinfrared atomic force microscopy. We propose that this altered\nsurface layer represents a novel sorption mode that is stabilized\nby conditions of sustained disequilibrium. This behavior may significantly\nimpact the transport of dissolved metals during disequilibrium processes\noccurring in acid mine drainage and subsurface CO<sub>2</sub> injection\nand, if appropriately accounted for, could improve the predictive\ncapability of geochemical reactive-transport models.