Dynamic Surface Incorporation of Pb²⁺ Ions at the Actively Dissolving Calcite (104) Surface

The reaction of dissolved Pb²⁺ with calcite surfaces at near-equilibrium conditions involves adsorption of Pb²⁺ and precipitation of secondary heteroepitaxial Pb-carbonate minerals. A more complex behavior is observed under far-from-equilibrium conditions, including strong inhibition of calcite dissolution, development of microtopography, and near-surface incorporation of multiple monolayers (ML) of Pb²⁺ without precipitation of secondary phases [where 1 ML ≡ 1 Ca/20.2 Ų, the crystallographic site density of the calcite (104) lattice plane]. However, the mechanistic controls governing far-from-equilibrium reactivity are not well understood. Here, we observe the interfacial incorporation of dissolved Pb²⁺ during the dissolution of calcite (104) surfaces at pH ∼ 3.7 in a flow-through reaction cell, revealing the formation of a ∼1 nm thick Pb-rich calcite layer with a total Pb coverage of ∼1.4 ML. These observations of the sorbed Pb distribution used resonant anomalous X-ray reflectivity, X-ray fluorescence, and nanoinfrared atomic force microscopy. We propose that this altered surface layer represents a novel sorption mode that is stabilized by conditions of sustained disequilibrium. This behavior may significantly impact the transport of dissolved metals during disequilibrium processes occurring in acid mine drainage and subsurface CO₂ injection and, if appropriately accounted for, could improve the predictive capability of geochemical reactive-transport models.