Synergistic Enhancement of Lead and Selenate Uptake at the Barite (001)–Water Interface

Abstract

The interactions of heavy metals with minerals influence the mobility and bioavailability of toxic elements in natural aqueous environments. The sorption of heavy metals on covalently bonded minerals is generally well described by surface complexation models (SCMs). However, understanding sorption on sparingly soluble minerals is challenging because of the dynamically evolving chemistry of sorbent surfaces. The interpretation can be even more complicated when multiple metal ions compete for sorption. In the present study, we observed synergistically enhanced uptake of lead and selenate on the barite (001) surface through two sorption mechanisms: lattice incorporation that dominates at lower coverages and two-dimensional monolayer growth that dominates at higher coverages. We also observed a systematic increase in the sorption affinity with increasing co-sorbed ion coverages, different from the assumption of invariant binding constants for individual adsorption processes in classical SCMs. Computational simulations showed thermodynamically favorable co-incorporation of lead and selenate by simultaneously substituting for barium and sulfate in neighboring sites, resulting in the formation of molecular clusters that locally match the net dimension of the substrate lattice. These results emphasize the importance of ion−ion interactions at mineral−water interfaces that control the fate and transport of contaminants in the environment.

Uptake of Pb and the Formation of Mixed (Ba,Pb)SO4 Monolayers on Barite During Cyclic Exposure to Lead-Containing Sulfuric Acid

“…For example, SrSO 4 has a relatively low mismatch with PbSO 4 and shows F-VdM growth . A similar growth mode was observed for PbSeO 4 on BaSO 4 . However, materials with higher lattice mismatches, closer to that of PbSO 4 on BaSO 4 (such as BaCrO 4 on BaSO 4 and SrSO 4 on BaSO 4 ), tend to form islands.…”

“…Thus, the multiple layers of incorporated lead seen by RAXR can be understood as a kinetically determined state involving exchange followed by overgrowth. This is consistent with recent studies on the interaction of Pb–selenate solutions with barite, which also showed a combination of direct exchange and overgrowth . Some additional mechanisms would be needed to understand the apparent lead incorporation into deeper layers, i.e., the third monolayer and below, albeit in a small quantity compared to the top two monolayers.…”

“…This is consistent with recent studies on the interaction of Pb−selenate solutions with barite, which also showed a combination of direct exchange and overgrowth. 18 Some additional mechanisms would be needed to understand the apparent lead incorporation into deeper layers, i.e., the third monolayer and below, albeit in a small quantity compared to the top two monolayers. For example, it is possible that the apparent Pb incorporation in deeper layers may reflect Pb sorbed into negative topographic features of the barite surface, such as etch pits or cleavage topography.…”

“…17 A similar growth mode was observed for PbSeO 4 on BaSO 4 . 18 However, materials with higher lattice mismatches, closer to that of PbSO 4 on BaSO 4 (such as BaCrO 4 on BaSO 4 14 and SrSO 4 on BaSO 4 16 ), tend to form islands. In this light, we expect that if PbSO 4 can be induced to form multilayers on barite they are likely to grow via an S−K mode.…”

“…An additional complexity is that epitaxial relationships between anglesite and barite may cause deviations from the most commonly used models of heterogeneous nucleation, such as Turnbull’s hemispherical cap model . Studies of similar heteroepitaxial pairs such as BaCrO 4 on BaSO 4 , SrSO 4 on CaSO 4 , SrSO 4 on BaSO 4 , PbSO 4 on SrSO 4 , and most recently PbSeO 4 on BaSO 4 have shown that epitaxial strain can induce a diversity of complex nucleation and growth modes. Depending on the relative values of the surface energies and the lattice mismatch, the active mode may be Volmer–Weber (V–W), Frank Van der Merwe (F-VdM), or Stranski–Krastanov (S–K) growth.…”