Sorption of ions at the mineral–water interface is an important factor that determines the fate of toxic metals in the environment. Here, we use barite as a model substrate to understand the interaction of toxic-metal lead (Pb) with ionic crystals. The coverage and location of Pb sorbed to the (001) surface was measured as a function of aqueous Pb concentration ([Pb]aq) using in situ specular resonant anomalous X-ray reflectivity (RAXR) to determine the sorption capacity and process. The results show that Pb sorption occurs via incorporation (primarily within the top barite layer ∼3 Å in depth) and adsorption (mostly as an inner-sphere complex at ∼2 Å in height) simultaneously. Both the incorporated and adsorbed Pb coverages increase with increasing [Pb]aq up to [Pb]aq ≈ 200 μM, above which the adsorbed fraction increases more rapidly than the incorporated fraction. This enhanced adsorption has a height distribution that is further extended (≥15 Å from the surface) than that observed in lower [Pb]aq. This change in distribution is interpreted as arising from additional sorption of outer-sphere species or Pb-bearing phases precipitated on the surface. Desorption experiments in Pb-free solutions show that the incorporated fraction is more resistant to removal than the adsorbed fraction, consistent with the speciation-dependent stabilities premised in the classical sorption models.
Jacquelyn N. Bracco, Sang Soo Lee, Inva Braha, Amanda Dorfman, Paul Fenter, Andrew G. Stack, “Pb Sorption at the Barite (001)–Water Interface,” J. Phys. Chem. C 124 (40), 22035-22045 (2020). DOI: 10.1021/acs.jpcc.0c03842 abstract
(a) XR and (b) normalized XR measured as a function of [Pb]aq in solution. The model fits are shown as the black lines in (a) and (b). The ED profiles derived from the model fits are shown in (c) and the asterisks correspond to the positions of the barium ions in [Pb]aq = 0 μM. The data points were excluded when the measured intensity was not distinguishable from background, when there was background scattering that obscured the signal, or at positions corresponding to forbidden Bragg peak reflections (Q ∼ 0.9, 2.65, and 4.4 Å–1). XR signals and ED profiles are offset from one another by an order of magnitude and a value of 1.5, respectively, for visual clarity. The normalized reflectivity is derived by dividing the reflectivity by the generic XR shape, 1/[Q sin(Qc/4)]2, where Q is the momentum transfer perpendicular to the (001) plane and c = 7.1538 Å is the (001) lattice spacing.