Abstract
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.
