Ion adsorption at mineral-water interfaces plays a significant role in geochemical processes such as mineral growth and dissolution and particle aggregation. Here, we use barite as a model mineral to understand divalent cation interactions with ionic crystals. The coverage and location of either lead (Pb) or strontium (Sr) sorbed to the (001) and (210) surfaces were studied as a function of [Pb]aq or [Sr]aq using in situ specular x-ray reflectivity (XR) and resonant anomalous x-ray reflectivity (RAXR). In the presence of Pb, the XR signals become progressively more distorted as [Pb]aq increases, implying that the presence of Pb alters the structure of the two surfaces. In contrast, the presence of Sr has smaller effects on the XR. The interfacial structural changes imply Pb and Sr sorption and that the extent of Pb sorption may be greater than that of Sr sorption. The specific Sr or Pb sorption behavior at the (001) and (210) surfaces was measured using RAXR. At both surfaces, Sr and Pb incorporate into the topmost barite layer and adsorb as an inner-sphere complex with a small outer-sphere fraction. Roughly half the sorbed Sr is incorporated and the other half is adsorbed, regardless of concentration. However, a larger amount of Sr sorbs to the (001) as compared with the (210). Pb is similarly partitioned half incorporated and half adsorbed at both surfaces at [Pb]aq ≤ 75 μM. At [Pb]aq ≥ 200-225 μM, less than a third of the Pb sorbed to the (001) is incorporated, but up to three-quarters of Pb sorbed to the (210) is incorporated. As with Sr, Pb coverage is greater at the (001) as compared with the (210). These results suggest that the (001) has a higher sorption capacity than the (210), despite sorption occurring through simultaneous incorporation and adsorption at both surfaces.

Cation sorption at the barite (001)- and (210)-water interfaces

ACS Spring Meeting 2021

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