Rare earth element sorption onto hydrous manganese oxide: a modeling study.

Manganese oxides are important scavengers of rare earth elements (REE) in hydrosystems. However, it has been difficult to include Mn oxides in speciation models due to the lack of a comprehensive set of sorption reactions consistent with a given surface complexation model (SCM), as well as discrepancies between published sorption data and predictions using the available models. Surface complexation reactions for hydrous Mn oxide were described using a two surface site model and the diffuse double layer SCM. The specific surface area, surface side density, and pH(zpc) were fixed to 746 m2/g, 2.1 mmol/g, and 2.2, respectively. Two site types (≡XOH and ≡YOH) were also used with pK(a2) values of 2.35 (≡XOH) and 6.06 (≡YOH). The fraction of the high affinity sites was fixed at 0.36. Published REE sorption data were subsequently used to determine the equilibrium surface complexation constants, while considering the influence of pH, ionic strength, and metal loading. LogK increases from light REE to heavy REE and, more specifically, displays a convex tetrad effect. At low metal loading, the ≡YOH site type strongly expresses its affinity toward REE, whereas at higher metal loading, the same is true for the ≡XOH site type. This study thus provides evidence for heterogeneity in the distribution of the Mn oxide binding sites among REE.