Rare earth element sorption onto hydrous manganese oxide: A modeling study

Olivier Pourret (1) , Mélanie Davranche (2)

1 Institut Polytechnique LaSalle Beauvais
2 GR - Géosciences Rennes

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 pHzpc were fixed to 746 m2/g, 2.1 mmol/g, and 2.2, respectively. Two site types (triple bond; length of mdashXOH and triple bond; length of mdashYOH) were also used with pKa2 values of 2.35 (triple bond; length of mdashXOH) and 6.06 (triple bond; length of mdashYOH). 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. Log K increases from light REE to heavy REE and, more specifically, displays a convex tetrad effect. At low metal loading, the triple bond; length of mdashYOH site type strongly expresses its affinity toward REE, whereas at higher metal loading, the same is true for the triple bond; length of mdashXOH site type. This study thus provides evidence for heterogeneity in the distribution of the Mn oxide binding sites among REE.

Domaines: Géochimie; Milieux et Changements globaux

Olivier Pourret, Mélanie Davranche. Rare earth element sorption onto hydrous manganese oxide: A modeling study. Journal of Colloid and Interface Science, 2013, pp.18-23. ⟨10.1016/j.jcis.2012.11.054⟩. ⟨hal-00801166⟩