Adsorption of rare-earth elements and yttrium (REE) with hydrous ferric oxide and hydrous aluminum oxide-Roles of sulfate and pH.

Rare-earth elements and yttrium (REE) form surface complexes (i.e., adsorption) with hydrous ferric oxides (HFO), hydrous aluminum oxides (HAO), and hydrous manganese (III-IV) oxides (HMO). However, the applicable reactions and equilibrium constants for REE binding to these environmentally relevant sorbents are not fully defined in available surface-complexation models. Titration experiments with iron (Fe) or aluminum (Al) in HCl or H2SO4 matrices were performed to investigate the potential for co-precipitation and/or adsorption of REE, and the effects of HFO and HAO sorbent availability, pH, and SO4. In solutions with Fe and without SO4, 100% of Fe was attenuated by pH 4.3, while the adsorption edges for 50% removal of Lu, Eu, and La were at pH 5.0, 5.3, and 5.7, respectively. In comparison, the adsorption edges for all REE decreased by 0.5 to 1.0 pH units in solutions with SO4. In solutions with Al and SO4, the adsorption edge decreased marginally; the shift was insignificant for light REE but was as much as 0.5 units lower for heavy REE. The empirical data for the HCl and H2SO4 matrices were used with parameter estimation software, coupled with geochemical modeling software, to calculate equilibrium constants for hypothetical surface-complexation reactions without and with SO4 (ternary complexes). The new constants for REE adsorption by HFO and HAO were incorporated into a comprehensive thermodynamic database file for use with PHREEQC. Geochemical models using the new thermodynamic database accurately simulate changes in REE concentrations in a mixed-metal synthetic solution and a real acid mine drainage (AMD) sample as functions of pH, aqueous and surface speciation, and sorbent availability and characteristics. This new database could be particularly useful in developing strategies to recover valuable REE during AMD treatment.