Sorption of Yttrium and Rare Earth Elements by Amorphous Ferric Hydroxide: Influence of PH and Ionic Strength

The sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide at low ionic strength (0.01 M ≤ I ≤ 0.09 M) was investigated over a wide range of pH (3.9 ≤ pH ≤ 7.1). YREE distribution coefficients, defined as _i K _Fe = [MS _i ] _T / (M _T [Fe ³⁺ ] _S ), where [MS _i ] _T is the concentration of YREE sorbed by the precipitate, M _T is the total YREE concentration in solution, and [Fe ³⁺ ] _S is the concentration of precipitated iron, are weakly dependent on ionic strength but strongly dependent on pH. For each YREE, the pH dependence of log _i K _Fe is highly linear over the investigated pH range. The slopes of log _i K _Fe versus pH regressions range between 1.43 ± 0.04 for La and 1.55 ± 0.03 for Lu. Distribution coefficients are well described by an equation of the form _i K _Fe = ( _S β ₁ [H ⁺ ] ^{− 1} + _S β ₂ [H ⁺ ] ^{− 2} ) / ( _S K ₁ [H ⁺ ] + 1), where _S β _n are stability constants for YREE sorption by surface hydroxyl groups and _S K ₁ is a ferric hydroxide surface protonation constant. Best-fit estimates of _S β _n for each YREE were obtained with log _S K ₁ = 4.76. Distribution coefficient predictions, using this two-site surface complexation model, accurately describe the log _i K _Fe patterns obtained in the present study, as well as distribution coefficient patterns obtained in previous studies at near-neutral pH. Modeled log _i K _Fe results were used to predict YREE sorption patterns appropriate to the open ocean by accounting for YREE solution complexation with the major inorganic YREE ligands in seawater. The predicted log _i K _Fe ′ pattern for seawater, while distinctly different from log _i K _Fe observations in synthetic solutions at low ionic strength, is in good agreement with results for natural seawater obtained by others.