Sorption of yttrium and rare earth elements by amorphous ferric hydroxide: Influence of pH and ionic strength

Abstract

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.