Sorption of yttrium and rare earth elements by amorphous ferric hydroxide: Influence of pH and ionic strength
✍ Scribed by Kelly A. Quinn; Robert H. Byrne; Johan Schijf
- Publisher
- Elsevier Science
- Year
- 2006
- Tongue
- English
- Weight
- 492 KB
- Volume
- 99
- Category
- Article
- ISSN
- 0304-4203
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✦ Synopsis
The sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide at low ionic strength (0.01 M V I V 0.09 M) was investigated over a wide range of pH (3.9 V pH V 7.1). YREE distribution coefficients, defined as
, 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 F 0.04 for La and 1.55 F 0.03 for Lu. Distribution coefficients are well described by an equation of the
, where S b 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 b 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 V 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.