Stability Constants for Sulfate Complexation of Yttrium and the Rare Earth Elements
Abstract
In natural waters, yttrium and the rare earth elements (YREE) are mostly complexed with inorganic anions such as carbonate and chloride. YREE complexation with sulfate is minor (<5%) in seawater and generally negligible in fresh waters, yet it can be quite important in situations where oxidation of pyrite leads to high dissolved sulfate concentrations. The development of a consensus pattern of stability constants for YREE monosulfato-complexes has a quite convoluted history. It is based on a 1974 'compilation' that was derived from a much older, somewhat questionable, dataset, by adding interpolations and omitting a few elements. During subsequent transcriptions, some errors were introduced and the omitted data were interpolated rather than replaced with the original values. All this has resulted in a consensus pattern that is essentially flat, without any structure or fractionation between light and heavy REE. It is widely quoted and has been used to argue that sulfate complexation does not cause YREE fractionation. Conversely, flat YREE patterns in natural waters have been taken as evidence for substantial complexation of the YREE with sulfate.
We have taken a new approach to measuring the stability constants of all YREE monosulfato-complexes with the greatest possible precision. Stability constants were determined by comparing the solubility of barium sulfate in YREE chloride solutions and in an ammonium chloride reference solution, all at the ionic strength of seawater. The reference solution and solutions of individual YREE chlorides were equilibrated with a small amount of high purity barium sulfate at t = 25°C for at least one week. The equilibrated solutions were then filtered and analyzed for concentrations of YREE and Ba (ICP-MS), chloride and sulfate (ion chromatography), and pH (glass electrode). A simple model was used to determine the stability constants from these measurements. Averaged results from four experiments, two with and two without added sulfate, have standard deviations of 0.03 log units or less. Within this precision, the pattern of stability constants is not flat, as has been assumed, but has a very distinct shape. The pattern is nearly flat from La to Gd, possibly with a slight maximum at Eu. From Gd to Lu it shows a gradual and almost linear decrease, with the stability constant of Lu being more than 0.2 log units below that of La. The stability constant of Y is close to that of Er. Our pattern is in broad agreement with several careful earlier studies that have been largely ignored in the recent literature. Comparison with the substantial body of existing work also indicates that our stability constants are well within the published range for individual YREE. This revised pattern may have significant consequences for the interpretation of YREE patterns in high-sulfate environments such as runoff from mine tailings and certain groundwaters.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.B72B0770S
- Keywords:
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- 1806 Chemistry of fresh water;
- 4805 Biogeochemical cycles (1615);
- 4807 Chemical speciation and complexation;
- 4835 Inorganic marine chemistry