Co-Precipitation of Double Carbonates of Yttrium and the Rare Earth Elements, Na2xM2(CO3)3+x, from Seawater-Like Electrolyte Solutions
Abstract
Co-precipitation of mixed carbonates of yttrium and the rare earth elements (YREEs) from perchlorate and chloride solutions at seawater ionic strength (0.7 molal, T = 25°C) was induced by raising pH under an atmosphere of 100% CO2. Solution pH was monitored with a glass combination electrode, and dissolved YREE concentrations by ICP-MS after filtration through 0.2 μm pore size membranes. Experiments were conducted (a) in NaClO4 and NaCl with equimolar YREE mixtures (all 1 mM); (b) in NaClO4 with equimolar YREE mixtures (10 μM) plus 1 mM Sm; and (c) with pure Sm and Yb solutions (10 mM). The applied pH range (4-8) resulted in a nearly 7 orders of magnitude variation of the free carbonate concentration. Double-logarithmic plots of individual YREE concentrations as a function of the free carbonate concentration are highly linear for all experiments, whereby the slopes represent carbonate:YREE ratios of the saturated solutions in equilibrium with the mixed co-precipitates, for each element. In virtually all cases (except for Y, Yb, and Lu in the equimolar mixture and Lu in the Sm-enriched mixture, both in NaClO4) the observed slopes are significantly larger than 1.5, the value expected for pure YREE carbonates (x = 0). The higher values suggest missing positive charge, which must be supplied by Na+, the only other positive ion in the solutions. Double carbonates of the form NaM(CO3)2 have long been known as the least soluble YREE salts in Na-rich solutions, yet their role in YREE geochemical cycles has been largely ignored. In addition, studies of their solubilities have focused mainly on precipitates of single YREEs, which do not occur in nature. The stoichiometry of the mixed co-precipitates appears quite complicated and variable. The pure Sm and Yb precipitates yield slopes of ~1.6, indicating minor incorporation of Na+. The mixed co-precipitate formed in NaClO4 yields slopes that smoothly vary from ~1.5 for Y and Lu to a maximum of ~1.8 for Sm. In NaCl the slopes are about 0.1 units higher and the maximum is shifted to slightly heavier elements, possibly due to YREE-chloride complexation. In the Sm-enriched co-precipitate the slopes range from ~1.5 for Lu to ~2.0 for Dy, displaying substantially more structure as a function of atomic number. Ion concentration products, calculated from the inferred stoichiometries, are similar in magnitude to published solubility products for the pure carbonates of single YREEs but show distinctly different patterns across the YREE series. These results could have a profound impact on interpretations of the pH-dependent incorporation of YREEs in marine carbonates, such as coral skeletons and pteropod shells, which are generally based on the solubility products of single-YREE carbonates precipitated from Na-free solutions. YREE abundance patterns and Nd isotope ratios in biominerals are likely to be increasingly used in studies of paleo-ocean circulation patterns, coastal inputs of dust due to mining and deforestation, and possibly ocean acidification.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS33D1497S
- Keywords:
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- 0419 BIOGEOSCIENCES / Biomineralization;
- 4807 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Chemical speciation and complexation;
- 4854 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Physical chemistry;
- 4875 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Trace elements