Complexation of Trivalent Rare Earth Elements (Ce, Eu, Gd, Tb, Yb) by Carbonate Ions

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Carbonate stability constants for five rare earth elements (Ce3+, Eu3+, Gd3+, Tb3+, and Yb3+) have been determined at t = 25°C and 0.70 ± 0.02 M ionic strength through solvent exchange techniques. Estimated stability constants for Ce, Eu, and Yb are in close agreement with previous work. Analyses using Gd and Tb provide the first carbonate stability constants for these elements based on direct measurements. Our measured stability constants were used to estimate carbonate stability constants for the entire suite of REEs. Our Eu, Gd, and Tb carbonate stability constants demonstrate the existence of a “Gd-break”: Carbonate stability constants for Gd are smaller than those for Eu and Tb. In analogy to Gd concentration anomalies reported in field observations, Gd stability constant anomalies have been defined in terms of the difference logLβn(Gd) − log {(Lβn(Eu) +Lβn(Tb))2}">logLβn(Gd) − log {(Lβn(Eu) +Lβn(Tb))2} , where Lβn(M) = [MLn][M3+]−1[L]n. Examinations of REE-organic stability constants demonstrate that 106 out of 125 organic ligands have negative Gd anomalies in their first stability constants. The magnitudes of negative Gd anomalies generally become greater with increasing magnitude in Gd-ligand stability constants. Field observations of positive anomalies in shale-normalized Gd concentrations can be explained in terms of REE scavenging by organic surface ligands, such as polyaminocarboxylic acids, which possess a more negative Gd anomaly than carbonate ligands. Our modeling efforts indicate that a mixture of strongly complexing organics such as polyaminocarboxylic acids, and weakly complexing organics such as mono- and dicarboxylic acids is consistent with the pattern of REE scavenging by marine particulate matter.

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Geochimica et Cosmochimica Acta, v. 57, issue 2, p. 295-302