Stability Constants for Mono- and Dioxalato-complexes of Y and the REE, Potentially Important Species in Groundwaters and Surface Freshwaters

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We present the first measured set of stability constants for mono- and dioxalato-complexes of yttrium and all rare earths except Pm (Y+REE), Oxβn = [MOxn3−2n] [M3+]−1 [Ox2−]−n(where [ ] ≡ concentrations, M ≡ Y+REE, and Ox2− ≡ C2O42−). Aqueous solutions of Y+REE were titrated with oxalic acid in the presence of a cation-exchange resin, and Y+REE concentrations in the solution phase were measured by ICP-MS. This method allows investigation of all Y+REE simultaneously under identical conditions and is thus very sensitive to subtle inter-element variations in log Oxβn. Experiments were performed at a single ionic strength (I = 0.05 M), but at two values of pH. Patterns of log Oxβ1 and log Oxβ2, determined from our experiments, are similar in shape and reminiscent of those for carbonato-complexes. The average ratio of stepwise stability constants K2/K1 = Oxβ2/(Oxβ1)2 is 0.05 ± 0.02 for Y+REE excluding La and Ce.

Literature values of Oxβ1(Eu) for 0.03 mol/L ≤ I ≤ 1 mol/L were used to derive the relation log Oxβ1(Eu) = log Oxβ10(Eu) − 6.132√I/(1 + 1.47√I) + 0.902I, where log Oxβ10(Eu) is the stability constant at infinite dilution. Applying this relation to all Y+REE, the following values of log Oxβ10 (at zero ionic strength) were found: 6.66 (Y), 5.87 (La), 5.97 (Ce), 6.25 (Pr), 6.31 (Nd), 6.43 (Sm), 6.52 (Eu), 6.53 (Gd), 6.63 (Tb), 6.74 (Dy), 6.77 (Ho), 6.83 (Er), 6.89 (Tm), 6.95 (Yb), 6.96 (Lu). These values, which are based on direct measurements for each individual Y+REE, agree quite well with published extrapolations that are mostly based on linear free-energy relationships.

Total oxalate concentrations of 10−5−10−3 M have been reported for soil solutions. Free oxalate ions persist at much lower pH than free carbonate ions and a simple speciation model demonstrates that oxalato-complexes can dominate Y+REE speciation in mildly acidic groundwaters of low-to-moderate alkalinity.

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Geochimica et Cosmochimica Acta, v. 65, issue 7, p. 1037-1046