Stability constants for mono- and dioxalato-complexes of Y and the REE, potentially important species in groundwaters and surface freshwaters

We present the first measured set of stability constants for mono- and dioxalato-complexes of yttrium and all rare earths except Pm (Y+REE), (Ox)beta (n) = [MOx(n)(3-2n)] [M3+](-1) [Ox(2-)](-n)(where [ ] drop concentrations, M drop Y+REE, and OX2- drop 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 (Ox)beta (n). Experiments were performed at a single ionic strength (I = 0.05 M), but at two values of pH. Patterns of log (Ox)beta (1), and log (Ox)beta (2), determined from our experiments, are similar in shape and reminiscent of those for carbonato-complexes. The average ratio of stepwise stability constants K-2/K-1 = (Ox)beta (2)((Ox)beta (1))(2) is 0.05 +/- 0.02 for Y+REE excluding La and Ce. Literature values of (Ox)beta (1)(Eu) for 0.03 mol/L less than or equal to I less than or equal to 1 mol/L were used to derive the relation log (Ox)beta (1)(Eu) = log (Ox)beta (0)(1)(Eu) - 6.132 rootI/(1 + 1.47 rootI) + 0.9021, where log (Ox)beta (0)(1)(Eu) is the stability constant at infinite dilution. Applying this relation to all Y+REE, the following values of log (Ox)beta (0)(1) (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 oil 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. Copyright (C) 2001 Elsevier Science Ltd.