Carbonate complexation of yttrium and the rare earth elements in natural waters

Potentiometric measurements of Yttrium and Rare Earth Element (YREE) complexation by carbonate and bicarbonate indicate that the quality of carbonate complexation constants previously obtained via solvent exchange analyses are superior to characterizations obtained using solubility and adsorptive exchange analyses. The results of our analyses at 25 C are combined with the results of previous solvent exchange analyses to obtain YREE carbonate complexation constants over a wide range of ionic strength (0 less than or equal to I less than or equal to 3 molal). YREE carbonate complexation constants are reported for the following equilibria, M3+ + nHCO(3)(-)reversible arrowM(CO3)(n)(3-2n) + nH(+), where n = 1 or 2. Formation constants written in terms of HCO3- concentrations require only minor corrections for ion pairing relative to the corrections required for constants expressed in terms of CO32- concentrations. Formation constants for the above complexation equilibria, (H)(CO3)beta(1) = [MCO3+][H+][M3+](-1)[HCO3-](-1) and (H)(CO3)beta(2) = [M(CO3)(2)(-)][H+](2)[M3+](-1)[HCO3-](-2), have very similar dependencies on ionic strength because the reaction MCO3+ HCO3 M(CO3)(2)(-) + H+ is isocoulombic. Potentiometric analyses indicate that the dependence of log(CO3)(H)beta(1), and lo(CO3)beta(2) on ionic strength at 25 degreesC is given as log(CO3)(H)beta(n) = log(CO3)(H)beta(n)(0) - 4.088 I-0.5/(1 + 3.033 I-0.5) + 0.042 I where (H)(CO3)beta(n)(0) denotes a formation constant at 25 degreesC and zero ionic strength. Recommended values for log(CO3)(H)beta(1), and log(CO3)(H)beta(2)(0), expressed in the form (element, -log(CO3)(H)beta(1)(0), -log(CO3)(H)beta(2)(0)), are as follows: (Y, 2.85, 8.03), (La, 3.60, 9.36), (Ce, 3.27, 8.90), (Pr, 3.10, 8.58), (Nd, 3.05, 8.49), (Sm, 2.87, 8.13), (Eu, 2.85, 8.03), (Gd, 2.94, 8.18), (Tb, 2.87, 7.88), (Dy, 2.77, 7.75), (Ho, 2.78, 7.66), (Er, 2.72, 7.54), (Tm, 2.65, 7.39), (Yb, 2.53, 7.36), (Lu, 2.58, 7.29). Copyright (C) 2004 Elsevier Ltd.