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

Abstract

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 ≤ I ≤3 molal). YREE carbonate complexation constants are reported for the following equilibria, M 3++nHCO 3 −⇌M(CO 3) n 3−2n+nH +, where n = 1 or 2. Formation constants written in terms of HCO 3 − concentrations require only minor corrections for ion pairing relative to the corrections required for constants expressed in terms of CO 3 2− concentrations. Formation constants for the above complexation equilibria, CO 3 Hβ 1=[MCO 3 +][H +][M 3+] −1[HCO 3 −] −1 and CO 3 Hβ 2=[M(CO 3) 2 −][H +] 2[M 3+] −1[HCO 3 −] −2, have very similar dependencies on ionic strength because the reaction MCO 3 ++HCO 3 −⇌M(CO 3) 2 −+H + is isocoulombic. Potentiometric analyses indicate that the dependence of log CO 3 Hβ 1 and log CO 3 Hβ 2 on ionic strength at 25°C is given as (A) log CO 3 H β n = log CO 3 H β n 0−4.088 I 0.5/(1+3.033 I 0.5)+0.042 I where CO 3 Hβ n 0 denotes a formation constant at 25°C and zero ionic strength. Recommended values for log CO3 Hβ 1 and log CO 3 Hβ 2 0, expressed in the form (element, −log CO 3 Hβ 1 0, −log CO 3 Hβ 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).