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 I 3 molal). YREE carbonate complexation constants are reported for the following equilibria, M nHCO3 oM(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 CO3 2 concentrations. Formation constants for the above complexation equilibria, CO3 H 1 [MCO3 ][H ][M ] [HCO3 ] 1 and CO3 H 2 [M(CO3)2 ][H ][M ] [HCO3 ] , have very similar dependencies on ionic strength because the reaction MCO3 HCO3 oM(CO3)2 H is isocoulombic. Potentiometric analyses indicate that the dependence of logCO3 H 1 and logCO3 H 2 on ionic strength at 25°C is given as logCO3 H n logCO3 H n 0 4.088 I/(1 3.033 I) 0.042 I (A) where CO3 H n 0 denotes a formation constant at 25°C and zero ionic strength. Recommended values for logCO3 H 1 and logCO3 H 2 , expressed in the form (element, logCO3 H 1 , logCO3 H 2 ), 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 © 2004 Elsevier Ltd