Stability constants for the formation of rare earth-inorganic complexes as a function of ionic strength

Abstract

Recent studies have been made on the distribution of the rare earths (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in natural waters relative to their concentration in shales. These metals have also been used as models for the behavior of the trivalent actinides. The speciation of the rare earths in natural waters is modelled by using ionic interaction models which require reliable stability constants. In this paper the stability constants for the formation of lanthanide complexes (kmx∗) with Cl−, NO3−, SO42−, OH−, HCO3−, H2PO4−, HPO42−, and CO32− determined in NaClO44 at various ionic strengths have been extrapolated to infinite dilution using the Pitzer interaction model. The activity coefficients for free ions (γM,γx) needed for this extrapolation have been estimated from the Pitzer equations. The thermodynamic stability constants (KMX) and activity coefficients of the various ion pairs (γMX) were determined from In (solKMX∗γMγx) = In Kmx+ In (γMX). The activity coefficients of the ion pairs have been used to determine Pitzer parameters (BMX) for the rare earth complexes. The values of BMX were found to be the same for complexes of the same charge. These results make it possible to estimate the stability constants for the formation of rare earth complexes over a wide range of ionic strengths. The stability constants have been used to determine the speciation of the lanthanides in seawater and in brines. The carbonate complexes dominate for all natural waters where the carbonate alkalinity is greater than 0.001 eq/L at a pH near 8.