Sorption of yttrium and rare earth elements by amorphous ferric hydroxide: Influence of solution complexation with carbonate

The influence of solution complexation on the sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide was investigated at 25 degrees C over a range of pH (4.0-7. 1) and carbonate concentrations (0 M <= [CO32-] <= 150 mu M). Distribution coefficients, defined as K-i(Fe)T = [MSi](T)/(M-T X [S-i]), where [MSi](T) is the total concentration of sorbed YREE, M-T is the total YREE concentration in solution, and [Si] is the concentration of amorphous ferric hydroxide, initially increased in magnitude with increasing carbonate concentration, and then decreased. The initial increase of iK(Fe)(T) is due to sorption of YREE carbonate complexes (MCO3+) in Fe addition to sorption of free YREE ions (M3+). The subsequent decrease of iK(Fe)(T), which is more extensive for the heavy REEs, is due Fe to the increasing intensity of YREE solution complexation by carbonate ions. The competition for YREEs between solution complexation and surface complexation was modeled via the equation: [GRAPHICS] where s beta(1) and s beta(2) are equilibrium constants for free YREE surface species, (CO3)(S)beta(1) is the equilibrium constant for the YREE-carbonate surface species, sKj is the surface protonation constant for amorphous ferric hydroxide, and HC03 #11 03 03 #2 are YREE solution complexation constants expressed in terms of bicarbonate concentrations. The equation, which includes (i) a single new constant ((CO3)(S)beta(1)) s for each YREE, (ii) previously published sorption coefficients (s beta(1) and s beta(2)) determined in the absence of carbonate, and (iii) previously published solution complexation constants, precisely predicts both the absolute magnitude of K-i(Fe)T and the pattern of K-i(Fe)T values over Fe Fe our range of experimental conditions. Experimentally observed K-i(Fe)T values, spanning more than five orders of magnitude, are accurately described by our surface/solution complexation model. The log(S)(CO3)beta(1) values determined for each YREE in this work are: s Y(-1.30 +/- 0.04), La(-0.39 +/- 0.02), Ce(-0.21 +/- 0.02), Pr(-0.22 +/- 0.02), Nd(-0.20 +/- 0.02), Sm(-0.20 +/- 0.02), Eu(-0.26 +/- 0.02), Gd(-0.38 +/- 0.02), Tb(-0.40 +/- 0.02), Dy(-0.51 +/- 0.02), Ho(-0.57 +/- 0.02), Er(-0.59 +/- 0.02), Tm(-0.56 +/- 0.02), Yb(-0.62 +/- 0.02), and Lu(-0.59 0.02). (c) 2006 Elsevier Inc. All rights reserved.