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

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

Aug 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006gecoa..70.4151q&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 70, Issue 16, p. 4151-4165.

Computer Science

7

Scientific paper

The influence of solution complexation on the sorption of yttrium and the rare earth elements (YREEs) by amorphous ferric hydroxide was investigated at 25 °C over a range of pH (4.0 7.1) and carbonate concentrations (0M⩽[CO32-]⩽150μM). Distribution coefficients, defined as iKFeT=[MS]/(M×[S]), where [MSi]T is the total concentration of sorbed YREE, MT 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 iKFeT is due to sorption of YREE carbonate complexes (MCO3+), in addition to sorption of free YREE ions (M3+). The subsequent decrease of iKFeT, which is more extensive for the heavy REEs, is due 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: iKFeT=(Sβ1[H]+Sβ2[H]+β1SCO×β1COH[HCO3-][H])(SK1[H]+1)×(1+HCOβ1[HCO3-]+β1COH[HCO3-][H]+β2COH[HCO3-]T2[H]) where Sβ1 and Sβ2 are equilibrium constants for free YREE surface species, β1SCO is the equilibrium constant for the YREE-carbonate surface species, SK1 is the surface protonation constant for amorphous ferric hydroxide, and HCOβ1, β1COH, and β2COH are YREE solution complexation constants expressed in terms of bicarbonate concentrations. The equation, which includes (i) a single new constant (β1SCO) for each YREE, (ii) previously published sorption coefficients (Sβ1 and Sβ2) determined in the absence of carbonate, and (iii) previously published solution complexation constants, precisely predicts both the absolute magnitude of iKFeT and the pattern of iKFeT values over our range of experimental conditions. Experimentally observed iKFeT values, spanning more than five orders of magnitude, are accurately described by our surface/solution complexation model. The logβ1SCOvalues determined for each YREE in this work are: 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).

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