Examination of Comparative Rare Earth Element Complexation Behavior Using Linear Free-energy Relationships

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The comparative behavior of rare earth formation constants, Lβn(M), where M a trivalent rare earth element (REE), has been assessed using compilations of rare earth-organic ligand stability constants. Linear free-energy relationships of the form log Lβn(M) = In(M) + Sn(M) log n(Gd), where In(M) and Sn(M) are constants, are used to quantitatively describe the relative magnitudes of n(M) for rare earth complexation by a given ligand L. Our characterizations provide a means of estimating formation constants for the entire suite of rare earths when formation constants n(M) = MLnM3+−1 [L]n) have been determined experimentally for at least one element. We have used the results of our analyses to calculate rare earth phosphate stability constants for the entire suite of rare earths. Estimated stability constants for each rare earth have been used to assess the relative importance of phosphate complexes, carbonate complexes, and hydrolyzed rare earths in groundwaters. Speciation calculations for representative groundwater environments indicate a strong dependence of Speciation on REE atomic number and pH. Between pH 7 and 9, heavy REE complexes, MPO40, can be important compared toMCO3+ andM(CO3)2. ThecomplexesM(PO4)23− become significant at higher phosphate concentrations than those considered in our model groundwater. For the light REEs at pH ≤9, the concentrations ofMCO3+ complexes can exceed the concentrations of MPO40 complexes. For pH ≤ 6, M3+ concentrations exceed the concentrations of complex ions. Phosphate complexes exhibit an insignificant role in REE Speciation in seawater. Using3 ×103 as a carbonate/phosphate ion concentration ratio representative of seawater ([CO32−[PO43−] ≈3103">CO32−[PO43−] ≈3103 ) calculations indicate that, for all rare earths, [MCO3+][MPO40]">[MCO3+][MPO40] 100, and [CeCO3+[CePO40]">[CeCO3+[CePO40] ≈ 450.

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Geochimica et Cosmochimica Acta, v. 56, issue 3, p. 1127-1137