Rare Earth Element Complexation by Carbonate and Oxalate Ions

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Rare earth carbonate and oxalate complexation constants have been determined through ex-amination of distribution equilibria between tributyl phosphate and an aqueous perchlorate phase. Carbonate complexation constants appropriate to the REE in seawater (25°C, 35%., 1 atm) can be described in terms of atomic number, Z. nlog swβ1 = 4.853 + 0.1135(Z − 57) − 0.003643(Z − 57)2log swβ2 = 80.197 + 0.1730(Z − 57) − 0.002714(Z −57)2 where swβ1=[MCO+3][M3+][CO2−3]T">swβ1=[MCO+3][M3+][CO2−3]T , swβ2=[M(CO3)−3][M3+][CO2−3]2'T">swβ2=[M(CO3)−3][M3+][CO2−3]2'T [M3+] is an uncomplexed rare earth concentration in seawater, [MCO+3] and [M(CO3)2] are carbonate complex concentrations, and [CO2−3]T is the total (free plus ion paired) carbonate ion concentration in seawater (molal scale). Our analyses indicate that in seawater with a total carbonate ion concentration of 1.39 × 10−4 moles/Kg H2O, carbonate complexes for the lightest rare earth, La, constitute 86% of the total metal, 7% is free La3+ and the remaining 7% exists as hydroxide, sulfate, chloride and fluoride complexes. For Lu, the heaviest rare earth, carbonate complexes are 98% of the total metal, 0.3% is uncomplexed and 1.5% is complexed with hydroxide, sulfate, chloride and fluoride. Oxalate and carbonate constants are linearly correlated. This correlation appears to be quite useful for estimating trivalent metal-arbonate stability constants from their respective oxalate stability constants.

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Geochimica et Cosmochimica Acta, v. 51, issue 3, p. 597-605