Rare Earth Element Complexation by Carbonate and Oxalate Ions

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 _sw β ₁ = 4.853 + 0.1135( Z − 57) − 0.003643( Z − 57) ² log _sw β ₂ = 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 [ M ³⁺ ] is an uncomplexed rare earth concentration in seawater, [ MCO ⁺ ₃ ] and [ M ( CO ⁻ ₃ ) ₂ ] are carbonate complex concentrations, and [CO ²⁻ 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 H ₂ O, carbonate complexes for the lightest rare earth, La, constitute 86% of the total metal, 7% is free La ³⁺ 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.