The Aqueous Complexation of Nd(III) with Molybdate: The Effects of Both Monomeric Molybdate and Polymolybdate Species

The complexation of Nd(III), with molybdate was studied by examining the solubility of three distinct solid phases, NdP04(c), NaNd(Mo04)2(c), and Nd2(Mo04)3 · JTH20 over wide ranges of concentrations of Na2Mo04 and NdCl3, and/or of pH values, and over equilibration times extending to 120 days. The solubility data for NdP04(c) in different concentrations of dissolved molybdate showed that at dissolved molybdate concentrations of > 1 0 " 3 Μ and pH values less than ~ 5 , the observed NdP04(c) solubility can increase several orders of magnitude. These enormous increases in solubility and their very strong pH dependence indicate that polymolybdate complexation of Nd is occurring in these solutions. It appears that Nd(III) complexation with polymolybdate species is much stronger than Nd(III) complexation with monomelic molybdate species. A preliminary aqueous thermodynamic model was developed based principally on the NdP0 4 solubility data in different molybdate concentrations. This thermodynamic model, which included both monomeric and polymolybdate complexes of Nd ' + , was then tested on, and found to be consistent with, the NaNd(Mo04)2(c) solubility data and the solubility data for the apparent Nd2(Mo04)3 · x H 2 0 phase.

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