Investigation of the Solution Complexation Behavior of the Rare Earth Elements with Naturally Occurring Organic Ligands in Natural Terrestrial Waters

Old Dominion University Research Foundation, Norfolk VA

Investigators

Abstract

0001086 Johannesson Understanding the chemical behavior of the rare earth elements (REE) and other heavy metals in the environment is critical to predicting their impact on, as well as their fate and transport within, the environment. One of the most important factor affecting these and other heavy metals in natural waters is solution complexation. Solution complexation, for example, may exert controls on the mobility, effective solubility, reactivity, and toxicity of heavy metals in the environment. Unfortunately, metal speciation with naturally occurring organic ligands, and its direct connection to those processes and total metal concentrations, is only poorly known for most trace metals, including the REEs. Although geochemical models do exist that allow predictions to be made of REEs complexation with inorganic ligands (i.e., CO32-, P 4 3-, SO42-, OH-, CI- in natural waters, their in situ complexation behavior with naturally occurring organic ligands in natural waters has not been specifically studied. This disparity in our understanding of REE complexation exists despite the fact that stability constants for REE complexes with many simple organic acids are of the same magnitude, or greater, than stability constants for the strongest REE-inorganic (i.e., carbonate) complexes. Therefore, it is possible that organic complexation also controls the speciation of the REEs in natural waters, as has been shown to be the case for many transition metal cations. Consequently, considering what little is known about the [in situ] complexation of REEs with organic ligands in natural waters, the chief objectives of our study are: (1) modify and develop a specific electrochemical technique to (a) measure the fraction of dissolved REEs complexed with naturally occurring organic ligands in natural waters, and (b) measure the strength (i.e., conditional stability constants) of these naturally occurring REE-organic ligand complexes; (2) apply the modified electrochemical method to measure organic complexation of REEs in waters that span the pH range of natural waters and contain various concentrations of inorganic complexing ligands and dissolved organic carbon concentrations; and (3) develop a qualitative model of REE solution complexation that addresses competition between inorganic ligands and naturally occurring organic ligands and that builds upon an earlier equilibrium thermodynamic model. These objectives will be addressed using a combined laboratory and field approach. We will modify the competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE/ACSV) technique currently used for transition metals to quantify the amount of dissolved REEs that occur in natural waters as solution complexes with naturally occurring organic ligands. Natural water samples will be collected to compare and contrast concentrations, where organic ligands must compete with inorganic carbonate complexes, neutral pH waters with moderate concentrations or inorganic complexing ligands and low DOC, and acidic waters with high DOC. Our results concerning REE complexation with natural organic ligands will be combined with an equilibrium (inorganic ligand speciation) model currently in use.