Speciation of Yttrium and the Rare Earth Elements in Seawater: Review of a 20-Year Analytical Journey

Due to the very low solubility of many of their salts, including phosphates, hydroxides, and fluorides, studies of the aqueous speciation of yttrium and the rare earth elements (YREE) require extremely low metal concentrations. In the past, these could almost exclusively be attained by using radiotracers, which are expensive, somewhat hazardous, and not available for all YREE. With the advent of ICP-MS in the early 1990s, our research groups started a dedicated program, still active today, to fully characterize complexation of the entire YREE series with inorganic natural ligands and some important organic ones. Since 1997, we have published high-quality data on the stability of YREE complexes with *fluoride, chloride, sulfate, *carbonate, hydroxide, and the organic ligands NTA, *oxalate, *desferrioxamine B, and *DTPA (Gd only). For ligands preceded by an asterisk, data were also obtained for higher-order complexes or for complexes with protonated forms of the ligand. Investigations of YREE complexation with silicate and phosphate are ongoing. A wide range of analytical techniques was employed, including cation-exchange, solvent extraction, solubility of secondary salts, rapid-scan spectrophotometry, static or dynamic potentiometry, and the use of competitive ligands or metals. Data were mostly obtained at 25°C and at seawater ionic strength (~0.7 M).

In this presentation, we will review what has been learned in the past 20 years and what is left to be done in the future. Our discussion will focus on the shapes of various stability constant patterns and how these have improved our understanding and modeling of the oceanic distribution and cycling of the YREE and of Nd isotopes. In addition, we will highlight some of the remaining analytical challenges that are particularly pertinent to marine geochemistry, such as complexation at low temperature, high pressure, or extremely high ionic strength (as found in brines and evaporative basins).