Solution chemistry of the rare earth elements in seawater

In seawater the REE(III) series exhibits gradual enrichment with increasing atomic number from 57La to 71Lu. The exception is 58Ce which is strongly depleted in seawater due to its unique oxidation to highly insoluble Ce(IV) state. Next to these major trends the elements 57La and 64Gd appear to be modestly enhanced relative to their neighbours in the REE series. Complexation in seawater is dominated by carbonates, increasingly so with atomic number and strongly dependent on pH shifts around the typical pH 8.2 in the ocean. Shifts in hydration at or about 64Gd might yield some discontinuity in the series. The actual removal of REE from seawater is through adsorption on particles. Organic ligands tend to have lower formation constants for 57La and 64Gd. Carboxylic surface functional groups on particles would remove La and Gd less effectively, thus promoting the observed anomalous enhancements of La and Gd in seawater. The proposed interaction with biological particles is also supported by the vertical and lateral distributions of the REE, whose similarity to nutrients suggests an involvement in the organic geochemical cycle. The effect of dissolved organic ligands on solution chemistry is yet to be investigated. 56 refs