Rare Earth Precipitation and Coprecipitation Behavior: The Limiting Role of POsub4/subsup3−/sup on Dissolved Rare Earth Concentrations in Seawater

Formation of rare earth phosphate coprecipitates, ( ΣM _i ) PO ₄ ( s ), may set limits on the maximum REE concentrations observed in seawater. Examination of rare earth phosphate solubility products, as well as oceanic { M ³⁺ } { PO ₄ ³⁻ } activity products, indicates that below approximately 300 m, the oceans are at saturation with respect to fresh rare earth phosphate coprecipitates and supersaturated with respect to aged coprecipitates. Removal of REEs from seawater through formation of mixed REE precipitates, ( ΣM _i ) PO ₄ ( s ), influences REE solution concentrations in a manner which closely resembles scavenging (adsorptive removal from solution). Laboratory observations demonstrate that rare earth coprecipitation preserves the odd/even abundance pattern derived from source materials and enriches HREEs in solution compared to the LREEs. Our observations indicate that phosphate and carbonate concentrations, in addition to pH, should be considered as essential parameters requisite to an understanding of environmental rare earth distributions.