SORPTION OF RARE-EARTH ELEMENTS FROM SEAWATER ONTO SYNTHETIC MINERAL PARTICLES - AN EXPERIMENTAL APPROACH

The sorptive properties of the rare-earth elements (REE) in seawater-pure synthetic mineral phase systems were investigated through standard batch equilibration experiments. Mineral phases associated with marine ferromanganese deposits such as vernadite, goethite and apatite were synthesized at low temperature and pressure and used in single solid-phase batch experiments. Results from our adsorption studies are in agreement with features of REE patterns in natural deposits including the Ce anomaly found in deep-sea marine ferromanganese minerals and a general trend of preferential uptake of light (LREE) relative to heavy (HREE) rare-earth elements. Although fractionation trends lead to a general LREE enrichment for all phases investigated, significant differences exist in the adsorptive properties of iron oxyhydroxides (alpha-FeOOH) and vernadite (delta-MnO2). These include a positive Ce anomaly on vernadite and a difference in the extent of REE fractionation between the two phases. In addition to the fractionation of LREE from HREE which was observed on vernadite and goethite, the REE sorption patterns on hydroxylapatite lead us to suggest that incorporation of the light to intermediate REE may also occur by crystal lattice substitution for Ca2+.