COMPARATIVE COMPLEXATION BEHAVIOR OF THE RARE-EARTHS

Linear free energy relationships (LFERs) have been used to assess the general character of rare earth element (REE) solution complexation. T hese analyses demonstrate that the coherence of interelement complexat ion behavior changes substantially across the fifteen-member series of REEs. The most coherent interelement relationships with a single rare earth reference element and the remaining rare earths as a group are obtained by referencing rare earth complexation behavior to Dy. The le ast coherent relationships are obtained using Pr or Nd as reference el ements. Across the rare earth series of elements a light rare earth pa ttern of complexation behavior (La-Eu) and a heavy rare earth pattern of complexation behavior (Tb-Lu) is observed. At the junction of these light (LREE) and heavy (HREE) rare earth element series, Gd appears t o behave as a HREE for weak (outersphere) complexation and as a LREE f or strong (innersphere) complexation. Due to the unique position of Gd among two trends of complexation behavior, the solution chemistry of Gd may appear anomalous with respect to the complexation behavior of i ts immediate neighbors. Other than that observed for Gd, our work prov ides no evidence for substantial ''breaks'', junction points, or appar ent anomalies in the average behavior of aqueous REE stability constan ts, The quantitative LFER results obtained in this work can be used to examine the complexation characteristics of individual ligands in the context of typical (average) REE complexation behavior, Such examinat ions reveal that substantial deviations from ''average'' behavior are common, This observation demonstrates, in a geochemical context, that detailed observation of REE complexation by organic ligands on particl e surfaces is required to understand REE behavior in the organic-rich marine environment.