Adsorption of rare earth elements (REEs) onto schwertmannite and basaluminite from acid mine drainage to estuary water

Rare earth elements (REEs) are scarce in surface water, although areas impacted by acid mine drainage (AMD) display REE concentrations that are several orders of magnitude higher than those in freshwater and seawater. AMD neutralization as a result of mixing with seawater in estuaries induces a spontaneous precipitation of Fe- and Al-oxyhydroxysulfate nanominerals (<i>i.e.</i>, schwertmannite and basaluminite, respectively). Although the affinity of REEs for these minerals under AMD conditions has been addressed, the effects of an increase in pH and ionic strength observed in AMD-impacted estuaries have not been investigated. In this work, REE adsorption onto schwertmannite and basaluminite has been studied in the pH range of 4.5-7 and ionic strength range of 0.25-0.5 M by batch experiments and extended X-ray absorption fine structure (EXAFS) analysis. Adsorption batch experiments show (1) that REEs have higher affinity for the schwertmannite surface than for basaluminite and (2) that the REE adsorption is strongly dependent on pH and weakly dependent on ionic strength. The log <i>K</i>_REE values calculated from the REE adsorption onto schwertmannite were implemented in a non-electrostatic surface complexation model (NESCM), reflecting that REEs are retained through monodentate and bidentate surface coordination at pH below and above 5.25, respectively. As for basaluminite, NESCM and EXAFS results indicate that REEs are retained by monodentate binuclear coordination in heavy and medium REEs, whereas light REEs form outer-sphere complexation with a resulting increase in the basaluminite adsorption capacity at higher ionic strength. Thus, the thermodynamic parameters provided in this study prove useful to predict the geochemical behaviour of REEs in AMD-impacted estuarine areas.