SORPTION MECHANISMS OF LANTHANUM ON OXIDE MINERALS

The retention of hazardous species, including many of the lanthanides, on soils and sediments is vital for maintaining environmental quality . In this study, high-resolution transmission electron microscopy (HRT EM) was used to identify surface precipitates of La and their degree o f atomic ordering on oxides of Mn (birnessite), Fe (goethite) and Ti ( rutile) over a pH range of 3 to 8. At pH > 5.5, the aqueous concentrat ion of La was fully depleted by all three metal-oxides. On birnessite, surface precipitation of La-hydroxide occurred at pH greater than or equal to 5 and appears to be the dominant sorption mechanism on this m ineral. Surface precipitation was not observed on rutile or goethite u ntil much higher pH values, 6.5 for rutile and 8.0 for goethite. Preci pitation is thus correlated with the points of zero charge (PZC) of th e minerals, 6.3 for rutile and 7.8 for goethite, and in each case was observed only at pH values above the PZC. Although La sorption was ext ensive on all of the minerals at the higher pHs, the depletion of La f rom solution by rutile and goethite at pH values well below the PZC in dicates that the sorption mechanism differs from that on birnessite. W hile surface precipitation was found to be the dominant sorption mecha nism of La on birnessite, surface complexation of monomeric or small m ultinuclear species appears to predominate in La retention on rutile a nd goethite at most commonly encountered pH values.