SORPTION OF NEODYMIUM VERSUS STRONTIUM ON RUTILE SURFACES TO ELEVATED TEMPERATURES

The interaction of terminal oxygen atoms on mineral surfaces with natural aqueous solutions results in the hydroxylation of surface sites, which impart a pH-dependent surface charge. The surface charge density (Coulombs per square meter of mineral surface) of a given crystal face varies widely as a function of temperature, pH, and solution composition. Furthermore, the charged mineral surface attracts cations and anions from the bulk solution, influencing phenomena such as mineral nucleation, dissolution and precipitation rates, colloid flocculation, and contaminant transport.

Bulk potentiometric surface titrations have become a standard technique for determining surface charge density; in addition, specific ion adsorption studies are also conducted as a function of pH. This presentation will summarize our experimental results characterizing the adsorption of neodymium and strontium ions on rutile surfaces to 250°C. Interest in Nd has been stimulated by its chemical similarity to trivalent actinides, and the need to understand the aqueous geochemistry of rare earths, actinides and radionuclides to predict their fate in the environment. Moreover, the temperature range investigated includes the extremes of natural environments, as well as field conditions associated with high-level radioactive waste storage.

Divalent and trivalent cations enhance dramatically the development of negative surface charge on rutile with respect to monovalent electrolyte ions. Moreover, surface charge development increases with increasing temperature and ionic strength. At elevated temperatures adsorbed Sr enhanced the negative surface charge beginning at pH values near the pHznpc, whereas even at 25°C Nd sorbs to the surface well below the pHznpc. Our experimental results will be rationalized with the temperature extended Multisite Complexation model, and complementary x-ray standing wave and crystal truncation rod experiments. The latter experiments suggest that the Sr and Nd ions are adsorbed as an inner-sphere tetradentate complex, within a condensed layer that also includes bound water molecules.

Research sponsored by: NSR – NIRT initiative EAR-0124001, and DOE contract #DE-AC05-00OR22725.