Surface complexation of neodymium at the rutile-water interface: A potentiometric and modeling study in NaCl media to 250°C

The adsorption of Nd³⁺ onto rutile surfaces was examined by potentiometric titration from 25 to 250°C, in 0.03 and 0.30m NaCl background electrolyte. Experimental results show that Nd³⁺ sorbs strongly, even at low temperature, with adsorption commencing below the pH_znpc of rutile. In addition, there is a systematic increase in Nd³⁺ adsorption with increasing temperature. The experimental results were rationalized and described using surface oxygen proton affinities computed from the MUlti SIte Complexation or MUSIC model, coupled with a Stern-based three-layer description of the oxide/water interface. Moreover, molecular-scale information was incorporated successfully into the surface complexation model, providing a unique geometry for the adsorption of Nd³⁺ on rutile. The primary mode of Nd³⁺ adsorption was assumed to be the tetradentate configuration found for Y³⁺ adsorption on the rutile (110) surface from previously described in situ X-ray standing wave experiments, wherein the sorbing cations bond directly with two adjacent "terminal" and two adjacent "bridging" surface oxygen atoms. Similarly, the adsorption of Na⁺ counterions was also assumed to be tetradentate, as supported by MD simulations of Na⁺ interactions with the rutile (110) surface, and by analogous X-ray standing wave results for Rb⁺ adsorption included binding constants for the tetradentate adsorption complex and capacitance values for the inner-sphere binding plane. In addition, hydrolysis of the tetradentate adsorption complex was permitted and resulted in significantly improved model fits at higher temperature and pH values. The modeling results indicate that the Stern-based MUSIC surface-complexation model adequately accommodates molecular-scale information to uniquely rationalize and describe multivalent ion adsorption systematically into the hydrothermal regime.