Abstract
The adsorption of Y3+ and Nd3+ onto rutile has been evaluated over a wide range of pH (3–11) and surface loading conditions,
as well as at two ionic strengths (0.03 and 0.3 m), and temperatures (25 and 50 C). The experimental results reveal the same
adsorption behavior for the two trivalent ions onto the rutile surface, with Nd3+ first adsorbing at slightly lower pH values. The
adsorption of both Y3+ and Nd3+ commences at pH values below the pHznpc of rutile. The experimental results were evaluated
using a charge distribution (CD) and multisite complexation (MUSIC) model, and Basic Stern layer description of the electric
double layer (EDL). The coordination geometry of possible surface complexes were constrained by molecular-level information
obtained from X-ray standing wave measurements and molecular dynamic (MD) simulation studies. X-ray standing wave measurements
showed an inner-sphere tetradentate complex for Y3+ adsorption onto the (110) rutile surface (Zhang et al., 2004b).
TheMDsimulation studies suggest additional bidentate complexes may form. The CD values for all surface species were calculated
based on a bond valence interpretation of the surface complexes identified by X-ray and MD. The calculated CD values
were corrected for the effect of dipole orientation of interfacial water. At low pH, the tetradentate complex provided excellent
fits to the Y3+ and Nd3+ experimental data. The experimental and surface complexation modeling results show a strong pH
dependence, and suggest that the tetradentate surface species hydrolyze with increasing pH. Furthermore, with increased surface
loading of Y3+ on rutile the tetradentate binding mode was augmented by a hydrolyzed-bidentate Y3+ surface complex. Collectively,
the experimental and surface complexation modeling results demonstrate that solution chemistry and surface loading
impacts Y3+ surface speciation. The approach taken of incorporating molecular-scale information into surface complexation
models (SCMs) should aid in elucidating a fundamental understating of ion-adsorption reactions.