Here,
we characterize oxalate adsorption by rutile in NaCl media
(0.03 and 0.30 m) and between pH 3 and 10 over a
wide temperature range which includes the near hydrothermal regime
(10–150 °C). Oxalate adsorption increases with decreasing
pH (as is typical for anion binding by metal oxides), but systematic
trends with respect to ionic strength or temperature are absent. Surface
complexation modeling (SCM) following the CD-MUSIC formalism, and
as constrained by molecular modeling simulations and IR spectroscopic
results from the literature, is used to interpret the adsorption data.
The molecular modeling simulations, which include molecular dynamics
simulations supported by free-energy and ab initio calculations, reveal
that oxalate binding is outer-sphere, albeit via strong hydrogen bonds.
Conversely, previous IR spectroscopic results conclude that various
types of inner-sphere complexes often predominate. SCMs constrained
by both the molecular modeling results and the IR spectroscopic data
were developed, and both fit the adsorption data equally well. We
conjecture that the discrepancy between the molecular simulation and
IR spectroscopic results is due to the nature of the rutile surfaces
investigated, that is, the perfect (110) crystal faces for the molecular
simulations and various rutile powders for the IR spectroscopy studies.
Although the (110) surface plane is most often dominant for rutile
powders, a variety of steps, kinks, and other types of surface defects
are also invariably present. Hence, we speculate that surface defect
sites may be primarily responsible for inner-sphere oxalate adsorption,
although further study is necessary to prove or disprove this hypothesis.