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Detailed analysis of the adsorption of oxalic acid ions, that is, oxalate and hydrogenoxalate, on the rutile (110) surface was carried out using molecular dynamics augmented by free energy calculations and supported by ab initio calculations. The predicted adsorption on perfect nonhydroxylated and hydroxylated surfaces with surface charge density from neutral to +0.208 C/m2 corresponding to pH values of about 6 and 3.7, respectively, agrees with experimental adsorption data and charge-distribution multisite ion complexation model predictions obtained using the most favorable surface complexes identified in our simulations. We found that outer-sphere complexes are the most favorable, owing to strong hydrogen binding of oxalic acid ions with surface hydroxyls and physisorbed water. The monodentate complex, the most stable among inner-sphere complexes, was about 15 kJ/mol higher in energy, but separated by a large energy barrier. Other inner-sphere complexes, including some previously suggested in the literature as likely adsorption structures such as bidentate and chelate complexes, were found to be unstable both by classical and by ab initio modeling. Both the surfaces and (hydrogen)oxalate ions were modeled using charges scaled to 75% of the nominal values in accord with the electronic continuum theory and our earlier parameterization of (hydrogen)oxalate ions, which showed that nominal charges exaggerate ion-water interactions.

A multistart optimization algorithm for surface complexation equilibrium parameters (MUSE) was applied to a large and diverse data set for chromate adsorption on iron (oxy)hydroxides (ferrihydrite and goethite). Within the Basic Stern and the charge-distribution multisite complexation (CD-MUSIC) framework, chromate binding constants and the Stern Layer capacitance were optimized simultaneously to develop a consistent parameter set for surface complexation models. This analysis resulted in three main conclusions regarding the model parameters: (a) There is no single set of parameter values that describes such diverse data sets when modeled independently. (b) Parameter differences among the data sets are mainly due to different amounts of total sites, i.e., surface area and surface coverages, rather than structural differences between the iron (oxy)hydroxides. (c) Unified equilibrium constants can be extracted if total site dependencies are taken into account. The implementation of the MUSE algorithm automated the process of optimizing the parameters in an objective and consistent manner and facilitated the extraction of predictive relationships for unified equilibrium constants. The extracted unified parameters can be implemented in reactive transport modeling in the field by either adopting the appropriate values for each surface coverage or by estimating error bounds for different conditions. The evaluation of a forward model with unified parameters successfully predicted chromate adsorption for a range of capacitance values.

The MUlti-start optimization algorithm for Surface complexation Equilibrium (MUSE) algorithm has been developed to optimize the fitting of thermodynamic constants for surface complexation modeling (SCM). Although there is a plethora of software to perform data fitting and determine intrinsic equilibrium constants, the algorithms used are highly dependent on initial values and choice of parameters. This limits their transferability to model other systems, for example, reactive transport processes. With this in mind, a hybridized optimization approach, based on a multistart algorithm combined with a local optimizer, has been developed to allow the simultaneous optimization of SCM parameters and to assess the sensitivity of these parameters to changes in the model assumptions. In this study, the CD–MUSIC formalism with a Basic Stern electrostatic model is utilized to model chromate adsorption on ferrihydrite, although the MUSE algorithm can be applied to any adsorption data set and be implemented in any mode...

Cations in natural solutions significantly impact interfacial processes, particularly dissolution and surface charge measurements for quartz and silica, which are amongst the most naturally abundant and technologically important solids. Thermodynamic parameters for cation-specific interfacial reactions have heretofore been mostly derived instead of directly measured experimentally. This work investigates the energetics of adsorption and exchange reactions of alkali metal (M+) and alkaline earth (M2+) cations with the quartz surface by flow adsorption microcalorimetry, in tandem with in-situ pH measurements. The magnitudes of the heats of adsorption and exchange were found to increase along the Hofmeister series i.e., Li+

Flow microcalorimetry was used to investigate the energetics associated with Rb+, K+, Na+, Cl–, and NO3– exchange at the rutile–water interface. Heats of exchange reflected differences in bulk hydration/dehydration enthalpies (Na+ > K+ > Rb+, and Cl– > NO3–) such that exchanging Na+ or Cl– from the surface was exothermic, reflecting their greater bulk hydration enthalpies. Exchange heats were measured at pH 2, 3.25, 5.8, and 11 and exhibited considerable differences as well as pH dependence. These trends were rationalized with the aid of a molecularly constrained surface complexation model (SCM) that incorporated the inner-sphere binding observed for the cations on the rutile (110) surface. Explicitly accounting for the inner-sphere binding configuration differences between Rb+, K+, and Na+, as well as accompanying differences in negative surface charge development, resulted in much better agreement with measured exchange ratios than by considering bulk hydration enthalpies alone. The observation that cal...

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.

The effect of particle size on the adsorption of Sr(2+) onto monodisperse nanometer diameter (4, 20, and 40 nm) anatase samples has been evaluated quantitatively with macroscopic experimental studies. The adsorption of Sr(2+) onto the anatase particles was evaluated by potentiometric titrations in NaCl media, at two ionic strengths (0.03 and 0.3 m), and over a wide range of pH (3-11) and surface loadings, at a temperature of 25 °C. Adsorption of Sr(2+) to the surface of the 20 and 40 nm diameter samples was similar, whereas the Sr(2+) adsorption titration curves were shallower for the 4 nm diameter samples. At high pH, the smallest particles adsorbed slightly less Sr(2+) than was adsorbed by the larger particles. At the molecular scale, density functional theory (DFT) calculations were used to evaluate the most stable Sr(2+) surface species on the (101) anatase surface (the predominant crystal face). An inner-sphere Sr-tridentate surface species was found to be the most stable. The experimental data were described with a charge distribution (CD) and multisite complexation (MUSIC) model, with a Basic Stern layer description of the electric double layer. The resulting surface complexation model explicitly incorporated the molecular-scale information from the DFT simulation results. For 20 and 40 nm diameter anatase, the CD value for the Sr-tridentate species was calculated using a bond valence interpretation of the DFT-optimized geometry. The CD value for the 4 nm sample was smaller than that for the 20 and 40 nm samples, reflecting the shallower Sr(2+) adsorption titration curves. The adsorption differences between the smallest and larger anatase particles can be rationalized by water being more highly structured near the 4 nm anatase sample and/or the Sr-tridentate surface species may require more well-developed surface terraces than are present on the 4 nm particles.

Adsorption of Rb+, Na+, Sr2+, and Cl– on hydroxylated (110) rutile surfaces was studied by molecular dynamics (MD) simulations. Our previous work was extended to the range of surface charge densities from −0.2 to +0.1 C/m2 (from −0.4 to +0.1 C/m2 for Sr2+) and to temperatures of 25, 150, and 250 °C. These conditions can be linked to experimental surface charge and pH values from macroscopic titrations of rutile powders with surfaces dominated by 110 crystal planes. Simulations revealed that Na+ and Sr2+ adsorb closer to the surface, shifting from predominately bidentate to tetradentate inner-sphere binding with increasing temperature, whereas Rb+ binding is predominately tetradentate at all temperatures. These differences are related to hydration energies, which must be partially overcome for inner-sphere binding and which decrease with increasing temperature and are lowest for Rb+. The interaction of Cl– with the rutile surface is generally less than that for cations because of repulsion by surface oxyge...

The acid-base properties of cassiterite (alpha-SnO2) surfaces at 10-50 degrees C were studied using potentiometric titrations of powder suspensions in aqueous NaCl and RbCl media. The proton sorption isotherms exhibited common intersection points in the pH range of 4.0-4.5 under all conditions, and the magnitude of charging was similar but not identical in NaCl and RbCl. The hydrogen bonding configuration at the oxide-water interface, obtained from classical molecular dynamics (MD) simulations, was analyzed in detail, and the results were explicitly incorporated in calculations of protonation constants for the reactive surface sites using the revised MUSIC model. The calculations indicated that the terminal SnOH2 group is more acidic than the bridging Sn2OH group, with protonation constants (log KH) of 3.60 and 5.13 at 25 degrees C, respectively. This is contrary to the situation on the isostructural alpha-TiO2 (rutile), apparently because of the difference in electronegativity between Ti and Sn. MD simulations and speciation calculations indicated considerable differences in the speciation of Na+ and Rb+, despite the similarities in overall charging. Adsorbed sodium ions are almost exclusively found in bidentate surface complexes, whereas adsorbed rubidium ions form comparable numbers of bidentate and tetradentate complexes. Also, the distribution of adsorbed Na+ between the different complexes shows a considerable dependence on the surface charge density (pH), whereas the distribution of adsorbed Rb+ is almost independent of pH. A surface complexation model (SCM) capable of accurately describing both the measured surface charge and the MD-predicted speciation of adsorbed Na+/Rb+ was formulated. According to the SCM, the deprotonated terminal group (SnOH(-0.40)) and the protonated bridging group (Sn2OH+0.36) dominate the surface speciation over the entire pH range of this study (2.7-10). The complexation of medium cations increases significantly with increasing negative surface charge, and at pH 10, roughly 40% of the terminal sites are predicted to form cation complexes, whereas anion complexation is minor throughout the studied pH range.

In this article, we review the sorption of multivalent cations on rutile (alpha-TiO2) powder surfaces in aqueous 1:1 electrolyte media from room temperature to 250 degrees C. All cations are shown to occupy 'inner sphere' sorption sites in contact with surface oxygens and hydroxyl groups, as well as the diffuse portion of the electrical double layer (EDL). Sorption is shown to increase strongly with increasing temperature, and the sorption affinity is strongly-related to cation radius and charge. Macroscopic powder pH-titration results obtained with ORNL's high temperature hydrogen electrode concentration cells can be rationalized with Gouy-Chapman-Stern models of the EDL, augmented by atomic-scale structural and proton affinity data from synchrotron X-ray studies and computational modeling approaches.

The rutile (1 1 0)–aqueous solution interface structure was measured in deionized water (DIW) and 1 molal (m) RbCl + RbOH solution (pH 12) at 25 � C with the X-ray crystal truncation rod method. The rutile surface in both solutions consists of a stoichiometric (1 · 1) surface unit mesh with the surface terminated by bridging oxygen (BO) and terminal oxygen (TO) sites, with a mixture of water molecules and hydroxyl groups (OH � ) occupying the TO sites. An additional hydration layer is observed above the TO site, with three distinct water adsorption sites each having well-defined vertical and lateral locations. Rb + specifically adsorbs at the tetradentate site between the TO and BO sites, replacing one of the adsorbed water molecules at the interface. There is no further ordered water structure observed above the hydration layer. Structural displacements of atoms at the oxide surface are sensitive to the solution composition. Ti

Adsorption of Zn{sup 2+} at the rutile TiO{sub 2} (110)-aqueous interface was studied with Bragg-reflection X-ray standing waves (XSW), polarization-dependent surface extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations to understand the interrelated issues of adsorption site, its occupancy, ion-oxygen coordination and hydrolysis. At pH 8, Zn{sup 2+} was found to adsorb as an inner-sphere complex at two different sites, i.e., monodentate above the bridging O site and bidentate between two neighboring terminal O sites. EXAFS results directly revealed a four or fivefold first shell coordination environment for adsorbed Zn{sup 2+} instead of the sixfold coordination found for aqueous species at this pH. DFT calculations confirmed the energetic stability of a lower coordination environment for the adsorbed species and revealed that the change to this coordination environment is correlated with the hydrolysis of adsorbed Zn{sup 2+}. In addition, the derived adsorption locations and the occupancy factors of both sites from three methods agree well, with some quantitative discrepancies in the minor site location among the XSW, EXAFS, and DFT methods. Additional XSW measurements showed that the adsorption sites of Zn{sup 2+} were unchanged at pH 6. However, the Zn{sup 2+} partitioning between the two sitesmore » changed substantially, with an almost equal distribution between the two types of sites at pH 6 compared to predominantly monodentate occupation at pH 8.« less

The adsorption of Nd{sup 3+} 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 Nd{sup 3+} sorbs strongly, even at low temperature, with adsorption commencing below the pHznpc of rutile. In addition, there is a systematic increase in Nd{sup 3+} 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 Nd{sup 3+} on rutile. The primary mode of Nd{sup 3+} adsorption was assumed to be the tetradentate configuration found for Y{sup 3+} 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{sup +} counterions was also assumed to be tetradentate, as supported by MD simulations of Na{sup +} interactions with the rutile (110) surface, and by analogous X-ray standing wave results for Rb{sup +}more » adsorption on rutile. Fitting parameters for Nd{sup 3+} 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.« less

A recently developed force field for interactions of water molecules with the (110) surface of rutile (α-TiO2) has been generalized for atomistically detailed molecular dynamics simulations of the interfacial structure of the uncharged mineral surface in contact with liquid SPC/E water at 298 K and 1 atm and for negatively charged surfaces in contact with SPC/E water containing dissolved electrolyte ions (Rb+, Sr2+, Zn2+, Na+, Ca2+, Cl-). Both hydroxylated (dissociative) and nonhydroxylated (associative) surfaces are simulated, since both types of water−surface interactions have been postulated from ab initio calculations and spectroscopic studies under near-vacuum conditions. The positions of water molecules at the interface were found to be very similar for both hydroxylated and nonhydroxylated surfaces, with either terminal hydroxyl groups or associated water molecules occupying the site above each terminal titanium atom. Beyond these surface oxygens, a single additional layer of adsorbed water molecul...

We describe an approach to directly image three-dimensional elemental distributions at the crystal–liquid interface with � 1 A spatial resolution. This method, based on the Fourier synthesis of X-ray standing wave data, is demonstrated by imaging the distribution of Sr 2þ ,Z n 2þ and Y 3þ adsorbed to the rutile (1 1 0)-water interface with no a priori assumptions. The approach resolves distinct sites and is robust for systems with single or multiple simultaneous adsorption sites. The observed ion distributions reveal unexpected differences in the adsorption sites of these cations that are needed to interpret electrical double-layer phenomena using surface complexation models. Published by Elsevier B.V.

The adsorption behavior of metal-(hydr)oxide surfaces can be described and rationalized using a variety of surface complexation models. However, these models do not uniquely describe experimental data unless some additional insight into actual binding mechanisms for a given system is available. This paper presents the results of applying the MUlti SIte Complexation or MUSIC model, coupled with a Stern-based three layer description of the electric double layer, to Ca2+ adsorption data on rutile surfaces from 25 to 250°C in 0.03 and 0.30 m NaCl background electrolyte. Model results reveal that the tetradentate adsorption configuration found for Sr2+ adsorbed on the rutile (110) surface in the in situ X-ray standing wave experiments of Fenter et al. (2000) provides a good fit to all Ca2+ adsorption data. Furthermore, it is also shown that equally good fits result from other plausible adsorption complexes, including various monodentate and bidentate adsorption configurations. These results amply demonstrate the utility of in situ spectroscopic data to constrain surface complexation modeling, and the ability of the MUSIC model approach to accommodate this spectroscopic information. Moreover, this is the first use of any surface complexation model to describe multivalent ion adsorption systematically into the hydrothermal regime.

The revised multisite complexation (MUSIC) model of T. Hiemstra et al. (J. Colloid Interface Sci. 184, 680 (1996)) is the most thoroughly developed approach to date that explicitly considers the protonation behavior of the various types of hydroxyl groups known to exist on mineral surfaces. We have extended their revised MUSIC model to temperatures other than 25 degrees C to help rationalize the adsorption data we have been collecting for various metal oxides, including rutile and magnetite to 300 degrees C. Temperature-corrected MUSIC model A constants were calculated using a consistent set of solution protonation reactions with equilibrium constants that are reasonably well known as a function of temperature. A critical component of this approach was to incorporate an empirical correction factor that accounts for the observed decrease in cation hydration number with increasing temperature. This extension of the revised MUSIC model matches our experimentally determined pH of zero net proton charge pH values (pH(znpc)) for rutile to within 0.05 pH units between 25 and 250 degrees C and for magnetite within 0.2 pH units between 50 and 290 degrees C. Moreover, combining the MUSIC-model-derived surface protonation constants with the basic Stern description of electrical double-layer structure results in a good fit to our experimental rutile surface protonation data for all conditions investigated (25 to 250 degrees C, and 0.03 to 1.0 m NaCl or tetramethylammonium chloride media). Consequently, this approach should be useful in other instances where it is necessary to describe and/or predict the adsorption behavior of metal oxide surfaces over a wide temperature range. Copyright 2001 Academic Press.

Calcium adsorption by rutile was studied potentiometrically from 25 to 250 C, at ionic strengths of 0.03 and 0.30 m in NaCl media, using two complementary experimental methodologies. In the first, net proton adsorption in the presence and absence of Ca{sup 2+} was monitored, and in the second, samples were periodically withdrawn during the course of a titration to determine Ca{sup 2+} adsorption directly. These experiments revealed that Ca{sup 2+} adsorption systematically increased with temperature relative to the pH of zero net proton charge in NaCl media alone (pH{sub znpc(NaCl)} - pH). That is, as temperature increased, Ca{sup 2+} adsorption commenced at progressively more positive pH{sub znpc(NaCl)} - pH values. Increasing ionic strength from 0.03 to 0.30 m NaCl suppressed Ca{sup 2+} adsorption at all temperatures as a result of either increased competition from Na{sup +} or greater complexation of Ca{sup 2+} by Cl{sup {minus}}. Finally, there was no apparent trend in the proton stoichiometric ratios (moles H{sup +} released/moles Ca{sup 2+} adsorbed) with increasing temperature. This suggests that the electrostatic and/or chemical processes involved in Ca{sup 2+} adsorption do not change greatly with increasing temperature. Favorable entropic effects, related to the increasing ease of releasing Ca{sup 2+} waters ofmore » hydration, are believed to be primarily responsible for the increase in adsorption with temperature.« less

Fluoroquinolone (FQ) antibacterials are aquatic contaminants of emerging concern (CEC), and adsorption to mineral surfaces is expected to play an important role in the fate, transport, and treatment of FQs. This study characterizes and models the adsorption of a zwitterionic FQ, ofloxacin (OFX), to goethite (α-FeOOH) over a wide range of pH (3–11), OFX concentration (20–500 μM), and electrolyte compositions (0.001–0.1 M NaCl and NaClO 4 ). Comparing OFX adsorption to structural analogues demonstrates that the carboxylate group is essential for binding to goethite. ATR-FTIR measurements indicate that FQs complex to goethite surfaces through carboxylate and carbonyl oxygen atoms, and that ClO 4 − co-adsorbs with OFX. Adsorption of the zwitterionic OFX increases with increasing ionic strength and is enhanced in NaClO 4 relative to NaCl electrolyte, whereas adsorption of a non-zwitterionic analogue is insensitive to ionic strength. A CD-MUSIC (charge distribution-multisite complexation) model, incorporating multiple modes of surface complexation constrained by spectroscopic measurements and the crystallographic distribution of goethite surface sites, yields accurate predictions over wide-ranging solution conditions. According to the model, OFX adsorbs predominantly by inner-sphere complexation on terminal surfaces of the rod-shaped goethite crystals in NaCl electrolyte, and OFX-ClO 4 − ion pairing in NaClO 4 induces formation of additional inner- and outer-sphere surface complexes on multiple crystal faces of goethite.

The concentration of H + which reacts with an adularia surface, [H S + ], was measured with acid-base titrations of adularia powder-water suspensions. Due to the complexity of feldspar surface reactions, it was necessary to calculate a H + mass balance in order to separate the fractions of H + involved in cation exchange reactions, [H ex + ]; dissolution reactions, [H dis + ]; and adsorption at surface hydroxyl sites, [H ads + ]. Reproducibility of acid and base titrations of H S + was pH-dependent, ranging from ±3 μmol H + m −2 at pH 4 to ±1.5 μmol H + m −2 at pH > 6.5. This departure was due to the exchange of K fsp + for H aq + , which was not completely reversible under the conditions of our experiment. Reproducibility of acid and base titration curves for [H ads + ] vs. pH was ± 1.5 μmol m −2 , suggesting the H + adsorption reaction was reversible. Fifteen μmol H + m −2 reacted with the washed feldspar surface during an acid titration from pH 10 to pH 4, in distilled water. 50–60% of the total is attributed to cation exchange, which is estimated to take place at >3 A depth within the surface, suggesting the near-surface is porous, and that H + reacts with sites within the surface pores as well as at the external surface. Less than 5% of [H S + ] was due to [H dis + ], and the remainder to [H ads + ]. [H ex + ] decreases with increasing concentrations of NaCl, presumably because of competition between the solution ions, H + and Na + , for K + exchange sites in the feldspar. [H ex + ] is independent of (CH 3 ) 4 NCl concentrations, suggesting that (CH 3 ) 4 N + cannot compete with H + for the K + exchange sites. The relatively large diameter of (CH 3 ) 4 N + probably prohibits it from penetrating the pores of the adularia surface; therefore, it cannot access exchange sites within the pores which are available to the smaller H + , Na + , and K + ions. Feldspar dissolution rates, often modeled as rate = k H [H ads + ] n , where k H = the rate constant, and n = the reaction order, have been observed to decrease with increasing ionic strength. Because we observe an ionic strength dependence in [H ex + ], rather than [H ads + ], we suggest a rate model where rate = k H [H ex + ] n . This expression emphasizes that dissolution rates are dependent upon K + −H + exchange at the feldspar surface, and that rates decrease with increasing {Na + } due to competition between Na + and H + for the surface exchange site.

The surface complexation behavior of ofloxacin (OFX), a zwitterionic fluoroquinolone antibiotic, to nano-anatase titanium dioxide (TiO(2)) was characterized. OFX adsorption in aqueous TiO(2) suspensions was measured as a function of pH, OFX concentration, and electrolyte type and concentration, and structural information was derived from in situ spectroscopic observations. An ultraviolet-visible spectral red shift upon OFX adsorption indicated formation of inner-sphere coordination complexes. Fourier transform infrared spectra of TiO(2)-adsorbed OFX were invariable over a wide concentration and pH range and were similar to measured spectra of dissolved species wherein the carboxylate group is deprotonated. A charge distribution surface complexation model constrained by spectroscopic observations was developed to describe macroscopic adsorption trends. A tridentate mode of adsorption involving bridging bidentate inner-sphere coordination of the deprotonated carboxylate group and hydrogen bonding through the adjacent carbonyl group on the quinoline ring resulted in successful predictions of observed adsorption trends. In NaClO(4) electrolyte, spectroscopic data and model fitting suggested that OFX ion pairing with ClO(4)(-) enhanced adsorption under acidic conditions. Moreover, comparison of OFX adsorption data with the pH trend in the kinetics of OFX degradation by visible light (λ400 nm) photocatalysis suggested that adsorbed OFX-ClO(4)(-) ion pairs inhibit photodegradation.

Liu and co-workers [Phys. Rev. B 82, 161415 (2010)] discussed the long-standing debate regarding whether H${}_{2}$O molecules on the defect-free (110) surface of rutile (\ensuremath{\alpha}-TiO${}_{2}$) sorb associatively, or there is dissociation of some or all first-layer water to produce hydroxyl surface sites. They conducted static density functional theory (DFT) and DFT molecular dynamics (DFT-MD) investigations using a range of cell configurations and functionals. We have reproduced their static DFT calculations of the influence of crystal slab thickness on water sorption energies. However, we disagree with several assertions made by these authors: (a) that second-layer water structuring and hydrogen bonding to surface oxygens and adsorbed water molecules are ``weak''; (b) that translational diffusion of water molecules in direct contact with the surface approaches that of bulk liquid water; and (c) that there is no dissociation of adsorbed water at this surface in contact with liquid water. These assertions directly contradict our published work, which compared synchrotron x-ray crystal truncation rod, second harmonic generation, quasielastic neutron scattering, surface charge titration, and classical MD simulations of rutile (110) single-crystal surfaces and (110)-dominated powders in contact with bulk water, and (110)-dominated rutile nanoparticles with several monolayers of adsorbed water.

The adsorption of Y 3+ and Nd 3+ 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 Nd 3+ first adsorbing at slightly lower pH values. The adsorption of both Y 3+ and Nd 3+ commences at pH values below the pH znpc 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 Y 3+ adsorption onto the (1 1 0) rutile surface ( Zhang et al., 2004b ). The MD simulation 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 Y 3+ and Nd 3+ 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 Y 3+ on rutile the tetradentate binding mode was augmented by a hydrolyzed-bidentate Y 3+ surface complex. Collectively, the experimental and surface complexation modeling results demonstrate that solution chemistry and surface loading impacts Y 3+ 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.

A stirred hydrogen-electrode concentration cell was used to follow hydrogen ion adsorption by the rutile surface in NaCl media (0.01–1.0 m) between 25 and 250°C. The pH of zero net surface charge (pHzpc) decreases to 200°C and then appears to increase. Away from the pHzpc, negative surface charge is screened more efficiently by Na+ ions than positive charge is by Cl− ions and this effect increases with temperature. Thus, Na+ moves closer to the rutile surface with increasing temperature relative to Cl−. Finally, [ pHzpc − 1 2pK w ] is approximately constant (− 1.1 ± 0.2) for this rutile sample to 250°C and is also equivalent to the equilibrium constant for an isocoulombic form of the “IpKa” surface ionization model. Thus, the constancy of [ pHzpc − 1 2pK w ] may also be useful for extrapolating rutile pHzpc values to higher temperatures and pressures than those studied here, and may also apply to many other oxide surfaces as well.

Macroscopic net proton charging curves for powdered rutile and cassiterite specimens with the (110) crystal face predominant, as a function of pH in RbCl and NaCl solutions, trace SrCl(2) in NaCl, and trace ZnCl(2) in NaCl and Na Triflate solutions, are compared to corresponding molecular-level information obtained from static DFT optimizations and classical MD simulations, as well as synchrotron X-ray methods. The similarities and differences in the macroscopic charging behavior of rutile and cassiterite largely reflect the cation binding modes observed at the molecular level. Cation adsorption is primarily inner-sphere on both isostructural (110) surfaces, despite predictions that outer-sphere binding should predominate on low bulk dielectric constant oxides such as cassiterite (ε(bulk) ≈ 11). Inner-sphere adsorption is also significant for Rb(+) and Na(+) on neutral surfaces, whereas Cl(-) binding is predominately outer-sphere. As negative surface charge increases, relatively more Rb(+), Na(+), and especially Sr(2+) are bound in highly desolvated tetradentate fashion on the rutile (110) surface, largely accounting for enhanced negative charge development relative to cassiterite. Charging curves in the presence of Zn(2+) are very steep but similar for both oxides, reflective of Zn(2+) hydrolysis (and accompanying proton release) during the adsorption process, and the similar binding modes for ZnOH(+) on both surfaces. These results suggest that differences in cation adsorption between high and low bulk dielectric constant oxides are more subtly related to the relative degree of cation desolvation accompanying inner-sphere binding (i.e., more tetradentate binding on rutile), rather than distinct inner- and outer-sphere adsorption modes. Cation desolvation may be favored at the rutile (110) surface in part because inner-sphere water molecules are bound further from and less tightly than on the cassiterite (110) surface. Hence, their removal upon inner-sphere cation binding is relatively more favorable.

Surface titrations were carried out on suspensions of monoclinic ZrO2 from 25 to 290 °C slightly above saturation vapor pressure at ionic strengths of 0.03, 0.1 and 1.0 mol⋅kg−1(NaCl). A typical increase in surface charge was observed with increasing temperature. There was no correlation between the radius of the cations, Li+, Na+, K+ and (CH3)4N+, and the magnitude of their association with the surface. The combined results were treated with a 1-pKa MUSIC model, which yielded association constants for the cations (and chloride ion at low pH) at each temperature. The pH of zero-point-charge, pHzpc, decreased with increasing temperature as found for other metal oxides, reaching an apparent minimum value of 4.1 by 250 °C. Batch experiments were performed to monitor the concentration of LiOH in solutions containing suspended ZrO2 particles from 200 to 360 °C. At 350 and 360 °C, Li+ and OH− ions were almost totally adsorbed when the pressure was lowered to near saturation vapor pressure. This reversible trend has implications not only to pressure-water reactor, PWR, operations, but is also of general scientific and other applied interest. Additional experiments probed the feasibility that boric acid/borate ions adsorb reversibly onto ZrO2 surfaces at near-neutral pH conditions as indicated in earlier publications.

Acid–base reactivity and ion-interaction between mineral surfaces and aqueous solutions is most frequently investigated at the macroscopic scale as a function of pH. Experimental data are then rationalized by a variety of surface complexation models. These models are thermodynamically based which in principle does not require a molecular picture. The models are typically calibrated to relatively simple solid-electrolyte solution pairs and may provide poor descriptions of complex multi-component mineral–aqueous solutions, including those found in natural environments. Surface complexation models may be improved by incorporating molecular-scale surface structural information to constrain the modeling efforts. Here, we apply a concise, molecularly-constrained surface complexation model to a diverse suite of surface titration data for rutile and thereby begin to address the complexity of multi-component systems. Primary surface charging curves in NaCl, KCl, and RbCl electrolyte media were fit simultaneously using a charge distribution (CD) and multisite complexation (MUSIC) model [Hiemstra T. and Van Riemsdijk W. H. (1996) A surface structural approach to ion adsorption: the charge distribution (CD) model. J. Colloid Interf. Sci . 179 , 488–508], coupled with a Basic Stern layer description of the electric double layer. In addition, data for the specific interaction of Ca 2+ and Sr 2+ with rutile, in NaCl and RbCl media, were modeled. In recent developments, spectroscopy, quantum calculations, and molecular simulations have shown that electrolyte and divalent cations are principally adsorbed in various inner-sphere configurations on the rutile 1 1 0 surface [Zhang Z., Fenter P., Cheng L., Sturchio N. C., Bedzyk M. J., Předota M., Bandura A., Kubicki J., Lvov S. N., Cummings P. T., Chialvo A. A., Ridley M. K., Benezeth P., Anovitz L., Palmer D. A., Machesky M. L. and Wesolowski D. J. (2004) Ion adsorption at the rutile–water interface: linking molecular and macroscopic properties. Langmuir 20 , 4954–4969]. Our CD modeling results are consistent with these adsorbed configurations provided adsorbed cation charge is allowed to be distributed between the surface (0-plane) and Stern plane (1-plane). Additionally, a complete description of our titration data required inclusion of outer-sphere binding, principally for Cl − which was common to all solutions, but also for Rb + and K + . These outer-sphere species were treated as point charges positioned at the Stern layer, and hence determined the Stern layer capacitance value. The modeling results demonstrate that a multi-component suite of experimental data can be successfully rationalized within a CD and MUSIC model using a Stern-based description of the EDL. Furthermore, the fitted CD values of the various inner-sphere complexes of the mono- and divalent ions can be linked to the microscopic structure of the surface complexes and other data found by spectroscopy as well as molecular dynamics (MD). For the Na + ion, the fitted CD value points to the presence of bidenate inner-sphere complexation as suggested by a recent MD study. Moreover, its MD dominance quantitatively agrees with the CD model prediction. For Rb + , the presence of a tetradentate complex, as found by spectroscopy, agreed well with the fitted CD and its predicted presence was quantitatively in very good agreement with the amount found by spectroscopy.

Gold(III)-chloride and gold(I)-thiosulfate adsorption by goethite was investigated as a function of pH (4 to 8), Cl− concentration (0 to 0.1 M), and ionic strength (0.01 and 0.1 M). Several observations suggest that Au(III)-chloride hydrolysis species with

Titration calorimetry is used to investigate the enthalpies associated with proton adsorption and desorption in aqueous alumina (γ-Al 2 O 3 ) suspensions. Adsorption enthalpies are exothermic and desorption enthalpies endothermic over the pH (4–10) and ionic strength (0.001, 0.01 and 0.1 M NaCl) ranges examined. Below pH 6, enthalpies are constant at about 30 kJ mol −1 H + adsorbed or desorbed (in absolute magnitude) for both alumina samples investigated and titration directions as well as with ionic strength. Above pH 6, enthalpies increase to 40–60 kJ mol −1 (in absolute magnitude) at pH 9.75 and significant differences are observed between ionic strengths, titration directions and the 2 alumina samples titrated. Residual heat and H + consumption or production resulting from the titration of supernatant blank solutions were only important above pH 7. Conceivable dissolved Al hydrolysis and solid-phase dissolution and precipitation reactions are also compared with these blank effects so that the precision and accuracy of the data can be ascertained with more accuracy. It is concluded that the data primarily reflect the enthalpy associated with the proton adsorption and desorption process and that data precision is adequately represented by the estimates resulting from averaging several separate titrations over narrow (0.5 pH unit) pH intervals. The enthalpy values also compare favorably with most of the several previous estimates available for alumina and with analogous dissolved Al hydrolysis reactions.

This chapter provides an overview of the experimental and modeling approaches that are used to study ion adsorption and zeta potential phenomena into the hydrothermal regime. There are many hydrothermal environments where the phenomena are important, including during the transport and deposition of ore-forming fluids, in the cooling circuits of fossil- and nuclear-powered steam generators, for the development and characterization of high temperature proton-exchange membrane fuel cells, and in the near-field environments of high-level nuclear waste repositories. The hydrogen electrode concentration cell (HECC) design and configuration are used in these studies. Multivalent cations and anions, and any other solution species that strongly adsorb at the mineral/water interface induce much larger release or uptake of H + than the relatively inert background electrolytes, MX, even at concentrations as low as 10 –4 to 10 –3 molal. Electrokinetic phenomena generally refer to the tangential motion of liquids with respect to charged solid surfaces, and the measurement of these phenomena provide important information on the structure of electrical double layers. Electrophoresis is the most suitable electrokinetic method to study particulate solids. A complete surface complexation model (SCM) consists of both mass-law expressions that describe ion binding to surface functional groups, and electrostatic correction terms for the mass law expressions derived from electrical double layer (EDL) theory, because the ion adsorption process results in charge development at the interface.

The X-ray standing wave technique was used to probe the sensitivity of Zn{sup 2+} and Sr{sup 2+} ion adsorption to changes in both the adsorbed ion coverage and the background electrolyte species and concentrations at the rutile ({alpha}-TiO{sub 2}) (110)-aqueous interface. Measurements were made with various background electrolytes (NaCl, NaTr, RbCl, NaBr) at concentrations as high as 1 m. The results demonstrate that Zn{sub 2+} and Sr{sub 2+} reside primarily in the condensed layer and that the ion heights above the Ti-O surface plane are insensitive to ionic strength and the choice of background electrolyte (with

A comprehensive picture of the interface between aqueous solutions and the (110) surface of rutile (alpha-TiO2) is being developed by combining molecular-scale and macroscopic approaches, including experimental measurements, quantum calculations, molecular simulations, and Gouy-Chapman-Stern models. In situ X-ray reflectivity and X-ray standing-wave measurements are used to define the atomic arrangement of adsorbed ions, the coordination of interfacial water molecules, and substrate surface termination and structure. Ab initio calculations and molecular dynamics simulations, validated through direct comparison with the X-ray results, are used to predict ion distributions not measured experimentally. Potentiometric titration and ion adsorption results for rutile powders having predominant (110) surface expression provide macroscopic constraints of electrical double layer (EDL) properties (e.g., proton release) which are evaluated by comparison with a three-layer EDL model including surface oxygen proton affinities calculated using ab initio bond lengths and partial charges. These results allow a direct correlation of the three-dimensional, crystallographically controlled arrangements of various species (H2O, Na+, Rb+, Ca2+, Sr2+, Zn2+, Y3+, Nd3+) with macroscopic observables (H+ release, metal uptake, zeta potential) and thermodynamic/electrostatic constraints. All cations are found to be adsorbed as "inner sphere" species bonded directly to surface oxygen atoms, while the specific binding geometries and reaction stoichiometries are dependent on ionic radius. Ternary surface complexes of sorbed cations with electrolyte anions are not observed. Finally, surface oxygen proton affinities computed using the MUSIC model are improved by incorporation of ab initio bond lengths and hydrogen bonding information derived from MD simulations. This multitechnique and multiscale approach demonstrates the compatibility of bond-valence models of surface oxygen proton affinities and Stern-based models of the EDL structure, with the actual molecular interfacial distributions observed experimentally, revealing new insight into EDL properties including specific binding sites and hydration states of sorbed ions, interfacial solvent properties (structure, diffusivity, dielectric constant), surface protonation and hydrolysis, and the effect of solution ionic strength.

Publisher Summary This chapter provides an overview of the recent developments in the understanding of the interaction of metal oxide and hydroxide minerals with hydrothermal solutions. With a few exceptions, metals exposed to aqueous solutions generally form an oxide surface film, which may or may not passivate the surface toward further oxidation. This phenomenon is driven by the relative Gibbs energies of the solid phases and the redox state of the system. Metal oxides formed in the presence of water are often hydrated regardless of whether the hydrous phase is thermodynamically stable relative to the pure oxide or a less hydrated phase. The ideal method of defining a solubility product is to demonstrate that the reaction is “reversible,” i.e., to show that the solution achieves the same IAP = K by approaching the equilibrium condition from under and super-saturation. The solubilities of metal oxides cannot be described without a detailed understanding of the hydrolysis and complexation of the aqueous reactants and products at elevated temperatures. At this time, there are no systems for which both dissolution/precipitation rate data and detailed information on the pH-dependent charging and adsorptive characteristics of the solid phase are known at elevated temperatures. However, the HECC and other emerging experimental techniques make such studies feasible and important targets for future research in hydrothermal oxide–water interactions.

The interaction of high temperature aqueous solutions with mineral surfaces plays a key role in many aspects of fossil, geothermal and nuclear energy production. This is an area of study in which the subsurface geochemical processes that determine brine composition, porosity and permeability changes, reservoir integrity, and fluid flow rates overlap with the industrial processes associated with corrosion of metal parts and deposition of solids in pipes and on heat exchanger surfaces. The sorption of ions on mineral surfaces is also of great interest in both the subsurface and ''above ground'' regimes of power production, playing a key role in subsurface migration of contaminants (nuclear waste disposal, geothermal brine re-injection, etc.) and in plant operations (corrosion mitigation, migration of radioactive metals from reactor core to heat exchanger, etc.). In this paper, results of the solubility and surface chemistry of metal oxides relevant to both regimes are summarized.