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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

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.

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

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.

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.

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