Monodentate Surface Complexes Research Articles

Enhanced arsenic removal by reusable hexagonal CeO2/Fe2O3 nanosheets with exposed (0001) facet

Arsenite in wastewater has caused increasing concern because of high toxicity and mobility. Iron oxides are widely available and regarded as effective adsorbents for arsenic. However, conventional iron oxides usually are only effective for arsenate (As(V)) adsorption by complexation, but not for As(III) adsorption because of their poor catalytic oxidation activities, which greatly limits arsenic removal efficiency. In this study, a uniform hexagonal FeCe bimetal oxide nanosheets (Fe0.21Ce0.29O) enclosed by high active (0001) planes was synthesized by a solvothermal method to improve the catalytic activity of Fe2O3. The experimental results showed that adsorption capacity of Fe0.21Ce0.29O reached 61.1 mg/g for arsenic and 70 % of that at equilibrium was achieved in <10 min. Based on characterization analyses and density functional theory simulation, the new insight in oxidation and complexation mechanism of arsenic was proposed. Firstly, As(III) was adsorbed to adsorbent surface by forming stable structure of Ce-O-As or Fe-O-As, and then converted into As(V) by dissolved oxygen under the catalysis of (0001) planes densely distributed on Fe2O3 and CeO2 surfaces. The formed As(V) species were bound on Fe0.21Ce0.29O surface by forming bidentate and monodentate surface complexes. Finally, the safety of As-containing solution treated with Fe0.21Ce0.29O was well proved by the zebrafish embryo developmental toxicity tests.

Kinetics and equilibrium adsorption of phosphate on lanthanum oxide supported on activated carbon

Understanding the thermodynamics and kinetics of phosphate adsorption is crucial in the design of packed-bed columns for phosphate removal, particularly in clinical applications such as dialysis, in which limited contact time is available. Studies have shown that the kinetics of phosphate adsorption on metal oxides is a two-step process: rapid initial adsorption that occurs in a few minutes to a few hours is followed by slower adsorption that may last for days. In this work, the thermodynamics and kinetics of phosphate adsorption on lanthanum oxide impregnated on activated carbon (La-AC) are investigated to explain its phosphate adsorption behavior. The phosphate adsorption mechanism was investigated by studying (1) the effects of temperature on phosphate adsorption capacity, (2) the dynamics and enthalpy of adsorption using flow microcalorimetry (FMC), (3) the kinetics of phosphate adsorption, and (4) the relationship between the amount of phosphate adsorbed and the amount of hydroxide released. Phosphate adsorption on La-AC followed the Langmuir isotherm. FMC results indicate that phosphate adsorbed through several mechanisms, principally through the formation of monodentate and bidentate inner-sphere surface complexes. A pseudo-second-order kinetic model is typically used to model phosphate adsorption; however, it was found that a kinetic model developed based on the assumption of two parallel surface adsorption reactions fits the measured adsorption kinetics significantly better than the pseudo-second order kinetic model that is almost universally used to describe phosphate adsorption kinetics. • Lanthanum-impregnated activated carbon was investigated as a phosphate adsorbent. • Phosphate adsorption was well described by the Langmuir isotherm. • Phosphate anions adsorbed on lanthanum surface via a parallel path mechanism. • A kinetic model based on the parallel path mechanism was developed. • Parallel path model fit kinetics much better than the pseudo-2nd-order model.

Radiotracer techniques (18F) and modeling of fluoride sorption on alumina

Chemical equilibria on the removal of excess fluoride from water by a classic solid alumina are presented. The fluoride concentrations used are representatives of those reported in borewell water at North region of Zacatecas State (Mexico). The percentage of fluoride removal was obtained as a function of pH using the radiotracer, 18F, from cyclotron of PET-UNAM. Proton binding sites of solid were characterized by potentiometric titrations yielding a point of zero charge, pHPZC, around 5.75. Batch experiments with alumina-equilibrated solutions were conducted to determine the mechanism of fluoride adsorption by this solid. The sorption isotherms show that fluoride removal is strongly dependent on pH showing a maximum value (∼98% adsorption) at pH near pHPZC; above this pH value, fluoride sorption falls sharply. This study concentrates on the fluoride sorption mechanism in terms of surface and solution equilibria constants. The experimental data were treated with the FITEQL 4.0 software program considering the existence of a single type of surface sites (hydroxyl groups on alumina). As a result of fitting fluoride removal data to model for the lowest initial fluoride concentration 1.0mg/L, the formation of monodentate surface complexes ≡Al–F dominate below pHPZC, when pH was above this value, fluoride adsorption decreased and the formation of the surface complexes ≡Al–OHF− became dominant. However, for the higher initial fluoride concentration of 9.3mg/L, the monodentate surface complexes ≡Al–F are predominant over the entire pH range.

Adsorption of Chromium (VI) on Calcium Phosphate: Mechanisms and Stability Constants of Surface Complexes

The adsorption of chromate on octacalcium phosphate (OCP) was investigated as a function of contact time, surface coverage, and solution pH [...]

Molecular orientation of molybdate ions adsorbed on goethite nanoparticles revealed by polarized in situ ATR-IR spectroscopy

The speciation of species adsorbed on nanoparticles is a major concern for several fields, as environmental pollution and remediation, surface functionalization, or catalysis. Attenuated total reflectance infrared spectroscopy (ATR-IR) was amongst the rare methods able to give in situ information about the geometry of surface complexes on nanoparticles. A new possibility using this technique is illustrated here with the MoO42−/goethite system. Using deuterated goethite to avoid spectral interferences, adsorption of molybdate ions on a spontaneous oriented film of nanoparticles has been followed using a polarized infrared beam. From the decomposition of spectra in the x, y and z directions, a monodentate surface complex on the {101} faces has been found as the most probable geometry. This result demonstrates that polarized ATR-IR allows to characterize in more details adsorption mode at the atomic scale, in comparison with usual ATR-IR spectroscopy.

Facet-Dependent Cr(VI) Adsorption of Hematite Nanocrystals.

In this study, the adsorption process of Cr(VI) on the hematite facets was systematically investigated with synchrotron-based Cr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, density-functional theory calculation, and surface complexation models. Structural model fitting of EXAFS spectroscopy suggested that the interatomic distances of Cr-Fe were, respectively, 3.61 Å for the chromate coordinated hematite nanoplates with exposed {001} facets, 3.60 and 3.30 Å for the chromate coordinated hematite nanorods with exposed {001} and {110} facets, which were characteristic of inner-sphere complexation. In situ ATR-FTIR spectroscopy analysis confirmed the presence of two inner-sphere surface complexes with C3ν and C2ν symmetry, while the C3ν and C2ν species were assigned to monodentate and bidentate inner-sphere surface complexes with average Cr-Fe interatomic distances of 3.60 and 3.30 Å, respectively. On the basis of these experimental and theoretical results, we concluded that HCrO4(-) as dominated Cr(VI) species was adsorbed on {001} and {110} facets in inner-sphere monodentate mononuclear and bidentate binuclear configurations, respectively. Moreover, the Cr(VI) adsorption performance of hematite facets was strongly dependent on the chromate complexes formed on the hematite facets.

Adsorption of arsenic with struvite and hydroxylapatite in phosphate-bearing solutions.

Arsenic sorption at above neutral pH is relevant when considering contaminant mobility in alkaline, phosphorus-bearing wastewaters, and may be viable in the presence phosphate minerals. Arsenic adsorption on struvite (MgNH4PO4 · 6H2O, MAP) and hydroxylapatite (Ca5(PO4)3OH, HAP) was evaluated at pH 8-11 from solutions with 2.7-0.125 mM phosphate and 0.05 mM As(III) or As(V). Over 7 d, As(III) removal from solution was minimal, but As(V) removal increased with pH, and was higher in the presence of MAP compared to HAP with a maximum of 74% removal in pH 11 MAP-bearing solutions. X-ray absorption fine structure spectroscopy (XAFS) analysis of solids recovered from pH 10-11 solutions revealed different mechanisms of As(V) sorption with MAP and HAP. Arsenic forms monodentate mononuclear surface complexes with MAP through the formation of a Mg-O-As bond, but is incorporated at the near-surface of HAP forming a johnbaumite-like (Ca5(AsO4)3OH) structure. Experiments using radioactive (33)P at pH 10-11 revealed faster exchange of P at the HAP surface, which could promote more facile As incorporation. Near-surface incorporated As in HAP may be less susceptible to remobilization compared to surface adsorbates formed with MAP. Overall, both MAP and HAP may sorb As at high pH in the presence of phosphate. This is relevant to the fate of As in alkaline contaminated waters in contact with phosphate mineral phases.

Arsenic Adsorption on Lanthanum-Impregnated Activated Alumina: Spectroscopic and DFT Study.

Rare earth-modified adsorbents (REMAs) have been widely used to remove oxyanion pollutants from water, including arsenic (As). However, the molecular-level structural information and reactions at the liquid/solid interface are still murky, which limits the design of applicable REMAs. Herein, a lanthanum-impregnated activated alumina (LAA) was synthesized as a representative REMA, and its As uptake mechanisms were explored using multiple complementary characterization techniques. Our adsorption experiments showed that LAA exhibited 2-3 times higher As adsorption capacity than AA. In contrast to the bidentate configuration formed on most metal oxide surfaces, our EXAFS and DFT results suggest that As(III) and As(V) form monodentate surface complexes on LAA through As-O-La coordinative bonding. In situ flow cell ATR-FTIR observed a strong dependence of As-O peak positions on pH, which could be interpreted as the change in the fractions of As(V) surface complexes with zero- to double-protonation on LAA, AA, and LaOOH. As(V) on LAA existed as singly and doubly protonated surface species, and the pKa of transition from double to single protonation (∼5.8) was lower than that for its soluble counterpart (6.97). The surface reaction and structural configuration were incorporated in a CD-MUSIC model to satisfactorily predict macroscopic As adsorption behaviors. The insights gained from the molecular-level reactions shed light on the design and application of REMAs in environmental remediation for As and its structural analogues.

Residence time and pH effects on the bonding configuration of orthophosphate surface complexes at the goethite/water interface as examined by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy

Identifying the mechanisms by which P is bound to soils and soil constituents is ultimately important as they provide information on the stability of bound species and their reactivity in the environment. EXAFS studies were carried out to provide information on how the local chemical environment of sorbed P changes as an effect of pH and time. Goethite was reacted with orthophosphate at a P concentration of 0.8mmolL−1 P at pH 3.0, 4.5 and 6.0. The residence time effect on the mechanisms of P sorption on goethite was also evaluated for two different reaction times, 5 and 18days, on goethite suspensions reacted at pH 4.5. The objective of this study was to understand how P sorption mechanisms change over a wide pH range when subjected to P concentrations above the P saturation ratio of goethite. Phosphorus K-edge EXAFS spectra were collected at 2150eV in fluorescence mode and the structural parameters were obtained through the fits of sorption data using Artemis. The monodentate surface complex was shown to be the predominant mechanism by which P sorbs at the goethite surface under the experimental conditions. The lack of a discrete Fe–P shell and the presence of highly disordered structures, particularly, at R-space ⩾3.5 suggested the formation of P surface precipitates at the goethite/water interface.

Treatment of multi-dentate surface complexes and diffuse layer implementation in various speciation codes

Spectroscopic studies and atomistic simulations of (hydr)oxide surfaces show that ionic aqueous adsorbates can bind to one, two, three, or four surface oxygen atoms (sites), forming multi-dentate species in surface complexation reactions. The law of mass action (LMA) for such reactions can be expressed in several alternative scales of surface concentration (activity). Unlike for mono-dentate surface complexes, the numerical value of the equilibrium constant is not independent of the choice of the surface concentration scale. Here, we show in a number of examples that the different formalisms implemented in popular speciation codes (MINEQL, MINTEQ, PHREEQC, and ECOSAT) yield different results for the same systems when the same parameters are used. We conclude that it is very important to generate general equations to easily transfer stability constants between the different concentration scales. It is of utmost importance for application of these models to reactive transport that the implementation in both the model fitting and speciation codes, and in the transport codes, is transparent to users.We also point to the problem that the implementation of the diffuse layer formalism in the various codes is not necessarily generally applicable. Thus, codes like VisualMinteq or MINEQL involve the Gouy–Chapman equation, which is limited to symmetrical (z:z) electrolytes, while PHREEQC and ECOSAT use general equations. Application of the former two to environmental problems with mixed electrolytes will therefore involve an inconsistency.

Surface complexation modeling of Eu(III) and phosphate on Na-bentonite: Binary and ternary adsorption systems

Abstract This study aims to investigate and model the adsorption of Eu(III) on bentonite in the presence of phosphate. Binary (phosphate/bentonite) and ternary (Eu(III)/phosphate/bentonite) systems were studied as a function of contact time, pH, solid-to-liquid ratio and Eu(III)/phosphate concentration by using a batch experimental method. The adsorption of phosphate on bentonite slightly increased in the pH range of 2.5–6.5, and decreased in the pH range of 6.5–9.4. This adsorption can be quantitatively interpreted by a model considering the formation of three monodentate surface complexes. In the ternary system, a synergistic adsorption was observed in the presence of both phosphate and Eu(III). In addition to the two sub-models describing Eu(III) and phosphate adsorption, the formation of ternary surface complexes had to be considered in order to explain the synergistic effect experimentally observed. The experimental data could be quantitatively explained when Eu(III) ( SOEuH2PO4+ and SOEuHPO4) or phosphate ( SPO4Eu+) are the bridged atoms. Complementary experiments carried out by X-ray photoelectron spectroscopy suggested that the second case is the most probable. The proposed model can be used in order to predict Eu(III) adsorption on buffer/backfilling material in the presence of phosphate.

Mechanisms of chromate adsorption on hematite

The fate of chromate in subsurface environments is strongly affected by adsorption reactions on iron oxide surfaces. Characterization of adsorption mechanisms is needed to formulate physically meaningful surface complexation reactions for thermodynamic models. In this study, chromate adsorbed on the surface of hematite was characterized by density functional theory, Cr K-edge extended X-ray absorption fine structure (EXAFS) measurements, and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Estimates of the adsorption energies of different surface configurations predicted that outer-sphere, monodentate, and corner-sharing bidentate complexes are the most thermodynamically favorable. Best-fit structural models of the EXAFS data included two second-shell Fe atoms at distances of approximately 3.3 and 3.6Å from the central Cr atom, characteristic of inner-sphere complexation. A less distant peak at 2.3Å in the Fourier transformed EXAFS was determined to be the result of multiple scattering in the chromate tetrahedron, as indicated by the fitting model. Results from in situ ATR–FTIR spectroscopy confirmed the presence of two inner-sphere surface complexes with C3v and C2v symmetry. The lack of significant peak shifts in spectra recorded using D2O as a solvent indicated that the complexes are nonprotonated. Based on these observations, the C3v and C2v species are assigned to nonprotonated monodentate and bidentate inner-sphere surface complexes with average Cr–Fe interatomic distances of 3.6 and 3.3Å, respectively. Based on simple geometric considerations, the latter distance indicates that the bidentate complex exists in a corner-sharing configuration on two adjacent surface sites. Speciation diagrams derived from multivariate curve resolution with alternating least squares optimization of the ATR–FTIR spectra showed that monodentate complexes are dominant above pH 6, below which bidentate complexes are dominant. The relative concentration of monodentate complexes was suppressed when the ionic strength was increased from 0.01 to 0.1M.

Nano-Adsorptive Effects Manifested in Arsenite Adsorption Onto the Nano-Sized Goethite

Goethite nanorods were synthesized to investigate the comprehensive nanoscale effects on As(III) sequestration by iron oxyhydroxide nanocrystallines. The specific adsorption-site accessible for As(III) was 12.4 sites/nm(2) on the goethite nanorods, and 6.9 sites/nm2 on the bulk phase of goethite. However, original goethite nanorods showed a weaker As(111) affinity in comparison with the bulk phase. Potent aggregation of nano-sized goethite led to a much slower rate of As(III) adsorption than its bulk relative and a very different profile of adsorption edge from its bulk phase. Surfactant activation of the nano-surface resulted in dramatic increase in both the affinity and the rate of As(III) adsorption onto the nano-sized goethite, as well as a significant improvement in As(III) adsorption over a broad pH range from 3 to 8. Both extended X-ray absorption fine structure (EXAFS) and X-ray photon spectra (XPS) found no As(Ill) oxidation on the surface of nano- or microsized goethite. Two As Fe shells, attributing to a bidentate binuclear and a monodentate surface complex between the As(III) and nano-goethite, fitted very well the experimental data of X-ray absorption fine structure (EXAFS).

An EXAFS study on the adsorption structure of phenyl-substituted organoarsenic compounds on ferrihydrite

Adsorption structures of mono- and diphenyl substituted organoarsenic compounds (PAA and DPAA, respectively) on ferrihydrite were analyzed by extended X-ray absorption fine structure (EXAFS), which suggested that PAA and DPAA form inner-sphere complexes with ferrihydrite regardless of their bulky functional groups. In addition, coexistence of two types of inner-sphere complex modes, i.e. bidentate–binuclear and monodentate surface complexes was suggested by EXAFS fitting with two type of the AsFe shell.

Effectiveness of Aluminum-based Drinking Water Treatment Residuals as a Novel Sorbent to Remove Tetracyclines from Aqueous Medium

Low levels of various veterinary antibiotics (VAs) have been found in water resources across the United States as a result of nonpoint-source pollution. As the first phase of developing a potential green sorbent for tetracycline (TTC) and oxytetracycline (OTC), we examined the effects of solution chemistry, pH, ionic strength (IS), sorbate:sorbent ratio (SSR), and reaction time on TTC and OTC sorption by a waste byproduct of the drinking-water treatment process, namely, Al-based drinking-water treatment residuals (Al-WTR). The sorption of TTC and OTC on Al-WTR increased with increasing pH up to pH 7 and decreased in the pH range of 8 to 11. A concentration of 20 g L was deemed as optimum SSR, where more than 95% of the initially added TTC and OTC were sorbed and equilibrium was reached in 2 h. A pseudo-second-order model ( = 0.99) was used for Al-WTR sorption for TTC and OTC. The data best fit the linearized form of the Freundlich isotherm ( = 0.98). No significant effect ( > 0.05) of IS on sorption of TTC and OTC was observed between 0.05 and 0.5 mmol L. However, at higher initial concentrations (>1 mmol L), IS dependence on TTC and OTC sorption was observed. Surface complexation modeling and Fourier transform infrared spectroscopy analysis indicated the possibility of TTC and OTC forming a mononuclear monodentate surface complex through strong innersphere-type bonds on Al-WTR. The results show promising potential of Al-WTR for use as a "green" and cost-effective sorbent to immobilize and stabilize TTC in soils and waters.

Understanding Arsenate Reaction Kinetics with Ferric Hydroxides

Understanding arsenic reactions with ferric hydroxides is important in understanding arsenic transport in the environment and in designing systems for removing arsenic from potable water. Many experimental studies have shown that the kinetics of arsenic adsorption on ferric hydroxides is biphasic, where a fraction of the arsenic adsorption occurs on a time scale of seconds while full equilibrium may require weeks to attain. This research employed density functional theory modeling in order to understand the mechanisms contributing to biphasic arsenic adsorption kinetics. The reaction energies and activation barriers for three modes of arsenate adsorption to ferric hydroxides were calculated. Gibbs free energies of reaction depended on the net charge of the complexes, which is a function of the system pH value. Physical adsorption of arsenate to ferric hydroxide proceeded with no activation barrier, with Gibbs free energies of reaction ranging from -21 to -58 kJ/mol. The highest Gibbs free energies of reaction for physical adsorption resulted from negative charge assisted hydrogen bonding between H atoms on the ferric hydroxide and O atoms in arsenate. The conversion of physically adsorbed arsenate into monodentate surface complexes had Gibbs free energies of activation ranging from 62 to 73 kJ/mol, and Gibbs free energies of reaction ranging from -23 to -38 kJ/mol. The conversion of monodentate surface complexes to bidentate, binuclear complexes had Gibbs free energies of activation ranging from 79 to 112 kJ/mol and Gibbs free energies of reaction ranging from -11 to -55 kJ/mol. For release of arsenate from uncharged bidentate complexes, energies of activation as high as 167 kJ/mol were encountered. Increasingly negative charges on the complexes lowered the activation barriers for desorption of arsenate, and in complexes with -2 charges, the highest activation barrier was 65 kJ/mol. This study shows that the slow kinetics associated with arsenic adsorption and desorption can be attributed to the high Gibbs free energies of activation for forming and breaking bonds with the ferric hydroxide.

Sorption of Eu(III) by amorphous titania, anatase and rutile: Denticity difference in surface complexes

To study the batch sorption of Eu(III) by different phases of titania viz. amorphous, anatase and rutile, amorphous titania was synthesized by organic route, followed by annealation to prepare anatase and rutile. Characterization of these phases was done by zeta potential measurement and potentiometric titration. Eu(III) sorption by titania phases increases with pH, pH50 (pH corresponding to 50% sorption) being lower in case of rutile than other two. Sorption edges for anatase and amorphous titania overlap on normalizing the data with specific surface area of particles. Surface complexation modelling of sorption data reveals the formation of higher hydrolysed Eu(III) surface species (TiEu(OH)2) on rutile compared to other two phases (TiOEu(OH)+). Consideration of bidentate surface complex for anatase and rutile gave equally good fit. Time resolved fluorescence spectroscopy showed higher lifetime for Eu sorbed on rutile, suggesting higher binding for Eu(III) in the surface complex. Keeping in view the regular arrangement of Ti octahedron on rutile surface, rutile surface can have more bidentate surface complex in comparison to other two phases. These results indicate the simultaneous existence of monodentate and bidentate surface complexes of Eu(III) on titania surfaces, with rutile having higher proportion of bidentate complex.

Adsorption of Uranium(VI) to Manganese Oxides: X-ray Absorption Spectroscopy and Surface Complexation Modeling

The mobility of hexavalent uranium in soil and groundwater is strongly governed by adsorption to mineral surfaces. As strong naturally occurring adsorbents, manganese oxides may significantly influence the fate and transport of uranium. Models for U(VI) adsorption over a broad range of chemical conditions can improve predictive capabilities for uranium transport in the subsurface. This study integrated batch experiments of U(VI) adsorption to synthetic and biogenic MnO(2), surface complexation modeling, ζ-potential analysis, and molecular-scale characterization of adsorbed U(VI) with extended X-ray absorption fine structure (EXAFS) spectroscopy. The surface complexation model included inner-sphere monodentate and bidentate surface complexes and a ternary uranyl-carbonato surface complex, which was consistent with the EXAFS analysis. The model could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. U(VI) adsorption to synthetic δ-MnO(2) appears to be stronger than to biogenic MnO(2), and the differences in adsorption affinity and capacity are not associated with any substantial difference in U(VI) coordination.

Variable Charge and Electrical Double Layer of Mineral–Water Interfaces: Silver Halides versus Metal (Hydr)Oxides

Classically, silver (Ag) halides have been used to understand thermodynamic principles of the charging process and the corresponding development of the electrical double layer (EDL). A mechanistic approach to the processes on the molecular level has not yet been carried out using advanced surface complexation modeling (SCM) as applied to metal (hydr)oxide interfaces. Ag halides and metal (hydr)oxides behave quite differently in some respect. The location of charge in the interface of Ag halides is not a priori obvious. For AgI(s), SCM indicates the separation of interfacial charge in which the smaller silver ions are apparently farther away from the surface than iodide. This charge separation can be understood from the surface structure of the relevant crystal faces. Charge separation with positive charge above the surface is due to monodentate surface complex formation of Ag(+) ions binding to I sites located at the surface. Negative surface charge is due to the desorption of Ag(+) ions out of the lattice. These processes can be described with the charge distribution (CD) model. The MO/DFT optimized geometry of the complex is used to estimate the value of the CD. SCM reveals the EDL structure of AgI(s), having two Stern layers in series. The inner Stern layer has a very low capacitance (C(1) = 0.15 ± 0.01 F/m(2)) in comparison to that of metal (hydr)oxides, and this can be attributed to the strong orientation of the (primary) water molecules on the local electrostatic field of the Ag(+) and I(-) ions of the surface (relative dielectric constant ε(r) ≈ 6). Depending on the extent of water ordering, mineral surfaces may in principle develop a second Stern layer. The corresponding capacitance (C(2)) will depend on the degree of water ordering that may decrease in the series AgI (C(2) = 0.57 F/m(2)), goethite (C(2) = 0.74 F/m(2)), and rutile (C(2) = ∞), as discussed. The charging principles of AgI minerals iodargyrite and miersite may also be applied to minerals with the same surface structure (e.g., sphalerite and würtzite (ZnS)).

Adsorption of Nitrate on Kaolinite Surfaces: A Theoretical Study

The interactions of HNO(3) and NO(3)(-) with kaolinite surfaces are important in the reaction of airborne clay mineral particles with atmospheric trace gases. Theoretical calculations at the B3LYP/6-311++G(d,p) level were performed on surface clusters of HNO(3) on kaolinite, on monodentate, on bidentate, and on two bridged complex structures of NO(3)(-) on kaolinite and on gas phase and solvated NO(3)(-). The results are compatible with the ranges assigned in the literature for NO(3)(-) on mineral surfaces for the frequency of the vibrational symmetric stretches (ν(1)) and for the low-frequency branch of the asymmetric stretches (ν(3)), with slightly higher frequencies reported for the bridged models. For the higher-frequency branch of the asymmetric stretches measured on kaolinite (Angelini et al.), our calculations match the assigned values for the monodentate surface complex, while predicting an inverse assignment for the frequencies of the bidentate and bridged complexes.