V23B-2137 INVITED
Quantum Mechanical Calculations to Interpret Vibrational and NMR Spectra of Organic Compounds Adsorbed onto Mineral Surfaces
Vibrational (e.g., ATR FTIR and Raman) and nuclear magnetic resonance (NMR) spectroscopies provide excellent information on the bonding and atomic environment of adsorbed organic compounds. However, interpretation of observed spectra collected for organic compounds adsorbed onto mineral surfaces can be complicated by the lack of comparable analogs of known structure and uncertainties about the mineral surface structure. Quantum mechanical calculations provide a method for testing interpretations of observed spectra because models can be built to mimic predicted structures, and the results are independent of experimental parameters (i.e., no fitting to data is necessary). In this talk, methodologies for modeling vibrational frequencies and NMR chemical shifts of adsorbed organic compounds are discussed. Examples included salicylic acid (as an analog for important binding functional groups in humic acids) adsorbed onto aluminum oxides, organic phosphoryl compounds that represent herbicides and bacterial extracellular polymeric substances (EPS), and ofloxacin (a common agricultural antibiotic). The combination of the ability of quantum mechanical calculations to predict structures, spectroscopic parameters and energetics of adsorption with experimental data on these same properties allows for more definitive construction of surface complex models.
V23B-2138 INVITED
Surface Complexation Modeling of Dual-Mode Adsorption of Organic Acids
Results from spectroscopic investigations and theoretical calculations provide evidence to suggest that organic acids bind to mineral surfaces simultaneously as inner- and outer-sphere complexes and that the relative importance of these species varies with solution chemistry (e.g., pH), mineral type, and organic acid. Constraining surface complexation models (SCMs) with this information has proven difficult; however, as efforts to tune the surface complex stoichiometry within the SCM to match the structures and speciation trends observed in the spectra as a function of solution chemistry are seldom successful. In this study we apply the extended constant capacitance model to describe phthalic acid adsorption on hematite in a manner wholly consistent with spectroscopic results. In order to do so, we investigate how the model fit and resulting species distribution respond to changes in the SCM adjustable model parameters (capacitance, site density, and charge distribution). As expected, similar fits to the macroscopic adsorption trends result from models with either a single outer-sphere complex (single-mode) or with an outer-sphere complex and an inner- sphere complex (dual-mode). To get good fits from the dual-mode model, which was most consistent with the spectroscopic results, requires careful selection of capacitance, site density values, and charge distribution. Relatively low values for the inner-layer capacitance or site density were needed to produce a significant concentration of the inner-sphere complex observed in the low pH spectra. This suggests that utilizing SCMs to accurately describe surface speciation over a wide range of experimental conditions relies on adjusting model parameters such that the surface complexation reactions remain consistent with specific direct knowledge of surface speciation.
V23B-2139 INVITED
X-ray, vibrational and NMR spectroscopic investigations of natural organic molecules and their complexes: Applications & Limitations
Naturally occurring organic molecules play an important role in a variety of environmental processes, such as mineral weathering, nutrient and elemental cycles, and contaminant transformations and transport. However, their polyfunctional behavior and macromolecular structure poses a challenge for geochemists when it comes to their characterization, and while evaluating the impact of these molecules in various aquatic and interfacial geochemical processes. A single spectroscopic technique can only provide part of the picture about the structure of a moiety, and may have serious limitations in regard to the estimation of its concentration and/or evaluation of its coordination and electronic state in the presence of other potential interfering elements that are common in the natural systems. Further, presence of water and interfaces could make analysis more difficult. Using a combination of X-ray, vibrational and multidimensional NMR spectroscopy techniques, we are studying the functional group composition of natural organic molecules. Our research focus is on the identification of the types of carboxylic acid structures in the natural organic molecules, their local coordination environments and their metal complexes, and on the other important N-, P-, and S-functional groups present in natural organics. A summary of our results, and the potential challenges involved in the applications of different spectroscopic techniques in the identification of different moieties and their structures in natural organic molecules in aqueous solutions and at interfaces will be presented.
V23B-2140
Mineral oxide transformation of antimicrobial contaminants
The quality of our water supply is dependent on the organic-mineral interface. Organics contain reactive groups that dissolve minerals, and release surface associated contaminants into aquifers and reservoirs. Conversely, minerals may transform organic pollutants, including antimicrobial drugs that are potentially deleterious to aquatic ecosystems or human health. Under aqueous conditions typical of soils and natural waters, the antibiotic agent sulfamethoxazole (SMX) is transformed in the presence of pyrolusite, presumably on the MnO2 surface. At least 50 percent loss of SMX was observed after 269 h, in both acidic and basic solutions (pH 3-9). Nearly 100 percent loss is recorded at pH 3 and 66 percent loss was recorded at circumneutral pH. Initial mass spectrometry of the reaction products suggests an oxidative pathway where hydroxylation and oxidation occurs at the aniline moiety and isoxazolamine ring of SMX. Concomitant increases in aqueous manganese concentrations suggest reductive transformation of the mineral surface. Ongoing electric force spectroscopy and force microscopy experiments probe potential mineral surface alteration associated with the SMX-MnO2 reaction. Coupling bulk aqueous observations and mass spectrometry with molecular-scale force microscopy should further elucidate sulfonamide reactivity as influenced by mineral surface chemistry and topography. Moreover, the observed transformation suggests manganese oxides likely play an important role in the fate of SMX in the environment.
V23B-2141
Distinguishing Multiple Surface Species of Glutamate on Hydrous Ferric Oxide (HFO)
Surface complexation models provide a way of integrating the results of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic studies with bulk adsorption measurements. Assuming that glutamate adsorbs similarly on HFO and amorphous titanium dioxide (ATD), we used ATR-FTIR results for glutamate on ATD [1] to construct a surface complexation model for glutamate on HFO [2]. Three surface complexes were distinguished following the ATR-FTIR study: a bridging-bidentate, a chelating-monodentate and a chelating species,corresponding to 4, 3 and 2 points of attachment of the glutamate to surface functional groups, respectively. In this regard, the ATR-FTIR spectroscopic results and the model surface complexes agree. However, the model surface complexes contain partly inner-sphere binding and partly H- bonding or outer-sphere binding. For example, the model bridging-bidentate species has one oxygen of each carboxylate functional group bound in an inner-sphere mode, whereas the other oxygen of each carboxylate functional group is bound with a H-bond or as an outer-sphere species to a surface >FeOH group. The selection of this species gave the best reaction stoichiometry for the model of the adsorption data. However, it was suggested in [1] that all four glutamate oxygens may bind in an inner-sphere mode to surface cations. Distinguishing between these possibilities should be a primary goal of combined spectroscopic and theoretical molecular calculations. [1] Roddick-Lanzilotta A.D. and McQuillan A.J. (2000) J. Colloid & Interface Sci. 227, 48-54. [2] Sverjensky, D. A. et al. (2008) Env. Sci. & Technology, 42, 6034-6039.
V23B-2142
Preferential Treatment: Interaction Between Amino Acids and Minerals
Amino acids are the building blocks of proteins and are important for some models of the origin of life. Polymerization of amino acids from dilute solution is unlikely without a scaffold or catalyst. The surfaces of early Earth minerals are the most likely candidates for this role. The surface adsorption behavior of 12 amino acids (L-alanine, L-serine, L-aspartic acid, L-proline, L- phenylalanine, L-valine, L-arginine, d-amino valeric acid, glycine, L-lysine, L-isoleucine, and B-alanine) on 21 minerals (quartz, calcite, enstatite, illite, olivine, pyrrhotite, pyrite, alkali basalt, albite, analcime, chlorite, barite, hydroxyl apatite, hematite, magnetite, aluminum hydroxide, kaolin, silica gel, corundum, rutile, and montmorillonite) was determined via batch adsorption experiments. Absorption was determined for concentrations between 10-4M and 10-6M in the presence of 0.1M NaCl, and between pH values of 3 and 9 at 25 degrees C. The equilibrated solutions were centrifuged, filtered, derivatized using a fluorescent amino group tag (dansyl-chloride) and analyzed by HPLC. Adsorption was standardized using BET surface area measurements for each mineral to give the number of mols of each amino acid adsorbed per square meter for each mineral. The results indicate an enormous difference in the adsorption of amino acids between minerals, along with major differences in the adsorption of individual amino acids on the same mineral surface. There is also a change in the absorbance of amino acids as the pH changes. Many previous studies of amino acid concentration and catalysis by minerals have used clay minerals because of their high surface areas, however, this data suggests that the surfaces of minerals such as calcite, quartz and pyrite have even higher affinities for amino acids. The results suggest mineral surfaces that could be optimal locations for the polymerization of molecules linked to the origin of life.
V23B-2143
Adsorption of Aspartic Acid onto Rutile: Implications for Biochirality
Mineral surfaces may have facilitated the concentration and polymerization of simple biomolecules into macromolecules while promoting the development of biochirality. In this study, rutile and aspartic acid (Asp) were investigated as a possible system in this scenario. Batch adsorption experiments were performed to examine the adsorption of Asp as a function of total concentration and pH. A constant background electrolyte of 0.1 M NaCl was applied to the system, and all solutions were purged with argon gas to eliminate carbon dioxide contamination. Asp adsorbs onto rutile to the highest extent over the pH range 3-5.5 suggesting that an acidic environment is required for the adsorption between Asp and rutile to occur in significant amounts. This pH range of maximum adsorption is constrained between the isoelectric point of Asp and the point of zero charge of rutile, which indicates that electrostatic effects are influencing Asp adsorption. Both the L- and D- enantiomers of Asp were individually adsorbed onto the rutile surface to determine the potential of the system for chiral selection. Preliminary results indicate that D-Asp may possibly adsorb in greater amounts than L-Asp at higher Asp total concentrations. This trend is unexpected as the growth planes dominating the rutile are achiral, and a more thorough study is required to validate this difference in adsorption. Nevertheless, this result may provide insight on the emergence of chiral selection in macromolecules within what might be a predominantly achiral prebiotic system.
V23B-2144
Infrared Spectroscopic Evidence of Surface Speciation of Amino Acids on Titanium Dioxide
Interactions that occur at the interface between molecules and mineral surfaces in the presence of water are integral to many chemical and physical processes, including the behavior of pollutants in the environment, metal implants in the human body, and perhaps the origin of life. During the emergence of life, mineral surfaces may have played a role in the selection of amino acids, leading to the formation of proteins that are essential building blocks of life. To investigate this hypothesis, we are studying two amino acids, glutamic (Glu) and aspartic (Asp) acid, and their adsorption to the rutile form of titanium dioxide as a function of pH and surface coverage in electrolyte solutions. The objective is to get a fundamental understanding of the speciation and coordination chemistry of these amino acids at the rutile surface. We used attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy to investigate the adsorption of Glu on rutile, and a previously published ATR-FTIR study [1] of Asp and Glu adsorption on an amorphous titanium dioxide film was used as a guide to peak assignment and interpretation of our FTIR spectra. Binding of Glu to both surfaces occurs primarily through one or both of the carboxyl groups, implying that at least two types of surface complexes are formed in a proportion presumably dependent on surface coverage and pH. The interpretation of our results suggests that Glu binds to rutile in a mixed chelating-monodentate fashion involving both carboxyl groups (Glu lying down at the surface), and in a chelating fashion involving only the gamma carboxyl group (Glu standing up at the surface). FTIR results also show that the intensity of the amine peak increases with sorption, which is possibly a consequence of the amine group being brought closer to the surface but not binding directly to it. Glu adsorption on rutile is favored at low pH, based on results from batch adsorption experiments. We have commenced a systematic investigation of Glu and Asp interactions with the rutile surface using potentiometric titrations, adsorption experiments and FTIR spectroscopy. The spectroscopic evidence integrated with quantitative adsorption data and potentiometric titration data are used to describe the adsorption with surface complexation models. [1] Roddick-Lanzilotta A.D. and McQuillan A.J. (2000) J. Colloid & Interface Sci. 227, 48-54.
V23B-2145
Kinetic Studies of Amino Acid Adsorption on Rutile
With the aim of getting a fundamental understanding of the coordination chemistry and speciation of amino acids on mineral surfaces we have undertaken a study of the kinetics of glutamate and aspartate adsorption on titanium dioxide. We have performed potentiometric titrations and batch adsorption experiments to investigate the dependence of adsorption of L-glutamate and L-aspartate on pH, total concentration and ionic strength. Results show that the adsorption is favored below the point of zero charge of the mineral (pH 5.2) and is dependent on ionic strength. The same methods were used for kinetic experiments to study the time dependence of adsorption. Adsorption reactions for glutamate at pH 4, occur within minutes reaching a maximum in about 15 minutes. The adsorption then decreases after 30 minutes and achieves steady state after approximately 4 hours. Precise pH measurements in a closed titration vessel indicate a small increase in pH during the first 75 minutes followed by a small decrease to steady state after about 11 hours. In contrast, pH values for aspartate decrease initially and then start to increase after 5 minutes with a maximum at about 30 minutes, followed by a decrease to steady state after about 8 hours. Taken together these results could indicate equilibration of competing surface complexes. This inference is consistent with results from ATR- FTIR spectroscopic studies indicating two or more surface species.