Molecular Modeling Study Of Sulfate And Phosphate Adsorption At The Mineral Water Interface

The transport and bioavailability of sulfate and phosphate are significantly affected by adsorption reactions with soil minerals. Elucidating the adsorption mechanisms and kinetics is critical to improving surface complexation models, which are used to simulate the reactive transport of nutrients in soils. The objective of this investigation was to utilize computational DFT methods to improve our understanding of sulfate and phosphate adsorption at the mineral-water interface. The effect of dehydration on sulfate adsorption at the hematite-water interface was investigated, using ATR-FTIR spectroscopy and DFT calculations. The DFT calculations were performed with edge-sharing dioctahedral Fe(III) cluster models of sulfate and bisulfate complexes. The DFT calculations suggested that sulfate formed a monodentate or bidentate bridging complex under hydrated conditions, but that bisulfate formed under dehydrated conditions (i.e., speciation change). A QMD simulation of monodentate bisulfate at the (101) goethite-water interface, however, suggested that a speciation change is probably reversible. The energies of sulfate adsorption pathways on edge-sharing dioctahedral Al(III) and Fe(III) cluster models were estimated with DFT calculations. The DFT-calculated adsorption energies were directly related to the proton/sulfate stoichiometry and the overall charge of the Al(III) and Fe(III) clusters. DFT-calculated adsorption energies for bidentate bridging and monodentate sulfate on a +1 charged Fe(III) cluster agreed reasonably well with experimental measurements of sulfate adsorption on goethite. The binding geometries of bidentate bridging and monodentate sulfate complexes at the Fe-(hydr)oxide-water interface were investigated, using cluster and periodic slab DFT calculations. The DFT cluster calculations were performed with edge-sharing dioctahedral Fe(III) models. The periodic DFT calculations were performed with a slab model of the (100) goethite surface. The cluster model predictions of the interatomic distances and angles of monodentate and bidentate bridging sulfate were in good agreement with the periodic slab model predictions. QMD simulations were performed to better understand the dynamical behavior of sulfate and phosphate complexes at the (101) goethite-water interface. The H-bonding interactions of sulfate and phosphate with goethite surface OH functional groups and with solvent water molecules were investigated. To explain why phosphate is a stronger competitor than sulfate for goethite surface sites, a proton-assisted ligand exchange mechanism was proposed.

Clearly explains how to more effectively decipher and predict contaminant fate in the environment by combining kinetic methods and molecular-scale spectroscopic and microscopic techniques to analyze mineral/water interfacial reactions in situ. The book begins with a broad overview, then continues with three sections written by internationally known expert. The first deals specifically with spectroscopic/microscopic techniques that can be used in combination with macroscopic approaches to glean mechanistic information on mineral/water reactions and processes. The second section emphasizes computer models that are used to elucidate surface mediated reaction mechanisms. The remainder of the volume is organized around reaction type, including sorption/desorption of inorganic species, sorption/desorption of organic species, precipitation/dissolution processes, heterogeneous electron transfer reactions, photochemically driven reactions, and microbially mediated reactions. Mineral-Water Interfacial Reactions will be a valuable resource for environmental scientists, geochemists, soil chemists, microbiologists, and marine engineers who need to be familiar with the most current and effective methods for testing and controlling the mobility, speciation, and bioavailability of contaminants in the environment.

Provides an introduction to the chemistry of the solid-water interface, progressing from the simple to more complex and applied. Discusses the important interfaces in natural systems, especially geochemistry, in natural waters, soils and sediments. The processes occurring at mineral-water, particle-water and organism-water interfaces play critical roles in regulating the composition and ecology of oceans and fresh waters, the development of soils and plant nutrient's supply, preserving the integrity of water repositories and in such applications as water technology and corrosion science.

Soil Physical Chemistry, Second Edition takes up where the last edition left off. With comprehensive and contemporary discussions on equilibrium and kinetic aspects of major soil chemical process and reactions this excellent text/reference presents new chapters on precipitation/dissolution, modeling of adsorption reactions at the mineral/water interface, and the chemistry of humic substances. An emphasis is placed on understanding soil chemical reactions from a microscopic point of view and rigorous theoretical developments such as the use of modern in situ surface chemical probes such as x-ray adsorption fine structure (XAFS), Fournier transform infrared (FTIR) spectroscopies, and scanning probe microscopies (SPM) are discussed.

An evolving, living organic/inorganic covering, soil is in dynamic equilibrium with the atmosphere above, the biosphere within, and the geology below. It acts as an anchor for roots, a purveyor of water and nutrients, a residence for a vast community of microorganisms and animals, a sanitizer of the environment, and a source of raw materials for co

This book provides a description of the generalized two layer surface complexation model, data treatment procedures, and thermodynamic constants for sorption of metal cations and anions on gibbsite, the most common form of aluminum oxide found in nature and one of the most abundant minerals in soils, sediments, and natural waters. The book provides a synopsis of aluminum oxide forms and a clearly defined nomenclature. Compilations of available data for sorption of metal cations and anions on gibbsite are presented, and the results of surface complexation model fitting of these data are given. The consistency of the thermodynamic surface complexation constants extracted from the data is examined through development of linear free energy relationships which are also used to predict thermodynamic constants for ions for which insufficient data are available to extract constants. The book concludes with a comparison of constants extracted from data for sorption on gibbsite with those determined previously for hydrous ferric oxide (HFO), hydrous manganese oxide (HMO), and goethite. The overall objective of this book is the development and presentation of an internally consistent thermodynamic database for sorption of inorganic cations and anions on gibbsite, an abundant and reactive mineral in soils, sediments, and aquatic systems. Its surface has a high affinity for sorption of metal cations and anions, including radionuclides. The gibbsite database will enable simulation and prediction of the influence of sorption on the fate of these chemical species in natural systems and treatment processes in which aluminum oxides are abundant. It thus will help to advance the practical application of surface complexation modeling.

Chromate is a common groundwater contaminant as a result of extensive industrial use. The fate and transport of chromate in subsurface environments are largely governed by adsorption to the surfaces of aluminum and iron oxides. Characterization of adsorption mechanisms is needed to constrain thermodynamic models, better predict the fate and transport of environmental pollutants, and expand our fundamental knowledge of surface geochemistry. However, the mechanisms of chromate adsorption in soils have remained unclear. This dissertation clarifies the mechanisms by which chromate adsorbs to aluminum and iron oxide minerals on the basis of molecular spectroscopy and quantum mechanical calculations. Chromate adsorption was first characterized on ferrihydrite using in situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR--FTIR) and theoretical frequency calculations. The effects of pH, aqueous chromate concentration, and ionic strength were investigated. It was determined that chromate primarily forms inner--sphere surface complexes on ferrihydrite. Monodentate complexes were observed to form at pH $>$ 6 and at low surface coverage under acidic pH conditions. Bidentate complexes are dominant below pH 6 and at high surface coverage. Theoretical infrared frequencies are in close agreement with those observed experimentally. Characterization of the chromate adsorption on hematite was performed using in situ ATR--FTIR spectroscopy and extended X--ray absorption fine structure (EXAFS) spectroscopy. The results were interpreted in the context of the optimized geometries and thermodynamics of the theoretical chromate--iron oxide cluster models. The results indicate that chromate binds to the surface of hematite in a similar manner as that of ferrihydrite; monodentate complexes dominate at high pH, and bidentate complexes dominate at low pH. The last system investigated was chromate adsorption on the surface of the aluminum oxyhydroxide boehmite. Both in situ ATR--FTIR and EXAFS spectroscopies were used to characterize chromate adsorption as a function of pH, ionic strength, and chromate and sulfate concentrations. It was determined that chromate primarily forms outer--sphere complexes on boehmite over a broad range of pH and chromate concentrations. Results from EXAFS and ATR--FTIR spectroscopy indicate the presence of a small fraction of inner--sphere chromate under acidic conditions.