Minerals and organic matter (OM) may form intricate associations via myriad interactions. In soils, the associations of OM with mineral surfaces are mainly investigated because of their role in determining the long-term retention of OM. OM “must decay in order to release the energy and nutrients that drive live processes all over the planet” ( Janzen, 2006 ). Thus, the processes and mechanisms that retain OM in soil are a central concern to very different branches of environmental research. An agronomist may want to synchronize periods of high nutrient and energy release with the growth stages of a crop. An environmental chemist may wish to either immobilize an organic soil contaminant or enhance its decomposition into less harmful metabolites, while climate scientists need to understand the processes that mediate the production of potent greenhouse gases from decomposing OM. Associations of OM with pedogenic minerals (henceforth termed mineral–organic associations (MOAs)) are known to be key controls in these and many other processes. Here we strive to present an overview of the current knowledge on MOAs and identify key questions and future research needs.

Chapter One – Mineral–Organic Associations: Formation, Properties, and Relevance in Soil Environments

The present review discusses the most important aspects to take into consideration to improve the properties of γ-Al2O3 as a support for catalytic applications. We show that the synthetic route to γ-Al2O3 is the starting point that determines the micro- and macrostructure of the oxide and, consequently, allows control of the support characteristics. The relevance of the adequate structural characterization of the oxide as well as of its surface sorption behavior through the proton-affinity distributions, are considered. The usefulness of the latter during supported catalyst preparation, for catalytic active sites characterization, and for stability evaluation after calcination are discussed for the Co/γ-Al2O3 system in particular. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

γ-Alumina as a Support for Catalysts: A Review of Fundamental Aspects

One of the first subjects introduced to students of environmental chemistry is the aqueous chemistry of aluminum. This metal is the third most abundant element in the shallow Earth, where it hydrolyzes in water to produce a rich array of solute molecules and solids, including clays and aluminum hydroxide phases. Although these materials are ubiquitous, we are just beginning to understand the kinetic properties of their surfaces at the molecular scale. The problem is experimental sthe solids are too unwieldy, even as colloids, for detailed spectroscopy. One recent approach has been to use 1 -2 nm aqueous Al(III) molecules as experimental models to determine reaction rates and pathways at a fundamental level. The 1 -2nm-sized clusters are useful because they expose functional groups that resemble those found on the minerals, yet reactions at these functional groups can be studied at the molecular scale using relatively simple methods of solution spectroscopy, such as NMR. The 1 -2 nm ions are sufficiently small that reactions can then be simulated at a high * E-mail: whcasey@ucdavis.edu. Phone: 530-752-3211. William Casey (born 1955) received his Ph.D. degree in Mineralogy and Geochemistry from The Pennsylvania State University in 1986 under the direction of Prof. Antonio Lasaga. After graduating, he worked as a research geochemist at Sandia National Laboratories in Albuquerque, New Mexico, for several years and jointed the faculty of the University of California in 1991. He has published over 130 scientific articles on subjects relating to aqueous solution chemistry of natural waters, mineral surface chemistry, and reaction kinetics. Volume 106, Number 1

Large aqueous aluminum hydroxide molecules.

The surface composition of a quartz surface reacted with various aqueous solutions of pH 0−10 was qualitatively and quantitatively evaluated using X-ray photoelectron spectroscopy (XPS). The positions and intensities of the recorded Si 2p and O 1s lines change depending on solution conditions. The O 1s line, where the position varies more significantly, was analyzed in detail showing three components corresponding to three surface species:  >SiOH2+, >SiOH0, and >SiO- (where > represents the bulk quartz). The changes in the Si 2p spectra support these findings. The atomic ratio between the surface oxygen and silicon atoms was found to be 1.8. These data allow proposing a two-step deprotonation model of the quartz surface where two surface oxygen atoms are bonded to one silicon surface atom and where the most deprotonated surface sites show the SiO- configuration. Physisorbed water in the amount of around 10% of the monolayer was also found on all samples under spectroscopic investigation. The density of th...

Evidence of the Existence of Three Types of Species at the Quartz−Aqueous Solution Interface at pH 0−10: XPS Surface Group Quantification and Surface Complexation Modeling

Dissolved organic matter (DOM) is often neglected as a factor in the formation of stable soil organic matter (OM). Precipitation of DOM by dissolved Al could contribute substantially to C retention in acidic forest soils; however, no information is available on the stability of precipitated OM against microbial decay. We investigated the stability of Al-OM precipitates against microbial decay as related to (i) DOM composition, (ii) Al speciation, and (iii) the dissolved Al/C ratio. We produced Al-OM precipitates by adding AlCl 3 (molar Al/C ratios: 0.05-0.3) at pH values of 3.8 and 4.5 to DOM solutions derived from Oi and Oa horizons, from either beech (Fagus sylvatica L.) or spruce [Picea abies (L.) Karst.] litter. Between 13 and 84% of the C was precipitated, depending on pH, Al/C ratio, and the type of DOM. Precipitates were found to be enriched in aromatic C and mostly depleted in N when compared with DOM. Only 0.5 to 7.7% of precipitated C was mineralized during 7 wk of incubation. Mineralization of Al-OM precipitates was up to 28 times less than that of the respective DOM solutions. The extent of mineralization of Al-OM precipitates formed at pH 3.8 was reduced by 50 to 75% when compared with those formed at pH 4.5. The stability of precipitates against microbial decay increased with larger aromatic C content and larger C/N ratios. Our study clearly demonstrated that a large fraction of DOM can be precipitated and is thereby substantially stabilized against microbial decay.

Precipitation of Dissolved Organic Matter by Aluminum Stabilizes Carbon in Acidic Forest Soils

Chemistry and Structure of Al(OH)/Organic Precipitates. A Small-Angle X-ray Scattering Study. 3. Depolymerization of the Al13 Polycation by Organic Ligands

The equations developed by us for the triple layer surface complexation approach, taking into account energetic heterogeneity of surface oxygens, are applied here to study the heterogeneity influences on the enthalpic effects accompanying ion adsorption at the silica/NaCl aqueous solution interface. That study is accompanied by the parallel experimental/theoretical study of the energetic heterogeneity of surface oxygens for adsorption of argon molecules which are known to interact practically only with surface oxygens. Such studies provide a simpler interpretation for information of the surface energetic heterogeneity. That additional study confirms a proper choice of the model of surface heterogeneity, accepted by us in our description of ion adsorption at the oxide/electrolyte interfaces. Our quantitative analysis also confirms quite different features of the silica/electrolyte interface, compared to those formed by other metal oxides. Namely, in the case of the silica/electrolyte interface, the adsorption of water molecules on the surface oxygens SO- is a process highly competitive with proton adsorption leading to that of formation of the neutral surface complexes SOH°.

Calorimetric Effects of Simple Ion Adsorption at the Silica/Electrolyte Interface: Quantitative Analysis of Surface Energetic Heterogeneity

Chemistry and structure of Al(OH)/organic precipitates. A small-angle X-ray scattering study. II: Speciation and structure of the aggregates

The theory based on the complexation model has been used to derive the adsorption isotherm equation describing proton adsorption at the solid/electrolyte interface. The present equation applies to ...

Application of the Theoretical 1-pK Approach to Analyzing Proton Adsorption Isotherm Derivatives on Heterogeneous Oxide Surfaces

Chemistry and Structure of Al(OH)/Organic Precipitates. A Small Angle X-ray Scattering Study. 1. Numerical Procedure for Speciation from Scattering Curves

Fabien Thomas

Papers

Chemistry and Structure of Al(OH)/Organic Precipitates. A Small-Angle X-ray Scattering Study. 3. Depolymerization of the Al13 Polycation by Organic Ligands

Calorimetric Effects of Simple Ion Adsorption at the Silica/Electrolyte Interface: Quantitative Analysis of Surface Energetic Heterogeneity

Chemistry and structure of Al(OH)/organic precipitates. A small-angle X-ray scattering study. II: Speciation and structure of the aggregates

Application of the Theoretical 1-pK Approach to Analyzing Proton Adsorption Isotherm Derivatives on Heterogeneous Oxide Surfaces

Chemistry and Structure of Al(OH)/Organic Precipitates. A Small Angle X-ray Scattering Study. 1. Numerical Procedure for Speciation from Scattering Curves