Mineral-mediated reaction

A primary goal of this volume is to outline strategies by which mineral spectroscopy can be used to elucidate mineral-mediated reactions. The chosen articles do directly... 

These properties and principles govern the transfer of organic chemicals between different environmental compartments. Of particular interest to halogenated hydrocarbon reactivity discussed in this treatise is the air-water interface (emissions, water column outgassing), and the solid-water interface (sorptive processes, sequestration, mineral-mediated reactions). For example, the role of air-water diffusive exchange in large aquatic systems may provide a source or sink for volatile compounds such as the halomethanes... 

Fe(II) goethite and Fe(II) with no mineral phase added. The degradation kinetics of CgCl NO in reactions and the change in sorbed Fe(II), [Fe(II)] as a function of pH are shown in Fig. 16.8. The primary degradation route for C Cl NO occurs through a surface-mediated reaction with Fe(II) the final product is C Cl NH, with an intermediate product believed to be phenylhydroxylamine (Cp NHOH). 

Klupinski et al. (2004) conclude that the reduction of nitroaromatic compounds is a surface-mediated process and suggest that, with lack of an iron mineral, reductive transformation induced only by Fe(II) does not occur. However, when C Cl NO degradation was investigated in reaction media containing Fe(II) with no mineral phase added, a slow reductive transformation of the contaminant was observed. Because the loss of C Cl NO in this case was not described by a first-order kinetic model, as in the case of high concentration of Fe(II), but better by a zero-order kinetic description, Klupinski et al. (2004) suggest that degradation in these systems in fact is a surface-mediated reaction. They note that, in the reaction system, trace amounts of oxidize Fe(II), which form in situ suspended iron oxide... 

When a mineral or Lewis acid replaces the carboxylic component in the Passerini reaction, the final products are usually a-hydroxyamides. Also in this case, when chiral carbonyl compounds or isocyanides are employed, the asymmetric induction is, with very few exceptions, scarce [18, 19]. For example, the pyridinium trifluoroacetate-mediated reaction of racemic cyclic ketone 14 with t-butyl isocyanide is reported to afford a single isomer [19] (Scheme 1.7). This example, together with those reported in Schemes 1.3 and 1.4, suggests that high induction may be obtained only by using rigid cyclic or polycyclic substrates. 

Unlike laboratory chemistry, geological chemistry occurs in thermodynamically open systems. That is to say, there is flagrant exchange of materials and energy between the system and the environment Not only is the system as a whole not isolated from the environment, but the materials of interest are rarely isolated from each other. To further the complexity, interaction of mineral catalysts with those forms of energy most likely to affect surface-mediated reactions has barely been characterized in model systems. 

Both these catalytic centers and the reaction mechanism on them have been better than average well-characterized by the spectroscopic methods discussed and applied in this volume. The comparison between anticipated locations of the catalytic centers for these materials also points up the importance of methodology for determining not only the number, but the accessibility of catalytic sites. Therefore, it is thought that comparisons and contrasts between thermal and photo chemistry, as well as many aspects of the state of present comprehension of mineral spectroscopy and mineral-mediated catalysis will be well-illustrated by a reexamination of published studies of change, even though this reaction is not directly relevant to applications of spectroscopy discussed in the succeeding papers. 

Biological activities, such as photosynthesis and respiration, physical phenomena, such as natural or induced turbulence with concomitant aeration, and above all processes such as the precipitation and dissolution of CaC03 and of other minerals influence pH regulation through their respective abilities to decrease and increase the concentration of dissolved carbon dioxide. Besides photosynthesis and respiration, other biologically mediated reactions affect the H" ion concentrations of natural waters. Oxygenation reactions often lead to a decrease in pH, whereas processes such as denitrification and sulfate reduction tend to increase pH. 

The electrolytes—both anions and cations—perform a number of vital roles in maintaining fluid balance and acid-base balance, membrane potentials, muscular functions, and nervous conduction. They act as cofactors in many enzyme-mediated reactions. In addition, calcium and phosphate are the main mineral constituents of the skeleton. 

The surface defines the interface between a mineral and its surroundings. In a dynamic context, reactions that occur between apatites and the environments in which they exist take place at or through their surfaces. This includes, but is not restricted to, crystal growth, dissolution, and surface-mediated reactions such as sorption, surface complexation, and catalysis (Hochella and White 1990). Because this interface is partly defined by the nature of the environment around the crystal, the properties, structure and chemistry of the crystal surface are always different than those of the bulk, and can be quite varied depending on the environment. For example, the crystal surface of apatite may have very different characteristics in contact with an aqueous solution as opposed to a polymerized silicate melt. 

An additional emphasis that we hope becomes discemable to the reader is in the presentation of work that uses evolving spectroscopic techniques to complement either experimental data obtained from the macroscopic behavior of surfaces or modeling efforts aimed at a mechanistic understanding of surface mediated reactions. Much of the mechanistic detail that is needed to attain a predictive capability towards mineral surface behavior is derived via combined studies of this type. 

While by common perception, mineral dusts may appear as particularly inert materials, inorganic particles can participate in a variety of chemical and cellular reactions, some of which are mediated by free radicals. 

The yeast-mediated enzymatic biodegradation of azo dyes can be accomplished either by reductive reactions or by oxidative reactions. In general, reductive reactions led to cleavage of azo dyes into aromatic amines, which are further mineralized by yeasts. Enzymes putatively involved in this process are NADH-dependent reductases [24] and an azoreductase [16], which is dependent on the extracellular activity of a component of the plasma membrane redox system, identified as a ferric reductase [19]. Recently, significant increase in the activities of NADH-dependent reductase and azoreductase was observed in the cells of Trichosporon beigelii obtained at the end of the decolorization process [25]. 

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