Catalysis by mineral surfaces

Even so, it is clear that certain structural features and special entities do influence catalysis by mineral surfaces, as well. The most common means by which they operate fall into four general classes, three of which have been extensively investigated. Covalent linkages in clays are less common than the others, being dependent on grafting to SiOH, which entities are not numerous in clays (10). 

Vorlicek T. P. and Helz G. R. (2002) Catalysis by mineral surfaces implications for Mo geochemistry in anoxic environments. Geochim. Cosmochim. Acta 66, 3679-3692. 

Catalysts played an important role in the emergence of life on Earth nearly 4 billion years ago. Catalysis by mineral surfaces and small molecules enabled the emergence of a proto-metabolic network that, in turn, enabled the emergence of the RNA world. The first macromolecular catalysts may have been ribozymes, an idea first proposed by Carl Woese that gained credence with the discovery of catalytic RNAs by Cech and Altman. Subsequently, ribozymes generated by in vitro evolution methods have been shown to catalyze a wide range of reactions involved in metabolism, including amino acid activation formation of coenzyme A (CoA), nicotinamide adenine dinucleotide (NAD), and flavin adenine dinucleotide (FAD)... 

Bell, JS, Palmer DA, Barnes HL, Drammond SE (1994) Thermal decomposition of acetate 111 Catalysis by mineral surfaces. Geochim Cosmochim Acta 58 4155-4177 Benson SW (1960) The Foundations of Chemical Kinetics. McGraw-Hill, New York Bemdt ME, Allen DE, Seyfried WE (1996) Reduction of CO2 during serpentinization of olivine at 300°C and 500 bars. Geolo 24 351-354... 

Barteau MA, Bowker M, Madix RJ (1981) Formation and decomposition of acetate intermediates on the Ag(llO) surface. J Catal 67 118-128 Barth T (1987) Quantitative determination of volatile carboxylic acids in formation waters by isotachophoresis. Anal Chem 59 2232-2237 Bell JLS (1991) Acetate decomposition in hydrothermal solutions. PhD Thesis, The Pennsylvania State University, University Park, 228 pp Bell JLS, Palmer DA, Barnes HL, Drummond SE (1993) Thermal decomposition of acetate. Part III. Catalysis by mineral surfaces. Geochim Cosmochim Acta (in press) Benziger JB, Madix RJ (1979) The decomposition of formic acid on Ni(lOO). Surface Sci 79 394-412... 

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

Contact with a mineral surface can in many cases allow a redox reaction to proceed at a rate considerably greater than attainable within an aqueous solution itself. The catalyzing mineral sorbs the electron donating and accepting species, then, within its structure or along its surface, conducts electrons from one to the other. Where electron transfer by this pathway proceeds more rapidly than via a direct transfer in solution between colliding molecules, the redox reaction proceeds preferentially by heterogeneous catalysis. 

Catalysis by such a mechanism can be accounted for in a kinetic rate equation by including as a factor the catalyzing mineral s surface area. Sung and Morgan (1981), for example, in studying the oxidation on Mn11 by dissolved dioxygen,... 

Chemical reactions of adsorbed species are of importance in vast areas of science, the involvement of adsorbed metal ions in catalysis being one example of great economic value. In addition reactions involving adsorbed species can sometimes produce products that may be either difficult or impossible to prepare away from the mineral surface. Therefore, an understanding of the chemical processes that occur in such systems is of potential economic benefit to industrial operations. Such knowledge is also of much wider significance, however, because the movement of ions in most environmental situations is controlled by sorption processes, and aluminosilicate minerals play a major role in many situations. 

Abiotic, for example by adsorption of reactants onto mineral surfaces, distinguished from biotic catalysis by the absence of a temperature optimum. 

As reviewed by Alexander (1985) and Morris Pritchard (1994), many studies suggest that bioavailability represents one of the most important factors influencing PAH biodegradability in the environment. This is because PAHs, especially the HMW, hydrophobic compounds, possess intrinsically low water solubilities and therefore tend to partition onto soil mineral surfaces and to sorb tightly to available organic materials (Hites et al., 1977 Means et al., 1980 Weber etal., 1993). When PAHs are adsorbed in this way they become physically unavailable to resident microorganisms and are therefore protected from microbial catalysis. 

Applying the foregoing thermodynamic and kinetic information to manganese behavior in natural water systems is considerably limited because the manganese system exemplifies the difficulties discussed earlier. On the thermodynamic side, the kinds of oxide phases in natural waters may not correspond to those for which equilibrium data are available. Also, cation exchange reactions are probably important (21). On the kinetic side, the role of catalysis by various mineral surfaces in suspension or in sediments is not really known. Of considerable importance may be microbial catalysis of the oxidation or reduction processes, as described by Ehrlich (7). With respect to the real systems, relatively... 

Gay minerals and zeolites are interesting with respect to possibilities for geometric influences. Activation can be produced, as in enzyme catalysis, by constraining the reactive molecule, via surface complexation, in a configuration in which it is destabilized relative to that of the free molecule, yet still accessible to other reactants. A possible example is hydrazine complexed with kaolinite. The conformation of hydrazine is flattened relative to that of the free molecule (See Giff Johnston s paper in Part m of this volume). It has been shown that hydrazine is readily air-oxidized by kaolinite (Coyne, submitted for publication). 

Sulfide-mineral oxidation by microbial populations has been postulated to proceed via direct or indirect mechanisms (Tributsch and Bennett, 1981a,b Boon and Heijnen, 2001 Fowler, 2001 Sand et al., 2001 Tributsch, 2001). In the direct mechanism, it is assumed that the action taken by the attached cell or bacterium on a metal sulfide will solubilize the mineral surface through direct enzymatic oxidation reactions. The sulfur moiety on the mineral surface is oxidized to sulfate without the production of any detectable intermediates. The indirect mechanism assumes that the cell or bacteria do not act directly on the sulfide-mineral surface, but catalyze reactions proximal to the mineral surface. The products of these bacterially catalyzed reactions act on the mineral surfaces to promote oxidation of the dissolved Fe(II) and S° that are generated via chemical oxidative processes. Ferrous iron and S°, present at the mineral surface, are biologically oxidized to Fe(III) and sulfate. Physical attachment is not required for the bacterial catalysis to occur. The resulting catalysis promotes chemical oxidation of the sulfide-mineral surface, perpetuating the sulfide oxidation process (Figure 1). 

However, amino acids are unlikely to form themselves into polymers without the help of some form of catalyst (Bada, 2004). Possible natural catalysts are mineral surfaces such as in the regular, repeating structure of clays although once bound to a clay the polymer has to be released. This is achieved in the laboratory with salt solutions and may, in nature, reflect an evaporative marine environment. An alternative venue is at hydrothermal vents where peptide bond formation is favored, and where catalysis may take place on sulfide mineral surfaces (Bada, 2004). Such a process has been described by Holm and Charlou (2001) who found linear saturated hydrocarbons with chain lengths of 16 to 29 carbon atoms in high-temperature hydrothermal fluids from a vent in the Mid-Atlantic Ridge. 

Even without atomic resolution, AFM has proved its worth as a technique for the local surface structural determination of a number of bio-inorganic materials, such as natural calcium carbonate in clam and sea-urchin shells [123]. minerals such as mica [124] and molybdenite [125] as well as the surfaces of inorganic crystals, such as silver bromide [126] and sodium decatungstocerate [127]. This kind of information can prove invaluable in the understanding of phenomena such as biomineralization, the photographic process or catalysis, where the surface crystallography, especially the presence of defects and superstructures, can play an important role, but is difficult to determine by other methods. AFM has the considerable advantage that it can be used to examine powdered samples, either pressed into a pellet, if the contact mode is employed, or loosely dispersed on a surface, if intermittent or non-contact AFM is available. 

Catalysis of hydrolytic reactions may also occur by surface-bound metals. Although there has been a greater focus on the study of metal catalysis by dissolved metal ions, there is increasing evidence to suggest that catalysis by surface-bound metals may be of greater importance in environmental systems such as groundwater aquifers. Stone (1989) postulates 3 mechanisms for catalysis at the mineral-water... 

The second model requires catalysis by adsorbed protons and is applied to silicate minerals with covalent, polymerized structures, like quartz. In this model, protons react quickly with oxide bonds at the surface, accelerate cleavage, and return to solution. As bonds are progressively cleaved, a monomer or small oligomer is released from the surface. The weakness of this model is the enormous difficulty in simultaneously determining rates of dissolution and proton adsorption densities on complicated multi-oxide mineral structures. Protons taken up by leaching alkaline-metal cations from the mineral must be separated from those involved in protonation of bonds in order to assign a value to the rate order. 

The application of IR spectroscopy to organic sorbates has been primarily limited to gas-phase a orption kinetics and gas-phase catalysis (21). The usefrilness of IR for investigating sorption/desorption processes in situ, has been demonstrated for the selectivity of conformer adsorption at mineral surfaces (22). Strong absorption of H O vibrational modes by IR radiation has been a major hindrance in the application of IR spectroscopy to study organic sorption at aqueous-mineral interfaces. Attenuated total reflectance-IR (ATR-IR) spectroscopy and the use of DjO in the aqueous phase minimizes the water absorption problem. Figure 3 details the ATR setup used in this study. Clay pastes were loaded into Teflon plaques and clamped to both sides of a vertical ATR prism. Silicon sealant around the edges of the plaques prevented water evaporation during extended data collection times (up to 2 days). The area of the D O... 

Long paired regions would have been difficult to synthesise in early biology, however. A simple, general, and processive RNA polymerase ribozyme (i.e., capable of proceeding base-by-base to replicate an entire RNA sequence) has not yet been found. Early RNAs were likely short, perhaps synthesised relatively slowly on mineral surfaces [35]. Since most known ribozymes are tens or hundreds of nucleotides long, and since most protein enzymes are composed of chains of hundreds of amino acids, it may seem that short nucleic acids are incapable of biologically relevant catalysis, beyond simple template-driven reactions. 

A heterogeneous natural system such as the subsurface contains a variety of solid surfaces and dissolved constituents that can catalyze transformation reactions of contaminants. In addition to catalytically induced oxidation of synthetic organic pollutants, which are enhanced mainly by the presence of clay minerals, transformation of metals and metalloids occurs with the presence of catalysts such as Mn-oxides and Fe-containing minerals. These species can alter transformation pathways and rates through phase partitioning and acid-base and metal catalysis. 

Itoh, H. Sugimoto,T. (2001) Synthesis of monodispersed magnetic particles by the gel-sol method and their magnetic properties. Stud. Surface Sci. Catalysis 132 251-254 Iwasaki, 1. (1978) Selective flocculation of fine grained iron bearing materials. In Somasun-darun, P. Arbiter, N. (eds.) Benefication of mineral fines. Problems and research needs. New York, 257-262... 

Transition metals play an important role in heterogeneous catalysis where reactions occur on the surfaces of metal or oxide crystals. Typical of these metals are V or Mo which exist in oxides with tetrahedral, tetragonal pyramidal, or octahedral coordination and which can change their oxidation states with minimal changes in their coordination environment. As in the case of soil minerals (Section 13.4.1), bond valences can be used to determine the bonding strength of the anions on the surface, by noting how far the valence sums around the surface ions fall short of 2.00 vu. 

These examples illustrate that biomolecules may act as catalysts in soils to alter the structure of organic contaminants. The exact nature of the reaction may be modified by interaction of the biocatalyst with soil colloids. It is also possible that the catalytic reaction requires a specific mineral-biomolecule combination. Mortland (1984) demonstrated that py ridoxal-5 -phosphate (PLP) catalyzes glutamic acid deamination at 20 °C in the presence of copper-substituted smectite. The proposed pathway for deamination involved formation ofa Schiff base between PLP and glutamic acid, followed by complexation with Cu2+ on the clay surface. Substituted Cu2+ stabilized the Schiff base by chelation of the carboxylate, imine nitrogen, and the phenolic oxygen. In this case, catalysis required combination of the biomolecule with a specific metal-substituted clay. 

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