Oxidation polymerization reactions mineral surfaces

Abiotic organic reactions, such as hydrolysis, elimination, substitution, redox, and polymerization reactions, can be influenced by surfaces of clay and primary minerals, and of metal oxides. This influence is due to adsorption of the reactants to surface Lewis and Br nsted sites. Temperature and moisture content are the most important environmental variables. Under ambient environmental temperatures, some reactions are extremely slow. However, even extremely slow transformation reactions may be important from environmental and geochemical viewpoints. 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

The fate of organic contaminants in soils and sediments is of primary concern in environmental science. The capacity to which soil constituents can potentially react with organic contaminants may profoundly impact assessments of risks associated with specific contaminants and their degradation products. In particular, clay mineral surfaces are known to facilitate oxidation/reduction, acid/base, polymerization, and hydrolysis reactions at the mineral-aqueous interface (1, 2). Since these reactions are occurring on or at a hydrated mineral surface, non-invasive spectroscopic analytical methods are the preferred choice to accurately ascertain the reactant products and to monitor reactions in real time, in order to determine the role of the mineral surface in the reaction. Additionally, the in situ methods employed allow us to monitor the ultimate changes in the physico-chemical properties of the minerals. 

Oxidation and polymerization of catechol, pyrogallol and 2,6-dimethylphenol (FTIR spectra) SEM coupled with energy dispersive X-ray spectrometry investigation of reaction products on surface of clay minerals, 13C NMR, MALDI MS study of reaction products... 

Studies on the photochemical stability achieved of polymeric materials upon addition of mineral fillers yielded in some unexpected results, since the decrease in properties is, in some cases, more intense after the UV exposure, in comparison with unfilled materials [80-82]. Thus, for PP and PE filled with clays (montmo-rillonite, MMT) [9] such behavior was explained considering the very small amounts of metal ions (i.e., Fe " ) present as impurities in clays and which can act as catalysts in UV-initiated oxidation reactions that entails accelerated degradation. At the same time, hydroperoxides formed during photo-oxidation are a source of free radical species able to further promote the degradation. Even more, light stabilizers effectiveness was found to be much reduced in polymer-MMT composites, maybe because these molecules were adsorbed on the surface of clay platelets so that their action was diminished or even blocked. Such results were reported for PC/MMT composites [5], as well as for PA6/MMT [83]. 

The rate order is given by a charge balance of protons at the surface this result is, however, not observed for complicated silicate minerals. There are many potential reasons for this, including the selective leaching of cations from the surface [e.g., 41, 45, 46], self-poisoning of the reaction [e.g., 47], or migration of protons deep into the mineral. We yet have virtually no fundamental understanding of the dissolution kinetics of complicated mixed oxide and aluminosilicate polymerized structures. 

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