Phyllosilicates, soil

F. Paris, P. Bonnaud, J. Ranger, M. Robert, and F. Lapeyrie, Weathering of ammonium- or calcium-saturated 2 1 phyllosilicates by ectomycorrhizal fungi in vitro. Soil Biol. Biochein. 27 1237 (1995). 

Clay minerals or phyllosilicates are lamellar natural and synthetic materials with high surface area, cation exchange and swelling properties, exfoliation ability, variable surface charge density and hydrophobic/hydrophilic character [85], They are good host structures for intercalation or adsorption of organic molecules and macromolecules, particularly proteins. On the basis of the natural adsorption of proteins by clay minerals and various clay complexes that occurs in soils, many authors have investigated the use of clay and clay-derived materials as matrices for the immobilization of enzymes, either for environmental chemistry purpose or in the chemical and material industries. 

Whereas studies have been carried out on the factors (surface coverage, residence time, pH) which influence the desorption of arsenate previously sorbed onto oxides, phyllosilicates and soils (O Reilly et al. 2001 Liu et al. 2001 Arai and Sparks 2002 Violante and Pigna 2002 Pigna et al. 2006), scant information are available on the possible desorption of arsenate coprecipitated with iron or aluminum. In natural environments arsenic may form precipitates or coprecipitates with Al, Fe, Mn and Ca. Coprecipitation of arsenic with iron and aluminum are practical and effective treatment processes for removing arsenic from drinking waters and might be as important as sorption to preformed solids. 

A weathering sequence for phyllosilicates has been established which indicates that their relative stability in soils is as follows mica,... 

Although many authors insist upon the sequential degradation of phyllosilicates, i.e., taking the same initial material and transforming it into the various types of expandable minerals in the weathering process, (Heaver and Jackson, notably) this is undoubtedly not the only mechanism by which these clay minerals are formed in soils and possibly not the dominant one. Studies on the weathering of granites and more basic rocks... 

There are zeolite-bearing rocks in which one mineral is apparently being replaced by another mineral under constant P-T conditions. This indicates a system in which certain chemical components appear to be perfectly mobile a system in which the total number of phases that can coexist at equilibrium is reduced as a function of the number of chemical components which ar e internal variables of the system. Two examples of this type of equilibrium concerning zeolites can be cited saline lakes and analcite-bearing soil profiles (Hay, 1966 Hay and Moiola, 1963 Jones, 1965 and Frankart and Herbillon, 1970). In both cases a montmorillonite-bearing assemblage becomes analcite or zeolite-bearing at the expense of the expandable phyllosilicate. Other phases remain constantly present. 

Figure 2.1. Basic structures of 1 1 and 2 1 phyllosilicates. Adapted from Schulze (1989). Reprinted with permission from the Soil Society of America.

Phyllosilicate amorphous Al and Si surface coatings Soil solution chemistry Schultess and Huang (1991)... 

Manganese(III/IV) oxides also play an important role in oxidizing cations such as As(III) to As(V) (Oscarson et al., 1980,1981a,b). This is a very beneficial effect, since As(III) is an extremely toxic pollutant and is more soluble and mobile (Deuel and Swoboda, 1972) than As(V). Iron(III) oxides in soils and sediments may also play a role in oxidizing As(III), but the kinetics of this reaction is very slow (Oscarson et al., 1980). Phyllosilicates and calcite are not effective in oxidizing As(III) to As(V) (Oscarson et al., 1981b). 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

Aluminosilicates or phyllosilicates are inorganic crystalline structures which make up a large part of the < 0.2 mm soil-sized particles. These minerals, commonly referred to as clay minerals, consist of Si-0 tetrahedrons, in which one silicon atom (Si4+) is... 

Paris, F., Bonnaud, P., Ranger, J. Lapeyrie, F. (1995). In vitro weathering of phlogopite by ectomycorrhizal fungi. I. Effect of K" " and Mg deficiency on phyllosilicate evolution. Plant and Soil, 177, 191-205. 

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