Phyllosilicates surface complexes

Surface complexes between metal cations and siloxane ditrigonal cavities on 2 1 phyllosilicates, shown in exploded view. 

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. 

In contrast to humic acids, proteins constitute a class of compounds that are chemically well defined, prevalent in soils, and able to form complexes with clay surfaces. Therefore, in order to study the interactions of anthropogenic chemicals with organic matter/clay complexes, we have started a series of MD simulations dealing with the adsorption of proteins by phyllosilicate surfaces. These investigations can be extended to humic acid coated clays, because many of the functional groups in proteins are the same as those in humic acids. 

More recent x-ray absorption studies of Co sorption complexes on quartz [77] suggest that for quartz at relatively low density coverage, first-sorbed Co species provide energetically favorable nucleation sites for the subsequent formation of multinuclear hydroxide-like surface complexes. This is also seen for Ni " " adsorbed onto silica [78] where the first species adsorbed are seen as nickel phyllosilicates and not hydroxides. Subsequent layers are either more nickel phyllosilicate or nickel hydroxide depending on the activity of the sdica. If the silica is more active, for example due to smaller particles or greater surface area, then more metal-sUicate species are formed either at the... 

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. 

He has contributed to research on the interface between soil chemistry and mineralogy and soil biology. His special areas of research include the formation mechanisms of aluminum hydroxides and oxyhydroxides, the surface chemistry and reactivities of short-range-ordered precipitation products of Al and Fe, the influence of biomolecules on the sorption and desorption of nutrients and xenobiotics on and from variable charge minerals and soils, the factors that influence the sorption and residual activity of enzymes on phyllosilicates, variable charge minerals, organomineral complexes, and soils and the chemistry of arsenic in soil environments. 

The three examples of the effects of pedochemical weathering on the surface structures in soil clays just described illustrate the complexity of the reactive solid materials in natural soils. To these examples can be added many others, including the formation of iron oxyhydroxide or calcium carbonate coatings on the external surfaces (as opposed to interlayer surfaces) of phyllosilicates, the development of thick envelopes of colloidal organic matter on aggregates of metal oxides and aluminosilicates, and the... 

Ca ", when complexed by phyllosilicates, are associated with interlayer hydration, whereas K, Rb, and Cs tend not to be. X-ray diffraction data show that, when interlayer hydration occurs, x determined as indicated in Fig. 1.12 actually corresponds to a single layer of intercalated water molecules.Therefore, x for water vapor adsorption refers to monolayer coverage of the external surfaces plus one half of the accessible internal surfaces of the quasicrystals. This physical interpretation of for water vapor, as well as, that of jr, measured by nitrogen gas adsorption, permits... 

FIGURE 4.17 Schematic view of the different modes of adsorption, classified per the location of the adsorbate. The two horizontal groups of atoms illustrated correspond to two typical phyllosilicate layers (Chapter 8). The sorbate may reach close contact with the surface giving an inner-sphere complex, may retain its hydration layer yielding an outer-sphere complex, or stay farther in the diffuse layer region. (Adapted from Wang, J., and Gutierrez, M. S., J. Nanomater., 1-13, 2010.)... 

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