Formation on clays

During the last 5 years, there have been several reports of multiblock copolymer brushes by the grafting-from method. The most common substrates are gold and silicon oxide layers but there have been reports of diblock brush formation on clay surfaces [37] and silicon-hydride surfaces [38]. Most of the newer reports have utilized ATRP [34,38-43] but there have been a couple of reports that utilized anionic polymerization [44, 45]. Zhao and co-workers [21,22] have used a combination of ATRP and nitroxide-mediated polymerization to prepare mixed poly(methyl methacrylate) (PMMA)Zpolystyrene (PS) brushes from a difunctional initiator. These Y-shaped brushes could be considered block copolymers that are surface immobilized at the block junction. 

Scheidegger AM, Strawn DG, Lamble GM, Sparks DL (1998) The kinetics of mixed Ni-AL hydroxide formation on clay and aluminum oxide minerals a time-resolved XAFS study. Geochim Cosmochim Acta 62 2233-2245... 

White, D. H., Kennedy, R. M. and Macklin, J. (1984). Acyl silicates and acyl aluminates as activated intermediates in peptide formation on clays. Origins of Life and Evolution ofBiosphers, 14, 273-8. 

Scheidegger, A.M., D.G. Strawn, G.M. Lamble, and D.L. Sparks. 1998. The kinetics of mixed Ni-Al hydroxide formation on clays and aluminum oxides A time-resolved XAFS study. Geochim. Cosmochim. Acta 62 2233- 2402. 

In hilly regions where gully formation on clay and loam soils is a common, or possible, occurrence there is often a need for catch and cover crops if these can be fitted in between main crops with very little disturbance of the soil. Of course sod crops are preferable for such soils but they may be impractical for the type of agriculture common to the region. The primary need here is to keep the soil covered with some kind of vegetation as near continuously as possible, and if a green manure crop aids in doing this, then it probably should be used. 

Gray, D.H. Rex, R.W. "Formation Damage in Sandstones Caused by Clay Dispersion and Migration", 1966 Fourteenth National Conference on Clays and Clay Minerals, 355-366. 

Coulter, G.R. Hower, W. "The Effect of Fluid pH on Clays and Resulting Formation Permeability", Proc. Annu. Southwest Pet. Short Course, April 1975. 

Extensive, horizontal sandstone plateaus occur in tropical shield areas. Well-known examples are the Precambrian Roraima sandstone formations on the Guiana Shield and the Voltaian sandstone formations in Western Africa. Major occurrences of consolidated sands are found in Northern Africa, in Guyana and Surinam, eastern Peru, northeastern Brazil and in Liberia (western Africa). These sandstone formations have a history of tropical weathering in common they all have a deep weathering mantle of bleached, white sands that are very rich in quartz, poor in clay and excessively drained. Electrolyte contents differ by region In arid and semi-arid areas where evaporation exceeds precipitation, salts and carbonates may accumulate at or near the surface of the soil. 

Metal oxides have a significant role in influencing physical, chemical, and biological properties of soils. They may exist as ciystalline minerals, as short-range ordered (SRO) mineral colloids, or as surface coatings on clay minerals and organic matter. Organic compounds influence the formation, transformation, and surface properties of these metal oxides. The SRO A1... 

For the understanding of the binding of heavy metals on clays one needs to consider - in addition to ion exchange - the surface complex formation on end-standing functional OH-groups. Furthermore, the speciation of the sorbate ion (free hydroxo complex, carbonato- or organic complex) and its pH-dependence has to be known. 

The Adsorption of Cs+on Clays - an ion with a simple solution chemistry (no hydrolysis, no complex formation) - can be remarkably complex. Grutter et al. (1990) have studied adsorption and desorption of Cs+ on glaciofluvial deposits and have shown that the isotherms for sorption and exchange on these materials are nonlinear. Part of this non-linearity can be accounted for by the collaps of the c-spacing of certain clays (vermiculite, chlorite). As illustrated in Fig. 4.23 the Cs+ sorption on illite and chlorite is characterized by non-linearity. 

Hydrolysis reactions occur by nucleophilic attack at a carbon single bond, involving either the water molecule directly or the hydronium or hydroxyl ion. The most favorable conditions for hydrolysis, e.g. acidic or alkaline solutions, depend on the nature of the bond which is to be cleaved. Mineral surfaces that have Bronsted acidity have been shown to catalyze hydrolysis reactions. Examples of hydrolysis reactions which may be catalyzed by the surfaces of minerals in soils include peptide bond formation by amino acids which are adsorbed on clay mineral surfaces and the degradation of pesticides (see Chapter 22). 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

The influence of sucrose and glycerol on the formation and transformation of iron oxides - the implication for soil formation. Appl. Clay Sci. 2 41-62... 

Early on, water existed at least periodically on the Martian surface. The formation of clay minerals in the Noachian and precipitation of sulfates and chlorides in the Hesperian were consequences of this water. Some fraction of the water evaporated and was lost to space, as indicated by the high D/H ratios in SNCs. Other water was apparently sequestered at the poles or underground as permafrost. The surface of Mars is effectively dry now, and chemical weathering of crustal rocks is minimal. 

The acidity of a clay can be either of the Brpnsted (H+ donor) or Lewis (electron pair acceptor) type. Even at temperatures below 100 °C, tertiary carbocation intermediates can be generated on clays with high Brpnsted acidity through protonation of the C=C double bond in secondary alkenes, as in the clay-catalyzed formation of MTBE from methanol and isobutene ... 

Figure 1. Computer simulation of the formation of clay tactoid by a process of aggregation of lCr particles. Ration length to thickness 9/1. In the three topmost diagrams each rectangle endorses a portion of the tactoid shown enlarged below, df is represented by the thickness of the line enclosing the sheets in contact. Adapted from ref. 6. N.B. With respect to the next section, the molar fraction xi consists of the water in the interlamellar space and in the closed spaces within the tactoid. The molar fraction xb is in a 10 A thick layer on the external contour of the tactoid.

These three types of surface species—inner-sphere complex, outer-sphere complex, and diffuse-layer—represent three modes of adsorption of small aqueous ions that contribute to the formation of the electrochemical double layer on clay mineral surfaces. No inference of special planes containing adsorbed ions is required by these surface chemical speciation concepts, nor is detailed molecular structure implied, other than the general notions of surface complexes and vicinal dissociated ions. It is sometimes convenient, although not necessary, to group the two types of surface complex into a Stern layer to distinguish them from diffuse-layer ions [18]. This geometric partitioning of surface species, however, should not be taken to mean that diffuse-layer ions necessarily approach a particle surface less closely than do Stern-layer ions. 

Gaines, G. L., and H. C. Thomas. 1953. Adsorption studies on clay minerals II. A formation of the thermodynamics of exchange adsorption. J. Phys. Chem. 21 714—718. 

Konya, J., N. M. Nagy, and M. Foldvari. 2005. The formation and production of nano and micro particles on clays under environmental-like conditions. J. Therm. Anal. Calorimetry 79 537-543. 

As shown in Chapter 2, Section 2.10.3, the system of lead ion/calcium-montmoril-lonite lead enrichments may form when, based on thermodynamic conditions, their formation is not expected. The process was considered as surface precipitation or coprecipitation, depending on the chemical composition of the lead enrichments. Since lead enrichments are fairly bulky (a few hundred nanometers to micrometers), their migration rate is practically zero. Therefore, if they form under natural conditions, they should be observed on the surface of natural clay. In fact, lead enrichments were found on clay mineral surfaces from lake sediments (Figure 3.6 Nagy et al. 2003a). 

Nemes, Z., N. M. Nagy, A. Komlosi, and J. Konya. 2006. The effect of mineral composition on the sorption of strontium ion on geological formations. Appl. Clay Sci. 32 172-178. 

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