The carbonate phase

The most common reagent for the extraction of trace metals from carbonate phases in soil is lmoll-1 sodium acetate acidified to pH 5 with acetic acid (Kunze, 1965). Carbonate phases effectively attacked include dolomite, but the presence of acetic acid also promotes the release of metals specifically sorbed on inorganic and organic substrates (Tessier et al., 1979). 

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Superheated water at 100°-240 °C, with its obvious benefits of low cost and low toxicity, was proposed as a solvent for reversed-phase chromatography.59 Hydrophobic compounds such as parabens, sulfonamides, and barbiturates were separated rapidly on poly(styrene-divinyl benzene) and graphitic phases. Elution of simple aromatic compounds with acetonitrile-water heated at 30°-130 °C was studied on coupled colums of zirconia coated with polybutadiene and carbon.60 The retention order on the polybutadiene phase is essentially uncorrelated to that on the carbon phase, so adjusting the temperature of one of the columns allows the resolution of critical pairs of... 

The protocol involving NaOAc-HOAc at pH 5 was first proposed and used by Jackson (1958) to remove carbonates from calcareous soils to analyze soil cation exchange characteristics (Grossman and Millet, 1961). Other researchers used HOAc for the extraction of metals from sediments and soils (Nissenbaum, 1972 Mclaren and Crawford, 1973). Tessier et al. (1979) first used the NaOAc-HOAc solution at pH 5 to dissolve the carbonate fraction from sediments. Since then, the NaOAc-HOAc buffer has been widely used as a specific extractant for the carbonate phase in various media (Tessier et al., 1979 Hickey and Kittrick, 1984 Rapin et al., 1986 Mahan et al., 1987 Han et al., 1992 Clevenger, 1990 Banin et al., 1990). Despite its widespread use, this step is not free from difficulties, and further optimization is required in its application. Questions arise with regard to this step in the elemental extraction from noncalcareous soils, the dissolution capacity and dissolution rates imposed by the buffer at various pHs, and the possibility that different carbonate minerals may require different extraction protocols (Grossman and Millet, 1961 Tessier et al., 1979). 

The following sections summarize the studies on the dissolution technique for the carbonate fraction from arid and semi-arid soils with different amounts and types of carbonate minerals. In addition, the selectivity and effectivity of the NaOAc-HOAc extraction technique at varying pHs to extract the carbonate phase, and only the carbonate phase, from soils is examined (Han and Banin, 1995). 

Calcium can be used as an indicator for selective carbonate dissolution. Its concentration in the extracts is limited either by its content in the carbonate phase (at low pHs and/or low carbonate content of the soil) or by the acid capacity of the extracting solution (at higher pHs and/or higher carbonate content in the soil). The interplay of these factors is clearly shown in Fig. 4.2. 

The specificity of the NaOAc-HOAc solutions for the carbonate phase may be measured by other major elements released during extraction. For example, only limited attack of the alumosilicate components in the soil... 

Equation of State Applied to the Carbon Phase Diagram. 

Parmar N, Warren LA, Roden EE, Eerris EG (2000) Solid phase capture of strontium by the iron reducing bacteria Shewanella alga strain BrY. Chem Geol 169 281-288 Pearson MJ (1974) Sideritic concretions from the Westphalian of Yorkshire a chemical investigation of the carbonate phase. Min Mag 39 696-699... 

While carbon-containing anodes are scientifically intriguing, it is difficult to see how one could achieve stable performance with these under practical operating conditions. For operation of cells at higher fuel utilization, parts of the anode will be exposed to high steam contents, conditions under which the carbon phases will not be stable. 

Copper, lead, cadmium and zinc have been found predominantly in potentially mobile forms by sequential extraction of material collected during road cleaning (Colandini et al, 1995). Cadmium and zinc were found to be more labile than lead and copper. A study of street dust and gully pot sediments confirmed this order of potential availability (Striebel and Gruber, 1997) and also suggested that lead levels in material of the types studied had decreased since the introduction of unleaded fuel in Germany. Lead has also been studied in street dusts from Brisbane, Australia. The element was found mainly in the carbonate phase and in the smaller particle size fraction (Al-Chalabi and Hawker, 1996) except where resuspension caused particle aggregation. 

Figure 8.20. Generalized relationship between Sr and Na in CaC03 for periplatform carbonate sediments at ODP site 716, Maldives Archipelago, Indian Ocean. In general, Sr and Na concentrations in the carbonate phase decrease with increasing burial depth at this site. (After Malone et al., 1990.)...

Figure 8, the carbon phase diagram, forms a basis for discussing the processes involved. Ideal graphite has a density of 2.2 and diamond, 3.52, so 1 ml of graphite becomes 0.63 ml of diamond, a relatively large change. Diamond is favored to form at pressures and temperatures where it is stable, but the carbon atoms must be in the proper environment, particularly at the milder conditions. 

X-ray diffraction (XRD) has been poorly used to characterize the carbon phase of intumescent structure. Indeed, as shown previously, the carbon structure resulting from the development of the intumescent system is mainly disordered whereas XRD characterizes ordered structure. However, this technique may be of interest to study the carbonization process in the case of flame-retardant systems containing layered additives, such as expandable graphite,28,42 or even more in the case of lamellar nanocomposites, such as MMT-based nanocomposites. 

The maximum current yield of the carbon phase in the melt NaCl - KC1 does not exceed 30%, that is connected apparently with cathode product loss due to passing reaction at temperatures above 600 °C ... 

Abstract. IR pyrolysis of PAN and PAN based composites yields ordered graphitelike structure as well as several carbon nanostructures. Metal-carbon nanocomposites, in which the nanosized metal particles were introduced into the structure of carbon matrix in the course of IR pyrolysis of composite-precursor on the basis of PAN and metal (Gd, Pt, Ru, Re) compounds were prepared. The carbon phase of metal-carbon nanocomposites was shown to include different types of nano structured carbon particles. Bamboo-like CNT were observed in the structure of pyrolized at 910 and 1000°C composite-precursor based on PAN and GdCl3. At T=1200°C the solid carbon spheres with diameter in the range of 50-360 nm and octahedral carbon particles with the size in the range of 300-350 nm were observed. These nanostructured particles consist of carbon only or they include Gd nanoparticles incapsulated in carbon shell. IR pyrolysis of composite-precursor based on PAN as well as H2PtCl6 and RuC13 or NH4Re04 (Pt Ru(Re)=10 l) allows the preparation of Pt-Ru and Pt-Re alloys nanoparticles with 2[Pg.577]

The carbon phase of obtained metal-carbon nanocomposites was shown to contain different types of nanostructured carbon particles in parallel with main graphite-like structures. Bamboo-like carbon nanotubes (CNT) with 14-30 nm in their outer diameter were observed in structured carbon material when GdCl3 was used as a component of composite-precursor (Fig. 4). In this case IR radiation intensity provides the heating of sample to 910 and 1000°C. 

Thus, the carbon phase of obtained metal-carbon nanocomposites represents in reality the carbon-carbon nanocomposite of main graphite-like structure with array of carbon nanostructures such as bamboo-like CNT, spherical or octahedral carbon nanoparticles. 

PNP poorly adsorbs onto silica surfaces.6,11 Therefore, it can be used to estimate the structural characteristics of the carbon phase in carbosils. An increase in the Cc value leads to a significant enhancement of PNP adsorption per m2 of carbosil surface (Figure 3A), which is greater for the A series (Table 3). 

In the stepwise process (5.18) at 900 K, the carbon phase formation is supposed to be the rate limiting step. The estimated stationary oversatura tion of an active species during the reaction in the latter condition is a 5. Hence, Ina as 1.6 and, therefore. 

Another approach to diamond synthesis involves energetic ion or laser beams to produce local areas where, for short duration, carbon atoms are subjected to pressure and temperature conditions that reach into the diamond stable region of the carbon phase diagram. After rapid quenching to ambient temperature and pressure, diamond thus formed remains metastable with respect to graphite. Attempts to deposit diamond by such techniques have been only moderately successful. 

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