Limiting mineral phases

The existing input file is extended by setting up equilibrium not only with pyrite but also with calcite. 2.621 mmol of calcite dissolve. The amount of pyrite dissolved is the same as in the absence of calcite (1.347 mmol). The pH value of 7.58 is in the neutral range. Thus, to neutralize the pH approximately 2 moles of calcite must be added for every mol of pyrite. The saturation index of gypsum is still clearly undersaturated (SI = -1.09), i.e. that gypsum is not a limiting mineral phase and hence the sulfate contents stay more or less invariable. 

Fig. 66 shows the undersaturation of some mineral phases of interest. If the saturation index is attained, the respective mineral is precipitated by the model and acts as a limiting phase (kinetics are not considered). The possible limitation by coffinite, uraninite, and pyrite from 500 days onwards (not distinguishable in the figure coffinite is not a limiting mineral phase any more from 2000 days on furthermore it is questionable that coeffinite forms under this conditions) is remarkable. Kaolinite is supersaturated after 2000, calcite after 7000, and Al(OH)3... 

The extent of substitution of magnesium and siUcon by other cations in the chrysotile stmcture is limited by the stmctural strain that would result from replacement with ions having inappropriate radii. In the octahedral layer (bmcite), magnesium can be substituted by several divalent ions, Fe ", Mn, or Ni ". In the tetrahedral layer, siUcon may be replaced by Fe " or Al ", leaving an anionic vacancy. Most of the other elements which are found in vein fiber samples, or in industrial asbestos fibers, are associated with interstitial mineral phases. Typical compositions of bulk chrysotile fibers from different locations are given in Table 3. 

Mossbauer Spectral Analysis and Analog Measurements. Mossbauer spectra were obtained in the temperature range between 200 and 270 K and in two different energy windows (14.4 and 6.4 keV), which provide depth selective information about a sample [346]. To compensate for low counting statistics due to limited integration time, all available spectra were summed for the integrations on the undisturbed and brushed surface, respectively. In addition to kamacite (a-(Fe,Ni)) ( 85%) and small amounts of ferric oxide (see Fig. 8.38), all spectra exhibit features indicative for an additional mineral phase. Based on analog measurements... 

Of the 53 elements determined, 48 had useful values above the method detection limits. The data for As and Cr are shown as dotplots (Figs. 1 and 2) as examples of metals and metalloids that occur in both soluble and acid-resistant mineral phases. Dotplots highlight the varying analytical precision for each material and each analytical protocol. 

There are probably several mineral phases, particularly for the highly alkaline systems, that remain to be discovered. Mixed hydroxides may control solubility. Calcium zincate (CaZn2(OH)6), for example, is thermodynamically more stable than Zn(OH)2 above pH 11.5 and may be important in cementitious systems. Another group of minerals is that of the hydrotalcite-like minerals, the layered double hydroxides (LDH, M2+2M3+l/yXy (OH)6 where X is an anion). Cobalt, Ni and Zn can form such minerals (Johnson Glasser 2003) under neutral to alkaline conditions. For the majority of species, however, solubility-limiting phases do not appear to control dissolved concentrations. 

If strontianite is assumed to be the limiting phase, significantly more strontium (activity approx. 2.4 10-4 mol/L) could be dissolved compared to that of the solid-solution mineral phase. 

The second most important mineral phase of the biological pump is biogenic opal, Si02. Opal shells are produced by diatoms, a marine alga, and by radiolarians, a heterotrophic protist. On a large scale, production of Si02 is limited by the availability of dissolved sihca, in the form of silicic acid, H4Si04. 

Simple thermodynamic calculations based on literature data (5-12) support the choice of phosphates as the optimum mineral phases for actinide immobilization. The calculations considered every relevant species reported (5-72) that contained protons, hydroxide, or the ligand in question for each metal ion. Where necessary, equilibrium constants were corrected to 0.1 M ionic strength using the Davies equation. As an example, the calculated solubility of europium, thorium, and uranium in various media at p[H] 7.0 (p[H] = - log of the hydrogen ion concentration), 0.001 M total ligand concentration, 0.1 M ionic strength, and 25 °C are shown in Table I. Within the constraints of the calculation, the solubility of thorium is limited by Th(OH)4, but the lowest europium and uranyl solubilities are observed for phosphates. 

---