Solubility controlling phases

Heavy metal cations precipitate readily as hydroxides or carbonates in alkaline media. Dissolved carbonate content will be limited by calcite precipitation or by conversion of hydroxyl AFm to carbonate AFm. Hydroxide ions, on the other hand, are abundant. Here only the solids that may be present under oxic conditions will be discussed. Figure 5a shows the total dissolved heavy metal cation concentrations that would prevail if hydroxide precipitation were to be the dominant solubility-controlling process. Figure 5b shows the solubility of Ca metallate species, as these are likely to act as solubility-controlling phases for oxyanionic species. 

Fig. 9. Possible solubility controlling phases for dissolved concentrations of Ca(II), Al(III), SO4- and Si(IV) in Landfill Lostorf leachate. The symbols represent measured concentrations for different rain events and the curves represent model calculations for the conditions given in Table 1.

Figure 9.13 Temperature dependence of the compositions of illitic solubility-controlling phases in the presence of kaolinite and quartz betweeen 25°C and 250°C. From S. U. Aja and P. E. Rosenburg in Clays Clay Minerals 40(3) 292-99. Copyright 1992. Used by permission.

In figures 11 to 12 the Mo-activity measmed over three waste/soil combinations— neutral ash/sand, neutral ash/peat and acidic ash/sand—are shown. In all cases, except the acidic ash, all Mo added is washed out in the percolation time used. In the acidic ash some 60% of the originally added Mo-activity is effectively retained in the ash. Evidence of the nature of the retained chemical phase is not available, although MoOj/s) might be the solubility controlling phase, since this form is known to dissolve veiy slowly... 

In the ash matrix the molybdate solubility controlling phases are probably MoOgfs),... 

Dang et al. (1994) observed that the experimentally determined solubility lines for Zn2+ in 14 soil solutions from southern Queensland with soil pH from 7.45-8.98 and 0.08-2.07% CaC03 were not undersaturated with respect to the solubility of any known mineral form of Zn. Therefore, they suggested that Zn2+ activity was mainly controlled by adsorption-desorption reactions in these soils. Similar observation on solubility of Cr(VI) in arid soils was reported by Rai et al. (1989). In the absence of a solubility controlling solid phase, Cr(VI) aqueous concentrations under slightly alkaline conditions may be primarily controlled by adsorption/desorption reactions (Rai et al., 1989). Chromuim(VI) is adsorbed by iron and aluminum oxides, and kaolinite and its adsorption decreases with increasing pH. 

Systematic solubility measurements of organometallic transition metal precatalysts were performed in our laboratories as part of the ConNeCat activities [25]. These activities focus on a multi-phase approach, in which product separation is to be achieved by a thermally or pressure-controlled phase separation, while the catalyst is completely precipitated. 

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. 

Fruchter, J. S., Rai, D. Zachara, J. M. 1990. Identification of solubility-controlling solid phases... 

The most frequendy used technique to shift the equilibrium toward peptide synthesis is based on differences in solubility of starting materials and products. Introduction of suitable apolar protective groups or increase of ionic strength decreases the product solubility to an extent that often allows nearly quantitative conversions. Another solubility-controlled technique is based on introduction of a water-immiscible solvent to give a two-phase system. Products preferentially partition away from the reaction medium thereby shifting the equilibrium toward peptide synthesis. 

Currently proposed licensing regulations for geologic nuclear waste repositories require a performance assessment involving long-term predictive capabilities. Previous work (J- 5) has shown the importance of solubility controls for modeling maximum actinide concentrations in repository groundwaters. However, until reliable data are available on the actinide solid phases that may be present or that may precipitate in the environment, the solubility of solid phases such as hydrous oxides that have fast precipitation kinetics can be used to initially set maximum solution concentration limits. 

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