Solubility limit phases

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. 

Figure 6 Eh-pH diagram for the Fe0-H2O system where total dissolved Fe = lxlCT6 M, Fe304 and Fe203 are assumed to be the solubility limiting phases, and [ox] = [red] for all redox active species. Other Eh-pH diagrams for Fe0-H2O-contaminant systems can be found in Refs. 42, 84, and 129-131.

Other alkaline earth fluorides (BaF2, SrF ) have been added to the model. However, they are less likely than their respective sulfates or carbonates to be solubility limiting phases. 

Actinide Solubility (M) Solubility-limiting phase Dominant aqueous phases... 

In the WIPP performance assessment calculations, it was assumed that redox conditions would be controlled by the presence of metallic iron and that U(VI) would be reduced to U(IV). An upper bound for the solubility of U(IV) was estimated from that of Th(IV), using the oxidation state analogy. Calculations by Wall et al (2002) suggest that this is a conservative assumption— that the solubility of U(IV) is much lower than that of Th(IV). This is because the solubility product constant of Th02(am)> the solubility limiting phase in the An(IV) model, is several orders of magnitude greater than that of U02(am)-... 

Figure 7 Calculated Np solubilities as a function of pH and Eh in J-13 groundwater variants (Table 5). Np205(g) and Np(OH)4(s) were assumed to be the solubility-limiting phases. Inset shows regions of solubility control versus redox control (shaded area) (Kaszuba and Runde, 1999) (reproduced by permission of American Chemical Society from Environmental Science and Technology 1999, 33, 4433).

Novak C. F. (1997) Calculation of actinide solubilities in WIPP SPC and ERDA-6 brines under MgO backfill scenarios containing either nesquehonite or hydromagnesite as the Mg-COs solubility-limiting phase. Sandia National Labs, WIPP Records Center. 

Figme XI-6 shows the stability field of Na6[Th(C03)5]12H20(s) compared to Th02(am, hyd), which is found to be the stable solubility limiting phase in many other solubility studies [19940ST/BRU], [1995R7H/FEL], [1997FEL/RAI],... 

The various kinds of solubility-limiting phase boundaries that exist in binary aqueous surfactant systems will now be considered. There are three classes of such boundaries. 

Cubic phases are often the solubility-limiting phase above the Krafft eutectic of monoglycerides [84] and possibly of other polyol surfactants. It is claimed that these cubic phases have an inverted (head group in) structure [85]. Often, they coexist with a more concentrated bicontinuous cubic phase [86] which lies next to the lamellar phase. Cubic phases are the solubility-limiting phase in the iV-acyl-JV-methylglucamines, but only at temperatures above the Ejafft eutectic (see later). Just above the Krafft eutectic, the solubility boundary of this surfactant is defined by the familiar hexagonal phase. 

In compounds that have relatively small and weakly hydrophilic functional groups, the lamellar phase is very often the solubility-limiting phase. The same is true of many surfactants containing two or more lipophilic groups—irrespective of the hydrophilicity of the fiindlional groups present. Catanionic surfactants (ionic surfactants in vduch-both ions are amphiphilic [87]) are also reported to display the lamellar liquid-crystal solubility boundary. Dioctadecylammonium cumenesulfonate (mentioned earlier) may be regarded as a cationic surfactant in which the anion has a hydrotropic molecular structure [78]. It displays a very unusual lamellar liquid-crystal solubility boundary above the Krafft eutectic temperature. 

Figure 6. Effect of increasing iminodiacetic acid and glyphosate concentrations on the reduction potential for the Fe (aq) + e Fe (aq) half reaction. Reaction conditions l.OxKT MFe 1.0x1MFe 10 mM NaNOs constant ionic strength medium. Fe(OH)s(amorphous) serves as the solubility-limiting phase for Fe The dashed line corresponds to the reduction potential in the absence of added ligand.

Figure 7. Speciation of 1.0x10 Min the presence of LOxlOf MNTA and LOxlCf MNTMP (10 mMNaNOs constant ionic strength medium). Fe(OH)3(amorphous) is used as the solubility-limiting phase. logK values for F -NTMP complexes were obtained from ref (72).

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