Hydrous oxides solubility effects

We will first describe a relatively simple scenario for the enhancement of the dissolution of Al203 by a (complex-forming) ligand. As we have seen ligands tend to become adsorbed specifically and to form surface complexes with the AI(III) Lewis acid centers of the hydrous oxide surface. They also usually form complexes with AI(III) in solution. Complex formation in solution increases the solubility. This has no direct effect on the dissolution rate, however, since the dissolution is surface-controlled. 

Effect of Aging on the Solubility and Crystallinity of Np(TV) Hydrous Oxide... 

Because of the longer half-life of Np compared with Pu, the alpha radiation effects should be significantly less. Therefore, Np(IV) hydrous oxide is expected to rapidly develop crystallinity resulting in a decrease in solubility and thus a decrease in the maximum predicted solution concentration. The objective of this study were to determine the solubility of Np(IV) hydrous oxide and to determine the effect of aging Np(IV) hydrous oxide on its solubility and crystallinity. 

Metal cations may be soluble, readily exchangeable, complexed with organic matter, or hydrous oxides, substituted in stoichiometric compounds, or occluded in mineral structures (see reviews by Brummer et al., 1986 Beckett, 1989 Forstner, 1991). The chemical factors that affect the retention of a specific chemical form of a trace metal (e.g. effects of pH and I on specific adsorption ) are well documented (Jones and Jarvis, 1981 Tiller, 1983 McBride, 1989 1991 Alloway, 1990 Forstner, 1991). When several components co-exist in a soil, the distribution of a trace metal among them will also depend on the type and relative quantities of the soil components how they change with pH, I, etc. and the extent of saturation of adsorption sites on soil adsorbents. 

This method can be applied to the preparation of single catalysts or, by coprecipitation, to multi-component catalysts. Hydrous oxides, sulphides, carbonates and phosphates can be used. After precipitation, the catalyst must be washed free of impurities which might have an adverse effect on the final catalytic properties. The presence of these impurities can be minimised by the use of dilute solutions. An alternative is the mixing of a metal salt with ammonia or an ammonium salt. Any ammonium nitrate remaining in the precipitate is readily removed by washing because of its high water solubility, and the last traces can be eliminated by calcination. 

From many possible precursors [e.g., metal nitrates or acetates (12), mono-dispersed metal hydrous oxides (549), oxides dissolved in alcohols (12), oxo-alkoxides (550), and alkoxides (12)], metal alkoxides were considered as specially suitable precursors (12, 31, 551) for the preparation of oxide ceramics since the 1950s, mainly due to the ease of their purification. This purification was generally distillation and in some cases crystallization, for example, Zr(0-i-Pr)4(i-Pr-OH), solubility in organic solvents such as the parent alcohols that are miscible with water, and their extremely facile hydrolyzability, which can be modulated effectively by substitution of some of the alkoxide groups with chelating ligands such as (3-diketonates (35, 536). The extraordinary future potential and possibilities of the SG process were reemphasized recently by Sakka (534) and Roy (552) in two review articles. 

It is well known that organic complex-formers, such as simple organic acids (citric, oxalic, tartaric, salicyclic) formed by microorganisms in soils and humic or fulvic acids, solubilize mineral iron(III) and aluminum. These complex-formers not only increase the solubility of these minerals but also are able to form chelates on hydrous oxide surfaces and thus, in turn, catalyze the dissolution of oxides and aluminum silicates (Kummert and Stumm, 1980 Sigg and Stumm, 1980). The downward vertical displacement of A1 and Fe, as it is observed in the podsolization of soils, can be accounted for by considering the effect of pH and of complex formers on both solubility equilibria and dissolution rates (Schnoor and Stumm, 1985 Schnoor and Stumm, 1986). 

PER/RIG] Peretroukhine, V., Riglet-Martial, C., Capdevila, H., Calmon, V., Bienvenu, P., Laszak, I., Effect of soluble silicates on the solubility of thorium(lV) hydrous oxide, J. Nucl. Sci. Technol, Suppl.3, (2002), 516-519. Cited on pages 379, 673. 

Such precipitation or colloidal behavior occurs under conditions that would be expected if the radionuclide hydrolyzed to form a hydrous oxide at higher concentration than inferred from radiation measurements. At a very small solubility product Ksp (see Section 3.1), any unknown small amount of stable ion in solution may be sufficient to cause such an effect. Coprecipitation of trace-level radionuclides with another insoluble ion, such as Ra+ with BaS04 and Pu+ with LaFs, incorporates the radioactive atoms within the crystal structure in various ways or sorbs it on particle surfaces (Kolthoff 1932). A precipitate such as Fe(OH)3 in neutral or slightly basic solution can scavenge from solution many tracer-level radionuclides that hydrolyze under the conditions of the procedure (see Table 3.1 for the effect of pH). 

If the modifying agent is in the form of the elemental metal or the metal oxide, hydrous oxide, hydroxide, or other insoluble compound, no problem of coagulation of the silica particles will ordinarily be encountered when it is added to the sol. However, if the metal modifier is in the form of a soluble salt, it it usually desirable to introduce it into the silica sol immediately before the drying step so that the extent to which it can promote coalescence of the silica is minimized. Since the proportion of metal compound is generally quite small the coalescence effect in any event is not very pronounced. 

Strickert, R. G. Rai, D. Fulton, R. W. "Effects of Aging on the Solubility and Crystallinity of Np(IV) Hydrous Oxide." In Geochemical Behavior of Disposed Radioactive Waste American Chemical Society Symposium Series 246 ACS Washington, DC, 1984 pp 135-145. 

Example 7.7. Effect of EDTA on the Solubility of Fe(OH)3(s) EOT A (ethyl-enediaminetetraacetate) is a very powerful complex former, a hexadentate (two nitrogen donor atoms and four oxygen donor atoms from the four carboxyl groups). A comparison of Figure 7.7a with Figure 6.8b shows that EDTA dissolves hydrous ferric oxide, under the conditions given, up to pH =9. It is important however, to consider that in the presence of an excess of Ca ", as typically occurs in natural waters, CaEDTA is formed. 

Lead compounds such as lead naphthalene tetracarboxylate, lead pyrophosphate, lead stannate, lead sulfide, and others which are not listed in Table 3, were proved to be effective uranium sorbents in sea water 131). However, sorbent materials containing toxic metals must be excluded with respect to sea contamination which would be inevitable even in the case of slight solubility. On the other hand, many metal compounds are decomposed in sea water by hydrolysis and carbonate formation. Among the inorganic sorbents presented in Table 3, hydrous titanium oxide and... 

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

PROBABLE FATE photolysis not important oxidation Pb (IV) readily reduces to Pb (II), solubility control by PbS04 low pH and by PbCOs high pH hydrolysis not important pH <11.5 volatilization importance of volatilization of (CH3)4Pb unknown in natural condition sorption Pb removed to sediments effectively by inorganic solids, hydrous iron oxides and crystalline structures biological processes bioaccumulation by aquatic organisms and biomethyl ati on by microbes... 

A film of. colloidal silica on the surface of fibers, as in carpets, greatly reduces the pickup of dirt and leaves a cleaner appearance after vacuuming. It has been postulated that the silica forms a smooth adherent film to which soil particles do not cling, especially because it fills crevices in the fiber surface that would otherwise be filled with dark particles of dirt. Similar effects are reported on painted surfaces, plastic fabrics, window shades, and wallpaper (575). This type of use for colloidal silica was patented by Cogovan and Frederic (576). The advantage of using two hydrous colloidal metal oxides together is claimed by Florio and Rainard (577). Soluble aluminum phosphate is claimed to improve the adhesion of the silica to the fiber (578). 

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