Hydrous oxides colloidal precipitates

Controlled hydrolysis is one of the most popular methods for processing silica spheres in the range of 10-1,000 nm. The method was developed by Stober, Fink, and Bohn (SFB) [226-229] and is based on the hydrolysis of TEOS in a basic solution of water and alcohol. Particle size depends on the reactant concentration, i.e., the TEOS/alcohol ratio, water concentration, and pH (>7). This method has been extended to other metal oxide systems with similar success, particularly for Ti02 synthesis [85,230]. The hydrous oxide particles precipitated by the hydrolysis of an alkoxide compound have the same tendency to agglomerate as that described for metal colloid systems. Different stabilizers can be used to stabilize these particles and prevent coagulation (step 2). These stabilizers control coagulation by electrostatic repulsion or by steric effects [44], similarly to the metal colloid systems. 

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

If a surface precipitate of metal hydroxy-polymer has formed on an adsorbent, the -pH relationship for the coated adsorbent should resemble closely that observed for particles consisting purely of the hydroxy-polymer or the hydrous oxide of the metal (15). This kind of evidence for Co(ll), La(lII), and Th(lV) precipitation on silica colloids was cited by James and Healy (15). It should be noted, however, that the increase in C toward a maximum value often occurs at pH values well below that required thermodynamically to induce bulk-solution homogeneous precipitation of a metal hydrous oxide (15, 16). If surface precipitation is in the incipient stage under these conditions, it must be a nucleation phenomenon. James and Healy (15) argue that the microscopic electric field at the surface of a charged adsorbent is sufficiently strong to lower the vicinal water activity and induce precipitation at pH values below that required for bulk-solution precipitation of a metal hydrous oxide. 

In the 1 to 2 pH range still other types of oxo species may be formed. At pH > 2 more condensed species and colloidal gels are formed leading to precipitation of the red brown gelatinous hydrous oxide. 

Hydrous oxides are commonly precipitated when NaOH is added to aqueous metal solutions, but they are difficult to free from alkali ions and sometimes readily become colloidal. When freshly precipitated, they may be soluble in acids, but only with great difficulty or not at all after aging. 

Adsorption is the principal source of contamination of precipitates that have large surfaces, for example, flocculated colloids (metal sulfides, silver halides, hydrous oxides). The extent of adsorption may be relatively small, as it usually is with silver halides, or severe, as it often is with hydrous oxides. 

Chang PL, Yen FS, Cheng KC, Wen HL (2001) Examirratiorrs on the critical and primary crystallite sizes during 0-to a-phase transformation of rrltrafine alumina powders. Nano Letters 1 253-261 Charlet L, Manceau A (1992) X-ray absorption spectroscopic study of the sorption of Cr(IIl) at the oxide/water interface. 11 Adsorptiorr, coprecipitation and surface precipitation on ferric hydrous oxides. J Colloid Interface Sci 148 425-442... 

Unfortunately, many precipitates cannot be formed as crystals under practical laboratory conditions. A colloidal solid is generally encountered when a precipitate has such a low solubility that S in Equation 12-1 always remains negligible relative to Q. The relative supersaturation thus remains enormous throughout precipitate formation, and a colloidal suspension results. For example, under conditions feasible for an analysis, the hydrous oxides of iron(lll), aluminum, and chromium(III) and the sulfides of most heavy-metal ions form only as colloids because of their very low solubilities.- ... 

M Precipitates that have very low solubilities, such as many sulfides and hydrous oxides, generally form as colloids. 

McFadyen, P., and Matijevic, E., Precipitation and characterization of colloidal copper hydrous oxide sols, J. Inorg. Nucl. Chem., 35, 1883. 1973. 

Over the decades that have passed since La Mer s work numerous examples of monodispersed particles of various composition, morphologies and properties, as well as methods for their preparation (not limited to condensational formation), were described in the literature. Extensive studies in this area were carried out by E. Matijevic and T. Sugimoto. Examples of monodisperse systems formed by precipitation from homogeneous solutions include dispersions of uniform particles of simple composition having different morphologies, such as metal halides, sulfides, phosphates, (hydrous) oxides, etc, various composite particles, including particles of internally mixed composition and coated particles. Both crystalline and amorphous materials can be obtained. Electron micrographs of some characteristic examples of monodispersed colloids are shown in Fig. IV-14. 

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). 

Karthikeyan, K. G., H. A. Elliott, and J. Chorover. 1999. Role of surface precipitation in copper sorption by the hydrous oxides of iron and aluminum. Journal of Colloid and Interface Science 209, no. 1 72-78. doi 10.1006/jcis.1998.5893. 

As pH rises, the metal content of drainage water tends to decrease. Some metals precipitate directly from solution to form oxide, hydroxide, and oxy-hydroxide phases. Iron and aluminum are notable is this regard. They initially form colloidal and suspended phases known as hydrous ferric oxide (hfo, FeOOH n O) and hydrous aluminum oxide (HAO, AlOOH nH.2O), both of which are highly soluble under acidic conditions but nearly insoluble at near-neutral pH. 

A number of reagents containing oxide components are used in zeolite manufacture [19]. Silica is provided by addihon of sodium or other alkali silicate solutions, precipitated, colloidal, or fumed silica, or tetraalkylorthosihcate (alkyl = methyl, ethyl) and certain mineral silicates such as clays and kaolin. Alumina is provided as sodium aluminate, aluminum sulfate soluhon, hydrous aluminum oxides such as pseudo boehmite, aluminum nitrate, or aluminum alkoxides. Additional alkali is added as hydroxide or as halide salts, while organic amines and/or... 

Charlet, L. Manceau, A.A. (1992a) X-ray absorption spectioscopic study of the sorption of Cr(III) at the oxide/water interface. II. Adsorption, coprecpitation, and surface precipitation on hydrous ferric oxide. J. Colloid Interface Sd. 148 443-458 Charlet, L. Manceau, A.A. (1992) X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface. J. Colloid Interface Sd. 148 425-442 Chatellier, X. Fortin, D. West, M.M. Leppard, G.G. Ferris, F.G. (2001) Effect of the presence of bacterial surfaces during the synthesis of Fe oxides by oxidation of ferrous ions. Fur. J. Mineral. 13 705-714 Cheetham, A.K. Fender, B.E.F. Taylor, R.I. (1971) High temperature neutron diffraction study of Fei. O. J. Phys. C4 2160-2165 Chemical Week (1988) Glidderfs anti rust secret is out." 15 10... 

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

The sol-gel process was first used for the preparation of silicates used in phase equilibrium studies. Often the sol-gel process makes use of a concentrated hydrous sol, a colloidal dispersion of (hydrated) oxide particles produced by controlled precipitation. It is also a precipitation process that makes use of immobilization of ions in a gel or glassy structure. 

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