Metal hydrous oxide particles

Since the hydrolysis of each metal ion depends on the concentration, pi 1. and temperature of the solution, both the composition and the rate of formation of the resulting solutes are affected by all these parameters. In turn, the natures of the final metal (hydrous) oxide particles are determined by the speeialion of the hydrolyzed intermediates. 

Fig. 1.5.11 Scanning electron micrograph (SEM) of mixed metal hydrous oxide particles generated from mixed Ti(OEt)4 and A1(a -OBu)3 vapors at flow rate of 1.51 dm3 min-1 and boiler temperatures of 75°C and I25°C. (From Ref. 71.)...

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. 

Processes controlling metal ion attenuation in add mine drainage streams. Geochim. Cosmochim. Acta 47 1957-1973 Charlet, L. A. Manceau (1993) Structure, formation, and reactivity of hydrous oxide particles Insights from x-ray absorption spectroscopy. In Buffle, J. van Leeuwen, H.P. 

Subsequent to these early studies, several techniques have been developed to produce well-dispersed metal (hydrous) oxides of different chemical compositions (single or biphasic), consisting of uniform particles in a variety of shapes, including spheres. These efforts are certainly justified in view of the ever-increasing recognition of the importance of such materials in numerous applications in various areas of modern technology and medicine. 

It is noteworthy that a rather convenient approach to produce monodispersed metal (hydrous) oxides is to hydrolyze metal alkoxides. As described elsewhere in this volume, this process is rather efficient, but it does involve the use of expensive chemicals and organic solvents. Furthermore, the hydrolysis rates of some metal alkoxides are so rapid as to preclude the preparation of uniform particles. 

This chapter summarizes the present state of the art of the forced hydrolysis approach by considering specific cations, particularly those of greatest practical and theoretical interest, using aqueous solutions of common salts. In addition to being economical in the manufacture of different products, the described procedure can also help in the development of a better understanding of different processes, such as corrosion of metals or formation of minerals, to mention a few. It should be emphasized that the focus of this chapter is on dispersions of narrow particle size distributions, normally designated as monodispersed systems. While a number of genera reviews have been published on monodispersed colloids (7,9-21), this chapter specifically addresses the problems related to metal (hydrous) oxides. 

The forced hydrolysis approach can be employed for the generation of monodispersed simple or internally composite metal (hydrous) oxides. In the latter case, the resulting particles are mostly internally inhomogeneous. 

Despite the fact that the hydrolysis of the ferric ion is exceedingly sensitive to various experimental parameters (temperature, pH, etc.), hematite (a-Fe203) and akageneite ((3-FeOOH) were apparently the first reasonably uniform colloidal metal (hydrous) oxides dispersions reported in the literature, as already indicated in the introduction. Since then, this family of compounds has been the most extensively investigated, with specific emphases on particle uniformity, composition, and morphology. 

Various metal alkoxides are ideal starting materials for the preparation of metal (hydrous) oxides by the described aerosol techniques, because many of these compounds are in the liquid state at room temperature, easily vaporized, and exceedingly reactive with water vapor. Additional advantage is the purity of the resulting powders, because the only products of the chemical reactions are the metal (hydrous) oxide and alcohol. The particles are, therefore, free of impurities, such as various ions, normally present in solids prepared from different salts. 

Chang, H.C.. Healy, T.W., and Matijevic, E., Interaction of metal hydrous oxides with chelating agents. III. Adsorption on spherical colloidal hematite particles,... 

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. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

A sample may be characterized by the determination of a number of different analytes. For example, a hydrocarbon mixture can be analysed by use of a series of UV absorption peaks. Alternatively, in a sediment sample a range of trace metals may be determined. Collectively, these data represent patterns characteristic of the samples, and similar samples will have similar patterns. Results may be compared by vectorial presentation of the variables, when the variables for similar samples will form clusters. Hence the term cluster analysis. Where only two variables are studied, clusters are readily recognized in a two-dimensional graphical presentation. For more complex systems with more variables, i.e. //, the clusters will be in -dimensional space. Principal component analysis (PCA) explores the interdependence of pairs of variables in order to reduce the number to certain principal components. A practical example could be drawn from the sediment analysis mentioned above. Trace metals are often attached to sediment particles by sorption on to the hydrous oxides of Al, Fe and Mn that are present. The Al content could be a principal component to which the other metal contents are related. Factor analysis is a more sophisticated form of principal component analysis. 

Matijevic, E. Scheiner, P. (1978) Ferric hydrous oxides sols. III. Preparation of uniform particles by hydrolysis of Fe(III)-chIoride, -nitrate, and -perchlorate solutions. J. Colloid Interface Sd. 63 509—524 Matijevic, E. (1980) Colloid chemical aspects of corrosion of metals. Pure Applied Chem. 52 1129-1193... 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Waste constituents may be immobilized in a soil system mainly by sorption and/or partitioning. Adsorption on soil particles is competitive, pH-dependent and, usually, inversely proportional to the solubility of the compound in water. Dry soils are better adsorbents than wet ones. HSs are able to form complexes with metal ions and hydrous oxides and also interact with minerals and a variety of organic compounds, including alkanes, fatty acids, dialkyl phthalates, pesticides and herbicides, and may therefore increase the concentration of these constituents in soil and natural waters. 

Particles—because of their high surface areas—are scavengers for metal ions and often are reactive elements in their transport from land to rivers and lakes and from continents to the floor of the oceans. Hydrous oxide and aluminum... 

Distribution coefficients based on adsorption equilibria are independent of the total concentrations of metal ions and suspended solids, as long as the metal concentrations are small compared with the concentration of surface groups. Examples of the Kq obtained from calculations for model surfaces are presented in Figure 10.14. A strong pH dependence of these Kq values is observed. The pH range of natural lake and river waters (7-8.5) is in a favorable range for the adsorption of metal ions on hydrous oxides and organic particles. 

The production of catalyst particles of suitable configuration and hardness is an essential part of catalyst manufacture . Most heterogeneous catalysts are produced by processes that involve formation of solids from aqueous solutions. Precipitation is frequently employed in preparation of hydrous oxide catalysts. To avoid occluded or adsorbed impurities, ammonia or ammonium salts are often used as well as nitrates of the desired metal constituents. Calcination removes the nitrogen-containing components. Anions such as Cl or 864 or cations such as Na are avoided these often are poisons if they are present in the final catalyst. 

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