Hydrous oxides of aluminum

There is a complex relationship among the oxide, hydroxide, and hydrous oxide of aluminum. Conversion between several phases is possible as a result of the reactions... 

Purification actually starts with the precipitation of the hydrous oxides of iron, alumina, siUca, and tin which carry along arsenic, antimony, and, to some extent, germanium. Lead and silver sulfates coprecipitate but lead is reintroduced into the electrolyte by anode corrosion, as is aluminum from the cathodes and copper by bus-bar corrosion. 

Since hot add hydrolysis was required to release practically all of the amino adds and amino sugars from the soils, it is likely that the amino adds occur in soils in the form of peptides, polypeptides, and proteins dosely assodated with and protected by SOM and inorganic soil constituents such as clay minerals and hydrous oxides of iron and aluminum. Similarly, amino sugars do not appear to exist in soils as free compounds. 

Kinniburgh, D.G. Syers, J.K. Jackson, M.L. (1975) Specific adsorption of trace amounts of calcium and strontium by hydrous oxides of iron and aluminum. Soil Sci. Soc. Am. J. 39 464-470... 

A sulfuric acid solution of the oxide (25-75% solution) can be reduced with tin, copper, zinc, and other reducing agents forming a blue solution of molybdenum blue which are hydrous oxides of non-stoichiometric compositions (see Molybdenum Blue). Reduction with atomic hydrogen under carefully controlled conditions yields colloidal dispersion of compounds that have probable compositions Mo204(OH)2 and Mo40io(OH)2. Reduction with lithium aluminum hydride yields a red compound of probable composition MosOtIOEOs. Molybdenum(Vl) oxide suspension in water also can be reduced to molybdenum blue by hydriodic acid, hydrazine, sulfur dioxide, and other reductants. 

Inorganic phosphorus Incorporated with easily reducible hydrous manganese oxides is typically less than that found for the hydrous oxides of iron and aluminum. 

Hydrous aluminum oxide is used frequently in chromatographic columns. Hydrous oxides of Ga, In, and the lanthanides may also be employed as inorganic ion exchangers. However, substantive research of these oxides has not been performed. 

Finally, finely divided hydrous oxides of iron, aluminum, manganese, and silicon are the dominant sorbents in nature because they are common in soils and rivers, where they tend to coat other particles. This is the reason why numerous laboratory researchers have been studying the uptake of trace elements by adsorption on hydrous oxides (Dzomback and Morel, 1990). Partition coefficients (concentration in solid/concentration in the solution) for a number of trace elements and a great variety of surfaces have been determined. The comparison of these experimental with natural values should give information on the nature of the material on which trace elements adsorb in namral systems and allow quantitative modeling. 

Figure 2 The fate of phosphorus during soil formation can be viewed as the progressive dissolution of primary mineral phosphorus (dominantly apatite), some of which is lost from the system by leaching (decrease in Ptotai). and some of which is reincorporated into nonoccluded, occluded, and organic fractions within the soil. Nonoccluded phosphorus is defined as phosphate sorbed to surfaces of hydrous oxides of iron and aluminum, and calcium carbonate. Occluded phosphorus refers to phosphorus present within the mineral matrix of discrete mineral phases. The initial buildup in organic phosphorus results from organic matter return to soil from vegetation supported by the soil. The subsequent decline...

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... 

Haseman, J.F., Brown, E.H. and Whitt, C.D., 1950. Some reactions of phosphate with clays and hydrous oxides of iron and aluminum. Soil Sci., 70 257—271. 

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.- ... 

Regardless of the method of treatment, a coagulated colloid is always contaminated to some degree, even after extensive washing. The error introduced into the analysis from this source can be as low as 1 to 2 ppt, as in the coprecipitation of silver nitrate on silver chloride. In contrast, coprecipitation of heavy-metal hydroxides on the hydrous oxides of trivalent iron or aluminum can result in errors as large as several percent, which is generally intolerable. 

Reprecipitation A drastic but effective way to minimize the effects of adsorption is reprecipitation. In this process, the filtered solid is redissolved and reprecipitated. The first precipitate ordinarily carries down only a fraction of the contaminant present in the original solvent. Thus, the solution containing the redissolved precipitate has a significantly lower contaminant concentration than the original, and even less adsorption occurs during the second precipitation. Reprecipitation adds substantially to the time required for an analysis but is often necessary for such precipitates as the hydrous oxides of iron(III) and aluminum, which have extraordinary tendencies to adsorb the hydroxides of heavy-metal cations such as zinc, cadmium, and manganese. 

This hydrolysis proceeds slowly at temperatures just below 100°C, and 1 to 2 hours is needed to complete a typical precipitation. Urea is particularly valuable for the precipitation of hydrous oxides or basic salts. For example, hydrous oxides of iron(III) and aluminum, formed by direct addition of base, are bulky and gelatinous masses that are heavily contaminated and difficult to filter. In contrast, when these same products are produced by homogeneous generation of hydroxide ion, they are dense and easily filtered and have considerably higher purity. Figure 12-5 shows hydrous oxide precipitates of aluminum formed by direct addition of base and by homogeneous precipitates with urea. Homogeneous precipitation of crystalline precipitates also results in marked increases in crystal size as well as improvements in purity. 

Reference has already been made to the fact that activated clays and synthetic compositions containing the hydrous oxides of silicon and aluminum were recognized, long before the development of a practical commercial process, to have catalytic activity toward hydrocarbons. Basically, these are the types of catalysts in commercial use today, although many modifications and improvements over the earlier catalysts... 

The nonlabile fraction of inorganic phosphorus not available to plants is sometimes divided into the occluded and reductant soluble forms. Occluded phosphorus consists of aluminum- and/or iron-bonded phosphates surrounded by an inert coat of another material such as oxides or hydrous oxides of iron or aluminum. Reductant soluble forms are covered by a coat that may be partially or totally dissolved under anaerobic conditions (Uehara and Gillman, 1981). The opportunities for occlusions to occur increase dramatically with soil age (Walker and Syers, 1976). This is because substantial amounts of Fe and Al oxides tend to be present only in heavily weathered soils in which the secondary silicate minerals have already dissolved (Fox et al., 1991). Data from tropical forest chronosequence studies in Hawaii are more or less in accordance with this view the fraction of P present in the occluded form increases with soil age (Crews et al, 1995). Nevertheless, that study also showed high amounts of nonoc-cluded (i.e., labile and accessible) inorganic phosphorus to be present, even in forests growing on the oldest soils. 

Letunova et al. (1987) calculated that the microorganism biomass contains from 0.012 to 3.24% of the iodine present in surface soil layers, though some fungi that occur in soils are known to accumulate much higher amounts of iodine. Soil acids favor iodine sorption by soil components such as organic matter, hydrous oxides of iron and aluminum. However, liming is known to reduce the solubility of iodides, iodates and iodine in soils and thus also to reduce iodine bioavailability. 

Reisenauer, H. M., Tabikh, A. A., and Stout, P. R. (1962). Molybdenum reactions with soils and the hydrous oxides of iron, aluminum and titanium. Soil Sci. Soc. Am. Proc. 26 23-7. 

Soils possjess anion exchange capacities for certain anions that increase as the pH decreases. This property of anion exchange has been shown to be greatest in soils that consist largely of 1 1 clay minerals and hydrous oxides of iron and aluminum the 2 1 clays have a lower anion-retaining capacity. 

The granular metal-oxide adsorbents discussed in this chapter are activated alumina (AAl), modified activated aluminas, iron-oxide-coated sand (IOCS), granular ferric hydroxide (GFH or GEH), and proprietary filter media such as ADI. Although they differ in physical appearance, they all involve hydrous oxides of iron or aluminum that remove arsenic by a process of ligand exchange. Generally, they are employed in packed beds or small filters containing 28 X 48 mesh (0.6-... 

Carriers that have proved effective for coprecipitation are the hydrous oxides of the metals, particularly of iron, other transition metals, and aluminum. Their efficacy is due to their large surface area, gelatinous character, and ability to coagulate. Some distinction among carried ions can be achieved because, as indicated by Table 3.1, ions are precipitated as hydroxides at various pH thresholds. 

J. A. Veith and G. Sposito, Reactions of aluminosilicates, aluminum hydrous oxides, and aluminum oxide with o-phosphate The formation of X-ray and amorphous analogs of variscite and montebrasite. Soil Sci. Soc. Am. J 41 870 (1977). 

The amorphous hydrous oxide of Cr(III) (HCO) has been studied extensively as a source of sols of uniform particle size and shape [30] and as an electrophoresis standard [31] however, it has not been studied extensively as an adsorbent of metal ions. Sen [32] found that chromium (III) hydroxide generally has a larger adsorption capacity than the hydroxides of other triple-charged metals. Simon et al. [33] reported that coprecipitation of Cd(II) with chromium(III) hydroxide removed Cd(Il) from solution at a pH that was two units lower than that obtained using aluminum(III) hydroxide as the coprecipitating hydroxide. Similarly, Packter and Panesar [34] used Cr(III) and Mg(II) mixtures to show enhanced removal for both ions compared with the individual metal systems. HCO has also been used as an adsorbing surface to effectively adsorb the proteins ovalbumin, y-globulin, and lysozyme as a separation technique [351. 

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. 

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