Aluminum hydrolysis

Table 1 Hsts a number of common inorganic coagulants. Typical iron and aluminum coagulants are acid salts that lower the pH of the treated water by hydrolysis. Depending on initial raw water alkalinity and pH, an alkah such as lime or caustic must be added to counteract the pH depression of the primary coagulant. Iron and aluminum hydrolysis products play a significant role in the coagulation process, especially in cases in which low turbidity influent waters benefit from the presence of additional colHsion surface areas.

The earlier work of Brosset (4), among others, indicates that formation of soluble aluminum hydrolysis products must be a rather slow process, even at the higher temperatures used in the laboratory investigations. For the solid phases such as Al(OH)3 and Al>0 3H >0, the aging process is also known to be slow, at least at low temperatures (9). 

With these kinetic factors in mind, it is still of interest to attempt to get some idea of the possible soluble species and their relationship to aluminum solubility. The data for polymeric aluminum hydrolysis products are those for 50°C. reported by Biedermann (2), who has described the two reactions... 

Over the last three decades, two general approaches have been proposed in the literature for describing the interactions of sulfate in soils. The first approach is that of a chemical nature where thermodynamic interrelationships with speciation of cations and anions present in soil solution and the interaction with the soil surface are the major mechanisms. These models may be referred to as chemical models. Examples of such models include that of Cosby et al. (1986), Reuss and Johnson (1986), De Vries et al. (1994), among others. A common feature of these models is that both ion exchange and aluminum hydrolysis reactions are similar. Their capability of quantifying these processes varies according to whether the interactions are... 

Hem, J. D., and Roberson, C. E. (1988) Aluminum Hydrolysis Reactions and Products in Mildly Acidic Aqueous Systems. In Chemical Modeling of Aqueous Systems, Vol. II, D. C. Melchior and R. L. Bassett, Eds., ACS Series 416, Washington, DC. 

In the field of transition metal catalysis, zeolites may offer opportunities for uniform active sites. With the discovery of both aluminosilicate and aluminophosphate, zeolites with a variety of transition-metal ions in tetrahedral firework positions may offer new possibilities. On the basis of existing zeolite chemistry dealing vrith aluminum hydrolysis and the formation of adsorption adducts in the zeolite pores, chemists may envision strategies aimed at the activation of tetrahedral transition metal ions, either by lattice oxide replacement or by the application of strong donor ligands. The demonstrated... 

As discussed above, the presence of AI13 is usually identified via a +62.5 ppm peak in the Al-NMR spectra that arises from the central Al(0)4 site in the s-Keggin-like AI13 structure. Fu et al. (1991) and Nazar et al. (1992) noticed that additional peaks for the A1(0)4 occur in the Al-NMR spectra when AI13 solutions were reacted for extended periods of time at temperatures of 85-90°C and polymerized by titration with base. Peaks in the Al-NMR spectra appear at +64.5, +70.2, and +75.6 ppm as a peak near 0 ppm increases in intensity. The 0 ppm >eak arises from the monomeric aluminum hydrolysis complexes, mostly Al + Al(OH) at 4 < pH < 5. The peak at +64.5 ppm is weak and transient and that at 75.6 ppm only appears after extended periods of reaction. The dominant peak is at 70.2 ppm. 

ALUMINUM SOLUBILITY IN SOILS 5.2a. The Chemistry of Aluminum Hydrolysis... 

From the aluminum hydrolysis work and electron microscopy, several conclusions can be reached relative to the behavior of aluminum in acid aqueous systems. These conclusions are ... 

Aluminum Hydrolysis Reactions and Products in Mildly Acidic Aqueous Systems... 

An understanding of the mechanisms of aluminum hydrolysis and the formation of crystalline species of aluminum hydroxide has been viewed as important in various fields of pure and applied chemistry, biochemistry, and geochemistry. In part, this interest results from the unique properties of certain hydrolysis species of aluminum that appear to be present as polymeric or polynuclear macro-ions. These ions have a strong positive charge and may interact with specific charge sites on surfaces they encounter. The polymeric species also may grow by accretion, and they may persist melastably for months or years under some conditions (1). 

Aluminum hydrolysis reactions arc particularly sensitive to pH and temperature, and to the concentrations of other ligands that might compete with OH in complex formation. The most important inorganic competitor is F (2). Certain organic ligands also can form strong complexes with aluminum (3). Hydrolysis reactions of aluminum also arc influenced by sulfate, and hydroxysulfate solids may control aluminum solubility in acidic solutions (4). 

This paper presents results of some open-system laboratory experiments in which aluminum hydrolysis behavior was studied in detail between pH 4.75 and 5.20, at 10°, 25°, and 35°C. Results arc compared with those of our earlier work and that of others on the hydrolysis of aluminum in dilute solutions below the pH of minimum aluminum solubility. 

Table III. Aluminum Hydrolysis Equilibrium Constants at Various Temperatures...

A later study by DeHek et al. (20) explored the effect of SO4 ions on the precipitation mechanism. These investigators suggested that SO4 could form surface groups on the Alb structures that catalyze the formation of precipitates. The role of SO4 in aluminum hydrolysis in natural systems also was thought by Nordstrom (4) to be an important one. 

Removal of color and turbidity caused by particulate matter is commonly accomplished, during the treatment of water for municipal and industrial purposes, by adding a solution of Al to the raw water in a holding tank or mixing facility while maintaining the pH at a level favorable for aluminum hydrolysis. The particulate material in the raw water is coagulated by the aluminum hydrolysis species and can be removed by filtration. Much of the literature on aluminum hydrolysis is directly or indirectly related to the chemistry involved in this process. 

A commonly observed effect of acidic precipitation is an increased concentration of aluminum in water of streams and lakes. Water of lakes that received acidic runoff may increase in transparency (26) owing to coagulation and settling of suspended organic and inorganic particulates. This effect is probably related to aluminum hydrolysis, as it is the same as the coagulation step in water-supply treatment cited earlier. 

Many workers have tended to restrict their interpretations to thermodynamic approaches. The application of thermodynamics to aqueous aluminum chemistry, however, may be restricted as follows (a) In many cases, a true equilibrium state may not have been reached, yet the rate of equilibration is too slow to be detected under ordinary laboratory conditions. (b) The position of equilibrium and/or the rate of equilibration may vary with the nature and concentration of the counter anions present, yet in most studies of aluminum hydrolysis the role of counter anions has not been considered. Therefore, the conclusion derived under one condition is not necessarily valid under another condition in which diflFerent counter anions are present. A failure to recognize these possible... 

The preparation and application of practical catalysts usually require exposure to thermal or hydrothermal conditions that induce some degree of framework cation hydrolysis. In the case of zeolites, the hydrothermal manipulation of the aluminum between crystal framework and extra framework sites is the preferred method to optimize zeolite acidity and catalytic performance. The chemistry of these materials is complex. Namely, the change from framework aluminum to nonframework-aluminum species affects the intrinsic acidity of the remaining framework aluminum sites. In addition, the nonframework aluminum usually displays a catalytic activity of its own. Therefore, the interpretation of catalytic data obtained with such catalysts requires a detailed knowledge of the crystal chemistry, including the amorphous debris formed from framework aluminum hydrolysis. 

Figure 1.5 Distribution of aluminum hydrolysis products (x,y) at ionic strength =...

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