Amphoteric behaviour of metal oxides

Behind the general parameters (viscosity, transmembrane pressure, temperature, flow velocity) which can influence cross-flow filtration with ceramic membranes two aspects must be considered to be more specific of this sort of membrane. One is related to the geometry (tubular multichannel or honeycomb) found for the major part of commercially available membranes, the other is the amphoteric behaviour of metal oxides used in the preparation of these ceramic membranes. 

The amphoteric behaviour of metal oxides in contact with water has thoroughly been described by many authors [22-24]. This basic property results in charged surfaces depending on pH condition. In a first approximation, connected porosity in ceramic membranes can be represented by an array of... 

Fig. 12.9. Mechanism of charged surface formation due to amphoteric behaviour of metal oxides.

Ellis Wilson (1991, 1992) examined cement formation between a large number of metal oxides and PVPA solutions. They concluded that setting behaviour was to be explained mainly in terms of basicity and reactivity, noting that cements were formed by reactive basic or amphoteric oxides and not by inert or acidic ones (Table 8.3). Using infrared spectroscopy they found that, with one exception, cement formation was associated with salt formation the phosphonic add band at 990 cm diminished as the phosphonate band at 1060 cm" developed. The anomalous result was that the acidic boric oxide formed a cement which, however, was soluble in water. This was the result, not of an add-base readion, but of complex formation. Infrared spectroscopy showed a shift in the P=0 band from 1160 cm" to 1130 cm", indicative of an interaction of the type... 

The corresponding As species in acid solution are less stable than those of P due to the effects of the 3d contraction. Further down the group, Sb and Bi show more metallic behaviour, with the two positive ions indicated in Table 6.11, and Sb shows amphoteric behaviour the + 3 oxide reacts with sodium hydroxide solution. 

There are many indications in the literature that surface chemistry plays an important role in photoelectrochemical reactions at extended electrodes and at particles. There are, however, only a few quantitative investigations on this problem (see e.g. [12, 57, 58]), probably due to the lack of sufficiently sensitive methods. In the case of metal oxide particles, the adsorption of H2O plays already an important role. Due to the amphoteric behaviour of most metal hydroxides, two surface equilibria have to be considered [18] ... 

One criterion for classifying an element as a metal, a non-metal or a metalloid is the acid-base behaviour of the oxide. In general metals form basic oxides, non-metals form acidic oxides and metalloids form amphoteric oxides. Basic oxides react with acid, acidic oxides react with bases and amphoteric oxides react with both acids and bases. 

As a general conclusion to this part dedicated to nanofiltration with ceramic membranes one can assume that the general behaviour of these membranes can be assimilated to the behaviour of electrically charged organic nanofiltration membranes. However some specificities exist with ceramic nanofilters due to a sintered metal oxide grains derived porous structure and an amphoteric character... 

Studies of the speciation of actinides in environmental waters are made difficult by the very low concentrations involved and the possibility that minor, undetected contaminants may dominate the binding of a particular metal ion. The environmental behaviour of the actinides has been reviewed. Americium and thorium exhibit simpler behaviour than other actinides since their oxidation states under such conditions are limited to Am and Th. Both are readily adsorbed by granitic rocks and tend to exhibit low solubilities, The thermodynamic solubility product of amorphous Am(OH)3 has been measured as log = 17.5 0.3 and no evidence for amphoteric behaviour or the formation of Am(OH)4 was found below pH 13. Stability constants for the binding of Am to humic acid have been found to vary with the degree of ionization, a, and were given by log = 10.58a -1-3.84 and log 2 = 5.32a -b 10.42. These were larger than the corresponding values for Eu. Humic acids also bind Th as described in Section 65.2.1. 

The insoluble oxides of aluminium and silicon show their acidic nature by reacting and dissolving in an alkaline solution, such as hot, concentrated sodium hydroxide solution, forming a soluble salt. This behaviour is typical of a covalently bonded oxide. However, aluminium oxide also reacts and dissolves in acidic solutions, forming a soluble salt - behaviour typical of a basic metal oxide with ionic bonding. This dual nature provides evidence that the chemical bonding in aluminium oxide is not purely ionic nor purely covalent, hence its amphoteric nature. 

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