Hydrolysis and adsorption

Hydrolysis and Adsorption. Some years ago, a theory was advanced, that hydrolyzed metal species, rather than free metal ions, are adsorbed to hydrous oxides. The pH-dependence of adsorption (the pH edge for adsorption is often close to the pH for hydrolysis) was involved to account for this hypothesis. As Figs. 2.7b and c illustrate, there is a correlation between adsorption and hydrolysis but this correlation is caused by the tendency of metal ions to interact chemically with the oxygen donor atoms with OH, and with S-OH. The kinetic work of Hachiya et al. (1984) and spectroscopic information are in accord with the reaction of (free) metal ions with the surface. 

Effects of hydrolysis and adsorption on the oxygenation of transition-metal ions. Arrows indicate lower limit. (From Wehrli and Stumm, 1989)... 

The predominant oxidation stale of the element is (V). There is some evidence that the (IV) state is obtained under certain reduction conditions. When the pentapositive form is not in the form of a complex ion it may exist in solution as PaC>2+. The compounds are very readily hydrolyzed in aqueous solution yielding aggregates of colloidal dimensions, thus showing marked similarity to niobium and tantalum in this respect. These properties play a dominant role in the chemical properties of aqueous solution, because the element is so easily removed from solution by hydrolysis and adsorption Protactinium coprecipilates with a wide variety of substances, and it seems likely that the explanation for this lies in the hydrolytic and adsorptive behavior. 

APMS and AAPS isotherms was similar. It was observed that the precision of a single adsorption experiment is very good, but the reproducibility of the entire isotherm is poor. The lack of reproducibility is again probably due to the wide variety of reactions undergone by the silanes. The partial formation of oligomers also makes the hydrolysis and adsorption process rather complicated. 

Since the hydrolysis and adsorption of the OTS occurs with the subsequent layers of water, an optimum level is necessary to form robust films too little water results in the formation of an incomplete monolayer, whereas a thick water layer causes a polymerization of the OTS with the water, resulting in a very poor adherence to the silica surface. 

Figure 10,26 Correlation plot for some metal cations, of their first hydrolysis constants ( /fii) versus intrinsic surface complex constants i Ku) for their adsorption by Si02(am) assuming the constant capacitance model. The equation of the solid line is log = 0.09 -( 0.62 log A. Hydrolysis and adsorption reactions are written A,i -t- H2O = +...

The models describing hydrolysis and adsorption on oxide surfaces are called surface complexation models in literature. They differ in the assumptions concerning the structure of the double electrical layer, i.e. in the definition of planes situation, where adsorbed ions are located and equations asociating the surface potential with surface charge (t/> = f(5)). The most important models are presented in the papers by Westall and Hohl [102]. Tbe most commonly used is the triple layer model proposed by Davis et al. [103-105] from conceptualization of the electrical double layer discussed by Yates et al. [106] and by Chan et al. [107]. Reviews and representative applications of this model have been given by Davis and Leckie [108] and by Morel et al. [109]. We will base our consideration on this model. 

Abstract This chapter explores the role of abiotic reactions such as acid catalysis (hydrolysis) as well as the adsorption of methyl tert-butyl ether (MTBE) and other fuel oxygenates in environmental issues as the remediation of these substances is notoriously difficult. First of all, these methods are briefly classified with other abiotic technologies. The suitability of hydrolysis and adsorption for the remediation of water contaminated by fuel oxygenates is then discussed in detail, with information being provided about the principle of the reactions, potential catalysts and sorbents, limitations of the reactions, and practical implications. To conclude, the possible application of hydrolysis and adsorption in combination with other remediation techniques is also examined. 

The exfenf fo which the processes of hydrolysis and adsorption contribute to the elimination of ether from contaminated water has still to be elucidated. To this end, detailed desorption experiments need to be performed and all the reaction products precisely analysed. 

The abiotic processes based on acid hydrolysis and adsorption have been evaluated for their applicability in the remediation of contaminated water containing fuel oxygenates. 

For both adsorption and coprecipitation mechanisms, the expected correlation between hydrolysis and adsorption (or coprecipitation) is seen in Figs. 3-5. Cr(III), which hydrolyzes at the lowest pH of the three metal ions, was also removed by adsorption and coprecipitation at the lowest pH. Conversely, Ni(II), which hydrolyzes at the highest pH, was removed by adsorption and coprecipitation at the highest pH. 

Tamura M, Sugihara M, Uragami T (1979), Ultrafiltration, hydrolysis, and adsorption characteristics of membranes from cellulose nitrate, urease stylite, activated charcoal , Augeiv Makromol Chem, 79,67-77. 

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