Adsorption isotherms surface oxygen complexes

Surface Oxygen Complexes Effects on Adsorption Isotherms... 

Surface polarity is of major concern with activated carbons particularly when used for adsorptions from aqueous solution. This can be assessed by water isotherms, but the process is tedious when compared to the use of an immersion calorimeter. The variation of enthalpies of immersion with coverage by surface oxygen complexes is as in Figure 5.47 which indicates the rapid increase in AHj from 32 J g , for a clean surface, to a maximum of 75J g for carbon surfaces possessing surface oxygen complexes, for steam activated olive stones (Section 4.7.4). 

Currently, because of the frequency of their occurrence in wastewaters, the adsorption of phenolic compounds on carbons and the influence of surface oxygen complexes on their uptake are the most frequently studied, Radovic et al. (1997). It is well established that an increase in surface acidity of AC, after an oxidation, causes a decrease in phenol adsorption from dilute aqueous solution. For example. Figure 8.8 shows the adsorption isotherms of phenol on oxidized carbons, Mahajan et al. (1980). There was a large decrease in phenol uptake after oxidation, the effect of oxidation not being trivial. This phenol uptake progressively increased as the surface acidity decreased, and the oxidized sample (heat treatment temperature (HTT) 950 °C) had the same adsorption capacity as the original carbon (about 1.5 xmolg" ). 

Fig. 5.12 (a) Water adsorption isotherms at 20°C on Graphon activated to 24-9 % burn-off, where its active surface was covered to varying extents by oxygen complex. (b) The results of (a) plotted as amount adsorbed per of active surface area (left-hand scale) and also as number of molecules of water per atom of chemisorbed oxygen (right-hand scale). (After Walker.)... 

Puri and coworkers 45 studied the adsorption of phenol and p.nitrophenol from aqueous solutions on a number of activated carbons and carbon blacks at low and moderate concentrations, and found that the adsorption was partly reversible and partly irreversible. At moderate concentrations, there was a small irreversible adsorption when phenol concentration was 0.12 M in the case of carbons associated with greater than 1.5% oxygen, which they attributed to the complexation of n electrons of the benzene nucleus with the carbonyl groups present on the carbon surface. The irreversible adsorbed amount, however, was only 3 to 4% of the total adsorption. The adsorption isotherms of reversibly adsorbed phenol for different activated carbons, and carbon blacks (Figure 7.11) were almost similar. In the case of carbon blacks, the isotherms showed a well-defined plateau followed by a distinct rise, indicating completion of the monolayer and starting of a second layer. However, in the case of activated carbons, there was no indication for the commencement of the second layer, although the formation of the monolayer was completed at about the same concentration of the phenol solution as in the case of carbon blacks. 

The adsorption of an inhibitor onto the metal surface slows the rate of corrosion by blocking part of the surface. The extent of inhibition depends on the equilibrium between the dissolved and adsorbed inhibitor species, expressed by the adsorption isotherm. This mechanism which is particularly important in acids will be discussed in the next section. (Sect. 12.4.2). Certain inhibitors promote the spontaneous passivation of a metal and thus drastically reduce the corrosion rate. Oxidizing species such as chromates fall in this category. Buffer agents that maintain a high pH at the metal surface also favor the passive state. Other inhibitors lead to the formation of surface films by precipitation of mineral salts or of weakly soluble organic complexes. These films reduce the ability of oxygen to reach the surface and, in addition, they may impede the anodic dissolution reaction. 

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