Carbon-Oxygen Surface Groups

Information about the number of primary adsorption centers of activated carbons, oxygenated surface groups, can be obtained form the analysis of the enthalpy of carbon immersion in water. The relationship between these two quantities was proposed by Stoeckli and co-workers [304]. 

Bansal et al. also studied the decomposition of carbon-oxygen surface groups formed on low temperature oxidation of ultra clean surfaces of activated graphon using a mass spectrometer and observed that both CO2 and CO were primary... 

Several spectroscopic techniques have been used to investigate the presence of carbon-oxygen surface groups on carbons. Infrared (IR) and electron spin resonance (ESR) are the two most commonly used spectroscopic techniques, although in more recent years, x-ray photoelectron spectroscopy (XPS) has been increasingly applied for the... 

Thus, it is apparent that active carbons, charcoals, carbon blacks, and carbon films before and after several different treatments such as oxidation, degassing, treatment with alkalies, adds, and methylation have been examined by different IR techniques, and meaningful results have been obtained, which help in the identification of carbon-oxygen surface groups. These results have been well reviewed. ... 

INFLUENCE OF CARBON-OXYGEN SURFACE GROUPS ON ADSORPTION PROPERTIES... 

Bansal, studied the adsorption desorption isotherms of benzene, toluene and o-xylene on sugar charcoal associated with varying amounts of the carbon-oxygen surface groups and observed that the area of the hysteresis loop decreases as the molecular dimensions of the adsorbate increase from benzene to o-xylene (Table 2.7). The point of inception of the hysteresis loop was also found to shift to lower relative vapor pressures as we move up the series of hydrocarbons, which is due to an increase in the molecular size of the adsorbate. The point of inception of hysteresis loop was calculated using Cohn postulates and compared with the values read from the experimental curves (Table 2.8). It is seen that the two values agree closely for all the adsorbate-adsorbent systems. Higute, from thermodynamic considerations, also proposed that the critical radius for the inception of capillary condensation is equal to four times the molecular radius of the adsorbate. 

Puri et al. ° determined the adsorption-desorption isotherms of water vapor on different samples of charcoals associated with different types of carbon-oxygen surface groups and observed that the presence of acidic surface groups, which are evolved as CO2 on evacuation, make the charcoal surface hydrophilic and increase the size of the hysteresis loop, the increase being proportional to the amount of acidic surface groups present (Figure 2.27). This was attributed to the chemisorption... 

FIGURE 3.6 Influence of carbon-oxygen surface groups on the adsorption of methylene blue by activated carbon. (After Goyal, M., Ph.D. dissertation. Panjab Univ., Chandigarh, India,... 

When the adsorption of same fatty acids from aqueous solutions was studied on a charcoal prepared under strongly oxidizing conditions (Figure 3.29), Traube s rule shows a reversal, which means that the magnitude of adsorption decreases as we ascend the homologous series. This has been attributed to the polar nature of the charcoal surface, due to the presence of polar carbon-oxygen surface groups, which... 

The adsorptive removal of organics and inorganics from waste water by activated carbons depends upon the surface area, the pore volume, and the pore-size distribution in carbons. Although adsorption capacity of an activated carbon depends on these parameters, it is strongly influenced by the surface chemistry of the activated carbons. Most of the as-received activated carbons are hydrophobic and are associated with small amonnts of nentral carbon-oxygen surface groups. Such activated carbons are more snitable for the adsorption of neutral or nonpolar organic compounds and they show little affinity for polar and ionic pollutants. An example is the adsorption of phenol for which the snrface chemistry " of a carbon is more important than the snrface area. 

---