Phenols irreversible adsorption

An assumption implicit in most adsorption studies is that adsorption is fully reversible. In other words, once the empirical coefficients are measured for a particular substance, Equations 20.6 to 20.10 describe both adsorption and desorption isotherms. This assumption is not always true. Collins and Crocker140 observed apparently irreversible adsorption of phenol in flowthrough adsorption experiments involving phenol interacting on a Frio sandstone core under simulated deep-well... 

Previous analysis of Uinta Basin bitumens (6,10) have shown that the predominant nitrogen types are pyrollic, amide, and aromatic nitrogen. Predominant sulfur types are sulfide, sulfoxide, and thiophenic sulfur predominant oxygen types are ketones, phenols, carboxylic acids, and possibly appreciable concentrations of furans. The aromatic (basic) nitrogen is expected to participate in irreversible adsorption on acidcracking catalysts (13) and it could require an increased cat-to-oil ratio. Previous analyses (10) have indicated also that Uinta Basin bitumen is high in naphthenic hydrocarbon and low in free paraffins. This is illustrated by the results given in Table II in which over 60% of the saturates... 

Flavonoids are a specific class of polyphenols. It is generally believed that flavonoids include a wide variety of phenolic compounds, such as flavones, flavonols, flavanones, flavanonols, anthocyanidins, flavan-3,4-diols, xanthones, flavan-3-ols, isoflavones, isoflavanones, chal-cones, dihydrochalcones, aurones, and homoisoflavones. Their separation poses special problems because there is often irreversible adsorption and even hydrolysis on solid supports. 

The irreversible adsorption of organic solutes, which is of great importance in the regeneration of the adsorbents, is due to stronger interactions than dispersion or hydrophobic interactions. In the case of aromatic compounds such as phenol, it could involve a charge-transfer mechanism between the carbon surface and the adsorbate and/or its polymerization under certain experimental conditions. Therefore, further research is warranted in this area. 

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 effects of surface functionalities on adsorption of phenols have been studied extensively. However, in light of the findings of Grant and King (1990), snch effects need to be re-examined, that is, reversible and irreversible adsorption should be treated separately. 

AC provide one of the most efficient materials for the removal of phenols from aqueous media. Traditionally, powdered AC and GAC have been used with new attention being given to the use of ACFs, Rodriguez-Reinoso (1997). It has been known for some time that adsorptions of phenols exhibits irreversibility and D browski et al. (2004) review this topic. Applications of AC to water purification must produce a pollution-free water supply, with little interest in phenol recovery. Hence, irreversible adsorption ensures an efficient way of trapping the phenols. 

S.1.4.9 Summary of review of Dgbrowski et al. (2004) irreversible adsorption Irreversibility of phenol adsorption is important because of the need (if possible) for regeneration of the AC, the adsorbed (trapped) phenol having little (or zero) commercial value. 

Grant TM, King CJ. Mechanisms of irreversible adsorption of phenolic compounds by activated carbons. Ind Eng Chem Res 1990 29 264-271. 

The basicity of silica gel was determined similarly by irreversible adsorption of acidic molecules. Acrylic acid (pAa 4.25) and phenol (pAa - 9.9) gave the basicity values of 0.093, 0.012 mmol g , respectively. ... 

The two mechanisms proposed by Coughlin can better explain many of the experimental results obtained to date. However, an electron donor-acceptor mechanism cannot be completely ruled out because it could explain tbe irreversible or chemical adsorption of phenohc compounds. Thus, it is well known that the adsorption of phenolic compounds is pardy physical and partly chemical. 

More recently, Terzyk [32] also suggested that the irreversibility of phenol adsorption is due to the creation of strong complexes between phenol and surface carbonyl and lactone groups and to phenol polymerization. Salame and Bandosz [33] studied phenol adsorption at 30 and 60°C on oxidized and nonoxidized activated carbons. They concluded, from analyses of the isotherms by the FreundUch equation and the surface acidity of the carbons, that phenol was physisorbed by tt—tt dispersion interactions, whereas it was chemisorbed via ester formation between the OH group of phenol and surface carboxyl groups. 

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