Phenol, adsorption

A number of phenols have proven to be valuable adsorbates for surface area determinations in organic pigments [10]. Phthalocyanines in particular have been evaluated successfully by phenol adsorption [11],... 

High activity associated with x = 0.5 composition demonstrates an optimum concentration of acid-base sites is needed for phenol adsorption and subsequent polarization of both phenol and isobutene as in the ease of other alkylations. It was proposed that in the phenol t-butylation, t-butyl carbocation ean attaek phenol from the adsorbed as well as from the gaseous state resulted in the formation of para t-butylated products such as 4-tBP and 2,4-tBP. The steric hindrance of t-butyl group prevents the sequential attack of t-butyl cation at ortho position for dialkylation and that demonstrated the negligible formation of 2,6-di-t-butyl phenol. 

Approximately 40 to 50% of the total amount of phenolics sorbed was retained by the organic matter fraction (27). In surface soil layers, organic matter is frequently intimately associated with the mineral components present, providing a large surface area and reactive sites for surface interaction. Soil acidity has a major influence on phenolic adsorption by the organic carbon fraction, since the degree of dissociation of the phenolic acids is pH-dependent. Whitehead and coworkers (28) observed that the extractability of several phenolic acids was highly dependent upon the extractant pH between pH 6 and 14. The amount extractable continually increased with extractant pH thus the extracted acids could not be readily classified into distinct fractions. 

Structural differences between the monolayers of anthraquinone derivatives self-assembled on silver and gold electrodes have been investigated using CV and in situ SERS spectroscopy [323]. Neves etal. ]324] have described Monte Carlo simulations of phenol adsorption on gold electrodes. 

Also, Naucke has reported a method for determining the surface area of cokes by phenol adsorption from solution (8). 

Economy and Lin [340] investigated the phenol adsorption characteristics of high-surface-area activated carbon fibers Fig. 17b shows that their correlation was not nearly as good, but the role of surface chemistry was not invoked. Additional evidence that the agreement shown in Fig. 17a is more often the exception [439] than the rule is contained in the study of Dondi et al. [342], who u.sed a chromatographic method to determine low-concentration phenol adsorption isotherms on four different carbons, as well as in many other investigations (see, for example. Refs. 356, 382, 384, and 436). 

Peel and Benedek [407] published a useful review of reported phenol adsorption isotherms on a popular commercial activated carbon and concluded that the observed differences [as low as 100 vs. as high as 200 mg/g at 100 mg/L] are greater in magnitude than might reasonably be attributed to [variations in carbon properties and environmental conditions such as temperature, pH, and buffer... 

FIG. 17 Illustration of the relationship between phenol adsorption capacity and surface area of a series of carbon materials (a) adapted from Ref. 341 (b) adapted from Ref. 340. , activated carbons A, carbon blacks T, degassed carbon blacks. 

The reversibility of phenol adsorption [341], which is of great importance in adsorbent regeneration (or reactivation) [343,347-349,352,355,453,454] was first discussed in detail by Magne and Walker [360]. They reported that weakly adsorbed (physisorbed) phenol can become chemisorbed in the course of time or by increasing the temperature. On the basis of the finding that chemisorption (on a commercial activated carbon) was inhibited by the presence of oxygen surface complexes, the authors concluded that the sites responsible for phenol chemisorption are carbon sites of the active surface area, i.e., oxygen-free sites... 

Liu and Pinto [437] have analyzed the effect of pH on the uptakes of phenol and aniline in an effort to as.sess the validity of the ideal ad,sorbed solution theory. The large decrease in the amount adsorbed from pH 6.3 to pH 11.35 was attributed to both greater solubility of dissociated phenol at pH > pK and increased repulsion between dissociated form of the adsorbate and the carbon surface, while the decrease in capacity from pH 6.3 to 3.07 is explained. .. as occurring due to increased proton adsorption on carbonyl oxygen sites, which suppresses phenol adsorption on these sites. The references provided for these interesting. statements [333,35,417] are rather inappropriate, however the cited studies are hardly representative, let alone the first ones, in which such arguments have been proven to be valid. 

It has long been known [16], and has more recently been confirmed by many authors [1], that phenol adsorption presents certain complexities, such as the occurrence of two distinct plateaus in the isotherm. In addition, the phenol uptake decreases upon oxidation of the activated carbons. 

To demonstrate this phenomenon (Table 25.2), sample A was oxidized in order to increase its surface acidity. Heat treatment of the oxidized sample progressively decreased the surface acidity, mainly through the removal of carboxyl and phenolic groups [19]. A sample heat-treated at 950°C had no acidity, and its surface area was lower than that of an as-received sample. Phenol adsorption isotherms on these carbons are shown in Fig. 25.3. There was a large decrease in phenol uptake after oxidation. This phenol uptake progressively increased as the surface acidity decreased, and the oxidized sample heat-treated at 950°C had the same adsorption capacity as the as-received one, despite the lower surface area of the former sample compared with the latter. 

Since the initial proposals by Coughlin and Mattson, many published papers have attempted to elucidate the most appropriate mechanism to explain the adsorption of phenolic compounds and of aromatic compounds in general on carbon materials. Perhaps the first experimental evidence of the Tr-ir dispersion interaction mechanism was provided by Mahajan and coworkers [19] in their study of phenol adsorption on graphite and boron-doped graphite samples. They reported that the presence of substitutional boron in the lattice of polycrystalhne graphite, which removes ir-electrons from the solid, results in a lowering of the phenol uptake from water. 

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. 

Magne, P. and Walker, P.L., Jr (1986). Phenol adsorption on activated carbons application to the regeneration of activated carbons polluted with phenol. Carbon, 24, 101-7. 

Terzyk, A.P. (2003). Further insights into the role of carbon surface functionahties in the mechanism of phenol adsorption. J. Colloid Interface Sd., 268, 301—29. 

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