Phenol adsorption mechanisms

It has been demonstrated that mixed oxides obtained from calcined LDHs have the ability to act as sorbents for a variety of anionic compounds from aqueous solution. This ability is because of the propensity for the mixed oxide to hydrate and re-form an LDH in such conditions and is of particular interest for the decontamination of waste-water. Hermosin et al. have found, for example, that MgAl-LDHs calcined at 500 °C are potential sorbents for the pollutants trinitrophenol and trichlorophenol from water [208, 209]. The adsorption mechanism was shown, using PXRD, to involve reconstruction of the LDH, with the uptake of the phenolate anions into the interlayers. Similarly, the ability of calcined MgAl-LDHs to remove nitriloacetate anions from solution has been demonstrated [210]. Calcined LDHs have been utilized also for the sorption of radioactive anions, such as 111, from aqueous solution [211]. A particularly attractive feature of the use of calcined LDHs for the remediation of waste-water is that the sorption capacity of the material may be regenerated via calcination of the rehydrated LDH. 

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. 

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. 

The quantitative analysis of the adsorption mechanism (cf Miller Kretzschmar 1991) shows a diffusion controlled adsorption over the whole concentration range with a slight change of the diffusion coefficient D with adsorption time and surfactant concentration. A detailed data analysis with butyl phenols of different chemical structure is in progress. 

The presence of ash can also influence the adsorption mechanism either via ion exchange or due to the catalytic effect of an inorganic matter. The adsorption feature of activated carbon combined with iron oxides in composites have been reported for a wide range of contaminants in water. The cmnposites materials show high adsorption capacities fiir phenol, chloroform, chlorobenzene and organic %es in aqueous solution [353]. 

Adsorption of SOC by activated carbon may involve various combinations of chemical, electrostatic, and physical (i.e. non-specific dispersion forces) interactions [59]. The overall adsorption interactions can be very complex for some SOCs. One good example is the adsorption of phenolic compounds, probably the most widely studied class of adsorbates in the activated carbon literature. Several possible mechanisms have been proposed for phenol adsorption [60-69]. These incluile (i) n-n dispersion interactions between the basal plane of activated carbon and the aromatic ring of the adsorbate, (ii) electrostatic attraction-repulsion interactions, (iii) hydrogen bonding between adsorbate and surface functional groups of activated carbons, (iv) electron acceptor-donor complex formation mechanisms between the carbonyl... 

Summary of review ofDqbrowski et al. (2004) adsorption mechanisms Phenolic compounds are to be found in contaminated water sources. At low concentrations, drinking water can be rendered unpalatable and contaminants may be carcinogenic to humans. Phenols are by no means rare on the industrial scene and are associated with oil refineries, with sites of coal gasification and coal carbonization, as well as the petrochemical industry. The production of plastics, colours, pesticides and insecticides are other examples of sources of phenols. Degradation of waste and used plastics produces phenols. 

M. Chorro, C. Chorro, O. DoUadiUe, S. Partyka, R. Zana, Adsorption mechanism of conventional and dimeric cationic surfactants on silica surface effect of the state of the surface. J. Colloid Interface Sci. 210(1), 134-143 (1999). doi 10.1006/jcis.l998.5936 R. Chaghi, L.-C. de Menorval, C. Chamay, G. Derrien, J. Zajac, Interactions of phenol with cationic micelles of hexadecyltrimethylammonium bromide studied by titration calorimetry, conductiraetry, and IH NMR in the range of low additive and surfactant concentrations. J. Colloid Interface Sci. 326(1), 227-234 (2008). doi 10.1016/j.jcis.2008.07.035... 

Umeyama, H., Nagai, T., andNogami, H. Mechanism of adsorption of phenols by carbon black from aqueons solution. Chem. Pharm. Bull, 19(8) 1714-1721, 1971. 

On co-adsorbing phenol and methanol, the protonation of methanol occurs on the active acid sites as the labile protons released from the phenol reacted with methanol. Thus protonated methanol became electrophilic methyl species, which undergo electrophilic substitution. The ortho position of phenol, which is close to the catalyst surface, has eventually become the substitution reaction center to form the ortho methylated products (Figure 3). This mechanism was also supported by the competitive adsorption of reactants with acidity probe pyridine [79]. A sequential adsorption of phenol and pyridine has shown the formation of phenolate anion and pyridinium ion that indicated the protonation of pyridine. 

Irrespective of the sources of phenolic compounds in soil, adsorption and desorption from soil colloids will determine their solution-phase concentration. Both processes are described by the same mathematical models, but they are not necessarily completely reversible. Complete reversibility refers to singular adsorption-desorption, an equilibrium in which the adsorbate is fully desorbed, with release as easy as retention. In non-singular adsorption-desorption equilibria, the release of the adsorbate may involve a different mechanism requiring a higher activation energy, resulting in different reaction kinetics and desorption coefficients. This phenomenon is commonly observed with pesticides (41, 42). An acute need exists for experimental data on the adsorption, desorption, and equilibria for phenolic compounds to properly assess their environmental chemistry in soil. 

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