Sorption reversibility is a phenomenon that is of considerable interest today in multiple contexts, including bioavailability of contaminants sorbed to soils and sediments, and increased emphasis on the use of sorbents for in-process recovery of industrial chemicals. One important class of chemicals that are both found in contaminated soils and sediments, and are widely used in industry, is the phenolic compounds. In this paper, we present research into the factors that govern the sorption reversibility of phenol from graphite surfaces. Graphite was chosen as a model sorbent to provide some relevance to both natural geo-sorbents and sorbents used in industrial processing, such as activated carbon. The use of graphite eliminates the confounding effects of microporosity characteristic of activated carbon. We hypothesize that the phenomenon of irreversible adsorption, which has been linked to oxidative coupling, hinders sorption reversibility. Oxidative coupling may cause sorption irreversibility resulting from chemisorption to specific sites; from strong binding of reaction products; or, in the case of microporous sorbents, from trapping of reaction products inside micropores. Uptake of phenol by graphite, and regeneration by methanol extraction, was measured to better understand the effects of oxidative coupling on irreversible adsorption of phenols to carbon surfaces. The isotherm data were well described by the Langmuir isotherm from pH 3 to pH 9. At pH 10 and above, modeling of uptake, q_e, and the Langmuir energy parameter, b, as a function of pH suggested reactions at the surface. One oxidative coupling product, 2,2'-dihydroxybiphenyl, was obtained exclusively after adsorption, at pH above 7, and appeared both in aqueous solution and in the regenerate. Dimer concentrations increased with pH showing a correlation between the oxidative coupling reaction and phenolate ion concentration. For a given pH, irreversible adsorption decreased with increasing equilibrium phenol concentration, suggesting preferential adsorption at high-energy sites; and increased with increasing pH, correlating with oxidative coupling and dimer formation. In addition to phenol adsorption, adsorption of 2,2-dihydroxybiphenyl under similar conditions from single solute solutions and from phenol/dimer bisolute solutions was measured.
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