Journal of Hazardous Materials 148 (2007) 436–445 The effect of functional groups on oligomerization of phenolics on activated carbon Qiuli Lu, George A. Sorial ∗ Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0071, United States Received 11 December 2006; received in revised form 25 February 2007; accepted 27 February 2007 Available online 3 March 2007 Abstract Adsorption of seven phenolic compounds, namely phenol, 2-methylphenol, 2-ethylphenol, 2-chlorophenol, 2-nitrophenol, 4-chlorophenol, and 4-nitrophenol on granular activated carbon (GAC) F400 and two activated carbon fibers (ACFs), ACC-10 and ACC-15 were conducted to study the impact of functional groups on oligomerization. Single solute adsorption and binary solute adsorption were conducted under anoxic (absence of molecular oxygen) and oxic (presence of molecular oxygen) conditions at 23 ± 1 ◦ C. For F400, the critical oxidation potential (COP), influenced by substitution of functional group, was found to be a limiting factor in oligomerization of adsorbates. For ACC-10, which has narrow pore size distribution (PSD) and small critical pore diameter (8.0 ˚ A), only phenol and 2-methylphenol showed occurrence of oligomerization under oxic conditions. The degree of oligomerization under oxic conditions was related to the PSD of the adsorbent. The micro-pore diameter of the ACFs effectively reduced the oligomerization process. The oxic and anoxic binary adsorption isotherms of phenol/2-ethylphenol and 2-methylphenol/2- nitrophenol on ACC-10 overlapped indicating hampering of oligomerization by the limited micro-pore diameter of ACFs. The Ideal Adsorbed Solution Theory (IAST), using the Myers equation for correlating the single-solute anoxic isotherms, well predicted both anoxic and oxic binary isotherms. © 2007 Elsevier B.V. All rights reserved. Keywords: Activated carbon fibers (ACFs); Adsorption; Anoxic; Granular activated carbon (GAC); IAST; Oligomerization; Oxic 1. Introduction Phenolic compounds are toxic to humans and aquatic life [1]. Some phenols, like chlorophenolic compounds, have been prescribed as priority pollutants by the US Environmental Pro- tection Agency (EPA) [2]. Activated carbon adsorption has been used to remove phenolic compounds in the past decades [3–7]. The major advantage of activated carbon adsorption is that the solid adsorbent can easily be separated from the treated liquid or gas streams thus easy and flexible process operation as well as a reduction in process costs can be achieved, especially if the pollutants are to be recycled [8,9]. Adsorption on activated carbon adsorption is influenced by both the adsorbate and the adsorbent properties [10]. Carbons are essentially hydrophobic and display a strong affinity for organic molecules which have a limited solubility in water, like phenols [11]. Adsorption is a manifestation of complicated interactions ∗ Corresponding author. Tel.: +1 513 556 2987; fax: +1 513 556 2599. E-mail address: George.Sorial@uc.edu (G.A. Sorial). among the three components involved, i.e. the adsorbent, the adsorbate and the solvent. Normally, the affinity between the adsorbent and the adsorbate is the main interaction force con- trolling adsorption. However, the affinity between the adsorbate and the solvent (i.e. solubility) can also play a major role in adsorption. Hydrophobic compounds have low solubility and tend to be pushed to the adsorbent surface and hence are more adsorbable than hydrophilic compounds. Phenolic compounds undergo oligomerization on the surface of activated carbon when molecular oxygen is present in the test environment. This has been confirmed by many researchers [12–15]. Phenolic radicals, formed from a phenol by the loss of a proton, form stable molecular products by coupling such as dimers. Dimers may couple with another phenolate radical to give a trimer and so forth [16]. Oligomerized phenols on the surface of the activated carbon are essentially irreversibly bound to the carbon surface [17]. As a consequence, the regen- eration efficiency of activated carbon after oligomerization of phenolic compounds is low. Regeneration efficiency has been a major environmental concern in activated carbon usage. To enhance the efficiency of regeneration as well as to design 0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2007.02.058