Journal of Colloid and Interface Science 292 (2005) 171–178 www.elsevier.com/locate/jcis Describing chlorophenol sorption on variable-charge soil using the triple-layer model Mara Cea a , J.C. Seaman b , Alejandra A. Jara c,∗ , M.L. Mora c , M.C. Diez d a Universidad de La Frontera, P.O. Box 54-D, Temuco, Chile b Savannah River Ecology Laboratory, The University of Georgia, Drawer E, Aiken, SC 29802, USA c Departamento de Ciencias Químicas, Universidad de La Frontera, P.O. Box 54-D, Temuco, Chile d Departamento de Ingeniería Química, Universidad de La Frontera, P.O. Box 54-D, Temuco, Chile Received 8 February 2005; accepted 19 May 2005 Available online 1 August 2005 Abstract The sorption of 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol by a variable-charge soil from southern Chile was stud- ied in a series of batch experiments. The chlorophenol sorption behavior was evaluated as a function of pH (pH range 4–8) at two different ionic strengths, 0.01 and 0.1 M KCl (25 ◦ C). Chlorophenol sorption depended on pH and a downward shift in the soil PZC was observed with increasing chlorophenol concentration. Chlorophenol sorption decreased with increasing pH, suggesting that the undissociated species is sorbed more readily and that electrostatic repulsion may inhibit partitioning as pH increases. Data from the sorption experiments were fitted by the triple-layer model, in which monodentate outer- and inner-sphere complexes were formed between deprotonated organic molecules and active sites on the variable-charge soil.  2005 Elsevier Inc. All rights reserved. Keywords: Variable charge soil; Chlorophenol; Surface complexation modeling 1. Introduction The release of xenobiotic substances derived from chem- ical processes and forest production constitutes an important source of environmental contamination, because many of these compounds are toxic and resistant to biodegradation. In soil, the fate of such contaminants are governed by their interactions with soil colloids and the biota. The physical, chemical, and biological characteristics of the soil, as well as the chemical properties of the contaminant, control its fate and transport. Adsorption is an important process affect- ing the fate and transport of chlorinated phenols, controlled largely by their degree of substitution and the resultant hy- drophobicity. In the normal soil pH range, most chlorophe- nols will totally or partially ionize, affecting their solubility, adsorption, transport, and bioavailability [1]. Many inves- * Corresponding author. Fax: +56 45 325053. E-mail address: aljara@ufro.cl (A.A. Jara). tigators have noted that pH plays an important role in the adsorption of compounds with acidic functional groups on activated carbon and soil because the neutral and ionic forms display very different adsorption properties [2–7]. However, chlorophenol sorption in soil has been generally described assuming hydrophobic partitioning of the neutral species to soil organic matter [6,8]. In a variable-charge soil, Diez et al. [5] demonstrated that the adsorption of phenolic compounds increase with decreasing pH, possibly as a consequence of electrostatic repulsion between the organic compound and the resulting negative surface charge as pH increases. The allophane–ferrihydrite associations found in Chilean andisols accumulate organic matter through the formation of stable complexes with FeOH and AlOH [9]. Based on spectroscopic data, Parfitt et al. [10] reported that spe- cific interactions take place via ligand exchange between the surface hydroxyls of metal oxides and the functional groups (hydroxyl and carboxylate) of humic acid. As a con- 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.05.074