Journal of Colloid and Interface Science 227, 132–140 (2000) doi:10.1006/jcis.2000.6886, available online at http://www.idealibrary.com on Benzenecarboxylate Surface Complexation at the Goethite (α -FeOOH)/Water Interface III. The Influence of Particle Surface Area and the Significance of Modeling Parameters Jean-Fran¸ cois Boily, 1,2 Per Persson, and Staffan Sj¨ oberg Department of Inorganic Chemistry, Ume˚ a University, S-901 87 Ume˚ a, Sweden Received October 25, 1999; accepted April 3, 2000 A surface complexation model describing the adsorption of three benzenecarboxylates (phthalate, trimellitate, and pyromellitate) on goethite (α-FeOOH) was calibrated on data using goethite particles of 37 and 43 m 2 /g surface area. The models predict potentiometric titration and batch adsorption data with the multisite complexation model coupled with the three-plane model to account for surface electrostatics. The modeling parameters were found to be similarto those calibrated on benzenecarboxylate adsorption data on goethite particles of 90 m 2 /g (Boily et al. Geochim. Cosmochim. Acta, in press). The significance of the benzenecarboxylate-dependent val- ues of the modeling parameters is also discussed. The values of the capacitances of the inner- and outer-Helmholtz planes were shown to be important modeling parameters to model the benzenecarboxylate-dependent slopes of the adsorption edges. It was shown that the larger the charge of the ligand, the larger the capacitance of the outer-Helmholtz plane. C  2000 Academic Press Key Words: goethite; carboxylate; adsorption; complexation; modeling. INTRODUCTION In most surface complexation studies, models are devised us- ing experimental data collected for one adsorbent sample with fixed particle surface areas. These models are expected to predict adsorption for a wide range of adsorbent/adsorbate ratios, in- cluding different adsorbate concentrations (in g/L) and specific surface areas (in m 2 /g). The ability of models to predict systems at various adsorbent/adsorbate ratios is ensured by collecting ex- perimental data for a wide range of concentration ratios. Models can however fail with samples for sorbents with different specific surface areas. This is especially important for models where the concentrations of the different surface sites are determined by the crystal planes present. Furthermore, preparations of differ- ent surface areas may exhibit different basic charging properties 1 To whom correspondence should be addressed. Fax: +41 01/632 1088. E-mail: boily@erdw.ethz.ch. 2 New address: Institut f¨ ur Mineralogie und Petrographie, Eidgen¨ ossiche Technische Hochschule, ETH-Zentrum, CH-8092 Z¨ urich, Switzerland. (1, 2) which must be taken into account. These differences may have repercussions on the extent of adsorption and potentially on the binding modes of surface complexes. The purpose of this paper is to compare adsorption data for three benezenecarboxylates (phthalate, trimellitate, and py- romellitate) on goethite particles with different specific surface areas and to evaluate the potential of determining a particle sur- face area-independent set of modeling parameters. MODELING APPROACH In the study of Boily et al. (3), the surface complexation of phthalate, trimellitate, and pyromellitate on goethite particles with a specific surface area of 90 m 2 /g was described using the charge distribution multisite complexation model (CD-MUSIC) (4). This model considers the abundance and the distribution of surface functional groups (namely, singly-, doubly-, and triply- coordinated surface oxygens with underlying Fe(III)) on the {110} and on the {001} crystal planes of goethite (Fig. 1). Only singly-coordinated sites were considered to participate in ben- zenecarboxylate surface complexes. A general surface complex- ation reaction is written with pH + + q [≡FeOH 0.5− ] {110}/{001} + rL z− → ←  H p (≡FeOH) q L ( p−0.5q −zr ) r  {110}/{001} β p,q ,r , [1] where p, q , and r are the stoichiometric coefficients, L z− is a totally deprotonated benecarboxylate of charge z −, β p,q ,r is the stoichiometric formation constant, and the subscript {110}/{001} refers to the crystallographic plane on which com- plexation takes place. The basic charging properties of the goethite surface, on the other hand, were described using singly- and triply-coordinated sites with the 1 p K approximation (4) with (2) [≡FeOH 0.5− ] {110}/{001} + H + → ←  ≡FeOH 0.5+ 2  {110}/{001} β 1 , [2a] 132 0021-9797/00 $35.00 Copyright C  2000 by Academic Press All rights of reproduction in any form reserved.