Colloids and Surfaces A: Physicochem. Eng. Aspects 303 (2007) 249–252 Brief notes A method for the determination of hydrophobicity of suspended soil colloids Sondra Klitzke ∗ , Friederike Lang Berlin University of Technology, Department of Soil Science, Salzufer 11-12, D-10587 Berlin, Germany Received 16 February 2007; received in revised form 21 March 2007; accepted 11 April 2007 Available online 13 April 2007 Abstract Colloids play a crucial role in the translocation of trace elements in soils. Recent studies provided hints that colloid hydrophobicity may be an important factor controlling colloid (im)mobilization in soils. However, existing methods for the determination of hydrophobicity are limited to the bulk soil. Therefore, we developed a method to determine the hydrophobicity of suspended colloids in aqueous soil suspensions, which was based on a distribution between a polar and a non-polar phase. The proposed method uses 30 mg of an unpolar solid phase (C18-column material) which are mixed with 10 mL of suspension for 2 h. The turbidity of the suspensions is measured before and after mixing. The ratio of the colloids in the hydrophilic aqueous and the hydrophobic solid phase is calculated as a measure of colloid hydrophobicity. This method was successfully tested on differently hydrophobized goethite particles. At DOC concentrations exceeding 20 mg L -1 , organic molecules sorbed to C18-material limit the applicability of the method. © 2007 Elsevier B.V. All rights reserved. Keywords: Water repellence; Zeta potential; Colloid; Soil drying 1. Introduction Colloids play a crucial role in the translocation of heavy metals [1–3] and organic contaminants [4–6] in soils. Their mobilization and stability in soils is controlled by physico- chemical factors, such as pH, ionic strength and DOM concentration in the soil solution, as well as steric effects [7]. Furthermore, there are hints in the literature that hydrophobicity is another important factor controlling colloid retention, stabil- ity and sorption capacity. Wan and Wilson [8] demonstrated an increased retention of colloidal latex particles and bacte- ria with increasing particle hydrophobicity in an unsaturated sand column experiment. Similarly, under saturated conditions hydrophobic colloids showed a lower recovery than hydrophilic colloids, allowing the authors to conclude hydrophilic col- loids are more mobile than hydrophobic ones [8] as they sorb less strongly to both the gas–water and solid–water interfaces. Breiner et al. [9] reported hydrophobic organic molecules affect ∗ Corresponding author. Tel.: +49 30 314 73546; fax: +49 30 314 73548. E-mail addresses: Sondra.klitzke@tu-berlin.de (S. Klitzke), Fritzi.lang@tu-berlin.de (F. Lang). colloid stability in aqueous solutions by altering the surface properties of colloids. They postulate an enhanced hydropho- bic nature of the colloids would reduce their transport in the environment. Another parameter hydrophobicity has an effect on is the sorption capacity of colloids. According to Liu and Lee [10] hydrophobicity is thought to enhance the sorption capac- ity of colloids for hydrophobic organic compounds. Breiner et al. [9] suggest organic matter-coated inorganic colloids facilitate the sorption of hydrophobic organic contaminants. Different conditions are reported to induce hydrophobicity, for instance drying [11], this being explained by changes in the molecular conformation of the organic matter [10,12]. In addition, McHale et al. [13] found wax coatings arising from vegetation to render the surface of small soil particles hydrophobic. Hydrophobicity is inversely correlated with the wettability of water [14]. For pure mineral colloids the surface charge is closely related to the hydrophobicity of colloids. In contrast, organo- mineral colloids of similar surface charge may vary strongly in hydrophobicity, depending on the accessibility of functional groups to surrounding water. For these colloids, hydrophobicity might be a more suitable indicator for the estimation of sorption capacity and mobility than surface charge. 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.014