Colloids and Surfaces A: Physicochemical and Engineering Aspects
Brief notesA method for the determination of hydrophobicity of suspended soil colloids
Introduction
Colloids play a crucial role in the translocation of heavy metals [1], [2], [3] and organic contaminants [4], [5], [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, stability and sorption capacity. Wan and Wilson [8] demonstrated an increased retention of colloidal latex particles and bacteria 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 colloids 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 colloid stability in aqueous solutions by altering the surface properties of colloids. They postulate an enhanced hydrophobic 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 capacity 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.
There are a number of methods for determining the hydrophobicity of the soil solid phase, such as the water droplet penetration time test (WDPT; [15], [16]), the Wilhelmy plate method as described by Bachmann et al. [17] and the molarity of ethanol droplet (MED) test [18]. All these methods, however, cannot be applied to dispersed colloids in soil suspensions. Recently, Guiné et al. [19] proposed a method to determine the hydrophobicity of colloidal bacteria in aqueous suspensions using hexadecane as an extracting solvent. We adopted this approach to colloidal soil suspensions (1:10 water extract) in a preliminary experiment (unpublished data) but results indicated that this method cannot be used for colloidal soil suspensions due to incomplete separation of the aqueous phase and the organic solvent. This incomplete separation may be attributed to the presence of surface-active substances in the soil suspension (such as dissolved organic matter), which cause an emulsion, rendering a complete phase separation impossible. Therefore, the aim of this paper is to discuss a newly developed method for the determination of the hydrophobicity of suspended soil colloids in soil extracts.
Section snippets
Material and methods
The two tested methods described in the following are based on a distribution of suspended hydrophobic goethite particles between (a) two liquid phases (dichlormethane-method) and (b) a hydrophobic solid phase (C18-method).
Dichlormethane-method
The results show a clear relation between the zeta potential and the hydrophobicity and are in line with our assumption (Fig. 1). The hydrophobicity determined by the two phase extractions of Al-treated goethite was higher than the Na- and Mg-treated versions at a similar absolute surface charge. These results may be explained by the interaction of Al with the organic solvent. Aluminium forms transition complexes with the chlorinated solvent as described in the Diels-Alder-Reaction [23].
Conclusion
The C18-method as described can be used for the determination of hydrophobic colloids in soil suspensions. The application of our method can help to better relate the properties of soil colloids to their function as carrier of pollutants in soils.
Acknowledgements
We thank Claudia Kuntz for her help with analyses, Christina Mikutta for providing the goethite samples, the German Research Foundation (DFG) for financial support of the project (La 1398/2) and David Meredith for proof reading the manuscript.
Glossary
- DOM
- dissolved organic matter
- WDPT
- water droplet penetration time
- MED
- molarity of Ethanol droplet
- DCM
- dichlormethane
- DOC
- dissolved organic carbon
- FNU
- formazin nephlometric unit (unit of turbidity)
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