A method for the determination of hydrophobicity of suspended soil colloids

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.