Investigation of physico-chemical features of soil colloidal suspensions.

The knowledge of surface properties of soil mobile colloids is of great importance to predict their role, but these properties are difficult to assess due to the heterogeneity and the complex nature of natural colloids. Physico-chemical features of natural colloids circulating in the soil solution were investigated with complementary approaches: microscopic techniques (TEM and AFM), potentiometric titrations and electrophoretic measurements. This study was realized on two colloidal suspensions isolated from soil solutions collected in situ from the plough layers of two different soils (MC and MPM). Colloidal particle properties including morphology, size distribution, specific surface area, electrophoretic mobility, and surface site concentration were compared before and after oxidation of the organic matter of the colloidal suspensions by H2O2 treatment. Statistical analysis of the geometrical features of TEM and AFM images revealed a comparable mean size of 170 70 nut for the two samples, and a mean height of 6.5 +/- 4.6 mn and 9.9 +/- 8.2 nm for the MPM and MC, respectively. From the 3D-analysis of AFM images, the specific surface areas were assessed at 203 m(2)/g and 260 m(2)/g for MC and MPM suspensions, respectively. Phyllosilicates (illite, interstratified clays and smectite) primarily present in the two samples seem to dominate the surface charge behaviour of the soil colloidal suspensions as seen by potentiometric or electrophoretic titrations. However, TEM after staining and AFM observations show also the presence of organic phases with different status according to the sample, i.e. surface coatings or interparticle bonds. The data clearly indicate the influence of organic matter on the main physico-chemical characteristics of colloids (morphology, size distribution and surface charge) related to the reactivity and the transfer of colloids in soils. (c) 2006 Elsevier B.V. All rights reserved