Colloidal facilitated transfer of metals in soils under different land use

In order to assess the importance of colloids in the metal transfer in soils under varying land use (a podzol under a forest, a cultivated luvisol and a luvisol under a metallophyte grassland), gravitational waters were collected in situ by zero-tension lysimeters. Dissolved and colloidal fractions of metals were separated by ultracentrifugation and colloids were studied by transmission electron microscopy (TEM) coupled with energy dispersive X-ray analysis (EDS). The stability of metal complexes in the collected solutions was operationally defined by an ion exchange method. The results show that in the two luvisols about 75% of Pb was always present in colloidal forms and a substantial fraction of Pb in solution was present as stable complexes and labile complexes with slow dissociation kinetics. Zn and Cd were found mostly in dissolved forms and mainly as free ions or labile complexes in solution. In the podzol, all metals were found in dissolved forms and as free ions or labile complexes. The Zn-bearing colloids were organic (biocolloids) or mineral (smectites, Fe-oxyhydroxides, Ca- or Al-rich phosphates) depending on soil type and land use. The results are discussed in terms of metal mobility to explain the different vertical metal distributions observed in the soil profiles. (C) 2002 Elsevier Science B.V. All rights reserved.