Adsorption of several metal ions onto a model soil sample: equilibrium and EPR studies

Soils play an important role in the control of metallic cations in the environment. Therefore, knowledge of the adsorption properties of soil is crucial in understanding and solving pollution problems. Adsorption isotherms provide a macroscopic view of the retention phenomena. The aim of this paper is to study iron, manganese, and chromium adsorption onto a soil sample as a function of the reaction time, pH, and metal concentration. The adsorption isotherms allow the determination of the affinity order of metals for the surface of the soil sample as such: Fe(3+)>Cr(3+)>Mn(2+). The equilibrium data fit well with the Langmuir and Freundlich models and confirm the affinity order of the soil sample for these metals. These adsorption data are combined with EPR spectroscopy to obtain structural information about the surface complexes formed. Iron is held in inner-sphere complexes. Manganese is simultaneously held in outer- and inner-sphere complexes. Due to poor resolution, chromium was not detected by EPR and thus it is impossible to infer coordination sphere and coordination number. Iron and manganese are in an octahedral environment.