The effect of removal of soil organic matter and iron on the adsorption of radiocaesium

Abstract

SummaryRadiocaesium is more available to biological systems in organic soils than in mineral soils. Part of the reason is that the adsorption of caesium on minerals, particularly illites, is very specific, whereas its adsorption on organic matter is non‐specific. However, increasing evidence shows that even organic upland soils contain enough illitic material to immobilize caesium effectively. The presence of organic matter may reduce the affinity of soil minerals for caesium.We have studied adsorption of radiocaesium on the clay fractions of a mineral soil obtained from a surface, and the corresponding C, horizon. The soil clay fractions were treated to remove organic matter and Fe oxides progressively.Adsorption isotherms of caesium were obtained in dilute suspension using trace amounts of 137Cs and stable caesium. In general, adsorption was greater when the organic matter content was less, namely on the soil from the C horizon rather than the surface horizon, or when organic matter had been removed. However, adsorption is not a simple function of organic matter content: Cs adsorption was greater on the topsoil fraction after organic matter removal than on the subsoil fraction. X‐ray diffraction analysis indicated that the surface soil clay contained more weathered minerals and less illite than the subsoil clay. The effect of organic matter removal was more marked at large Cs concentrations than when only radiocaesium was added, suggesting that very selective Cs adsorption sites are less sensitive to the presence of organic macromolecules. Iron oxides do not play a role in the affinity of soil clays for Cs. The form of the Freundlich isotherm was unchanged by any of the chemical treatments.