FIXATION OF RADIOCESIUM IN AN ACID BROWN FOREST SOIL

The OAh and Ah horizons of acid brown and podzolic forest soils are re ported to hx more radiocaesium than the mineral B horizons beneath the m. We determined the respective influence of organic matter and clay m inerals on the magnitude of Cs+ retention in a strongly acid brown for est soil in Belgium. The soil contained mica throughout the profile. V ermiculite was identified in the OAh and Ah horizons, and hydroxy inte rlayered vermiculite (HIV) in the Bw horizon. The OAh and Ah clay frac tion retained much more Cs+ than the Bw horizon. The extraction of Al interlayers by Na-citrate resulted in a marked increase in Cs+ fixatio n in the Bw clays as well as the collapse of the vermiculitic layers a fter K+ saturation. Organic matter had a strong but indirect effect on Cs+ fixation. In the Bw horizon, acid weathering of layer silicates r eleases free Al and produces HIV minerals in which Al polymers block t he access of radiocaesium onto Cs+-specific sites. In OAh and Ah horiz ons, free Al is complexed by organic acids. Consequently, the interlay er specific sites remain accessible for Cs+ fixation.