Fixation of radiocaesium traces in a weathering sequence mica -> vermiculite -> hydroxy interlayered vermiculite

Radiocaesium fixation in soils is reported to occur on frayed edge sites of micaceous minerals. The weathering of mica in acid soils may therefore inf luence the Cs+ fixation process and thereby the mobility of the radiopollut ant. We produced a laboratory weathering model biotite --> trioctahedral ve rmiculite --> oxidized vermiculite --> hydroxy interlayered vermiculite (HI V) and quantified the Cs' fixation of each mineral both in a fixed K+-Ca2background and in acid conditions. The transformation process was achieved through K depletion by Na-tetraphenylboron, oxidation with Br-2 and Al-inte rcalation using NaOH and AlCl3. In a constant K+-Ca2+ background, vermiculi te fixed 92-95% of the initial Cs-137(+) contamination while biotite and HI V fixed only 18-33%. In acid conditions, the interlayer occupancy by either potassium (biotite) or hydroxy-Al groups (HIV) strongly limited Cs+ fixati on to 1-4% of the initial Cs-137(+) contamination. Cs+ fixation occurred on vermiculitic sites associated with micaceous wedge zones. Though both oxid ized and trioctahedral vermiculites fixed similar Cs+ amounts in a constant K+-Ca2+ background (92-95%), the oxidized vermiculite retained much more r adiocaesium in acid conditions (78-84% against 54-59%), because of its dioc tahedral character.