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 influence the Cs+ fixation process and thereby the mobility of the radiopollutant. We produced a laboratory weathering model biotite --> trioctahedral vermiculite --> oxidized vermiculite --> hydroxy interlayered vermiculite (HIV) and quantified the Cs' fixation of each mineral both in a fixed K+-Ca2+ background and in acid conditions. The transformation process was achieved through K depletion by Na-tetraphenylboron, oxidation with Br-2 and Al-intercalation using NaOH and AlCl3. In a constant K+-Ca2+ background, vermiculite fixed 92-95% of the initial Cs-137(+) contamination while biotite and HIV fixed only 18-33%. In acid conditions, the interlayer occupancy by either potassium (biotite) or hydroxy-Al groups (HIV) strongly limited Cs+ fixation to 1-4% of the initial Cs-137(+) contamination. Cs+ fixation occurred on vermiculitic sites associated with micaceous wedge zones. Though both oxidized and trioctahedral vermiculites fixed similar Cs+ amounts in a constant K+-Ca2+ background (92-95%), the oxidized vermiculite retained much more radiocaesium in acid conditions (78-84% against 54-59%), because of its dioctahedral character.