Specific Retention of Radiocesium in Volcanic Ash Soils Devoid of Micaceous Clay Minerals

Abstract

The environmental availability of trace radiocesium ( 137 Cs) was studied in soils devoid of weathered micas. The soils were developed from basaltic ash, within a sequence Udand → Tropept → Udalf → Udult from Cameroon. Tropepts and Udalfs are halloysite‐rich and they exhibit a large cation exchange capacity (CEC) and a strong exchange selectivity for K probably due to the presence of halloysite‐smectite mixed‐layered clays. The 137 Cs mobility was evaluated in the B horizons through (i) a physicochemical approach using the radiocesium interception potential concept (RIP) and a sequential sorption–desorption procedure, (ii) a biological test assessing the 137 Cs rhizospheric mobilization ( 137 Cs‐RM). In a constant K + –Ca 2+ background solution, one of the Tropepts and the Udalfs fixed about 76% of the initial 137 Cs loading. The second desorption phase in acidic conditions was more discriminating: the Udalfs fixed about 40%, while the other soils fixed 5 to 20% of the initial 137 Cs + loading. The 137 Cs‐RM was generally small (3–15%) in all samples and was negatively correlated with the RIP (439–1836 μmol g −1 ). The specific retention of 137 Cs in these soils was thus largely similar to that obtained in soils that contain weathered micas. It demonstrates the presence of 137 Cs specific sites in halloysitic soils devoid of such minerals. These sites might be associated with halloysite‐smectite mixed‐layered clays. They were probably formed following wetting‐drying cycles in soils heavily fertilized with K.