Eu(III) sorption on granite was assessed using combined microscopic and macroscopic approaches in neutral to acidic conditions where the mobility of Eu(III) is generally considered to be high. Polished thin sections of the granite were reacted with solutions containing 10 μM of Eu(III) and were analyzed using EPMA and LA-ICP-MS. On most of the biotite grains, Eu enrichment up to 6 wt % was observed. The Eu-enriched parts of biotite commonly lose K, which is the interlayer cation of biotite, indicating that the sorption mode of Eu(III) by the biotite is cation exchange in the interlayer. The distributions of Eu appeared along the original cracks of the biotite. Those occurrences indicate that the prior water-rock interaction along the cracks engendered modification of biotite to possess affinity to the Eu(III). Batch Eu(III) sorption experiments on granite and biotite powders were conducted as functions of pH, Eu(III) loading, and ionic strength. The macroscopic sorption behavior of biotite was consistent with that of granite. At pH > 4, there was little pH dependence but strong ionic strength dependence of Eu(III) sorption. At pH < 4, the sorption of Eu(III) abruptly decreased with decreased pH. The sorption behavior at pH > 4 was reproducible reasonably by the modeling considering single-site cation exchange reactions. The decrease of Eu(III) sorption at pH < 4 was explained by the occupation of exchangeable sites by dissolved cationic species such as Al and Fe from granite and biotite in low-pH conditions. Granites are complex mineral assemblages. However, the combined microscopic and macroscopic approaches revealed that elementary reactions by a single mineral phase can be representative of the bulk sorption reaction in complex mineral assemblages.

This study investigated the distribution and mineralogy of radioactive Cs in a core sample from reservoir sediment affected by the Fukushima Daiichi Nuclear Power Plant accident. The sample was obtained 19 months after the accident from a reservoir in Iitate -mura, Fukushima Prefecture. The vertical concentrations of radioactive Cs in the sediment core varied little from the surface to 9 cm depth: 20 -30 Bq/g for 137 Cs. The concentration decreased abruptly at depths greater than 9 cm. The radioactive Cs concentrations were particle -size dependent: finer particles exhibited higher concentrations of radioactive Cs. X -ray diffraction analyses of the size -fractionated sample revealed that the contribution of clay minerals (14 Å minerals, kaolin mineral and mica) increased concomitantly with the decrease of particle size, implying a close relation between radioactive Cs concentration and clay mineralogical compositions. Periodical airborne monitoring showed that the 137 Cs inventory in the reservoir catchment area decreased significantly with time after the accident. Results of a 137Cs inventory in the reservoir sediment were almost identical to those for the catchment area immediately after the accident, but were significantly higher than those in the core sampling period. Results might indicate that the reservoir sediments serve as a sink for radioactive Cs.