Asian dust increases radiocesium retention ability of serpentine soils in Japan

Radiocesium (RCs) is selectively adsorbed on interlayer sites of weathered micaceous minerals, which can reduce the mobility of RCs in soil. Therefore, soils developed from mica-deficient materials (e.g. serpentine soils) may have a higher risk of soil-to-plant transfer of RCs. Soils were collected from three serpentine soil profiles; Udepts in Oeyama, Japan, and Udepts and Udox in Kinabalu, Malaysia. Soil was sampled every 3 cm from 0 to 30 cm depth and sieved to isolate soil particles of ≤20 μm diameter for the assessment of radiocesium interception potential (RIP) after a series of pretreatments. One subset was treated with H2O2 to remove organic matter (OM). Another subset was further treated with hot sodium citrate to remove hydroxy-Al polymers (Al(OH)x). RIPuntreated was <0.4 mol kg-1 whereas mica-K content was <0.02% by weight for ≤20-μm soil particles from Udepts and Udox in Kinabalu, Malaysia, values as low as those of non-micaceous minerals (e.g. kaolinite and smectite). Neither OM nor Al(OH)x removal resulted in a large increase in RIP value for these soils. These results clearly indicated that serpentine soils in Malaysia have very few RCs selective adsorption sites due to the absence of micaceous minerals. In contrast, soil from Udepts in Oeyama, Japan showed average RIPuntreated of 5.6 mol kg-1 and mica-K content of 0.72% by weight for the ≤20-μm particles. Furthermore, the RIP value was significantly increased to an average of 22.5 mol kg-1 after removing both OM and Al(OH)x. These results strongly suggest that weathered micaceous minerals primarily control the ability to retain RCs. These micaceous minerals cannot originate from serpentine minerals, and are probably incorporated as an exotic material, such as Asian dust. This hypothesis is supported by the δ18O value of quartz isolated from the ≤20-μm soil particles from Oeyama, Japan (+16.13‰±0.11‰), very similar to that of Asian dust. In conclusion, serpentine soils in Japan may exhibit a reduced risk of soil-to-plant transfer of RCs due to the historical deposition of Asian dust.