Estimation of desorption ratios of radio/stable caesium from environmental samples (aerosols and soils) leached with seawater, diluted seawater and ultrapure water

To understand the dissolutive behaviour of radio Cs discharged to the ocean environment as a result of the Fukushima Dai-ichi Nuclear Power Plant accident, an aerosol sample collected on the 15th of March 2011 at Kawasaki City (Kanagawa) was sequentially leached with seawater for 30 days. In addition, a surface soil sample collected from Kawamata Town (Fukushima) two months after the accident, was leached for three days with natural seawater, diluted seawaters and ultrapure water to observe the effect of the ionic strength of the waters on the respective leaching ratios and apparent distribution coefficient (Kd) values. Furthermore, the soil sample was subjected to a 223-day continuous sequential leaching with a natural seawater and with a 1:1 mixture of ultrapure water and seawater. When leaching the aerosol sample in seawater, about 40% of the total 137Cs was extracted in the first three days, and a further 20% of the total 137Cs was extracted within 30 days. Lower Kd values for 137Cs between the soil and leachates were obtained with seawater and diluted seawater compared to ultrapure water. For the long-term experiment (223 days) using the three leaching solutions, approximately 0.1–2% of the total 137Cs was leached in the first three days. Eventually, more than 15% of total 137Cs in the surface soil sample was efficiently desorbed by seawater leaching. In comparison, about 9% of the total 137Cs was leached with 1:1 diluted seawater and less than 1% of the total 137Cs was leached with ultrapure water over the 223 days. In general, there were some similarities between the leaching behaviour for natural 133Cs and radio Cs. In the surface soil, radio Cs species was eventually incorporated into the clays after undergoing solubilisation as fallout aerosols in natural waters. Thereafter, the insoluble or less soluble forms of radio Cs in the soil would be partially extracted by seawater after the transport of contaminated surface soils to the ocean via rivers.