Speciation of cesium in tree tissues and its implication for uptake and translocation of radiocesium in tree bodies

Since the Chernobyl and Fukushima nuclear power plant accidents, extensive research has focused on the distribution and cycle of radiocesium in forest systems. Nevertheless, direct chemical speciation analyses of Cs by spectroscopic methods are limited by the low abundances of stable Cs as well as radiocesium in trees. In this study, we investigated coordination structures of Cs in ¹³³Cs-doped bark, sapwood, heartwood, needle, and branch samples of trees collected in Fukushima by extended X-ray absorption fine structure (EXAFS) spectroscopy. We examined four representative tree species in Fukushima, Cryptomeria japonica, Pinus densiflora, Quercus serrata, and Eleutherococcus sciadophylloides. EXAFS spectra suggested that Cs was adsorbed as an outer-sphere complex on all parts of the four species, with electrostatic binding to negatively charged functional groups in components of tree tissues. These results were supported by extraction experiments where more than 98.5% of the sorbed Cs was desorbed from all parts of each tree species using 1 M CH₃COONH₄. Sorption experiments of Cs on cellulose, an important component of plant cell walls, were carried out in ultrapure water, NaCl, and KCl solutions. The K_d values for cellulose and solutions were not high enough to fix Cs, considering the composition of sap in trees. Overall, the results of this study are consistent with previous field observations indicating that radiocesium is translocated in mobile form to metabolically active tree parts.