Accumulation mechanisms of radiocaesium within lichen thallus tissues determined by means of in situ microscale localisation observation.

Many lichens are well known to accumulate radiocaesium and, thus acting as biomonitors of contamination levels. However, the actual localisation and chemical forms of radiocaesium in contaminated lichens have not yet been elucidated because, despite their high radioactivity, these forms are present in trace amounts as chemical entities. Here, we use autoradiography and demonstrate for the first time in situ microscale localisation of radiocaesium within thallus tissues to investigate the radiocaesium forms and their accumulation mechanism. Radiocaesium distributions showed similar trends in lichen tissues collected two and six years after the Fukushima nuclear accident. The radiocaesium was localised in the brown pigmented parts i.e., melanin-like substances, in the lower cortex of lichen thallus. Quantum chemical calculations showed that functional group of melanin-like substances can chelate Cs+ ion, which indicates that the Cs+ ions form complexes with the substances. Based on these findings, we suggest that radiocaesium ions may be retained stably in melanin-like substances for long periods (two to six years) due to steric factors, such as those seen in porphyrin-like structures and via multimer formation in the lower cortex. In addition, electron microscopy and autoradiography were used to observe radiocaesium-bearing microparticles (CsMPs) on/in the upper cortex and around the medullary layer. Micron-sized particles appeared to adhere to the surface tissue of the thallus, as shown by electron microscopy, suggesting that the particles were trapped by development of an adhesive layer; that is, CsMPs were trapped both physically and physiologically. These findings provide information on in situ localisation of two chemical forms of radiocaesium, cations and particles, in lichen thallus tissues and their accumulation mechanisms.

Radiocaesium accumulation capacity of epiphytic lichens and adjacent barks collected at the perimeter boundary site of the Fukushima Dai-ichi Nuclear Power Station.

We investigated the radiocaesium content of nine epiphytic foliose lichens species and the adjacent barks of Zelkova serrata (Ulmaceae, "Japanese elm") and Cerasus sp. (Rosaceae, "Cherry tree") at the boundary of the Fukushima Dai-ichi Nuclear Power Station six years after the accident in 2011. Caesium-137 activities per unit area (the 137Cs-inventory) were determined to compare radiocaesium retentions of lichens (65 specimens) and barks (44 specimens) under the same growth conditions. The 137Cs-inventory of lichens collected from Zelkova serrata and Cerasus sp. were respectively 7.9- and 3.8-times greater than the adjacent barks. Furthermore, we examined the radiocaesium distribution within these samples using autoradiography and on the surfaces with an electron probe micro analyzer (EPMA). Autoradiographic results showed strong local spotting and heterogeneous distributions of radioactivity in both the lichen and bark samples, although the intensities were lower in the barks. The electron microscopy analysis demonstrated that particulates with similar sizes and compositions were distributed on the surfaces of the samples. We therefore concluded that the lichens and barks could capture fine particles, including radiocaesium particles. In addition, radioactivity was distributed more towards the inwards of the lichen samples than the peripheries. This suggests that lichen can retain 137Cs that is chemically immobilised in particulates intracellularly, unlike bark.

Quantum chemical calculation studies toward microscopic understanding of retention mechanism of Cs radioisotopes and other alkali metals in lichens.

We evaluate stability of cesium (Cs) and other alkali-metal cation complexes of lichen metabolites in both gas and aqueous phases to discuss why lichens can retain radioactive Cs in the thalli over several years. We focus on oxalic acid, (+)-usnic acid, atranorin, lecanoric acid, and protocetraric acid, which are common metabolite substances in various lichens including, e.g., Flavoparmelia caperata and Parmotrema tinctorum retaining Cs in Fukushima, Japan. By performing quantum chemical calculations, their gas-phase complexation energies and aqueous-solution complexation free energies with alkali-metal cations are computed for their neutral and deprotonated cases. Consequently, all the molecules are found to energetically favor cation complexations and the preference order is Li[Formula: see text]Na[Formula: see text]K[Formula: see text]Rb[Formula: see text]Cs[Formula: see text] for all conditions, indicating no specific Cs selectivity but strong binding with all alkali cations. Comparing complexation stabilities among these metabolites, lecanoric and protocetraric acids seen in medullary layer are found to keep higher affinity in their neutral case, while (+)-usnic acid and atranorin in upper cortex exhibit rather strong affinity only in deprotonated cases through forming stable six atoms' ring containing alkali cation chelated by two oxygens. These results suggest that the medullary layer can catch all alkali cations in a wide pH range around the physiological one, while the upper cortex can effectively block penetration of metal ions when the metal stress grows. Such insights highlight a physiological role of metabolites like blocking of metal-cation migrations into intracellular tissues, and explain long-term retention of alkali cations including Cs in lichens containing enough such metabolites to bind them.

Dissolution behaviour of radiocaesium-bearing microparticles released from the Fukushima nuclear plant.

Radiocaesium-bearing microparticles (CsMPs) composed of silicate glass were released by the Fukushima Daiichi Nuclear Power Plant accident in March 2011. Since CsMPs contain a high concentration of radiocaesium, their dynamics and fate in the environment are urgent issues to be investigated. Here, we show that CsMPs are dissolved by weathering in the environment and that their radioactivity is more rapidly decreased by dissolution than the physical decay of radiocaesium. We conducted dissolution experiments with CsMPs in pure water that absorbed CO2 from the atmosphere and in artificial seawater at several temperatures. The dissolution progress was monitored by the decrease in the 137Cs radioactivity in CsMPs, and the dissolution rate was estimated. The activation energy for the dissolution of CsMPs was estimated to be 65 and 88 kJ/mol and the dissolution rate at 13 °C (approximate annual mean temperature in Fukushima City) was 0.014 and 0.140 µm/y for pure water and seawater, respectively, assuming that radiocaesium is uniformly distributed in spherical CsMPs. The shapes of the CsMPs dissolved in pure water were considerably altered, suggesting that the dissolution proceeded without maintaining the geometry. Tin oxide and iron oxide nanoparticulates formed on the surfaces. Such features were similar to those observed in CsMPs recently collected in Fukushima Prefecture, indicating that weathering dissolution of CsMPs is also occurring in the environment. For the CsMPs dissolved in seawater, a crust of secondary Mg- and Fe-rich minerals was formed, and the glass matrix inside the crust decreased, creating space between the crust and the glass matrix.

Inner structure and inclusions in radiocesium-bearing microparticles emitted in the Fukushima Daiichi Nuclear Power Plant accident.

Radiocesium-bearing microparticles (CsMPs), consisting substantially of silicate glass, were released to the environment during the Fukushima Daiichi Nuclear Power Plant accident in March 2011. Since the CsMPs were formed inside the damaged reactors during the accident, we investigate the inner structures of several CsMPs by transmission electron microscopy to understand the events within the reactors. Elemental mapping of the CsMPs shows a distinct radial distribution of Cs with a higher concentration near the surface of the CsMPs, implying that Cs was in a gaseous state in the reactor atmosphere and diffused into the glass matrix after formation of the glass particles. In some CsMPs, Zn and Fe also showed a similar radial distribution to Cs, suggesting that those elements may have diffused outward where Cs was abundant. In addition, submicron crystals were present as inclusions in several of the CsMPs and were identified as chromium spinels ((Fe2+,Zn)(Cr,Fe3+)2O4), acanthite (Ag2S), molybdenite (MoS2) and hessite (Ag2Te). The spinels contained ferrous iron (Fe2+), suggesting that the atmosphere inside the reactors was reductive to some extent. Also, boron was not detected in the glass matrix of the CsMPs despite using electron energy-loss spectroscopy, indicating that most of the control rods made of B4C might have created a eutectic alloy without vaporization. These detailed investigations of the inner structures in the CsMPs may offer information on the damaged reactors that are difficult to access because of the high radiation fields.

Loss of radioactivity in radiocesium-bearing microparticles emitted from the Fukushima Dai-ichi nuclear power plant by heating.

Radiocesium-bearing microparticles (CsPs) substantially made of silicate glass are a novel form of radiocesium emitted from the broken containment vessel of Fukushima Dai-ichi nuclear power plant. CsPs have a potential risk of internal radiation exposure caused by inhalation. Radiation-contaminated waste including CsPs is being burned in incinerators; therefore, this study has investigated the responses of CsPs to heating in air. The radioactivity of CsPs gradually decreased from 600 °C and was almost lost when the temperature reached 1000 °C. The size and spherical morphology of CsPs were almost unchanged after heating, but cesium including radiocesium, potassium and chlorine were lost, probably diffused away from the CsPs. Iron, zinc and tin originally dissolved in the glass matrix were crystallized to oxide nanoparticles inside the CsPs. When the CsPs were heated together with weathered granitic soil that is common in Fukushima, the radiocesium released from CsPs was sorbed by the surrounding soil. From these results, it is expected that the radioactivity of CsPs will be lost when radiation-contaminated waste including CsPs is burned in incinerators.

Radiocaesium activity concentrations in parmelioid lichens within a 60 km radius of the Fukushima Dai-ichi Nuclear Power Plant.

Radiocaesium activity concentrations ((134)Cs and (137)Cs) were measured in parmelioid lichens collected within the Fukushima Prefecture approximately 2 y after the Fukushima Dai-ichi Nuclear Power Plant accident. A total of 44 samples consisting of nine species were collected at 16 points within a 60 km radius of the Fukushima Dai-ichi Nuclear Power Plant. The activity concentration of (134)Cs ranged from 4.6 to 1000 kBq kg(-1) and for (137)Cs ranged from 7.6 to 1740 kBq kg(-1). A significant positive correlation was found between the (137)Cs activity concentration in lichens and the (137)Cs deposition density on soil (n = 44), based on the calculated Spearman's rank correlation coefficients as r = 0.90 (P < 0.01). The two dominant species, Flavoparmelia caperata (n = 12) and Parmotrema clavuliferum (n = 11), showed strong positive correlations, for which the r values were calculated as 0.92 (P < 0.01) and 0.90 (P < 0.01) respectively. Therefore, Flavoparmelia caperata and Parmotrema clavuliferum are suggested as biomonitoring species for levels of radiocaesium fallout within the Fukushima Prefecture.

¹³7Cs concentrations in foliose lichens within Tsukuba-city as a reflection of radioactive fallout from the Fukushima Dai-ichi Nuclear Power Plant accident.

(137)Cs concentrations in ten species of foliose lichens collected within Tsukuba-city in August 2013 ranged from 1.7 to 35 kBq/kg. The relationships between (137)Cs in two dominant species, Dirinaria applanata and Physcia orientalis, and the air dose rate (µSv/h) at the sampling sites were investigated. (137)Cs in P. orientalis measured about 1 year after the Fukushima nuclear accident was correlated (r(2) = 0.80) more closely with the air dose rate than those measured after about 2 years (r(2) = 0.65), possibly demonstrating its continued value as a biomonitor to reflect ambient fall-out levels. In contrast, those of Dirinaria applanata were not correlated with the air dose rate in either year.