Characterization of particulate matters and river sediments

Figure 1 shows XRD patterns of particulate matters and sediments obtained from Chernobyl and Fukushima watersheds. Two peaks at 27° and 28° observed for all sediments and particulate matters are attributed to quartz and plagioclase, respectively25. Small peaks of mica and feldspar were observed at 19.7° and 35°, respectively. Compared with that in particulate matters obtained from Pripyat River, higher contents of micaceous minerals were found at 8.5° and 12.2° in sediment and particulate matters obtained from Kuchibuto River (Fukushima): these minerals include vermiculite, montmorillonite, and chlorite. Sediment obtained from Kuchibuto River contain larger amounts of plagioclase compared with particulate matters obtained from Kuchibuto River, suggesting that clay minerals, given their small particle size, are the main minerals existing in particulate matters (Fig. 1).

Figure 1 XRD patterns of particulate mattes (PM) and sediments from Chernobyl (Pripyat River) and Fukushima (Kuchibuto River, Ooyado Pond, Matsuzawa-Kami Pond, and Motomiya Pond). Q: Quartz; Vt: vermiculite; It: illite; Mt: montmorillonite; Ct: chlorite; Kt: kaolinite; M: mica; Pl: plagioclase; Fs: feldspar. Full size image

To understand the effect of NOM on the solid-water partitions of RCs, sediments in two irrigation ponds in Fukushima were also studied here (Ooyado and Motomiya Ponds)26: the Ooyado Pond sediment sample has similar mineralogy to the sediment and particulate matters of Kuchibuto River, whereas main components of Motomiya Pond sediment are feldspar and quartz. Major ions and 137Cs concentrations and other water quality data for the rivers and ponds were summarized in Tables 1 and 2. As a viewpoint of radiation protection, activities of 137Cs in water samples are much lower than that in WHO guideline for drinking water quality (=10 Bq/kg; WHO, 2011)27.

Table 1 Sampling sites and dates with compositions of major cations, anions, total dissolved solids (TDS), 133Cs, 137Cs and DOC in river and pond waters. Full size table

Table 2 Physicochemical and mineralogical properties of particulate matter and sediment samples. Full size table

Organic carbon contents of particulate matters from Pripyat River were 16.7 Cwt.% and 14.3 Cwt.% for two particulate matter samples with different particle sizes (PSP-1 (0.45–63 µm) and PSP-2 (>63 µm) in Table 2), respectively, and this result was similar to that obtained from Motomiya Pond sediment (Table 2). Such high NOM content can lead to weak adsorption affinity for Cs as confirmed by the small partition coefficients (K d ) calculated from 137Cs concentrations in solid phase (particulate matters or sediments) relative to those in aqueous phase in river or pond systems (Table 2). Generally, such high organic carbon content makes detection of mineral peaks in XRD difficult. Thus, XRD patterns after removal of NOM through H 2 O 2 treatment (Fig. 1) were measured, and two peaks were observed at approximately 8.5° and 12.2°, suggesting presence of phyllosilicate minerals also in particulate matters of Pripyat River.

The Fukushima catchment is granite-rich7,8,28, whereas organic peat soils cover approximately 47% of the Pripyat catchment (total catchment area is 200 km2) in Pripyat marshes in Ukraine-Belarus border20,21. High NOM content possibly with lower content of clay minerals indicated that K d and capacity of Cs is relatively small in particulate matters of Pripyat River (and in Motomiya Pond sediment).

Possible effects of mineralogy and NOM on radiocesium interception potential (RIP), or Cs adsorption

Figure 2 shows the radiocesium interception potential (RIP) and K d of RCs sediments and particulate matters28 obtained from Fukushima and Chernobyl catchments. The RIP of particulate matters from Pripyat River is considerably smaller than those of particulate matters and sediments from Kuchibuto River and Ooyado Pond. RIP is considered as the most important factor for the retention of RCs in terrestrial and river systems15,16,17,28,29. Clay minerals normally display large RIP (above 6000 mmol/kg), except montmorillonite (~1000 mmol/kg)29. Variation in K d is correlated with the variation in RIP (Fig. 2). Adsorbed fraction of 137Cs onto sediments and particulate matters is considerably larger in Kuchibuto River than in Pripyat River. Based on the definition of RIP, FES capacity is directly related to the difference in mineralogy of sediments and particulate matters obtained from Chernobyl and Fukushima, which includes effect of NOM content to inhibit formation of inner-sphere (IS) complex of Cs into the interlayer of the minerals12,16,25. The higher organic carbon content of particulate matters in Pripyat River (16.7 wt.% for PSP-1) than in Kuchibuto River (=10.6 wt.%) can explain the smaller RIP and K d values for particulate matters in Pripyat River. The difference in organic carbon content is possibly caused by the larger dissolved organic content (DOC; Table 1) in Pripyat River (=19.0 mg/L) than in Kuchibuto River (=1.2 mg/L), reflecting their provenances. Motomiya Pond sediment also exhibits lower RIP and K d possibly due to the higher abundance of NOM. These results suggest that high NOM content in the particulate matters or sediment is important in reducing Cs adsorption, which will be discussed below based on EXAFS results.

Figure 2 Relationship between RIP and K d for particulate matters (PM) and sediments. Full size image

X-ray spectroscopic study on Cs speciation: EXAFS

Adsorption species of Cs normally controls the mobility of Cs in the environment depending on the mineralogy and NOMs in particles7,12,29. In the present study, the adsorbed Cs species in sediments and particulate matters were estimated through EXAFS from the speciation of Cs added into the sample. The added concentration is much higher than natural level, but if IS complex is observed in this EXAFS analysis, the result will at least show that the particulate matters or sediment is capable of fixing Cs at lower Cs level such as for 137Cs in environment, since Cs should be adsorbed at more stable site when its concentration is lower. Figure 3A,B respectively show the Cs L3-edge EXAFS in k pace (k 3 χ(k) function) and the corresponding radial structural functions (RSFs; phase shift uncorrected) in R space for Cs adsorbed onto particulate matters and sediments. EXAFS spectra of CsNO 3 solution (hydrated Cs+ ion) and Cs adsorbed on vermiculite are considered as two end members representing OS and IS complexes, respectively. Previous studies have confirmed that IS complex of Cs adsorbed onto vermiculite12,30.

Figure 3 Cesium L3-edge EXAFS spectra in (A) k space and (B) R space. (a) hydrated Cs+ in water; (f) Cs adsorbed on vermiculite; Cs adsorbed on particulate matter in Pripyat River (b) before and (c) after the removal of organic matter; Cs adsorbed on particulate matter in Kuchibuto River (d) before and (e) after the removal of organic matter. Full size image

The k 3 χ(k) function of Cs adsorbed onto particulate matters in Kuchibuto River (spectrum (d) in Fig. 3A; before removal of NOM) is obviously different from that of hydrated Cs+ ion (Fig. 3A), and this finding was also observed in Cs adsorbed onto sediments in Kuchibuto River. For example, the peak at approximately 3.7 Å−1 for the hydrated Cs+ ion shifted to approximately 3.3 Å−1 for the particulate matters in Kuchibuto River, and this phenomenon is due to the increase in peak intensity for IS Cs+ complex as seen in Cs adsorbed onto vermiculate, suggesting that contribution of IS complex is evident in Cs adsorbed onto particulate matters in Kuchibuto River. Similar results were found in RSF in Fig. 3B: (i) only one coordinated shell at (R + ∆R) ~2.4 Å was found in hydrated Cs+ ion, which originated from the hydration sphere of Cs+ and assigned to the OS complex; (ii) an intense shell at (R + ∆R) ~3.5 Å was determined in the spectrum (f) in Fig. 3B 12,30,31, suggesting that the IS complex is responsible for the high stability of Cs on minerals, such as vermiculite and illite. In practice, the two characteristic shells of the IS and OS complexes are successfully applied to distinguish Cs adsorption species in soils and sediments7,12,29. Expectedly, two shells in RSFs were observed for Cs adsorbed on particulate matters in Kuchibuto River (in this study) and Kuchibuto River sediments as reported by Fan et al.25 at R + ∆R ranges of 2.2–2.8 Å and 3.6–3.8 Å, respectively.

EXAFS for Cs adsorbed on particulate matters in Pripyat River showed contrasting results (spectrum (b) in Fig. 3). Absence of a small peak around k = 4.2 Å−1 in k space and lower contribution of the second shell in R space resulted in lower contribution of IS complex in the sample (Fig. 3). Relative ratio of the two shells (=CN IS /CN OS ) is indicated in the coordination number (CN) of Cs-O shells in the second and first shells12,29. The CN IS /CN OS ratio for particulate matters in Kuchibuto River was 0.78, considerably larger than 0.42 and 0.13 for particulate matters (PSP-1 and PSP-2, respectively) in Pripyat River (Table 3), demonstrating that IS complex of Cs is responsible for the higher K d and RIP values of Cs in Kuchibuto River than in Pripyat River.

Table 3 Local structure of Cs adsorbed on river sediments and vermiculite using Cs L III -edge EXAFSa. Full size table

X-ray spectroscopic study of NOM in particulate matters and sediments: STXM

Normally, chemical and mineralogical characteristics of particulate matters are similar to those of surface soil layer surrounding the rivers5,32. The Fukushima area is geologically classified as weathered granite and the Chernobyl as peat wetland1,20,33. Correspondingly, higher content of NOM content or DOC was observed in Pripyat River than in Fukushima watershed (Tables 1 and 2)20,21. Figure 4 shows the relationship between the natural K d of 137Cs and DOC content in rivers and ponds (two ponds in Table 1 with data of Matsuzawa-kami Pond in Yoshimura et al.26) in Fukushima and Chernobyl areas, as well as the results reported in Sakaguchi et al.5 K d of 137Cs adsorbed on particulate matters sharply decreases with increased DOC possibly due to the inhibition effect of NOM on the access of Cs to strong adsorption sites, such as FES and interlayer site in phyllosilicate minerals12,14,20,34.

Figure 4 Natural partition coefficients of 137Cs as a function of dissolved organic carbon content (DOC) in Fukushima and Pripyat rivers and ponds. Full size image

For better understanding of the association between NOM and particulate matters, STXM analysis for the particulate matters obtained from Pripyat and Kuchibuto Rivers and Motomiya Pond was conducted. For the STXM analysis, particulate matter samples were completely dispersed on a Si 3 N 4 membrane after thorough sonication treatment of the particulate matters in pure water35. Mappings of C and Al were obtained through STXM based on subtraction of optical density images taken at post- and pre-edges for C and Al K-edges, respectively (Fig. 5). In addition, potassium (K) shows a distribution pattern similar to that of Al in the particles (Fig. S2 in Supporting Information), suggesting that Al mostly exists as phyllosilicates in particulate matters. NOM are associated with Al-bearing particles collected from Pripyat River, demonstrating the strong affinity between NOM and clay minerals in Pripyat River, and such strong affinity in turn reduces Cs adsorption. By contrast, STXM images of the particulate matters collected from Kuchibuto River and Motomiya Pond showed that C and Al are more or less independently distributed as particulate matters. The difference between the two rivers is more clearly demonstrated in scattered plots of the absorption intensities of C and Al in their mapping data (Fig. 5): relatively high correlation was found in particulate matters of Pripyat River but not in Kuchibuto River and Motomiya Pond.

Figure 5 Distributions of Al and C in particulate matters collected in the Kuchibuto and Pripyat Rivers and the Motomiya Pond with their scatter plots. Full size image

In investigating the speciation of C in the particulate matters, near-edge X-ray absorption fine structure (NEXAFS) spectra at C K-edge were obtained through STXM by using the image stacking method (Fig. 6)36. NEXAFS spectra at 5–6 spots are basically similar to those of fulvic and humic acids (HAs), wherein C displays characteristic peaks at 285.1 eV (aromatic C), 286.8 eV (phenolic or ketonic C), and 288.6 eV (carboxylic C). Moreover, spectra of the particulate matters are more similar to those of fulvic acid with lower contribution at 286.8 eV peak. This result is reasonable considering that the particulate matters were collected from freshwater, which contains more soluble fraction of humic substances, or fulvic acid. Hence, NEXAFS confirmed that the dominant species of NOMs covering the particulate matters are humic substances. In addition, absorption of potassium L-edge was prominent only in sample collected from Pripyat River (Fig. 6), further demonstrating the close association between NOM and clay minerals in Pripyat River. Such submicron-scale characterization of NOM is indispensable in revealing the nature of particulate matters in rivers.

Figure 6 Carbon K-edge NEXAFS spectra at various carbon-rich spots in STXM images for natural organic matter in particulate matters recovered from Pripyat and Kuchibuto Rivers. Spectra of fulvic (FA; Suwannee River fulvic acid47) and humic acids (HA; Suwannee River humic acid47) were also shown as reference materials. Absorption peaks of 285.1, 286.8, and 288.6 eV correspond to aromatic, phenolic, and carboxylic carbons, respectively. Absorption of potassium L-edges were also found at 297.1 and 299.9 eV for the samples from Pripyat River. Full size image

Although C K-edge NEXAFS showed that NOM detected in the particulate matters in Kuchibuto and Pripyat Rivers are humic substances, association of humic substances and clay mineral particles was considerably weaker in Kuchibuto River than in Pripyat River. Coagulation of humic substances and clay minerals are strongly enhanced in the presence of divalent cations, such as Ca2+ and Mg2+, in natural waters37,38,39. Based on the data on water quality (Table 1), Ca2+ concentration was higher in Pripyat River (approximately 1.6 mM) than in freshwater samples obtained in Fukushima by a factor of 4.6. This difference was also found as general trends in major ion concentrations in Japanese and Black Sea rivers40. In particular, the critical concentration of Ca2+ that induces aggregation of humic substances was 1 mM38,39. These results can explain the strong association of humic substances and clay minerals in Pripyat River.

EXAFS for Cs adsorbed on particulate matters after treatment with H 2 O 2

To confirm the blocking effect of NOM on the particle surfaces to Cs adsorption, changes in Cs adsorption species before and after H 2 O 2 treatment to remove NOM were evaluated based on EXAFS (Fig. 3). Before the treatment, the CN IS /CN OS ratio (=relative importance of IS complex to OS complex in the RSF; Table 3) was considerably larger in Kuchibuto River than in Pripyat River. Even after H 2 O 2 treatment, the CN IS /CN OS ratio for particulate matters in Kuchibuto River was 0.83, slightly higher than that before H 2 O 2 treatment (0.78; Table 3), suggesting that the effect of NOM is not very important in Cs adsorption on particulate matters in Kuchibuto River. However, the ratio increased dramatically from 0.42 to 1.2 after H 2 O 2 treatment of the particulate matters (PSP-1) obtained in Pripyat River. These results revealed that (i) Cs is more strongly fixed onto the particulate matters in Kuchibuto River due to the formation of IS complex of Cs with phyllosilicate minerals; (ii) Cs adsorption on particulate matters obtained from Pripyat River is weak due to the inhibition effect of NOM; (iii) phyllosilicate minerals are actually included in the particulate matters in Pripyat River; and (iv) incorporation of Cs into the interlayer can be inhibited by the presence of humic substances. In addition, the XRD patterns displayed the characteristic peaks of phyllosilicate minerals at 6.7° (or 8.5°) and at 12.2° after NOM removal in samples obtained both in Fukushima and Chernobyl (Fig. 1), consistent with the occurrence of high adsorption affinity for Cs after removal of NOM in EXAFS analyses.

Other factors, including complexation with humic substances, and other types of organic matter

Another possible effect of NOM on solid-water partition is the retention of Cs+ in aqueous phase through complexation with humic substances in aqueous phase. Thus, a dialysis method using diffusion cell and HA extracted from paddy soil in Tochigi Prefecture (adjacent to Fukushima Prefecture) was conducted to test the hypothesis41,42. This method is detailed in the experimental section, wherein Cs introduced as 137Cs gradually diffuses from tracer cell to HA cell (HA concentration: 60 mg/L) as a function of elapsed time (Figs S3 and S4). After equilibrium was established, radioactivity in HA cell finally became higher than that in tracer cell. Blank experiment (n = 2), that is, without HA, reached equal 137Cs concentrations between tracer and HA cells. The difference between the two experiments in the absence and presence of HA allows us to determine the stability constant of HA-Cs complex (β MA ), which is defined as42:

$${\beta }_{{\rm{MA}}}=\frac{[\mathrm{MA}]}{{([{\rm{M}}}^{{\rm{z}}+}][{\rm{A}}])},$$ (1)

where z and A are the charges of metal ion and dissociated ligand of HA, respectively. Determination of β MA is detailed in Supporting Information. logβ CsA was determined to be 4.4 (pH = 7), which is considerably lower than the logβ MA values determined for divalent cations (e.g., logβ CaA = 7.0 for Ca2+) and trivalent cations (12–16 for rare earth ions and Fe3+ at pH = 7) for the same HA sample42. A logβ CsA value lower than those of multivalent cations is reasonable considering its weak electrostatic attraction between Cs+ and HA42. The logβ CsA with reported logβ MA values for other cations allows us to conduct speciation calculation of Cs in natural waters in the presence of humic substances including the competitive effect of major multivalent cation (Ca2+) on humate formation. The calculation for Cs in Pripyat River was conducted assuming that (i) DOC = 50 mg/L, which is higher than that in Pripyat River and (ii) [Ca2+] is 64 mg/L following the value in the Pripyat River (Table 1). The estimated Cs-HA fraction in Pripyat River is at most 1% relative to total Cs in water, because complexation of Cs+ with HA is strongly inhibited by the presence of Ca2+ due to its much higher concentration than Cs+ in natural waters. Thus, complexation with HA cannot explain the higher concentration of dissolved Cs+ in river water with high DOC, such as in Pripyat River.

Naulier et al.43 have recently suggested that particulate NOM is an important carrier of RCs in Fukushima Rivers. They suggested that NOM consists mainly of initially contaminated dead leaves and/or other allochtonous organic materials, which are abundant in autumn, or during the sampling season in this study. In our study, NOM in particulate matters was characterized as humic substances by STXM, indicating that the NOM examined in this study is not plant debris.