Sequestration and remobilization of radioiodine (129I) by soil organic matter and possible consequences of the remedial action at Savannah River Site

In order to investigate the distributions and speciation of (129)I (and (127)I) in a contaminated F-Area groundwater plume of the Savannah River Site that cannot be explained by simple transport models, soil resuspension experiments simulating surface runoff or stormflow and erosion events were conducted. Results showed that 72-77% of the newly introduced I(-) or IO(3)(-) were irreversibly sequestered into the organic-rich riparian soil, while the rest was transformed by the soil into colloidal and truly dissolved organo-iodine, resulting in (129)I remobilization from the soil greatly exceeding the 1 pCi/L drinking water permit. This contradicts the conventional view that only considers I(-) or IO(3)(-) as the mobile forms. Laboratory iodination experiments indicate that iodine likely covalently binds to aromatic structures of the soil organic matter (SOM). Under very acidic conditions, abiotic iodination of SOM was predominant, whereas under less acidic conditions (pH ≥5), microbial enzymatically assisted iodination of SOM was predominant. The organic-rich soil in the vadose zone of F-Area thus acts primarily as a "sink," but may also behave as a potentially important vector for mobile radioiodine in an on-off carrying mechanism. Generally the riparian zone provides as a natural attenuation zone that greatly reduces radioiodine release.     

Dependence of Lanthanide-Ion Binding Performance on HDEHP Concentration in HDEHP Impregnation to Porous Sheet

An octadecylamino-group-introduced polymer chain grafted onto a porous sheet was impregnated with bis(2-ethylhexyl)hydrogen phosphate (HDEHP). A mixture of HDEHP and ethanol of various HDEHP concentrations was used for the impregnation. The porous sheet into which a C₁₈H₃₇NH group was introduced was immersed in HDEHP/ethanol solution before ethanol evaporation. The liquid permeability of a cartridge charged with the HDEHP-impregnated porous sheet in disk form prepared in 50 (v/v)% HDEHP/ethanol solution was 96% that of the starting-porous-disk-packed cartridge. The equilibrium binding capacity of the HDEHP-impregnated porous disk for yttrium ions was 0.32 mol per kg of the disk. In addition, the HDEHP-impregnated-porous-disc-packed cartridge was found to be applicable to the preconcentration of trace amounts of lanthanides in a multielement solution prior to their measurement by inductively coupled plasma mass spectrometry.     

Seven-year temporal variation of Caesium-137 discharge inventory from the port of Fukushima Daiichi Nuclear Power Plant: continuous monthly estimation of Caesium-137 discharge in the period from April 2011 to June 2018

ABSTRACT

After direct discharges of highly contaminated water of the Fukushima Daiichi Nuclear Power Plant (1 F) from April to May 2011, Kanda suggested that relatively small amounts of run-off of radionuclides from the 1 F port into the Fukushima coastal region subsequently continued by his estimation method. However, the estimation period was limited to up to September 2012. Therefore, this paper estimatesthe discharge inventory up to June 2018. In the missing period, the Japanese government and Tokyo Electric Power Company Holdings have continued efforts to stop the discharge, and consequently, the radionuclide concentration in seawater inside the 1 F port has gradually diminished. We show the monthly discharge inventory of ¹³⁷Cs up to June 2018 by two methods, i.e., Kanda method partially improved by the authors and a more sophisticated method using Voronoi tessellation reflecting the increase in the number of monitoring points inside the 1 F port. The results show that the former always yields overestimated results compared with the latter, but the ratio of the former to the latter is less than one order of magnitude. Using these results, we evaluate the impact of the discharge inventory from the 1 F port into the coastal area and radiation dose upon fish ingestion.     

Bacterial production of organic acids enhances H2O2-dependent iodide oxidation

To develop an understanding of the role that microorganisms play in the transport of (129)I in soil-water systems, bacteria isolated from subsurface sediments were assessed for iodide oxidizing activity. Spent liquid medium from 27/84 bacterial cultures enhanced iodide oxidation 2-10 fold in the presence of H(2)O(2). Organic acids secreted by the bacteria were found to enhance iodide oxidation by (1) lowering the pH of the spent medium, and (2) reacting with H(2)O(2) to form peroxy carboxylic acids, which are extremely strong oxidizing agents. H(2)O(2)-dependent iodide oxidation increased exponentially from 8.4 to 825.9 μM with decreasing pH from 9 to 4. Organic acids with ≥2 carboxy groups enhanced H(2)O(2)-dependent iodide oxidation (1.5-15-fold) as a function of increasing pH above pH 6.0, but had no effect at pH ≤ 5.0. The results indicate that as pH decreases (≤5.0), increasing H(2)O(2) hydrolysis is the driving force behind iodide oxidation. However, at pH ≥ 6.0, spontaneous decomposition of peroxy carboxylic acids, generated from H(2)O(2) and organic acids, contributes significantly to iodide oxidation. The results reveal an indirect microbial mechanism, organic acid secretion coupled to H(2)O(2) production, that could enhance iodide oxidation and organo-iodine formation in soils and sediments.     

Factors controlling mobility of 127I and 129I species in an acidic groundwater plume at the Savannah River Site

In order to quantify changes in iodine speciation and to assess factors controlling the distribution and mobility of iodine at an iodine-129 (¹²⁹I) contaminated site located at the U.S. Department of Energy's Savannah River Site (SRS), spatial distributions and transformation of ¹²⁹I and stable iodine (¹²⁷I) species in groundwater were investigated along a gradient in redox potential (654 to 360 mV), organic carbon concentration (5 to 60 μmol L^− 1), and pH (pH 3.2 to 6.8). Total ¹²⁹I concentration in groundwater was 8.6 ± 2.8 Bq L^− 1 immediately downstream of a former waste seepage basin (well FSB-95DR), and decreased with distance from the seepage basin. ¹²⁷I concentration decreased similarly to that of ¹²⁹I. Elevated concentrations of ¹²⁷I or ¹²⁹I were not detected in groundwater collected from wells located outside of the mixed waste plume of this area. At FSB-95DR, the majority (55-86%) of iodine existed as iodide for both ¹²⁷I and ¹²⁹I. Then, as the iodide move down gradient, some of it transformed into iodate and organo-iodine. Considering that iodate has a higher K_d value than iodide, we hypothesize that the production of iodate in groundwater resulted in the removal of iodine from the groundwater and consequently decreased concentrations of ¹²⁷I and ¹²⁹I in downstream areas. Significant amounts of organo-iodine species (30-82% of the total iodine) were also observed at upstream wells, including those outside the mixed waste plume. Concentrations of groundwater iodide decreased at a faster rate than organo-iodine along the transect from the seepage basin. We concluded that removal of iodine from the groundwater through the formation of high molecular weight organo-iodine species is complicated by the release of other more mobile organo-iodine species in the groundwater.     

Detection and activity of iodine-131 in brown algae collected in the Japanese coastal areas

Iodine-131 (physical half-life: 8.04 days) was detected in brown algae collected off the Japanese coast. Brown algae have been extensively used as bioindicators for radioiodine because of their ability to accumulate radionuclides in high concentration factors. The maximum measured specific activity of ¹³¹I in brown algae was 0.37 ± 0.010 Bq/kg-wet. Cesium-137 was also detected in all brown algal samples used in this study. There was no correlation between specific activities of ¹³¹I and ¹³⁷Cs in these seaweeds. The specific activity of ¹³⁷Cs ranged from 0.0034 ± 0.00075 to 0.090 ± 0.014 Bq/kg-wet. Low specific activity and minimal variability of ¹³⁷Cs in brown algae indicated that past nuclear weapon tests were the source of ¹³⁷Cs. Although nuclear power stations and nuclear fuel reprocessing plants are known to be pollution sources of ¹³¹I, there was no relationship between the sites where ¹³¹I was detected and the locations of nuclear power facilities. Most of the sites where ¹³¹I was detected were near big cities with large populations. Iodine-131 is frequently used in diagnostic and therapeutic nuclear medicine. On the basis of the results, we suggest that the likely pollution source of ¹³¹I, detected in brown seaweeds, is not nuclear power facilities, but nuclear medicine procedures.