Evolution of pH, organic matter and (226)radium/calcium partitioning in U-mining debris following revegetation with pine trees

Substratum influences uptake of radium-226 by plants

Radium-226, an alpha emitter with half-life 1600 years, is ubiquitous in natural environments. Present in rocks and soils, it is also absorbed by vegetation. The efficiency of ²²⁶Ra uptake by plants from the soil is important to assess for the study of heavy metals uptake by plants, monitoring of radioactive pollution, and the biogeochemical cycle of radium in the Critical Zone. Using a thoroughly validated measurement method of effective ²²⁶Ra concentration (EC_Ra) in the laboratory, we compare EC_Ra values of the plant to that of the closest soil, and we infer the ²²⁶Ra soil-to-plant transfer ratio, R_SP, for a total of 108 plant samples collected in various locations in France. EC_Ra values of plants range over five orders of magnitude with mean (min-max) of 1.66 ± 0.03 (0.020-113) Bq kg^-1. Inferred R_SP values range over four orders of magnitude with mean (min-max) of 0.0188 ± 0.0004 (0.00069-0.37). The mean R_SP value of plants in granitic and metamorphic context (0.073 ± 0.002; n = 50) is significantly higher (12 ± 1 times) than that of plants in calcareous and sedimentary context (0.0058 ± 0.0002; n = 58). This difference, which cannot be attributed to a systematic difference in emanation coefficient, is likely due to the competition between calcium and radium. In a given substratum context, the compartments of a given plant species show coherent and decreasing R_SP values in the following order (acropetal gradient): roots > bark > branches and stems ≈ leaves. Oak trees (Quercus genus) concentrate ²²⁶Ra more than other trees and plants in this set. While this study clearly demonstrates the influence of substratum on the ²²⁶Ra uptake by plants in non-contaminated areas, our measurement method appears as a promising practical tool to use for (phyto)remediation and its monitoring in uranium- and radium-contaminated areas.

Assessment of radium-226 bioavailability and bioaccumulation downstream of decommissioned uranium operations, using the caged oligochaete (Lumbriculus variegatus)

The present study investigated the integrated effects of several geochemical processes that control radium-226 ((226) Ra) mobility in the aquatic environment and bioaccumulation in in situ caged benthic invertebrates. Radium-226 bioaccumulation from sediment and water was evaluated using caged oligochaetes (Lumbriculus variegatus) deployed for 10 d in 6 areas downstream of decommissioned uranium operations in Ontario and Saskatchewan, Canada. Measured (226) Ra radioactivity levels in the retrieved oligochaetes did not relate directly to water and sediment exposure levels. Other environmental factors that may influence (226) Ra bioavailability in sediment and water were investigated. The strongest mitigating influence on (226) Ra bioaccumulation factors was sediment barium concentration, with elevated barium (Ba) levels being related to use of barium chloride in effluent treatment for removing (226) Ra through barite formation. Observations from the present study also indicated that (226) Ra bioavailability was influenced by dissolved organic carbon in water, and by gypsum, carbonate minerals, and iron oxyhydroxides in sediment, suggestive of sorption processes. Environmental factors that appeared to increase (226) Ra bioaccumulation were the presence of other group (II) ions in water (likely competing for binding sites on organic carbon molecules), and the presence of K-feldspars in sediment, which likely act as a dynamic repository for (226) Ra where weak ion exchange can occur. In addition to influencing bioavailability to sediment biota, secondary minerals such as gypsum, carbonate minerals, and iron oxyhydroxides likely help mitigate (226) Ra release into overlying water after the dissolution of sedimentary barite. Environ Toxicol Chem 2015;34:507-517. © 2014 SETAC.

Distribution and relationship of uranium and radium along an allochthonously dominated wetland gradient

Uranium mining may pose a large threat for freshwater ecosystems, caused by elevated concentrations of metals/radionuclides in drainage water. Important pollutants of such waters are uranium (U) and radium (Ra), because of their impact due to both radio- as well as chemo-toxicity. Despite the comprehensive knowledge about specific element speciation as well as fixation processes, less is known about the retention of U and Ra at a higher level of complexity (within allochthonous ecosystems as predominant for low order streams). Consequently, we investigated the distribution and retention potential of allochthonous ecosystems regarding U and Ra as well as changing U/Ra ratios. We found U predominantly transported over long distances, whereas Ra mainly precipitates immediately after reaching the surface, i.e. in the spring area. Although high U accumulation in organic rich sediments is found, still high transport rates are detected. Low overall fixation of U within the allochthonously dominated wetland results in an U transport over long distances. Consequently, large areas are affected by U mining activities and its post-mining impact, with U being more relevant compared to Ra.

Vegetation composition and ²²⁶Ra uptake by native plant species at a uranium mill tailings impoundment in South China

A field investigation was conducted for the vegetation composition and (226)Ra uptake by native plant species at a uranium mill tailings impoundment in South China. 80 species belonging to 67 genera in 32 families were recorded in the sampling sites. The Poaceae and Asteraceae were the dominant families colonizing the impoundment. The number of the plant species and vegetation community composition in the sampling sites seemed most closely related to the activities of (226)Ra and the pH value of the uranium tailings. The plant species in the sampling sites with relatively low activities of (226)Ra and relatively high pH value formed a relatively stable vegetation community. The plant species in the sampling sites with medium activities of (226)Ra and medium pH value formed the transitional vegetation community. The plant species in the sampling sites with relatively high activities of (226)Ra and relatively low pH value formed a simple unstable vegetation community that was similar to that on the unused grassland. The activities of (226)Ra and transfer factors (TFs) varied greatly with the plant species. The high activities of (226)Ra and TFs were found in the leaves of Pteris multifida (150.6 Bq/g of AW; 9.131), Pteridium aquilinum (122.2 Bq/g of AW; 7.409), and Dryopteris scottii (105.7 Bq/g of AW; 6.408). They satisfied the criteria for a hyperaccumulator for (226)Ra. They may be the candidates for phytoremediation of (226)Ra in the uranium mill tailings impoundment areas and the contaminated soils around.

Removal of radium from groundwater using a modified bauxite refinery residue

Radium (Ra) removal by an unconventional sorbent, a modified bauxite refinery residue (MBRR), is investigated for a groundwater extracted in Missouri, USA. The MBRR treatment causes substantial reductions of both gross α and combined Ra activities from 0.955 ± 0.005 and 0.66 ± 0.005 Bq L to below detection limits (0.037 Bq L or 1 pCi L). Column breakthrough occurs at 0.555 Bq L for gross α and 0.185 Bq L for combined Ra (15 and 5 pCi L; USEPA's maximum contaminant levels) after 54 and 40 d run time, respectively. At 84 d the MBRR media continues to remove 24.3% of raw water gross α and 39.7% of the combined Ra. The treatment effluent has an initial pH of 10.9, outside the USEPA guides (6.5-8.5); this may be readily mitigated by posttreatment acid injection, or by raw water blending. The MBRR simultaneously removes other potentially hazardous trace elements (e.g., Cu, Zn, and Fe) to extremely low concentrations. In addition, toxicity characteristic leaching procedure testing of spent MBRR suggests that metals are bound tightly, such that it is nonhazardous, permitting cost-effective disposal to landfill without special confinement or storage. Consequently, MBRR may be utilized as an alternative adsorbent for treating Ra-contaminated groundwater.