Diffusion of radionuclides in concrete and concrete-bentonite systems

Various construction materials are under consideration for nuclear waste repositories. Two important materials are concrete and bentonite clay, which will act as mechanical barriers and prevent convective water flow. These barriers will also retard transport (diffusion controlled) of dissolved radionuclides by a combination of mechanical constraints and chemical interactions with the solid.An important issue is the possible change of the initial sodium bentonite into the calcium form due to interaction with calcium from the concrete. The initial leaching of concrete was studied using radioactive spiked concrete in contact with compacted bentonite.Measurement were made of the diffusion of Cs, Am and Pu into 5 different types of concrete in contact with pore water. The diffusivity measured for Cs agrees reasonably well with data found in the literature. No movement could be measured for Am and Pu (< 0.2 mm), even though the contact times were extremely long (2.5 and 5 yr, respectively). The diffusion of Na, Ca and Cs from concrete into bentonite was also measured.     

Modelling radionuclide distribution and transport in the environment

Mathematical models of radionuclide distribution and transport in the environment have been developed to assess the impact on people of routine and accidental releases of radioactivity from a variety of nuclear activities, including: weapons development, production, and testing; power production; and waste disposal. The models are used to estimate human exposures and doses in situations where measurements have not been made or would be impossible or impractical to make. Model results are used to assess whether nuclear facilities are operated in compliance with regulatory requirements, to determine the need for remediation of contaminated sites, to estimate the effects on human health of past releases, and to predict the potential effects of accidental releases or new facilities. This paper describes the various applications and types of models currently used to represent the distribution and transport of radionuclides in the terrestrial and aquatic environments, as well as integrated global models for selected radionuclides and special issues in the fields of solid radioactive waste disposal and dose reconstruction. Particular emphasis is placed on the issue of improving confidence in the model results, including the importance of uncertainty analysis and of model verification and validation.     

Studies on oxygen toxicity after simultaneous administration of paraquat and chelate-forming agents in rats

The influence of intravenously administered mixtures of paraquat and the chelating agents CaNa₂-EDTA or dithiocarb on the toxicity of oxygen at normal pressure was studied in rats. No increase in toxic effects of the combination paraquat-oxygen was seen by additionally administered chelate-forming agents.This paper completes the investigations on oxygen toxicity after administration of various chelate-forming agents in mice¹.     

Synergistic cytotoxicity between pentachlorophenol and copper in a bacterial model.

Both pentachlorophenol (PCP) and copper compounds have been widely used as wood preservatives, and are commonly found not only in the area near wood-preserving facilities, but also in body fluids and tissues of people who are not occupationally exposed to them. In this study, we found that exposing bacteria to a combination of PCP and copper at non- or sub-toxic concentrations resulted in enhanced cytotoxic effect in a synergistic mode as indicated by both the inhibition of growth and the lowering of the colony-forming ability. The toxicity of the combination PCP/Cu(II) was relieved by hydrophilic chelating agents, thiol compounds and adventitious proteins, but was markedly potentiated by low levels of the lipophilic metal chelating agents.     

Seasonal variation of radionuclides in Fucus vesiculosus L. from the Oresund, Southern Sweden

The activity concentrations of radionuclides (60Co, 54Mn, 137Cs, 40K) in whole plants of Fucus vesiculosus of different ages, and also in tissues of different ages from the same plants were investigated in the vicinity of the Barseb?ck nuclear power plant in southern Sweden. Activity concentration differed between the tissues of different age of the plants, depending on the radionuclide as well as the time of year. The highest concentrations of 60Co and 54Mn were measured in the older parts of the plants in spring and summer, while the activity concentrations of 137Cs and 40K were highest in receptacles and new vegetative fronds. The discharge of radionuclides from the nuclear power plant was reflected in the measured activity concentration in the algae. For 60Co, the response seemed to be smoothed out when considering the whole plant, while the new vegetative tissue better reflected the discharge during the autumn. Thus, it is important to have knowledge about the autecology of the organism when it is used as an indicator for radionuclides.     

Translocation studies of 137Cs and 90Sr in bean plants (Phaseolus vulgaris): simulation of fallout

In case of an accident at a nuclear power plant with liberation of radioactive material into the atmosphere, knowledge about the behavior of plant species when in contact with radionuclides is indispensable for safety reasons. The leaf-fruit translocation is an important route through which agricultural products are contaminated by radionuclides. To quantify the leaf-fruit translocation factors for 137Cs and 90Sr in common beans (Phaseolus vulgaris) an experiment was carried out in a greenhouse with a randomized block design. 137Cs activity was determined by gamma-ray spectrometry, while chemical separation followed by beta counting of 90Y was used for 90Sr determination. The model applied for translocation indicated functional dependence between the moment of tracer application and the physiological development of the bean plant. Translocation factors obtained for 137Cs and 90Sr were 0.16 and less than 0.005, respectively.     

Chelate assisted phytoextraction of heavy metals from soil. Effect, mechanism, toxicity, and fate of chelating agents

The low-cost, plant-based phytoextraction technique has often been described as a promising technique to remediate heavy metal contaminated agricultural land. The application of chelating agents has shown positive effects in increasing the solubility of heavy metals in soil and therefore in enhancing phytoextraction. This paper gives an overview of the chelating agents applied in recent studies. Various synthetic aminopolycarboxylic acids, such as ethylene diamine tetraacetic acid, and natural ones such as, ethylene diamine disuccinate and nitrilotriacetic acid, are described. Additionally, results of the application of natural low molecular weight organic acids, such as citric and tartaric acid are given. The effectiveness of these different chelating agents varies according to the plant and the heavy metals used. Furthermore, a focus is laid on the chelating agents fate after application and on its toxicity to plants and soil microorganisms, as well as it degradation. The rate of degradation is of great importance for the future of chelate assisted phytoextraction as it has a direct impact on the leaching probability. An effective prevention of leaching will be crucial for the acceptance and the economic breakthrough of enhanced phytoextraction, but a satisfactory solution to this key issue has so far not been found. Possibly further experiments in the field of enhanced phytoextraction will be able to solve this major problem, but over decades various greenhouse experiments and recently field experiments have resulted in different observations. Therefore, it is questionable if further research in this direction will lead to a promising solution. Phytoextraction has possibly reached a turning point in which it should distance itself from chelate assisted phytoextraction and focus on alternative options.     

Study of the contaminant transport into granite microfractures using nuclear ion beam techniques

Hydrated bentonite is a very plastic material and it is expected to enter in the rock microfractures at the granite/bentonite boundary of a deep geological high-level waste repository. This process is enhanced by the high swelling pressure of the clay. Since bentonite has a very good sorption capability for many radionuclides, the displacement of the clay might lead to a "clay-mediated" contaminant transport into the rock. The aim of this work is to study the contaminant transport into granite microfractures using nuclear ion beam techniques, and to determine to what extent the clay can favour it. To do so, bentonite previously doped with uranium, cesium and europium was put in contact with the surface of granite sheets. Granite sheets contacted with non-doped bentonite and with radionuclide solutions were also prepared as references. This allowed analysing the differences in the diffusion behaviour of the three systems: clay, radionuclides and clay plus radionuclides. A combination of Rutherford backscattering spectrometry (RBS) and other nuclear ion-beam techniques such as particle-induced X-ray emission (PIXE) and microPIXE was used to study the depth and lateral distribution of clay and contaminants inside granite. It was also tried to evaluate not only the diffusion depth and diffusion coefficients but also the different areas of the granite where the diffusants have a preferential access.     

Enhanced uptake of As, Zn, and Cu by Vetiveria zizanioides and Zea mays using chelating agents

Vetiveria zizaniodes (vetiver) is commonly known for its effectiveness in soil and sediment erosion control. It can tolerate to extreme soil conditions and produce a high biomass even growing in contaminated areas. Zea mays (maize) can also produce a very high biomass with a fast growth rate and possesses some degree of metal tolerance. A greenhouse study was conducted to investigate the feasibility of using vetiver and maize for remediation of arsenic (As)-, zinc (Zn-), and copper (Cu)-amended soils and evaluate the effects of chelating agents on metal uptake by these plants. Vetiver had a better growth (dry weight yield of root and shoot) than maize under different treatment conditions. The effects of different chelating agents on As, Zn, and Cu extraction from soil to soil solution were studied. Among the nine chelating agents used, it was noted that 20 mmol NTA could maximize As and Zn bioavailability, while 20 mmol HEIDA could maximize Cu bioavailability in the soil solution. The surge time in maximizing metal uptake ranged from 16 to 20 days which indicated that timing on plant harvest was an important factor in enhanced metal accumulation. In general, vetiver was a more suitable plant species than maize in terms of phytoextraction of metals from metal-contaminated soil. Application of NTA in As-amended soil and HEIDA in Cu-amended soil at the rate of 20 mmol kg(-1) increased 3-4-fold of As and Cu in shoot of both plants, whereas application of NTA (20 mmol kg(-1)) increased 37- and 1.5-fold of Zn accumulation in shoot of vetiver and maize, respectively. The potential environmental risk of metal mobility caused by chelating agents used for phytoextraction should not be overlooked.     

Degradation of chelating agents in aqueous solution using advanced oxidation process (AOP)

This article presents an overview with critical analysis of technical applicability of advanced oxidation process (AOP) in removing chelating agents from aqueous solution. Apart from the effect of metals for chelating agents as a major influencing factor, selected information such as pH, oxidant’s dose, concentrations of pollutants and treatment performance is presented. The performance of individual AOP is compared. It is evident from our literature survey that photocatalysis with UV irradiation alone or coupled with TiO₂, ozonation and Fenton’s oxidation are frequently applied to mineralize target pollutants. Overall, the selection of the most suitable AOP depends on the characteristics of effluents, technical applicability, discharge standard, regulatory requirements and environmental impacts.     

60Co retention by a freshwater planktonic alga Scenedesmus obliquus

This paper presents the results of various decontamination experiments aimed at evaluating (60)Co retention by Scenedesmus obliquus, and the respective roles played by absorption and adsorption in the contamination of the alga by this radionuclide. The physiological condition of cells is not involved in radiocobalt desorption, which seems to indicate that the phenomenon is of a passive nature. When the precontamination time is extended, the proportion of adsorbed (60)Co decreases, and the final percentage of radionuclide retained increases. These results are confirmed by bringing cells into contact with a strong chelating agent. The elimination of (60)Co in the presence of EDTA undergoes a sharp decrease which is correlated with the increase in the duration of the precontamination phase. The use of a weaker complexing agent, such as NaCl, leads to the hypothesis of there being 2 types of (60)Co receptor on cell membranes.     

Combined use of a transformed red mud reactive barrier and electrokinetics for remediation of Cr/As contaminated soil

A reactive barrier (RB) of transformed red mud (TRM), a by-product of the refinement of bauxite in alumina production, was placed adjacent to the anode of an electrokinetic (EK) system with the aim of enhancing removal of chromium or arsenic, added singly to a low permeability clayey soil, and favouring entrapment. The innovative study focused on evaluation of the synergic interaction between the EK system and the RB, and of the efficiency when compared to traditional EK remediation (control tests). The results obtained underlined the successful outcome of treatment of the Cr(VI)-contaminated soil. In presence of the TRM RB, 19.4% wt. of total Cr content was detected in the anolyte and 20.6% wt. trapped in the anodic RB after 6 d, versus 6.6% wt. in the anolyte and 8.8% wt. in the soil adjacent to the anode following the control run without RB. On increasing duration of treatment up to 12 d, 60.8% wt. of total initial Cr was found in the anolyte and 25.5% wt. trapped in the RB, versus 9.1% wt. and 5.3% wt., respectively, after a control run of the same duration. Finally, on increasing the mass of TRM in the RB, 60.6% wt. of initial Cr content was found to have accumulated in the RB, with Cr being completely absent from the anodic chamber. Conversely, combined treatment was much less effective on As contaminated soil, at least under the operative conditions applied. Low initial As concentration and interference with iron oxides in the soil were likely the reasons underlying low efficiency while attempting As decontamination.     

pH-Dependent fate and transport of NTA-complexed cobalt through undisturbed cores of fractured shale saprolite

The codisposal of toxic metals and radionuclides with organic chelating agents has been implicated in the facilitated transport of the inorganic contaminants away from primary waste disposal areas. We investigated the transport of Co(II)NTA through undisturbed cores of fractured shale saprolite. Experiments were conducted across the pH range 4 to 8 by collecting cores from different locations within the weathering profile. Aqueous complexation, adsorption, dissociation and oxidation reactions influenced Co(II)NTA transport. The suite of reaction products identified in column effluent varied with experimental pH. At low pH and in the presence of abundant exchangeable aluminum, Co transport occurred predominantly as the Co2+ ion. At higher pH, Co was transported primarily as Co(II)NTA and the Co(III) species Co(III)(HNTA)2 and Co(III)(IDA)2. The formation of the geochemical oxidation products (Co(III) species) has far reaching implications as these compounds are kinetically and thermodynamically stable, are transported more rapidly than Co(II)NTA, and are resistant to biodegradation. These results demonstrate that natural minerals, in the physical structure encountered naturally, can be more important in the formation of mobile, stable contaminant forms than they can be for the retardation and dissociation of the contaminants.     

Comparison of electrokinetic soil remediation methods using one fixed anode and approaching anodes

During the cation exchange membrane (CEM) enhanced electrokinetic (EK) soil remediation, the nearer to the anode, the higher are the H+ concentrations and the redox potentials. As both low pH and high redox potential are helpful to speed-up Cd electro-migration, soils near the anode can be quickly remedied. Usually EK process is operated with one fixed anode (FA). A novel CEM enhanced EK method with approaching anodes (AAs) is proposed to accelerate electro-migration effect. Several mesh Ti/Ru anodes were inserted as AAs in the treated soil. They were switched in turn from the anode towards the cathode. Thus high H+ ions concentrations and high redox potentials quickly migrate to the cathode. Consequently, soil remediation is accelerated and nearly 44% of energy and 40% of time can be saved. The mechanism of Cd electro-migration behavior in soils during CEM enhanced EK is described as the elution in an electrokinetically driven chromatogram.     

Potential of Hemidesmus indicus for phytoextraction of lead from industrially contaminated soils

Many sites in the industrial region of Kattedan near Hyderabad, Andhra Pradesh (AP), India are contaminated with high concentrations of lead. The use of plants to remove toxic metals from soils (phytoremediation) is emerging as a potential strategy for cost-effective and environmentally sound remediation of contaminated soils. We studied remediation of soils contaminated with lead using a lead hyperaccumulating plant, Hemidesmus indicus. The ability of this plant to accumulate lead in shoots and roots was studied with pot experiments. The results showed that accumulation was maximum in roots for the first 1-3 weeks and later for a contact period of three months, the accumulation rate was maximum in shoots. In addition we used various chelating agents such as EDTA, HEDTA, DTPA and CDTA to determine the best chemical modifier for efficient lead removal from contaminated soils. The effect on lead accumulation of plant in the presence of various metal co-ions was also studied. An attempt was made for the decontamination of lead from five different "Real-life" soils of Kattedan using H. indicus.     

国内外重金属废水处理新技术的研究进展

有毒重金属对环境的严重威胁正逐渐成为全球性问题，着重综述了近年来国内外重金属污染处理技术的研究与进展。除了改进传统的化学、物理方法外，各种廉价高吸附性能的吸附剂、新型可选择性重金属捕集剂都是研究的焦点。特别是随着生物技术的发展，利用功能菌和植物处理含重金属污染已取得相当成效，并在工业上进行了实际应用。研究和发展新型天然吸附剂、重金属捕集剂和生物技术，加强多种技术的综合应用，是治理重金属污染的有效途径。     

Targeting of observations for accidental atmospheric release monitoring

In the event of an accidental atmospheric release of radionuclides from a nuclear power plant, accurate real-time forecasting of the activity concentrations of radionuclides is acutely required by the decision makers for the preparation of adequate countermeasures. Yet, the accuracy of the forecasted plume is highly dependent on the source term estimation. Inverse modelling and data assimilation techniques should help in that respect. However the plume can locally be thin and could avoid a significant part of the radiological monitoring network surrounding the plant. Deploying mobile measuring stations following the accident could help to improve the source term estimation. In this paper, a method is proposed for the sequential reconstruction of the plume, by coupling a sequential data assimilation algorithm based on inverse modelling with an observation targeting strategy. The targeting design strategy consists in seeking the optimal locations of the mobile monitors at time t + 1 based on all available observations up to time t.The performance of the sequential assimilation with and without targeting of observations has been assessed in a realistic framework. It focuses on the Bugey nuclear power plant (France) and its surroundings within 50 km from the plant. The existing surveillance network is used and realistic observational errors are assumed. The targeting scheme leads to a better estimation of the source term as well as the activity concentrations in the domain. The mobile stations tend to be deployed along plume contours, where activity concentration gradients are important. It is shown that the information carried by the targeted observations is very significant, as compared to the information content of fixed observations. A simple test on the impact of model error from meteorology shows that the targeting strategy is still very useful in a more uncertain context.     

Radionuclide release and transport from nuclear underground tests performed at Mururoa and Fangataufa--predictions under uncertainty

In the context of a study by the International Geomechanical Commission (IGC) and the International Atomic Energy Agency (IAEA) on the effects of nuclear tests at the atolls of Mururoa and Fangataufa, release to the biosphere is estimated for 35 radionuclides originating from 147 nuclear underground tests. Based on a qualitatively characterised hydrogeological situation of atolls and relatively scarce site-specific data, a model chain was developed to conservatively estimate the radionuclide fluxes via groundwater, from their sources, the explosion cavities, towards the biosphere, the ocean or lagoon. Finite element hydro-thermal modelling was used to describe water flow. Parameters were calibrated by a very few measured pre-test temperature profiles in bore holes. The impact of the tests on groundwater flow and mechanical impact on rock was considered. Estimates were made to quantify spatial extensions and temporal evolution of impact by using measurements on refilling rate of the cavities. Tests were categorised according to their specific yield and location although detailed data were missing. A base case parameter set was defined for the hydraulic conditions and for the initial radionuclide inventory of individual tests. Models were used to describe the concentration of radionuclides in the cavities as a function of time. Radionuclide transport from the cavities to the biosphere was represented by two different approaches: a double porosity model for the fractured volcanic rock and a single porosity model for the overlaying, highly porous carbonates. Results consist of conservative estimates on radionuclide release into the environment, or concentration in the lagoon or ocean water. Their sensitivity was investigated using different models and parameters. A few measured data (concentrations in a few cavities, in the deep carbonates and in the lagoons for selected radionuclides, such as 3H, 14C, 36Cl, 90Sr, 129I, 137Cs239 240Pu and 241Am) were available for a comparison with the calculations. In view of the lack and uncertainty of site-specific data, the agreement is of acceptable quality.     

The effect of operating variables on chelant-assisted remediation of contaminated dredged sediment

The paper shows the results from a number of lab-scale washing treatments using the four chelating agents EDTA, NTA, citric acid and [S,S]-EDDS aiming at the remediation of a real heavy metal-contaminated sediment. Investigation of the influence of chelant type and concentration as well as solution pH was the major focus of the work. The analysis of speciation of metals and chelating agents in solution was carried out through geochemical speciation modelling in order to identify the optimal conditions for the washing process as well as to evaluate the competition phenomena of metal-chelant complexes in solution. The major competing cations were found to be Ca above all and Mg under specific conditions. Among the investigated chelating agents, EDDS appeared to be less affected by competition by major cations while ensuring adequate heavy metal extraction efficiencies. For a 1:1 chelant/metal ratio, the following ranking was observed: EDDS>Cit>NTA>EDTA for As, EDDS>NTA congruent withEDTA>Cit for Cu, EDDS congruent withEDTA congruent withNTA>Cit for Zn, EDTA>NTA>EDDS>Cit for Pb at pH 5 and EDTA congruent withEDDS congruent withNTA>Cit for Pb at pH 8. For a 10:1 chelant/metal ratio geochemical modelling indicated that at the equilibrium the extracting solutions were dominated by the free form of the chelating agents, indicating the inability of such species to complex trace metals due the strong interactions existing between heavy metal ions and sediment constituents.     

EDDS and EDTA-enhanced zinc accumulation by Solanum nigrum inoculated with arbuscular mycorrhizal fungi grown in contaminated soil

The effect of two different chelating agents [EDTA and EDDS (S,S-ethylenediaminedissucinic acid)] on Zn tissue accumulation in Solanum nigrum L. grown in a naturally contaminated soil was assessed. Under those conditions, the response of the plant to the inoculation with two different isolates of arbuscular mycorrhizal fungi (AMF)--Glomus claroideum and Glomus intraradices--was also studied. Plants grown in the local contaminated soil (Zn levels of 433mg kg(-1)) accumulated up to 1191mg kg(-1) of Zn in the roots, 3747mg kg(-1) in the stems and 3409mg kg(-1) in the leaves. S. nigrum plants grown in the same soil spiked with extra Zn (Zn levels of 964mg kg(-1)) accumulated up to 4735, 8267 and 7948mg Zn kg(-1) in the leaves, stems and roots, respectively. The addition of EDTA promoted an increase in the concentration of Zn accumulated by S. nigrum of up to 231% in the leaves, 93% in the stems and 81% in the roots, while EDDS application enhanced the accumulation in leaves, stems and roots up to 140, 124 and 104%, respectively. In the stems, the presence of Zn was predominantly detected in the cortex collenchyma cells, the starch sheath and the internal phloem and xylem parenchyma, and the addition of chelating agents did not seem to have an effect on the localisation of accumulation sites. The devise of a chelate-enhanced phytoextraction strategy, using chelating agents and AMF, is discussed.