[en] The kinetics and reversibility of radiocaesium solid/liquid partitioning in sediments have been reviewed and interpreted in terms of a mechanistic framework. This framework is based on the premise that radiocaesium is almost exclusively and highly-selectively bound to the frayed particle edges of illitic clay minerals in sediments. Several processes with distinctly different rates can be distinguished in radiocaesium sorption to sediments. 2- and 3-box kinetic models can describe both the overall solid/liquid partitioning in sediments and the reversible (exchangeable) and irreversible (non-exchangeable or 'fixed') fractions of radiocaesium in sediments over time scales relevant for natural aquatic systems. The obtained rate parameters indicate that reversible partitioning of radiocaesium dominates over the first few days following a contamination event, whereas irreversible kinetics become important over time scales of weeks to months. The slow process, which reduces the exchangeability of sediment-bound radiocaesium over time, is believed to result from a migration of radiocaesium from exchangeable sites on the frayed edges of illite towards less-exchangeable interlayer sites. Long-term extraction of radiocaesium from historically contaminated sediments has given evidence for a reverse (remobilization) process with a half-life of the order of tens of years. These findings suggest that the long-term exchangeability of radiocaesium in sediments may be higher than the few percent which is generally assumed. 5 figs., 3 tabs., 19 refs

[en] The overall objective of the title multidisciplinary project is to improve both the applicability to dynamic situations and transportability between sites of mathematical models of radionuclide transport through the aquatic environment, by replacing the present black-box models with functional models which incorporate fundamental scientific principles within them. The objectives for the reporting period (1992-1995) are (1) to test the general validity of the in-situ K_D(¹³⁷Cs)/NH⁴⁺ ion-exchange relationship and its power to predict radiocaesium mobility in (and remobilisation from) freshwater sediments; (2) determination of 'exchangeable' in-situ radiocaesium-K_D's, in addition to 'total' in-situ radiocaesium-K_D's, by multiple extraction of the sediment with NH⁴⁺-solutions and subsequent preconcentration/low-background measurement of the extracted-¹³⁷Cs; and (3) search for possibilities (method) to measure the reverse rate constants of radiocaesium sorption (remobilisation) on natural samples, to be included in kinetic models for radiocaesium sorption. 1 fig., 3 tabs., 8 refs

[en] ECOPRAQ (ECOlogical PROcesses in AQuatic systems) refers, together with the full title of the project, to the process- or mechanistically-based approach to modelling the fluxes and the availability of radionuclides to the biosphere in the aquatic environment. The global objectives of the project are the development of mechanistic and general applicable whole-ecosystem models. Because of their mechanistic basis, these models enable the advancement of a sound theoretical basis for chemical and hydrological countermeasures. The models can also be applied to test the effectiveness of existing countermeasures. The project focuses on radiocaesium and radiostrontium, with emphasis on the former radionuclide, and is subdivided into the following six 'work packages': (1) Development of mechanistic submodels for the solid/liquid partitioning and (bio)availability of particle-bound radiocaesium and radiostrontium (WP2); (2) Development of mechanistic submodels for the accumulation and elimination of radiocaesium and radiostrontium by aquatic biota (WP3); (3) Development and assessment of chemical and/or hydrological countermeasures (WP4); (4) Development of mechanistically-based whole-ecosystem models (WP5); (5) Model validation of sediment-water exchange and bioavailability of radionuclides in a large-scale controlled laboratory setup and in situ (WP6); and (6) Model validation of radionuclide scavenging by suspended particles and uptake by aquatic plants (WP7) The development of mechanistic submodels for the basic aquatic processes such as partitioning of the radionuclides with inorganic (suspended and sediment) particles, accumulation and elimination by biota, transport processes in lakes/rivers and their catchments, and the incorporation of these basic processes/models in whole-ecosystem models, is organised in the first 4 work packages. Useable versions of these (sub)models are available and presented and discussed in Chapter 3 of this report. lt is clearly shown that these models can predict transport, solid/liquid partitioning, and biological uptake and biological elimination of radiocaesium much more accurately and over a range of very different aquatic environments, compared to the (more empirical) models that were available at the start of the project. This is the case both at the submodel ecosystem level and at the whole-ecosystem level. The key parameters that affect these basic transport and availability processes have been identified and the (sub)models are tested for systems that span the range over which these parameters may be encountered in different aquatic environments. Contrary to many earlier projects/studies, all laboratory experiments, large-scale tests, and field measurements have been designed to identify the role of the major environmental variables on basic model-parameters such as the solid/liquid distribution coefficient (K_D), the Concentration Factor (CF), and the rates of radionuclide 'fixation'/ remobilization by sediments and uptake/elimination by aquatic plants and fauna. All experimental and field data have been collected and interpreted in the framework of the above (sub)models. lt is shown that these monitoring data have been used for a successful model validation and parametrisation. 13 refs

[en] Study of ¹³⁷Cs in rivers implies knowledge of its adsorption behaviour towards particulate matter and sediments and the possibility of low-level counting of water and solids. This paper focuses on the low-level counting of ¹³⁷Cs. The use of flat-ended Ge(Li) detectors is compared to that of an anticoincidence array after preconcentration of the cesium ions into a ≤ 0.5 ml solid aliquot. The lower limits of determination at a relative standard deviation of 10% are ≅ 10 Bq · kg^-1 for aliquots received on a flat Ge(Li) detector and ≅ 10 mBq in case of preconcentration from water and counting in the anticoincidence facility. The eventual improvement of the accuracy by applying 'moving average' and 'smoothing' procedures are considered. (author) 11 refs.; 1 fig.; 3 tabs

[en] The kinetics and reversibility of radiocaesium solid/liquid partitioning in sediments have been reviewed and interpreted in terms of a mechanistic framework. This framework is based on the premise that radiocaesium is almost exclusively and highly-selectively bound to the frayed particle edges of illitic clay minerals in the sediments. Several processes with distinctly different rates can be distinguished in radiocaesium sorption to sediments. 2- and 3-box kinetic models can describe both the overall solid/liquid partitioning in sediments and the reversible (exchangeable) and irreversible (nonexchangeable or 'fixed') fractions of radiocaesium in sediments over time scales relevant for natural aquatic systems. The obtained rate parameters indicate that reversible partitioning of radiocaesium dominates over the first few days following a contamination event, whereas irreversible kinetics becomes important over time scales of weeks to months. The slow process, which reduces the exchangeability of sediment-bound radiocaesium over time, is believed to result from a migration of radiocaesium from exchangeable sites on the frayed edges of illite towards less-exchangeable interlayer sites. Long-term extraction of radiocaesium from historically contaminated sediments has given evidence for a reverse (remobilization) process with a half-life of the order of tens of years. These findings suggest that the long-term exchangeability of radiocaesium in sediments may be higher than the few % which is generally assumed. (orig.)

[en] In order to prevent CO2 concentrations in the atmosphere rising to unacceptable levels, carbon dioxide can be separated from the flue gas of, for example, a power plant and subsequently sequestrated. Various technologies for carbon dioxide sequestration have been proposed, such as storage in depleted gas fields, oceans and aquifers. An alternative sequestration route is the so-called 'mineral CO2 sequestration' route in which CO2 is chemically stored in solid carbonates by the carbonation of minerals. As mineral feedstock, rocks that are rich in alkaline earth silicates can be used. Examples are olivine (MgSiO4) and wollastonite (CaSiO3). Mineral CO2 sequestration has some fundamental advantages compared to other sequestration routes. The formed products are thermodynamically stable and therefore the sequestration of CO2 is permanent and safe. Furthermore, the sequestration capacity is large because large suitable feedstock deposits are available worldwide. Finally, the carbonation reactions are exothermic and occur spontaneously in nature. The reaction rates of the process at atmospheric conditions, however, are much too slow for an industrial process. Therefore, research focuses on increasing the reaction rate in order to obtain an industrial viable process. Optimisation of the process conditions is constrained by the thermodynamics of the process. Increasing the temperature and CO2 pressure accelerates the reaction rate, but gaseous CO2 is favoured over mineral carbonates at high temperatures. Using water or another solvent to extract the reactive component from the matrix accelerates the process. Pre-treatment of the mineral by size reduction and thermal or mechanical activation and optimisation of the solution chemistry result in major improvements of the reaction rate. During recent years, laboratory-scale experiments have shown major improvements of the conversion rates by developing various process routes and optimising process conditions. The most promising route available seems to be the direct aqueous route, for which reasonable reaction rates at feasible process conditions have been shown. Important aspects of mineral CO2 sequestration are the transport of the materials involved and the fate of the products. Transport costs can be minimised by transporting the carbon dioxide towards a mineral sequestration plant situated near the feedstock mine. The carbonated products can be used for mine reclamation and construction applications. Unfortunately, only few rough cost estimates have been published and detailed cost analyses of the most promising process routes are absent in the literature. Therefore, at present, there is insufficient knowledge to conclude whether a cost-effective and energetically acceptable process will be feasible. Mineral carbon sequestration is a longer-term option compared to other sequestration routes, but its fundamental advantages justify further research. Major issues that need to be resolved in order to enable large-scale implementation are the energy consumption of the process, the reaction rates and the environmental impact of mineral CO2 sequestration. Finally, the use of alkaline solid wastes as an alternative feedstock for calcium or magnesium is acknowledged and warrants further research

[en] The contents of the overhead sheets on the title subject concern the principles of mineral CO2 sequestration, an example of alkaline waste carbonation, carbonation experiments with steel slag, characterization of the carbonation process and the rate-limiting step, comparison of the carbonation of steel slag and wollastonite, leaching of sequestration residues, and conclusions

[en] The increasing interest in mineral CO2 sequestration caused the need for an update of the ECN literature review on this subject (February 2003). The present report reviews literature published in 2003 and 2004 on the carbonation of solid Ca/Mg-silicates for CO2 sequestration. This review update confirms the selection in the previous report of the so-called aqueous mineral carbonation route as the most promising process route. Much progress has been made on this route in recent years resulting in a system study that showed this approach to be both technologically and energetically feasible. However, sequestration costs are (still) too high compared to other CO2 storage options and in view of expected CO2 market prices. Cost reductions might be achieved by adding suitable additives to enhance the reaction rate in an (two-step) aqueous process, by developing large-scale continuous reactors and (on a limited scale) by using low-cost feedstock such as industrial alkaline solid residues. A breakthrough in further cost reduction of mineral sequestration probably has to come from totally new concepts, such as a mineral sequestration process integrated within a power plant. Very limited research has been published on such approaches. Beneficial re-use of carbonated products could also reduce sequestration costs for the first mineral CO2 sequestration (demonstration) plants. Mineral CO2 sequestration (still) is a longer-term option compared to other sequestration options, and probably has limited potential in the short term. Further technology development and cost reduction are needed for mineral CO2 sequestration to become part of a broad portfolio of employable CO2 sequestration technologies. In the present report, the need for a new study on mineral CO2 sequestration by the International Energy Agency (IEA) is also assessed. In 2000, the IEA published an evaluation of the technological and economical feasibility of a number of CO2 mineralisation process routes. The main conclusion of that evaluation was that none of the process routes studied proved to be energetically and economically feasible. With regard to the process-routes evaluated in the IEA study, the present report confirms this conclusion. However, the IEA report did not include the most promising carbonation process route available today (i.e., the aqueous carbonation route). Therefore, an update of the IEA-assessment on 'CO2 storage as carbonate minerals' is, in principle, advisable. However, since a (cost-)evaluation of the aqueous carbonation route has been published recently, it is questionable whether a new IEA assessment study would provide sufficient new insights at this moment. Furthermore, more information is required on the feasibility of some potentially attractive developments that are currently in a conceptual state. In view of these considerations, the IEA is advised to repeat this literature review, with a similar scope, in 2-3 years. The developments over this period may provide the necessary new insights to warrant a new assessment on mineral CO2 sequestration by the IEA

[en] A study of ¹³⁷Cs in rivers implies a knowledge of its adsorption behaviour towards particulate matter and sediments and the possibility of low-level counting of water and solids. This text focuses on the low level counting of ¹³⁷Cs. The use of flat-ended Ge(Li) detectors is compared to that of an anticoincidence array after preconcentration of the cesium ions into a≤0.5 ml solid aliquot. The eventual improvement of the accuracy by applying 'moving average' and 'smoothing' procedures are considered. (author) 10 refs.; 1 fig.; 4 tabs

[en] The enormous and worldwide production of coal fly ash cannot be durably isolated from the weathering cycle, and the weathering characteristics of fly ash must be known to understand the long-term environmental impact. The authors studied the weathering of two coal fly ashes and compared them with published data from weathered volcanic ash, it's closest natural analogue. Both types of ash contain abundant aluminosilicate glass, which alters to noncrystalline clay. However, this study reveals that the kinetics of coal fly ash weathering are more rapid than those of volcanic ash because the higher pH of fresh coal fly ash promotes rapid dissolution of the glass. After about 10 years of weathering, the noncrystalline clay content of coal fly ash is higher than that of 250-year-old volcanic ash. The observed rapid clay formation together with heavy metal fixation imply that the long-term environmental impact of coal fly ash disposal may be less severe and the benefits more pronounced than predicted from previous studies on unweathered ash. Their findings suggest that isolating coal fly ash from the weathering cycle may be counterproductive because, in the long-term under conditions of free drainage, fly ash is converted into fertile soil capable of supporting agriculture