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Applied Geochemistry (v.27, #2)

Preface by Lara Duro (pp. 377-377).

The effects of humic substances on the transport of radionuclides: Recent improvements in the prediction of behaviour and the understanding of mechanisms by Nick D. Bryan; Liam Abrahamsen; Nick Evans; Peter Warwick; Gunnar Buckau; Liping Weng; Willem H. Van Riemsdijk (pp. 378-389).

Some recent developments made during the European Union 6th Framework Integrated Project FUNMIG in the understanding and prediction of behaviour in ternary systems of radionuclides, humic substances and mineral surfaces are described. These developments are placed in the context of the existing literature. The aim is to describe the current understanding of humic substance mediated radionuclide transport as it may be applied to calculations in support of Radiological Performance Assessment. Some improvements in experimental techniques that provide the raw data to calibrate metal ion binding models are explained. The various metal ion binding models that are available are described and contrasted, before the recent development of ternary system models, in particular the Ligand Charge Distribution model that can predict metal ion and humic substance behaviour in ternary systems. The kinetic effects in ternary systems are described along with the models that are used to describe them. Finally, the remaining challenges in making predictions of radionuclide transport for the Radiological Performance Assessment of radioactive waste repositories are discussed.

Nanoparticles and their influence on radionuclide mobility in deep geological formations by Thorsten Schäfer; Florian Huber; Holger Seher; Tiziana Missana; Ursula Alonso; Michael Kumke; Sascha Eidner; Francis Claret; Frieder Enzmann (pp. 390-403).

► Inorganic nanoparticle concentrations in deep geological groundwaters are quantified. ► Fulvic acids are detected in crystalline environments and plastic Clay formations. ► Sources of inorganic nanoparticles from the repository near-field are investigated. ► Compacted bentonite erosion rates are quantified. ► Divalent cations strongly influence nanoparticle mobilization/stabilization.This article gives an overview of the current status of knowledge concerning the role of nanoparticles (inorganic and organic) in deep geological host rocks and the potential influence of these nanoparticles on radionuclide migration in far-field systems. The manuscript is not intended to be a full review paper or overview paper concerning nanoparticles, here the intention is to refer to recent publications but to highlight the progress made in the 6th framework project IP FUNMIG (Fundamental processes of radionuclide migration) and the open literature over the past 5a concerning the process understanding of nanoparticle related issues in the three host rock formations investigated, namely: claystones, crystalline rocks and salt rock overburden. The results show inter alia that the inorganic nanoparticle concentration in deep groundwaters of advection dominated systems rarely exceeds 1mgL⁻¹ and is expected to be in the ngL⁻¹ range in diffusion controlled systems. For organic nanoparticles DOC concentrations up to tens of milligrams in diffusion-controlled indurated clays with molecular sizes mostly <500Da have been found. Fulvic acid type organics have been identified in crystalline environments and plastic Clay formations (Boom Clay) with molecular sizes ⩽300kDa. Additional sources of inorganic nanoparticles from the repository near-field (compacted bentonite) were identified and the initial erosion rates were determined. The results indicate under stagnant conditions ∼38mgcm⁻²a⁻¹ for bi-distilled water, ∼20mgcm⁻²a⁻¹ for glacial melt water (Grimsel groundwater) and very low rates ∼0.02mgcm⁻²a⁻¹ for 5mM CaCl₂ contact water. The low critical coagulation concentration (CCC) indicative for purely diffusion controlled coagulation of 1mML⁻¹ Ca²⁺ found in bentonite nanoparticle stability analysis matches the low nanoparticle mobilization from compacted bentonite found in these systems.

Redox-active phases and radionuclide equilibrium valence state in subsurface environments – New insights from 6th EC FP IP FUNMIG by C. Bruggeman; N. Maes; B.C. Christiansen; S.L.S. Stipp; E. Breynaert; A. Maes; S. Regenspurg; M.E. Malström; X. Liu; B. Grambow; Th. Schäfer (pp. 404-413).

Within the 6th EC FP Integrated Project “Fundamental Processes of Radionuclide Migration” (FUNMIG), progress has been made to improve knowledge about the phases and reaction mechanisms involved in complex reduction processes of radionuclide contaminants in natural subsurface environments. This review paper gives an overview of the achievements made by the research groups involved in this project, and puts the scope and results of the studies in a more global context. Firstly, both thermodynamic and experimental evidence show that green rust is present and reactive in subsurface groundwater with a composition that spans the Fe(II)/Fe(III) redox boundary. Green rust has been shown to reduce Np(V), Se(VI) and Se(IV), but the pathways for the redox processes and the reaction products that result are complicated, and change as a function of the reaction parameters. Secondly, considerable evidence has emerged that Se(IV) is reduced on Fe(II)-bearing minerals which are ubiquitous in subsurface environments. The stable Se valence state in the presence of FeS₂ has been shown to be Se(0). Also, natural dissolved humic substances that contain sufficient electron donating capacity are capable of interacting with, and possibly reducing, Se(IV) to lower valence states. Thirdly, the influence ofHCO3- and organic ligands on the uptake and reduction of U(VI) on Fe(II)-bearing minerals was investigated. While it appeared thatHCO3- decreased the extent of U(VI) uptake by the reducing surface, the fraction of reduced U(IV) in the solid phase increased with increasingHCO3- concentration. In contrast with the observations forHCO3-, organic ligands decreased both the extent of U uptake, as well as the fraction of U(IV) found in the solid phase.The studies performed within FUNMIG show that investigating reduction-oxidation mechanisms require (1) a detailed control over reaction conditions (anoxic atmosphere, purification of solid phases, initial radionuclide speciation), (2) a rigorous follow-up of reaction products (both solution chemistry and spectroscopic methods), and (3) the consideration of slow kinetics in the setting up of an experiment. These requirements make the study and assessment of redox processes one of the most demanding scientific challenges for geochemists who are asked to make predictions for radionuclide transport behaviour in the environment.

Thermodynamic data provided through the FUNMIG project: Analyses and prospective by Pascal E. Reiller; Thomas Vercouter; Lara Duro; Christian Ekberg (pp. 414-426).

In this paper some of the needs for good quality thermodynamic data in radioactive waste management are highlighted. A presentation of the thermodynamic data produced within the 6th EC framework programme integrated project FUNMIG (Fundamental processes of radionuclide migration) and how these have helped in filling relevant thermodynamic data gaps is given. The manuscript does not intend to be a complete review of thermodynamic data, but a short overview of the aqueous complexation of Am(III), lanthanides (III), U(VI), and Th(IV) by sulfates, silicates, carbonates and phosphates. The work presented is based on the latest developments published in the literature and specifically addressed within the IP FUNMIG.

Summary of studies on (ad)sorption as a “well-established” process within FUNMIG activities by Johannes Lützenkirchen (pp. 427-443).

This paper summarizes the activities within the European project FUNMIG (FUNdamental Processes of Radionuclide MIGration) concerned with sorption as a “well-established” process. Within FUNMIG, many, if not all, work-packages in some way involved sorption processes. In order to avoid repetition within this special issue and to have a clear separation, the present contribution only gives an impression of the work done within the one particular work-package with the above title. The various subprojects from that work-package were regrouped into studies on model sorbents, on sorbents directly pertaining to host rock formations and on studies primarily concerned with sorption. For each of the issues a short summary with an aspect that may be of particular interest to readers from outside the nuclear waste community is given. The work on sorption covers experimental and modeling work on model sorbents (silica and alumina), sorbents related to the engineered barrier (clay minerals) and identified host rock formation related sorbents (clay minerals in France, Switzerland and Belgium; granite in Sweden and Finland). Experimental and modeling results on these systems are addressed in some detail. Pure modeling studies on database building and evaluation of highly simplified adsorption models were also carried out and these will be discussed along with a suggestion to relate a model to a certain purpose to facilitate justification of the model approach chosen.

From aqueous solution to solid solutions: A process oriented review of the work performed within the FUNMIG project by Jordi Bruno; Vanessa Montoya (pp. 444-452).

The Fundamentals of Radionuclide Migration (FUNMIG) project was organised in some Research and Technological Development Components (RTDCs) integrated by a number of Work Packages (WP’s) attempting to cover the key migration processes at different scales (laboratory to repository) and in different geological media, mainly clay and granite, but also some salt rock related work was performed. The main RTDCs of interest for this review paper are numbers 1 and 2, which concerned laboratory studies of well-established and less-established migration processes. Some additional interesting outcomes arise also from RTDCs 3, 4 and 5 devoted to the upscaling and applicability of migration processes to clay, granite and salt rock environments, respectively. This review discusses the contribution of the various parts of the FUNMIG project in the process oriented context of understanding the mechanistic approximation to what can be seen as the final and thermodynamically stable stage of the long chain of processes which are covered under the general term of sorption and that culminate in the formation of solid solutions.

Implementation of microbial processes in the performance assessment of spent nuclear fuel repositories by Thilo Behrends; Evelyn Krawczyk-Bärsch; Thuro Arnold (pp. 453-462).

Present strategies for the long-term disposal of high-level nuclear wastes are based on the construction of repositories hundreds of meters below the earth surface. Although the surrounding host-rocks are relatively isolated from the light at the earth surface they are by no means lifeless. Microorganisms rule the deep part of the biosphere and it is well established that their activity can alter chemical and physical properties of these environments. Microbial processes can directly and indirectly affect radionuclide migration in multiple ways. Within 6th FP IP FUNMIG the interplay between microbial biofilms and radionuclides and the effect of microbially induced redox transformations of Fe on radionuclide mobility have been investigated. For the first time, formation of U(V) as a consequence of microbial U(VI) reduction in a multi-species biofilm was detected in vivo by combining laser fluorescence spectroscopy and confocal laser scanning microscopy. Furthermore, it was demonstrated that addition of U(VI) can lead to increased respiratory activity in a biofilm. Increased respiration in a biofilm can create microenvironments with lower redox potential, and hence induce reduction of radionuclides. Transient mobilization of U was observed in experiments with Fe oxides containing adsorbed U(VI) in which the activity of SO₄-reducing organisms was mimicked by sulfide addition. Faster reaction of sulfide with Fe oxides compared to U(VI) reduction, and decreasing U(VI) adsorption due to the transformation of Fe oxides into FeS can explain the observed intermittent U mobilization. The presented research on microbe-radionuclide interactions performed within FUNMIG addresses only a few aspects of the potential role of microorganisms in the performance assessment of nuclear waste repositories. For this reason, additionally, this article provides a cursory overview of microbial processes which were not studied within the FUNMIG project but are relevant in the context of performance assessment. The following aspects are presented: (a) the occurrence and metabolic activity of microorganisms of several proposed types of host-rocks, (b) the potential importance of microorganisms in the near-field of nuclear waste repositories, (c) indirect effects of microbial processes on radionuclide mobility in the repository far-field, (d) binding of radionuclides to microbial biomass, (e) microbial redox transformations of radionuclides, and (f) the implementation of microbial processes in reactive transport models for radionuclide migration.

Diffusion-driven transport in clayrock formations by Scott Altmann; Christophe Tournassat; Florence Goutelard; Jean-Claude Parneix; Thomas Gimmi; Norbert Maes (pp. 463-478).

Clay mineral-rich sedimentary formations are currently under investigation to evaluate their potential use as host formations for installation of deep underground disposal facilities for radioactive waste (e.g. Boom Clay (BE), Opalinus Clay (CH), Callovo–Oxfordian argillite (FR)). The ultimate safety of the corresponding repository concepts depends largely on the capacity of the host formation to limit the flux towards the biosphere of radionuclides (RN) contained in the waste to acceptably low levels. Data for diffusion-driven transfer in these formations shows extreme differences in the measured or modelled behaviour for various radionuclides, e.g. between halogen RN (³⁶Cl,¹²⁹I) and actinides (^238,235U,²³⁷Np,²³²Th, etc.), which result from major differences between RN of the effects on transport of two phenomena: diffusion and sorption. This paper describes recent research aimed at improving understanding of these two phenomena, focusing on the results of studies carried out during the EC Funmig IP on clayrocks from the above three formations and from the Boda formation (HU).Project results regarding phenomena governing water, cation and anion distribution and mobility in the pore volumes influenced by the negatively-charged surfaces of clay minerals show a convergence of the modelling results for behaviour at the molecular scale and descriptions based on electrical double layer models. Transport models exist which couple ion distribution relative to the clay–solution interface and differentiated diffusive characteristics. These codes are able to reproduce the main trends in behaviour observed experimentally, e.g.D _e(anion)_e(HTO)_e(cation) andD _e(anion) variations as a function of ionic strength and material density. These trends are also well-explained by models of transport through ideal porous matrices made up of a charged surface material. Experimental validation of these models is good as regards monovalent alkaline cations, in progress for divalent electrostatically-interacting cations (e.g. Sr²⁺) and still relatively poor for ‘strongly sorbing’, highK _d cations.Funmig results have clarified understanding of how clayrock mineral composition, and the corresponding organisation of mineral grain assemblages and their associated porosity, can affect mobile solute (anions, HTO) diffusion at different scales (mm to geological formation). In particular, advances made in the capacity to map clayrock mineral grain-porosity organisation at high resolution provide additional elements for understanding diffusion anisotropy and for relating diffusion characteristics measured at different scales. On the other hand, the results of studies focusing on evaluating the potential effects of heterogeneity on mobile species diffusion at the formation scale tend to show that there is a minimal effect when compared to a homogeneous property model. Finally, the results of a natural tracer-based study carried out on the Opalinus Clay formation increase confidence in the use of diffusion parameters measured on laboratory scale samples for predicting diffusion over geological time–space scales.Much effort was placed on improving understanding of coupled sorption–diffusion phenomena for sorbing cations in clayrocks. Results regarding sorption equilibrium in dispersed and compacted materials for weakly to moderately sorbing cations (Sr²⁺, Cs⁺, Co²⁺) tend to show that the same sorption model probably holds in both systems. It was not possible to demonstrate this for highly sorbing elements such as Eu(III) because of the extremely long times needed to reach equilibrium conditions, but there does not seem to be any clear reason why such elements should not have similar behaviour. Diffusion experiments carried out with Sr²⁺, Cs⁺ and Eu(III) on all of the clayrocks gave mixed results and tend to show that coupled diffusion–sorption migration is much more complex than expected, leading generally to greater mobility than that predicted by coupling a batch-determinedK _d and Fick´s law based on the diffusion behaviour of HTO. If theK _d measured on equivalent dispersed systems holds as was shown to be the case for Sr, Cs (and probably Co) for Opalinus Clay, these results indicate that these cations have aD _e value higher than HTO (up to a factor of 10 for Cs⁺). Results are as yet very limited for very moderate to strongly sorbing species (e.g. Co(II), Eu(III), Cu(II)) because of their very slow transfer characteristics.

Mixing induced reactive transport in fractured crystalline rocks by Lurdes Martinez-Landa; Jesus Carrera; Marco Dentz; Daniel Fernàndez-Garcia; Albert Nardí; Maarten W. Saaltink (pp. 479-489).

In this paper the solute retention properties of crystalline fractured rocks due to mixing-induced geochemical reactions are studied. While fractured media exhibit paths of fast flow and transport and thus short residence times for conservative solutes, at the same time they promote mixing and dilution due to strong heterogeneity, which leads to sharp concentration contrasts. Enhanced mixing and dilution have a double effect that favors crystalline fractured media as a possible host medium for nuclear waste disposal. Firstly, peak radionuclide concentrations are attenuated and, secondly, mixing-induced precipitation reactions are enhanced significantly, which leads to radionuclide immobilization. An integrated framework is presented for the effective modeling of these flow, transport and reaction phenomena, and the interaction between them. In a simple case study, the enhanced dilution and precipitation potential of fractured crystalline rocks are systematically studied and quantified and contrasted it to retention and attenuation in an equivalent homogeneous formation.

Real system analyses/natural analogues by Ulrich Noseck; Eva-Lena Tullborg; Juhani Suksi; Marcus Laaksoharju; Václava Havlová; Melissa A. Denecke; Gunnar Buckau (pp. 490-500).

This paper gives an overview of the behaviour of U in two natural systems, the Forsmark site (a granitic system) in Sweden and the Ruprechtov site (a Tertiary sedimentary system) in the Czech Republic, which have been investigated in the frame of the FUNMIG project. It is not a full review paper on U geochemistry. It shows how different approaches and methods have been used to derive information on U solubility and speciation, on characteristics of key processes as well as on timescales of these processes and accordingly information on the long-term stability of U phases in the natural systems. The results are set in a wider context by relation to selected results from other sites.

FUNMIG Integrated Project results and conclusions from a safety case perspective by B. Schwyn; P. Wersin; J. Rüedi; J. Schneider; S. Altmann; T. Missana; U. Noseck (pp. 501-515).

The scope of the FUNMIG Integrated Project (IP) was to improve the knowledge base on biogeochemical processes in the geosphere which are relevant for the safety of radioactive waste repositories. An important part of this project involved the interaction between data producers (research) and data users (radioactive waste management organisations in Europe). The aim thereof was to foster the benefits of the research work for performance assessment (PA), and in a broader sense, for the safety case of radioactive waste repositories. For this purpose a specifically adapted procedure was elaborated. Thus, relevant features, events and processes (FEPs) for the three host rock types, clay, crystalline and salt, were taken from internationally accepted catalogues and mapped onto each of the 108 research tasks conducted during the FUNMIG project by a standardised procedure. The main outcome thereof was a host-rock specific tool (Task Evaluation Table) in which the relevance and benefits of the research results were evaluated both from the PA and research perspective. Virtually all generated data within FUNMIG are related to the safety-relevant FEP-groups “transport mechanisms” and “retardation”.Generally speaking, much of the work within FUNMIG helped to support and to increase confidence in the simplified PA transport and retardation models used for calculating radionuclide (RN) transport through the host rock. Some of the studies on retardation processes (e.g. coupled sorption-redox processes at the mineral–water interface) yielded valuable data for all three rock types dealt within the IP. However, most of the studies provided improved insight regarding host-rock specific features and processes, the majority of this work being dedicated to clay-rich and crystalline host rocks. For both of these host rock types, FUNMIG has significantly contributed to improving understanding on a conceptual level, both by providing new experimental data at different spatial scales and by developing improved modelling approaches. The disposal concept in salt host rocks differs from those in other host rock types in that the host rock is, at least in part, regarded as near-field from which under normal scenarios no release of radionuclides is possible. Corresponding investigations in FUNMIG concerned therefore not salt host rocks but an example of an overburden under the premise of radionuclide escape from the salt barrier.Selected highlights with regard to FUNMIG’s achievements include: For argillaceous host rocks, the systematic effort of investigating and comparing diffusion and sorption processes at different scales in different clay rocks using a variety of methods has substantially increased the knowledge basis for future safety cases. For crystalline host rocks, valuable data on the generation, transport and filtration of clay colloids from the near-field and their impact on RN transport under realistic conditions have been obtained. The results from studies on organic colloids and on biofilms, including their interaction with radionuclides, have been shown to be of interest for future safety cases of salt-host rocks. Among the main research issues from a PA perspective that need to be addressed in the future, the following are noted: (i) the question of irreversibility of RN sorption to colloids in fractures of crystalline rocks, (ii) a comprehensive model for cation and anion diffusion in clays for different scales and (iii) the applicability of mechanistic retardation models for strongly sorbing radionuclides in intact clay and crystalline host rocks.An important lesson learnt from the interaction between research and PA is that it would be desirable to apply a similar evaluation procedure for proposed research tasks before the start of the research work. In this regard, the procedures developed within FUNMIG are a useful tool for planning future Integrated Projects.

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Applied Geochemistry (v.27, #2)

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Applied Geochemistry (v.27, #2)