The role of organics on the migration of radionuclides in the geosphere

European Commission Directorate-General For The Information Society And Media Directorate-General For Research And Innovation

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Nuclear energy and safety
Energy research

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• Publié par: EUROPEAN-COMMISSION-DIRECTORATE-GENERAL-FOR-THE-INFORMATION-SOCIETY-AND-MEDIA-DI
• Nombre de lectures: 8
• Langue: English
• Poids de l'ouvrage: 2 Mo

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Commission of the European Communities
uclear science
nd technology
The role of organics on the migration
of radionuclides in the geosphere
\
Report
EUR 12024 EN t
Commission of the European Communities
nuclear science
and technology
The role of organics on the migration
of radionuclides in the geosphere
L. Carlsen
Chemistry Department,
Risø National Laboratory
DK-4000 Roskilde
Contract No FI1W/0066
Final report
Work performed in the frame of the indirect action programme
(1986-89) of the European Atomic Energy Community:
Management and storage of radioactive waste
Task: 4 Migration of radionuclides in the geosphere (Mirage 2)
PARL. FinOP. B-L'ioih.
Directorate-General
N.C./EU.?
Science, Research and Development
CL~7 1989 ! EUR 12024 EN Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
L-2920 Luxembourg
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting
on behalf of then is responsible for the use which might be made of
the following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1989
ISBN 92-825-9334-7 Catalogue number: CD-NA-12024-EN-C
© ECSC-EEC-EAEC, Brussels · Luxembourg, 1989
Panted in Belgium CONTENTS
1. INTRODUCTION 1
2. ORGANIC COMPOUNDS IN THE GEOSPHERE 3
2.1. Naturally occurring organic compounds
2.1.1. Humic and fulvic acids
2.1.2. Low molecular organics 9
2.2. Artificially introducedc compounds 1 0
3. BEHAVIOUR OF ORG ANICS IN THE TERRESTRIAL
ENVIRONMENT 15
3.1. Sorption
3.2. Persistence in the terrestrial environment 7
3.2.1. Chemical degradation 8
3.2.2. Microbiological degradation 20
4. COMPLEX FORMATION 3
4.1. Stability constants: Low-molecular-weight ligands 31
4.2.y: Humic- and fulvic acids 3
4.2.1. Complexation of divalent metal ions4
4.2.2.n of trivalent iron 37
4.2.3.n of europium8
4.2.4. Complexation of actinides 40
4.2.5. Effect of ionic strength on humic- and fulvic
acid complexation3
4.2.6. Effect of pH on humic-and fulvic acid complexation 4
5. INFLUENCE OF ORGANICS ON THE MIGRATION BEHAVIOUR
OF RADIONUCLIDES IN THE TERRESTRIAL ENVIRONMENT .. 45
5.1. The migration behaviour of complexed metal ions:
A theoretical study 46
5.2. Technetium in the terrestrial environment 55
5.3. Iodine in the terrestrialt 7
6. SUMMARY 6 0
ACKNOWLEDGMENTS 5 4
REFERENCES5
• III 1. INTRODUCTION
The importance of chemical and physico-chemical reactions as radioactive
decay, sorption/desorption, precipitation/dissolution, filtration, redox
equilibria, degradations, substitutions, hydrolysis, complex formation and
colloid formation, which a priori can be expected to operate when
radionuclides are released to the ground water and subsequently transported
in the terrestrial environment is emphasized. The possible role of the
operation of chemical and/or physico-chemical processes in the migration of
radionuclides has recently been discussed from a theoretical point of view by
Carlsen et al. [1,2]. The chemistry, possibly influencing the migration of a series
of radionuclides (the "Migration Chemistry"), comprising technetium, cobalt,
strontium, and the rare earth elements has recently been reported by Nilsson,
Jensen, and Carlsen [3-6]. The geochemical behaviour of radionuclides, i.e.
metal ions, has during the past decade constantly received considerable
attention [7-17].
As stated in the reports on migration chemistry [3-6], complex formation with
ligands present in ground water may significantly influence the migration
behaviour of the radionuclides, e.g. owing to differences in charge for the free
and complexed species. In recent years an increased awareness of the possible
importance of organic ligands has developed [18-28]. The importance of a
detailed knowledge of the actual species, i.e. correct determination of
formation constants for the possibly involved complexes has been emphasized
[3-6,29].
Studying the possible role of organics in ground water on the migration
behaviour of radionuclides, the ligands should a priori be classified according
to their origin:
• Naturally Occurring Organics as humic acids (HA) and fulvic acids (FA) as
well as low molecular species, and
• Artificially Introduced Organics, e.g. due to agricultural use or pollution. The types of compounds to be taken into consideration can most probably be
(and are in the present report) limited to those being able to form complexes
with radionuclides (metal ions). Species interacting with metal ions only by
relatively weak electrostatic forces are to be left out of consideration in the
present context. The strength of the latter type of forces are typically so low that
only very minor effects, if any at all, on the migration behaviour of metal ions
can be expected.
Based on a literature survey, personal contacts, as well as on results obtained
in the author's laboratory this report will cover aspects of interest in relation
to elucidation of the role of organic compounds in the migration of
radionuclides in the geosphere. This comprises subjects as presence and
behaviour of organics in the terrestrial environment, complex formation,
sorption/migration of metal ions in the presence of organic complexing agents
as well as theoretical consideration of the migration behaviour. 2. ORGANIC COMPOUNDS IN THE GEOSPHERE
As mentioned in the introductory section, the organic compounds in the
geosphere, which may possibly act as ligands in radionuclide complexes and,
hence, influence the transport of the radionuclides, have to be classified
according to their origin, i.e. naturally occurring and artificially introduced
organics. In contrast to the omnipresence of naturally occurring organics, the
possible presence of the artificially introduced compounds may be considered
as being strongly site dependent.
2.1 Naturally Occurring Organic Compounds
A wide variety of organic compounds can be found in the geosphere ranging
from simple species as methan and low molecular aliphatic acid to highly
complex polymeric species as humic- and fulvic acids. The organic
constituents in soils have been reviewed by Flaig [30], In deep geological
formations the content of dissolved organic carbon has been studied by Means
et ah [31-34], They found that the content of dissolved organic carbon ranged
from few tenths to several hundreds of mg/L, the occurrence, however, being
strongly site dependent. Leenheer et al. claim that most ground waters contain
as little as only a few mg/L of dissolved organic carbon [35]. Based on the
analyses of 100 ground water samples from sandstone, limestone and
crystalline rocks, taken from various locations and depths (10-400 m), they
determined an average values of 1.2 mg/L. In general, however, informations
on the organic content of ground waters are sparse.
2.1.1. Humic and fulvic acids.
Far the most abundant fraction of organic matter in soils and sediments is the
humic substances, i.e. humic and fulvic acids. These highly polymeric species,
which are widely distributed over the earth's surface, are often referred to as
'colloidal humus', a name which apparently can be associated with the fact that these species to a certain extent is 'soluble' in aqueous solutions. The high
contribution of these compounds to the dissolved organic carbon content of
ground waters is a result of their content of functional group as amino-,
hydroxy-, and carboxylate moieties. In the present context the humic
substances are of special interest, due to their pronounced ability to form
rather stable complexes with metal ions [18,23].
In contrast to the low molecular weight organic compounds found in the
geosphere, humic substances, i.e. humic- and fulvic acids and humine, are not
well-defined substances, due to their lack of specific physical and chemical
properties that characterize organic compounds in general. Thus, the humic
substances are mixtures of dark-colored, partly aromatic, hydrophilic,
chemically rather complex, polyelectrolytic materials that range in molecular
weight from a few hundreds to tenths, or even hundreds of thousands,
depending of origin. However, despite the lack of knowledge of the actual
chemical structure of these substances, it appears that they possess several
characteristic features, which can be used to characterize the single samples. A
compilations of methods of characterization of humic substances has recently
been reported by Choudhry [36] and Lesourd-Moulin [37] (cf. also [18,23]).
Scrutiny of the available literature reveals that humic- and fulvic acids
exhibits differences in composition from sample to sample, which, however,
taking their rather complicated route of formation into account does not seem
surprising [18,23,36,37]. Nevertheless, it appears that several features, like
elemental composition, functional group content etc, to a certain extent can be
described by values derived theoretical