Provenance and characterization of aquatic actinide colloids [Elektronische Ressource] : interaction of actinides with aluminosilicate and humate colloids / vorgelegt von Domnica Cristina Breban

Domnica Breban

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2007
• Nombre de lectures: 14
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

INAUGURAL – DISSERTATION

zur
Erlangung der Doktorwürde
der
Naturwissenschaftlich-Mathematischen
Gesamtfakultät
der
Ruprecht-Karls-Universität
Heidelberg









Vorgelegt von

Diplom-Chem. Domnica Cristina Breban
aus: Bukarest, Rumänien







Tag der mündlichen Prüfung: 20 April 2007






Thema





Provenance and Characterization of Aquatic Actinide Colloids:
Interaction of Actinides with Aluminosilicate and Humate
Colloids







Gutachter: Prof. Dr. Thomas Fanghänel
Gutachter: Dr. Margot Isenbeck- Schröter
















Acknowledgements


The experiments described in this work were performed at the Institute of Radio-
chemistry (RCM), Technical University München (the radiometric experiments) and Insti-
tute of Nuclear Waste Management (INE), Forschungszentrum Karlsruhe (the spectro-
scopic measurements).
I would like to express my deepest gratitude to Prof. Dr. Jae-Il Kim, the initiator of
this research subject, for offering me the chance to work on the topic and to Dr. Maria-
Anna Kim, my direct supervisor, who shaped my work through her ideas, scientific guid-
ing and critical advices. They both were my mentors and the accomplishment of this the-
sis would not have been possible without their help.
Special consideration goes to my doctoral supervisor, Prof. Dr. Thomas Fanghänel
(FZK), for providing me the opportunity to work at INE-FZK and for his permanent sup-
port. I would also like to thank PD Dr. Petra Panak (INE), for the help during the EXAFS
measurements, many valuable discussions (regarding TRLFS and not only) and sugges-
tions for revising the thesis.
I am very grateful to Dr. Andrey Priemyschev (RCM) for sharing with me his ex-
perience and knowledge and very good collaboration.
Special thanks are due to the colleagues from the INE-EXAFS group: Dr. Melissa
Denecke for the very useful discussions and revision of the EXAFS chapter of the thesis
and Drs. Kathy Dardenne and Jörg Rothe for assistance during the EXAFS measurements,
data analysis as well as for helpful discussions.
A number of other people contributed to the realization of the present work.
Thanks are due to Dr. Christian Marquardt and Dr. Gunnar Buckau from INE, for provid-
ing the radioactive standard solutions and the humic acid, as well as to Dr. Alexander
Mansel (IIIF Leipzig) for labeling the humic acid. I would also like to thank Dr. Reinhard
Klenze (INE) for his very kind assistance regarding the formalities in obtaining the Ger-
man visa and those at the University of Heidelberg.
It was a chance, a pleasure and great experience for me to be one of the co-
workers of the RCM and INE; I would like to thank from all my heart to all people from
the two institutes for the very friendly atmosphere and their helpful attitude. I own special
thanks to Jörg Aign (RCM), for his patience in answering my numerous technical ques-
tions.
ANKA Angstromquelle Karlsruhe is acknowledged for the provision of the beam-
time for the EXAFS measurements. The financial support for the Ph.D grant provided by
the Helmholtz Association, FZK Germany, is also gratefully acknowledged.































Summary

One of the major uncertainties in the process of actinide migration is due to the
limited knowledge about the mechanism of formation and stability of the actinide col-
loids. During the formation of hydroxy-aluminosilicate (HAS) colloids as kernels of natu-
ral aquatic colloid, trace actinide ions may be incorporated and become colloid-borne spe-
cies.
The present work investigates the behaviour of actinides (An) of different oxida-
tion states in the process of aquatic colloid formation, e.g. at the generation of alumi-
nosilicates (HAS) by co-nucleation of Si and Al in absence or in presence of natural hu-
mic acid (HA). The study comprises three parts.
The first part of the work concentrates on the interaction of tri-, tetra-, penta- and
hexavalent actinide ions with HAS colloids. The HAS colloids are synthesized through
heterogeneous nucleation of Si and Al in the pH region 4 to 9 at room temperature and
atmospheric pressure. Si concentration is maintained either below or above the saturation
-3 -3concentration of amorphous silica (2x10 M). In the former case 10 M Si is mixed with
-5 -210 M Al and Si is present in solution as monosilicic acid. In the latter case, 10 M Si is
-4 mixed with 10 M Al and polysilicic acid prevails in solution. The following tracer nu-
241 234 237clides are introduced in the co-nucleation process: Am(III), Th(IV), Np(V) and
233U(VI). Colloids are separated from solution and precipitate by sequential filtration and
ultrafiltration. Definition of the optimum conditions for the formation of colloid-borne
actinide species is ascertained by radiometric assay, determining the actinide fraction in
the colloidal phase as a function of pH, conditioning time, concentration and concentra-
tion ratio of the involved components. Several methods are further applied for the ap-
praisal of the chemical binding state of actinides in the HAS colloids as well as of Al in
aluminosilicate solutions: TRLFS (time resolved laser fluorescence spectroscopy),
EXAFS (extended X-ray absorption fine structure) spectroscopy and ligand displacement
method (using EDTA as competing ligand). The radiometric and spectroscopic results
show that the co-nucleation of An with Si and Al forming HAS colloid-borne actinides is
more favourable with increasing pH, and concentration of Si and Al, parameters favouring
the element hydrolysis. Accordingly, Np does not interact with the HAS colloids in its
non-hydrolysis pH range ( ≤ 9), whereas maximum incorporation of An(III, IV, VI) into
the colloidal phase is observed for HAS generated from polysilicic acid in the pH range

7-9. Under these conditions, the desorption experiments show that the Al-O-Si binding as
well as well as the Th(IV)/U(VI)-HAS colloid binding are EDTA resistant, whereas
Am(III)-HAS colloid binding is not EDTA resistant. The affinity for conucleation with
HAS (Th(IV) ~ U(VI) > Am(III)) appears to be higher for the elements with higher ten-
dency toward hydrolysis. The less hydrolyzing tendency of Am(III) might be also the rea-
son of its discrimination against Al(III), Th(IV) and U(VI) at the co-nucleation with
polysilicic acid.
The second part of the work further concerns the behaviour of An(III, IV, VI) at
14C-the interaction with humate colloids. In addition to actinide activity measurements,
labelled natural humic acid is used in order to facilitate the tracing of humic colloids be-
haviour. The conditions of colloid formation are investigated by radiometric assay as a
-5 -4function of pH (6.6-7.8), time, concentration of HA (0.6-8 mg/L), Al (1x10 -1x10 M)
-8 -5and An (5x10 -1x10 M). Formation of humate-colloid-borne An species appears to be
very efficient for trace amount of actinides at HA concentration close to the average level
encountered in natural waters (approximately 5 mg/L), as long as the proton exchange ca-
pacity of HA for complexation of Al species is not exceeded. The generation of humate-
colloid-borne species is distinctively favored for the non-hydrolyzed ionic species. Ac-
cordingly, due to the higher hydrolysis tendency of Al(III), its ionic species have lower
tendency to complex with humic colloids. Among the An(III, IV, VI) present at tracer
concentration, the less hydrolyzing Am(III) shows higher affinity for formation of hu-
mate-colloid-borne species in the neutral pH range and for HA concentration at the aver-
age level of natural waters. Generation of humate-colloid-borne An(IV, VI) is enhanced
by increasing HA concentration or decreasing the An/HA concentration ratio.
The third part of the work follows the conditions for formation of colloid-borne
An(III, IV, VI) in the mixed system containing the competing HAS and humic colloids.
The simultaneous presence of HAS and HA generally enhances the stability region (in
respect with pH and Si/Al concentration) of the colloid-borne An species. The formation
of mixed HAS-humic colloids with synergic binding appears to be responsible for the sta-
ble incorporation of An into the colloidal phase, as ascertained for Cm(III) by TRLFS.
Such a synergic effect is assumed to be related to the two different and complementary
mechanisms exhibited by the HAS and HA colloids in binding the actinides. Formation of
mixed HAS-humic colloid-borne An could be expected under environmental conditions
favouring the presence of partially hydrolyzed An species, interacting with both HAS and
HA.
Zusammenfassung

Eine der größten Unsicherheiten im Migrationsprozess der Act