dc.contributor.advisor | Martínez Fernández, José Manuel | es |
dc.contributor.advisor | Sánchez Marcos, Enrique | es |
dc.creator | Pérez Conesa, Sergio | es |
dc.date.accessioned | 2019-10-18T07:14:21Z | |
dc.date.available | 2019-10-18T07:14:21Z | |
dc.date.issued | 2019-09-27 | |
dc.identifier.citation | Pérez Conesa, S. (2019). Computational chemistry of actinoids in solution and confinedMedia. (Tesis Doctoral Inédita). Universidad de Sevilla, Sevilla. | |
dc.identifier.uri | https://hdl.handle.net/11441/89735 | |
dc.description | Existe un contrato de cesión de derechos que afecta a los capítulos 4-9. | es |
dc.description.abstract | The nuclear power industry accounts for around 10% of the electricity
production worldwide and up to 70% in some countries.1 One of the problems
of this otherwise clean energy source is the generation of high level
radioactive waste which remains harmful for centuries.2 Spent nuclear fuel is
reprocessed to extract the actinoids that are still fissile (U and Pu) from highly
radiotoxic minor actinoids (Np and Am). 2–4 This is done typically through the
PUREX process which is based on liquid-phase extraction of actinoids based
on their physico-chemical properties. Some of the most important species
in this process are the actinyl hydrated cations, [AnO2
¢(H2O)5]2+/+(aq)] for
An=U,Np,Pu,Am. The actinyls are linear oxo-cations formed by the oxidation
states V and VI of the metal. High level radioactive waste resulting from the
PUREX process are destined to be kept underground in permanent geological
repositories for the centuries to come.2 These repositories use clays as liner
materials to prevent potential diffusion of radioelements to the environment.5
The main clay component is montmorillonite clay.5 In this thesis we will study
the physico-chemical properties of actinyl cations in aqueous solution and in
clays using computational chemistry .
In order to run molecular dynamics simulations (MD), ab initio force fields
were developed for U(VI), Np(VI), Np(V), Pu(VI) and Am(VI) in water. One
additional force field was developed for the interaction of uranyl with the
montmorillonite clay aluminosilicate layers. The force fields are based on the
hydrated ion model6–8 developed by the group in the mid 90’s. This model accounts for many-body effects like polarization and charge transfer in a nonpolarizable
framework. Its main characteristic is to consider the hydrated ion
and not the naked ion as the solute. In this way, first-shell water molecules
and bulk water molecules are different species. This allows the assignment of
different atom types, partial charges and interaction potentials to the first-shell
than to bulk water molecules. It additionally parametrizes explicitly hydrated
ion bulk-water molecule interactions.
Once the force fields were developed, MD simulations of the actinyls in
water were run. The simulations reproduced satisfactorily a wide variety of
physico-chemical properties of the system: hydration enthalpy, vibrational
spectra, diffusion coefficients, XAS spectra etc. This was a sign of the robustness
of our force field development strategy. The first conclusion drawn from
the simulations is that the solvation structure of the different actinoids is almost
indistinguishable one from the other. Furthermore, despite the charge
difference between [NpO2]2+ and [NpO2]+, their solvation resembled strongly.
We observed that the equatorial solvation of the actinyls was equal to most
conventional cations: the first-shell forms two hydrogen bonds with bulk
water molecules. In contrast, the Oyl atom solvates hydrophobically: water
molecules surround it forming hydrogen bonds with other solventmolecules
but not with Oyl . We concluded that the actinyl cations are highly anisotropic
amphiphillic cations that have a conventional hydration sphere caped at the
poles by hydrophobic solvation regions.
The theoretical EXAFS spectra of the actinyls were calculated and compared
to experiment. Except for uranyl, the spectra had an improvable correspondence
with experiment. The force fields for these cations were developed
at the DFT level of theory. With the aimof improving performance, the explicit
treatment of static correlation was then taken into account. A NEVPT29–11
force field was developed as well as a strategy to modify the DFT force fields to
include the small changes in distances of the higher level. The effect of this
increase in level of theory was studied, and the decomposition of the complex
EXAFS signal was shown to be useful in the understanding of themain EXAFS
spectrumfeatures.
Due to its chemical instability, americyl ([AmO2]2+), has never been isolated
in aqueous solution. As a result, the only EXAFS spectrum of [AmO2]2+
corresponds to a 70/30 mixture of americyl and Am3+.13 We simulated the EXAFS
spectra of both species fromtheir respectiveMDsimulations andweighted
them into a single spectrum to produce a simulated EXAFS of a mixture of species. The good comparison of the simulated spectrum and experiment
allowed us to predict theoretically the structural parameters and EXAFS spectrumof
a pure americyl solution, a solution yet to be obtained experimentally.
The same procedure was applied to XANES spectra. This work was featured in
the cover of Inorganic Chemistry, Figure 1.
The MD simulations of the uranyl hydrated ion in the aqueous interlayers
of montmorillonite clays gave interestingmicroscopical details of the system
hard to obtain experimentally. The simulations reproduced the few experimental
microscopical information of the system: the uranyl hydrated ion
interacts with the surface through the formation of an outer-shell complex14
and the uranyl axis is neither perpendicular nor parallel to the surface.15 We
calculated for the first time from aMD simulation the constrictivity factor, ±int ,
which was found to be near the right order of magnitude to experiment. Strong
interaction sites for uranyl were found on the clay. These sites are groups of
three magnesium substitutions around which uranyl cations appear to have
deep free energy wells. As a consequence the diffusion of uranyl in the clay exhibits a hopping diffusion mechanism. Because of this, the diffusion of
uranyl increases with increasing uranyl concentration due to cation-cation
interactions and a larger coverage of surface sites. This work was featured in
the cover of Inorganic Chemistry. Finally, a simple local fingerprint for hydrophobicity/hydrophilicity was
developed. This fingerprint is inspired by the expansion of the entropy of a
system as a sumof terms of increasing correlational order.17,18 The fingerprint
measures the hydrophobicity/hydrophillicity of individual atoms of a solute
taking as input its radial distribution function with water. The fingerprint classifies
satisfactorily, the atoms of the amino acids. Nevertheless, the fingerprint
hasmixed results in classifying the atoms of the actinyl pentahydrates. A future
improved fingerprint should probably make use of orientational pair entropy
in addition to some technique to consider the anysotropicity of the solute in
complex environments. Additionally, the fingerprint proved to be a useful
solvation/desolvation collective variable for enhanced sampling simulations. | es |
dc.format | application/pdf | es |
dc.language.iso | eng | es |
dc.rights | Attribution-NonCommercial-NoDerivatives 4.0 Internacional | * |
dc.rights.uri | http://creativecommons.org/licenses/by-nc-nd/4.0/ | * |
dc.title | Computational chemistry of actinoids in solution and confinedMedia | es |
dc.type | info:eu-repo/semantics/doctoralThesis | es |
dcterms.identifier | https://ror.org/03yxnpp24 | |
dc.type.version | info:eu-repo/semantics/publishedVersion | es |
dc.rights.accessRights | info:eu-repo/semantics/openAccess | es |
dc.contributor.affiliation | Universidad de Sevilla. Departamento de Química Física | es |
idus.format.extent | 122 p. | es |
dc.description.awardwinning | Premio Extraordinario de Doctorado US | |