Computational investigations of the speciation of Sr2+ in aqueous solution, and its interactions with the hydrated brucite (0001) surface

The fundamental objective of this research project was to develop a computational model, using high-level quantum chemical techniques based on density functional theory (DFT), which is able to describe the aquo and hydroxide complexes of strontium and their inter- actions with hydrated brucite surfaces, aiming to create a general approach which can be subsequently modified for the investigation of other radioactive ions/surfaces. The first two chapters of this PhD thesis are a general introduction on the project’s industrial relevance and on the computational methodology used. The subject of this study is strongly related to the decommissioning of the UK’s nuclear legacy fuel storage ponds and therefore the the- sis is organised such that, through the three main steps of the computational investigation, it eventually leads to an industrially relevant main conclusion. In the third chapter, the possible strontium hydroxide complexes in aqueous environment have been investigated, in order to establish likely candidate species for the interaction of nu- clear fission-generated strontium with the hydrated brucite surfaces in high pH spent nuclear fuel storage ponds. A combination of the COSMO continuum solvation model and one or two shells of explicit water molecules are employed for describing accurately the hydrolysis of Sr2+. The next chapter presents the periodic electrostatic embedded cluster model, developed for the brucite (0001) surface to be employed in the study of the adsorption reactions. Using the periodic electrostatic embedded cluster method (PEECM), implemented in the TURBOMOLE code, we have created a quantum chemically treated cluster in an infinite array of point charges and validated this surface model by exploring the adsorption of Sr2+ and other s block cations on bare and hydrated surfaces, comparing the PEECM data with those from a periodic DFT study using the CRYSTAL code. In the fifth chapter, the results of the previous two chapters are combined to describe the Sr-surface interactions as realistically as possible. A theoretical reaction was created, in which the energy of the adsorbed Sr2+ ion on a hydrated brucite surface was compared with the energy of a solvated Sr2+ in the bulk solution, i.e. with the previously identified strontium complexes in aqueous phase. To achieve this, the PEECM model was extended with one and two layers of water molecules both in the quantum mechanical and point charge region, whose geometries are based on previous molecular dynamics studies. Several possible complexes are identified both in the presence or absence of solvatedOH− groups with different Sr-surface distances and complex conformation, and their adsorption energies were calculated in order to evaluate the general strength of the possible ion-surface interactions

Ionic adsorption on the brucite (0001) surface:a periodic electrostatic embedded cluster method study

Density functional theory (DFT) at the generalised gradient approximation level is employed within the periodic electrostatic embedded cluster method (PEECM) to model the brucite (0001) surface. Three representative studies are then used to demonstrate the reliability of the PEECM for the description of the interactions of various ionic species with the layered Mg(OH)2 structure, and its performance is compared with periodic DFT, an approach known to be challenging for the adsorption of charged species. The adsorption energies of a series of s block cations, including Sr2+ and Cs+ which are known to coexist with brucite in nuclear waste storage ponds, are well described by the embedded cluster model, provided that basis sets of triple-zeta quality are employed for the adsorbates. The substitution energies of Ca2+ and Sr2+ into brucite obtained with the PEECM are very similar to periodic DFT results, and comparison of the approaches indicates that two brucite layers in the quantum mechanical part of the PEECM are sufficient to describe the substitution. Finally, a detailed comparison of the periodic and PEECM DFT approaches to the energetic and geometric properties of differently coordinated Sr[(OH)2(H2O)4] complexes on brucite shows an excellent agreement in adsorption energies, Sr–O distances, and bond critical point electron densities (obtained via the quantum theory of atoms-in-molecules), demonstrating that the PEECM can be a useful alternative to periodic DFT in these situations

Lack of Small Intestinal Dysbiosis Following Long-Term Selective Inhibition of Cyclooxygenase-2 by Rofecoxib in the Rat

Intestinal dysbiosis is linked to numerous gastrointestinal disorders, including inflammatory bowel diseases. It is a question of debate if coxibs, selective inhibitors of cyclooxygenase (COX)-2, cause dysbiosis. Therefore, in the present study, we aimed to determine the effect of long-term (four weeks) selective inhibition of COX-2 on the small intestinal microbiota in the rat. In order to avoid mucosal damage due to topical effects and inflammation-driven microbial alterations, rofecoxib, a nonacidic compound, was used. The direct inhibitory effect of rofecoxib on the growth of bacteria was ruled out in vitro. The mucosa-sparing effect of rofecoxib was confirmed by macroscopic and histological analysis, as well as by measuring the intestinal levels of cytokines and tight junction proteins. Deep sequencing of bacterial 16S rRNA revealed that chronic rofecoxib treatment had no significant influence on the composition and diversity of jejunal microbiota. In conclusion, this is the first demonstration that long-term selective inhibition of COX-2 by rofecoxib does not cause small intestinal dysbiosis in rats. Moreover, inhibition of COX-2 activity is not likely to be responsible per se for microbial alterations caused by some coxibs, but other drug-specific properties may contribute to it

The importance of second shell effects in the simulation of hydrated Sr2+ hydroxide complexes

Density functional theory at the meta-GGA level is employed to study the microsolvation of Sr2+ hydroxides, in order to establish likely candidate species for the interaction of nuclear fission-generated strontium with corroded Magnox fuel cladding in high pH spent nuclear fuel storage ponds. A combination of the COSMO continuum solvation model and one or two shells of explicit water molecules is employed. Inclusion of only a single explicit solvation shell is unsatisfactory; open regions are present in the strontium coordination shell which would not exist in real aqueous complexes, and many optimised structures possess unavoidable energetic instabilities. Incorporation of a second shell of explicit waters, however, yields energetically minimal structures without open regions in the first strontium coordination shell. The most stable systems with one, two or three hydroxide ions are all 6-coordinated with a distorted trigonal antiprismatic geometry, whereas systems with four OH− ions have a most stable coordination number of five. Transformation, via a proton transfer mechanism, from one coordination mode to another (e.g. from a system with two hydroxides bound directly to the strontium to one in which a hydroxide ion migrates into the second coordination shell) is found to be energetically facile. It is concluded that the most likely strontium-hydroxide complexes to be found in high pH aqueous solutions are mono- and dihydroxides, and that these coexist

Depth-dependent oxygen redox activity in lithium-rich layered oxide cathodes

Lithium-rich materials, such as Li1.2Ni0.2Mn0.6O2, exhibit capacities not limited by transition metal redox, through the reversible oxidation of oxide anions. Here we offer detailed insight into the degree of oxygen redox as a function of depth within the material as it is charged and cycled. Energy-tuned photoelectron spectroscopy is used as a powerful, yet highly sensitive technique to probe electronic states of oxygen and transition metals from the top few nanometers at the near-surface through to the bulk of the particles. Two discrete oxygen species are identified, On− and O2−, where n < 2, confirming our previous model that oxidation generates localised hole states on O upon charging. This is in contrast to the oxygen redox inactive high voltage spinel LiNi0.5Mn1.5O4, for which no On− species is detected. The depth profile results demonstrate a concentration gradient exists for On− from the surface through to the bulk, indicating a preferential surface oxidation of the layered oxide particles. This is highly consistent with the already well-established core–shell model for such materials. Ab initio calculations reaffirm the electronic structure differences observed experimentally between the surface and bulk, while modelling of delithiated structures shows good agreement between experimental and calculated binding energies for On−