Abstract
Comprehending adsorption mechanisms and the thermodynamics that underlie the interaction between nanoparticulate minerals and fluids in the environment that contain organic compounds is important for a range of fields. Understanding this interaction can help us recover more oil from reservoirs, develop remediation strategies for contaminated soils, optimise bacterially controlled biocrystallisation, explore the circumstances under which the prebiotic polymerisation unfolded and optimise many industrial processes where nanoparticulate minerals play a role. The goal of this thesis is to elucidate nanoscale processes underlying the interaction between nanoparticulate minerals and organic compounds. These interactions are important because nanoparticulate minerals, such as some phyllosilicates and iron oxides, are among the most common particles on the surface of Earth and thus have a large impact on the mobility and the geochemical cycling of organic compounds. To further our understanding of the processes that control the properties and behaviour of nanoparticulate minerals when they are associated with organic compounds, it is important to establish the strength of the mineral-organic compound interaction. The strength can be established by: i) assessing the stability of mineral-organic compound complexes under degradation conditions, ii) measuring how surface charge density of the nanoparticulate minerals controls the composition in the interface with the aqueous fluids containing organic molecules and iii) understanding how the surface structure and composition influence the strength of the bond between the nanoparticulate minerals and the organic molecules. A range of phyllosilicate and iron oxide mineral standards were used to represent the nanoparticulate minerals found in nature. Organic compounds were represented by complex mixtures, such as the crude oil or the extracellular polymeric substances extracted from bacteria, or purified compounds such as the low molecular weight organic ligands or the polysaccharide alginate. The minerals were natural or synthesised in the lab and both types were used to measure chemical and physical properties of the nanoparticulate mineral-organic compound interface. The main analytical techniques applied in this thesis were X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). XPS was used to probe the composition and AFM to probe the interaction forces in the nanoparticulate mineral-organic compound interface. The thesis is written as a synopsis, based on five studies that are grouped in three parts by their topics: The first part contains one study showed that organic compounds can be so tightly bound to minerals that even cleaning with solutions used in standard laboratory cleaning methods, such as Soxhlet solvent extraction and oxidation with hydrogen peroxide, cannot produce clean mineral surfaces. After cleaning, organic material is left at the mineral surfaces that is resistant to the cleaning treatments. The resistant organic material was found to most likely be associated with the clay minerals. This study confirms previous studies that claim that it is very likely that all pore surfaces of geologic materials have adsorbed organic compounds and are not simply a mineral termination of the atomic structure .
The second part of the thesis contains two studies. Both demonstrated that the lower the surface
charge, the lower the solution concentration at which the composition of the clay mineral-solution
interface changes from being enriched in anions to being enriched in cations. It has long been
assumed from theoretical calculations and macroscopic experiments that the surface charge of
minerals controls the electric potential profile in the interface and thus the concentration of ions,
including the organic ligands. This work showed it directly. The distribution of ions adsorbed at the
surface of the clay minerals alters the forces between the surface and the outersphere organic
ligands and as the solution concentration changes, the ion distribution changes, affecting the
mobility of anions.
The third part contains two studies that demonstrated how the Gibbs free energy of binding, ΔGbu,
between minerals and organic compounds affects the nucleation of iron oxides and the
polymerisation of biomonomers on clay minerals. The first study showed that organic compounds
facilitate mineralisation of iron oxides by decreasing the interfacial free energy, γ, for their
nucleation. However, this happens only for ferrihydrite but not for nanoparticulate hematite. There
are indications that the enhanced nucleation of ferrihydrite is influenced by the interfacial water
content and the hydration properties of the newly formed crystals, which can explain the decrease of
the interfacial energy for nucleation.
The second study of the third part of the thesis investigated how the mineral surfaces affect the
formation of polymers in the presence of nanoparticulate minerals. It is known that hydration shells
prevent many organic monomers from interacting in the bulk solution but once adsorbed to mineral
surfaces, the hydration shells are disrupted and e.g. the formation of polymers is favourable. The
strength of the mineral-organic compound bond determines the magnitude of disruption of the
hydration shells which can in turn affect conformational freedom of the adsorbed monomeric
compounds. In this way, the binding of the monomers can lead to their enhanced polymerisation. It
is shown that it is equally thermodynamically favourable for nucleotides to polymerise at iron and
aluminium rich phyllosilicate edges.
The manuscripts based on these studies are appended at the end of the thesis.
Overall, the results presented in this thesis demonstrate the complexity of interactions between
nanoparticulate minerals and organic compounds and add important information to our
understanding of the processes where minerals and organic compounds interact such as: i) the
magnitude of surface charge of clay minerals affects the composition of the clay mineral-organic
compound interface and ii) the Gibbs free energy of binding between nanoparticulate minerals and
organic compounds affects the interfacial free energy of iron oxide nucleation and the energetics of
monomer polymerisation at clay mineral surfaces.
The second part of the thesis contains two studies. Both demonstrated that the lower the surface
charge, the lower the solution concentration at which the composition of the clay mineral-solution
interface changes from being enriched in anions to being enriched in cations. It has long been
assumed from theoretical calculations and macroscopic experiments that the surface charge of
minerals controls the electric potential profile in the interface and thus the concentration of ions,
including the organic ligands. This work showed it directly. The distribution of ions adsorbed at the
surface of the clay minerals alters the forces between the surface and the outersphere organic
ligands and as the solution concentration changes, the ion distribution changes, affecting the
mobility of anions.
The third part contains two studies that demonstrated how the Gibbs free energy of binding, ΔGbu,
between minerals and organic compounds affects the nucleation of iron oxides and the
polymerisation of biomonomers on clay minerals. The first study showed that organic compounds
facilitate mineralisation of iron oxides by decreasing the interfacial free energy, γ, for their
nucleation. However, this happens only for ferrihydrite but not for nanoparticulate hematite. There
are indications that the enhanced nucleation of ferrihydrite is influenced by the interfacial water
content and the hydration properties of the newly formed crystals, which can explain the decrease of
the interfacial energy for nucleation.
The second study of the third part of the thesis investigated how the mineral surfaces affect the
formation of polymers in the presence of nanoparticulate minerals. It is known that hydration shells
prevent many organic monomers from interacting in the bulk solution but once adsorbed to mineral
surfaces, the hydration shells are disrupted and e.g. the formation of polymers is favourable. The
strength of the mineral-organic compound bond determines the magnitude of disruption of the
hydration shells which can in turn affect conformational freedom of the adsorbed monomeric
compounds. In this way, the binding of the monomers can lead to their enhanced polymerisation. It
is shown that it is equally thermodynamically favourable for nucleotides to polymerise at iron and
aluminium rich phyllosilicate edges.
The manuscripts based on these studies are appended at the end of the thesis.
Overall, the results presented in this thesis demonstrate the complexity of interactions between
nanoparticulate minerals and organic compounds and add important information to our
understanding of the processes where minerals and organic compounds interact such as: i) the
magnitude of surface charge of clay minerals affects the composition of the clay mineral-organic
compound interface and ii) the Gibbs free energy of binding between nanoparticulate minerals and
organic compounds affects the interfacial free energy of iron oxide nucleation and the energetics of
monomer polymerisation at clay mineral surfaces.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Number of pages | 208 |
| ISBN (Print) | 978-87-93510-23-4 |
| Publication status | Published - 25 Apr 2018 |