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An experimental and computational investigation of soil water repellency

Daniel, Nicholas (2020) An experimental and computational investigation of soil water repellency. PhD thesis, Murdoch University.

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The ability for soil to restrict the transport of water from the topsoil to deeper subsoil horizons is not a new problem. There has been approximately 70 years of research investigating soil water repellency (SWR). However, the chemical/molecular mechanisms associated with this phenomenon remain poorly understood. The aim of the work presented in this thesis was to develop a better understanding of the role of organic matter in inducing SWR, and how the mineral component of soils affects the degree of repellency. In order to achieve these aims; laboratory experiments were coupled with computational simulations.

As a prelude, common extraction methods were compared to identify the most suitable methodology for SWR studies. A comparison of Soxhlet, sonication and accelerated solvent extraction methods revealed that Soxhlet extraction is the most suitable method for removing the organic compounds associated with SWR.

Solvent extraction of 20 soil samples revealed that four compound classes suspected of inducing SWR (long-chain saturated carboxylic acids, alkanes, alcohols and steroids) have a significant relationship with SWR (p < 0.05). Furthermore, in the field, the concentrations of these compound classes vary with season.

Experiments involving the loading of individual organic compounds (hexadecanoic acid, hexadecanol, cholesterol, hexadecane, and a series of palmitate salts) onto mineral substrates revealed the efficacy of the different compounds for inducing SWR. Maximum SWR (> 4 M) as measured with the Molarity of Ethanol (MED) test, was induced on acid-washed quartz sand at a loading of 2.3 palmitic acid molecules per nm2. Molecular dynamics simulations (Materials Studio) revealed that the palmitic acid molecules at this loading form two distinct layers on amorphous silica/quartz surfaces under neutral and dry conditions.

Hydrogen-bonding was identified from the simulations as the dominant organo-mineral interaction on amorphous silica sand and clay under neutral and dry conditions. In particular, palmitic acid was a hydrogen-bond donor to both sand and kaolinite surfaces. In comparison, the amorphous silica only acted as a H-bond acceptor, whereas kaolinite was both a H-bond donor and acceptor. Improved simulation models incorporated mineral surface charges, dissociation of the organic acid and solvation from water. In these more complex models hydrogen bond interactions were limited between the organic acid and amorphous silica (sand) due to deprotonation at neutral pH. Thus, cation-bridging was identified as the principal binding mechanism of the organic species to sand. In comparison, hydrogen-bonding continued to occur on kaolinite, with the mineral acting as the H-bond donor. Furthermore, the incorporation of surface charge effects to the molecular dynamics simulations led to significant differences in the orientation of the palmitate molecules on the two mineral surfaces.

Prior to this work, SWR was considered to be caused by the perpendicular arrangement of non-polar organic tails of amphiphilic compounds. These tails were suspected of inhibiting the movement of water molecules due to the unfavourable organo-water interactions. Further, it was proposed that SWR is overcome by the inversion of the organic molecules, allowing for favourable interactions between the water and polar head-groups of the organic molecules. The work presented in this thesis suggests that a lateral arrangement of organic molecules is required to both cover more of the mineral surface and prevent the possibility of pathways through the organic bulk structure. In addition, inversion of the organic molecules is not necessary, but rather a change in orientation from lateral to perpendicular is sufficient. This revised theory also accounts for the reestablishment of SWR during wetting and drying cycles.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Chemistry and Physics
Supervisor(s): Henry, David
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