Dynamics of Saline Water Evaporation from Porous Media

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Abstract

Saline water evaporation from porous media with the associated salt precipitation patterns is frequently observed in a number of industrial and environmental applications and it is important in a variety of topics including, but not limited to, water balance and land-atmosphere interaction, terrestrial ecosystem functioning, geological carbon storage, and preservation of historical monuments. The excess accumulation of salt in soil is a global problem and is one of the most widespread soil degradation processes. Thus, it is important to understand the dominant mechanisms controlling saline water evaporation from porous media. This process is controlled by the transport properties of the porous medium, the external conditions, and the properties of the evaporating fluid. During saline water evaporation from porous media, the capillary induced liquid flow transports the solute towards the evaporation surface while diffusive transport tends to spread the salt homogeneously thorough the porous medium. Therefore, the solute distribution is influenced by the competition between the diffusive and convective transport. As water evaporates, salt concentration in the pore space increases continually until it precipitates. The formation of precipitated salt adds to the complexity of the description of saline water evaporation from porous media. In this dissertation, the effects of salt concentration, type of salt, and the presence of precipitated salt, on the evaporation dynamics have been investigated. The obtained results show that the precipitated salt has a porous structure and it evolves as the drying progresses. The presence of porous precipitated salt at the surface causes top-supplied creeping of the evaporating solution, feeding the growth of subsequent crystals. This could be visualized by thermal imaging in the form of appearance and disappearance of cold-spots on the surface of the porous medium, brought about by preferential water evaporation through the salt crust. My results show that such a phenomenon influences the dynamics of saline water evaporation from porous media. Moreover, a simple but effective tool was developed in this dissertation capable of describing the effects of ambient temperature, relative humidity, type of salt and its concentration, on the evaporative fluxes. Additionally, pore-scale data obtained by synchrotron x-ray tomography was used to study ion transport during saline water evaporation from porous media in 4D (3D space + time). Using iodine K-edge dual energy imaging, the ion concentration at pore scale with a high temporal and spatial resolution could be quantified. This enabled us to reveal the mechanisms controlling solute transport during saline water evaporation from porous media and extend the corresponding physical understanding of this process. Within this context, the effects of particle size distribution on the dispersion coefficient were investigated together with the evolution of the dispersion coefficient as the evaporation process progresses. The results reported in this dissertation shed new insight on the physics of saline water evaporation from porous media and its complex dynamics. The results of this dissertation have been published in 3 peer-reviewed journal papers together with one additional manuscript which is currently under review.

Keyword(s)

Evaporative Fluxes; Pore-scale Imaging; Saline Water Evaporation from Porous Media; Solute Transport; Thermal Imaging

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