[Thesis]. Manchester, UK: The University of Manchester; 2018.
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.