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Pore-Scale Investigation of Salt Precipitation during Evaporation from Porous Media

Norouzi Rad, Mansoureh

[Thesis]. Manchester, UK: The University of Manchester; 2015.

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Abstract

Understanding the physics of water evaporation from saline porous media is important in many processes such as soil salinity, terrestrial ecosystem functioning, vegetation and crop production, biological activities in vadose zone, and CO2 sequestration. Precipitation of salt is one of the possible outcomes of the evaporation process from saline porous media which may either enhance or interrupt the desired process depending on the localization and pattern of the precipitated salt. In the present study X-ray micro tomography was used to study the 3D dynamics and patterns of salt deposition in drying porous media under different boundary conditions and the effects of salt concentration, particle size distribution and shape of grains on the precipitation patterns and dynamics at pore-scale have been investigated. Evaporation process from porous media involves preferential invasion of large pores on the surface while the fine pores remain saturated serving as the evaporation sites to supply the evaporative demand. This results in increasing salt concentration in fine pores during evaporation. Precipitation starts when salt concentration exceeds the solubility limit in the preferential evaporation sites. At the early stages, the precipitation rate increases with time until all evaporation sites at the surface reach the solubility limit and turn into the precipitation sites. This is followed by a constant rate of precipitation proportional to the evaporation rate. We show that the formation of salt crust at the surface does not immediately interrupt the evaporation process due to the porous nature of the precipitated salt investigated using the scanning electron microscopy. Also, our results confirmed the formation of discrete efflorescence at the surface of porous media due to the presence of pores with different sizes. Distribution of these fine pores on the surface directly influences the patterns of salt precipitation and thickness of the salt crust such that in the media with more fine pores, precipitated salt forms a thinner crust as the solute transferred to the surface is distributed among more evaporation sites. In contrast, in the media with fewer evaporation sites at the surface the salt crust will be more discrete but thicker. A simple equation is also proposed to estimate the evolution if the thickness of the salt crust on the surface of porous media. Our results provide new insights regarding the physics of salt precipitation and its complex dynamics in porous media during evaporation.