Fundamental Investigation of Saline Water Evaporation and Salt Precipitation in Porous Media

Abstract

Saline water evaporation from porous media is of great importance to a variety of environmental, hydrological and engineering applications, including but not limited to CO2 sequestration, preservation of buildings and historical monuments, and land-atmosphere interaction. Excess accumulation of salt in soil has significant impacts on water cycle, vegetation and plat growth, and the issue of soil salinization, which is a global problem. Soil with high level of salinity is considered as ecosystem under stress affecting crop production and many biological activities in soil. Thus, it is important to understand the physical mechanisms controlling saline water evaporation from porous media under given boundary conditions. Natural soils may include various layers and textural contrasts due to the depositional and scouring processes, shrinking cycles and local plant alternations. Moreover, wettability of natural soil may be modified due to the presence of organic matter, partly decomposed plant material, and alternation of surface characteristics due to the biogeochemical processes occurring in soil. The presence of such textural and wettability heterogeneity in porous media may give rise to complex preferential flow causing spatially abrupt changes in liquid phase distribution and solute transport and deposition patterns. During saline water evaporation from porous media, as water evaporates the salt concentration increases until it substantially exceeds the solubility limit when it precipitates. In the present dissertation, a comprehensive series of experimental and theoretical investigations were conducted to reveal the effects of mixed wettability condition and the presence of interfaces with sharp textural contrasts on the dynamics salt deposition and precipitation patterns during saline water evaporation from porous media. Our results show that salt deposition in porous media containing sharp textural interfaces is significantly influenced by the complex interaction between the so-called water characteristic curves of porous media and salt concentration. When salt concentration is lower than the solubility limit, salt is preferentially deposited at the coarse-textured domain of the heterogeneous porous surface due to the preferential water evaporation and solute deposition supplied by the capillary induced liquid flow connecting the wet zone to the surface of porous media. The results show that as the textural contrasts between the capillarity-coupled textural domains increases, salt precipitation occurs earlier and faster on the surface of coarse-textured region. Using 4D pore-scale images obtained by X-ray micro-tomography, laboratory column-scale experiments combined with physically-based theoretical analysis, the mechanisms controlling this counter-intuitive behaviour were revealed in this dissertation. Moreover, a comprehensive series of macro-scale evaporation experiments were performed in order to quantify how the mixed wettability conditions influence salt deposition patterns in porous media with varying wettability conditions. The obtained results enabled us to quantify the evaporative fluxes and the macro-scale precipitation patterns observed during evaporation from porous media with mixed wettability. The results reported in this dissertation extend the physical understanding of the processes controlling the complex dynamics of saline water evaporation from heterogeneous porous media together with the associated precipitation patterns.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Thomas Vetter (Supervisor)