Experimental Study of Evaporation Flux, Salt Precipitation, and Surface Temperature on Homogeneous and Heterogeneous Porous Media

Abstract

Salt precipitation in porous media is widespread, which has garnered great research attention. However, the mechanisms governing the salt precipitation, water flux, and surface temperature changes in homogeneous and heterogeneous porous media remain unclear. This study investigated the dynamics of salt precipitation, evaporative loss, and surface temperature in homogeneous fine sand (0.1–0.25 mm), coarse sand (1-2 mm), and a heterogeneous column with fine and coarse sand. All sand columns were initially saturated with NaCl solution. The experimental results showed that the salt was precipitated as efflorescence above the surface of the fine sand, whereas it was mainly precipitated as subflorescence below the surface of the coarse sand, causing the unconsolidated sand to form a strong stone-like mass. The evaporated loss was significantly higher in heterogeneous than in homogeneous sand, but this difference in evaporation was insignificant in the stage where vapor diffuses through the precipitated salt to the external air. The salt crust formed not only decreased the surface temperature due to increased albedo by salt precipitation, but also resulted in a more discrete temperature distribution in the porous media. Our research results can promote further understanding of salt precipitation and evaporation in porous media.

Effect of salt types on salt precipitation and water transport in saline sandy soil

“…For progressively drying soils, the first stage of evaporation is a constant evaporation stage that occurs when liquid water reaches the vaporization interface under surface tension, whereas the second stage of evaporation is a rapid decrease stage controlled by water vapour diffusion (Lehmann & Or, 2009; Or et al, 2013; Shokri & Or, 2011). When the subflorescence of Na 2 SO 4 appears, the porosity of the cemented soil particles decreases and the water transport rate is reduced, which shortens the time of the first stage of evaporation, leading to a rapid decrease in the evaporation rate in the second stage of evaporation, water vapour diffusion successively occurs (Li & Guo, 2022; Ruiz‐Agudo et al, 2007). However, the subflorescence layer of the cemented soil still has large porosity and low resistance to water vapour transport (Figure 2b), due to which the evaporation rate is still high during the second stage of evaporation.…”

“…As evaporation shifted to the second stage of water vapour diffusion after salt crystallization covered the soil surface, the inhibition of evaporation by salt crystallization was mainly reflected in the increased resistance to water vapour diffusion (Fujimaki et al, 2006; Li & Guo, 2022; Nachshon, Shahraeeni, et al, 2011). The water vapour diffusion capacity (WVDC) can be expressed by the water vapour diffusion coefficient.…”

“…For estimating the vapour diffusion coefficient (Equation ), the effect of the crystallization structure (Desarnaud et al, 2015; Li & Guo, 2022) should be taken into account, which is calculated as follows: $$$$ {D}_{eff}=2.29\times {10}^{-5}{\left(\frac{T_k}{273.15}\right)}^{1.75}\xi $$$$ where ξ is the structure factor of the salt crystallization layer. This structure factor parameter is employed to characterize the structure condition of the crystallization layer (Li & Guo, 2022; Nachshon, Weisbrod, et al, 2011). The vapour diffusion coefficient of the salt crystallization layer after stabilization under the different salt solution treatments (Table 3) showed that the greater resistance to water vapour diffusion induced by the efflorescence for salt mixture solution treatments compared to mono‐salt solution treatments.…”

“…In this study, when water vapour passed through the salt crystallization layer, the water vapour concentration difference between the surface and the bottom of the salt crystallization layer can be expressed as ∆ρ v in Equation (), and the thickness of the salt crystallization layer is expressed as ∆z . For estimating the vapour diffusion coefficient (Equation ), the effect of the crystallization structure (Desarnaud et al, 2015; Li & Guo, 2022) should be taken into account, which is calculated as follows: $$$$ {D}_{eff}=2.29\times {10}^{-5}{\left(\frac{T_k}{273.15}\right)}^{1.75}\xi $$$$ where ξ is the structure factor of the salt crystallization layer. This structure factor parameter is employed to characterize the structure condition of the crystallization layer (Li & Guo, 2022; Nachshon, Weisbrod, et al, 2011).…”

“…According to previous studies, variations in salt types (Li et al, 2022; Shokri‐Kuehni, Vetter, et al, 2017), soil textures (Eloukabi et al, 2013; Li et al, 2022; Li & Guo, 2022; Norouzi Rad et al, 2015), and environment conditions (Li & Shi, 2021a; Shokri‐Kuehni, Rad, et al, 2017) can affect the evolution and patterns of crystallization. However, the impact of salt type on the relationship between crystallization and water transport remains unclear, particularly for mixed salts in natural soils.…”