Beyond the Salt Crust: On Combined Evaporation and Subflorescent Salt Precipitation in Porous Media

Abstract

The impact of salt precipitation, within or on top of porous media, on water evaporation from the media is an important issue with practical implications for agricultural practices, civil engineering and construction materials, the food industry and more. Even though the evaporation of saline solutions from porous media has been widely studied in recent years, there are still many uncertainties about the associated physical and chemical mechanisms. Moreover, most studies have focused on the impact of efflorescent salt precipitation on evaporation. This work focuses on subflorescent salt precipitation and its connection to evaporation from porous media. The different impacts of subflorescent and efflorescent salt precipitation on porous media evaporation are discussed, and the results of experiments that measured and quantified the impact of crystallization patterns on evaporation and porous media hydraulic conductivity are presented. A simple model was used to better understand the limiting factors that control liquid transport within porous media, with and without crystallized salts. The experimental results showed a major impact of efflorescent salt crust on evaporation, while for the subflorescent salt precipitation, the measured evaporation rates were similar to those measured for salt-free conditions. The model indicated that even though subflorescent-precipitated salt reduced the hydraulic conductivity of the medium by several orders of magnitude, it was not the limiting factor for evaporation as the diffusive vapor transport between the matrix surface and atmosphere was the slowest transport mechanism in the system, thus controlling evaporation rates.

NaCl Crust Architecture and Its Impact on Evaporation: Three‐Dimensional Insights

“…This was demonstrated experimentally, as the transition to third stage evaporation appeared earlier with lower cumulative evaporation for the sand with the lower number of nucleations (Figure 2), in agreement with other studies (Eloukabi et al, 2013;Gupta et al, 2014;Nachshon & Weisbrod, 2015;Shokri-Kuehni, Vetter, et al, 2017) that have shown that as long as the salt crust effectively delivers water by capillarity toward the salt atmosphere interface, its impact on evaporation is minor. These pillars probably were nucleation centers at the beginning of the evaporation process, and they have two important features: (1) to support the ESC structure and (2) to deliver solutes from the matrix to the growing ESC.…”

“…In the first repetition (A), approximately 50 nucleations were counted at the onset of salt precipitation and this number was stable for 5 hr. At this point, sand coverage by ESC was in the order of 40%, and as reported by Nachshon and Weisbrod (2015), cumulative evaporation was in the order of 1.3 mm and stage three of evaporation began. Five hours after the experiment onset, the number of centers started to decline as adjacent nucleations merged one into the other.…”

Modelling evaporation processes in soils from the Huasco salt flat basin, Chile

“…Table presents the values of the parameters obtained in the calibration process. After calibrating the soil hydraulic and thermal properties, the model results agree fairly well with the experimental data and suggest that the only way to represent water and heat transport in these experiments is by accounting for the soil properties spatial variability, which are likely to be modified as a consequence of salt transport and precipitation/dissolution reactions (Vásquez et al, ; Fierro, ; Nachshon and Weisbrod, ).…”

“…According to the electrical conductivity profiles, salt crystallized in the smallest pores in the soil as subflorescent precipitation. This crystallization decreases the pore space in the soil matrix and therefore decreases the K s (Nachshon et al, ; Vásquez et al, ), reduces the vapour flow (Nachshon and Weisbrod, ), and also modifies the water retention characteristics (as described below) (Fierro, ). Although subflorescent precipitation has been widely investigated in other disciplines, few investigations have reported this type of precipitation in soils under drying conditions (Sghaier et al, ; Weisbrod et al, ).…”

Drying of Porous Media

“…Norouzi Rad et al (2015) used X-ray tomography to quantify the effect of grain and pore size on salt precipitation during evaporation from porous media and its consequences on the dynamics and patterns of salt precipitation in saline porous media. Nachshon and Weisbrod (2015) addressed the topic of subflorescent salt precipitation during evaporative drying and subsequent feedback on the evaporation process from saline porous media. The various impacts of efflorescent and subflorescent on evaporation rates and on modification of hydraulic characteristics of the porous medium due to salt precipitation are quantified.…”