Nonlinear Effects of Dissolved NaCl Concentrations on Water Evaporation From Porous Media.

Transport properties of porous media, thermodynamics of the evaporating solution and atmospheric conditions are the key factors affecting the evaporation rate from porous media. Capillary-induced liquid flow from a receding drying front toward surface plays a key role in supplying the evaporative demand. Presence of salt in water modifies the interfacial processes at the air-liquid-solid interface, thus affects the capillary flow. Also, it alters the saturation vapor pressure above the surface, which directly affects the evaporation rate. To study the effects of salt concentration on evaporation dynamics, we conducted a series of experiments under constant atmospheric conditions using cylindrical transparent columns packed with sand saturated by NaCl solutions with different concentrations ranging from 0 to 23% by weight. The columns were mounted on digital balances to record the evaporative water losses automatically. The dynamics of water evaporation were monitored for a period of 400 hours. Although it is commonly believed that an increase in salt concentration results in reduction of the evaporation rate, our results support this statement only up to a certain salt concentration. Exceeding this critical concentration leads to an increase of the evaporation rate. We describe this nonlinear behavior from a thermodynamic point of view. Additionally, obtained results reveal a nonlinear correlation between the onset of the salt precipitation at the surface and its concentration. Using convection-diffusion equation, we obtained an analytical solution to predict the initiation of the salt precipitation at the surface which was verified by the experimental data. Our results provide new insights into the nonlinear effects of the presence of salt on the evaporation rates and salt deposition patterns during evaporation from porous media. The findings of our work are particularly useful when addressing the evaporation from salt marshes and lakes with high salt concentration.