Solar-driven interfacial evaporation and salt precipitation from porous media with in-situ magnetic resonance characterization

• NMR relaxation time variation reflects solar evapotranspiration of porous media. • Precipitated crusty/patchy salts are characterized by NMR and MRI/SEM imaging. • Salt crust formed inside layered porous media hinders capillary water transport. • Activation energy of confined brine among pores is determined from T 2 curve shift. Salty water transport and evaporation-induced salt precipitation are of vital importance for sustainable desalination and saline agriculture processes. However, there are limited physical insights into capillary transport of saline water, solar-driven interfacial evaporation and salt precipitation in opaque and heterogeneous porous media. To probe solar-driven evapotranspiration dynamics of salty wet porous media, in-situ magnetic resonance characterization is conducted under continuous day and night cycles by monitoring shifts in transverse relaxation time T 2 and imaging cross-sections. Our findings indicate that the crusty salt grows with saline water evaporation from tight porous media, and it blocks the vapor escape to the ambient air while T 2 distribution shifts towards the left. As for coarse porous media, the growth of patchy salt takes place at the free surface of the porous media while T 2 distribution shifts towards the right. More interestingly, crusty salt precipitation has been observed at the interface of fine and large particles in layered porous media with coarse one on the top. By analyzing the fluctuations in T 2 distribution owing to periodic solar heating, we can even determine the activation energy of confined brine in homogeneous or heterogeneous porous media. Our in-situ characterization results unveil the intertwined transport physics of solar-driven interfacial evaporation and salt precipitation for broad water-energy-agriculture nexus applications.