Combined effects of salt and microplastics on evaporation and crystallization dynamics in porous media

This study investigates the combined effects of salinity and microplastic contamination on the evaporation process and salt crystallization in porous media through complementary column- and pore-scale experiments. Laboratory soil columns were packed with either pure sand or sand mixed with 5% (w/w) poly(vinyl chloride) (PVC) microplastics and subsequently saturated with freshwater or saline (NaCl) solution. Evaporation and crystallization dynamics were monitored by using mass loss measurements, surface optical and thermal imaging, and synchrotron X-ray tomography. Results show that salinity consistently suppressed evaporation by roughly 25–30%, whereas PVC microplastics enhanced it, generating substantial differences in cumulative water loss across treatments. Moreover, thermal imaging revealed distinct surface responses: NaCl samples developed salt crusts that progressively reduced local temperature contrasts and led to more spatially uniform surface conditions, whereas PVC-NaCl samples exhibited lower mean surface temperatures but substantially higher spatial variability, reflected in larger and more persistent temperature anomalies during drying. Pore-scale μCT imaging further confirmed that microplastics altered crystallization patterns by redistributing salt deposition over the upper part of the sand profile and modifying the nucleation behavior. Together, these findings underscore the complex interplay between microplastics and salinity, with implications for soil moisture regulation, surface energy flux, and environmental monitoring strategies.