Mechanistic Analysis of Microplastic Incorporation during Supersaturation-Driven NaCl Crystallization

Microplastic (MP) contamination in edible salt raises increasing food-safety and environmental concerns, yet the mechanisms governing their incorporation during crystallization remain poorly understood. In this study, targeted experiments and mechanistic modeling were combined to quantify and control MP entrapment in NaCl crystals. A synthetic seawater feed (200 g L –1 NaCl) was pretreated using ceramic ultrafiltration (20 nm pores, 0.358 m 2 ), removing >97% of MPs and reducing their concentration from 6,760 to 188 particles L –1 . The clarified brine was then evaporatively concentrated and subjected to batch crystallization under varied saturation levels, solution volumes, and cooling rates. MPs in harvested salt were quantified using Nile Red fluorescence microscopy coupled with automated MATLAB image analysis (median size deviation 2.95%). A first-principles dynamic model incorporating evaporation-driven concentration changes and heterogeneous nucleation kinetics reproduced salt yield and MP incorporation within 8% error. Results show that higher supersaturation and slower cooling increase particle-assisted nucleation, whereas larger solution volumes reduce mass-normalized MP capture.