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Mechanistic Analysisof Microplastic Incorporationduring Supersaturation-Driven NaCl Crystallization
Summary
Scientists found that tiny plastic particles get trapped inside salt crystals during the salt-making process, and the conditions used—like how concentrated the saltwater is and how fast it cools—directly affect how much plastic ends up in your table salt. The good news: filtering the saltwater before crystallization removed over 97% of microplastics, suggesting salt producers could use this method to make cleaner, safer salt for consumers.
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 m2), 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.