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Aggregation and Aggregate Strength of Microscale Plastic Particles in the Presence of Natural Organic Matter: Effects of Ionic Valence
Summary
Calcium ions promote stronger aggregation of polyethylene microplastic particles in the presence of natural organic matter through divalent bridging, producing more cohesive aggregates than monovalent potassium ions. Understanding how microplastics aggregate under different water chemistry conditions is important for predicting their transport, sedimentation, and bioavailability in natural aquatic environments.
Considering the recent plastic loads in water bodies we studied the aggregation, charging, and aggregate strength of polyethylene microsphere (PEM) particles in the presence of natural organic matter (NOM) as a function of KCl and CaCl2 concentrations. We used the Suwannee river humic acid (SRHA) as NOM and PEM as microplastics. The aggregation was triggered in the presence of CaCl2 solutions more effectively than KCl due to the divalent bridging and strong electrostatic attraction between Ca2+ and SRHA as well as between PEM particles. We found that the maximum aggregate strength around 1.87 nN in the presence of 100 mg/L SRHA at 0.5 M CaCl2 solution. The aggregate strength of PEM particles in the KCl solution was lower than that of CaCl2 solution, manifesting the more effective bridging flocculation and divalent bridging due to the Ca2+ ions.