Heteroaggregation mechanisms of microplastics with suspended sediments in a reservoir system under variable hydraulic conditions using laboratory flume experiments.

In the Three Gorges Reservoir, interactions between microplastics (MPs) and sediments govern MP-sediment transport, but floc formation under varying flows remains unclear. Heterogeneous coagulation of polymethyl methacrylate (PMMA) and polystyrene (PS) with sediment was investigated using a custom water tank fitted with a continuously adjustable motor-driven agitating valve to generate shear rates (G=10-160 s-1). The results indicated that the mean floc size initially increased and then leveled off with time under shear. At the end of the 180 min experiments, the floc sizes reached 107.03μm (PMMA-sediment, G=30 s-1), and 103.99 μm (PS-sediment) and 101.86 μm (sediment) at G=40 s-1. The fractal dimension first decreased and then increased with increasing G, ranging from 2.36 to 2.52. The floc size balance ranged from 60.53 to 105.36 μm across all shear conditions. XPS revealed electron transfer and chemical shifts, including C-O-C and O-C=O peak shifts, during PMMA and PS aggregation with sediment, forming unsaturated C=C bonds. PMMA showed larger binding energy changes than PS, indicating greater electron loss and stronger sediment adsorption. DLVO theory revealed a lower energy barrier for PMMA-sediment (4437.22 kT) than PS-sediment (4680.38 kT); AFM analyses showed higher adhesive force for PMMA-sediment (48.05 nN) than PS-sediment (30.27 nN), indicating weaker repulsion and stronger cohesion for PMMA, which facilitated more stable floc formation. These results indicate that PMMA-sediment are more stable than PS-sediment. This study elucidates MP-sediment flocculation under varying hydraulics, revealing hydrodynamic controls and providing a scientific basis for environmental pollution risk assessment and riverine MPs management.