Unravelling the pH-driven multiscale cascade of hematite flocculation: From interfacial tuning to structural assembly and sedimentation dynamics

• Alkaline pH enhances APAM adsorption on hematite, increasing from 0.106 to 0.186 mg/m2, due to surface deprotonation and stronger Fe–OOC coordination. • Alkaline conditions promote the formation of larger and denser flocs, with size increasing from 56 to 982 μm and fractal dimension from 1.44 to 1.87. • Improved floc structure leads to significantly better sedimentation, reducing turbidity from 436.8 to 76.7 NTU and increasing settled solids from 35.94% to 52.43%. • A multi-scale correlation model quantitatively links interfacial chemistry, floc structural evolution, and settling behaviour, providing a unified mechanistic basis for pH-regulated hematite flocculation.