<p>The coordinated recovery of iron and aluminum from red mud for preparing inorganic polymeric flocculants represents a critical pathway to enhance red mud’s resource utilization value, yet the inhibition mechanisms of Na⁺ and Si impurities generated during the acid leaching process on the flocculation performance remain unclear. This study therefore aims to elucidate impurity-mediated regulation of Polyaluminum Ferric Sulfate (PAFS) structure–performance relationships through comparative synthesis of pure reagent systems (PAFS-C) and red mud acid leachate systems (PAFS-J). Experimental results demonstrate that in PAFS-J, Na⁺ occupies Fe/Al active sites via ion exchange to inhibit hydroxyl bridging, while SiO₂ in calcined red mud transforms into reactive silicic acid, fragmenting polymer structures through Al–O–Si bonding and Si–O–Si networks, leading to significantly inferior flocculation efficiency compared to PAFS-C. The flocculation mechanism of PAFS-J was further investigated through Zeta potential analysis and polarized light microscopy. The results demonstrate that PAFS-J relies on electric double-layer compression and weak charge neutralization to form loose flocs, with its flocculation process following a multi-stage cooperative mechanism involving charge neutralization, adsorption bridging, and sweep flocculation. This study deeply elucidates the inhibition mechanisms of impurities on the flocculation performance of PAFS and their flocculation mechanism, providing a theoretical foundation for the precise design of red mud-based flocculants and the high-value utilization of industrial solid waste.</p>