Unraveling the Structure–Activity Relationship of Polysilicic Acid Aluminum Composite Coagulants for Multipollutant Elimination and Residual Aluminum Minimization

The structure and hydrolysis of Al-based coagulants critically depend on the synthesis conditions. In this work, a series of polyaluminum silicate chloride (PASiC) coagulants with different basicity ( B ) and Al/Si ratios were prepared and used to remove multiple pollutants and minimize residual Al. FTIR and particle size analyses revealed that increasing the Si content promoted the formation of Si–O–Al bonds and enhanced the degree of polymerization of hydrolysates. Elevated B values facilitated the generation of mesopolymeric hydrolysates with a high charge density and reactivity. Concurrently, the low content of oligomeric aluminum species significantly reduced the residual dissolved Al. However, excessive Si dosage and B values induced overhydrolysis and gelation, deteriorating coagulation efficacy. Comparative analysis revealed that PASiC outperformed conventional polyaluminum chloride (PAC), achieving high turbidity removal efficiency (93.04%) and maintaining residual total Al below 0.200 mg/L (versus 0.259 mg/L for PAC) and organically bound dissolved Al under 0.026 mg/L. This work establishes a “synthesis–structure–activity” framework for the rational design of inorganic composite coagulants, providing theoretical reference for multipollutant elimination and residual aluminum minimization.