Synthesis pathway governs Si-O-Fe bonding and unlocks pH-switchable multi-mechanism synergy in silane-grafted hybrid coagulants

Synthesis pathway governs Si-O-Fe bonding uniformity and functional group exposure. • GPFC-H unlocks pH-switchable synergy: charge neutralization (acidic) and hydrophobic association (alkaline). • GPFC-H achieves 91.2% HA removal at pH 6.0 and 74% at pH 9.6, versus < 41% for PFC. • The GPFC-H route requires no post-treatment and is compatible with spray drying. • Spray-dried powder retains > 90% coagulation activity after 28-day storage. Long-chain silane coupling agents like dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (TPODAC) promise enhanced coagulation performance through hydrophobic interactions, yet their practical application is crippled by uncontrolled hydrolysis and detrimental self-condensation during conventional synthesis. This study demonstrates that the synthesis pathway fundamentally governs Si-O-Fe bonding configuration, colloidal stability, and the resulting coagulation mechanisms. By systematically comparing four synthesis strategies with modulated silane hydrolysis states and component addition sequences, a clear structure-property hierarchy is established through multi-method characterization (FTIR, XPS, XRD, zeta potential) and coagulation performance evaluation. The optimal route, pre-hydrolyzed silane grafted onto pre-formed polymeric ferric chloride (GPFC-H), suppresses silane self-condensation and yields uniform Si-O-Fe covalent bonding with fully exposed quaternary ammonium and C18 hydrophobic chains. This structural perfection unlocks a rare pH-switchable multi-mechanism synergy. At pH 6.0, GPFC-H achieves 91.2% humic acid removal via charge neutralization. Remarkably, at pH 9.6 where conventional coagulants fail (<41% removal), GPFC-H maintains 74% removal through hydrophobic association mediated by its pH-insensitive C18 chains. GPFC-H also achieves 78% Sb(V) removal at pH 7.0, versus 65% for PFC. Furthermore, this synthesis route requires no post-treatment and is compatible with spray drying; the obtained solid powder retains > 90% of its initial activity after 28-day storage, demonstrating its practical potential for industrial production and long-distance transport. By establishing causal links from synthesis pathway to bonding structure to pH-adaptive performance, this work transforms hybrid coagulant preparation from empirical optimization into a designable strategy for advanced water purification. Download: Download high-res image (173KB) Download: Download full-size image