Engineered cake layer via organosilicon-modified coagulant aid for regulating fouling behavior and enabling ultra-low-pressure microfiltration of PFOA and NOM

Ultra-low-pressure ceramic microfiltration (ULPCM) is fundamentally constrained by the permeability-selectivity trade-off: the accumulation of compact cake layers increases hydraulic resistance, whereas the intrinsic large pore size of microfiltration membranes limits the rejection of trace micropollutants. In this study, a tri-functional quaternary ammonium silane (AC) was introduced during polyaluminum chloride (PACl) pre-coagulation to regulate floc architecture and engineer the structure of the coagulation-derived cake layer. During 360 h (15 d) of continuous operation in both synthetic surface water and actual surface water, the AC-integrated system achieved a stable normalized flux (J/J₀ = 0.38-0.40) and exceptional PFOA sequestration (>93.71%), significantly outperforming conventional systems. Molecular dynamics (MD) simulations and spectroscopic analyses (XPS/FT-IR) elucidated that the C₁₈ alkyl chains and quaternary ammonium groups dictate PFOA capture through synergistic hydrophobic and electrostatic interactions, while hydrophilic silanol groups enhance dissolved organic carbon (DOC) removal via hydrogen bonding. At the interfacial level, XDLVO theory and dynamic light scattering (DLS) quantification revealed that AC effectively regulates the thermodynamics of floc adhesion, promoting the assembly of a highly branched, low-fractal-dimension (D_f) scaffold. Morphology measurements confirmed that the engineered cake layer maintained a high porosity (up to 62.79%) with a pore size distribution dominated by micro-channels (<0.5 μm), which enhanced the water permeability of the cake layer and its retention capacity for foulants. Integrated fouling analysis using Hermia, Tansel, and saturation decay models, together with statistical evaluation, demonstrated that floc size, cake layer porosity, pore size distribution, and AC-induced hydrophobic interactions played key roles in mitigating membrane fouling and enhancing contaminant retention. These findings highlight that regulating floc architecture via amphiphilic coagulant aids provides an effective strategy for engineering cake layer microstructure and mitigating membrane fouling in low-pressure microfiltration systems.