Elucidating floc physicochemical properties governing cake layer formation and growth in coagulation-ultrafiltration hybrid processes

Magnetically enhanced coagulation currently faces dual constraints: limited organic removal efficiency (primarily occurring during rapid mixing despite improved collision efficiency) and insufficient understanding of magnetic floc/cake layer aging in ultrafiltration (UF) systems. Crucially, after obtaining highly effective control of membrane fouling, the reduced need for frequent backwashing invariably leads to heavy consolidation and prolonged accumulation of the cake layer. This study prepared four nanomagnetic seeds (Fe₃O₄, Fe₃O₄@SiO₂(FS), Fe₃O₄@SiO₂-NH₂(FSN), MFe₃O₄@CS-g-PIA(MFCS)), demonstrating MFCS's superior organic removal capacity. Furthermore, during the slow stirring phase, sustained adsorption and complexation capabilities were exhibited by the coagulation process, particularly for aromatic proteins. The control of membrane fouling in combination with ultrafiltration also exhibited significant differences: the mechanism of cake layer formation was advanced with retainment of more tryptophan-like proteins. With respect to the aging problem, the flocs and cake layers formed by conventional polymeric alumina chloride primarily consist of amorphous Al(OH)₃ with low polymerization. Within 3 to 7 days, a substantial release of highly aromatic humic acid-like substances will occur, accompanied by a gradual increase in the release rate of all organic matter. This enhanced release is attributed to hydroxyl bridging reactions and recrystallization occurring during aging. Conversely, magnetic flocs or cake layers markedly suppress the release of all classes of organic compounds. Key factors underlying this suppression include a lower proportion of lattice oxygen, higher relative abundance of C-O/C=O functional groups, and the presence of abundant nanoscale Al(OH)_3cr. These compositional features enhance π-π and hydrophobic interactions and promote the formation of a hydrogen-bonding network which effectively constrains/immobilizes organic compounds. Molecular dynamics confirm MFCS aggregates' elevated binding affinity stems from both physicochemical bonding and reduced non-bonding (Coulombic/Lennard-Jones) interactions.