A modified coagulation-ultrafiltration process for silver nanoparticles removal and membrane fouling mitigation: The role of laminarin

Silver nanoparticles (AgNPs) in surface water are highly toxic to humans and difficult to remove due to their adsorption to humic acid (HA). In this study, laminarin (LA) was used as a coagulant aid in a coagulation-ultrafiltration (C-UF) system to improve AgNPs-HA removal efficiency. C-UF efficiency, membrane flux, and flocs properties were investigated to explore the AgNPs-HA removal mechanism and membrane fouling. Results showed that when poly aluminum chloride (PAC) was dosed with LA, AgNPs-HA removal was 10–15% higher than when using PAC alone. The C-UF system using only PAC improved the AgNPs-HA removal efficiency through increased coagulation but resulted in membrane fouling. LA application helped mitigate membrane fouling, and the highest normalized permeate flux and smallest resistance values (0.573 and 2.180 × 1010 m−1, respectively) were achieved when 0.1 mg/L of LA was applied with 5 mg/L of PAC. The alleviating mechanism was related to flocs with large sizes and small fractal dimension (Df) values, generating a cake layer with porous morphology. This cake layer was easily removed by flushing and backwashing, which resulted in minimal resistance and fouling of the UF membrane.