Revealing key meso-particles responsible for irreversible membrane fouling in an integrated oxidation-coagulation ultrafiltration system: Fouling behavior and interfacial interaction mechanism

• The particles scale was regulated by in-situ Fe(VI) oxidation-hydroxide coagulation. • Meso-particle in range of 20-100 nm was positive related to irreversible fouling. • The meso-particle had more stable and tight structure than micro-particle. • The interfacial interaction between membrane and multi-scopic particles were revealed. This study applied a ferrate in-situ oxidation-coagulation ultrafiltration integrated system in drinking water treatment, illustrating the influence of meso-particles on membrane fouling behavior by oxidation-coagulation regulating. Under neutral conditions, the meso-particles between 20 and 100 nm were the mainly components at initial oxidation phase. Subsequently small molecules gathered to meso-particles of 200-500 nm under the flocculation, and 40% of micro-particles aggregates were detected finally. According to the statistical correlation between multiscale particles and membrane flux and resistance, wherein meso-particle especially in range of 20-100 nm principally contributed to the irreversible fouling while the correlation coefficients above 0.9, and reversible fouling related to the synergy of meso- and micro-particles. Additionally, the fouling behavior of meso-particles on the membrane surface and the structural evolution of the fouling layer were illustrated based on the interfacial interaction, UV-vis reflectance spectroscopy and molecular dynamics simulation. The changes in hydrophilicity and charge of the meso-particle (20-100 nm) increased the attractive interaction with the membrane, causing severe membrane fouling. As flocculation progressed, the polynuclear Fe-hydroxide interacted with HA molecular fragments, and aggerated to meso-particles in range of 200-500 nm, which had smaller molecular gap and more tight structure than micro-particle (500-1000 nm), due to the HA molecular fragments were more strongly bound on the polynuclear Fe-hydroxide compared to iron hydroxide cluster. The stable and compact structure induced the meso-particles deposited on membrane surface and hardly be physically cleaned. The investigation of meso-particle on membrane fouling was provided new insight into the membrane separation process.