Modification of ultrafiltration membrane with iron/aluminum mixed hydrolyzed precipitate layer for humic acid fouling reduction

Iron/aluminum mixed flocs were pre-deposited onto an ultrafiltration (UF) membrane surface to test their antifouling properties in the presence of humic acid (HA). Membranes modified with higher aluminum mole ratios in the mixture were more effective in controlling membrane fouling. Relatively strong adhesion was observed between the mixed flocs and the UF membrane used. The differences in the structure of the deposited layer were an important contributing factor for antifouling properties, which was significantly influenced by solution pH. Scanning electron microscopy showed that dense deposition layers were formed by small flocs at pH 6.0, while loose deposition layers were formed by large flocs at pH 8.0. Atomic force microscopy showed that the surface roughness of the deposition layer was much larger at pH 6.0 than that at pH 8.0. The corresponding positive charge of the mixed flocs was also higher at pH 6.0, resulting in stronger adsorption ability to the negatively charged HA molecules. These factors reduced the amount of HA getting to the membrane surface and thus helped increase membrane flux at lower pH values. In addition, HA molecules with various molecular weights were removed much faster at pH 6.0 than at pH 8.0.