However, the cake layer formed by oxides with lower df can worsen the performance of membrane filtration because of increased specific cake resistance. In this study, the df of Fe–Mn oxides formed at a constant mixing intensity is greater than that of Fe oxides. But, permeate flux is reduced by the formation of Fe–Mn oxides at a constant mixing intensity, as illustrated in Fig. 5. Cake permeability can thus be affected by the composition of oxides. In/during the oxidation of Fe and Mn, a bulk/prevalence of Fe–Mn oxides can easily be formed in the presence of Mn oxides [14]. As a result, the surface roughness of cake layers formed by Fe and Fe–Mn oxides differs, resulting in varied cake permeability. To verify this, the contact angle of the cake surface on a fouled membrane was determined. The contact angle of the cake layer formed with Fe–Mn oxides is greater than that with Fe oxides at a fixed mixing intensity, as shown in Table 2, indicating that Fe–Mn oxides are more hydrophobic than Fe oxides. Surfaces with increased hydrophobicity have greater roughness than less hydrophobic surfaces [24] and [25]. It is likely that the smaller Mn oxides adsorb on the surface of Fe oxides and enhance the roughness of the cake, resulting in a surface with higher hydrophobility. This process could also worsen the performance of membrane filtration, and as a result, the mean permeate flux of dead-MF membrane in the filtration of Fe–Mn oxides significantly declines.