Exploring the Growth Pattern and Hydraulic Resistance of BioMnO_x Cake Layer under Different Fluxes in Ultrafiltration Processes

Abstract

Biogenic manganese oxides (BioMnOx) are known to self-catalyze Mn(II) oxidation and are unfavorable in conventional ultrafiltration (UF) processes for Mn(II)-containing water purification because they can accumulate on membrane surfaces and cause severe cake fouling. However, recent studies based on gravity-driven membrane (GDM) have found that the fouling resistance of the BioMnOx cake layer is surprisingly low under low flux conditions (several L m–2 h–1), but the underlying mechanism is not clear. Therefore, in this study, we investigated the formation and hydraulic resistances of the naturally formed BioMnOx cake layers at various fluxes (5, 10, 15, and 20 L m–2 h–1). The results showed that the BioMnOx could form a heterogeneous cake layer with many mounds, and these BioMnOx cake layers could remove nearly 100% of 2 mg L–1 Mn(II) after 34, 49, 55, and 64 days of dead-end filtration at fluxes of 5, 10, 15, and 20 L m–2 h–1, respectively. The hydraulic resistance of the BioMnOx cake layer rapidly increased at high fluxes of 10, 15, and 20 L m–2 h–1, while it remained very low (<5 × 1012 m–1) at a low flux of 5 L m–2 h–1. In-situ optical coherence tomography (OCT) observation revealed that at low flux, the BioMnOx mounds of the cake layer had a higher height–width ratio and lower coverage and thus more porous structures. We attributed this structure to the unique growth pattern of BioMnOx at low fluxes. Specifically, because the advection velocity of Mn(II) was low at low fluxes, new BioMnOx was mainly grown in the upper parts of the cake layer with enough time for Mn(II) diffusion and oxidation. Then, the upper parts depleted Mn(II), and consequently, the lower parts maintained a high porosity with few formations of BioMnOx at their confined pores.