Influence of water quality factors on cake layer 3D structures and water channels during ultrafiltration process

The three-dimensional (3D) structure of the cake layer, which could be influenced by water quality factors, plays a significant role in the ultrafiltration (UF) efficiency of water purification. However, it remains challenging to precisely reveal the variation of cake layer 3D structures and water channel characteristics. Herein, we systematically report the variation in the cake layer 3D structure at the nanoscale induced by key water quality factors and reveal its influence on water transport, in particular the abundance of water channels within the cake layer. In comparison with pH and Na⁺, Ca²⁺ played more significant role in determining cake layer structures. The sandwich-like cake layer, which was induced by the asynchronous deposition of humic acids and sodium alginate (SA), shifted to an isotropic structure when Ca²⁺ was present due to the Ca²⁺ bridging. In comparison with the sandwich-like structure, the isotropic cake layer has higher fractions of free volume (voids) and more water channels, leading to a 147% improvement in the water transport coefficient, 60% reduction in the cake layer resistance, and 21% increase in the final membrane specific flux. Our work elucidates a structure-property relationship where improving the isotropy of the cake layer 3D structure is conducive to the optimization of water channels and water transport within cake layers. This could inspire tailored regulation strategies for cake layers to enhance the UF efficiency of water purification.