Cake layer 3D structure regulation to optimize water channels during Al-based coagulation-ultrafiltration process

The variation in cake layer three-dimensional (3D) structures and related water channel characteristics induced by coagulation pretreatment remains unclear; however, gaining such knowledge will aid in improving ultrafiltration (UF) efficiency for water purification. Herein, the regulation of cake layer 3D structures (3D distribution of organic foulants within cake layers) by Al-based coagulation pretreatment was analyzed at the micro/nanoscale. The sandwich-like cake layer of humic acids and sodium alginate induced without coagulation was ruptured, and foulants were gradually uniformly distributed within the floc layer (toward an isotropic structure) with increasing coagulant dosage (a critical dosage was observed). Furthermore, the structure of the foulant-floc layer was more isotropic when coagulants with high Al₁₃ concentrations were used (either AlCl₃ at pH 6 or polyaluminum chloride, in comparison with AlCl₃ at pH 8 where small-molecular-weight humic acids were enriched near the membrane). These high Al₁₃ concentrations lead to a 48.4% higher specific membrane flux than that seen for UF without coagulation. Molecular dynamics simulations revealed that with increasing Al₁₃ concentration (Al₁₃: 6.2% to 22.6%), the water channels within the cake layer were enlarged and more connected, and the water transport coefficient was improved by up to 54.1%, indicating faster water transport. These findings demonstrate that facilitating an isotropic foulant-floc layer with highly connected water channels by coagulation pretreatment with high-Al₁₃-concentration coagulants (having a strong ability to complex organic foulants) is the key issue in optimizing the UF efficiency for water purification. The results should provide further understanding of the underlying mechanisms of coagulation-enhancing UF behavior and inspire precise design of coagulation pretreatment to achieve efficient UF.