Fouling Mechanisms of Nanofiltration Membranes Induced by Residual Aluminum and Natural Organic Matter in Drinking Water Treatment

Though complexes formed between residual active aluminum (Al_a) and natural organic matter (NOM) have been identified as key contributors to the aggravation of nanofiltration membrane fouling, the fouling behaviors and mechanisms of Al_a-NOM complexes remain poorly understood. In this study, the unique role of Al_a in inducing membrane organic fouling and the interspecies interactions within Al_a-NOM complexes were investigated. The results showed that complexes formed between Al_a and fulvic acid (FA) created a compact foulant layer attaching to the membrane surface, which contributed more than 90% of the total fouling resistance. This surface-attached foulant layer was attributed to the high electronegativity of Al_a, which induced conformational rearrangement within FA molecules and exposed additional reactive functional groups which would be involved in membrane fouling. Moreover, the Al_a within the Al_a-FA complexes bridged to carboxyl groups on the membrane surface through strong hydrogen bonds and continuous charge-transfer channels, reinforcing the stability and adhesion of this fouling layer. Unexpectedly, the addition of sodium alginate at only one-seventh the concentration of FA significantly mitigated Al_a-FA fouling by encapsulating the Al_a-FA complexes and disrupting the formation of the surface-attached foulant layer. This study provides new mechanistic insights into residual aluminum-involved organic fouling faced by nanofiltration membranes in drinking water treatment.