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

Though complexes formed between residual active aluminum (Ala) and natural organic matter (NOM) have been identified as key contributors to the aggravation of nanofiltration membrane fouling, the fouling behaviors and mechanisms of Ala-NOM complexes remain poorly understood. In this study, the unique role of Ala in inducing membrane organic fouling and the interspecies interactions within Ala-NOM complexes were investigated. The results showed that complexes formed between Ala 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 Ala, which induced conformational rearrangement within FA molecules and exposed additional reactive functional groups which would be involved in membrane fouling. Moreover, the Ala within the Ala-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 Ala-FA fouling by encapsulating the Ala-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.