Impacts of distinct residual aluminum on gypsum scaling in nanofiltration: Identification of key species

Membrane scaling severely hinders practical application of nanofiltration (NF) for brackish water treatment. However, the contributions of residual aluminum (Al), especially various species with differing polymerization degrees and sizes, to gypsum scaling remain unclear. Herein, the complex interactions between three Al species (AlCl₃, Al₁₃ and Al₃₀) and gypsum scaling were investigated, including surface and bulk crystallization processes. Results demonstrated that Al species adhering to the membrane surface promoted surface crystallization and altered crystal morphology. Notably, the Al₁₃ species induced the formation of dense, complete rosette crystals causing the most severe flux decline. For bulk crystallization, Al₃₀ was the primary species to promote nucleation. This promotion resulted from two factors: a lower nucleation energy barrier and the promoted aggregation of small crystals. Furthermore, molecular dynamics (MD) simulations revealed that the inhibition of nucleation due to the competition between Al species and calcium ions at nucleation sites was negligible compared to the strong promotion effect. These findings demonstrated that PACl exhibit higher scaling potential during NF than monomeric Al coagulants. Ultimately, the competition between surface and bulk crystallization determined the final membrane flux decline. This study offers valuable insights into the mechanisms of NF scaling caused by residual Al and guides mitigation strategies.