Impact of residual aluminum on nanofiltration gypsum scaling: Mitigation roles played by different species

Residual aluminum (Al) is a growing pollutant in nanofiltration (NF) membrane-based drinking water treatment. To investigate the impact of distinct Al species fouling layers on gypsum scaling during NF, gypsum scaling tests were conducted on bare and three Al-conditioned (AlCl₃-, Al₁₃, and Al₃₀-) membranes. The morphology of gypsum, the role of Al species on Ca²⁺ adsorption during gypsum scaling, and the interactions between gypsum crystals and Al-conditioned membranes were investigated. Results indicated that Al-conditioned membranes had lower flux decline than the bare membrane, with the order of AlCl₃-<Al₃₀-<Al₁₃-conditioned membrane, due to a lower heterogeneous crystallization tendency. The membrane surface charge was the major factor determining heterogeneous crystallization. Based on the analyses of quartz crystal microbalance with dissipation (QCM-D), it was found that surface Al species inhibited the adsorption of Ca²⁺ on the membrane surface thereby mitigating heterogeneous crystallization. FTIR further indicated that this inhibition was due to the competition between Al species and Ca²⁺ for binding sites on the membrane surface. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analyses showed that Al-conditioned membranes had a higher gypsum adhesion tendency than the bare membrane, and hydrophobic attraction dominated the interaction. This study provides new insights into how residual Al species impact mineral scaling during NF.