Molecular-level interaction mechanisms of humic acid and residual aluminum species in gypsum scaling during nanofiltration

Gypsum scaling is a major challenge limiting the nanofiltration (NF) desalination of high-salinity water. However, the impact of residual coagulants after pretreatment on mineral scaling in membrane systems has rarely been studied. Herein, the effects of humic acid (HA) and different aluminum (Al) species on gypsum scaling during NF were investigated. HA and Al species reduced the crystal growth rate in the bulk solution by 27.3%-64.9%. Molecular dynamics and quantum chemical calculations revealed strong interactions between HA/Al species and scaling ions, suppressing calcium sulfate diffusion and lowering the probability of particle collisions, particularly for the HA-Al system. In contrast, HA and Al species significantly accelerated membrane flux decline during NF. Their coexistence caused a 55.3%-66.3% greater flux reduction than the control, attributed to intensified concentration polarization and secondary crystallization on the membrane. Compared with Al₁₃, detachment of CaSO₄ clusters from the colloidal Al surface required a lower free-energy barrier, suggesting that more gypsum nuclei could detach from the colloidal Al surface and migrate toward the membrane surface, subsequently inducing severe secondary crystallization. These findings reveal the detrimental effects of residual Al and HA on gypsum scaling during NF, offering new insights into mixed fouling mechanisms in membrane desalination.