Insights into iron-accelerated gypsum scaling mitigation strategies in nanofiltration membrane process

In this study, the impact of ferric ions on gypsum crystallization and NF membrane performance was investigated to mitigate iron-accelerated gypsum scaling. Gypsum scaling in the presence of ferric ions led to a significant decline in the water flux, but a relatively higher reversible fouling ratio was observed compared to that under individual gypsum and iron-oxide scaling. Ferric ions influenced the crystallization of gypsum on NF membranes by serving as additional nucleation sites, leading to accelerated gypsum scaling. Various mitigation methods (e.g., feed spacers and cleaning processes) were evaluated to effectively control iron-accelerated gypsum scaling. The introduction of spacers exhibited a high mitigation efficiency by enhancing the mass transfer of scalant ions due to changes in the water channel hydrodynamics, but the overall efficiency decreased over time. To enhance the mitigation efficiency, different cleaning methods (e.g., CO₂ oversaturated solution, HCl, and NaCl flushing) were conducted with feed spacers. A CO₂ saturated solution demonstrated high cleaning efficiency owing to the high turbulence induced by CO₂ bubbles, whereas HCl flushing showed negligible efficiency. NaCl flushing exhibited the highest cleaning efficiency through mono/divalent ion-exchange effects on iron-gypsum scaling. This study provides comprehensive insights into iron-accelerated gypsum scaling mitigation strategies for nanofiltration membranes, highlighting the impact of coexisting ferric compounds on gypsum scaling.