Role of humic acid aromaticity in mitigating gypsum-induced membrane wetting in membrane distillation

Aromaticity is associated with membrane wetting mitigation. • Humic acid (HA)––CaSO 4 interactions form larger-sized complexes. • Aromaticity strengthens HA–CaSO 4 affinity for PVDF and forms protective layers. • HA–CaSO 4 deposition redirects gypsum growth and reduces pore intrusion. Gypsum (CaSO 4 ∙2H 2 O) scaling is a critical issue that impedes the application of membrane distillation (MD) for treating hypersaline water and wastewater. However, the impact of ubiquitously present humic acid (HA) remains unclear. Herein, this study systematically investigates the effects of HA on both bulk and surface gypsum crystallization and elucidates the molecular-level mechanisms by which HA mitigates gypsum-scaling-induced membrane wetting. Bulk crystallization results indicated that HA retarded crystal growth and formed larger-sized HA–CaSO 4 complex, which was associated with membrane scaling behavior. Tailored HA pretreatments of a bench-scale MD system demonstrated that aromaticity showed a stronger positive correlation with membrane wetting control (R 2  = 0.920), compared with molecular weight and negative charge. Guided by this structure–performance relationship, membrane surface scaling behavior revealed that higher HA aromaticity enhanced hydrophobic interactions between HA–CaSO 4 complex and the polyvinylidene difluoride (PVDF) membrane. This interaction facilitated the deposition of larger-sized HA–CaSO 4 complex on the membrane surface and altered the growth orientation of gypsum, which suppressed crystal growth within membrane pores and consequently mitigated membrane wetting. While carboxyl groups are important for scaling mitigation by organic matter, the aromaticity of organic matter shapes scaling behavior in membrane processes treating saline water. Download: Download high-res image (124KB) Download: Download full-size image