Simultaneous fouling and scaling-resistant membrane based on glutamic acid grafting for robust membrane distillation

Membrane fouling and scaling are two critical challenges in membrane distillation (MD). The traditional superhydrophobic anti-scaling strategy is contradictory to the hydrophilic anti-fouling coating, creating a crucial challenge to combine these two properties into one material. In this study, glutamic acid (GA) was grafted on a polyvinylidene fluoride (PVDF) membrane surface to achieve fouling and scaling resistance simultaneously. The hydrophilic GA modification endowed the membrane with a larger underwater oil contact angle without affecting the surface morphology. The direct contact MD (DCMD) tests demonstrated that the GA-modified membrane had excellent oil fouling resistance due to the formation of a hydration layer by the GA molecules. In addition, the surface carboxyl groups offered plenty of coordination sites for metal ions, such as calcium ions. The surface coordination of calcium resulted in a lower free calcium concentration on the membrane surface, thereby postponing the scale nucleation and preventing the scales from intruding into the membrane matrix. The scaling tests also showed that the gypsum scales grown on the modified membrane were smaller than those on the pristine PVDF membrane. Besides, membrane cross-section analyses revealed that no scale intrusion was observed in the GA-modified membrane. Long-term stability tests and practical application with ethylene plant spent caustic further confirmed the promising application potential of the membrane. These results demonstrated that anti-fouling and anti-scaling properties can be integrated using the GA modification. This study provides a new surface modification strategy for designing and fabricating a robust anti-fouling and anti-scaling MD membrane.