Metal–Organic Frameworks Supported on Nanofiber for Desalination by Direct Contact Membrane Distillation

Among other applications, metal-organic frameworks (MOFs) are slowly gaining grounds as fillers for desalination composite membranes. In this study, superhydrophobic poly(vinylidene fluoride) nanofibrous membranes were fabricated with MOF (iron 1,3,5-benzenetricarboxylate) loading of up to 5 wt % via electrospinning on a nonwoven substrate. To improve the attachment of nanofibers onto the substrate, a substrate pretreatment method called "solvent basing" was employed. The iron content in the nanofiber, measured by energy-dispersive X-ray spectroscopy, increased proportionally with the increase of the MOF concentration in the spinning dope, indicating a uniform distribution of MOF in the nanofiber. The water contact angle increased up to 138.06 ± 2.18° upon the incorporation of 5 wt % MOF, and a liquid entry pressure of 82.73 kPa could be maintained, making the membrane useful for direct contact membrane distillation experiments. The membrane was stable for the entire operating period of 5 h, exhibiting 2.87 kg/m²·h of water vapor flux and 99.99% NaCl (35 g/L) rejection when the feed and permeate temperature were 48 and 16 °C, respectively. Immobilization of MOF on nanofibers with the enhanced attachment was proven by inductively coupled mass spectrometry analysis, by which no Fe²⁺ could be found in the permeate to the detection limit of ppt.