As we all know, membrane distillation (MD) process has gained increasing popularity and attention for saline treatment especially because of its extremely high salt rejection (the theoretical value up to 100%). In MD, the hydrophobic membrane serves core and significant part to realize two phases separation. However, the existing membrane wetting phenomenon always restricts the long-term stable operation and separation efficiency.
Zhaoliang Cui, a professor comes from the College of Chemical Engineering of Nanjing Tech University, and the co-workers of the present study, found one promising solution to solve this problem by preparing hydrophobic poly(vinlidene fluoride-co-hexafluoro propylene) (PVDF-HFP) copolymer membrane to improve surface hydrophobicity. This study was published online in Frontiers of Chemical Science and Engineering on March 28, 2022.
In this study, one hydrophobic fluorinated polymer material, PVDF-HFP, was chosen as the membrane material to prepare porous membrane for membrane distillation application. The thermally induced phase separation (TIPS) method was employed to prepare membrane, which is a common fabrication way. In order to decrease harmfulness due to the volatilization of used solvent at high temperature during TIPS process, the green and environmentally friendly diluent, acetyl tributyl citrate (ATBC) was used.
According to the phase diagram of PVDF-HFP/ATBC system, there was a wide liquid–liquid phase region among the range of prepared polymer concentration (25–40 wt%), indicating that the liquid–liquid phase separation happens. As a result, the final bi-continuous structure can be obtained. The pore connectivity, permeability and mechanical properties of such structure will benefit the mass transfer during separation process. The surface contact angle, porosity, tensile strength and elongation at break of these prepared membrane in this study were up to 123.87o, 60.43%, 5.89 MPa and 396.85%, respectively.
Then the prepared PVDF-HFP membranes were applied in membrane distillation process treating 3.5 wt% NaCl saline. Firstly, the based desalination performance was tested at different feed temperatures, which showed that the permeate flux kept proportional to the feed temperature. This is consistent with common sense. Moreover, for a longer testing period, the prepared membrane had a stable membrane distillation flux and was not wetted (the permeated conductivity was always around 2–3 mS·cm−1). These are all exciting and satisfied results.
Cui and his team will continue their study on the preparation of hydrophobic or superhydrophobic membrane for membrane distillation application. They hope to prepare enough excellent hydrophobic membranes and achieve the final goal of zero liquid discharge of wasted saline to recycle the industrial raw materials and water resource.