Preparation And Properties Of Pervaporation Composite Membrane For Salty Water Desalination

As a result of the fast development of economy and worsening environmental pollution, the water shortage is now a worldwide problem. China is one of the countries which have least water resources per capita, and the lack of fresh water has seriously affected our economic development and the life of the people. Conventional salty water desalination technologies have some limits in application because of their characteristics, so the development of new salty water desalination technologies has become a research hotspot. Pervaporation desalination is a new desalination technology developed in the recent years, and its significant advantage is that it combines the advantages of membrane desalination and thermal desalination technologies while eliminating the disadvantages of them, so it is becoming one of the most potential technologies after multistage flash evaporation, low temperature multi-effect distillation and reverse osmosis, to release the increasingly serious water crisis. But present relatively low efficiency of pervaporation desalination limited its actual applications. Therefore, the redesign of pervaporation membrane material and structure was carried out to increase the pervaporation desalination efficiency. In this paper, pervaporation composite membranes with high permeate efficiency were prepared by redesigning the membrane material and structure, and the relationship between pervaporation desalination efficiency and the membrane material and structure.1. Electrospun nanofiber has a unique structure of high porosity and through pores. Furthermore, polymer skin layer prepared by electrospraying has low crystallinity, and its amorphous structure enables the fast diffusion of small molecules inside the polymer layer. Based on these, the membrane was designed in a unique structure. Polyvinyl alcohol (PVA) skin layer was prepared by electrospraying, polyacrylonitrile (PAN) nanofiber was prepared by electrospinning as the support layer, polyester nonwoven was used as base membrane to provide mechanical strength to the composite membrane, and by simple immerse crosslinking, PVA/PAN nanofiber composite membrane was eventually prepared. This composite membrane has excellent pervaporation desalination efficiency. Under room temperature, when separating 35,000 ppm sodium chloride (NaCl) solution, the flux was 7.36 L·m-2·h-1, and the rejection was over 99.5%. Experimental results proved that this membrane can efficiently separate salty water with different concentrations. The separation efficiency is mostly determined by the skin layer thickness, and the flux was inversely proportional to the skin layer thickness. A continuous 50 h operation of the composite membrane proved that the membrane has good stability under normal operation conditions.2. By simple coating method and using 4-sulfophthalic acid (SPTA) as crosslinking agent, SPTA-PVA skin layer was prepared on a PAN ultrafiltration membrane surface, then SPTA-PVA/PAN pervaporation composite membrane was obtained after thermal crosslinking, and the relationship between the skin layer structure and the pervaporation desalination efficiency was studied in detail. The results showed that the water molecules had very high diffusion ability in the skin layer containing sulfonic acid groups. For 35,000 ppm NaCl solution at 70℃, the water flux was 46.3 L·m-1·h-1 and the rejection was over 99.8%. When the sulfonic acid group content increased, the crystallinity of the skin layer reduced and the content of amorphous structure increased, and because that the sulfonic groups have transport ability for hydrated proton, the pervaporation mass transfer efficiency increased. The skin layer thickness still had significant effect on the separation efficiency, and the separation efficiency reduced with thicker skin layer. A continuous 100 h operation of the composite membrane proved that the membrane has good stability under high temperature.3. Based on the unique structure and separation characteristics of graphene oxide (GO), by simple vacuum filtration, GO skin layer was prepared on a PAN ultrafiltration membrane to form a GO/PAN pervaporation composite membrane, and the relationship between the structure of GO and the pervaporation performances of the composite membrane was characterized in detail. The results showed that the GO layer had unique structure and separation performances. GO laminas contain some hydrophilic groups like carboxyl and hydroxyl groups, and the space between GO layers is 7.36 A, which makes water molecules able to transfer fast between GO layers while effectively blocking hydrated ions. For 35,000 ppm NaCl solution at 90℃, the water flux was 65.1 L·m-2·h-1 and the rejection was over 99.8%. The composite membrane can treat NaCl solution with a concentration of up to 100,000 ppm. The temperature and the mass transfer efficiency followed the Arrhenius law.