Positively charged and flexible SiO₂@ZrO₂ nanofibrous membranes and their application in adsorption and separation

Tiny particles with a negative charge in water can be removed effectively by inorganic positively charged nanofiber membranes due to their physical separation and electrostatic adsorption properties. However, the charged positive and isoelectric points (IEP) on the surface of the positively charged nanofiber membranes need to be further improved to meet the requirements of the Environmental Protection Agency for virus filters (excellent adsorption and separation properties in solutions with pH between 5 and 9). The positively charged SiO₂@ZrO₂ nanofibrous membrane was fabricated by electrospinning combined with the impregnation method and calcined in an anoxic atmosphere. The effects of the impregnating solution concentration and centrifugal speeds on the morphology, pore size and nanofiber diameter of the composite nanofibrous membrane were investigated. The phase composition, the element valence state, the surface charge and the adsorption separation properties of the composite nanofibrous membranes were characterized. Flexible SiO₂@ZrO₂ nanofibrous membranes with high specific surface area and water flux were obtained. The surface isoelectric point is 7.3 when calcined in a mixed atmosphere of N₂ (96%)/H₂ (4%), which is higher than that when calcined in air and vacuum. Some oxygen vacancies were present on the surface of the ZrO (0 < x < 2) shell, resulting in the tetragonal zirconia still being available at room temperature. The interception rate of the positively charged SiO₂@ZrO₂ nanofibrous membranes for a titan yellow solution with a concentration of 10 mg L^-1 reaches 99.996%. Its maximum adsorption capacity can reach 63.27 mg cm^-3 when filtering a 1 L titan yellow solution. The resulting nanofibrous membranes have potential application for the separation of bacteria and viruses from water.