An experimental study was conducted on the problem of low salt rejection of tubular hydrophobic ceramic membranes for membrane distillation. First, the both ends of a ceramic membrane support were uniformly coated with mixture of self-made oxide AB and deionized water in the given ratio and sintered at a high temperature, and the ends were vitrified and sealed in order to solve the problem of the short circuit flow from the end face. Then, at room temperature, hydrophobic modification of the tubular support was carried out by using the isopropanol solution of 1H,1H,2H,2H-perfluorodecyltriethyloxysilane (C16H19F17O3Si, FAS) with the solution volume of 50 mL and a concentration of 0.01 mol/L to prepare a hydrophobic FAS-ZrO2-Al2O3composite membrane. By increasing the number of grafting modification times (3 days per time), the hydrophobic properties of the membrane tubes were investigated by using the pure water penetrating pressure of the hydrophobic tubular ceramic membrane as the index. And the modified membrane tubes were used to carry out AGMD experiments. The analysis of SEM, IR and water contact angle test were used to observe and determine the hydrophobic nature of the inner surface of the tubular membranes. The results showed that under the conditions of NaCl thermal solution temperature of 75 ℃, bulk flow rate of 25 L/h, concentration of 2%, cooling water temperature of 15 ℃ and flow rate of 50 L/h, the best comprehensive performance was 3#membrane tube whose membrane distillation flux reached 4.29 kg/(m2·h) and rejection rate was 99.83%. The two hydrophobic groups CxF2x+1and Si-CH2CH2·CxF2x+1in the fluorosilane were successfully grafted on the inner surface of the tubular ZrO2-Al2O3ceramic membrane by using 1H,1H,2H,2H-perfluorodecyltriethoxysilane to modify the ceramic membrane tube. The inner surface of the tube formed a very good hydrophobic layer and the water contact angle was of 142°. Therefore, the FAS-ZrO2-Al2O3composite membrane met the requirements for membrane distillation, which makes it possible to desalinate high-salt brackish water by AGMD in order to obtain high-purity water.
