The invention discloses a method for preparing positively charged
nanofiltration membranes. The positively charged
nanofiltration membranes are formed by porous support
layers and functional
layers which are formed by copolymers containing cations and hydroxy. The preparation process is as follows: firstly, obtaining functional copolymers through free radical copolymerization, preparing the copolymers into water solution with certain concentration,
coating the prepared water solution on the support
layers and
drying the support layers; and secondly, immerging the support layers into solution containing cross-linking agents, and finally carrying out heating and curing to obtain the positively charged
nanofiltration membranes. Under the operation pressure of 0.6MPa, the positively charged nanofiltration membranes have water flux of 12-18L / m<2>.h, show very high
retention ratio which is generally 75-95% to bivalent cations and show
retention ratio which is generally lower than 65% to
monovalent cations. The prepared positively charged nanofiltration membranes have excellent separation property, and the method is simple and feasible, low in cost and easy for industrial production.