The removal of acid sulphate pollution by nanofiltration

Mine water, specifically from gold and coal mines in South Africa, contains large quantities (up to 3 g/l = 3000 ppm) of sulphate and can be very acid ic (pH 2-4). A study was done to investigate the performance of commerciall y available nanofiltration (NF) membranes for the removal of sulphates and other ions at acidic conditions. The NF measurements were carried out in a dead-end membrane set-up at pressures between 3 and 20 bars. At neutral pH, membranes were identified with a sulphate rejection of 95-99% and a water flux of 2-7 l/m(2).h.bar. At lower pH values, the performance decreases due to the presence of a higher fraction of monovalent HSO4- ions and as well as a possible change in the membrane charge from negative to positive. Howe ver, there are still some membranes (from Filmtec) which are suitable for m ine water with a pH of 4 and a salt loading of 2500 ppm. The water flux of these membranes is, however, below 4 l/m(2).h.bar. Surprisingly multi-charg ed ions like Ca2+ and SO42- as well as single-charged ions like Na+ and Cl- are rejected with values larger than 90%.