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 acidic (pH 2-4 ). A study was done to investigate the performance of commercially 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/m2.h.bar. At lower pH values, the performance decreases due to the presence of a higher fiaction of monovalent HSCY4ions and as well as a possible change in the membrane charge from negative to positive. However, there are still some membranes (from Filmtec) which are suitable for mine water with a pH of 4 and a salt loading of 2500 ppm. The water flux of these membranes is, however, below 4 l/m2.h.bar. Surprisingly multi-charged ions like Ca2+and S~4- as well as single-charged ions like Na+ and CI- are rejected with values larger than 90%.

Commercially available NF membranes were tested on their performance for the removal of sulphate ions from acid mine water. The water flux of these membranes is between 2 and 101/m2.h.bar. The membranes with a flux of 61/m2.h.bar and lower performed excellently (95-99% rejection) for the removal of sulphate ions at neutral pH. However, if the pH is decreased using sulphuric acid and sulphuric acid/sodium sulphate mixtures, the performance decreases drastically, especially for the membrane with the highest flux. Two causes could be recognized for this decrease: (1) an increase of single-charged HSO] ion fraction with decreasing pH; and (2) a change in membrane charge from negative to positive with decreasing pH. Only the membrane with the lowest flux (NF90, 2.51/m2.h.bar) is hardly influenced by acid concentration and pH and is classified as a RO type of membrane with relatively high flux. The low-flux NF membranes performed excellently (>90% rejection) for an actual mine water sample with pH of 4.1 and a salt loading of 2500 ppm. Surprisingly, NaCI as a single-charged salt was also rejected by these low-flux NF membranes.