Treatment of acid mine drainage by forward osmosis: Heavy metal rejection and reverse flux of draw solution constituents

Acid mine drainage (AMD) is one of the most relevant potential environmental liabilities of mining activities. They are formed when the excavated ore is exposed to rain and other natural erosion factors. This research evaluated the application of forward osmosis (FO) for the treatment of AMD. Rejection of several metals was determined, using a thin-film composite FO membrane, and two types of draw solutions: NaCl and NH₄HCO₃. When using NaCl as draw solution, results showed high levels of rejection, exceeding 97%. Individual rejections were Mg: 98.9%, Al: 99.8%, Si: 97.1%, Ca: 99.3%, Mn: 99.6%, Fe: 100%, Co: 99.4%, Cu: 98.9%, Zn: 99.5%. When using NH₄HCO₃ as draw solute, precipitates were observed over membrane surface, preventing normal operation. Results show that transport of ions from draw solution (Ammonia and bicarbonate) is the cause of such phenomenon. FO has the potential to become an alternative for the treatment of AMD, based on the high levels of rejection for metals provided by FO membranes. However, attention must be paid to the reverse flux of ions from the draw solution to the feed, especially when they can react with feed solutes.