Canada Research Chair in Bioresources Engineering, Ph.D., P.Eng., D.WRE, F.ASCE, F.EWRI
Director of Beaty Water Research Center
Cross-Appointed to Chemical Engineering
Kingston, ON K7L 3N6
Keynote: "Carbon dioxide switchable draw agents for forward osmosis"
Sarah Ellis, Anna Riabsteva, Alex Cormier, Philip G. Jessop, Michael F. Cunningham and Pascale Champagne*
Current methods for obtaining potable water from wastewater or seawater include distillation, pervaporation, reverse osmosis (RO) and forward osmosis (FO). Forward osmosis utilizes a draw solution containing a draw agent, which is usually a dissolved salt that can be easily removed. The water to be treated and the draw solution are placed on opposite sides of a membrane. The draw solution has a higher osmotic pressure than the feed water so that water will diffuse, without added pressure, from the feed solution to the draw solution, thereby diluting the draw solution and concentrating the feed solution. The draw agent is then removed from the draw solution to give fresh water. In existing FO systems, the major energy costs are associated with the separation of the draw agent from the diluted draw solution. An ideal draw agent would require little energy to be removed or the energy required could be supplied largely by low-grade waste heat or even solar heat. To meet these demands we are developing novel draw agents based on CO2 switchable polymers, whose properties can be changed simply with the addition or removal of CO2 gas at atmospheric pressure. Under air or inert atmosphere, the polymers are neutral and hydrophobic but under a CO2 atmosphere the polymers become positively charged and dissolve in water with bicarbonate counteranions. Osmotic pressure is generated by the bicarbonate ions in solution plus the swelling pressure of the polymers, allowing the FO process to occur. CO2-responsive tertiary amine-containing polymer, namely poly(N,N-dimethylallylamine), with targeted molecular weight were synthesized. Laboratory-scale FO tests were performed to confirm the efficiency of a synthesized poly(N,N-dimethylallylamine) as a draw agent for wastewater purification. Control of the polymer molecular weight allows for lower viscosity draw solutions with high concentration (up to 35 wt. %) of the polymer and prevents diffusion across the membrane. It was shown that the osmotic pressure of a carbonated solution of poly(N,N-dimethylallylamine) significantly increases with an increase in polymer concentration, while the osmotic pressure of the non-carbonated polymer solution remains low even at high concentration of the polymer. After the FO step has been completed, the CO2 is removed and the polymers, now neutral and insoluble in the water, can be readily separated to yield purified water.