CO2-Responsive Agents for Water Filtration

Abstract

Water purification is essential for modern human life. Without it we could not produce clean water, process wastewater or concentrate desired products. Unfortunately, standard water purification techniques are energy intensive, especially for concentrated feeds. Forward osmosis (FO) is a promising filtration method, with the ability to process concentrated feeds at lower energy costs than conventional methods. FO operates by placing a concentrated “draw solution” opposite the water to be filtered (the feed), separated by a semipermeable membrane. The osmotic pressure (π) difference across the membrane causes water to flow from the feed to the draw solution. After filtration is complete the draw solute is removed, leaving behind clean water. While the initial filtration requires minimal energy, removing the draw solute from the clean water after filtration can be energy intensive due to the draw solutes’ high π. The efficiency of FO hinges on the nature of the draw solute. An ideal draw solute must possess two seemingly contradictory properties: a high π state for efficient filtration and a low π state for easy draw solute removal post-filtration. CO2-responsive materials are attractive draw solutes as they possess a high π under CO2 (πCO2) and a low π under air (πair). The most successful draw solute reported to date, trimethylamine (TMA), exerts a very high πCO2 but is also flammable, malodorous and can cross the membrane. This work investigated new CO2-responsive materials which could be safe alternatives to TMA. Diverse CO2-responsive materials were considered, including polymers, hydrogels and magnetic particles. In each case, materials were designed and synthesized, then their performance as a draw solute was assessed by measuring their πCO2 and πair or their flux and recovery. Fluid draw agents outperformed solid draw agents due to improved mass transfer, and are easier to implement industrially. Among all the draw solute studied, soluble poly(N,N-dimethylallylamine) (PDMAAm) showed the most promise due to its high πCO2 and low πair. The use of PDMAAm’s dual CO2- and thermo-responsive character was found to facilitate its separation from water after filtration. Future work will study the performance of PDMAAm on larger scales for industrial use.