Carbon Dioxide-Mediated Desalination

Water is arguably the most important molecule for mankind. It is becoming more critical to secure water supply, which is increasingly difficult. Desalination is an ideal solution – taking abundant seawater and turning it into potable water – however requires energy for distillation and/or membrane-based reverse osmosis (RO) processes even though the technologies were highly optimized. Modern desalination technologies, namely RO membranes, is based on the rejection of ions (mostly sodium and chloride) on the phase boundary is the main mechanism. Meanwhile, the development of supramolecular chemistry showed that selective recognition and sequestration of ions can reduce energy input by taking advantage of pre-organized “host” environments. However, this approach renders difficulties in aqueous media due to high hydration energy of ions. Moreover, a regeneration of the host can be cumbersome due to the high affinity between the host and guest ions, which is prerequisite for the recognition. We found a completely different approach: stimuli mediated ion-exchange for removing chloride ions. This is based on well-known Solvay process where baking soda is produced from brine (NaCl + H2O), CO2 and ammonia. After ion exchange between ammonium bicarbonate and salt, ammonium chloride is produced, which is water soluble. By employing hydrophobic amines, now we can mediate Solvay-like process while removing in situ generated ammonium chloride from brine solutions and real seawater samples up to 1 L scale. This method can be applied in a continuous flow system with amine-decorated resin, producing desalinated water by using only CO2 and acid treatment to decompose produced NaHCO3.