Geothermal Water Desalination in Greece’s Islands, Coupled with Extracting Precious Metal Salts from the RO Retentate

Many Greek islands host geothermal springs whose waters can be desalinated to produce drinking water. Some of these waters contain meaningful concentrations of the valuable Rb+ and Cs+ ions, which, when extracted from the desalination brine as RbCl and CsCl salts, can yield revenues exceeding the freshwater production costs. We demonstrate the use of reverse osmosis (RO) to produce freshwater and apply theoretical simulations to assess a proven extraction method applied to the RO retentate of geothermal water from Samothrace, characterized by [Rb+] = 2.72, [Cs+] = 0.55, [K+] = 514, [Na+] = 3759 (all in mg/L) and pH 6. The extraction method, developed by the authors, relies on ion exchange using a PES-coated Zn-hexacyanoferrate sorbent with high affinity for monovalent cations (no affinity for multi-valent cations), followed by a unique ion-chromatography separation. We show that the production cost remains