Redox flow desalination for tetramethylammonium hydroxide removal and recovery from semiconductor wastewater

As part of humankind’s path towards more sustainable water technologies, redox flow desalination (RFD) has emerged as a promising technology due to its high energy efficiency and easy operation. So far, RFD research has focused on removing and recovering inorganic salts such as lithium-ions, heavy metal ions, or phosphate and nitrate ions. Thus, the potential of RFD in water desalination and resource recovery processes has not been fully demonstrated. Therefore, this study aimed to assess RFD for the valorization of tetramethylammonium hydroxide (TMAH) as value-added organic compounds from wastewater beyond inorganic elements, which is widely being used as an etching solvent, photoresist developer, and surfactant in semiconductor and display industries. By applying a low cell voltage (<1.2 V), a reversible redox reaction allowed a continuous removal of TMAH from the wastewater stream and a simultaneous recovery for reuse as a form of tetramethylammonium cation. The TMAH removal rate was approximately 4.3 mM/g/h with a 40% recovery ratio. With various operational conditions (i.e., TMAH concentration, cell voltage, and flow rate), our system exhibited a high potential for the valorization of TMAH with 60% reduction in capital cost compared to conventional desalination processes.