Simultaneous water softening and selective ammonia nitrogen recovery from coal gasification gray water using an innovative four-chamber flow-electrode electrochemistry system

Capacitive deionization-based technologies such as flow electrode capacitive deionization (FCDI) have shown great promise for desalination and resource recovery, but simultaneous desalination and resource recovery using one FCDI device has thus far received minimal attention. In this study, a novel four-chamber FCDI coupled with a monovalent cation exchange membrane (MCEM) was proposed to realize simultaneous desalination (including water softening) and ammonia-rich solution production from coal gasification gray water (CGGW). The results showed that under optimal conditions, up to 91.2 ± 0.1 % ammonia nitrogen, 88.9 ± 1.0 % Ca2+, and 92.6 ± 5.5 % total organic carbon of real CGGW were removed by the desalination chamber. Furthermore, NH4+ passed through the MCEM and entered the recovery chamber, while Ca2+ was effectively blocked from entering the recovery chamber by MCEM. Thus, a multipurpose ammonia-rich solution with purity of 98.0 % was obtained. The process was superior since two typical FCDI cells are needed to realize water softening and recover ammonia-rich solutions. The mechanistic insights into the proposed four-chamber FCDI coupled with MCEM can be expanded to other simultaneous desalination, ion separation and recovery processes. Thus, the four-chamber FCDI is a promising technology for simultaneous desalination and resource recovery with the advantages of being chemical-free, capital efficient and operable.