Ammonia-rich solution production from coal gasification gray water using Chemical-free Flow-Electrode capacitive deionization coupled with a monovalent cation exchange membrane

The conventional technologies for ammonia recovery from wastewaters usually requires high chemicals and energy input. And if the wastewater contains hardness, there is a risk of scaling with increasing in cost. Flow electrode capacitive deionization (FCDI) coupled with monovalent cation exchange membrane (MCEM) was employed to solve abovementioned problem. Our method was an effective strategy for the selective production of an ammonia-rich solution from coal gasification gray water by simply adjusting the charging and discharging processes. During the charging process, both NH4+ and Na+ were transported to the cathode due to electrosorption of the electrical double layers (EDLs) on charged carbon particles and electrodialysis of potential gradient, while Ca2+ was retained by the MCEM. The Faradaic reactions induced the generation of OH- and led to the conversion of charged NH4+ into uncharged NH3. When the electrode polarity was reversed, charged species such as Na+ would be mostly pushed back into the spacer chamber, yet NH3 was more likely to be trapped in the cathode due to sluggish pH variations, thereby producing an ammonia-rich solution. Under optimal operating conditions (i.e., a carbon content of 5 wt%, charge and discharge voltages of 4.5 and 0.2 V, respectively, charge 80 min and discharging 15 min, and six continuous cycles), a stable recovery efficiency (∼205 mg/L per cycle) and product purity of 70.4% were attained at a relatively low electrical energy consumption (11.1 kWh/kg N). Additionally, Ca scale was effectively avoided by MCEM. These results demonstrated that FCDI coupled with MCEM could be a promising technology for efficient NH4+ removal, recovery as an ammonia-rich solution and TOC removal from industry wastewaters with alleviation of scaling.