Ammonia Recovery from Wastewater as a Fuel: Effects of Supporting Electrolyte on Ammonium Permeation through a Cation-Exchange Membrane

Electrodeionization (EDI) is used to recover ammonia from wastewater as a fuel, but how its performance for ammonia recovery is affected by the supporting electrolyte is not very clear. This study involved experimental tests and theoretical calculations on NH3 recovery, NH4+ permeation, and NH4+ and Na+ interacting with the functional groups in a cation exchange membrane (CEM) using Na2SO4 as the supporting electrolyte. The results demonstrated that a low concentration (<= 0.250 mol L-1 of Na2SO4) was conducive to NH4+ permeation, while the a concentration (0.750 mol L-1 of Na2SO4) hindered NH4+ permeation. A maximum recovery efficiency of ammonia of 80.00%, a current efficiency of 70.10%, and an energy balance ratio of 0.66 were obtained at 0.250 mol L-1 of Na2SO4. Numerical results indicated that an increase in Na2SO4 concentration caused severe concentration polarization that resisted NH4+ migration in the CEM. The DFT results demonstrated that competitive adsorption of Na+ to the CEM hindered NH4+ migration. The weaker interacting force between NH4+ and the sulfonate functional group (-SOH3) in comparison to that between Na+ and -SOH3 might be related to the geometric and orientation effects, which generated an additional energy barrier for NH4+ transport. Therefore, this study suggests that the supporting electrolyte concentration should be matched with that of the desalted ions.