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.