A tartrate-EDTA-Fe complex mediates electron transfer and enhances ammonia recovery in a bioelectrochemical-stripping...

Traditional bioelectrochemical systems (BESs) coupled with stripping units for ammonia recovery suffer from an insufficient supply of electron acceptors due to the low solubility of oxygen. In this study, we proposed a novel strategy to efficiently transport the oxidizing equivalent provided at the stripping unit to the cathode by introducing a highly soluble electron mediator (EM) into the catholyte. To validate this strategy, we developed a new kind of iron complex system (tartrate-EDTA-Fe) as the EM. EDTA-Fe contributed to the redox property with a midpoint potential of -0.075 V (vs. standard hydrogen electrode, SHE) at pH 10, whereas tartrate acted as a stabilizer to avoid iron precipitation under alkaline conditions. At a ratio of the catholyte recirculation rate to the anolyte flow rate (R_C-A) of 12, the NH₄ ⁺-N recovery rate in the system with 50 mM tartrate-EDTA-Fe complex reached 6.9 ± 0.2 g N m^-2 d^-1, approximately 3.8 times higher than that in the non-EM control. With the help of the complex, our system showed an NH₄ ⁺-N recovery performance comparable to that previously reported but with an extremely low R_C-A (0.5 vs. 288). The strategy proposed here may guide the future of ammonia recovery BES scale-up because the introduction of an EM allows aeration to be performed only at the stripping unit instead of at every cathode, which is beneficial for the system design due to its simplicity and reliability.