(Invited) Electrochemical Stripping for Ammonia Recovery: Mechanisms and Impacts of Chlorine Evolution

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Electrochemical stripping is an attractive alternative to converting NH4 + to NH3(g) via the OH− produced by the hydrogen evolution reaction at the cathode. However, conventional electrochemical systems suffer from relatively low ammonia recovery (<80%) and need of external acid solution to absorb ammonia. To address the limitations, this work has developed a novel stacked system that uses anodic oxygen evolution reaction (OER) to in−situ produce acid and achieve chemical free ammonia recovery. To solve the issue of ammonia escape caused by constant current charging in conventional electrochemical systems, pulsed electric field (PEF) has been applied to regulate side reactions and interfacial mass transfer, weakening the sweeping effect of hydrogen gas produced by the cathodic hydrogen evolution reaction on ammonia gas. With an ammonia removal rate of 35.1 g−N m−2 h−1 and an electrical energy consumption of 28.9 kWh kg−N−1, our chemical−free system in PEF mode has achieved significantly higher ammonia recovery (>90%) from high-strength streams.With regard to a high concentration of chloride, high potentials during electrolysis may lead to the generation of strong active chlorine. The non−specific oxidation of ammonia by active chlorine results in lower ammonia recovery. This study therefore combined in−situ Raman spectroscopy to explore the effects of chloride ion concentration and pH on ammonia speciation and recovery. Under strong acidic conditions, active chlorine mainly escaped in the form of Cl2(g), thus having a negligible effect on ammonia recovery. The proposed system maintained good stability after 390 h continuous operation. While the effect of active chlorine on ammonia oxidation was negligible, the reaction of chlorine with dissolved organic matter (DOM) can form more toxic by-products. This work further realized the effective control of by-products through the integration with activated carbon adsorption process.Overall, the total cost to recover 1 kg of NH3–N using the electrochemical system from real waste stream is $15.9, which includes $11.1 for capital expenses (CapEx) and $4.8 for operating expenses (OpEx). This highlights the great potential for realizing high ammonia recovery in practical applications, and provides insights to innovations in treatment and resource recovery.