Electrodialysis and Nitrate Reduction to Synthesize and Recover Ammonia from Wastewaters
Authors: Liu Matthew Junjie , Guo Jinyu , Blair Sarah J. , Nielander Adam C. , Jaramillo Thomas F , Tarpeh William Abraham 
Published: December 2020

Humans have drastically altered the global nitrogen cycle, asymmetrically doubling its throughput due to industrial Haber-Bosch fertilizer production.1,2 Fertilizer runoff and inadequate wastewater removal cause waterborne emissions of reactive nitrogen (e.g., NH4
+, NO3
-) that induce harmful algal blooms.3,4 Meanwhile, both water purification and ammonia synthesis are conducted separately in centralized facilities with severe limitations: only 20% of wastewater is collected at treatment plants,5 and ammonia synthesis requires high temperature (400 °C), pressure (200 atm), energy input (1.2% of global energy), and greenhouse gas emissions (5% of industrial CO2).6,7 To couple water purification and ammonia production more efficiently, the following two reactions can be leveraged to synthesize and recover ammonia from nitrogen-polluted wastewater.

Ammonia synthesis via electrochemical nitrate reduction: NO3
- + 9H++ 8e- → NH3 + 3H2O

Ammonia recovery via electrodialysis and electrochemical pH control: NH4
+ + OH- ↔ NH3 + H2O

In our previous work, we have successfully demonstrated electrochemical stripping (ECS), a novel process combining electrodialysis and membrane stripping that selectively extracts (NH4)2SO4 from wastewater with > 93% recovery efficiency.8 We have also constructed a nitrogen mass transfer model for this process to inform rational decision of operating parameters.9 Building on these results, we reimagine nitrogen pollutants as products and propose electrodialysis and nitrate Reduction (EDNR). This novel unit process consists of three stages: (1) electrodialysis to separate influent NO3
- and NH4
+; (2) electrocatalytic reduction of NO3
- to NH3 (NO3RR), along with migration of undesired anions; and (3) product purification via migration of undesired cations (see Figure 1). We have built a proof-of-concept EDNR reactor using Ti/IrO2-Ta2O5 mesh (left and middle chambers) and stainless steel electrodes (right chamber). With recirculating batches of synthetic ion exchange brine (NaNO3/NH4Cl solution) and LiCl supporting electrolyte, we demonstrated that (1) over 99% of influent NH4
+ is recovered to the right chamber; (2) 12% of initial NO3
- reacts, reaching 32% peak Faradaic efficiency towards NH3; (3) Cl- migrates out of the left chamber during NO3RR as desired and Cl2 gas evolution is minimal; and (4) ideal pH values are sustained in all three chambers to maximize NH3 synthesis and recovery.

Aiming at selective NO3RR to NH3, we have also explored the effect of varying electrochemical conditions (e.g., pH, NO3

-
concentration, applied potential) at a titanium foil cathode, which has shown high NH3 selectivity and corrosion resistance. Moving forward, we plan to use Ti foil electrode in the left chamber of EDNR, in which it will perform the oxygen evolution reaction (OER) in stage 1 and NO3RR in stage 2. To better understand the evolution of the catalyst during reactions and rationally identify EDNR operating potentials and cycle durations that maximize active catalyst stability, we perform characterizations of the Ti surface.

By enabling automated, distributed ammonia manufacturing with minimal environmental impacts from prevalent wastewater contaminants, EDNR furthers our endeavor to re-engineer the nitrogen cycle.

References

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Figure 1