Ammonia
in wastewater can be a valuable resource if recovered sustainably
and with sufficient purity to compete with commercial ammonia products.
A new electrochemical method employing a proton-mediated redox couple
was developed to selectively recover ammonia from domestic wastewater
by simultaneous separation of NH4
+ and conversion
into NH3 at an elevated pH. Using hydroquinone (HQ) and
benzoquinone (BQ), solution pH increased by H+ uptake (BQ
+ 2H+ + 2e– → HQ) or decreased
by H+ release (HQ → BQ + 2H+ + 2e–). This redox reaction facilitated the separation of
ammonium from synthetic wastewater (5 mM NH4
+ + 20 mM Na+) at a nitrogen flux of 33.8 g N m–2 day–1 while increasing the solution pH to 9.6.
A low voltage of 0.47 V at 9.6 A m–2 resulted in
a 3.2 kWh kg–1 N energy consumption. The separated
ammonia was recovered in a membrane contactor at a nitrogen flux of
12.0 g N m–2 day–1 (9.1 kWh kg–1 N). This proof-of-concept demonstrated the use of
a proton-mediated redox couple as a new platform to enable ammonia
recovery from wastewater.
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