Stale urine catalysed resource recovery from source separated urine using magnesium air fuel cell: Insights into the mechanism, its implications and challenges

The current study presents a new concept of "stale urine catalysed resource recovery (SCRR)" in a magnesium air fuel cell (MAFC) to enhance the nutrients (phosphate and ammonia) and energy recovery from source - separated urine. The MAFC consists of a sacrificial Mg anode (AZ31) and a gas diffusion electrode (GDE) as a cathode. The wettability and surface morphology of GDE were examined by water contact angle and FESEM-EDAX analysis. The addition of stale urine to fresh urine catalyzed urea hydrolysis, as a result, struvite production in MAFC was accelerated due to alkaline pH conditions and high NH4+: PO43- ratio. The influential parameters for struvite precipitation were optimised as: (i) pH – 9; (ii) HRT – 24 h; (iii) NH4+: PO43- ratio – 90; (iii) Ca2+: Mg2+ ratio < 0.2. The X-ray diffraction pattern of the recovered precipitates revealed the formation of high-quality struvite. Besides, the FT-IR spectra displayed the presence of N–H and H-O–H bonds, and PO4 units. Moreover, the surface morphology depicted various morphologies (pyramid, wedge-shaped, rod-shaped and disc-shaped) of the crystal structure of struvite. The elemental composition revealed the presence of Mg, P, N, O, Ca, Na, and K. Furthermore, the addition of stale urine increased power density from 3.19 to 4.87 Wm−2 due to increased electrolyte conductivity, high NH4+:PO43- ratio, reduced internal resistance, and spontaneity of Mg anode dissolution. A current efficiency (CE) > 100 % indicated the passivation of anode, however, a stable CE was achieved with high NH4+: PO43- ratios, indicating the suitability of SCRR in the MAFC.