Plasma-Enzyme Catalysis and Processing for Nitrogen Fixation from Artificial Urine in ‘Plasma Bubbles’

Artificial urine decomposition was studied in a gas–liquid microplasma reactor (‘plasma bubble reactor’) for nitrogen (N)-fixation, synthesising nitrate, nitrite, and ammonium in aqueous solution. The testing includes varying experimental parameters of non-catalytic and catalytic plasma operation. Longer reaction time and higher voltage, as expected, increased the N-fixation yield. At the highest input voltage (240 V) investigated, the N-fixation selectivity switches from nitrate to ammonium, which is the homologous N-product of urea. The nitrate yield almost doubles by isothermal operation at room temperature. Use of nitrogen plasma gives lower yield than for air plasma. Micro- and nano-scale refractory oxide catalysts were able to change the selectivity from nitrate towards ammonium, while maintaining the N-fixation yield. Enzyme catalysis by urease provides by far the highest ammonium yields (without nitrate or nitrite). Urease processing is most effective at 60 °C but has lower performance at 25 °C. In combination with plasma, the urease catalysis provides a mixture of ammonium, nitrate or nitrite, which demonstrates additive performance of both processes. Plasma processing of urea in the presence of organic N-compounds, typical for artificial urine, achieves much higher (up to a factor of four) nitrate and ammonium yields, seemingly boosting the urea conversion. The environmental impacts of the three process variants (enzyme, plasma-enzyme, plasma) are scoped, using circularity metrics, green chemistry metrics, and life cycle assessment, providing a holistic sustainability view. Main result is that combine plasma-enzyme operation is predicted to lower the environmental impact as compared to plasma-only operation via the practice in the AU system.