(42ad) Gas-Diffusion Cathode Expedites Ammonia Removal from Aqueous Phase

Ammonia is a pollutant in many types of aqueous streams such as industrial wastewater from coking, chemical fertilizer, coal gasification, petroleum refining, pharmaceutical and catalyst factory. It is also a byproduct in processes such as microbial cultivation, fish aquaculture, and livestock/poultry feed digestion. Ammonia is toxic to fishes and microorganisms in bioreactors, and has a tendency of causing eutrophication and depletion of dissolved oxygen due to nitrification. It is critical to protect water-borne species and aquatic environments by removing ammonia from entering aquatic environments. The  reactors designed and manufactured in this study aimed to expedite the efficiency of ammonia removal from aqueous phase by utilizing the principle of membrane contactor and modifying the cathode microcosm environment via electrochemical or microbial electrochemical reduction of oxygen. Our experiment evaluated cathode materials and structures that favored the catalytic reduction of oxygen as well as the improved ammonia volatilization via hydrophobic gas-diffusion layers. The volatized ammonia gas was eventually collected and concentrated in sulfuric acid solution.

Several designs of cathode materials and structures were tested. The cathode reaction, oxygen reduction reaction (ORR), was tested under three types of catalysts, i.e., Pt/carbon black, activated carbon, and biofilm. The gas diffusion layers were prepared in laboratory from different materials, i.e., poly (tetrafluoroethylene) (PTFE), poly (dimethylsiloxane)(PDMS), poly (vinylidene fluoride)(PVDF) by natural drying or phase inversion. The structures of cathode were optimized by studying the impact on ammonia removal of omitting the macro-porous substratum, the micro-porous layers, the position of chemical catalyst layer, and the application of conductive polymer layer (polyaniline). Different cathode materials and structures were evaluated by linear scanning voltammetry (LSV) for the catalytic activity of ORR, under both dissolved oxygen saturated and depleted conditions. Ammonia removal was further studied in the cathodes that showed high ORR catalytic effects, at initial ammonia concentrations of from 200 to 1000 mg/L with an increment of 200 mg/L, pH of 6, 6.5, 7, 7.5 and 8, and under different cathode potentials of -200 mV, -100 mV, 0 mV, +100 mV, and +200 mV vs. Ag/AgCl (3 M NaCl). Further experiments were run by sustainably providing energy that pushed ORR through the operation of microbial fuel cell mode and cultivation of biofilm cathode (biocathode). This design effectively removed ammonium/ammonia nitrogen from contaminated aqueous phase with a benefit of pure ammonium enrichment, and will have potential applications in real wastewater operated in an cost-effective and energy-sustainable way.