Combination of Pd-Cu Catalysis and Electrolytic H2 Evolution for Selective Nitrate Reduction Using Protonated Polypyrrole as a Cathode.

Pd-Cu catalysis is combined with in situ electrolytic H2 evolution for NO3- reduction with protonated polypyrrole (PPy) as a cathode. The surface of PPy is not only beneficial for H2 evolution, but exclusive for NO3- adsorption, and thus inhibits NO3- reduction. Meanwhile, the in situ H2 generation exhibits a much higher utilization efficiency because of the smaller bubble size and higher dispersion. The Pd-Cu catalysts with the ratios of 6:1 and 4:1 exhibit the highest NO3--N removal (100%) and N2 selectivity (93-95%) after 90 min. In comparison with the results obtained with other cathode materials (Ti, Cu, Co3O4, and Fe2O3) and obtained by other researchers, the new process shows a faster NO3--N reduction rate and much higher N2 selectivity. However, the O2 generated on the anode can oxidize Cu to Cu2O that may work as the catalyst for NO3--N reduction to NH4+-N by H2, resulting in more than 60% NH4+-N generated without a proton exchange membrane. Both the PPy film and Pd-Cu catalyst exhibit good stability and there is no Cu2+ or Pd2+ in solution after reaction. Real industrial wastewater is further treated in this system, the NO3--N is reduced from 670 mg L-1 to less than 100 mg L-1 in 90 min, and only little amount of NH4+-N is generated.