Selective reduction of NO3--N from wastewater to N2 by Zn/Ag bimetallic particles combined with wet ammonia oxidation process

High N-2 selectivity was obtained by the employment of a novel two-step method to treat NO3--N containing wastewater. In this process, NO3--N was selectively reduced to NO2--N and NH4+-N (molar ratio of 1:1) by Zn/Ag bimetallic material, and the produced NO2--N and NH4+-N were further converted to N-2 gas at high temperature (210 degrees C) and high pressure (1.1 MPa). Batch experiments indicated that at Zn/Ag bimetallic reduction, the NO3--N reduction efficiency of 90.9% was obtained under the condition of temperature of 40 degrees C, Zn/Ag bimetal dosage of 60 g L-1, reaction time of 30 min and the initial pH of 3.0. The optimal products molar ratio of the first reduction step was obtained by adjusting the experimental parameters (e.g. solution pH and reaction time) to achieve a satisfactory N-2 selectivity during the wet ammonia oxidation process, in which the maximum N-2 selectivity of 98.0% was achieved with 99.2% of NO2--N and 98.5% of NH4+-N were removed, respectively. The two-step method was further applied in the NO3--N reduction from membrane filtration concentrate, and 84.9% of TN removal efficiency with the N-2 selectivity of 94.5% was obtained.