Selective oxidation of ammonia nitrogen to nitrogen gas by S-doped carbon nanotubes activated persulfates

The selective oxidation of ammonia nitrogen (NH4+-N) to dinitrogen (N2) by persulfates (PS)-based advanced oxidation processes (SR-AOPs) without the input of light or electrons is desired, but presents enormous challenges. In this study, S-doped carbon nanotubes (SCNTs) were prepared via a high-temperature sintering process and used to activate PS for NH4+-N oxidation in the presence of magnesium oxide (MgO). Compared with pristine carbon nanotubes (CNTs), SCNTs improved the N2 selectivity of NH4+-N oxidation by SR-AOPs, which was attributed to the lower impedance of SCNTs than that of CNTs, enhancing the direct electron transfer pathway of NH4+-N oxidation. In the SCNTs/PS/MgO process, the non-radical pathway dominated the oxidation of NH4+-N. When SCNTs/PS/MgO process treated polluted lake water, 84.0% of NH4-N removal and 85.8 % selectivity towards N2 were achieved at initial pH of 6, initial NH4+-N concentration of 30 mg N/L, SCNTs dosage of 8 g/L, the mass ratios of SCNTs to PS of 1: 1, and reaction time of 60 min, highlighting its potential for practical applications.