The role of nitrate in simultaneous removal of nitrate and trichloroethylene by sulfidated zero-valent Iron

Sulfidated zero-valent iron (S-ZVI) is commonly used to degrade trichloroethylene (TCE). The reactivity of S-ZVI is related to not only the properties of S-ZVI but also the geochemical conditions in groundwater, such as coexisted NO3-. Therefore, the effect of NO3- on TCE degradation by S-ZVI and its mechanism were systematically studied. 95.17% of TCE was degraded to acetylene, dichloroethene, ethene, ethane and multi‑carbon products via β-elimination by fresh S-ZVI that contained 85.31% Fe0 and 14.69% FeS in the presence of NO3-, demonstrating that NO3- did not affect the degradation pathway of TCE. While high concentration of NO3- (> 10 mg/L) competed for electrons at the Fe/FeOx interface with degradation products, leading to a continuous rising of acetylene. Moreover, the rapid reduction of NO3- to NH4+ (89.79%) at the Fe0 interface contributed to the release of 5.08 mM Fe2+ from S-ZVI, which promoted the formation of Fe3O4 with excellent electron conduction properties on the surface of S-ZVI. Accordingly, NO3- improved the degradation and electron selectivity of TCE by 51.07% and 2.79 fold, respectively. This study demonstrated that S-ZVI could remediate the contamination of NO3- and TCE simultaneously and the presence of NO3- could effectively enhance the degradation of TCE in groundwater.