| Hydroxyl radical (·OH) is a highly reactive oxidant, which can indiscriminately oxidize most organic compounds and redox-sensitive elements, and has significant influence on cycle of elements in the environment as well as transport and transformation of pollutants. Mackinawite(FeS) commonly occurs inreducing underground sediments at low temperature where sulfate-reducing bacteria and Fe are present. Under reducing conditio.ns, FeS can immobilize many heavy metals and radionuclides by surface complexation or isomorphous substitution, as a strong reducing substance, it can degrade efficiently organic pollutants via reductive transformation. However, due to the impact of natural factors and human activities, FeS wolud be oxidized as soon as reducing underground environments are disturbed by oxygen. As reported in recent years, oxygenation of FeS could simultaneously lead to the transformation of As(III) to As(V), but the oxidation mechanisms are not clear. To this end, the purpose of this study is to explore the reactive oxygen species produed from FeS oxidation by O2, and reveal the oxidation mechanisms of pollutants caused by this process.In this study, the quantitative formation of hydroxybenzoic acid resulted from the oxidation of sodium benzoate by OH was used as a probe reaction, and ·OH concentrations produced from FeS oxygenation under different conditions were quantitatively detected. Experimental results show that the cumulative concentrations of ·OH produced increase with oxidation time prolonging, the cumulative concentrations of ·OH produced increased dramatically up to 117.2 μM within 4 h at 1 g/L FeS. The characteristic 1:2:2:14-peak in electron spin resonance spectrum further supported the production of ·OH. Moreover, with an increase from 0.1 to 3 g/L in FeS dosages, the cumulative concentrations of ·OH elevated from 13.7 to 266.0 μM, showing a good linear relationship between them(R2=0.98). The slope is 84.7, indicating the production of 84.7 umol ·OH from the complete oxidation of 1 g FeS.In order to ascertain the relative contribution of oxidation of different forms of Fe(II) to production of ·OH, BPY, which can effectively chelating dissolved Fe2+ so that it cann’t be oxidized, was added to mask the Fenton reaction process. The addition of BPY had little influence on ·OH yield, indicating oxidation of structural Fe(II) mainly contributed to the formation of ·OH. To explore the number of electrons transferred from FeS surface to O2 that was reduced to produce H2O2, NBT and SOD was added to quench the generation of O2·- generated via a one-electron transfer process, respectively. The addition of NBT and SOD had little impact on ·OH yield, illustrating it was a two-electron transfer process when O2 was reduced to produce H2O2 on FeS surface. The X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) patterns of FeS oxidation products supported structural Fe(II) oxidation predominantly contribute to Fe(II) oxidation.Experimental results indicate that FeS oxygenation in 1 g/L can oxidize As(III) to As(V) with an amount of 95.9%within 4 h. In order to ascertain the contribution of ·OH to As(Ⅲ) oxidation, BA, a specific quencherof ·OH, is added, leading to a significant reduction of As(III) oxidation, which indicates ·OH dominantly contributed to As(Ⅲ) oxidation. However, to eliminate the impact of Fe(Ⅳ), methanol,which can simultaneously scavenge ·OH and Fe(Ⅳ), was added, giving rise to a similar reduction of As(III) oxidation with that of BA, which further supports ·OH produced from FeS oxygenation predominantly contributed to As(Ⅲ) oxidation.This study demonstrated FeS oxidation under neutral pH conditions can efficiently produce ·OH, and prove that structural Fe(II) oxidation mainly contributed to the formation of ·OH, and H2O2 formed via a two-electron transfer process. Finally, this study demonstrated directly ·OH produced from oxidation of FeS can quickly and effectively oxidize adsorbed As(III). Based on results of this study, we believe that in oxic-anoxic boundary of soil, ·OH generated from oxidation of FeS may have great impact on migration, transformation of elements and pollutants. |
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