Degradation of Nitrobenzene by Mackinawite through a Sequential Two-Step Reduction and Oxidation Process.

Mackinawite (FeS) has gained increasing interest due to its potential application in contaminant removal by either reduction or oxidation processes. This study further demonstrated the efficiency of FeS in degrading nitrobenzene (ArNO2) via a sequential two-step reduction and oxidation process under neutral conditions. In the reduction stage, FeS rapidly reduced ArNO2 to aniline (ArNH2), with nitrosobenzene (ArNO) and phenylhydroxylamine (ArNHOH) serving as the intermediates. X-ray photoelectron spectroscopy (XPS) analysis indicated that both Fe(II) and S(II) in FeS contributed electrons to the reduction of ArNO2. In the subsequent oxidation stage with oxygen, by addition of 0.5 mM tripolyphosphate (TPP), ArNH2 generated in the reduction process could be effectively oxidized to aminophenols by hydroxyl radicals (•OH), which would undergo eventual mineralization via ring-cleavage reactions. TPP exerted a favorable role in enhancing •OH production for ArNH2 degradation by promoting the formation of the dissolved Fe(II)-TPP complex, thus enhancing the homogeneous Fenton reaction. Additionally, TPP adsorption inhibited the surface oxidation reactivity of FeS due to the change of Fe(II) coordination. Finally, the effective degradation of ArNO2 by FeS in actual groundwater was demonstrated by using this sequential reduction and oxidation approach. These research findings provide a theoretical basis for a new FeS-based remediation approach, offering an alternative way for comprehensive removal of ArNO2.

Dramatically enhanced phenol degradation upon FeS oxygenation by low-molecular-weight organic acids.

Oxidizing potential of FeS for organic contaminants degradation due to hydroxyl radicals (•OH) production has been recently documented, but the oxidizing efficiency was limited. Here, we revealed that low-molecular-weight organic acids (LMWOAs) can immensely enhance phenol degradation during FeS oxygenation due to increased utilization efficiency of FeS electron for •OH production. Upon oxygenation of 0.5 g/L FeS, phenol degradation boosted from 7.1% without LMWOAs to 91.5%, 84.6% and 95.0% with the addition of 1 mM oxalate, citrate and EDTA, respectively. Electron utilization efficiency of Fe(II) for •OH production dramatically rose from 0.3% with FeS alone to respective 2.0%, 2.5% and 2.7% in the LMWOAs systems. An increase in oxalate concentrations benefited •OH formation and phenol degradation. Coexisting oxalate led to an additional •OH production pathway from Fe(II)-oxalate oxidation, which expanded the O2 reduction to H2O2 from a two- to one-electron transfer process. Meanwhile, electron transfer from FeS to dissolved Fe(III)-oxalate promoted the redox cycling of Fe(III)/Fe(II), thus supplying the Fe(II) oxidation for •OH production. Moreover, the presence of oxalate decreased the crystallinity and particles size of lepidocrocite generated from FeS oxidation. Consequently, this study shed lights on the LMWOAs-enhanced contaminant degradation in either natural or engineered FeS oxidation systems.