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 (& BULL;OH) production has been recently documented, but the oxidizing efficiency was limited. Here, we revealed that low-molecularweight organic acids (LMWOAs) can immensely enhance phenol degradation during FeS oxygenation due to increased utilization efficiency of FeS electron for & BULL;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 & BULL;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 & BULL;OH formation and phenol degradation. Coexisting oxalate led to an additional & BULL;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 & BULL;OH production. Moreover, the presence of oxalate decreased the crystallinity and particles size of lepidocrocite generated from FeS oxidation.