Production Of Hydroxyl Radicals From O₂/H₂O₂ Activation By Fe-bearing Clay Minerals For Degradation Of Organic Pollutants

Fe-bearing clay minerals are widely distributed in soil and sediment.In anoxic environment,structural Fe（III）in clay minerals can be reduced to Fe（II）abiotically by dissolved Fe（II）,sulfide,and reduced natural organic matter,and biotically by several types of naturally occurring metal-and sulfate-reducing microorganisms.Structural Fe（II）in clay minerals is reductive and can reduce a variety of contaminants.There were extensive investigations on element cycling,pollutants transformation by Fe（II）-bearing clay minerals under anoxic conditions.In redox-dynamic subsurface environment,the anoxic conditions are often disturbed by natural processe and human activities and suffer from O₂.So,the Fe（II）-bearing clay minerals can contact with O₂.According to Haber–Weiss theory,Fe（II）oxidized by O₂ can produce·OH.In a recent report,abundant·OH is produced when subsurface sediments are oxygenated,which is mainly attributed to Fe（II）-bearing clay minerals in sediment.Nevertheless,mechanisms of·OH produced from oxygenation of Fe（II）-bearing clay minerals is poorly understood.Therefore,this study aimed to reveal the mechanisms of electron transfer from structrual Fe（II）in reduced nontronite to O₂ for production of·OH,by using a series of modern instrument analytical technique such as FTIR,M?ssbauer spectra,XPS and UV.Meanwhile,oxidation of TCE by·OH produced from oxygenation of reduced nontronite was conducted.Then,on the basis of·OH produced from oxygenation of reduced nontronite,we further investigated Fe（II）-bearing clay minerals efficiently activates H2O2 at neutral pH for contaminant degradation.Finally,we used sediment containing Fe（II）-bearing clay minerals activate H2O2 to degrade phenol,to elaborate the mechanism of H₂O₂ decomposition by sediment containing Fe（II）-bearing clay minerals for phenol degradation.The detailed results were shown as the following.1.In order to identify the structural variation of Fe（II）entities during the oxidation of Fe（II）-bearing clay minerals by O₂,and to unravel the mechanisms of electron transfer within the mineral structure and from mineral to O₂ for·OH production,·OH production from oxygenation of Fe（II）-bearing clay minerals was studied.Nontronite（NAu-2,23%Fe）which was chemically reduced to 54.5%Fe（II）in total Fe was used as a model Fe（II）-bearing clay mineral.Production of·OH and oxidation of Fe（II）were measured during the oxidation of reduced NAu-2 by O₂.A wide spectrum of spectroscopic techniques,including FTIR,M?ssbauer spectra,and XPS,were employed to explore the structural variation of Fe（II）entities in NAu-2 and the electron transfer within NAu-2 and from NAu-2 to O₂.For 180 min oxidation of 1 g/L reduced NAu-2,a biphasic·OH production was observed,being quick within the initial 15 min and slow afterwards.Production of·OH correlates well with oxidation of Fe（II）in the reduced NAu-2.Within the initial 15 min,trioctahedral Fe（II）-Fe（II）-Fe（II）entities and edge Fe（II）in the reduced NAu-2 were preferentially and quickly oxidized,and electrons from the interior Fe（II）-Fe（II）-Fe（II）entities were most likely ejected from the basal siloxane plane to O₂.Meanwhile,trioctahedral Fe（II）-Fe（II）-Fe（II）entities were mainly transformed to dioctahedral Fe（II）-Fe（II）entities.When the time of oxygenation was longer than 15 min,dioctahedral Al-Fe（II）,Fe（II）-Fe（II）and Fe（II）-Fe（III）entities were slowly oxidized,and the interior electrons were transported through Fe（II）-O-Fe（III）linkages to edges and then ejected to O₂.In the slow stage of oxidation,electrons from interior Fe（II）accumulated towards the near surface layers and fueled the regeneration of edge Fe（II）for·OH production.In both stages,one-electron transfer mechanism with the involvement of O₂^·-and H₂O₂ applies for·OH production from the oxidation of structural Fe（II）by O₂.The mechanisms unraveled in this study advance the understanding of reactive oxygen species（ROS）production and structural Fe variation when Fe（II）-bearing clay minerals are oxygenated in redox-dynamic systems.2.On the basis of understanding of·OH production from oxygenation of reduced nontronite under oxic conditions,the natural process of CHCs transformation by oxygeration of Fe-bearing phyllosilicate was further studied.This study reveals that trichloroethylene（TCE）can be efficiently oxidized during the oxygenation of reduced nontronite at pH 7.5,whereas the reduction was negligible under anoxic conditions.The maximum oxidation of TCE（initially 1 mg/L）attained 89.6%for 3 h oxygenation of 2g/L nontronite with 50%reduction extent.TCE oxidation is attributed to the strongly oxidizing·OHproduced by the oxygenation of Fe（II）in nontronite.Fe（II）on the edges is preferentially oxygenated for·OH production,and the interior Fe（II）serves as an electron pool to regenerate the Fe（II）on the edges.Oxidation of TCE could be sustainable through chemically or biologically reducing the oxidized silicate minerals.Our findings present a new mechanism for the transformation of CHCs and other redox-active substances in the redox-fluctuation environments.3.To overcome the drawbacks such as acid wastewater production in pyrite Fenton system and low stoichiometric efficiency of H₂O₂ in Fe（III）-bearing minerals Fenton system,Fenton-like process catalyzed by reduced nontronite at circumneutral pH was investigated.This study reveals that the stoichiometric efficiencies of H₂O₂ in reduced nontronite and the classic Fenton reaction were comparable,4.7%and 5.0%at 2.5 min respectively.The maximum oxidation of TCE（initially 2 mg/L）attained 95.3%by the nontronite Fenton reaction between 0.6 g/L nontronite with 40%reduction extent and0.5 mM H₂O₂.TCE oxidation is attributed to the strongly oxidizing·OHproduced by the reaction between Fe（II）in nontronite and H₂O₂.The Fenton reaction catalyzed by reduced nontronite can effectively remove various organic pollutants.The stoichiometric efficiency of H₂O₂ for phenol degradation catalyzed by reduced nontronite was 20-800 times more than Fenton reaction catalyzed by Fe（III）-bearing minerals and aquifer materials in the previous studies.This study offers a new strategy of enhancing efficiency and saving costs for H₂O₂-based chemical oxidationin both insitu and ex remediation for soil and groundwater contaminants,and also sheds light on the environmental effects of reduced clay minerals.4.To further investigate the oxidation mechanism of Fe（II）-bearing minerals in subsurface,the mechanism of H₂O₂ catalyzed by sediment containing Fe（II）-bearing clay minerals for phenol degradation was studied.The results indicated that phenol can be efficiently and quickly degraded during H₂O₂ was catalyzed by sediment in the neutral pH.The oxidation of phenol（initially 10 mg/L）attained 84%when 15 g/L reduced sediment activated H₂O₂ at pH 6,whereas the degradation of phenol was negligible during H₂O₂ was decomposed by Fe（III）-bearing sediment.The phenol degradation was attributed to·OH production from H₂O₂ catalyzed by both dissolved Fe（II）and Fe（II）-bearing clay minerals in sediment.