Abiotic Transformation Of Nanosized Iron Sulfide In Groundwater Under Oxic Conditions And Its Performances For Cr(Ⅵ) Removal

Dissolved oxygen in groundwater is generally at a low level in natural environment,and the increase of its concentration may affect the chemical species and concentration of pollutants in groundwater.Oxygen（O₂）in surface water or air can be transferred into the groundwater,leading to the remarkable increase of concentration of dissolved oxygen and the change from anoxic to oxic conditions under anthropogenic or natural disturbances such as interaction between surface water and groundwater,fluctuation of groundwater table,artificial recharge of groundwater as well as groundwater extraction and remediation.Dissolved oxygen plays complex roles such as promotion and inhibition in Cr（Ⅵ）removal.FeS minerals are widely distributed in subsurface anoxic environments of sediment and groundwater.Under reducing conditions,the biological reduction of sulfate and Fe（ⅡI）may lead to the formation of FeS as nanoparticles（NPs）.FeS can be transformed into the multicomponent minerals with the performance of Cr（Ⅵ）reduction and sorption in oxygenated aqueous solution,including greigite,pyrite,green rust,lepidocrocite,goethite,etc.At the same time,reactive oxygen species（ROS）is generated in the oxygenation of FeS,and has the potential to oxidize Cr（Ⅲ）.At present,some studies have been carried out on the reaction process and the composition of minerals transformed from FeS under the oxidation of air and dissolved oxygen,at the same time,and it has been gradually focused on the degradation and transformation of pollutants during the oxygenation of FeS through the generation of strong oxidizing substances.However,the abiotic dynamic transformation process of FeS NPs in groundwater is unclear under anoxic to oxic transitional conditions,and the research on the influence of their transformation process and transformed minerals on Cr（Ⅵ）removal is still not enough in-depth.In this study,dynamic batch experiment,electron microscopy and spectroscopy,probe reaction,hydrochemical analysis and mass balance calculation were adopted.Typical natural reoxygenation scenario of groundwater from anoxic to oxic transitional conditions was simulated,and the experimental researches on abiotic dynamic transformation of FeS NPs in groundwater and its theoretical analysis were conducted.Firstly,this study investigated the effects of environmental conditions such as p H,temperature,cation and anion,and fulvic acid（FA）,on the abiotic transformation process of FeS NPs in groundwater and the composition of their transformed minerals under natural reoxygenation conditions.The law for transformation of chemical composition and phase of FeS NPs in groundwater was proposed.Secondly,the production kinetics of typical ROS of H₂O₂ and·OH radical from the oxygenation of FeS NPs in groundwater was studied.The pathways of formation of H₂O₂ and·OH from the oxygenation of FeS NPs was defined.The effects of groundwater environmental conditions on the production of H₂O₂ and·OH were explored such as p H,temperature,concentration of FeS NPs,reoxygenation mode and FA.In addition,the formation kinetics of Cr（Ⅵ）during the oxygenation of FeS NPs in groundwater containing Cr（Ⅲ）was investigated.The effects of groundwater environmental conditions on the oxidation of Cr（Ⅲ）were examined such as p H,molar ratio of FeS-to-Cr（Ⅲ）,temperature,cation,anion,and FA.Mechanisms of the conversion between Cr（Ⅲ）and Cr（Ⅵ）during the oxygenation of FeS NPs were clarified.Finally,the study investigated the removal kinetics of Cr（Ⅵ）in aqueous solution using minerals transformed from FeS NPs,analyzed the chemical species of chromium and phase composition in solid products from Cr（Ⅵ）removal using minerals transformed from FeS NPs,and clarified the ability of minerals transformed from FeS NPs for Cr（Ⅵ）reduction and sorption.Through this research,the main conclusions are as follows:（1）The transformation of FeS NPs in groundwater under natural reoxygenation conditions could go through many physicochemical reaction processes,such as the synergistic oxidation of Fe（Ⅱ）and S（-Ⅱ）,the proton-dissolution and oxidation,and the surface-mediated oxidation of FeS NPs solid,and was significantly affected by p H.During the synergistic oxidation of Fe（Ⅱ）and S（-Ⅱ）,Fe（Ⅱ）is oxidized by dissolved oxygen to Fe（ⅡI）,which generate Fe（Ⅱ）again under the reduction of S（-Ⅱ）,and S（-Ⅱ）is converted to S⁰.Under acidic conditions,the oxidation of FeS NPs is mainly controlled by the proton dissolution and oxidation process.FeS NPs was dissolved by H⁺,oxidized by dissolved oxygen and transformed into intermediates of pyrrhotite and green rust,finally into the major minerals of S⁰,goethite and amarantite with a small amount of lepidocrocite.There was no Fe-containing substance in the solution and S mainly existed in the form of SO₄^2-at the reaction equilibrium.Under basic conditions,the oxidation of FeS NPs is mainly controlled by the solid surface-mediated oxidation.The surface layer of FeS NPs was gradually oxidized to amorphous substance and lepidocrocite shell in sequence.The internal substances distributed from inside to outside were FeS,greigite and pyrite,which were successively transformed into lepidocrocite and S⁰ by dissolved oxygen.There was no material containing Fe in the solution,and the dissolved S existed mainly in the form of SO₄^2-at the reaction equilibrium.At neutral p H,the oxidation of FeS NPs is controlled by both the proton-dissolution and oxidation,and the solid surface-mediated oxidation.FeS NPs was finally transformed by dissolved oxygen into S⁰ andγ-Fe OOH through the intermediate substances of Fe₇S₈ and FeS₂.（2）The production of H₂O₂ and·OH from the oxygenation of FeS NPs showed a two-stage kinetic process,which first increased rapidly and then decreased slowly,and groundwater environmental conditions such as p H,temperature,reoxygenation mode,concentration of FeS NPs and FA can significantly affect the production process of H₂O₂ and·OH.The production processes of H₂O₂ include not only the reaction of structural Fe（Ⅱ）with O₂,but also the reaction of structural S（-Ⅱ）with O₂.The contribution of the oxygenation of structural S（-Ⅱ）to generate H₂O₂ was about 9.7%.Generation of·OH involves mainly three processes.H₂O₂ is activated by Fe（Ⅱ）to form·OH.The structural Fe（ⅡI）and adsorbed H₂O react and produce·OH.The oxygenation of Fe（Ⅱ）and SO₃^2-/S₂O₃^2-/S⁰ transformed from S（-Ⅱ）oxidation produce·OH.Acidic conditions,natural reoxygenation conditons and increasing the temperature were beneficial to the production of H₂O₂ and·OH,and there was a linear correlation between FeS NPs concentration and the cumulative yield of·OH.The effect of FA on the production of H₂O₂ and·OH was closely related to p H.FA could promote the production of H₂O₂ and·OH under acidic conditions and FA had no significant effect on the production of H₂O₂ and·OH under neutral conditions;FA could inhibit the production of H₂O₂ and·OH under basic conditions.（3）The oxygenation of FeS NPs had an oxidative mobilization effect on Cr（Ⅲ）,and groundwater environmental conditions such as p H,molar ratio of FeS NPs-to-Cr（Ⅲ）could change the oxidation and reduction reaction rate of Cr,thus significantly affecting the chemical form of oxidation products.The oxidative mobilization of Cr（Ⅲ）during the oxygenation of FeS NPs includes two processes of the oxidation of Cr（Ⅲ）to Cr（Ⅵ）and the reduction of the newly generated Cr（Ⅵ）to Cr（Ⅲ）.·OH generated from the oxygenation of FeS NPs oxidized Cr（Ⅲ）to Cr（Ⅵ）mainly through the surface oxidation process of FeS NPs or Cr（OH）₃ solid,and then the newly generated Cr（Ⅵ）was reduced to Cr（Ⅲ）.When the formation rate of Cr（Ⅵ）was higher than the reduction rate,Cr（Ⅵ）could be accumulated.When the reduction rate of Cr（Ⅵ）exceeded the production rate,the newly formed Cr（Ⅵ）could be reduced to Cr（Ⅲ）.p H and molar ratio of FeS NPs-to-Cr（Ⅲ）could affect the reaction rate of Cr（Ⅲ）oxidation and the chemical species of oxidation products.At relative high molar ratio of FeS NPs-to-Cr（Ⅲ）or under acidic conditions,Cr（Ⅲ）will be oxidized to Cr（Ⅵ）rapidly,whereas Cr（Ⅵ）still existed in the final product of Cr（Ⅲ）oxidation at relative low molar ratio of FeS NPs-to-Cr（Ⅲ）.Cr（Ⅲ）was more easily oxidized to Cr（Ⅵ）under acidic conditions and the yield of Cr（Ⅲ）oxidized to Cr（Ⅵ）was increased by increasing molar ratio of FeS NPs-to-Cr（Ⅲ）.Increasing the temperature can enhance the oxidation of Cr（Ⅲ）to Cr（Ⅵ）.The effect of FA on the oxidation of Cr（Ⅲ）to Cr（Ⅵ）was related to p H and Cr（OH）₃（s）was more easily oxidized to Cr（Ⅵ）than Cr_xFe_1-x（OH）₃（s）.The oxidation of Cr（Ⅲ）to Cr（Ⅵ）could be inhibited by HCO₃^-,NO₃^-and Ca²⁺.（4）The minerals transformed from FeS NPs such as Fe₃S₄、Fe₇S₈、FeS₂、GR、γ-Fe OOH andα-Fe OOH had the performances for Cr（Ⅵ）reduction and sorption,which was significantly affected by the time of oxygenation of FeS NPs and p H.The kinetics of Cr（Ⅵ）removal using the minerals transformed from FeS NPs showed two stages,and the concentration of Cr（Ⅵ）in the solution first decreased rapidly,then decreased slowly and tended to be stable.The time of oxygenation of FeS NPs can affect the processes of Cr（Ⅵ）removal,the composition of Cr-bearing substances in the products of Cr（Ⅵ）removal,the Cr（Ⅵ）removal efficiency,reduction performance and sorption performance of Cr（Ⅵ）.The Cr（Ⅵ）removal by the minerals transformed from FeS NPs in the initial stage of the oxygenation involved both Cr（Ⅵ）reduction and sorption,and the Cr-bearing substances in the products existed mainly in the form of ferric chromate,chromimum-iron oxide and（or）hydroxy chromium oxide.The minerals transformed from FeS NPs in the later stage of the oxygenation could remove Cr（Ⅵ）through sorption and ferric chromate was the main Cr-bearing substance in the products.With the increase of the time of the oxygenation of FeS NPs,the Cr（Ⅵ）sorption performances of their transformed minerals decreased first and then increased.Under acidic conditions,with increasing the time of oxygenation of FeS NPs,a decreasing trend was shown for Cr（Ⅵ）removal efficiencies and reduction performances using their transformed minerals such as Fe₇S₈、GR、γ-Fe OOH andα-Fe OOH.Under neutral and basic conditions,as the time of the oxygenation of FeS NPs increased,Cr（Ⅵ）removal efficiencies and reduction performances increased at first and then decreased using their transformed minerals such as Fe₃S₄、Fe₇S₈、FeS₂、γ-Fe OOH.This study elucidated the abiotic mechanism of transformation of FeS NPs in groundwater under anoxic to oxic transitional conditions,clarified the generation of·OH from oxygenation of FeS NPs,revealed the mechanisms of oxidative mobilization of Cr（Ⅲ）during the oxygenation of FeS NPs,and clarified the Cr（Ⅵ）reduction and sorption performances of minerals transformed from FeS NPs.The findings in this work enrich the basic theory of the oxygenation of nanosized FeS and redox conversion between Cr（Ⅲ）and Cr（Ⅵ）,and at the same time provide the theoretical basis for remediation and control of Cr（Ⅵ）pollution in groundwater.