A Preliminary Study Of The Characteristics And Mechanisms Of Iron Oxide Abotic Reduction

The process of reductive dissolution of iron(III)oxides which the high abundance and redox sensitivity strongly impacts the biogeochemical cycle of iron in natural environment.The study object were two similar crystalline structure iron(III)oxides:Goethite(?-Fe O-OH)and lepidocrocite(?-FeO-OH).The current study mainly focused on the synergetic effects of electron shuttle(anthraquinone-2-sulphonate,AQS)and organic acid(oxalate)on the reductive dissolution process of iron(III)oxides(goethite and lepidocrocite)under irradiated/nonirradiated and anaerobic/aerobic experimental conditions.Firstly,we discusses the iron oxides reductive dissolution in anaerobiciradiated experiments,the results showed that the extent and rate of reductively dissolution of iron oxides was influenced by of the content of iron oxides AQS,oxalate,and ion strength.We find that concentration of dissolve iron increase 0.24 mM to 0.31 m M the for goethite and 0.21 mM to 0.42 mM for lepidocrocite with increased of Fe(III)oxides initial content from 0.25 mM to 1.0mM.but increase more iron oxides would decease concentration of dissolved iron.meanwhile,in reaction systems containing AQS or oxalate alone,the reductive dissolution of goethite cannot occur under irradiated condition.As for lepidocrocite,the presence of AQS can make the reductive dissolution process happened even in the absence of oxalate under irradiated conditions.While in the presence of AQS and oxalate,the reductive dissolution process of goethite and lepidocrocite could both be furtherly accelerated under irradiated conditions.Goethite initial reductive rate presence linar icreased,As improving AQS concentration from 0mM to 0.2 mM,this tendency is more clear in lepidocrocite reaction.In oxalate concentration controlling group,we could not detected dissolve iron in 0M oxalate in goethite system.Dissolved Fe(II)quickly increased from 0.08 mM to 0.68 m M whenoxalate concentration increase from 0.25 mM to 0.40 mM.concentration of dissolved Fe(II)was 0.1 mM at 0 M oxalate in lepidocrocite system and reach to maximum value0.21 mM at 1.5 mM oxalate.Further more oxalate would not increase dissolve iron content.In ion strength experiment,the concentration of dissolved Fe(II)was 0.2 mM to0.3 mM from 0 to 50 m M NaClO4,the result of different ion strength experiment showed that the 10 mM ion strength was peak value for promoting iron oxides reductive dissolution.However,the above mentioned reductive dissolution processes cannot be occurunder nonirradiated conditions.As comparison,iron oxides reductive dissolution processes on aerobic was researched,both goethite and lepidocrocite dissolution are not occured on aerobic conditions out of the anaerobic tank even under light illuminated conditions.We find the concentration of adsorbed iron slightly higher than thedissolve iron,but both value nearly not change compared with initial concentration during reaction process.While all of concentration of detected iron not beyond 0.09 mM under nonirradiated and 0.01 mM underirradiated in goethite system,not beyond 0.10 mM under nonirradiated and 0.12 m M underirradiated in lepidocrocite system.Phenanthroline was also added for the purpose of research the effected of adsorption iron on the anaerobic irradiated condition of goethite,the result showed that Phenanthroline inhibited the light induced reductively dissolution and decrease the reaction rate.The final concentration of dissolved iron was less than 0.3 mM with Phenanthroline and more than 0.4 mM without phenanthroline.our experiment results indicated that the light induced and deaerated environment are necessary experiment condition for the process of iron oxides reductive dissolution.Iron oxides reductive dissolution in the presence of electron shuttle(AQS)and organic acid(oxalate)are mainly comprised of two related steps,one is reduce goethite surface crystal formed Fe(III)-Fe(II)bond less stable structure like lepidocrocite and slow release Fe(II).The presence of electron shuttle speedily reduce and detach Fe(II)on deaerated.These coupled process indicate that in this system detachment of surface Fe(II)from the crystal lattice is the rate-determining step of the overall dissolution process rather than electron transfer.The potential synergism between ligand and reductant for iron(hydr)oxide dissolution may have important implications for ironcycling in environments.