The Change Of Mechanism In FeS Remediation Of Heavy Metal Under Oxidizing Environments And The Regeneration Of FeS Reactivity By Sulfate Reducing Bacteria

Ferrous sulfide(FeS),as an emerging remediation material,has been widely used in the remediation of heavy metals in soil and groundwater.However,due to the complexity of pollutants and the shifting redox potential in soil and groundwater,the fate of FeS in heavy metal remediation is still uncertain due to the fact that FeS can by easily oxidized in oxidizing environments.This study first investigated possible(mineralogical,surface speciation)changes of FeS during heavy metal removal under different oxidizing environments and the involved mechanisms to reveal the potential environmental risks caused by FeS oxidation.In addition,sulfate-reducing bacteria(SRB)were utilized either to extend the reactivity of FeS towards heavy metals or to regenerate spent FeS of different oxidation time.This part aims to provide a novel approach to soil and groundwater heavy metal removal from the perspective of microbiotic-abiotic coupled remediation.The main conclusions are as follows:(1)The mechanism of heavy metal remediation by FeS is mainly reduction and precipitation.However,the mineral phase composition and surface speciation of FeS may change in oxidizing environments.The increase in oxygen concentration promoted the dissolution of FeS and increased the proportion of S(-?)oxidation state(S22-and Sn2-)on the surface.When the reaction time of FeS with Pb2+ or Cd2+solution was extended to 30 days,PbS or CdS was gradually oxidized with Pb-O or Cd-O appeared on the surface.In the presence of residual FeS,the oxidation of PbS and CdS may be delayed,which showed increased crystallinity of heavy metal sulfides and iron oxids(lepidocrocite and goethite)after 30 days.The mineral phase composition and reducing ability of FeS were also affected by aerobic drying.Aerobic drying converted surface FeS to lepidocrocite,and concomitantly decreased the reducing ability of FeS compared to the original non-dried material(aerobic drying:57%,non-drying:100%).(2)Within 30 days,SRB solution can extend the reactivity of FeS.The SRB impregnated FeS kept a high removal efficiency of Cr(VI)and Pb2+(up to 84%)up to 30 days.As affected by the SRB,the mineral phase on FeS surface changed with the appearance of Fe3S4 and elemental S phases.(3)The SRB could regenerate the reactivity of air-oxidized FeS with a short exposure time(less than 24 hours).A longer impregnation time in the SRB solution might promote the recovery of the reactivity(and reducing ability)of FeS.Nevertheless,for FeS with a long oxidation time(exceeding 24 hours),the reactivity(and reducing ability)of FeS was barely restored even after 30 days of SRB impregnation.The 30-day SRB impegnation led to small amount of elemental sulfur and pyrite(Fe7S8)formed on the surface,despite lepidocrocite still being the dominant surface phase.