The Evolution Of Mineralogy And Repartitioning Of As During Reduction Of Jarosite By SRB

Jarosite[KFe₃（SO₄）₂（OH）₆],an iron-hydroxysulfate mineral that is commonly found in acid mine drainage（AMD）environments.With its remarkable isomorphism,jarosite can incorporate many contaminants such as AsO₄^3-into its mineral structure.However,jarosite is metastable in water,and may dissolve or transform to other thermodynamically more stable minerals such as goethite or vivianite when environmental conditions change,leading to the release of bound As.Micro-biological decomposition is considered an important factor on the mineralogical transformation of jarosite mineral.Sulfate reducing bacteria（SRB）are strict anaerobic microorganisms that are commonly found in AMD environmnets.SRB can take low-molecular-weight forms of organic carbon such as lactate as the electron donor with the reduction of SO₄^2-to S^2-,which then subsequently reduces the Fe³⁺oxides.Besides indirect reduction of iron oxide through the production of S^2-,some known SRB are capable of reducing Fe³⁺via an enzymatic mechanism.Theoretically,both the Fe³⁺and SO₄^2-in jarosite have the potential to serve as the terminal electron acceptors for SRB.Comprehensive knowledge of the reductive dissolution of jarosite by SRB will further our understanding of the geochemical cycling of a range of elements including iron and sulfur,and the transport and repartitioning of As in AMD environments.Nevertheless,the mechanism of jarosite reduction by SRB,the characteristic of jarosite transformation,and the associated AsO₄^3-behavior have not been fully elucidated so far.In this study,pure jarosite and AsO₄^3--bearing jarosite were chemical synthesized in laboratory.The mechanism for the reductive dissolution of jarosite by SRB,the evolution of mineralogy and the repartitioning of As during reductive dissolution of jarosite mineral by SRB were explored.During the experiment,the variation of the concentrations of K⁺,SO₄^2-,Fe²⁺and As in the solution,the total Fe^II and elemental sulfur（S⁰）in the suspension,the crystallinity,morphology and functional group for the mineral,the speciation of element Fe、S and As on the mineral surface,and the evolution of the microbial communities,were examined by HPLC、IC、XRD、FTIR、SEM-EDS、XPS and 16S-r RNA.The mian results are listed as follows:The chemical synthesized AsO₄^3--bearing jarosite presented the diffraction lines of jarosite as well.The incorporation of AsO₄^3-did not change the crystallinity of the jarosite mineral.The incorporation of AsO₄^3-decreased the particle size of the mineral,and increased the specific surface area.The partial size of“AJ0”,“AJ0.6”and“AJ5.0”is 2～3,～1 and0.5～1.0μm;and the specific surface area is 2.10、3.76 and 5.96 m²/g,respectively.Compared with the pure jarosite,the AsO₄^3--bearing jarosite had a weaker SO₄^2-peaks and an additional AsO₄^3-peak,which indicated that the AsO₄^3-had occupied the position of SO₄^2-in jarosite.In the presence of dissolved SO₄^2-,the concentration of lactate decreased with the increase of acetate concentration.In addition,the concentration of K⁺,SO₄^2-and Fe²⁺increased as well,which indicated that jarosite reduction by SRB proceeded via an indirect,sulfur-dependent pathway.SRB incompletely oxidized lactate to acetate coupled with the reduction of dissolved SO₄^2-to S^2-,which then chemically reduced the Fe³⁺in jarosite,forming secondary minerals including vivianite[Fe₃（PO₄）₂?8H₂O],mackinawite（Fe S）and pyrite（Fe S₂）.In the absence of dissolved SO₄^2-,the variation of solution parameters showed a similar tendency to that in systems with dissolved SO₄^2-.This indicated that the reductive dissolution of jarosite occurred as well.In the system without dissolved SO₄^2-,K⁺and SO₄^2-concentrations started to increase on day 10～15,and Fe²⁺concentrations began to increase on the 25th day,which indicated that jarosite dissolution occurred prior to reduction.The dissolution and transformation of jarosite was inhibited by the secreted extracellular polymeric substance（EPS）of SRB.EPS contained the founctional group of phosphorus.The maximum amounts of EPS-P adsorption by jarosite was 0.2459 mg/m²,and the adsorption constant for Langmuir isotherm models was 0.9920.PO₄^3-played an essential role on the reduction and transformation of jarosite by SRB.In the low PO₄^3-concentration treatment,the decrease of lactate concentration was the fastest,and the increase of K⁺,SO₄^2-and Fe²⁺concentration was the earliest,which indicated that the activity of SRB was the strongest,and the jarosite was the most unstable compared to that in the other treatments.FTIR result presented that the adsorption of PO₄^3-could bind to the Fe O₆ octahedron,formed Fe-O-P bond and stabilized the jarosite mineral.With the increase of PO₄^3-concentration,vivianite became increasingly important during reductive dissolution jarosite by SRB.The reductive dissolution of AsO₄^3--bearing jarosite by SRB led to a release of the As in the solid phase into the aqueous phase,which then partially be re-adsorbed by the newly formed secondary minerals.In the systems with and without dissolved SO₄^2-,the As concentration in solution was 4.94 and 6.08μmol/L,respectively.The addition of dissolved SO₄^2-increased the amount of secondary Fe²⁺minerals and mitigated the mobilization of As into the aqueous phase.For the synthetic AsO₄^3--bearing jarosite,most of As was associated with fraction of crystalline iron oxide-bound phase.The proportion of As in the crystalline iron oxide-bound phase decreased,with the increase of the fractions of As in the iron oxide surface-bound phase and aqueous phase during the reductive dissolution of AsO₄^3--bearing jarosite by SRB.The S^2-produced by SRB is a powerful reductant that can chemically reduce the Fe³⁺in AsO₄^3--bearing jaroste and accelerate the reductive dissolution of mineral.SRB rather than S^2-converted the As（V）on mineral to As（III）under neutral conditions.