Reduction And Transformation Mechanism Of Fe-Oxides And Fe(?)-Oxyhydroxysulfate Minerals By Citrobacter Sp.

Acid mine drainage?AMD?has a great threat to surrounding river,sediment and soil ecology.The secondary Fe?III?-oxyhydroxysulfate minerals?such as schwertmannite and jarosite?and ferrihydrite formed in AMD environment have the ability to adsorb and co-precipitate with toxic and harmful substances.The reduction of the Fe-S elements in minerals by sulfate reducing bacteria?SRB?accelerates the dissolution and transformation of these secondary minerals,resulting in the re-release of co-precipitations.The study on the reduction and transformation mechanism of Fe-S minerals by functional microorganisms is helpful to understand the Fe and S element cycle in AMD environment,and has theoretical significance for the control and management of heavy metal risk in AMD impacted environment.The main research results were as follows:?1?Isolation and characterization analyze of SRB strain in AMD-contaminated sediments:In present study,a functional strain with sulfate reduction capacity was isolated by solid-liquid medium shifting culture from the SRB community.The strain could rapidly grow under aerobic conditions but couldn't reduce sulfate while sulfate reduction and black Fe S occurred under anoxic condition.It can grow with lactate as carbon source and reduce 10mmol/L SO₄^2-in a week;and can also grow with pyruvate and citrate but unable to reduce sulfate.The biomass was ordered as following:lactate>pyruvate>citrate;EBS8 cannot use acetate as electron donor.The suitable initial p H for growth and sulfate reduction was 6,7 and8,in which p H=6 was the optimal p H.Both acidic and alkaline conditions seriously inhibited the growth of EBS8.It was identified as Citrobacter sp.and named EBS8?the login number:MN420979?after 16S r RNA sequencing.?2?Reduction mechanism of Citrobacter sp.EBS8 on iron oxide?ferrihydrite?:EBS8couldn't directly reduce ferrihydrite without SO₄^2-.In the presence of SO₄^2-,the sulfide produced by EBS8 rapidly attacked the surface of ferrihydrite,resulting in the collapse of mineral structure and the reduction of Fe?III?.The minerals were finally transformed into mackinawite and greigite.The proportion of Fe and S in systems will affect the rate of sulfate reduction which was increased when the ratio of Fe and S was 2:1.However,more ferrihydrite?Fe:S=10:1?inhibited the rate of sulfate reduction.The reason may be that more ferrihydrite leads to damage cells resulting in decreased activity.At the same time,more dispersive distribution of sulfide was occurred and more dissolved organic matter was absorbed with more ferrihydrite,which promoted the electron transfer between Fe and S.Then the formation rate of Fe?II?was faster and the concentration of reduced Fe?II?was also higher than 2 Fe:1 S treatment in the end.?3?The reductive and transformation of EBS8 to Fe?III?-oxyhydroxysulfate minerals:Under the neutral environment,the spontaneous dissolution of Fe-S minerals and the biological dissolution leaded to the release of SO₄^2-,Fe³⁺and K⁺in minerals.EBS8 reduced SO₄^2-to S^2-,then promoting the reduction of Fe³⁺and further dissolution of minerals.The reduction products of Fe-S and the co-existing ions in the solution recombined and precipitated to form Fe related products under the combined action of biology and chemistry.The type of new mineral was related to the structure of the original mineral and the release rate of key elements.The main transformation products of schwertmannite were siderite and vivianite identified by XRD.Jarosite were transformed into vivianite and mackinawite.In summary,SO₄^2-will promote the dissolution of iron oxides by SRB in AMD-contaminated sediments.The final reduction amount of iron oxides and the transformed products are related to the proportion of Fe and S.Citrobacter sp.EBS8 can directly act on Fe and S minerals,reducing and transforming them into new minerals.Meanwhile,the transformation products of schwertmannite and jarosite was different.This study helps to our understanding of the influence of biological factors on the transformation of Fe and S minerals in AMD-contaminated sediments and provides theory for AMD environmental remediation.