Supergenic Oxidation Of Pyrite And The Study For Microbial Oxidation Mechanism

It is always been concerned of the most serious environmental problems caused by the mining industry because of that sulfide(such as pyrite,galena,sphalerite,chalcopyrite)oxidation plays an important role in the release of heavy metals and the formation of acid mine drainages(AMD).The oxidation of sulfides is responsible for the generation of acid mine drainage and acid rock drainage,which further leads to contaminations of soils and waters.Adding limestone could neutralize acidity of AMD and was commonly used to retain heavy metals by the formation of a variety of secondary minerals which supported the contribution to the heavy metal pollution control.Since calcite and pyrite are widespread in all kinds of rock types,surface weathering of outcrops sulfide deduced the formation of acid rock drainage(ARD),the decomposition of carbonate minerals,and the formation of secondary minerals.This process is also relevant in the geochemical behavior of heavy metals that has been given high priority.Therefore,it is provided important theoretical and practical significance in the remediation of AMD and tailings pollution,as well as understanding the significance of environmental geochemistry of ARD by detailed research of the weathering process of carbonate-sulfide and the transformation of secondary minerals.In such processes,microbial oxidation usually prevails over chemical oxidation.Pyrite is a commonly sulfide minerals in mine wastes,while Acidithiobacillus ferrooxidans(A.ferrooxidans)is one of the most important and intensively studied bacteria in bioleaching.So it is vitally to disclose the mechnism and the process of microbial oxidation of pyrite,which has important theoretical and practical significance in the remediation of AMDIn our study,field samples inclouding waste ores,mine tailings,AMD and the sulfate-bearing carbonates were analyzed by Raman,XRD,FTIR,SEM/EDS,TEM,EPMA,XANES,and so on.By determining which kind of secondary minerals were presented in the samples,the morphology,and the composition differences as well as the ability of retaining heavy metals,we discussed the chemical heterogeneity,formation mechanism and the occurrence of element As to provide evidences for which minerals control the acidification of water and which soluble minerals contain metals that could be released.Based on this,we selected A.ferrooxidans isolated from AMD to interact with pyrite sampled in Shizishan mine in laboratory experiments.We investigated the surface properties of pyrite leached by A.ferrooxidans,and the secondary minerals generated in this process by using scanning electron microscopy(SEM),X-ray diffraction(XRD)and Fourier transform infrared(FTIR)spectra.The oxidation state of the element sulfur in the interaction has been disclosed with the aids of X-ray photoelectron spectroscopy(XPS)depth profiles which further to disclose the microcosmic decomposition mechanism of pyrite.Firstly,we have identified the types of secondary minerals and influencing factors by surface weathering of metal-rich sulfide waste ore.The analysis results of the waste sulfide-bearing ores show that pyrite was oxidized firstly from the flaw spot such as fissure,and then gradually oxidized to the whole sample.Before goethite formed,the intermediary ferrihydrite possibly had produced.Due to its unstable property,ferrihydrite would transform to goethite or hematite quickly under different conditions.There were various configurations of goethite.The acerose goethite was formed and then it recrystallized to become globose and rhombic dipyramid goethite.In the end,the layer of dusty goethite formed above the waste ore's surface.Secondary highly complex Fe-minerals are generated from microbial activities in the circumstance.SEM observation shows there maybe biominerals in pyrite surface and hypha in the surface of the waste ores,which indicate that the organism might have been played certain role in the weathering process of the waste ores.It is further revealed the types,distribution and the forming condition of secondary minerals under the weathering process of carbonate-sulfide,which provided geological analogy in understanding the remediation of AMD and tailings pollution by using carbonate.Surface weathering process of outcrops of sulfate-bearing carbonate veins could reflect the formation of acid rock drainage(ARD)and the environmental damaging phenomenon by minerals weathering.It could be summarized as follows:at the beginning,weathering of sulfide minerals resulted in the acidification of the environment and the dissolution of calcite.Since the formation of CO32-could consume H+by sulfide(e.g.,pyrite)oxidation,thereby it neutralized acidity of the environment.Ca2+ and SO42-were binding to form gypsum,or anhydrite,while Fe3+ could transform to a variety of mineral phases at different pH conditions.The more nearby calcite,the more alkaline in pH,and it decreased gradually keep away from calcite.From the high pH to the end,4 mineral belts of calcite-gypsum/anhydrite-oxides and hydroxides minerals-secondary highly complex Fe-minerals were sequentially formed.Because of that carbonates could neutralize acidity of the environment,it thus prevented main contributor of sulfides weathering to the acidification and the formation of acid mine drainage(AMD).The formation of jarosite,especially schwertmannit could effectively adsorb heavy metals such as element As,thus reducing the environmentally damaging of soil and water known as the releasing of element As.Results of laboratory experiments and XPS analysis have revealed the changes of chemical states of element sulfur by bio-oxidation of pyrite.It is found that oxidation of sulfur is mainly involved in the process of electrons loss in couples.Based on the results of morphology,solution feature and the analysis of XPS depth profiles,a 3-staged model is proposed to interpret the kinetics of microbial oxidation of pyrite,which involves an abiotic oxidation dominant stage,surface erosion by attached bacteria cells,and cooperative oxidation by biofilm and ferric ions.It is found that abundant bacteria cells attached onto pyrite surfaces and formed biofilms,which greatly enhances the surface corrosion and results in two modes of etching pits,i.e.,bacteria-driven rod-shaped and chemically driven hexagonal etching pits.The details of XPS depth profiles on a reacted pyrite surface reveal that the surface sulfur was oxidized firstly into elemental sulfur.Thereafter,elemental sulfur was further oxidized as intermediate species of S2O32-,SO32-,and ultimately SO42-.Meanwhile,the remnant ferrous iron in the surface layer was released into solution and subsequently oxidized into Fe3+ by A.ferrooxidans and dissolved oxygen,which in turn enhances the oxidation of sulfur.Fe3+ sulfate and other ions(such as K+,Na+,NH4+)in the solution precipitate as jarosite,hydroniumjarosite,and ammoniojarosite.