Experimental Studies On The Phase Transformation Of Bimessite

Birnessite is a ubiquitous mineral on the earth surface,and is also the main component in marine Fe-Mn nodules.Recent researches have disclosed that the forming and transformation of birnessite play important roles in the geochemical cyclings of various elements.The mineral composition determines the processing of Fe-Mn nodules.As the main mineral,birnessite tends to change into other mineral phase as its environment changes,in which microbe interaction and hydrothermal activities are the two most important factors.Based on the mineralogical analysis and microscopic observation of a marine Fe-Mn nodules,we designed and conducted two series of simulation experiments,including microbial reduction and hydrothermal reflux of birexperiment.By employing several spectrometric techniques and microscope apparatus,the phase transformation and mechanism of birnessite have been disclosed.Based the simulation experiments with Shewanella oneidensis MR-1,a common heterotrophic metal-reducing bacteria,the reduction of vernadite,an important kind of birnessite,was studied in anaerobic condition by using field emission scanning electron microscope(FESEM),field emission transmission electron microscope(FETEM),X-ray diffraction(XRD)and Raman apparatus.The results show that Shewanella oneidensis MR-1 can reduce vernadite into secondary minerals with lower-valent manganese,including fiberiform todorokite,manganese phosphate,plate-like manganite,rhodochrosite,and so on.The phenomena of surface cell attachments onto vernadite and cell connection to vernadite through nanowires are commonly observed.However,in the experiments with heated bacteria cells only todorokite and manganite can be found,and triclinic birnessite and trace todorokite formed in the experiments without inoculatation of bacteria.Mineralogical analysis and microscopic observation indicate that the degree of reduction determines the components of secondary minerals.In another series of simulation experiments,the reduction and phase transformation of triclinic and hexagonal birnessite under reflux condition have been investigated and compared.The results indicate that triclinic birnessites transforms into todorokite more easily than orthorhombic birnessites,and straight firber todorokite crystal and plate-like todorokite aggregation can be formed.The transformation is commonly determined by the crystal structure.Triclinic birnessite presents a orthorhombic layer symmetry composed by regular arrangement of two Mn(?)-O octahedron rows and Mn(?)-O octahedron row,which is similar to the structure of todorokite,and thus easily transforms into todorokite.While hexagonal birnessite has more lattice defects,and exhibits small and unregular plat-like forms.Under the condition of of hydrothermal reflux,hexagonal birnessite tends to recrystalize firstly followed by phase transformation,and thus shows a delayed transformation.The phenomena of microbial reduction and reflux experiments reveal that bacteria can reduce birnessite into various manganese minerals with lower valence,while hydrothermal reflux only produced todorokite with different appearances.Two kinds of birnessite in hydrothermal reflux experiments show different dynamic properties.Triclinic birnessite can transform into todorokite with better crystal form.The findings help to understand the transformation of birnessite under various environments,and also provide a potential mineral sign for identifying the mediation of microbial reduction in birnessite reduction.