Research On Mechansim Of Iron Mineral Recrystallization Drived By Interfacial Electron Transfer Coupling Environmental Behavior Of Heavy Metal

The speciation,mobility and transform of nutrient elements and contaminants in the soil and groundwater are controlled by Fe oxides with high surficial reactivity and redox ability.The water-iron mineral interfacial electron transfer is central of the interaction between Fe oxide and surrounding environmental substances,such as Fe（ⅡI）reducing bacteria and Fe（Ⅱ）.To clarify the electron transfer pathway and its microcosmic mechanism at water-mineral interface can provide scientific basis for assessing and predicting the effects of Fe oxides on the environmental behaviors of nutrient element and contaminant.It is widely recognized about the importance and extensiveness of the water-mineral interfacial process,however,the studies on how different mineralogical characteristics such as crystal facets and crystallite size affect the reactivity,and the speciation of heavy metal（s）induced by interfacial electron transfer are still limited.In this study,hematite and goethite,the most thermodynamic stable and ubiquitous minerals,were selected as the research object,in order to exploring（i）the effects of different crystal facets on the bioavailability of microbially mediated reduction;（ii）the interaction between crystallite size and microbially mediated reduction and（iii）the mechanism of aqueous Fe（Ⅱ）(Fe（Ⅱ）_aq)-catalyzed recrystallization of goethite coupled As（ⅡI）oxidation.The results obtained in this works will aid in understanding the mechanism of electron transfer and dynamic interaction between Fe oxides and the surrounding environmental substances at water-mineral interface.The major findings are as follows:（1）The bioavailability of microbially mediated reduction of different crystal facets in facet-combined system were clarified.In the present study,⁵⁷Fe M?ssbauer,stable Fe isotope tracer,and high resolution transmission electron microscopy（HRTEM）were employed to investigate the Fe reduction kinetics of different facets in the facet-combined and-separate system.The results indicated that the initial～72%and final～63%of reduced atoms from hematite{0 0 1}nanoplate（HNP）when the HNP and hematite{1 0 0}nanprod（HNR）combined.The pseudo-first-order rate constants（k）of reduced Fe（Ⅱ）for HNP and HNR were 3.2 and 2.0×10^-2 d^-1,respectively.HNP was reduced 1.8 times more significantly than HNR when the two facets combined,whereas HNP was reduced only 1.2 times more significantly than HNR in separate reactors.Based on the results of switching the hematite isotope labels and comparing isotope fractionation factors,we found that the effect of the isotope fractionation on the facet preferential reduction was negligible,thus the facet preferential reduction of HNP was ascribed to the intrinsic differences between HNP and HNR.We attributed the more efficient reduction of HNP than HNR to their differences in surface hydroxyl groups,surface charges,ligand-bound conformation and steric effects.These findings provide new insights into microbe-mineral interaction based on the crystal facet and the overall role of Fe oxide nanocrystals in the environments.（2）The dynamic interactions between goethite crystallinity and microbially mediated reduction,and the mechanism of increase in goethite crystallite size during dissimilatory Fe reduction process were disclosed.X-ray diffraction,TOPAS Rietveld refinements and chemical extraction experiments suggested that the crystallite sizes of obtained goethite particles range from 12.0 to 26.6 nm in the present study.The crystallite size of goethite strongly depend on the synthesis conditions.After incubation,the reduced Fe（Ⅱ）density decreased with the increase of goethite crystallite size.Because the goethite with smaller crystallite size exhibits larger specific surface area and more surface oxhydryl group,which is beneficial for interfacial electron transfer between bacteria and Fe oxide.Initial Fe（ⅡI）reduction rates increased with the crystallite size of goethite,the Fe（ⅡI）of（0 2 1）facet on goethite particle tip was believed to be more favorable for microbially mediated reduction than that of（1 1 0）facet on goethite particle edge.Goethite with higher crystallite size has more Fe（ⅡI）in（0 2 1）facet,so its initial iron reduction rate is higher than goethite with lower crystallite size.Fe（Ⅱ）reduced from goethite by Fe（ⅡI）reducing bacteria can futher catalyze the dissolution and reprecipitation of goethite,resulting in the increase of crystallite size.So more Fe（Ⅱ）reduced from goethite with lower crystallite size can result in larger increases of crystallite size.This study can help us to elucidate the effect of the dissimilatory Fe reduction process on the morphology of Fe oxide,and the geochemical behavior of Fe in the surface environment.（3）The process and mechanism of generation of Fe（Ⅳ）intermediate during Fe（Ⅱ）_aq-catalyzed recrystallization were clarified.Both the ⁵⁷Fe M?ssbauer,X-ray absorption near edge structure spectroscopy and methyl phenyl sulfoxide（PMSO）-based probing experiments evidenced the generation and quenching of Fe（Ⅳ）intermediate.The results indicate that the Fe（Ⅱ）_aq-catalyzed recrystallization of goethite is of oxidizability,resulting in oxidation of the coexisted As（ⅡI）.But XRD and TEM examinations suggest that no new phase was produced in the Fe（Ⅱ）_aq-goethite system.Numerous Fe atoms in goethite undergo atom exchanges with Fe（Ⅱ）_aq in the presence of As（ⅡI）,consequently,the oxidation of As（ⅡI）should be attributed to the interplay between Fe（Ⅱ）_aq and goethite.Based on the spectroscopic analyses,we evidenced the generation and quenching of Fe（Ⅳ）intermediate during Fe（Ⅱ）_aq-catalyzed recrystallization.According to the isotopic labeled tracing,the Fe（Ⅳ）was originated from the Fe（Ⅱ）_aq rather than from the structural Fe（ⅡI）in goethite.In addition,H radical（H·）signals were detected in the Fe（Ⅱ）_aq-goethite suspension and supernatant,combining with generation of Fe（Ⅳ）signals,we proposed that a proton-coupled electron transfer process could be responsible for Fe（Ⅳ）generation in Fe（Ⅱ）_aq-goethite system.These findings are expected to provide new insight into the anoxic oxidative transformation processes of matters in non-surface environments on earth.