Microscopic Mechanism Of Arsenate And Organic Carbon Sequestration During The Biotic And Abiotic Transformation Of Iron Minerals

The mobility of heavy metals in the natural environments is closely related to its chemical species,which are usually affected by the iron(Fe)minerals transformation and natural organic matter(NOM).The dynamic interaction among Fe minerals,NOM,and heavy metals affects the biogeochemical cycle of organic carbon and heavy metals,and clarifying the dynamic coupled reaction mechanism among Fe minerals,NOM,and heavy metals can provide scientific basis for accurately predicting the heavy metals and NOM sequestration.Although the inhibitory effect of heavy metals or NOM on the transformation processes of Fe minerals has been studied,there is still a lack of mechanism and quantitative understanding of the coupled reaction mechanism of heavy metals and organic carbon during the Fe minerals transformation processes.In this study,the metalloid,arsenic(As),is selected to explore the microscopic mechanism of arsenic and NOM sequestration during the abiotic transformation of ferrihydrite under the impact of NOM,and the microscopic mechanism of the ferrihydrite transformation coupled with arsenic reaction in the presence of arsenic-reducing microorganisms.The results obtained this study will help us to understand the microscopic mechanisms of the heavy metal and NOM sequestration at multi-component interfaces.The major findings are as follows:(1)The effect of a typical NOM,humic acid,on the ferrihydrite transformation rate and the microscopic mechanism of arsenate and NOM sequestration during the Fe minerals transformation process were clarified.In this study,spherical correction scanning transmission electron microscopy(Cs-STEM),synchrotron radiation X-ray absorption spectroscopy(XAS),chemical extraction experiments,and stirred-flow experiments were used to explore the dynamic coupled reaction of arsenate,humic acid,and Fe minerals.The results showed that the hematite was the main transformed product at pH 10.0 and 75 ? during the abiotic transformation of ferrihydrite under anaerobic condition.Arsenate was gradually doped into the hematite lattice,while humic acid was mainly adsorbed on the hematite surface.The change rate of arsenate species was positively related to the ferrihydrite transformation rate.The humic acid adsorbed on the Fe mineral surface slowed down the ferrihydrite transformation rate and reduced the content of doped arsenate.Based on the above experimental results,a kinetic model was developed,which could be used to predict the dynamic changes of As(V)species under different components of Fe minerals and solution chemistry conditions.The results obtained in this study are helpful for us to accurately understand the ferrihydrite transformation coupled with arsenate and organic carbon sequestration.(2)The microscopic mechanism of As(V)and organic carbon sequestration during the Fe(?)induced the transformation of ferrihydrite-arsenate-fulvic acid(Fh-As(V)-FA)coprecipitates under anoxic condition was clarified.X-ray diffraction,XAS,and Cs-STEM results showed that arsenate or fulvic acid slowed down the ferrihydrite transformation rate,and led to a higher production of lepidocrocite than goethite during the coprecipitates transformation.In addition to surface adsorption,a portion of arsenate would be doped into the lepidocrocite and goethite lattice through isomorphism,and the As(V)sequestration was positively related to the content of lepidocrocite and goethite.Fulvic acid was mainly adsorbed on the goethite surface or entered into the nanopores of lepidocrocite,which may be an effective way to sequester carbon.The above results reveal a new mechanism of organic carbon and arsenate sequestration at the molecular scale and nanoscale,which provides a scientific basis for accurately predicting the geochemical cycles of metal elements and NOM.(3)The reaction mechanism of arsenate adsorption,desorption,reduction,and sequestration at the microbe-mineral interface during the dissimilatory Fe reduction process was clarified.The XAS quantitative results showed that arsenate slowed down the ferrihydrite transformation rate,which decreased with the increase of the As/Fe molar ratios.The ferrihydrite transformation products mainly consisted of magnetite with a small amount of goethite,and the magnetite production decreased with the As/Fe molar ratios.During the dissimilatory Fe reduction process,aqueous As(V)was rapidly reduced to As(?)by bacteria,and the generated As(?)can compete for the adsorption sites of As(V)on ferrihydrite,which caused the As(V)initially combined with ferrihydrite to release into solution.The desorbed As(V)was further reduced to As(?).Part of As(V)was incorporated into the magnetite lattice and As(V)sequestration was positively correlated with the formed magnetite content,while As(III)was mainly adsorbed on the Fe mineral surface.The quantitative real-time PCR results showed that the activity of bacteria was low in the initial stage of Fe minerals transformation,and then was quickly activated and reached to the peak.Subsequently,the activity of bacteria began to decrease with the decrease of aqueous As(V)concentration.Moreover,the expression of arsenate reduction genes(ars C and arr A)was promoted by the increase of As/Fe molar ratios,which was positively related to the As reduction rate.The results are helpful to understand the effects of various As/Fe molar ratios on the dissimilatory Fe reduction process and the coupled microscopic mechanisms of arsenate adsorption,desorption,reduction,and sequestration at the microbe-mineral interface.