Facet Dependent Environmental Contaminants Removal Behaviors Of Hematite And Their Environmental Effects

Iron ranks the 4th most abundant element in the Earth's Crust and is ubiquitous in atmospheric aerosols,natural waters and soils as well as animals and plants.Iron could play an important role in the processes of the biogeochemical cycling in the environment and chemical dynamics in organisms because of its abundant existence and redox properties.Iron cycling and its coupling interactions can directly drive the geochemical cycling of some major and trace elements,such as carbon,nitrogen and sulfur,and have strong influence on the fate,transport,transformation and bioavailability of natural contaminants.Iron cycling has great significant characteristics,such as the important spatial location,functional diversity and strategic position.Thus,research of the iron cycling and its corresponding environmental implications has become the internationally cutting-edge topic and recognized as the key scientific issue to uncover the geochemical cycling processes in the Critical Zone.The key scientific issues of the iron cycling are regarded as the atomic or molecular-scale understanding of the adsorption/desorption,redox and electron transfer processes of contaminants at the surfaces/interfaces of iron(hydr)oxides and further elucidating their corresponding environmental effects.Iron oxide minerals are known to be widely used in the geochemical control and environmental remediation because of their abundant existence,environmentally benign nature and rich active sites on the solid surfaces.Among all the iron oxide minerals,hematite is the most thermodynamically stable form of iron oxides,and also environmentally friendly and abundant in nature.Therefore,the exploration of the processes about the transport and transformation of contaminants involving hematite has important environmental implications.Unfortunately,most of the previous studies have been carried out on irregular iron oxide particles to explore the contaminants transformation dynamics but remaining serious lack of efficient characterization and experimental and theoretical investigation,making it difficult to understand the fundamental adsorption/desorption,redox and electron transfer processes at the atomic or molecular scale.In this study,the adsorption/desorption,redox and electron transfer processes of model contaminants on different hematite facets,such as Rhodamine B,alachlor,chromate and uranyl,were systematically investigated by multiple spectroscopic characterizations and theoretical calculations as well as various complexation models and kinetics to clarify the intrinsic environmental implications of hematite on atomic or molecular scale.This doctoral dissertation will help us to understand the complicated elements geochemical cycling in the Critical Zone and clarify the internal relationship between iron cycling as well as their impacts on the mobility and transformation of contaminants,and also utilize iron cycling to realize geochemical control and environmental remediation.Thus,the specific research contents of this doctoral dissertation are as follows:1.The processes of iron redox involving iron oxides can strongly affect some geochemical cycling of redox-active elements and the fate and transport of model environmental contaminants,which could help us deeply understand the implications of the iron geochemical cycling.In this part,the hematite nanocrystals were synthesized via hydrothermal/solvothermal methods and employed to study the interactions between Fe(?)and hematite facets.We found that Fe(?)confined on the hematite facets could more efficiently decompose the hydrogen peroxide into hydroxyl radicals to degrade organic pollutants than the Fe(?)alone.Moreover,the hematite {110} facets exhibit better Fe(?)confining effects than the hematite {001} facets.The hydrogen peroxide transformation efficiencies were not only affected by the density of surface confined Fe(?),but also governed by the binding mode of Fe(?)on the hematite facets.We interestingly found that the polar {110} facets could confine ferrous ions of higher density with a five-coordination binding mode and thus lower the hydrogen peroxide decomposition energetic span more efficiently than the nonpolar {001} facets to confine ferrous ions with a six-coordination binding mode.Given that the reductive dissolution of iron oxides are the key process of iron geochemical cycling,the interaction of ascorbate and hematite facets was systematically investigated with attenuated total reflectance Fourier transform infrared spectroscopy,density-functional theory calculation,and kinetics model.We found that the nonprotonated inner-sphere bidentate mononuclear and monodentate mononuclear iron-ascorbate complexes were respectively formed on the hematite {001} and {012} facets.The kinetics study suggested that the bidentate mononuclear iron-ascorbate complexes on the {001} facets favored the reductive dissolution processes than the monodentate mononuclear iron-ascorbate counterparts on the{012} facets.These results revealed that the the reductive dissolution of hematite facets with ascorbate were strongly dependent on the iron-ascorbate complexes formed on the hematite facets.The two types of iron-ascorbate complexes on the hematite facets were then used to catalyze the hydrogen peroxide decomposition for the degradation of herbicide alachlor.The bidentate mononuclear iron-ascorbate complexes on the {001} facets were found to exhibit better performance than the monodentate mononuclear counterparts on the {012} facets.2.As the fate,transport and bioavailability of actinide elements in the subsurface environments are strongly influenced by its adsorption on iron minerals.In this study,we systematically investigated the molecular-scale structures of uranyl complexes formed at the interfaces of hematite and water with periodic density-functional theory calculation,synchrotron-based U L?-edge extended X-ray absorption fine structure spectroscopy,and attenuated total reflectance Fourier transform infrared spectroscopy.The combined theoretical and experimental results revealed that uranyl was complexed on three hematite facets in inner-sphere coordination,but edge-sharing bidentate mononuclear(2E)complexes were formed on {001} facets,and corner-sharing bidentate binuclear(2C)ones were on both {012} and {110} facets.Moreover,the uranyl adsorption site densities on the {012} and {110}facets of hematite were nearly the same,but significantly higher than that of {001} counterpart These results revealed that the uranyl adsorption performance of hematite facets was strongly dependent on the local coordination environments of uranyl on the hematite facets.3.The iron-bearing minerals are ubiquitous in nature,which could have strong influence on the fate and transformation of toxic heavy metal ions.In this study,we employee synchrotron-based Cr K-edge extended X-ray absorption fine structure spectroscopy,attenuated total reflectance Fourier transform infrared spectroscopy,density-functional theory calculation,and surface complexation models to investigate the adsorption process of Cr(?)on the hematite facets.We found that the inner-sphere monodentate mononuclear and bidentate binuclear chromate surface complexes were formed on the hematite {001} and {110} facets,respectively.Moreover,the site density of Cr(?)on the {001} facets of hematite was significantly lower than that of {110} counterparts.These results revealed that the Cr(?)adsorption performance of hematite facets was strongly dependent on the chromate complexes formed on the hematite facets.Meawhile,low molecular weight carboxylic acids,such as oxalate,could be adsorbed on the mineral surfaces,which can greatly affect the adsoption of chromate on hematite.We found that the monodentate mononuclear iron-oxalate surface complexes could be formed on hematite {001} facets,while the bidentate mononuclear side on iron-oxalate with five membered ring surface complexes could be formed on the hematite {012} facets.The adsorption kinetics study suggested that the monodentate mononuclear iron-oxalate surface complexes on the{001} facets favored the oxalate adsorption processes than the bidentate mononuclear side on iron-oxalate counterparts on the {012} facets.These results revealed that the adsorption of oxalate were strongly dependent on the iron-oxalate complexes formed on the hematite facets.The two types of iron-oxalate complexes on the hematite facets were then used to adsorb the chromate.The bidentate mononuclear side on iron-oxalate complexes on the {001} facets were found to exhibit better adsorption performance than monodentate mononuclear counterparts on the {012} facets.