The Ad/desorption,Redox,and Crystallization Of Metals On Typical Iron (Oxyhydr) Oxides

Iron?oxyhydr?oxides are critical components in the Earth system.Iron?oxyhydr?oxides are usually nanosized with large specific surface area and high surface reactivities;hence,they are recognized as ubiquitous geosorbents and geocatalysts,regulating the immigration,transformation,crystallization,and bioavailability of metal ions in the environment through adsorption or redox reactions.At present,a plentiful of studies provided useful insights about the interactions between iron?oxyhydr?oxides and metals.However,as the environment is quite complex,it is important to consider the effects of multiple factors?e.g.,co-existing pollutants,sunlight irradiation,etc.?on the interactions between metal ions and iron?oxyhydr?oxides,hence to understand the geochemical reactions affecting the cycle of metals.In this study,chemical experiments,in-situ spectroscopy,and electron microscope were employed to investigate the adsorption and redox reactions of metals?e.g.,Zn,Cd,Cr,Mn?on typical iron?oxyhydr?oxides as influenced by the co-existing ions and sunlight irradiation.The main understandings gained in this study are listed below:1.The adsorption characteristics and mechanisms of Zn???in the presence and absence of phosphate?P?on ferrihydrite?Fhd?were investigated.Fhd showed higher affinity to P over Zn???in the single solute reaction systems;the presence of P significantly enhanced the adsorption of Zn on Fhd,and the two ions were synergistically adsorbed on Fhd.In situ attenuated total reflectance Fourier transform infrared?ATR-FTIR?observed increased formation of outer-and inner-sphere complexes of P in the co-adsorption system.On the other side,X-ray photoelectron spectroscopy?XPS?revealed significant chemical shifts of the binding energies of Zn2p in the simultaneous adsorption systems as compared to those in the single adsorption system;whereas the binding energies of P 2p were not evidently changed;hence,the adsorbed Zn in the presence of P should both bond to the surface sites of Fhd and the adsorbed P forming ternary complexes.The synergistic adsorption of Zn and P could be attributed to the formation of ternary complexes as well as electrostatic interactions,while surface precipitation could not be completely ruled out.2.The influences of sulfate?S?and P on the interactions between Cd???and Fhd were individually explored and compared.The ability of P in enhancing the adsorption of Cd???was higher than S at anion concentrations of 2 mM.The increase of the ionic strength could weaken the adsorption of Cd on Fhd in the presence of S,but had negligible effect on that in the presence of P.In addition,the percentages of desorbed Cd???and oxyanions in P+Cd systems were smaller than those in S+Cd systems,indicating a stronger long-term immobilization of P and Cd???when they were co-existed.The results of ATR-FTIR spectroscopy in combination with 2D-COS analysis indicate that the synergistic adsorption of S and Cd???was due to electrostatic interaction and the formation of cation-bridged ternary complexes.In contrast,P formed anion-bridged ternary complexes with Cd???in most of the co-adsorption systems,in addition to the electrostatic interaction.3.The adsorption and co-adsorption behaviors of Cd???and P on Fhd,goethite?Gth?,and hematite?Hem?were compared.In the single and co-adsorption systems,Fhd had the strongest capacity in the uptake of both P and Cd???,followed by Gth and Hem.The P species adsorbed on the three minerals were varied in the single solute adsorption systems.In the co-adsorption systems,synergistic adsorption of P and Cd???on the three minerals were observed,and the adsorbed amounts of P were higher than Cd???in the co-adsorption system.The relative adsorbed amounts of P/Cd decreased and approached to 1 with the increase of surface coverage and the decrease of surface charge.The 2D-COS analysis in combination with FTIR spectra suggest that electrostatic interaction and the formation of P-bridged ternary complexes and/or surface precipitation could contribute to the synergistic adsorption of P and Cd???on the three minerals,but the contributions of these mechanisms were different on the three minerals:the formation of surface precipitation were more significant on Fhd than those on Hem and Gth,and electrostatic attraction was the main mechanism contributing to the synergistic adsorption of P and Cd on Gth.4.The redox and immobilization characteristics of Cr?Cr???or Cr????by Hem under simulated sunlight irradiation were systematically studied.Both the oxidation of Cr???and the reduction of Cr???were accelerated by Hem under sunlight irradiation,resulting in the enhanced immobilization of Cr on Hem.In the Cr???reaction systems,the adsorbed Cr???could be heterogeneously oxidized by the photo-generated holes on hematite and the produced Cr???on hematite surfaces could stimulate further adsorption of aqueous Cr???;on the other hand,aqueous Cr???could be homogeneously oxidized to highly toxic Cr???by the hydroxyl radicals yielded through interaction between O₂ and the photo-generated-electrons.At pH 5,the formation of Cr-riched surface precipitates were confirmed by TEM,which should be the surface precipitation of Cr on hematite since the suspension was under saturated.In the Cr???enriched environment,as a result of the weak interaction between Cr???and the surfaces of hematite,small amounts of Cr???were adsorbed and reduced to Cr???by accepting the photo-generated electrons in hematite.The reduction of Cr???was weak as O₂ would compete with Cr???for the electrons and reactive oxygen species generated through the interaction of O₂ with electrons would hinder the reduction of Cr???.Nevertheless,the produced Cr???were completely immobilized on hematite surfaces,and the removal of Cr increased by sunlight irradiation with the environmental toxicity being decreased.5.The surfaces of Hem,Gth,and Fhd could catalyze the oxidation of Mn???and growth of MnO_x nanofibers at room temperature under environmental atmosphere.As revealed by TEM and selected area electron diffraction?SAED?,the fibers grown in the presence of Hem and Fhd were manganite nanofibers;whereas in the presence of Gth,manganite-hausmannite core-shell structures were found in the produced fibers.The elongation direction of manganite grown in the presence of the FeO_x was[101]and was parallel to the MnO₆ octahedra chains.The heterogeneous oxidation of Mn???and surface nucleation of MnO_x on FeO_x were responsible for the growth of MnO_x fibers;meanwhile,self-catalyzed oxidation also played an role as revealed by the removal kinetics of Mn???and SEM observation.Fluffy fabric membranes consisting of MnO_x fibers and FeO_x formed on the inner surfaces of the reaction containers,which could be the possible formation pathway of MnO_x and FeO_x coatings in sediments that played critical role in the fate of chemicals in the nature.