Iron-based Nanomaterials Preparation And Its Removal Mechanism For Uranium And Chromium In Water

With the rapid development of industry,the demand of energy is increasing quickly.Nuclear energy has attracted more and more attention for the meeting of increasing energy demand and the replacement of traditional energy.However,the radioactive waste produced in nuclear power plant and other radionuclides generated in the mine process will inevitably enter into the ecological environment,and bring environmental pollution and harmful to human health.Among different kinds of radionuclides,uranium(U(?))is a common representative radioactive element with potential toxic to aquatic organism and human health.The wide use of chromium in variety of industrial activities like pigments,paints,leather tanning,stainless steel manufacturing and electroplating,has resulted in large release of Cr(?)into the natural environment and thereby results in an alarming concern to the environmental and human health issues.Radionuclides have similar properties to heavy metal ions,both of them exist as inorganic metal ions in water,and have high toxicity,persistence,and difficult in degradation.Therefore,the efficient elimination of these metal ions from water is a hot topic U(?)is usually existed as cation ions whereas Cr(?)is existed as anion ions in water.Thereby it is of great significance to develop nanomaterials with synergistic elimination of cation ions and anion ions.In addition,both U(?)and Cr(?)are oxidized ions with high valent which can be reduced to lower states with low toxicity in the presence of reductants.Therefore,it is of great significance to study the efficient removal of U(?)and Cr(?)by sorption-reduction processes.Fe has three common valence states of 0,+2 and+3,both 0 and+2 ferric ions have strong reductivity.which make them ideal materials for the reduction of U(?)and Cr(?)to U(?)and Cr(?),respectively.In addition,iron-based materials have good magnetic properties,which can be separated from aqueous solutions by magnetic separation process.Therefore,three types of iron-based nanomaterials(nanosized zero valent iron,NZVI),iron oxide(Fe3O4)and iron sulfide(Fe7S8)were selected as representative materials,and the iron-based nanocomposites were prepared by various methods from low cost carbon fly ash(CFA)and clay mineral zeolite(MCM-41).A variety of characterization methods were used to characterize the interface characteristics and composition structures of iron-based nanomaterials.The effects of time,temperature,pH and ionic strength on the removal of U(?)and Cr(?)were investigated.The micro interaction mechanisms and sorption-reduction of U(?)and Cr(?)by iron-based nanomaterials were studied systematically by the combination of characterization results of spectroscopic analysis,theoretical calculation and batch experimental results.The main results and innovations of this thesis are as follows:(1)The experimental results showed that Fe3O4,Fe3O4/CFA,NZVI/CFA and Fe7S8 had good removal abilities on U(?)ions from solutions.Among them,Fe7S8 had the highest removal ability for U(?)with 1.48mmol/g at pH=5.0 and T=298K,and then was Fe3O4/CFA>NZVI/CFA>NZVI.The removal of Cr(?)ions follows the sequences of Fe7S8>NZVI/MCM>NZVI.The maximum Cr(?)removal amount for Fe7S8 was calculated to be 9.84mmol/g at pH=5.0,T=298K.Based on the abovementioned results,the combination with fly ash and MCM-41 could increased the dispersity and activity of Fe3O4 and NZVI,and provide more active sites for the binding of the metal ions,which can effectively improve the uptake of U(?)and Cr(?)onto iron-based nanomaterials.(2)At low pH values,the removal of U(?)to Fe3O4/CFA and NZVI/CFA was significantly affected by pH and less affected by ionic strength,indicating that the sorption of U(?)on Fe3O4/CFA and NZVI/CFA were mainly dominated by inner-sphere surface complexation.At high pH values,the sorption of U(?)on Fe3O4/CFA was significantly affected by ionic strength,but the sorption of U(?)on NZVI/CFA was weakly affected by ionic strength.The results suggested that the sorption of U(?)on Fe3O4/CFA was mainly dominated by electrostatic attraction and hydrogen bonding,and the sorption on NZVI/CFA was mainly affected by electrostatic interaction and physical sorption.(3)From the results of density functional theory(DFT)calculation,the binding energy of UO22+ and UO2 towards Fe2O3 were calculated to be 2.86 eV and 5.23 eV,respectively,indicating that both U(?)and U(?)can form stable complexes with Fe3O4.Among which U(?)forms much stronger complexes than U(?),illustrating that the reduction of U(?)was helpful to improve its removal ability.All of NZVI,NZVI/CFA and Fe7S8 can reduce U(?)to U(?)efficiently and thereby improved U(?)removal effectively.The X-ray absorption near-edge structure spectroscopy(XANES)spectrum of Cr on Cr(?)-reacted NZVI/MCM was similar with that of Cr(?)reference sample,the XANES spectrum of Cr on Cr(?)-reacted MCM-41 was similar with Cr(?)reference sample,while the XANES spectrum of Cr on Cr(?)-reacted NZVI showed the co-existing Cr(?)and Cr(?)typical spectra,illustrating the fact that both NZVI and NZVI/MCM could reduce Cr(?)to Cr(?),and the sorption-reduction ability of NZVI/MCM was higher than NZVI and MCM-41.The results showed that the simultaneous sorption-reduction of high valent Cr(?)and U(?)to low valent Cr(?)and U(?)by the composites is efficient method for the elimination of U(?)and Cr(?)in U(?)and Cr(?)pollution treatment.(4)The X-ray absorption fluorescence spectroscopy(XAFS)spectra of Cr(?)-reacted analysis showed that Cr(?)on Cr(?)-reacted NZVI/MCM was coordinated with RCr-O 1.99A,RCr-Cr/Fe 3.02A and RCr-Al/Si 3.65A,indicating that MCM-41 could combine with Cr(?)and achieved the synergic removal of Cr(?)by the two components on the composites.The reduction and removal of Cr(?)by NZVI/MCM was higher than that of Cr(?)by NZVI.Similarly,the removal of U(?)by NZVI/CFA and Fe3O4/CFA also confirmed that the removal percentages of U(?)by the composites were much higher than that of U(?)by NZVI or Fe3O4.There results were related to the synergistic removal by the components of the composites.(5)Besides the simultaneous removal of U(?)and Cr(?)from aqueous solutions to the composites,the synergistic removal of the anionic Cr(?)ions and cationic U(?)ions on Fe3O4 and Fe7S8 nanomaterials were also investigated.It was found that the addition of Cr(?)could significantly improve the removal rate of U(?)by Fe3O4 and Fe7S8,and the addition of U(?)can also improve the removal rate of Cr(?)by Fe7S8.The nanomaterials could remove cationic U(?)ions and anionic Cr(?)ions simultaneously,which bring them good applications potentials in environmental pollution cleanup.(6)All of NZVI,NZVI/MCM,Fe3O4 and Fe7S8 showed the sorption-reduction ability of Cr(?),and all of NZVI,NZVI/CFA and Fe7S8 also showed the sorption-reduction ability of U(?)from aqueous solutions.The iron-based nanomaterials showed good application prospect in the reduction-sorption of U(?)and Cr(?).(7)According to the results of spectroscopy analysis,Fe(0)and Fe(?)were two main reduction components in iron-based nanomaterials,while S(-?)and S(?)were two main components during the reduction of U(?)and Cr(?)in Fe7S8.In this thesis,different kinds of iron-based nanomaterials were synthesized and characterized in detail.The synthesized iron-based nanomaterials were applied as adsorbents for the simultaneous removal of cationic U(?)ions and anionic Cr(?)ions.The mechanisms of U(?)and Cr(?)elimination from aqueous solutions on the iron-based nanomaterials were studied intensively by the combination of batch experiments,theoretical calculations and advanced spectroscopy characterization.This thesis confirmed the efficient sorption-reduction of U(?)and Cr(?)by iron-based nanomaterials,especially iron-based nanocomposites and iron sulfide nanoparticles.The iron-based nanomaterials are promising materials for the remediation of radionuclides and heavy metal ions from multiple perspectives.This thesis highlights the sorption-reduction of high valent metal ions and radionuclides to low valent states,which is an effective method for the in-situ solidification and elimination of metal ions in pollution management.