Structure Mediation Of Iron-based Nanomaterials For Highly Efficient Low Concentration Of Arsenic Adsorption And Mechanism

Arsenic and its compounds are more toxic and widespread in nature.Excessive intake and long-term continuous exposure can lead to acute and chronic poisoning and even cancer,respectively,which cause serious damage to human health.According to statistics,the concentration of arsenic in groundwater of china and other countrie ranges from 0.01 to 10mg·L^-1,which is a low-concentration pollution problem.This has prompted us to put forward higher requirements for future groundwater arsenic removal technology as a source of drinking water.Generally,arsenic pollution in water generally possesses two different valence forms of inorganic arsenic(As(?)and As(?)),it is very urgent to develop high-efficiency inorganic arsenic removal technology.At present,adsorption technology has been considered to be the most economical,efficient and suitable water purification technology for arsenic pollution.Due to the improved requirements of the country's standard,traditional adsorption materials can no longer meet the treatment standards for inorganic arsenic.Hence,it is necessary to add pre-oxidation technology to solve these problems,but it brings the operation difficulty and increased processing cost.In this thesis,we selected respectively inorganic As(?)and As(?)as target pollutants,synthesizing and characterizing a series of iron-based nanomaterials with highly efficient adsorptive performance for removing As-contained pollutants.Based on the advantage that the materials have two-component synergistic effect,the use of adsorption technology to achieve efficient removal of low concentration arsenic,and on this basis,the synergistic enhancement mechanism of adsorption by using iron-based nanomaterials to remove arsenic pollutants is further explored.At last,we hope to provide theoretical basis and technical support for iron-based nanomaterials in the applications of arsenic pollution treatment and environmental remediation.Based on this,the thesis carried out the following researches:(1)Through the solvothermal reduction method,the monodisperse and highly ordered manganese ferrite spinel(Mn Fe2O4)nanomaterial was prepared to achieve the efficient removal of low concentration As(?)in water.The adsorption results showed that Mn Fe2O4 exhibited higher adsorption capacity(20.79 mg g^-1)and adsorption affinity(KL 1.11 L·g^-1)for low concentrations of As(?).Comparing to Mn Fe2O4,The adsorption effect and affinity of its isomorphic Mg Fe2O4 for low concentration As(?)were significantly reduced.Through systematic experimental research,and combined with X-ray photoelectron spectrum(XPS)and X-ray absorption fine structure(EXAFS)analysis,the results showed that the doping of Mn element increases the number of hydroxyl groups on the surface of the material.In addition,on the basis of the original As-Fe bidentate binuclear coordination form,the monodentate mononuclear coordination mode between As-Mn was found.It was revealed for the first time that the coordination of the surface hydroxyl group formed by metal atoms of M-OH to As was a key factor that determined the performance of the entire adsorption reaction,and showed the low concentration of As(?)efficient adsorption depending on metal atoms.(2)A series of manganese ferrite spinel adsorption materials with different Mn contents(Mnx Fe3-x O4)were constructed by adjusting the Mn/Fe doping ratio based on solvothermal method,and researching deeply the influence of Mn content and its distribution in the Mnx Fe3-x O4 matrix for the adsorption performance of As(?).XRD refinement results showed that Mn was simultaneously doped into the tetrahedral and octahedral sites in the spinel structure to form manganese ferrite nanomaterial with a mixed spinel structure,and as the doped content of Mn increased,the material gradually increases in size.When x<1.8,Mn atoms were mainly distributed in the center of spinel octahedron;but when x>1.8,the structure of the manganese ferrite spinel was reversed,and Mn gradually diffused into the tetrahedron.The results from adsorption experiments showed that for Mnx Fe3-x O4 with different Mn content,the material showed the best adsorption performance for As(?)when x=1.8.According to the fitting calculation result of Langmuir,the maximum theoretical As(?)adsorption capacity of Mn1.8Fe1.2O4was 7.86 mg g^-1 when the original concentration of As(?)was 10 mg L^-1 and the adsorbent dosage was 1 g L^-1.XPS results showed that the main reason of that Mn atom doping enhanced the ability of spinel to adsorb As(?)was to increase the content of hydroxyl groups on the surface of the materials.Based on the EXAFS result,As(?)-adsorbed Mn1.8Fe1.2O4 simultaneously formed a complexes As-Fe bidentate binuclear corner-sharing(²C)complexes and As-Mn monodentate corner-sharing(¹V)complexes.(3)As is well known,comparing to inorganic As(?),some unique properties such as more toxic,easier to migrate,difficult to regulate are existed in inorganic As(?)pollutions.In this chapter,a new sulfide-modified nanoscale zero-valent iron(namely S-n ZVI)was prepared to remove toxic As(?)in water by one-step and two-step methods.The structure and chemical properties of elements in materials were characterized by using XRD and inductively coupled plasma emission spectrometer(ICP)methods and et al.The distribution of sulfuric ion species(S^2-,S2^2-and S0)at different etching depths and materials surface in S-n ZVI-1 and S-n ZVI-2prepared with different synthetic methods was characterized by XPS technology,which was further used to select S-n ZVI nanomaterials with the best removal performance on As(?).On this basis,the effect of amorphous phase and crystal phase structure on S-n ZVI crystal structure and sulfur distribution was investigated,which was beneficial for improving the understanding of how these properties affect As(?)removal reactivity and selectivity.Through the XPS and XRD structure analysis of the As-adsorbed material,and combined with the determination of the valence state of the residual arsenic in the adsorbents after the reaction,it confirmed that As(?)was underwent the oxidation process during the adsorption process.The mechanism of S-n ZVI's adsorption and oxidation of arsenic was confirmed that the active radicals generated during the reaction process of oxidize As(?)to As(?)according to the free radical shielding experiment.