Construction Of Heterostructured Iron-based Hydrotalcite-like Materials And Its Multi-stage Recycling Treatment Of Wastewater

With the development of chemical industry,large amount of industrial wastewater was produced.It is of great significance to develop new photocatalyst materials and treat various kinds of wastewater by improving photocatalysis efficiency for multistage recycling and reducing photocatalysis operation cost.The divalent and trivalent metals in the the layered structure of iron-based hydrotalcite-like material(Fe³⁺-LDHs)can be mixed on the atomic level,making the metal active centers more evenly dispersed.Isomerized Fe³⁺-LDHs can make up for the defects of large band gap of single metal oxide and easy recombination of holes and electrons.Through the synergistic combination of efficient adsorption of catalyst and photocatalysis,phenol containing wastewater and organic dye（methyl orange）wastewater can be degraded,and fluoride ions in wastewater can be adsorbed in stages.Magnetic Fe³⁺-LDHs is convenient to to reuse,and it can be recycled through the memory effect of LDHs,so as to achieve the purpose of multi-level recycling of Fe³⁺-LDHs.However,Fe³⁺-LDHs directly prepared by conventional methods have the defects of low crystallinity,no magnetism and inability of regeneration.These drawbacks greatly hinder the application of Fe³⁺-LDHs in wastewater treatment.The sodium fluoride were used as a complexing agent for complexing with Fe³⁺,which can regulate the p H value of Fe³⁺precipitation,and further optimized the matching and selectivity of the fluoride ion with Fe³⁺and transition metal ions,which was found to be effective to guide the synthesis of three-dimensional porous magneticγ-Fe₂O₃-Cd-Ni-Fe-LDHs.The processes of construction and regulation,and the formation mechanism of this new material were systematically studied.In order to enlarge the specific surface area,improve efficiency of the separation and transfer between the carriers and charges,enhance the adsorption of fluorine to achieve the purpose of multi-stage recycling at the same time,ultrathin Ni-Al-LDHs was supported on the surface ofγ-Fe₂O₃-Cd-Ni-Fe-LDHs by homogeneous precipitation.Six kinds of Ni-Al-LDH/γ-Fe₂O₃-Cd-Ni-Fe-LDHs composites（S1-S6）were fabricated with three dimentional heterostructure and different ratio by controlling reaction conditions,using homogeneous precipitation technology and spatial restriction effect.The following effects of the loading mass of the Ni-Al-LDHs on the substrate ofγ-Fe₂O₃-Cd-Ni-Fe-LDHs were investigated,including,the specific surface area,the magnetic properties,and forbidden band width of the composites.Both phenol and methyl orange were degraded by the Ni-Al-LDHs/γ-Fe₂O₃-Cd-Ni-Fe-LDHs（S3）with a larger specific surface area,lower forbidden band width and certain magnetic recycling ability for multi-stage recycling research.Afterwards,through calcination and topological transformation,the as-prepared compositescould adsorbed fluoride ions in the wastewater,and was regenerated under alkaline conditions,so as to realize the multi-stage recycling.Finally,the photocatalytic mechanism and F^-ions adsorption mechanism of the S3 material are revealed.（1）CdCl₂·H₂O,Ni Cl₂·6H₂O,Fe Cl₃·6H₂O were used as raw materials,and the molar ratio of Cd,Ni,and Fe was 1:6:2.Theγ-Fe₂O₃-Cd-Ni-Fe-LDHs material with its defined magnetic（given byγ-Fe₂O₃）properties was prepared by a homogeneous precipitation technique assisted by fluoride ions as complexing agent.,As a complexing agent,fluoride ions ions play an important role in the crystallinity ofγ-Fe₂O₃-Cd-Ni-Fe-LDHs,since the p H value of Fe³⁺deposition can effectively increase by adjusting the concentration of fluoride ions.Meanwhile,fluoride ions can also make Fe³⁺close to divalent metal ions(Ni²⁺,Cd²⁺),which was favorable for the formation ofγ-Fe₂O₃-Cd-Ni-Fe-LDHs material with high crystallinity.The crystallinity ofγ-Fe₂O₃-Cd-Ni-Fe-LDHs was the highest when the optimal amount of fluoride ions was between 0.6 and 1.8 MM.The amount of the magneticγ-Fe₂O₃ particles on the surface of the LDHs was gradually increased with an increase in the total metal ion concentration from 22.5to 225 mmol/L.This increase of fluoride ions concentration caused an enlargement of the raster-shaped holes on the LDHs surface due to conversion of the octahedral Fe（OH）₆ to theγ-Fe₂O₃ particles in the host layers of the LDHs.Theγ-Fe₂O₃/Cd-Ni-Fe-LDHs with sheet-like structure（6-hour product）and grating structure（24-hour product）was obtained by controlling the reaction time.（2）The formation mechanism of the porousγ-Fe₂O₃/Cd-Ni-Fe-LDHs materials was studied based on experimental results.In the initial stage of the reaction,the majority of Fe³⁺ions underwent hydrolysis（due to an p H）increased,and generated the amorphous,filamentous Fe（OH）₃.A small portion of iron cations transformed intoγ-Fe₂O₃（magnetic）particles,and this process was induced by the Cd²⁺ions.A portion of the iron ions and fluoride ions formed[Fe F₆]^3-complexes,which resulted in an increased deposition p H of Fe³⁺ions that promoted the co-precipitation of other metal ions,and the formation of the LDHs.The slow hydrolysis of urea gradually increased the p H of the solution,which resulted in a stepwise reaction of[Fe F₆]^3-with Ni²⁺and Cd²⁺,and the formation of large plate-like Cd-Ni-Fe-LDHs.The dispersity and quantity of the heterogeneousγ-Fe₂O₃ was regulated by adjusting the amount of Cd²⁺.At higher p H value,octahedral Fe（OH）₆ in LDHs layer was partially transformed intoγ-Fe₂O₃ also leads to the LDHs with raster-shaped morphology,making moreγ-Fe₂O₃ particles disperse on the Cd-Ni-Fe-LDHs surface and give them the magnetic properties.（3）Three-dimensional（3D）Ni-Al-LDHs/γ-Fe₂O₃-Cd-Ni-Fe-LDHs structures（S1–S6）with defined magnetic properties,and different（γ-Fe₂O₃-Cd-Ni-Fe-LDH s）/(Ni²⁺-Al³⁺-LDHs)ratios were prepared.The microstructure,crystallinity,specific surface area,the forbidden band width,and the saturation magnetic field strength of Ni-Al-LDH/γ-Fe₂O₃-Cd-Ni-Fe-LDHs composites were controlled by adjusting the relative ratios of Ni-Al-LDHs sandγ-Fe₂O₃/Cd-Ni-Fe-LDHs,which eventually achieved precise control of material properties.With an increase in the dosage ofγ-Fe₂O₃/Cd-Ni-Fe-LDHs,the Ni-Al-LDHs on theγ-Fe₂O₃/Cd-Ni-Fe-LDHs matrix gradually decreased,which was effective to the growth of Ni-Al-LDHs.With an increase in the relative proportion of the Ni-Al-LDHs,the average pore size and the magnetic field of the composites gradually decreased,while the specific surface area increased.Although the increased loading of the Ni-Al-LDHs increased the specific surface area of the composites,too much loading of Ni-Al-LDHs led to a decrease in the saturation magnetization of the composite products.Due to this,the separation of the materials could not be achieved by the magnetic field,thus hindering the recycling.When Ni²⁺and Al³⁺concentrations were 37.5 and 12.5 mmol/L,respectively,and the dosage ofγ-Fe₂O₃/Cd-Ni-Fe-LDHs was 1.0 g/L,the prepared Ni-Al-LDHs/γ-Fe₂O₃-Cd-Ni-Fe-LDHs（S3）composite had a large specific surface area（164 m²/g）,a low forbidden band width（1.60 e V）,and a suitable saturation magnetic field strength,which is suitable for multi-stage wastewater treatment.（4）In the multi-stage utilization process,S3 could effectively degraded phenol,methyl orange and adsorbed fluoride ions subsequently.When the amount of S3 was 0.3 g/L,the degradation rate of the phenol reached 98.9%,further,S3 was used to degrade the methyl orange after degrading phenol when the S3 was magnetically recovered.On this aspect of degradation,previous photocatalytic kinetics studies have shown that the methyl orange degradation reaction is the first-order kinetics reaction with a large rate constant k（0.3185/min）,which indicates that S3 has higher photocatalytic activity.During the S3 adsorption of the fluoride ions,an increase in the calcination temperature,the adsorption amount of S3material to fluoride ions also increased.When the calcination temperature was 300°C,the fluoride ions adsorption capacity of the S3 was the greatest（49.25 mg/g）.During the 10consecutive photodegradation cycles（methyl orange）,present study the activity of the material for degrading organic compounds did not decrease（in fact,decreased only by 1%）,and the photocatalytic performance of the material basically remained unchanged.During the regeneration of material S3,fluoride ions were not completely desorbed,and spinel was irreversibly formed during the repeated calcination,which affected the subsequent adsorption of the fluoride ions.The fluoride ions removal rate of this material could also reach 89.5%after10 cycles.These experimental data sufficiently demonstrated that material S3 many merits including overall chemically stable,easy separation,and excellent reusability,which all make it promising material for fluoride ions removal.The recycling study of the composite material showed that changing the Ni/Al ratio did not affect the recycling ability of the S3 to degrade the methyl orange,but this ratio had an important effect on the surface adsorption sites of the composites.When the ratio was 2 of the composites,the fluoride ions removal rate decreased the most（the decrease was 19.3%）during the 10 cycles,while when the ratio was 3,the decrease of the fluorine removal rate of the composites was the least（the decrease was 8.6%）.The kinetic study of material S3 adsorbing fluoride ions showed that the kinetic equation of the adsorption was the pseudo-second-order kinetic equation/reaction,and the activation energy of the reaction was Ea=25.571 k J/mol,which showed that the adsorption type of S3 adsorbent was of physical and chemical adsorption type.The experimental data had proved that S3 has good chemical stability,easy separation and good reusability,which was a new material for wastewater treatment.（5）The photocatalytic mechanism in this study has shown that:the recombination of Ni-Al-LDHs andγ-Fe₂O₃-Cd-Ni-Fe-LDHs enhanced the photoinduced electron-hole separation and transfer,and significantly improved the charge transfer efficiency.Due to the existence of oxygen vacancies（VOs）in the composite material,and the formation of S-type heterojunction betweenγ-Fe₂O₃-Cd-Ni-Fe-LDHs and Ni-Al-LDHs,a charge transfer of S-type heterojunction existed at the interface of theγ-Fe₂O₃-Cd-Ni-Fe-LDHs and Ni-Al-LDHs.The photogenerated electrons were transferred from the CB of Ni-Al-LDHs to the VB ofγ-Fe₂O₃-Cd-Ni-Fe-LDHs,which suppressed the fast recombination rate,and thus prolonging the carrier lifetime.Therefore,the photocatalytic activity could be enhanced.It is worth mentioning that the obtained OVs material in S3 acted as the bridge between the CB of Ni-Al-LDHs and the VB ofγ-Fe₂O₃-Cd-Ni-Fe-LDHs which furtherly accelerated the electron transfer rate.（6）This study on the adsorption mechanism of fluorine has shown that:LDHs compound（S3）is a layer of positively charged anion compound,which has a strong adsorption performance for fluoride ions.In the experimental cycles,Fe,Al,Ni,and Cd were evenly distributed on the LDHs layer.These metal ions also formed internal complexation with fluoride ions.Among them,the contribution of aluminum hydroxide,iron hydroxide,and nickel hydroxide was 66.85%,30.40 and 2.75%,respectively.Thus,on these bases,chemisorption is an important process for S3 to adsorb fluoride ions.According to the analysis results of NMR and XPS data,there are at least four kinds with a total of seven adsorption fluoride ion sites of Ni-Al-LDHs/γ-Fe₂O₃-Cd-Ni-Fe-LDHs composites.75 Figures,16 Tables,and 202 references are contained in this thesis.