Complexes Of MOF-Fe Modified By Iron Oxides: Preparation, Characterization And Adsorption Se(Ⅳ)

Objective Metal organic framework materials（MOFs）are the ideal adsorbents with large specific surface area,high porosity and abundant active sites.Compared with small molecular adsorbents,the porosity utilization of MOFs is relatively low due to the large pore size.Iron oxides,a kind of widely used and environmentally friendly pollutant adsorbents,can be mixed into the pore structure of MOFs to improve the porosity utilization and adsorption performance of MOFs.However,the research and application of loading inorganic metal oxides into the pore structure of MOFs are very limited.In this study,the complexes of iron-based MOF（MOF-Fe）and iron oxides were prepared by growing iron oxides in the original structure of the MOF-Fe,and then the excessive selenium in the waters will be effectively removed using these adsorbents.Methods First,MOF-Fe was prepared by hydrothermal method,then MOF-Fe was dispersed in ferric chloride（FeCl₃）solution,and the temperature and proportion of Fe content were adjusted（R,R is the molar ratio of Fe in FeCl₃ and MOF-Fe）.A series of complexes of MOF-Fe doped using ferrihydrite,akaganeite or hematite were prepared in situ,which were denoted as MOF-Fe@Fer,MOF-Fe@Aka and MOF-Fe@Hem,respectively.The basic structure and properties of the samples were studied by means of X-ray diffraction（XRD）,attenuated total reflection-fourier infrared spectroscopy（ATR-FTIR）,N₂ isothermal adsorption/desorption,comprehensive thermal analysis and Zeta potential analysis.Moreover,the adsorption characteristics for Se（Ⅳ）of the samples were studied using batch adsorption experiments.Results（1）The different MOF-Fe@iron oxides were all the materials with both microporous and mesoporous.Compared with MOF-Fe,the specific surface area,total pore volume and micropore volume of the complexes decreased,but the micropore volume fraction increased.The pore volume of the complexes at 1.1,2.0 and 2.5 nm decreased significantly,but that at 1.3¹.8 nm increased.（2）The isoelectric points（IEP）of MOF-Fe,MOF-Fe@Aka_0.8,MOF-Fe@Hem_0.1.1 were 3.3,7.4 and 4.6,respectively,and the IEP of MOF-Fe@Fer_0.8was greater than 8.It can be seen that the IEP of the MOF-Fe@iron oxide complexes was significantly higher than that of MOF-Fe.（3）When the initial concentration of Se（IV）was 200 mg/L and the initial pH was 5,the adsorption capacity of MOF-Fe for Se（IV）was 57.04 mg/g.The ratio of iron oxide to MOF-Fe significantly affected the adsorption for Se（IV）by the complexes.The adsorption capacity for Se（IV）by MOF-Fe@Aka_0.8,MOF-Fe@Fer_0.8.8 and MOF-Fe@Hem_0.1.1 were 80.52,70.81 and 66.66mg/g,respectively,and the amounts were the highest among the three types of complexes.（4）Langmuir model was suitable to fitting the isothermal adsorption data for Se（Ⅳ）by MOF-Fe（R²=0.9810）,the adsorption data of MOF-Fe@Aka_0.8,MOF-Fe@Fer_0.8,MOF-Fe@Hem_0.1.1 could be well fitted by Freundlich model（R²=0.9712-0.9903）.The maximum adsorption capacity（Q_m）for Se（IV）of the four samples were 84.14,86.22,87.15 and 86.94 mg/g,respectively.（5）The adsorption process of Se（IV）by MOF-Fe@Aka_0.8,MOF-Fe@Fer_0.8.8 and MOF-Fe@Hem_0.1.1 could be described by the second-order kinetic equation（R²>0.9995）,indicating that the kinetic adsorption process of Se（IV）by three samples was affected by various factors such as physical adsorption,chemical adsorption and diffusion process.（6）With the temperature increasing,the adsorption of Se（IV）by MOF-Fe@Aka_0.8,MOF-Fe@Fer_0.8.8 and MOF-Fe@Hem_0.1.1 showed a trend of decreasing firstly and then increasing.At the temperature of 288 K,298 K and 308 K,the fitting parametersΔG^θwere less than zero,thetaΔH^θwere 2.47,4.98 and 0.72 kJ/mol,ΔS^θwere 54.72,77.17,and 56.55 J/（mol·K）.This showed that the Se（IV）adsorption thermodynamics of the three samples were spontaneous and exothermic.Conclusions MOF-Fe used as a microreactor,the complexes of MOF-Fe and iron oxides were prepared by introducing Fe³⁺into MOF-Fe,and thus the porosity utilization and stability of MOF-Fe were improved,and the excess selenium in the waters could be high efficiently removed by the adsorption of the complexes..