Room-temperature Synthesis And Adsorption Performance Study Of Metal-organic Frameworks And Their Aerogels Composites

With the large-scale development of urbanization and industrialization,water pollution has become increasingly severe.Therefore,it is urgent to develop efficient technology to address water pollution.Adsorption is considered one of the most promising techniques due to its simplicity,convenience,and high efficiency.Metal-organic frameworks（MOFs）,as a novel type of porous high-crystalline material,have characteristics such as ultra-high surface area,tunable pore structure,easy modification and design flexibility,making them ideal adsorbents for wastewater treatment.However,MOF synthesis often requires long reaction time,complex fabrication process,acid modulation and high energy consumption.Additionally,MOFs’s powder form and fragility pose challenges in practical applications,including difficulties in separation,shaping and secondary pollution.This study employed a room-temperature conversion strategy to tackle with the problems encountered in MOF synthesis.Two efficient MOF adsorbents（HKUST-1and NH₂-MIL-53（Fe））were synthesized with low cost,low energy consumption,and simple process conditions,using hydroxy double salts and copper-based MOF with poor stability as templates.Additionally,to overcome the limitations of MOFs as powders in practical applications,hydroxy double salts and ZnO were incorporated into alginate aerogel beads as precursors to achieve the in-situ growth of HKUST-1 and ZIF-8 into the substrate with a high loading rate at room temperature.The prepared MOFs and MOFs composites were used as adsorbents to study the adsorption performance and related mechanism on water pollutants（heavy metals,dyes and antibiotics）.The main conclusions of this study are as follows:（1）Submicron HKUST-1 has been successfully synthesized in one minute at room temperature using（Zn,Cu）hydroxy double salts as a template.The prepared HKUST-1 showed excellent adsorption performance towards Pb²⁺with maximum uptake capacity of 819.28 mg·g^-1at 298K.Besides,HKUST-1 exhibited a stronger binding affinity to Pb²⁺than the coexisting ions.The adsorption data fitted well with the pseudo-second order model（R²=0.9962）and Langmuir model（R²=0.9919）,suggesting the adsorption behavior was dominated by monolayer chemisorption.Further,the destruction of HKUST-1 crystal structure after Pb²⁺exposure and strong correlation between the released Cu²⁺and adsorbed Pb²⁺collectively confirmed the ion exchange could be the potential mechanism.Besides,the coordination interaction between Pb²⁺ and the free-standing carboxylate groups in the ligands of HKUST-1 also played an important role in the adsorption process.This work not only provides a new idea for Pb²⁺removal by unmodified MOFs based on cation substitution,but also has great implications for the construction of low-energy and low-cost MOFs for wastewater treatment.（2）NH₂-MIL-53（Fe）was synthesized through a room-temperature conversion strategy using unstable Cu（BDC-NH₂）as a template.The prepared NH₂-MIL-53（Fe）showed a significantly higher adsorption capacity and faster adsorption rate towards congo red（CR）than the other selected dyes,with more than 99%of CR being removed by NH₂-MIL-53（Fe）within 20 min.The maximum adsorption capacity of NH₂-MIL-53（Fe）for CR was 1877 mg·g^-1,which was superior to most reported adsorbents.In addition,NH₂-MIL-53（Fe）showed strong resistance to ions interference in CR removal.After six adsorption-desorption cycles,NH₂-MIL-53（Fe）displayed high removal efficiency and good reusability towards CR.Mechanistic studies suggested that the removal of CR was attributed to the synergistic effects of theπ-πstacking interactions,hydrogen bonds and electrostatic interactions between CR and NH₂-MIL-53（Fe）.（3）（Zn,Cu）HDS/Alg was prepared through one-step method and was used as precursor,which lead to fast HKUST-1 loading into alginate matrix at room temperature.The obtained HKUST-1/copper alginate aerogel beads（HKUST-1/Alg）was used to remove methylene blue（MLB）from water.This strategy allowed for the high dispersion and loading rate of HKUST-1（66.7%）and improved the stability of HKUST-1 in water.Compared with the individual components,HKUST-1 and copper alginate aerogel beads,HKUST-1/Alg showed higher adsorption capacity towards MLB,indicating good interaction between the substrate and fillers.The p H effect study showed that HKUST-1/Alg had an optimal p H value of 9 for MLB removal,and the coexistent ions could interfere with the MLB removal ability of HKUST-1/Alg to some extent.The relative adsorption data matched well with the Langmuir model and pseudo second-order model,implying that the adsorption behavior of MLB was dominated by mono-layer chemistry adsorption.The saturated adsorption capacity of HKUST-1/Alg towards MLB was 571.4 mg·g^-1,which was higher than many reported adsorbents.After 4 adsorption-desorption cycles,the adsorption capacity of MLB onto HKUST-1/Alg was more than 150 mg·g^-1,indicating good reusability.Mechanism studies showed that MLB removal by HKUST-1/Alg was mainly attributed toπ-πstacking,hydrogen bonds,coordination and electrostatic interaction.（4）ZnO/Fe₃O₄/Alg-Ca was used as precursor,which lead to the uniform and stable ZIF-8 loading into a magnetic calcium alginate aerogel substrate at room temperature.The obtained ZIF-8/Fe₃O₄/Alg-Ca effectively improved the substrate’s adsorption ability towards tetracycline（TC）and enabled efficient recovery of the adsorbent.Based on Langmuir model,the saturated adsorption capacity could reach 333.3 mg·g^-1,which was higher than many reported MOFs adsorbents.p H-dependence study showed that the optimal p H value was 4 and the coexistent ions could interfere with the TC removal by ZIF-8/Fe₃O₄/Alg-Ca.Interestingly,the relative adsorption data matched well with both pseudo first-order and pseudo second-order models,implying that the adsorption rate of TC was controlled by surface diffusion and chemistry adsorption process.After four adsorption-desorption cycles,ZIF-8/Fe₃O₄/Alg-Ca still showed good TC removal ability.Mechanism studies suggested that TC removal by ZIF-8/Fe₃O₄/Alg-Ca was mainly attributed toπ-πstacking and hydrogen bonds interaction.