Construction Of Hollow/porous Amidoxime Functionalized Adsorbent Based On A Gas In Liquid Drop Reactor And Its Selective Uranium Extraction

Controlling the use of fossil fuels and promoting the development of alternative new and clean energy are in line with the theme of coordinated development of resource development and environmental protection.Nuclear energy,as a green energy with high energy density,its wide application can greatly alleviate the energy shortage in our chuntry.The proven uranium resource content in seawater is more than 1000 times higher than that in uranium mines.Extracting uranium from seawater is a potential method to ensure long-term uranium resource supply and sustainable development of nuclear energy,and is a strategic enrichment to improve energy security.Adsorption has become one of the effective methods for extracting uranium from seawater due to its advantages of high adsorption efficiency,simple operation,low cost,and environmentally friendly.However,the adsorption method faces many challenges when extracting uranium from seawater,such as extremely low concentrations of uranium in seawater（only 3.3 ppb）and stable existence in the form of Ca₂UO₂（CO₃）₃or[UO₂（CO₃）₃]^4-,as well as a large variety and quantity of coexisting ions.Therefore,the preparation of high-performance adsorbents to achieve efficient and selective separation and enrichment of uranium in seawater is of great strategic significance and is one of the important research topics in the field of environmental science.Amidoxime functionalized adsorbents have attracted much attention due to their highly specific adsorption performance for uranum.The resulting amidoxime functionalized hollow or porous adsorbents and amidoxime functionalized hydrogel adsorbents have emerged.However,the existing functional adsorbents for uranium extraction from seawater still have the following problems unsolved:（1）The preparation and washing process of the adsorbent is complex,which is easy to cause secondary pollution and does not conform to the development concept of green chemistry;（2）The adsorption rate and capacity of the adsorbent for uranium still need to be further improved;（3）The recognition activity of adsorbents for uranium still needs to be strengthened,and the corresponding mechanisms need to be further studied.Based on this,this work focuses on the preparation of hollow or porous amidoxime functionalized adsorbents in a gas in liquid droplet reactor.Three types of adsorbents for selective uranium extraction were designed and synthesized,elucidating the mechanisms of adsorbent structure regulation and performance enhancement,providing theoretical and technical support for promoting research on uranium extraction.The main research contents of this thesis are as follows:1、Preparation of functionalized hollow adsorbents based on a gas in liquid droplet reactor induced by solvent mixing and its selective adsorption of uranium（1）Firstly,polydopamine（PDA）nanobowls were prepared using a gas in liquid droplet reactor induced by solvent mixing based on the adhesion effect and self-polymerization of dopamine.The effects of homogeneous speed,types of alcohols,amount of alcohols,and use of mixed alcohols on the morphology of PDA nanoparticles were explored.Secondly,2-amino-3,5-divinylbenzonitrile was selected as the functional monomer,and amino-amidoxime functionalized bowl shaped nano-adsorbent（NH₂-AO-PDAbawl）was prepared through photoinitiated free radical polymerization（FRP）using the photothermal nano confinement effect of PDA nanobowl and its own free radicals,and was used for selective extraction of uranium.Finally,the adsorption behavior of NH₂-AO-PDAbawl on uranium was investigated through static adsorption experiments,including the effect of p H,adsorption kinetics,adsorption equilibrium,adsorption thermodynamics,adsorption selectivity,and adsorption regeneration.The results showed that NH₂-AO-PDAbawl had the maximum adsorption capacity for uranium at a p H of 6.0,and the adsorption equilibrium could be achieved within 30 min.The adsorption kinetics data followed a pseudo-second-order kinetic model,and the Langmuir adsorption model showed that its maximum single-layer adsorption capacity for uranium was 507.0 mg/g at 298 K.In addition,NH₂-AO-PDAbawl exhibits excellent adsorption selectivity for uranium in the presence of competitive ions,and still exhibits high adsorption efficiency after 6 adsorption-desorption cycles,indicating that NH₂-AO-PDAbawl has good adsorption regeneration ability for uranium.（2）Graphite oxide（GO）nanosheets were used as carriers,and hollow templates were generated on the nanosheets in situ by the adhesion of gas in liquid droplet reactor induced by solvent mixing.Based on the self-polymerization of dopamine and the covalent bond formed by the amino group and the carboxyl group on the surface of GO,a composite material with stable structure was prepared for in situ growth of nanoparticles on nanosheet.A functionalized adsorbent（ad AO-HPDA/RGO）was prepared using Schiff base reaction and FRP,and its adsorption performance for uranium was investigated through static adsorption experiments.The results show that the adsorption process conforms to Langmuir model and pseudo-second-order model,indicating that the adsorption of uranium by ad AO-HPDA/RGO is dominated by single-layer chemisorption,and the adsorption thermodynamic results indicate that the adsorption process is a spontaneous endothermic process with entropy increase.At 298K and p H=6.0,the maximum adsorption capacity of ad AO-HPDA/RGO was 581.25mg/g within 35 min.Under simulated seawater conditions,ad AO-HPDA/RGO exhibited adsorption selectivity for uranium.After 7 adsorption-desorption cycles,the adsorption capacity of the adsorbent for uranium decreased by only 7.4%compared to the first cycle,and its structure remained unchanged,indicating that the adsorbent has good adsorption regeneration and stability.The XPS results indicate that the adsorption mechanism is the coordination between the O-containing and N-containing functional groups of the adsorbent and uranium.2、Preparation of functionalized hollow porous adsorbent based on CO₂ in water Pickering dropper reactor and its selective uranium extraction（1）Conventional methods for preparing amidoxime-based hollow porous sorbents（HPS）for uranium extraction usually use a solution or emulsion process with surfactant and expensive pore template,so they cannot meet requirements of economic and environmentally friendly evaluation.Here,hollow porous microspheres functionalized with abundant amidoxime（AO-HP-MF）are prepared via CO₂ in water Pickering dropper reactor interfacial polymerization and assembly of site chains for highly efficient uranium extraction.For this strategy,in situ generated CO₂ bubbles are utilized as a core template,and Si O₂ nanoparticles serve dual roles of an emulsion stabilizer and pore template for polymer shell.Meanwhile,assembled polyvinyl polyamine（PEA）chains provide lots of sites for post-modification with amidoxime.Taking the advantages of well-defined hollow porous morphology,uniform size,and high density of amidoxime,AO-HP-MF is expected to possess excellent adsorption capacity,fast equilibrium rate,and high selectivity.The equilibrium time and maximum adsorption capacity are 1.0 h and 162.6 mg/g at 298 K,respectively,and the monolayer chemisorption based on chelation between uranium and amidoxime is the main mechanism of selective uranium extraction.Their uniform size may be associated with the pressure-dependent formation condition of in situ generated CO₂ bubbles,which is different from traditional emulsion-templated microspheres with fairly irregular sizes.Most importantly,AO-HP-MF has remarkable selectivity for uranium,even if massive amounts of Ca²⁺,Mg²⁺,Na⁺,Zn²⁺,and other ions exist in the simulated solution.Therefore,this work not only presents a new clue to fabricating high-performance sorbents via a simple gas in liquid dropper reactor but also provides a novel platform for uranium extraction.（2）Simultaneous construction of porous and hollow adsorbent,especially from gas in water Pickering dropper reactor,is vital for improving mass transfer kinetics and uptake amount.Inspired by the formation process of stalagmites in karst cave,amino,and amidoxime bi-functionalized lotus root-type microsphere with the porous surface（NH₂@AO-PLRMS）is prepared by the silica nanoparticles stabilized CO₂-in-water Pickering dropper reactor and subsequent two-step grafting polymerization.The important roles of silica nanoparticles acting as Pickering emulsifiers,surface pore-forming agents,and adjusting internal lotus root structure are confirmed.Lotus root-type pores are dependent on the interface intensity and the permeability of compressed CO₂bubbles in the CO₂-in-water Pickering dropper reactor.Benefitting from the lotus root-type structure and abundant affinity sites,the maximum uranium adsorption capacity of NH₂@AO-PLRMS is 1214.5 mg/g at 298 K,and an ultrafast uptake process can be achieved in the first 30 min.Both thermodynamic and kinetic studies indicate a spontaneous,entropy-increased,and endothermic chemisorption process,and the synergies of amidoxime and amino groups can enhance the adsorption selectivity.Remarkably,NH₂@AO-PLRMS displays a high uranium adsorption capacity and desorption efficiency after seven cycles.3、Preparation of porous hydrogel-based adsorbent functionalized with amidoxime and its selective uranium extraction in air-in-water droplet reactor（1）A convenient design of amidoxime（AO）functionalized hydrogel-based adsorbent containing interconnected pores is critical for facilitating fast and selective uranium extraction.To address this,the AO functionalized macroporous hydrogel sorbents（GMPAO）are constructed by air-in-water high internal phase dropper reactor（HIPEs）,and used for highly efficient extraction of uranium from aqueous solution.Curled gelatin methacryloyl（Gel MA）chains,as amphiphilic Pickering emulsifier and molecular surfactant,endow self-emulsifying ability to stabilize the air/water interface.GMPAO exhibits an open-cell structure（diameter is about 80%between 200μm and300μm）with interconnecting pores（distribution is about 70%between 55μm and 75μm）,and the chain entanglement between polyamidoxime（PAO）and Gel MA enhances mechanical strength.Benefitting from the macroporous structure and abundant affinity sites,GMPAO displays a spontaneous uranium adsorption capacity of 386.6 mg/g at298 K and faster uptake within 60 min,which are superior to better than the previously reported hydrogel-based adsorbent.In addition,GMPAO still achieves the highest capacity and remarkable removal rate of almost 100%towards uranium in the presence of competitive ions,as well as excellent recyclability toward uranium capture.Except for the coordination between uranium with amidoxime groups,the carboxylates from Gel MA also cooperate as a synergistic effect for uranium uptake by charge interaction,which enhances the binding affinity of the adsorbent to uranium.Most importantly,this work demonstrates a new strategy for preparing macroporous hydrogel-based adsorbent with enough stability and provides a new perspective for recovering uranium from an aqueous solution.（2）Firstly,this work prepared the polymerizable microgel nanoparticles with the ability to stabilize the liquid/gas interface by hydrophobic modification of hydrophilic microgel and grafting double bonds,which were used as Pickering particles,and a novel porous hydrogel-based adsorbent with abundant active sites was prepared based on the air in water Pickering high internal phase droplet reactor.The porous hydrogel-based adsorbent（PAOmgel）with underwater adhesion characteristics was obtained by dopamine immersion.The underwater adhesion of the adsorbent is based on the hydrophobicity of the microgel at a higher temperature,while the microgel is hydrophilic at a lower temperature,which avoids the influence on the adsorption performance.The functionalized hydrogel was prepared by winding the polymerized chain with the PAO chain,which also was conducive to enhancing the mechanical properties of the hydrogel-based adsorbent.The prepared adsorbent can quickly（<50min）achieve high adsorption efficiency（97.8%）and high adsorption capacity（475.3mg/g）for uranium,which is much faster than the adsorption kinetic of other amidoxime functionalized hydrogel-based adsorbents.In addition,PAOmgel also exhibits high adsorption selectivity and adsorption regeneration for uranium.