Development Of Covalent Organic Frameworks Based Solid Phase Extraction And Its Application In The Samples Pretreatment Of Environmental Pollutants

Objectives:（1）Covalent organic frameworks（COFs）with high adsorption ability were synthesized and the COFs were applied as disperse solid phase extraction（dSPE）adsorbents for the enrichment of nitroaromatic compounds（NACs）.COFs-based HPLC was applied to the effective extraction of NACs from actual water samples.（2）Development of COFs with the property of good adsorption performance towards fluoronitrobenzenes（FNBs）,the COFs-based dSPE coupled with HPLC method was appropriated for effective extraction of FNBs in water samples.（3）Development of magnetic covalent organic frameworks（MCOFs）with the properties of excellent adsorption capacities and fast adsorption,and the MCOFs were applied as magnetic solid-phase extraction（MSPE）adsorbents for the enrichment of DPA and its analogs in actual samples.Methods:（1）By using DMSO as solvent,acetic acid as catalyst,a simple and quick one-pot synthesized method for the fabrication of COFs by using2,3,5,6-tetrafluoroterephthalaldehyde and 1,3,5-tris（4-aminophenyl）benzene as two building blocks at room temperature.The physicochemical property of the COFs were characterized by scanning electron microscopy（SEM）,energy dispersive spectrum（EDS）,fourier transform infrared spectroscopy（FT-IR）,X-ray diffraction（XRD）,the nitrogen adsorption-desorption isotherms and thermogravimetric analysis（TGA）.Further,the prepared COFs were applied as dSPE adsorbents for the adsorption of NACs（2,4-DNT,1-F-2-NTB,2-NP,2,4-F-NTB,3-NP,4-NP）and the adsorption mechanisms of NACs on the COFs were speculated.Then,by combining COFs-based dSPE with HPLC analysis,some parameters（including adsorbent dosage,extraction time,p H,ion strength,desorption solvent,desorption time and desorption frequency）which influenced the extraction efficiencies of NACs were investigated in details.Under the above optimized dSPE conditions,the COFs were applied to the determination of NACs from three kinds of actual water samples.（2）COFs-SWMU were synthesizedviaafacileapproachbyusing2,5-dihydroxy-1,4-benzenedicarboxaldehyde and 4,4’,4’’-methylidynetri-anilin as precursors,the mixture solution of mesitylene and 1,4-dioxane as solvent,acetic acid as catalyst under ambient temperature,the COFs-SWMU had good adsorption abilities for fluoronitrobenzenes（FNBs,including o-fluoronitrobenzene,m-fluoronitrobenzene,p-fluoronitrobenzene,2,4-difluoronitrobenzene,3,4-difluoronitrobenzene and 3,4-dinitrofluorobenze）.The morphology and elements of COFs-SWMU were characterized by using SEM、FT-IR、XPS、XRD、TGA and et al.The COFs-SWMU were applied as dSPE adsorbents for the adsorption of FNBs,and some parameters that affected the extraction efficiencies in dSPE process were systematically explored.Finally,combined COFs-SWMU based dSPE with HPLC-DAD analysis was applied to the effective extraction and detection of FNBs from environmental samples.（3）Core-shell structured MCOFs were synthesized by using2,5-dibromo-1,4-benzenedicarboxaldehydeand1,3,5-tris（4-aminophenyl）benzene as the building blocks and Fe₃O₄nanoparticles as the magnetic core.The MCOFs were characterized by SEM、TEM、FT-IR、XRD、VSM、TGA、XPS and et al to ensure that the MCOFs were successfully synthesized.The static and dynamic adsorption experiment was performed to investigate the adsorption abilities of DPA and its analogs（benzidine,1-naphthylamine,4-phenylphenol,and o-tolidine）on MCOFs.Finally,some parameters that affected the extraction efficiencies were optimized,then,the composite was successfully used to enrich and determine DPA in apple peel,matches and lake water.Results:（1）The COFs were successfully synthesized under room temperature.The result of SEM showed that the structure of COFs was like pellets and the average diameter was 230 nm.The results of FT-IR demonstrated that the strong peak at 1616.2 cm^-1 was ascribed to C=N stretching of imine in COFs,suggesting the successful formation of COFs via the condensation of TFA and TAPB.The Brunauer-Emmett-Teller（BET）surface area and average pore diameter of COFs were calculated 107.36 m²?g^-1and 4.07 nm,respectively.The results of TGA showed that the COFs were stable up to 350°C and about 22.36%weight-loss was observed between 350 and 700°C.The phenomenon indicated that the prepared COFs had good thermal stability.The static adsorption experiments were performed to evaluate the adsorption capacity of the COFs.It can be seen from the results that the COFs had good adsorption efficiencies for NACs,the experimental maximum adsorption capacities for 2,4-DNT,1-F-2-NTB,2-NP,2,4-F-NTB,3-NP,4-NP were calculated at 239.88,159.43,158.25,137.99,98.50 and 78.63 mg?g^-1,respectively.In comparison with the correlation coefficient（R²）in the regression analysis and the calculated maximum adsorption capacity,the Langmuir isotherm was a better model for NACs adsorption on COFs than the Freundlich isotherm.The results of dynamic adsorption experiment showed that the adsorption capacities of NACs had a rapid increase in the first 30 min,after 60 min,the adsorption almost reached equilibrium.Thereofore,60 min was selected as the optimal adsorption time.Then,extraction parameters were optimized,the results showed that the optimal adsorbent dosage was 4 mg,the optimal extraction time was 90 min,the optimal solution p H was 7.0,the optimal desorption solvent was acetonitrile,the optimal desorption time was 10 min,the optimal desorption frequency was three times.Finally,COFs-based dSPE coupled with HPLC method was used to extract and determine NACs in three kinds of water samples.The recoveries of NACs were in the range of 84.00-112.29%,the inter-day and intra-day RSDs were in the range of 2.04-4.78%and 1.76-3.56%,indicating the acceptable precision.（2）Imine-linked COFs-SWMU were successfully obtained in the mixture solvent of mesitylene and 1,4-dioxane under the catalysis of acetic acid in a short time,and the yield of COFs-SWMU was 79.75%.The result of SEM、FT-IR and XPS showed that COFs-SWMU had a typical spherical structure.A new characteristic peak at 1603.1 cm^-1 corresponding to the C=N stretching wereexistedviathecondensationof2,5-dihydroxy-1,4-benzenedicarboxaldehyde and 4,4’,4’’-methylidynetri-anilin.Furthermore,the high resolution version of N1s also demonstrating the existance of imine stretching bonds in COFs-SWMU.The results of TGA showed that the COFs-SWMU had good thermal and chemical stability.The static adsorption experiments were performed to evaluate the adsorption capacity of the COFs-SWMU,results of static adsorption experiments demonstrated that COFs-SWMU had good adsorption abilities for fluoronitrobenzenes（FNBs,including2,4-difluoronitrobenzene,3,4-difluoronitrobenzene,3,4-dinitrofluorobenze,o-fluoronitrobenzene,m-fluoronitrobenzene and p-fluoronitrobenzene）,the maximum adsorption capacities of them were 89.5,141.9,123.4,77.6,121.4 and 89.8 mg?g^-1,respectively.According to the isotherm models,the Langmuir isotherm was a better model for FNBs adsorption on COFs-SWMU than the Freundlich isotherm.The results of dynamic adsorption experiment showed that the adsorption capacities of FNBs came to equilibrium over 60 min.The optimal conditions of dSPE were as followed:the optimal adsorbent dosage was 5 mg,the optimal extraction time was 60 min,the optimal solution p H was 7.0,the optimal desorption solvent was tetrahydrofuran,the optimal desorption time was 15 min,optimal desorption frequency was three times.The COFs-SWMU were applied as dSPE adsorbents coupled with HPLC-DAD detection method was propitious to the enrichment of FNBs from environmental water and soil with good recoveries（ranging from 90.6%-109.91%）.（3）By using the mixture solvent of mesitylene and 1,4-dioxane as solvent,acetic acid as catalyst,core-shell structured MCOFs were synthesized at room temperature.The morphologies of Fe₃O₄@COFs were examined by SEM and TEM,a typical core-shell structure,with a dark Fe₃O₄ nanoparticle core and a gray COF layer,can be clearly seen.The results of FT-IR and XPS showed that imine stretching bonds were formed in COFs,suggesting the Schiff reaction was completed via the condensation of TAPB and DBDA.The BET surface area of Fe₃O₄ and Fe₃O₄@COFs were calculated as 45.13 m²?g^-1、141.94 m²?g^-1,respectively.The results of VSM showed that Fe₃O₄@COFs had good magnetic(32.49 emu?g^-1).Various characterization indicated that the Fe₃O₄@COFs were successfully prepared.The results of static adsorption experiment showed that Fe₃O₄@COFs had adsorption capacity for DPA and its analogs(The maximum adsorption capacities of DPA,1-NA,4-PP,BZ,O-TD were calculated as 246.25,95.20,85.85,107.20,123.55 mg?g^-1,respectively).Dynamic adsorption experiment showed that the Fe₃O₄@COFs had faster dynamic adsorption performance（The equilibration time was 20 min）.The MSPE extraction parameters were optimized,the results show that the optimal adsorbent dosage was 4 mg,the optimal extraction time was 20 min,the optimal solution p H was 7.0,the optimal desorption solvent was tetrahydrofuran,the optimal desorption time was 10 min,the optimal desorption frequency was three times.Hence,the Fe₃O₄@COFs was employed as the adsorbents for MSPE was used to enrich and determine DPA and its analogs in apple peel,matches and lake water.The MSPE method was successfully applied to the extraction of DPA and its analogs in actual samples,and good recoveries were obtained（ranging from79.97-122.52%）.Conclusions:（1）The COFs were prepared via one-pot method at room temperature and employed as the adsorbents for dSPE of NACs.COFs demonstrated good thermal and chemical stability,excellent reusability and good adsorption performance towards NACs.Further,the COFs were applied for extraction and determination of six NACs in actual samples,the method illustrated good precision and recoveries.It indicated that the COFs had great potential to be employed as adsorbents of dSPE to effective extract and detect NACs from complex samples.（2）The COFs-SWMU has a better selective to FNBs were successfully synthesized by using 4,4’,4’’-methylidynetri-anilin and2,5-dihydroxy-1,4-benzenedicarboxaldehyde as two building blocks.Combined with dSPE-HPLC,a fast and sensitive detection method was developed for the analysis of FNBs in actual samples.（3）The MCOFs were employed as the adsorbents for MSPE and demonstrated good adsorption performance towards DPA and its analogs,so it can be applied to enrich and determine DPA and its analogs.Combining Fe₃O₄@COFs based on MSPE with HPLC-DAD,the Fe₃O₄@COFs had great potential for the detection of trace concentrations of FNBs.Meanwhile,the preparation of the composite materials and research results provided reference for the detection of other trace compounds.