Applied Research On MOFs And Its Derivatives Activate Persulfate For Degradation Of Emerging Pollutants In Water

With the rapid development of industry,agriculture and medicine,as well as the improvement of the detection level of ultra-low concentration pollutants in the environment,a large number of emerging pollutants have been widely detected in the aquatic environment such as surface water,rivers and groundwater.The continuous input of emerging pollutants will have a serious impact on the terrestrial and aquatic ecosystems,especially the water environment of headwaters ground.Emerging pollutants are difficult to be effectively removed by traditional water treatment technology because of their indegradability.In order to solve this technical problem,the advanced oxidation processes of sulfate radical based on metal-organic frameworks（MOFs）has gradually become a research hotspot.This study focuses on the functional modification of MOFs to solve the problems of poor activation effect of persulfate（PS）,poor water stability and low utilization rate of free radicals of existing MOFs materials.The structural characteristics of MOFs and their functional derivatives were analyzed by various characterization methods,and the catalytic performance was studied by degrading the typical emerging pollutants sulfamethoxazole（SMX）,simultaneously the activation mechanism of PS and the degradation mechanism of SMX were further revealed by theoretical calculation and analysis of active species according to the structural characteristics of different MOFs and their derivatives.It provided theoretical basis for the design and optimization of heterogeneous catalysts for PS advanced oxidation processes in the future,and provides theoretical basis and technical reference for the development and engineering application of PS advanced oxidation processes.The specific study content and main conclusions are listed as follows:（1）A new trimetallic MOFs material NH₂-MOF（Fe,Cu,Mn）was assembled by means of metal doping to effectively solve the problems of unsatisfactory PS activation effect of MOFs and limited degradation efficiency improvement of bimetallic doping MOFs.The results showed that NH₂-MOF（Fe,Cu,Mn）could degrade 85.40%of SMX within 120 min.Compared with NH₂-MOF（Fe）and NH₂-MOF（Fe,Cu）,SMX removal rates were increased by 21.67%and 12.73%,respectively.NH₂-MOF（Fe,Cu,Mn）could make full use of coordinatively unsaturated metal sites of iron and doped copper and manganese,so that persulfate could produce strong oxidizing sulfate radical(SO₄^-˙)under the synergic catalysis of iron,copper and manganese to effectively degrade emerging pollutants in water.Compared with similar mono-metal MOFs and bimetallic MOFs,the catalytic degradation efficiency was improved to remove SMX quickly and effectively.NH₂-MOF（Fe,Cu,Mn）is a more excellent environment-friendly material,which has great engineering application prospect in degrading emerging pollutants in water,especially in headwaters ground.（2）A new trimetallic carbon nanomaterial C-Fe-Cu-Mn-Nx was assembled by using NH₂-MOF（Fe,Cu,Mn）as the precursor through pyrolysis in an inert gas atmosphere to solve the problem of poor water stability of MOFs.In the system of C-Fe-Cu-Mn-Nx/PS,the removal rate of SMX could reach 92.75%within 100 min.Compared with precursor NH₂-MOF（Fe,Cu,Mn）,SMX removal rate was increased by 12.45%.The dissolved metal ions of the reaction system were reduced by 62.50%,which significantly improves the water stability of NH₂-MOF（Fe,Cu,Mn）.In addition,C-Fe-Cu-Mn-Nx had good cycling performance.After 7 cycles,C-Fe-Cu-Mn-Nx still maintained good catalytic performance.Free radical quenching experiments showed that SO₄^-˙plays a major role in the process of activation of PS and degradation of SMX by C-Fe-Cu-Mn-Nx.The intermediates produced in C-Fe-Cu-Mn-Nx/PS system were identified by LC-MS,and five possible degradation pathways of SMX were proposed.（3）A new trimetallic molecularly imprinted targeted catalytic material C-Fe-Cu-Mn-Nx@MIP was assembled by using C-Fe-Cu-Mn-N as the precursor through molecular imprinting technology to solve the problem of low free radicals utilization rate of MOFs.In the C-Fe-Cu-Mn-Nx@MIP/PS system,the removal rate of SMX within 50 min was92.75%.Compared with the precursor C-Fe-Cu-Mn-Nx,SMX removal rate increased by36.01%.This was due to the fact that there are imprinting holes in C-Fe-Cu-Mn-Nx@MIP imprinting layer that can identify SMX molecules qualitatively,which can quickly recognize SMX and enrich SMX onto C-Fe-Cu-Mn-Nx@MIP surface,greatly shortening the mass transfer distance between SMX molecules and various active species.Thus,targeted and efficient degradation of SMX could be achieved.Free radical quenching experiments showed that SO₄^-˙plays a major role in the process of activation of PS and degradation of SMX by C-Fe-Cu-Mn-Nx@MIP.The formation of molecular imprinting layer could not only efficiently and quickly identify target pollutant molecules,but also produce spatial confinement effect,which effectively improves the removal efficiency of target pollutants.The intermediate products produced in C-Fe-Cu-Mn-Nx@MIP/PS system were identified by LC-MS,and five possible degradation pathways of SMX were proposed with density functional theory（DFT）calculation.Toxicity evaluation software was used to evaluate the toxicity of the intermediate products.The results showed that C-Fe-Cu-Mn-Nx@MIP/PS system could effectively reduce the toxicity of SMX and its intermediates and degrade the pollutants into safe degradation products.（4）The practical application potential of the above-mentioned newly prepared MOFs and their derivatives was explored through the actual water purification experiment.By coupling NH₂-MOF（Fe,Cu,Mn）,C-Fe-Cu-Mn-Nx and C-Fe-Cu-Mn-Nx@MIP,new ecological floating beds which can effectively remove new pollutants in the actual water body were assembled.The results indicated that NH₂-MOF（Fe,Cu,Mn）,C-Fe-Cu-Mn-Nx and C-Fe-Cu-Mn-Nx@MIP have good engineering application prospect in advanced oxidation processes.