Study On Targeted Degradation Of Aqueous Dimethyl Phthalate By Molecularly Imprinted MOFs With High Water Stability

Dimethyl phthalate(DMP),a type of refractory organic pollutant,is widely exists in water,which seriously threatens human health and water environment safety.Based on generating free radicals with strong oxidizing capability,persulfate based advanced oxidation technology(AOPs)can completely mineralize DMP.MOFs(metal organic frameworks)have shown great performance for PS activation.However,the application of MOFs in advanced oxidation is in the initial stage,and there are many problems to be solved.For example,the free radical utilization rate and selectivity are relatively low,water stability of MOFs is weak,and how would the non-target continents effect the system is uncertain.To solve the above issues,MOFs was modified by molecular imprinting to achieve targeted degradation of target pollutants DMP.Improving the water stability of MOFs by growing Si O₂ layer on its surface.On this basis,successfully synthesizing molecularly imprinted metal-organic framework with enhanced water stability,besides,the influences of operating variable and environmental factors on targeted degradation were studied.Conclusions were drawn from the above research,as follow:(1)Adsorptive and catalytic properties of an Fe-based metal-organic framework(Fe-MOF-74)have been improved by molecular imprinting technique.Hydrothermal method was used to synthesize the catalyst.Adsorption behaviors have been evaluated by the Freundlich and pseudo-second-order model.The results have shown that selective adsorption ability of the material for DMP was highly enhanced.A 1.5-fold increase in catalytic rate after being modified by molecular imprinting indicated that the selective adsorption is crucial.FI-IR measurement was employed to study the mechanism on DMP degradation,indicating that targeted recognition was mainly due to the hydrogen bonding between the ester C-O in DMP and carboxylic C-OH in MAA.Besides,there were electrostatic interactions between DMP and MIP.(2)A water stable MOF core-Si O₂ shell nanomaterial(Fe-MOF-74@Si O₂)was synthesized by a facile hydrothermal method.The Fe-MOF-74 presented a flower-like three-dimensional structure and smooth surface,with the size of 10-12?m.Si O₂ coating did not change the main size,while led to a rougher surface.Fe-MOF-74@Si O₂ had a higher surface area and pore volume than Fe-MOF-74,suggesting that pores formed in the Si O₂ layer were larger than that of pure Fe-MOF-74.Thus,the Si O₂coating would not affect the mass transfer process.According to FT-IR,the coating of the Si O₂ layer mainly occurred after 20 h,with the iron acted as a nucleation site to form Fe–O–Si chemical bond and to grow Si O₂ coating layer.The Si O₂ growth promoted bonding,which alleviated MOF dissociation and improved the catalyst water stability.Based on the XPS analysis,it was found that the redox reaction occurred in Si O₂ layer during the catalytic process.Resistance values of materials were obtained by EIS test,it was deduced that the silica layer endowed the catalyst with smaller impedance,decreased electron transfer resistance,finally,improved DMP degradation efficiency.The results showed that the Si O₂ coating enhanced the water stability and catalytic performance of the catalyst.(3)The influences of operating conditions on DMP degradation were investigated.The degradation rate of DMP improved when the addition of catalyst<0.1g.However,with further increasing Fe-MOF-74@Si O₂@MIP,lower degradation rates were observed.High temperature accelerated PS decomposition,enhanced specific adsorption efficiency and promoted electron transfer between catalyst and PS,which was beneficial to the targeted degradation of DMP.Based on the Arrhenius equation,the activation energy(Ea)of DMP degradation in Fe-MOF-74@Si O₂@MIP+PS system was evaluated to be 21.9 k J/mol.This Ea value was lower compared to that in Fe-MOF-74@Si O₂+PS system(28.7 k J/mol),suggesting the designed Fe-MOF-74@Si O₂@MIP+PS system was suitable for targeted DMP removal.Acid environment promoted the DMP degradation,while alkaline condition had negative effects.DMP degradation with the initial p H around 7,but without p H adjustment during the reaction,was also analyzed.It is found that the system can activate PS effectively under this condition,indicating the proposed system had a practical application prospect.(4)Environmental interference factors in the actual water,such as Cl^-,CO₃^2-,HCO₃^-and HA,have dual effects on DMP degradation,which were concentration dependent.The results showed that these continents involved in the initiation,propagation and termination of the free radical chain reactions,as a result,changed the species and distribution of free radicals in the system.New types of free radicals were generated in this process,which would influence the active sites of the target pollutants,degradation intermediates,pathways and efficiency.In addition,the generated new free radicals would affect the properties and catalytic activity of the catalytic sites in MOFs as well.In summary,this study improved the selectivity and water stability of MOFs advanced oxidation system,and provided theoretical guidance for the application of advanced oxidation technology in the field of water treatment.