Removal Mechanism And Performance Of The Fly Ash Based Magnetic Material For Co-pollution Aquatic System With Microplastics And Heavy Metals

Microplastics,as a kind of high-molecular polymer,have caused a series of water pollution problems due to its wide distribution,strong toxicity and small size,especially polystyrene（PS）plastic.Meanwhile,the polluted aquatic system with microplastics is often accompanied by heavy metals,especially both the Cu and Pb.Therefore,a new type of co-pollution with microplastic and heavy metal pollution is emphasized.However,how to remove microplastics and heavy metals from complex polluted water bodies has become a hot research topic in the field of environment and ecology.In view of this,a new type of magnetic porous material（magnetic material for short）was synthesized by co-precipitation based on the fly ash,and the environmental safety of heavy metals in magnetic materials was evaluated.On this basis,both the fly ash and magnetic materials are characterized to explore the changes of microstructure and functional groups.Based on the laboratory experiments and theoretical calculations,the synchronous removal effect,influence factors,and removal mechanism of PSNPs and Cu/Pb in mixed solution were discussed,and the synchronous removal of microplastics and Cu/Pb in actual domestic sewage was investigated by using magnetic materials.The results of this study will provide effective treatment methods and suggestions for the combined pollution of microplastics and heavy metals in water bodies.The main conclusions of this study are as follows:（1）It was proved that an environmentally friendly magnetic material has been successfully prepared from fly ash by a co-precipitation technology.Based on the different characterization analysis,it was found that the surface of the magnetic material is loose and porous,and a large amount of iron oxides are formed.The specific surface area of magnetic materials is increased from 1.74 m²/g to 23.4 m²/g,and the magnetism is enhanced from0.0189 emu/g to 4.23 emu/g.Meanwhile,a large number of Fe-O and Si（Al）-O functional groups is exposed.These changes in physicochemical properties of fly ash are beneficial to the removal of pollutants.Besides,the toxic leaching experiments showed that the maximum leaching concentrations of Cd,Cu,Pb,As and Hg in magnetic materials are 0.408,2.34,3.74,27 and 0.856μg/L,respectively,which were all lower than the national environmental risk values,indicating that the magnetic materials could be used for water environmental pollution treatment,and will not cause secondary pollution to the natural environment.（2）The simultaneous removal of PSNPs and Cu/Pb from mixed solution by magnetic materials was described.The batch adsorption experiments results showed that the magnetic material presented a well removal effect on the PSNPs and Cu/Pb,and the adsorption amount of pollutants in the mixed pollution is higher than that in single pollution.Based on the kinetic analysis,the pseudo-second-order was well fitted by Cu and Pb adsorption process,and the pseudo-first-order was well fitted by PSNPs adsorption behavior.For the adsorption isothermal,it was found that the Freundlich model could better fit the adsorption the single and synchronous adsorption process of Cu/Pb by magnetic materials,while Langmuir model could better fit the single adsorption behavior of magnetic materials on PSNPs.Besides,the thermodynamic and adsorption-desorption experiments results showed that the adsorption process of PSNPs and Cu/Pb by magnetic materials is spontaneous and endothermic,and magnetic materials have better service life and recycling performance.（3）The influence of environmental factors on the synchronous removal of PSNPs and Cu/Pb by magnetic materials was clarified.In the complex contaminated solution,cations(Na⁺,Ca²⁺,Fe²⁺,Fe³⁺)could enhance the removal of PSNPs by magnetic materials,while inhibiting the removal of Cu/Pb.Increasing solution pH and humic acid concentration promote the removal of Cu/Pb by magnetic materials but inhibit the removal of PSNPs.When the solution pH was 7,the magnetic material has a better removal effect on microplastics and Cu/Pb.Besides,the coexistence of inorganic matter（ammonium nitrogen,phosphate）and organic matter（methylene blue and tetracycline）will also affect the synchronous removal of PSNPs and Cu/Pb by magnetic materials.The actual water quality parameters also affect the synchronous removal ability of magnetic materials for PSNPs and Cu/Pb.（4）The potential removal mechanism of PSNPs and Cu/Pb by magnetic materials in mixed solution was revealed.The experimental results found that the adsorption behavior of magnetic materials on PSNPs and Cu/Pb in binary pollution is different.In the mixed solutions,magnetic materials adsorbs Cu/Pb through ion exchange,surface complexation,cation-πinteraction and precipitation,while the removal of PSNPs by magnetic materials are mainly electrostatic attraction,hydrogen bonding and surface complexation.In mixed solution,the presence of Cu/Pb can promote the adsorption of PSNPs by magnetic materials through ion bridging.In the simultaneous removal process of PSNPs and Cu/Pb,the interaction occurs between Cu/Pb and Si（Al）-O groups,while PSNPs mainly interact electrostatically with iron oxides.（5）The simultaneous removal efficiency of microplastics and Cu/Pb from domestic wastewater by magnetic materials was verified.The simultaneous removal of microplastics and Cu/Pb in actual domestic wastewater was studied by using magnetic materials as the filter media of the filled column.The magnetic materials achieved 94.8%,80.4%and 95.5%removal of microplastics,Cu and Pb from actual domestic wastewater,respectively.The filling heights of the materials and the flow rates of effluent significantly affect the removal of pollutants.The removal of microplastics in domestic sewage by magnetic materials is different from that in laboratory simulation experiment,which mainly due to the effects of the polymer type,shape type,abundance of microplastics and other substances.Through the evaluation of adsorption stability,economy and removal performance of microplastics and heavy metals by different processes,it was confirmed that magnetic materials have a better removal ability of microplastics and Cu/Pb in domestic sewage.To sum up,magnetic materials synthesized from fly ash could effectively remove microplastics and heavy metals from the mixed pollution aquatic system,which is mainly attributed to the multi-functional morphology and functional groups of magnetic materials.Based on the safety evaluation,removal effect,influence factors,response mechanism,and practical application of magnetic materials,the study provides a new perspective for the combined pollution of microplastics and heavy metals.