Preparation Of Manganese-Doped Magnetic Biochar And Evaluation Of Its Adsorption Performance On Tetracycline Hydrochloride

As a country with relatively short water resources in the world,water quality safety is an important aspect of maintaining China’s water security.At present,China’s water pollution situation is still very serious.As a country with a large use of antibiotics,the ecological toxicity and human health hazards of antibiotics cannot be ignored.At the same time,as a large agricultural country,a large amount of agricultural and forestry waste must be effectively utilized.At present,biochar produced by pyrolysis to assist in the purification of water pollution is one of the high-efficiency uses of agricultural and forestry waste.However,the direct preparation of biochar is limited by the surface structure and properties,the adsorption effect on pollutants is poor,so the surface properties of biochar must be optimized to improve the adsorption capacity.In this study,typical agricultural wastes—walnut shells and Fe/Mn mixed salts were used as raw materials.The original biochar（BC）was prepared by direct pyrolysis,and the corresponding magnetic biochar composite materials（MDBC,BOBC）were further prepared by melting method and impregnation-pyrolysis method.The effects of the two preparation methods on the properties of biochar were characterized by SEM-EDS,XRD,BET,FTIR,XPS and other means.Taking tetracycline hydrochloride（TCH）as the characteristic pollutant,the effects of dosage,pH,temperature and coexisting ion factors on the adsorption effect were explored.The adsorption mechanisms of the two biochars were explored by combining isothermal adsorption,adsorption kinetics,adsorption thermodynamics,and FTIR and XPS characterization results before and after adsorption.And evaluate the reuse effect of biochar.It is hoped that it will become an efficient adsorbent for removing TCH and provide a reference for the removal of similar pollutants.It mainly has the following conclusions:（1）The surface of MDBC and BOBC is rougher than that of original biochar（BC）,and the pore structure is more developed.Compared with BC,the specific surface area increases by about 3.5 times and 8.9 times,respectively,and the average pore diameter and total pore volume are larger than BC,both the specific surface area and pore structure of biochar were improved during the modification process.The composition and ratio of main elements（C,O,Fe,Mn）on the surface of MDBC and BOBC changed,and the content of Fe and Mn elements on the surface increased significantly compared with BC,indicating that Fe/Mn elements had been successfully loaded on biochar.The crystallinity of the surface material of manganese-doped magnetic biochar is improved.Fe and Mn elements exist in different forms on the surface of the two kinds of manganese-doped magnetic biochar.Fe on the surface of MDBC mainly exists in the form of Fe₃O₄,and Mn mainly exists in the form of MnO,and MnFe₂O₄exists on the surface of BOBC.Compared with BC,MDBC and BOBC have abundant functional groups on the surface,such as C-O,C=O,-OH,-COOH,-COOR,（C）-O-（C）and M-O,etc.The saturation magnetic induction of MDBC and BOBC are 47.18 and 33.69 emu g^-1 respectively,and the two kinds of manganese-doped magnetic biochars have strong magnetism.（3）The adsorption capacity of manganese-doped magnetic biochar(MDBC 229.50 mg g^-1,BOBC 287.11 mg g^-1)on TCH was better than that of BC(95.21 mg g^-1).The adsorption process of manganese-doped magnetic biochar to TCH conforms to pseudo-second-order kinetics.Intra-particle diffusion is not the only rate-controlling step in the adsorption process.There is the influence of liquid film diffusion,and the liquid film diffusion rate is faster and occupies a dominant position.Both the Langmuir and Freundlich models can describe the adsorption process of TCH by manganese-doped magnetic biochar well.The adsorption process involves both single-layer adsorption and multi-layer complex adsorption,and the adsorption rate is controlled by both chemical and physical effects.（4）The adsorption mechanism of manganese-doped magnetic biochar on TCH is complex,mainly involving hydrogen bonding,π-πinteraction and electrostatic interaction,in addition to pore filling.And the change of the relative content of Fe/Mn oxide also indicated that it participated in the adsorption process.（5）After 5 cycles,the adsorption capacities of the two manganese-doped magnetic biochars showed a downward trend.Compared with the first adsorption capacity,the fifth adsorption capacity decreased by 30.69%and 34.41%,respectively.But in general both MDBC and BOBC have good recycling performance.