Preparation Of Magnetic Sludge-based Biochar And Its Adsorption Performance For Lead Ions In Wastewater

As my country’s domestic sewage treatment capacity is increasing,the remaining sludge output of municipal sewage treatment plants continues to increase,and its treatment and disposal issues have attracted much attention.The carbon-based material prepared from the surplus sludge has a relatively developed pore structure and abundant functional groups,and has good adsorption performance for heavy metal ions in water,and has a broad application prospect in the field of wastewater treatment engineering.In this paper,the remaining sludge of the sewage treatment plant is used for pyrolysis with different activators at different temperatures,and the sludge-based biochar with the best adsorption effect on simulated Pb²⁺wastewater is selected as the precursor.The Fe₃O₄ colloid is prepared by the co-precipitation method.The sludge-based biochar is magnetized to prepare a magnetic sludge-based biochar that has excellent Pb²⁺adsorption performance and can be recycled.According to the adsorption effect of sludge-based biochar on Pb²⁺,the influence of sludge-based carbon material preparation,activation,magnetization and other processes on the adsorption performance of heavy metals was determined,and the preparation conditions were optimized;the prepared material was investigated The adsorption isotherm,kinetic equation and adsorption mechanism of Pb²⁺in water;through the comparison test of adsorption performance between simulated wastewater and actual wastewater,it provides theoretical and technical guidance for the practical application of the prepared carbon-based functional materials.ZnCl₂ plays the role of dehydrating agent and tar inhibitor in the activation process,which promotes the aromatization of the carbon skeleton.With the volatilization of ZnCl₂,the biochar produced a more porous structure.In the pyrolysis process of ZnCl₂biochar,the biochar prepared by activation of 5mol/L ZnCl₂ at 450℃has the best adsorption performance for Pb²⁺.Therefore,this condition was selected as the precursor carrier for the later preparation of Fe₃O₄-BC-ZnCl₂ in this experiment.The best experimental conditions for the adsorption performance of Pb²⁺on Fe₃O₄-BC-ZnCl₂ prepared by activated pyrolysis biochar（BC-ZnCl₂）after negative magnetization are:negative magnetism,0.09g/g;Water bath temperature for negative magnetism 80℃;Stirring time,30min;Ammonia dosage,5ml.The main influencing factors of Fe₃O₄-BC-ZnCl₂ adsorption on Pb²⁺include adsorption time,system temperature,system p H,initial concentration of Pb²⁺and dosage of Fe₃O₄-BC-ZnCl₂.When the initial concentration was 250 mg/L,the temperature was 50℃,p H was 4,and the dosage was 0.2 g,the adsorption of Fe₃O₄-BC-ZnCl₂ for Pb²⁺was reached in 4 H,the adsorption capacity of Fe₃O₄-BC-ZnCl₂ was 59.36 mg/g,and the removal rate of Pb²⁺was 95.25%.With the increase of initial concentration,the adsorption capacity increased continuously,while the removal rate decreased due to the decrease of available active sites.With the increase of initial concentration,the adsorption capacity increased,while the removal rate decreased due to the decrease of available active sites.When the initial concentration of Pb²⁺was 400mg/L,the adsorption of Pb²⁺by Fe₃O₄-BC-ZnCl₂ was almost saturated,and the adsorption capacity and removal rate were 96.31mg/g and 94.87%,respectively.With the increase of Fe₃O₄-BC-ZnCl₂ dosage,the adsorption capacity of Fe₃O₄-BC-ZnCl₂ on Pb²⁺decreased gradually,while the removal rate increased,when the dosage was 0.3g,the removal rate could reach 99.9%.After a series of characterization tests,it was found that the specific surface area of Fe₃O₄-BC-ZnCl₂after negative magnetic activation was 291.49m²/g,which was significantly improved compared with BC-ZnCl₂ and BC,thus providing more adsorption sites.In the preparation process of the Fe₃O₄-BC-ZnCl₂,reducing groups on the surface are converted into carboxyl groups or carbon-oxygen double bonds,so that the surface of the Fe₃O₄-BC-ZnCl₂ is rich in hydroxyl groups,hydroxyl groups and carbon-oxygen double bonds.The saturation magnetization of Fe₃O₄-BC-ZnCl₂ is 6.398 emu/g,and the magnetization of Fe₃O₄-BC-ZnCl₂ is 4.966 emu/g after desorption,which satisfies the recovery conditions from the aqueous solution with an external magnetic field.Biochar with negative magnetic BC showed a Fe peak and a Pb peak after adsorption of Pb²⁺,indicating that Fe₃O₄-BC-ZnCl₂ could successfully adsorb Pb²⁺.The adsorption isotherm model and kinetic model fitting of the three carbon-based materials showed that the adsorption of Pb²⁺by Fe₃O₄-BC-ZnCl₂ accorded with Langmuir isotherm model and quasi-second-order kinetic model.The main adsorption process was surface monolayer chemisorption,and the maximum adsorption capacities of Fe₃O₄-BC-ZnCl₂,BC-ZnCl₂ and BC were 98.65 mg/g,59.93 mg/g and 55.93 mg/g,respectively.The adsorption capacity of Fe₃O₄-BC-ZnCl₂ is higher than that of BC-ZnCl₂ and BC,and it also has a certain adsorption capacity for Cu²⁺,Cd²⁺and Cr⁶⁺.In the separate wastewater containing metal ions,the adsorption capacity of Fe₃O₄-BC-ZnCl₂ for Cu²⁺was 22.86mg/g,and the removal efficiency was 96.85%.The adsorption capacity of Cd²⁺was 6.86mg/g,with a removal efficiency of 31.69%.The adsorption capacity of Cr⁶⁺was 11.95mg/g,with a removal efficiency of 47%.In the mixed wastewater containing Pb²⁺,Cu²⁺and Cd^2+,the adsorption capacity is 61.01mg/g,23.55mg/g and9.98mg/g respectively,and the removal efficiency could reach 97.89%,99.75%and46.08%respectively.The leaching rate could be reached in 7%HNO₃ when Pb²⁺is adsorbed to saturation by Fe₃O₄-BC-ZnCl₂,the leaching amount was 58.13mg/g,and the leaching efficiency was 95.84%.Furthermore,Fe₃O₄-BC-ZnCl₂was adsorbed by four cycles of Pb²⁺,and the adsorption capacities were 59.64mg/g,54.71mg/g,51.17mg/g,42.81mg/g,respectively.After four cycles of adsorption,the adsorption capacity of Fe₃O₄-BC-ZnCl₂ decreased gradually.Nevertheless,the fourth adsorption capacity could still reach 42.81mg/g,and the removal efficiency was 68.69%,explaining the magnetic sludge-based biochar has multiple recycling and stable adsorption performance,could be used as a regenerated adsorbent,which had certain economic benefits.