A Study On Adsorption Mechanism Of Heavy Metal Lead Ion In Water Body By Sludge-based Biochar Under Microplastics Occurrence Condition

With the continuous improvement of the domestic sewage treatment rate in our country,the effective treatment and disposal of the excess sludge is an urgent problem to be solved.The traditional sludge landfill technology occupied a lot of land,and the incineration technology produced a lot of harmful substances such as dioxins,which not only causes great harm to the environment,but also wastes a lot of energy.Therefore,there is an urgent need to explore a green,efficient,and cheap surplus sludge treatment technology.The sludge is composted and fermented and then thermally cracked to produce biochar,which can effectively realize the"reduction"and"harmlessness"of the sludge.Due to the relatively high volatile content of the sludge itself,the biochar prepared by pyrolysis and carbonization has more pore structure,which can be used to remove heavy metal ions in water bodies and achieve the purpose of"resource"utilization.In this study,the composted fermented sludge was pyrolyzed at different temperatures to produce biochar.To improve the structure of biochar and increase its removal efficiency of lead ions,potassium permanganate was used to modify the biochar.To improve the separation performance of sludge biochar,magnetic nanoparticles and biochar were coupled to give the biochar magnetic properties.The prepared biochar and its modified materials were used as adsorbents to remove lead ions in water bodies,and the focus is on the characteristics and mechanism of its removal of lead ions in water bodies.The specific research work and results of this article can be summarized into the following four aspects:In the first part,after the excess sludge was composted and anaerobic fermentation,it is pyrolyzed to produce biochar at different temperatures.The surface physical and chemical properties of the biochar were analyzed,and the adsorption performance of heavy metal lead ions was studied.The kinetics,thermodynamics,and isothermal models were used to fit and analyze experimental data and reveal the adsorption mechanism.The research results showed that the biochar prepared at a temperature of 600 degrees has a larger specific surface area.When the pH is less than 6,the adsorption amount of lead ions increases continuously with the pH value.KCl,CaCl₂,and NaCl all have an impact on the adsorption process,and CaCl₂ and NaCl have the greatest impact on the adsorption process.The pseudo-second-order kinetic model can better describe the adsorption process of three kinds of sludge biochar.Langmuir model can fit SDBC400 and SDBC600 adsorption isotherm data well,and Freundlich model can better fit SDBC800 adsorption isotherm data for lead ions.Lead ions interact with the hydroxyl and carboxyl groups on the biochar.The SiO₂ in the biochar also reacts with the lead ions.The second part uses potassium permanganate oxidation to modify the sludge biochar to increase its specific surface area and pore volume,and enhance its ability to adsorb lead ions.Explore the effects of sludge biochar dosage and solution pH on the adsorption process,use isotherms and kinetic models to fit adsorption data,and combine BET,XRD,Zeta potential,element composition and Fourier infrared analysis,etc.to characterize the results and explore the adsorption mechanism of lead ions on the surface of the material.The research results show that after potassium permanganate oxidation modification,the specific surface area of SDBC-M400 is nearly 30 times higher than that of original sludge biochar SDBC400.The pseudo-second-order kinetic model can better describe the adsorption process of SDBC-M400 and SDBC-M800 sludge biochar.The maximum adsorption capacity of SDBC-M800 is 303.19 mg g^-1.Increasing the amount of sludge biochar will increase the removal rate of Pb（Ⅱ） by sludge biochar.SDBC-M600 and SDBC-M800 sludge biochar have the best adsorption effect when the pH value is about 6-7.Increasing the background ion concentration of the adsorption environment can affect the adsorption effect of the three types of sludge biochar to a certain extent.The higher the concentration of sodium humate,the greater the adsorption capacity of SDBC-M400and SDBC-M600 on Pb（Ⅱ）,while the effect of SDBC-M800 on the adsorption of Pb（Ⅱ） ions is smaller.In the third part,the sludge biochar was activated with Zn Cl₂to improve its adsorption capacity,and then the sludge biochar was loaded with magnetic nanoparticles through the hydrothermal method to give the sludge-based biochar magnetism,to improve the separation performance of the sludge biochar and explore its effect on water bodies.Adsorption performance and mechanism of heavy metal lead ions.The research results show that the BET specific surface area and total pore volume ratio of Fe/Zn-SDBC600 are significantly higher than Fe/Zn-SDBC400.The adsorption capacity of Fe/Zn-SDBC600 is higher than that of Fe/Zn-SDBC400.When the pyrolysis temperature continues to increase,the adsorption capacity of Fe/Zn-SDBC800 will decrease to a certain extent.With the increase in the dosage of the adsorbent Fe/Zn-SDBC600,the adsorption removal rate of Pb²⁺ has been continuously improved.The pseudo-first-order kinetic model can better simulate the process of Fe/Zn-SDBC600 adsorption of Pb²⁺.The adsorption isotherm data of lead ions can be better fitted by the Langmuir model.As the pH increases from 3 to 5,the adsorption amount of lead ions also gradually increases.When 5<pH<7,the adsorption amount of lead ions slowly increases.Cl^-and HCO₃^- have a certain promoting effect on the adsorption of heavy metal Pb²⁺,while SO₄^2- has a slight inhibitory effect on the adsorption of Pb²⁺.Fe/Zn-SDBC600 has good regeneration performance.The fourth part uses microplastics collected from natural fresh water（Xiangjiang）to study the adhesion potential and adsorption mechanism of naturally aged microplastics to heavy metal Pb（Ⅱ）.On this basis,the performance and mechanism of magnetized modified sludge-based biochar activated persulfate on the removal of lead ions in microplastics were explored.The results showed that the surface of naturally aged microplastics is much rougher than that of the original microplastics,and the surface shows a fluffy and porous structure.The porous structure of the aged microplastics also provides a larger specific surface area for the microplastics.The specific surface area after aging is 8.4 m²g^-1.Infrared spectroscopy and X-ray powder diffraction（XRD）data prove that most of the naturally aged microplastics are PE in this experiment,with an organic film and SiO₂ attached to the surface.The fitting results of the adsorption isotherm model show that the R²（0.9989） of the Langmuir model is closer to 1 than the R²（0.8380）of the Freundlich model.It can be inferred that the adsorption between the microplastics and Pb（Ⅱ）after natural aging is simple Layer molecular adsorption,the maximum adsorption capacity q_m(mg g^-1)is 13.60mg g^-1.As the concentration of NaCl increases,the adsorption efficiency decreases.This result shows that the squeezing effect is stronger than the salt-out effect during the adsorption process of the naturally aged microplastics on Pb（Ⅱ）.As the concentration of NaCl increases,the advantage of the extrusion effect is gradually weakening.In the presence of HCO₃^-,the concentration of Pb（Ⅱ） drops rapidly at the beginning of the reaction.When SO₄^2- exists in the solution,the adsorption efficiency increases.The presence of HA significantly reduces the concentration of Pb（Ⅱ） in the solution,which is shown to promote the adsorption process of microplastics and Pb（Ⅱ）.The Zeta potential and adsorption capacity under different pH values indicate that the electrostatic force on the surface of the microplastics and Pb（Ⅱ）play a leading role in the adsorption process.In different water media,the main factors affecting the capture process are pH value and organic matter content.The adsorption kinetic curves of ultrapure water and rainwater are basically the same,the adsorption efficiency of surface water and tap water is higher,and the adsorption process efficiency of tap water is the highest.The organic film on the surface of the microplastics plays a vital role in the adsorption of Pb（Ⅱ）.There is a chemical bond interaction between the naturally aging microplastics and Pb（Ⅱ）.According to FTIR and XPS analysis,the two functional groups-OH and-COOH are the main functional groups that play an important role in the adsorption process.In the Fe/Zn-SDBC600+PS treatment group,the lead content on the microplastics was greatly reduced,which was 78%lower than that of the original microplastics that adsorbed Pb（Ⅱ）.This may be because Fe/Zn-SDBC600 activates PS to generate free radicals,which destroys the organic layer on the surface of the microplastic or the bonding bond between Pb（Ⅱ） and the surface of the microplastic,causing Pb（Ⅱ）to fall off the surface of the microplastic.NaCl simulated Fe/Zn-SDBC600 activated PS to remove the attached lead ions of microplastics.Me OH and TBA increased the Pb（Ⅱ） content on the surface of the microplastics to varying degrees,indicating that sulfate radicals and hydroxyl radicals were generated during the reaction,and these active substances decomposed organic matter on the surface of the microplastics.And the use of strong hydrophobic scavenger phenol to capture sulfate radicals and hydroxyl radicals,in the presence of phenol,the removal efficiency of Pb（Ⅱ）on the microplastic surface dropped from 78% to 34%,which also shows that sulfuric acid Root radicals and hydroxyl radicals play a role in the removal of Pb（Ⅱ） on the surface of microplastics.