Study On Adsorption And Immobilization Of Heavy Metals In Water And Soil By Magnetic

In recent years,the discharge of some industrial and agricultural sewage and domestic wastewater has caused surroundings polluted by heavy metals,which seriously threatened the growth of animals and plants and human health.Due to the porous structure and rich surface functional groups of activated carbon,it is often used to remediate heavy metals in water bodies and soil.However,in practical applications,there is a widespread problem that solid-liquid and solid-solid separation is difficult,and the residual adsorbent may cause secondary pollution in the environment.Therefore,on the basis of retaining the original characteristics of activated carbon,the modified activated carbon can be separated and recovered from the solution and the soil,which has important theoretical significance for the remediation of heavy metal pollution in practical application.In this study,agricultural waste biomass rape straw was used as the raw material to prepare the precursor of porous carbon.The activated carbon was obtained by pyrolyzing rape straw powder activated by phosphoric acid at 300,400,and 500?,respectively.Then,the activated carbon was magnetized by hydrothermal synthesis method to obtain magnetic activated carbon,and labeled as MAC-T?T=300?400?500??.The technologies,such as SEM,FTIR,XRD,XPS,BET and pore size distribution,magnetic induction strength and Zeta potential analysis,were carried out to analyze the physical and chemical properties of MAC-T materials.The results showed that compared with the pristine carbon?PC-300?,MAC-T materials have large specific surface area and rich oxygen-containing functional groups,and confirmed the existence of manganese ferrite and magnetic properties in the composite materials.In water experiments,to explore the removal effect of Pb???and Cd???in water by PC-300 and MAC-T?T=300?400?500??materials,the Langmuir and Freundlich models were used to fit the adsorption isotherms,and the fitting results showed that MAC-300 had the largest adsorption capacity with Pb???and Cd???,reaching 253.2mg/g and 73.3 mg/g respectively,and it had the best adsorption capacity on Pb???,and the Freundlich model could better fit the isothermal adsorption process.Meanwhile,the effects of reaction time,temperature,initial p H value and coexisting ions on the adsorption of Pb???and Cd???by MAC-300 were analyzed.The results showed that the adsorption kinetics of Pb???and Cd???on MAC-300 could be described by the pseudo-second-order kinetic model,the adsorption mainly depended on the non-uniform active sites on the surface of magnetic porous carbon materials.According to thermodynamic calculations,it could be seen that the adsorption of Pb???and Cd???by MAC-300 was a spontaneous endothermic reaction,and higher the reaction temperature was favorable for adsorption experiments.Improving the p H of the original solution could significantly increase the amount of Pb???and Cd???adsorbed by MAC-300.when high-valent ions were present in the solution would occupy the active sites of the adsorbent,and had a significant inhibitory effect on MAC-300 adsorption of Pb???and Cd???.Combined with the various characterizations of the samples before and after adsorption,magnetic activated carbon had a high adsorption capacity for Pb???and Cd???,in addition to physical adsorption,it was mainly due to electrostatic attraction and complexationon the surface of the composite materials.At the same time,since Pb???was relatively easier to combine iron oxides to form Pb Fe₆O₁₀precipitates,and Cd???was relatively easier to combine manganese oxides to form Cd Mn O₃precipitates,so there is also an ion exchange-precipitation mechanism.Furthermore,at the same initial p H value,the hydrated ionic states of lead was easier to form than the hydrated ionic states of cadmium,and the carbon-oxygen and phosphorus-oxygen functional groups had greater affinity for Pb???than Cd???.This is the two main reasons that the adsorption capacity of Pb???by MAC-300 was higher than that of Cd???.In soil remediation experiments,PC-300 and MAC-T?T=300,400,500??were added to paddy field soil for the cultivation experiments of 30 and 180 days,respectively.The Pb,Cd,Cu and Zn in the tested soil samples were analyzed by BCR three step sequential extraction procedure,and the remediation effect of the soil was estimated with the Toxicity Characteristic Leaching Procedure?TCLP?.The results showed that MAC-T?T=300,400,500??had a better effect on remediation of heavy metals in soil than PC-300,and the MAC materials obtained by pyrolysis at high temperature had a stronger ability to immobilize heavy metals in contaminated soil.With the Lengthening of the culture time of the tested materials in the soil,the remediation effect of Pb and Cd in the soil was enhanced,but the remediation effect of Cu and Zn was not significant.The remediation effect of heavy metals in RMAC-T?T=300,400,500??after using permanent magnet to separate MAC-T?T=300,400,500??in the soil was significantly improved.From the above,the MAC-T material not only immobilized heavy metals in soil,but also adsorbed different forms of heavy metals in contaminated soils.