Study On The Removal Of Heavy Metals In Water By Starch-stabilized Iron-carbon Materials

With the rapid development of industry,the discharge of heavy metals in the environment continues to increase,resulting in increasingly serious pollution.Heavy metal pollutants are non-degradable in the environment,have the characteristics of strong mobility,high concealment,and strong toxicity.They can enter the human body through direct contact or food chain,seriously threatening the ecosystem and public health.For the removal of heavy metals in the environment,commonly used methods include chemical methods,physical methods,and biological methods.Among them,chemical methods have attracted much attention because of their high efficiency and fast speed.Nano zero-valent iron(nZVI)has high reducibility,so it is one of the hot materials in the field of chemical repair,but nZVI has shortcomings such as easy oxidation and easy agglomeration,which affects its actual repair effect.In order to overcome this problem,this paper used rice straw biochar as a carrier and soluble starch as a stabilizer to prepare modified iron-carbon materials(nZVI/SS/BC)by chemical deposition and applied it to the removal of heavy metals in water.A variety of characterization analysis methods were used to study the microscopic morphology of the material and its removal mechanism for contaminants.The main research results obtained in this paper are as follows:(1)This study showed that soluble starch can effectively improve the microstructure of nZVI/SS/BC.Firstly,soluble starch can slow down the agglomeration of nZVI particles and make them evenly loaded on rice straw biochar.Secondly,XRD images showed that soluble starch can significantly reduce the formation of iron oxides during the preparation of materials and improve the reactivity of the materials.Finally,the addition of soluble starch can increase the specific surface area of the material,thereby improving the material's adsorption capacity.(2)nZVI/SS/BC had good removal potential for Cr(?)in aqueous solution.First,the material had a good effect on the removal of Cr(?).Compared with the control group,the removal rate of Cr(VI)by nZVI/SS/BC600 was increased by 46%.This is because soluble starch effectively improves the microstructure of nZVI/SS/BC,which improves the adsorption capacity and reaction capacity of the material,and thus improves the removal rate of Cr(?).Secondly,the material can overcome the limitation of pH in the solution to the reaction.As the pH value increased from 1.0 to 10.0,the removal rate of Cr(?)was higher than 90%.Even at pH=12,the removal rate of Cr(?)still reached 70%.Finally,multiple reaction mechanisms coexist.XPS analysis before and after the reaction of the material showed that the removal process of Cr(?)by nZVI/SS/BC involved both physical adsorption and chemical reduction reactions,accompanied by co-precipitation of iron and chromium.Among them,the redox reaction is the main reason for removing Cr(VI).In addition,the material ratio and the addition amount of the composite material have a great influence on the removal rate of Cr(?).The results of this study showed that the optimal condition was the material ratio of 3:3:1 and the addition amount of 1.25 g·L-1.(3)nZVI/SS/BC had good removal potential for Pb(?)in solution.First,the material had a good effect on the removal of Pb(?).Compared with the control group,the removal rate of Pb(?)by nZVI/SS/BC500 was increased by 44%.Secondly,the material can well overcome the effect of the initial pH value in the solution on the removal of Pb(?).When the pH rised from 2 to 6,the removal rate of Pb(?)gradually increased from 63.90%to 99.92%;when it was further increased to 12,the removal rate of Pb(?)gradually decreased to 70.51%.Even at pH=12,the removal rate of Pb(?)still reached 70%.Finally,the fitting of the intra-particle diffusion model showed that the reaction process involved both surface diffusion and intra-particle diffusion,and surface diffusion was the main rate-controlling step.In addition,kinetic studies showed that pseudo-secondary kinetics can better describe the removal process of Pb(?)by composite materials,with R2 value of 0.99978;isothermal adsorption studies showed that Langmuir model can more accurately describe the adsorption and removal process of Pb(?),with R2 value of 0.99487.(4)nZVI/SS/BC had a good effect on the removal of Cu(?)in the solution.Compared with the control group,nZVI/SS/BC500 significantly improved the removal rate of Cu(?),and the removal rate was as high as 99.89%.The initial pH value of the solution can affect the removal rate of Cu(?)by nZVI/SS/BC.When the pH rised from 2 to 6,the removal rate of Cu(?)gradually increased from 48.73%to 99.10%;when the pH value further increased,the OH-in the solution will react with Cu(?)to produce precipitation of Cu(OH)2,which adhered to the surface of nZVI/SS/BC through adsorption,thereby affecting the removal of Cu(?)by nZVI/SS/BC.In addition,the removal process of Cu(?)in solution by nZVI/SS/BC was more in line with pseudo-second-order kinetics,with an R2 value of 0.99983;compared with the Freundlich model,the Langmuir model can describe the Cu(?)adsorption process more accurately,with an R2 value of 0.98728.