Surface Modification Of Biomass-derived Porous Carbon Supported Nano-zerovalent Iron For Hexavalent Chromium Adsorption And Reduction In Water

Chromium is widely used in many industrial processes.Due to insufficient storage and improper disposal,chromium ions enter the environment and cause water pollution incidents.The hexavalent chromium has fluidity,solubility and toxicity and other hazards,which can also cause a series of human health problems.Zero-valent iron can effectively reduce hexavalent chromium,and has the characteristics of low cost and non-polluting.It has become an important method for the treatment of hexavalent chromium in water bodies.Research in recent years has found that nano-scale zero-valent iron(n ZVI)particles have small particle size,large specific surface area,and many reactive sites,which can significantly improve the efficiency of hexavalent chromium.Due to inherent magnetism and high surface energy,n ZVI particles are easily agglomerated spontaneously,which reduces their contact area with contaminants,which in turn leads to a rapid decrease in reaction activity.More critically,n ZVI easily reacts with oxygen and water,causing self-corrosion,and the formation of a passivation layer on the surface will also hinder the electron transmission between it and the target pollutants,thereby reducing the reaction life.Therefore,how to modify n ZVI to improve its stability during the reduction process and ensure its high activity and long-term performance has become a research hotspot and difficulty in n ZVI reduction technology in recent years.This paper proposes to synthesize a series of n ZVI-based composites with high reduction performance from the perspectives of n ZVI structure construction and surface modification,and study its removal performance and mechanism for hexavalent chromium.The main research contents and conclusions are as follows:(1)Using biomass as a carbon source,combined with self-foaming technology to prepare a hierarchical porous carbon support,and through a one-step vulcanization method to load the sulfide modified n ZVI on the hierarchical porous carbon surface to obtain S-n ZVI@ABC composite material.The supporting effect of graded porous carbon on n ZVI particles can reduce the agglomeration of nanoparticles and increase their dispersibility,while surface vulcanization modification can alleviate the surface passivation of n ZVI.Structural tests show that S-n ZVI@ABC has significantly improved specific surface area and porosity,and maintains a better n ZVI crystal structure.Based on the above characteristics,the removal efficiency and anti-aging ability of S-n ZVI@ABC for hexavalent chromium are far better than n ZVI and unvulcanized modified n ZVI@ABC.At the same time,the molar ratio of iron to sulfur,the dosage,the initial concentration of hexavalent chromium and the p H value of the solution will all affect the removal of hexavalent chromium.(2)Using the above graded porous carbon as a carrier,Fe Ni@ABC particles are obtained by in-situ loading of Fe-Ni bimetallic nanoparticles on the surface by a liquid-phase reduction method.The supporting effect of the hierarchical porous carbon carrier can improve the dispersibility of the iron-nickel bimetallic nanoparticles,and the metal nickel can form a galvanic cell with n ZVI to accelerate electron transfer and inhibit the formation of a passivation layer.The research results show that compared with unloaded Fe Ni nanoparticles(Fe Ni NPs),Fe Ni@ABC has significantly improved dispersibility and specific surface area,and the zero-valent iron phase can exist stably.Compared with Fe@ABC,Fe Ni NPs and Fe Ni@ABC significantly improves the removal efficiency and service life of hexavalent chromium,and the reaction activation energy is significantly reduced.Changing the amount of nickel doping,the dosage,the initial concentration of hexavalent chromium,the reaction temperature and the p H value of the solution will all affect the removal of hexavalent chromium.At the same time,it is proved that zero-valent iron,divalent iron and active hydrogen participate in the reduction of hexavalent chromium.(3)The above graded porous carbon support is impregnated with iron salt,the iron/carbon composite material is prepared by carbothermic reduction technology,the calcination process is adjusted,and an iron carbide intermediate layer is introduced in situ between the zero-valent iron and the graded porous carbon layer to obtain core-shell Fe⁰@Fe₃C@ABC composite material.The graded porous carbon support ensures the dispersibility of iron-based nanoparticles,and the iron carbide intermediate layer can protect the inner layer of Fe⁰from being oxidized,and the iron carbide layer has high conductivity and catalytic ability,which can accelerate electron transfer.The research results show that Fe⁰@Fe₃C@ABC composite material has significantly improved specific surface area and oxidation resistance,and the crystallinity of zero-valent iron is higher.The comparative experiment of hexavalent chromium reduction also shows that the reduction performance of Fe⁰@Fe₃C@ABC is far better than that of ABC-n ZVI and n ZVI.At the same time,the experimental conditions were optimized,and it was found that under acidic conditions,low initial concentration and high reaction temperature are more conducive to the removal of hexavalent chromium.After a long period of aging,Fe⁰@Fe₃C@ABC can still maintain a high ability to remove hexavalent chromium,and its reduction mechanism can be attributed to the combined action of zerovalent iron,divalent iron and active hydrogen.The multifunctional structure with the functions of reduction,catalysis,oxidation resistance and immobilization is more conducive to the preparation of n ZVI with high stability and oxidation resistance,and is more conducive to the application in the restoration of hexavalent chromium.