The Application Of Zero-Valent Iron-Coconut Shell Activated Carbon In The Removal Of Cr(Ⅵ) In Groundwater By Permeable Reactive Barrier Technology

Chromium（Cr）is widely used in electroplating and printing industries due to its corrosion resistance.However,due to improper treatment and discharge,a large amount of chromium-containing waste is discharged into soil and water,causing serious environmental pollution.Hexavalent chromium（Cr（Ⅵ））is a common form of chromium discharged into the natural environment.It has strong toxicity and carcinogenicity to the human body.Therefore,it is urgent to control Cr（Ⅵ）pollution in groundwater.The permeable reactive barrier technology（PRB）with zero-valent iron as the reaction medium is an in-situ and efficient repair technology.However,due to its easy passivation and blockage,it usually needs to be modified or mixed with other substances.Zero-valent iron and activated carbon can usually be mixed to remove Cr（Ⅵ）pollution in PRB.However,the optimal proportion of zero-valent iron and activated carbon mixed materials used in PRB to remove Cr（Ⅵ）is not very clear,and the removal effect and effective operation time in different aquifers are also lack of quantitative analysis.These problems are very important for the application of mixed materials in PRB to repair actual contaminated sites.The purpose of this paper is to study the removal effect of Cr（Ⅵ）by the mixture of zero-valent iron and coconut shell activated carbon with different particle sizes as PRB filling materials in aquifers,as well as the optimal ratio and influencing factors of the mixed material for removing Cr（Ⅵ）.In this paper,three kinds of zero-valent iron and coconut shell activated carbon with particle sizes of 0.1-0.15 mm,0.4-0.8 mm,and1.0-2.5 mm were selected as research materials.The effects of mixed materials of zero-valent iron and coconut shell activated carbon with different particle sizes on the removal of Cr（Ⅵ）in different aquifer media were studied by static and dynamic experiments.The specific research conclusions are as follows:（1）When the mass ratio of zero-valent iron to coconut shell activated carbon is6:1,2:1,the removal rate of Cr（Ⅵ）by mixed materials of zero-valent iron and coconut shell activated carbon with different particle sizes is higher than that of zero-valent iron or coconut shell activated carbon alone.When the particle size of the mixed material is0.4-0.8 mm and the mass ratio of iron to carbon is 6:1,the removal efficiency of Cr（Ⅵ）is the best,up to 100%.It was found that the primary battery was formed into the removal of Cr（Ⅵ）by the mixed material,and the removal mechanism mainly included adsorption,reduction and coprecipitation.The adsorption of Cr（Ⅵ）on the mixed material conforms to the Langmuir isotherm adsorption.The removal of Cr（Ⅵ）conformed to the quasi-secondary kinetic model.（2）It was found that the removal efficiency of Cr（Ⅵ）by mixed materials was affected by the initial Cr（Ⅵ）concentration and p H of the reaction solution and the dosage of mixed materials.The temperature of the reaction system has a certain influence on the removal efficiency of Cr（Ⅵ）by the mixed material,but it is not significant.Ca²⁺、Mg⁺、Cl^-、NO₃^-、HCO₃^-,these five different anions and cations will inhibit the removal of Cr（Ⅵ）by the mixed material.Among them,Ca²⁺、Mg⁺and Cl^-have relatively small inhibitory effects,while NO₃^-and HCO₃^-have strong inhibitory effects.Therefore,the mixed material is not suitable for alkaline or groundwater environments containing a large amount of NO₃^-and HCO₃^-.（3）The dynamic simulation column study of mixed materials with different iron-carbon massed ratios as filling media showed that Cr（Ⅵ）had a good removal effect in the early stage,but the removal rates decreased after a period of time.When the Cr（Ⅵ）concentration of the solution was 20 mg/L and the p H was 6.0,the column filling material with a mass ratio of iron to carbon of 6:1 had the longest time to be completely penetrated,which was 348 h,and the experimental results could be well fitted using the Thomas model.According to the permeability of coarse sand,medium sand and fine sand,different sizes of mixed materials were selected to explore their removal effect and effective running time in different simulated aquifer media.When the iron-carbon ratio was 6:1,the time required for 1.0-2.5 mm mixed material to be completely penetrated was 288h,the time required for 0.4-0.8 mm mixed material was 348 h,and the time required for 0.1-0.15 mm was 372 h,which indicated that the effective running time of the simulated experimental column with small particle size mixed material as the filling material was the longest.Therefore,in practical engineering applications.According to the formation conditions and permeability of the aquifer,the mixed material with small particle size should be selected as the reaction medium of PRB.