Preparation Of Porous Carbon Fiber Loaded Nano Zero-valent Iron And Application Research On Cr(VI) Removal By In-situ Synthesized Gel System In Soi

The heavy metal chromium is widely used in papermaking,electroplating,dye manufacturing,leather tanning,and paint production.A large amount of waste containing chromium（Cr）is discharged into the natural environment,causing serious pollution.Hexavalent chromium（Cr（Ⅵ））(such as Cr₂O₇^2-and HCr₂O₇^-)has strong toxicity,high mobility and it is easy to accumulate in the human body,causing skin irritation,lung cancer and a series of health problems.Therefore,Cr（Ⅵ）pollution control is very necessary.Nanoscale zero-valent iron（nZⅥ）has excellent chemical properties,low price,high reactivity,large specific surface area,strong surface adsorption capacity and reduction capacity,making it an efficient and simple material for Cr（Ⅵ）removal.However,nZⅥ itself has some problems such as easy oxidation,easy agglomeration and instability.The results show that nZⅥ loaded with porous carbon fiber can not only improve the stability,dispersity and oxidation resistance of nZⅥ,but also improve the removal rate of Cr（Ⅵ）.Therefore,nZⅥ loaded with porous carbon fiber carrier can lay a foundation for its application in the field.However,the application of nZⅥ in in-situ remediation of Cr（Ⅵ）contaminated groundwater is limited due to the high fluidity and easy loss of nZⅥ and powdered carbon based nZⅥ composites in the environment.Therefore,this paper proposes the combination of porous carbon fiber modified nZⅥ and in-situ synthesis of calcium alginate gel to achieve in-situ synthesis of soil calcium alginate coated with porous carbon fiber loaded nZⅥ composite system,which has been successfully used in the treatment of wastewater containing Cr（Ⅵ）.The research results are listed as follows:（1）Carbonized melamine foam-supported nano-zero-valent iron composite（nZⅥ-CMF）was prepared by one-step carbothermic reduction method to effectively remove Cr（Ⅵ）.The microstructure characterization of nZⅥ-CMF composite showed that nZⅥ-CMF had better dispersibility,stability and reactivity than nZⅥ.nZⅥ-CMF can effectively remove Cr（Ⅵ）from the environment,but the removal efficiency of Cr（Ⅵ）is affected by preparation conditions（carbonization temperature and iron dosage）and environmental factors（storage time and initial concentration of Cr（Ⅵ））.According to the analysis of zebrafish toxicity,nZⅥ-CMF composite material is non-toxic to the environment and has great potential as a green and pollution-free environmental remediation material.（2）Carbon nanofiber-supported nano-zero-valent iron composite（nZⅥ-CNFs）was synthesized by electrostatic spinning process for Cr（Ⅵ）removal.The electrospun nanofibers were successfully prepared from ferric chloride/polyacrylonitrile（Fe Cl₃/PAN）,and the nZⅥ-CNFs composites were carbonized at 800^oC.The surface morphology and internal structure of nZⅥ-CNFs before and after the reaction with Cr（Ⅵ）were characterized.The results showed that the nZⅥ particles coated with carbon layer were evenly dispersed,and the three-dimensional porous fiber structure could effectively trap the reaction products.In addition,nZⅥ-CNFs has higher activity and Cr（Ⅵ）removal efficiency than nZⅥ in a wider p H range and longer storage time,showing good stability.In addition,nZⅥ-CNFs can also be directly used as membrane filtration material for mobile phase Cr（Ⅵ）removal.（3）Based on the in-situ activated calcium alginate gel synthesis process,the in-situ calcium alginate coated nZⅥ composite system（SA@nZⅥ）was constructed for the treatment of wastewater containing Cr（Ⅵ）.Calcium alginate gel coated nZⅥ can effectively enhance the dispersion and suspension of nZⅥ particles,and contribute to the uniform distribution of nZⅥ in the three-dimensional space of sand column.Meanwhile,in-situ synthesis of calcium alginate gel helps to fix nZⅥ in sand and reduce the loss caused by hydraulic seepage,as the loss rate is reduced from 45%to0%.In addition,the SA@nZⅥ sand column had the longest breakout curve and the highest total Cr（Ⅵ）removal.Compared with other single components,the breakout curve time（78 h）was 78 times longer,and the total removal of Cr（Ⅵ）was 4 times higher（1344μg）.（4）Based on the in-situ soil calcium alginate gel synthesis process,the system of porous carbon fiber based nZⅥ composites（SA@nZⅥ-CMF and SA@nZⅥ-CNFs）coated by in-situ soil calcium alginate was constructed.The results show that,compared with SA@nZⅥ-CNFs,SA@nZⅥ-CMF system has higher gel strength,stable structure and higher removal rate of wastewater containing Cr（Ⅵ）,and the removal efficiency is still greater than 99%after 96 h operation.At the same time,the addition of carbon material significantly improves the durability of the system in open environment.After 14 days in laboratory environment,the removal efficiency of Cr（Ⅵ）wastewater from SA@nZⅥ-CMF composite sand column decreased by 21%,which was 1.8 times higher than that of SA@nZⅥ composite sand column system.