A Study On The Application Of Nanoscale Zero-valent Iron/biochar Composite On Remediation Of Contaminated Water In Both Reducing And Oxidizing System

With the explosion of urban industry,environmental pollution is becoming more serious than ever before.Thus,it is urgent to explore a fine environmental functional material.In recent years,nanoscale zero-valent iron（nZVI）has been widely used in contaminants remediation owning to its large specific surface and high reactivity.However,iron nano particles can easily aggregate.In this work,biochar was chosen to be a supporter of nZVI to stabilize nZVI,therefore,lower the incidence of agglomeration.The possibility of applying nanoscale zerovalent iron/biochar composites（nZVI@BC）in pollutant removal was studied.Based on removal mechanism,the application system is divided into two parts,namely reducing system and oxidizing system.In the experiment of reducing system,three types of modified biochar（BC）were produced respectively with acid（HCl）treatment（HCl-BC）,base（KOH）treatment（KOH-BC）and oxidation（H₂O₂）treatment（H₂O₂-BC）of raw biochar.Both the raw biochar and modified biochars supported zero valent iron nanopartilces（nZVI）（i.e.nZVI@BC,nZVI@HCl-BC,nZVI@KOH-BC and nZVI@H₂O₂-BC）were synthesized and their capacities for Cr（VI）removal were compared.The results showed that the nZVI@HCl-BC exhibited the best performance and the underlying mechanisms were discussed.The surface elemental distribution maps of the nZVI@HCl-BC after reaction with Cr（VI）showed that Fe,Cr and O elements were deposited on the surface of HCl-BC evenly,indicating that the formed Cr（III）/Fe（III）could settle on the surface of HCl-BC uniformly rather than coated only on the nZVI surface.This reveals that the supporter HCl-BC could also play a role in alleviating the passivationof nZVI.Besides,the effects of mass ratio（nZVI/HCl-BC）,pH,and initial Cr（VI）concentration on Cr（VI）removal were examined.At lower mass of HCl-BC,nZVI aggregation cannot be fully inhibited on the surface of HCl-BC,whereas excessive biochar can block the active sites of nZVI.Additionally,it was found that Cr（VI）removal by nZVI@HCl-BC was dependent on both pH and initial Cr（VI）concentration.In oxidizing system,nZVI@BC composite was used as an activator for Fenton-like removal of sulfamethazine（SMT）.The possible removal mechanisms in the reaction system were proposed.nZVI was mainly responsible for H₂O₂ decomposing to generate·OH for the degradation of SMT,while BC played multiple roles,i.e.,preventing nZVI aggregation,adsorbing SMT,activating H₂O₂,and alleviating nZVI passivation.The effects of various factors（i.e.,the mass ratio of nZVI to BC,solution pH,H₂O₂ concentration and nZVI@BC dosage）on SMT removal were evaluated.The highest removal efficiency（74.04%）of SMT（10 mg/L）was achieved at the optimal conditions（the mass ratio of nZVI and BC=1:5,pH=3,H₂O₂=20mM and nZVI@BC=1.2 g/L）.Additionally,the feasibility of recycling of nZVI@BC composites was examined.It was found that the removal efficiency of SMT decreased significantly from 74.04%to 53.28%and 38.02%,respectively,in the second and third run of experiments.X-ray diffraction analysis of the recycled composites demonstrated the gradual loss of Fe⁰ content after each run of experiment,which resulted in the decrease of catalytic activity of nZVI@BC composites and thus the drop of SMT removal.