Study On The Mechanism Of Lotus Root Biochar Loaded With Sulfurized Nano-zero-valent Iron To Activate Persulfate To Degrade Decabromodiphenyl Ether In Contaminated Sites

Decabromodiphenyl ether（BDE209）is a type of refractory organic pollutant with 10bromine atoms.It usually enters the soil environment in the form of sedimentation and dust adsorption.It is insoluble in any solvent and difficult to degrade,which has serious harm to the human and the ecological environment.At present,The persulfate advanced oxidation technology degrades organic matter more thoroughly and the product has no secondary pollution.Nano-zero-valent iron（nZVI）not only has a large specific surface area and excellent adsorption performance,it can also be used as an iron source to continuously transport Fe²⁺to persulfate.However,nZVI is very magnetic and easy to agglomerate and oxidize,which has a relatively restrictive effect on practical application and development.Biochar-loaded modified nZVI can effectively avoid the above problems,and can improve the stability and dispersibility of nZVI.In this paper,lotus root biochar is used to load modified sulfurized nano-zero-valent iron（LRBC@S-nZVI）,and the composite material is combined with persulfate（PS）to degrade BDE209 in contaminated sites.This paper not only identifies and analyzes the morphology,size,functional group composition,element valence of LRBC@S-nZVI,and compares the degradation effects of BDE209 under different reaction systems,but also studies the different experimental parameters such as biochar（BC）calcination temperature,sulfur-iron ratio,iron-carbon ratio,p H on the degradation of BDE209 by LRBC@S-nZVI/PS to determine the optimal degradation reaction conditions.In addition,this article also analyzes the quenching experiment to identify the main free radical species in the degradation process,and explains the reaction mechanism of LRBC@S-nZVI activation of PS and the degradation path of BDE209.The related research results are as follows:（1）SEM and TEM were used to characterize the morphology of LRBC@S-nZVI material,and it was found that spherical S-nZVI was uniformly dispersed on the surface of BC,and the morphology was relatively rough.The surface of nZVI is covered with a corrugated iron pyrite layer material,which not only effectively slows down the agglomeration and oxidation of Fe0,but also enhances electron transfer.In addition,according to the XRD and FTIR characterization results,it was found that there are sulfide compounds and ferrite species on the surface of LRBC@S-nZVI,indicating that nZVI had been sulfided and modified and slightly oxidized.The XPS split peak results not only confirmed the occurrence of nZVI vulcanization and oxidation,but also confirmed the successful loading of nZVI by biochar.（2）In the comparative experiment of PS,LRBC/PS,nZVI/PS,S-nZVI/PS,LRBC@nZVI/PS and LRBC@S-nZVI/PS system,the degradation effect of BC@S-nZVI/PS on BDE209 is the best,and the removal rate reached 72.018%.In addition,the study found that the best reaction parameters for LRBC@S-nZVI/PS to degrade BDE209 are:BC calcination temperature is 600℃,the ratio of sulfur to iron and the ratio of iron to carbon are 1:4 and 1:5,the concentration of PS solution is 0.2mol/L and the p H value is 3.At this time,the maximum degradation rate of BDE209 can reach 87.23%.The degree of vulcanization and the dosage of biochar should not be too much or too little.Too much will easily cause loading and block the reaction site,and too little will easily cause nZVI to be oxidized and deactivated.Humic acid（HA）has a significant inhibitory effect on the degradation reaction of BDE209.（3）In an acidic environment,LRBC@S-nZVI/PS has the best degradation effect on BDE209,and it conforms to pseudo first-order kinetics,and the reaction rate also reaches0.0137min-1.At the same time,sulfate radical（SO₄^-·）is the dominant free radical in the degradation reaction under acidic environment.ethanol and SO₄^-·have high reaction activity,which can inhibit the degradation of BDE209 to a certain extent.Tert-butanol is an effective quencher of HO·,and its inhibitory effect is more significant under alkaline conditions.（4）The concentration of ferrous ions and soluble iron in the reaction system rose rapidly within the first 30 minutes,and then remained relatively stable during the subsequent period.At the same time,the soil p H first showed a trend of sharp decline,and then slowly increased.The PS content and bromide ion in the system both rise faster in the first half an hour,and then the growth trend slows down,which is consistent with the change trend of the degradation effect of BDE209.This is mainly because a large amount of acidic PS solution is added to the reaction system,the acid-base balance of the soil is broken to a certain extent,the corrosion rate of LRBC@S-nZVI is accelerated,the release of Fe²⁺is accelerated,and PS is activated in a large amount to generate SO₄^-·degrade BDE209.However,with the massive consumption of H⁺in the solution,the corrosion degree of nZVI weakens,the formation and conversion of Fe²⁺are relatively balanced,and the debromination rate of BDE209 slows down due to the decrease of PS activated objects.（5）The main ways of LRBC@S-nZVI/PS to degrade BDE209 are stepwise debromination,nucleophilic substitution and ring opening,etc.The strong attack of SO₄^-·and HO·causes the Br atom on the benzene ring of BDE209 to be replaced by H atom and generate Br^-.And the ether bond of the low bromination intermediate generated after debromination is also attacked and broken,or the benzene ring is opened to reduce the toxicity of the benzene compound,and finally BDE209 is mineralized to small molecular substances,such as CO₂,H₂O,Br^-and short chain carboxylic acids.