Effect Of Biochar Supported Nano Zero-valent Iron On Nitrogen Removal In Bioretention System

In recent years,climate change and rapid urbanization have led to an increase in extreme rainfall events,exacerbating the problem of non-point source pollution,and it is necessary to take measures to alleviate the current situation.Bioretention facilities are widely used in the field of runoff pollution control.However,traditional bioremediation facilities often find it difficult to stably remove nitrogen and phosphorus substances from rainfall runoff due to issues such as fillers and structures,and may even become pollution sources in extreme events.To solve the problem of unstable nitrogen removal efficiency in traditional bioretention facilities,this study constructed traditional bioretention facilities and new biochar loaded nano zero-valent iron（nZVI@BC）bioretention facility model was used to study its nitrogen removal efficiency and influencing factors under different operating conditions,revealing the migration path of nitrogen in the bioretention facility and the mechanism of action of microorganisms and media,providing reference for subsequent related research.The main research content and conclusions of the paper are as follows:（1）A large number of nano zero-valent iron particles were loaded on the nZVI@BC surface.When the mass ratio of nano zero-valent iron（nZVI）to biochar（BC）was 1/2,1/4,and 1/8,the proportion of surface Fe⁰ was 52.81%,50.11%,and 43.53%,respectively.Due to the occupation of the porous structure of biochar by iron nanoparticles The nZVI@BC specific surface area of the composite material（42.77㎡/g-61.31㎡/g）is smaller than BC（86.75㎡/g）.NO₃^--N adsorption test showed that the mass ratio of nZVI/BC would lead to different selectivity of nitrate nitrogen Product selectivity.When the mass ratio of nZVI/BC was 1/2,the selectivity of product nitrogen was the highest（62.19%）.With the decrease of the mass ratio of nZVI/BC（1/2-1/8）,the selectivity of nitrate nitrogen product NH₄⁺-N gradually increased.In addition,the loading of nZVI increased the active sites on the surface of the composite material,improving its adsorption capacity for NO₃^--N.（2）The removal rates of NO₃^--N,NH₄⁺-N,COD,and TN in traditional bioretention facilities are 20.33%-44.36%,68.98%-88.05%,69.18%-81.28%,and 44.17%-58.00%,respectively.The cumulative amount of NO₂^--N is 0.88mg/L-1.59mg/L;The corresponding removal rates in nZVI@BC bioretention facilities are 57.59%-98.51%,47.03%-79.56%,39.00%-73.97%,62.63%-84.43%,and the cumulative amount of NO₂^--N is 0.02mg/L-0.37mg/L,respectively.The data results indicate that,The nZVI@BC bioretention facilities significantly improved the removal efficiency of NO₃^--N and TN（37.26%-54.15%and 18.46%-26.43%）,but the removal efficiency of NH₄⁺-N and COD decreased（8.49%-21.95%and 7.31%-30.18%）.besides,nZVI@BC also alleviated the accumulation of NO₂^--N（0.86 mg/L-1.22 mg/L）.（3）The changes in the nZVI/BC mass ratio,influent pollutant load,and organic carbon source have a significant impact on the removal efficiency of various pollutants,The nZVI@BC dosage has no significant difference in the removal efficiency of various pollutants.The mass ratio of nz VI/BC has a significant impact on the removal of COD and NH₄⁺-N.When the mass ratio of nz VI/BC is 1/4,the COD removal rate is poor（39.00%）,while when the mass ratio is 1/8,the removal rate is higher（70.85%）;The NH₄⁺-N removal rate decreases with the decrease of nZVI/BC mass ratio.With the increase of rainfall runoff load（COD=300 mg/L,NO₃^--N=15 mg/L,NH₄⁺-N=13 mg/L）,the contact between NH₄⁺-N and COD with soil media and microorganisms was enhanced,resulting in a significant increase in their removal rate（12.46%-21.81%and15.68%-23.78%）.After the influent organic carbon source is limited,the traditional bioretention facility denitrification process is limited,but The nZVI@BC bioretention facilities can still maintain good removal efficiency（69.70%-57.59%）,significantly higher than traditional bioretention facilities（20.33%-20.82%）,alleviating the negative impact of organic carbon limitation on NO₃^--N removal efficiency.When the nZVI/BC mass ratio is 1/2,The removal efficiency of NO₃^--N,NH₄⁺-N,COD,and TN in nZVI@BC bioretention facilities is good（98.22%,79.56%,72.01%,84.43%）,indicating that the optimal mass ratio of nZVI/BC is 1/2.（4）Analysis shows that the redox reaction products of Fe⁰ and NO₃^--N are mainly composed of Fe₃O₄,The nZVI@BC composite material significantly enhanced the denitrification effect of the bioretention facility,and could transform NO₃^--N into NH₄⁺-N and N₂ through redox reaction;The migration paths of NO₃^--N in nZVI@BC bioretention facilities mainly include Fe⁰ reduction,denitrification,soil medium non microbial fixation,microbial fixation and pore water retention.（5）Add to nZVI@BC will significantly increase the microbial community abundance in the bioretention facility,which is conducive to the growth of denitrification bacteria such as Gammaproteobacteria and Azospira.In addition,Azospira is the main dominant microbial genus in the process of NO₃^--N reduction coupled with ferrous oxidation,which is conducive to NO₃^--N reduction;Meanwhile,nZVI@BC can increases the relative abundance of Actinobacteria,which has the ability of iron reduction and promotes the iron cycle in the system.Alphaprotobacteria is related to the removal of NH₄⁺-N.The relative abundance of Alphaprotobacteria in traditional bioretention facilities is relatively high（10.45%-17.31%）,resulting in better NH₄⁺-N removal efficiency.Nitrospira,as the main oxidizing bacteria of NO₂^--N The reduction of NO₂^--N by Fe⁰ in nZVI@BC bioretention facilities reduces its content,alters the growth environment of Nitrospira,and leads to a decrease in its relative abundance.Bacteroidia and Bacillus are related to organic matter degradation,and their relative abundance in traditional bioretention facilities（4.76%to 5.17%）is significantly higher nZVI@BC bioretention facilities（0.99%to 2.95%）,resulting the removal rate of COD in nZVI@BC bioretention facilities is relatively low.Correlation analysis shows that the R² values of TN,NO₃^--N,and COD range from 0.548-0.608,indicating a strong correlation with microbial communities.