Removing Nitrate In Groundwater By Zero-Valent Iron Coupled With Modified Biochar:Performance And Mechanism

Groundwater is an important composing of water resources,with many advantages such as stable water supply,excellent water quality and convenient to exploit,which has become an important water source for agricultural irrigation,industrial production and urban domestic water in China.However,industrial wastewater discharge and the extensive use of nitrogen-containing fertilizers lead to NO₃^--N serious pollution in groundwater.Drinking groundwater containing excessive NO₃^--N can cause diseases,such as blue baby syndrome（methemoglobinemia）,liver cancer and gastric cancer,posing a potential threat to public health.How to remove NO₃^--N in groundwater has become an urgent problem to be solved at present.Aiming at removing NO₃^--N in groundwater,economical and practical zero valent iron material（Fe~0）coupled with biological denitrification technology was adopted in this study firstly.Sequential batch experiments were carried out on Fe~0 supported autotrophic and mixotrophic denitrification for groundwater treatment,respectively.The influence factors of nitrogen removal process,material transformation,variation of microbial community and functional gene were studied.Finally,in order to strengthen the denitrification,Fe~0 coupled with modified biochar two-stage PRB continuous flow system was built to study the effect of groundwater denitrification and the enhancement mechanism.The main conclusions of the paper were as follows:（1）Research on batch tests of autotrophic denitrification of groundwater based on Fe~0（iron powder reduced）Basing on Fe~0（iron powder reduced）-driven autotrophic denitrification,the reactor had the best denitrification performance when the influent p H was 6,and the TN removal efficiency（NRE）was86.84%.In a single batch process,the TN concentration showed a uniform downward trendency and the nitrogen removal rate（NRR）was 1.36 mg N/L·h.Concurrently,Iron dissolution was obvious and the final total iron concentration in the effluent was 18.60 mg/L.The discovery of Bioorganic-Fe complexes and Fe OOH on the surface of Fe~0（iron powder reduced）confirmed microbial corrosion of Fe~0 for denitrification.At the same time,Fe~0（iron powder reduced）can enhance the abundance of species and make the species of microorganisms more concentrated.The abundance of Thauera is 8.7%,which can use the electrons produced by Fe~0 to promote efficient denitrification in groundwater autotrophic denitrification system based on Fe~0（iron powder reduced）.（2）Research on batch tests of mixotrophic denitrification of groundwater based on Fe~0（iron scraps）Under the synergistic effects of Fe~0（iron scraps）,autotrophic and heterotrophic denitrifying bacteria,the reactor had the best denitrification performance when the influent C/N is 2.78～3.08,and the NRE was 81.43%.The complete reaction cycle contained two stages:mixotrophic denitrification stage in 0-12 h and autotrophic denitrification stage in 12-24 h.The NRR of mixotrophic denitrification was 9.5 times that of autotrophic denitrification.What’s more,the contribution ratio of autotrophic denitrification and heterotrophic denitrification was stable（about4:6）.Most of the total iron in effluent existed in the form of Fe³⁺.Fe OOH,the main corrosion product on the surface of Fe~0（iron scraps）,can efficiently transfer electrons and strengthen biological mixotrophic denitrification.At the same time,Fe~0（iron scraps）coupled with carbon source promoted growth and accumulation of heterotrophic,autotrophic and endogenous denitrifiers,such as Zoogloea,Erythrobacter,Terrimonas,Dechloromonas,among which,Zoogloea,as the dominant,plays a key role in the denitrification process.（3）Study on NO₃^--N adsorption mechanism of modified biocharFe-RBC600 modified by using ferric chloride solution had strong adsorption characteristics for NO₃^--N.Through adsorption kinetics and isothermal experiment,it indicated that the adsorption of NO₃^--N by Fe-RBC600 was mainly multi-layer chemical adsorption.In addition,the modification method which using ferric chloride solution made the surface pore size of Fe-RBC600 larger.Simultaneously,the surface is loaded with electron withdrawing groups Fe-O,which can further strengthen the adsorption of NO₃^--N through electrostatic attraction.（4）Research on Fe~0（iron powder reduced）supported autotrophic denitrification coupled with modified biochar two-stage PRB for removing nitrate in groundwaterWhen the influent p H=6 and HRT=24 h,the two-stage PRB system had the best denitrification performance.At this time,the NRE of first-stage and second-stage were 89.06%and 98.21%,and the effluent met the levelⅠin standard of Groundwater Quality Standard.When the influent p H=6,by increasing protein,humic acid content in EPS and ETSA,it promoted sludge granulation and effectively increased the electron transport rate to achieve high-efficiency denitrification.What’s more,p H changes had no effect on the effluent SMP.Lastly,p H=6 can increase the abundance of denitrification-related functional genes,the phylum related to the denitrification process such as Gemmatimonadetes and Bacteroidota and autotrophic Ellin6067,which enhanced the effect of denitrification furtherly.（5）Research on Fe~0（iron scraps）supported mixotrophic denitrification coupled with modified biochar two-stage PRB for removing nitrate in groundwaterWhen the influent C/N=2 and HRT=24 h,the two-stage PRB system has the best denitrification performance.At this time,the first-stage and the second-stage NRE were 92.17%and 96.91%,and the effluent met the levelⅠin standard of Groundwater Quality Standard.The properties of the sludge in the reactor are greatly affected by C/N.When the influent C/N=2,the protein,humic acid content in the EPS and ETSA of the sludge were the highest,which promoted the aggregation of microorganisms and promotes electron transfer.At the same time,Under the condition of C/N=2,the abundance of functional genes for denitrification is the highest.Fe~0（iron scraps）together with carbon source increased the abundance of Proteobacteria,Bacteroidota,and Gemmatimonadetes,Pseudomonas at phylum level and Ellin6067,Azospira,Thermomonas at genus level.Due to growth and enrichment of heterotrophic and autotrophic denitrifiers,mixotrophic system achieved high denitrifying efficiency finally.（6）Feasibility study on synergistic removal of nitrogen,emerging pollutants and heavy metalsFe~0（iron scraps）supported mixotrophic denitrification coupled with modified biochar two-stage PRB can synergistically remove nitrogen（NO₃^--N、NO₂^--N、NH₄⁺-N）,emerging pollutants（DEHP）and heavy metals（Cd,Cr,Cu,Zn）.Among them,TN removal efficiency,NO₃^--N removal efficiency and NO₂^--N removal efficiency are all greater than 80%,while NH₄⁺-N removal efficiency is relatively poor,only 33.56%;removal efficiency of Cd,Cr,Cu and Zn were 91.33%,97.77%,92.75%and 64.31%,respectively;it can effectively remove DEHP.