Carbon Fiber Facilitates Zero-valent Iron Based Biological Denitrification To Remove Nitrate In Groundwater:Performance And Mechanism

Nitrate contamination of groundwater resources was a common phenomenon,which posed a serious threat to ecological environment,human health,and even social sustainable development.Iron-based autotrophic denitrification with low cost,less additional carbon source,has become a promising technology in the field of groundwater denitrification.However,in the process of removing NO₃^--N contamination from groundwater by iron-based biological denitrification technology,zero-valent iron（Fe~0）is easy to be hardened and passivated,which limits the denitrification process.In this paper,firstly,a batch experimental research was conducted to verify the feasibility of carbon fiber facilitated Fe~0 bio-denitrification to remove NO₃^--N from groundwater.Then,based on the simulated groundwater quality,an experimental research of PRB on the removal of NO₃^--N from groundwater by a carbon fiber facilitated Fe~0 based biological denitrification was carried out.The microbiological mechanism of carbon fiber facilitation was analyzed by material transformation,variations of Fe~0 morphology and crystal form,variations of sludge characteristic,microbial community and functional gene evolution.Finally,through a series of characterization of carbon fiber materials,the in-depth mechanism of carbon fiber facilitated biological denitrification of Fe~0 was clarified.The main results as follows:（1）Batch experimental research on removal of NO₃^--N from groundwater by carbon fiber facilitated zero valent iron bio-denitrificationUnder different C/N ratio or HRT,the denitrification efficiency of carbon fiber facilitated Fe~0 denitrification was significantly improved,and the highest nitrogen remova l efficiency was 84.06%.Carbon fiber reduced the DO concentration of the system and ensured the efficient reaction of the denitrification process by promoting Fe~0 corrosion.The results of material transformation showed that the nitrogen removal contribu tion ratio of autotrophic to heterotrophic in carbon fiber enhanced Fe~0 denitrification system was about 4:5.The research on the variations of sludge characteristics showed that the input of carbon fiber was beneficial to the enrichment and growth of micr oorganisms and the attachment of biomass,thus increasing the biomass,EPS and ETSA.The analysis of microbial community and functional genes showed that the input of carbon fiber could increase the relative abundance of iron autotrophic denitrifying bacte ria such as Acinetobacter,Comamonas and Pseudomonas,and could increase the gene abundance of denitrification function in the system.（2）PRB experimental research on removal of NO₃^--N from groundwater by carbon fiber facilitated zero-valent iron bio-denitrificationUnder different C/N ratio or HRT,the denitrification effect of the carbon fiber facilitated Fe~0denitrification was significantly improved,and the nitrogen removal efficiency was up to 92.02%,and the average accumulation of NO₂^--N was even less.The variations of pH value and DO concentration show that the input of carbon fiber maintained the DO concentration in the suitable range of denitrification and increased the pH v alue of the system by promoting Fe~0 corrosion.The variations of Fe~0morphology and crystal form before and after used show that carbon fiber can not only promote the deep corrosion of Fe~0,but also facilitated the transformation of corrosion products to b ioorganic-Fe complexes.The study on the variations of sludge characteristics shows that the input of carbon fiber facilitated the synthesis of EPS and the promotion of ETSA,which are beneficial to the denitrification process of the system.According to the variations of micro bial community and functional genes,the increase of C/N ratio promoted the diversity of denitrifying microorganisms in the system,enhanced the abundance of heterotrophic denitrifying bacteria,and significantly facilitated the enrichment of denitrifying genes in the system.The input of carbon fiber materials helped to strengthen the stability of denitrification community,improved the diversity of denitrification microorganisms,increased the abundance of iron autotrophic denitrification bacteria such as Acinetobacter and Pseudomonas,and made the denitrification reductase activity at a higher level.（3）Mechanism of carbon fiber facilitated zero-valent iron based bio-denitrificationAccording to the variations of apparent morphology of carbon fiber,iron and iron oxides are attached to the surface of carbon fiber,which may be beneficial to the adsorption of carbon fiber to NO₃^--N and adhesion to microorganisms.The XRD patterns of carbon fiber show that,the granular material on the surface of carbon fiber is iron oxide,mainly Fe₃O₄,bioorganic-Fe complexes and Fe~0.Carbon fiber materials can not only facilitated the corrosion of Fe~0,but also enhance the capture of NO₃^--N and the enrichment of functional microorganisms by modifying the surface with iron oxides.The results of X-ray photoelectron spectroscopy of carbon fiber show that iron element forms composites with carbon fiber through adhesion,and the iron element in carbon fiber is mainly Fe³⁺.According to the fourier infrared spectra of carbon fiber,the variations of functional groups on the surface of carbon fiber before and after used were not significantly related to the process of carbon fiber facilitates Fe~0 based biological denitrification.The cyclic voltammograms and electrochemical impedance spectroscopy of carbon fiber confirmed that the carbon fiber material had strong chemical stability but limited reversible redox capabilities,moreover the impedance of carbon fiber material decreased and the electrical conductivity increased after reaction.