Research On Biocathode Remediation Of Combined Pollution Of Trichloroethylene And Nitrate In Groundwater

The sources of pollutants in groundwater are extensive and complex.Among them,nitrate and trichloroethene（TCE）,as common inorganic and organic representative pollutants,coexist widely in groundwater,posing great threats to human health and ecological safety.It is a hot issue that urgently needs to be resolved worldwide.Since TCE and nitrate are both oxidizing pollutants,they require external electron donors in the process of biological metabolism.Bioelectrochemical systems（BESs）can use biocathodes as electron donors,and have a wide range of applications in the removal of TCE and nitrate.However,the dechlorination and denitrification processes on the biocathode will compete with each other for electrons,resulting in poor removal efficiency of TCE and nitrate.It is a feasible method that the addition of external electron donors can relieve or weaken the electronic competition between dehalogenated respiratory bacteria and denitrifying bacteria.Microscale zero-valent iron（mZVI）is an inexpensive metal material that can be used as an electron donor for the denitrification and dechlorination of groundwater.This paper first explores the efficiency and key influencing factors of the simultaneous removal of nitrate and TCE by the biocathode,and then uses the mZVI assisted biocathode to achieve the simultaneous and efficient removal of TCE and nitrate under the condition of low concentration of organic carbon source.First,a bioelectrochemical reactor was constructed in this paper to study the effect of different concentrations of nitrate on the removal of TCE from the biocathode.When the cathode potential is-0.26V（vs.SHE）and the sodium acetate concentration is5mmol/L,the removal efficiency of TCE can reach 99.27%in 24h.Adding different concentration gradients of nitrate（0～100mg/L）to the feed water of the reactor that stably removes TCE.As the nitrate concentration increases,the TCE removal efficiency shows a significant downward trend,but the different nitrate concentration gradients did not change the dechlorination pathway of TCE and the products are cis-1,2-dichloroethylene（75.47～80.22%）,vinyl chloride（3.74～7.51%）and ethylene（2.59～5.52%）.Microbial community structure analysis showed that with the increase of nitrate concentration,the abundance of organohalide-respiring bacteria such as Peptococcaceae and Desulfovibrio and mutually beneficial symbiotic bacteria such as Acetobacterium and Eubacterium on the cathode biofilm decreased,while the abundance of denitrifying bacteria such as Acidovorax,Brachymonas,Pseudomonas rise.The existence of denitrifying bacteria has preempted the niche of some dehalogenation-related bacteria,and may have ecological competition with organohalide-respiring bacteria,leading to a decrease in the dehalogenation rate.The effect of the organic carbon source concentration on the simultaneous removal of TCE and nitrate by the biological cathode was further studied under the conditions of NO₃^--N concentration of 50 mg/L and TCE concentration of 1 mmol/L combined pollution.When the sodium acetate concentration was 5mmol/L and 3mmol/L,the removal efficiency of TCE and nitrate hardly changed.When the sodium acetate concentration drops to 1 mmol/L,compared with the reactor with 3 mmol/L influent sodium acetate concentration,the 24h TCE removal efficiency decreases by 19.44%,and the 6h nitrate removal efficiency decreases by 14.53%.The decrease of sodium acetate concentration increases the accumulation of denitrification products nitrite and nitrous oxide,and delays the appearance of the maximum nitrite concentration.It further illustrates the limited denitrification function.However,when no organic carbon source is added,the biocathode still has a certain ability to remove TCE and nitrate.The removal efficiency of TCE in 24h reaches 64.73%,and the removal efficiency of nitrate in 24h reaches 77.71%.This may be because organohalide-respiring bacteria and denitrifying bacteria can use electrodes as the only electron donor for respiratory metabolism.Microbial community structure analysis showed that with the decrease of sodium acetate concentration,the abundance of dehalogenation-related bacteria such as Acetobacterium,Eubacterium,Peptococcaceae,and denitrifying bacteria such as Brachymonas decreased significantly.In the reactor without sodium acetate,the abundance of autotrophic bacteria Xanthobacter was found to increase to 2.4%.Finally,when the sodium acetate concentration is 1 mmol/L,the addition of mZVI enhances the research on the simultaneous denitrification and dehalogenation of biological cathodes.When the cathode potential is-0.26V（vs.SHE）,the addition of mZVI makes the removal efficiency of TCE reach 99.75%in 24h,and the removal efficiency of nitrate reaches 93.76%in 6h.At the same time,the TCE removal efficiency of the reactor without mZVI was only 82.26%,and the nitrate removal efficiency was only 78.42%.The addition of mZVI increased the ethylene in the TCE dechlorination product by 54.69%.It may be that part of the TCE and its incomplete dechlorination product were directly chemically reduced by mZVI.The influence of potential regulation on the simultaneous removal of TCE and nitrate by mZVI-assisted biocathode was explored by changing the cathode potential.It was found that the removal efficiency of TCE and nitrate was better at-0.26V and-0.46V.The nitrate removal efficiency is less affected by the cathode potential,and it can be inferred that the denitrification part is transferred to the biofilm on the surface of mZVI.This study provides a theoretical basis for the efficient and sustainable simultaneous removal of halogenated hydrocarbons and nitrates in groundwater.