Study On The Nitrogen Removal And Mercury Resistance Mechanism Of Mercury-resistance Aerobic Denitrifying Bacteria LV3 For Flue Gas Denitrification

China’s energy resource displayed the characteristics of―coal-rich,lean oil and low gas‖.China accounted for 16 million tons of oil equivalent of the increment in global coal consumption in 2018,ranking second in the world.Environmental pollution caused by nitrogen oxides（NO_x）emissions from coal-fired flue gas has been one of the hot topics of concern.In response to this,national policies have continued to strengthen the limits on NO_xemissions.Therefore it is important to explore flue gas denitrification technologies with high denitrification efficiency and low cost.Current researches show that biological treatment of NO_x in flue gas has the advantages of low cost and no secondary pollution.The essence is the conversion of NO_x to N₂ by the action of denitrifying bacteria.In consideration of the low mass transfer efficiency of NO in the liquid phase,which is predominant in flue gas,and the merely 4%～10%average oxygen content in the flue gas,our group proposed the oxidative absorption-bioreduction method to remove NO_x.However,heavy metals,represented by mercury,are also present in the actual flue gas,and heavy metals can adversely affect the growth and denitrification performance of denitrifying bacteria.Taking this into consideration,in this study,an aerobic denitrifying strain with mercury tolerance was screened from mercury-contaminated soil as a source of strain for oxidative absorption-bioreduction flue gas denitrification system,and its growth and denitrification performance in the presence of mercury were investigated.The screened strain was named LV3.The colonies were small,creamy white opaque dots with a transparent halo,smooth and moist surface easy to be picked up,and regular edges.After PCR amplification of 16S rRNA,the obtained base sequences were compared and the strain was identified as Acinetobacter sp.The tolerance of the strain to mercury and denitrification performance under mercury stress were first investigated in denitrification media with different mercury concentrations.Next,the effects of carbon source,C/N,temperature,dissolved oxygen,pH,ionic strength and heavy metal on the denitrification performance of the strain under mercury-containing conditions were investigated.Then the denitrification performance of the strain under mercury-containing conditions with different nitrogen sources was investigated under the optimal conditions of the experiment.Finally,the mechanism of mercury tolerance of strain LV3 was analyzed by volatile development technology,infrared spectroscopy and scanning electron microscopy.The main conclusions of this paper are as follows:（1）The maximum tolerable concentration of Hg²⁺for strain LV3 was 6 mg/L.Within this range,the strain was able to achieve complete denitrification and complete mercury removal,and the inhibitory effect of Hg²⁺on the strain was mainly manifested in the prolongation of the lag phase.（2）The results of single-factor shake flask experiments at 4 mg/L Hg²⁺showed that the optimal denitrification conditions for strain LV3 were sodium pyruvate as the only carbon source,C/N of 16,temperature of 30°C,and speed of 120 rpm.Strain LV3 could grow normally and denitrify in the pH range of 6～9,and the lag phase of the strain was shortened as the pH increased.The appropriate addition of both Mg²⁺and Mn²⁺promoted the growth of the strain,and the promoting effect of Mn²⁺was stronger than that of Mg²⁺.When Hg²⁺and other heavy metals Pb²⁺and Cr⁶⁺in the flue gas were added to the culture solution at the same time,both of them synergistically inhibited the growth of the strain,and the inhibitory effect of Cr⁶⁺was much stronger than that of Pb²⁺.（3）Strain LV3 could utilize either nitrate or nitrite nitrogen as the only nitrogen source under 4 mg/L Hg²⁺.Under optimal conditions,the nitrogen removal rate was higher than 93%for anyone.Analysis of the nitrogen balance results when nitrate as the only nitrogen source showed that the percentage of initial nitrogen conversion to gaseous nitrogen was close to and greater than 60%for strain LV3 at 0～4 mg/L Hg²⁺concentration.Secondly,the initial nitrogen is mainly converted to intracellular nitrogen.This indicates that the strain mainly remove nitrogen by denitrification and assimilation.The activities of NIR and NAR were inhibited in the experimental range of 0～6 mg/L Hg²⁺concentration.But even in 6 mg/L Hg²⁺concentration,the extremely inhibited NIR activities were yet greater than some denitrifying strains,which combined with the previous study showed that the strains were still able to denitrify efficiently under Hg²⁺stress.In contrast,the results of the effect of Hg²⁺on FDA hydrolase activity indicated that low concentrations of Hg²⁺could promote the metabolic activity of the strains to maintain normal cell growth,while high concentrations of Hg²⁺could damage the cells hence reducing enzyme activity.（4）The results of the volatile ability analyses,infrared spectroscopy and the scanning electron microscopy indicate that the strain LV3 removes mercury via the reduction reaction and adsorption by cell.The secretion of EPS can make the strains reduce the contact with Hg²⁺.This three factors make the strain LV3 have certain mercury tolerance.