Study On The Mechanism Of Electron Donor Effect On Simultaneous Denitrification And Chromium Removal In Denitrification System

Groundwater is a rich water resource on earth,and is one of the important water sources for industrial and agricultural production and domestic use.At present,nitrate and chromate complex pollution in groundwater is common,which seriously affects the safety of human drinking water.In this paper,autotrophic,heterotrophic and mixotrophic denitrification systems were constructed with pyrite,glucose and pyrite+glucose as electron donors,respectively,to investigate the differences of simultaneous nitrate and chromate removal under different electron donor denitrification systems;furthermore,a mixotrophic reaction permeation wall was constructed to investigate the effectiveness of its long-term operation with a view to guiding practical engineering applications.The study concluded as follows:（1）The effect of Cr（Ⅵ）on denitrification was different in the three systems.When the Cr（Ⅵ）concentration increased from 2 mg/L to 6 mg/L,the denitrification performance of the autotrophic system was significantly inhibited,while the mixotrophic and heterotrophic systems were not significantly affected;this was consistent with the trend of NAR enzyme activity levels.Meanwhile,when C/N（0,1,2）was less than 4 in the heterotrophic and mixotrophic systems,the mixotrophic system presented better denitrification and Cr（Ⅵ）removal ability;this could be explained by the chemical reduction of Cr（Ⅵ）by the genus of autotrophic denitrifying bacteria,pyrite under abiotic conditions in the system.In addition,the rate of Cr（Ⅵ）removal increased to some extent in all three groups of systems in the absence of NO₃^--N presence,however,the mixotrophic system degraded the fastest,further indicating the promotion effect of pyrite on Cr（Ⅵ）removal in the mixed culture system.（2）The addition of Cr（Ⅵ）changed the EPS fraction of microorganisms and their community structure characteristics.3D-EEM analysis showed a decrease in microbial by-products and tyrosine-like substances in EPS and a significant increase in tryptophan;the zeta potential of EPS showed a significant decrease,all these changes were beneficial to the structural stability of sludge and its impact resistance to Cr（Ⅵ）.The relative abundance of the dominant genera Thiobacillus（1.65%～2.81%）,Denitratisoma（2.57%～8.70%）and Aegiribacteria（3.65%～8.43%）in the autotrophic,heterotrophic and mixotrophic systems was significantly increased after the addition of Cr（Ⅵ）.（3）During the long-term operation of the reaction permeation wall,when the influent Cr（Ⅵ）concentration gradually increased from 0.5 mg/L to 3 mg/L,the average removal rates of nitrate and Cr（Ⅵ）decreased from 90.85±7.41%to 62.32±8.19%and 92.13±2.24%to 55.75±10.44%,respectively.With the increase of C/N,the removal rates of both nitrate and Cr（Ⅵ）recovered,however,the recovery period of nitrate removal was much shorter than that of Cr（Ⅵ）,indicating that nitrate has a clear competitive advantage when nitrate and Cr（Ⅵ）coexist.SEM-EDS and XPS results showed that both sludge and pyrite surfaces contained Cr（III）precipitates,indicating that Cr（Ⅵ）was removed by biological and chemical reduction.The reduced state of Fe（II）/S₂^2-/S⁰ changed to Fe（III）and SO₄^2-after pyrite use,and it is inferred that Fe（II）and S₂^2-/S⁰are involved in the biological reduction process of nitrate nitrogen and Cr（Ⅵ）.In addition,the main dominant genera of the system after long-term operation were Pseudoxanthomonas,Acidovorax and Simplicispira.