Study On Performance And Optimizing Conditions For Pyrite-based Autotrophic Denitrification

The sulfur autotrophic denitrification process has been widely used due to its stability,high efficiency,and low cost.However,it also has disadvantages such as a significant drop in pH and excessive sulfate production.In recent years,sulfides such as pyrite and ferrous sulfide have been proved to be available as electrons for denitrification.But there is a lack of comparative analysis of denitrification characteristics between different sulfides.The purpose of this study was to analyze the differences in denitrification performance and bacterial community structure between pyrite,sulfur and ferrous sulfide,and optimize the operating conditions of pyrite-packed columns.In this study,pyrite was the main research object to conduct autotrophic denitrification for simulate nitrate-contaminated groundwater,and sulfur and ferrous sulfide were used as controls.The advantages and characteristics of pyrite as an electron donor for nitrate removal were investigated.Then the experimental conditions such as pretreatment,dosage of pyrite and ceramsite addition were optimized.An upflow reactor with pyrite was constructed,and the denitrification effect and operation conditions like the hydraulic retention time（HRT）were discussed.The main research results are as follows:Nitrate degradation rate constants for different electron donors were 18.85（sulfur）,10.20（pyrite）,and 4.95 mg·L^-1·d^-1（ferrous sulfide）,respectively.It showed that the nitrate degradation rate of pyrite was slightly lower than that of sulfur,and compared with ferrous sulfide,pyrite had a higher nitrate removal capacity.Both pyrite and ferrous sulfide had low sulfate yield（Pyrite:420.56 mg/L,Ferrous Sulfide:271.80 mg/L Sulfur:531.24 mg/L）.The pH of the pyrite system was stable at about 7.50,and it was more suitable for the growth and reproduction of denitrifying bacteria.Through the microbial community analysis,dominant strains in the pyrite system were T.denitrificans and S.denitrificans,while the dominant species in the sulfur system was T.denitrificans.And the dominant genus of ferrous sulfide system was Brevundimonas and Pseudoxanthomonas,but Pseudoxanthomonas would reduce nitrite to N₂O.And Stenotrophomonas,which was also present in both sulfur and ferrous sulfide systems,was capable of reducing nitrite to N₂O as the final product.The two N₂O-producing strains were only present with a very small amount in the pyrite system,and it was concluded that pyrite-based denitrification systems would produce less N₂O as greenhouse gas.Studies have shown that washing the pyrite or adding ceramsite could reduce the efficiency of nitrogen removal,and the optimum dosage of pyrite in this study was 75 g/L.It was found that denitrification columns constructed by mixing pyrite with maifanite,dolomite and quartz sand had different nitrate reduction abilities.When HRT was extended from 10 h to 24 h,maifanite could significantly increase the nitrogen removal efficiency,and the nitrate nitrogen removal efficiency increased from 54.1%to 82.3%.And the dolomite could effectively reduce the effluent sulfate（maifanite:256.87 mg/L;quartz sand:239.80 mg/L;dolomite:185.87 mg/L）.Therefore,maifanite and dolomite were ideal fillers.This study clarified that the pyrite-based denitrification system had obvious advantages in terms of denitrification performance,acid production and microbial composition.And it provided a new approach for nitrate bioremediation in groundwater.