Removal Of Nitrate From Water By Autotrophic Denitrification Based On Sulfur And Iron Substrates

Nitrate is one of the most common pollutants in water today.Excessive NO₃^-enters the water and poses a huge threat to the natural environment and human health.Autotrophic denitrification technology has high application value in the denitrification process of water bodies with low C/N and lack of nutrients.Sulfur and iron substrate autotrophic denitrification is favored by researchers as a novel and efficient method to remove nitrate from nutrient-poor waters.To investigate the contribution of Fe（II）to the biological and chemical processes of NO₃^-degradation in the system of sulfur and iron coexistence,this study used Fe²⁺and S₂O₃^2-as electron donors to investigate the action patterns and mechanisms of sulfur autotrophy,iron autotrophy,and sulfur-iron co-substrate autotrophic denitrification of microorganisms under pure and mixed bacterial conditions,respectively,and the main findings are as follows.A facultative anaerobes strain with denitrification function was isolated and purified in this experiment.By 16S r DNA sequence analysis,it was Citrobacter genus,named Citrobacter sp.ZY630.The strain was probed for their ability to reduce nitrate by ferrous oxidation,and the results showed that the strain ZY630 could reduce NO₃^-with Fe²⁺as the electron donor in the absence of additional organic matter.After 120 h of reaction,about 1.1mmol·L^-1of NO₃^-in the Fe²⁺+bacterias group was completely degraded,and the products included NO₂^-,N₂O and N₂.The control analysis revealed that the conversion of nitrate to nitrite in the system was dominated by biological action.It was also found that the 6 mmol·L^-1Fe²⁺+bacterias group carried out denitrification more completely than the 3 mmol·L^-1Fe²⁺+bacterias group,indicating that Fe²⁺helped to promote the denitrification process in this system.Further studies on the biological and chemical pathways in the NO₂^-degradation process by strain ZY630 using Fe²⁺.It was found that the molar ratio of total amount of Fe²⁺oxidation to total amount of NO₂^-reduction was about 2.22 after 120 h,which was slightly higher than that of the system with only Fe²⁺,indicating that the chemical reaction between NO₂^-and Fe²⁺was dominant.The anaerobic ferrous oxidative nitrate reduction by strain ZY630 is a biochemically coupled process.During the investigation of the sulfur autotrophic denitrification process of the strain ZY630,it was found that although the strain ZY630 can undergo sulfur autotrophic denitrification,it requires organic matter to maintain the long-term normal growth of the strain.Strain ZY630 could not obtain enough energy to sustain the growth of the strain and the denitrification reaction when using Fe²⁺and S₂O₃^2-as electron donor.Sulfur autotrophic denitrifying sludge（mixed bacteria）was domesticated with S₂O₃^2-as the only applied electron donor,and after 35 days of incubation,the sludge is enriched with sulfur autotrophic bacteria group mainly composed of Sulfurimonas and Thiobacillus.Sequencing batch experiments were conducted to verify the sulfur autotrophic denitrification performance of microorganisms in the domesticated sludge.It was found that S₂O₃^2-could be used within 48 h to convert about 1 mmol·L^-1of NO₃^-into N₂and form SO₄^2-accumulate in System.At the same time,the iron autotrophic denitrification function of the bacteria in the domesticated sludge was investigated.It was found that the microorganisms in the domesticated sludge could use Fe²⁺as electron donor for iron autotrophic denitrification.Based on the confirmation that microorganisms in domesticated sludge can perform autotrophic denitrification with S₂O₃^2-and Fe²⁺alone as electron donors,the sludge was inoculated in a sulfur-iron coexistence system,and it was found that microorganisms can continue denitrification using Fe²⁺as electron donor when the S₂O₃^2-content is not sufficient to reduce all NO₃^-to N₂.Therefore,the domesticated sulfur autotrophic sludge can use S₂O₃^2-and Fe²⁺as electron donors and has sulfur-iron co-substrate autotrophic denitrification.