Research On Nitrogen Removal Performance And Mechanism Of Iron Driving Ammonium Oxidation Mediated By Biochar

Anaerobic ammonium oxidation coupled with Fe（III）reduction（Feammox）is a hot topic in recent years.Feammox related bacteria can use the reduction of Fe³⁺and the oxidation of NH₄⁺-N to remove nitrogen from water.However,due to the low nitrogen removal rate and efficiency and the unknown structure of functional bacteria,Feammox has not been applied in engineering on a large scale.Due to its abundant surface functional groups and large specific surface area,biochar has been added to bioreactors as electron shuttles in recent years to accelerate the removal of pollutants.However,the effects of biochar on the performance and community structure of iron driving ammoxidation are rarely studied.Therefore,the research focus on the study of the physical and chemical properties of different biochar,the enrichment of iron driving ammonia oxidation sludge,and the influence of different feedstocks and dosages of biochar on the nitrogen removal performance of iron driving ammonia oxidation.The changes of nitrogen,iron valence,sludge characteristics and microbial community structure in iron driving ammoxidation system were studied,and the mechanism of nitrogen removal of iron driving ammoxidation mediated by biochar was studied in combination with the physicochemical properties of biochar.The main conclusions are as follows:（1）The study on iron driving ammonium oxidation mediated by biochar showed that the ammonia removal efficiency（ARE）of different biochar systems was:corn straw biochar（YBC）>Control group（BLK）>Wood biochar（WBC）>Sludge biochar（NBC）,which is consistent with its electrochemical properties（phenolic-OH）.At the same time,the higher hydrophobicity of YBC can increase the contact with bacteria and promote its electron transfer.SEM test results showed that the surface of WBC and NBC were heavily damaged and covered with iron phosphate compounds,indicating that the aging of biochar and the reduction of accessible redox active functional groups would lead to a gradual decline in biochar performance.The fluorescence intensity of humic acids in NBC system was the highest,indicating that heavy metal ions Cr,Mn and excessive Se in NBC may lead to the deterioration of its performance.（2）Research on the ammonia oxidation performance with different biochar dosage showed that 5g/L YBC promotes the best ammoxidation performance,and 10g/L YBC has little difference in the ammoxidation performance compared with BLK,indicating that it is not more biochar dosage that lead to better ammonia oxidation performance.The lower protein-polysaccharide ratio（PN/PS）in YBC10 may reduce the contact between biochar and bacteria,thereby reducing its electrochemical stimulating effect.In addition,the EEM intensity of humic acids in YBC10 was stronger than that of YBC5,indicating that the inhibition effect of biochar was higher than its electrochemical promotion effect due to the high dosage.（3）The effects of different feedstocks and dosage of biochar on the changes of community structure showed that YBC had little effect on the changes of major microbial community structure,while WBC and NBC inhibited the presence of major functional bacteria Nitrospira.The sum of Nitrospira and Rhodanobacter abundance decreased significantly in the system supplemented with 10g/L YBC,indicating that excessive dosage of biochar may inhibit the enrichment of functional bacteria and lead to performance degradation.The addition of biochar enriched OLB17,indicating that the electroactivity of biochar could stimulate the growth of electrochemically active bacteria.In addition,Rhodanobacter capable of producing O₂ may be the cause of continuous ammonia oxidation and feammox reactions under oxygen limited conditions.