Migration And Transformation Of Endogenous Nitrogen In Remediation Of Black And Odorous Water

River pollution is currently one of the widespread environmental problems worldwide.In long-term polluted rivers,a large number of toxic and harmful pollutants accumulate in the substrate,which will be re-released into the overlying water under certain exchange effects,and is the main secondary source of pollution affecting and restricting the overlying water quality.In the process of black smelly water pollution remediation and treatment,the remediation of substrate pollution has become one of the difficult points of river pollution control.Nitrogen exceedance is currently an urgent problem in many rivers and lakes,and when exogenous nitrogen is effectively controlled,the release of endogenous nitrogen in the substrate becomes an important source of contamination of water bodies.In this experiment,sediment from the Tongjia River in Lezhi County,Sichuan Province,was collected and tested in a home-made reaction column for the migration and transformation of endogenous nitrogen.The release and transformation patterns of nitrogen from Ca O₂ and peroxyacetic acid in contaminated substrates under different environmental conditions（dosing amount,dosing depth,temperature and aeration）were investigated.The main findings of the study are as follows.（1）The application of Ca O₂ promoted the process of nitrification of the overlying water and converted the ammonia nitrogen of the overlying water into nitrite and nitrate nitrogen.The reduction rate of ammonia nitrogen was 47.89%,53.78%,58.33%and 61.49%respectively.The ammonia nitrogen in the overlying water was significantly and positively correlated with the total nitrogen in the substrate,the nitrate nitrogen in the overlying water was significantly and positively correlated with the total nitrogen in the interstitial water,and the nitrate nitrogen in the interstitial water was significantly and positively correlated with the total nitrogen in the interstitial water in the four test groups with Ca O₂.The addition of peroxyacetic acid inhibited the nitrification process,and ammonia nitrogen increased in the initial stage of each test group,while nitrification gradually prevailed as the reaction progressed,promoting the nitrification of ammonia nitrogen into nitrate and nitrite nitrogen.At the same time the nitrite nitrogen of the interstitial water undergoes oxidation under aerobic conditions to produce nitrate nitrogen,and releases nitrate nitrogen to the overlying water.（2）The removal rate of ammonia nitrogen from the overlying water was 53.78%and54.69%when the amount of Ca O₂ was 10g and the depth of application was 10cm and 20cm respectively.39.55%and 43.66%of the total nitrogen in the substrate was reduced in the10cm and 20cm groups respectively.In the 10cm and 20cm groups,nitrate-nitrogen was mainly diffused from the interstitial water,while nitrite-nitrogen was produced from the overlying water.The ammonia nitrogen in the overlying water was significantly and positively correlated with the total nitrogen in the overlying water and the total nitrogen in the substrate.When the amount of peroxyacetic acid was 10g and the depth was 10cm and20cm,the organic nitrogen in the substrate formed ammonia nitrogen through ammonification,and the concentration difference migrated to the interstitial water,which increased the concentration of total nitrogen in the interstitial water and overlying water.20cm group of overlying water ammonia nitrogen was mainly diffused from the interstitial water,and nitrification occurred to produce nitrite nitrogen.The total nitrogen in the sediment is dominated by ammonia nitrogen,which continuously diffuses into the interstitial water,thus affecting the total nitrogen in the interstitial water.（3）High temperature favours nitrification rate-denitrification rate.In this experiment,the control temperature of 25°C in the simulated summer group resulted in a significant reduction in ammonia nitrogen concentration in the overlying water compared to the control group in both the simulated summer group and the simulated winter group.The difference in ammonia removal between the two groups was not significant,mainly due to the diffusion of ammonia from the substrate and interstitial water to the overlying water.However,the increase rate of nitrate nitrogen in the simulated summer group was higher than that in the simulated winter group,and the nitrification effect in the simulated summer group was stronger than that in the simulated winter group.The reduction of ammonia nitrogen in the overlying water was 61.22%in the simulated winter group and 81.29%in the simulated summer group with the addition of peroxyacetic acid,and the removal of ammonia nitrogen in the simulated summer group was better than that in the simulated winter group.The reduction of ammonia nitrogen in the interstitial water took place through nitrification or ammonia oxidation processes in addition to molecular diffusion.The ammonia nitrogen in the overlying water did not accumulate,mainly because the nitrification in the overlying water converted the ammonia nitrogen into nitrate or nitrite nitrogen.（4）Dissolved oxygen in the water of the aeration group was basically maintained between 8.1 and 9.23.The removal rate of ammonia nitrogen from the overlying water was50.52%and 81.24%in the non-aerated and aeration groups respectively after the addition of Ca O₂.The removal rates of total nitrogen from the overlying water were 34.84%and46.89%respectively,and the removal of ammonia nitrogen from the overlying water in the aeration group was much higher than that in the non-aeration group.The higher dissolved oxygen not only promoted nitrification,but also ammonification.The organic nitrogen in the substrate was ammoniated to produce ammonia nitrogen,which continuously diffused into the interstitial water,making the interstitial water nitrogen rise.The removal rate of ammonia nitrogen from the overlying water was 53.41%in the aeration group after the application of peroxyacetic acid.In contrast,the ammonia nitrogen in the overlying water increased by 4.08%in the non-aerated group.It is believed that under anaerobic conditions the organic nitrogen of the substrate mineralised into ammonia nitrogen and diffused into the interstitial water or overlying water driven by the concentration gradient,resulting in an increase in ammonia nitrogen concentration in the water column.