Research On Immobilization Of Biochar-based Heterotrophic Nitrifying Bacteria For Ammonia Nitrogen Removal And N₂O Emission In Water

The object of this study was to explore NH₄⁺-N removal ability of biochar-immobilized heterotrophic nitrifying bacteria from water with low NH₄⁺-N concentration.Adsorption of NH₄⁺-N by biochar as basic research,a strain of heterotrophic nitrifying bacteria,Pseudomonas putida strain-N3,was isolated from the sludge of a wastewater treatment plant,the microbial immobilized body were prepared by using unmodified rice husk biochar and 1 mol·L^-1NaOH/5%H₂O₂/10%H₂O₂/20%H₂O₂/30%H₂O₂modified rice husk biochars(designated as BC/NaOH-BC/5%H₂O₂-BC/10%H₂O₂-BC/20%H₂O₂-BC/30%H₂O₂-BC)with the adsorption method.Then NH₄⁺-N removal kinetics by the microbial immobilization body were studied.The effect of H₂O₂-modified biochar on the ammonia oxidation process and N₂O emission of heterotrophic nitrifying bacteria was further explored.Compared with BC,NaOH modification increased the pH and pH_pzcby1.76 and 1.61,respectively.The conductivity decreased 12.33 m S·m^-1,the specific capacitance decreased 0.75 F·g^-1,and total basic oxygen-containing functional groups increased 0.611 mmol·g^-1,the total acid oxygen-containing functional groups decreased 0.030 mmol·g^-1,and the amount of microorganisms adsorbed and fixed on the surface of NaOH-BC increased by 129.77 nmol P·g^-1(Pseudomonas putida strain-N3);H₂O₂modification decreased pH?pH_pzcand conductivity,5%,10%,20%,30%H₂O₂modified biochar decreased in sequence,the number of acidic oxygen-containing functional groups of biochar increased by 0.064,0.124,0.159,0.177 mmol·g^-1,and the amount of microorganisms adsorbed and fixed on the surface of H₂O₂modified biochar decreased by 55.62,86.52,98.88,111.24 nmol P·g^-1(Pseudomonas putida strain-N3).The selected three strains of N1,N2,and N3 have a removal rate of52.65%,64.18%,and 72.02%for NH₄⁺-N(NH₄⁺-N concentration of 9.43 mg·L^-1)in heterotrophic nitrification medium at 72 h,N3 has better heterotrophic nitrification capacity than N1 and N2,N3 was identified as Pseudomonas putida.Results showed that the removal effect of NH₄⁺-N by biochar-based heterotrophic nitrifying bacteria immobilized body is better than that of biochar.Among them,the NaOH-modified biochar immobilized body has the best removal effect on NH₄⁺-N,the maximum removal rate of NH₄⁺-N is 89.08%;the concentration of NH₄⁺-N is less than 1.5 mg·L^-1(ammonia-nitrogen limit in the Environmental Quality Standards for Surface Water Class IV)after 48 h;In addition,the removal process of NH₄⁺-N by biochar-based microbial immobilized body is more in line with the quasi-second-order kinetic equation.Experiments on the effects of unmodified biochar and 20%,30%H₂O₂-modified biochar on ammonia oxidation process and N₂O emissions,the results showed that the aromaticity of biochar modified by H₂O₂weakened,which resulted in the weakening of the electron-charging ability and oxidation activity of biochar,and the specific capacitance reduced,not conducive to electron transfer between NH₄⁺-N and microorganisms adsorbed on biochar,and has an inhibitory effect on the ammonia oxidation process;There is a good correlation between the cumulative emission of N₂O and the concentration of NH₂OH,the NH₂OH concentration of H₂O₂-modified biochar was generally higher than that of BC during the experiment,so the cumulative N₂O emissions of 20%,30%H₂O₂-modified biochar were higher than BC.This study provides a certain theoretical basis for the application of the biochar-based heterotrophic nitrifying bacteria immobilized body in the treatment of low-concentration ammonia nitrogen wastewater.It has a positive significance to study the impact of N₂O emissions in the ammonia oxidation process in water in order to understand its mechanism.