Removal Effect Of Ammonium And Phosphate From Simulated Livestock And Poultry Wastewater By Chitosan Modified Magnesium Impregnated Straw Biochar

Livestock wastewater is the main source of agricultural non-point source pollution in China,which is easy to cause serious pollution to water bodies.As important agricultural wastes in China,the resource utilization of wheat straw and corn straw has always been the focus of attention.Biochar prepared from wheat straw and corn straw can be used for the treatment of ammonium and phosphorus and other pollutants in livestock wastewater,which can not only improve agricultural non-point source pollution,but also improve the resource utilization efficiency of agricultural waste such as straw,which has important strategic significance for China to achieve carbon neutrality by 2060.In this study,corn straw and wheat straw were used as raw materials,and under different pyrolysis temperature,pyrolysis time,solid-liquid ratio and MgCl₂ concentration conditions,the optimal conditions for preparing biochar by impregnating corn straw（MgCBC）and wheat straw（Mg WBC）with MgCl₂ were explored.Then the magnesium impregnated straw biochar was modified by chitosan,and the physicochemical properties,surface structure and functional groups of the biochar were characterized and analyzed.The removal effect of NH₄⁺-N and PO₄^3--P in simulated livestock wastewater by chitosan modified magnesium impregnated straw biochar was analyzed,and the adsorption mechanism was revealed,providing a reference for the removal of NH₄⁺-N and PO₄^3--P in livestock wastewater and the resource utilization of straw.The main results are as follows:（1）The preparation conditions of MgCBC and Mg WBC were investigated by single factor experiment.The experimental results show that the optimal preparation conditions of Mg WBC are as follows:pyrolysis temperature 500°C,pyrolysis time 5 h,MgCl₂ concentration 2.5 mol/L and solid-liquid ratio 1:10.The optimum preparation conditions of MgCBC were pyrolysis temperature 400°C,pyrolysis time 3 h,MgCl₂concentration 2 mol/L and solid-liquid ratio 1:10.Too high pyrolysis temperature and too long pyrolysis time will inhibit the adsorption performance of biochar.（2）The adsorption experiments of NH₄⁺-N and PO₄^3--P by MgCBC and Mg WBC showed that the adsorption processes of the two biochar were consistent with the Second-order kinetic model and Langmuir model.Langmuir simulation data showed that the maximum adsorption capacity of PO₄^3--P for MgCBC and Mg WBC at room temperature（298K）was 155.39 mg/g and 143.50 mg/g,respectively,which was 7.65%higher than that of Mg WBC.The maximum adsorption capacity of NH₄⁺-N was 28.53mg/g and 26.01 mg/g,respectively,and MgCBC was 8.83%higher than Mg WBC.The adsorption and regeneration experiment results of MgCBC and Mg WBC showed that after three desorption times,the removal rates of PO₄^3--P by MgCBC and Mg WBC were 90.28%and 90.19%,and the removal rates of NH₄⁺-N were 42.97%and 33.61%,respectively.The results showed that the reproducibility of MgCBC was significantly better than that of Mg WBC.Meanwhile,the main adsorption mechanisms of NH₄⁺-N and PO₄^3--P by MgCBC and Mg WBC are ion exchange and precipitation.（3）As the adsorption performance of MgCBC was better than that of Mg WBC,the CS-MgCBC was further investigated by modifying MgCBC with chitosan,and the removal effect of NH₄⁺-N and PO₄^3--P in simulated livestock wastewater was studied.The results show that the adsorption of NH₄⁺-N and PO₄^3--P by CS-MgCBC conforms to the Second-order kinetic model and Langmuir model.Langmuir model simulation data showed that the maximum adsorption capacity of CS-MgCBC on NH₄⁺-N and PO₄^3--P was 35.59 and 221.89 mg/g,respectively.Compared with the maximum adsorption capacity of MgCBC,the maximum adsorption capacity of CS-MgCBC for NH₄⁺-N and PO₄³-P increased by 19.8%and 29.8%,respectively.Compared with the adsorption capacity of other adsorbents for NH₄⁺-N and PO₄³-P,the maximum adsorption capacity of CS-MgCBC increased by 48.3%and 68.9%,respectively.When p H=9,the removal rate of NH₄⁺-N reaches the highest,which is 48.4%.When p H=6,the removal rate of PO₄^3--P reached 99.6%.Chitosan modification improved the surface area of biochar,and formed abundant functional groups（-OH,-CH₃,-NH₂）on the surface of biochar,thus improving the adsorption capacity of CS-MgCBC.The main mechanisms of adsorption of NH₄⁺-N and PO₄^3--P by CS-MgCBC are electrostatic adsorption,precipitation and ligand exchange.After three times of desorption,the removal rate of NH₄⁺-N and PO₄^3--P reached 42.97%and 90.19%,respectively.CS-MgCBC has good adsorption and environmental performance,and is an effective adsorbent for removing NH₄⁺-N and PO₄^3--P from livestock wastewater.（4）MgCl₂ impregnated biochar solves the problem of poor adsorption capacity of original biochar.Chitosan modified MgCl₂ impregnated biochar can improve the specific surface area of biochar,and further improve the adsorption capacity of biochar.This study not only provides a reference for the treatment of NH₄⁺-N and PO₄^3--P in livestock wastewater,but also effectively reduces the disposal pressure of agricultural solid waste and achieves the purpose of treating waste with waste.