Removal Mechanism Of Phosphorus From Sewage By Constructed Wetland Mediated By Biochar

Nitrogen and phosphorus is one of the main pollutants in sewage and wastewater.Excessive presence of nitrogen and phosphorus will cause excessive plant growth and may lead to eutrophication of water.Among the common methods of sewage treatment,adsorption method has become a research hotspot in recent years because of its high efficiency,accuracy and good economy.The base materials of the adsorption method are mainly from agricultural and forestry wastes such as plant roots,stems and leaves,animal bones and microorganisms.The cost of pollutant removal can be greatly reduced by adsorbents made from cheap materials.However,the adsorption capacity of single biological substrate is usually low,and reasonable modification can solve this problem.In this study,chestnut shell agricultural and forestry waste was used as raw material to form biochar by pyrolysis,and modified by mixed solution of ferric chloride and potassium permanganate to form modified chestnut shell biochar（MBC）.The effect of MBC on phosphorus adsorption was explored,and the material was characterized,adsorption kinetics and isothermal adsorption analysis to explore its adsorption mechanism.Finally,MBC was applied to the simulated constructed wetland device to test its effect in practical application.The experimental results are as follows:（1）compared with chestnut shell,gravel and sand,the adsorption effect of BC and MBC on P was better.The maximum adsorption capacity of MBC reached 0.7mg·g-1 and reached equilibrium at about 14h.The maximum adsorption capacity of BC to P was 0.2mg·g-1,and trace desorption occurred after equilibrium.（2）By studying the adsorption mechanism through adsorption kinetics and adsorption isotherm,it can be concluded that the adsorption process of BC on P is monomolecar chemisorption,and the adsorption process is easy to carry out,and the saturated adsorption capacity is 0.8275mg·g^-1.The adsorption of P by MBC is also homogeneous monolayer chemisorption,with a saturated adsorption capacity of 0.788mg·g^-1.The coexistence of antibiotics in the solution had little effect on the adsorption of P by MBC.（3）In order to further understand the adsorption mechanism of P by biochar,the characterization of chestnut shell（BL）,chestnut shell biochar（BC）and modified chestnut shell biochar（MBC）materials with the same particle size and all washed and dried with deionized water were carried out.The results showed that:SEM images show that MBC has a rougher surface than BC and BL,and the material surface is modified into new functional groups,which provides a more favorable physical and chemical basis for the adsorption and purification of pollutants.EDS results show that the modified materials are indeed loaded with new substances and increase the oxygen-containing functional groups.Fourier transform spectra showed that the functional groups on the surface of BC and MBC mainly included c-H,C=C,C=O,O-H and C-O.The number of functional groups on the surface of MBC changed before and after adsorption,and the number of functional groups decreased after adsorption,which also confirmed that the adsorption of phosphorus by MBC was caused by the reaction of its surface functional groups with pollutants.BET results show that the specific surface area of BC is larger than that of BL,while the pore size of MBC decreases,which may be due to the modification of metal salt ions filling the pores.XRD shows that BC and MBC have obvious crystal structure.XPS made a qualitative and quantitative analysis of the elements contained in the substance,and the results showed that the carbon content of BC increased and the oxygen content decreased compared with BL,which was similar to the results characterized by EDS in the above experiment.By observing the atomic ratio of each element in MBC,it can be found that the content of oxygen element increases,because BC is modified by strong oxidant to add oxygen-containing functional groups on the surface of biochar.Meanwhile,the content of manganese and iron element also increases significantly.All of the above characteristics provide theoretical support for MBC as a constructed wetland substrate for sewage purification.（4）the MBC is applied to simulate artificial wetland plant for sewage purification experiment results show that the MBC exists more excellent purification effect,the total phosphorus removal rate increased by 3.4%,on the average of total nitrogen removal rate was increased by 23%and the water is more stable,for ammonia nitrogen removal rate increased by 5%,increased by an average of 5%for COD.In order to further study of electron transfer in the system and the influence of microorganism on pollutants removal,artificial wetland simulator experiment to join and coupling control microbial fuel cells,and plant operation after a period of time samples from the matrix analysis of high flux,the results show that,after joining a microbial fuel cell,strengthened the electron transfer in the system,It promoted the growth and reproduction of many new microorganisms and enhanced the removal of pollutants.In conclusion,the method of using modified chestnut shell biochar to strengthen constructed wetlands is feasible for villages with excessive sewage discharge at present,and provides reference for solving the same problem in the future.