Study On The Performance And Mechanism Of Phosphorus Adsorption By Cow Dung Biochar Modified With Magnesium On Farmland Soil

Phosphorus is one of the main factors leading to eutrophication of water body,and the source of phosphorus entering the water is mainly industrial and living point source emissions and agricultural non-point source loss.With the effective control of point source pollution,the non-point source phosphorus loss has become the focus of treatment.In the solution,by adding adsorbent in the soil from the source of phosphorus pollution control is the most direct and effective method,which can avoid the loss of phosphorus,but also as a slow release fertilizer to provide nutrients to the plant in later period.In recent years,biochar has been widely studied for its special environmental functions and is used for the control of nutrient loss in farmland.In order to further explore the potential of biochar in controlling the pollution of non-point source phosphorus in farmland,this study used Shangri-La yak dung as bio-raw material and modified with magnesium ion to prepare the modified biochar which has a strong adsorption capacity for dissolved inorganic phosphorus and organic phosphorus in farmland soil.Finally,soil column simulation experiments were conducted to determine the effects of phosphorus absorption and soil improvement.At the same time,the adsorption mechanism of the modified biochar on the adsorption kinetics and adsorption isotherm model of inorganic phosphorus and organic phosphorus was studied.The main conclusions are as follows:(1)The dried and crushed cow dung was thoroughly immersed in a magnesium ion solution with a concentration of 2 mol/L for 2 hours and then pyrolysised by a muffle furnace at an elevated temperature of 5?/min and final temperature of 300?,450?,600? for 3 hours under anoxic conditions.To be natural cooling after the experiment can be obtained the modified biochar(recorded as MBC300,MBC450 and MBC600,respectively).The results showed that the adsorption capacity of phosphorus was MBC600>MBC450>MBC300.Compared with the biochar which without modified with magnesium ion.the adsorption capacity of the modified biochar to phosphorus is more stronger.And it's adsorption capacity to inorganic phosphorus and organic phosphorus was increased by 39.89 times and 1.33 times,respectively.Theoretically,it can be used to control the loss of dissolved inorganic phosphorus and organic phosphorus in farmland soils.(2)Based on the surface morphology.surface area.elemental analysis and pH of the modified biochar were studied,it was found that the higher the pyrolysis temperature,the more rough of surface,the larger of specific surface area,the less element content of C?H?O?N and the higher of pH value.(3)It was found that many MgO crystals and magnesium chloride hydrate crystals were formed on the surface by characterized with XRD.The MgO grains were positively charged in the solution,and then the negatively charged phosphorus groups were adsorbed.And the hydrated magnesium chloride was chemically bonded to phosphorus to form magnesium hydrogen phosphate dihydrate(Mg(H2PO4)2)and magnesium hydrogen phosphate(MgHPO4)and attached to the surface of the biochar.Theoretically,it explained that magnesium modified biochar can effectively adsorb phosphorus and control the loss of phosphorus.(4)The adsorption capacity of MBC600 to organic phosphorus and inorganic phosphorus were determined.And it was found that they are more in line with the quasi two level dynamic and Langmuir isotherm model,the adsorption of MBC600 on phosphorus is mainly controlled by chemical adsorption with single molecule layer of uniform adsorption method.Besides,the theoretical maximum adsorption capacity to organic phosphorus and inorganic phosphorus can reach 19.18 mg/g and 26.12 mg/g,respectively.(5)The leaching experiment was carried out by adding the magnesium modified biochar to the soil column for leaching experiment.The analysis of the volume,pH and phosphorus content of the collected leachate was carried out to analyze the control of phosphorus loss after inputting farmland.The results showed it not only can reduce the loss of phosphorus in the early stage,but also provide nutrients for plants(the release of phosphorus is 100%)slowly in the last period.Therefore,it has played dual role in controlling phosphorus pollution at first and then as a slow-release fertilizer to provide nutrition for plants.The optimized result is that the best input of MBC600 in the surface soil application is only 0.05%.Therefore,MBC600 can be used as a good adsorbent for inorganic phosphorus and organic phosphorus to control phosphorus loss in agricultural soil.(6)The basic data and theoretical results obtains by modifies cow dung biochar and its study of phosphorus adsorption can open up a new path for the application of high added value of Plateau Yak dung.It is applied to the control of phosphorus loss in farmland soil,to lay the theoretical foundation for the application of modified biochar to prevention and control phosphorus pollution in farmland,and to provide the technical support for the restoration of degraded wetland and phosphorus pollution prevention.