Effects Of Biochars On Black Soil Adsorbing The Nutrients And Heavy Metals In Pig Manure Biogas Slurry

There are a lot of nutrient elements in the biogas slurry.If it is directly discharged or applied to the soil,it will cause the loss of nitrogen,phosphorus and potassium nutrient elements.The heavy metals in the biogas slurry will also cause secondary pollution of farmland water.Biochar has porous and strong adsorption characteristics.It has a good effect on retaining soil nutrients and passivating heavy metals,but it contains less mineral nutrients.At present,there are many problems in Heilongjiang Province,such as the uneven distribution of black soil resources,the hardening of black soil,the low content of organic matter,and the decrease of the thickness of arable layer,which seriously affect the grain yield and restrict the development of agricultural economy.The purpose of this paper is to provide theoretical basis for the study of biochar and biogas slurry to improve the properties of black soil.It is of great significance to reduce the loss of nitrogen,phosphorus,potassium and other nutrients in black soil,to clarify the adsorption law between black soil and heavy metals,to reduce the pollution of heavy metals on black soil,to solve the problem of drying black soil in Northeast China,to accelerate the development of agricultural economy and increase grain output.The main contents and conclusions are as follows:(1)Poplar charcoal and bamboo charcoal were added to loam,sandy loam and sandy soil respectively.The particle size and proportion of biochar were 0.25mm,0.5mm,1mm and 2%,5%,10%respectively.The field water holding capacity was measured;Two kinds of biochar(10%,0.25mm)were added to three kinds of black soil,and the law of volume moisture content of black soil was studied by changing the centrifugal time and suction;The VG model and BC model of the volume moisture content of black soil at the best centrifugal time were proposed to be combined and compared,the water characteristic curve of black soil was constructed to systematically study the influence of biochar on the water holding characteristics of black soil.The results showed that the order for the field water holding capacity from large to small was loam,sandy loam and sandy soil;poplar charcoal increased the field water holding capacity of black soil more than bamboo charcoal,10%,0.25mm biochar had the best effect.The volume moisture content of black soil was significantly related to suction,which decreases with the increase of suction and centrifugation time.100min was the optimal centrifugation time.The VG model fitting effect of water characteristic curve was better because of the R~2 value was greater than 0.99.(2)Activated carbon,poplar charcoal,bamboo charcoal and loam,sand soil were selected to study the effects of biochar particle size and proportion,initial mass concentration of biogas slurry,oscillation time and temperature on the absorption of nutrients and heavy metals in biogas slurry by black soil.The results showed that the adsorption capacity of black soil to nitrogen,potassium,copper,zinc and chromium could be improved by adding three kinds of biochar.The adsorption capacity increased with the decrease of the particle size of biochar and the increase of the proportion of biochar.The best effect was to add 10%,0.25mm biochar.Adding biochar could reduce the ability of black soil to absorb phosphorus,and the more proportion was added,the stronger the inhibition was;With the increase of the initial mass concentration of biogas slurry,the adsorption capacity of black soil to nitrogen,phosphorus,potassium,copper,zinc and chromium increased significantly.When the same kind of carbon was added,the adsorption capacity of each experimental group of loam was higher than that of each group of sandy soil.When the same initial mass concentration of biogas slurry,the same soil,and different kinds of carbon were added,the adsorption capacity of activated carbon was slightly better than that of poplar charcoal,and poplar charcoal was better than bamboo charcoal.The process of nutrient and heavy metal adsorption by black soil experienced rapid,slow and stable stages.With the increase of temperature,the adsorption capacity of black soil to ammonia nitrogen,phosphorus,potassium,copper,zinc and chromium in pig manure biogas liquid increased gradually,which was endothermic reaction,while the adsorption process of black soil to nitrate nitrogen was exothermic reaction,and the adsorption capacity decreased gradually.(3)The adsorption kinetics model and isothermal adsorption model of nitrogen,phosphorus,potassium,copper,zinc and chromium in pig manure biogas slurry by adding activated carbon,poplar charcoal and bamboo charcoal on black soil were established,and the adsorption characteristics of nutrients and heavy metals in biogas slurry by adding biochar on black soil were studied in depth.The results showed that the quasi second-order adsorption model was more suitable for describing the kinetic process of adsorption of nitrogen,phosphorus,copper,zinc and chromium in biogas slurry by adding biochar on black soil,while the quasi first-order adsorption model had the highest fitting for the kinetic process of quick acting potassium;Freundlich model could better simulate the reaction characteristics of nitrogen,phosphorus,potassium and copper in biogas slurry adsorbed by biochar on black soil under isothermal condition,and Langmuir model could better simulate the reaction characteristics of zinc and chromium in biogas slurry adsorbed by black soil;Both chemical and physical adsorption reactions take place that were in the process of adsorption of nitrogen,phosphorus,potassium,copper,zinc and chromium.(4)Active carbon,poplar charcoal,bamboo charcoal and loam,sandy soil were selected to study the law of nutrient and heavy metal desorption from biogas slurry by black soil.The results showed that for the desorbing process of nitrogen,phosphorus,potassium,copper,zinc and chromium in biogas slurry,the desorption capacity was far less than the effective adsorption capacity.On the one hand,the order of desorption capacity,desorption rate and effective adsorption capacity was the same as the adsorption capacity.On the other hand,a positive correlation was between the desorption capacity and adsorption capacity.Desorption capacity and effective adsorption capacity of activated carbon were the largest when it was added to black soil.The effective adsorption capacity of loam was higher than that of sand soil when adding the same biochar.