Preparation And Release Characteristics Of Biochar-based Urea Fertilizer With Core-shell Structure

Urea is widely used as the nitrogen fertilizer for its high nitrogen content.But nitrogen use efficiency is less than 30%.The biochar has rich pore structure and good adsorption capacity,so it can be used as fertilizer carrier to improve the utilization rate of the nitrogen fertilizer and soil environment.Also the waste of the agricultural and forestry resources and the pollution to the environment can be avoided.In this research,the preparation and release characteristics of core-shell biochar-based urea fertilizer were studied.The mechanical properties and release properties of biochar fertilizer were studied by pressure test and leaching experiment,and the contact angle and SEM,were used to study the mechanical properties and release performance of biochar fertilizer.The K / P equation fitting of the release process revealed the mechanism of nutrient release.In this paper,the difference of physical and chemical properties between rice husk biochar and cottonseed shell biochar was analyzed to screen out the more suitable biochar materials for shell.The fertilizer forming properties of modified acetate starch and chitosan binder were investigated,and the biochar production temperature,biochar particle size and biochar fertilizer ratio were analyzed.The effects of fertilizer parameters such as binder concentration on maximum anticrushing pressure,nutrient release performance and water holding capacity.Biochar based urea fertilizer with core shell structure was prepared by using rice husk biochar,cottonseed shell biochar and 2% mass fraction chitosan as binders.SEM of biochar fertilizer particles before and after leaching,contact angle and nutrient release model were used to study the release mechanism of biochar fertilizer from core-shell structure The research contents and conclusions of this research were as follows:1.The difference between the appearance structure,the specific surface area,the industrial analysis,the functional group and the hydrophobicity of the rice husk biochar and the cottonseed husk biochar prepared at 450℃ was analyzed.The results showed the porosity of the rice husk biochar was more and more permeable,the pores were exposed,and the pores of the cottonseed husk biochar were small,shallow and most of the biochar residue was covered by the biochar residue.The specific surface area of the rice husk biochar was larger than that of the cottonseed husk biochar,and was 37 times that of the cottonseed husk biochar.The difference of water and volatile of the two kinds of biochar was not significant,the fixed biochar content of the cottonseed husk biochar was 1.7 times of the rice husk biochar,and the ash content of the rice husk biochar was 6.65 times of that of the cottonseed husk biochar.The cottonseed husk biochar retained more functional groups with respect to the rice husk biochar,and the hydrophilicity of the cottonseed husk biochar was stronger than that of the rice husk biochar.2.The effect of the biochar prepared temperature,the biochar particle size,the biochar fertilizer ratio and the binder concentration on the anti-crushing pressure and the release performance of the fertilizer was studied.The results showed the pyrolysis temperature of the biochar has little effect on the maximum anti-crushing pressure of the fertilizer.With the decrease of the biochar particle size,the maximum anti-crushing pressure of the fertilizer was gradually increased.When the biochar particle size was less than 0.198,the maximum anticrushing pressure of the fertilizer was 25 N.With the decrease of the urea to biochar ratio,the maximum anti-crushing pressure of the fertilizer was gradually increased,and the maximum anti-crushing pressure of the biochar fertilizer with the urea to biochar ratio of 1:4 was 15 N.The effect of temperature on the release characteristics of rice husk was not great,the release performance of the biochar fertilizer prepared by the rice husk biochar prepared at 450℃ was the best,and the fertilizer release characteristics of the rice husk biochar prepared at the temperature of 550℃ and 650℃ were basically the same.The effect of biochar particle size on the release characteristics of the fertilizer was not great,and the release characteristic of the biochar with 60-mesh(more than 0.245 mm)was the best.With the decrease of the urea to biochar ratio,the release rate of the biochar fertilizer is gradually reduced,and the release period is prolonged.With the increase of the binder concentration,the release rate of the fertilizer was decreased firstly,and the starch binding with the mass fraction of 10% had the best fertilizer release property.3.The biochar prepared at 450℃ was used as the raw material,and the 2% mass fraction of the chitosan solution was used as the binder to prepare the chitosan-binder biochar fertilizer.The effects of the biochar to urea ratio and the chitosan binder amount on the anti-crushing pressure,the release property and the water-holding capacity of the fertilizer were studied.The results showed the maximum anti-crushing force of the fertilizer granules with the biochar to urea ratio of 1:1 and the addition of the chitosan binder was 29.42 N,and the maximum anti-crushing force of all the other fertilizer-forming particles was over 70 N.With the increase of the biochar to urea ratio and the chitosan binder amount,the maximum anti-breaking force,the release property and the water-holding capacity of the fertilizer were gradually enhanced.When the biochar to urea ratio was 1:3 and the chitosan binding amount was 4%,the maximum anti-breaking force of the fertilizer was the highest,reaching 285.42 N.At this time,the release characteristic of the fertilizer was also the best,and the release period was increased by 4 times as compared with the pure urea.4.The single-structure fertilizer(RH and CSH)and core-shell-structure fertilizer(CR and RR)was prepared using the rice husk biochar,the cottonseed husk biochar prepared at 450 and the 2% mass fraction chitosan solution are used as a binder.The influence of the biochar types and the core-shell structure on the release characteristics of the fertilizer was studied and its release mechanism was explored.The results showed the release performance of the biochar fertilizer with the core-shell structure was better than that of the singlestructure.The fertilizer which core and the shell was the rice husk biochar has better release characteristics than the fertilizer which the core and the shell was different biochar.Compared with the pure urea,the urea release rate in the initial stage of the core-shell structure biochar fertilizer was reduced by 8 times,and the release period was prolonged by four times.The release of urea in the core-shell biochar fertilizer was divided into two stages,and the initial stage was a large amount of urea release stage and a slow release phase of a small amount of urea in the later stage.The fitting model showed the urea release in the early stage was a framework dissolving mechanism,and the urea broke through the physical barrier of the biochar particles and the chitosan binding to form a membrane under the transportation of water molecules,and the later release model was the Fick diffusion mechanism,The urea,which entered the pores of the biochar,was slowly released under the carrying of water molecules,through the barrier of the biochar pore,the barrier of the biochar particles and the membrane formed by the chitosan adhesive.