Preparation Of Bio-based Composite Coated Controlled-release Urea And Its Nitrogen Release And Residual Film Degradation Mechanism

Bio-based materials are environmentally friendly and renewable,which is an important means to realize the green development of the controlled-release fertilizer(CRF)industry.However,due to its porous structure and poor controlled-release performance,and the influence of core on coating formation is often ignored,the nutrient release period is usually only extended by increasing the coating rate in the traditional single-material multi-layer coating process,resulting in high coating rate and low nutrient release accuracy of CRF.In addition,the characteristics and mechanisms of degradation of biomaterials are lack of systematic research,and their overuse will lead to potential environmental risks.In this study,urea core surface polishing technology was developed to reduce the surface roughness and save the amount of coating material and the cost of coating;secondly,starch was used to prepare bio-based polyurethane(BPU)coating by mutual modification with castor oil to improve the porous structure and hydrophilicity of starch-based coating;and then polyolefin wax-bio-based polyurethane-epoxy resin multilayer heterogeneous composite coating technology was constructed,and response surface models of different production process parameters and nutrient release characteristics were established to regulate the release rate of nutrients at different stages;finally,the degradation and accumulation characteristics of different coatings were clarified through soil degradation tests;photoreactor experiments were used to reveal the key factors and mechanisms driving the degradation of CRF residual film.The main findings are as follows:(1)Preparation of starch/castor oil mutually modified bio-based polyurethane coating and determination of its nutrient controlled release mechanism.Liquefied starch-based polyol(SP)was prepared from starch as raw material,and mutually modified bio-based polyurethane coated urea(BCPCU)was prepared by using castor oil(CO)and SP to modify each other.The hydrophobicity of starch-based coating was increased by 36.4%by CO modification;and the high-temperature wrinkling characteristics of CO coating were improved by SP;while the particle hardness of BCPCU was increased by 17.1% compared with that of single-component coated urea,and the ultra-long release period of 7 months of bio-based material coated CRF was realized.The response surface model revealed the mechanism of nutrient release regulation: a)CO reduced hydrophilic groups and the exudation of impurities,improved the compactness,and delayed the penetration of water molecules;b)the ductility of the cross-linked network was improved after CO modification to avoid the fracture of the polymerization network caused by the swelling;c)The mutual modification technology affected the nutrient flux and controlled release ability by controlling the pore structure of the coating.(2)Establishment of polyolefin wax,bio-based polyurethane and epoxy resin composite coating technology and revealing nutrient regulation mechanism.On the basis of BPU coating,polyolefin wax(PW)was used as the base coat and epoxy resin(EP)as the outer control layer to prepare multi-layer heterogeneous composite coated urea.PW increased the fluidity of urea by 14.2% and reduced the heat loss during the coating process,while EP increased the hardness of coated urea particles by 16.8%,increased the hydrophobicity by 11.6%,and improved the wear resistance of the particles.The response surface model revealed the mechanism of nutrient release regulation by the composite coating process: PW isolated the direct contact between water molecules and urea,which reduced the initial release rate by more than 57.6% and delayed the initial release,while EP reduced the 28-day cumulative release rate by more than 39.3% by reducing the surface hydrophilic groups,improving the compactness of the coating and regulating the release of nutrients in the middle and late stages.Under the condition of the same coating rate,the release period of the composite coating process increased by 3 times compared with that of the single-material multi-layer coating process.(3)Research of abrasive-based fertilizer core surface modification technology and clarify its influence on the amount and cost of coatings.The surface of urea core was polished by 6mm spherical Alumina porcelain beads,and the EDEM discrete element simulation showed that the use of abrasive increased the resultant force of urea by 20.0% and the number of collisions by 13.0% compared with self-friction,which improved the polishing efficiency.The technique reduced the surface roughness by 64.43% and increased the ratio of urea to high roundness(0.950-0.975)by 8.72% by means of friction surface protrusions and filling depressions,so that the film was tightly and evenly adhered and the uniformity of nutrient release was improved.This technology prolonged the release period of BPU coated urea by more than 15%,reduced the amount of coating material by more than 13.0% under the same release period,which can save the cost of coating material by 157 yuan/ton.If the technology is applied,1,400 tons of overused film residue can be prevented from being put into the soil each year.(4)The characteristics of degradation,accumulation,microbial community and metabolism of residual film of CRF in soilThe 807-day buried bag test showed that the degradation rate of SPU was the highest(6.84%),followed by CPU(4.45%)and EP(2.79%).After degradation,the porous structure produced in the coating,and the average particle size of SPU decreased the most(21.3%),the contact angle decreased by 22.3%,the surface carbon content decreased by 47.9%,and the oxygen content increased by 134.6%.Taking BPU as an example,with the continuous input of residual film,its accumulation in the soil will gradually reach a dynamic equilibrium with the amount of degradation,and the residual amount will not exceed 0.88wt?.16 S r RNA sequencing and non-targeted metabolomics analysis showed that the microbial richness and diversity on the residual film surface were significantly lower than those in soil,and bacteria such as Chujaibacter and Ralstonia,as well as fungi such as Fusarium and Penicillium,which were related to polymer degradation,biofilm formation and soil fertility,were significantly enriched.Biofilms composed of lipids,proteins and living cells were formed on the surface of the coating,which significantly enhanced the metabolism of amino acids and polymers,and significantly increased metabolites such as organic acids,organic heterocyclic compounds and benzene compounds,thereby promoting the biodegradation of the residual film.(5)Revealing the key factors driving the degradation of CRF residue film and the mechanism of microbial degradation A strain of Streptomyces sp.B2 was isolated,screened and purified from the surface of the polyurethane residual film that had remained in the soil for a long time,and the degradation process of bio-based and petroleum-based polyurethane plastics under three conditions:microbial,light and light/microbial coupling was simulated by photoreactor.Microbial conditions had the greatest impact on the degradation rate,which was 3.3 ～ 4.4 times that of light,and the degradation time was shortened by more than 9 times compared with natural deep burial conditions,while the material had the least impact.The main mechanism of residual film degradation is as follows: microorganisms colonize the surface of plastics and produce organic acids to provide negative charges to the culture medium,break and age the original carbon chain through hydrogen peroxide induction or oxygen radical attack,and promote the formation of oxidative functional groups such as-OH,C=O,etc.,thereby destroying the structure of the polymer,causing local fragmentation and denudation of the microstructure,and producing secondary microplastic(MPs)particles,resulting in the weightlessness of the original plastic.In summary,this study prepared a composite coating controlled-release urea,and realized the multi-stage segmented regulation of nutrients by regulating the pore structure of different coatings,which delayed the nutrient release rate and saved the amount of coating material and the cost of coating compared with single-material multi-layer coating,and the study reveald the degradation mechanism of residual film under the action of microorganisms,clarified the good degradation characteristics of bio-based coatings,and provided a theoretical basis for the green development and safe application of controlled-release fertilizer industry.