Preparation And Characterization Of Water-retention And Slow-release Fertilizers Based On Halloysite Nanotubes

Fertilizer is an important factor in increasing grain production.In recent years,the utilization rate of fertilizer in China has continued to rise,and the effect of increasing crop yields is obvious.However,the unreasonable application of fertilizer not only reduces the utilization rate of fertilizer,but also brings about many environmental problems such as groundwater pollution,soil salinization and lake eutrophication.On the other hand,water is also an important factor affecting agricultural production.Water not only affects the growth of crops,but also affects the performance of fertilizer.How to improve the utilization of water resources is also an important topic for researchers.Water-retaining slow-release fertilizer based on slow-release fertilizer and water-retaining agent can increase the utilization of water and fertilizer at the same time,which is particularly eye-catching.How to reduce the cost of water-retaining slow-release fertilizer and improve its performance is a problem that scientists need to solve.To this end,this study uses halloysite nanotubes as inorganic fillers and urea as nitrogen source,using respectively sweet potato starch,sodium alginate and sodium carboxymethyl cellulose as natural materials,three kinds of composite water-retaining slow-release fertilizers were prepared and the structure and properties of water-retaining slow-release fertilizers were characterized.The dissertation mainly includes the following three aspects of work:(1)First,the cavity of the halloysite nanotubes was enlarged by acidification,and then urea was added into the cavity of the halloysite with the help of vacuum to prepare an urea-loaded halloysite nanofiller.Then,using the low-cost β-cyclodextrin as a starting material,the butyric acid monoesterified β-cyclodextrin(MAH-β-CD)was synthesized by esterification reaction.Finally,using MAH-β-CD,acrylamide and acrylic acid as monomers,urea as a nitrogen source,N,N-methylenebisacrylamide as a crosslinking agent,urea-loaded halloysite nanotubes as a filler,sweet potato starch as a natural polymer material,under the action of a redox initiation system,a new composite water-retaining slow-release fertilizer based on starch was synthesized by free radical polymerization.The as-prepared fertilizer had excellent water retention capacity,and the water retention rates in soils on the 4th and 10 th day were 46.6 % and 21.6%,respectively.Studies suggested that the addition of halloysite helps to improve the release properties of the water-retaining slow-release fertilizers.(2)Selecting low-cost sodium alginate as a natural polymer material,urea-loaded halloysite as a nano-filler,MAH-β-CD,acrylamide and acrylic acid as monomers,polyethylene glycol dimethacrylate as a crosslinking agent,the system of ammonium persulfate and ascorbic acid as an initiator,a mixed solution was obtained by dissolving them in water.The mixed solution was added dropwise into a calcium chloride solution at a preset temperature by a syringe pump to prepare a pellet fertilizer containing double networks which formed by ion crosslinking and covalent crosslinking based on free radical polymerization.The single-factor experiment showed that the double-network composite water-retaining slow-release fertilizer had the swelling ratio of 107.9 g/g under the optimal reaction conditions.The swelling properties of slow-release fertilizers in three salt solutions(NaCl,CaCl2 and FeCl3)were studied and the maximum swelling ratios were respectively 50.6 g/g,33.8 g/g and 14.6 g/g when the concentration of the salt solutions was 0.1 wt%.By comparing the cumulative release rates of three different types of fertilizers,it was found that the release rate of slow-release fertilizers containing halloysite was significantly slower than that of slow-release fertilizers without halloysite.(3)Applying the above similar process,the mixed solution containing sodium carboxymethyl cellulose,MAH-β-CD,acrylamide,acrylic acid,polyethylene glycol dimethacrylate,initiator and urea-loaded halloysite was prepared.The solution was dropped into a solution of aluminum trichloride at a prescribed temperature to fabricated pellet fertilizer containing double networks which formed respectively by ion crosslinking and covalent crosslinking based on free radical polymerization.The pellet fertilizer possessed more surface ion crosslinking which could enhance the release performance of the slow-release fertilizer.The swelling ratio of the water-retaining slow-release fertilizer under the optimal reaction conditions was 110.2 g/g.The swelling properties of slow-release fertilizers in three salt solutions(NaCl,CaCl2 and FeCl3)were studied and the results showed that the maximum swelling ratios were respectively 33.1 g/g,29.9 g/g and 23 g/g when the concentration of the salt solution was 0.1 wt%.Soil release experiments showed that the culminulative release rates of the water-retaining slow-release fertilizers were 51.2%,64.1% and 95.2% on the 1st,2nd and 5th day,respectively,which indicated the fertilizer had good sustained-release properties.