Preparation And Properties Of Humic Acid Based Hydrogel Sustained-release Material

The low utilization rate of chemical fertilizers and insufficient water supply have limited the development of agriculture in China,leading to rise costs associated with agricultural development,reduced the economic benefits and elevated environmental pollution.Hydrogel as an agricultural and horticultural water-retaining material has received more and more attention.Hydrogel can be used as a carrier to load fertilizer to regulate water and fertilizer integration.Meanwhile,it helps to reduce irrigation frequency,improve the physical and biological properties of soil would increase crop yield.At present,most studies on hydrogel slow-release material for agricultural inclined to be basic applications,and seldom involve the mechanism of release of fertilizer.Based on this,this study uses lignite as raw material to extract humic acid,and then graft copolymerized with sodium alginate and polyvinyl alcohol to synthesize two hydrogel materials,which are used as carriers for urea release.The preparation conditions,structure characterization,swelling and release mechanism,and pot application performance were discussed.The specific studies are as follows:(1)Humic acid(HA)was extracted using lignite as raw material by the combination of oxidation method and alkali dissolved and acid out analysis method.At the same time,the optimal process conditions were determined by response surface experiment:the ratio of volume(mL)of the H2O2 solution to the mass(g)of coal sample was 5.87:1,the concentration of KOH was 3.98%,and the reaction temperature was 60.55℃.Under these conditions the yield of HA could reached up to 57.98%.The results of FT-IR,TGA,UV Vis and DLS showed that HA contains a variety of functional groups,similar to the thermogravimetric changes of coal samples,with low molecular weight and good activity,and the smallest particle size could reached 491 nm.(2)Humic acid and sodium alginate have similar structures,which are commonly used as biostimulants.The KHA/SA/PAA hydrogel as slow-release materials was synthesized from potassium humate(KHA),sodium alginate(SA)and acrylic acid(AA).The best synthesis conditions were as follows:AA,KHA,SA,KPS and MBA were 10g,10%,12%,1.0%,0.12%,respectively.The polymerization temperature was 60℃ and the neutralization degree of AA was 60%.The FT-IR and SEM analysis showed that KHA,SA and PAA undergo the graft copolymerization with each other,and the polymer network has a porous structure.The swelling mechanism of the material at different pH can be described by Scott’s second-order kinetic model.The diffusion behavior of water belongs to non-Fickian diffusion,and the equilibrium swelling ratio was 621.46%at pH=7.By studying the absorption and release behavior of urea,it could be seen that diffusion behavior of water belongs to non-Fickian diffusion,the release rate of urea was quickly in acidic environment.The release mechanism conforms to the first-order kinetic equation and Higuchi equation.The cumulative release rate of urea was 35.6%after 30 days and the release of potassium humate was 0.17 mg/5mL(9.50%).The degradation rate of the product was 1.21%/day.The pot experiment showed that when the soil-to-product ratio was 500:1 after 14 days,the soluble protein content of greens leaves could be increased by 31.4%,the chlorophyll content could be increased by 33.2%,and the MDA content was decreased by 21.1%,indicated that product can effectively improve the quality of crop.(3)Because humic acid has structural characteristics of unsaturated amphiphiles,the KHA/PVA/PAA hydrogel as slow-release materials was synthesized from potassium humate(KHA),polyvinyl alcohol(PVA)and acrylic acid(AA)as raw materials.The best synthesis conditions were as follows:AA/PVA/KHA mass ratio,AA neutralization degree,polymerization temperature,initiator dosage and crosslinker dosage were 10:1:1,65%,80℃,0.25 g,0.06 g,respectively.The FT-IR,SEM,XRD analysis showed that KHA reacted with PAA and participated in the polymerization reaction,and then PVA with them form the interpenetrating network structure through hydrogen bonding,the surface of the product was porous.The swelling and diffusion mechanism of the product was the same as that of KHA/SA/PAA at different pH,but the swelling rate was faster,the equilibrium swelling rate was 876.37 g/g at pH=7.By studying the absorption and release behavior of urea,it could be seen that diffusion behavior of water belongs to non-Fickian diffusion,the release rate of urea was quickly in alkaline environment.The release mechanism conforms to Higuchi equation.The cumulative release rate of urea was 37.5%after 30 days and the release of potassium humate was 0.206 mg/5mL(11.2%).The degradation rate of the product was 1.06%/day.The pot experiment showed that when the soil-to-product ratio was 750:1 after 14 days,the soluble protein content of greens leaves could be increased by 21.9%,the chlorophyll content could be increased by 23.1%,and the MDA content was decreased by 30.1%,indicated that the product have contributed to improve the quality of the crop.