Preparation And Properties Of Humic Acid/Urea Aldehyde Biodegradable Polymer Materials

Urea aldehyde(UF)slow-release nitrogen fertilizer,as a kind of polycondensation,has high crystallinity and is difficult to be degraded by microorganisms,resulting in a long slow-release period of nutrients.The typical inverted "L" type nutrient release curve is inconsistent with the growth law of most crops,so the nutrient utilization rate is low.As a growth regulator,humic acid(HA)not only improves soil structure and physical and chemical properties significantly,but also improves soil fertility by increasing nitrogen,resolving phosphorus and promoting potassium,and can stimulate the growth and development of crops and promote physiological metabolism in crops.In this paper,UF was modified by HA,and humic acid modified urea aldehyde(CHAUF)was prepared by in situ reactive extrusion process,so as to adjust the nutrient release rule of UF and make it more suitable for crop growth.The preparation mechanism,morphology and composition of CHAUF,nutrient release capability in water,biodegradation capability in soil,and effects on soil physicochemical properties and plant growth of CHAUF were systematically studied.Specific research contents and results are as follows:(1)First,formaldehyde was used to modify HA,so that the modified humic acid(CHA)hydroxyl group was increased to a certain extent compared with HA;Subsequently,the UF precursor hydroxymethylurea(MU)was dehydrated and condensed during the extrusion process to generate UF.At the same time,the generated UF was grafted onto the surface of CHA,and finally CHAUF was obtained.The optimum process parameters were obtained by orthogonal experiment and single factor optimization experiment.The results of Fourier infrared spectroscopy(FTIR),thermogravimetric(TG),X-ray diffraction(XRD)and scanning electron microscopy(SEM)tests showed that CHAUF fertilizer was synthesized successfully and the two groups are not physically mixed,but a crosslinking reaction occurs,forming a loose and dense network structure.The introduction of HA destroys the ordered arrangement and regular structure of UF macromolecular chains,reduces the crystallinity of UF and reduces the crystallization region of UF.Secondly,the nitrogen nutrient release performance of CHAUF with different HA content and different urea-formaldehyde ratio was studied by static water release experiment.The results showed that after 90 days of treatment,the cumulative nitrogen release rates of10% CHAUF-1.2,20% CHAUF-1.2,30% CHAUF-1.2,20% CHAUF-1.4,20% CHAUF-1.6were 49.90%,38.48%,48.71% and 64.79%,respectively.(2)The biodegradability of CHAUF in soil and its effects on soil physical and chemical properties and plant growth were studied by pot experiment system.The results showed that:(1)the modification of HA significantly improved the release rate of UF nutrients.After 90 days,the cumulative nitrogen release rates of 10% CHAUF-1.2,20% CHAUF-1.2,30%CHAUF-1.2,20% CHAUF-1.4 and 20% CHAUF-1.6 reached 67.86% and 61.04.HA affects the structure and molecular chain length of UF,and then affects the nutrient release performance of fertilizer.(2)CHAUF polymer slow-release fertilizer significantly improved the physical and chemical properties of the soil after it was applied to the soil.After CHAUF fertilizer is applied to the soil,firstly,the soluble small molecular components(a very small amount of urea and MU which are not completely reacted)are released into the soil quickly,which increases the contents of N and C in the soil and the content of organic matrix in the soil(mainly humic acid).Secondly,the basic physical and chemical properties of soil microorganisms are seriously affected by affecting the p H value,cation exchange capacity and conductivity of the soil,which can provide a good environment for the growth and reproduction of soil microorganisms.Thirdly,the activity of enzymes which are beneficial to plant growth and promote fertilizer decomposition in the soil is stimulated.Fourthly,the addition of HA makes heavy metal ions widely existing in the soil and humic acid generate insoluble precipitation of heavy metal-humic acid through coprecipitation,which promotes the combination of heavy metal ions and humic acid and the transformation to a stable residual state,thus reducing the toxic effects of heavy metal ions on plants and soil.(3)After CHAUF polymer slow-release fertilizer was applied to the soil,the N content,C content,dry weight and yield of small rapeseed were significantly improved.Under the treatment of 20%CHAUF-1.4(humic acid content is 20wt%,urea-formaldehyde molar ratio is 1.4:1),the plant yield is the highest,but the plant nitrogen and carbon content is the highest,and the dry weight is the highest at 20%CHAUF-1.6(humic acid content is 20wt%,urea-formaldehyde molar ratio is 1.6:1).One is that the root system is the most developed under the treatment of 20%CHAUF-1.6,which can absorb water and nutrients in the soil faster;The second reason is that when the humic acid content is 20% and the molar ratio of urea to formaldehyde is 1.6:1,the soil p H value and conductivity are more stable and suitable for microbial reproduction,and the soil enzyme activity will be higher,which will accelerate the decomposition of longer molecular chains,shorten the release cycle,and more fully provide the increased nutrient demand of small rapeseed in the later stage,thus leading to increased production rates.The reason why the yield of 20%CHAUF-1.4 is higher may be that under this treatment,small rapeseed absorbs more water and has greater water content,which has its own advantages and disadvantages in terms of quality.