| Water and fertilizer have been the important limiting factors of agricultural development. China is the country that water natural resources is in short supply, also a major fertilizer consumer country. Problems of low utilization of fertilizer and environmental problems caused by fertilizer use improper and backward applied technology cannot be ignored. Water-retaining agent is a kind of typical functional polymer material and a crosslinked three-dimensional networks of hydrophilic polymer that can absorb large amounts of water and retain water under high temperature and high pressure. The characteristics of water-retaining agent in structure are that it has a large number of hydrophilic groups and moderate crosslinking network structure. Due to the physical structure, water absorption and retention function of water-retaining agent, it is widely used in many specialized applications, such as farmland drought defence, fertilizer synergist and improve soil etc. In order to improve the utilization rate of water and fertilizer resources, a slow release fertilizer that both have water absorption and retention function, also have the nutrients release properties will be prepared which have a great significance on high yield and high benefit agriculture, and reduce environmental pollution.A novel distarch phosphate/polyacrylic acid(DSP/PAA) water-retaining agent containing phosphorus and potassium was prepared by direct grafting of acrylic acid neutralized by KOH onto distarch phosphate(DSP) in the presence of a crosslinker N, N-methylenebisacrylamide(MBA), thermal initiator ammonium persulfate(APS) and photoinitiator 2, 2-Dimethoxy-2-phenylacetophenone(BDK) by UV irradiation. The optimal condition of synthesis was defined by single-factor experiments. The results showed that the optimal condition was that the weight ratio among the MBA, APS, BDK and DSP to AA was 1.00%, 1.50%, 1.25% and 50%, neutralization degree of AA 80%. The maximum water absorbency of the synthesized water retaining agent under this experimental condition was 1185.4 g/g in distilled water and 110.2 g/g in 0.9%(w/w) NaCl solution, respectively. The resultant water-retaining agent was characterized by Fourier transform infrared spectroscopy(FTIR), thermogravimetric analysis(TG), Scanning electron microscope(SEM) and energy dispersive spectrometer(EDS). FTIR showed that distarch phosphate and acrylic acid realized a copolymerization reaction. TG showed that the resultant composite had excellent thermal stability. SEM micrographs of DSP/PAA exhibited a comparatively smooth and porous surface. EDS showed that the composite contained plant nutrient elements P and K.In order to investigate the kinetic mechanism of the water-retaining agent in distilled water and 0.9%(w/w) NaCl solution, the pseudo-first-order and pseudo-second-order models would be considered. The results showed that the swelling process both in distilled water and 0.9 wt% NaCl solution were followed the pseudo-second-order swelling kinetic model. Moreover, the water retention rate of DSP/PAA hydrogel under different conditions were investigated. Under natural conditions for 15 days, water retaining rate was still 50%. The filtration rate is 10.4% carried down centrifugal at rotate speed of 6000 rpm in 60 minutes. The water retention rate at 60, 80 and 100℃ maintained 67.1%, 60.1% and 48.1% after 360 minutes. As a result, the resultant composite hydrogel had good water retention properties.In this paper, a slow release urea with water absorption and retention function was prepared by mixing granulation of urea and distarch phosphate/polyacrylic acid water-retaining agent. With the increasing of water-retaining agent, the cumulate nitrogen release rate of slow release urea in water after 7 days decreased. But taking cost and slow release effect into account, the weight ratio of water retaining agent to AA was 5%. The experimental results of sand column leaching showed that the product exhibited a good slow release property. After 1 day, the cumulate nitrogen release rate of slow release urea was 14.8%, and it was 71.0% after 15 days. Moreover, the slow release urea had a good water retention ability. When added to soil, the water evaporation rate was 35% after a week, and 62% after 20 days, which were much lower than the soil without slow release urea. |
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