One-Pot Synthesis and Hydrolysis Behavior of Highly Water-Soluble Ammonium Polyphosphate

Ammonium polyphosphate (APP) exhibits high water solubility, mobility, and slow-release properties, which can greatly improve the utilization of phosphorus by alleviating soil phosphorus fixation. However, obtaining an efficient method of preparing APP that involves low energy consumption and production cost remains a challenge in current industrial production. Herein, by using urea phosphate (UP) as the main feedstock, we established a simple one-pot method for synthesizing highly water-soluble APP and explored the regulatory mechanism of the reaction temperature, the ratio of urea phosphate to monoammonium phosphate (MAP)/potassium dihydrogen phos-phate (MKP), and the reaction time on APP performance. After analyzing the products obtained from different reaction conditions, we classified the polymerization process into the following stages: chain generation and chain growth. The chain generation reaction occurs simultaneously with the chain growth reaction, and the chain growth reaction tends to dominate with further reaction time. Furthermore, the optimum processing conditions for industrial production were obtained by comparing product performance, and the resulting APP products exhibit the following characteristics: the N and P contents were 20.6% and 58.8%, respectively, the polymerization rate (Pr) was 93.6%, the degree of polymerization was 4.22, and the solubility was 263.91 g/100 g H2O. Finally, to characterize the stability of the APP liquid fertilizer, we further investigated the hydrolysis behavior of APP and obtained the apparent activation energy (Ea = 93.17 kJ/mol) of APP hydrolysis and the evolution of polymerization degree distribution of APP at different pH values. Overall, our results indicate that low temperature and alkaline conditions can greatly prolong the storage time of ammonium polyphosphate-based liquid fertilizers, which provides a basis to compound highly concentrated ammonium polyphosphate-based liquid fertilizers.