Research On Permeability And Thermodynamic Properties Of N-,P-,and K- Compound Fertilizer Solution In Water

With the development of fertilizer polymer coating material as background, a novel developing method was proposed based on the experimental measurements and data documentation of N-, P-, and K-nutrient permeability through the polymer membrane as well as the mathematical model. Using the "solution-diffusion" theory,these permeability data was analyzed and interpreted by measuring the thermodynamic properties of the N-, P-, and K-nutrient solutions in water including apparent molar volume and solubility in polymer membrane. The feasibility of applying chemical oxidative degradation in the polymer coating of fertilizer was preliminarily studied.For the KH2PO4 solution in water with the same molarity,the addition of urea would lead to increase in apparent molar volume. The limit partial molar volume of KH2PO4 showed an increase with urea concentration or temperature, suggesting that the solute-solvent interaction became stronger with addition of urea. The solubility of KH2PO4 solution, KH2PO4, and water in polystyrene (PS) membrane were experimentally determined. These three solubilites demonstrated increases with increase in KH2PO4 concentration in the solutions. Additionally, the ratio of solubility of KH2PO4 to water in the PS membrane was much higher than those in the penetrant solutions,suggesting a selective solution of KH2PO4 over water in the PS membrane.At nominal temperature of 298 K, the permeability of N-, P- and K-nutrient through the PS and polycaprolactone (PCL) membrane showed an "increase-decrease"trend with the nutrient concentration in the solution for the aqueous solutions of KH2PO4, KH2PO4-urea, NaH2PO4-urea-KCl, which can be closely related to the solubility and diffusion coefficient of the fertilizer nutrients in the polymer membrane.In the region of low nutrient concentration, the increase in nutrient solubility in polymer membrane with nutrient concentration dominated leading to the increase in permeability;while in the high nutrient concentration region, the molecule/ion interaction became stronger with nutrient concentration resulting in possible formation of molecule-ion complexes with larger sizes, which would lead to the decrease in the diffusion coefficient, thus the permeability.With the permeability data of water and N-, P-, and K-nutrient through the PS and PCL membrane that was generated in this study, the Shaviv's mathematical model was applied to calculate the release of N-, P-, and K-nutrient from the polymer-coated fertilizer granule. The results of calculation showed that the lag term could be prolonged with the lower permeability of water to obtain longer release term thus better slow release. The increase in value of r ×l would also prolong the lag term and the linear term, suggesting the increases in diameter of the fertilizer granule and the thickness of polymer coating would prolong the fertilizer nutrient release term. Under the same condition, the release term of urea was the shortest and K-nutrient was the longest,suggesting that the composition of the compound fertilizer needed to be adjusted according the nutrient requirements of the crops.The preliminarily study of the chemical oxidative degradation of fertilizer polymer coating material showed that the degradation of PS membrane in 15wt% hydrogen peroxide solution was the fastest while the PMMA membrane degraded faster in the 30wt% H2O2 solution. In the Fenton agent, the degradation degree of PMMA demonstrated an "increase-decrease" with ratio of Fe2+to H2O2 and the maximum was reached at 1:10. For the same degradation time, the ability of Fenton agent to degrade the PMMA, PS, and PLA membrane was better than H2O2 solution.