Permeability of P and K-nutrient through polystyrene membrane from aqueous solutions of urea + KH₂PO₄

With the polymer-coated fertilizer as background, the permeability of P- and K-nutrient through a representative polymer membrane-polystyrene membrane were investigated by measuring their permeability in the solutions of KH₂PO₄-water and urea-KH₂PO₄-water at nominal temperature of 298 K using the Ussing chamber method. To analyze and interpret the variation of permeability with solute concentration, the solubility of permeate in polymer membrane were determined experimentally and the permeate diffusion coefficient were assessed by the measurements of density and apparent molar volume of the aqueous fertilizer solutions. An interesting “increase-decrease” trend for the permeability of both phosphorous (P)-nutrient, and potassium (K)-nutrient fertilizer with permeate concentration was observed, in which the increases in permeability at low concentrations of permeate could be attributed to the increase in solubility of KH₂PO₄ in polymer while the decreases in permeability at high concentrations was due to the decrease in diffusion coefficient of permeate in polymer membrane. Finally, the release kinetics of these nutrients from a PS-coated urea-KH₂PO₄ compound fertilizer granule was predicted using the Shaviv’s model along with the permeability data of P- and K-nutrient generated.