SOLUTE DIFFUSION-COEFFICIENT IN THE INTERNAL MEDIUM OF A NEW GEL BASED CONTROLLED-RELEASE FERTILIZER

The diffusion of solutes in a new controlled release device was invest igated and in situ measurements of constant and variable diffusion coe fficients were obtained. The new controlled release device consists of a dry mixture of fertilizer and gel forming thickener contained in a nonpermeable coating having at least one opening. Water penetrates int o the device through the opening, forms a gel and dissolves the fertil izer, which is then released by Fickian or non-Fickian diffusion mecha nisms. Based on measurements of the penetration of water and the disso lution of fertilizer, the pseudo-steady state transport equation was s olved and the solute diffusion coefficient was calculated. The computa tion of the diffusion coefficient was possible solely because a dual b oundary condition exists at the dissolution front. An analytical solut ion was developed assuming a constant diffusion coefficient. A numeric al solution was obtained for the case where the diffusion coefficient is concentration dependent. Two thickeners were tested: sodium carboxy methylcellulose (Na-CMC) and sodium polyacrylamide (Na-PAM). It was fo und that the solute diffusion mechanism in Na-CMC gels is Fickian-like and can be approximated by a constant diffusion coefficient. The solu te diffusion in Na-PAM gels showed non-Fickian behavior and was estima ted numerically using the variable diffusion coefficient. For comparis on with the theoretical solutions, a dialysis cell was used to simulat e the conditions in the gel formed inside the device and to evaluate t he effects of thickener concentration on the diffusion coefficient. Re asonable agreement was found between the pseudo-steady state solutions and the dialysis cell experimental results.