Superabsorbent hydrogels as carriers for the controlled-release of urea: Experiments and a mathematical model describing the release rate

Hydrogel, prepared by swelling superabsorbent in a urea solution and used as carrier for controlled-release, can significantly reduce the leaching losses of urea. Static release experiments showed that the size and the degree of crosslinking of the hydrogel had a significant effect on the release rate of urea. A mathematical model based on the logistic growth model was developed to describe the change in concentration of urea with release time in given volumes of water. In this model, a damping coefficient related to the rate of mass transfer in the hydrogel was introduced to modify the instantaneous release rate. By fitting the model with the results of static release experiments, the damping coefficients were found to have a good linear relationship with the minimum volume fraction of hydrogel (coefficient of correlation R = 0.9680). The relative median error of the model prediction was in general less than 10%. The experimental and modelling methods used in the study could provide a convenient method for designing and preparing superabsorbent hydrogels with suitable release rates to meet the practical requirements of the plant, and thus produce economic and environmental benefits.