Slow-Release Fertilizer Based on Microalgae Chlorella sp Microencapsulated with Urea–Formaldehyde: Potassium Release Kinetics

In this work a detailed kinetic study of the K2O release of a slow release biofertilizer based on Chlorella sp microalgae encapsulated with urea–formaldehyde (UF). Two main release processes were found, besides the burst release. One of them that takes place in the first hours of release, is related with the nutrient release from the UF matrix and is a first-order process and releases little K2O with a reaction rate k ≅  − 0.405 h−1, although this process had low cumulative lock-off release rlo≈ 7.4%. The other process, which is more significant, is a zero-order process and corresponds to the release from the encapsulated Chlorella sp microparticles. Long-term K2O release of UF-microencapsulated Chlorella sp with reaction rate kUF+Chlo ≅  − 0.00238 h−1 was compared with non-encapsulated and UF-microencapsulated conventional fertilizer with reaction rates kCF ≅  − 0.00145 h−1 and kUF+CF ≅  − 0.0022 h−1 respectively. Burst, lag and lock-off stages were characterized and analyzed for all the processes and formulations.