New directions for agricultural wastes valorization as hydrogel biocomposite fertilizers

The tremendous amount of waste is an environmental and social problem worldwide. The agri-food sector is the largest producer of waste and requires the extensive use of fertilizers, which entails the need to look for innovative solutions in waste management. Properly recycled bio-waste can be reused as fertilizer. Polymer capsules with immobilized waste biomass can be applied as carriers for fertilizer nutrients. The amount of components exerts a certain influence on the effectiveness of copper ions binding. The most important physicochemical properties of biocomposites, such as swelling, SEM (Scanning Electron Microscopy) and FTIR (Fourier Transform Infrared Spectroscopy) were investigated. FTIR analyzes revealed that carboxyl and hydroxyl groups play a key role in Cu²⁺ ion binding. Morphology analysis showed that ion binding leads to homogenization of the composite surface, while coating the structure makes it more regular and cohesive. The sorption kinetics and the determination of the process's equilibrium parameters (Q_max = 29.4 ± 0.493 mg g^-1) play an important role. The study of Cu²⁺ ion release in different media showed that the chitosan layer slowed down the diffusion of cations by about 50% in NaNO₃ (1% m/m) solution. Preliminary studies of the applicability of the capsules in germination tests demonstrate that the biocomposites have no phytotoxic effects on the test plant. The chitosan coating slows the release of Cu²⁺ ions by about 20% compared to uncoated capsules. New fertilizer formulations containing chitosan-encapsulated hydrogel with biomass-immobilized micronutrients can be applied for precision agriculture to minimize the loss of fertilizer nutrients to the environment. These fertilizers could be used to cultivate houseplants and greenhouse crops.