Modifying soluble NPK release with hydrophobized nanocellulose-based hydrogels for sustainable enhanced efficiency fertilizers

Enhancing the delivery efficiency of NPK fertilizers benefits both crops and the environment through moderating the supplied dosage of nutrients in the soil, avoiding side reactions, maximizing absorption by the plant, and minimizing leaching and runoff. Bio-based materials such as cellulose are ideal scaffolds for nutrient delivery due to their inherent biocompatibility, biodegradability, and significant water uptake. In this work, nanocellulose-based hydrogels were regenerated from mixed softwood in acidic media and loaded with NPK by immersion in varied concentrations of an NPK-rich fertilizer solution. High loading of NPK was achieved within the hydrogel, but immersion in the matrix provided only slight slowing of nutrient release compared to rapid solubility of conventional formulations. Densification, crosslinking, and coating of the hydrogels with beeswax were ineffective strategies to further slow NPK release. Following these results, both gas and solution-phase esterification reactions of the cellulosic matrix with hexanoyl chloride were performed after NPK loading to introduce a hydrophobic surface layer. While solution-phase modification led to phosphorus leaching and was overall ineffective in altering nutrient release, the gas-phase modification slowed the release of P and K by more than an order of magnitude. Moreover, it was found that varying both the properties of the hydrophobic surface layer and the nutrient loading provide a means to tune release rates. Overall, this work demonstrates the potential of nanocellulose-based hydrogels to be used as an environmentally safe and sustainable vehicle for the controlled release of nutrients in agricultural applications.