Tunable NPK Release from Surface-Esterified Nanocellulose-Based Prills

Polysaccharides represent an ideal delivery platform for agrochemicals due to their biodegradability, biocompatibility, and abundance. However, hydrophilicity renders native polysaccharides ineffective at controlling the release of water-soluble agrochemicals. To overcome this limitation, we used a solvent-free, vapor-phase modification strategy to generate hydrophobic shells on the surface of nanofibrillated cellulose (CNF) prills and evaluated the effects of these tunable diffusion barriers on fertilizer release behavior. Hydrophobic shells of different thicknesses were created on CNF prills by esterification with acyl chlorides of varying alkyl chain lengths, although esterification did not hinder the inherent biodegradability of the CNFs. Fertilizer release rates were largely invariable to pH and NPK loading but were tunable over 3 orders of magnitude by varying the alkyl chain length and the degree of substitution (i.e., shell thickness). However, NPK release rates did not increase monotonically with increasing shell thickness; for long-chain (6 carbons or longer) esters, increases in shell thickness beyond optimal values increased release rates; SEM revealed that this originated from the introduction of fractures to the prills which act as diffusion channels. This work demonstrates the potential of controlled surface modification to generate a mineralizable and tunable NPK release platform from naturally sourced and sustainable feedstocks.