From hollow lignin microsphere preparation to simultaneous preparation of urea encapsulation for controlled release using industrial kraft lignin via slow and exhaustive acetone-water evaporation

Abstract

Lignin nano/microparticles have recently attracted growing interest for various value-additive applications of lignin, especially encapsulation. In this study, in order to establish a highly efficient and highly productive preparation process to effectively utilize technical lignin, a brand-new, slow and exhaustive solution evaporation process following a simple, self-assembly principle was developed using industrial softwood kraft lignin (SKL) from a starting acetone-water (80/20, v/v) solution to recover 100% of the lignin as homogeneous and well-shaped microspheres. The prepared microspheres had a typical average diameter of 0.81 ± 0.15 μm and were hollow with very thin shells (of nanoscale thickness). Based on this developed technique, encapsulation of urea by these lignin microspheres was directly achieved using the same process as hollow lignin microspheres with urea attached to the outside and entrapped inside of the wall. Two distinct urea release rates were observed for the urea-encapsulated microspheres: a fast release of the urea outside the shell wall and a slow (controlled) release of the urea inside the shell wall. The encapsulation efficiency was as high as 46% of the trapped urea as encapsulated inside the lignin microspheres. The slow and exhaustive solution evaporation procedure reported here is a simple and straightforward method for the valorization of industrial kraft lignin as hollow microspheres with controllable, homogeneous and desired morphologies, and especially for the direct preparation of lignin-based encapsulating fertilizers for controlled release.