Controlled release of a herbicide from matrix granules based on solvent-fractionated organosolv lignins

An organosolv (Alcell) lignin was fractionated with both sequential solvent-extraction and solvent-mixture precipitation using various organic solvent systems. The resulting fractionated lignins were used to prepare matrix granules with bromacil as a model compound using a melt process. The release rates of bromacil in vitro from the granular matrix system were influenced by both the lignin used as carrier and the solution fractionation. The variations in release rates were related to the high proportion of low-molecular-weight fraction and its associated water-soluble lignin in the fractionated lignins. Fickian diffusion was a predominant factor in controlling the release of bromacil from the matrix granules based on the low-molecular-weight fraction (L(fr1)). For the matrix granules based on the other fractionated lignins, the release of bromacil was of super-case II transport. The fine structure of the matrix granules before and after release was also studied. An unique release pattern of bromacil was observed for the matrix granules based on the low-molecular-weight fraction (L(fr1)), showing a fast release followed by zero-order release, which was a result of remarkable changes in the structure of the matrix granules during the course of release.