Revealing the effects of the ball milling pretreatment on the ethanosolv fractionation of lignin from walnut and pistachio shells

In this work, a detailed study on the ethanosolv lignin fractionation from endocarps, aiming at understanding the role played by the ball milling (BM) pretreatment, is presented. Walnut and pistachio shells were chosen as a case study, since they exhibit excellent mechanical properties and their tissues have been extensively studied in terms of structure, composition, and lignin distribution. Here, it was demonstrated that the BM pretreatment is not particularly suited for these biomasses, since it is not able to promote the lignin fractionation from the whole tissue, and additionally, it triggers the oxidation of the substrate. Although in the case of walnut shell this oxidation can be prevented thanks to the remarkable radical scavenger activity (RSA) of its extractives, in the case of pistachio shell the BM is responsible for a partial lignin condensation. Pistachio shells resulted a particularly attractive feedstock since they can be easily delignified using mild ethanosolv processes, leading to highly regular lignin fractions, rich in S units (86%) and beta-O-4 linking motif (>60% C-9 units), so particularly suited for the chemical upgrading to added-value aromatics.