Lignin from mixed fruit waste valorised with a green deep eutectic binary solvent system: recovery, process optimization and characterisation

Abstract

The present study reports the recovery of lignin from combined pomegranate and banana peels through the process of valorisation with a binary deep eutectic solvent (DES) system comprising of choline chloride (CC) and oxalic acid (OA). The valorisation process was further optimized by Response Surface Methodology-based Central Composite Design, yielding an enhanced lignin recovery of 52.75 ± 0.55%. The interactions between the valorisation process variables and their effect on lignin recovery was analysed by developing a predictive quadratic model. Temperature and time had a profound influence on the valorisation process. The purity of the lignin was measured as 85.70 ± 1.62%. UV-Visible and Fourier Transform Infrared Spectroscopy confirmed the presence of the major functional groups of lignin. Field Emission Scanning Electron Microscopy images revealed agglomerated and spherical lignin structure; whereas X-Ray diffraction analysis identified its amorphous nature. Energy Dispersive X-ray Spectroscopy showed its higher and lower amounts of carbon and sulphur content compared to the commercially available lignin. ¹H Nuclear Magnetic Resonance Spectroscopy (NMR) depicted characteristic signals for methoxy, aliphatic, and aromatic protons of the lignin. The major interunit linkages of lignin along with their relative contents were identified by 2D Heteronuclear Single Quantum Coherence NMR spectra. The thermal stability of the lignin was confirmed by Differential Scanning Calorimetry measuring glass transition temperature of 88.75 ℃. The recovered lignin demonstrated higher antioxidant and antibacterial activities compared to commercial lignin. Overall, the RSM method was efficient in optimizing mixed fruit waste valorisation with CC-OA DES, resulting an enhanced lignin yield and purity.