Lignin Extraction from Date-Palm Waste for Structural and Thermal Applications: A Comparative Study of Alkaline and Deep Eutectic Solvent Pretreatments

Abstract

Lignin, a major component of abundant wood from date-palm waste, remains largely underutilized despite its potential as a valuable bioresource. The improper disposal of this waste contributes to environmental hazards, underscoring the need for sustainable valorization strategies. While prior studies have explored lignin extraction from different biomasses using either deep eutectic solvents or alkaline methods, limited work has provided a direct comparison of these techniques on the same date-palm waste feedstock, nor has the effect of the deep eutectic solvent pretreatment time on lignin characteristics been systematically investigated. This study addresses these gaps by evaluating lignin extraction from date-palm waste using a green deep eutectic solvent composed of choline chloride and formic acid, while varying pretreatment time (2-8 h) and alkaline pretreatment methods. The extractions' performances were evaluated by calculating lignin yield and purity. The extracted lignins were characterized using FTIR, TGA, DSC, SEM, XRD, molecular weight determination, and ESR. While the deep eutectic solvent lignin exhibited better purityup to 63.44%, alkaline pretreatment produced a higher lignin yield of 36.7% but with a considerably reduced purity of 27.42%. SEM and FTIR results verified that the deep eutectic solvent lignin maintained a more intact molecular structure with less carbohydrate fragmentation. Moreover, TGA results indicated that the deep eutectic solvent lignin exhibited enhanced thermal stability. A deep eutectic solvent pretreatment time of 4 h was found to be optimal, yielding the highest lignin yield (22.85%) with high purity (58.89%). Extending the duration to 8 h led to increased fragmentation, a reduced molecular weight (4.90 kDa), and a slight decline in thermal stability due to possible degradation. These results highlight the potential of deep eutectic solvent pretreatment as a sustainable substitute for conventional lignin extraction methods to produce high-purity, thermally stable lignin.

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