Abstract

Lignin present in black liquor represents a major renewable resource that can be valorized into high-value bio-based materials; however, its recovery typically relies on strong mineral acids that pose environmental and corrosion concerns. In this study, citric acid was employed as a greener and less corrosive alternative for lignin precipitation from black liquor. Laboratory-scale experiments were conducted to optimize recovery conditions, identifying a precipitation pH of 2.0 and a lignin-to-deionized water washing ratio of 1:100 as the optimal parameters. Under these conditions, the recovered lignin exhibited high purity (91.69 ± 0.23%), with very low ash (0.11 ± 0.02%) and volatile content (5.01 ± 0.51%). Bench-scale validation confirmed the reproducibility of the optimized process, yielding lignin with comparable purity (91.30 ± 1.28%) and recovery yield (14.05 ± 0.21%). Gel permeation chromatography revealed that the isolated lignin possessed a relatively high molecular weight (Mw ≈ 45,000–47,000 g mol−1) with a narrow molecular weight distribution (PDI = 1.09), which remained consistent during scale-up. Structural characterization by elemental analysis, FTIR, and 1H NMR further confirmed that the chemical composition and functional groups of lignin were preserved across both processing scales. These findings demonstrate that citric-acid-based precipitation is a promising approach for recovering structurally consistent lignin from kraft black liquor and provide valuable insights into the bench-scale implementation of green lignin recovery processes.

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