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Dissolution process and structural characteristics of residual lignin during soda pulping of sugarcane bagasse at high liquid-to-solid ratio

This study demonstrates that optimizing soda pulping of sugarcane bagasse using a high liquid-to-solid ratio and a new non-wood displacement digester system yields high-quality pulp by significantly degrading syringyl units and cleaving key lignin linkages, thereby facilitating the dissolution of lignin primarily from the compound middle lamella and cell corner middle lamella.

Original authors: Jiali Pu, Zhaoxia Chen, Xin Wang, Baojie Liu, Chenrong Qin, Shuangquan Yao, Chen Liang

Published 2026-06-29

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CC BY 4.0

Original authors: Jiali Pu, Zhaoxia Chen, Xin Wang, Baojie Liu, Chenrong Qin, Shuangquan Yao, Chen Liang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Turning Sugar Cane Waste into Paper

Imagine sugarcane as a giant, fibrous plant. After the juice is squeezed out to make sugar, what's left is a dry, straw-like waste called bagasse. This paper is about a new way to turn that waste into high-quality paper pulp.

The researchers wanted to fix a problem: traditional methods for making paper from wood don't work perfectly on sugarcane. They often leave the paper weak or uneven. So, the team tried a new approach called "High Liquid-to-Solid Ratio" cooking.

The Analogy: The Soup vs. The Stew
Think of traditional cooking like making a thick stew. You have a small amount of liquid and a lot of solid ingredients. The heat and chemicals (alkali) have to fight to get through to the center of the ingredients. This often leads to the outside being overcooked while the inside is still raw.

The new method is like making a giant pot of thin soup. You use a massive amount of liquid compared to the amount of sugarcane. This ensures the "soup" (the cooking chemicals) can reach every single nook and cranny of the sugarcane fibers instantly and evenly.

What They Did

The researchers took sugarcane bagasse and cooked it in a special machine that circulates this "thin soup" of chemicals. They tested different amounts of chemicals (alkali) to find the "Goldilocks" zone—not too weak, not too strong, but just right.

The Results:

High Yield: They got a lot of usable pulp (about 51%) without wasting much of the plant.
Strong Paper: The resulting paper fibers were long and strong (high viscosity).
Clean Pulp: The pulp had very little "gunk" left in it (low Kappa number), meaning the lignin (the natural glue holding the plant together) was removed very well.

The Secret Ingredient: How the "Glue" (Lignin) Was Removed

Lignin is the natural glue that holds plant fibers together. To make paper, you have to dissolve this glue without destroying the fibers themselves.

1. The "Onion" Layers
The sugarcane fiber is like an onion with different layers:

The Outer Skin (Middle Lamella): This is the glue between the fibers.
The Inner Core (Secondary Wall): This is the strong fiber itself.

The Discovery:
Using a special microscope (CLSM), the researchers watched the cooking process like a time-lapse video. They found that the chemicals first dissolved the outer skin (the glue between fibers). Once that glue was gone, the fibers separated.

Crucial Finding: Even after the fibers separated, a tiny bit of stubborn glue remained stuck to the inner core. The researchers found that using a higher concentration of chemicals helped dissolve even this last bit of stubborn glue.

2. The "Re-Adsorption" Problem
In old cooking methods, the dissolved glue (lignin) sometimes floats around and sticks back onto the fibers, like mud splashing back onto a clean car.

Because this new method uses so much liquid, the "mud" gets very diluted. It washes away cleanly and doesn't stick back to the fibers. This is why the final paper is so clean.

Breaking Down the Molecular "Lego"

The researchers also looked at the molecular structure of the leftover glue (residual lignin) using a machine called NMR (think of it as a super-precise molecular X-ray).

The Structure: Lignin is built like a wall made of different types of Lego bricks connected by specific bonds.
The Breakage: They found that the high-chemical cooking broke specific connections (called β-O-4 bonds) by about 44%.
The Selectivity: It broke the "bad" connections that hold the glue together but left the "good" connections that hold the paper fibers strong. This is called delignification selectivity. It's like removing the mortar from a brick wall without breaking the bricks.

The Trade-Off

There is one catch. Because they used so much liquid and chemicals to get this perfect result, they used more chemicals than usual.

The Paper says: This creates a lot of "dirty water" (black liquor) with high chemical content. However, they suggest this dirty water can be recycled and reused in the next batch, solving the waste problem.

Summary

This paper proves that cooking sugarcane bagasse in a "thin soup" of chemicals works better than the traditional "thick stew" method. It removes the natural glue (lignin) more evenly, prevents the glue from sticking back to the fibers, and breaks the glue's molecular structure efficiently. The result is a stronger, higher-quality paper pulp made from agricultural waste.

Technical Summary: Dissolution Process and Structural Characteristics of Residual Lignin during Soda Pulping of Sugarcane Bagasse at High Liquid-to-Solid Ratio

Problem Statement
Sugarcane bagasse is a promising non-wood raw material for pulp and paper production due to its high cellulose content and low lignin levels. However, traditional wood-based alkaline pulping processes (e.g., standard soda or kraft pulping) often yield bagasse pulps with quality instability and poor performance. These limitations stem from the structural heterogeneity of bagasse (fibers, parenchyma cells, vascular bundles) and its high reactivity toward alkali. Conventional low liquid-to-solid ratio cooking leads to uneven liquor penetration, resulting in localized overcooking, uneven delignification, and excessive degradation of hemicelluloses and lignin-carbohydrate complexes (LCCs). While the "deep delignification" theory suggests that uniform liquor impregnation and timely removal of dissolved lignin are key to improving selectivity, the application of Displacement Digester Systems (DDS) with high liquid-to-solid ratios for sugarcane bagasse remains under-investigated, particularly regarding the specific mechanisms of lignin dissolution and the structural evolution of residual lignin.

Methodology
This study employed a high liquid-to-solid ratio (15:1) soda pulping process using a recirculating digester to simulate DDS conditions. The research utilized a three-factor, three-level Box-Behnken response surface methodology (RSM) to optimize the cooking process, varying initial alkali concentration (16–32 g/L NaOH), maximum temperature (150–170°C), and holding time (15–105 min).

To elucidate the lignin dissolution mechanism and structural changes, the authors employed a multi-technique characterization approach:

Microscopy: Confocal Laser Scanning Microscopy (CLSM) using LM11 antibodies to visualize lignin distribution in cell walls (rejects vs. screened pulp) at different alkali concentrations.
Surface Analysis: Scanning Electron Microscopy (SEM) for morphology and X-ray Photoelectron Spectroscopy (XPS) to quantify surface lignin content via C1s peak analysis.
Chemical Analysis: UV spectroscopy and ion chromatography for black liquor composition; Thermogravimetric Analysis (TGA) and Gel Permeation Chromatography (GPC) for residual lignin properties.
Structural Elucidation: 2D HSQC NMR spectroscopy on enzymatic acid lignin (EAL) isolated from the pulp to semi-quantitatively analyze lignin substructures, linkages, and unit compositions.

Key Results

Process Optimization: The RSM model identified initial alkali concentration as the most significant factor influencing cooking outcomes. Under optimized conditions (approx. 30 g/L NaOH, 165°C), the process achieved a screened pulp yield of 50.9%, a low kappa number of 8.5, and a high viscosity of 1146 mL/g. The resulting pulp exhibited superior mechanical properties (tensile, tear, and burst indices) compared to literature values for conventional bagasse pulping methods.
Lignin Dissolution Mechanism: CLSM analysis revealed a distinct spatial dissolution pattern. Lignin in the Secondary Wall (S layer) was removed first. Fiber dissociation occurred only after the lignin in the Composite Middle Lamella (CML) and Cell Corner Middle Lamella (CCML) was sufficiently degraded. At high alkali concentrations, the S layer became porous, allowing alkali to penetrate and further dissolve lignin in the CML and CCML.
Surface Lignin Reduction: XPS analysis showed that increasing the alkali concentration significantly reduced the surface lignin content of the pulp fibers, decreasing from approximately 73.5% to 58.4%. This reduction is attributed to the effective removal of lignin from the CML/CCML and the low concentration of dissolved lignin in the black liquor, which minimized lignin re-adsorption onto fiber surfaces.
Structural Changes in Residual Lignin: NMR analysis of the residual lignin (EAL) indicated that the S-type (syringyl) units were predominantly degraded compared to G-type units, leading to a decreased S/G ratio at higher alkali concentrations. The study quantified a 44% degradation of the critical $\beta$ -O-4 linkages. Furthermore, $\beta$ -1 and $\alpha,\beta$ -diaryl ether bonds were significantly broken or eliminated at high alkali levels. Notably, no new lignin condensation structures were detected, suggesting the high liquid-to-solid ratio prevented the condensation reactions often seen in conventional cooking.

Significance and Claims
The authors claim that this study demonstrates the feasibility of applying high liquid-to-solid ratio soda cooking (mimicking DDS) to sugarcane bagasse. The primary contribution is the detailed elucidation of the lignin dissolution pathway, showing that uniform liquor distribution enables a more selective delignification process that preserves carbohydrate integrity while effectively removing lignin from specific cell wall layers.

The paper posits that this approach offers a reference for studying high liquid-to-solid ratio pulping from the perspective of lignin dissolution. By clarifying the structural characteristics of residual lignin and the mechanism of fiber separation, the work supports the potential of deep delignification theory to produce high-quality bagasse pulp. However, the authors modestly acknowledge that the study is a preliminary exploration; challenges such as high rejected pulp content, excessive alkali consumption, and the practical utilization of the resulting black liquor remain to be solved for full industrial applicability.