Lignocellulosic biomass, particularly lignin from agricultural waste like rice straw, is a potential renewable energy resource. Vietnam generates a large amount of rice straw annually, much of which is burned, causing harmful emissions. Recovering lignin from rice straw offers a sustainable biofuel alternative and mitigates environmental impact. Rice straw consists of cellulose, hemicellulose, and lignin, with silica being a significant component that impedes its utilization. Silica's interaction with other components in the rigid lignocellulosic network hinders biorefinery processes like bioethanol fermentation and lignin conversion into valuable chemicals. Existing desilication methods often require harsh conditions or specialized equipment, lacking efficiency. Alkaline pretreatment, effective for herbaceous plants like rice straw, is favored for lignin recovery. This process releases silica into the alkaline medium, creating black liquor, a mixture containing dissolved lignin, silica, and hemicellulose. Acidification is a common method for lignin recovery from black liquor, with sulfuric acid offering a balance of efficiency and practicality for industrial applications. This research aims to develop a novel, efficient, and selective method for reducing silica content in lignin recovered from rice straw black liquor by optimizing the acidification process.

Numerous studies have explored lignin extraction and purification from various sources, including rice straw. These studies investigated different pretreatment methods and lignin recovery techniques, highlighting the challenges posed by high silica content. Previous research has explored the use of different acids for lignin precipitation, with varying degrees of success. Sulfuric acid has emerged as a promising choice due to its cost-effectiveness and suitability for large-scale operations. However, a comprehensive investigation of the effect of pH on silica and lignin precipitation during acidification of rice straw black liquor remained lacking, motivating this study.

Rice straw was collected, cleaned, and pulverized. Its composition (cellulose, hemicellulose, lignin, ash) was determined using the NREL TP-510-42618 method. Alkaline pretreatment was conducted using 1% NaOH solution at 90°C for 2 hours to produce black liquor. Single-step acidification using 20% sulfuric acid was performed at various pH values (10-1) to analyze precipitation behavior. Precipitates were characterized through physical observation, weight measurement, ash content determination, and chemical structural analysis using FTIR, XRD, and TGA. A two-step acidification process was developed to optimize lignin recovery and silica removal. The process involved initial acidification to pH 9 for silica precipitation, followed by acidification to pH 3 for lignin recovery. The effects of different NaOH concentrations (0.5, 1, 2, 4 w/v%) during pretreatment on lignin purity, recovery yield, and silica removal were also investigated. Lignin yield was calculated using a formula considering the mass of obtained lignin, its purity, and the total lignin content in the rice straw.

The study found that the total mass of precipitate increased gradually with decreasing pH from 10 to 6, peaking at pH 5. Ash content (silica) increased marginally from pH 10 to 8 and remained stable until pH 5, then decreased significantly at lower pH values. Non-ash content (lignin) increased significantly from pH 5-7 and showed lignin recovery at pH 3 or lower. FTIR analysis showed the presence of both lignin and silica bands in precipitates at all pH values, with silica bands decreasing at pH values lower than 7. XRD analysis revealed amorphous silica in precipitates from pH 10 to 6 and an amorphous structure dominated in precipitates at low pH (pH<3). TGA analysis showed varying thermal degradation patterns depending on pH, with the highest weight loss observed for samples obtained at pH 3 or lower, indicating lignin decomposition. The two-step acidification process (pH 9 for silica removal, then pH 3 for lignin recovery) resulted in a high silica reduction rate (94.38%) and lignin recovery yield (66.75%) with a lignin purity of 65.74%, using a 1% (w/v%) NaOH pretreatment. The optimal NaOH concentration for the two-step process was 1 w/v%, offering a balance of lignin recovery yield, silica removal, and lignin purity.

The findings demonstrate the effectiveness of the two-step acidification method for selectively removing silica and recovering high-purity lignin from rice straw black liquor. The optimized process, using mild alkaline pretreatment and a two-step acidification, provides a practical approach for industrial-scale lignin production. The results contribute significantly to the efficient utilization of rice straw, a readily available and sustainable resource. The achieved high silica removal rate and lignin recovery yield demonstrate the potential of this method to enhance the economic viability of lignin production and its value-added applications.

This study successfully developed a novel two-step acidification process for efficiently reducing silica content and recovering high-purity lignin from rice straw. The process offers a cost-effective and environmentally friendly approach for large-scale lignin production. Future research could explore further optimization of the process parameters or investigate the application of different acid types or catalysts. Investigating the potential value-added uses of the recovered lignin and silica would also be valuable.

The study focused on a specific type of rice straw from a particular region in Vietnam. The generalizability of the findings to other types of rice straw or different geographical locations might require further investigation. The study primarily focused on chemical characterization of the extracted lignin. More comprehensive studies on the properties and applications of the recovered lignin might be needed. The scale-up of the process to a fully industrial setting will need further experimentation and optimization.