The novel method to reduce the silica content in lignin recovered from black liquor originating from rice straw

Lignocellulosic biomass is a renewable feedstock for fuels and chemicals, with rice straw abundantly available in Vietnam. Rice straw contains cellulose, hemicellulose, lignin, and significant silica that complicates lignin recovery and utilization. Silica originates from plant uptake of monosilicic acid and polymerizes, and it can associate with lignin and carbohydrates, contributing to biomass recalcitrance. Conventional desilication methods can be harsh, equipment-intensive, or inefficient. Acidification is a common approach for lignin recovery from alkaline black liquor, but the effect of pH on selective precipitation of silica versus lignin from rice straw black liquor has not been comprehensively mapped. This study aims to develop an effective, selective method to reduce silica content in recovered lignin by elucidating precipitation behavior across a wide pH range and establishing a two-step acidification that first removes silica and then recovers lignin, enabling higher-quality lignin suitable for fuels and aromatics, and scalable integration with carbohydrate-oriented processes.

Black liquor from alkaline pretreatment contains dissolved lignin, silica, and some hemicellulose, providing an opportunity for fractionation. Lignin recovery methods include acidification, ultrafiltration, and electrolysis, with acidification being dominant. Different mineral acids show varying efficiencies: phosphoric acid can yield high lignin precipitation but requires high concentration to reach low pH; hydrochloric acid is effective at lab scale but is corrosive and less feasible industrially; sulfuric acid offers reasonable lignin recovery with better economic and operational feasibility. Prior studies explored lignin precipitation from black liquors of grass and wood, reporting color changes and selective precipitation behaviors, but a detailed examination of pH-dependent co-precipitation of silica and lignin for rice straw black liquor and an industrially applicable desilication-first strategy had not been reported.

Materials: Rice straw from Cu Chi District (HCMC, Vietnam) was washed, sun-dried to <15% moisture, cut to 0.5–2 mm, and stored. Composition (cellulose, hemicellulose, lignin, ash, dry matter) was determined per NREL TP-510-42618. Reagents: NaOH and H2SO4 (reagent grade, Merck); solutions in distilled water. Pretreatment: 300 g rice straw mixed with 4.5 L NaOH 1 w/v% in a 10 L boiler. Preheated to 60 °C, then heated to 90 °C in 15 min, maintained 2 h at 90 °C with agitation (150 rpm). After cooling to 40 °C, solids were removed by vacuum filtration to obtain ~4.3 L black liquor (pH 12.4). Single-step acidification: Ten aliquots of 200 mL black liquor were acidified with 20 w/v% H2SO4 to target pH 10–1. After 24 h settling, solids were filtered, washed with deionized water, dried at 90 °C to constant mass, and ground. Precipitates were analyzed for mass, ash, and non-ash and characterized by FT-IR, XRD, and TGA to elucidate precipitation behavior versus pH. Two-step acidification: Bulk black liquor was adjusted first to pH 9 with 20 w/v% H2SO4 and left 36 h to precipitate silica gel, which was removed by vacuum filtration. The filtrate was then acidified to pH 3 with 20 w/v% H2SO4 to precipitate lignin. Effect of NaOH concentration: Black liquors from pretreatments using NaOH at 0.5, 1, 2, and 4 w/v% underwent the two-step acidification (pH 9 then pH 3). Lignin purity, ash content, and recovery yield were determined; experiments were triplicated and averaged. Analytical methods: Ash and non-ash contents by calcination at 900 ± 25 °C for 6 h. FT-IR (KBr pellets, 400–4000 cm⁻1, 4 cm⁻1 resolution; PerkinElmer Frontier IR). XRD (Bruker-D8, 2θ 10–80°, step 0.019°, step time 43 s; Cu Kα, λ=1.5406 Å, 40 kV). TGA (Linseis TGA PT 1600; RT to 800 °C, 20 °C/min, Ar). pH measured with Thermo Scientific Expert pH meter. Lignin content by NREL/TP-510-42618. Lignin recovery yield calculated as: Yield (%) = (m_raw lignin × purity) / (total lignin in rice straw).

- Rice straw composition (wt% of dry matter): cellulose 45.70±0.16, hemicellulose 22.45±0.15, lignin 19.60±0.18, ash 12.25±0.15. Black liquor enriched in lignin (51.81±0.35 wt%) and ash (25.14±0.22 wt%). - Single-step acidification (pH 10→1): Total precipitate mass increased from pH 10 to peak at pH 5 (~2.89 g/200 mL). Ash fraction peaked near pH 8 (~80%) and remained high to pH 5, indicating silica precipitation as silicic acid hydrates/silicates. From pH 4 to 3, ash content dropped markedly (≈35% to ≈16%), consistent with silica re-dissolution at low pH and lignin-dominant precipitation at pH ≤3. - Visual indicators: Precipitates light golden-brown at pH 10–8 (dense gel; silica-rich), turning dark brown at pH ≤7 (lignin co-precipitation). Filtrates remained dark brown down to pH 5, then became opaque reddish-brown at lower pH. - FT-IR: Precipitates at pH 10–8 dominated by silica bands (Si–O–Si 950–1000 cm⁻1, 458–561 cm⁻1). Lignin aromatic bands (∼1510 and ∼1605 cm⁻1) became prominent at pH 3–1; silica band influence negligible at low pH, supporting lignin recovery at pH 3. - XRD: Broad amorphous hump (17–30°, maximum ∼22.5° 2θ) in pH 10–6 precipitates indicating amorphous silica. Precipitates at pH ≤3 showed broad patterns (10–45° 2θ) indicative of amorphous organic-dominated material (lignin). - TGA: Three groups identified. Precipitates from pH 10–7 had ~25 wt% total mass loss; pH 6–4 had 41–53% loss; pH 3–1 had ~80% loss, consistent with increasing organic (lignin) content at lower pH. Major lignin decomposition occurred between 260–480 °C, with up to ~50 wt% loss at pH 3–1. - Optimal separation windows established: silica recovers at basic pH 8–10; lignin at acidic pH 1–3 (particularly pH 3). - Two-step acidification performance: Pre-acidification to pH 9 to remove silica followed by pH 3 for lignin recovery produced lignin with low ash; FT-IR showed clear lignin bands; XRD still showed peaks around 22°, indicating minor residual silica. Measured lignin ash content 3.46% after two-step process. - Comparative pH for first step (8, 9, 10 then to 3): Highest silica recovery at first-step pH 8; however, the 9→3 sequence gave higher lignin yield with similar purity and was economically favored. - Effect of NaOH concentration (two-step process): Using NaOH 1 w/v% yielded 66.75% lignin recovery with 3.46% silica in lignin (94.38% silica removal). NaOH 2 w/v% gave 3.22% silica (99.98% silica removal) but lower lignin yield. Highest lignin purity (78.91%) was observed for NaOH 0.5 w/v% pretreatment, but with lower recovery yield (52.64%).

Mapping precipitation versus pH revealed that silica and lignin can be selectively separated from rice straw black liquor by controlling pH. Silica precipitates predominantly at basic pH (8–10) as silicic acid hydrates/silicates, while lignin precipitates at acidic pH (≤3) with minimal silica co-precipitation. Building on this, a two-step acidification (first to pH 9 for silica removal, then to pH 3 for lignin recovery) effectively reduces silica contamination in recovered lignin, addressing the key challenge that limits lignin recovery yield and downstream applications. Spectroscopic (FT-IR), structural (XRD), and thermal (TGA) analyses corroborate the compositional shifts across pH and confirm the enhanced lignin purity at low pH. From an industrial perspective, sulfuric acid (20 w/v%) provides a practical acidulant, and optimizing NaOH pretreatment concentration balances efficiency and safety: 1 w/v% NaOH enables high lignin recovery (66.75%) with substantial silica reduction (94.38%), suitable for scale-up and integration with carbohydrate-focused biorefinery operations.

The study establishes a selective, scalable method to reduce silica in lignin recovered from rice straw black liquor by exploiting pH-dependent precipitation. A two-step acidification—pre-acidifying to pH 9 to precipitate and remove silica, followed by acidifying to pH 3 to recover lignin—achieves low ash lignin. With NaOH 1 w/v% pretreatment and 20 w/v% H2SO4 acidification, the process yielded a lignin recovery of 66.75% and a silica reduction of 94.38%. Comprehensive FT-IR, XRD, and TGA analyses validate the separation windows (silica at pH 8–10; lignin at pH 1–3) and the improved lignin quality. This simple, low-chemical-intensity approach supports industrial implementation and integration with bioethanol or other carbohydrate valorization processes. Potential future work includes further improving lignin purity while maintaining yield, refining first-step pH to maximize silica recovery without compromising lignin yield, and scaling pilot demonstrations with process optimization for energy and water use.

Minor residual silica remained in the recovered lignin after the two-step process, as indicated by XRD peaks around 22°, despite low ash content measurements. Trade-offs were observed between NaOH pretreatment concentration, lignin purity, and recovery yield. Co-precipitation of lignin–carbohydrate complexes at low pH was indicated by thermal analysis, potentially affecting purity.