Abstract

Reed was used as a raw material to develop a low-temperature oxygen-alkali pulping process, and a mathematical prediction model was established to evaluate the impact of process variables on pulping performance. Additionally, the dissolution behavior of reed’s chemical components was analyzed to elucidate the synergistic mechanism of oxygen-alkali pulping. The results revealed that optimal pulp performance was achieved at a maximum temperature of 114.5 °C, a retention time of 3.88 hours, and a sodium carbonate-to-sodium hydroxide molar ratio of 1/10. Under these conditions, the pulp yield reached 50.5%, with a brightness of 55.5% ISO, a viscosity of 465 mL/g, and a kappa number of 16.2. Notably, most of the silicon remains in the pulp, with only 34.1% migrating to the black liquor, thereby mitigating silica-related interference during alkali recovery. Variance analysis of the response surface confirmed that the investigated variables had significant impacts on all response indices, and the developed predictive model demonstrated high accuracy in forecasting the pulping performance of reed. Furthermore, the physical properties of the paper produced under these optimal conditions were superior to those of the paper obtained by using conventional high-temperature oxygen-alkali pulping processes.

Full Article

Optimization of Low-Temperature Oxygen-Alkali Pulping Process for Reed and Analysis Using Response Surface Methodology

Jinze Li, Kai Zhang ,* Peng Gan, Yu Zhao, Guihua Yang,* Qixi Xu, Baobin Wang, Lei Zhang, Jiachuan Chen *

Reed was used as a raw material to develop a low-temperature oxygen-alkali pulping process, and a mathematical prediction model was established to evaluate the impact of process variables on pulping performance. Additionally, the dissolution behavior of reed’s chemical components was analyzed to elucidate the synergistic mechanism of oxygen-alkali pulping. The results revealed that optimal pulp performance was achieved at a maximum temperature of 114.5 °C, a retention time of 3.88 hours, and a sodium carbonate-to-sodium hydroxide molar ratio of 1/10. Under these conditions, the pulp yield reached 50.5%, with a brightness of 55.5% ISO, a viscosity of 465 mL/g, and a kappa number of 16.2. Notably, most of the silicon remains in the pulp, with only 34.1% migrating to the black liquor, thereby mitigating silica-related interference during alkali recovery. Variance analysis of the response surface confirmed that the investigated variables had significant impacts on all response indices, and the developed predictive model demonstrated high accuracy in forecasting the pulping performance of reed. Furthermore, the physical properties of the paper produced under these optimal conditions were superior to those of the paper obtained by using conventional high-temperature oxygen-alkali pulping processes.

DOI: 10.15376/biores.20.2.4068-4095

Keywords: Oxygen-alkali method; Agricultural straw; Pulping process; Low temperature; Dissolution; Mechanism

Contact information: State Key Laboratory of Biobased Materials and Green Papermaking, Key Laboratory of Pulp & Paper Science and Technology of Education Ministry, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China;

* Corresponding authors: zhangkai2018@qlu.edu.cn; ygh@qlu.edu.cn; chenjc@qlu.edu.cn

Graphical Abstract

INTRODUCTION

As the world’s largest producer and consumer of paper and paperboard, China has faced the dual challenges of raw material shortages and increasing environmental pressures (Ma et al. 2023; Zhang et al. 2023), severely impacting the sustainable development of the industry. As an agricultural powerhouse, China possesses abundant non-wood fiber resources, such as wheat straw, rice straw, reed, and bamboo. The rational development and utilization of these raw materials can partially substitute wood resources, alleviate the raw material shortage crisis, reduce reliance on imported wood chips, and mitigate the environmental pollution associated with straw waste disposal (Höller et al. 2021; Yimlamai et al. 2023).

Reed, a perennial grass fiber raw material, holds significant potential in the pulp and paper industry due to its wide distribution, rapid growth, high yield, and strong adaptability. With high cellulose and hemicellulose content, robust fiber structure, and excellent crystallinity, reed is a highly competitive papermaking material (Shatalov and Pereira 2007; Zhao et al. 2021). However, mainstream reed pulping methods, such as the soda process or soda-anthraquinone process, face challenges including severe carbohydrate degradation under high-temperature operations and silicon interference in alkali recovery (Jahan et al. 2021; Frazier et al. 2024), which significantly restricts the broader application of reed resources in pulping and papermaking. Therefore, developing green, low-carbon pulping technologies tailored to reed fiber materials is of critical importance.

Since the 1960s, oxygen-alkali pulping technology has received widespread attention due to its environmentally friendly characteristics and high efficiency. This technology facilitates lignin degradation through the radical reaction between oxygen molecules and lignin, where the synergistic effect of hydroxyl radicals (HO·), superoxide radicals (O₂⁻·), and hydroperoxide radicals (HOO·) promotes lignin dissolution (Gierer 1997; Gierer et al. 2001). Oxygen-alkali pulping has the dual benefits of pulping and bleaching, with advantages including stronger adaptability, less silicon interference, and higher pulp brightness. Specifically, oxygen-alkali pulping is particularly suited for fiber materials with loose structures (Yue et al. 2016), such as straw, as it encounters less resistance to oxygen and alkali penetration and diffusion compared to wood chips, leading to more uniform pulps and superior paper physical properties. Additionally, the oxidation process leads to the formation of micro-nano-sized silicate crystals in the black liquor, which are deposited on the fiber surface, effectively reducing the silicon content in the black liquor and alleviating technical difficulties in alkali recovery (Xu et al. 2021). Furthermore, oxygen-alkali pulping combines pulping and bleaching in one step, resulting in pulp brightness that is approximately 20% ISO higher than that achieved by traditional methods. However, while the oxygen-alkali synergy significantly removes lignin, the poor selectivity of oxygen radicals can lead to the oxidative degradation of carbohydrates, reducing pulp yield and quality (Håkansdotter and Olm 2002). Therefore, further improvements and optimizations are still needed for oxygen-alkali pulping technology.

This study develops a green low-temperature oxygen-alkali pulping technology based on reed as the raw material, using a cooking liquor composed of sodium carbonate and sodium hydroxide, combined with single-factor optimization and response surface methodology. By optimizing variables such as cooking temperature, time, and the sodium carbonate-to-sodium hydroxide mass ratio, a mathematical model was developed to investigate the dissolution behavior of the chemical components of reed and to analyze the pulping mechanism of the low-temperature oxygen-alkali process. The pulp properties and physical characteristics of the resulting paper were characterized and compared with those obtained from other pulping processes to assess the advantages and applicability of the proposed method. This research aims to provide theoretical support and data reference for the development of green pulping technology suitable for reed fiber resources, while also opening up new technological pathways for the efficient utilization of agricultural straw and the sustainable development of the pulp and paper industry.

EXPERIMENTAL

Materials

Sodium hydroxide, anhydrous sodium carbonate, potassium permanganate, sodium thiosulfate, and copper ethylenediamine were purchased from McLin Chemical Reagent Co., Ltd. The reed was sourced from a paper mill in Shandong, cut into 3- to 5-cm segments, washed, air-dried, and then stored in sealed bags for future use.

Determination of the Chemical Composition and Its Content in Reed

The experiment selected air-dried reed as the research subject, which was ground into a powder with a particle size of 40 to 60 mesh using a hammer mill. The contents of total cellulose, lignin, ash, and moisture in the reed were determined according to the relevant national standards GB/T 2677.10 (1995), GB/T 2677.8 (1994), GB/T 2677.3 (1993), GB/T 2677.3 (2005), GB/T 2677.2 (1993), and GB/T 2677.2 (2011). The results were as follows: total cellulose content 76.9%, cellulose content 46.9%, lignin content 24.7%, and ash content 4.86%.

Oxygen-Alkali Pulping Experiment

Sodium hydroxide was used as the primary alkali source, and sodium carbonate served as a buffer. The oxygen-alkali cooking of reed was carried out in a 1L×4 oxygen delignification vessel, equipped in a 15-LZQS type electric rotary cooking pot. A total of 50 g of oven-dried reed was impregnated with alkali solution at a specific solid-liquid ratio and then transferred to the oxygen delignification vessel. After evacuating the air, high-purity oxygen was introduced within a defined pressure range. The vessel was then placed in the cooking pot for oxygen-alkali pulping. At the end of the experiment, the pulp and black liquor were collected for testing pulp properties and chemical composition analysis, providing data for optimizing the low-temperature oxygen-alkali pulping process.

First, a systematic study was conducted using a single-factor experimental method to investigate the effects of alkali dosage, the Na₂CO₃/NaOH ratio, maximum temperature, and holding time on the pulp properties of reed prepared by low-temperature oxygen-alkali pulping.

The oxygen-alkali pulping process of reed was optimized using response surface methodology (RSM). The experimental design is shown in Table S1, with alkali dosage (X₁), the Na₂CO₃/NaOH ratio (X₂), maximum temperature (X₃), and holding time (X₄) as independent variables, and pulp yield, viscosity, brightness, and Kappa number as dependent variables. Based on the experimental data, a second-order polynomial model (Eq. 1) was developed using regression analysis. By analyzing the influence of each factor on pulp quality, the pulping process was further optimized. This model provides a theoretical basis for optimizing the low-temperature oxygen-alkali pulping of reed and contributes to improving pulp properties and pulping efficiency.

(1)

where Y represents the response values (pulp yield, Kappa number, brightness, and viscosity), K denotes the number of independent variables; X_i and X_j represent the independent factors, β₀ is the intercept, and β_i, β_ii, and β_ij represent the coefficients for linear terms, quadratic terms, and interaction terms, respectively. A second-order model was established using Design Expert 10, and multivariate linear regression analysis was applied to process the data, generating 3D surface plots and 2D contour plots.

Characterization of Pulp Properties

The pulp obtained from cooking was placed into a flat-screen pulp washer to separate the fines, and the yield of the fines was calculated using Eq. S2. The Kappa number, ash content, and silicon content of the reed chemical pulp were measured according to the standards GB/T 1546 (2018), GB/T 1548 (2004), GB/T 1548 (2016), and TAPPI T 245 cm-98 (1998), respectively. As shown in Eq. S3, the crystallinity values of both the raw material and the pulp were determined using the methods of French and Cintrón (2013) and Ruan et al. (2016). The fiber morphology of the pulp, including fiber length, width, length-to-width ratio, and fine fiber content, was measured using the Fiber Quality Analyzer (L&W Fiber Tester Plus) based on Pasquier’s method (Pasquier et al. 2024). The surface morphology of the regenerated cellulose was observed using a field-emission scanning electron microscope (Regulus 8220).

Papermaking and Characterization of Paper Properties

A total of 30 g of oven-dried pulp was prepared, and the consistency was adjusted to 10%. The pulp was then evenly applied to the inner walls of a PFI mill (SR-D). The number of revolutions was adjusted to achieve a freeness of 40 ± 2 ºSR. The target grammage of the paper was set to 70 g/m². A sheet former was used to mold the paper, which was then dried and placed in a constant temperature and humidity environment (25 ºC, 50% relative humidity) for 24 h to equilibrate the moisture content. Once the paper reached equilibrium moisture, it was used for subsequent testing. The paper’s tensile index, tear index, burst index, folding endurance, and brightness were measured according to national standards: GB/T 12914 (2008), GB/T 12914 (2018), GB/T 455 (2002), GB/T 2679.8 (1995), GB/T 2679.8 (2016), GB/T 454 (2002), GB/T 454 (2020), GB/T 7974 (2002), and GB/T 7974(2013).

Characterization of Black Liquor Properties

The determination of black liquor pH was conducted according to GB/T 6920 (1989) and GB/T 6920 (1986), while the measurement of solid content and silica in the black liquor followed the TAPPI T625 cm-14 standard (2014).

RESULTS AND DISCUSSION

Optimization of Pulping Process Based on Single-Factor Analysis

The effects of alkali dosage, sodium carbonate/sodium hydroxide ratio, temperature, and time on the properties of low-temperature oxygen-alkali chemical pulp from reed were investigated through single-factor experiments.

The effect of alkali dosage on the properties of reed oxygen-alkali pulp was first investigated. As shown in Fig. 1, with an increase in alkali dosage, the pulp yield and brightness initially increased and then decreased, while the Kappa number exhibited a decrease followed by an increase. At the same time, the viscosity of the pulp gradually decreased. During the oxygen-alkali cooking process, oxygen molecules within the cooking system react with lignin, facilitating its oxidative degradation. Simultaneously, hydroxide ions initiate nucleophilic attacks on the lignin structure, breaking ether and carbon-carbon bonds. This dual mechanism promotes the depolymerization and solubilization of lignin macromolecules, thereby enhancing its removal from the fiber matrix (Li et al. 2022). Moreover, the synergistic effect of alkali and oxygen generates highly active oxygen species (AOs), whose concentration is a key factor influencing pulp properties (Li et al. 2015). As the alkali dosage increases, the concentration of AOs rises, accelerating their reaction with phenolic hydroxyl groups and ether bonds in lignin molecules, causing the cleavage of lignin macromolecular chains and disrupting its chemical structure. This promotes selective lignin removal, resulting in high yield and brightness of the reed pulp (Guay et al. 2000). However, when the alkali dosage is further increased, AOs may attack the glycosidic bonds of cellulose, leading to partial cleavage and the formation of new reducing glucose units. Additionally, under the combined action of oxygen and alkali, the glucose units at the cellulose chain ends are removed (Guay et al. 2001), in the course of peeling reactions. These reactions not only reduce the pulp yield but also significantly lower the pulp viscosity, thereby affecting the overall pulp quality. At the same time, the viscosity of the pulp gradually decreased.

Fig. 1. The effect of alkali dosage on the yield and viscosity (a), as well as the brightness and kappa number (b). (Cooking conditions: liquid-to-solid ratio of 8:1, the Na₂CO₃/NaOH ratio of 1/9, 120 °C, 4 h)

Figure 2 illustrates the effect of the Na₂CO₃/NaOH ratio on the properties of low-temperature oxygen-alkali chemical pulp from reed. Under the same alkali dosage conditions, as the Na₂CO₃/NaOH ratio decreased, indicating a higher proportion of sodium hydroxide, the pulp yield initially increased and then decreased. Both the pulp viscosity and Kappa number decreased, while the brightness increased. During the oxygen-alkali cooking process, AOs are the main reactive species, including O₂⁻·, HO·, and HOO·. In an alkaline environment, oxygen reacts with the phenolic structure of lignin to generate O₂⁻·, which then accepts a proton in the alkaline solution to form the more stable HOO·. Eventually, HOO· can convert to HO· by gaining a proton (Gierer 1997). The HO· is the most oxidative, attacking the phenolic hydroxyl groups and ether bonds in lignin, promoting lignin degradation and dissolution. However, excessively high concentrations of HO can also lead to the degradation of carbohydrates (Zhang et al. 2024). In contrast, O₂⁻· and HOO· have weaker oxidative abilities and cause less damage to carbohydrates, but they are less effective at removing lignin. Therefore, they need to work synergistically with HO· to effectively promote lignin removal. When the Na₂CO₃/NaOH ratio is large, the amount of sodium hydroxide is low, resulting in a weaker alkaline environment. In this case, the synergistic effect of HO· and O₂⁻· promotes lignin removal, which is reflected in the improved pulp yield and brightness. However, when the amount of sodium hydroxide is excessively increased, it will promote the transformation of O₂⁻·, raising the ratio of HO·/O₂⁻· and enhancing the oxidation potential of the cooking system, which accelerates the degradation of carbohydrates and leads to difficulties in pulp formation (Gierer et al. 2001).

Fig. 2. The effect of the Na₂CO₃/NaOH ratio on the yield and viscosity; (a) as well as the brightness and kappa number (b) Cooking conditions: liquid-to-solid ratio of 8:1, alkali dosage of 21%, 120 °C, 4 h

The effect of temperature on the properties of low-temperature oxygen-alkali chemical pulp from reed is shown in Fig. 3. As the cooking temperature increased, pulp yield, brightness, and viscosity initially increased and then decreased, while the Kappa number first decreased and then increased. When the cooking temperature reached 115 °C, the pulp properties reached their optimal values, with a pulp yield of 50.4%, viscosity of 468 mL/g, brightness of 55% ISO, and Kappa number of 18.6. Temperature is one of the key factors influencing the oxygen delignification process (Leh et al. 2008; Pasquier et al. 2023). At lower cooking temperatures, the chemical reaction rate is slow, which may inhibit the generation of AOs. In this case, the dominant reaction in the cooking system is lignin removal. As the temperature rises, the activity of free radical reactions increases, and the concentration of AOs in the cooking system also increases. This not only promotes lignin removal but also intensifies the peeling of the cellulose in the fibers, leading to a decrease in pulp yield and viscosity (Ma et al. 2013). Furthermore, when the cooking temperature is too high, lignin may undergo excessive oxidation, and the colored by-products formed can easily deposit on the fiber surface, negatively affecting the pulp brightness and leading to a decrease in brightness (Sun et al. 2019).

Fig. 3. The effect of temperature on the yield and viscosity (a), as well as the brightness and kappa number (b). (Cooking conditions: liquid-to-solid ratio of 8:1, alkali dosage of 21%, the Na₂CO₃/NaOH ratio of 1/9, 4 h)

Fig. 4. The effect of time on the yield and viscosity (a), as well as the brightness and kappa number (b). (Cooking conditions: liquid-to-solid ratio of 8:1, alkali dosage of 21%, the Na₂CO₃/NaOH ratio of 1/9, 115°C)

As shown in Fig. 4, with the increase in holding time, pulp yield, brightness, and pulp viscosity initially increased and then decreased, while the Kappa number showed the opposite trend. During the initial stage of holding, as the holding time increased, lignin in the middle lamella and cell walls was gradually removed effectively, and the separation of cellulose also improved (Jia et al. 2022; Karp et al. 2014). During this process, pulp yield, brightness, and viscosity increased, while the Kappa number decreased. However, as the holding time continued to extend, AOs in the oxygen-alkali reaction system persisted in their activity, which may lead to intensified carbohydrate degradation. At the same time, OH· free radicals may nucleophilically attack lignin, generating chromophore groups such as quinones and conjugated double bonds (Zhang et al. 2018). These changes collectively result in a decrease in pulp yield, brightness, and viscosity, while the Kappa number increases.

Response Surface Model Fitting and Statistical Analysis

Based on the results of the single-factor experiments, a RSM experiment was designed using Design Expert software. The results of the response surface experiment are shown in Table S2. Within the experimental range, the pulp performance was excellent. Specifically, the pulp yield ranged from 42.6% to 51%, viscosity ranged from 409 to 482 mL/g, brightness ranged from 51.6% ISO to 57% ISO, and Kappa number ranged from 15.9 to 22.4, validating the rationality and effectiveness of the experimental design. Given the different responses of the four indicators to the interactions between variables and their varying optimal conditions, it is necessary to construct and analyze mathematical models for each response to better understand their relationship with the experimental variables. This will provide support for further optimizing pulp performance.

Mathematical Model and Analysis of Variance for Fine Pulp Yield

By correlating the pulp yield with different independent variables and performing regression analysis on the resulting data, a second-order multiple regression equation (Eq. 2) for the pulp yield in reed oxygen-alkali pulping was obtained. The equation only displays the terms that have a significant correlation with the pulp yield.

(2)

The suitability and accuracy of the model were validated through F value testing and the error probability value (p-value) (Nnaemeka et al. 2021). As shown in Table S3, the F-value was 17.57, and the p-value was less than 0.0001, indicating that the independent variables in the model have a significant effect on pulp yield, and the model demonstrates good reliability (Sekhar et al. 2021). Additionally, the correlation coefficient R² was 0.9461, and the difference between the adjusted R² (Adj-R²) and the predicted R² (Pre-R²) was 0.1769, which is less than 0.2, indicating that the model achieved a good fit and can be used for predicting and optimizing the changes in pulp yield in the reed oxygen-alkali pulping process.

In the second-order multiple regression equation, when the coefficients of linear and interaction terms are positive, it indicates that the factor promotes the response variable; when the coefficients are negative, it suggests that the factor has an inhibitory effect on the response variable. According to Eq. 2, the coefficients for X₁ and X₄ were positive, indicating that the increase of these factors helps to improve pulp yield. According to the analysis of variance table (Table S3), the p-values for X₁², X₃² and X₄² were all less than 0.001, indicating that the effects of these parameters were highly significant. Additionally, the p-values for X₁, X₄, X₁X₃ and X₂² were all less than 0.01, indicating that the effects of these parameters were highly significant, while the p-value for X₃X₄ was less than 0.05, indicating that its effect was also significant.

The response results for the interactive effects of various factors on the pulp yield in reed oxygen-alkali pulping are shown in Fig. 5. With a cooking temperature of 115 °C and holding time of 4 h, Fig. 5a illustrates the interaction between alkali dosage and the Na₂CO₃/NaOH ratio on pulp yield. The curvature of the surface related to alkali dosage was more pronounced, indicating that alkali dosage had a greater effect on pulp yield than the Na₂CO₃/NaOH ratio. This conclusion is supported by the results from the analysis of variance. Figure 5b shows the interaction between alkali dosage and cooking temperature on pulp yield. When both alkali dosage and cooking temperature were low, the pulp yield was low, which may be because low alkali dosage and low temperature hinder the generation of AOs, making it difficult for reed to pulp at low temperatures (Steffen et al. 2024). However, when alkali dosage and cooking temperature were both high, the pulp yield was still low. This may be due to the higher alkali dosage and cooking temperature generating more HO·, whose strong oxidizing effect enhances the breakdown of carbohydrates, leading to their degradation and lowering the pulp yield (Guay et al. 2002). Similar to the results in Fig. 5a, alkali dosage had a slightly greater impact on pulp yield than holding time.

Both excessively long or short holding times had a negative effect on pulp yield (Fig. 5c). Figures 5d and 5e indicate that when the Na₂CO₃/NaOH ratio was high, lower temperatures or shorter holding times were unfavorable for pulping, resulting in lower pulp yield. This is likely because Na₂CO₃ has weaker alkalinity, leading to lower concentrations of AOs and weaker oxidative capacity during cooking. At low temperatures and short holding times, lignin cannot be sufficiently dissolved, making pulping difficult and reducing pulp yield (Yang et al. 2021). Conversely, when the Na₂CO₃/NaOH ratio was low, higher temperatures and longer holding times also resulted in lower pulp yield. This may be because excessively high temperatures and long holding times lead to excessive degradation of carbohydrates (Nieminen et al. 2014). The response surfaces for temperature and holding time, shown in Fig. 5f, are spherical, and the contour lines are approximately circular. This indicates that the effects of temperature and holding time on pulp yield were similar. The maximum pulp yield was achieved at a cooking temperature of 115.1 °C and a holding time of 3.8 h.

Fig. 5. 3D response surface and contour plot of the effect of independent variables on fine pulp yield. (a) X₁-X₂; (b) X₁-X₃; (c) X₁-X₄; (d) X₂-X₃; (e) X₂-X₄; (f) X₃-X₄

Mathematical Model and Analysis of Variance for Pulp Viscosity

The second-order response surface equation for pulp viscosity obtained from reed oxygen-alkali cooking is shown in Eq. 3. In this equation, the positive coefficients of X₁ and X₄ indicate that both factors contributed to improved pulp viscosity. Additionally, the positive coefficient of the quadratic interaction term X₁X₂ suggests a favorable interaction between these two factors, which enhances viscosity. This interaction implies that optimizing both alkali dosage and temperature can improve pulp viscosity, benefiting the pulp’s quality and processing characteristics in paper-making. Fine-tuning these factors within their optimal ranges will yield better pulp viscosity, enhancing overall pulp performance.

(3)

From the analysis of variance Table S4, the F value of the model was 15.28, and the p-value was less than 0.0001, indicating that the model was both significant and effective. The R² value was 0.9386, and the coefficient of variation (C.V.%) was 1.67%, suggesting that the model was able to effectively predict the pulp viscosity variation. Specifically, the p-values for X₂, X₄, X₁², X₂², X₃², and X₄² were all less than 0.001, indicating that these factors had an extremely significant impact on the pulp viscosity. Additionally, the p-value for X₁ was less than 0.01, and the p-value for X₁X₂ was less than 0.05, showing that these factors also had a significant effect on pulp viscosity. These results highlight the importance of carefully optimizing these key factors and their interactions to achieve the desired pulp viscosity for better performance in the subsequent paper-making process.

Figure 6 presents the 3D response surface and contour plots of the interaction between the independent variables and their effect on pulp viscosity. Fig. 6a demonstrates the influence of the interaction between the alkali dosage and the Na₂CO₃/NaOH ratio on the pulp viscosity. Under conditions of high alkali dosage and low Na₂CO₃/NaOH ratio, the pulp viscosity was relatively low. This could be because, at high alkali dosages, NaOH as the alkali source leads to an increased concentration of HO·, which accelerates the degradation of carbohydrates and causes the breakdown of cellulose macromolecules, ultimately lowering the pulp viscosity (He et al. 2021).

Fig. 6. 3D response surface and contour plot of the effect of independent variables on pulp viscosity. (a) X₁-X₂; (b) X₁-X₃; (c) X₁-X₄; (d) X₂-X₃; (e) X₂-X₄; (f) X₃-X₄

The contour lines for any two factors are concentric (Figs. 6b-6f), with approximately equal distances between adjacent contours, indicating that the interaction between factors did not significantly affect the viscosity. This finding is consistent with the results from the analysis of variance. Under more severe cooking conditions—such as high alkali dosage, high Na₂CO₃/NaOH ratio, high cooking temperature, and long holding time—the pulp viscosity tended to be lower. This is because, under these conditions, cellulose molecular chains may break, resulting in the degradation of cellulose macromolecules, which further reduces viscosity. These insights highlight the importance of optimizing the experimental parameters to prevent excessive degradation of cellulose during the cooking process (Lapierre et al. 2006).

Mathematical Model and Analysis of Variance for Pulp Brightness

The quadratic multiple regression equation for the relationship between the independent variables and brightness is shown in Eq. 4. The model’s p-value was less than 0.001, with a coefficient of determination R² of 0.9477, and the difference between Adj-R² and Pre-R² is less than 0.2, indicating a high degree of fit and predictive ability for brightness trends (Table S5). Among the variables, X₁, X₂, X₃, X₃², and X₄² (p<0.001) had a highly significant effect on brightness, while X₁² (p<0.05) had a moderately significant influence. This highlights the critical impact of these factors on optimizing the brightness of reed oxygen-alkali pulp and provides a robust basis for understanding the interactions between variables.

(4)

The influence of the interaction of each independent variable on the whiteness is shown in Fig. 7. Figure 7a illustrates the interaction effects of alkali dosage and the Na₂CO₃/NaOH ratio on pulp brightness.

Fig. 7. 3D response surface and contour plot of the effect of independent variables on brightness. (a) X₁-X₂; (b) X₁-X₃; (c) X₁-X₄; (d) X₂-X₃; (e) X₂-X₄; (f) X₃-X₄

When the Na₂CO₃/NaOH ratio was within 0 to 0.25, increasing the alkali dosage enhanced brightness. This is likely due to the higher activity of HO· generated during oxygen-alkali cooking, which promotes lignin dissolution. However, as the Na₂CO₃ proportion increased, brightness declined, possibly because the weaker alkalinity of Na₂CO₃ inhibits HO· generation, thereby reducing the oxidative capability of the cooking system and lowering brightness (Guay et al. 2000). Figures 7b and 7c demonstrate that excessive cooking temperatures or prolonged holding times reduced pulp brightness, which may result from carbohydrate degradation or the formation of chromophoric groups (Yang et al. 2022). In Figs. 7d and 7e, the 3D response surfaces for the interaction between the Na₂CO₃/NaOH ratio and temperature or time exhibited concave shapes, tilted toward regions with lower Na₂CO₃/NaOH ratios. This indicates that the Na₂CO₃/NaOH ratio had a greater influence on brightness than temperature or time. Higher NaOH levels yield pulp with improved brightness. Figure 7f shows that the contour lines for the interaction between temperature and time were concentric and evenly spaced, suggesting that their interaction had minimal impact on brightness, consistent with the variance analysis results. This provides additional evidence supporting the relative importance of chemical composition and cooking conditions in optimizing pulp brightness.

Mathematical Model and Analysis of Variance for Kappa Number

By performing regression analysis on the quadratic polynomial equation for the relationship between the independent variables and the Kappa number, Equation 5 was derived.

(5)

The analysis of variance results is presented in Table S6. The model exhibited an F-value of 20.23 and a p-value of less than 0.0001, indicating that the model parameters had significant impacts and high reliability. The R² was 0.9529, and the difference between the Adj-R² and Pre-R² coefficients of determination was less than 0.2, demonstrating the model’s strong predictive capability for the Kappa number of the pulp. Further analysis revealed that the p-values for the linear terms (X₁,X₂,X₃) and quadratic terms (X₃²,X₄²) were less than 0.001, indicating their highly significant effects on the Kappa number. Additionally, X₁X₂, X₁² and X₂² had a p-value less than 0.01, and X₁X₃ and X₂X₃ had a p-value less than 0.05, confirming their significant impact.

Figure 8 shows the response surface results of the interaction effects between any two independent variables on the pulp Kappa number.

Fig. 8. 3D response surface and contour plot of the effect of independent variables on kappa number. (a) X₁-X₂; (b) X₁-X₃; (c) X₁-X₄; (d) X₂-X₃; (e) X₂-X₄; (f) X₃-X₄

As shown in Fig. 8a, with temperature and time fixed, the interaction between alkali dosage and the Na₂CO₃/NaOH ratio on the pulp Kappa number was explored. When the alkali dosage was high and the Na₂CO₃ ratio was low, the Kappa number of the pulp was lower. The stronger alkalinity of NaOH facilitates lignin removal, leading to a lower Kappa number pulp. Increasing the Na₂CO₃ ratio or reducing the alkali dosage hinders lignin removal, resulting in a higher Kappa number (Moradbak et al. 2016). Figures 8b and 8c present the interaction effects of alkali dosage and temperature or time on the Kappa number. When the cooking temperature or time was at the central level, increasing the alkali dosage helped lower the Kappa number. Figures 8d and 8e show the interaction between the Na₂CO₃/NaOH ratio and temperature or time on the Kappa number, indicating that with a decreasing Na₂CO₃ ratio, the Kappa number also decreased. This suggests that the Na₂CO₃ ratio had a greater impact on the Kappa number than the cooking temperature. Fig. 8f illustrates the interaction between the maximum temperature and the holding time. The contour lines are similar to concentric circles, indicating that their interaction has no significant impact on the Kappa number.

Validation of Optimal Process for Oxygen-Alkali Pulping of Reed

To obtain a high yield of refined pulp, pulp viscosity, and brightness, as well as a low Kappa number, the cooking conditions were optimized using Design Expert software. The optimal cooking conditions (R1) were found to be: alkali dosage of 21.0%, Na₂CO₃/NaOH ratio of 1/10, cooking temperature of 114.5°C, and holding time of 3.88 hours. Under these conditions, the predicted pulp yield was 50.5%, brightness was 55.6% ISO, pulp viscosity was 468 mL/g, and Kappa number was 16.2. To validate the model’s predictions, three parallel experiments were conducted, and the results are shown in Table 1. After cooking reed under R1 conditions, the average refined pulp yield was 50.5%, pulp viscosity was 465 mL/g, brightness was 55.5% ISO, and Kappa number was 16.2. The relative errors compared to the predicted values were 0.1%, 0.63%, 0.31%, and 0.37%, respectively. These results demonstrate that the response surface model has good predictive ability.

Table 1. Pulp Properties under Optimal Conditions

Comparison with Other Pulping Processes

This study compared the optimized cooking process with the traditional alkaline-oxygen (TAO) cooking process. Under the R1 conditions, the refined pulp yield, viscosity, and brightness of the pulp obtained were increased by 4.06%, 14.8 mL/g, and 0.98% ISO, respectively, compared to the TAO method. Additionally, the Kappa number was reduced by 1.27. These results indicate that the addition of a small amount of sodium carbonate significantly improved pulp quality. This could be attributed to the weaker alkalinity of sodium carbonate, which improved the alkaline environment of the cooking system, thereby protecting the carbohydrates from degradation while promoting the dissolution of lignin (Marin et al. 2017).

Table 2. Comparison of Pulp and Black Liquor from R1 Pulping and the TAO Pulping

*TAO：alkali dosage of 21.05%, sodium hydroxide amount of 13.58 g, oxygen pressure of 0.8 MPa, cooking temperature of 114.5 °C, and holding time of 3.88 h.

The chemical composition of the black liquor has a significant impact on the subsequent alkali recovery process (Jia et al. 2022). Therefore, the chemical composition of the black liquor obtained from the two cooking processes was analyzed. As shown in Table 2, compared to the TAO cooking process, the black liquor obtained from the R1 cooking process had a pH value reduced to 9.35. The organic matter content was relatively higher, and the inorganic matter content was lower, with a silica content of 1.57%, which was 0.9 times that of the black liquor from the TAO method. Due to the weaker alkalinity of sodium carbonate, some silicates were less soluble during the cooking process and remained in the solid phase (Zhang and Chen 2016). As a result, the silica content in the black liquor after R1 cooking was lower, while the silica content in the pulp was higher. According to formula S4, 65.9% of the silicon remained in the pulp.

According to Table 3, the cellulose, hemicellulose, and lignin content in reed raw material were 46.9%, 29.9%, and 24.7%, respectively. After oxygen-alkali pulping, the cellulose content of R1 pulp and TAO pulp increased to 76.4% and 73.0%, respectively, indicating that both pulping methods significantly enhanced the cellulose component. However, the cellulose content of R1 pulp was slightly higher than that of TAO pulp, while its lignin content was significantly lower, suggesting that the introduction of sodium carbonate may have improved the lignin removal efficiency during pulping (Xia et al. 2020). In addition, the hemicellulose retention percent of R1 pulp was higher than that of TAO pulp, indicating that the use of sodium carbonate helped protect hemicellulose to some extent, preventing its excessive degradation (Geng et al. 2014).

The synergistic mechanism of oxygen-alkali pulping is primarily reflected in the combined action of the alkaline environment and the oxidant. Sodium hydroxide and sodium carbonate provide an alkaline condition that promotes the cleavage of ether and ester bonds in the lignin structure, allowing it to dissolve in the cooking liquor (Zheng et al. 2018). Meanwhile, oxygen, as an oxidant, further disrupts the aromatic ring structure of lignin, enhancing its hydrophilicity and improving lignin removal efficiency (Yang et al. 2022). Additionally, the alkaline environment inhibits excessive degradation of cellulose, allowing it to be efficiently retained (Pavasars et al. 2003). Hemicellulose retention is achieved through the optimization of reaction conditions during oxygen-alkali pulping, preventing excessive damage to its structure by strong alkali and high temperature. In summary, oxygen-alkali pulping, through the synergistic mechanism of alkalinity and oxidation, achieves efficient cellulose retention, effective lignin removal, and moderate protection of hemicellulose, providing a theoretical basis for the efficiency and environmental friendliness of the pulping process.

Table 3. Chemical Composition of Teed, R1 Pulp and TAO Pulp

Fig. 9 (a-c) SEM images of reed (a, d), R1 pulp (b, e), and the TAO pulp (c, f). (d-e) Fiber size distribution of R1 pulp (g) and TAO pulp (h). (f-h) Average fiber length (i), width (j), and fines content (k) of R1 pulp and TAO pulp

The morphology of pulp fibers is closely related to paper properties. In this study, the surface morphology of the pulp fibers obtained from reed under the R1 cooking process and TAO cooking process was analyzed using SEM. As shown in Figs. 9a-9c9f, the fibers in the raw reed material were neatly arranged, uniform, and dense in structure. During the R1 cooking process, lignin degradation and gradual dissolution occurred, causing the fibers to dissociate and exhibit a rod-like shape. At the same time, the supporting role of the plant cell wall weakened (Chen et al. 2021), resulting in surface indentations on the fibers. In contrast, the fibers obtained from the TAO cooking process were rougher, with more significant damage and deeper indentations. This may be due to the stronger alkalinity of sodium hydroxide. The higher concentration of AOs in the cooking system, caused by the use of more sodium hydroxide, can lead to oxidative attack on the glucose units in cellulose (Guay et al. 2002), exacerbating the fiber peeling reaction and causing greater fiber damage, which in turn increases the depth of the surface indentations. Furthermore, the fiber size distribution of the two pulps is shown in Figs. 9d-9h. The average fiber length, average fiber width, and fines content of the pulp obtained from the R1 cooking process were 0.864 mm, 19.1 μm, and 36.2%, respectively. Compared to the TAO cooking process, the R1 pulp exhibited an increase in average fiber length by 0.04 mm and a reduction in fines content by 3.5%. Furthermore, the fiber size distribution of the two pulps is shown in Figs. 9g-9k. The fiber length of the pulp obtained through the R1 cooking process ranged from 0 to 2.7 mm, with widths between 10 and 38 μm. The average fiber length, average fiber width, and fines content were 0.864 mm, 19.1 μm, and 36.2%, respectively. In comparison, the traditional TAO pulp exhibits a narrower fiber distribution, with lengths ranging from 0 to 2.6 mm and widths from 10 to 40 μm. Additionally, the R1 pulp had an average fiber length that is 0.04 mm longer and a fines content that is 3.5 percentage points lower than that of the TAO pulp. These changes may be beneficial for improving the paper properties, as longer fibers and lower fines content generally contribute to higher strength and formation of the paper (Ji et al. 2018; Larsson et al. 2018).

Fig. 10. (a) XRD patterns of reed, R1 pulp and the TAO pulps; (b) physical and optical properties of the hand-sheets made from the pulps

XRD analysis was performed to examine the crystalline structure of the reed before and after cooking. As shown in Fig. 10a, the crystalline phase of cellulose remained unchanged after cooking, indicating that the cooking process did not alter the crystalline structure of cellulose (Chen et al. 2024). Under the same cooking conditions, the crystallinity of reed pulp obtained from the R1 cooking process was 67%, while the crystallinity of reed pulp from the TAO process was 65%. This difference is likely due to the relatively mild disruption of the cellulose crystalline regions by sodium carbonate during the R1 cooking process, which primarily affects the amorphous regions of the fibers. In contrast, the generation of a large amount of HO· during the TAO cooking process not only leads to the degradation of carbohydrates but may also damage the crystalline regions of cellulose (Carrillo-Varela et al. 2019; Korhonen et al. 2019).

By comparing the physical properties of the papers produced by the two methods, Fig. 10b shows that the paper made using the R1 cooking process exhibited higher performance in terms of tensile index (72.6 N·m·g⁻¹), tear index (3.38 mN·m²·g⁻¹), burst index (4.05 kPa·m²·g⁻¹), and ring crush index (8.46 N·m·g⁻¹), which were 1.04, 1.17, 1.06, and 1.11 times higher, respectively, than the values obtained from the TAO cooking process. This improvement can be attributed to the milder alkalinity of sodium carbonate in the R1 process, which results in less damage to cellulose and fiber bonds compared to the stronger alkaline conditions of pure sodium hydroxide cooking. The latter leads to a faster rate of polysaccharide degradation reactions (such as peeling reactions and alkaline hydrolysis), which can exceed the rate of lignin removal, thereby compromising the integrity of the fibers and reducing the physical properties of the paper (Puitel et al. 2015). Additionally, the lower whiteness of the paper produced by the TAO method is likely associated with the formation of chromophores, which may result from the excessive degradation of lignin during the cooking process (Sadeghifar and Ragauskas 2020).

CONCLUSIONS

Reed was utilized as the raw material, and a low-temperature oxygen-alkali pulping process was developed to produce high-quality chemical pulp. A systematic optimization of the cooking conditions was conducted by combining single-factor experiments and response surface methodology.

1. Under the conditions of 21.05% alkali dosage, a Na₂CO₃/NaOH molar ratio of 1/10, a cooking temperature of 114.5°C, and a holding time of 3.88 h, both the reed alkali pulp and the paper produced achieved optimal performance. Specifically, the yield of fine pulp was 50.5%, viscosity was 465 mL/g, brightness was 55.5% ISO, and the kappa number decreased to 16.16.
2. The paper’s physical properties were excellent, with a tensile index of 72.6 N·m·g⁻¹, tear index of 3.38 mN·m²·g⁻¹, burst index of 4.05 kPa·m²·g⁻¹, and ring crush index of 8.46 N·m·g⁻¹.
3. The high-quality chemical pulp obtained by this low-temperature novel alkaline oxygen pulping process can be attributed to two key factors: first, the low-temperature conditions effectively suppress alkaline hydrolysis of cellulose, thereby protecting the carbohydrates; second, the addition of sodium carbonate optimized the ratio of AOs in the cooking system, significantly enhancing the selective removal of lignin.
4. Further analysis showed that 65.9% of the silicon content was retained in the pulp, reducing the silicon content in black liquor and effectively mitigating potential silicon interference issues during alkali reuse.
5. This process not only significantly reduces pulping energy consumption but also facilitates the reuse of part of the green liquor, helping to reduce the capacity of the causticizing kiln, minimizing secondary white mud pollution, and producing high-quality reed chemical pulp, thereby realizing the high-value utilization of agricultural waste.

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (Grant No. 32101464, 22108137, 22208177), Shandong Provincial Key Research and Development Program (2024CXGC010412), Jinan Science and Technology Bureau project (Grant No.20233046), and Taishan Industrial Experts Program.

The authors declare no competing financial interest.

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Article submitted: December 19, 2024; Peer review completed: February 1, 2025; Revised version received: February 14, 2025; Accepted: February 28, 2025; Published: April 14, 2025.

DOI: 10.15376/biores.20.2.4068-4095

APPENDIX

Determination of the Yield of Fine Reed Pulp

Put the pulp obtained by cooking into a flat screen pulper. After screening, the fine pulp is obtained. Then, spin-dry the fine pulp and put it into a polyethylene bag to balance the moisture for 24 hours. Calculate the yield B% of the fine pulp according to formula S2.

(S2)

Analysis of the Crystallinity of Reed and Pulp

Take an appropriate amount of ground reed and pulp powder, spread them evenly on the glass sample stage, and then use an X-ray diffractometer for determination. The determination conditions are as follows: the diffraction angle ranges from 5° to 60°, the scanning speed is 4°/min, and the voltage is 40 Kv. Calculate the crystallinity index (CrI) of the reed and the pulp according to formula S3.

(S3)

Among them, represents the integrated area of all crystalline peaks, while represents the integrated area of all amorphous peaks.

Analysis of Silicon Content in Pulp

The silicon content in the pulp and black liquor was determined according to TAPPI T 245 cm-98 (1998). The silicon content (C%) in the pulp was calculated using the formula S4.

(S4)

Table S1. Independent Process Variables with Experimental Ranges and Levels in Response Surface Methodology

Table S2. Response Surface Experimental Design and Experimental Results

Table S3. Analysis of Variance for Fine Pulp Yield

Notes: Std. Dev. = 0.76, Mean = 46.56, C.V. % = 1.64%, Predicted residual error sum of squares (PRESS) = 43.29, R² = 0.9461, Adj-R² = 0.8923, Pre-R² = 0.7154, Adequate precision = 15.175

Table S4. Analysis of Variance for Pulp Viscosity

Notes: Std. Dev. = 7.34, Mean = 439.45, C.V. % = 1.67 %, Predicted residual error sum of squares (PRESS) =3800.05, R² = 0.9386, Adj-R² = 0.8771, Pre-R² = 0.6908, Adequate precision = 12.474

Table S5. Analysis of Variance for Brightness

Notes: Std. Dev. = 0.46, Mean = 53.84, C.V. % = 0.85 %, Predicted residual error sum of squares (PRESS) =15.28, R² = 0.9477, Adj-R² =0.8954, Pre-R² = 0.7293, Adequate precision = 13.868

Table S6. Analysis of Variance for Kappa Number

Notes: Std. Dev. = 0.57, Mean = 18.75, C.V. % = 3.02 %, Predicted residual error sum of squares (PRESS) =22.90, R² = 0.9529, Adj-R² =0.9058, Pre-R² = 0.7602, Adequate precision = 13.958

Table S7. Properties of Pulp and Hand-made Paper Obtained by R₁ Cooking were Compared with Other Studies

Fig. S1. The linear relationship between the predicted value of the model and the experimental value