Limitations on the protective action of MgSO4 for cellulose during kraft pulp oxygen delignification

Abstract

Magnesium sulfate (MgSO4) is the most widely used protector for alleviating the effects that metal ions have on cellulose degradation. However, the efficiency of MgSO4 is limited by the oxygen delignification conditions. This work discusses the factors influencing MgSO4 efficiency in terms of cellulose protection and delignification. The type and concentration of metal ions, delignification rate, additions order, and mixing degree of MgSO4 should affect the cellulose degradation during oxygen delignification in the presence of MgSO4. The most adverse effects on cellulose are observed with Mn2+ and Fe2+ ions followed by Cu2+ and Fe3+. MgSO4 addition can diminish such negative effects; however the protection becomes reduced in the presence of higher concentrations of metal ions. In addition, the optimum MgSO4 application level is closely dependent on the delignification rate and metal ions concentration. Adding MgSO4 is optional for pulps with trace metal ions at relatively low delignification levels, but it is essential for pulps with concentrated metal ions or when the oxygen delignification rate is relatively high. More simply, when the added MgSO4 is thoroughly mixed with the pulp before the addition of NaOH, it exhibits a prominent effect on cellulose protection.

Full Article

Limitations on the Protective Action of MgSO₄ for Cellulose during Kraft Pulp Oxygen Delignification

Hai Huang, Yuantao Hu, Hui Zhang, Shilin Cao,* and Xiaojuan Ma *

Magnesium sulfate (MgSO₄) is the most widely used protector for alleviating the effects that metal ions have on cellulose degradation. However, the efficiency of MgSO₄ is limited by the oxygen delignification conditions. This work discusses the factors influencing MgSO₄ efficiency in terms of cellulose protection and delignification. The type and concentration of metal ions, delignification rate, additions order, and mixing degree of MgSO₄ should affect the cellulose degradation during oxygen delignification in the presence of MgSO₄. The most adverse effects on cellulose are observed with Mn²⁺ and Fe²⁺ ions followed by Cu²⁺ and Fe³⁺. MgSO₄ addition can diminish such negative effects; however the protection becomes reduced in the presence of higher concentrations of metal ions. In addition, the optimum MgSO₄ application level is closely dependent on the delignification rate and metal ions concentration. Adding MgSO₄ is optional for pulps with trace metal ions at relatively low delignification levels, but it is essential for pulps with concentrated metal ions or when the oxygen delignification rate is relatively high. More simply, when the added MgSO₄ is thoroughly mixed with the pulp before the addition of NaOH, it exhibits a prominent effect on cellulose protection.

Keywords: MgSO₄; Oxygen delignification; Metal ions; Cellulose degradation

Contact information: College of Material Engineering, Fujian Agriculture and Forestry University Fuzhou 350002 China; *Corresponding authors: Shilin Cao (scutcsl@163.com) and Xiaojuan Ma (1212juanjuan@163.com); Huang Hai and Hu Yuantao contributed equally to this work

INTRODUCTION

Since it offers a way to avoid release of toxic chlorinated aromatic compounds, oxygen delignification has become widely popular. It is the most commonly used process in totally chlorine-free bleaching sequences because of its economic and ecological feasibility. The compatibility of the effluent from oxygen delignification with the kraft chemical recovery process, in addition to the reduction in the operating costs, have given the oxygen delignification process an advantage over other bleaching processes (McDonough 1995; van Heiningen et al. 2003; Bajpai 2005; van Heiningen et al. 2018). Oxygen delignification is typically conducted at moderate temperatures under an alkaline environment, where the oxygen reacts through complex radical chain reactions with both the lignin and carbohydrates. However, the delignification/cellulose degradation selectivity deteriorates in an oxygen delignification process at a higher delignification rate, or when the initial pulp exhibits a high kappa number (Tao et al. 2011; Jafari et al. 2014a). The poor selectivity is primarily due to the formation of hydroxyl radicals during oxygen delignification, and these can easily cleave the glycosidic bonds in cellulose via direct attack on the anomeric carbon (Guay et al. 2001; Guay et al. 2002). Transition metal ions in the pulp may facilitate the generation of highly reactive hydroxyl radicals, which in turn cause severe cellulose degradation (Gierer et al. 2001; Guay et al. 2001).

An effective approach to protect cellulose from degradation is therefore to control the generation of hydroxyl radicals in the oxygen delignification system. An effective method of removing transition metal ions from pulp is by washing the pulp with a chelating agent or mineral acid prior to oxygen delignification (Jones and Williams 2002; Granholm et al. 2009; Jafari et al. 2014b). However, considering the additional washing machinery and increase in caustic consumption, such technology is difficult to industrialize. Another excellent way to control the generation of hydroxyl radicals is by adding transition metal ion controllers or radical-scavenging biopolymers during the oxygen delignification process, e.g., Na₂EDTA (Zhao et al. 2018), zeolite (Hoang and Long 2017), chitosan (Li et al. 2015), guar-galactomannan (van Heiningen and Violette 2003), phenol (Chen and Lucia 2002), and anthraquinone (Liu et al. 2013). Among these approaches, the utilization of MgSO₄ is definitely the most effective relative to protecting cellulose from degradation. During oxygen delignification, the alkaline will cause MgSO₄ to be precipitated as Mg(OH)₂, which can absorb transition metal ions or form complexes, thereby inhibiting peroxide decomposition. However, it is doubted that Mg²⁺ can form a stable complex with carbonyl groups of cellulose to reduce the rate of cellulose degradation (Lapierre et al. 2003).

However, the protective benefits provided by the MgSO₄ protector to cellulose are limited. With the addition of the Mg protector, the degree of kappa reduction in a single oxygen delignification stage must be maintained at below 50% for softwood kraft pulps, to prevent excessive cellulose degradation and carbohydrate losses (van Heiningen et al. 2018). It has been found that an improper addition sequence of the Mg protector, e.g., the addition of NaOH before MgSO₄ or the addition of insoluble Mg(OH)₂, could also have a negative effect on cellulose protection (Bouchard et al. 2011). The proportion of metal ions to Mg is another important factor that affects the efficiency of Mg protection. According to Yokoyama et al. (1999), Mg²⁺ displays the best protection effect on cellulose when the Mg²⁺ to Mn²⁺ ratio is greater than 30 mol/mol. Moreover, studies have revealed that the integration of MgSO₄ and other additives, e.g., phenol (Fu et al. 2004) or EDTA (Lapierre et al. 2003), yields more effective cellulose protection. These studies have uncovered additional factors affecting the MgSO₄ efficiency, e.g., metal ions and Mg²⁺ addition methods; however, there has been a need to comprehensively evaluate these factors in terms of MgSO₄ efficiency.

To further enhance the performance of MgSO₄, the authors continuing effort is to explore the factors influencing MgSO₄ efficiency in terms of cellulose protection in order to provide theoretical guidance for the efficient utilization of MgSO₄. The present work, combined with various factor analyses, e.g., transition metal ions in the pulp, delignification rates, chemical addition sequences, and pre-mixing, will provide a comprehensive analysis and evaluation of the factors which affect the MgSO₄ performance.

EXPERIMENTAL

Pulps

The pulps with different kappa numbers (KP-1, KP-2, KP-3, and KP-4) were self-made kraft pulps prepared in a laboratory. The KP-1 and KP-2 samples were prepared from Eucalyptus grandis×urophylla, while the KP-3 and KP-4 samples were prepared from Pinus radiata D. Don. The pulping experiment was carried out in a laboratory digester (M/K 609-2-10, M/K Systems, Williamstown, MA). The parameter information, namely the active alkali (AA, expressed as Na₂O), sulfidity, liquor to wood ratio, heating rate, and holding time are listed in Table 1. The kappa number, viscosity, and metal ion content of the above kraft pulp samples are listed in Table 2. NaOH and H₂SO₄ were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. The MgSO₄, Fe₂(SO₄)₃, FeSO₄, MnSO₄, CuSO₄, and the chelating agent diethylenetriaminepentaacetic acid (DTPA) were purchased from Sinopharm Chemical Reagent Co. Ltd. The metal ion contents were determined via inductively coupled plasma mass spectrometry (ICP-MS) analysis. The detailed procedure was described in the authors previous work (Cao et al. 2014).

Table 1. Pulping Conditions of the Lab Kraft Pulp

Pulp Pre-treatment

The lab pulp was treated with DTPA and sulfuric acid in that sequence in order to remove the metal ions. The chelating treatment was conducted at a pH of 4, with a pulp consistency of 1% and an addition of 0.5% DTPA, for 60 min at 60 °C. After the chelating treatment, the pulp was treated with 2 mol/L sulfuric acid with a pH of 2. The other parameters were similar to those employed in the former chelating treatment. Subsequently, the pulp was washed several times with deionized water until the pH of the filtrate reached approximately 7. The residual metal ions in the pulps are shown in Table 2.

Table 2. Kappa Number, Viscosity, and Metal Ion Content of the Lab Kraft Pulp

Introduction of Metal Ions

In order to investigate the effects of metal ions on cellulose degradation, the transition metal ions, i.e., Fe²⁺, Mn²⁺, Fe³⁺, and Cu²⁺ ions, were added to the treated pulp as required for oxygen delignification. In this case, Fe₂(SO₄)_3, FeSO₄, MnSO₄, and CuSO₄ were added at 4 different concentrations (Table S1). Before adding the metal ions solutions, the pH of the kraft pulp slurry was adjusted to 2 by adding 0.1 mol/L H₂SO₄. After that, the pH of the pulp was adjusted to 9 with 0.1 mol/L NaOH in order to ensure the consistency of the alkali dosage, which was required for the subsequent oxygen delignification process.

Chemical Addition

The following processes outline the chemical additions for the pulp samples:

NaOH process: did not include the addition of MgSO₄.

MgSO₄ + NaOH process: First, MgSO₄ was added into the pulp and mixed for 2 min (hand kneading). After that, NaOH was added into the pulp slurry. Hand kneading was necessary for thoroughly mixing the chemicals and pulp.

NaOH + MgSO₄ process: First, NaOH was added into the pulp slurry, and after being mixed for 2 min, MgSO₄ was added and was hand kneaded before undergoing oxygen delignification.

Mg(OH)₂ + NaOH process: First, Mg(OH)₂ was added into the pulp and mixed for 2 min. After that, NaOH was added into the pulp slurry. Hand kneading was necessary for thoroughly mixing the chemicals and pulp.

MgSO₄ premixing process: Before undergoing oxygen delignification, the pulp slurry was mixed with MgSO₄ by stirring at 200 rpm for 60 min in order to thoroughly mix the pulp. The NaOH was added before the pulp underwent oxygen delignification.

Oxygen Delignification

The oxygen delignification experiments were performed in a Parr autoclave (Parr 4577, Parr Instrument Co., Moline, IL, USA). Oxygen bleaching was conducted at a pulp consistency of 10% at 100 °C at an oxygen pressure of 1.0 MPa; the added NaOH and MgSO₄ contents were 4% and 0.5%, respectively. As required, the pulps with different kappa numbers and viscosities were obtained by delignifying the pulp for different time periods.

Particle size analyses of Mg(OH)₂

To describe the effects of the MgSO₄ addition sequence on the properties of Mg(OH)₂, and therefore oxygen delignification, Mg(OH)₂ was in situ formed from the reaction between MgSO₄ and NaOH with different addition sequences. The process in which MgSO₄ was added first is described as: NaOH with a concentration of 80 g/L (20 mL) was added drop-wise to 20 mL of MgSO₄ solution (2.4 g/L). By contrast, the process in which NaOH was added first is describe as: MgSO₄ with a concentration 2.4 g/L (20 mL), was added drop-wise to 20 mL of NaOH (80 g/mL) solution. For comparison, purchased Mg(OH)₂ powder was also used. The in situ produced and purchased Mg(OH)₂ suspension was then stirred for 1 h at 200 rpm. After that, the suspension was diluted several times to measure the particle size via a Malvern nanoparticle size analyzer (Malvern Instruments, Nano-ZS90, Malvern, UK) (Ning et al. 2019).

Pulp Property Analysis

The kappa number and intrinsic viscosity of the pulp were determined in accordance with TAPPI test methods T236 om-13 (2013) and T230 om-13 (2013), respectively. The kappa number of the pulp was used to refer to the lignin content level, while the pulp viscosity was used to evaluate the level of cellulose degradation. While the kappa number change was calculated by the following equation,

 (1)

where k₀ is the kappa number of the pulp without the addition of MgSO₄ and k₁ is the kappa number of the pulp with the addition of MgSO₄. The viscosity increase was calculated by the following equation,

 (2)

where, v₀ is the viscosity of the pulp without the addition of MgSO₄ and v₁ is the viscosity of the pulp with the addition of MgSO_4.

RESULTS AND DISCUSSION

Transition Metal Type and Content Level

As shown in Figs. 1a and 1b, the effect of metal ions such as Fe²⁺, Fe³⁺, Mn²⁺, and Cu²⁺ on lignin removal was marginal. It has been reported that metal ions, including Cu²⁺, Mn²⁺, and Fe³⁺ can slightly promote delignification. Cu²⁺, which acts as an electron acceptor, can accelerate the formation of phenoxy radicals, as well as the radical reaction, thus increasing the degradation rate of lignin. Fe³⁺ ions can form Fe³⁺-complexes with phenolic compounds that link loosely to oxygen to form an O₂–Fe³⁺-lignin intermediate, which catalyzes the degradation of lignin (Johansson and Ljunggren 1994; Wu and Heitz 1995). However, cellulose depolymerization, observed as a DP (degree of polymerization) drop, occurs with an increased intensity with the increase of metal ions concentration in pulp. In this case, Mn²⁺ and Fe²⁺ ions are the most offensive ions, even at concentrations as low as 20 ppm, followed by Cu²⁺ and Fe³⁺ ions. Notably, Fe³⁺ has little effect on cellulose degradation. A carbohydrate model study revealed that in the presence of hydrogen peroxide, Fe²⁺ ions are the most harmful species to carbohydrates (Kishimoto and Nakatsubo 1998). In the oxygen delignification process, autoxidation leads to generation of the superoxide (•O₂^–) by electron transfer from the substrate to oxygen, demonstrated in Eq. 1.

O₂+e^–(from the substrate) →·O₂^– (1)

The superoxide is the anionic form of the hydroperoxyl radical (HO₂•). Superoxide and the hydroperoxyl radical react at an almost diffusion-controlled rate giving oxygen and hydrogen peroxide (Gierer et al. 2001), as shown in Eq. 2,

·O₂^–+ HO₂• →O₂+HO₂^– (2)

The Fe²⁺ ions can catalyze the decomposition of H₂O₂ molecules to form hydroxyl radicals, demonstrated in Eq. 3,

2 H₂O₂ + Fe²⁺ → HO• + OH⁻ + Fe³⁺ (3)

This is followed by the reduction of Fe³⁺ to Fe²⁺ ions by hydrogen peroxide, demonstrated in Eq. 4 (Sixta et al. 2006),

Fe³⁺ + H₂O₂ → Fe²⁺ + 2O₂•⁻ + 2H⁺ (4)

The formation of hydroxyl radicals by the reaction of H₂O₂ with Fe²⁺ ions is inhibited as the Fe³⁺ concentration increases, and hence the impact of Fe³⁺ on cellulose degradation is small.

Figure 1c shows the correlation between the kappa number and the viscosity, and a high delignification selectivity was observed (selectivity is expressed as the ratio between viscose drop to kappa number change) in the presence of Cu²⁺ or Fe³⁺. In contrast, Fe²⁺ cannot promote delignification, but it can accelerate cellulose degradation.

Fig. 1. Effects of the metal ions on oxygen delignification (a) Kappa number; (b) cellulose viscosity; and (c) relationship between the Kappa number and viscosity. Eucalyptus (KP-1) pulp was used for the oxygen delignification in this experiment, which was subjected to sequential acid and chelating treatments prior to oxygen delignification.

Figure 2a suggests that the addition of MgSO₄ can retard the delignification process. A positive kappa number change (defined as the ratio between the gain in the kappa number with the addition of MgSO₄ to the kappa number of the pulp without the addition of MgSO₄) is observed if the Mn²⁺ is out of consideration. However, the presence of Mn²⁺ ions can reduce the negative effect of MgSO₄ on delignification. Figure 2b reveals that MgSO₄ can effectively prevent the degradation of cellulose, but the protection effect of MgSO₄ on cellulose is limited and varies with the metal concentration level and the metal type. Table S2 lists the viscosity loss per ppm of transition metal ions with and without the addition of MgSO₄. As the metal ion content increases, the viscosity loss per ppm of transition metal ions with and without the addition of MgSO₄ tend to be similar. This means that the effect of the transition metal ions on cellulose degradation could not be reduced by adding MgSO₄, as the concentration of transition metal ions continued to increase. However, MgSO₄ can significantly protect cellulose from degradation when the concentrations of Mn²⁺, Fe²⁺ and Cu²⁺ are at low levels, according to the viscosity loss per ppm of transition metal ions with and without the addition of MgSO₄, or when Fe³⁺ concentrations are at high levels (up to 800 ppm) considering the value of viscosity (see Fig. S1). The Mg²⁺ ion exhibits a positive effect on cellulose protection. It has been proposed that at a pH of greater than 10, the Mg(OH)₂ precipitate formed from MgSO₄ and an alkaline species can encapsulate Mn²⁺ ions to form a complex of variable composition (Mg_1–xMn_x)(OH)₂(ss), therefore effectively binding some of the transition metal ions (Lidén and Öhman 1998; Wiklund et al. 2001). This extra action between Mn and Mg might account for the significant effects observed. Moreover, MgSO₄ is more efficient in eliminating cellulose degradation induced by Fe²⁺ ions, followed by the degradation induced by Mn²⁺, Cu²⁺, and Fe³⁺ ions, in descending order.

Fig. 2. Effect of MgSO₄ on reducing the metal ion effects on oxygen delignification (a) Kappa number; (b) cellulose viscosity; and (c) relationship between the Kappa number and viscosity. Eucalyptus (KP-1) pulp was used in this experiment.

Delignification Rate

Oxygen delignification is a competitive reaction between oxygen and oxygen species with residual lignin and carbohydrate. During this process, the accumulation of the condensed phenolics and p-hydroxyphenol units depends on the extent of oxygen delignification (Fu and Lucia 2003; Yang et al. 2003), and the functional units of lignin are resistant to further degradation. Therefore, as the oxidation resistance of lignin increases with the increase in the extent of oxygen delignification, the oxygen and oxygen species are more likely to react with carbohydrates than with lignin.

Figures 3a and 3b illustrate the Mg protection effects on oxygen delignification for pulp at both high and low metal concentrations. As shown in Fig. 3a, when the starting pulp only contains a trace amount of metal ions, pre-treatment can remove most of the metal ions from the kraft pulp. Whether a Mg protector is added or how it is added has little effect on the cellulose degradation rate when the oxygen delignification is limited to less than 53%. It has been reported that an initial large Kappa loss is accompanied by a small loss in the cellulose viscosity, while a larger viscosity loss is observed at the end of the delignification process (van Heiningen et al. 2018).

Fig. 3. Selectivity at (a) relative low delignification (less than 53%); and (b) relative high delignification (62% to 67%). Pre-treatment involves sequential acid washing and chelation treatments. Without treatment means no pre-treatment. The KP-2 and KP-4 samples were used in this experiment.

When the pulp undergoes extensive oxygen delignification (greater than 60%), MgSO₄ is efficient in protecting cellulose from degradation in the metal ion-concentrated pulp. This protection is more significant in pulp with trace amounts of metal ions. The result is consistent with the Bouchard’ study (2011) that the selectivity of oxygen delignification of the acid-washed pulp was improved when MgSO₄ is added prior to the oxygen stage.

Addition Sequence

The data in Table 3 illustrates that the NaOH and MgSO₄ addition sequence has effects on oxygen delignification.

Table 3. Addition Sequence of the Chemicals

The addition of MgSO₄ prior to oxygen delignification has been shown to decrease the efficiency of the delignification process; under identical conditions, to provide pulp with a slightly higher kappa number than no Mg²⁺ added (Kappa Number is 10.5 ± 0.3). In contrast, the addition of MgSO₄ has a positive influence on the viscosity of cellulose. In this case, the protection effect is independent of the amount of MgSO₄ added and both Mg(OH)₂ and MgSO₄ display a protection effect. However, MgSO₄ is more effective than Mg(OH)₂; moreover, when MgSO₄ is added before NaOH is added, it displays the most prominent protection effect, exhibited as the highest cellulose viscosity. The superior performance of MgSO₄ over Mg(OH)₂ can be reasonably explained as the difference between the addition of soluble Mg²⁺ ions and insoluble Mg(OH)₂ molecules (Bouchard et al. 2011). The different size of the Mg(OH)₂ molecules, resulting from the various MgSO₄ additions, could be one of the reasons. To investigate the effect of the chemical addition sequence on the size of Mg(OH)₂, a controlled experiment was conducted in the presence of MgSO₄ and NaOH with different addition sequences. A commercial Mg(OH)₂ sample was used for comparison. The size distribution of the Mg(OH)₂ colloidal particles is illustrated in Fig. 4, while the average sizes are listed in Table 3. The data revealed that the commercial Mg(OH)₂ sample presented a larger average size (2953 nm). When MgSO₄ was added before NaOH, it was inclined to form small Mg(OH)₂ particles, which were smaller than those formed when NaOH is added first. The oxygen delignification process is usually conducted under alkaline conditions, which has a higher pH than the isoelectric point of the Mg(OH)₂ (pH of 12) in water. Accordingly, when Mg is added to NaOH, a pH value greater than 12 is reached in the reacting mixture. Therefore, the electric charge on the surface of the particles is expected to be negative, resulting in an extremely fast nucleation generation process (Song et al. 2011). Thus, the size of the Mg(OH)₂ particles formed by the NaOH + MgSO₄ process were larger than those from the MgSO₄ + NaOH process. The small particle size of Mg(OH)₂ affords better performance of the adsorption properties of the transition metal ions, according to the adsorption mechanism of the Mg protector.

Fig. 4. Size of the Mg(OH)₂ colloidal particles based on the different chemical additions

Pre-mixing

As mentioned in the above section, the prior addition of MgSO₄ is important for the Mg²⁺ ions to be evenly distributed throughout the pulp fiber mixture. In this part of the study, thorough premixing was introduced prior to oxygen delignification and the resultant effects were investigated. The data in Table 4 revealed that premixing with MgSO₄ significantly increased the cellulose viscosity for all of the pulp samples, confirming the importance of the uniform distribution of the Mg ions in the fiber suspension or cell walls. According to the Donnan equilibrium, Mg²⁺ ions can be effectively distributed in the pulp fiber suspension, fiber surface pores, and fiber cell cavities after reaching mass transfer equilibrium. In this case, small-sized Mg(OH)₂ molecules were synthesized in-situ with the addition of NaOH; this is beneficial for cellulose protection. It has been proposed that the Mg ions is a prerequisite for effective physical encapsulation of the transition metal hydroxides within the Mg(OH)₂ precipitate when NaOH is added (Bouchard et al. 2011); where, the Mg ions can complex with the carboxylic acids and carbonyl groups present in the cellulose, thus preventing cellulose degradation. In all, it is demonstrated that the thorough mixing of the MgSO₄ and pulp can help cellulose from degradation. Compared to the other factors such as metal ions, delignification rate, addition order of the chemicals, premixing procedure is readily to be achieved as “MgSO₄ added before NaOH process” in a pulp mill.

Table 4. Comparison of the Performance of Oxygen Delignification between the Process With and Without Pre-mixing of MgSO₄

CONCLUSIONS

1. The efficiency of MgSO4 in terms of cellulose protection during oxygen delignification varied with the types and levels of the metal ions, delignification rates, pre-mixing, and addition sequence of the chemicals.
2. MgSO₄ was most effective in eliminating the cellulose degradation resulting from Fe²⁺, but exhibited a feeble influence relative to the detrimental effects of other metals.
3. The addition of MgSO₄ is optional for the pulps with prior acid washing and EDTA chelation at relatively low delignification rates (＜53%), but it is essential for pulps with higher concentration of metal ions independent of the extent of delignification.
4. The addition order of MgSO₄ and NaOH showed more influence on cellulose degradation than delignification. MgSO₄ added before NaOH process was demonstrated to be effective for cellulose protection.
5. Thorough mixing of MgSO₄ and the pulp helps protect cellulose from degradation and is readily incorporated into industrial processes.

CONFLICTS OF INTEREST

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No. 31770632 and No. 31270638), and the innovation fund from Fujian Agriculture and Forestry University CXZX2017296 and CXZX2017037.

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Article submitted: October 9, 2020; Peer-review completed: November 27, 2020; Revised version received and accepted: December 31, 2020; Published: January 7, 2021.

DOI: 10.15376/biores.16.1.1438-1452

APPENDIX

Supplemental Information

Table S1. Addition Amount of Transition Metal Ions

Table S2. Viscosity Loss Brought by per ppm Transition Metal Ions

Fig. S1. Effect of MgSO₄ addition on viscosity under different kinds and levels of transition metal ions. Eucalyptus (KP-1) pulp is employed for oxygen delignification in this experiment, which is subjected to sequential acid and chelating treatments prior to oxygen delignification.