Influence of Nonprocess Elements on Lignin Structure During Oxygen Delignification

A series of kraft pulps were prepared in which the level of calcium was selectively enriched and depleted. These pulps were then oxygen delignified, and the pulps were characterized according to kappa number and viscosity. The residual lignin samples were isolated via acid-hydrolysis and characterized by NMR techniques. The results indicated that the presence of nonprocess elements could impact not only the extent of delignification but also the types of lignin removed. INTRODUCTION Oxygen delignification is widely used in many bleached pulp-grade operations to diminish the levels of residual lignin in kraft pulps (1). The chemistry of oxygen delignification has undergone a renaissance in the last decade. Recent studies have indicated that oxygen delignification occurs primarily via the free phenolics of lignin (2-5). Although the chemical structure and reactivity of residual lignin play a role in its effective removal, a variety of metal ions, frequently referred to as nonprocess elements (NPEs), that are either originated in pulps or introduced are also suggested to strongly affect the oxygen delignification process (6). Nonprocess elements commonly include magnesium, manganese, barium, calcium, iron, potassium, aluminum, and silicon (7). The incoming wood supply is the principal source of most NPEs for kraft pulps. Under the hot alkaline conditions of pulping, the fiber is able to act as a metal complexing reagent, retaining certain metals and exchanging others for sodium. It is generally believed that a variety of functional groups in the pulp can bind nonprocess elements, including carboxyl groups, hexenuronic acids, catechols, and phenoxy groups. Werner and Ragauskas have examined the metal binding capacity of kraft lignin (6). The role of metal ions in oxygen delignification has been a matter of debate for a long time (10,11). It has been demonstrated that peroxides were generated in oxygen delignification from both phenolic lignin structures and carbohydrate components (8,9). In the presence of metal ions, the peroxides formed are decomposed and likely form radical species (8,10). These radical species react with lignin leading to lignin degradation and solubilization, but also with carbohydrates leading to loss of pulp strength. Hydroxyl radical was identified among these to be a strong oxidative species and capable of attacking both lignin and carbohydrate (11). Although the reactions between lignin and hydroxyl radical facilitate lignin degradation, the oxidation of the carbohydrates may lead to the formation of carbonyl group in the C2 or C3 position (12,13). The carbonyl group then initiates the cleavage of the cellulose chain through an n—elimination reaction, and this reaction results in a lower pulp viscosity (14). Results have shown that iron and copper are able to induce the generation of harmful radicals that create a detrimental effect on the pulp viscosity (15). Magnesium is considered to be beneficial for oxygen delignification. The protective nature of magnesium has long been known, and magnesium compounds are the only generally accepted reagents for improving the selectivity of the process. However, the mechanism by which magnesium functions is still not completely clear. Several hypotheses have been proposed to account for its positive effect: a. formation of stable peroxide complexes (9); b. deactivation of the transition metal ion by absorption or by formation of complexes (16,17); c. formation of complexes with carbonyl group in C2 or C3 position of a glucose unit and thus being able to eliminate the further degradation (18). Calcium seems to have attracted little attention in oxygen delignification, although it has distinct solubility profiles that can lead to scaling problems in the bleach plant or recovery cycle. Brown et al. have reported that calcium has a detrimental effect on the selectivity of oxygen delignification (19). In contrast, Yang et al. have found that there is no such effect observed in their study (20). Although it is well appreciated that nonprocess elements such as manganese, magnesium, calcium, and iron influence oxygen delignification, the impact on the structure of lignin is largely unknown. As part of the work in the study of extended oxygen delignification, this manuscript summarizes our studies directed at determining how nonprocess elements influence the oxidative degradation chemistry of an 0-stage on lignin. A series of kraft pulps were prepared in which the level of calcium was selectively enriched and depleted and the effects of calcium on oxygen delignification was examined. Changes in lignin structure were analyzed by characterizing the residual lignin before and after the 0-stage via NMR. EXPERIMENTAL Materials All reagents and solvents were commercially purchased and used as received except for pdioxane, which was freshly distilled over NaBH4 prior to usage. A commercial softwood kraft pulp (kappa number 27.6, viscosity 32.6 mPa.$) was employed in this study. The brownstock pulp was extensively washed until the filtrate was pH neutral and colorless prior to usage. Chelation Treatment Chelation treatment of the pulps was carried at 10% consistency at 90°C for 1.00 hour according to the procedure (21). Q-Q treatment was performed with addition of 0.6% EDTA for each time of treatment as chelating agent and Ca-Q treatment with addition of 0.2% EDTA and 0.2% CaSO4. The pH of the slurry was adjusted to 5-7 with NaOH solution. Pulp was placed in a sealed polyethylene bag and thoroughly mixed by kneading. After chelation, the pulp was completely washed with deionized water. Oxygen Delignification Oxygen delignification was performed in a Parr reactor equipped with a continuous mixing device. The conditions were as follows: pulp consistency 10%, MgSO4 0.05%, NaOH 1.6%, temperature 104 °C, 02 pressure 100 psig, time 1.00 h. In a typical experiment, the reactor was warmed up to 80°C and charged with a mixture of pulp, water, and reaction agents. The reactor was then sealed and purged with oxygen; the air was driven out. It was pressurized with 02 and rapidly heated to the desired reaction temperature. Each oxygen delignified pulp was thoroughly washed until the effluent was colorless and pH neutral. The delignified pulps were used for kappa, viscosity analysis, and lignin isolation. Metals Analysis Metal ions were analyzed by using the Inductively Coupled Plasma Emission Spectroscopy technique. Lignin Isolation Lignins from the brownstock and post-0 delignified pulps were isolated by the acid hydrolysis method (22). In general, the pulp was adjusted to 4% consistency in a 2000-mL threenecked round-bottom flask with 0.1 N HC1 solvent solution containing 90% freshly distilled 1,4dioxane and 10% water (volume percentage). After refluxing for two hours under an argon atmosphere the pulp slurry was cooled and filtered through celite; the filtrate was neutralized to around pH of 6 by using a saturated Na2CO3 aqueous solution. The solution was then concentrated under reduced pressure to remove almost all the dioxane. The concentrated lignin solution was then added to acidic water (pH 2-3), frozen, and subsequently thawed. The lignin suspension was centrifuged and the supernatant was decanted. Newly prepared HC1 solution at pH 2-3 was used to wash the precipitated lignin three times following the freezing-thawing process. The washed lignin was then freeze dried and stored for use. NMR Analysis The isolated lignin samples were analyzed using a 400 MHz Bruker DMX spectrometer. Quantitative 13 C NMR spectra were acquired and analyzed in accordance with established literature methods (23). Lignin was dissolved in 500 !IL of DMSO-d6 before being transferred into a 5-mm NMR tube. 13C NIVIR spectra were recorded with an inverse gated decoupling sequence, 90° pulse angle, 14-s pulse delay, 23,000-Hz sweep, 10-12,000 transients, at 50.0°C. The fourier-transformed spectra were integrated according to reported chemical shifts for lignin functional groups. The integrals were normalized to the aromatic signals, which were assumed to have a value of 6 carbons. Lignin samples were also derivatized with 2-chloro-4,4,5,5-tetramethy1-1,3,2dioxaphospholane and analyzed by 31 P NMR following literature methods (24,25). 31 P NMR spectra were recorded using an inverse gated decoupling sequence, 90° pulse angle, 25-s pulse delay, 13,000-Hz sweep, and 300 transients, at room temperature. RESULTS AND DISCUSSION Metal Profiles after Chelation Treatment Softwood kraft pulps were subjected to EDTA chelation (Q-Q treatment) to remove the metal ions and especially to lower the content of calcium. EDTA&CaSO4 treatment (Q-Ca treatment) was carried out to enrich the calcium content in the pulp. Table 1 summarized the results of metal profiles of the pulp samples before and after chelation treatment. Table 1. Metal ion contents of pu1ps before and after chelation and calcium enrichment treatment (mg/kg pulp). Metal Ion Non-Q Q-Q Q-Ca Mg 330 174 263 Mn 97.2 0.33 2.63 Ca 1090 106 1360 Cu <0.5 <0.1 <0.1 Fe 6.03 3.99 4.20 K 45.4 13.2 19.5 Zn 10.5 0.32 0.35 Al 11.4 4.30 7.52 Si 80.7 6.98 20.7 Ba 7.38 7.40 6.08 Sr 4.90 4.33 4.02 After Q-Q treatment the decrease of nonprocess element contents in the pulp was observed. Manganese and zinc in the pulps was drastically reduced by more than 95%, whereas iron was reduced by nearly 30%. The reductions of magnesium, potassium, aluminum, calcium, and silicon are between these two extremes. There was no significant change in barium and strontium contents after chelation. The difference in the chelation efficiency for the ions is probably attributed, in part, to their ability to form complexes with EDTA. As demonstrated by Lapierre, magnesium ion has a lower tendency to form complexes with EDTA than does ferric ion (26). Judging from the formation constant of iron-EDTA complex (K eg), iron should be removed by EDTA in an efficient manner. However, the data in Table 1 indicate that there are still significant amounts of iron left in the pulp after EDTA treatment. Yang et al. have also observed the same effect and suggested that the failure to completely remove this metal ion may be due to the intimate connection between iron and fiber components, which limits its removal from the pulp during the Q-stage (20). The reduction of nonprocess element contents in the pulp was also observed after Q-Ca treatment, except calcium with an increase of its content. Since there is a lower charge of EDTA and just a single Q stage in the Q-Ca treatment, the metal removal is a little less effective than QQ treatment. The two posttreatment pulps had comparable contents of nonprocess elements, apart from the much higher content of calcium in the Q-Ca treatment pulp. Influence of Chelation Treatment on Oxygen Delignification In order to evaluate the effect of the metal ion profile on oxygen delignification, the treated and untreated pulps were subject to oxygen delignification with the addition of magnesium sulfate. The results are summarized in Figure 1 and Table 2. As indicated in Figure 1, the pulp after Q-Q treatment displays lower delignification than that without the pretreatment. The enrichment of calcium after EDTA treatment results in a lower lignin removal than the Q-Q chelated pulp. High pulp viscosity was obtained after the chelation treatment and enrichment of calcium. Oxygen delignification of the pulp after Q-Q and Q-Ca treatment resulted in around 23% reduction in pulp viscosity, whereas in the case of the untreated sample a 30.1% viscosity reduction was found. Figure I. Influence of chelation treatment and enrichment of calcium on pulp delignification and viscosity in oxygen bleaching. It is evident from these results that the removal of metal ions or the enrichment of calcium can retard carbohydrate degradation. However, the price for the maintenance of pulp viscosity is the reduction in delignification. This observation is in agreement with the basic theory of oxygen delignification. The enrichment of calcium was found to slightly reduce delignification and maintain similar viscosity of the pulp as compared to the depletion of calcium. Significant removal of calcium can result in higher selectivity of oxygen delignification (Table 2). The amounts and types of metals in the pulp can clearly influence both delignification and carbohydrate degradation. Table 2. Kappa and viscosity of oxygen delignified nonchelated and chelated pulps. Oxygen delignified pulp Brownstock Non-Q Q-Q Q-Ca