Acid Precipitation of Kraft Lignin from Aqueous Solutions: The Influence of pH, Temperature, and Xylan

Abstract

In this study, the manners in which temperature (45-77 C) and the addition of xylan (5 g/95 g lignin) influence the onset of precipitation and evolution of the particle size distribution during acid precipitation of softwood kraft lignin were investigated in situ. No systematic trend between the onset pH of precipitation and the temperature or the addition of xylan could be observed at these conditions: the average onset pH was found to be 9.3. However, the size of the agglomerates increased as the temperature was increased, but added xylan rendered a decrease in agglomerate size. A higher onset pH was measured at increased Na ion concentration. The results indicate that the ionization degree of the phenolic groups influences the precipitation at 1 M Na ions but it is also probable that the degree of ionization of the carboxylic groups (on kraft lignin and xylan) influences precipitation (particle numbers and sizes).

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“…The increased aggregation observed below pH 6, and the reduction in ƺ-potential between pH 4-6, coincide with the augmentation in hydroxyl groups with reduced pH ( Table 2.2), implying stronger ionic and molecular hydrogen bonding interactions at pH 4, consequently making the molecules more stable in solution. These observations agree with the observations by Serwing et al [123], they reported that there was no significant increase in mass yield between pH 4-6. Howell and Thring [98] also observed some stagnation in yield between pH 6-7.…”

“…Similarly, the Mw increased with increasing pH at 85 °C and 1.5 hours residence time ( Table 2.2). This trend agrees with previous results [38,123], and could be due larger molecules being more unstable and prone to hydrophobic aggregations and subsequent precipitation. Our results show that the Mw decreases with residence time, but the Đ increases ( Table 2.2) These results complement the trend in the titration cure ( Figure 1), and the data in Figure 3 A-I, indicating that increased time allows for prolonged unspecific condensations of molecular fragments, and the sorption of smaller molecules onto larger chains, resulting in a lower Mw and higher Đ.…”

“…The color of the kraft lignin varied at different experimental conditions ( Figure A1.3 A-F , Table S3), it was observed that at higher temperatures, the product becomes very compact and sticky ( Figure A1.3 E) , confirming what has been reported by other authors [36,85,98], this characteristic in-turn, makes the sample difficult to process [98]. The amount of xylose decreased with increasing temperature, and this could be one factor inducing the closely-packed precipitate ( Figure A1.3 D-F) as it has been discerned that the absence of xylose promotes particle nucleation growth during kraft lignin acidification [123]. It is known for organic molecules, that the more double bonds a molecule contains, and the more functional groups attached to it, the more light it can absorb [214].…”

“…It is equally imperative to develop a deep understanding of the mechanisms behind the solubility, colloidal stability and precipitation of lignin in black liquor, and how is affected by the process variables. However, the work published on precipitation mechanisms is very scant and superficial [123]. Furthermore, most studies on lignin precipitation have been conducted either on softwood or mixed softwood and hardwood lignin, but there are few studies on pure hardwood kraft lignin obtained through the LignoBoost process [123] 1.4.…”

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“…Acid precipitation is a common and simple method to obtain lignin fractions that uses strong acid to gradually decrease the pH of black liquor [ 81 , 82 , 83 , 84 ]. This technique is based on the dissociation equilibria of weak acid groups which affect the solubility behavior of lignin-related chemical species.…”