The properties of unmodified and hydroxymethylated lignin are shown in Table 1. As can be seen, the molecular weight of H-lignin was marginally larger than the molecular weights of kraft lignin. Formaldehyde was reported to crosslink phenolic groups and lignin in acidic or basic environment (Gonçalves and Benar, 2001; Saito et al., 2012). This may also verify that polymerization occurred through the formaldehyde reagent despite optimum conditions designed to discourage this undesirable side reaction (Pang et al., 2008). The polymerization of formaldehyde with phenols and/or lignin was reported to occur via reacting at the available para or ortho positions of the phenyl propene subunits to form a tertfunctional based network (Zoumpoulakis and Simitzis, 2001).
Lignin Mw (g/mol) Mn (g/mol) Mw/Mn Elemental composition (wt%) Charge density (meq/g) SO3- group (meq/g) C H N O S Kraft lignin 9679 2141 4.52 64.20 5.12 0.07 28.52 1.46 0±0.03 0±0.04 H-lignin 14 092 3090 5.56 58.61 5.62 0.04 35.11 0.62 1.55±0.02 0±0.01 SAH-lignin ND ND ND 46.75 3.46 0.03 40.88 7.73 0.46±0.01 0.16±0.01 SSH-lignin 8243 2001 4.53 59.45 5.53 0.04 32.18 2.15 1.22±0.01 1.01±0.01 Notes: All samples were created under optimum conditions for charge density and solubility. ND, Not detected.
Table 1. Properties of lignin in this study
The elemental analysis facilitated the determination of chemical formulas for lignin samples. To allow for ease of comparison, a basis of 9 units of carbon was used. The chemical formula of softwood kraft lignin and H-lignin were determined to be C9H8.545O2.994S0.075 and C9H10.356O4.044S0.035, respectively. The presence of sulfur in both kraft lignin and H-lignin may be indicative of impurities in unmodified kraft lignin, as unmodified kraft lignin was acid- washed with sulfuric acid prior to use. It is noted that hydroxymethylation increased the hydrogen and oxygen contents of lignin with grafting formaldehyde to the phenyl propene subunits of lignin. However, chemical formulas of lignin considering its methoxyl group is recommended for readers and will be in fact used by the authors in future studies.
The H-lignin exhibited a solubility of 55%. The charge density of H-lignin is higher than that of kraft lignin due to the addition of formaldehyde to the aromatic ring.
Figure 1a shows the charge density of soluble lignin (CDSL), charge density of insoluble lignin (CDIL), and solubility of sulfonated H-lignin (SAH-lignin) by H2SO4 as a function of sulfuric acid/lignin molar ratio. A maximum value of 0.4 meq/g was achieved for the CDSL, while solubility and the CDIL were marginal. Furthermore, the CDSL, CDIL and solubility exhibit no significant changes as the H2SO4/lignin molar ratio increased.
Figure 1. Charge density of soluble lignin (CDSL), charge density of insoluble (CDIL) and solubility of SAH-lignin as a function of (a) H2SO4/H-lignin molar ratio (140℃ and 1 h); (b) reaction time (6.33 molar ratio of H2SO4/H-lignin and 140℃) and (c) reaction temperature (6.33 molar ratio of H2SO4/H-lignin and 1 h)
Figure 1b exhibits the CDSL, CDIL and solubility of SAH-lignin as a function of time of sulfonation reaction. It is evident that the CDSL, CDIL, and solubility remained low tested all the times. The CDIL was negligible and CDSL did not exceed 0.27 meq/g, regardless the reaction time.
Figure 1c presents the CDSL, CDIL and solubility of SAH-lignin as a function of temperature of sulfonation. Evidently, with an increase in temperature from 80℃ to 140℃, the CDSL decreased from a value of 0.43 meq/g to 0.26 meq/g. The solubility and the CDIL remained negligible over the evaluated reaction temperature. This confirms the literature results that condensation reactions rely heavily on temperature and increase with time (Xie and Shi, 2011).
However, H-lignin has an initial charge density of 1.2 meq/g and a solubility of 55% prior to sulfonation. Therefore, it can be concluded that the sulfonation had a detrimental effect on charge density and solubility, as the charge density and solubility of the H-lignin significantly dropped after sulfonation. In this respect, self-condensation of lignin under strong acidic conditions might be the main reason for such results (Matsushita et al., 2004a; Matsushita et al., 2004b; Matsushita et al., 2009).
Figure 2a shows the CDSL and the solubility of sulfonated H-lignin by Na2SO3 (SSH-lignin) as a function of the Na2SO3/H-lignin molar ratio. It is seen that the solubility increased from 75% to a maximum of 95.6% at the Na2SO3/H-lignin molar ratio of 0.33. A decrease in solubility is noted as the sulfonation molar ratio was increased to 0.82 mol/mol. Condensation has been found to occur in alkaline conditions (Hedjazi et al., 2009), and elemental sulfur in the solution reported to act as an inhibiting agent for alkaline condensation of lignin (Gierer, 1964; Gierer and Petterson, 1977). The charge density decreased from an initial charge density of 1.6 meq/g to 1.3 meq/g once the ratio increased from 0.67 to 1.20 Na2SO3/H-lignin. This decrease may be due to the presence of hydroxymethyl group on phenyl propene group being sulfonated at lower Na2SO3/H-lignin ratio, and occurrence of condensation at a higher Na2SO3/ H-lignin ratio.
Figure 2. Charge density of soluble lignin (CDSL), charge density of insoluble (CDIL) and solubility of SSH-lignin as a function of (a) Na2SO3/H-lignin molar ratio (95℃, 2 h and a lignin concentration of 11.1 g/L); (b) reaction time (Na2SO3/H-lignin molar ratio of 0.37 mol/mol, 95℃ and a H-lignin concentration of 11.1 g/L) and (c) reaction temperature (Na2SO3/H-lignin molar ratio of 0.37 mol/mol, 2 h and a H-lignin concentration of 11.1 g/L)
Figure 2b illustrates the impact of time on the CDSL and solubility of SSH-lignin. The CDSL increased from 1.08 meq/g to 1.60 meq/g as time of reaction extended from 1 h to 5 h. At shorter reaction times, the SSH-lignin was more soluble. However, extending the reaction time led to a decrease in solubility. As stated earlier, an increased reaction time favors condensation over sulfonation, which has been stated earlier to occur in alkaline conditions (Gierer, 1964; Gierer and Petterson, 1977).
Figure 2c presents the influence of reaction temperature on the CDSL and solubility of SSH-lignin. Through the examination of solubility and the CDSL, a temperature of 95℃ was the optimum with the highest charge density of 1.4 meq/g and solubility of 96wt%. The optimized temperature for the sulfomethylation of sodium lignosulfonate was 95℃ in the literature (Yu et al., 2013). However, a lower temperature of 85℃ was reported as optimum for the sulfonation of woody biomass (Xie and Shi, 2011).
The optimum conditions for the SAH-lignin and SSH- lignin were the temperatures of 80℃ and 95℃; reaction time of 1.0 and 3.0 h; H2SO4/lignin molar ratio of 14.8 mol/mol and Ns2SO3/lignin molar ratio of 0.49 mol/mol, respectively.
The properties of sulfonated lignin prepared under optimum conditions are shown in Table 1. The molecular weight of H-lignin decreases dramatically after sulfonation for SSH- lignin. This decrease in molecular weight can be attributed to the cleavage of large amount of ether bonds by the sulfonation of phenolic β-O-4 structure of lignin in alkaline solution (Matsushita, 2015), which will be discussed later. The molecular weight of SAH-lignin could not be measured, due to limited solubility of the lignin samples in tetrahydrofuran after acetylation. The derived chemical formula of SAH-lignin and SSH-lignin were C9H7.954O6.908S0.557 and C9H9.98O3.66S0.12, respectively. It is seen that oxygen is greater in the sulfonated lignin samples than kraft and H-lignin. The SAH had a greater amount of sulfur than SSH-lignin, which is due to the presence of sulfuric acid after treatment in the samples, and condensation of lignin using sulfuric acid as a catalyst would increase its overall molecular weight (Helander et al., 2013).
Lignin condensation under strong acidic conditions could be the reason for lower charge density of the SAH- lignin than that of H-lignin (Table 1). The sulfonation amount and charge density of SSH-lignin was determined to be over two times higher than those of the SAH-lignin (Inwood et al., 2017). The lower charge density of the SSH-lignin than that of H-lignin could be attributed to the loss of hydroxyl groups in condensation reaction under alkaline conditions (Gierer, 1964; Gierer and Petterson, 1977), which are necessary for substitution by -SO3- groups.
Figure 3 shows the solubility (1wt%) of lignin samples as a function of pH. Unmodified kraft lignin was only soluble under alkaline conditions at a pH higher than 10. The H-lignin exhibited improved solubility of 55% at pH 7. The SSH-lignin exhibited complete solubility over all pH ranges, because it has a large polarity originating from hydroxymethyl and sulfonated groups, which both would aid in maintaining a high solubility of lignin.
The reaction schemes for sulfonation (via sulfuric acid or sodium sulfite) of hydroxymethylated are presented in Fig. 4. Lignin is an amorphous, polyphenolic, highly cross- linked polymer consisting of polyphenyl propene units joined by carbon-carbon and ether bonds. The subunits of lignin are referred to as p-hydroxyphenyl (H), guaiacyl (G), and sinapyl (S) subunits, respectively. As major components of softwood lignin are G units with trace amounts of S and small amount of H units, G and H units were drawn as the representatives in the following reaction schemes.
With the aliphatic chain occupying the para position, the formaldehyde would react solely on the ortho position relative to the hydroxide group on lignin's ring (Yasuda et al., 1998). Only one type of H-lignin (i-a in Fig. 4) can form via hydroxymethylaion of G subunit, while two types of H-lignin (i-b and i-c in Fig. 4) could be induced from hydroxymethylaion of H subunit according to the number of ortho position substituted.
As discussed in our previous study, aromatic sulfonic group formed through the sulfonation by sulfuric acid on the ortho position of lignin's aromatic ring under strongly acidic conditions (Cerfontain et al., 1985; Gao et al., 2019). Therefore, after the hydroxymethylation pretreatment, the occupation of both ortho positions on H-lignin i-a and i-b prevented introduction of sulfonic groups by sulfuric acid treatment, while H-lignin i-c presented the only possibility with sulfonation on its one available ortho position. This explains the relatively low charge density and sulfonic group content in the SAH-lignin (Table 1).
On the other hand, the sulfonation by sodium sulfite treatment mainly occurs on aliphatic hydroxyl groups (e.g., α-position) as well as the phenolic β-O-4 structure of lignin under alkaline conditions (Matsushita, 2015; Aro and Fatehi, 2017; Gao et al., 2019). The sulfite group is not expected to react at the ortho position (relative to the hydroxide group) as stronger conditions are needed for aromatic substitution (Cerfontain et al., 1985). On the hydroxymethylated lignin (i.e., H-lignin i-a, i-b, i-c in Fig. 4), new reactive sites on both G and H subunits can occur for the sulfonation on the newly added hydroxymethyl group associated with the aromatic ring (Alonso et al., 2005). Thus, hydroxymethylation was demonstrated to be an effective pretreatment for the sodium sulfite treatment of lignin, resulting in its high sulfonic group content (Table 1) and solubility (Fig. 3).
Fillers have been used in composites and papermaking to reduce the cost of the products and to improve their properties. However, a major problem of fillers is their compatibility with other components of composites and papermaking. Previously, various lignin species were adsorbed on activated carbon, calcium carbonate, and calcium oxide; and the products were reported to be used as modified fillers in various applications such as papermaking and composites (Fatehi et al., 2010; Fatehi et al., 2013). Therefore, the adsorption performance of the unmodified kraft lignin and the SSH-lignin samples was evaluated on kaolinite (Fig. 5) for serving for this purpose.
It can be seen from Fig. 5 that kraft lignin did not adsorb on kaolinite but did the sulfonated lignin samples. Generally, charge density, molecular weight, hydrophilicity and molecular structure of organic materials as well as the properties of adsorbents affect their adsorption performance on adsorbents. Increasing the sulfur content and molecular weight of lignin were reported to improve the adsorption of lignin (Matsushita and Yasuda, 2005). In the literature, increased charge density was noted to improve the adsorption capacity of lignocellulosic materials on calcium carbonate (Cerfontain et al., 1985). This is verified in this study, as the SSH-lignin exhibits the greatest charge density and the greatest adsorption affinity. The results showed that modified lignin samples had better adsorption than unmodified lignin. The modified kaolinite can potentially be used as fillers in composites or papermaking (e.g., container board).
Lignin presents itself as an interesting compound to be converted to a coagulant as it degrades naturally into non-toxic monomers and shows great potential due to its large and complex polyphenolic structure (Hedges et al., 1985; Doherty et al., 2011; Zhang et al., 2013). The lignin samples synthesized in this study were also evaluated as a coagulant to remove cationic dye. Ethyl violet was chosen as a representative of cationic dyes with a charge density of 2.57 meq/g and a molecular weight of 492.14 g/mol. Figure 6 shows the impact of the SSH-lignin dosage on ethyl violet dye removal. An optimum dye removal of 82.6% was reached at the SSH-lignin dosage of 0.2 g/L. Increasing the lignin concentration from 0.2 g/L to 0.3 g/L would produce overcharged complexes by the excess amount of available lignin in solution. These complexes would repel each other and prevent coagulation, leading to reduced dye removal (Fang et al., 2010). Kraft lignin, H- and SAH-lignin were not shown in this set of experiments due to their ineffectiveness and insolubility in neutral solutions.