Dilute lignin solution was successfully digested into colorless and clarified liquor under microwave-assisted oxidative digestion with hydrogen peroxide. High dosage of hydrogen peroxide is needed to effectively digest lignin, but excessive hydrogen peroxide may lead to recondensation of formed fragments in digested lignin. Microwave irradiation greatly facilitates the oxidative digestion of lignin. Compared with conventional heating technique, microwave-assisted digestion achieves the same or higher digestion rate within a shorter time and/or at lower temperature. After digestion, total organic carbon content of lignin solution decreases by 93.9%, and a small amount of aliphatic alkane, alcohol, acid and ester are formed via the cleavage of aromatic rings as well as the deprivation of side chains in original lignin. This work provides an alternative way to efficiently treat spent pulping liquor.

INTRODUCTION

The effluents from pulp and paper mill contain high volume of chemical oxygen demand (COD) and total organic carbon (TOC), mainly because it has large amount of lignin and its derivatives (Mishra & Thakur 2010). The environmental impact of this wastewater results not only from its chemical nature, but also from its dark coloration that reduces oxygen availability and negatively affects aquatic fauna and flora (Lara et al. 2003). The adequate treatment of this wastewater is necessary before discharging. Up to now, lignin from pulping process is mostly recovered by acid precipitation, but the resulting effluent cannot meet environmental discharge regulations because about 7 wt% of lignin residues still remain in the spent liquor even with an enhanced recovery process (Kim et al. 2007).

Coagulation-flocculation is also considered to be an effective method for removal of lignin, and 90% recovery of lignin is achieved by using a mixture of oxotitanium sulfate and aluminium sulfate (Nawaz et al. 2014). No matter which kind of recovery method of lignin is used, there is still a small amount of lignin resistant to natural biodegradation (Bhargava et al. 2007) remaining in the wastewater. To reduce environmental problem, the residual lignin is needed to be further digested for TOC and color removal from wastewater. However, relatively little attention has been paid to this further treatment, which may become necessary if legislation becomes more stringent (Thompson et al. 2001).

Advanced oxidation and biologic processes are widely investigated for degradation of lignin. A catalytic wet air oxidation process was used for the removal of a lignin model compound (ferulic acid) from synthetic wastewater, in which ca. 70% of COD was removed at 120 °C (Yadav & Garg 2012). However, COD removal from real wastewater containing lignin cannot reach this level, because lignin is more difficult to degrade than the model compound. Biodegradation has been considered to be an environmentally friendly way to treat lignin in spent pulping liquor. A lignin model compound was completely degraded within 13 days under sulfate reducing conditions, and the presence of additional carbon source enhanced this process (Pareek et al. 2001), but the prolonged process is a constraint to commercial utilization. In addition, photocatalytic degradation of lignin using Pt/TiO2 as the catalyst under ultraviolet light irradiation was reported to achieve a good color removal efficiency of lignin wastewater (Ma et al. 2008), and may be a promising method to treat lignin if the removal of TOC was further improved. Therefore, complete degradation of lignin is still a huge challenge.

In the present work, a high dosage of hydrogen peroxide was used to investigate the complete degradation of lignin, and the degraded products were characterized by TOC measurement, UV–visible spectroscopy (UV), Fourier transform infrared spectroscopy (FTIR), elemental analysis and gas chromatography–mass spectrometry (GC-MS). The purpose of this work is to seek an efficient and environmentally friendly method for color and TOC removal from spent pulping liquor.

EXPERIMENTAL

Materials

Lignin (Quanlin Paper Co., Shandong, China) was recovered from soda pulping liquor by acid precipitation. Lignin fraction with low molecular weight, inorganic salt and carbohydrate were removed by ultrafiltration (Wuxi Membrane Science and Technology Co., Wuxi, China) with a 30,000 Da cutoff membrane, and the retentate was dried in a vacuum oven to obtain purified lignin powder. H2O2 solution (30 wt%), NaOH and HCl were purchased from Damao Chemical Reagent Factory in Tianjin, China. All chemicals used in this work were of analytical grade and used without further purification.

Digestion of lignin

A mixture containing 0.20 g of lignin, 9.80 g of water and 0.08–2.00 g of H2O2 (30 wt%) was added into a 100-ml cylindrical reactor made of modified Teflon. The pH of the mixture was adjusted to 3–11 by NaOH or HCl. The closed reactor was placed into an Ethos-1 microwave digestion system (Milestone Inc., Sorisole, Italy). Then, the mixture was digested under microwave irradiation with an initial power of 300 W at 80–160 °C for 15–150 min. The digested products were cooled down to room temperature and then stored in a refrigerator before further analyses. A conventional heating process was also carried out under the same conditions except without microwave irradiation in order to address the effect of microwave irradiation on the digestion of lignin.

Analytical methods

UV-visible spectra were recorded on a UV-2450 UV-visible spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The relative molecular weights of lignin and digested lignin were determined by an Agilent 1100 series gel permeation chromatography (GPC) instrument (Agilent Technologies Inc., Santa Clara, CA, USA) using tetrahydrofuran as a mobile phase (0.6 mL/min) and polystyrene as a calibration standard. FTIR spectra were recorded on a Nicolet 380 Infrared Spectrophotometer (Thermal Fisher Scientific Inc., Waltham, MA, USA) using potassium bromide pellet technique. Carbon content in lignin and digested products was determined using a PE 2400 II Elemental Analyzer (PerkinElmer Corporation, Waltham, MA, USA).

TOCs of all samples were measured with a Sievers InnovOx Total Organic Carbon Analyzer (General Electric Company, Fairfield, CT, USA), which is based on the oxidation of organic compounds into carbon dioxide using sodium persulfate. The digestion rate of lignin was calculated as follows: 
formula
where TOC0 and TOC1 are the TOC content of lignin solution before and after digestion, respectively.

All liquid digested products were analyzed in a GC-MS QP2010 system (Shimadzu Corporation, Kyoto, Japan). The column connected to the system was a PE-5MS capillary column (30 m × 0.25 mm). Helium was used as a carrier gas at a flow rate of 1 mL/min. During analysis, the column temperature was first held at 50 °C for 2 min before ramping up to 250 °C at 10 °C/min and then subjected a final hold at 250 °C for 10 min. The temperatures of transfer line and ion source were maintained at 200 °C and 250 °C, respectively. A solvent delay of 3.0 min was selected. In the full-scan mode, electron ionization mass spectra in the range of 30–300 (m/z) were recorded with an electron energy of 70 eV.

RESULTS AND DISCUSSION

Influence of oxidation temperature and time on the digestion of lignin

The effect of oxidative digestion temperature and time on the TOC content of digested lignin is summarized in Table 1.

Table 1

Comparison of the effect of microwave irradiation and conventional heating at various reaction temperatures and times

Reaction type Reaction temperature (°C) Reaction time (min) TOC (mg/L) Digestion rate % 
Microwave irradiation 80 120 289 78.0 
95 120 218 83.4 
110 90 206 84.3 
110 120 195 85.2 
110 150 196 85.1 
130 60 90 93.2 
130 30 92 93.0 
150 30 80 93.9 
150 15 94 92.9 
160 15 92 93.0 
Conventional heating 150 15 196 85.1 
Reaction type Reaction temperature (°C) Reaction time (min) TOC (mg/L) Digestion rate % 
Microwave irradiation 80 120 289 78.0 
95 120 218 83.4 
110 90 206 84.3 
110 120 195 85.2 
110 150 196 85.1 
130 60 90 93.2 
130 30 92 93.0 
150 30 80 93.9 
150 15 94 92.9 
160 15 92 93.0 
Conventional heating 150 15 196 85.1 

Digestion conditions: 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5.

Under microwave irradiation for 120 min, the TOC content of digested lignin decreases from 289 to 195 mg/L upon increasing reaction temperature from 80 to 110 °C, indicating that high-oxidation temperature benefits the digestion of lignin. As listed in Table 1, when the digestion was carried out at 80 °C for 120 min, the digestion rate of lignin is 78.0%. When the reaction temperature is further increased to 150 °C, the rate reaches as high as 93.9% within 30 min. This suggests that the digestion time can be greatly shortened at high-oxidative digestion temperature to achieve a certain digestion rate. It is well recognized that the digestion process of lignin includes the digestion of lignin macromolecules and recondensation of degraded intermediates simultaneously. At the early treatment stage, the digestion reaction is dominant. However, with increasing reaction time, especially at high-reaction temperature, the digested intermediates accumulate and recondense, resulting in a lower digestion rate.

Compared with oxidative digestion of lignin at 150 °C for 15 min under conventional heating, microwave irradiation leads to a significant decrease in TOC content in digested lignin solution from 196 mg/L with conventional heating to 94 mg/L with microwave irradiation, indicating that microwave irradiation effectively facilitates the digestion of lignin. Microwave irradiation enables better temperature homogeneity of reactants (de la Hoz et al. 2005), and hence accelerates the digestion of lignin compared with conventional heating reaction. Besides this thermal effect, microwave irradiation is considered to increase the pre-exponential factor and decrease the activation energy in a chemical reaction (Lidström et al. 2001). Consequently, under microwave irradiation, a higher extent of lignin digestion can be realized within a shorter time period. It is worth noting that although extra microwave energy is induced in this process, the energy utilization and specific energy requirements for microwave reaction are reportedly better than conventional techniques (Gude et al. 2013), and it is potentially available in wastewater treatment at large-scale application.

Influence of H2O2 dosage on the digestion of lignin under microwave irradiation

The effect of H2O2 dosage on the digestion of lignin under microwave irradiation at 110 °C for 120 min is shown in Table 2. TOC content of digested lignin solution decreases from 1,003 to 195 mg/L with increasing H2O2 dosage from 0.08 to 1.60 g. However, further increasing H2O2 dosage to 2.00 g leads to an increase in TOC content from 195 to 337 mg/L. As a strong oxidant, H2O2 produces hydroxyl radicals, which can attack ether bonds and carbon–carbon bonds in lignin molecules, and thus accelerates the digestion of lignin. However, excessive hydroxyl radicals generated from excess H2O2 result in a recondensation of digested products.

Table 2

Influence of H2O2 dosage on relative molecular weight and TOC content of digested lignin

Dosage of H2O2, g 0.08 0.40 0.80 1.60 2.00 
Relative molecular weight 13,490 13,140 8,800 2,560 4,410 
TOC content, mg/L 1,003 793 671 195 337 
Dosage of H2O2, g 0.08 0.40 0.80 1.60 2.00 
Relative molecular weight 13,490 13,140 8,800 2,560 4,410 
TOC content, mg/L 1,003 793 671 195 337 

Digestion conditions: 0.2 g lignin, 9.8 g water, at pH 5.5 and 110 °C for 120 min.

The relative molecular weights of digested lignins determined by GPC are listed in Table 2. They decrease from 13,490 to 2,560 when H2O2 dosage increases from 0.08 to 1.60 g. However, the relative molecular weight of digested lignin increases to 4,410 when H2O2 dosage further increases to 2.00 g. The increase in relative molecular weight at high H2O2 dosage (2.00 g) is an indication of the occurrence of product recondensation in excessive H2O2.

Influence of pH on digestion of lignin under microwave irradiation

Figure 1 plotted the TOC content in digested lignin as a function of pH. The TOC content slightly decreases from 209 to 195 mg/L upon increasing the pH from 3.0 to 5.5. However, the TOC content grows monotonically to 473 mg/L when pH increases from 5.5 to 11, indicating that alkaline condition is constraint to the digestion of lignin. H2O2 is facile to produce hydroxyl radicals and superoxide ions under alkaline conditions (Sanz et al. 2003). The excessive radicals formed at high pH may react either with each other or with H2O2 to produce water and oxygen before reacting with lignin, and therefore make the process less efficient in digesting lignin. Conversely, under alkaline conditions, H2O2 is unable to attack phenolic structure of lignin (Agnemo & Gellerstedt 1979). However, under acidic conditions, hydroxyl peroxide is protonated and forms a cationoid species which services as an electrophile. The resulting electrophile will react with the π electrons of the benzene ring, resulting in the cleavage of aromatic ring (Xiong & Lee 2000). Therefore, weakly acidic conditions favor color removal from spent lignin solution.

Figure 1

Influence of pH on the TOC content in digested lignin. Digestion conditions: 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 110 °C for 120 min.

Figure 1

Influence of pH on the TOC content in digested lignin. Digestion conditions: 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 110 °C for 120 min.

Characterization of digested products

UV-visible spectra of lignin and digested lignin were recorded to identify the decomposition of aromatic structure and color removal of lignin. As shown in Figure 2(a), lignin before digestion exhibits a strong absorbance peak at 280 nm in the UV spectrum, which is attributed to the characteristic peak of lignin originated from non-conjugated phenolic groups (Sun et al. 2000). However, the peak at 280 nm disappears in the UV spectrum of digested lignin by microwave irradiation, indicating that aromatic rings in lignin are mostly cleaved or opened after digestion. Figure 2(b) also shows that, for digested lignin by microwave irradiation, there is no absorbance in the visible-light region ranging from 400 to 900 nm consistent with the fact that lignin has been completely converted into clarified and colorless products (Figure 3). Nevertheless, digested lignin by conventional heating shows absorbance both at 280 nm and in the region of visible light, suggesting that aromatic ring in lignin molecule cannot destruct completely.

Figure 2

UV-visible spectra of lignin and digested lignin (a) UV region; (b) visible region. Digestion conditions 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 150 °C for 15 min; sample concentration (a) 60 mg/L and (b) 2 g/L.

Figure 2

UV-visible spectra of lignin and digested lignin (a) UV region; (b) visible region. Digestion conditions 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 150 °C for 15 min; sample concentration (a) 60 mg/L and (b) 2 g/L.

Figure 3

The color appearance of lignin solution before (left) and after digestion by conventional heating (middle) as well as by microwave irradiation (right). Digestion conditions 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 150 °C for 15 min.

Figure 3

The color appearance of lignin solution before (left) and after digestion by conventional heating (middle) as well as by microwave irradiation (right). Digestion conditions 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 150 °C for 15 min.

After digestion, carbon element content decreases by about 90% from 42.4 to 4.13 wt% according to the analysis of element, which means that lignin is digested mainly by oxidative digestion under microwave irradiation. However, the TOC content goes down 93.9% from 1315 mg/L before digestion to 80 mg/L after digestion. Clearly the decrease of TOC is greater than that of the carbon element content. This suggests that most of the carbon is converted into carbon dioxide and released from the liquid product, but there is still a small amount of carbon being converted into inorganic carbonates or small organic molecules. In addition, digested products contain water, resulting in slightly lower measured value of TOC in digested lignin.

The FTIR spectra of lignin and digested lignin are presented in Figure 4. Aromatic skeletal vibration at 1,500 cm−1 and aromatic methyl group vibrations at 1,460 cm−1 disappear in the spectrum of digested lignin by microwave irradiation, confirming that the aromatic rings in the lignin molecule have been destructed. Interestingly, the IR band at 1,706 cm−1, attributed to conjugated C = O, is enhanced in intensity after conventional heating digestion, indicating that the formation of aromatic compounds with small molecular weight results in an enhanced conjugated effect. However, this band disappears after microwave-assisted digestion, implying the destruction of aromatic ring. The band at 1,640 cm−1 associated with C = O conjugated or stretch aryl ketone still exists in digested lignin by microwave irradiation with lower intensity, demonstrating that the digestion of lignin produces some aliphatic carbonyl, such as acid or esters. Compared with lignin, digested lignin products by microwave irradiation show stronger IR band at 1,120 cm−1 (assigned to C-O stretch of secondary alcohols), but weaker IR band at 1,050 cm−1 (assigned to C-O stretch of primary alcohols) (Gabov et al. 2014), suggesting that the digested products contain more secondary alcohols and less primary alcohols. For digested lignin products by microwave irradiation, a weak but distinct peak at 993 cm−1 due to unconjugated carbonyl stretch is also observed, which supports the idea that any aryl structure has been destructed.

Figure 4

FTIR spectra of lignin (a), digested lignin by conventional heating (b), and digested lignin by microwave irradiation (c). Digestion conditions 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 150 °C for 15 min.

Figure 4

FTIR spectra of lignin (a), digested lignin by conventional heating (b), and digested lignin by microwave irradiation (c). Digestion conditions 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 150 °C for 15 min.

Since digested lignin is a mixture of several components, its FTIR spectrum is rather complex and difficult to be understood thoroughly. Therefore, GC-MS analysis was conducted to identify the composition of digested products. The total ion current (TIC) chromatogram of a digested product is plotted in Figure 5. After microwave-assisted digestion, four main compounds were detected, among other trace components. According to their characteristic fragments combined with National Institute of Standards and Technology (NIST) MS Database search for matching compounds, the peaks at retention time of 9.970 min, 12.828 min, 13.113 min and 22.238 min are identified as 2,2-dimethoxybutane, 1,3-dimethyl-1,3,5-pentanetriol, 2,2,4,4-tetramethyl-pentanoic acid and 10-methyl-dodecanoic acid methyl ester, respectively (Table 3). The results from GC-MS indicate that aliphatic alkane, alcohol, acid and ester are formed in digested lignin via the cleavage of aromatic rings and the deprivation of side chains in original lignin. However, conventional heating digestion produces more complicated components, among which eight digested products are identified, mostly containing aromatic compounds as well as a small amount of aliphatic acid.

Table 3

Compounds in digested lignin identified by mass spectroscopy

 Retention time (min) Identified compounds Main ions (m/z
Microwave irradiation 9.970 2,2-Dimethoxybutane 103, 89, 87, 71, 57, 55, 43, 29 
12.828 1,3-Dimethyl-1,3,5-pentanetriol 118, 101, 89, 69, 59, 43 
13.113 2,2,4,4-Tetramethyl-pentanoic acid 159, 143, 103, 91, 71, 69, 59, 58, 41, 29 
22.238 10-Methyl-dodecanoic acid methyl ester 228, 185, 171, 143, 129, 97, 87, 81, 74, 69, 55, 43, 31 
Conventional heating 8.275 Levulinic acid 116, 101, 73, 56, 43, 29 
8.845 Monom-ethyl succinate 114, 101, 87, 73, 59, 55, 43, 29 
13.220 Vanillin 152, 137, 123, 109, 93, 81, 65, 53, 39, 29 
14.370 Acetovanillone 166, 151, 136, 123, 108, 93, 77, 65, 43 
14.900 4-Hydroxy-3-methoxyphenylacetone 180, 137, 122, 107, 94, 77, 66, 43 
15.660 4-Hydroxy-3-methoxybenzoic acid 168, 153, 125, 108, 97, 79, 63, 51, 39 
16.595 3, 5-Dimethoxy-4-hydroxybenzaldehyde 182, 167, 153,139, 123, 111, 93, 79, 65, 51, 39 
17.450 Acetosyringone 196, 181, 153, 138, 123, 108, 93, 79, 65, 43 
 Retention time (min) Identified compounds Main ions (m/z
Microwave irradiation 9.970 2,2-Dimethoxybutane 103, 89, 87, 71, 57, 55, 43, 29 
12.828 1,3-Dimethyl-1,3,5-pentanetriol 118, 101, 89, 69, 59, 43 
13.113 2,2,4,4-Tetramethyl-pentanoic acid 159, 143, 103, 91, 71, 69, 59, 58, 41, 29 
22.238 10-Methyl-dodecanoic acid methyl ester 228, 185, 171, 143, 129, 97, 87, 81, 74, 69, 55, 43, 31 
Conventional heating 8.275 Levulinic acid 116, 101, 73, 56, 43, 29 
8.845 Monom-ethyl succinate 114, 101, 87, 73, 59, 55, 43, 29 
13.220 Vanillin 152, 137, 123, 109, 93, 81, 65, 53, 39, 29 
14.370 Acetovanillone 166, 151, 136, 123, 108, 93, 77, 65, 43 
14.900 4-Hydroxy-3-methoxyphenylacetone 180, 137, 122, 107, 94, 77, 66, 43 
15.660 4-Hydroxy-3-methoxybenzoic acid 168, 153, 125, 108, 97, 79, 63, 51, 39 
16.595 3, 5-Dimethoxy-4-hydroxybenzaldehyde 182, 167, 153,139, 123, 111, 93, 79, 65, 51, 39 
17.450 Acetosyringone 196, 181, 153, 138, 123, 108, 93, 79, 65, 43 
Figure 5

TIC chromatogram of digested lignin by conventional heating (a), and by microwave irradiation (b), digestion conditions: 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 150 °C for 15 min.

Figure 5

TIC chromatogram of digested lignin by conventional heating (a), and by microwave irradiation (b), digestion conditions: 0.2 g lignin, 9.8 g water, 1.6 g H2O2, at pH 5.5 and 150 °C for 15 min.

CONCLUSIONS

Dilute lignin solution is successfully digested into colorless and clarified liquor via oxidative digestion with the assistance of microwave irradiation. Effective digestion of lignin needs to be carried out with a high dosage of hydrogen peroxide, but excessive hydrogen peroxide may lead to recondensation of digested lignin fragments. Microwave irradiation greatly enhances the digestion of lignin, and the same or higher digestion rate can be obtained within a shorter reaction time and/or at lower temperature relative to conventional heating technique. The enhanced process can achieve a TOC reduction of 93.9%, which provides a promising lead in efficient treatment of spent pulping liquor. Comparing the changes in TOC and carbon element of lignin before and after digestion, it can be concluded that the lignin is converted into carbon dioxide, water, carbonates and a small amount of aliphatic compounds with low molecular weight. The results from UV-visible, FTIR and GC-MS show that aliphatic alkane, alcohol, acid and ester are formed in microwave digested lignin via cleavage of aromatic rings and the deprivation of side chains in original lignin, whereas after conventional heating digestion, there are still more aromatic compounds existing in the digested products.

ACKNOWLEDGEMENTS

This work is financially supported by the National Basic Research Program of China (No. 2012CB215302) and the Guangdong Provincial Natural Science Fund (No. 9151064101000082).

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