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Whitening sulfonated alkali lignin via H2O2/ UV radiation and its application as dye dispersant Xueqing Qiu, Jue Yu, Dongjie Yang, Jingyu Wang, Wenjie Mo, and Yong Qian ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03369 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Whitening sulfonated alkali lignin via H2O2/UV radiation and its application as dye dispersant Xueqing Qiu,†,‡ Jue Yu,† Dongjie Yang,† Jingyu Wang,† Wenjie Mo,† and Yong Qian*,†,‡ †

School of Chemistry and Chemical Engineering, South China University of

Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, P. R. China ‡

State Key Laboratory of Pulp and Paper Engineering, South China University of

Technology, 381 Wushan Road, Tianhe District, Guangzhou 510640, P. R. China E-mail: [email protected] (Y. Qian)

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ABSTRACT: Sulfonated alkali lignin (SAL) can be used as dye dispersants, but its dark color may lead to severe staining problem, thus hindering its application. Here, a UV/H2O2 whitening process was applied to decolorize SAL and the color was faded by 50%. The structural changes of SAL before and after whitening were characterized by GPC, UV-vis, FTIR, fluorescence, and potentiometric titration, and the mechanism of decolorization was deduced. The results show that the contents of aromatic ring, methoxyl and phenolic hydroxyl groups in SAL decreased by 37%, 64%, and 78%, respectively, while the content of carboxylic groups increased by 187% after being radiated in H2O2 for 20 h. The changes in chromophoric group and the contribution of each chromophoric group to the overall color before and after whitening were also investigated. Due to the removal of chromophores, the staining phenomenon of light-colored SAL (LSAL) was reduced effectively when it was used as a dye dispersant. KEYWORDS: Sulfonated alkali lignin, Whitening, UV radiation, Hydrogen peroxide, Dye dispersant.

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INTRODUCTION Dye, which provides people with a variety of colors to choose on clothes, is an important raw material in the textile industry. Dye can be further classified into various types including, but not limited to, direct dyes, reactive dyes, disperse dyes, vat dyes, sulfur dyes, basic dyes, acid dyes, and solvent dyes

[1]

. Among them,

disperse dyes are the most widely-used [2-4], occupying 40%-50% of the global dye market [5]. This type of dyes is difficult to disperse in water without dispersants. The three

most

commonly

used

dye

dispersants

are

sulfonated

lignin,

naphthalene-sulfonated formaldehyde condensate and acid-phenol-formaldehyde condensate [6]. Lignin, a renewable biomass, is the second most abundant component in plants [7-9]. In addition to lignin from enzymatic hydrolysis in biorefinery, industrial lignin can be divided into two types according to its pulping processes, namely lignosulfonate (LS) that is obtained from sulfite process and alkali lignin (AL) that is obtained from kraft process

[10, 11]

. Due to the presence of hydrophobic aromatic skeletons and the

hydrophilic sulfonate and carboxylic groups, LS has an excellent amphipathy and is a natural dispersant [12,13]. As early as 1909, studies found that LS in the waste liquor of sulfite process could be used as auxiliaries in the processing of dye up-taking From then on, researchers such as Dilling

[15-17]

, Lin

[14, 18]

and Falkehag

[19]

[14]

.

have

made many significant contributions to improve the performance of LS as dye dispersant. On the other hand, AL has poor solubility in water and cannot be used as dispersant directly sulfomethylation

[20]

. It is often used after modification by sulfonation or

[21, 22]

. The dispersibility of sulfonated alkali lignin (SAL) is usually

greatly improved compared to that of AL [22]. Both LS and SAL are well-qualified as dye dispersants. However, sulfite process produces a large amount of waste acid and water, which caused severe pollution and a high processing cost. In addition to Borregaard Co., Ltd in Norway and some pulping factories in Russia, sulfite process has been gradually replaced by Kraft process. Nowadays, most of the industrial lignin

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comes from Kraft process [23]. Lignin-based dye dispersants have good thermal stability and they are also green and renewable materials. However, their dark color poses a big obstacle for their application. In the sulfite process, lignin is mainly sulfonated, degraded and polymerized, the resulting LS possesses a higher molecular weight and contains fewer phenolic hydroxyl groups and carboxyl groups, and thus shows a light brown color generally. In the Kraft process, a large number of phenolic hydroxyl groups are introduced due to the breakage of methoxy groups and ether bonds in the lignin structure

[24]

, thus the alkali lignin is dark brown or even black, and would get even

darker after being sulfonated. Therefore, LS and SAL will stain the fiber and distort the dye color when used as dye dispersants. In addition, the phenolic hydroxyl groups easily reduce the azo dyes. It is necessary to whiten the color and produce lignin-based dye dispersants with better qualities and less staining. There are a large amount of chromophore and auxochrome groups in industrial lignin. The main chromophore groups are quinone groups, conjugated double bonds, carbonyl groups, and free radicals, while the auxochrome groups include phenolic hydroxyl groups, hydroxyl groups, and carboxyl groups

[25, 26]

. To reduce the color of

industrial lignin, these chromophore and auxochrome groups should be blocked or cleared. The blocking method usually consists of two steps, by first blocking the phenolic hydroxyl groups, followed by oxidizing the lignin sample by air, oxygen, hydrogen peroxide or chlorine dioxide, etc. Lin used chloromethane sulfonate or propylene oxide to block at least 90% of phenolic hydroxyl groups, then oxidized SAL or LS with air, oxygen or hydrogen peroxide [27]. The color was successfully reduced by 80% and the obtained production can be used as dispersants for disperse dyes and vat dyes. Falkehag

[19]

used alkylene oxide or halogen-containing alkylene

alcohol as blocking agents and whitened SAL by 44%. As for non-sulfonated lignin, Dilling

[28]

first methylolated or crosslinked the lignin, then blocked the phenolic

hydroxyl groups, followed by oxidizing with chlorine dioxide. Dispersants obtained in this process had advantages such as low staining, low azo dye reduction, good heat

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stability, dye grinding efficiency, and dispersion stability. An alternative way of whitening lignin is breaking or clearing the chromophore and auxochrome groups by photocatalysis methods

[29-31]

. Researchers have tried H2O2/UV, TiO2/H2O2/UV or

Fe2+/H2O2/UV to degrade azo dyes in the sewage for the purification treatment of colored wastewater [32-34]. For lignin, Wang et al.[35] dissolved alkali lignin in THF and put the solution under UV irradiation for a period of time, thus achieving a color-fading rate of 65%, turning the dark-brown alkali lignin into light yellow color. THF is slightly toxic and the residuals have potential hazards, it should be replaced by safer oxidants. H2O2 can oxidize methoxyl groups and benzene rings

[36]

, enabling

it to be a lignin-whitening agent cooperating with UV irradiation. In this work, we use a H2O2/UV radiation method to reduce the color of SAL and obtain a product of which the color is faded by 50% from the untreated SAL. The effect of H2O2/UV radiation on the structure of SAL is investigated and the mechanism of decolorization is deduced. The whitened SAL is also successfully used as a dye dispersant.


EXPERIMENTAL SECTION Materials. Sulfonated alkali lignin (SAL) was obtained from Shenhua Forestry Co., Ltd (Huaihua, China). 30 wt% hydrogen peroxide (H2O2, AR) was obtained from Guanghua Technology Co., Ltd (Guangdong, China). Ferric chloride (FeCl3, AR), sodium hyposulfite (Na2S2O4, AR), sodium borohydride (NaBH4, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). GC grade methyl iodide was purchased from Sigma-Aldrich (Shanghai, China). Deionized water (resistivity ≥ 18 MΩ/cm) was obtained from a Millipore water purification system. C.I. disperse blue 79, a commercial dye, was obtained from Runtu Co., Ltd (Zhejiang, China). Other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China) and used as received.

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H2O2/UV whitening modification process. The SAL solution with concentration of 10.0 g/L was prepared and hydrogen peroxide, which is equivalent to 1.5 times the mass of SAL, was then added and mixed well. The solution was placed in a UV irradiation box (produced by Wanjia Instrument Co., Ltd., Dongguan, China) and irradiated for 20 h. The light-colored product, LSAL, was collected, washed with deionized water and freeze-dried for 24 h. SAL was treated solely by UV irradiation or 1.5 times of H2O2 for 20 h as controlled trials. The effects of solution pH and H2O2 dosage on the H2O2/UV process were also investigated by adding hydrochloric acid, sodium hydroxide solutions and 30 wt% H2O2.

Influence of chromophoric groups on the overall color [37]. Preparation of lignin-iron chelate. 1.0 g SAL (or LSAL) was dissolved in 40 mL 3wt% FeCl3 aqueous solution. After being shaken for 24 h, the pH of the mixture was adjusted to 2.0 by HCl and the precipitate was obtained. The product, lignin-iron chelate, was washed and dried and referred as CSAL (or CLSAL). A dark-color effect would occur if SAL was chelated with ferric iron. Therefore, the UV-absorbance difference of lignin before and after being chelated with iron can be reckoned as the absorbance of phenolic hydroxyl. Preparation of Na2S2O4-reduced lignin. 1.0 g SAL (or LSAL) was dissolved in 40 mL 4 mol/L NaOH aqueous solution and then 75 mL 70 g/L Na2S2O4 was added. After being shaken for 24 h, the mixture was filtered, and the collected filtrate was dialyzed in deionized water with the help of a membrane with cutoff molecular weight of 1000 Da for a week. The purified solution was then freeze-dried into solids, which was referred as rSAL (or rLSAL). Na2S2O4 reduced the quinone structure in lignin. Preparation of NaBH4-reduced lignin. 1.0 g SAL (or LSAL) was dissolved in 20 mL 0.1 mol/L NaOH aqueous solution with the addition of 50 ml 50% (vol/vol) ethanol aqueous solution. 1 g NaBH4 was then added in batches and the mixture was shaken for a week. The mixture was filtered to remove the filter residue, and the

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filtrate was dialyzed in deionized water followed by freeze-drying. The product was referred as RSAL (or RLSAL). NaBH4 reduced the quinone and conjugated carbonyl structures of SAL (or LSAL) at the same time. UV absorbance of chromophoric groups. 10 mg SAL (or rSAL, RSAL) was dissolved in 10 mL deionized water and its UV absorbance at 450 nm, marked as

A(SAL) (or A(rSAL), A(RSAL)), was measured. 10 mg SAL (or CSAL) was dissolved in 10 mL NaOH aqueous solution (0.2 mol/L) and the UV absorbance at 450 nm, marked as A '(SAL) (or A(CSAL)), was measured. The absorbance of each chromophoric group can be calculated by following formula: A1 = A (CSAL) − A ' (SAL)

(1)

A2 = A (SAL) − A (rSAL)

(2)

A3 = A(rSAL) − A(RSAL)

(3)

A4 = A(SAL) − A2 − A3

(4)

Aw = A(SAL) + A1

(5)

Where Aw represents the overall color absorbance of lignin, A1 represents the absorbance of phenolic hydroxyl, A2 represents the absorbance of ortho-quinone;

A3 represents the absorbance of conjugated carbonyl; A4 represents the absorbance of other factors.

Performance tests of dye dispersants. Preparation of dye slurry. Dye dispersants, C.I. disperse blue 79, and 5 mm agate beads were mixed with a mass ratio of 1:1:20. Then, the pH of the mixture was adjusted to 5.5. The dye slurry was then obtained by ball-milling the mixture at a rate

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of 400 rpm for 4 h. Dyeing process. A high-temperature program-controlled dyeing machine (GRY-12, Quanrun Machinery Co., Ltd., Wuxi, China) was used to dye with following procedures. The temperature was set at 30 °C initially, then increased to 130 °C at a rate of 3.0 °C/min and kept for 30min; after that the temperature was reduced to 85 °C at the same rate and was also kept for 30 min. Thermal stability of dye dispersants. A laser particle size analyzer (MS2000, Malvin Co., UK) was used to determine the particle sizes of dye slurry with different dispersants at 25 °C and 130 °C (after dye process). Staining performance of dispersant on polyester fibers. 2.0g/L dispersant solutions were prepared and the pH was adjusted to 5.2. Polyester fibers, mixed with dispersant solutions, were put into dyeing cups and dyed by the dyeing process above. The dyed polyester fibers were washed and dried. A color photometer instrument (Dataclolor110TM, Datacolor Co., USA) was used to measure the K/S values and staining rates of dye dispersants. The computational formulas are shown below: K/S=(1-R2)/2R

(6)

Fiber staining rate/%=[(R0-Ri)]/R0×100

(7)

where K/S is the apparent color depth value, R0 is the reflectivity of unstained polyester fiber at 450 nm and Ri is the reflectivity of the polyester fibers stained by dye dispersants at 450 nm.

Other characterizations. The UV-Vis absorption spectra of lignin samples in the range of 200-800 nm were scanned by a UV-Vis spectrophotometer (UV-2450, Shimadzu Co., Japan). The Fourier transform infrared spectra (FTIR) in the range of 4000-400cm−1 were obtained by a FTIR spectrometer (PerkinElmer Inc., USA). Disks were prepared by mixing 3 mg of SAL (or LSAL) with 300 mg KBr and then pressed at 10 MPa for 3 min. The fluorescence spectra in the range of 350-750 nm were

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obtained by a fluorescence spectrometer (F-4500, Hitachi Ltd., Japan). The pH of SAL (or LSAL) solution was adjusted to 5.2 and the excitation wavelength was 340 nm. The molecular weight was determined by gel permeation chromatography (GPC). A chromatographic column, made up of UltrahydrageTM120 and UltrahydrageTM250 in series, was used as the stationary phase, while the mobile phase was a 0.1 mol/L NaNO3 aqueous solution. Polystyrene sulfonate was used as the standard substance. The content of the methoxyl, phenolic hydroxyl and carboxyl groups of SAL and LSAL was measured by the following methods. The determination of methoxyl content was determined by headspace gas chromatography (HS-GC) according to the method described in Reference 38. Methyl iodide was applied as the standard substance. A typical process is as follows: 10 mg lignin sample is dissolved in 0.5 mL hydroiodic acid (57%) and heated at 130 °C for 2 hours, followed by injection of 0.5 mL sodium hydroxide solution (6mol/L) into the sealed vial. The content of phenolic hydroxyl and carboxyl content was determined by an automatic potentiometric titrator (905 Titrando, Metrohm AG, Switzerland)

[39]

. P-hydroxybenzoic acid, the internal

standard substance, was dissolved in water to reach a concentration of 2.0 g·L-1 with a KOH-concentration of 5.6g·L-1. 5 mL of this solution and 5 mL lignin solution were mixed and titrated with known concentration of HCl standard solution. Particle size distribution was obtained by dynamic light scattering (DLS) using a ZetaPALS instrument (Brookhaven Instruments Co., America). Experimental samples were filtered with 0.45 µm syringe filter before being measured at 25 °C.


RESULTS AND DISCUSSION Whitening SAL in H2O2 by UV radiation. After being exposed to UV radiation in H2O2 for 20 h, the dark brown SAL solution turned into light yellow solution. Light-colored SAL (LSAL) was then obtained after freeze-drying, as shown in Figure 1. The structural changes of SAL before and after UV radiation were characterized by FTIR, shown in Figure 2. Compared with SAL, the vibration of the aromatic ring in

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the range of 1420-1600cm-1 still exists after H2O2/UV whitening process, but tends to be less obvious. It means the aromatic structure of SAL was retained, but the content decreased. The contents of methyl and methylene increased in LSAL due to the breakage of the aromatic rings, which can be confirmed by the more obvious stretching vibration at 2940cm-1 and 2845 cm-1. The detail view is shown in Figure S1. A stretching vibration of the carbonyl group at 1710 cm-1 appears after the whitening process, indicating more carbonyl groups are formed. To explore the exact structural changes of SAL, the molecular weight distribution and the functional group content are measured, as shown in Table 1. After modification, the contents of methoxyl group and phenolic hydroxyl group decreased from 3.46 and 2.84 mmol/g to 1.24 and 0.62 mmol/g, respectively, while the content of carboxyl group increased from 0.75 mmol/g to 2.15 mmol/g. The Mw and Mn of SAL decreased from 6500 and 3000 Da to 4800 and 650 Da, respectively, after the whitening process. The UV-vis absorptions at 280 nm and 450 nm are shown in Figure 3, which represent the content of aromatic rings and the color degree of lignin. After H2O2/UV whitening process, the content of aromatic rings decreased by 43% and the color faded by 50%. In the control experiments, the Mw of SAL processed solely by UV radiation or H2O2 stayed the same and the Mn decreased to 1400 and 1200 respectively, as shown in Table S1. The decrement of methoxyl and phenolic hydroxyl groups and the increment of carboxyl group of SAL processed by H2O2 were more obvious than those of SAL processed by UV radiation. However, the functional group changes of both controlled processes were much less than that of SAL processed by H2O2/UV radiation together. Therefore, the decolorization of SAL processed solely by UV radiation or H2O2 was less obvious than that of SAL processed by UV/H2O2 together, as shown in Figure S2. Based on the above results, the whitening mechanism of SAL is deduced. During the H2O2/UV radiation process, the methoxyl groups of SAL were oxidized and thus catechol structures were formed. Catechol structures were not stable under UV radiation and further oxidized to quinone structures

[35]

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. Finally the aromatic ring

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opened and the unstable quinone structures were oxidized into carboxylic acid by H2O2, which made the color of SAL lighter. The structural changes of SAL being whitened into LSAL is shown in Figure 4. Since the oxidation efficiency of H2O2 is highly dependent on the solution pH and the amount of H2O2, their influences on the decolorization of SAL were investigated. As shown in Figure S3, the color reduction reached to 72% and 60%, respectively, when the solution pH was adjusted to 12 or the H2O2 dosage was increased to 2 times of the mass of SAL. Depolymerization of lignin always leads to its disaggregation

[35]

. As shown in

Figure 5, both molecular and aggregate peaks of the particle size distribution of SAL shifted to smaller size after it was whitened to LSAL. In addition, the blue shifts of characteristic peaks in both UV-vis (inset picture in Figure 3) and fluorescence spectra (Figure 6) indicate that not only intermolecular disaggregation, but also intramolecular disaggregation happened during the whitening process, which may also contributed to the decolorization of SAL. Influence of whitening process on the chromophoric groups of SAL. The chromophoric group changes of SAL before and after whitening were characterized by UV-vis spectra and summarized in Table 2. Be consistent with the color change of SAL observed, the contents of all chromophoric groups decreased when it was whitened to LSAL. The contents of phenolic hydroxyl groups, ortho-quinone structures, and conjugated carbonyl groups decreased by 36%, 19% and 4%, respectively. The contents of other chromophoric groups such as methoxyl groups decreased by 41%. The contribution of each chromophoric group to the overall color before and after whitening is also calculated, as shown in Figure 7. Compared with SAL, the contributions of phenolic hydroxyl and methoxyl groups to the color of LSAL decreased by 17% and 14%, respectively. The contributions of ortho-quinone and conjugated carbonyl structures increased by 7% and 24% correspondingly, which are consistent with the functional groups changes shown in Table 1. Since phenolic

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hydroxyl groups are the main sources of the color of LSAL, their reduction can be a feasible way if a further whitening process of LSAL is needed. From the ratio of the color change value of each chromophoric group to that of the overall color, it can be confirmed that phenolic hydroxyl groups affected the color of SAL most during H2O2/UV whitening process. In contrast, the effect of the conjugated carbonyl groups was not obvious. That is why the color of LSAL was not deep when its contribution increased from 22% to 46%. The application of LSAL as dye dispersant. LSAL was applied as dye dispersant and its staining performance on polyester fibers was investigated, as shown in Figure 8. After whitening modification, the staining rate of SAL decreased from 24% to 13%. The color of polyester fiber stained by LSAL was almost white, while that stained by SAL was earthy yellow. In addition, the high temperature stability of SAL before and after H2O2/UV modification was almost unchanged, as shown in Figure 9. It means LSAL has potential to be a natural alternative for the commercial synthetic dye dispersant.


CONCLUSION Industrial sulfonated alkali lignin (SAL) was successfully whitened via one-step H2O2/UV radiation method and the color was faded by 50%. During the whitening process, the methoxyl groups in SAL were photo-oxidized into phenoxyl radicals and formed catechol structures. The catechol structures were further oxidized to unstable quinone structures and turned into aliphatic acid structures soon afterwards. After whitening, SAL was not only practically depolymerized, but also disaggregated. The cleavage of phenolic hydroxyl and methoxyl groups and the weakening of π-π interaction in SAL also contributed to its decolorization. Light-colored SAL (LSAL) was used as a dispersant and showed lower staining rate than SAL, while its high temperature stability was almost unchanged. This work provides a potential natural alternative for the commercial synthetic dye dispersant.

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ASSOCIATED CONTENT Supporting Information The molecular weight distributions and functional group content changes of SAL processed by UV radiation and H2O2. FTIR spectra of SAL and LSAL between 3000-2800 cm-1. The decolorization and decrease of aromatic rings of SAL treated by UV+H2O2 , UV radiation and H2O2. Color and aromatic ring changes of SAL during decolorization processes under different pH conditions and H2O2 dosages.


AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

Notes The authors declare no competing financial interest.


ACKNOWLEDGEMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21606089), the Guangdong Province Science and Technology Research Project of China (2014B050505006) and The Fundamental Research Funds for the Central Universities (2015ZM149).


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(10) Li, Z. L.; Xiao, D.; Ge, Y. Y.; Koehler, S. Surface-functionalized porous lignin for fast and efficient lead removal from aqueous solution. ACS Appl. Mater. Inter., 2015, 7(27), 15000-15009. (11) Zhou, H. F.; Lou, H. M.; Yang, D. J.; Zhu, J. Y.; Qiu, X. Q. Lignosulfonate to enhance enzymatic saccharification of lignocelluloses: role of molecular weight and substrate lignin. Ind. Eng. Chem. Res., 2013, 52(25), 8464-8470. (12) Qin, Y. L.; Qiu, X. Q.; Liang, W. S.; Yang, D. J. Investigation of adsorption characteristic of sodium lignosulfonate on the surface of disperse dye using quartz crystal microbalance-dissipation. Ind. Eng. Chem. Res., 2015, 54(49) , 12313-12319. (13) Qin, Y. L.; Yu, L. X.; Wu, R. C.; Yang, D. J.; Qiu, X. Q.; Zhu, J. Y. Biorefinery lignosulfonates from sulfite-pretreated softwoods as dispersant for graphite. ACS Sustainable Chem. Eng., 2016, 4(4), 2200-2205. (14) Lin, S. Y. A New Lignosulfonate dispersant for dyes. Text. Chem. Color., 1981, 13(11), 24-28. (15) Dilling, P. The effect of cation type on lignosulfonate dispersant performance in disperse dye systems. Text. Chem. Color., 1988, 20(5), 17-22. (16) Dilling, P. Low electrolyte sodium lignosulfonates. U.S. Patent 4,590,262, May 20, 1986. (17) Dilling, P.; Loeffler, V. R.; Prazak, G.; Thomas, K. U. Production of lignosulfonate additives. U.S. Patent 4,892,588, Jan. 9, 1990. (18) Lin, S. Y. Method for polymerization of lignosulfonates. U.S. Patent 4,332,589, Jun. 1, 1982. (19) Falkehag, S. I.; Moorer, H. H.; Bailey, C. W. Alkylene chlorohydrin, oxide or carbonate modified sulfonated lignins in a disperse or vat dye cake. U.S. Patent 3,672,817, Jun. 27, 1972.

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(20) Sun, R.; Tomkinson, J.; Mao, F. C.; Sun, X. F. Physicochemical characterization of lignins from rice straw by hydrogen peroxide treatment. J. Appl. Polym. Sci., 2001, 79(4), 719-732. (21) Qin, Y. L.; Yang, D. J.; Guo, W. Y.; Qiu, X. Q. Investigation of grafted sulfonated alkali lignin polymer as dispersant in coal-water slurry. J. Ind. Eng. Chem., 2015, 27,192-200. (22) Ouyang, X. P.; Ke, L. X.; Qiu, X. Q.; Guo, Y. X.; Pang, Y. X. Sulfonation of alkali lignin and its potential use in dispersant for cement. J. Disper. Sci. Technol., 2009, 30(1), 1-6. (23) Chakar, F. S.; Ragauskas, A. J. Review of current and future softwood kraft lignin process chemistry. Ind.Crop. Prod., 2004, 20(2), 131-141. (24) Nada, A. M. A.; El‐Diwany, A. I.; Elshafei, A. M. Infrared and antimicrobial studies on different lignins. Acta Biotechnol., 1989, 9(3), 295-298. (25) Qian, Y.; Zhong, X. W.; Li, Y.; Qiu, X. Q. Fabrication of uniform lignin colloidal spheres for developing natural broad-spectrum sunscreens with high sun protection factor. Ind. Crop. Prod., 2017, 101, 54-60. (26) Qian, Y.; Deng, Y. H.; Li, H.; Qiu, X. Q. Reaction-free lignin whitening via a self-assembly of acetylated lignin. Ind. Eng. Chem. Res., 2014, 53(24), 10024-10028. (27) Lin, S. Y. Process for reduction of lignin color. U.S. Patent 4,184,845, Jun. 22, 1980. (28) Dilling, P. Color reduction process for non-sulfonated lignin. U.S. Patent 4,486,346, Dec. 4, 1984. (29) Sarathy, S. R.; Mohseni, M. The impact of UV/H2O2 advanced oxidation on molecular size distribution of chromophoric natural organic matter. Environ. Sci. Technol., 2007, 41(24), 8315-8320.

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(30) Coca, M.; Pena, M.; Gonzalez, G. Variables affecting efficiency of molasses fermentation wastewater ozonation. Chemosphere, 2005, 60(10), 1408-1415. (31) Muruganandham, M.; Swaminathan, M. Photochemical oxidation of reactive azo dye with UV–H2O2 process. Dyes Pigments, 2004, 62(3), 269-275. (32) Gupta, V. K.; Jain, R.; Mittal, A.; Saleh, T. A.; Nayak, A.; Agarwal, S.; Sikarwar, S. Photo-catalytic degradation of toxic dye amaranth on TiO2/UV in aqueous suspensions. Mat. Sci. Eng. C, 2012, 32(1), 12-17. (33) Lucas, M. S.; Peres, J. A. Decolorization of the azo dye Reactive Black 5 by Fenton and photo-Fenton oxidation. Dyes Pigments, 2006, 71(3), 236-244. (34) Hao, O. J.; Kim, H.; Chiang, P. C. Decolorization of wastewater. Crit. Rev. Env. Sci. Tec., 2000, 30(4), 449-505. (35) Wang, J. Y.; Deng, Y. H.; Qian, Y.; Qiu, X. Q.; Ren, Y.; Yang, D. J. Reduction of lignin color via one-step UV irradiation. Green Chem., 2016, 18: 695-699. (36) Orita, H.; Shimizu, M.; Hayakawa, T.; Takehira, K. Oxidation of methoxy-and/or methyl-substituted benzenes and naphthalenes to quinones and phenols by H2O2 in HCOOH. B. Chem. Soc. Jpn., 1989, 62(5), 1652-1657. (37) Li, Y. M.; Chen, Z. H. The chromophoric behavior of the undissolved lignin in Na2SO3 treatment and H2O2 bleaching. Transaction of China pulp and paper, 2000, 15(B12), 75-78. (38) Li, H. L.; Chai, X. S.; Liu, M. R.; Deng, Y. H. Novel method for the determination of the methoxyl content in lignin by headspace gas chromatography. J. Agr. Food Chem., 2012, 60(21), 5307-5310. (39) Zhou, M. S.; Huang, K.; Qiu, X. Q.; Yang, D. J. Content determination of phenolic hydroxyl and carboxyl in lignin by aqueous phase potentiometric titration. CIESC J., 2012, 63(1), 258-265.

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Captions of Tables and Figures Table 1. The molecular weight distribution and functional group content changes of SAL before and after radiation. Table 2. UV absorbance of chromophoric groups at 450 nm before and after whitening process and the change of the contribution of each chromophoric group to the overall color. Figure 1. The solid particles and aqueous solutions of SAL and LSAL. The concentrations of both solutions were 0.5 g/L. Figure 2. FTIR spectra of SAL and LSAL. Figure 3. UV-vis spectra of SAL and LSAL. The insert image: the absorption spectra from 300 to180 nm. The concentrations of both solutions were 0.1 g/L. Figure 4. The structural mechanism of SAL being whitened into LSAL during the radiation process. Figure 5. Particle size distributions of SAL and LSAL in water. The concentrations of SAL and LSAL solutions were 0.5 g/L. Figure 6. Fluorescence spectra of SAL and LSAL. The concentrations of SAL and LSAL in water were 0.5 g/L. The excitation wavelength was 340 nm. Figure 7. The contributions of each chromophoric group to the overall color before and after whitening process. Figure 8. Fiber staining rates of SAL and LSAL. Insert images are polyester fibers stained by SAL and LSAL. Figure 9. The thermal stability of SAL and LSAL.

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Table 1.

Sample

Mw/Da

Mn/Da

Methoxy content (mmol·g-1)

Phenolic hydroxyl

Carboxyl

content

content

-1

(mmol·g )

(mmol·g-1)

SAL

6000

3000

3.46± ±0.02

2.84± ±0.09

0.75± ±0.03

LSAL

4800

650

1.24± ±0.01

0.62± ±0.02

2.15± ±0.08

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Table 2

Color

Phenol

O-quinone

Conjugated

Other

Aw

hydroxyl A1

A2

carbonyl A3

A4

SAL

2.35

0.57

0.56

0.49

0.73

LSAL

0.98

0.07

0.30

0.44

0.17

∆Ai ×100% ∆Aw

/

36.37

18.98

3.65

40.88

Sample

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Figure 1.

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Figure 2.

SAL

LSAL

2845

2940

1710

1420

1600 4000

3500

3000

2500

2000

1500 -1

Wavenumber (cm )

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1000

500

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Figure 3.

3.0 2.5

279

215 Absorbance (a.u.)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0 1.5

207 274

1.0

200

220

240

260

280

Wavelength/nm

0.5 0.0 200

SAL LSAL 300

400

500

600

Wavelength (nm)

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700

800

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Figure 4.

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Figure 5.

100

SAL LSAL

80

Intensity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

0 0

500

1000

1500

Diameter (nm)

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2000

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Figure 6.

160

LSAL SAL

510

120

FL Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

40

0

350

523 400

450

500

550

600

650

Wavelength (nm)

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700

750

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Figure 7.

50

40

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Phenol hydroxyl O-quinone Conjugated carbonyl Other

30

20

10 0 SAL

LSAL

Dispersants

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Figure 8.

40 35 30 Fiber staining (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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25 20 15 10 5 0

SAL

LSAL

Samples

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Figure 9.

20 25℃ 130℃ Particle size (µm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

15

2 0.60

0.56

0

SAL

LSAL

Samples

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For Table of Contents Use Only Synopsis: Sustainable sulfonated alkali lignin was whitened via one-step UV radiation and thus present lower staining rate when it was used as dye dispersant.

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