Novel Process for Generating Cationic Lignin Based Flocculant

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Applied Chemistry

A novel process for generating cationic lignin based flocculant Shoujuan Wang, Fangong Kong, Weijue Gao, and Pedram Fatehi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05381 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Industrial & Engineering Chemistry Research

A novel process for generating cationic lignin based flocculant Shoujuan Wang,a,b Fangong Kong, a,b* Weijue Gaob and Pedram Fatehib* a

Key Laboratory of Paper Science and Technology of Ministry of Education, Qilu University of

Technology, Jinan, China, 250353 b

Department of Chemical Engineering, Lakehead University, 955 Oliver Road, Thunder Bay,

ON, Canada, P7B 5E1 *Corresponding authors: F. Kong ([email protected]), P. Fatehi ([email protected]) ABSTRACT Kraft lignin is currently under-utilized since it is primarily used as fuel in pulping processes. However, it can be used as a raw material to prepare a functional polymer, such as flocculant. A cationic lignin polymer was synthesized via graft polymerization of lignin and [2(methacryloyloxy)ethyl] trimethylammonium chloride (METAC) in an acidic environment (i.e., a heterogeneous system). The reaction, which was optimized under the conditions of pH 4.0, METAC/lignin ratio of 1.8 mol/mol, 3 h, 80 °C, 0.3 mol/L of lignin concentration and 1.5 wt.% of initiator dosage, generated a lignin-graft-PMETAC polymer having 2.93 meq/g charge density and 1.53×106 g/mol molecular weight. The lignin-graft-PMETAC polymer can be dissolved in water (at 10 g/L concentration) and a pH range of 0.5-13. The flocculation performance of the polymer was evaluated in a 0.25 wt.% kaolin suspension using a particle dispersion analyzer and the results demonstrated the superiority of the lignin-graft-PMETAC polymer to unmodified lignin and PMETAC. These results confirmed that lignin-graft-PMETAC polymer could replace PolyMETAC (i.e., PMETAC, a synthetic polymer) as a flocculant, which promotes the use of 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

sustainable products in industry. This paper introduces a new process for inducing a cationic lignin-based flocculant. KEYWORDS: polymerization, kraft lignin, sustainable product, green process, flocculation INTRODUCTION Today, the production of sustainable and renewable polymeric materials to substitute oilbased polymers is extensively studied. Lignin is the third plentiful natural polymer in the world after cellulose and chitin,1-5 which is not currently well utilized despite its vast availability. Kraft is currently the dominant pulping process. Currently, holocelluloses of woody materials are taken into account as value-added products, while lignin is burned in the recovery operation of kraft pulping process. Kraft lignin can be separated from the black liquor of the Kraft pulping process and considered as the raw material for the production of other chemicals. LignoboostTM and LignoForceTM technologies are currently used for extracting kraft lignin from black liquor, promoting the creation of lignin based materials.6 Lignin has phenolic aliphatic hydroxyl as well as carboxylic groups.7,8 These groups are active and can be used for modifying lignin via esterification, 9 etherification,10 sulfonation,11 chlorination12 and graft polymerization13-16 to create products with high values.10, 17 The cationic modification of lignin via grafting (in Mannich reaction) was studied in solvent media in the past.18-20 The polymerization of lignin using various cationic monomers, such as N,N-dimethyldiallyl ammonium chloride (DADMAC),21-23 N,N′-methylenebisacrylamide13 and co-monomers, such as methyl methacrylate, acrylamide24 or vinyl acetate8 following free radical polymerization was also studied in 2 ACS Paragon Plus Environment

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solvent media. However, the high cost and complexity of solvent recovery make it unattractive to industry. In this regard, the incentives for developing sustainable processes for generating lignin-based products are high in industry. In previous studies, polymerization reactions were initiated with UV,25 microwave,26 enzymes27-29 and initiators, such as cereus nitrate, calcium chloride, potassium persulfate or redox.30-32 Potassium persulfate was selected as it is an inexpensive and effective initiator.33, 34 One objective of this study was to evaluate the creation of a novel cationic lignin via polymerizing kraft lignin with [2- (Methacryloyloxy)ethyl] trimethylammonium chloride (METAC). In this work, a process that could readily be implemented in the kraft pulping process to produce lignin-graft-PMETAC polymer is introduced. As water is the primary medium in the pulping industry, an aqueous-based system was selected for this polymerization system. It is the first time that the graft polymerization of kraft lignin and a cationic monomer is reported in an acidic aqueous system. As kraft lignin is not soluble under acidic condition, this system is a heterogeneous reaction operation. Lignin based products were considered as oil well dispersants,30 water thickening agents,35 water reducers,36 antioxidants,37, 38 drilling mud thinners39, 40 and hydrogels.13, 16 Flocculants play important roles in water purification and wastewater treatment for several industries, such as, textile, food, and papermaking.41, 42 The commercially available flocculants are primarily synthetic polymers, which are inefficient, nonbiodegradable and sometimes toxic. 41-43 There is a great incentive for generating environmentally friendly flocculants.44, 45 Lignosulfonates have long been exploited as flocculants.44, 46-52 In one study, lignosulfonates were used for flocculating sulfur slurry of 3

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copper heap leaching.53 In another study, lignosulfonates in combination with xanthan gum and carrageenan were applied to various food processing wastewater streams as flocculants.54 He et al.55 prepared a lignosulfonate–acrylamide–chitosan ternary copolymer as a flocculant, which showed a high colour removal efficiency (>95%) for anionic and neutral dyes and a moderate efficiency (>50%) for cationic dyes from solutions.55 Fang et al. synthesized a cationic lignin-based flocculant by reacting dimethylamine, acetone and formaldehyde and enzymatically hydrolyzed corn stalk lignin, and the product was used for dye removal.56 It was claimed that the removal of acid black dye reached 95 % at 70 mg/L dosage of the cationic polymer.56 Rong et al.57 prepared an alkali lignin-acrylamide polymer and used it as a flocculant. The ligninacrylamide polymer significantly decreased the organic matters of a humic acid-kaolin wastewater solution.57 In another report, alkali lignin was reacted with acrylamide, starch, and acrylic acid; and the resulting copolymer was used for treating organics surface runoff.58 Kraft lignin has also been grafted with glycidyl-trimethylammonium chloride (GTMAC), and the generated polymer was applied for removing anionic dyes from a simulated wastewater effluent.59 Laszlo and coworkers also evaluated the reaction of kraft lignin and 3-chloro-2-hydroxypropyl-trimethyl ammonium chloride (CHMAC) to induce a cationic polymer for eliminating dye from a simulated solution.60 The sulfomethylated kraft lignin was also modified via nitric acid oxidation and sulfomethylation, and the modified lignin was utilized as a flocculant for treating a simulated wastewater containing ethyl violet dye.61 However, the production of kraft lignin based cationic flocculants with high molecular weights has not been studied in detail. Another objective of this study was 4 ACS Paragon Plus Environment

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to evaluate and compare the performance of a lignin-based cationic flocculant with a synthetic one. This paper contributes to the development of a green and industrially attractive process which can be adapted at the kraft pulping process for the production of a sustainable product (e.g., lignin-based flocculants). MATERIALS AND METHODS Materials Undried (44% moisture content) softwood kraft lignin was obtained by LignoForceTM technology of FPInnovations located in Thunder Bay, Ontario, and was used as received.62 [2- (Methacryloyloxy) ethyl] trimethylammonium chloride solution (METAC), 80 wt.% solution, potassium persulfate (K2S2O8, purity of ≥99.0%), potassium permanganate (analytical grades), ferrous ammonium sulfate (analytical grades), sodium hydroxide (NaOH, 97%), hydrochloric acid (HCl, 37%) and kaolin were purchased from Sigma-Aldrich Company. Anionic polyvinyl sulfate (PVSK, 97.7% esterified) with a Mw of 100,000–200,000 g/mol was purchased from Wako Pure Chem. Ltd., Japan. Ethanol (purity of 95 vol.%) was purchased from Fisher Scientific Company. Preparation of Lignin-graft-PMETAC and PMETAC polymers. In this experiment, 2.2 g dried kraft lignin was dissolved in water in a 250 mL three-neck glass flask. The flask was maintained in a water bath and nitrogen gas was purged into the suspension for 30 min. METAC was placed into the solution and the pH of the medium was adjusted to 2, 4, 6, 8, 10 or 12. K2S2O8 (0.03 g) was also dissolved in 5 mL of water. The total volume of the reaction medium was 35 mL, which contained 0.3 mol/L lignin concentration. The polymerization was carried out at different temperatures (50-90 °C) for 1-8 h. A 5



continuous purging of nitrogen was supplied during the reaction. When the reaction was finished, the flask was cooled by tap water to room temperature. After that, the solution was poured into 200 mL of ethanol (80 vol. % in water) to coagulate lignin-graftPMETAC polymer.34 The suspension was then centrifuged for 10 min at 3500 rpm (Sorvall ST 16 Laboratory Centrifuge, Thermo Fisher) to separate the lignin-graftPMETAC polymer from the suspension. To optimize the reaction, the aforementioned analyses were conducted under different conditions such as pH, METAC/lignin ratio, temperature and time. All calculations in this work are based on lignin’s C9 unit using the molecular weight of 185 g/mol.63 The homopolymer (PMETAC) was also produced under the reaction conditions of pH 4.0, 5.4 mol/L of METAC, 3 h, 80 °C, which were optimal conditions for preparing lignin-graft-PMETAC polymer. Upon completion, the solution was neutralized using 0.1 mol/L NaOH solution, and then kept in a dialysis tube for 48 h. Water of the dialysis process was replaced every 4 h in the first 24 h and every 6 h in the following 24 h. The solution from the dialysis was dried at 105 oC for 24 h, and considered as PMETAC in this paper. The lignin-graft-PMETAC polymer and PMETAC homopolymer, which were prepared at the optimal conditions, were characterized by FIIR, TGA, GPC, 1H-NMR and elemental analyses. Purification of Lignin-graft-PMETAC. It was discovered that the addition of ethanol to the reaction media could separate lignin polymers from the mixture. To determine the efficiency of this process, 2 g of lignin, 6 g of PMETAC and 6 g of METAC were added to 200 mL of 80 % ethanol solution. After shaking for 60 min at room temperature, the mixture was centrifuged. The UV absorption at 280 nm was selected to evaluate the mass 6 ACS Paragon Plus Environment

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of lignin remaining in solution after the isolation process. The 1H-NMR was used to testify that METAC and PMETAC remained in the supernatant and no METAC and PMETAC precipitated. Based on the results, this separation/precipitation was used for removing homopolymers and unreacted monomers from the lignin-graft-PMETAC polymer. The lignin-graft-PMETAC polymer was then dried at 105 °C in an oven. Quantification Analysis of Lignin and METAC Polymerization. In this set of experiments, the polymerization reactions were carried out at pH 4.0, METAC/lignin molar ratio of 1.8 (based on the C9 unit of lignin), 80 °C, lignin concentration of 0.3 mol/L, initiator dosage of 1.5 wt.% (based on dried lignin) and reaction time of 1 h, 2 h, 3 h, 4 h, 5 h and 8 h. After completion, three cycles of dissolving in water and ethanol precipitation/centrifugation process were performed as stated previously. The precipitate collected in this process was dried in the oven at 105 oC for 24 h to measure the mass of lignin-graft-PMETAC in the precipiate, m0 (g). In this process, the supernatants of centrifugation were collected to identify the amounts of the products (ligningraft-PMETAC, PMETAC and METAC contents). The amount of lignin-graft-PMETAC polymer, m1 (g/L), in the supernatants was measured by using UV at the wavelength of 280 nm. The same supernatants were dried in an oven at 105 °C for 24 h to evaluate the total mass concentration, m2 (g/L), of the supernatants. One part of the same supernatant was then dialyzed for 48 h to eliminate unreacted METAC. The solution from the dialysis was dried in the oven at 105 oC for 24 h in order to measure the concentration, m3 (g/L), of supernatant after dialysis. The percentages of unreacted METAC and PMETAC produced in the reaction were then calculated using equation (1) and (2), respectively. The product yield, METAC conversion, and grafting efficiency were determined following equations (3) to (5). 35, 64 7



Percentage of unreacted METAC = × Percentage of PMETAC = × Product yield =

× 100%

(1)

− × 100% (2)

' + × 100% (3) + )

METAC Conversion = Grafting Efficiency =

− × (- − ) × 100% (4)

0 123 4 0 12 4

× 100% (5)

In these equations, V (L) is the total volume of supernatant collected, and m (g) is the mass of METAC used in the reaction, and mL (g) is the mass of lignin used in the reaction. Analysis of Initiator Effect. The graft polymerization reactions were carried out as explained earlier at 80 °C for 3 h in nitrogen in 250 mL three-necked flasks, but without adding METAC. At first, 25 mg of lignin was suspended in 50 mL of deionized water. Afterwards, the initiator was added into the flasks at an initial K2S2O8 concentration of 0.75 g/L. The final pH of the suspensions was adjusted to 4 or 10 by adding 1.0 mol/L NaOH or HCl solutions. The nitrogen gas was purged continuously during the reaction. Similar reactions were carried out only with K2S2O8 at pH 4 and 10 (i.e., control samples). The concentrations of initiator under acidic and alkaline conditions before and after the reactions was analyzed by back titration with potassium permanganate with a ferrous ammonium sulfate solution.65 10 mL of persulfate containing solution was added to a 100 mL Erlenmeyer flask. Then, 10 mL of the 0.5 M H2SO4 solution and 10 mL of the 0.025 M ferrous ammonium sulfate solution were added to the flaks while stirring continuously. After one min of stirring, the solution was titrated against 0.002 M KMnO4 8 ACS Paragon Plus Environment

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to a permanent pink endpoint. A blank titration was conducted on 10 mL of ferrous ammonium sulfate solution in 10 mL of the 0.5 M H2SO4. The persulfate concentration in the solution was identified following equation (6) g : × (; − ; < ) × 3 Persulfate content 6 8 = 9 × (6) L 2′

where E is the volumes (L) of KMnO4 solution assigned for the endpoint of the blank sample. E’ is the volumes (L) of KMnO4 solution assigned for the endpoint of the actual sample, M is the molarity (mol/L) of KMnO4, V’ is the volume (L) of the sample and m4 is the molar mass of K2S2O8. Acid Hydrolysis of PMETAC and Lignin-graft-PMETAC Polymer. To investigate the stability of lignin-graft-PMETAC polymer and PMETAC in pH 4.0 (acidic solution), 2 g of lignin-graft-PMETAC polymer or PMETAC prepared under the optimal conditions was dissolved in 40 mL of water by agitating for 20 min and 300 rpm at room temperature in a flask, and then the pH was adjusted to 4.0 using sulfuric acid (1 mol/L). The flask was then put into a water bath with 80 °C for 6 h while stirring at 300 rpm. After the hydrolysis, the solution was neutralized to pH 7 using NaOH solution (1 mol/L) and then placed into a dialysis tube for 48 h to remove the inorganic salt generated by the pH adjustment. Dialysis water was replaced every 4 h in the first 24 h and then every 6 h in the next 24 h. The product from the dialysis tube was dried in an oven at 105 oC for 24 h, and the acid hydrolyzed products were analyzed by GPC and 1H-NMR analysis. Charge Density Analysis. Approximately, 0.1 % lignin-graft-PMETAC polymer solution was prepared by dissolving 0.05 g polymer in 50 g of water, and the solution was placed into a water bath shaker (Innova 3100, Brunswick Scientific, Edison, NJ, USA) and 9



shaken at 100 rpm for 1 h at 30 ˚C. After that, the suspension was centrifuged at 1000 rpm for 5 min. The supernatant collected was used to measure the charge density of the samples. A Particle Charge Detector, Mütek PCD 04 titrator (Arzbergerstrae, Herrsching, Germany) was used for identifying the charge density of lignin-graft-PMETAC using a PVSK solution (0.005 mol/L). The measurement was repeated three times and the average values were reported in this study. Furthermore, the concentration of lignin-graftPMETAC polymer in the filtrate was identified by drying it at 105 °C in an oven overnight. Elemental Analysis. Kraft lignin used in this paper had a very low nitrogen content. The nitrogen in the lignin-graft-PMETAC polymer is primarily originated from METAC attached to lignin. Therefore, the nitrogen content of the polymer is directly related to the grafting of METAC on kraft lignin. The higher nitrogen content, the higher grafting ratio of the lignin polymer would be.66 The nitrogen content of the samples were measured to optimize the reaction conditions, which was performed with an Elementar Vario EL elemental analyzer. The grafting ratio of METAC to lignin was calculated according to equation 7. In the past, this equation was used for the grafting ratio analysis of acrylamide and enzymatic hydrolyzed lignin67 as well as for xylan and METAC.34 Grafting ratio (wt. %) = ?%/AB×C

HIJKLM NO KPQOMIR STUVW

HIJKLM NO XJKYJY JY MLI ZNX[\IP

D = AEE?%/AB×C × AEE D

× 100

(7)

N (wt.%) is the nitrogen content of samples and Mw is the molecular weight of METAC (207.7 g/mol). 10 ACS Paragon Plus Environment

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Hydrodynamic Diameter Measurement (Hy). The hydrodynamic diameter of lignin, PMETAC and lignin-graft-PMETAC polymer was determined by a dynamic laser light scattering analyzer (DLSA), BI-200SM Brookhaven Instruments, USA. The solid state laser light was set at 35 mW and a wavelength of 637 nm. The scattering angle was set at 90°. The operating procedure of Yan et al 68 was followed in this analysis. The samples were dissolved in a NaCl solution (1 mg/mL) at a 0.1 % concentration and pH 9.0. The solution was then agitated for 30 min, 300 rpm and 25 °C for 24 h. The solution (20 mL) was filtered using a 0.45 µm disposable syringe filter. Each sample analysis was carried out for 2 min and repeated 5 times, and the mean values were reported. The temperature of the sample solution was kept at 25 ± 0.02 °C. Analysis of Molecular Weight. For kraft lignin, about 10 mg air-dried kraft lignin was acetylated using 2.5 mL of acetyl bromide/acetic acid 8/92 (V/V) at 50 °C for 2 h. Then, the sample was placed into a freeze dryer to remove the solvent. The acetylated KL was dissolved in 10 mL of tetrahydrofuran (THF) and filtered with a PTFE 13 mm diameter filter (pore size of 0.2 µm). After the filtration, the sample solution was injected into a high performance liquid chromatography system, Agilent Model 1200 with UV diode-array detector, multiangle laser light scattering detector and RI detector, to measure the molecular weight of samples. Waters Styragel HR4 (WAT044225), HR4E (WAT044240) and HR1 (WAT 044234) columns and precolumn (P/N WAT054405) were used, and a 1.0 mL/min of THF flow rate was considered at 25 °C in this assessment. In this system, polystyrenes were chosen as standards. For lignin-graft-PMETAC polymer analysis, 2 mg of air dried polymer sample was dissolved in 10 mL of 5 % acetic acid solution. The solution was stirred at 300 rpm for 6 h. After that, the 11



solution was filtered using 0.2 µm nylon filter. The solution, after filtration, was injected into a gel permeation chromatography system, Malvern GPCmax VE2001 Module and Viscotek TDA305 with multi-detectors (UV, RI, viscometer, low angle and right angle laser detectors) for assessing the molecular weight. A 0.50 mL/min flow rate of 5 % acetic acid solution was used with columns of PAC103 and PAC101 in this system. The column temperature was set at 35 °C, and pullulan, 47,300 g/mol, was used as standard. Based on the number average molecular weight of kraft lignin and lignin-graft-PMETAC polymer, the average degree of polymerization (DP) of the PMETAC-side chain of lignin-graftPMETAC polymer can be calculated theoretically, assuming each phenolic group of lignin molecules can initiate the polymerization and anchor a PMETAC side chain, following equation (8). In equation 8, it is assumed that only one mole of lignin participates in one mole of ligningraft-PMETAC production. The DP of PMETAC polymer is calculated following equation (9). DP =

:^_ − :^) (8) :^) × `_abc × 207.7

DP =

:^_fghij (9) 207.7

where MnP is the number average molecular weight of lignin-graft-PMETAC polymer, g/mol. MnL is the number average molecular weight of lignin (g/mol). MnPMETAC is the number average molecular weight of PMETAC (g/mol). CPhOH is the phenolic group content in lignin (mol/g), 1.73× 10 mol/g. 51 207.7 is the molecular weight of METAC, g/mol. Fourier Transform Infrared (FTIR). The FTIR spectra of lignin, lignin-graft-PMETAC and PMETAC (0.1 g) were recorded using a Bruker Tensor 37 FT-IR spectrophotometer. Each spectrum was recorded in transmittance mode with 32 scans in the wavenumber range of 6004000 cm-1. The resolution was set at 4 cm-1. 12 ACS Paragon Plus Environment

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1

H-NMR Analysis. The structure of lignin, lignin-graft-PMETAC and PMETAC were analyzed

with 1H-NMR. The samples were dissolved in D2O at pH 9.5 and 40-50 mg/mL concentration while being stirred for 30 min. 1H-NMR spectra of lignin, lignin-graft-PMETAC and PMETAC were recorded by an INOVA-500 MHz machine (Varian, USA) with a 45o pulse and a 1s relaxation delay time. Thermogravimetric Analysis (TGA). The thermogravimetric analysis of lignin, lignin-graftPMETAC and PMETAC was carried out using a TGA analyzer, Instrument Specialist, i1000, for characterizing the decomposition temperature of the samples. The temperature was increased from room temperature to 700 °C at a 10 °C/min heating flow rate with a 100 mL/min nitrogen flow rate. Water Solubility Measurement. The solubility of kraft lignin and lignin-graft-PMETAC polymer was measured according to the method adopted by Lappan et al.69 In this experiment, 0.5 g of kraft lignin or lignin-graft-PMETAC polymer was dissolved in 50 mL of deionized water to prepare 10 g/L sample solutions at different pH using 1.0 mol/L NaOH or 1.0 mol/L H2SO4 solution. The solutions were placed in a water bath shaker (Innova 3100, Brunswick Scientific, Edison, NJ, USA) and shaken (100 rpm) at 30 °C for 4 h. After that, the suspension or solution was centrifuged at 1000 rpm for 5 min. The concentration of kraft lignin or lignin-graftPMETAC polymer in the filtrates was assessed by drying the filtrates at 105 °C. The mass of NaOH or H2SO4 used for adjusting pH was taken into account in measuring solubility. Flocculation of Kaolin Suspension. The flocculation of the polymers in a kaolin suspension was analyzed using a photometric dispersion analyzer (PDA, PDA 3000, Rank Brothers, UK) connected with a dynamic drainage jar (DDJ). In this set of experiments, 450 mL of deionized 13



water was firstly poured into the jar without any mesh. The system circulated water through PDA and DDJ for 10 min to reach a steady flow. A 50 mL of 2.5 % kaolin suspension was stirred at 200 rpm. The kaolin suspension was circulated in the system at a 50 mL/min flow rate. After reaching steady state, the lignin-graft-PMETAC polymer, PMETAC or lignin solution (0.1 g/L concentration) was added into the DDJ to induce the flocculation process. The degree of flocculation was presented as a relative turbidity, which was calculated according to the equation (10):70

lm =

no np

=

q XY ( 0 ) qo q XY( 0 ) qp

(10)

τi is the initial turbidity of the kaolin suspension (prior to adding lignin based samples); τf denotes the final turbidity of the kaolin suspension (after adding lignin based samples); V0 is initial base DC voltage (water solution); Vi stands for the DC voltage of the kaolin suspension (without lignin based products); and Vf is the DC voltage of the kaolin suspension after adding lignin based polymers.

RESULTS AND DISCUSSION Polymerization of Kraft Lignin and METAC. The polymerization reaction of lignin and METAC was performed in an aqueous solution via free radical polymerization. In this system, the potassium persulfate was considered as the initiator. The reaction scheme of the polymerization is shown in Figure 1.

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Figure 1. Polymerization scheme of lignin and METAC to produce lignin-graftPMETAC polymer (top) and PMETAC (bottom).

In this polymerization reaction, the sulfate radicals can initially be generated through heat decomposition, which can take unstable hydrogen from phenolic hydroxyl groups of lignin to generate phenoxy radicals. These phenoxy radicals can also form resonance radicals. The phenoxy radicals and their resonance radicals form on the lignin backbone 15



(free radical reaction sites); and then they react with the monomer (METAC) or propagate monomers to form lignin-graft-PMETAC polymer as shown in reaction (1) of Figure 1.71 The poly METAC (PMETAC) chain segments, which contain quaternary ammonium groups, existing on lignin-graft-PMETAC polymer would offer cationic charges, water solubility and a high molecular weight to lignin. In addition, the homopolymer, PMETAC, can be produced through a side reaction, as shown in reaction (2) of Figure 1. Both polymerization and homopolymerization can be influenced by polymerization conditions. In order to optimize the copolymerization reactions, the charge density and grafting ratio were considered as means to quantify the properties of lignin-graftPMETAC in this work. Purification of Lignin-graft-PMETAC Polymer. The products of lignin and METAC polymerization consist of lignin-graft-PMETAC polymer, homopolymer (PMETAC), unreacted monomer (METAC) and some inorganic, salts such as potassium sulfate. To investigate the separation performance of these products in 80 vol % ethanol solution, a mixture of lignin, PMETAC and METAC was prepared and then mixed with ethanol to generate 80 vol% mixture. UV analysis confirmed that 92.7 % of lignin was precipitated in the mixture, and only 7.3 % of lignin remained in the solution. The NMR analysis also confirmed that no METAC and PMETAC were observed in the final precipitates of the ethanol/water mixture, illustrating that ethanol/water mixture could successfully separate PMETAC and the unreacted METAC monomer from lignin-graft-PMETAC polymer. Therefore, mixing the reaction solution with ethanol was effective in purifying ligningraft-PMETAC polymer.


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pH Effect. Figure 2 shows the effect of reaction pH on the charge density and grafting ratio of lignin-graft-PMETAC polymer. It is observed that, when the reaction pH was increased, the charge density and grafting ratio of lignin-graft-PMETAC polymer declined. At pH 2.0, the highest values of 2.94 meq/g and 184 % were obtained for charge density and grafting ratio, respectively. However, when the pH was higher than 2, the grafting ratio and charge density declined dramatically. Under alkaline conditions, the hydrogen on β-carbon adjacent to quaternary ammonium group in METAC segment can be attacked by hydroxyl ions to modify quaternary ammonium groups into tertiary ammonium groups, which would decrease the charge density.72 Therefore, under strong alkaline circumstances, the quaternary ammonium groups of METAC segment was unstable, and their decompositions declined the charge density. The results showed that lignin-graft-PMETAC polymer with the higher charge density and grafting ratio was obtained at a pH between 2 and 4. As pH close to neutral is more industrially attractive, pH 4.0 was selected as the optimum pH.

200

3

Charge density

2.5

150 Grafting ratio

2 1.5

100

1 0.5

50

0 -0.5 -1

0 0

2

4

6

8

10

12

14

pH

17


Grafting ratio, wt.%

3.5

Chage density, meq/g

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



Figure 2. Charge density and grafting ratio of lignin-graft-PMETAC polymer as a function of reaction pH (METAC/lignin molar ratio 1.8, 80 ºC, 3 h, initiator 1.5 wt.% and 0.3 mol/L lignin concentration).

Furthermore, the homolytic scission of potassium persulfate depends strongly on pH.73 Thus, the efficiency of the initiator to produce two anion-radicals may be different in acidic, alkaline and neutral conditions. Also, radicals could be generated by the redox system of persulfate-phenolic moieties of lignin, which would be considerably affected by pH.7 To investigate the mechanism of free radical formation, the behavior of persulfate decomposition was studied in the presence and absence of lignin under acidic and alkaline conditions at 80 °C for 3 hours (as stated in the experimental section). Figure 3 shows K2S2O8 concentration in the reaction media as a function of time in the presence and absence of lignin at pH 4 and 10. In the absence of lignin, the decomposition of the initiator occurred as time elapsed very similarly at pH 4 and 10. In the sample containing lignin, the decomposition of K2S2O8 was significantly enhanced at pH 4, however, only a slight reduction in K2S2O8 concentration was observed at 10. Therefore, polymerization was limited under alkaline conditions due to the fact that lignin inhibited the consumption of the initiator. More radicals were produced under acidic conditions by the redox systems with lignin, promoting more polymerization.


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0.9 0.8 K2S2O8 concentration, g/L

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


0.7 0.6 0.5 0.4 0.3 pH 10 pH 10 with lignin pH 4 pH 4 with lignin

0.2 0.1 0 0

50

100 Time, min

150

200

Figure 3. Decomposition of K2S2O8 at acidic and alkaline conditions as a function of reaction time.

METAC/Lignin Molar Ratio. The influence of METAC/lignin molar ratio on the charge density and grafting ratio of lignin-graft-PMETAC polymer is shown in Figure 4. An increase in METAC/lignin molar ratio from 0.7 to 1.8 increased the charge density and grafting ratio rapidly. Further increase in this ratio insignificantly increased the charge density and grafting ratio. This behavior is ascribed to the fact that a rise in METAC/lignin molar ratio increased the METAC concentration, which would react with the lignin macro radicals and produce more polymer of lignin-graft-PMETAC.74 Based on these results, 1.8 of METAC/lignin molar ratio was chosen as an optimum in this study.

19



3.5

250

3 200

2.5 2

150

1.5 1

100

0.5 Charge density

0

50

Grafting ratio

-0.5 -1

Grafting ratio, wt. %

Charge density, meq/g

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

0 0

0.5

1

1.5

2

2.5

3

METAC/lignin molar ratio

Figure 4. Charge density and grafting ratio of lignin-graft-PMETAC polymer as a function of METAC/lignin molar ratio (pH 4.0, 80 ºC, 3 h, initiator 1.5 wt.% and 0.3 mol/L lignin concentration).

Reaction Temperature. The effect of reaction temperature on the charge density and grafting ratio is shown in Figure 5. The increase in the reaction temperature from 50 to 80 °C dramatically increased the charge density and grafting ratio. The higher the production rate of free radicals, the higher polymerized product was obtained.33 The results also suggested that the reaction was endothermic and a temperature increase caused more free radical generation and hence a higher polymerization rate. However, no further increase in the charge density and grafting ratio were observed when the temperature raised from 80 °C to 90 °C, which was attributed to more chain termination and chain transfer reactions at a temperature higher than 80 °C.33,75


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3.5

200

3 2.5

150

2 1.5

100 Charge density

1 0.5

Grafting ratio, %


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


50

Grafting ratio

0 -0.5

0 40

50

60

70

80

90

100

Temperature, ºC

Figure 5. Charge density and grafting ratio of lignin-graft-PMETAC polymer as a function of reaction temperature (METAC/lignin molar ratio 1.8, pH 4.0, 3 h, initiator 1.5 wt.% and 0.3 mol/L lignin concentration).

Reaction Time. The influence of reaction time on the charge density and grafting ratio of lignin-graft-PMETAC polymer is presented in Figure 6a, while the effect of time on METAC consumption and PMETAC production is presented in Figure 6b. The charge density and grafting ratio enhanced with extending the reaction time from 0.5 h to 3.0 h, and reached the highest values of 2.93 meq/g and 178.5 %, respectively. The greater values of charge density and grafting ratio are attributed to the fact that with prolonging reaction time, more radicals are formed and more of unreacted METAC are polymerized. The results also depicted that, during the first 3 h of reaction, the amount of unreacted METAC decreased dramatically with a slight increase in PMETAC formation (Figure 6b). However, when the reaction time was over 3 h, the grafting ratio and charge density of lignin-graft-PMETAC polymer decreased, while the production of PMETAC 21



accelerated and the amount of METAC did not decrease significantly (Figure 6a and 6b). These results demonstrated that some of the METAC that was originally grafted on lignin-graft-PMETAC polymer was cleaved due to the cleavage of ether linkage between lignin and METAC segment, or the cleavage of ester existing on METAC under acidic conditions.

3

200


150

2.6 100 2.4 Charge density

Grafting ratio, %

a 2.8

50

2.2 Grafting ratio 2

0 0

2

4

6

8

10

Time, h

50

b

unreacted METAC

40

Percentage, wt.%

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

PMETAC 30 20 10 0 0

2

4

6

8

Reaction time, h

Figure 6. a) Charge density and grafting ratio of lignin-graft-PMETAC polymer, and b) unreacted METAC and PMETAC produced as a function of reaction time


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(METAC/lignin molar ratio 1.8, pH 4.0, 80 ºC, initiator 1.5 wt.% and 0.3 mol/L lignin concentration).

To further examine the hydrolysis of the lignin-graft-PMETAC polymer, the lignin-graftPMETAC polymer and PMETAC, prepared under optimal conditions were treated separately at pH 4.0 and 80 oC for 6 h in water, and it was confirmed by 1H-NMR that the amount of METAC segment on lignin-graft-PMETAC polymer was reduced by 21 % after treatment, and the molecular weight was reduced from 1.53×106 g/mol to 1.32×106 g/mol. However, PMETAC didn’t show any change in either ammonium group content or molecular weight. This analysis confirmed that the decreases in charge density and grafting ratio were due to the cleavage of ether linkage between lignin and METAC, and not to the ester linkage in METAC. This mild acid hydrolysis was also reported by Chen and coworkers in creating grafted polyacrylic chains from lignosulfonate-acrylonitrilemethyl methacrylate polymer.76 From the above experiments, the optimal reaction conditions were selected to be pH 4.0, METAC/lignin molar ratio of 1.8, 3 h, 80 °C and lignin concentration of 0.3 mol/L. The polymer that was prepared under these conditions was assessed using GPC, DLSA, FTIR, 1

H-NMR and TGA analyses. The charge density of this polymer was 2.93 meq/g, and its

nitrogen content was 4.32 wt. %, which corresponded to a grafting ratio of 178.5 %. This grafting ratio implies 64 wt.% METAC and 36 wt.% kraft lignin proportions in the ligningraft-PMETAC polymer. The charge density of resulting polymer can be theoretically calculated using the nitrogen amount of the polymer considering that 1 mol of quaternary ammonium groups grafted onto lignin backbone has 1 eq charge density.59 The theoretical 23



charge density of lignin-graft-PMETAC with a 4.32 % nitrogen content was 3.08 meq/g, which was similar to that of the experimental value (2.93 meq/g) measured by using the PCD analysis.

The molecular weight and hydrodynamic diameter of kaft lignin, PMETAC and ligningraft-PMETAC polymer are listed in Table 1. The molecular weight of lignin-graftPMETAC polymer was 1.53×106 g/mol, which was significantly higher than that of kraft lignin (26,100 g/mol) and that of PMETAC (1.61×105 g/mol) prepared under the same reaction conditions, demonstrating the successful polymerization of METAC and lignin. Despite the smaller DP of lignin-graft-PMETAC than PMETAC, lignin-graft-PMETAC had a large molecular weight due to the contribution of the molecular weight of lignin to that of lignin-graft-PMETAC. Also, the hydrodynamic diameter of lignin-graft-PMETAC polymer increased to 43.9 nm (from 6.1 nm for kraft lignin and 11.6 nm for PMETAC). It was observed in the literature that the hydrodynamic diameter of kraft lignin is directly related to its molecular shape. Lignin with a higher hydrodynamic diameter would have a looser molecular shape.77, 78 These results postulate that the grafted PMETAC chain onto lignin backbone (to produce lignin-graft-PMETAC) not only raised the molecular weight but also changed the compact shape and conformation of lignin molecules due to the dissociation of its ionizable functional groups (e.g., ammonium group).79, 80 This phenomenon was also reported by Sarkar and coworker81 in the polymerization of amylopectin and poly (acrylic acid). The relatively small hydrodynamic diameter of PMETAC (compared with that of lignin-graft-PMETAC polymer) may imply that PMETAC had either a branched and/or coil shape conformation.82,83 The grafting 24 ACS Paragon Plus Environment

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efficiency was 95%, which shows the remarkable involvement of METAC in the reaction. The METAC conversion and product yield of lignin-graft-PMETAC were similar to those of PMETAC implying that METAC’s similar engagement in both reactions.

Table 1. Characteristics of lignin, PMETAC and lignin-graft-PMETAC polymer Sample

lignin

PMETAC lignin-graft-PMETAC polymer

Mn, g/mol

17,300 1.34☓105

1.14×106

Mw, g/mol

26,100 1.61☓105

1.53×106

Mw/Mn

1. 510

1.596

2.740

DP

--

645

177


--

4.80

2.93

Grafting ratio, %

--

--

178

METAC conversion, % --

92.2

93.1

Grafting efficiency, %

--

--

94.7

Product yield, %

--

92.0

93.5

Hy, nm

6.1

11.9

43.9

Characterization of Lignin-graft-PMETAC Polymer FTIR. The FTIR spectra of lignin-graft-PMETAC polymer, unmodified kraft lignin and PMETAC are presented in Figure 7. The spectra depicted a wide band around 3400 cm-1, 25



which was attributed to the O-H absorption in the phenolic and aliphatic compounds, and a band at 2900 cm-1, which was the C-H stretching in the methyl groups.84-86 In the spectrum of kraft lignin, peaks at 1225 cm-1 and 1140 cm-1 were attributed to the C-O stretching of guaiacyl unit and C-H stretching of guaiacyl unit, respectively, illustrating that the kraft lignin was a softwood lignin.87 The characteristic peaks of the aromatic skeletal vibration of kraft lignin were observed at 1591, 1510 and 1245 cm-1, respectively.87 In the spectrum of lignin-graft-PMETAC polymer, three new peaks appeared at 1728 cm-1, 1472 cm-1 and 960 cm-1, which did not appear in the spectrum of kraft lignin. These three peaks were assigned to the C=O stretching vibration, C-N bending and methyl groups of quaternary ammonium in PMETAC, respectively.66, 74, 88 These peaks were observable on the spectrum of PMETAC, implying the successful polymerization of lignin and METAC. Furthermore, the intensity of the peak at 1028 cm-1, which belonged to non-etherified phenolic hydroxyl groups of lignin,71 was weaker in lignin-graft-PMETAC polymer than in kraft lignin. The results suggests that lignin polymerization occurred through the phenolic hydroxyl group of lignin.


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1.01 1.00

Lignin-graft-PMETAC

0.99 0.98

Transmittance,%

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


Lignin

0.97 0.96

PMETAC

0.95 0.94 0.93

2900cm-1 3400cm-1

0.92

1472cm-1 0.91

960cm-1

1728cm-1

0.90

3600

3000

2400

Wavenumber,

1800

1200

600

cm-1

Figure 7. FTIR spectra of lignin-graft-PMETAC, lignin and PMETAC. Elemental Study. The elemental composition of lignin and lignin-graft-PMETAC is listed in Table 2. It is evident that the carbon and oxygen contents of the lignin-graftPMETAC polymer were lower than those of kraft lignin, which was due to the lower carbon and oxygen contents in METAC segment of the lignin-graft-PMETAC polymer. The nitrogen content of the polymer was 4.32 %, but that of kraft lignin was 0.01. These changes confirm the polymerization of lignin and METAC. The C9 unit formulas of lignin and lignin-graft-PMETAC were also determined based on the elemental analysis (Table 2). Table 2. Elemental analysis of lignin and lignin-graft-PMETAC polymer N,

C,

H,

O,

wt.%

wt.%

wt.%

wt. %

Formula

27



Lignin

0.01

60.45 5.97

31.40

C9H10.67O3.51N0

4.32

55.14 7.71

21.18

C9H15.1O2.59N0.6

Lignin-graftPMETAC

1

H-NMR. The NMR spectra of kraft lignin, lignin-graft-PMETAC and PMETAC are

shown in Figure 8. The peak at 9.35 ppm is assigned to the formyl protons in cinnamaldehyde unit. The peak at 8.46 ppm is associated with unsubstituted phenolic protons. The peak at 7.42-6.00 ppm is associated with aromatic protons including certain vinyl protons on the carbons connected to aromatic rings. The peak at 5.75-5.15 ppm is associated with aliphatic protons including Hα and Hβ. The peak at 4.05-3.45 ppm is associated with protons in –OCH3 groups of lignin. And the peak at 3.2-1.75 ppm is associated with the aliphatic protons of lignin.89-91 Peak at 4.75 ppm belongs to D2O. In Figures 8, it can be found that the characteristic peaks for PMETAC segment appeared at 1-1.2 ppm, 2.0 ppm, 3.30 ppm, 3.85 ppm and 4.53 ppm, respectively. Peaks at 1-1.2 ppm are assigned to the protons of –CH3 of PMETAC segment. The peak at 2 ppm is associated with the –CH2 protons of PMETAC. The peak for protons corresponding to the CH3 on the ammonium group of –N+(CH3)3 was observed at 3.3 ppm. The peak at 3.85 ppm originates from the methylene protons of –CH2-N+(CH3)3 in METAC. The peak at 4.53 ppm is assigned to the protons of –CH2 in the ester group of METAC.92, 93


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FINAL L i g n i n LIGNIN.ESP

10

8

6 Chemical Shift (ppm)

4

2

0

4

2

0

July-7-2014-sample-R1-1H P M E T A C

10

8


ligin-MeTAC-3.txt L ig n in - g r a f t- P M E T A C

10

8


4

2

0

Figure 8. The 1H-NMR spectra of lignin, PMETAC and lignin-graft-PMETAC

The peaks for the kraft lignin segment was also present at 3.45-3.75 ppm, 5.15-5.75 ppm, 6-7.42 ppm, 8.46 ppm and 9.35 ppm in the spectrum of lignin-graft-PMETAC polymer, which illustrate the successful polymerization of lignin and METAC. In addition, the peak at 4.1 ppm is observed in the spectrum of lignin-graft-PMETAC polymer, which is absent in that of kraft lignin and assigned to the protons of –CH2- connected to aromatic structure through ether bond (-CH2-O-C6H5).94, 95 This also ascertained that the phenolic 29



hydroxyl groups are the active sites participated in the polymerization, which agrees with the FTIR results. TGA Analysis. Thermogravimetric analysis (TGA) can provide information about the thermal decomposition/stability of kraft lignin and the lignin-graft-PMETAC polymer.96 The weight loss and weight loss rate of kraft lignin, lignin-graft-PMETAC and PMETAC are shown in Figure 9. As shown in Figure 9a, there is a weight loss at a temperature lower than 200 °C, which is due to the evaporation of water.97 The weight loss of kraft lignin was 40 %, when the temperature was increased to 400 °C. However, the weight loss reached 59 wt.% for lignin-graft-PMETAC polymer and 84 wt.% for PMETAC at the same temperature. The weight loss of lignin-graft-PMETAC polymer was more significant than kraft lignin, which was due to the decomposition of the main chain of the polymer. Therefore, lignin-graft-PMETAC polymer showed a lower stability than kraft lignin at a higher temperature. A lower thermal stability was obtained for METAC-cotton polymer compared with unmodified cotton.98 When the temperature raised higher than 700 °C, the weight of PMETAC reached 0 %, but the lignin had 17 % of its original weight, which was due to the higher ash content of kraft lignin. As kraft lignin extracted from black liquor was not acid washed after separation, it probably contained some salts that contributed to its ash content. The weight loss rate of kraft lignin shows one peak at 395 °C (Figure 9b). However, for PMETAC and lignin-graft-PMETAC polymer, the peaks occur at 250 °C and 405 °C (Figure 9b). The peak occurring at 250 °C is due to the decomposition of quaternary ammonium groups in METAC segment as reported in the literature,98 and the peak at 405 °C showed the decomposition of the main chain of lignin-graft-PMETAC polymer and 30 ACS Paragon Plus Environment

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PMETAC.66 Figure 9c also shows the impact of the grafting ratio of lignin-graftPMETAC polymers on its thermal stability. It is seen that, by increasing the ratio of METAC, the thermal stability of lignin-graft-PMETAC is reduced and becomes closer to that of PMETAC. However, the thermal stability of lignin-graft-PMETAC polymers is generally higher than that of PMETAC.

lignin

100

Lignin-graft-PMETAC

Weigh, wt/%

80

PMETAC

60 40

a

20 0 20

220

420

620

Temperature, ℃ 1 Lignin Weight loss rate, %/oC

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


Lignin-graftPMETAC

0.8

0.6

b

0.4

0.2

0 20

220 Temperature,

420

620

oC

31



100

grafting ratio 101 % grafting ratio 145 %

80

grafting ratio 198 % Weight, %

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

60 40 20

c

0 20

120

220

320 420 Temperature, oC

520

620

Figure 9. a) Weight loss, b) weight loss rate of lignin, lignin-graft-PMETAC and PMETAC, (c) Weight loss of lignin-graft-PMETAC polymer with different grafting ratios.

Water Solubility of Lignin-graft-PMETAC Polymer. The solubility of lignin-graftPMETAC polymer and kraft lignin is presented as a function of pH in Figure 10. As can be seen, at a pH higher than 10, the solubility of kraft lignin significantly increased to 10 g/L concentration. The increase in the solubility is attributed to the deprotonation of the phenolic groups of kraft lignin.90 However, lignin-graft-PMETAC polymer was soluble at a wide pH range of 0.5-13.0 at a 10 g/L concentration, which illustrated that the water solubility of kraft lignin was markedly improved via polymerization.


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12 10

Solubility, g/L

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


8 lignin 6

lignin-graft-PMETAC

4 2 0 0

1

2

3

4

5

6

7 8 pH

9

10 11 12 13 14

Figure 10. The solubility of lignin-graft-PMETAC polymer and lignin as a function of pH at 10 g/L concentration.

Flocculation Characteristics in Kaolin Suspension. The flocculation performance of lignin, lignin-graft-PMETAC polymer and PMETAC was assessed in a 0.25 wt.% kaolin concentration, and the results are shown in Figure 11. With an increase in the flocculant concentration, the flocculation efficiencies of lignin-graft-PMETAC and PMETAC were enhanced, but better results were obtained for lignin-graft-PMETAC polymer than PMETAC. As reported in the literature, flocculation can be promoted via charge neutralization, bridging, and hydrophobic/hydrophobic interaction.99,100 The reason for the better flocculation efficiency of lignin-graft-PMETAC polymer than PMETAC may be due to 1) its higher molecular weight (and hy), and thus higher bridging efficiency, and 2) hydrophobic/hydrophobic interaction, where lignin part of lignin-graft-PMETAC polymer 33



provides this interaction with kaolin.99 Also, unmodified kraft lignin didn’t flocculate kaolin, which is because i) at pH 6.5, kraft lignin is not soluble, ii) kraft lignin has a slightly negative charge density due to its carboxyl groups, which provides a poor flocculation efficiency (Figure 11), and iii) it has a very small molecular weight.

1

Relative turbidity, Tf/Ti

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

0.8

lignin lignin-graft-PMETAC

0.6 PMETAC 0.4 0.2 0 0

0.5

1

1.5

2

2.5

3

3.5

Concentration, mg/L

Figure 11. Flocculation performance of lignin, lignin-graft-PMETAC and PMETAC in kaolin suspension of 0.25 wt. % at pH 6.5. Development of a Sustainable Process. The results presented in this work confirmed that lignin-graft-PMETAC polymer with a high molecular weight (i.e., 1.53×106 g/mol) and charge density (i.e., 2.93 meq/g) can be generated successfully. As the reaction medium to produce this polymer is an aqueous acidic system (i.e., a non-solvent-based system), the reaction process can be adapted at kraft pulping and LignoForceTM processes. Figure 12 shows a block diagram of the proposed process. In this process, kraft lignin will be received in dried and powder form from the LignoForceTM process. The reaction will be conducted in 34 ACS Paragon Plus Environment

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an acidic environment. The results in Table 1 show that lignin-graft-PMETAC polymer and PMETAC have very different molecular weights and hydrodynamic sizes. Although ethanol precipitation can be used for separating the lignin-graft-PMETAC polymer from the reaction medium (as was used in this work for academic purposes), ultrafiltration may be a more feasible option for use in industry. After the reaction, the lignin-graft-PMETAC polymer can be isolated from the reaction medium via ultrafiltration. Then, it can be spray dried as a marketable product after adjusting the pH of its solution (Figure 12). PMETAC can also be separated from the filtrate of the second filtration system, its pH can be adjusted, spay dried and considered as a by-product. The filtrate of the second filtration process can be recycled or treated in wastewater treatment systems. The water evaporated in the spray drying systems can also be recycled (Figure 12). It should also be stated that the pore opening of the first ultrafilter should be larger than the Hy of PMETAC. However, it should be smaller than that of lignin-graft-PMETAC polymer so that lignin-graft-PMETAC is concentrated in this filtration system. The second filtration system should have a pore opening smaller than Hy of PMETAC for concentrating PMETAC polymer. It should also be stated that the primary attention of this work was on the development of the reaction system for ligningraft-PMETAC production. The use and impact of ultrafiltration for separating lignin-graftPMETAC and PMETAC from the reaction medium is out of the scope of this study. This process could be readily implemented in the pulping industry to directly use lignin as a raw material. On the other hand, the application of ultrafiltration in this process could help tailor the size of lignin-graft-PMETAC polymers isolated as flocculants for use in industry. However, the drying process using a spray dryer may be costly. 35



The developed process shown in Figure 12 can be exploited to produce lignin-graftPMETAC with different properties. In this process, lignin with altered properties can be utilized to produce flocculants. However, the characteristics of lignin as the raw material may influence the performance of the polymerization reactions and the properties of generated lignin-graft-PMETAC polymers implying that the flocculation efficiencies of these lignin-graft-PMETAC may vary.

Figure 12. Block diagram of a process for producing lignin-graft-PMETAC polymer Development of sustainable polymers. The results in Figure 11 revealed that lignin-graftPMETAC polymer was more effective than PMETAC. To achieve a relative turbidity of 0.2, about 0.5 mg/L of lignin-graft-PMETAC or 1 mg/L of PMETAC was required. As discussed earlier, lignin-graft-PMETAC contained 64% METAC and 36% lignin. Therefore, it can be concluded that 0.32 mg/L of METAC was needed to achieve the relative turbidity of 0.2 when lignin-graft-PMETAC was used, but 1 mg/L of METAC was needed to achieve the same turbidity when PMETAC was used. This analysis implies that the application of 36 ACS Paragon Plus Environment

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lignin-graft-PMETAC instead of PMETAC could reduce the use of synthetic chemicals (i.e., METAC) to 1/3, which is a significant contribution to the development and use of sustainable chemicals. Furthermore, a lower relative turbidity of 0.1 was possible to achieve when lignin-graft-PMETAC was used, but it was impossible to achieve when PMETAC was used.

CONCLUSIONS The optimal polymerization conditions of kraft lignin and METAC were pH 4.0, METAC/lignin molar ratio 1.8, 3 h, 80 °C, lignin concentration of 0.3 mol/L; which resulted in the charge density of 2.93 meq/g and the grafting ratio of 178.5%. The FTIR, 1

H-NMR and elemental analyses confirmed the polymerization of lignin and METAC.

The molecular weight of lignin-graft-PMETAC polymer was increased from 26,140 g/mol (for kraft lignin) to 1.53×106 g/mol, and the hydrodynamic diameter was increased from 6.1 nm (for kraft lignin) to 43.9 nm (for lignin-graft-PMETAC). TGA results showed that the polymerization reduced the thermal stability of the kraft lignin, but the thermal stability of lignin-graft-PMETAC was higher than that of PMETAC. Moreover, the lignin-graft-PMETAC polymer was soluble at a 10 g/L concentration in the pH range of 0.5-13. The flocculation analysis confirmed that lignin-graft-PMETAC polymer was indeed an excellent flocculant for kaolin suspension (better than METAC and kraft lignin) and the use of lignin-graft-PMETAC could significantly reduce the use of METAC (i.e., a synthetic chemical) in flocculating kaolin.

ACKNOWLEDGEMENTS 37



The authors would like to thank NSERC-Canada and FPInnovations for supporting this research. Also, supports from Canada Research Chair, Canada Foundation for Innovation and Northern Ontario Heritage Fund Corporation are acknowledged.

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Ghosh, S.; Sen, G.; Jha, U.; Pal, S. Novel biodegradable polymeric flocculant based on polyacrylamide-

grafted tamarind kernel polysaccharide. Bioresour. Technol. 2010, 101, (24), 9638-9644.

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Industrial & Engineering Chemistry Research L ig n in

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

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Novel Process for Generating Cationic Lignin Based Flocculant

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