Cationic High Molecular Weight Lignin Polymer: A

molecules Article

Cationic High Molecular Weight Lignin Polymer: A Flocculant for the Removal of Anionic Azo-Dyes from Simulated Wastewater Shoujuan Wang 1 , Fangong Kong 1, *, Pedram Fatehi 2, * 1 2 3

*

ID

and Qingxi Hou 3

Key Laboratory of Paper Science and Technology of Ministry of Education, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China; [email protected] Department of Chemical Engineering, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin 300222, China; [email protected] Correspondence: [email protected] (F.K.); [email protected] (P.F.); Tel.: +1-807-343-8697 (P.F.); Fax: +1-807-346-7943 (P.F.)

Received: 31 July 2018; Accepted: 7 August 2018; Published: 11 August 2018

Abstract: The presence of dyes in wastewater effluents made from the textile industry is a major environmental problem due to their complex structure and poor biodegradability. In this study, a cationic lignin polymer was synthesized via the free radical polymerization of lignin with [2-(methacryloyloxy) ethyl] trimethyl ammonium chloride (METAC) and used to remove anionic azo-dyes (reactive black 5, RB5, and reactive orange 16, RO16) from simulated wastewater. The effects of pH, salt, and concentration of dyes, as well as the charge density and molecular weight of lignin-METAC polymer on dye removal were examined. Results demonstrated that lignin-METAC was an effective flocculant for the removal of dye via charge neutralization and bridging mechanisms. The dye removal efficiency of lignin-METAC polymer was independent of pH. The dosage of the lignin polymer required for reaching the maximum removal had a linear relationship with the dye concentration. The presence of inorganic salts including NaCl, NaNO3 , and Na2 SO4 had a marginal effect on the dye removal. Under the optimized conditions, greater than 98% of RB5 and 94% of RO16 were removed at lignin-METAC concentrations of 120 mg/L and 105 mg/L in the dye solutions, respectively. Keywords: lignin-METAC; lignin modification; azo dye; flocculation; COD

1. Introduction Dyes are readily found in wastewater effluents of various industries including: dye manufacturing, textile, cosmetic, pharmaceuticals, food, rubber, leather, printing, and pulp & paper. Dyes are classified into acidic, basic, azo, diazo, disperse, metal complex, and anthraquinone-based categories [1,2]. There are approximately 8000 dyes and 10,000 commercial dye-based products in the market [3,4]. Azo-dyes are aromatic compounds with one or more azo bonds (–N=N–) and currently represent 60–70% of commercially used dyes in the world [5,6]. They are generally used for coloring plant fibers including cotton, hemp and linen, wool fibers, as well as inorganic particles (e.g., clay) [5,6]. In the coloring process, not all of the dye is adsorbed onto the end-use products, and as a result, some remain in the process effluents (i.e., wastewater), which must then be treated [7]. In addition, dyes may cause serious health problems, such as allergy, dermatitis, skin irritation, and cancer [8]. The identification of an effective chemical treatment for the removal of dyes from wastewater is currently needed.

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Presently, many chemical and physical treatment methods are used for treating effluents, including chemical oxidation (using H2 O2 , ozone), electrolysis, biodegradation (aerobic and anaerobic), adsorption (activated carbon or biosorbents) [9–11], coagulation, flocculation, and their combinations [5,7,12–14]. However, the removal of dyes is a challenging task and many small organic molecules remain in the effluent after partial decomposition using the above degradation treatment processes. Coagulation/flocculation can be used as an effective method for dye removals, as it can precipitate the soluble dye molecules (through charge neutralization and/or bridging), is cost-effective, energy efficient, and easy to use [14–16]. When separated from the solution in floc forms, the isolated flocs can be oxidized chemically or biochemically to decompose. In other words, the interaction of a flocculant and dye in solution facilitate the separation of dyes, which can subsequently be decomposed by oxidation, for instance. Commercial coagulants and flocculants include inorganic salts as well as synthetic and natural organic polymers. Inorganic coagulants consisting of aluminum sulfate, ferric chloride, and polyaluminum chloride usually require a large dosage and produce a high amount of sludge [17,18]. Synthetic flocculants, e.g., polyacrylamides, are extensively used because of their superior performance and limited sludge production [19]. However, the high price of the synthetic polymers and their limited biodegradability are their main drawbacks [19]. Natural polymers, such as chitosan and polysaccharides, are also used as flocculants because of their low price and biodegradability, but they are not shear stable, have limited removal efficiency, and require a high dosage to reach an acceptable flocculation efficiency [20]. To overcome the shortcomings of both synthetic and natural flocculants, the syntheses of semi-natural polymers have been investigated [21–24]. In one study, a carboxymethyl chitosan-graft-polyacrylamide copolymer with a 74% grafting ratio was able to remove 92% of dyes from an aqueous solution [21]. In another study, the grafting of (2-methaceyloyloxyethyl) trimethyl ammonium chloride on chitosan was produced, and the product was used as a flocculant for wastewater treatment (95.2% turbidity removal) [22]. Xylan-METAC copolymer with grafting ratio of 198% has also been prepared and used to remove dyes from wastewater (97.8% dye removal efficiency) [25]. Lignin is the second most abundant natural material and is currently an under-valued co-product of pulping and biofuel industries. Lignin has great potential to be converted into a flocculant for the dye removal [26,27]. Alternatively, a cationic lignin with a small molecular weight was prepared by grafting glycidyl-trimethylammonium chloride onto lignin and was used to remove anionic dyes from wastewater with a 95% dye removal efficiency [26]. In the present study, lignin and [2-(methacryloyloxy) ethyl] trimethylammonium chloride (METAC) were polymerized in an acidic aqueous solution through a heterogeneous reaction to produce a high molecular weight cationic lignin [27]. The first objective of this study was to investigate the impact of lignin-METAC polymer as a flocculant for dye removals. It was comprehensively discussed in the literature that the properties of polymers significantly affect their interaction with colloidal particles in solutions/suspensions [28]. In this regard, it is unclear how the properties of lignin-METAC polymer would affect its flocculation performance. The second objective of this study was to investigate the impact of lignin-METAC polymer’s properties on its flocculation efficiency. The main novelty of this work was the application of lignin-METAC polymer as a flocculant in simulated dye solutions. As stated earlier, the removal of dyes was affected by the pH, salt, and dye concentration in solutions [27,29,30]. In this work, the effects of pH, salt, dye concentration, charge density and molecular weight of lignin-METAC polymer on the dye removal efficiency were fundamentally examined.

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2. Results and Discussion 2.1. Cationic Lignin Preparation Lignin-METAC was prepared in an aqueous heterogeneous reaction by the free radical polymerization of kraft lignin and METAC under mild acidic condition initiated by K2 S2 O8 . During this reaction, METAC monomers, which contain cationic quaternary ammonium groups, were grafted onto the lignin backbone, yielding a cationic lignin with a high molecular weight. As shown in Table 1, increasing the METAC/lignin molar ratio in the polymerization reaction increased both the charge density and the molecular weight of lignin-METAC. It has been reported in the literatures that dye removal through flocculation occurs via charge neutralization, bridging, and hydrophobic/hydrophobic interaction [17,31–33]. Therefore, an increase in both the charge density and molecular weight of lignin after polymerization will affect its flocculation performance for dye molecules. Table 1. Reaction conditions and physical properties of Lignin-METAC polymer. Lignin-METAC Copolymer

METAC/Lignin Molar Ratio

Charge Density, meq/g

Mn , ×106 g/mol

Mw , ×106 g/mol

Mw /Mn

Sample 1 Sample 2 Sample 3 Sample 4

0.8 1.0 1.3 1.6

1.36 2.12 2.67 2.93

0.32 0.45 0.96 1.15

0.55 0.83 1.38 1.65

1.718 1.844 1.438 1.434

2.2. Effects of Dosage and pH The interaction between dye and polymer segments can be affected by the pH of the effluent wastewater [23]. The impacts of the dosage of lignin-METAC (sample 4) and the pH of the solution on the dye removal are shown in Figure 1. Regardless of the pH, the dye removal efficiency reached a maximum at 120 mg/L and 105 mg/L of lignin-METAC concentration for RB5 and RO16, respectively. In this case, the sulfonate groups (i.e., anionic groups) of the dye were neutralized by the cationic ammonium groups of lignin-METAC polymer forming large flocs that settled [34]. When the concentration of lignin-METAC polymer was higher than 150 mg/L for RB5 and 120 mg/L for RO16, more lignin-METAC interacted with the dye segments generating coagulates that were probably positively overcharged by lignin-METAC. These coagulates could repel each other in the solutions as they had a net charge. The balance of the repulsion of the coagulates in the solutions would yield the stabilization of the coagulates (and thus, dye segments) in the solutions, decreasing the dye removal efficiency [24]. The limited impact of pH on dye removal is due to the fact that, although the positive charge of lignin-METAC would be reduced with increasing pH as the lignin-METAC are surrounded with OHcounter ions, the cationic charge density of the lignin-METAC polymer would still be sufficiently high to act as an effective flocculant under alkaline conditions. Moreover, the high molecular weight of lignin-METAC can also facilitate the removal of dye through a bridging effect. A similar trend was reported using carboxymethyl cellulose-graft-poly[(2-methacryloyloxyethyl) trimethyl ammonium chloride] (CMC-g-METAC) as a flocculant to remove acid green dye, and pH had a minimal effect in the dye removal [24]. It can be claimed that the dye removal efficiency of lignin-METAC polymer with a high charge density and a high molecular weight is independent of pH.

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120 120 100 100 80 80 60 pH2 60 40 pH4 pH2 pH6 40 pH4 20 pH8 pH6 20 pH8 0 50 100 150 0 0 0 100 150 Dosage50 of lignin-P(METAC) , mg/L Dosage of lignin-P(METAC) , mg/L (a)

120 120 100 100 80 80 60 60 40 40 20 20 0 0 0 0

Dye removal, % Dye removal, %


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(a)

pH2 pH2 pH4 pH4 pH6 pH6 pH8 pH8 50 100 150 Dosage of lignin-P(METAC) , mg/L 50 100 150 Dosage of lignin-P(METAC) , mg/L (b)

200 200

(b)

Figure 1. Effect Effect of of pH pHon onthe thedye dyeremoval removalofof RB5; RO16 (from a dye concentration of mg/L) 100 mg/L) Figure 1. (a)(a) RB5; (b)(b) RO16 (from a dye concentration of 100 Figure 1. Effect4.of pH on the dye removal of (a) RB5; (b) RO16 (from a dye concentration of 100 mg/L) using Sample using Sample 4. using Sample 4.

The total cationic charge (meq/L) of lignin-METAC polymer in the dye solution was determined and Thetotal totalcationic cationiccharge charge(meq/L) (meq/L) lignin-METACpolymer polymer inthe the dye solution was determined The ofof lignin-METAC dye solution was determined and is shown in Figure 2. The total cationic charges introduced to theinsolution were increased by the addition and is shown in Figure 2. The total cationic charges introduced to the solution were increased by isofshown in Figure 2. The total to thei.e., solution increased by the addition the lignin polymer into thecationic solution.charges Chargeintroduced neutralization, point were of zero charge, will occur when the addition of the lignin polymer intoCharge the solution. Charge neutralization, i.e.,charge, point ofwill zero charge, ofthe thetotal lignin polymer into the solution. neutralization, i.e., point of zero occur when number of charges introduced by the polymer will be equal to the total charges of the dye in the will occur whenofthe total number of charges introduced the polymer willcharges be equal the total the total number charges by the polymer will by be equal theshown total of to the the solutions (i.e., the crossingintroduced points of lignin-METAC line and dye to line in Figure 2). dye Thisinpoint charges of the dye in the solutions (i.e., the crossing points of lignin-METAC line and dye line shown solutions (i.e., the crossing points of lignin-METAC line and dye line shown in Figure 2). This point corresponds to the lignin-METAC concentration of 145.7 mg/L for RB5 and 110 mg/L for RO16, in Figure 2). This point corresponds to the lignin-METAC concentration of 145.7 mg/L for RB5 and corresponds theoptimal lignin-METAC concentration of 145.7 mg/L for RB5 for and RO16, respectively.to The experimental concentration of lignin-METAC wasand 120 110 mg/Lmg/L for RB5 105 110 mg/L for RO16, respectively. The optimal experimental concentration of lignin-METAC was respectively. The(Figure optimal lignin-METAC was 120 mg/L for RB5 and 105 mg/L for RO16 1),experimental respectively, concentration which is lowerofthan that of the theoretical concentrations (Figure 120 mg/L for (Figure RB5 and1),105 mg/L for RO16 1), respectively, is lower than that of(Figure the mg/L for RO16 respectively, which(Figure istolower than that of thewhich theoretical concentrations 2). This provides evidence that, in addition charge neutralization, other factors, such as bridging, theoretical concentrations (Figure This provides evidence in addition chargesuch neutralization, 2). This provides that, in2).addition to charge other factors, as bridging, contributed to theevidence interaction of lignin-METAC with theneutralization, dyethat, molecules in theto solutions [25–27]. other factors, such as bridging, contributed to the interaction of lignin-METAC with the dye molecules contributed to the interaction of lignin-METAC with the dye molecules in the solutions [25–27]. in the solutions [25–27].

Charges in solution, meq/L Charges in solution, meq/L

0.6 0.6

negative charge negative charge

0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2

lignin-METAC copolymer lignin-METAC copolymer RB5 dye solution RB5 dye solution RO16 dye solution RO16 dye solution

0.1 0.1 0 0

0 0

50 100 150 50 100 150 Lignin-METAC concentration in dye solution, mg/L Lignin-METAC concentration in dye solution, mg/L

200 200

Figure 100 mg/L) as function of ligninFigure 2. 2. Overall Overallcationic cationiccharges chargesofofthe thedye dyesolutions solutions(dye (dyeconcentration, concentration, 100 mg/L) as function of Figure 2. Overall cationic charges of the dye in solutions (dye concentration, 100 mg/L) as function of ligninMETAC polymer (sample 4) concentration the solutions. lignin-METAC polymer (sample 4) concentration in the solutions. METAC polymer (sample 4) concentration in the solutions.

It is also apparent that a lower dosage of lignin-METAC polymer was needed to remove RO16 than Itisisalso alsoapparent apparentthat thata alower lowerdosage dosage of lignin-METAC polymer was needed remove RO16 It lignin-METAC needed to to remove RO16 than RB5. This higher efficiency is most likelyofattributed to thepolymer differentwas molecular structures and charge than RB5. This higher efficiency is most likely attributed to the different molecular structures and RB5. This higher efficiency is most likely attributed to the different molecular structures and charge charge densities of RB5 and RO16. As shown in Figure 3, RO16 has two sulfonate groups, while RB5 contains four sulfonate groups, requiring a higher concentration of lignin-METAC polymer to

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densities of of RB5 RB5 and and RO16. RO16. As As shown shown in in Figure Figure 3, 3, RO16 RO16 has has two two sulfonate sulfonate groups, groups, while while RB5 RB5 contains contains densities four sulfonate groups, requiring a higher concentration of lignin-METAC polymer to neutralize its four sulfonate groups, requiring a higher concentration of lignin-METAC polymer to neutralize its neutralize its charge. A similar phenomenon has also been observed by Szygula and coworkers in charge. A A similar similar phenomenon phenomenon has has also also been been observed observed by by Szygula Szygula and and coworkers coworkers in in removing removing sulfonated sulfonated charge. removing sulfonated azo-dyes by using chitosan from solutions [23]. azo-dyes by using chitosan from solutions [23]. azo-dyes by using chitosan from solutions [23].

RB5 RB5

RO16 RO16 Figure 3. 3. Structure Structure of of RB5 RB5 and and RO16. RO16. Figure Figure 3. Structure of RB5 and RO16.

2.3. Effect Effect of of Dye Dye Concentration Concentration 2.3. 2.3. Effect of Dye Concentration The effect effect of of dye dye (25, (25, 50, 50, 100, 100, 200 200 mg/L) mg/L) and and lignin-METAC lignin-METAC concentration concentration on on the the removal removal of of RB5 RB5 The The effect of dye (25, 50, 100, 200 mg/L) and lignin-METAC concentration on the removal of RB5 and RO16 RO16 at at pH pH 6, 6, which which is is the the typical typical pH pH of of dye dye effluent, effluent, is is shown shown in in Figure Figure 44 [35]. [35]. The The concentration concentration and and RO16 at pH 6, which is the typical pH of dye effluent, is shown in Figure 4 [35]. The concentration of lignin-METAC polymer was varied in order to determine the best concentration required for the the of lignin-METAC polymer was varied in order to determine the best concentration required for of lignin-METAC polymer was varied in order to determine the best concentration required for the maximum dye removal. The dye removal efficiency increased and then decreased with increasing the maximum dye removal. The dye removal efficiency increased and then decreased with increasing the maximum dye removal. The dye removal efficiency increased and then decreased with increasing the concentration of of lignin-METAC lignin-METAC polymer polymer for for both both dye dye solutions solutions and and an an optimum optimum dosage dosage of of 120 120 mg/L mg/L and and concentration concentration of lignin-METAC polymer for both dye solutions and an optimum dosage of 120 mg/L 105 mg/L for RB5 and RO16, respectively, was determined. 105 mg/L for RB5 and RO16, respectively, was determined. and 105 mg/L for RB5 and RO16, respectively, was determined. 120 120 120 120 100 100 100 100

Dye removal,%% Dyeremoval,

80 80

Dye removal,%% Dyeremoval,

80 80

60 60

60 60

25 mg/L mg/L 25

40 40

50 mg/L mg/L 50

00 50 100 150 200 250 50 100 150 200 250 Concentration of of lignin-P(METAC), lignin-P(METAC), mg/L mg/L Concentration

(a) (a)

100 mg/L mg/L 100 200 mg/L mg/L 200

20 20

200 mg/L mg/L 200 00

50 mg/L mg/L 50

40 40

100 mg/L mg/L 100

20 20

25 mg/L mg/L 25

300 300

00 00

50 100 150 200 250 50 100 150 200 250 Concentration of of lignin-P(METAC), lignin-P(METAC), mg/L mg/L Concentration

300 300

(b) (b)

Figure 4. 4. Effect Effect of of dye dye concentration concentration on on dye dye removal removal(a), (a),RB5; RB5;(b) (b)RO16 RO16at atpH pH666using usingsample sample4. 4. Figure 4. Effect of dye concentration on dye removal (a), RB5; (b) RO16 at pH using sample 4. Figure

From the the results results shown shown in in Figure Figure 4, 4, the the amount amount of of lignin-METAC lignin-METAC needed needed to to remove remove the the maximum maximum From From the results shown in Figure 4, the amount offunction lignin-METAC needed to remove the maximum amount of dye was determined and presented as a of dye concentration in Figure 5. In In this this amount of dye was determined and presented as a function of dye concentration in Figure 5. amount oftheoretical dye was determined and presented as a function of dyefor concentration in Figure 5. Inshown. this figure, the amount of lignin-METAC polymer required dye neutralization is also figure, the theoretical amount of lignin-METAC polymer required for dye neutralization is also shown. figure, the theoretical amount of lignin-METAC polymer required for dye neutralization is also shown. It is apparent that, at the same dye concentration, less lignin-METAC polymer was needed to remove RO16 than RB5. As listed in Table 2, RO16 had a lower anionic charge density than RB5, which is due to its smaller number of sulfonate groups. Therefore, a lower concentration of lignin-METAC

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It Molecules is apparent that, at the same dye concentration, less lignin-METAC polymer was needed to remove 2018, 23, 2005 6 of 13 RO16 than RB5. As listed in Table 2, RO16 had a lower anionic charge density than RB5, which is due to its smaller number of sulfonate groups. Therefore, a lower concentration of lignin-METAC polymer was polymerto was necessary to anionic neutralize the anionic charges RO16. The theoretical and experimental necessary neutralize the charges of RO16. The of theoretical and experimental correlations in correlations in Figure 5 depict that (1) at a lower dye concentration, charge neutralization was thefor main Figure 5 depict that (1) at a lower dye concentration, charge neutralization was the main cause dye cause for removal, asand theexperimental theoretical and experimental were(2) very and concentration, (2) at a high removal, as dye the theoretical values were veryvalues close; and at aclose; high dye dye concentration,was lessnecessary lignin-METAC was necessary (than theoretically) to remove the less lignin-METAC experimentally (thanexperimentally theoretically) to remove the dye, illustrating that dye, illustrating that bridging played a significant role in dye removal at higher concentrations. bridging played a significant role in dye removal at higher concentrations. Table 2. Physical properties of dyes. Table 2. Physical properties of dyes. Dye Dye

Molecular Formula Molecular Formula

Mw, g/mol Mw, g/mol

Purity, % Purity, %

λmax, nm λmax, nm

RB5 RB5 RO16 RO16

N5 Na C26CH2621H N215Na 4O19 S619 S6 4O N3 Na 2O C20CH2017H N173Na 2O11 S311 S3

991.82 991.82 617.54 617.54

55 55 ≥70 ≥70

597 597 493 493

Dosage of lignin-METAC, mg/L

350

AnionicCharge Charge Anionic Density,meq/g meq/g Density, 4.27 4.27 3.24 3.24

experimental-RB5 theoretical-RB5 experimental-RO16 theoretical-RO16

300 250 200 150 100 50 0 0

50

100 150 Dye concentration, mg/L

200

250

Figure dosage and and dye dye concentration. concentration. Figure 5. 5. Relationship Relationship between between optimum optimum lignin-METAC lignin-METAC dosage

The correlation between the dye removal and lignin-METAC concentration is listed in Table 3. A The correlation between the dye removal and lignin-METAC concentration is listed in Table 3. linear correlation with a high regression was obtained in both cases. A stoichiometric correlation was A linear correlation with a high regression was obtained in both cases. A stoichiometric correlation obtained between lignin-METAC and dye, indicating a close to one to one charge interaction. was obtained between lignin-METAC and dye, indicating a close to one to one charge interaction. Table 3. Correlation between dye removal and lignin-METAC concentration. Table 3. Correlation between dye removal and lignin-METAC concentration.

Dye Dye RB5

Linear Correlation Correlation yLinear = 1.1443x+ 15.217 yy==0.9426x+ 1.1443x+ 4.1304 15.217

R² 2 0.9960 R 0.99630.9960

RB5RO16 RO16 y = 0.9426x+ 4.1304 mg/L; R0.9963 2: linear correlation coefficient. x: dye concentration, mg/L; y: lignin-METAC concentration, x: dye concentration, mg/L; y: lignin-METAC concentration, mg/L; R2 : linear correlation coefficient.

2.4. Effect of Charge Density and Mw of Lignin-METAC Polymer

2.4.The Effect of Charge Density and M Polymer effect of charge density and MLignin-METAC w of lignin-METAC on dye removal was presented in Figure 6. A w of 100 mg/L sample of RB5 and RO16 solutions were used as the simulated dye wastewater in this The effect of charge density and Mw of lignin-METAC on dye removal was presented in Figure 6. experiment. At the maximum dye removal, a smaller dosage of lignin-METAC polymer with a higher A 100 mg/L sample of RB5 and RO16 solutions were used as the simulated dye wastewater in this charge density and higher Mw was needed for both RB5 and RO16 solutions. experiment. At the maximum dye removal, a smaller dosage of lignin-METAC polymer with a higher charge density and higher Mw was needed for both RB5 and RO16 solutions.

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120 100

120 100

10080

10080


120


120

8060

8060 sample 1

6040 4020 20 0 0 0 0

50

100

150

4 200sample250

Concentration lignin-P(METAC), mg/L 50 100 of150 200 250

(a) Concentration of lignin-P(METAC), mg/L

sample 1

6040

sample 2 sample 1 sample 3 sample 2 sample 4 sample 3

sample 2 sample 1 sample 3 sample 2 sample 4 sample 3

4020

300 300

20 0 0 0 0

50

100

150

sample 4 200 250

Concentration of lignin-METAC, mg/L 50 100 150 200 250

300 300

Concentration of(b) lignin-METAC, mg/L

Figure 6. Effect of dosage of lignin-METAC samples with different charge densities on dye removal (100 (a) (b) Figure 6. Effect of dosage of lignin-METAC samples with different charge densities on dye removal mg/L RB5 (a) and RO16 (b) dye solution, pH 6, 30 °C). ◦ (100 mg/L RB5 and RO16 (b) dye solution, pH 6,with 30 different C). Figure 6. Effect of(a) dosage of lignin-METAC samples charge densities on dye removal (100 mg/L RB5on (a)the andcharge RO16 (b) dye solution, 6, 30in°C). Based densities of thepH dyes Table 2 and that of the lignin-METAC in Table 1, the

Based on of thelignin-METAC charge densities of thethat dyescould in Table 2 and of the lignin-METAC in determined Table 1, concentration polymer result in a that maximum dye removal was Based on the charge densities of the dyes inthat Table 2 and that in of the lignin-METAC in Table was 1, the the concentration of lignin-METAC polymer could result a maximum dye removal and is shown as a function of dye removal in Figure 7. The concentration of lignin-METAC to obtain the concentration of lignin-METAC polymer that could result in a maximum dye removal was determined determined andremoval is shownwas as a 270 function of165 dyemg/L, removal Figureand 7. The lignin-METAC maximum dye mg/L, 125inmg/L, 110concentration mg/L for RB5ofand 225 mg/L, 140 and is shown as a function of removal dye removal in Figure 7.165 Themg/L, concentration ofand lignin-METAC to RB5 obtain to obtain the maximum dye was 270 mg/L, 125 mg/L, 110 mg/L for andthe mg/L, 105 mg/L, and 90 mg/L for RO16 for samples 1 to 4, respectively. Interestingly, the concentration maximum wasmg/L, 270 mg/L, mg/L, mg/L, samples and 1101mg/L for RB5 and Interestingly, 225 mg/L, 140 225 mg/L,dye 140removal mg/L, 105 and 90165 mg/L for125 RO16 totheoretically 4, respectively. of lignin-METAC used experimentally was lower than for that required to interact with the mg/L, 105 mg/L, and 90 mg/L for RO16 for samples 1 to 4, respectively. Interestingly, the concentration the concentration of lignin-METAC lowereffect thanisthat requiredin theoretically dyes, which was due to the bridgingused effectexperimentally of dyes as the was bridging discounted the theoretical oftolignin-METAC used experimentally was lower than that required theoretically to interact with isthe interact with the dyes, which the wascharge due to the bridging effect ofweight dyes as the bridging effect estimation. Moreover, the higher density and molecular of lignin-METAC, the larger dyes, which was duetheoretical to the bridging effect ofMoreover, dyes as the bridging effect is discounted in themolecular theoretical discounted in the the higherconcentrations, the charge density and difference there was between estimation. the experimental and theoretical further illustrating that estimation. the higher the charge density and molecular weight of lignin-METAC, the larger weight of Moreover, lignin-METAC, thepronounced larger difference there was between the experimental and theoretical the bridging effect was more when the polymer had a higher molecular weight. The bridging difference there was between the experimental and theoretical further illustrating that concentrations, further illustrating thathas thealso bridging effect wasinconcentrations, more pronounced when the polymer effect in dye removal from wastewater been reported the literature when removing acid violet the bridging effect was more pronounced when the polymer had a higher molecular weight. The bridging had a higher molecular weight. bridging effect in dye removal from wastewater has also been 5, methyl orange, and acid black The 1 from dye wastewater [21,27,36]. effect in dyeinremoval from wastewater has also been reported in theorange, literature when acid dye violet reported the literature when removing acid violet 5, methyl and acidremoving black 1 from 5,wastewater methyl orange, and acid black 1 from dye wastewater [21,27,36]. [21,27,36]. 340

experimental-RB5

concentration of lignin-METAC, mg/L concentration of lignin-METAC, mg/L

340 290

theoretical-RB5 experimental-RB5 experimental-RO16 theoretical-RB5 theoretical-RO16 experimental-RO16

290 240 240 190

theoretical-RO16

190 140 14090 9040 40 -10 1

1

1.5

2 2.5 charge density, meq/g

3

3.5

1.5 2 2.5 3 3.5 0.55 0.83 3.0 charge density, meq/g1.38 1.65 5 molecular weight, ×10 g/mol -10 0 0.55 0.83 1.38 1.65 3.0 5 g/mol Figure 7. Relationship between molecular charge density, MW, and optimum concentration of lignin-METAC weight, ×10 0

copolymer in dye removal. Figure 7. Relationship between charge density, MW, and optimum concentration of lignin-METAC Figure 7. Relationship between charge density, MW, and optimum concentration of lignin-METAC copolymer in dye removal. copolymer in dye removal.

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2.5. Effect of Inorganic Salts 2.5. Effect of Inorganic Salts Common inorganic salts, such as chloride, sulphate, carbonate, and nitrate, present in textile Common inorganic salts, such as chloride, sulphate, carbonate, and nitrate, present in textile effluents may affect dye removal [23,29]. For this reason, the effect of NaCl, Na2 SO4 , Na2 CO3 , and effluents may affect dye removal [23,29]. For this reason, the effect of NaCl, Na2SO4, Na2CO3, and NaNO3 NaNO3 on the dye removal of RB5 (100 mg/L), which has a higher dye removal percentage than RO16, on the dye removal of RB5 (100 mg/L), which has a higher dye removal percentage than RO16, was was investigated in Figure 8. The addition of NaCl, Na2 SO4 , and NaNO3 to the solution did not have investigated in Figure 8. The addition of NaCl, Na2SO4, and NaNO3 to the solution did not have a a significant effect on the removal of RB5 dye. The percentage of dye removal remained constant at significant effect on the removal of RB5 dye. The percentage of dye removal remained constant at high high salt concentrations, illustrating the high efficiency of lignin-METAC as a flocculant. However, the salt concentrations, illustrating the high efficiency of lignin-METAC as a flocculant. However, the removal of RB5 was more influenced by Na2 CO3 . In this case, the dye removal efficiency decreased removal of RB5 was more influenced by Na2CO3. In this case, the dye removal efficiency decreased from from 98.8% to 92.2%. This has also been reported in the removal of azo-dyes using chitosan and is 98.8% to 92.2%. This has also been reported in the removal of azo-dyes using chitosan and is attributed attributed to a charge screening effect and/or change of the double layer surrounding the flocculated to a charge screening effect and/or change of the double layer surrounding the flocculated molecules [24]. molecules [24]. 100

98 NaCl

Dye removal, %

96

Na2SO4 NaNO3

94

Na2CO3

92

90 0

0.02

0.04

0.06

0.08

0.1

0.12

Concentration of salt, mol/L

Figure 8. Effect saltdosage dosageon ondye dyeremoval removal (100 (100 mg/L RB5 dye Figure 8. Effect of of salt mg/L RB5 dye solution, solution,110 110mg/L mg/Llignin-METAC lignin-METAC ◦ C). dosage (sample 4), pH 6, 30 dosage (sample 4), pH 6, 30 °C).

In Figure 9, 9, thethe effect 2CO 3 and NaCl of lignin-METAC lignin-METAC In Figure effectofofNa Na NaClon onthe thehydrodynamic hydrodynamic diameter diameter (Hy) of 2 CO 3 and andand RB5RB5 are are shown. The hydrodynamic diameter of lignin-METAC decreased with an increase in Na2CO shown. The hydrodynamic diameter of lignin-METAC decreased with an increase in 3 andNa NaCl demonstrating that the charge of the the charge lignin-METAC and dye segments and NaCl concentrations, demonstrating that of the lignin-METAC and were dye 2 CO3concentrations, partially screened increasing ionic strength. The reduced Hy implies that the polymer andthat dye the had segments were with partially screened with increasing ionic strength. The reduced Hy implies coiled structures at high salt concentration Na2CO3 was added to the polymer and dye had coiled structures at [11,37]. high saltWhen concentration [11,37]. When Na2lignin-METAC CO3 was addedand to dyethe solutions, a smaller hydrodynamic diameter was observed compared to the containing NaCl lignin-METAC and dye solutions, a smaller hydrodynamic diameter wassolution observed compared to of the bothsolution the polymer andNaCl dye. of This a stronger screening effectaof CO32− than Cl−, effect whichofis containing bothindicates the polymer and dye. This indicates stronger screening 2 − − consistent with Cl the ,results in Figure 8. results The decrease in the removal byinNa 2CO 3 is CO3 than which presented is consistent with the presented indye Figure 8. Theinduced decrease the dye ascribed to induced two facts: charges of lignin-METAC and dye are partially screened, resulting in removal by (i) Nathe CO is ascribed to two facts: (i) the charges of lignin-METAC and dye are 2 3 weakened electrostatic interactions between the polymer and the dye, and (ii) a coiled molecule partially screened, resulting in weakened electrostatic interactions between the polymer and the dye, conformation (smaller Hy). These factors would affect neutralization and affect bridging the polymer. and (ii) a coiled molecule conformation (smaller Hy).the These factors would the of neutralization This behavior was alsopolymer. reportedThis on the application of areported cellulose-based flocculant for anionic and bridging of the behavior was also on the application of aremoving cellulose-based dyeflocculant acid green [29]. for25removing anionic dye acid green 25 [29].

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nm Hy, nm Hy,

180 180

lignin-METAC-NaCl lignin-METAC-NaCl

160 160

lignin-METAC-Na2CO3 lignin-METAC-Na2CO3

140 140

RB5-NaCl RB5-NaCl

120 120

RB5-Na2CO3 RB5-Na2CO3

100 100 80 80 60 60 40 40 20 20 00 00

0.02 0.02

0.04 0.06 0.08 0.04 0.06 0.08 Concentration Concentration of ofsalt, salt,mol/L mol/L

0.1 0.1

0.12 0.12

Figure 9. Hydrodynamic diameter of lignin-METAC (sample 4) and RB5 in solutions Figure 9. diameter (Hy) of (sample 4) RB5 in containing Figure 9. Hydrodynamic Hydrodynamic diameter (Hy)(Hy) of lignin-METAC lignin-METAC (sample 4) and and dye dye RB5dye in solutions solutions containing ◦ C. containing NaCl or Na CO , pH 6, 30 2 3 NaCl or Na 2 CO 3 , pH 6, 30 °C. NaCl or Na2CO3, pH 6, 30 °C.

2.6. COD Removal 2.6. COD Removal 2.6. COD Removal The chemicaloxygen oxygendemand demand(COD) (COD)is isan an indicator indicator of The chemical load of organics present wastewater The chemical oxygen demand (COD) is an of the the load loadof oforganics organicspresent presentinin inwastewater wastewater effluents, which closely monitored determining quality of wastewater worldwide effluents, which is closely monitored for determining the quality of worldwide [3]. The effluents, which is is closely monitored forfor determining thethe quality of wastewater wastewater worldwide [3].[3]. The The impact of lignin-METAC on the COD removal from the dye solutions are presented in Figure 10. impact of lignin-METAC on the COD removal from the dye solutions are presented in Figure 10. The impact of lignin-METAC on the COD removal from the dye solutions are presented in Figure 10. The The concentration of lignin-METAC was based on the optimum dosage obtained in Figure 1. A dosage concentration of was on optimum dosage obtained in 1. of concentration of lignin-METAC lignin-METAC was based based on the the optimum dosage obtained in Figure Figure 1. A A dosage dosage of 120 120 of 120 of lignin-METAC in mg/L amg/L 100 mg/L RB5solution dye solution to 96.4% COD removal.Alternatively, Alternatively, aa mg/L of lignin-METAC in RB5 led to COD removal. mg/L ofmg/L lignin-METAC in aa 100 100 RB5 dye dye solution led led to 96.4% 96.4% COD removal. Alternatively, a dosage of 105 mg/L of lignin-METAC a 100 mg/L RO16 solution to 95.5% COD removal. dosage of mg/L of in mg/L RO16 solution led to COD removal. The dosage of 105 105 mg/L of lignin-METAC lignin-METAC in aain100 100 mg/L RO16 solution led led to 95.5% 95.5% COD removal. The The significant decrease in COD isto due the removal of from dye from the solution. Figure 10 also illustrates significant decrease in is the of the Figure 10 illustrates that significant decrease in COD COD is due due to thetoremoval removal of dye dye from the solution. solution. Figure 10 also also illustrates that thatremained little remained the flocculant, lignin-METAC, in treated the treated solutions as the COD levels little of flocculant, lignin-METAC, in dye solutions as COD levels of the little remained of the theof flocculant, lignin-METAC, in the the treated dyedye solutions as the the COD levels ofof the the treated samples were negligible. treated treated samples samples were were negligible. negligible. 160 160 COD CODbefore beforetreatment, treatment,mg/L mg/L 140 140

COD CODafter aftertreatment, treatment,mg/L mg/L

120 120

mg/L COD, mg/L COD,

100 100 80 80 60 60 40 40 20 20 00 RB5 RB5

RO16 RO16

Figure 10. The COD 120 mg/L lignin-METAC dosage (sample Figure 10. The COD removal of 100 mg/L RB5 and RO16 by by120 120mg/L mg/Llignin-METAC lignin-METACdosage dosage(sample (sample 4) Figure 10. The CODremoval removalof of100 100mg/L mg/LRB5 RB5and and RO16 RO16 by 4)4) for RB5 and 105 mg/L lignin-METAC (sample 4) dosages for RO16 pH 6, 30 °C. ◦ forfor RB5 and 105 mg/L lignin-METAC (sample 4) dosages for RO16 pH 6, 30 °C. RB5 and 105 mg/L lignin-METAC (sample 4) dosages for RO16 pH 6, 30 C.

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3. Materials and Methods 3.1. Materials Softwood kraft lignin was produced by LignoForceTM technology of FPInnovations in its pilot plant facility located in Thunder Bay, ON, Canada [38]. [2-(Methacryloyloxy) ethyl] trimethylammonium chloride solution (METAC), 80 wt.% in H2 O, potassium persulfate (K2 S2 O8 , ACS reagent ≥ 99.0%), NaCl, NaNO3 , Na2 SO4 , Na2 CO3 , and the dyes were all purchased as reagent grade from Sigma-Aldrich (Darmstadt, Germany) company and used as received. The details of reactive black 5 (RB5) and reactive orange 16 (RO16) dyes are presented in Table 2. Anionic polyvinyl sulfate (PVSK) with an Mw of 100,000–200,000 g/mol (97.7% esterified) was purchased from Wako Pure Chem. Ltd., Osaka, Japan. Ethanol (95 vol.%) was received from Fisher Scientific (Waltham, MA, USA). 3.2. Preparation of Cationic Lignin-METAC The preparation of lignin-METAC polymer was carried out according to our previously described methods [27]. We comprehensively discussed that METAC would graft to the phenolic OH of lignin and then proceed with chain extension in a free radical polymerization system. We also illustrated that lignin-METAC and xylan-METAC were more effective than polyMETAC as flocculants for clay suspensions [25,27]. In this set of experiments, 1 g of lignin was mixed with 30 mL of deionized water in a 250 mL three-neck glass flask at 80 ◦ C in a water bath. The suspension was purged with nitrogen gas for 30 min, and then a determined amount of METAC was added to the suspension based on the molar ratio of METAC to lignin (the molecular weight unit of lignin was assumed 180 g/mol) [27]. Then, the pH was adjusted to 4. A 5 mL solution of K2 S2 O8 (0.03 g) was then added dropwise to the reaction mixture to initiate the polymerization. The reaction was heated to 80 ◦ C for 3 h and then cooled to room temperature. Subsequently, the reaction mixture was poured dropwise into a 95 vol.% ethanol solution in order to precipitate the lignin-METAC polymer from the rest of the reaction medium [27]. The suspension was then centrifuged at 2100× g for 10 min using a Sorvall ST 16 laboratory centrifuge in order to separate the lignin-METAC polymer from the suspension. The lignin-METAC polymer was then dried in a 105 ◦ C oven prior to use. However, drying may affect the properties of lignin-METAC, and thus drying of this polymer using other methods, e.g., freeze drying or vacuum drying, is suggested. The properties of lignin-METAC polymers with varying amount of METAC along with the reaction conditions are listed in Table 1. 3.3. Charge Density Analysis Approximately 0.05 g of lignin-METAC polymer and dye were separately dissolved in 50 g of water, the solutions were then immersed in a water bath shaker (Innova 3100, Brunswick Scientific, Edison, NJ, USA) and shaken at 150 rpm and 30 ◦ C for 2 h. The charge densities of the samples were then measured using a Particle Charge Detector, Mütek PCD 04, with a 0.005 M PVSK solution. The charge densities of the dyes can be found in Table 2, while that of lignin-METAC polymer are shown in Table 1. 3.4. Preparation of Dye Solutions The dye solutions were prepared by dissolving a specific amount of dye in deionized distilled water to make up the dye solutions with varying concentrations (25, 50, 100, 200 mg/L) at different pH (2, 4, 6, 8). The solutions were kept overnight stirring at 200 rpm and room temperature. The dye solutions were considered as the simulated wastewater effluents in this work. 3.5. Hydrodynamic Diameter (Hy) Measurement The hydrodynamic diameter (Hy) of the RB5, RO16 and lignin-METAC polymer was determined using a dynamic light scattering analyzer (DLSA), BI-200SM Brookhaven Instrument, Holtsville, NY,

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USA at a scattering angle of 90◦ . The light source for the DLSA is a power solid state laser with a maximum power of 35 mW and a wavelength of 637 nm. To measure the hydrodynamic diameter, a 0.02 g/L sample of lignin-METAC polymer and 100 mg/L of dye solution were stirred for 30 min and then a specific amount of salt was added into the solutions. The salt-containing solutions were kept at room temperature for 24 h. Subsequently, the solutions were filtered with 0.45 µm Acrodisc syringe filters and then tested with the instrument. 3.6. Dye Removal Analysis In this set of experiments, 1 g/L aqueous solution of lignin-METAC polymer was prepared with deionized distilled water at room temperature. Different amounts of lignin-METAC were then added to 30 mL of dye in centrifuge tubes as seen in Figures 4 and 6. The tubes were then immersed in a water bath shaker at 30 ◦ C and 150 rpm for 10 min. The tubes were then centrifuged at 1500× g for 10 min using a Sorvall ST 16 centrifuge. The filtrates were collected and the concentration of dye remaining in the filtrates was measured using calibration formulas by a UV/Vis spectrophotometer (Genesys 10s). The dye removal was calculated based on equation 1 [20,39,40]: Dye removal =

A0 − A × 100 A0

(1)

where A0 and A are the absorbance of the dye solutions (Table 2) before and after the addition of lignin-METAC. The chemical oxygen demand (COD) of the simulated dye solutions (100 mg/L of dye concentration) was measured before and after the addition of lignin-METAC using YSI CR2200 COD thermo reactor. The COD determination is based on the amount of potassium dichromate reduced in concentrated sulfuric acid after 2 h at 150 ◦ C. The test tubes used were manufactured by the Hach Company (Loveland, CO, USA). The COD removal was determined using equation 2: COD removal =

C0 − C × 100 C0

(2)

where C0 and C are the COD of dye solutions before and after lignin-METAC polymer treatment, respectively. 4. Conclusions The cationic lignin-METAC polymer was an effective flocculant for removing anionic dye from simulated wastewater. The results showed that charge neutralization and bridging effects were the main mechanisms for the dye removal. Increasing the charge density and molecular weight of the lignin-METAC polymer improved the efficiency of lignin-METAC polymer for the dye removal. The presence of inorganic salts including NaCl, NaNO3 , and Na2 SO4 in the dye solution did not affect the dye removal efficiency of lignin-METAC, whereas Na2 CO3 did have a slight affect and decreased the dye removal efficiency from 98.8% to 92.2%. The pH had a minimal impact on dye removal and the lignin-METAC polymer was more effective in removing RO16 than RB5. The relationship between the optimum dosage of lignin-METAC and dye concentration was linear and there was a stoichiometric interaction between the dye and lignin-METAC. Furthermore, more than 95% of COD was removed by treating the dye solutions with lignin-METAC polymer at the optimized dosages. Author Contributions: S.W. was the main contributor of this work. F.K. and P.F. were the main leaders of this project. Q.H. was the advisor of S.W. Funding: The research was funded by Natural Science and Engineering Research Council, NSERC, Canada Foundation for Innovation and Canada Research Chair programs. Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds are available from the authors. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).