Production of Flocculant from Thermomechanical Pulping Lignin via

Feb 22, 2016 - There is a growing need to utilize lignin (i.e., wasted material) from the pulping industry in the production of value-added products a...
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Research Article pubs.acs.org/journal/ascecg

Production of Flocculant from Thermomechanical Pulping Lignin via Nitric Acid Treatment Robin L. Couch,† Jacquelyn T. Price,†,‡ and Pedram Fatehi*,†,§ †

Chemical Engineering Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada Bio-Economy Technology Centre, 2001 Neebing Avenue, Thunder Bay, Ontario P7E 6S3, Canada § Chemical Engineering Department, University of New Brunswick, 15 Dineen Drive, Fredericton, New Brunswick E3B 5A3, Canada ‡

ABSTRACT: There is a growing need to utilize lignin (i.e., wasted material) from the pulping industry in the production of value-added products and to develop cost-effective and environmentally friendly processes for removing dyes from wastewater effluents. In this context, lignin can be modified to gain anionic charges, which can successfully remove cationic dyes from wastewater. In this study, lignin was extracted from thermomechanical pulp (softwood) via periodate treatment, and then the extracted lignin was oxidized using 30 wt % nitric acid concentration at 80 °C for 1.5 h, which resulted in oxidized lignin with the charge density and solubility of 3.02 mequiv/g and 97% (at a 1 wt % lignin concentration), respectively. The oxidized lignin was used for removing ethyl violet and basic blue cationic dyes from simulated wastewater effluents. It was observed that the dye removals were in the ranges of 70−80 wt % for ethyl violet and of 80−95 wt % for basic blue, while the COD removals were in the ranges of 60−70% for ethyl violet and 70−85% for basic blue when the concentrations of dyes varied between 50 and 400 mg/L. The dye removal was pH dependent, and the removal of basic blue decreased from 84 wt % (in the absence of salt) to 77% in the presence of 3 g/L NaCl, whereas salt had a marginal effect on the removal of ethyl violet from the solution. KEYWORDS: Lignin, Oxidation, Dye removal, Colloid, Charge density, Biorefinery

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INTRODUCTION Recently, the demand for the production of value-added products from pulping processes has been increased in order to help the overall economy of the pulping industry.1 A cellulosebased product can be produced from a thermomechanical pulp (TMP), but the lignin-based material produced in this process cannot presently be utilized.2 Although a cellulose-derived material is the value-added product of this process, the ligninbased material produced in this process has no industrial value currently. To produce value-added products from lignin, one alternative is to modify its structure.3 Lignin is generally a water insoluble product with limited practical applications in aqueous systems.3 The increase in the number of carboxylate groups in lignin increases lignin’s anionic charge density and allows lignin to be more hydrophilic.4,5 Various mild oxidation pathways, i.e., hydrogen peroxide,5 metal oxides,6 nitrobenzene,6 and oxygen with catalysts,7 were performed on lignin to improve its hydrophilicity. In the developed cellulosebased process, sodium periodate treatment is used to separate lignin from cellulose and hemicellulose of TMP. Therefore, mild oxidation treatments would not be effective to oxidize lignin, and a stronger oxidizing agent is needed to oxidize the lignin-based material produced in this process.2,8 On the other hand, nitric acid8−10 and potassium permanganate11 have been used to oxidize organic materials in the past. Alvarez et al.9 © XXXX American Chemical Society

observed that the nitric acid treatment of coal caused an increase in the carboxylate and amino groups of coal, which improved its hydrophilicity. It was also found that the treatment of carbon nanotubes with 8 M nitric acid at 40 °C for 1 h improved the dispensability of the carbon nanotubes by approximately 30%.10 Therefore, nitric acid treatment may be used as a method to increase the number of carboxylate groups and to improve the hydrophilicity of lignin. The first objective of this work was to increase the water solubility of lignin via oxidation using nitric acid.12 Today, dyes are used in the textile, pulp and paper, tannery, and paint industries on a large scale.13 Approximately, 10−15% of dyes are released to the environment from the effluents of these industries.13 Dyes can be toxic and harmful to aquatic ecosystems,13 and these effluents should be treated prior to being released to the environment.13 Many studies were performed removing dyes from effluents using either filtration and adsorption.14−17 Tang and Chen17 used a film composite nanofilter to remove reactive black 5 dye from simulated wastewater. However, filter blockage and high maintenance costs hamper the application of these techniques at industrial scales. Activated carbon is the most commonly used material Received: September 20, 2015 Revised: January 30, 2016

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DOI: 10.1021/acssuschemeng.5b01129 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering for the removal of dyes from effluents.14−16 Although a biomass-based adsorbent can potentially be inexpensive, its removal efficiency may not be acceptable at industrial scales.18 A coagulation concept has been comprehensively used for removing dyes from effluents. Generally, salts, such as alum, are used in the wastewater treatment for coagulating dissolved components. In one study, FeSO4 and H2O2 were used to coagulate and decompose simulated Reactive Black 5 dye from wastewater effluents.19 This process was effective in dye removal (99% removal from a 100 g/L dye concentration), but the sludge produced in this process contained high amounts of Fe(III) and Al(III), which might create an environmental concern.19 Similarly, coagulation and flocculation were conducted for treating wastewater using an inorganic coagulant and synthetic polymers. Joo et al.20 used a synthesized polymer from cyanoguanidine and formaldehyde with alum and ferric salts in a coagulation/flocculation process to remove reactive dye from a wastewater effluent. However, the presence of inorganic salt in the effluent was an issue due to its ineffectiveness and the large sludge production.20 Alternatively, Wang et al.21 produced an aminated lignin, and its application as a flocculant led to 96% removal of an anionic dye from wastewater effluents. Similarly, Fang et al.22 used enzymatically generated lignin and rendered it cationic for anionic dye removal. The lignin-based product removed over 95% of dyes from dye effluent with 50−250 mg/L concentrations.22 The study also found that 89% of chemical oxygen demand (COD) was removed from a 100 mg/L simulated dye wastewater.22 In principle, anionically charged biomaterials could be used as a flocculant for cationic dye removal from effluents in the textile industry.8 The second objective of this study was to investigate how the anionic lignin produced in this study could be used as a flocculant for isolating dyes from simulated dye effluents. The production of a flocculant from lignin was evaluated in this work in an effort to add values to lignin, which is currently a byproduct of the newly developed cellulose production process and to convert a TMP process to a fully integrated biorefinery with cellulose-based products and flocculants as value-added products.2 The lignin-based product was used as a flocculant for removing dyes from simulated dye effluents. The main novelties of this work were (1) the production of water soluble and anionically charged material from lignin that was separated from TMP pulp via periodate treatment for the first time; and (2) the investigation on the use of the lignin-based material as a flocculant for removing dyes from simulated dye effluents.

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Nitric acid (69−70 wt %) and sulfuric acid (95−98 wt %), reagent grades, were obtained from Sigma-Aldrich Company. Polydiallyldimethyl ammonium chloride (PDADMAC) with the Mw of 100−200 kg/mol, sodium hydroxide (reagent grade), sodium chloride (bioreagent grade ≥99.5%), dimethyl sulfoxide-d6 (DMSO-d6), deuterium dioxide (D2O), and ethyl violet cationic triarylmethane dye were obtained from Sigma-Aldrich Company. Ethyl violet and basic blue 41 had molecular weights of 492.14 and 482.57 g/mol, and the chemical formulas of C31H42N3Cl and C20H26N4O6S2, respectively. Ethyl violet had an absorption wavelength (λmax) at 595 nm and an extinction coefficient of 113 M−1 cm−1. Basic blue dye had a λmax at 617 nm and an extinction coefficient of 156 M−1 cm−1. All of the chemicals were used without any further purification in this study. Dialysis membrane tubes with the molecular weight cut off of 1000 g/mol were obtained from SpectrumLabs (Houston, TX). Furthermore, chemical oxygen demand (COD) kits were received from CHEMetrics Company (Midland, VA) and used for COD analysis. Potassium polyvinyl sulfate (PVSK) was obtained from Wako Pure Chemical Industries Ltd., Dallas, TX. Nitric Acid Treatment. In this set of experiments, 1 g of UL and 35.8, 28.6, 21.5, 14.3 g of deionized water were added to different 250 mL three-neck flasks. The flasks were equipped with thermometers and Graham condensers and sealed with stoppers. The flasks were heated to 100 °C, and then 14.2, 21.4, 28.5, and 35.7 g of preheated 70 wt % nitric acid (HNO3) were added to make the nitric acid concentrations 20, 30, 40, and 50 wt % in the flasks which were maintained for 1 h under stirring at 100 rpm, respectively. In another set of experiments, the analysis was repeated at 30 wt % nitric acid concentration for 1 h but at various temperatures (60, 70, 80, 90, and 100 °C). In another set of experiments, independent samples were maintained at 80 °C and 100 rpm, but at different time intervals of 20, 40, 60, 90, and 120 min. After the reaction, the flasks were cooled, and the pH of the solutions was adjusted to 7 with a 20 wt % NaOH solution. The solution was placed into dialysis membranes and sealed. The membranes were put into deionized water for 2 days while water was replaced twice a day. Finally, the samples were dried at 105 °C for 12 h. The collected samples were denoted as oxidized lignin (OL). Solubility and Charge Density Analyses. In this set of experiments, a 1 wt % solution of samples was prepared with distilled deionized water at room temperature. Samples were then placed in a water bath shaker (Boekel Scientific, Feasterville, PA) for 1 h at 30 °C and 100 rpm. Generally, OL samples were soluble in water after 20 min; however, to ensure that all lignin segments had sufficient time to dissolve, 1 h of treatment was selected for solubility analysis. In industry, a shorter period of time may be beneficial. Afterward, samples were centrifuged at 1000 rpm for 5 min using a Sorvall ST 16 laboratory centrifuge (Fisher Sicentific) to separate the soluble part of OL from insoluble parts of OL samples. The soluble part was dried at 105 °C in order to measure its lignin concentration, and by considering the volume of the solution, the total amount of soluble lignin in 1 wt % solution was determined according to eq 1: solubility (wt %) conc of solub le OL

EXPERIMENTAL SECTION

=

Materials. Unbleached pine thermomechanmical pulp (TMP) was kindly received from Resolute Forest Products in Thunder Bay, Ontario. A 10 g portion of the pulp was mixed with 13.2 g of sodium periodate and 29.25 g of sodium chloride under stirring at 100 rpm and room temperature for 6 days in the dark.2 After completion, the reaction was quenched with ethylene glycol (obtained from SigmaAldrich company), and the solid residues were washed a few times with water.2 Then, 2 g of solid residues was heated with water at 80− 90 °C in an oil bath for 6 h.2 Once cooled, the sample was filtered using filter papers, and the insoluble solid residues were dried at 105 °C. This material contained about 76 wt % lignin and 24% chemically bound hemicellulose to lignin (the analysis was conducted in the previous work),2 but denoted as an unmodified lignin (UL) in this work.

( Lg ) × volume of solution (L)

initial mass of OL (g) (1)

× 100

The charge density of OL was determined via a direct titration with PDADMAC solution (∼0.005 M). In this set of experiments, a known mass (approximately 0.02−0.05 g dried weight) of a soluble part of OL was placed into the cell of a particle charge detector (Mütek PCD 04 titrator, Herrsching, Germany), and the charge density of the soluble OL was determined using eq 2: charge density of soluble OL (meq/g) = B

volume of titrant (mL) × conc of titrant (mol/L) dried mass of solub le OL (g)

(2)

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30 °C and 100 rpm. After shaking, the tubes were centrifuged at 2500 rpm for 10 min. The dye content of supernatants in each tube was analyzed using a spectrophotometer (Genesys 10s UV−vis spectrophotometer, Madison, WI) at the wavelengths of 595 nm for ethyl violet and of 617 nm for basic blue. The extinction coefficients of the dyes were measured using the Beer−Lambert law with absorbance at different concentrations, which were used for determining dye concentrations in supernatants. Dye removal was calculated using eq 5.24

In this analysis, it was assumed that 1 mol of PDADMAC or PSVK had 1 equivalent (eq) charge density. Furthermore, the charge density of the dye solution was determined with the charge density detector using a PVSK solution (0.0049 M) in order to calculate the total charges of the dye segments in solutions. Phenolic Hydroxyl and Carboxylate Group Analysis. In this set of experiments, a solution was made using 0.06 g of OL samples with 1 mL of 0.8 M potassium hydroxide, 4 mL of 0.5% 4hydroxybenzoic acid, and 100 mL of deionized water. The carboxylate groups were measured by means of the aqueous potentiometric titration using an automatic potentiometric titrator (785-DMP Titrino, Metrohm, Herisan, Switzerland) with 0.1 M HCl standard solution as a titrating solution. The phenolic hydroxyl group contents were calculated according to eq 3, and carboxylate group contents were calculated according to eq 4:

dye removal (%) =

((EP′2 − EP′1) − (EP2 − EP1)) × C m

(3)

⎛ mmol ⎞ carboxylate group ⎜ ⎟ ⎝ g ⎠ =

((EP′3 − EP′2 ) − (EP3 − EP2)) × C m

(5)

Here C0 and C were the dye concentrations of supernatants before and after treating with OL1, respectively. As the pH of commercial cationic dye wastewater effluents can vary from 4 to 7,25 it is important to investigate if OL1 is effective at different pHs in removing dyes. To investigate the impact of pH on dye removals, dye solutions of 100 mg/L were prepared at different pHs; 135 mg/L of OL1 was added to the ethyl violet solution, or 313 mg/L OL1 was added to the basic blue solution (these OL1 dosages generated the maximum removals in pH 7). The removal was determined using eq 5. It was reported that 2.5 g/L of salt was used in a dying process of acrylic fibers with a basic cationic dye,23 while 0−6 g/L salt was added to the methylene blue and malachite green dye solutions to simulate the effluents of synthetic wastewaters.24 Therefore, it is important to investigate how salt can impact the performance of OL in dye removal. To investigate the impact of salt on the dye removal, dye solutions of 100 mg/L that contained 0.3 or 3 g/L of NaCl were prepared; 135 mg/L of OL1 was added to the ethyl violet solution, or 313 mg/L OL1 was added to the basic blue solution. The dye removal was determined using eq 5. Chemical Oxygen Demand (COD). Two sets of the dye solutions with different concentrations (50, 100, 200, and 400 mg/L) were produced using ethyl violet or basic blue dyes with deionized water in 50 mL centrifugal tubes. In one set of experiments, 44, 143, 260, and 479 mg/L of OL1 were added to the ethyl violet solutions with different concentrations (50, 100, 200, and 400 mg/L), and 111, 313, 600, and 1153 mg/L of OL1 were added to the basic blue solutions with different concentrations (50, 100, 200, and 400 mg/L), respectively. In another set of experiments, control samples were prepared with the addition of water to dye solutions (i.e., no OL1 was added to the solution). Each centrifugal tube was placed in a water bath shaker at 100 rpm and 30 °C for 10 min. After shaking, the tubes were centrifuged at 2500 rpm for 10 min. Then, 2 mL of the supernatants was placed into a COD vial, and its COD content was analyzed using COD kits via following the standard procedure supplied by CHEMetrics Company, Midland, VA. The COD removal was calculated using eq 5, where C0 and C were the COD (mg/L) of the samples before and after treating with OL1, respectively.

⎛ mmol ⎞ phenolic hydroxyl group ⎜ ⎟ ⎝ g ⎠ =

C0 − C × 100 C0

(4)

Here C is the molar concentration of HCl (i.e., titrant); EP1, EP2, and EP3 are the consumed volume of HCl solution (mL) for the first, second, and third end points when the blank solutions were titrated. EP′1, EP′2, and EP′3 are the consumed volume of HCl solution (mL) for the first, second, and third end points when OL solutions were titrated, and m is the dried weight (g) of lignin samples. Molecular Weight Analysis. As unmodified lignin (UL) was not soluble in water, and oxidized lignin (OL) was soluble at pH 7, the determination of molecular weight of both samples was not possible with gel permeation chromatography. Therefore, the molecular weight of UL and OL samples was analyzed with a dynamic light scattering instrument that was attached to a goniometer, Brookhaven BI-200SM, Holtsville, NY. The solutions of UL and OL were prepared at different concentrations of 0.1, 0.2, 0.3, 0.4, and 0.5 wt % in water at pH 12.5 (samples were soluble at this pH). Potassium nitrate (KNO3) was added to the lignin solutions at a concentration of 10 mM, and the solutions were maintained for 24 h prior to measurement. The solutions were filtered using a nylon syringe filter with 0.45 μm pore size and 30 mm diameter (Celltreat Scientific Products). The laser polarized light wavelength was set at 637 nm, and the intensities of the scattered light were measured at different angles for each solution. The data was compiled and analyzed using BIC Zimm Plot software with the specific refractive index increments (dn/dC) set at 0.18 mL/g. FTIR Analysis. The 0.05 g portions of oven-dried (105 °C) UL and OL samples, which were produced under the conditions of 1−1.5 h, 80−100 °C, and 30−50 wt % nitric acid concentrations, were used for Fourier transform infrared spectroscopy (FTIR) analysis with a Bruker Tensor 37, Ettlingen, Germany, ATR accessory. The spectra were recorded in a transmittance mode in the range 600−4000 cm−1 with the resolution of 4 cm−1; 32 scans per sample were conducted. Thermogravimetric Analysis (TGA). The thermal behaviors of the UL and OL samples were assessed under nitrogen gas (N2). In this set of experiments, 5−7.5 mg of the samples was analyzed with a TGA analyzer (Instrument Specialist i1000 series system, Madison, WI). The weight loss and weight loss rate of the samples were determined in the temperature range 25−750 °C with N2 flow rate of 30−40 mL/ min and the heating rate of 10 °C/min. Dye Removal. The dye solutions with different concentrations (50, 100, 200, and 400 mg/L) were produced with dissolving ethyl violet and basic blue dyes in deionized water in 50 mL centrifugal tubes. Similarly, a 3 g/L solution of OL1 was made. Different amounts of OL1 solution were added to the dye solutions in centrifugal tubes. Each centrifugal tube was placed in a water bath shaker for 10 min at

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RESULTS AND DISCUSSION Optimization of Nitric Acid Treatment. Reaction Scheme. Figure 1 shows the reaction scheme of the oxidation of lignin with nitric acid. In softwood, greater than 95% of

Figure 1. Oxidation reaction scheme and resulting products for softwood lignin.31 C

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ACS Sustainable Chemistry & Engineering phenyl propane units are gauiacyl units.26 The oxidation of lignin creates more active sites on the aliphatic part of lignin. Under severe acidic conditions, the lignin segments may decompose and form low molecular weight lignin fragments,27−29 such as vanillin (4-hydroxy-3-methoxybenzaldehyde) and in lesser amounts acetovanillone (4′-hydroxy-3′methoxyacetophenone).26 Figure 1 also illustrates the potential products that may be produced as a result of lignin reacting with nitric acid, which includes the addition of aromatic nitro groups (NO2) and in the case of excess nitric acid, the addition of nitrate ester groups (ONO2) to lignin.4,8,30,31 Effect of Nitric Acid Concentration. Figure 2 shows the charge density and solubility of OL as a function of nitric acid

Figure 3. Effect of temperature on the charge density and solubility of lignin. Reaction conditions were 30% wt HNO3 concentration and 1 h.

dissolved oxygen and found that, as temperature increased, the degradation of lignin accelerated. The carboxylate group analysis of OL showed a decrease from 2.0 to 1.2 mmol/g when the temperature was increased from 80 to 100 °C. It is proposed that the degradation of OL was promoted at a higher temperature than 80 °C, which was supported by the decrease in the carboxylate group content. Effect of Reaction Time. Figure 4 shows the effect of time on the charge density and solubility of lignin, which was

Figure 2. Effect of HNO3 concentration on the charge density and solubility of lignin. Reaction conditions were 100 °C and 1 h.

concentration. An observation was that the solubility of ligninbased product was independent of nitric acid concentration, but the charge density of the product was at a maximum in 40 wt % concentration. It was previously reported that an increase in the solubility of coal (i.e., an insoluble organic compound) was observed with an increase in the nitric acid concentration from 20% to 30%.9 For the same lignin samples, the carboxylate group increased from 1.2 to 2.2 mmol/g with nitric acid concentration varying from 30 to 40 wt % implying that the increase in the charge density is related to the addition of carboxylate group to lignin (up to a 40 wt % nitric acid concentration). The carboxylate group concentration of lignin treated with 50 wt % nitric acid was 0.80 mmol/g, which would imply that nitric acid with the concentrations of greater than 50% might not have oxidized lignin but rather decomposed it. Therefore, the decrease in the charge density may be due to the decomposition of lignin at a very high nitric acid concentration.4 Alternatively, the decrease in charge density can be due to the addition of nitro groups onto the lignin by the excess amount of nitric acid (at 50 wt %).4,8,30 Although 40% nitric acid treatment provided a higher charge density, the exploitation of 40% nitric acid in the experimental setup was impractical (and may not be industrially attractive), and thus, 30% nitric acid treatment was selected for further studies. In another study on the nitric acid treatment of coal, 20−30 wt % nitric acid concentration was selected as an appropriate concentration for oxidizing coal.9 Effect of Temperature. Figure 3 demonstrates the impact of reaction temperature on the solubility and charge density of lignin. It was observed that the charge density of OL was at a maximum, and the solubility reached 100% when the reaction was conducted at 80 °C; this temperature was selected as an optimal temperature. Kindsigo and Kallas3 studied the effect of temperature on the degradation of alkali lignin in water using

Figure 4. Effect of reaction time on the charge density and solubility of lignin. The reaction conditions were 30 wt % HNO3 concentration and 80 °C temperature.

conducted under the conditions of 80 °C and 30 wt % nitric acid concentration. It is observable that the charge density increased from 1 to 3 mequiv/g by prolonging reaction time to 2 h while the lignin remained soluble over this period. Since the charge density reached a plateau at 1.5 h, this reaction time was selected as the optimum. The properties of OL samples produced under different conditions are presented in Table 1. As can be seen, by increasing temperature, or concentration of nitric acid, the phenolic and carboxylate groups as well as the molecular weight of OL were reduced, which clearly showed the degradation of OL samples under serve conditions. These results are in harmony with the results in Figures 3 and 4. In opposition to carboxylate group content of lignin, the phenolic group content of lignin was affected by the nitric acid treatment to a less extent. The OL1 sample selected for further characterization and application in dye removal was produced under the conditions of 30% nitric acid concentration, 80 °C, and 1.5 h treatment. This OL sample, which is denoted as OL1, had a charge density of 3.02 mequiv/g, carboxylate group content of 2.9 mmol/g, and solubility of 97% (at a 1 wt % lignin concentration). It should be noted that only 60 wt % of UL was soluble at pH 12.5, whereas 97.5 wt % of OL1 was soluble at D

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ACS Sustainable Chemistry & Engineering Table 1. Properties of Soluble UL and OL Samples sample UL OL1 OL2 OL3 OL4

temp, °C 80 80 100 100

time, h 1.5 1 1 1

HNO3 conc, wt %

Mw, g/mol

solubility, wt %

phenolic group, mmol/g

carboxylate group, mmol/g

30 30 30 50

14 400 6270 12 500 6400 5120

60 97 100 95 100

0.6 0.9 0.8 0.7 0.8

0.6 2.9 2.0 1.2 0.8

Figure 5. FTIR spectra of unmodified (UL) and modified lignins (OLs).

Figure 6. Weight loss and weight loss rate of UL and OL1 conducted under N2 with the flow rate of 30 mL/min and a heating rate of 10 °C/min. The solid line shows the weight loss while the dashed lines show the weight lost rate.

pH 12.5. The insoluble parts of UL probably had a higher molecular weight; thus, analysis (including the phenolic and carboxylate group amounts) of the insoluble part of UL was excluded from this study due to the insolubility of UL. It should be stated that lignin is treated with nitric acid to produce OL1, and the produced OL1 is separated from the reaction medium using membrane dialysis. If produced at large scales, the nitric acid should be recovered and recycled to the reaction process, which may be conducted with distillation. OL1 can be separated from the solution using an ultrafilter at industrial scales. Other liquid wastes generated in this process

should be treated in the wastewater treatment systems while any gaseous wastes should be either incinerated or scrubbed. Detailed environmental and economic studies should be conducted prior to the industrial implementation of OL1. FTIR Analysis. The FTIR spectra of UL and OL samples are shown in Figure 5. The broad peak at 3400 cm−1 in UL and OL1 is due to OH stretching, and the peak at 2934 cm−1 was attributed to CH stretching. The peak at 1600 cm−1 is assigned to CC aromatic functional groups and CO stretching. The peak at 1580 cm−1 was due to benzene ring skeletal vibrations.32−34 The peak at 1350 cm−1 was a result of E

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Figure 7. Removal of (a) ethyl violet and (b) basic blue from dye solutions (salt free) with different concentrations (50, 100, 200, and 400 mg/L) as a function of dosage of OL1.

were due to moisture removal.4,35 In the literature, it was stated that the polymeric structure of lignin generally started to decompose in the temperature range 120−275 °C with the degradation of propanoid side chains.35,36 The separation of these side chains forms formic acid, formaldehyde, carbon dioxide, and water.35 The degradation of major linkages such as β−β and C−C normally starts around 275−350 °C and ends around 480 °C.35,36 Side chains in the UL started to degrade at 210 °C, while the degradation of OL1 started at 150 °C. This means less energy was needed to degrade the OL1 side chains. The degradation of C−C and β−β linkages started at 275 °C for OL1 and at 300 °C for UL. Flocculation Analysis of Cationic Dye. Dosage Effect. It was stated in the literature that the main mechanism in flocculation−coagulation systems with lignin copolymers for dye removal is charge neutralization.21,22 Charge neutralization would occur between the anionic charges of OL and cationic charges of dyes.22 Figure 7 shows the impact of the dosage of OL1 in removing ethyl violet and basic blue from simulated dye solutions. Generally, there was a maximum in dye removals with respect to OL1 concentration, regardless of the dye concentration.21,22,37 By adding OL1 to dye solutions, OL1

NO stretching, which was caused by the addition of nitro groups (nitrate ester group RONO2) to lignin.4,35 Furthermore, the increase in the intensity of the peak for CO implies an increase in carboxylate groups (UL vs OL1). It should be stated that all OL samples had OH groups (OH of phenolic and carboxylate groups in Table 1), but as the spectra were stretched in Figure 5, the bands in the range 3400−3000 cm−1 corresponding to OH were not obvious. As seen, UL did not have any peak at 1350 cm−1 (NO stretching), but the intensity of the peak increased when the concentration of nitric acid increased from 30 to 50 wt %. The possible addition of nitro groups to the lignin (OL3 vs OL1) and increased oxidation (resulting in a higher number of carbonyl groups) might be reasons for a decrease in the charge density of samples in Figure 2. TGA Analysis. The thermal behavior of unmodified lignin (UL) and modified lignin (OL1) is illustrated in Figure 6. UL degraded completely above 570 °C, while 41 wt % of the OL1 remained after it reached a temperature that was higher than 750 °C. It is possible that some inorganic salts remained in the final OL1 product, which did not decompose at 750 °C. The peaks below 120 °C in the weight loss rate for UL and OL1 F

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Figure 8. Relationship between optimum concentration of OL1 and the concentration of ethyl violet and basic blue in solutions conducted experimentally and theoretically.

blue dye solutions were almost half of the amount needed experimentally. The removals of COD from the dye solutions with 50, 100, 200, and 400 mg/L concentrations at the optimum dosages of OL1 are shown in Figure 9. The COD removals were 60.6%,

interacted with dye components in solutions and formed large flocs, and these flocs were settled and thus removed from the solutions.22 When the dosage of OL1 was higher than the optimum, more OL1 (than required stoichiometrically) would interact with dye components and form flocs, which would have a net anionic charge density.22,37 Furthermore, there would be some uninteracted OL1 in the solutions when the dosage of OL1 was very high. The anionically charged flocs and uninteracted OL1 would generate repulsive force in the solutions, and this would stabilize the formed flocs in the solutions.22,37 The restabilization of flocs was the reason for the decrease in dye removal when the dosage of OL1 was relatively high.22,36 The highest removals for ethyl violet with 50, 100, 200, and 400 mg/L concentrations were 71%, 77%, 78%, and 71%, respectively. The highest removals for basic blue with 50, 100, 200, and 400 mg/L concentrations were 76%, 88%, 93%, and 95%, respectively. In the past, lignin-based products were also used for removing simulated dyes from wastewater effluents. In one study, lignin-based copolymer (cationic polyelectrolyte) with the charge density of 2.55 mequiv/g and molecular weight of 6143 g/mol was used as a flocculant, and more than 95 wt % of three anionic dyes (acid black 1, reactive red 2, and direct red 23) were removed from simulated dye solutions with the concentrations of 50 mg/L and 250 mg/ L at 6.5 pH in the absence of salt.22 It is also seen in Figure 7 that OL1 removed more basic blue than ethyl violet from the solutions, but to reach the maximum removal, more OL1 was used in basic blue solutions (than in ethyl violet solutions). Also, ethyl violet was more sensitive than basic blue to the dosage of OL1. The difference in behavior of OL1 in dye solutions is related to the altered structures of ethyl violet and basic blue dyes, but further analysis is required to confirm this hypothesis. On the basis of the charge densities and the concentrations of the dye and OL1 in solutions, the theoretical amounts required for obtaining the maximum dye removals via charge neutralization were calculated and plotted in Figure 8. In this figure, the experimental values that generated the highest dye removal in Figure 7 were also plotted. The analysis showed that there was a linear correlation between the concentration of dye in solutions and the optimum dye removals. Fang et al.22 also claimed a direct relationship between the dye and lignin-based flocculant concentrations. Figure 8 shows that the theoretical amounts of OL1 needed to neutralize ethyl violet and basic

Figure 9. COD removals from ethyl violet and basic blue dye solutions with the different concentrations. The concentrations of OL1 in this analysis were at the optimum shown in Figure 7.

70.0%, 71.7%, and 71.5% for ethyl violet and 68.4%, 86.1%, 86.0%, and 80.5% for basic blue with 50, 100, 200, and 400 mg/ L concentrations, respectively. In one report, 10 or 30 mg/L of aminated lignin concentrations caused 62.9% or 90.4% of COD removals as well as 95.5% or 96.2% of Congo red and Eriochrome blue dye removals, respectively.21 Alternatively, 89%, 96%, and 94% of COD removals, and 97% of Acid black, 98% of Reactive Red, and 99% of Direct Red dye removals were obtained via having 75, 50, and 35 mg/L concentrations of hydroxymethylated lignin in solutions, respectively.22 Effect of pH on Dye Removal. Figure 10 shows the removals of ethyl violet and basic blue dye at a 100 mg/L concentration, when OL1 was added at 135 in ethyl violet and at 313 mg/L in basic blue solutions as a function of pH. It is observable that OL1 was more effective in the pH range 5−11. A study on the use of a lignosulfonate-based flocculant was pH dependent for cationic dye removals in that the removal decreased below pH 6, and the efficiency of the flocculant was only half at pH 4 than at pH 7. The pH of the solution affects the charge density of OL1 because lignin contains hydroxyl and carboxylate groups, and these groups are deprotonated in alkaline solutions.39 The deprotonation of the hydroxyl and carboxylate groups causes an G

DOI: 10.1021/acssuschemeng.5b01129 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 10. Dye removal as a function of pH of dye solutions (100 mg/L). OL1 was added at the concentrations of 135 mg/L in ethyl violet and of 313 mg/L in basic blue.

that when the amount of acid or temperature was high, more nitro groups were grafted on lignin, while carboxylate group was not attached to lignin. However, when the amount of acid or temperature was low, more carboxylate group was grafted to lignin. The TGA analysis confirmed that the oxidation of lignin improved its thermal behavior. The results also showed that the highest dye and COD removals were 77% and 70% for ethyl violet and 88% and 86% for basic blue, respectively, for the dye solutions with the concentration of 100 mg/L. The removal of dyes was pH dependent, and salt slightly hampered the efficiency of OL1 in the removal of basic blue from the solution.

increase in the negative charge density of lignin, which causes an increase in the electrostatic interactions between the groups and the cationic dye particles.38,39 Therefore, there would be an increase in dye removal with the deprotonation of the hydroxyl and carboxylate groups in the OL1. Effect of Inorganic Salt on Dye Removal. Inorganic salts may exist in the wastewater effluents of the textile industry, which may hinder the performance of OL1 in dye removal.22 In this set of experiments, the impact of NaCl at 0.03 and 3 g/L concentrations on dye removals was assessed. In the presence of 3 g/L salt, the efficiency of OL1 in removing basic blue decreased from 84% to 77%; however, the addition of salt had an insignificant effect on the flocculation of ethyl violet (i.e., drop from 77% to 75%). The difference in impact of salt on dye removal is most likely due to the different structures of these dyes, but further analysis is needed to prove this hypothesis. The reason for the decreased removal in basic blue may be the screening of charged groups of OL1 and dye in the solution.39,40 It has been previously stated that when the mechanism of flocculation is only electrostatic (i.e., charge neutralization) and/or the charge density of the two materials are low, the interaction between these materials is in the screening reduced regime in the presence of salts.40 In the screening reduced regime, small salt ions compete with polymers having similar charges to form flocs with oppositely charged materials.39 Fatehi and Xiao39 observed a similar decrease in the adsorption of cationic-modified polyvinyl alcohol on pulp fibers when the ionic strength of the solution was increased with NaCl.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 807-343-8697. Fax: 807-346-7943. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Canada Research Chair, Canadian Foundation for Innovation, and NSERC programs and Northern Ontario Heritage Fund Corporation for supporting this research.

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REFERENCES

(1) Yuan, Z.; Browne, T. C.; Zhang, X. Biomass fractionation process for bioproducts. US Patent 2011, 0143411 A1. (2) Alam, M. N.; van de Ven, T. G. M. An aqueous process for the separation of cellulose from biomass. J-FOR 2014, 4 (3), 22−26. (3) Kindsigo, M.; Kallas, J. Degradation of lignins by wet oxidation: model water solutions. Proc. Estonian Acad. Sci. Chem. 2006, 55, 132− 144. (4) Meister, J. J. Modification of lignin. J. Macromol. Sci., Polym. Rev. 2002, C42, 235−289. (5) Pang, Y. X.; Qiu, X. Q.; Yang, D. J.; Lou, H. M. Influence of oxidation, hydroxymethylation and sulfomethylation on the phys-

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CONCLUSIONS The selected conditions for the nitric acid oxidation of lignin used in this study were 30 wt % concentration, 80 °C temperature, and 1.5 h, which generated OL1 with the charge density of 3.02 mequiv/g, carboxylate group content of 2.9 mmol/g, and solubility of 97% (at a 1 wt % lignin concentration). The titration and FTIR analyses confirmed H

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ACS Sustainable Chemistry & Engineering icochemical properties of calcium lignosulfonate. Colloids Surf., A 2008, 312, 154−159. (6) Villar, J. C.; Caperos, A.; Garcia-Ochoa, F. Oxidation of hardwood kraft-lignin to phenolic derivatives. Nitrobenzene and copper oxide as oxidants. J. Wood Chem. Technol. 1997, 17, 259−285. (7) Villar, J. C.; Caperos, A.; Garcia-Ochoa, F. Oxidation of hardwood kraft-lignin to phenolic derivatives with oxygen as oxidant. Wood Sci. Technol. 2001, 35, 245−255. (8) Morrow, L. R.; DaGue, M. G.; Whittington, L. E. Alkylated oxidized lignins as surfactants. US Patent 1988, 4739041. (9) Alvarez, R.; Clemente, C.; Gómez-Limón, D. The influence of nitric acid oxidation of low rank coal and its impact on coal structure. Fuel 2003, 82, 2007−2015. (10) Tchoul, M. N.; Ford, W. T.; Lolli, G.; Resasco, D. E.; Arepalli, S. Effect of mild nitric acid oxidation on dispersability, size, and structure of single-walled carbon nanotubes. Chem. Mater. 2007, 19, 5765− 5772. (11) Javor, T.; Buchberger, W.; Faix, O. Capillary electrophoretic determination of lignin degradation products obtained by permanganate oxidation. Anal. Chim. Acta 2003, 484, 181−187. (12) Laurichesse, S.; Averous, L. Chemical modification of lignins: Towards biobased polymers. Prog. Polym. Sci. 2014, 39, 1266−1290. (13) Gupta, V. K.; Suhas. Application of low-cost adsorbents for dye removal − A review. J. Environ. Manage. 2009, 90, 2313−2342. (14) Annadurai, G.; Juang, R. S.; Lee, D. J. Use of cellulose-based wastes for adsorption of dyes from aqueous solutions. J. Hazard. Mater. 2002, 92, 263−274. (15) Ó rfão, J. J. M.; Silva, A. I. M.; Pereira, J. C. V.; Barata, S. A.; Fonseca, I. M.; Faria, P. C. C.; Pereira, M. F. R. Adsorption of a reactive dye on chemically modified activated carbonsinfluence of pH. J. Colloid Interface Sci. 2006, 296, 480−489. (16) Tahir, S. S.; Rauf, N. Removal of a cationic dye from aqueous solutions by adsorption onto bentonite clay. Chemosphere 2006, 63, 1842−1848. (17) Tang, C.; Chen, V. Nanofiltration of textile wastewater for water reuse. Desalination 2002, 143, 11−20. (18) Kharub, M. Use of various, technologies, methods and adsorbents for the removal of dye. J. Environ. Res. Dev. 2012, 6, 879−883. (19) Meriç, S.; Kaptan, D.; Ö lmez, T. Color and COD removal from wastewater containing Reactive Black 5 using Fenton’s oxidation process. Chemosphere 2004, 54, 435−441. (20) Joo, D. J.; Shin, W. S.; Choi, J. H.; Choi, S. J.; Kim, M. C.; Han, M. H.; Ha, T. W.; Kim, Y. H. Decolorization of reactive dyes using inorganic coagulants and synthetic polymer. Dyes Pigm. 2007, 73, 59− 64. (21) Wang, X.; Zhang, Y.; Hao, C.; Dai, X.; Zhou, Z.; Si, N. Ultrasonic-assisted synthesis of aminated lignin by a Mannich reaction and its decolorizing properties for anionic azo-dyes. RSC Adv. 2014, 4, 28156−28164. (22) Fang, R.; Cheng, X. S.; Xu, X. R. Synthesis of lignin-base cationic flocculant and its application in removing anionic azo-dyes from simulated wastewater. Bioresour. Technol. 2010, 101, 7323−7329. (23) Choudhury, A. K. R. Textile Preparation and Dyeing; Science Publishers: Enfield, 2008; pp 491−804. (24) Kushwaha, A. K.; Gupta, N.; Chattopadhyaya, M. C. Removal of cationic methylene blue and malachite green dyes from aqueous solution by waste materials of Daucus carota. J. Saudi Chem. Soc. 2014, 18, 200−207. (25) Crini, G. Kinetic and equilibrium studies on the removal of cationic dyes from aqueous solution by adsorption onto a cyclodextrin polymer. Dyes Pigm. 2008, 77, 415−426. (26) Wong, Z.; Chen, K.; Li, J. Formation of vanillin and syringaldehyde in an oxygen delignification process. BioRes. 2010, 5, 1509−1516. (27) Ouyang, X. P.; Ke, L. X.; Qiu, X. Q.; Guo, Y. X.; Pang, Y. X. Sulfonation of alkali lignin and its potential use in dispersant for cement. J. Dispersion Sci. Technol. 2009, 30, 1−6.

(28) Santos, R. B.; Hart, P. W.; Jameel, H.; Chang, H. M. Wood based lignin reactions important to the biorefinery and pulp and paper industries. BioResources 2012, 8, 1456−1477. (29) Wu, H.; Chen, F.; Feng, Q.; Yue, X. Oxidation and sulfomethylation of alkali-extracted lignin from corn stalk. BioRes. 2012, 7, 2742−2751. (30) Wool, R. P. Lignin polymers and composites. In Bio-based Polymers and Composites; Wool, R. P., Sun, X. S., Eds.; Elsevier: London, 2005; pp 155−193. (31) Sobolev, I. Lignin model compounds, nitric acid oxidation of 4methylguaiacol. J. Org. Chem. 1961, 26, 5080−5085. (32) Pandey, K. K. A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy. J. Appl. Polym. Sci. 1999, 71, 1969−1975. (33) Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 3rd ed.; Harcourt Inc., 2001; pp 13−101. (34) Sahoo, S.; Seydibeyoglu, M. O.; Mohanty, A. K.; Misra, M. Characterization of industrial lignins for their utilization in future value added applications. Biomass Bioenergy 2011, 35, 4230−4237. (35) Fenner, R. A.; Lephardt, J. O. Examination of the thermal decomposition of kraft pine lignin by Fourier transform infrared evolved gas analysis. J. Agric. Food Chem. 1981, 29, 846−849. (36) Brebu, M.; Vasile, C. Thermal degradation of lignina review. Cell. Chem. Technol. 2010, 44, 353−363. (37) Wang, S.; Hou, Q.; Kong, F.; Fatehi, P. Production of cationic xylan−METAC copolymer as a flocculant for textile industry. Carbohydr. Polym. 2015, 124, 229−236. (38) Liu, M. H.; Huang, J. H. Removal and recovery of cationic dyes from aqueous solutions using spherical sulfonic lignin adsorbent. J. Appl. Polym. Sci. 2006, 101, 2284−2291. (39) Fatehi, P.; Xiao, H. Adsorption characteristics of cationicmodified poly (vinyl alcohol) on cellulose fibers−A qualitative analysis. Colloids Surf., A 2008, 327, 127−133. (40) Van de Steeg, H. G.; Cohen Stuart, M. A.; De Keizer, A.; Bijsterbosch, B. H. Polyelectrolyte adsorption: a subtle balance of forces. Langmuir 1992, 8, 2538−2546.

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DOI: 10.1021/acssuschemeng.5b01129 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX