Oxidation of Kraft Lignin with Hydrogen Peroxide and its Application

The production of oxidized kraft lignin and its application as a green dispersant will help ..... At this stage, the operation cost of OKL cannot be d...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10597-10605

Oxidation of Kraft Lignin with Hydrogen Peroxide and its Application as a Dispersant for Kaolin Suspensions Wenming He, Weijue Gao, and Pedram Fatehi*

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Chemical Engineering Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada, P7B 5E1 ABSTRACT: Lignin is an underutilized byproduct of pulping and cellulosic ethanol production plants. However, if utilized efficiently, it can facilitate the development of sustainable processes. In this work, oxidized kraft lignin (OKL) was prepared via treating kraft lignin (KL) with hydrogen peroxide, an environmentally friendly and industrially attractive oxidizing agent, under alkaline conditions. The oxidized kraft lignin with a carboxylate group content of 1.53 mequiv/g was obtained under the optimal oxidation conditions of 80 °C, 2 h treatment, at a 0.77 molar ratio of NaOH/H2O2, and 2.85 molar ratio of H2O2/lignin, which was then employed as an anionic dispersant for kaolin suspensions. The zeta potential, particle size, and specific surface area as well as the relative turbidity and flocculation index of the kaolin suspension were affected by the pH of the suspension. By increasing the dosage of OKL to 40 mg/L, the relative turbidity of the suspension was increased to 1.18 at pH 5 and the kaolin concentration of 4 g/L, which made its performance superior to that of commercially produced lignosulfonate. KEYWORDS: Oxidized kraft lignin, Hydrogen peroxide, Dispersant, Kaolin suspension, Relative turbidity

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INTRODUCTION The pulp and paper industry has faced significant challenges in recent years due to global competition and the low price of pulp products. Forest biorefining is considered as the conversion of underutilized raw materials of the pulping industry to value-added chemicals such as polymers,1,2 carbon fibers,3 and biodiesel4 in addition to the traditional pulp and paper products. Forest biorefining is believed to promote the profitability and global competitiveness of the pulping industry. Lignin is a complex, natural, and three-dimensional phenylpropanoid polymer mainly linked by ether bonds.5 Currently, lignin is either burnt in pulping processes or treated in wastewater systems, which introduces extra environmental and operational challenges to the pulping processes.6 However, lignin has been recognized as a potential raw material for preparing high-value products. Lignosulfonate obtained from the spent liquor of the sulfite pulping process has been used as a dispersant in the past.7 However, the supply of lignosulfonate has significantly decreased as the production of sulfite pulp has been substantially reduced in many countries.8 Alternatively, kraft lignin (KL) can be modified and used for the production of value-added products.9 Since KL is a water insoluble product, it has a limited practical application in industry.10 In this regard, the oxidation of lignin has been practiced to increase its water solubility11−13 via various oxidation pathways, i.e., nitric acid,13 metal oxides,14 nitrobenzene,14 and oxygen with catalyst.15 However, these methods usually require a high temperature and pressure and may result in many undesirable byproducts. Hydrogen peroxide is widely available in pulp mills and extensively used for bleaching pulp worldwide. As an oxidant, it © 2017 American Chemical Society

can also be used for oxidizing lignin to introduce carboxylate groups.16 A study on the oxidation of hydrolyzed nonwood lignin showed that hydrogen peroxide (H2O2) treatment of lignin can increase its carboxylate group and thus its water solubility.17 In another report, the black liquor of Caribbean Pine wood species was oxidized with hydrogen peroxide in the presence of ferrous ion.18 Since nonwood and wood species have different properties, the value-added products generated from these wood species would respond differently to chemical reactions. Therefore, the results reported on the hydrogen peroxide of nonwood species may not be achieved for kraft lignin of softwood species. One objective of this study was to investigate how hydrogen peroxide would impact the properties of softwood kraft lignin. The stabilization of particles in suspensions is important in many processes, including the mining,19 which depends on particle size, type, pH, and concentrations of the particles.20 Synthetic polymers such as polyacrylates and polystyrenesulfonates were introduced as dispersants for particles in suspensions in the past.21 These synthetic polyelectrolytes, however, are nonbiodegradable and may cause filtration problems (blocking).22 Natural polyelectrolytes, such as starch, cellulose, and guar gum, have also been considered to be used as dispersants. As the polymers adsorb onto particles and introduce steric and electrostatic repulsions between the particles, it leads to the stabilization of colloidal suspensions.23 Received: July 28, 2017 Revised: September 11, 2017 Published: October 9, 2017 10597

DOI: 10.1021/acssuschemeng.7b02582 ACS Sustainable Chem. Eng. 2017, 5, 10597−10605

Research Article

ACS Sustainable Chemistry & Engineering ⎛ mmol ⎞ carboxylate group ⎜ ⎟ ⎝ g ⎠

However, some of these natural polymers can be used as food, and some others are widely used in the production of other products, such as composites, papers, and boards. The production and application of lignin-based dispersants have not been thoroughly investigated and, in fact, is the second objective of this paper. The work presented herein focused on the oxidation of KL with hydrogen peroxide in order to prepare oxidized kraft lignin (OKL). The structural changes of KL induced by hydrogen peroxide treatment were investigated, as well. Furthermore, the impact of OKL on the dispersion of clay particles was studied under different conditions for the first time. The main novelty of this work was the assessment of producing and applying OKL as an anionic dispersant for a clay suspension. In other words, this paper shows a method to utilize an environmentally friendly and industrially attractive chemical, H2O2, to oxidize kraft lignin to produce a water-soluble product that can be used as a green dispersant. The production of oxidized kraft lignin and its application as a green dispersant will help develop sustainable processes for exploiting the potential of lignin use in different industries.

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Materials. Softwood kraft lignin was produced via pilot plant facilities of FPInnovations that uses LignoForce technology in Thunder Bay, Ontario.24 Hydrogen peroxide solution (30%), polydimethyl diallyl ammonium chloride (PDADMAC), potassium hydroxide, sodium nitrate, para-hydroxybenzoic acid, hydrochloric acid (0.1M), acetic anhydride, pyridine, tetrahydrofuran (THF), lignosulfonate, and kaolin particles were all purchased from Sigma-Aldrich company and used as received. Membrane dialysis with the molecular weight cut off of 1000 g/mol was received from Spectrum Laboratories company. Preparation of OKL. The KL was oxidized under different reaction temperatures (60−100 °C) and times (1−3 h) with sodium hydroxide and hydrogen peroxide solutions using a 500 mL three-neck flask equipped with a stirrer and a thermometer. After oxidation reaction, the solution was cooled to ambient temperature and the pH was adjusted to 7 with 4 M HCl. The oxidized kraft lignin (OKL) was dialyzed with a membrane dialysis (MW cut off of 1000 g/mol) for 24 h, while exchanging water every 12 h, and then dried in an oven overnight at 105 °C. Box−Behnken Experimental Design. Box−Behnken design (BBD) was employed as a statistical tool to investigate the impact of process parameters on the properties of OKL. The preliminary studies with several variables using one-variable-at-a-time approach revealed the importance of time, temperature, NaOH/H2O2 molar ratio, and H2O2/lignin molar ratio on the oxidation reaction of KL. This approach was employed to study the impact of nitric acid treatment on the properties of oxidized lignin in the past.25 Carboxylate and Phenolate Group Analysis. The charge density of OKL and lignosulfonate was measured using a Mütek particle charge detector (PCD-04, Germany) according to the procedure described previously.3 The carboxylate and phenolate groups of KL and OKL were measured with a potentiometric titration using an automatic potentiometric titrator (785-DMP Titrino, Metrohm, Switzerland) with 0.1 M HCl standard solution as a titrant. The phenolate hydroxyl group content was determined according to eq 1 and carboxylate group content was calculated according to eq 2:

ns =

V (C0 − C) m

(3)

where C0 and C are the concentrations of OKL in the suspensions before and after treating with clay (g/L), V is the initial volume (L) of the OKL, and m is the weight of clay (g). Particle Size and Specific Surface Area Analysis. The size and specific surface area of clay particles were determined using a laser light diffraction (MasterSizer 2000 particle analyzer). In this study, 2 g of the clay suspension was added to 100 mL of deionized water. A clay

⎛ mmol ⎞ phenolic hydroxyl group ⎜ ⎟ ⎝ g ⎠ ((EP′2 − EP′1) − (EP2 − EP)) ×C 1 m

(2)

where C is the concentration of titrant, HCl (mol/L), EP1, EP2, and EP3 are the consumed volumes of HCl solution (mL) at the first, second, and third end points, respectively, when titrating the control sample (blank solution). EP1′ , EP2′ , and EP3′ are the consumed volumes of HCl solution (mL) at the first, second, and third end points when the OKL sample was titrated, and m is the dried weight of OKL used in the analysis (g). Molecular Weight Analysis. To determine the molecular weight of kraft lignin, the sample was first acetylated. A 100 mg sample of kraft lignin was air-dried and then suspended in 4.0 mL of acetic anhydride/pyridine 1/1 (V/V) solution, which was stirred for 30 min at 300 rpm, 25 °C and then kept in the dark at 25 °C for 24 h. The resulting solution was then poured into an excess amount of ice water (50 mL) and centrifuged/washed 3 times. The solvent was then removed from the mixture under reduced pressure using a freezedryer. The acetylated KL was then dissolved in 10 mL of THF stirring at 300 rpm for 30 min at room temperature and then filtered using a PTFE filter (13 mm diameter and 0.2 μm pore size). The filtered samples were then used for molecular weight analysis. To determine the molecular weight of OKL, 100 mg of oven-dried OKL was dissolved in 10 mL of 0.1 mol/L NaNO3 solution and filtered by 0.2 μm nylon filter (13 mm diameter). The molecular weight of KL and OKL was determined using a gel permeation chromatography (GPC), Malvern GPCmax VE2001 Module + Viscotek TDA305 with multidetectors (UV, RI, viscometer, low angle and right angle laser detectors). For KL measurement, the organic columns of PolyAnalytic PAS106M, PAS103, and PAS102.5 were used, and HPLC-grade tetrahydrofuran (THF) was used as the solvent and eluent. The flow rate was set at 1.0 mL/min. For OKL, the columns of PolyAnalytic PAA206 and PAA203 were used, and 0.1 mol/L NaNO3 solution was used as solvent and eluent. The flow rate was set at 0.70 mL/min. The column temperature was set at 35 °C for both systems. Polystyrene polymers were used as standards for the organic system and poly(ethylene oxide) for the aqueous system. FTIR Analysis. A Fourier transform infrared (FTIR) analysis of KL and OKL was carried out at room temperature. The samples were dried in an oven at 105 °C overnight. A 0.05 g sample of KL and OKL was used for analysis by a Bruker Tensor 37 FT-IR (Germany, ATR accessory). The spectra were recorded in transmittance mode in the range of 600 to 4000 cm−1 with 4 cm−1 resolution, and 32 scans per sample were conducted. Adsorption of OKL on Kaolin Particles. A 0.6 g sample of clay was placed into an Erlenmeyer flask (125 mL). OKL aqueous solutions (10 mL) with varying concentrations (0.1−0.7 g/L) were prepared and added to the flask to generate different concentrations of OKL in the clay suspension. The suspensions were then stirred at 150 rpm for 120 min at 30 °C and then centrifuged for 10 min at 3000 rpm using a Thermofisher centrifuge (Sorvall ST16). The supernatants were then collected and the concentration of OKL in the supernatants was determined using an already stablished calibration curve at 280 nm by a UV/vis spectrophotometer (Genesys 10S UV−vis, Thermo Fisher scientific). The adsorption of OKL on the surface of clay particles (ns) was determined according to eq 3:26

MATERIALS AND METHODS

=

((EP′3 − EP′2) − (EP3 − EP2)) × C m

(1) 10598

DOI: 10.1021/acssuschemeng.7b02582 ACS Sustainable Chem. Eng. 2017, 5, 10597−10605

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ACS Sustainable Chemistry & Engineering suspension of 100 g/L in deionized water was used as a dispersive medium in this analysis.27,28 Zeta Potential Analysis. A 0.5 g sample of clay suspension (100 g/L) was added to 100 mL of deionized water and incubated in water bath shaker at 30 °C and 150 rpm for 2 h. After the incubation, the zeta potential of the samples was analyzed in the presence and absence of OKL (50 mg/g) at varying pH values (2−6) using a compact automatic zeta meter, Laval laboratories Inc.29 All the measurements were carried out at room temperature with a constant electric field of 8.4 V/cm. The pH of the solution was adjusted using 0.1 M HCl or 0.1 M NaOH. Dispersion Analysis. A clay suspension of 100 g/L was used as a model sample in this work. The dispersion analysis was conducted using a photometric dispersion analyzer (PDA 3000, Rank Brothers Ltd.), which was connected to a dynamic drainage jar (DDJ) fitted with a 70 mm mesh screen.30 Based on the variations in the direct current (DC) voltage of the PDA instrument, the dispersion of the clay particles in the aqueous system was measured.31 In this set of experiments, 500 mL of distilled water were added into the DDJ container. The initial base DC voltage of the instrument was recorded as V0. Then, a certain amount of a 20 wt % clay dispersion was added into DDJ to make a 0.1−0.5 wt % of kaolin concentration. V0 was reduced to the voltage of Vi after adding the clay suspension. After adjusting the pH, a predetermined amount of OKL (4−20 mg/g) was added into the DDJ, and the final suspension DC voltage (Vf) was recorded 300 s after OKL addition. The DDJ was then kept at 500 rpm and the suspension was circulated through the PDA at a flow rate of 38 mL/min by a peristaltic pump. The relative turbidity (τr) was determined via considering the turbidity of the suspension after adding OKL and that of clay suspension before adding OKL according to eq 4:32

τr =

ln(V0/Vf ) τf = τi ln(V0/Vi )

Figure 1. Oxidation scheme of KL (R is H or OCH3).

would promote the solubility of KL.41 The hydrogen peroxide treatment may generate several different products; however, the analysis of these products is out of the scope of this study. Optimization Conditions of Oxidation. The impact of hydrogen peroxide reaction on the charge density of OKL is shown in Figure 2. As seen, by increasing the NaOH/H2O2 molar ratio, the charge density of OKL is increased. A higher pH generates more perhydroxyl anions, which promotes the oxidation efficiency of hydrogen peroxide. However, when the amount of NaOH is very high in the reaction medium, it promotes the decomposition of hydrogen peroxide,41 which accounts for the lower charge density of OKL at very high NaOH/H2O2 ratios (e.g., 0.85 mol/mol). The results also showed that the temperature of 80 °C was the optimum for obtaining the maximum charge density. As the hydrogen peroxide oxidation is an endothermic reaction, the higher temperature increased the reactivity of lignin, accelerated the oxidation reaction and enhanced the rate of lignin depolymerization.42 Meanwhile, at a higher temperature, e.g. 90 °C, the decomposition of hydrogen peroxide into hydroxyl radicals and other side reactions becomes more likely.41 In one study, more than 80% of kraft lignin was degraded in hydrogen peroxide treatment at 110 °C and pH 11.34 Furthermore, by increasing the H2O2/lignin ratio, the charge density of OKL was increased, which may be due to the fact that more hydrogen peroxide would generate more perhydroxyl anions that can facilitate the oxidation of KL. It is apparent from the results in Figure 2 that time had a marginal effect on the oxidation, which may imply that the oxidation is a fast process and time extension may minimally promote the oxidation. The surface response analysis predicted that the highest charge density of 2.26 mequiv/g would be obtained for OKL under the conditions of 79.88 °C, time of 2.18 h, NaOH/H2O2 molar ratio of 0.79, and H 2 O 2 /lignin molar ratio of 2.85. Experimentally, a charge density of 2.22 mequiv/g was obtained for OKL that was produced under the conditions of 80 °C, 2 h, NaOH/H2O2 ratio of 0.77 mol/mol, and H2O2/lignin ratio of 2.85 mol/mol. Characterization of KL and OKL. Table 1 shows the properties of KL and OKL produced under the optimized conditions as stated above. As can be seen, the charge density of lignin was increased substantially. As stated earlier, kraft lignin is a complex polymer and H2O2 would cleave some of its bonds, decrease its molecular weight and introduce some carboxylate group on lignin’s structure (Figure 1). It is observable in Table 1 that the carboxylate group of lignin was increased by 0.52 mmol/g via H2O2 treatment. As 1 mol of carboxylate group could be ideally generated via reacting 1 mol of H2O2, the theoretical amount of H2O2 required for introducing carboxylate group onto lignin would be 0.52 mmol/g. Assuming 180 g/mol as the average molecular mass of the unit of lignin, the reaction needs an H2O2/lignin molar ratio of 0.09. The results in Figure 2 showed that the optimum

(4)

where τf is denoted as the final dispersion turbidity and τi is denoted as the initial dispersion turbidity. The flocculation index (FI) of the suspension was measured according to eq 5. The PDA monitors the average of transmitted light intensity (DC) and also the root-mean-square fluctuations in transmittance (RMS value) of the suspension.33 The RMS/DC ratio is referred to as a flocculation index (FI) and provides a qualitative measure of the aggregation of particles. The flocculation index increases as coagulation occurs and provides an indication of the rate of coagulation and of the floc size.34 The FI value is strongly correlated to the respective particle size and decreases as the particle size becomes smaller, accordingly. In this set of experiments, after the FI value reached a steady state, the dispersant was added to the suspension and the changes were monitored.35 The FI value was calculated using eq 5. FI =

RMS(V ) DC(V )

(5)

The effect of OKL dosage as well as the pH and concentration of the suspension on the performance of OKL were investigated in this work.

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RESULTS AND DISCUSSION Oxidation of KL. Oxidation Scheme of KL. The oxidation reaction of KL with hydrogen peroxide is shown in Figure 1. Softwood KL is mainly guaiacyl lignin, composed of coniferyl alcohol units.36 In the hydrogen peroxide reaction of KL under alkaline conditions, the perhydroxyl anion cleaves the side chains of KL, opens benzene ring, and produces new compounds, which have carboxylate or chromophore groups.37−40 Under severe conditions, these groups may undergo ring cleavage reaction and further degrade to form a variety of low molecular weight compounds, such as oxalic acid, formic acid, and malonic acid.40 These changes in KL structure 10599

DOI: 10.1021/acssuschemeng.7b02582 ACS Sustainable Chem. Eng. 2017, 5, 10597−10605

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ACS Sustainable Chemistry & Engineering

Figure 2. Response surface design conducted for analyzing the impact of oxidation temperature, time, NaOH/H2O2, and H2O2/lignin molar ratios on the charge density of OKL.

structure via cleavage of ether bonds of KL45 and probably for partial decomposition of H2O2 during reaction.46 Figure 3 shows the FTIR spectra of KL and OKL. The peaks at 1600 and 1510 cm−1 are related to the vibrations of aromatic rings present in KL.47 The relative intensity at 1600 cm−1 on the spectrum of OKL was smaller than that of KL, which is the evidence of lignin decomposition (aromatic cleavage). The relative intensity for the peak at 1375 cm−1,48 belonging to phenolic hydroxyls group of KL, was smaller in the spectrum of OKL than in that of KL, which may further indicate that the phenolic hydroxyl groups of KL were degraded in the oxidation reaction. The relative intensity of the absorption band at 1226 cm−1, which belongs to CO deformation, in the spectrum of OKL was larger than in that of KL, and this indicated the increment in the carboxylate group (−COOH) of KL after oxidation.49 The strong and broad band at around 3370 cm−1 is a characteristic of OH groups of lignin.7 The FTIR results are

Table 1. Functional Groups, Charge Density, and Molecular Weight of KL and OKL sample

phenolic hydroxyl group, mmol/g

carboxylate group, mmol/g

charge density, mequiv/g

Mw, g/mol

Mn, g/mol

KL OKL

1.725 0.892

1.012 1.526

0.970 2.217

16770 14825

13859 11273

molar ratio of H2O2/lignin was 2.85. The analysis revealed that much more hydrogen peroxide was used in the experiment than that theoretically needed for increasing the carboxylate group of lignin by 0.52 mmol/g. It is also observable that the molecular weight of lignin was decreased via H2O2 treatment (Table 1). It was reported that hydrogen peroxide generally decomposed the phenolate group, whereas it introduced carboxylate group to lignin.43,44 This analysis provides evidence for the fact that the majority of H2O2 was used for partially decomposing the lignin 10600

DOI: 10.1021/acssuschemeng.7b02582 ACS Sustainable Chem. Eng. 2017, 5, 10597−10605

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ACS Sustainable Chemistry & Engineering

Figure 3. FTIR spectra of KL and OKL.

well in agreement with the results presented in Table 1 and further proves the changes in the structure of KL. Adsorption Analysis of OKL. Adsorption isotherm of OKL on clay is shown in Figure 4. The adsorption amount of OKL

Figure 5. Effect of pH and OKL on zeta potential of clay suspensions.

effectively increased the repulsion force between the particles and thus improved the performance of clay particles in the suspension. The effect of pH on particle size and specific surface area of kaolin before and after OKL treatment is shown in Figure 6. As

Figure 4. Adsorption isotherms of OKL.

reached saturation of 2.3 mg/g on clay particles. The main driving force for adsorption is the electrostatic interaction between the cationic charges associated with clay and anionic charges (carboxylate group) attached to OKL. In the previous work, the adsorption of nitric acid treated with KL was 3 mg/g on cement particles at pH 12. It was comprehensively discussed in the past that the chemistry of suspensions impacts the adsorption capacities of polymers on particles significantly.50−53 The difference in the adsorption capacities reported in this work and the previous work is due to the differences in the properties of lignin products as well as the chemistry of suspensions (clay suspension vs cement admixture). Zeta Potential Analysis. The zeta potential analysis of clay suspension is presented as a function of pH in Figure 5. It is mainly used for describing the ionic strength of colloidal particles, which is directly related to the stability of particles in the suspensions.54,55 The zeta potential of clay particles decreased from −17 to −37 mV via increasing pH from 2 to 6. The decrease in the zeta potential is due to the presence of more hydroxyl ions in the suspension at higher pHs, and shows that the overall zeta potential of the suspension was more negative. Accordingly, the particles repelled each other more strongly at a higher pH, and formed more stable suspension. The adsorption of OH− ions on clay water interface results in a large diffuse double layer at a higher pH, with a higher zeta potential value.56 The results also depict that the addition of OKL reduced the zeta potential of the clay suspension, regardless of the pH of the suspension. Therefore, OKL

Figure 6. Effect of pH on particle size and specific surface area of clay before and after OKL addition.

shown, the particle size of clay was decreased with the pH increment, but the specific surface area of kaolin was increased with the pH increase. Moreover, the zeta potential of clay suspension was reduced at a higher pH and introduced a more intensive repulsion force between particles.27 This repulsion force prevented clay particles from self-agglomeration57 and produced particles that were smaller and had a higher surface area.32 In other words, particles were more dispersed and less agglomerated at a higher pH.56,58 10601

DOI: 10.1021/acssuschemeng.7b02582 ACS Sustainable Chem. Eng. 2017, 5, 10597−10605

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ACS Sustainable Chemistry & Engineering Effect of Dosage of OKL on Flocculation Index and Relative Turbidity. The effect of dosage of OKL on flocculation index and relative turbidity of clay suspension is shown in Figure 7. Although the flocculation index was large for a clay

achieved.27 The results also show that the OKL was more effective than lignosulfonate in dispersing the clay particles. Effect of pH on Flocculation Index and Relative Turbidity. The effect of pH on the flocculation index and relative turbidity of clay suspension is shown in Figure 8. The carboxylate group

Figure 7. Effect of dosage of OKL on flocculation index and relative turbidity at pH 5, flow rate of 38 mL/min, 500 rpm, and clay concentration of 4 g/L.

Figure 8. Effect of pH on (a) flocculation index and (b) relative turbidity (4 g/L of clay concentration and at an OKL dosage of 20 mg/g).

suspension (initial peaks in Figure 7), it was reduced remarkably via adding OKL to the suspension and became stable after time extension. In the dispersion analysis using the PDA instrument, the flocculation index is directly related to the size of particles in suspensions.34 At the same flow rate and particle concentration, the smaller the particles, the smaller the flocculation index, which would confirm the decrease in the size of particles via adding OKL.34 Furthermore, the relative turbidity of clay suspension increased when the dosage of OKL increased (Figure 7b). As shown previously, a limited amount of OKL can adsorb on the surface of the clay particles (Figure 4). Zaman et al. reported that the negatively charged dispersant adsorbed on the adsorption cites of aluminum oxide particles presented on the edge of kaolin-clay particles and produced a barrier, which might prevent the agglomeration of the particles.59 As the dosage of OKL increased in the suspension, the amount of unadsorbed OKL in the suspension also increased, which introduced a net negative charge in the suspension and thus steric forces between clay particles, while improving their dispersion.58 The high relative turbidity of the suspension suggests that a high degree of dispersion was

is the main contributor to the increased negative charge density of OKL, which facilitates its performance as a dispersant. The pKa of carboxylic acid is around 4.75 above which the disassociation of carboxylate group will occur. This study focused on the dispersion analysis of OKL in the pH range of 2 and 6 to investigate the impact of protonation/deprotonation of carboxylate group of OKL on its dispersion performance. A higher pH than 7 would increase the negative zeta potential of the kaolin suspensions and probably stabilize the suspensions, which would reduce the need for a dispersant. The flocculation index of the clay suspension varied significantly at different pHs (Figure 8a). When OKL was added, the slope of the dispersion process was the lowest at −0.008 s−1 at pH 2, and then increased to around −0.021 s−1 at other pH values, indicating that the dispersion was faster at pH 2 than other higher pH conditions. As explained earlier, the zeta potential of the clay suspension was reduced at a high pH, and this higher zeta potential resulted in a lower flocculation index of the clay suspension in Figure 8a. It is reported that clay (kaoline) particles can have different structures at different pH values, as there is a significant change in charges of the basal plane and the edge of the kaolin particles.60,61 At low pH values (e.g., pH 10602

DOI: 10.1021/acssuschemeng.7b02582 ACS Sustainable Chem. Eng. 2017, 5, 10597−10605

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ACS Sustainable Chemistry & Engineering 4.5), a card-house structure was probably formed on the edge (positively charged) and basal (negatively charged) structure of kaolin particles.61 Thus, the higher interparticle attractions at pH below 5 could be the reason for the higher flocculation index. At higher pH values, card-house structure transforms to a bandlike structure when both basal and edge faces are negatively charged.61 Therefore, the relatively low flocculation index at pH 6 could be induced by more repulsion introduced instead of attraction between particles in the kaolin suspension. After the addition of OKL, the relative turbidity of the suspension increased to a maximum value of 1.18 at pH 5 (Figure 8b), and this is consistent with the zeta potential values at different pHs in Figure 5. The increase in the relative turbidity is in agreement with both particle size and zeta potential measurements. In the past, pH 2.2 was reported to be the point of zero charge (pzc) for kaolin, implying its overall charge reversal from positive to negative.62 At pH 2, the clay suspension was not well dispersed by the anionic dispersant because the surface charge of kaolin was close to zero and the adsorption of OKL on clay did not help. However, the negative charge on the kaolin surface increased with pH. With the addition of OKL, the repulsion between kaolin particles was significantly increased between pH 2 and 5, which is the cause of its improved dispersion. At pH 6, the flocculation index of the suspension without OKL was relatively low (Figure 8a), indicating a well-dispersed suspension. When OKL was added, the relative turbidity was insignificantly increased, as the turbidity of the suspension itself was high (Figure 8b). Effect of Concentration of Suspension on Flocculation Index and Relative Turbidity. The effect of concentration of suspension on flocculation index and relative turbidity of clay suspension is shown in Figure 9. In the absence of OKL, with the increment in the concentration of clay in the suspensions, its flocculation index increased. This behaver is due to the selfassembly of clay particles in suspensions resulting from the interactions between the edges and the basal planes of adjacent particles.63 In the presence of OKL, the flocculation index and relative turbidity were increased with concentrating the suspension. This may be due to the fact that the higher concentration of the suspension, the more the dosage of OKL, and the closer the particles and OKL would be, which would promote the efficiency of OKL.64 Preliminary Economic Analysis. The production costs of OKL can be included as the summation of the raw material and operation costs. At this stage, the operation cost of OKL cannot be determined as this evaluation requires a pilot scale study. However, for producing each ton of OKL under the optimized conditions of NaOH/H2O2 ratio of 0.77 mol/mol and H2O2/ lignin ratio of 2.85 mol/mol; approximately 1.20 tons of 50% H2O2 and 0.49 tons of NaOH would be consumed. The price for kraft lignin, H2O2, and NaOH are approximately $500− 700/ton,65 $50−500/ton,66 and $200−500/ton,67 respectively. Therefore, the estimated raw material costs of OKL could range between $858 and $1545, assuming that no chemical is recycled in this process.69 The unused chemicals and basic/acidic solutions are typically recycled in chemical industries, which reduces the raw material costs, and this recycling may be applicable for OKL production at large scales. In comparison, the price of commercial lignosulfonate, which accounts for 90% of commercial lignin in the market,68 is between $400 and $2000/ton depending on its quality, purity, and applications. Despite uncertainty in the overall production costs of OKL, the preliminary analysis may conclude that it would be possible to

Figure 9. Effect of concentration of kaolin (4 g/L) with OKL dosage of 20 mg/g at pH 5 on the (a) flocculation index and (b) relative turbidity.

produce OKL with an attractive price for some applications, for which quality and performance are crucial factors. However, pilot plant studies are necessary to estimate the production costs of OKL more accurately, which is out of the scope of this study.

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CONCLUSIONS The charge density and carboxylate group of OKL were 2.22 and 1.53 mequiv/g, respectively, which was produced under the conditions of 80 °C, 2 h, NaOH/H2O2 molar ratio of 0.77, and H2O2/lignin molar ratio of 2.85. The reaction reduced the phenolate compounds of KL, while introducing more carboxylate group on KL. The dispersion analysis confirmed that the zeta potential and surface area of particles in the clay suspension increased via OKL addition. The addition of OKL increased the relative turbidity of the clay suspension, and better results were obtained for OKL than commercial lignosulfonate at pH 5. Under the optimal conditions of 4 g/ L concentration of suspension, 40 mg/L of OKL, and pH 5, the relative turbidity was at maximum (1.18).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 807-343-8697. Fax: 807346-7943. ORCID

Pedram Fatehi: 0000-0002-3874-5089 10603

DOI: 10.1021/acssuschemeng.7b02582 ACS Sustainable Chem. Eng. 2017, 5, 10597−10605

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ACS Sustainable Chemistry & Engineering Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank NSERC-Canada, Canada Research Chair, Canada Foundation for Innovation, and Northern Ontario Heritage Fund Corporation for funding this project.

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DOI: 10.1021/acssuschemeng.7b02582 ACS Sustainable Chem. Eng. 2017, 5, 10597−10605