Production and Application of Triblock Hydrolysis Lignin-Based Anionic Copolymers in Aqueous Systems.

Although lignin is currently an under-utilized biopolymer, it has the potential to be valorized through different modification pathways to yield alternative products to petroleum-based ones. In this work, hydrolysis lignin (HL) was copolymerized with acrylamide (AM) and acrylic acid (AA) under acidic conditions to generate the lignin/AM polymer (HM), lignin/AA polymer (HA), and lignin/AM/AA copolymer (HAM) with different negative charge densities and molecular weights. Lignin-based polymers characterized by advanced tools, such as proton nuclear magnetic resonance (1H NMR), gel permission chromatography (GPC), and elemental analysis confirmed the successful polymerization of HL with AM, AA, or AM/AA monomers. The adsorption analysis using a quartz crystal microbalance (QCM) revealed that compared to diblock HM and HA, the triblock copolymers of HAM adsorbed more on the Al2O3 surface and generated a bulkier adsorbed layer, which is important for lignin-based coating formulation. HAM1 with a lower charge density yielded a higher surface excess density, while HAM2 with a larger R h occupied more space (153.7 Å2) at the interface of water and Al2O3. In suspension systems, because of the higher M w, R h, and adsorption affinity, the bridging performance of HAM2 was more remarkable than that of the other lignin derivatives for Al2O3 particles via forming stronger flocs (with a deflocculation parameter, T df, of 80.6 s). However, the diblock lignin-AA (HA1) polymer showed the fastest floc regrowth capability after reducing the shear forces (with a reflocculation parameter, T rf, of 62.5 s). The high thermal stability, T g, and rheological characteristics of the HAM copolymer proved that it can be an excellent material for coating formulations and flocculants for wastewater treatment systems.

Introduction

Lignin, a highly branched aromatic polymer, is now considered the main aromatic renewable resource with attractive properties, such as biodegradability, as well as antimicrobial and UV activities.[1,2] Enzymatically hydrolysis lignin (HL) is derived from the biofuel production process and has been recognized to have poor hydrophilicity and low reactivity, which impede its conversion to value-added products.[3−5] Therefore, the advancement in the applications of hydrolysis lignin (HL) necessitates the modification of its chemical structure. Numerous studies demonstrated that the polymerization of lignin with functional monomers is a promising approach for lignin valorization, which can enhance both the molecular weight and charge density of the lignin macromolecule.[6−8] Previously, the polymerization reaction of kraft lignin (KL) with acrylamide (AM)[9] and acrylic acid (AA)[10,11] was studied for producing a lignin-based material with multifunctional applications. However, the production of HL with both AM and AA monomers in a ternary reaction system has not been investigated, which is covered in the present study.

Industrial wastewaters generally contain dissolved colloids, organic matter, and other impurities.[12,13] The removal of these suspended particles is recognized as a serious challenge. The flocculation process has extensively been adapted to treat various wastewaters, which exhibited many advantages including the low cost of operation and effectiveness.[12,13] Previously, the novel lignin–AA polymer was synthesized, which was very efficient in flocculating the aluminum oxide suspension.[11] Similarly, the HL–polyacrylamide (polyAM) showed promising results for flocculating azo dye particles from model wastewater.[14] Although the attachment of polyAM to natural polymers (i.e., lignin) can improve the grafting and bridging efficiencies of lignin in the flocculation process, it may have some disadvantages, such as a long dissolution time and a low efficiency for charge neutralization.[15,16] It was reported that the lignin-based flocculant prepared through grafting two monomers to lignin can significantly increase the solubility, molecular weight, and flocculation efficiency of lignin compared with that prepared through grafting one monomer to the lignin backbone.[17] For instance, the charge density, molecular weight, and solubility of KL was increased significantly via copolymerizing lignin with AM and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (DMC), which showed a high flocculation efficiency for both kaolin and bentonite suspensions.[8] However, there is no information available for the potential application of a triblock copolymer of HL, AM, and AA as a flocculant for solid particles in suspensions, which is the second objective of this work.

The adsorption of polymers on particles is an important factor in the flocculation process of the suspension systems. Previous studies showed that the adsorption of a high-molecular-weight (Mw) lignin polymer was greater than that of the low-Mw polymer on a model substrate.[18−22] It is also evident that the high charge of lignin polymers can develop more electrostatic attraction forces with particles leading to a higher level of adsorption, which is crucial for solid/liquid separation processes.[23] However, it is not clear how both the high negative charge density and molecular weight of lignin-based copolymers can affect the viscoelastic behavior of the adsorbed layer on particles, which is studied in this work.

The thermal stability of a flocculant is governed by its chemical architecture, molecular composition, and molecular weight properties, which are essential for wastewater treatment systems of the chemical processes operating at an elevated temperature (i.e., the mining process).[24,25] In this regard, thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) analysis can provide information on the thermal decomposition profiles of lignin macromolecules,[26] which facilitate the investigation on the physicoochemical alternation of the lignin polymer that occurs at a high temperature.[27] Moreover, high-molecular-weight flocculants have a high softening point and glass-transition temperature (Tg).[28] It was stated that at a temperature higher than the Tg, the molecular structure of lignin started to soften due to the Brownian motion, which would subsequently alter the physicochemical properties of lignin in solution and thus its performance as a flocculant.[28] Thus, the thermal properties of lignin-based polymers were fundamentally investigated in this study to probe if the lignin copolymers can be used as flocculants in wastewater treatment systems at an elevated temperature.

It should be stated that HL is physiochemically different from KL: HL has covalently bound carbohydrates, but KL does not have them, and HL has larger particles and is more insoluble than KL. Therefore, the results available on the polymerization of KL would not be representative of HL. To valorize HL, its polymerization should be delicately assessed. The main novelty of the present study was the copolymerization of HL, AM, and AA in a ternary reaction system. The adsorption behavior and flocculation efficiency of the triblock HL/AM/AA copolymers (HAMs) were evaluated and compared with those of the diblock HL/AA polymer (HA) and the HL/AM polymer (HM). In addition, the structural, thermal, and rheological properties of the produced lignin-based copolymers in the threefold reaction system were compared with those produced in the twofold systems.

Results and Discussion

HL Properties

The compositions of HL as the raw material used in this study were determined previously.[29] The constitutions of HL are presented in Table S1. It showed that more than 60 wt % of HL was composed of lignin (acid-insoluble and acid-soluble lignin), while the carbohydrates accounted for 32.8 wt % of the raw material. The results indicated that HL contained 0.6 wt % ash. Moreover, more detailed results of HL analysis are available elsewhere.[29−31]

1H Nuclear Magnetic Resonance Analysis

The 1H nuclear magnetic resonance (NMR) spectra of HL, HM, HA, and HAM are depicted in Figure S1a. The broad resonance at 7–8.2 ppm is associated with the aromatic protons, including the vinyl protons, on the carbon atom next to the aromatic ring.[32] The peak between 3.7 and 4.4 ppm is attributed to protons in the methoxy group of lignin,[33,34] and the signals at 3.38 ppm are assigned to the methylene protons in the β–β structure.[35] Moreover, the resonance at 3.1 ppm is attributed to the protons of anhydroxylose units of hemicelluloses.[36] The new peaks for the PAA chain segment (Figure S1b,c) occurred at 1.5, 1.96, and 2.33 ppm, which were assigned to C-1, C-2, and the hydroxyl end, respectively.[37] Moreover, the signal at around 4.15 ppm, which was absent in that of HL, corresponded to the protons of −CH2 attached to the aromatic structure through the ester bond (−CH2–O–C6H5). In the case of the HM polymer, the additional peaks presented at 1.6 and 2.1 ppm (Figure S1d) were attributed to the protons of Cα and Cβ, respectively, connecting to the amide group of HM.[38] The peak at 4.3 ppm also corresponded to the −CH2 protons connecting to the phenolic hydroxy group of lignin (i.e., via ester bonding) and indicating that Ph-OH of lignin was the active site for the polymerization reaction.[39]

Examining the 1H NMR spectrum of HAMs illustrated that the resonance from the AM and AA portions of the copolymers was observed at 1.5–1.8 ppm and 2.2–2.5 ppm, respectively. Moreover, a decline in the residual content of the phenolic hydroxy group in HAMs was evident as a conclusion of the reduced peak intensity at 7–8.2 ppm, which confirmed that the Ph-OH group of HL was the active site for the polymerization of HL and AM/AA (Figure S1e,f).

Properties of Lignin Polymers

In this work, the free-radical copolymerization of HL, AM, and AA (HAM) was carried out in an aqueous solution. To find out how AM or AA monomers work separately in a polymerization reaction with lignin, HL was polymerized with AA or AM to generate HA and HM, respectively. Since the amount of AM applied for producing HAMs was constant (5.1 mol/mol), but AA was used at different dosages (3.8 and 10.1 mol/mol), one HM and two HA (namely, HA1 and HA2) were synthesized following the free-radical polymerization. Then, HA and HM were considered as the control samples for HAM production throughout this work. The details of the polymerization mechanism and the proposed reaction scheme are available in Scheme S1.

Table lists the properties of the HL derivatives. HL had a charge density of −0.43 μeq/g and an Mw of 6300 g/mol. It is evident that the charge density, solubility, and molecular weight of HA and HAM polymers were increased after polymerization, confirming the successful grafting of the monomers onto the lignin structure. The successful polymerization of HL and AM was confirmed by enhancing the molecular weight of HM from 63 × 102 to 141 × 103 g/mol. Also, the high amount of nitrogen (1.09 wt %) in the HM polymer, originating from the amide group (−CONH2) of the AM monomer, was also an indicator of the successful polymerization of HL and AM. Although the charge density increment of the HM polymer was negligible, the AM monomer increased the solubility of the lignin polymer (from 25 to 47 wt %) due to the attachment of the amide group to the lignin backbone. A decrease in the phenolic hydroxy group of HM and HAs was mainly attributed to the participation of Ph-OH of HL in the polymerization. Increasing the carboxylate group content of HAs, stemming from the AA monomer, was associated with the decrease in the Ph-OH group of lignin.[11] Compared to HA1, the lower amount of Ph-OH and the higher amount of the carboxylate group, the charge density, the solubility, and the molecular weight of HA2 reflected that the polymerization reaction was accelerated more greatly for HA2. It is worth noting that by increasing the carboxylate group and the molecular weight of HAs, their oxygen content increased, while the carbon and hydrogen contents decreased.

Table 1

Properties of HL and HAM Polymers

sample	HL	HM	HA1	HA2	HAM1	HAM2
charge density, meq/g	–0.43 ± 0.11	–0.45 ± 0.1	–0.8 ± 0.07	–1.35 ± 0.2	–1.6 ± 0.2	–2.2 ± 0.1
carbon, wt %	62.6 ± 1.8	58.4 ± 0.2	59.4 ± 0.3	52.2 ± 0.9	48.9 ± 2.1	47.4 ± 1.2
hydrogen, wt %	7.5 ± 1.2	7.4 ± 0.1	7.3 ± 0.1	6.6 ± 0.4	6.5 ± 1.3	6.4 ± 0.06
nitrogen, wt %	0	1.09 ± 0.1	0.3 ± 0.1	0.3 ± 0.1	1.83 ± 0.1	2.52 ± 0.1
oxygen, wt %	27.4 ± 1.3	31.06 ± 0.6	31.1 ± 0.3	39.6 ± 0.5	42.2 ± 1.1	44.61 ± 0.4
phenolic hydroxyl group, mmol/g	1.6 ± 0.3	0.9 ± 0.1	0.85 ± 0.04	0.66 ± 0.03	0.58 ± 0.05	0.37 ± 0.03
carboxylate content, mmol/g	0.38 ± 0.05	0.38 ± 0.05	1.07 ± 0.04	1.45 ± 0.02	2.2 ± 0.03	3.1 ± 0.2
solubility, wt %	25 ± 0.5	47 ± 1.1	52 ± 0.9	64 ± 1.2	73 ± 2.3	81 ± 2.5
Mw, g/mol	44 ± 5.1 × 102	141 ± 3.7 × 103	131 ± 2.9 × 103	228 ± 5.5 × 103	277 ± 7.1 × 103	351 ± 4.2 × 103
Mn, g/mol	28 ± 1.3 × 102	59 ± 3.1 × 103	59 ± 2.5 × 103	95 ± 3.9 × 103	124 ± 4.4 × 103	140 ± 3.4 × 103
Mw/Mn	1.6 ± 0.3	2.4 ± 0.6	2.2 ± 0.7	2.4 ± 0.5	2.2 ± 0.3	2.5 ± 0.2
Rh, nm	17.2 ± 0.7	55 ± 1.1	34.3 ± 0.72	48.6 ± 0.8	64.3 ± 1.7	75 ± 2.1

A comparison between the products of the three-component reaction systems showed that HAM2 had a higher charge density and nitrogen content than HAM1 did, indicating that the copolymerization reaction was performed more efficiently for HAM2 than for HAM1. Moreover, the phenolic OH group content of the lignin copolymer was reduced to 0.78 and 0.37 mmol/g (from 1.63 mmol/g) for HAM1 and HAM2, respectively, illustrating that the phenolic group of lignin was involved in the reaction with AM and AA monomers. Unmodified HL had a carboxylate content of 0.38 mmol/g. However, the carboxylate content of HAMs increased to 2.2 and 3.1 mmol/g for HAM1 and HAM2, respectively. As stated in the literature,[11] an increase in the carboxylic acid content of lignin is an indicator of grafting of the AA group on the phenolic OH of lignin, improving the water solubility of lignin macromolecules. AM facilitated more bridging of HL and AA, promoting the water solubility of HAM polymers. According to Table , increasing the water solubility of lignin macromolecules after polymerization reaction could enhance the practical applications in aqueous systems (i.e., flocculation). Considering the constant amount of AM in the polymerization reaction, HAM2 with more attachment of AA had a higher molecular weight (351 × 103 g/mol) compared to HAM1 (277 × 103 g/mol). The results in Table also revealed that the hydrodynamic radius of modified lignin was larger than that of HL due to the presence of AA and AM segments on lignin. However, in the ternary reaction system, the attachment of both AA and AM made these copolymers with larger hydrodynamic sizes.

Adsorption Analysis

The adsorption kinetics of lignin polymers on the aluminum oxide-coated quartz crystal microbalance (QCM) sensors is shown as a function of time in Figure . The limited adsorption performance of HL implied that HL had limited interactions with aluminum oxide. However, by introducing the diblock lignin polymers (i.e., HM and HAs) to the surface, the frequency started to decrease due to the deposition of the polymer and water molecules on the surface. Of diblock polymers, the larger negative frequencies of the sensors (−11.4 Hz and −7.6 Hz) before buffer rinsing were observed for HA2 and HM, respectively, which illustrated higher adsorption affinity of those polymers to the Al2O3 surface. The greater deposition of HA2 can be due to the higher charge density and molecular weight (Table ) of HA2 compared to that of HM. Additionally, the high dissipation values of 8.6 and 6 for HM and HA2, respectively, confirmed that they created a soft and viscoelastic adsorbed layer on the sensor upon the course of adsorption. In this case, HA2 generated a more packed adlayer. However, the lower adsorption affinity and the less dissipative adsorbed layer were observed for HA1, which was attributed to its limited surface functional groups to interact with the Al2O3 surface.

Figure 1

Adsorption of HL, HA, HM, and HAM on the aluminum oxide sensor: (a) frequency changes and (b) dissipation changes (the vertical dashed line shows buffer rinsing).

In contrast to diblock polymers, a higher adsorption rate was observed for HAM copolymers. The changes in frequency and dissipation of the QCM sensor upon the adsorption of different lignin-based polymers were also statistically evaluated using one-way analysis of variance (ANOVA). The P-value of <0.001 (P < 0.05) implied that the lignin derivatives (diblock polymers and triblock copolymers) were significantly affected by the QCM sensor responses. Since HAM2 had a higher charge and molecular weight, it interacted more significantly than HAM1 with the aluminum oxide surface (P-value < 0.001) for both frequency and dissipation alternations.[40,41]

The adsorbed mass and adsorbed adlayer thickness are also illustrated in Table . The higher overall mass and thickness of the adsorbed layer of HAM2 (68.5 mg/cm2, 108.2 nm) than those of HAM1 (48.3 mg/cm2, 72.2 nm) on the aluminum oxide surface confirmed the more intense interaction and self-assembly of the HAM2 polymer, which was probably due to the greater size (Rh = 75 nm) of this polymer than other polymers.[42,43] When the adsorption reached a saturation level (at the time of 650 s), buffer rinsing was initiated, which removed the weakly adsorbed mass of HAMs from the sensors and thus reduced the dissipation of the sensor. The desorption of HAM molecules from the aluminum oxide surface was due to the loose binding of HAM molecules to the surface in the presence of strong electrostatic repulsion forces developed between the negatively charged deposited HAM polymers on the surface. The insignificant changes in dissipation were the indicator of the flat configuration of lignin polymers on the surface that was more pronounced for diblock polymers.[44] A similar behavior was also observed in a study on the adsorption of carboxymethylated lignin particles on the gold surface, in which some of the loosely bound lignin polymers desorbed during the buffer rinsing stage.[23]

Table 2

Adsorption Properties of Lignin Macromolecules on the Aluminum Oxide Surface at Equilibrium

sample	Δm (mg/cm2)	Δd (nm)
HL	7.3	7.8
HM	28.2	32.8
HA1	19.5	29
HA2	32.8	12.5
HAM1	48.3	72.2
HAM2	68.5	108.2

The changes in the frequency, Δf, and dissipation, ΔD, in the adsorption of lignin derivatives on the aluminum oxide sensor are illustrated in Figure S2 to understand the effect of polymer adsorption on the viscoelastic behavior of the adsorbed layer. Presenting the ΔD/Δf ratio illustrates the induced energy dissipation per coupled unit mass.[45] The results depicted a less viscous adsorbed layer for HM and HAs on the surface at a low adsorption level. However, HAM polymers with a steeper slope indicated a more viscous adsorbed layer on the sensor. According to the literature, the higher ΔD/Δf ratio may also imply the entrapment of water molecules within the adsorbed layer, which creates a more viscous film.[46] Thus, HAM2 with a more three-dimensional structure (Rh = 75 nm) may entrap more water molecules within its adsorbed polymer and generate a bulkier adsorption portfolio on the surface.

The fundamentals of the HAM polymer adsorption on the aluminum oxide surface were further studied via the determination of the surface occupancy of the HAM molecule on the surface to develop the first layer of the polymer on the sensor. To calculate the surface activity of each molecule on the desired surface, the surface excess density (Γ) of the molecule should be evaluated. Considering the slope of surface tension (σ) of water containing HAMs and the polymer concentration (C), the surface excess density can be calculated (Figure S3) following the Gibbs adsorption isotherm (eq )wherein R and T represent the universal gas constant [J/(mol K)] and the absolute temperature (K), respectively.

The surface tension results (Figure S3) depicted that the surface tension of HAMs reduced linearly with the increase of the natural logarithm of polymer concentration. Accordingly, at a 7 g/L HAM concentration, ∂ of lignin copolymers reached 65.3 and 63.9 mN/m for HAM1 and HAM2, respectively (from ∂water = 72.8 mN/m). Considering the results of Figure S3, the maximum surface concentration of polymers was determined to be 1.24 × 10–6 and 1.08 × 10–6 mol/m2 for HAM1 and HAM2 solutions at the air/water interface, respectively. As stated in the literature, the surface activity of a polymer is correlated with its surface access density (Γ).[47] Hence, the smaller the Γ value of HAM1, the smaller the surface activity of this polymer would be. The surface area occupied by each molecule (a) at the interface of the aluminum oxide sensor can be assessed following eq where N is the Avogadro constant.

Owing to the larger Rh (Table ), the HAM2 polymer occupied a larger area (153.7 Å2) at the interface than did HAM1 (133.8 Å2). In other words, the extended configuration and higher surface occupancy of HAM2 reflected a more three-dimensional structure of this polymer compared to that of HAM1, when the first layer of polymer segments coated the surface.

Rheological Characteristics

The intermolecular forces of chemically modified lignin can significantly impact the rheological properties of the polymer due to its three-dimensional structure as well as molecular chain length and conformation.[48] To understand how the structure of the lignin derivatives would impact their properties in solution, dynamic rheological studies of the polymers were carried out. Figure a illustrates the plot of storage modulus (G′) and loss modulus (G″) over the frequency sweep (ω). It is generally suggested that G′ represents the magnitude of the energy stored and recovered per cycle, which indicates the entropy elasticity for polymer chains.[49] Also, the G″ is a measure of the energy dissipated in a cycle of deformation.[49] All the lignin samples exhibited a higher storage modulus than the loss modulus at the higher range of frequencies, indicating their solid-like behavior (Figure a). At lower values of frequency (Figure b), the intersection points were present between G′ and G″ of all lignin samples, illustrating that the studied systems at a lower frequency (≥20 rad/s) manifested weak gel-like structures. In other words, the intersection point between G′ and G″, namely, crossover frequency, which shows the beginning of the elastic behavior of the gel state, was observed at a lower frequency value of lignin samples. At the crossover point, the sample property changes from viscous behavior to predominant elastic behavior due to the formation of an interconnected network of polymer chains. However, the HAM2 polymer with a higher molecular weight showed the highest modulus compared with the other samples (P-value = 0.002). Such behavior indicated strong interactions between HAM2 polymer chains, which reflected more elastic behavior and the existence of a three-dimensional network of HAM2 in an aqueous system. The results also revealed that no significant difference was identified between G″ of all lignin samples (P-value = 0.22).

Figure 2

Storage and loss modulus as a function of angular frequency in the ranges of (a) 0–100 (rad/s) and (b) 0–20 (rad/s) for lignin samples (G′ and G″ are represented as solid and open symbols, respectively).

Thermal Properties

The weight loss and weight loss rate of HL, HM, HAs, and HAMs can be found in Figure . It was observed that HL had different stages of decomposition. The initial weight loss of HL below 200 °C is mainly due to the release of adsorbed and bound water. In the temperature range of 100–180 °C, the plasticization phenomenon occurred, which indicated the cleavage of weak ether or aryl–alkyl bonds.[50] Generally, the decomposition of the polymeric structure of lignin starts in the temperature range of 120–275 °C, in which the propanoid side chains will degrade. Moreover, at a temperature of 275–350 °C, β–β and C–C linkages start to break down.[51,52] According to Figure , the HL sample continuously decomposed above 200 °C with a 34% weight loss at 400 °C. HL was significantly more stable than HM and HAs; however, the HAM copolymers were slightly more thermally stable. The statistical analysis also indicated a significant difference in the weight loss of HL and diblock polymers (P-value = 0.001 < 0.05). The higher thermal stability of HAMs can be due to the more resistance of ether bond linkages formed during the polymerization reaction[53] as a result of the wrapping of the lignin backbone by AM and AA chains. Compared to diblock polymers (i.e., HM and HAs), triblock HAM copolymers had three weight-loss events in the temperature ranges of 200–350, 350–450, and 450–700 °C. The first decomposition event (200–350 °C) was linked to the loss of water, ammonia, and small quantities of CO,[54] while the polymer network remained intact, and the main degradation phenomenon arose at the pendant amide groups on the AM part of the HAM copolymers.[17] In the second weight-loss event (350–450 °C), the main polymeric chains started to decompose, releasing carbon dioxide, nitrile compounds, AA from depolymerization, and imides.[17,55,56] At a temperature above 700 °C, only 1.5% of HAM1 and 5% of the HAM2 copolymer remained.

Figure 3

(a) Weight loss and (b) weight loss rate of HL, HM, HAs, and HAMs at a heating rate of 10 °C/min.

Table S2 lists the glass-transition temperature (Tg) of lignin samples. It was observed that the HAM copolymers had higher Tg values compared with HM, HA1, and HA2 polymers. The higher Tg of HAM1 (181 °C) and HAM2 (160 °C) is related to the presence of a lignin macromolecule that solidified the structure of the former copolymer. In other words, the free ends of AA and AM monomers attached to the rigid lignin macromolecule in HAMs can suppress the molecular motion of the grafted AA/AM chains, elevating the Tg values of HAM copolymers.[57]

It is worth mentioning that the knowledge of thermal properties of lignin-based polymers might be beneficial for the use in wastewater treatment systems of some processes, where oxidation or acid/alkaline treatments at a high temperature are required.[25,58] In such a process, the functionality and integrity of polymers are crucial.[24]

Behavior of Lignin Macromolecules in the Flocculation Process

Zeta Potential

The effect of HL, HM, HA, and HAM polymers on the zeta potential (ζ) of the aluminum oxide suspension as a function of polymer dosages is demonstrated in Figure S4. The addition of HL and HM reduced the ζ of the aluminum oxide suspension slightly due to the limited surface charge density of the polymers. The zeta potential of the aluminum oxide suspension increased significantly and changed from positive to negative when the concentration of HAs and HAMs increased in the suspension. ANOVA also demonstrated that the zeta potential of aluminum oxide particles presented a significant difference when treated with HAs and HAMs (P-values of 0.014 and 0.018, respectively). However, the aluminum oxide particles attained a higher zeta potential in the presence of HAM2 than HAM1 owing to their higher charge density and adsorption of HAM2 (Figure ), which created a more compact diffuse double layer around aluminum oxide particles.[59] It was also observed that the suspension reached the isoelectric point at a slightly lower dosage of HAM2 than HAM1.

Floc Formation

The weighted chord length distributions (CLDs) of the formed flocs in the aluminum oxide suspension are depicted in Figure S5 when different dosages of lignin polymers were added. By increasing the dosage of the samples to the aluminum oxide suspension, the number of counts decreased, and the CLD slightly shifted toward larger sizes. Moreover, considering the area under the count-CLD curve, the total counts dropped by 25, 24, 37, 45, and 47% when 2.4 mg/g of HM, HA1, HA2, HAM1, and HAM2 was added to the aluminum oxide suspension, respectively. This behavior reflected the aggregation of small particles to larger ones.[60,61] However, various dosages of HL did not change the chord length size of aluminum oxide particles, implying that HL was not effective in flocculating the particles. Since the maximum increment in the chord length and reduction in the number of counts of aluminum oxide particles treated with HM, HAs, and HAMs occurred at the dosage of 2.4 mg/g, this dosage was selected as the optimum dosage for further flocculation analysis. The details of the focused beam reflectance measurement (FBRM) results at 2.4 mg/g in various moments of the aggregate size distribution are available in Table S3.

Floc Strength and Recoverability

The variations in the mean chord length (MCL) of the suspension systems as a function of time for lignin samples at different shear rates are shown in Figure . The aluminum oxide suspension was stirred for 5 min at 200 rpm for stabilizing the system before initiating the trial. At the time of 300 s, different lignin polymers were added to the suspension, which induced larger flocs by increasing the chord length. HL did not impact the flocculation performance of the aluminum oxide suspensions owing to the limited anionic charge density and adsorption on the aluminum oxide surface (Figure ). However, the addition of HM, HA1, and HA2 to the suspension slightly increased the chord length of the aluminum oxide suspension to 14.4, 16.9, and 17.7 μm, respectively (from an initial size of 12.1 μm). Compared with diblock polymers, three-component HAM copolymers generated larger flocs at 2.4 mg/g, leading to the lower number of particles in the suspensions (P-value < 0.001). HAM2 with the higher molecular weight (Table ) created larger flocs with a larger chord length (28.5 μm) compared with HAM1 (22.8 μm). However, HAM1 created more stable flocs as the variation in the size of the formed flocs was less than that of HAM2 at 200 rpm. Considering the constant amount of the grafted AM to HAM polymers, the higher content of AA monomers grafted to HAM2 increased the charge density, Mw (Table ), and three-dimensional structure (i.e., Rh) of the HAM2 copolymer, facilitating the adsorption of this copolymer on the aluminum oxide particles, which subsequently improved its flocculation performance. Moreover, the higher surface occupancy of the HAM2 polymer when adsorbing on the aluminum oxide particles could be another reason for better flocculation characteristics of the aluminum oxide/HAM2 system. As stated in the literature, the higher surface occupancy of the polymer could contribute to the flocculation efficiency most probably through the bridging mechanism (i.e., tail and loop configurations).[23,62]

Figure 4

Change in the chord length of flocs in the aluminum oxide suspension in the presence of a 2.4 mg/g polymer dosage at different shear rates.

After 700 s, the mixing speed in the FBRM analysis was increased to 700 rpm (from 200 rpm), which reduced the mean chord length of particles, providing evidence of deflocculation (i.e., floc breakage). Once the shear force dropped to the initial level (200 rpm), the crushed particles agglomerated and the chord length increased accordingly.[63,64] As illustrated in Figure , by reducing the shear rate (i.e., from 700 to 200 rpm), the chord length of the aluminum oxide suspension treated with HM, HA1, and HA2 increased to 13.4, 15.9, and 16.7 μm, respectively, which were very similar to their chord length obtained at the flocculation step prior to the shear rate elevation. However, only partial reflocculation was achieved for the aluminum oxide/HAMs. To have further insight into the deflocculation and reflocculation phenomena, the obtained data from Figure were fitted into eqs and 4, respectivelyHere, C0 and C∞ are numerical constants (μm), A and K are pre-exponential factors, Tdf represents the deflocculation parameter (s), Trf stands for the reflocculation parameter (s), t is the time (s), and Y shows the mean chord size.

Table lists the parameters of the deflocculation and reflocculation processes. As stated in the literature, there is a dynamic equilibrium between the formation and the breakage of flocs in the flocculation process.[64] According to Table , the higher value of Tdf in aluminum oxide/HAM2 (80.6 s) is attributed to the higher strength of the generated flocs. The properties of flocs may be attributed to their main flocculation mechanism. Since the patching mechanism can form flocs with relatively weak strength, but bridging creates the stronger flocs,[65] it might be implied that the HAM2 copolymer formed flocs mainly through bridging rather than the patching mechanism.

Table 3

Deflocculation and Reflocculation Parameters of Aluminum Oxide/HAM Systems under Different Shear Rates

sample	Tdf, s		Trf, s
HL	58.1	R2 = 0.98	48.5	R2 = 0.97
HM	51.8	R2 = 0.96	71.4	R2 = 0.98
HA1	62.9	R2 = 0.98	62.5	R2 = 0.99
HA2	49.02	R2 = 0.97	76.3	R2 = 0.96
HAM1	71.4	R2 = 0.98	78.7	R2 = 0.98
HAM2	80.6	R2 = 0.99	91.7	R2 = 0.97

The Trf parameter elucidates the recovery of the broken flocs after reducing the shear rate. Accordingly, the lower Trf value of the aluminum oxide/HAM1 suspension indicates the higher tendency of this system to reflocculate and relatively faster regrowth capability.[66] However, the reformed flocs via the HAM2 polymer in the aluminum oxide suspension could not reach their initial sizes after the deflocculation process. In this case, the formed bridges between HAM2 and aluminum oxide particles may undergo scission, and hence, a part of the detached HAM2 would reconfigure on the surface of aluminum oxide particles and lose their bridging efficiency.[8,67] Among lignin polymers, the HA1 polymer formed smaller flocs with more reversible affinity in the floc size after the deflocculation and reflocculation processes, which confirmed the patching flocculation mechanism.[68]

Comparison

The flocculation performance of HAMs was compared with those of other bio-based flocculants, and the results are tabulated in Table . Compared to our previous work,[6] the molecular weight of HL increased significantly after polymerizing with two monomers (i.e., AM and AA) and thus yielded higher particle removals by generating a larger chord length. Although cationic HL-graft-[2-(methacryloyloxy)ethyl]trimethyl ammonium chloride (HL-METAC) was used at a lower dosage, it was not as efficient as HAMs in particle removal from model wastewater.[69] The higher molecular weight of HAM than cationic cellulose (29 × 103 to 103 × 103 g/L) resulted in larger floc production when interacting with colloidal particles.

Table 4

Comparison of Various Flocculants for Wastewater Treatment Systems

flocculant	charge density, meq/g	Mw, g/mol	wastewater	pH	dosage, mg/g	chord length, μm
HAM1	–1.6	277 × 103	aluminum oxide	7	2.4	22.8
HAM2	–2.2	351 × 103				28.5
HL-METAC	+2.1	55 × 103	kaolin	6	1.2	10.9
cationic starch[69]	+2.3	1–2 × 106	digested sludge	7.0–7.5	12	19.2
cationic cellulose[70]		29 × 103 to 103 × 103	digested sludge	7.5	4–12	15–20
cationic chitosan (low, medium, and high Mw)[71]	+3.8–5.4	77 × 103 to 444 × 103	calcium carbonate	7.6	5–10.8	<20
chitosan-graft-3-chloro-2-hydroxypropyltrimethylammonium chloride (Chito-CTA)[72]		10 × 106	oil sand tailings	7	120	25

Despite the high molecular weight of cationic starch[69] (1–2 × 106) and Chito-CTA[72] (107 g/mol), the chord length generated by these flocculants was smaller than that of HAM copolymers. Additionally, the optimum dosage of HAM1 and HAM2 achieved in this study was 50 times as much as that reported for Chito-CTA.[72] Interestingly, the high surface charge density of cationic chitosan[71] (3.8–5.4 meq/g) was not highly efficient in removing suspended particles (i.e., calcium carbonate) compared to HAM copolymers with a medium charge density. Overall, the results elucidated that the application of HAM copolymers for different wastewater treatment systems is promising.

Conclusions

HL was copolymerized with AM and AA to synthesize anionic copolymerized HL-based polymers. Compared with the two-component HA and HM polymers, the hydrodynamic radius of three-component HAM polymers was significantly larger after polymerization, indicating their more three-dimensional structures. The QCM studies indicated the greater adsorption performance of HAM polymers with a higher charge density and molecular weight, which created a more dissipative adlayer (i.e., more viscoelastic properties). Owing to its greater adsorption performance and three-dimensional structure, the bridging affinity of HAM was more remarkable compared with that of HM and HAs. However, the larger surface occupancy (153.7 Å2) and Rh (75 nm) of HAM2 led to its higher flocculation performance than HAM1 for treating the aluminum oxide suspension. Furthermore, HAM copolymers generated stronger flocs in the aluminum oxide suspension, which were more resistant to the external shear forces (higher Tdf values). However, the flocculation of aluminum oxide particles using diblock HM and HA polymers was more reversible (lower Trf values). The more three-dimensional structure of HAMs, which was manifested by larger Rh, resulted in the higher elastic characteristics of HAMs than diblock HM or HAs. HAM copolymers were more thermally stable than other polymers, which is advantageous for their application as flocculants for wastewater systems that partly operate at a high temperature (i.e., the mining industry).

Experimental Methodology

Materials

Enzymatically produced hardwood HL was supplied by FPInnovations (Thunder Bay, ON). AA, AM (99.0%), KOH, parahydroxy benzoic acid, potassium persulfate (K2S2O8) (analytical grades), sodium hydroxide and sulfuric acid, hydrochloric acid (37%, reagent grade), potassium hydroxide (8 M), potassium chloride (KCl), polydiallyldimethyl-ammonium chloride (PDADMAC, a molecular weight of 100–200 kg/mol, 20 wt % in water), sodium azide (NaN3, 99.5%), trimethylsilyl propanoic acid (TSP), d6-dimethyl sulfoxide (DMSO-d6), deuterium oxide (D2O), and aluminum oxide (Al2O3) were all purchased from Sigma-Aldrich Company. Potassium polyvinyl sulfate (PVSK) was provided by Wako Company. Fisher Scientific Company provided ethanol (95 vol %). Cellulose acetate dialysis membrane tubes (a molecular weight cutoff of 1000 g/mol) were obtained from Spectrum Laboratories. Deionized water with a resistivity of less than 18 MΩ/cm was produced using a Millipore water purification system and used throughout this work.

Synthesis and Purification of the HAM Copolymer

A 2 g sample of lignin was suspended in 20 mL of deionized water at room temperature and 300 rpm for 20 min in a 250 mL three-neck glass flask. After that, the desired amounts of AA (AA-to-lignin molar ratios of 3.8 and 10.1) and AM (an AM-to-lignin molar ratio of 5.1) were added to the flasks, and the pH of the medium was adjusted to 3 using 1.0 M NaOH solution. Subsequently, the predetermined amount of K2S2O8 (0.03 g) was added to the flasks as an initiator, and the medium was deoxygenated by purging with nitrogen gas for 20 min. The polymerization reaction was carried out at 80 °C for 3 h, while a continuous purging of N2 was supplied during the reaction. After completion, the reaction medium was first cooled to ambient temperature and then acidified to a pH of 1.5 to collect the final product from the solution. After mixing, the mixture was centrifuged at 3500 rpm for 10 min to precipitate the HL/AM/AA copolymer (HAM) and to remove the homopolymers (i.e., PAM and PAA) and unreacted monomers (i.e., AM and AA) present in the supernatants. Further purification of the HAM polymer macromolecules was achieved using membrane dialysis for 48 h to remove any unreacted monomers and salts from the HAM polymers. Then, the dialyzed anionic lignin was dried at 105 °C. In another set of experiments, HL was polymerized with AA or AM and they were named HA and HM, respectively, using the above-mentioned method, and the final products were considered as the control samples to compare with HAM copolymers.

Characterization of Lignin Polymers

The charge density of the lignin copolymers was measured using a Particle Charge Detector, Mütek PCD 04 titrator (Herrsching, Germany). In this analysis, the lignin samples (1 mL) at a 1 wt % concentration were titrated against the PDADMAC solution (0.0050 M) at pH 7. The charge density of the samples was then determined according to the previously established procedure.[10]

The molecular weight of the HL, HM, HA, and HAM polymers was analyzed by gel permeation chromatography (Malvern, GPCmax VE2001 Module + Viscotek TDA305) equipped with multidetectors. In this measurement, the organic columns of PolyAnalytic PAS106M, PAS103, and PAS102.5 were used, and NaNO3 (0.1 mol/L) was used as a solvent and eluent. The flow rate and column temperature were set at 0.7 mL/min and 35 °C, respectively. Poly(ethylene oxide) was used as the standard solution for the aqueous system, and the refractive index (RI) detector of the instrument was used to determine the molecular weight of the polymers.

The phenolic hydroxy group and carboxylate group contents of lignin samples were measured using an automatic potentiometer, Metrohm, 905 Titrando, Switzerland. In this analysis, 0.06 g of samples was mixed with 1 mL of KOH (0.8 M), and 4 mL of parahydroxybenzoic acid (0.5 wt %) was used as an internal standard solution. The prepared samples were then titrated against 0.1 M HCl solution, and the mean value of three measurements was reported.[73]

The elemental analysis was performed for lignin polymers using an Elemental Analyzer (Vario EL Cube, Elemental Analyzer, Germany). Approximately, 5 mg of the oven-dried samples was transferred into the carousel chamber of the instrument and combusted at 1200 °C to reduce the generated gasses to examine their carbon, hydrogen, and oxygen contents. Considering the carboxyl group content of the samples, the grafting ratio of HM, HA, and HAM polymers was calculated according to the equation described by Bayazeed and co-workers.[74]

The molecular structures of HL, HM, HA, and HAM polymers were analyzed using 1H NMR spectroscopy. In this set of experiments, 30 mg of dried samples and 8 mg of trimethylsilyl propanoic acid (TSP) were dissolved in 450 μL of DMSO-d6 and 50 μL of D2O with stirring overnight at room temperature. The 1H NMR spectra of samples were recorded using an INOVA-500 MHz instrument (Varian, USA) with a 45° pulse after 64 scans and a relaxation delay time of 1.0 s.

The hydrodynamic radius (Hy) of the HL, HM, HA, and HAM polymers was measured via a dynamic light scattering (DLS) instrument, BI-200SM Brookhaven Instruments, USA, equipped with a 35 mW laser power source. In this analysis, 1 g/L of HL and HAM were prepared in 1 mM KCl solution to avoid aggregation and stirred at 300 rpm for 24 h and at room temperature. After mixing, 20 mL of the sample was filtered using a 0.45 μm disposable syringe filter, and the hydrodynamic radius of lignin samples was determined using the method described previously.[75] The scattering angle was set at 90°, and the analysis was conducted at the wavelength of 637 nm. The average value of three repetitions was then reported in this study.

Quartz Crystal Microbalance with Dissipation (QCM-D) Studies

The adsorption of lignin polymers (1 wt %) on the aluminum oxide substrate was assessed using a QCM with dissipation (QCM-D). The adsorption analysis was monitored by introducing the buffer solution (i.e., Mili-Q water) to the chamber of the QCM instrument at the controlled temperature of 22 ± 0.1 °C to generate the baseline of the experiments. After reaching the equilibrated baseline, the buffer solution was switched to the lignin polymer solutions and the shifts in frequency and dissipation of the sensor at the fifth harmonic (n = 5) were recorded as a function of time. Solutions were pumped at the flow rate of 0.15 mL/min throughout the experiments. The viscoelastic properties of an adsorbed layer were also monitored by the dissipation of the sensor’s energy in the oscillating quartz crystal system following eq where ED represents the energy dissipated during oscillation and ES is the amount of energy stored in the oscillating system. For the rigid adsorbed layer, the dissipation value is very low. However, the viscoelastic film shows higher energy dissipated through the adsorbed layer, indicating more deformation during the oscillation.[76] The characteristic of the adsorbed lignin polymer on the aluminum oxide surface was determined by the Q-tools software in the QCM-D instrument.

Rheological Studies

The rheological properties of lignin samples (HL, HM, HA, and HAM) were determined using a hybrid rheometer (TA Instruments) equipped with a cylindrical geometry (cone length, 41.96 mm; cone diameter, 28.03 mm; gap, 5500 μm; angle, 1°) at 22 °C. In this set of experiments, 25 mL of the lignin solution (4 wt %) was placed inside the cell of the instrument and a 3 min pre-shear at 100 1/s was applied to the samples prior to the measurement. To determine the linear viscoelastic region (LVR), the dynamic strain sweep measurement was carried out at the frequency of 6.28 rad/s. The frequency sweep measurements were then performed in the range between 0.01 and 100 rad/s, while the strain value from the LVR was set at 0.1%.

Zeta Potential Analysis

The zeta potential of the aluminum oxide suspension was analyzed in the presence of various dosages (2–64 mg/L) of lignin polymers using a NanoBrook PALS (Brookhaven Inc., USA). The zeta potential measurements were carried out at room temperature and a constant electric field (8.4 V/cm). The reported data in this experiment was the average of three repetitions.

Flocculation Analysis

The flocculation behavior of the aluminum oxide suspension and the properties of the formed flocs were determined via monitoring the CLD of particles in the suspension in a real-time scenario using an FBRM, Mettler Toledo, E25. In this experiment, 200 mL of the aluminum oxide suspension (25 g/L at pH 6) was stirred at 200 rpm and then the laser probe (25 mm diameter) was submerged in the medium. After reaching the stable condition, the desired volume of lignin polymers was added to the suspension and the CLD was assessed by using 90 log-channels over the range between 1 and 1000 μm using the IC-FBRM software. To study the reflocculation performance of flocs, the stirring rate of the mixture was increased to 700 rpm for 1 min to break down the generated flocs (i.e., the deflocculation process). Subsequently, the agitation speed decreased to 200 rpm once again to analyze the reflocculation of broken flocs. The CLD of the particles in the aluminum oxide suspension was recorded every 3 s.

Thermogravimetric Analysis

TGA of HL, HM, HA, and HAM was evaluated using a thermogravimetric analyzer, Instrument Specialist, i1000, to determine the thermal behavior of lignin samples. In this set of experiments, the samples were first dried in an oven (105 °C) overnight prior to analysis. Then, they were heated from room temperature to 700 °C under a 20 mL/min nitrogen flow rate. The heating flow rate was adjusted at 10 °C/min.

Differential Scanning Calorimetry

The glass-transition temperature (Tg) and heat capacity (Cp) values of the lignin samples were analyzed using differential scanning calorimetry (DSC, TA Instruments Q2000). The experiment was conducted according to the methods published previously.[77,78] First, a 5 mg dried lignin sample was placed in a DSC pan, and then, the samples were treated in the temperature range between 0 and 200 °C, while the heating rate was set at 3 °C/min.[78] After heating the samples to 200 °C, they were cooled down to 0 °C and reheated again to 200 °C. The values of Tg and Cp of lignin samples were assessed in the second heating cycle.[77]

Statistical Analysis

The significance of reported data was examined using variance analysis (one-way ANOVA, Fisher’s test) considering a significance level of 95% (α = 0.05). Microsoft Excel software was used for the statistical analysis.