Preparation and Coagulation Performance of Carboxypropylated and Carboxypentylated Lignosulfonates for Dye Removal

In this work, 1-carboxypropyled (1-CPRLS) and 5-carboxypentyled lignosulfonates (5-CPELS) were synthesized using 2-chlorobutanoic acid and 6-chlorohexanoic acid as carboxylate group donors via SN1 and SN2 mechanisms, respectively. 1-Carboxypropyl and 5-carboxypentyl lignosulfonates with the charge densities of −3.45 and −2.94 meq g−1 and molecular weights of 87,900 and 42,400 g·mol−1 were produced, respectively, under mild conditions. The carboxylate content and degree of substitution (DS) of the 1-CPRLS product were 2.37 mmol·g−1 and 0.70 mol·mol−1, while those of 5-CPELS products were 2.13 mmol·g−1 and 0.66 mol·mol−1, respectively. The grafting of carboxypropyl and carboxypentyl groups to lignosulfonate was confirmed by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (1H-NMR and 13C-NMR) spectroscopies. In addition, 1-CPRLS and 5-CPELS were applied as coagulants for removing ethyl violet (EV) dye from a simulated solution, and their performance was related to their charge densities and molecular weights. Furthermore, fundamental discussion is provided on the advantages of (1) producing 1-CPRLS and (2) the superior properties and performance of 1-CPRLS to carboxyethylated lignosulfonate.

Lignin is a lignocellulosic biomass-derived multifunctional molecule that is considered a useful by-product with high valorization potential for producing value-added products [6,7]. In this regard, lignosulfonate as a by-product of spent sulfite liquor [8][9][10][11] can be recovered and valorized into high-grade commercial products using novel derivatization and green engineering approaches [12][13][14][15][16][17][18][19][20]. Lignosulfonate derivatives were reported to be used as emulsifiers/emulsion stabilizers, surfactants for pesticides, oil recovery and drilling muds, concrete cure retarders and plasticizers, and binders for foundry and wood adhesives [21]. However, the use of lignosulfonate for dye removal may expand its application to wastewater treatment in the textile industry.
Currently, conventional biological treatment processes are not very effective in treating dye wastewaters, and the research community is exploring opportunities for identifying an alternative process for removing dyes from wastes [22,23]. Today, the effluents of textile, painting, paper and pulp, leather, and cosmetic industries contain significant amounts of natural and synthetic dyes. The high solubility of these toxic dyes in water and the environmental pollution caused by them makes these dyes harmful to the health of aquatic life, humans, and other mammals [24,25]. Therefore, the application of acid) under altered times and temperatures in 250 mL three-neck round-bottom glass flasks under constant stirring at 300 rpm [62]. The product was extensively washed with ethanol/water (40:10, v/v) and recovered by centrifugation (3500 rpm, 10 min). After three cycles of washing/centrifugation, the precipitate was dissolved in deionized water (50 mL), then purified using dialysis tubes in deionized water for two days, and finally dried in an oven at 105 • C overnight. Different 1-carboxypropyl lignosulfonate and 5-carboxypentyl lignosulfonate samples were prepared by employing different reaction times (1.0, 1.5, and 2.0 h), temperatures (60,70, and 80 • C), ClCBA i /LS ratios (1.0, 1.5, and 2.0 mol·mol −1 ), and water/isopropyl alcohol ratios (10%, 15% and 20%). It should be stated that 1-CPRMLS and 5-CPEMLS were produced following the same procedures of 1-CPRLS and 5-CPELS under the conditions of 1.0 ratio of ClCBA i /MLS by mol·mol −1 , 80 • C, 2.0 h, and 15% water/isopropyl alcohol ratio.

Methylated Lignosulfonate
For this modification route, 2.0 g of LS was dissolved in 30 mL of 0.7 mol·L −1 NaOH solution in a 250 mL three-neck glass flask at room temperature by stirring at 200 rpm for 30 min. In a glass beaker, 2.5 mmol of dimethyl sulphate was added per each mmol of total phenolic hydroxyl groups of LS, and the solution was stirred at room temperature for 30 min. The mixture was then heated to 80 • C for an additional 2 h. During the reaction, the pH of the mixture was maintained at 11.0-11.5 by continuous addition of 0.7 mol·L −1 NaOH solution. After the reaction, the suspension was acidified to pH 2.5 by adding 2.0 mol·L −1 HCl solution, washed with excess deionized water, dialyzed for two days, and finally dried in a freeze-dryer. The final product was denoted as methylated LS (MLS) [20,63,64].

Taguchi Experimental Design
Taguchi's orthogonal array (OA) was used to obtain the maximum charge density and molecular weight of 1-CPRLS and 5-CPELS under the optimized conditions [31,[57][58][59][60]. In this study, a total of nine runs (L 9 ) with three factors (each at four levels) were conducted based on an orthogonal design to investigate the effect of parameters on the carboxyalkylation reactions [20].

Charge Density Analysis
In preparing the samples for this analysis, LS, MLS, 1-CPRLS, 5-CPELS, 1-CPRMLS, and 5-CPEMLS were firstly dried in a 105 • C oven overnight to remove moisture. A 0.1 g sample of each lignosulfonate derivative was added to 10 mL of deionized water, then it was used for determining their charge density using a particle charge detector (Mutek, PCD 04, BTG Instruments, GmbH, Weßling, Germany) with a PDADMAC standard solution (0.006 M) [31]. The reported data in this paper are the average of three repetitions. Viscotek TDA305 with viscometer detectors. About 20-30 mg of the dried samples were dissolved in 10 mL of 0.1 mol·L −1 NaNO 3 solution and filtered with a 0.2 µm nylon filter. The filtered solutions were used for molecular weight analysis [30]. PolyAnalytic PAA206 and PAA203 columns were used at the column temperature of 35 • C. A 0.1 mol·L −1 NaNO 3 solution was used as solvent and eluent. The flow rate was set at 0.70 mL·min −1 . Polyethylene oxides were used as standard [31,63].

Elemental Analysis
Elemental analysis of LS, MLS, 1-CPRLS, 5-CPELS, 1-CPRMLS, and 5-CPEMLS was assessed using an Elementar Vario EL Cube elemental analyzer by the combustion method [66]. In order to remove any moisture, all samples were firstly dried in a 105 • C oven overnight. A 0.002 g quantity of each sample was used for determining their carbon, hydrogen, nitrogen, and oxygen contents.

13 C-NMR Analysis
All NMR spectra were recorded using an INOVA-500 MHz instrument (Varian, USA), operated at 50 • C utilizing a mixture of DMSO-d 6 and D 2 O as the solvent. The chemical shifts were referred to the solvent signal of DMSO-d 6 at 39.5 ppm. The major disadvantage of 13 C-NMR spectroscopy is its inherent low sensitivity, which requires that a concentrated sample/solvent solution be made [67]. Therefore, in this study, for qualitative 13 C NMR, 125 mg of the samples was dissolved in 500 µL of DMSO-d 6 and 50 µL of D 2 O at 50 • C overnight at 100 rpm. Chromium (III) acetylacetonate (5 mg) was also added to the solution to provide complete relaxation of all nuclei. Line broadening of 1 Hz was applied to FIDs before Fourier transform [68]. To obtain a satisfactory signal-to-noise ratio, minimizing baseline and phasing distortions, the 13 C NMR spectra of LS, 1-CPRLS, and 5-CPELS were recorded at 50 • C (in order to reduce the viscosity of the solution) with a pulse angle of 90 • C, a pulse delay of 2 s, an acquisition time of 1.1 s, and a transient number of about 30,000. The total experiment time was 28-30 h. Finally, the inverse-gated decoupling sequence was used, which involved turning off the proton decoupler during the recovery between pulses so that the NOE effect was avoided.

Dye Removal Analysis
At first, 100 mg·L −1 of EV dye solution was prepared by dissolving the dye in deionized distilled water, and the solution was shaken for 10 min. This dye solution was considered as a model wastewater sample of the textile industry. In this set of experiments, 1 g·L −1 samples of LS, 1-CELS, 1-CPRLS, and 5-CPELS solutions were made by mixing each of them with deionized distilled water at room temperature. The mixtures were added to 10 mL of a 100 mg·L −1 dye solution in centrifuge tubes. Then, the tubes were maintained in a water bath shaker at 30 • C and 150 rpm for 10 min. The tubes were then centrifuged at 3000 rpm for 10 min using a Sorvall ST 16 Laboratory Centrifuge. The filtrates were collected for analysis, while precipitates were discarded. The concentrations of dye in the mixtures before and after treatments were determined using a Genesys 10 s UV-Vis spectrophotometer at the wavelength of 595 nm for EV. The dye removal was calculated by the following equation: where C 0 (g·L −1 ) and C (g·L −1 ) are the concentrations of the dye solution before and after the LS, 1-CELS, 1-CPRLS, and 5-CPELS treatment, respectively.

Mechanism of 1-Carboxypropylation of Lignosulfonate
The reaction scheme of 1-carboxypropyled lignosulfonate is shown in Scheme 1. The carboxypropylation of lignosulfonate was performed using 2-chlorobutanoic acid as the carboxylate group donor in an alkali medium. Because 2-chlorobutanoic acid is a secondary halide, the reaction would take place via an S N 1 mechanism in a water-isopropanol mixture as a polar protic solvent. Under alkali conditions, NaOH reacts with the hydroxyl group of lignosulfonate and generates a strong nucleophile. The dissociation of the carbon-halogen bond in 2-chlorobutanoic acid creates a planar carbocation (Scheme 1a). The alkoxide ion from the alkali lignosulfonate attacks the carbocation intermediate, resulting in the carboxypropylation reaction (Scheme 1b) [66,69]. The degree of carboxypropylation depends on the number of hydroxyl groups substituted by carboxypropyl groups. Furthermore, 2-hydroxybutanoic acid sodium salt can be generated as a by-product at high concentrations of 2-chlorobutanoic acid and NaOH (Scheme 1c) [20,66,69]. at room temperature. The mixtures were added to 10 mL of a 100 mg·L −1 dye solution in centrifuge tubes. Then, the tubes were maintained in a water bath shaker at 30 °C and 150 rpm for 10 min. The tubes were then centrifuged at 3000 rpm for 10 min using a Sorvall ST 16 Laboratory Centrifuge. The filtrates were collected for analysis, while precipitates were discarded. The concentrations of dye in the mixtures before and after treatments were determined using a Genesys 10 s UV-Vis spectrophotometer at the wavelength of 595 nm for EV. The dye removal was calculated by the following equation: where C0 (g·L −1 ) and C (g·L −1 ) are the concentrations of the dye solution before and after the LS, 1-CELS, 1-CPRLS, and 5-CPELS treatment, respectively.

Mechanism of 1-Carboxypropylation of Lignosulfonate
The reaction scheme of 1-carboxypropyled lignosulfonate is shown in Scheme 1. The carboxypropylation of lignosulfonate was performed using 2-chlorobutanoic acid as the carboxylate group donor in an alkali medium. Because 2-chlorobutanoic acid is a secondary halide, the reaction would take place via an SN1 mechanism in a water-isopropanol mixture as a polar protic solvent. Under alkali conditions, NaOH reacts with the hydroxyl group of lignosulfonate and generates a strong nucleophile. The dissociation of the carbon-halogen bond in 2-chlorobutanoic acid creates a planar carbocation (Scheme 1a). The alkoxide ion from the alkali lignosulfonate attacks the carbocation intermediate, resulting in the carboxypropylation reaction (Scheme 1b) [66,69]. The degree of carboxypropylation depends on the number of hydroxyl groups substituted by carboxypropyl groups. Furthermore, 2-hydroxybutanoic acid sodium salt can be generated as a byproduct at high concentrations of 2-chlorobutanoic acid and NaOH (Scheme 1c) [20,66,69]. Scheme 1. Proposed reaction mechanism for the carboxypropylation of lignosulfonate via an S N 1 pathway: (a) formation of carbocation; (b) reaction of carbocated chemical with bulky nucleophile [66,69]; (c) undesired 2-hydroxybutanoic acid sodium product.

Mechanism of 5-Carboxypentylation of Lignosulfonate
Since 6-chlorohexanoic acid is a primary halide, it can be predicted that the synthesis of the 5-carboxypentylated lignosulfonate proceeds through an S N 2 nucleophilic substitution reaction [33,66,69]. The scheme of this reaction is depicted in Scheme 2. Under alkali conditions, NaOH reacts with the hydroxyl group of lignosulfonate and creates an alkoxide ion. This strong nucleophile approaches the alkyl halide from the back side of the C-Cl bond; as the C-nucleophile bond forms, the C-Cl bond breaks (Scheme 2a). At the transition state of the reaction, there is a 5-coordinate trigonal bipyramidal carbon for a femtosecond [66,69]. There is a possible competing side reaction, including the production of sodium 6-hydroxycaproate, which lowers the overall yield of the 5-CPELS as illustrated in Scheme 2b.

Mechanism of 5-Carboxypentylation of Lignosulfonate
Since 6-chlorohexanoic acid is a primary halide, it can be predicted that the synthesis of the 5carboxypentylated lignosulfonate proceeds through an SN2 nucleophilic substitution reaction [33,66,69]. The scheme of this reaction is depicted in Scheme 2. Under alkali conditions, NaOH reacts with the hydroxyl group of lignosulfonate and creates an alkoxide ion. This strong nucleophile approaches the alkyl halide from the back side of the C-Cl bond; as the C-nucleophile bond forms, the C-Cl bond breaks (Scheme 2a). At the transition state of the reaction, there is a 5-coordinate trigonal bipyramidal carbon for a femtosecond [66,69]. There is a possible competing side reaction, including the production of sodium 6-hydroxycaproate, which lowers the overall yield of the 5-CPELS as illustrated in Scheme 2b. Scheme 2. Scheme representation: (a) carboxypentylation reaction via the SN2 pathway; (b) production of sodium 6-hydroxycaproate (6-hydroxyhexanoic acid sodium salt as a side effect).

Mechanisms of Methylation of Lignosulfonate
The selectivity of the phenolic and aliphatic hydroxyl groups of lignosulfonate for the purpose of carboxypropylation and carboxypentylation can be evaluated by methylation processes [63,64]. Upon these reactions, firstly, LS was methylated, which protected all phenolic hydroxy groups of lignosulfonate without impacting the other parts of lignosulfonate structure [70], as shown in Scheme 3a, and then the methylated lignosulfonate (MLS) was reacted with 2-chlorobutanoic acid and 6chlorohexanoic acid. The possible products of 1-CPRMLS and 5-CPEMLS, which may have been resulted from the substitution of hydroxy groups on the aliphatic chains of MLS with carboxyalkyl functions, are exhibited in Scheme 3b,c [20].

1 H-NMR Analysis
The 1 H-NMR spectra of LS, 1-CPRLS, and 5-CPELS in D2O or DMSO-d6 were studied for the qualification of the structures of these lignin derivatives ( Figure 1). In all spectra, protons in water Scheme 2. Scheme representation: (a) carboxypentylation reaction via the S N 2 pathway; (b) production of sodium 6-hydroxycaproate (6-hydroxyhexanoic acid sodium salt as a side effect).

Mechanisms of Methylation of Lignosulfonate
The selectivity of the phenolic and aliphatic hydroxyl groups of lignosulfonate for the purpose of carboxypropylation and carboxypentylation can be evaluated by methylation processes [63,64]. Upon these reactions, firstly, LS was methylated, which protected all phenolic hydroxy groups of lignosulfonate without impacting the other parts of lignosulfonate structure [70], as shown in Scheme 3a, and then the methylated lignosulfonate (MLS) was reacted with 2-chlorobutanoic acid and 6-chlorohexanoic acid. The possible products of 1-CPRMLS and 5-CPEMLS, which may have been resulted from the substitution of hydroxy groups on the aliphatic chains of MLS with carboxyalkyl functions, are exhibited in Scheme 3b,c [20].

1 H-NMR Analysis
The 1 H-NMR spectra of LS, 1-CPRLS, and 5-CPELS in D 2 O or DMSO-d 6 were studied for the qualification of the structures of these lignin derivatives ( Figure 1). In all spectra, protons in water appear at 4.7 (in D 2 O solutions) or 3.33 ppm (in DMSO-d 6 solutions), and the sharp peak at 2.54 ppm corresponds to the protons of dimethyl sulfoxide [33]. In 1-CPRLS' spectrum, the small broad peaks Additionally, dimethyl sulfate (Scheme 3a) was used for synthesizing methylated lignosulfonate (MLS) [20,63,64]. The representative qualitative 1 H-NMR spectrum supports the theory that dimethyl sulfate can afford the selective methylation of the phenolic hydroxyl groups in lignosulfonate [20]. Alternatively, the aliphatic hydroxyl groups in lignosulfonate remained unaffected [20]. In the carboxypropylation and carboxypentylation of MLS, the aliphatic hydroxyl groups may be substituted with carboxypropyl or carboxypentyl functions, generating 1-CPRMLS and 5-CPEMLS (Scheme 3b,c). 1 H-NMR spectra of MLS, 1-CPRMLS, and 5-CPEMLS in D 2 O are shown in Figure 2. In contrast to that of 1-CPRLS, two weak peaks located at δ ≈ 0.75-1.15 and 1.5-2.0 ppm ranges are observable in 1-CPRMLS spectrum, and this demonstrates that the formation of 1-carboxypropyl MLS was ineffective. This limited efficiency can be attributed to two reasons: (1) the ionization efficiency of the aliphatic hydroxyl group is about 80 times lower than that of their phenolic counterparts [64]; and (2) steric hinderance may affect the reactivity of the nucleophile and the carbocation intermediate through the S N 1 pathway in the production of 1-CPRMLS (Scheme 3b). Similarly, some broad and weak peaks in the range of δ ≈ 1.25-2.30 ppm were detected in the 5-CPEMLS spectrum. In this case, the reduction in the reactivity of chemical and thus limited sulfoproplyation can be related to the large gap between the halogen and the carboxyl group. Additionally, dimethyl sulfate (Scheme 3a) was used for synthesizing methylated lignosulfonate (MLS) [20,63,64]. The representative qualitative 1 H-NMR spectrum supports the theory that dimethyl sulfate can afford the selective methylation of the phenolic hydroxyl groups in lignosulfonate [20]. Alternatively, the aliphatic hydroxyl groups in lignosulfonate remained unaffected [20]. In the carboxypropylation and carboxypentylation of MLS, the aliphatic hydroxyl groups may be substituted with carboxypropyl or carboxypentyl functions, generating 1-CPRMLS and 5-CPEMLS (Scheme 3b, c). 1 H-NMR spectra of MLS, 1-CPRMLS, and 5-CPEMLS in D2O are shown in Figure 2. In contrast to that of 1-CPRLS, two weak peaks located at δ ≈ 0.75-1.15 and 1.5-2.0 ppm ranges are observable in 1-CPRMLS spectrum, and this demonstrates that the formation of 1-carboxypropyl MLS was ineffective. This limited efficiency can be attributed to two reasons: (1) the ionization efficiency of the aliphatic hydroxyl group is about 80 times lower than that of their phenolic counterparts [64]; and (2) steric hinderance may affect the reactivity of the nucleophile and the carbocation intermediate through the SN1 pathway in the production of 1-CPRMLS (Scheme 3b). Similarly, some broad and weak peaks in the range of δ ≈ 1.25-2.30 ppm were detected in the 5-CPEMLS spectrum. In this case, the reduction in the reactivity of chemical and thus limited sulfoproplyation can be related to the large gap between the halogen and the carboxyl group.

FT-IR Spectrum Analysis
To analyze the carbonyl groups of LS, 1-CPRLS, 5-CPELS, 1-CPRMLS, and 5-CPEMLS more accurately, FT-IR spectra of these samples were compared, as in Figure 4. Compared with the peak of LS, one noticeable difference in peak intensities at 1595 cm −1 appeared in the spectra of 1-CPRLS, 5-CPELS, 1-CPRMLS, and 5-CPEMLS, which is assigned to C = O stretching [20]. These results document the presence of the carboxypropyl and carboxypentyl side chains and successful formations of 1-CPRLS, 5-CPELS, 1-CPRMLS, and 5-CPEMLS. In addition, it can be seen that this peak for 1-CPRLS and 5-CPELS was stronger than that for 1-CPRMLS and 5-CPEMLS, which confirms the reduction of reactivity of aliphatic hydroxyl groups compared with phenolic groups in reacting with 2-chlorobutanoic acid and 6-chlorohexanoic acid in the carboxypropylation and carboxypentylation reactions (i.e., confirming the results of 1 H-NMR analysis). Additionally, the peak corresponding to the C-O stretching of C-O-C was observed at 1042 cm −1 [95,96]. The results show an increase in the absorption of this peak for 1-CPRMLS and 5-CPEMLS compared to for 1-CPRLS and 5-CPELS. Based on this analysis, it is claimed that the -OH of the phenolic groups of LS was substituted with -OCH 3 groups after methylation treatment. of LS, one noticeable difference in peak intensities at 1595 cm −1 appeared in the spectra of 1-CPRLS, 5-CPELS, 1-CPRMLS, and 5-CPEMLS, which is assigned to C = O stretching [20]. These results document the presence of the carboxypropyl and carboxypentyl side chains and successful formations of 1-CPRLS, 5-CPELS, 1-CPRMLS, and 5-CPEMLS. In addition, it can be seen that this peak for 1-CPRLS and 5-CPELS was stronger than that for 1-CPRMLS and 5-CPEMLS, which confirms the reduction of reactivity of aliphatic hydroxyl groups compared with phenolic groups in reacting with 2-chlorobutanoic acid and 6-chlorohexanoic acid in the carboxypropylation and carboxypentylation reactions (i.e., confirming the results of 1 H-NMR analysis). Additionally, the peak corresponding to the C-O stretching of C-O-C was observed at 1042 cm −1 [95,96]. The results show an increase in the absorption of this peak for 1-CPRMLS and 5-CPEMLS compared to for 1-CPRLS and 5-CPELS. Based on this analysis, it is claimed that the -OH of the phenolic groups of LS was substituted with -OCH3 groups after methylation treatment.

Taguchi Method and Optimization
The Taguchi method is a systematic approach to finding the optimal combination of input parameters. A four-factor, three-level Taguchi orthogonal array (OA) design was used to study the

Taguchi Method and Optimization
The Taguchi method is a systematic approach to finding the optimal combination of input parameters. A four-factor, three-level Taguchi orthogonal array (OA) design was used to study the effect of reaction parameters on the carboxypropylation and carboxypentylation of LS. The selected process variables were temperature (60, 70, and 80 • C), ClCBA i /LS (1.0, 1.5, and 2.0 mol·mol −1 ), reaction time (1.0, 1.5, and 2.0 h), and water/isopropyl (H 2 O/IPA) ratio (10%, 15%, and 20% v/v). ClCBA i was used as an abbreviation for chloro carboxylic acid including ClCBA 1 for 2-chlorobutanoic acid, and ClCBA 2 for 6-chlorohexanoic acid in the synthesis of 1-CPRLS or 5-CPELS, respectively. The results of these experiments are reported in Table 2.
The dependent variables were the charge density (CD) and molecular weight (Mw). The optimal reaction conditions for the production of 1-CPRLS and 5-CPELS with the highest CD and Mw could be obtained by systematically analyzing the results of CD and Mw in Table 2. Interestingly, the optimal conditions for the carboxypropylation and carboxypentylation of lignosulfonate were NaOH   * ClCBA i = Chloro carboxylic acid including ClCBA 1 (2-chlorobutanoic acid for 1-CPRLS production) and ClCBA 2 (6-chlorohexanoic acid for 5-CPELS production). In all reactions, NaOH was 30 wt.%. Figure 5 shows the relationship between the reaction conditions and the DS of 1-CPRLS and 5-CPELS. As can be seen, the DS significantly depended on the reaction conditions. Maximum DSs of 0.70 and 0.66 mol·mol −1 were obtained under the conditions of 1.0 mol mol −1 of ClCBA i /LS ratio, 80 • C, 2 h, 15 vol.% of H 2 O/IPA ratio, and 30 wt.% NaOH for 1-CPRLS and 5-CPELS, respectively. As the reactions were endothermic, the increase in temperature facilitated the reaction and thus increased the DS [97]. Additionally, the highest DS was found for the 15 % H 2 O/IPA ratio of the reaction medium. Water plays a significant role in the carboxypropylation and carboxypentylation of LS by facilitating the diffusion of reagents and making them more accessible to lignosulfonate [98,99]. However, the decrease in the DS at the water content of 20% could be due to a dilution effect. It has been reported that a mixture of isopropanol, water, and NaOH forms a two-phase liquid system consisting of IPA, water, and a small quantity of NaOH in the solvent-rich layer; and sodium hydroxide, water, and a very small amount of IPA in the water-rich one [97,100]. In this case, increasing the water proportion to 20 vol.% in the reaction medium would lower the concentrations of both the reagent and the nucleophile in the organic phase, hampering the reaction rate [97]. Furthermore, the maximum DS was obtained for the samples reacted for 2 h. Thus, prolonging the reaction time provided favourable conditions for carboxyalkylation (i.e., better contact between the etherifying agent and lignosulfonate) [31,98]. Finally, the best ratio of ClCBA i /LS in this work was 1.0 mol.mol −1 for both reactions. With further increase of this ratio from 1.0 to 2.0 mol·mol −1 , the undesired products were probably produced more dominantly, and the DSs of 1-CPRLS and 5-CPELS decreased [31].  Table 2 for the production of 1-CPRLS and 5-CPELS. Table 3 summarizes the properties of LS, MLS, 1-CPRLS, 5-CPELS (produced under the optimal conditions), 1-CPRMLS, 5-CPEMLS, as well as 1-CELS reported in the literature [20]. As can be seen, there was a higher carboxylic acid group content for 1-CPRLS and 5-CPELS compared with LS (2.37 and 2.13 vs. 0.08 mmol·g −1 , respectively), which reflects the success of the carboxyalkylation of LS. The higher carboxylate group content of 1-CPRLS compared to 5-CPELS may be due to the lower reactivity of 6-chlorohexanoic acid compared to 2-chlorobutanoic acid, as stated earlier. The longer chain hydrocarbon of 2-chlorobutanoic acid than in 2-chloropropionic acid (i.e., the carboxyl donor in 1-CELS production) is a possible reason for the lower carboxylic acid group content of 1-CPRLS compared to 1-CELS [20]. Furthermore, 1-CPRLS with a higher molecular weight (87,900 g·mol −1 for 1-CPRLS compared with 42,400 and 46,500 g mol −1 for 5-CPELS and 1-CELS) may equip the polymer with a coagulating affinity for solution systems (e.g., dye solutions).
The empirical formula of LS, MLS, 1-CPRLS, 5-CPELS, 1-CPRMLS, and 5-CPEMLS are also given in Table 3. Elemental analysis included the C 9 formula of C 9.00 H 9.47 S 0.45 O 5.71 , C 9.00 H 10.87 S 0.35 O 6.53 , and C 9.00 H 10.98 S 0.34 O 6.01 for LS, 1-CPRLS, and 5-CPELS, respectively. It is evident that the oxygen and hydrogen contents were higher for 1-CPRLS and 5-CPELS than for LS, confirming the success of carboxypropylation and carboxypentylation [31]. The yield of the products was also in a similar order, even though a slightly higher yield was obtained for 1-CELS (Table 3).

Dye Removal
In this work, a 100 mg·L −1 dye concentration was chosen as a model wastewater, which is based on the available literature (i.e., 50-250 mg L −1 dye concentration in the wastewater of the textile industry) [101]. Figure 6 depicts the impact of lignosulfonate derivatives on the removal efficiency of EV cationic dye from the model wastewater samples. As is evident, the maximum EV removal was found between 82 and 98 wt.% for LS, 1-CELS, 1-CPRLS, and 5-CPELS ( Figure 6). The results confirm the improved performance of modified lignosulfonate samples-particularly 1-CPRLS-in removing the EV dye.

Carboxyalkylated Lignosulfonate Comparison
The results of this work confirmed that changing the type of carboxylic acid group donor will have a considerable impact on the mechanism of the carboxyalkylation reaction of lignosulfonate, the reaction conditions required for achieving the best results, and hence on the properties of carboxyalkylated lignosulfonate. In addition, due to the differences in charge density and molecular weights of these modified lignosulfonates, they would have different flocculation performance and dye removal efficiency if used as coagulants ( Figure 6). In this work, we introduced a more appealing reaction system using less solvent (i.e., 30 vol% in this work vs. more than 90 vol% in our previous work) [20] for producing a carboxyalkylated lignosulfonate with better properties. The use of less solvent for modifying lignosulfonate, while generating a better lignosulfonate derivative, will have less significant environment footprints and be more accommodative for the chemical industry.

Conclusions
In this work, 1-carboxypropyl and 5-carboxypentyl lignosulfonates were successfully produced under different conditions, and the mechanism of the grafting reactions and the properties of the products were comprehensively analyzed by FT-IR, 1 H-NMR, and 13 C-NMR analyses. The methylation experiment confirmed the limited attachment of the carboxyethylate group to the aliphatic hydroxy groups of lignosulfonate. The impacts of the reaction conditions using Taguchi orthogonal design on the charge density, molecular weight, and degree of substitution for 1-CPRLS and 5-CPELS were also studied. The maximum CDs of  According to Table 3, lignosulfonate derivatives had a higher charge density and molecular weight than LS (except for methylated samples), and these properties have profound impacts on their interaction with dye molecules in solutions [28,29,102,103]. In this study, the presence of carboxylate groups along with different carboxyalkylated chains (1-CELS, 1-CPRLS, and 5-CPELS) provided a large number of available adsorption/interaction sites for dyes [104]. The removal mechanism could be explained by the combination of charge neutralization and polymer bridging, which led to their agglomeration and thus removal from the solution. The higher efficiency for 1-carboxypropyl lignosulfonate at the optimum dosage (0.75 g·g −1 ) of 1-CPRLS/EV was due to its higher charge density and molecular weight ( Table 3). The reduction in dosage needed for the removal and increased removal efficiency are critical factors in the dye removal, as a smaller dosage would generate a smaller amount of precipitated wastes. A combination of higher removal and lower dosage would also lead to precipitated flocs with a higher proportion of dyes. This lesser use of lignin polymer would help to reduce the cost of this flocculation process. It was also seen that the optimized dosages for lignosulfonate derivatives were in a smaller range than that for lignosulfonate, which stems from the higher charge density of the lignosulfonate derivatives. At a higher charge density, a small dosage increase may lead to the repulsion of dye segment or dye/lignosulfonate flocs with anionic charges that may repel each other, leading to more limited dye removal [102]. Compared with 1-CELS, 1-CPRLS was more effective at a lower required dosage, which is attributed to the higher molecular weight of 1-CPRLS.

Carboxyalkylated Lignosulfonate Comparison
The results of this work confirmed that changing the type of carboxylic acid group donor will have a considerable impact on the mechanism of the carboxyalkylation reaction of lignosulfonate, the reaction conditions required for achieving the best results, and hence on the properties of carboxyalkylated lignosulfonate. In addition, due to the differences in charge density and molecular weights of these modified lignosulfonates, they would have different flocculation performance and dye removal efficiency if used as coagulants ( Figure 6). In this work, we introduced a more appealing reaction system using less solvent (i.e., 30 vol.% in this work vs. more than 90 vol.% in our previous work) [20] for producing a carboxyalkylated lignosulfonate with better properties. The use of less solvent for modifying lignosulfonate, while generating a better lignosulfonate derivative, will have less significant environment footprints and be more accommodative for the chemical industry.

Conclusions
In this work, 1-carboxypropyl and 5-carboxypentyl lignosulfonates were successfully produced under different conditions, and the mechanism of the grafting reactions and the properties of the products were comprehensively analyzed by FT-IR, 1 H-NMR, and 13 C-NMR analyses. The methylation experiment confirmed the limited attachment of the carboxyethylate group to the aliphatic hydroxy groups of lignosulfonate. The impacts of the reaction conditions using Taguchi orthogonal design on the charge density, molecular weight, and degree of substitution for 1-CPRLS and 5-CPELS were also studied. The maximum CDs of −3.45 and −2.94 meq g −1 , Mws of 87,938 and 42,364 g·mol −1 , and DSs of 0.70 and 0.66 mol·mol −1 were obtained for 1-CPRLS and 5-CPELS, respectively, under the optimum conditions of 1.0 mol·mol −1 of 2-chlorobutanoic acid or 6-chlorohexanoic acid/lignosulfonate ratio, 80 • C, 2 h, and 15% v/v of H 2 O/IPA. The 1-CPRLS and 5-CPELS (produced under the optimum conditions), as well as 1-CELS, were applied for removing EV dyes, and the results confirmed the superior efficiency of 1-CPRLS as a coagulant for the dye removal due to its higher charge density and M W .