An Early Study on the Synthesis of Lignin-Graft-(Net-Poly(acrylamide-co-N,N (cid:48) methylenebisacrylamide)), Characterization of the Produced Copolymer, and Evaluation of Its Performance as Adsorbent for Lead Removal from Wastewater Purposes

: A lignin-graft-(net-poly(acrylamide-co-N,N (cid:48) methylenebisacrylamide)) copolymer was synthesized by conventional free-radical crosslinking copolymerization using conventional and microwave heating. Grafting of the polymer network onto lignin was conﬁrmed by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), and elemental analysis. The performance of the modiﬁed materials for the removal of lead from water was evaluated. The materials obtained by the two types of heating showed excellent removal efﬁciencies: sample HLigAM4h, 96%; and sample HLigAMMW5, 86%. The maximum adsorption capacity of HLigAM4h was 209.82 mg g − 1 . The obtained copolymer (sample HLigAM4h) was characterized by X-ray photoelectron spectroscopy (XPS) and SEM/EDS after its evaluation as an adsorbent, which conﬁrm the adsorption of Pb 2+ . This is the ﬁrst of a series of studies on the topic, of a preliminary nature, with several other ones coming up in due time.


Introduction
The use of natural biopolymers, such as lignocellulosic biomasses or its components, in different important technological areas is a viable option to decrease waste generation problems and reduce or replace the use of synthetic polymers. One of the main components of lignocellulosic biomasses is lignin, which is the second most abundant source of renewable materials in nature, after cellulose. Lignin has a heterogeneous structure composed mainly of three phenylpropane units: guaiacyl (G unit), syringyl (S unit), and p-hydroxyphenyl (H unit), where the difference is the degree of methoxyl substitution on the phenolic ring [1,2]. Although vast amounts of lignocellulosic biomasses are used in low added-values applications, such as composting, animal feeding, or burnt for energy producing purposes, their use for more appealing applications has grown in importance in the last few years [3]. If more sophisticated biopolymers are required, some of these applications require to chemically modify the starting biopolymers obtained from the lignocellulosic biomasses. Numerous physical and chemical methods can be used to perform chemical modifications to add industrial value to lignin, or develop new polymeric materials, due to the large number of functional groups present in lignin and its physical properties [4,5].
In this contribution, we report the synthesis of a grafted copolymer based on lignin and PAM, using conventional and microwave heating. PAM was grafted onto lignin by crosslinking free radical copolymerization using N,N'methylenebisacrylamide as crosslinker. A crosslinked structure was chosen to prevent dissolution in water of the obtained material. The grafted copolymer was characterized by FTIR, NMR, TGA, SEM/EDS, XPS, and elemental analysis.
The grafted copolymer was evaluated in batch adsorption experiments for removal of Pb 2+ from aqueous solutions. The adsorption efficiencies of Pb 2+ in unmodified lignin, PAM, and in our crosslinked grafted material were compared, obtaining a significantly better performance when our material is used.

Conventional Heating
The modified lignin was produced by combined grafting and crosslinking, following a free radical copolymerization chemical route [19]. First, lignin (1.0 g) was dissolved in 85 mL of NaOH (1.25 M). Ammonium persulfate (0.1 g) and N, N -methylenebisacrylamide (0.2 g) were added as initiator and crosslinker, respectively. The solution was heated at 60 • C and stirred for 30 min. Then, acrylamide (2.0 g) was added and allowed to react for 2, 4, and 24 h. The reaction was terminated by reducing the temperature to 20 • C. After finishing the reaction, 200 mL of water was added and acidified with H 2 SO 4 until pH 1.5 was obtained. The precipitated material was separated by centrifugation (5000 rpm), washed with water, and dried at 60 • C, for 24 h [17,19,31]. The synthetized materials, which correspond to a 2, 4, and 24 h reaction time, were named HLigAM2h, HLigAM4h, and HLigAM24h, respectively. The homopolymer (PAM) was synthesized under the same conditions, without lignin.
where W 0 and W 1 are masses (g) of lignin and AM, respectively, and W 2 is mass of copolymer (g). V 1 in Equation (2) is nitrogen gain obtained from elemental analysis measurements by subtracting nitrogen content present in unmodified lignin (N, %). MW AM and MW N are acrylamide and nitrogen molecular weights (g mol −1 ), respectively.

Microwave Heating
The same amounts of lignin, ammonium persulfate, N,N-methylene bisacrylamide and a solution of NaOH (0.5 M) as a solvent were added into a flask. After stirring for 5 min, acrylamide was added; stirring continued for 5 more min. Lignin modification was carried out in a glass container placed in a microwave oven (StartSYNTH, Milestone, Shelton, Connecticut, USA) at a power of 100 W, for 5 min. Reactions at other times were also carried out, but 5 min provided the best results. Once the reaction time was reached, the reaction was stopped, and the same separation procedure was followed. The synthetized material was named HLigAMMW5.

Characterization
Kraft Lignin, PAM and the modified materials were characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and elemental analysis, to confirm that grafting took place, and to evaluate changes in the structure of lignin. FTIR spectra were recorded in a range of wave numbers from 4000 to 650 cm −1 , using an M series, Midac Corporation spectrometer (Westfield, MA, USA). Potassium bromide (KBr) pellets containing the samples were prepared by mixing and pressing 2 mg of dried sample with 200 mg of KBr. 1 H NMR spectra were recorded using 400 MHz, Varian, MR, USA, and 600 MHz, JEOL, ECZ600R spectrometers, and 13 C NMR spectra were recorded also using a 400 MHz, Varian, MR, Santa Clara, CA, USA, spectrometer, operating at 151 MHz. The samples were dissolved in deuterated dimethyl sulfoxide (DMSO-d6) and analyzed. SEM images and energy dispersive spectroscopy (EDS) were obtained using a JSM-5900-LV, JEOL microscope, Peabody, MA, USA. TGA analyses were performed under nitrogen atmosphere by increasing temperature from 30 to 900 • C at a heating rate of 5 • C min −1 , using a TGA550, Discovery Series, equipment, USA. XPS was used to calculate elemental compositions and analyze chemical states; a VERSAPROBE II, Physical Electronics, Chanhassen, MI, USA, spectrometer was used. A Flash 200, FPD 800, Thermo Scientific elemental analyzer, Waltham, MA, USA, was also used for elemental analysis of samples.

Adsorption of Pb 2+
Adsorption of Pb 2+ batch experiments were performed at 25 • C, pH of 5.5, stirring of 240 rpm, as well as variable Pb 2+ initial concentration and contact time. Several reports confirm that the optimal pH for Pb 2+ removal lies between 5 and 6 [26,[34][35][36]. Adsorption capacity at low pHs is limited due to the protonation of -NH, -OH and -COOH, which are potential adsorption sites in lignin [31,36]. On the other hand, a high pH results in the precipitation of Pb(OH) 2 and Pb(OH) 4 2− , which may adversely affect the adsorption process and reduce the adsorption capacity [26,36,37]. Samples of the modified lignin copolymer (0.1 g) were soaked in a lead nitrate solution (100 mL) using a flask, for 2 h. Samples of 5 mL supernatant solution were separated and then filtered through 110 mm filter paper Whatman ® every 20 min. The concentrations of Pb 2+ after adsorption were measured by using an atomic absorption spectrophotometer (PinAAcle 900 Serie AA, Perkin Elmer Spectrometers). The adsorption capacity (q, mg g −1 ) and the removal efficiency (R, %) of Pb 2+ ions were calculated according to the following expressions: where C 0 and C e (mg L −1 ) represent the initial and equilibrium concentrations of Pb 2+ ions, respectively; V (L) is the volume of the solution and m (g) is the amount of lignin-modified copolymer (adsorbent).

Lignin Modifications
Lignin-graft-(net-poly(acrylamide-co-N,N methylenebisacrylamide)) copolymers with different compositions were synthesized by crosslinking free radical copolymerization. As explained earlier, a polymer network structure of the grafted copolymer was sought to prevent dissolution in water of the modified material. The proposed reaction route is shown in Figure 1. Thermal (conventional) heating has been used in several reports where the reaction time is typically 24 h [14,16,38]. The use of microwave heating has been reported in the synthesis of some copolymers, decreasing the reaction time, and obtaining higher conversions [39,40]. In this contribution, both routes were used. Thermal heating was carried out at three different reaction times (2, 4, and 24 h). Microwave heating was conducted at 1, 3, 5, and 7 min; but the best results were obtained at 5 min of reaction. The yields obtained in samples HLigAM4h and HLigAMMW5 were 26.19 and 25.63%, respectively. Likewise, the corresponding grafting ratios obtained for those samples were 4.56 and 4.01%, respectively.
Lignin-graft-(net-poly(acrylamide-co-N,N′methylenebisacrylamide)) copolymers with different compositions were synthesized by crosslinking free radical copolymerization. As explained earlier, a polymer network structure of the grafted copolymer was sought to prevent dissolution in water of the modified material. The proposed reaction route is shown in Figure 1. Thermal (conventional) heating has been used in several reports where the reaction time is typically 24 h [14,16,38]. The use of microwave heating has been reported in the synthesis of some copolymers, decreasing the reaction time, and obtaining higher conversions [39,40]. In this contribution, both routes were used. Thermal heating was carried out at three different reaction times (2, 4, and 24 h). Microwave heating was conducted at 1, 3, 5, and 7 min; but the best results were obtained at 5 min of reaction. The yields obtained in samples HLigAM4h and HLigAMMW5 were 26.19 and 25.63%, respectively. Likewise, the corresponding grafting ratios obtained for those samples were 4.56 and 4.01%, respectively.

Characterization of Starting and Modifed Lignin Copolymers
The FTIR spectra for PAM (A), Lignin (B), HligAM4h (C), HLigAM24h (D), and HLi-gAMMW5 (E) are shown in Figure 2. The bands around 1600, 1515, and 1420 cm −1 for lignin were assigned to the skeletal vibration of aromatic rings [41,42] and the absorption bands at 1261, 1241, and 1125 cm −1 were attributed to the guaiacyl, gyringyl, and ether bonds of lignin [30]. These absorption bands are clearly seen in the spectra for HLigAM4h, HLigAM24h, and HLigAMMW5, which indicates that the aromatic structure of lignin was not destroyed or changed during de copolymerization [36]. In the case of PAM, the bands at 2960, 1720, and 1660 cm −1 were attributed to N-H, C=O and C=C stretching vibrations, respectively [43]. Notably, in the spectra corresponding to HLigAM4h, HligAM4h and HLigAMMW5 a new adsorption peak around 1720 cm −1 was observed, which was attributed to the carbonyl bond from an amide bond, was observed. This confirms that PAM was successfully grafted onto lignin [8,16,43].

Characterization of Starting and Modifed Lignin Copolymers
The FTIR spectra for PAM (A), Lignin (B), HligAM4h (C), HLigAM24h (D), and HLigAMMW5 (E) are shown in Figure 2. The bands around 1600, 1515, and 1420 cm −1 for lignin were assigned to the skeletal vibration of aromatic rings [41,42] and the absorption bands at 1261, 1241, and 1125 cm −1 were attributed to the guaiacyl, gyringyl, and ether bonds of lignin [30]. These absorption bands are clearly seen in the spectra for HLigAM4h, HLigAM24h, and HLigAMMW5, which indicates that the aromatic structure of lignin was not destroyed or changed during de copolymerization [36]. In the case of PAM, the bands at 2960, 1720, and 1660 cm −1 were attributed to N-H, C=O and C=C stretching vibrations, respectively [43]. Notably, in the spectra corresponding to HLigAM4h, HligAM4h and HLigAMMW5 a new adsorption peak around 1720 cm −1 was observed, which was attributed to the carbonyl bond from an amide bond, was observed. This confirms that PAM was successfully grafted onto lignin [8,16,43].   Figure 3A) and DTGA ( Figure 3B) results for lignin, PAM, HLigAM4h, and HLigAMMW5. Weight losses in the range 40 to 150 °C correspond to water, solvent, and other small molecules. As observed, lignin decomposes over a wide temperature range, from 150 to 500 °C, due to its heterogeneity and the absence of a de-    Figure 3A) and DTGA ( Figure 3B) results for lignin, PAM, HLigAM4h, and HLigAMMW5. Weight losses in the range 40 to 150 • C correspond to water, solvent, and other small molecules. As observed, lignin decomposes over a wide temperature range, from 150 to 500 • C, due to its heterogeneity and the absence of a defined structure [17]. In this temperature range, the carbon-carbon bonds between lignin units and the side chain of aromatic rings are broken [44,45]. At 500 • C, lignin has lost 56.72% of its mass, while copolymers HLigAM4h and HLigAMMW5 have lost 35.69 and 35.76% of their masses, respectively. This result indicates that the modified lignin copolymers have better thermal stabilities than lignin and PAM. The maximum mass loss temperature is 554 • C for lignin, 824 • C for HLigAM4h and 831 • C for HLigAMMW5. It is also observed in Figure 3 that lignin losses mass much more pronouncedly than modified lignin copolymers. This result suggests that unmodified lignin may be severely affected by the harsh conditions of the grafting process.    Figure 6 shows 600 MHz 1 H-NMR spectra for lignin and HLigAMMW5. Chemical shifts of the aromatic rings (pcoumaryl, coniferyl, and sinapyl), vinylic protons [2], and methoxy group (OCH3) [46] were identified in the lignin spectra. In the spectra for the modified copolymers, chemical shifts from 7.03 to 7.21 ppm are attributed to the presence of PAM. Shifts between 4.0 and   H-NMR spectra for lignin and HLigAMMW5. Chemical shifts of the aromatic rings (pcoumaryl, coniferyl, and sinapyl), vinylic protons [2], and methoxy group (OCH 3 ) [46] were identified in the lignin spectra. In the spectra for the modified copolymers, chemical shifts from 7.03 to 7.21 ppm are attributed to the presence of PAM. Shifts between 4.0 and 4.6 are attributed to different PAM branches grafted onto lignin. The shifts related to the CH 2 group of PAM linked to the phenolic O atom of lignin [8,18], or to the CH 2 group linked to O in the crosslinker, are also observed. Shifts at 4.6 ppm can be attributed to protons associated with grafted PAM, but bonded to other phenolic structures, different from that proposed in Figure 1. The new signals between 2.7 and 2.2 ppm can be attributed to the CH and CH 2 groups of PAM. Sample HLigAMMW5 exhibits new shifts between 6.2 to 6.0 ppm, compared to bare lignin. These shifts are associated with hydroxyl groups, mainly in sugar units (HO-) [47]. Since Kraft lignin has no shifts in this region, they should have appeared after the grafting process, possibly by formation of a carboxylate group from hydrolysis of the amide group. These shifts might be attributable to the crosslinking process through the phenolic group of the crosslinking agent with the immediate breaking of the bond, where pendant HO-groups remain. Aromatic rings change with this bonding and breaking process, acquiring a quinoid conformation. Protons associated with these structures may show shifts from 6.2 to 5.7 ppm, according to the neighbor functional groups.    The 13 C-NMR spectra for Lignin, HLigAM24h, and HLigAMMW5 are shown in Figure 7. The signal at 40 ppm corresponds to DMSO-d6. Methoxy groups are observed at 55 ppm. The carbon in the aromatic skeleton of lignin is observed from 110 to 170 ppm. The signals at 112 and 115 ppm are related to C2 and C5 of the guaiacyl unit; the signal at 130 ppm corresponds to C1, in the guaiacyl ring [2]. The new signals at 35, 38, 49, 57, and 66 ppm in the spectrum for HLigAM24h can be attributed to carbons from the CH and CH 2 groups of PAM [26]. The increase in the signals at 166, 171, and 172 ppm may be attributed to the carboxyl group [36].
SEM images and EDS analyses for lignin, HLigAM4h, and HLigAM4h with adsorbed lead (HLigAM4h-Pb) are shown in Figure 8. As observed, the surface of lignin was irregular and relatively fragmented. After grafting/crosslinking modification, the surface was more regular, with larger particles. An increase in oxygen and sodium is observed in the EDS spectrum. It is reported in different publications that the hydroxyl groups of lignin react with NaOH under alkaline conditions, generating a strong nucleophile, which serves as a catalyst for carboxylation reactions [36,48,49]. The increase in the percentage of Na in the modified materials suggests that this bond is present.
The results of elemental analysis for lignin, HLigAM4h, and HLigAMMW5 are shown in Table 1. As observed, the content of nitrogen and oxygen increased, which may be attributed to the amide groups (-CONH 2 ) of PAM, which was grafted onto the lignin backbone.  [2]. The new signals at 35, 38, 49, 57, and 66 ppm in the spectrum for HLigAM24h can be attributed to carbons from the CH and CH2 groups of PAM [26]. The increase in the signals at 166, 171, and 172 ppm may be attributed to the carboxyl group [36]. SEM images and EDS analyses for lignin, HLigAM4h, and HLigAM4h with adsorbed lead (HLigAM4h-Pb) are shown in Figure 8. As observed, the surface of lignin was irregular and relatively fragmented. After grafting/crosslinking modification, the surface was   SEM images and EDS analyses for lignin, HLigAM4h, and HLigAM4h with adsorbed lead (HLigAM4h-Pb) are shown in Figure 8. As observed, the surface of lignin was irregular and relatively fragmented. After grafting/crosslinking modification, the surface was more regular, with larger particles. An increase in oxygen and sodium is observed in the EDS spectrum. It is reported in different publications that the hydroxyl groups of lignin react with NaOH under alkaline conditions, generating a strong nucleophile, which serves as a catalyst for carboxylation reactions [36,48,49]. The increase in the percentage of Na in the modified materials suggests that this bond is present. The results of elemental analysis for lignin, HLigAM4h, and HLigAMMW5 are shown in Table 1. As observed, the content of nitrogen and oxygen increased, which may be attributed to the amide groups (-CONH2) of PAM, which was grafted onto the lignin backbone.

Effect of material and likely removal mechanism
Pb 2+ adsorption experiments were performed to evaluate the effect of lignin modification on Pb 2+ removal efficiency from a water solution (80 mg/L of Pb 2+ ). Results are shown in Figure 9. Unmodified lignin had a removal efficiency of 43% while the removal efficiency of PAM was 73%, but unlike lignin, PAM dissolves in water. Copolymer HLigAM4h reached a 92% removal efficiency, almost 50% higher than unmodified lignin, with the additional advantage that it does not dissolve in water and its separation is easier. Adsorption capacities at 120 min of contact are 28.02, 55.40, and 74.33 mg g −1 for unmodified lignin, PAM, and HLigAM4h, respectively. Figure 8c shows images corresponding to HLigA4h-Pb. It is observed that lead particles adhered to the copolymer surface. Sodium content in HLigAM4h-Pb decreased almost entirely, which suggests that sodium may favor the removal of Pb 2+ , as an ion exchange device [50,51]. Figure 10a shows XPS spectra before and after adsorption of Pb 2+ . Characteristic peaks corresponding to carbon and oxygen are observed. Pb4f and Pb4d peaks can be observed after adsorption of Pb 2+ , which indicates that Pb 2+ has been adsorbed on the surfaces of the adsorbent in different binding

Effect of Material and Likely Removal Mechanism
Pb 2+ adsorption experiments were performed to evaluate the effect of lignin modification on Pb 2+ removal efficiency from a water solution (80 mg/L of Pb 2+ ). Results are shown in Figure 9. Unmodified lignin had a removal efficiency of 43% while the removal efficiency of PAM was 73%, but unlike lignin, PAM dissolves in water. Copolymer HLigAM4h reached a 92% removal efficiency, almost 50% higher than unmodified lignin, with the additional advantage that it does not dissolve in water and its separation is easier. Adsorption capacities at 120 min of contact are 28.02, 55.40, and 74.33 mg g −1 for unmodified lignin, PAM, and HLigAM4h, respectively. Figure 8c shows images corresponding to HLigA4h-Pb. It is observed that lead particles adhered to the copolymer surface. Sodium content in HLigAM4h-Pb decreased almost entirely, which suggests that sodium may favor the removal of Pb 2+ , as an ion exchange device [50,51]. Figure 10a shows XPS spectra before and after adsorption of Pb 2+ . Characteristic peaks corresponding to carbon and oxygen are observed. Pb4f and Pb4d peaks can be observed after adsorption of Pb 2+ , which indicates that Pb 2+ has been adsorbed on the surfaces of the adsorbent in different binding forms [52]. XPS spectra for O 1s is shown in Figure 10c,d. The three peaks at 531.78, 530.61, and 529.59 eV that result from the deconvolution of O1s, for sample HLigAM4h, can be assigned to O-H, C=O, and C-O, respectively. The O-H and C=O peaks had a chemical shift at 533.01 and 531.37 eV after the adsorption experiment, which suggests that they are involved in the adsorption of Pb 2+ [53]. A new peak at 527.27 eV was observed and assigned to the oxygen atom in Pb-O. The Pb4f XPS spectrum of Figure 10b can be divided into Pb4f5/2 (143.55 eV) and Pb47/2 (138.67 eV); these two peaks may be related to Pb-O and Pb-O-C, respectively [52,54]. assigned to O-H, C=O, and C-O, respectively. The O-H and C=O peaks had a chemical shift at 533.01 and 531.37 eV after the adsorption experiment, which suggests that they are involved in the adsorption of Pb 2+ [53]. A new peak at 527.27 eV was observed and assigned to the oxygen atom in Pb-O. The Pb4f XPS spectrum of Figure 10b can be divided into Pb4f5/2 (143.55 eV) and Pb47/2 (138.67 eV); these two peaks may be related to Pb-O and Pb-O-C, respectively [52,54].   assigned to O-H, C=O, and C-O, respectively. The O-H and C=O peaks had a chemical shift at 533.01 and 531.37 eV after the adsorption experiment, which suggests that they are involved in the adsorption of Pb 2+ [53]. A new peak at 527.27 eV was observed and assigned to the oxygen atom in Pb-O. The Pb4f XPS spectrum of Figure 10b can be divided into Pb4f5/2 (143.55 eV) and Pb47/2 (138.67 eV); these two peaks may be related to Pb-O and Pb-O-C, respectively [52,54].

Effect of Lignin Modification Time
Pb 2+ removal experiments from water using lignin-modified copolymers obtained at 2, 4, and 24 h, with initial Pb 2+ concentrations of 80 and 220 mg L −1 , are shown in Figure 11. Removal efficiencies higher than 85% are obtained after 20 min for materials HLigAM2h and HLigAM4h. Maximum removal efficiencies are reached at about 60 min. These efficiencies are 85, 91 and 77% for HLigAM2h, HLigAM4h, and HLigAM24h at 80 mg L −1 , respectively; and 87, 96, and 94% for HLigAM2h, HligAM4h, and HligAM24h at 220 mg L −1 , respectively. Removal efficiencies and adsorption capacities are improved by increasing the initial concentration of Pb 2+ . These improvements may be explained by better interaction and increased mass transfer between Pb ions and the surface of the materials [26,27]. The three lignin-modified copolymers have high removal efficiencies; however, the best performance is obtained with HLigAM4h. The maximum adsorption capacity for HLigAM4h was obtained using an initial concentration of 220 mg L −1 , which corresponds to 209.82 mg g −1 . This adsorption capacity is higher than the capacities reported for bioadsorbents synthesized by different polymerization techniques, such as the case of lignin microspheres prepared by inverse suspension copolymerization with polyethylenimine, where an adsorption capacity of 33.9 mg g −1 was reported [29]. In the case of graft copolymerization of cellulose acetate with acrylic acid and acrylamide, the adsorption capacity was 66.67 mg g −1 [55]. Finally, in the case of poly(acrylic acid) grafted onto an acid-pretreated alkali lignin-hydrogel prepared by crosslinking free radical copolymerization, the adsorption capacity was 1.076 mmol g −1 [19].
2, 4, and 24 h, with initial Pb 2+ concentrations of 80 and 220 mg L −1 , are shown in Figure  11. Removal efficiencies higher than 85% are obtained after 20 min for materials HLigAM2h and HLigAM4h. Maximum removal efficiencies are reached at about 60 min. These efficiencies are 85, 91 and 77% for HLigAM2h, HLigAM4h, and HLigAM24h at 80 mg L −1 , respectively; and 87, 96, and 94% for HLigAM2h, HligAM4h, and HligAM24h at 220 mg L −1 , respectively. Removal efficiencies and adsorption capacities are improved by increasing the initial concentration of Pb 2+ . These improvements may be explained by better interaction and increased mass transfer between Pb ions and the surface of the materials [26,27]. The three lignin-modified copolymers have high removal efficiencies; however, the best performance is obtained with HLigAM4h. The maximum adsorption capacity for HLigAM4h was obtained using an initial concentration of 220 mg L −1 , which corresponds to 209.82 mg g −1 . This adsorption capacity is higher than the capacities reported for bioadsorbents synthesized by different polymerization techniques, such as the case of lignin microspheres prepared by inverse suspension copolymerization with polyethylenimine, where an adsorption capacity of 33.9 mg g −1 was reported [29]. In the case of graft copolymerization of cellulose acetate with acrylic acid and acrylamide, the adsorption capacity was 66.67 mg g −1 [55]. Finally, in the case of poly(acrylic acid) grafted onto an acid-pretreated alkali lignin-hydrogel prepared by crosslinking free radical copolymerization, the adsorption capacity was 1.076 mmol g −1 [19].

Effect of Heating Source
It has been reported that the use of microwave irradiation as a heating source in polymerization studies decreases considerably the time required to complete the polymerization [40]. That is why we also conducted lignin modification using microwave irradiation as heating source. Figure 12 shows a comparison of Pb 2+ adsorption efficiencies using modified materials obtained with thermal (H-LigAM4h) and microwave promoted heating (HLigAM-MW5). Removals efficiencies of 81% and 86% were archived at 20 and 60 min when the initial concentration of Pb 2+ was 80 mg L −1 , using HLigAM-MW5. Similar results were obtained with HLigAM4h. In the case of Pb 2+ = [220 mg L −1 ], efficiencies of 72 and 80% were obtained at 20 and 60 min, respectively. It is observed that in the case of modified lignin copolymers obtained by using microwave irradiation their adsorption capacity decreases as the initial concentration of Pb 2+ increases. In contrast, the opposite effect is observed in modified copolymers synthesized by thermal heating, where the Figure 11. Comparation of removal efficiencies of the synthesized copolymers.

Effect of Heating Source
It has been reported that the use of microwave irradiation as a heating source in polymerization studies decreases considerably the time required to complete the polymerization [40]. That is why we also conducted lignin modification using microwave irradiation as heating source. Figure 12 shows a comparison of Pb 2+ adsorption efficiencies using modified materials obtained with thermal (H-LigAM4h) and microwave promoted heating (HLigAM-MW5). Removals efficiencies of 81% and 86% were archived at 20 and 60 min when the initial concentration of Pb 2+ was 80 mg L −1 , using HLigAM-MW5. Similar results were obtained with HLigAM4h. In the case of Pb 2+ = [220 mg L −1 ], efficiencies of 72 and 80% were obtained at 20 and 60 min, respectively. It is observed that in the case of modified lignin copolymers obtained by using microwave irradiation their adsorption capacity decreases as the initial concentration of Pb 2+ increases. In contrast, the opposite effect is observed in modified copolymers synthesized by thermal heating, where the reaction time is longer. This dissimilar behavior may be caused by a lower grating degree or by smaller grafted branches. reaction time is longer. This dissimilar behavior may be caused by a lower grating degree or by smaller grafted branches.

Conclusions
Modified lignin copolymers obtained by grafting of crosslinked PAM onto Kraft lignin using conventional and microwave heating were successfully synthesized. The obtained materials were evaluated as adsorbent materials for the removal of Pb 2+ from a solution of lead in water. Copolymerization time does not seem to be a determining factor in the adsorption of Pb 2+ since high removal efficiencies were observed at the lowest evaluated time of 2 h. A removal efficiency of 94% at 20 min of the adsorption experiment was obtained when using HLigAM4h at an initial Pb 2+ concentration of 220 mg L −1 . A maximum removal efficiency of 96% and a maximum adsorption capacity of 209.82 mg g −1 were obtained. The separation of copolymers after the adsorption experiment was simple due to the fact that the material did not dissolve and almost all the material sedimented when the agitation was turned off, which is a plus in the potential use of these materials for lead removal purposes. The modified lignin copolymer synthesized by microwave heating showed good Pb 2+ removal performance, with a removal efficiency of 86%.

Conclusions
Modified lignin copolymers obtained by grafting of crosslinked PAM onto Kraft lignin using conventional and microwave heating were successfully synthesized. The obtained materials were evaluated as adsorbent materials for the removal of Pb 2+ from a solution of lead in water. Copolymerization time does not seem to be a determining factor in the adsorption of Pb 2+ since high removal efficiencies were observed at the lowest evaluated time of 2 h. A removal efficiency of 94% at 20 min of the adsorption experiment was obtained when using HLigAM4h at an initial Pb 2+ concentration of 220 mg L −1 . A maximum removal efficiency of 96% and a maximum adsorption capacity of 209.82 mg g −1 were obtained. The separation of copolymers after the adsorption experiment was simple due to the fact that the material did not dissolve and almost all the material sedimented when the agitation was turned off, which is a plus in the potential use of these materials for lead removal purposes. The modified lignin copolymer synthesized by microwave heating showed good Pb 2+ removal performance, with a removal efficiency of 86%.

Data Availability Statement:
The data generated in this study are provided in the text, tables, and figures within the manuscript. Further details can be obtained from the corresponding author.