Synthesis of Magnetic Lignin-Based Hollow Microspheres: A Highly

Research Article pubs.acs.org/journal/ascecg

Synthesis of Magnetic Lignin-Based Hollow Microspheres: A Highly Adsorptive and Reusable Adsorbent Derived from Renewable Resources Yinliang Li, Miao Wu, Bo Wang, Yuying Wu, Mingguo Ma, and Xueming Zhang* Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China S Supporting Information *

ABSTRACT: Lignin, a byproduct of the wood-pulping industry, is mostly treated as a noncommercialized waste product. Therefore, it is significant to study its potential for the conversion of this renewable and sustainable resource into high-valued chemicals and materials. In this study, a renewable lignin-based material with high performance in wastewater treatment has been explored on account of its satisfactory properties and being environmentally friendly. Herein, lignin hollow microspheres (LHM) were facilely prepared from esterified organosolv lignin with maleic anhydride (MA) via self-assembly in the mixed tetrahydrofuran−Fe3O4 nanoparticles aqueous media. Moreover, the magnetic lignin spheres (MLS) were also successfully fabricated by introducing Fe3O4 nanoparticles. The structural changes of esterified lignin polymers were identified and morphology and property of obtained LHM and MLS were characterized by means of TEM, SEM, VSM and FT-IR. In addition, the adsorption capacities of MLS for methylene blue and Rhodamine B from aqueous solutions were also comparatively investigated. It was observed from SEM and TEM images that the LHM from larch lignin exhibited uniform spherical and dense surface, but that from poplar lignin was not rigid enough to keep the perfect spherical shape and partially collapsed. The adsorption capacity results showed that the MLS from larch lignin exhibited better adsorption properties for methylene blue (31.23 mg/g) and Rhodamine B (17.62 mg/g) than that from poplar lignin (25.95 and 15.79 mg/g, respectively). Simultaneously, the adsorption kinetics and adsorption isotherm experiments indicated that the data were agreed well with the pseudo-second-order and Langmuir model, respectively. Moreover, after three cycles of desorption, the removal efficiencies of the MLS from larch and poplar lignin could still reach more than 98% and 96%, respectively. Therefore, the developed magnetic lignin-based hollow microspheres has shown a great potential as a low-cost, highly adsorptive and reusable adsorbent for the applications in the wastewater treatments. KEYWORDS: Lignin, Magnetic hollow microspheres, Dye, Methylene blue, Rhodamine B, Adsorption, Desorption

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INTRODUCTION

of the lignin-based adsorbent brings in a new area for their applications on the field of wastewater treatments.7 Organic dyes are widely applied by clothing manufacturer and dyeing industries. However, the presence of dyes in polluted water has a negative effect on photosynthetic rate of aquatic flora,10 and it could also cause a serious health threat to human beings and animals.11,12 Therefore, the total or partial removal of organic dyes is vital for the treatment of effluent.13 Recently, various techniques for removal of dyes from wastewater have been reported, such as precipitation, adsorption, flocculation and biological processes. Adsorption techniques have been shown to be useful methods.13 In recent years, the biosorbents derived from biomass and natural materials have attracted considerable interest because of their low cost, ease of preparation and high effectiveness for the

Lignin, the second most abundant biomacromolecule next to cellulose, is a phenolic polymer derived primarily from phenylpropanoid units, specifically syringyl (S), guaiacyl (G), and p-hydroxyphenol (H) monomers and its potential structure is shown in Figure 1. It gives plants resistance against chemical or microbial attack and protects plants from other environmental stresses.1 The estimated annual production of commercial lignin is more than 70 million tons.2 Most commercial lignins are produced as a side product in pulping industries and biorefineries,3,4 and the price of lignin is only about $200 per ton in the Chinese market.5 Thus, scientists have made a lot of efforts to explore the high value-added utilization of lignin resources. However, the application of lignin is mainly concentrated in the general industrial fields, for example, resin products,6 concrete water reducers,7 hydrogels8 and pesticide dispersants.9 Developing new lignin-based materials with high performance is encouraged, and preparation © XXXX American Chemical Society

Received: June 7, 2016 Revised: August 1, 2016

A

DOI: 10.1021/acssuschemeng.6b01244 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Potential structure of lignin macromolecule. (b) Three primary lignin monomers: sinapyl alcohol (syringyl, S), coniferyl alcohol (guaiacyl, G) and p-coumaryl alcohol (p-hydroxyphenol, H).

removal of dyes.12−14 Lignin, as a nontoxic and inexpensive natural material,15 a great deal of research has been recently focused on the development of the removal of heavy metal ions and toxic dyes using lignin or its derivatives as adsorbents (Table 1). It has been shown that the adsorption capacity of the

generate the secondary pollution.21−23 Therefore, further investigation into the preparation of recyclable and reusable lignin-based adsorbents with higher adsorption capacities toward wastewater treatment is required. It has been shown that magnetic separation technology is the ideal way to solve this problem because adsorbents can be easily separated from the solution by introducing an external magnetic field.24−26 Moreover, the magnetic sorbents can improve the adsorption capacity of the sorbents, on account of that the magnetic nanoparticles embedded in polymer materials can improve the electrostatic interaction.14 In general, the sphere structures of adsorbents have been proven to be excellent adsorbents in wastewater treatment due to their suitable size, delivering ability and higher surface area.27 There are different techniques for producing lignin microspheres, such as emulsification,28 templated synthesis and selfassembly.5 Lignin supracolloids have been synthesized from water-in-oil (W/O) microemulsions formulated with a mixture of nonionic surfactants and a colloidal dispersion of a low molecular weight alkali lignin.28 In addition, Qian et al.7 has reported that nanocapsules were obtained from lignin via selfassembly. In a typical self-assembly, the process is very simple, only mixing two kinds of antisolvents is needed to obtain the products. Therefore, in the present study, the renewable lignin hollow microspheres (LHM) were prepared from esterified organosolv lignin (OL) by an inverse suspension copolymerization method with minor modifications.7,29 To solve the recyclability of adsorbents, the magnetic lignin hollow microspheres (MLS) were fabricated by introducing Fe3O4 nanoparticles. These adsorbents were characterized by means of FT-

Table 1. Applicability of Sorbent Based on Lignin in the Removal of Pollutants sorbent

pollutants

comments (mg/g)

lignin modified lignin

Cr(III) Pb(II) Cd(II) Cu(II) Cd(II) methylene blue reactive dye brilliant red HE-3B brilliant red HE-3B methylene blue Rhodamine B crystal violet

17.97 8.2−9.0 6.7−7.5 87.05 137.14 34.20 10.173 11.61 7.215 7.309 12.594

kraft lignin modified lignin industrial lignin sawdust

ref. 15 16 17 18 13 19

lignin-based materials depends on their oxygen functionalities, such as phenolic and alcoholic hydroxyl, carbonyl, carboxyl, methoxyl and aldehyde groups.20 However, it has been shown that the saturated adsorption capacity for organic dyes was not high enough for the most of lignin-based adsorbents. In addition, the adsorbents could not be easily separated and recycled, which would limit its applications in many fields and B


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The obtained LHM were assigned as SL1, SL2, SL3, SP1, SP2 and SP3 according to previously esterified lignin samples, respectively. Moreover, the LHM obtained from controlled larch and poplar lignin samples (without esterification) were named as SL0 and SP0, respectively. Preparation of Fe3O4 Nanoparticles. Fe3O4 nanoparticles were prepared according to the reported method with minor modifications.22 Briefly, 4.335 g of FeCl3·6H2O and 1.99 g of FeCl2·4H2O were added into 50 mL of water to form a solution, then 500 mL of ammonium hydroxide (1 M) was injected into the solution under vigorous stirring. After ultrasonic dispersion, the solution was heated at 70 °C for 30 min with stirring. Subsequently, 5.10 g of sodium citrate was added, and then the solution was also stirred for 2 h at 40 °C. Finally, the Fe3O4 nanoparticles were separated by a magnet and washed three times with deionized water. Moreover, Fe3O4 nanoparticles were dispersed into water to prepare a solution for further use, and the concentration of Fe3O4 nanoparticle was 8 × 10−2 mg/ mL. Preparation of Magnetic Lignin Hollow Microspheres (MLS). For a typical run, the esterified lignin (20 mg) was first dissolved in THF (20 mL) to form a clear solution. Then antisolvent (water, 16 mL) was added dropwise into the initial clear solution for the formation of lignin hollow microspheres, and subsequently, Fe3O4 nanoparticles solution (64 mL) was added into the mixture solution, in which the magnetic particles were embedded in lignin particles or attached to particle’s surface. The generated magnetic lignin hollow microspheres (MLS) were separated by a magnet, and washed three times with deionized water. Finally, the obtained magnetic lignin hollow microspheres were freeze-dried and collected for the utilization of adsorption of organic dyes. Similarly, the MLS were also assigned as MSL0, MSL1, MSL2, MSL3, MSP0, MSP1, MSP2, and MSP3 according to above-described lignin hollow microspheres (LHM), respectively. Characterizations. The results are shown in Electronic Supporting Information. Adsorption Experiments. The adsorption capacities of the MLS were evaluated by the adsorption of model organic dyes: methylene blue and rhodamine B. Kinetic studies for methylene blue dyes were conducted by using 350 mg MLS in 25 mL of the dyes solution (150 mg/L) at 40 °C, and the flasks were placed into a thermostatic shaker at 40 °C at a rate of 100 rpm. Then the MLS were removed from the solution by magnetic separation in the different time from 10 to 360 min. Adsorption isotherms for methylene blue dyes were conducted in the different concentrations from 100 to 600 mg/L at 40 °C for 24 h. Rhodamine B was also be used to perform the same adsorption experiments as described above and the results are shown in Electronic Supporting Information. In this process, the concentration of the methylene blue dyes in the solution was always determined with an UV−vis spectrophotometer. The adsorption capacity (qe, mg of dye/g of adsorption) of dyes was calculated by the following equation combined with the above measured concentrations:

IR, SEM, TEM and VSM, and the adsorption capacity along with reusability for organic dyes were also investigated to evaluate the application prospective in dealing with wastewater.

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EXPERIMENTAL SECTION

Materials. The poplar and larch wood samples were obtained from the experimental farm of Beijing Forestry University. Tetrahydrofuran (THF), ethanol, acetone and sodium citrate were purchased from Beijing Chemical Works (Beijing, China). Maleic anhydride (MA) and iron(II) chloride tetrahydrate were purchased from XiLong Chemical Co., Ltd. (Guangzhou, China). Ferric chloride came from Tianjin Institute of Fine Chemicals (Tianjin, China). Methylene blue (MB) was purchased from Sinopharm Chemical Reagent Co., Ltd. The MB solutions were prepared by dissolving MB powder in distilled water, and their concentrations were analyzed by an UV spectrophotometer (UV-2300 Shanghai, China). All chemicals are analytical grade and used without further purification. Separation of Lignin Fractions from Poplar and Larch Wood. The extraction of lignin fractions were based on an organosolv method proposed by Wu.30 Briefly, the wood powder was extracted with 90% (v/v) acetone/water in a Soxhlet apparatus for 24 h. The dewaxed wood powder (40 g) was dispersed in aqueous ethanol (400 mL, ethanol/water: 80/20, v/v), which was then added into the Parr reactor (Parr Instrument Company, USA), and heated to 220 °C and maintained for 30 min. At the end of the reaction, the treated wood powder was collected using a Buchner funnel and washed several times with ethanol. The filtrate was evaporated under vacuum pressure to remove the ethanol. Subsequently, the obtained residue was poured into water and the pH of solution was adjusted to the required value (pH = 1.5−2) by adding 1 M HCl to precipitate lignin. Finally, the lignin fractions were collected by centrifugation and freeze-dried. The yield (%, Klason lignin) of organosolv lignin was 72.1% and the impurity of total sugars was 1.41%.30 Preparation of Lignin Hollow Microspheres (LHM). In general, lignin macromolecules are hydrophilic due to the presence of hydroxyl and carboxyl groups, which is difficult for lignin molecules to aggregate when antisolvents added. Therefore, with the aim of obtaining lignin spheres, the lignin polymer should be treated by hydrophobic modification.7 Chen et al.31 have reported that the hydroxyl group in lignin can be blocked by incorporating new carbonyl (CO) groups, which enhanced the hydrophobic properties of the modified lignin samples. Therefore, organosolv lignin fractions were esterified with different weight of maleic anhydride (MA) according to MA to lignin weight ratios of 0.7:1, 1.0:1 and 1.3:1, respectively. In a typical procedure, 0.6 g of lignin and 0.4 g of MA were dissolved in 8 mL of acetone, and the reaction system was heated at reflux temperature (60 °C) for 7 h. Then, the reaction mixture was evaporated under vacuum pressure to remove acetone. The obtained lignin was dissolved in 8 mL of ethanol and the pH of solution was adjusted to the required value (pH = 2) by adding of 1 M HCl solutions, then the reaction system was again heated at reflux temperature (60 °C) for 12 h. Finally, the solvent was evaporated under vacuum at 55 °C (Figure S1), the esterified organosolv lignin samples were washed with distilled water to remove unreacted anhydrides. Thus, the esterified lignin samples from larch and poplar with the different MA to lignin weight ratios of 0.7:1, 1.0:1 and 1.3:1 were coded as L1, L2, L3 and P1, P2, P3, respectively. In addition, the controlled lignin samples (without esterification) from larch and poplar were noted as L0 and P0, respectively. The degree of esterification was calculated with weight percent gain (WPG), in which reaction degree was determined by the differences in freeze-dried weight of the sample before (W1) and after (W2) modification according to the following equation:31

WPG (%) = (W2 − W1)/W1 × 100

qe =

(co − ce) × v m

(2)

Where co (mg/L) and ce (mg/L) are the initial and the equilibrium concentrations of dye. v (L) is the volume of aqueous solution, and m (g) is the weight of the adsorbents. Desorption and Reusability. According to the previous literature,4,32 the desorption experiments have been carried out using 2 M NaCl+0.05 M NaOH or 0.4 M NaCl+0.2 M NaOH. Moreover, it has been proved that the lignin microspheres are stable and will not disaggregate when the pH < 12.7 Therefore, the desorption and regeneration processes were studied using the elution solution containing 0.4 M NaCl + 0.002 M NaOH (pH < 12). The adsorption−desorption cycle was carried out only in the 100 mg/L concentration of methylene blue. The regeneration efficiency of the MLS was calculated by the ratio of the amount of the adsorbed dyes at the nth time to that at the first time.

(1)

According to the typical self-assembly, the resulting lignin was immediately dissolved in THF to prepare 1 mg/mL lignin solution for spheres preparation. Then, 80 mL of water was added dropwise into the mixture solution with a dropping speeds of 10 mL/h, thus the lignin hollow microspheres (LHM) were prepared via self-assembly. C


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RESULTS AND DISCUSSION Characterization of Esterified Lignin Polymers. The organosolv lignin (OL) is used as an amphiphilic polymer to

and 1.3:1, respectively. The results indicated that the extent of reaction was increased with the rise of MA to lignin weight ratios for larch lignin samples. Similarly, the esterified poplar lignin exhibited the same trend. 31 P NMR methodology is a direct analytical method for quantifying the major hydroxyl groups in lignin.33 To investigate further the modification on the effects of hydroxyl groups in the lignin polymers, quantitative 31P NMR technique was applied (Figure S2 and Table S2). Signals from guaiacyl hydroxyl group were clearly observed in the larch lignin, whereas signals from guaiacyl and syringyl type hydroxyl groups were all qualitatively detected in the poplar lignin. The content of aliphatic OH for both larch and poplar lignin was found to be decreased greatly after modification. By contrast, the hydroxyl groups from total phenolic and carboxylic units were found to be decreased mildly. These results demonstrated that the esterification was successfully achieved, while it mainly occurred in aliphatic OH groups instead of aromatic OH groups. The FT-IR spectra of the virgin and esterified lignin samples are shown in Figure S3. Judging from the absorption peak at 3427 cm−1 attributed to the stretching of hydroxyl groups in phenolic and aliphatic structures, esterification reactions with MA indeed resulted in the decrease in the hydroxyl groups of lignin. In addition, it could also be observed that the intensity of band at 1713 cm−1 (carbonyl group) in the esterified lignin samples was higher than that in the unmodified samples, which demonstrated that the successful anhydride modifications were achieved in this process.34 The bands at 1328, 1267 and 1227 cm−1 related to ring breathing with C−O stretching, and the 1328 cm−1 band was associated with syringyl units and 1267 cm−1 band with guaiacyl units, which indicated that the lignin fraction from poplar was classified as syringyl-guaiacyl lignin type whereas that from larch was defined as guaiacyl lignin type. Characterization of the Lignin Hollow Microspheres (LHM). To compare the different morphology and adsorption

Table 2. Weight Percent Gain (WPG %) Values of Esterified Lignin Sample lignin samples

WPG (%)

L1 L2 L3 P1 P2 P3

11.61 17.66 18.58 16.23 16.70 22.66

prepare lignin hollow microspheres (LHM) via self-assembly because it could provide enough chemical reactivity for further utilization due to the presence of phenolic units and aliphatic hydroxyl groups.29 To synthesize lignin spheres, the lignin polymers was esterified by maleic anhydride to enhance their hydrophobic properties. First, the organosolv and esterified lignin factions were characterized by means of molecular weight, degree of esterification and 31P NMR spectra. As shown in molecular weight results (Table S1), the weight-average molecular weights (Mw) of larch and poplar lignin increased from 2050 to 3200 g/mol and from 2380 to 4130 g/mol, respectively. The dramatic increase of molecular weights revealed the successful cross-linking between the hydroxyl group and maleic anhydride was achieved, which would be beneficial for the formation of lignin spheres. In addition, degree of esterification was evaluated by weight percent gain (WPG) value.31 The results for esterified lignin samples from larch and poplar with the different MA to lignin weight ratios are summarized in Table 2. It was noted that the respective WPG values from larch were 11.61%, 17.66% and 18.58% with different MA to lignin weight ratios of 0.7:1, 1.0:1

Figure 2. TEM and SEM images of lignin hollow microspheres (LHM) prepared with unmodified larch (a, c) and poplar (b, d) lignin. D


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Figure 3. Size distribution of lignin hollow microspheres (LHM), (a) LHM from larch; (b) LHM from poplar.

Figure 4. (a) TEM images and the size distribution of Fe3O4 nanoparticles. (b) Magnetization curves of Fe3O4 and MLS; the inset is the separation of MLS by a magnet.

Figure 5. SEM images of magnetic lignin spheres (MLS). (a) MSL0; (b) MSL1; (c) MSL2; (d) MSL3; (e) MSP0; (f) MSP1; (g) MSP2 and (h) MSP3.

capacity between different substructures of lignin, the lignin hollow microspheres from both larch and poplar organosolv

lignin were fabricated. As shown in Figure 2, the chaplet-like lignin hollow colloidal spheres were formed through gradual E


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Figure 6. (a) Effect of contact time on the adsorption capacity of MLS for methylene blue; The insets are the original dye solution and the separation of the dye-loaded magnetic lignin spheres (MLS) with an external magnetic field. (b) Pseudo-second-order kinetic model.

subunits and poplar lignin consists mainly syringyl (S) and guaiacyl (G) subunits.35 It has been reported that only the acetylated alkali lignin (ACL) in THF could be used to prepare colloidal spheres with the addition of water due to the hydrophobic interaction.7 However, it was noted that the nanoscaled lignin hollow microspheres were also constructed from unmodified organosolv lignin fractions (Figure 2). Furthermore, it could be clearly found that the lignin microspheres possessed single hole, and the thickness of the shell was about 200 nm (Figure 2a,c). Therefore, it could be presumably concluded that the self-assembly mechanism for lignin spheres not only depended on hydrophobic interactions including van der Waals and π−π interactions due to modifications, but also was determined by the molecular weight and structure subunits of lignin fractions. To understand the mechanism for the formation of the LHM, we monitored the assembly processes using SEM, in which the morphology changes of aggregates were characterized as a function of added water content in the THF/water solutions (Figure S4). On the basis of the above observations and general features of the LHM, we proposed a mechanism for the formation of the lignin hollow microspheres as shown in Figure S5. Although the method for preparing LHM was similar to that of some other colloids made by amphiphilic polymers, the assembly process for lignin hollow microspheres has its own unique characteristics, such as hollow structure with holes in shell. It have been suggested that lignin is a sheet like macromolecule in solution and the π−π interactions among the aromatic groups play important roles when the lignin molecules gathered mutually to form the aggregates.5,36 Therefore, at the early stage of the reaction, a few esterified lignin molecules with stronger hydrophobic properties connected with each other in the manner of a head-to-tail arrangement (Figure S5a), which was

Table 3. Adsorption Kinetic Constants Modeled by a Pseudo-Second-Order Equation sample

qe, exp (mg/g)

qe, calc (mg/g)

K (g/mg/min)

R2

MSL1 MSP1

9.94 9.37

10.11 9.72

0.01044 0.00621

0.9995 0.9997

Figure 7. Adsorption isotherm for the adsorption of methylene blue onto MLS at 40 °C.

hydrophobic aggregation of the polymeric chains in THF-H2O dispersion media. It was noted that LHM from larch organosolv lignin exhibited uniform spherical and dense surface, while that from poplar organosolv lignin was not rigid enough to keep the perfect spherical shape and partially collapsed. This was presumably caused by the different molecular weights and structure subunits as larch lignin mainly comprises guaiacyl (G)

Table 4. Langmuir and Freundlich Isothermal Adsorption Equation Parameters for the Adsorption of Methylene Blue Langmuir parameters

Freundlich parameters 2

sample

qmax, exp (mg/g)

qmax, calc (mg/g)

b (mg/L)

R

MSL1 MSP1

31.23 25.95

33.78 28.90

0.066 0.032

0.9917 0.9929

F

1/n

KF(mg/g)

R2

0.3521 0.3788

5.81 3.52

0.9791 0.9903


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Figure 8. (a) Effect of recycling of magnetic lignin spheres (MLS) on dyes adsorption. (b) SEM images of magnetic lignin spheres (MSL1). (c) SEM images of magnetic lignin spheres after 6 days desorption (MSL1).

beneficial to help the lignin molecules to form the shells of the LHM.5,7 As water content was further increased, the hydrophobic blocks aggregated to form the two-dimensional (2D) polymer network sheets with hole (imperfection sites), which could be observed clearly in the corresponding SEM image (marked with red circles and blue circles in Figure S4) and the possibly internal structure was proposed in Figure S5b. Meanwhile, the total interfacial energy increased with further increasing the water content, so the sheets started to bend to reduce its total energy and further reaction between the sheets generated a loosely cross-linked hollow sphere as shown in the yellow square of Figure S4b.37,38 In addition, some remaining less hydrophobic sheets participated in additional flat bridge formation between hollow spheres or still aggregated on the surface of the already formed LHM (marked with green circles in Figure S4). The drive for formation of LHM was to minimize the total interface surface area and the total energy due to the increase of water content in THF solution. Therefore, the similarly shaped dimpled hollow microspheres were finally formed as shown in Figures S4f and S5e. The effect of esterification on the average diameter of lignin colloidal spheres was studied as shown in Figure 3. It was noted that the respective average diameter of LHM from esterified larch lignin was 929, 1530 and 1019 nm with different MA to lignin weight ratios of 0.7:1, 1.0:1 and 1.3:1, respectively (Figure 3a). Moreover, the average sphere diameter of controlled sample was comparatively low with 449 nm. Interestingly, the average sphere diameter increased with the rise of MA to lignin weight ratios for organosolv lignin samples. Increasing the MA to organosolv lignin weight ratios from 0.7:1 to 1.0:1 gave a large increment; however, the average diameter decreased notably as the ratio further increased. This might due to the stronger hydrophobic properties that made it much easier for the association of lignin molecular and then resulting in formation of lignin spheres. The trend of the change of average sphere diameter from poplar lignin fractions was almost the same as those from larch lignin fractions. However, it was noted that the average diameter of LHM from larch lignin was smaller than that from poplar lignin. Characterization of the Magnetic Lignin Spheres (MLS). To recover easily adsorbents and avoid secondary

pollution, the magnetic lignin spheres were fabricated by introducing Fe3O4 nanoparticles. The TEM image of the Fe3O4 nanoparticles in water is shown in Figure 4a. It could be clearly found that the average diameter of Fe3O4 nanoparticles was about 11 nm. The magnetic lignin spheres (MLS) was synthesized and the magnetic hysteresis loops of MLS and Fe3O4 nanoparticles were comparatively measured with a LakeShore 730T vibrating sample magnetometer at room temperature as shown in Figure 4b. The MLS and Fe3O4 nanocomposites showed extremely small hysteresis loop, and the saturation magnetization intensities of MLS and Fe3O4 nanoparticles were about 22.7 and 73.9 emu/g, respectively. The magnetization loss of the MLS was probably on account of the presence of nonmagnetic organic components.39 It was noted that the water became clear with introducing magnetic field as shown in the down inserts of Figure 4b, suggesting that the Fe3O4 nanoparticles were immobilized well in the MLS. Therefore, the MLS were expected to respond well to magnetic fields, in which MLS could be easily separated from the solution by a magnet owing to the presence of attached Fe3O4 nanoparticles. The SEM and TEM images of the MLS are shown in Figures 5 and Figures S6. It could be clearly found that some Fe3O4 nanoparticles were attached to the surface of the magnetic lignin spheres as shown in the Figure 5 (marked with yellow circle). It has been reported that Fe3O4 nanoparticles can be embedded strongly in the lignin-based materials via electrostatic interactions, which are expected to interact with electronrich oxygen atoms of hydroxyl groups of lignin.14 Moreover, the Fe3O4 nanoparticles could also be immobilized by entering the holes of MLS as shown in Figure 5 (marked with green circle), resulting in formation of tight adhering composites between Fe3O4 nanoparticles and lignin spheres. The MLS from larch still exhibited smooth and regular surface, which was like one by one “chaplet”, whereas obviously the MLS from poplar partially looked like “collapsed chaplet”. Moreover, it was further confirmed that the lignin microspheres was hollow microspheres with single hole structure as shown in the down inserts of Figure 5c,d. The FT-IR spectra of the LHM and MLS samples are shown in Figure S7. The appearance of a new band at 580 cm−1 was assigned to Fe−O group in Fe3O4, which G


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determined and listed in Table 3. The extremely high correlation coefficients (R2 > 0.999) between t/qt and t indicated the applicability of the pseudo-second-order model to describe the sorption process, which indicated that the adsorption process could be considered as chemisorptions. In addition, the calculated qe values of MLS from esterified poplar and larch lignin (9.72 and 10.11 mg/g, respectively) were also close to the experimental data (9.37 and 9.94 mg/g, respectively) as listed in Table 3, which further confirmed the feasibility of this equation. The effect of contact time on the adsorption of Rhodamine B had the same result as shown in Electronic Supporting Information, and the results revealed that the MLS possessed high adsorption capacity for methylene blue and Rhodamine B dyes. Adsorption Isotherm. It is well-known that the adsorption capacity is associated with the concentration of dye. The effect of the initial concentration on the adsorption of MLS for methylene blue was presented in Figure 7. It could be distinctly found that the adsorption capacity of methylene blue would increase with the increase of initial concentration. However, it trended toward steady state beyond a certain concentration. The Langmuir and Freundlich models were often successfully used to describe the adsorption process equilibrium adsorption isotherms. The Langmuir adsorption equations is given as

obviously indicated Fe3O4 nanoparticles were successfully embedded in MLS. Based on the above results, it was demonstrated that a facile method for the preparation of the MLS was presented, and this process was promising for a largescale production. Adsorption Experiments. The excellent adsorption capacity of the adsorbents depends on both its large surface areas and the number of active sites.27 Spheres size would be superior to other shapes in terms of adsorption capacity, and it has been reported that the smaller spheres exhibited the higher adsorption capacity.22 As a consequence, in order to better investigate the adsorption capacity of MLS on organic dyes, the larch and poplar lignin modified with MA to lignin weight ratios of 0.7:1 were chosen on account of their wonderful magnetic response and sufficiently small diameters. The insets of Figure 6a shows the adsorption capacity of MLS for methylene blue and Rhodamine B when 350 mg MLS was poured in 25 mL dye solution (60 mg/L) at 40 °C for 6 h. Obviously, the dyes of methylene blue and Rhodamine B could be effectively adsorbed by the MLS. After 6 h, the wastewater containing methylene blue became almost colorless, which indicated that MLS had immense potential in removal of organic dyes from wastewater. The adsorption mechanism of dyes binding onto esterified lignin adsorbent was revealed by infrared spectra.13 The FT-IR spectra of the MLS before and after the adsorption of dyes are shown in Figure S8, and the main relevant IR peaks and their assignment are presented in Table S3. In comparison, the spectra exhibited that the characteristics peaks for the dyes loaded MLS were either minimized or slightly shifted. These results demonstrated that the negatively charged functional groups of the esterified lignin were bound to dyes via electrostatic attraction as both methylene blue and Rhodamine B are positively charged. Simultaneously, van der Waals force derived from numerous benzene rings in the dye molecule and esterified lignin polymer could also play an important role. Furthermore, the adsorption isotherms and kinetic experiments, as well as the reusability property were also explored to evaluate the adsorption capacity of the MLS. Adsorption Kinetics. The effect of contact time on the adsorption of methylene blue by MLS was presented in Figure 6a. As it can be obviously observed, more than 60% of the equilibrium adsorption capacity for methylene blue occurred within 20 min, which indicated that the adsorption took place mainly on the surface of the MLS. The adsorbed amount of the dyes progressed rapidly within 60 min during the adsorption process, then the uptake rate decreased noticeably after 100 min, and there was an obviously difference in the adsorption capacity of poplar and larch esterified lignin. To investigate the kinetic mechanism, the pseudo-second-order equation was used to describe the adsorption process, and the adsorption/time data were plotted in pseudo-second-order model graphs as shown in Figure 6b. The pseudo-second-order model equation is given as t 1 t = + 2 qt q K 2qe e

ce c 1 = + e qe qmax b qmax

(4)

The Freundlich adsorption equations is given as ln qe =

1 ln ce + ln KF n

(5)

Where qmax and b are the saturated adsorption capacity (mg/g) and the constant of adsorption equilibrium (mg/L), respectively. KF and 1/n are the Freundlich constants, which represent the adsorption capacity and adsorption intensity, respectively.27,42 The isotherm parameters are summarized in Table 4. The value of the maximum adsorption capacity for the adsorption of dye was near to the calculated value. The correlation coefficient of Langmuir isotherm was higher than that of Freundlich isotherm, which manifested the adsorptive behaviors of MLS on methylene blue could be better represented by the Langmuir equation. In other words, the monolayer coverage process of MLS was approved by the best fit of equilibrium data. The Freundlich constant (1/n) gave indication for the sorption intensity of the sorbent, which meant 0.1 < 1/n ≤ 0.5 was easy to adsorb; 0.5 < 1/n ≤ 1, was difficult to adsorb; 1/n > 1 was quite difficult to adsorb.14 Hence, the values of 1/n of dye on the MLS lay between 0.3 and 0.4, indicating that the adsorption of MLS on dyes was much easier. The qmax values of MLS from esterified poplar and larch lignin were 25.95 and 31.23 mg/g, which indicated that the MLS possessed high adsorption capacity for the organic dyes. Desorption and Reusability. The recycling of adsorbents is also a key parameter for evaluating the potential application in wastewater treatment. Regenerative functions of MLS are shown in Figure 8a, in which the adsorbed magnetic lignin spheres were recovered after regeneration in elution solution. Notably, almost no decrease for the adsorption ability was detected after the second cycle. In addition, after three cycles of desorption, the removal efficiencies of the organic dyes for

(3)

Where k2 (g/mg/min) is the pseudo-second-order rate constant, and qe and qt are the amounts of dye adsorbed at equilibrium and at time t, respectively.40,41 The pseudo-second-order rate constant k and the calculated and experimental equilibrium sorption capacities qe were H



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ACKNOWLEDGMENTS The work was supported by the Fundamental Research Funds for the Central Universities (No. 2015ZCQ-CL-03), Natural Science Foundation of China (No. 31470606, 31070557) and China Ministry of Education (No. 111).

esterified larch and poplar lignin still could reach more than 98% and 96%, respectively. Interestingly, the magnetic lignin hollow microspheres still exhibited uniform spherical after 6 days desorption as shown in Figure 8c. Therefore, the results indicated that the MLS possessed good recyclability as it could be easily separated from the wastewater by introducing an external magnetic field, avoiding the secondary pollution of the adsorbents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01244. Characterizations; formula for lignin reacted with MA for esterification; molecular weights and polydispersity (Mw/ Mn) of the lignin fractions; quantitative 31P NMR and FT-IR spectra of the esterified lignin fractions; SEM images of self-assembly structures of LHM at different water contents in THF/water solution; proposed mechanism of the LHM formation in THF/water medium; TEM images of magnetic lignin spheres (MLS); FT-IR spectra of different samples: (L1) SL1; (F1) MSL1; (Y1) SP1; (F2) MSP1; FT-IR spectra and peak assignments for the lignin before and after sorption of dye; the effect of contact time on the adsorption capacity of MLS for Rhodamine B; pseudo-second-order and pseudo-first-order kinetic model; adsorption kinetic constants; adsorption isotherm for the MLS of Rhodamine B; Langmuir and Freundlich isotherm model; and the isothermal adsorption equation parameters (PDF).

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REFERENCES

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CONCLUSIONS In this study, lignin hollow microspheres (LHM) from unmodified and esterified organosolv lignins were all successfully fabricated using a facile method. The LHM from larch exhibited perfect chaplet-like spherical shape but that from poplar partially collapsed. Moreover, the magnetic lignin spheres (MLS) were also successfully fabricated by introducing Fe3O4 nanoparticles, and they could be easily separated from the effluent by introducing an external magnetic field. The adsorption results revealed that the MLS could efficiently adsorb organic dyes from the effluent and it could also be easily regenerated. The esterified larch and poplar MLS had respective values of the maximum adsorption capacities of 31.23 and 25.95 mg/g for methylene blue, respectively. The adsorption kinetics and adsorption isotherm were agreed well with the pseudo-second-order and Langmuir model, respectively. In addition, the MLS offered outstanding cycling capability for the removal of organic dyes. This work demonstrated that the MLS provides high utility value and attractive application prospects in wastewater treatments.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*X. Zhang. E-mail: [email protected]. Tel/Fax: +86-1062336189. Notes

The authors declare no competing financial interest. I


Research Article

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Synthesis of Magnetic Lignin-Based Hollow Microspheres: A Highly

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