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sodium alginate beads as highly effective adsorbents for cationic organic dyes
BIOMAC-12999; No of Pages 7 International Journal of Biological Macromolecules 139 (2019) xxx
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Lignin-containing cellulose nanocrystals/sodium alginate beads as highly effective adsorbents for cationic organic dyes Mingshuai Ma a,c, Zhong Liu a,⁎, Lanfeng Hui a, Zhen Shang a,c, Shaoyu Yuan a, Lin Dai a,c,⁎, Pengtao Liu a,b, Xinliang Liu b, Yonghao Ni a,c a b c
Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China Guangxi Key Laboratory of Clean Pulp & Paper and Pollution Control, Guangxi University, Nanning 530004, China Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
a r t i c l e
i n f o
Article history: Received 14 July 2019 Received in revised form 29 July 2019 Accepted 1 August 2019 Available online 2 August 2019 Keywords: Lignin Cellulose nanocrystals Adsorption
a b s t r a c t Cellulose nanocrystals (CNCs) is an exciting class of sustainable and carbohydrate material, which has great potential applications in molecular adsorption. However, the complex preparation process and limited adsorption capacity of CNCs hinder its commercial application. In this study, we design a novel functional cellulose nanocrystals-based adsorbent by an ingenious mixing of lignin-containing cellulose nanocrystals (LCNCs), sodium alginate (SA), and calcium chloride solution. Benefiting from the sulfonate groups of lignin, carboxyl groups of SA, the maximum adsorptive capability of LCNCs/SA beads for methylene blue was found to be 1181 mg g−1, which was significantly higher than previously reported biomass-based adsorbents. More importantly, LCNCs/SA beads can be reused several times. This strategy can not only improve the adsorption performance of CNCs-based materials, but also simplify the production technology of CNCs, which greatly promote the commercial application of CNCs materials. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Synthetic dyes are widely used in printing, textile, biomedicine, and other industries. While providing various colorful products, synthetic dyes also make people face the challenge of serious environmental problems [1]. Methylene blue (MB) and its derivatives have been explored in various medical applications including cell staining and imaging, antimalarial agents, rejuvenation, and anticancer research [2–4]. Compared to other hazardous dye molecules, MB is a lower toxic compound. However, relatively high dose of MB (above 7 mg/kg) can also cause harm to the human bodies, such as dizziness, nausea, chest pain, and even Heinz body anemia [5]. Moreover, under the white light, MB can be freely photo-sensibilized and produce singlet oxygen, which can destroy the DNA structures, consequently, impact up the food chain [6]. Therefore, it is necessary to develop effective approaches for removing MB and its derivatives. Adsorption process has many advantages for the removal of organic pollutants, such as low cost, convenient, controllable, simplicity of optimizing, and especially suitable for liquid environment [7–9]. United States Environmental Protection Agency (USEPA) classified adsorption as one of the top control methods [10,11]. In general, the adsorption process can be divided into physisorption and chemisorption. A single ⁎ Corresponding authors at: No.29 at 13th Avenue, TEDA, Tianjin 300457, China. E-mail addresses: [email protected] (Z. Liu), [email protected] (L. Dai).
physisorption process has a fast adsorption rate, but poor robustness and selectivity of adsorption. Thus, for MB and some other organic pollutants, we need to design a multi-functional adsorbent to simultaneously achieve physisorption and chemisorption. Cellulose nanocrystals (CNCs) have emerged as an exciting class of sustainable and carbohydrate materials. CNCs are rod-like nanoparticles having a width less than 10 nm and a length between 200 and 400 nm [12]. Because of the good mechanical strength, large specific surface area, and numerous surface functional groups, they have great potential applications in material reinforcement and molecular adsorption [13,14]. For the production of CNCs, whether chemical treatment (acid/enzymatic hydrolysis) or physical treatment (mechanical treatment), lignin removing is always an important process to obtain pure CNCs. However, recently, some studies reported that a certain remaining of lignin on the surface of CNCs could provide some additional features such as antibacterial activity and thermal stability [15–21]. It is also worth noting that lignin contains many aromatic structures, which not only have various applications [22–28] but also can attract and aggregate with the dye molecules (similar structure) by π-π interaction. Moreover, during the preparation of CNCs by sulfuric acid hydrolysis, the lignin residues have many sulfonyl hydroxide groups, which can provide more adsorption sites and improve adsorption performance. Based on the above background, we prepared lignin-containing CNCs, and developed a green and low-cost technique by the new
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lignin-containing CNCs-based beads for MB removing. This strategy can not only simplify the production technology of CNCs, but also improve the adsorption performance of CNCs-based materials. In this work, steam explosion poplar chips were used as raw materials to produce lignin-containing cellulose nanocrystals (LCNCs) via acidic hydrolysis. Then, LCNCs were mixed with sodium alginate (SA), and dropped into a calcium chloride solution to obtain LCNCs/SA beads (Scheme 1). Finally, the adsorption efficiency and kinetics of MB were systematically investigated. 2. Materials and methods 2.1. Materials Poplar chips were kindly provided by Sun Paper Industry Joint Stock Co., Ltd. (Shandong, China). Sulfuric acid was purchased from China National Pharmaceutical Group Co., Ltd. (Tianjin, China). Calcium chloride (CaCl2), methylene blue (MB), and sodium alginate (SA) were purchased from Sigma-Aldrich, USA. All Chemicals in this study were analytical grade and used without further purification. 2.2. Preparation of lignin-containing cellulose nanocrystals (LCNCs) LCNs were prepared from poplar chips by the acid-catalyzed method. First, poplar chips underwent a steam explosion pretreatment process (210 °C, 10 min), and then rinsed with deionized water until colorless. The obtained poplar power was dried and milled as a brown powder. 8.0 g poplar powders were added into 160 mL 60 wt% sulfuric acid solution and stirred under a fixed agitation of 150 rpm at 48 °C for 2 h. Sulfuric acid hydrolysis was stopped by adding 10-fold volume cold distilled water into the reaction mixture. The mixture was allowed to settle down for 12 h. The insoluble substance was collected by centrifugation (7000 rpm, 5 min) and washed by deionized water (1600 mL, five times) to the neutral condition. The obtained powder was homogenized by the use of a probe-type ultrasonic cell disruptor (Scientz, Ningbo, China) for 20 min in an ice bath. After that, the insoluble part (non-fibrillated cellulose and lignin with a large size) was removed by centrifugation. Finally, LCNCs suspension was dialyzed against distilled
water for three days and then diluted to a concentration of 0.75 wt% with deionized water. The composition in LCNCs was determined based on NREL/TP-51042618 [29]. 0.30 g ± 0.05 g freeze-dried LCNCs sample was treated with 3 mL of 72% (w/w) H2SO4 at 30 °C for 60 min under water bath and stirred every 5–10 min without removing the sample from the bath via glass rod and then diluted the acid to 4% (w/w) by adding deionized water. The suspension was hydrolyzed at 121 °C for 60 min in an autoclave, the residue was filtered through porcelain filter crucibles with sufficient deionized water under vacuum and then dried in an oven at 105 ± 0.2 °C until reaching constant weight. The weight of the residue was Klason lignin. Structural carbohydrates in filtrate was then subjected to do HPLC (Agilent 1200 with a Biorad Aminex HPX87H column) analysis with calibration curves of corresponding sugars. The amount of cellulose and hemicellulose were calculated based on the corresponding monomeric sugars, using a correction of 0.90 (or162/180) for glucose and an anhydro correction of 0.88 (or 132/ 150) for xylose. There were 78.98 wt% cellulose, 14.87 wt% Klason lignin, and 1.38 wt% hemicellulose in the obtained LCNCs. 2.3. Preparation of LCNCs/SA beads 0.3 g SA powder was dissolved into 40 mL deionized water to get a concentration of 0.75 wt% under constant stirring at room temperature. Equal amount of 0.75 wt% LCNCs suspension and SA solution were mixed for 1 h. Then, the mixture (0.375 wt% LCNCs and 0.375 wt% SA) was dropwise added into 2 wt% CaCl2 solution without stirring. The beads were allowed to cross-link in CaCl2 solution for 2 h. After washing with deionized water and removing the excess Ca2+, LCNCs/SA beads were frozen and dried for batch adsorption studies. 2.4. Characterization Surface and cross-section morphologies of the as-prepared lightweight bead were investigated using a Field Emission Scanning Electron Microscopy (FESEM, Hitachi SU-70). The chemical compositions of the resulting LCNCs were analyzed via a two-step acid hydrolysis method and Fourier transform infrared (FT-IR) spectroscopy (NICOLET iS5, Thermo Scientific) was conducted in the range of 4000–500 cm−1
Scheme 1. Preparation of LCNCs, LCNCs/SA beads, and MB adsorption of LCNCs/SA beads.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx
with a resolution of 4 cm−1 and 32 scans. The specific charge quantity q (μeq·g−1) was detected by conductometric titration, and calculated according to the following equation: q¼
Vc m
The relationship between LCNCs/SA beads and MB can be explained by adsorption isotherm parameters [30]. Two-parameter isotherm models, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich could be used to describe adsorption isotherm.
ð1Þ
where V and c are the volume (L) and concentration (μeq·L−1) of titrant, m is the mass of the sample (g).
Langmuir isotherm : qe ¼
ðC −C e ÞV qe ¼ i m
ð2Þ
where qe is the amounts of MB adsorbed at the equilibrium (mg·g−1), ci and ce represent initial and equilibrium concentrations of MB (mg·L−1), respectively. m and V are the weight of beads (g) and volume of the solution (L), respectively.
qm bC e 1 þ bC e
ð3Þ
Freundlich isotherm : qe ¼ aC 1=n e
2.5. Batch MB adsorption studies MB concentrations can be detected by using UV–visible spectrophotometer (Thermo Scientific, EVOLUTION 201) at 463 nm. The absorption quantity of MB can be calculated by the following equation:
3
Temkin isotherm : qe ¼
ð4Þ
RT ln ðAT C e Þ bT
ð5Þ (
"
−β RT ln
Redlich−Peterson isotherm : qe ¼ qm e
1 2 1þ Þ Ce
) ð6Þ
where qe and qm are the absorption quantity of MB per unit adsorbent (mg·g−1) at equilibrium and maximum, respectively. Ce is the equilibrium concentration of MB (mg·L−1). b, a, bT, and β are the constant of Langmuir (L·mg−1), Freundlich (mg1–1/n L1/n·g−1), Temkin (kJ·g·mol−2) and Dubinin-Radushkevich (mg·g−1) isotherm,
Fig. 1. (a) Illustration of the procedure for the preparation of LCNCs/SA beads. Mechanisms of cross-linking of (b) SA/Ca2+, (c) LCNCs/SA/Ca2+, (d) hydrogen bonds between LCNCs and SA.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx Table 1 Isotherm model parameters for adsorption. Isotherm
Parameters
AL NPs
Langmuir
qm (mg·g−1) b (L/mg) R2 a (mg1–1/n L1/n/g) n R2 bT (kJ g/mol2) AT (L/mg) R2 qm (mg g−1) β (mol2/kJ2) R2
1374.85 0.0079 0.9891 90.65 2.47 0.9836 0.081 0.07 0.9936 1065.08 1.50 0.9503
Freundlich
Temkin
Dubinin–Radushkevich
respectively. R and T are the ideal gas constant (8.3145 J·mol−1·K−1) and thermodynamic temperature (K), respectively. n is the constant of the adsorption intensity for the Freundlich model. AT represents the constant of equilibrium binding corresponding to the maximum binding energy. The time-dependent adsorptive processes of MB were analyzed by applying the pseudo-first- and second-order equations as followed [7]: k1 t Pseudo−first−order : logðqe −qt Þ ¼ logqe − 2:303 Pseudo−second−order :
Fig. 2. (a) FT-IR spectra of LCNCs, SA, and LCNCs/SA beads. (b) Photographs of swollen (left top) and dry LCNCs/SA beads (right top). SEM images of the surface (left bottom) and porous morphologies (right bottom) of LCNCs/SA beads.
t 1 t 1 t ¼ þ ¼ þ qt k2 q2e qe v0 qe
ð7Þ ð8Þ
where qe and qt are the absorption quantity of MB per unit adsorbent (mg·g−1) at equilibrium and time t, respectively. k1 (h−1) and k2 (h·g·mg−1) are the rate constant for the pseudo-first and secondorder adsorption, respectively. The parameters of qe and k1, k2 can be obtained experimentally from the slope and intercept of the plot of log(qe - qt) or t/qt versus t. v0 is the initial adsorption rate (h·mg·g−1) which can be calculated as v0 = k2q2e.
Fig. 3. (a) Equilibrium adsorption isotherms of LCNCs/SA beads. (b) Adsorption kinetics, linear fitting of (c) pseudo-first- and (d) pseudo-second-order kinetic models of LCNCs/SA beads. (e) Effect of temperature on MB adsorption capacity and plot of ln Kc vs 1/T. (f) Effect of the pH values on the adsorptive capacity of LCNCs/SA beads.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx Table 2 Thermodynamic parameters of MB adsorption. Adsorbents
LCNCs/SA
ΔG (kJ mol−1) 25
35
45
−6.31
−5.39
−4.47
ΔH (kJ mol−1)
ΔS (kJ mol−1 K−1)
−33.84
−0.092
2.6. Regeneration of absorbents In order to recycle present beads, we used the method reported in the literature with few modifications [31]. In brief, the MB-absorbed beads were incubated stir-treated with 40 mL of 1.0 N HCl for 3 h at 25 °C to desorb the loaded MB. After desorption, the beads were washed with sufficient distilled water and then removed left the water and reused for adsorption. The adsorption/desorption cycles were continuously repeated five times under 25 °C. The amount of MB absorbed onto the beads was determined and calculated as mentioned above.
3. Results and discussion Alginate hydrogel beads can be easily formed in the presence of divalent cations cross-linking agents. Calcium chloride (CaCl2) is one of the most frequently used agents to ionically cross-link alginate. However, the mechanical properties of Ca2+ cross-linked alginate beads are highly depended on the chemical structure of alginate. Alginates are composed of (1–4)-linked α-L-guluronic acid (G units) and β-Dmannuronic acid (M units) monomers which vary in amount and sequential distribution along the polymer chain depending on the source of the alginate. The Ca2+ is believed to bind solely to G units, as the structure of the G units allows a high degree of coordination of the Ca2 + (Fig. 1b) [32–34]. Therefore, in this study, the introducing of lignincontaining CNCs into the traditional alginate beads, one can expect the improvement of mechanical properties and adsorption performance by CNCs and lignin, respectively.
3.1. Preparation of LCNCs/SA beads
Fig. 2a shows the FT-IR spectra of LCNCs, SA, and LCNCs/SA beads. As shown in FT-IR spectrum of LCNCs, the characteristic peaks of cellulose obtained at 1425, 1162, 1112, 1055 and 898 cm−1 [35–37]. The characteristic peaks at 1510 cm−1 and 1456 cm−1 corresponded to the vibration of C_C aromatic skeletal and C−H deformation of the lignin, respectively [15,38]. The LCNCs/SA beads showed the changes in the peak positions, especially observable from 1415 cm−1 (Na-alginate) to 1436 cm−1 (Ca-alginate) for symmetric carboxylate (–COO−) group stretching, and confirmed the complex formation after crosslinking with Ca2+ ions [39]. Photographs of the sample obvious a yellowishbrown and light brown color of the swollen and dry LCNCs/SA beads, respectively. The surface and porous morphologies of the beads were shown in Fig. 2b revealed that LCNCs/SA beads have rough and fold surface and the porous internal structure, which could help to improve the adsorption performance. 3.2. Sorption isotherms The effects of different initial MB concentrations on the adsorption process by LCNCs/SA beads were studied. Fig. 3a shows that with the increasing of initial MB concentration from 41 to 574 mg·L−1, the adsorption quality of MB by LCNCs/SA beads ranged from 309 to 1180 mg·g−1, which could be due to the higher contact probability and stronger force between LCNCs/SA and dye molecules [31]. The model fitting curves and parameters of Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm for MB adsorption on LCNCs/SA beads were shown in Fig. 3a and Table 1. The Langmuir and Temkin isotherm models gave better fits than the Freundlich and Dubinin–Radushkevich isotherm models. According to the Langmuir equation, the maximum uptake capacity (qm) for MB is 1374.85 mg·g−1, which is similar to the experiment value (1180 mg·g−1). In order to reveal the actual adsorption mechanism, some thermodynamic parameters (ΔH, ΔS, ΔG) can be deduced by using the following equations [34]. MB adsorption processes were monitored under 25, 35 and 45 °C, respectively. The amount of MB absorbed onto the asprepared beads was determined and calculated as mentioned above. Kc ¼
Cellulose and sodium alginate (SA) are polysaccharides with a similar chemical structure that provide good chemical compatibility in the resultant composite. LCNCs with hydroxyl (−OH) and sulfate (–SO− 3 ) functional groups can involve in the complexations with Ca2+ crosslinked SA, and form additional hydrogen bonding, resulting in the nano-composite structure (Fig. 1c, d) in LCNCs/SA beads. The lignincontaining CNCs and SA have many sulfate, sulfonyl hydroxide, hydroxyl, and carbonyl groups. When adding the LCNCs/SA mixture into Ca2+ solution, the colloid stability was broken immediately, and Ca2+ was like a magnetic particle to attract LCNCs and SA together. The charge densities of LCNCs and LCNCs/SA were 260 and 400 μeq·g−1, respectively, which were detected by conductometric titration. The negative charge surface of LCNCs/SA beads can contribute to the MB adsorption process.
5
Ca Ce
ð9Þ
ΔG ¼ ΔH−TΔS ln K c ¼
ð10Þ
ΔS ΔH − R RT
ð11Þ
where Kc is the distribution coefficient for the adsorption, Ca and Ce are the amount of dye adsorbed on the beads per liter of the MB solution at equilibrium (mg/L) and the equilibrium concentration (mg/L) of the dye in the solution, respectively. T is adsorption temperature (K). ΔH and ΔS were calculated from the slope and intercept of ln Kc versus 1/T (Fig. 3e), and the results are summarized in Table 2. Negative ΔH values indicated the exothermic nature of the process, whereas a small negative value ΔS suggests that no noticeable change in the structure of
Table 3 Kinetic model parameters for adsorption at 25 °C. Time (min)
0–180 0–15 15–180 a b
Pseudo-first-order model k1 (h−1)
q ea (mg g−1)
0.019 0.065 –
175.34 308.92 –
Pseudo-second-order model R2
k2 (h·g mg−1)
q ea (mg g−1)
v0 (h·mg g−1)
0.9058 0.9999 –
8.24 × 10−4 – 8.25 × 10−4
320.51 – 316.46
29.51 – 34.98
R2 0.9996 – 0.9999
q eb (mg g−1)
310.67
Calculated results. From experiments.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx
adsorption sites at low pH environment. With the increasing pH values of the solution, more anion on the surface of LCNCs/SA beads increased the adsorption performance of the cationic dye. 3.5. Reusability and reproducibility
Fig. 4. Adsorption desorption cycles for LCNCs/SA beads.
LCNCs/SA beads occurs during the adsorption process, which was in agreement with previous works [40,41]. We observed that ΔG increased with increasing temperature, suggesting a reduction in the adsorption with temperature. The decrease in adsorption capacity with increasing temperature may be influenced by the weakening hydrogen bonding and van der Waals interaction between the LCNCs/SA beads and MB molecules [42]. 3.3. Adsorption kinetics The adsorption kinetics of LCNCs/SA beads were analyzed by using the pseudo-first- and second-order equations. Results were shown in Fig. 3b–d and Table 3. Obviously, during the whole adsorption process, the pseudo-second-order kinetic model (R2 = 0.9996) presented a better correlation than the first order (R2 = 0.9058). However, it is worth noting that, at the beginning of the adsorption (before 15 min), the adsorption process demonstrated more excellent correlation by the pseudo-first-order kinetic model (R2 = 0.9999), which could be due to the physisorption in the initial stage. The following process (15–180 min) fit well by the pseudo-second-order model. These results indicated that the MB adsorption process by LCNCs/SA beads included physisorption as well as chemisorption. 3.4. Effect of pH on adsorption The effect of pH on dye removal was studied by changing the pH conditions of the MB solution from 3 to 11 at 25 °C. Fig. 3f shows that the adsorption quality of MB by LCNCs/SA beads increased significantly from 261.9 to 309.9 mg g−1 with the increasing of pH from 3 to 7, then become steady. This phenomenon could be attributed to the changes in the ionic characteristics of the LCNCs and SA chains. The functional groups on the surface of LCNCs/SA beads were protonated or deprotonated under different pH environment. With many sulfate (lignin) and carboxyl (SA) groups, LCNCs/SA beads processed negative surface charge (400 μeq·g−1), which decreased as pH decreases. Excess of hydrogen ions competed with MB molecules (cationic dye) and invaded Table 4 Comparison of adsorption capacity of various bio-based adsorbents for MB. Adsorbent
qmax (mg g−1)
Reference
Cellulose nanocrystal-alginate hydrogel beads Carbon-alginate beads Polyacrylamide/sodium alginate microspheres GO/CA MCMFCs Cr(OH)3-NPs-CNC hybrid nanocomposite LCNCs/SA beads
256 287 1070 182 303 204 1181
[34] [43] [42] [44] [45] [46] This study
From an economic point of view, reusability is an essential property for scale-up of the adsorbent application in practical wastewater treatment. Here, the adsorption-desorption cycles were studied for five times. Fig. 4 shows that the adsorption capacity of LCNCs/SA beads slightly decreased from 1181 to 971 mg g−1 after the fifth repeated regeneration, which confirmed that the LCNCs/SA beads could reuse to adsorb MB with high and stable adsorption capacity. For further investigating the application of LCNCs/SA beads, the maximum adsorptive capacity of MB in this work was compared with various adsorbents in the previous study. From Table 4, LCNCs/SA beads show a superior adsorption performance than that of other bio-based adsorbents. Based on the above results, LCNCs/SA beads can be a promising, eco-friendly, and effective adsorbent for removing MB and other cationic dyes with a very easy and convenient recovering method. 4. Conclusions A novel lignin-containing CNCs-based adsorbent was designed by an ingenious mixing of LCNCs, SA and calcium chloride solution. Benefiting from the sulfonate groups of lignin, carboxyl groups of SA, the maximum adsorptive capability of LCNCs/SA beads for MB was found to be 1181 mg g−1, which was significantly higher than previously reported biomass-based adsorbents. Furthermore, LCNCs/SA beads simultaneously achieve physisorption and chemisorption. The kinetic studies denoted that the initial (before 15 min) and the following stages (15–180 min) of MB adsorption by LCNCs/SA beads followed pseudofirst and second order kinetic model, respectively. More importantly, LCNCs/SA beads can be reused with high adsorption capacity. In summary, this strategy can not only simplify the production technology of CNCs, but also improve the adsorption performance of CNCs-based materials. Acknowledgments We gratefully acknowledge the financial support from the National Key Research and Development Program (Grant No. 2017YFB0307901), Young Elite Scientists Sponsorship Program by Tianjin (TJSQNTJ-2017-19), and the Opening Project of Guangxi Key Laboratory of Clean Pulp and Papermaking and Pollution Control (No. KF201716). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijbiomac.2019.08.022. References [1] M.A. Rauf, S.S. Ashraf, Chem. Eng. J. 151 (2009) 10–18. [2] T. Simon, M. Potara, A.-M. Gabudean, E. Licarete, M. Banciu, S. Astilean, ACS Appl. Mater. Interfaces 7 (2015) 16191–16201. [3] R.H. Schirmer, B. Coulibaly, A. Stich, M. Scheiwein, H. Merkle, J. Eubel, K. Becker, H. Becher, O. Müller, T. Zich, W. Schiek, B. Kouyaté, Redox Rep. 8 (2003) 272–275. [4] S.D. Bukkitgar, N.P. Shetti, Mater. Sci. Eng. C 65 (2016) 262–268. [5] M. Oz, D.E. Lorke, M. Hasan, G.A. Petroianu, Med. Res. Rev. 31 (2011) 93–117. [6] A.B. Albadarin, M.N. Collins, M. Naushad, S. Shirazian, G. Walker, C. Mangwandi, Chem. Eng. J. 307 (2017) 264–272. [7] L. Dai, Y. Li, R. Liu, C. Si, Y. Ni, Int. J. Biol. Macromol. 132 (2019) 478–486. [8] R. Liu, L. Dai, C.-L. Si, ACS Sustain. Chem. Eng. 6 (2018) 15756–15763. [9] Y. Duan, A. Freyburger, W. Kunz, C. Zollfrank, ACS Sustain. Chem. Eng. 6 (2018) 6965–6973. [10] R.S. Blackburn, Environ. Sci. Technol. 38 (2004) 4905–4909. [11] G. Crini, Bioresour. Technol. 97 (2006) 1061–1085. [12] Y. Habibi, L.A. Lucia, O.J. Rojas, Chem. Rev. 110 (2010) 3479–3500.
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Lignin-containing cellulose nanocrystals/sodium alginate beads as highly effective adsorbents for cationic organic dyes Mingshuai Ma a,c, Zhong Liu a,⁎, Lanfeng Hui a, Zhen Shang a,c, Shaoyu Yuan a, Lin Dai a,c,⁎, Pengtao Liu a,b, Xinliang Liu b, Yonghao Ni a,c a b c
Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China Guangxi Key Laboratory of Clean Pulp & Paper and Pollution Control, Guangxi University, Nanning 530004, China Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
a r t i c l e
i n f o
Article history: Received 14 July 2019 Received in revised form 29 July 2019 Accepted 1 August 2019 Available online 2 August 2019 Keywords: Lignin Cellulose nanocrystals Adsorption
a b s t r a c t Cellulose nanocrystals (CNCs) is an exciting class of sustainable and carbohydrate material, which has great potential applications in molecular adsorption. However, the complex preparation process and limited adsorption capacity of CNCs hinder its commercial application. In this study, we design a novel functional cellulose nanocrystals-based adsorbent by an ingenious mixing of lignin-containing cellulose nanocrystals (LCNCs), sodium alginate (SA), and calcium chloride solution. Benefiting from the sulfonate groups of lignin, carboxyl groups of SA, the maximum adsorptive capability of LCNCs/SA beads for methylene blue was found to be 1181 mg g−1, which was significantly higher than previously reported biomass-based adsorbents. More importantly, LCNCs/SA beads can be reused several times. This strategy can not only improve the adsorption performance of CNCs-based materials, but also simplify the production technology of CNCs, which greatly promote the commercial application of CNCs materials. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Synthetic dyes are widely used in printing, textile, biomedicine, and other industries. While providing various colorful products, synthetic dyes also make people face the challenge of serious environmental problems [1]. Methylene blue (MB) and its derivatives have been explored in various medical applications including cell staining and imaging, antimalarial agents, rejuvenation, and anticancer research [2–4]. Compared to other hazardous dye molecules, MB is a lower toxic compound. However, relatively high dose of MB (above 7 mg/kg) can also cause harm to the human bodies, such as dizziness, nausea, chest pain, and even Heinz body anemia [5]. Moreover, under the white light, MB can be freely photo-sensibilized and produce singlet oxygen, which can destroy the DNA structures, consequently, impact up the food chain [6]. Therefore, it is necessary to develop effective approaches for removing MB and its derivatives. Adsorption process has many advantages for the removal of organic pollutants, such as low cost, convenient, controllable, simplicity of optimizing, and especially suitable for liquid environment [7–9]. United States Environmental Protection Agency (USEPA) classified adsorption as one of the top control methods [10,11]. In general, the adsorption process can be divided into physisorption and chemisorption. A single ⁎ Corresponding authors at: No.29 at 13th Avenue, TEDA, Tianjin 300457, China. E-mail addresses: [email protected] (Z. Liu), [email protected] (L. Dai).
physisorption process has a fast adsorption rate, but poor robustness and selectivity of adsorption. Thus, for MB and some other organic pollutants, we need to design a multi-functional adsorbent to simultaneously achieve physisorption and chemisorption. Cellulose nanocrystals (CNCs) have emerged as an exciting class of sustainable and carbohydrate materials. CNCs are rod-like nanoparticles having a width less than 10 nm and a length between 200 and 400 nm [12]. Because of the good mechanical strength, large specific surface area, and numerous surface functional groups, they have great potential applications in material reinforcement and molecular adsorption [13,14]. For the production of CNCs, whether chemical treatment (acid/enzymatic hydrolysis) or physical treatment (mechanical treatment), lignin removing is always an important process to obtain pure CNCs. However, recently, some studies reported that a certain remaining of lignin on the surface of CNCs could provide some additional features such as antibacterial activity and thermal stability [15–21]. It is also worth noting that lignin contains many aromatic structures, which not only have various applications [22–28] but also can attract and aggregate with the dye molecules (similar structure) by π-π interaction. Moreover, during the preparation of CNCs by sulfuric acid hydrolysis, the lignin residues have many sulfonyl hydroxide groups, which can provide more adsorption sites and improve adsorption performance. Based on the above background, we prepared lignin-containing CNCs, and developed a green and low-cost technique by the new
https://doi.org/10.1016/j.ijbiomac.2019.08.022 0141-8130/© 2019 Elsevier B.V. All rights reserved.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx
lignin-containing CNCs-based beads for MB removing. This strategy can not only simplify the production technology of CNCs, but also improve the adsorption performance of CNCs-based materials. In this work, steam explosion poplar chips were used as raw materials to produce lignin-containing cellulose nanocrystals (LCNCs) via acidic hydrolysis. Then, LCNCs were mixed with sodium alginate (SA), and dropped into a calcium chloride solution to obtain LCNCs/SA beads (Scheme 1). Finally, the adsorption efficiency and kinetics of MB were systematically investigated. 2. Materials and methods 2.1. Materials Poplar chips were kindly provided by Sun Paper Industry Joint Stock Co., Ltd. (Shandong, China). Sulfuric acid was purchased from China National Pharmaceutical Group Co., Ltd. (Tianjin, China). Calcium chloride (CaCl2), methylene blue (MB), and sodium alginate (SA) were purchased from Sigma-Aldrich, USA. All Chemicals in this study were analytical grade and used without further purification. 2.2. Preparation of lignin-containing cellulose nanocrystals (LCNCs) LCNs were prepared from poplar chips by the acid-catalyzed method. First, poplar chips underwent a steam explosion pretreatment process (210 °C, 10 min), and then rinsed with deionized water until colorless. The obtained poplar power was dried and milled as a brown powder. 8.0 g poplar powders were added into 160 mL 60 wt% sulfuric acid solution and stirred under a fixed agitation of 150 rpm at 48 °C for 2 h. Sulfuric acid hydrolysis was stopped by adding 10-fold volume cold distilled water into the reaction mixture. The mixture was allowed to settle down for 12 h. The insoluble substance was collected by centrifugation (7000 rpm, 5 min) and washed by deionized water (1600 mL, five times) to the neutral condition. The obtained powder was homogenized by the use of a probe-type ultrasonic cell disruptor (Scientz, Ningbo, China) for 20 min in an ice bath. After that, the insoluble part (non-fibrillated cellulose and lignin with a large size) was removed by centrifugation. Finally, LCNCs suspension was dialyzed against distilled
water for three days and then diluted to a concentration of 0.75 wt% with deionized water. The composition in LCNCs was determined based on NREL/TP-51042618 [29]. 0.30 g ± 0.05 g freeze-dried LCNCs sample was treated with 3 mL of 72% (w/w) H2SO4 at 30 °C for 60 min under water bath and stirred every 5–10 min without removing the sample from the bath via glass rod and then diluted the acid to 4% (w/w) by adding deionized water. The suspension was hydrolyzed at 121 °C for 60 min in an autoclave, the residue was filtered through porcelain filter crucibles with sufficient deionized water under vacuum and then dried in an oven at 105 ± 0.2 °C until reaching constant weight. The weight of the residue was Klason lignin. Structural carbohydrates in filtrate was then subjected to do HPLC (Agilent 1200 with a Biorad Aminex HPX87H column) analysis with calibration curves of corresponding sugars. The amount of cellulose and hemicellulose were calculated based on the corresponding monomeric sugars, using a correction of 0.90 (or162/180) for glucose and an anhydro correction of 0.88 (or 132/ 150) for xylose. There were 78.98 wt% cellulose, 14.87 wt% Klason lignin, and 1.38 wt% hemicellulose in the obtained LCNCs. 2.3. Preparation of LCNCs/SA beads 0.3 g SA powder was dissolved into 40 mL deionized water to get a concentration of 0.75 wt% under constant stirring at room temperature. Equal amount of 0.75 wt% LCNCs suspension and SA solution were mixed for 1 h. Then, the mixture (0.375 wt% LCNCs and 0.375 wt% SA) was dropwise added into 2 wt% CaCl2 solution without stirring. The beads were allowed to cross-link in CaCl2 solution for 2 h. After washing with deionized water and removing the excess Ca2+, LCNCs/SA beads were frozen and dried for batch adsorption studies. 2.4. Characterization Surface and cross-section morphologies of the as-prepared lightweight bead were investigated using a Field Emission Scanning Electron Microscopy (FESEM, Hitachi SU-70). The chemical compositions of the resulting LCNCs were analyzed via a two-step acid hydrolysis method and Fourier transform infrared (FT-IR) spectroscopy (NICOLET iS5, Thermo Scientific) was conducted in the range of 4000–500 cm−1
Scheme 1. Preparation of LCNCs, LCNCs/SA beads, and MB adsorption of LCNCs/SA beads.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx
with a resolution of 4 cm−1 and 32 scans. The specific charge quantity q (μeq·g−1) was detected by conductometric titration, and calculated according to the following equation: q¼
Vc m
The relationship between LCNCs/SA beads and MB can be explained by adsorption isotherm parameters [30]. Two-parameter isotherm models, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich could be used to describe adsorption isotherm.
ð1Þ
where V and c are the volume (L) and concentration (μeq·L−1) of titrant, m is the mass of the sample (g).
Langmuir isotherm : qe ¼
ðC −C e ÞV qe ¼ i m
ð2Þ
where qe is the amounts of MB adsorbed at the equilibrium (mg·g−1), ci and ce represent initial and equilibrium concentrations of MB (mg·L−1), respectively. m and V are the weight of beads (g) and volume of the solution (L), respectively.
qm bC e 1 þ bC e
ð3Þ
Freundlich isotherm : qe ¼ aC 1=n e
2.5. Batch MB adsorption studies MB concentrations can be detected by using UV–visible spectrophotometer (Thermo Scientific, EVOLUTION 201) at 463 nm. The absorption quantity of MB can be calculated by the following equation:
3
Temkin isotherm : qe ¼
ð4Þ
RT ln ðAT C e Þ bT
ð5Þ (
"
−β RT ln
Redlich−Peterson isotherm : qe ¼ qm e
1 2 1þ Þ Ce
) ð6Þ
where qe and qm are the absorption quantity of MB per unit adsorbent (mg·g−1) at equilibrium and maximum, respectively. Ce is the equilibrium concentration of MB (mg·L−1). b, a, bT, and β are the constant of Langmuir (L·mg−1), Freundlich (mg1–1/n L1/n·g−1), Temkin (kJ·g·mol−2) and Dubinin-Radushkevich (mg·g−1) isotherm,
Fig. 1. (a) Illustration of the procedure for the preparation of LCNCs/SA beads. Mechanisms of cross-linking of (b) SA/Ca2+, (c) LCNCs/SA/Ca2+, (d) hydrogen bonds between LCNCs and SA.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx Table 1 Isotherm model parameters for adsorption. Isotherm
Parameters
AL NPs
Langmuir
qm (mg·g−1) b (L/mg) R2 a (mg1–1/n L1/n/g) n R2 bT (kJ g/mol2) AT (L/mg) R2 qm (mg g−1) β (mol2/kJ2) R2
1374.85 0.0079 0.9891 90.65 2.47 0.9836 0.081 0.07 0.9936 1065.08 1.50 0.9503
Freundlich
Temkin
Dubinin–Radushkevich
respectively. R and T are the ideal gas constant (8.3145 J·mol−1·K−1) and thermodynamic temperature (K), respectively. n is the constant of the adsorption intensity for the Freundlich model. AT represents the constant of equilibrium binding corresponding to the maximum binding energy. The time-dependent adsorptive processes of MB were analyzed by applying the pseudo-first- and second-order equations as followed [7]: k1 t Pseudo−first−order : logðqe −qt Þ ¼ logqe − 2:303 Pseudo−second−order :
Fig. 2. (a) FT-IR spectra of LCNCs, SA, and LCNCs/SA beads. (b) Photographs of swollen (left top) and dry LCNCs/SA beads (right top). SEM images of the surface (left bottom) and porous morphologies (right bottom) of LCNCs/SA beads.
t 1 t 1 t ¼ þ ¼ þ qt k2 q2e qe v0 qe
ð7Þ ð8Þ
where qe and qt are the absorption quantity of MB per unit adsorbent (mg·g−1) at equilibrium and time t, respectively. k1 (h−1) and k2 (h·g·mg−1) are the rate constant for the pseudo-first and secondorder adsorption, respectively. The parameters of qe and k1, k2 can be obtained experimentally from the slope and intercept of the plot of log(qe - qt) or t/qt versus t. v0 is the initial adsorption rate (h·mg·g−1) which can be calculated as v0 = k2q2e.
Fig. 3. (a) Equilibrium adsorption isotherms of LCNCs/SA beads. (b) Adsorption kinetics, linear fitting of (c) pseudo-first- and (d) pseudo-second-order kinetic models of LCNCs/SA beads. (e) Effect of temperature on MB adsorption capacity and plot of ln Kc vs 1/T. (f) Effect of the pH values on the adsorptive capacity of LCNCs/SA beads.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx Table 2 Thermodynamic parameters of MB adsorption. Adsorbents
LCNCs/SA
ΔG (kJ mol−1) 25
35
45
−6.31
−5.39
−4.47
ΔH (kJ mol−1)
ΔS (kJ mol−1 K−1)
−33.84
−0.092
2.6. Regeneration of absorbents In order to recycle present beads, we used the method reported in the literature with few modifications [31]. In brief, the MB-absorbed beads were incubated stir-treated with 40 mL of 1.0 N HCl for 3 h at 25 °C to desorb the loaded MB. After desorption, the beads were washed with sufficient distilled water and then removed left the water and reused for adsorption. The adsorption/desorption cycles were continuously repeated five times under 25 °C. The amount of MB absorbed onto the beads was determined and calculated as mentioned above.
3. Results and discussion Alginate hydrogel beads can be easily formed in the presence of divalent cations cross-linking agents. Calcium chloride (CaCl2) is one of the most frequently used agents to ionically cross-link alginate. However, the mechanical properties of Ca2+ cross-linked alginate beads are highly depended on the chemical structure of alginate. Alginates are composed of (1–4)-linked α-L-guluronic acid (G units) and β-Dmannuronic acid (M units) monomers which vary in amount and sequential distribution along the polymer chain depending on the source of the alginate. The Ca2+ is believed to bind solely to G units, as the structure of the G units allows a high degree of coordination of the Ca2 + (Fig. 1b) [32–34]. Therefore, in this study, the introducing of lignincontaining CNCs into the traditional alginate beads, one can expect the improvement of mechanical properties and adsorption performance by CNCs and lignin, respectively.
3.1. Preparation of LCNCs/SA beads
Fig. 2a shows the FT-IR spectra of LCNCs, SA, and LCNCs/SA beads. As shown in FT-IR spectrum of LCNCs, the characteristic peaks of cellulose obtained at 1425, 1162, 1112, 1055 and 898 cm−1 [35–37]. The characteristic peaks at 1510 cm−1 and 1456 cm−1 corresponded to the vibration of C_C aromatic skeletal and C−H deformation of the lignin, respectively [15,38]. The LCNCs/SA beads showed the changes in the peak positions, especially observable from 1415 cm−1 (Na-alginate) to 1436 cm−1 (Ca-alginate) for symmetric carboxylate (–COO−) group stretching, and confirmed the complex formation after crosslinking with Ca2+ ions [39]. Photographs of the sample obvious a yellowishbrown and light brown color of the swollen and dry LCNCs/SA beads, respectively. The surface and porous morphologies of the beads were shown in Fig. 2b revealed that LCNCs/SA beads have rough and fold surface and the porous internal structure, which could help to improve the adsorption performance. 3.2. Sorption isotherms The effects of different initial MB concentrations on the adsorption process by LCNCs/SA beads were studied. Fig. 3a shows that with the increasing of initial MB concentration from 41 to 574 mg·L−1, the adsorption quality of MB by LCNCs/SA beads ranged from 309 to 1180 mg·g−1, which could be due to the higher contact probability and stronger force between LCNCs/SA and dye molecules [31]. The model fitting curves and parameters of Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm for MB adsorption on LCNCs/SA beads were shown in Fig. 3a and Table 1. The Langmuir and Temkin isotherm models gave better fits than the Freundlich and Dubinin–Radushkevich isotherm models. According to the Langmuir equation, the maximum uptake capacity (qm) for MB is 1374.85 mg·g−1, which is similar to the experiment value (1180 mg·g−1). In order to reveal the actual adsorption mechanism, some thermodynamic parameters (ΔH, ΔS, ΔG) can be deduced by using the following equations [34]. MB adsorption processes were monitored under 25, 35 and 45 °C, respectively. The amount of MB absorbed onto the asprepared beads was determined and calculated as mentioned above. Kc ¼
Cellulose and sodium alginate (SA) are polysaccharides with a similar chemical structure that provide good chemical compatibility in the resultant composite. LCNCs with hydroxyl (−OH) and sulfate (–SO− 3 ) functional groups can involve in the complexations with Ca2+ crosslinked SA, and form additional hydrogen bonding, resulting in the nano-composite structure (Fig. 1c, d) in LCNCs/SA beads. The lignincontaining CNCs and SA have many sulfate, sulfonyl hydroxide, hydroxyl, and carbonyl groups. When adding the LCNCs/SA mixture into Ca2+ solution, the colloid stability was broken immediately, and Ca2+ was like a magnetic particle to attract LCNCs and SA together. The charge densities of LCNCs and LCNCs/SA were 260 and 400 μeq·g−1, respectively, which were detected by conductometric titration. The negative charge surface of LCNCs/SA beads can contribute to the MB adsorption process.
5
Ca Ce
ð9Þ
ΔG ¼ ΔH−TΔS ln K c ¼
ð10Þ
ΔS ΔH − R RT
ð11Þ
where Kc is the distribution coefficient for the adsorption, Ca and Ce are the amount of dye adsorbed on the beads per liter of the MB solution at equilibrium (mg/L) and the equilibrium concentration (mg/L) of the dye in the solution, respectively. T is adsorption temperature (K). ΔH and ΔS were calculated from the slope and intercept of ln Kc versus 1/T (Fig. 3e), and the results are summarized in Table 2. Negative ΔH values indicated the exothermic nature of the process, whereas a small negative value ΔS suggests that no noticeable change in the structure of
Table 3 Kinetic model parameters for adsorption at 25 °C. Time (min)
0–180 0–15 15–180 a b
Pseudo-first-order model k1 (h−1)
q ea (mg g−1)
0.019 0.065 –
175.34 308.92 –
Pseudo-second-order model R2
k2 (h·g mg−1)
q ea (mg g−1)
v0 (h·mg g−1)
0.9058 0.9999 –
8.24 × 10−4 – 8.25 × 10−4
320.51 – 316.46
29.51 – 34.98
R2 0.9996 – 0.9999
q eb (mg g−1)
310.67
Calculated results. From experiments.
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M. Ma et al. / International Journal of Biological Macromolecules 139 (2019) xxx
adsorption sites at low pH environment. With the increasing pH values of the solution, more anion on the surface of LCNCs/SA beads increased the adsorption performance of the cationic dye. 3.5. Reusability and reproducibility
Fig. 4. Adsorption desorption cycles for LCNCs/SA beads.
LCNCs/SA beads occurs during the adsorption process, which was in agreement with previous works [40,41]. We observed that ΔG increased with increasing temperature, suggesting a reduction in the adsorption with temperature. The decrease in adsorption capacity with increasing temperature may be influenced by the weakening hydrogen bonding and van der Waals interaction between the LCNCs/SA beads and MB molecules [42]. 3.3. Adsorption kinetics The adsorption kinetics of LCNCs/SA beads were analyzed by using the pseudo-first- and second-order equations. Results were shown in Fig. 3b–d and Table 3. Obviously, during the whole adsorption process, the pseudo-second-order kinetic model (R2 = 0.9996) presented a better correlation than the first order (R2 = 0.9058). However, it is worth noting that, at the beginning of the adsorption (before 15 min), the adsorption process demonstrated more excellent correlation by the pseudo-first-order kinetic model (R2 = 0.9999), which could be due to the physisorption in the initial stage. The following process (15–180 min) fit well by the pseudo-second-order model. These results indicated that the MB adsorption process by LCNCs/SA beads included physisorption as well as chemisorption. 3.4. Effect of pH on adsorption The effect of pH on dye removal was studied by changing the pH conditions of the MB solution from 3 to 11 at 25 °C. Fig. 3f shows that the adsorption quality of MB by LCNCs/SA beads increased significantly from 261.9 to 309.9 mg g−1 with the increasing of pH from 3 to 7, then become steady. This phenomenon could be attributed to the changes in the ionic characteristics of the LCNCs and SA chains. The functional groups on the surface of LCNCs/SA beads were protonated or deprotonated under different pH environment. With many sulfate (lignin) and carboxyl (SA) groups, LCNCs/SA beads processed negative surface charge (400 μeq·g−1), which decreased as pH decreases. Excess of hydrogen ions competed with MB molecules (cationic dye) and invaded Table 4 Comparison of adsorption capacity of various bio-based adsorbents for MB. Adsorbent
qmax (mg g−1)
Reference
Cellulose nanocrystal-alginate hydrogel beads Carbon-alginate beads Polyacrylamide/sodium alginate microspheres GO/CA MCMFCs Cr(OH)3-NPs-CNC hybrid nanocomposite LCNCs/SA beads
256 287 1070 182 303 204 1181
[34] [43] [42] [44] [45] [46] This study
From an economic point of view, reusability is an essential property for scale-up of the adsorbent application in practical wastewater treatment. Here, the adsorption-desorption cycles were studied for five times. Fig. 4 shows that the adsorption capacity of LCNCs/SA beads slightly decreased from 1181 to 971 mg g−1 after the fifth repeated regeneration, which confirmed that the LCNCs/SA beads could reuse to adsorb MB with high and stable adsorption capacity. For further investigating the application of LCNCs/SA beads, the maximum adsorptive capacity of MB in this work was compared with various adsorbents in the previous study. From Table 4, LCNCs/SA beads show a superior adsorption performance than that of other bio-based adsorbents. Based on the above results, LCNCs/SA beads can be a promising, eco-friendly, and effective adsorbent for removing MB and other cationic dyes with a very easy and convenient recovering method. 4. Conclusions A novel lignin-containing CNCs-based adsorbent was designed by an ingenious mixing of LCNCs, SA and calcium chloride solution. Benefiting from the sulfonate groups of lignin, carboxyl groups of SA, the maximum adsorptive capability of LCNCs/SA beads for MB was found to be 1181 mg g−1, which was significantly higher than previously reported biomass-based adsorbents. Furthermore, LCNCs/SA beads simultaneously achieve physisorption and chemisorption. The kinetic studies denoted that the initial (before 15 min) and the following stages (15–180 min) of MB adsorption by LCNCs/SA beads followed pseudofirst and second order kinetic model, respectively. More importantly, LCNCs/SA beads can be reused with high adsorption capacity. In summary, this strategy can not only simplify the production technology of CNCs, but also improve the adsorption performance of CNCs-based materials. Acknowledgments We gratefully acknowledge the financial support from the National Key Research and Development Program (Grant No. 2017YFB0307901), Young Elite Scientists Sponsorship Program by Tianjin (TJSQNTJ-2017-19), and the Opening Project of Guangxi Key Laboratory of Clean Pulp and Papermaking and Pollution Control (No. KF201716). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijbiomac.2019.08.022. References [1] M.A. Rauf, S.S. Ashraf, Chem. Eng. J. 151 (2009) 10–18. [2] T. Simon, M. Potara, A.-M. Gabudean, E. Licarete, M. Banciu, S. Astilean, ACS Appl. Mater. Interfaces 7 (2015) 16191–16201. [3] R.H. Schirmer, B. Coulibaly, A. Stich, M. Scheiwein, H. Merkle, J. Eubel, K. Becker, H. Becher, O. Müller, T. Zich, W. Schiek, B. Kouyaté, Redox Rep. 8 (2003) 272–275. [4] S.D. Bukkitgar, N.P. Shetti, Mater. Sci. Eng. C 65 (2016) 262–268. [5] M. Oz, D.E. Lorke, M. Hasan, G.A. Petroianu, Med. Res. Rev. 31 (2011) 93–117. [6] A.B. Albadarin, M.N. Collins, M. Naushad, S. Shirazian, G. Walker, C. Mangwandi, Chem. Eng. J. 307 (2017) 264–272. [7] L. Dai, Y. Li, R. Liu, C. Si, Y. Ni, Int. J. Biol. Macromol. 132 (2019) 478–486. [8] R. Liu, L. Dai, C.-L. Si, ACS Sustain. Chem. Eng. 6 (2018) 15756–15763. [9] Y. Duan, A. Freyburger, W. Kunz, C. Zollfrank, ACS Sustain. Chem. Eng. 6 (2018) 6965–6973. [10] R.S. Blackburn, Environ. Sci. Technol. 38 (2004) 4905–4909. [11] G. Crini, Bioresour. Technol. 97 (2006) 1061–1085. [12] Y. Habibi, L.A. Lucia, O.J. Rojas, Chem. Rev. 110 (2010) 3479–3500.
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Please cite this article as: M. Ma, Z. Liu, L. Hui, et al., Lignin-containing cellulose nanocrystals/sodium alginate beads as highly effective adsorbents for ca..., , https://doi.org/10.1016/j.ijbiomac.2019.08.022