Biosorption is considered a promising technique for removing heavy metals from water. However, a biosorbent is usually prepared in the form of biomass powder that has drawbacks in stability and uniformity. Herein, a spherical lignocellulose-based anion exchanger (LCB-AE) was prepared from sugarcane bagasse through the method of dissolution-regeneration of biomass followed by quaternary ammonium modification. Dissolution-regeneration conditions of raw biomass were optimized, and the prepared materials were characterized by element composition analysis, pore-structure analysis (mercury intrusion porosimetry), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and carbon-13 nuclear magnetic resonance (13C-NMR) analyses. The LCB-AE has a macro-porous structure and a rough surface occupied mainly by quaternary ammonium and hydroxyl groups. Adsorption selectivity of LCB-AE follows the order of CrO42- > PO43- > SO42- > NO3-, and adsorption isotherms agree well with the Langmuir model, which suggests the experimental exchange abilities are approximately 0.8 to 0.9 mEq/g. These results show that LCB-AE as a new spherical biosorbent has the potential to be used for anions removal from water.
Preparation and Characterization of Spherical Lignocellulose-based Anion Exchanger from Sugarcane Bagasse
Wei Cao,* Hao Li, Yixia Hong, Ziyi Yang, and Minglai Fu
Biosorption is considered a promising technique for removing heavy metals from water. However, a biosorbent is usually prepared in the form of biomass powder that has drawbacks in stability and uniformity. Herein, a spherical lignocellulose-based anion exchanger (LCB-AE) was prepared from sugarcane bagasse through the method of dissolution-regeneration of biomass followed by quaternary ammonium modification. Dissolution-regeneration conditions of raw biomass were optimized, and the prepared materials were characterized by element composition analysis, pore-structure analysis (mercury intrusion porosimetry), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and carbon-13 nuclear magnetic resonance (13C-NMR) analyses. The LCB-AE has a macro-porous structure and a rough surface occupied mainly by quaternary ammonium and hydroxyl groups. Adsorption selectivity of LCB-AE follows the order of CrO42- > PO43- > SO42- > NO3–, and adsorption isotherms agree well with the Langmuir model, which suggests the experimental exchange abilities are approximately 0.8 to 0.9 mEq/g. These results show that LCB-AE as a new spherical biosorbent has the potential to be used for anions removal from water.
Contact information: Xiamen Key Laboratory of Municipal and Industrial Solid Waste Utilization and Pollution Control, College of Civil Engineering, Huaqiao University, Xiamen, Fujian 361021, PR China;
The ion exchange process is a rapid and efficient method for removal and recovery of ionic pollutants from water and wastewater. Ion exchange material plays an important role in determining process efficiency and cost. The desired material needs to have a low cost and high sorption capacity. Further, it ideally ought to be renewable (Bashir et al. 2019). Ion exchange resins that originate from polystyrene and polyacrylic acid are popular in water treatment because of their excellent mechanical properties and exchange sorption performance. However, these resins as petrochemical derivatives are expensive, and they face the problem of depletion of petrochemical resources. Many industries are committed to develop alternative products to reduce the use of petrochemical resources, thereby helping to cope with climate change and reduce carbon emissions.
Over recent decades, a great deal of research has been motivated by the need to develop novel, low-cost, and efficient ion exchangers for separating ionic species from wastewater. Both organic and inorganic materials are involved, such as functional zeolites, double layer hydroxides, cellulose- and chitin-based materials, and metal-organic frameworks (MOFs) (Kim and Kuga 2002; Liang et al. 2013; De Gisi et al. 2016). A cellulose-based exchanger has been considered as an important alternative of ion exchange resins because cellulose is the most abundant natural polymer in the earth (Hokkanen et al. 2016). Additionally, the presence of many hydroxyl groups in its structure makes chemical modifications available. To prepare an anion exchanger, amine and quaternary ammonium groups must be introduced into the cellulose structure to form positively charged sorption sites (Gao et al. 2009; Cao et al. 2016; Hokkanen et al. 2016). Several anion exchangers based on cellulose or cellulose derivatives have been successfully developed previously, and related commercial products have been established, such as the Whatman DE series. Although the exchange capacity of cellulose-based anion exchanger is less than that of commercial polystyrene and polyacrylic resins, it shows good potential for protein separation and in the field of chromatographic analysis (Hokkanen et al. 2016). However, it has limitations for application in wastewater treatment, for which it is desirable that the treatment cost be as low as possible. The main challenge may be that cellulose extraction and purification from lignocellulosic biomass are too complicated and costly, making it not available in large quantities for water treatment. Considering wastewater treatment does not require the use of pure cellulose material, researchers turned to raw natural cellulosic materials, especially lignocellulosic biomass wastes, which may have almost no cost (Tran et al. 2019).
Therefore, rice straw, corn stalk, wheat straw, walnut, and other agricultural residues are becoming increasingly attractive as new biosorbents for removing aquatic pollutants, in particular heavy metals, because they are abundant, have low cost, and are renewable annually. Generally, crop straw is a type of lignocellulose material, mainly containing cellulose, lignin, and hemicellulose. To convert crop straw into anion sorbent, it needs to be washed and ground, followed by chemical modifications based on hydroxyl group, which is similar to non-homogeneous modification of cellulose. In earlier times, Laszlo reported an anion sorbent with amine groups prepared from sugarcane bagasse powders (Laszlo 1996). Subsequently, rice hull, soy beans, corn stalk, wheat straw, and peanut shell were also selected for preparing anion biosorbents after modification with triethylamine, trimethylamine, or quaternary ammonium salts, i.e., trimethyl dihydroxypropyl ammonium chloride (Marshall and Wartelle 2004; Anirudhan et al. 2006; Blagojev et al. 2021). These low-cost anion biosorbents exhibit good potential for removing anionic pollutants, such as nitrate, sulfate, phosphate, arsenate, and chromate from wastewater. In the authors’ previous studies, anion biosorbents from rice straw and corn stalk were also prepared, and they were used to remove sulfate and chromate from aqueous solution (Cao et al. 2011, 2018). However, some drawbacks of biosorbents were observed in previous reports and the authors’ own experiments. Most biosorbents were prepared in the form of biomass powder, which has limitations in dynamic adsorption and repeated use because of the lack of stability and uniformity. A fine particle means large hydraulic resistance in bed operation and lacks sufficient mechanical strength in repeated regeneration-adsorption cycles. Apart from lab trials, this may be the main reason behind why few practical applications of biosorbent have been reported in wastewater treatment. The other problem for a powder biosorbent is stability. It has been demonstrated that some organic components of biosorbent powder could be dissolved into solution during the adsorption process (Cao et al. 2018; Wen et al. 2018). In the authors’ own experiment, a sorbent prepared from rice straw particles started to degrade after a month at an extreme situation (anaerobic expanded bed operation). Based on the above analysis, the authors believe that the key to biosorption technique is to develop biosorbents having a similar morphology and stability as the commercial resins in present use.
Therefore, the authors consider the conversion of raw lignocellulosic biomass into spherical beads, which can be used as precursors to produce anion exchangers. The most difficult part is dissolution and regeneration of total components of lignocellulosic biomass in a clean and sustainable way (Huang et al. 2019). Ionic liquids makes it possible for natural lignocellulosic materials, such as crop straw, to be shaped into an ideal form without the extraction and purification process of cellulose. Recent studies (Chen et al. 2016; Wang et al. 2017; Huang et al. 2019) showed that ionic liquid could dissolve wood or straw powders directly and then the lignocellulose can be regenerated in the form of short fibers containing much higher cellulose than that of the raw material.
In the present work, sugarcane bagasse was dissolved using an ionic liquid solvent for preparation of lignocellulose bead (LCB), which subsequently was modified into an anion exchanger with epichlorohydrin and trimethylamine in a pyridine reaction system. Synthesis conditions of LCB were optimized with single factor experiments. Elemental analysis, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), carbon-13 nuclear magnetic resonance (13C-NMR), and X-ray photoelectron spectroscopy (XPS) were employed to characterize the physicochemical structure of prepared materials. Adsorption performance of LCB-AE was also measured by batch adsorption experiments, including adsorption isotherms and the selectivity for common anions, in order to evaluate its application potential.
Materials and Chemicals
Sugarcane bagasse was collected from a fresh sugarcane juice shop located at Jimei District, Xiamen, Fujian Province, China. Raw material was cleaned with tap water, and dried at 60 °C. Then, it was ground to obtain biomass particles with 40- to 80-mesh size using a plant disintegrator. The biomass particles were subsequently ground with a ball grinder for 2 h to achieve a powder consistency, which was used to prepare lignocellulose beads in this work.
Ionic liquid, 1-allyl-3-methylimidazolium chloride ([Amim]Cl), with a purity of more than 98%, was bought from Lanzhou Institute of Chemical Physics (Lanzhou, China). Other chemicals used in this study, such as sodium dicarbonate (NaHCO3), pyridine, dimethylformamide (DFM), trimethylamine, epichlorohydrin (ECH), ethanol, sodium hydroxide (NaOH), and hydrochloric (HCl), were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the reagents were of analytical grade.
Preparation of LCB
A certain weight of sugarcane bagasse powder was dissolved in [Amim]Cl solvent under optimized temperature and time conditions. The mass ratio of sugarcane bagasse to [Amim]Cl solvent should be controlled at approximately 3%, as suggested by the authors’ pre-experiments. When sugarcane bagasse is completely dissolved in ionic liquid, the yielded viscose liquid is a homogeneous solution with slight brown color. Then, the solution was injected dropwise into a water coagulation bath using a syringe to form spherical lignocellulose beads (LCBs) under a low-speed stirring conditions (50 rpm). After consolidation, the formed LCBs were separated from the water coagulation bath and washed with large volumes of deionized water. The beads can be dried at 60 °C to obtain dry samples for calculation of yield and further characterization analysis. In addition, when a pore-forming agent, NaHCO3, was added to sugarcane bagasse/[Amim]Cl solution, the water coagulation bath needs to be substituted by 0.1 M HCl solution to improve the pore structure of LCB.
Preparation of LCB-AE
Surface chemical modification was conducted with the LCBs for preparing spherical lignocellulose-based anion exchanger. According to previous studies (Cao et al. 2016; Hokkanen et al. 2016) and preliminary experiments, relative optimal reaction steps and conditions were adopted and described in the following implementation case. First, the LCB (0.2 g) was reacted with ECH (3 mL) in a four-neck flask, which was loaded with DFM as solvent and a little pyridine as catalyst in advance. This reaction step was continued for 1 h at 65 °C. Then, trimethylamine solution (5 mL) was added into the flask, and the reaction remained for 3 h at 100 °C. After separation, the solid product was washed by ethanol solution (1:1), NaOH solution (0.1 M), HCl solution (0.1 M), and deionized water in that order. Consequently, the final product as LCB-AE was converted into Cl–-resident form. The expected reaction route based on hydroxyl groups in lignocellulose structure for preparing LCB-AE is schematically shown in Fig. 1.
Fig. 1. The expected synthetic route of LCB-AE based on hydroxyl groups in lignocellulose structure
Total nitrogen contents (N %) of LCB and LCB-AE were determined by Vario EL III element analyzer (Elementar Co., Ltd., Germany) with CHNS pattern. Mercury intrusion porosimetry (MIP) was used to measure the pore structure of the prepared materials. The MIP experiments were conducted using an AutoPore IV 9500 (Micromeritics Instrument Corporation, USA). The main operation parameters included a sample weight 0.035 g, penetrometer volume of 5 mL, Hg-contacting angle 130°, low pressure 0.52 psia, and high pressure 33000 psia with both equilibrium time of 10 s. Scanning electron microscopy was used to probe the surface morphology of prepared materials. The experiment equipment is a field emission SEM (Sigma 300, Carl Zeiss, Germany). X-ray photoelectron spectroscopy was used to probe surface composition and group structure of the prepared materials. The experiments were conducted on an X-ray scanning micro-prober photoelectronic energy spectrometer (Quantum2000, Physical electronics Co., Ltd., USA). The X-ray source was a monochromatic Al K-alpha radiation anode target. The survey spectra and high-resolution spectra were recorded with constant pass energy of 150 and 50 eV, respectively. The XPS determination depth was approximately 0.5 to 7.5 nm. The XPS data were calibrated with C1s peak at 285.0 eV. The high-resolution XPS spectra were decomposed into sub-components, and the curve fitting deviation should be as little as possible. Solid-state 13C nuclear magnetic resonance was used to probe the cellulose structure of the prepared materials. Experiments were performed on a Bruker AVANCE 400 spectrometer (Bruker Co., Ltd., Fällanden Switzerland) with static field strength of 2.3 T (100 MHz 1H) at 298 K. The proton 908 pulse time was 5.5 ms. The acquisition time was 0.03 s and the delay time after acquisition of signals was 3 Ys. The chemical shift data were calibrated with methyl group (methyl benzene) resonance peak at 17.3 ppm.
Batch Adsorption Experiment
Batch adsorption experiments were used to determine adsorption isotherms of LCB-AE for anion species such as sulfate, chromate, nitrate, and phosphate. Adsorption was conducted in a 100 mL conical flask containing 0.1 g LCB-AE and 50 mL solution containing selected anions. The flasks were put into an orbital shaker at conditions of 150 rpm and 25 ℃. In batch adsorption experiments, all of the samples were duplicated. After shaking for 24 h, the supernatant liquid was separated from adsorbents by filtration, and the filtrate was collected for chemical analysis. Concentrations of the anions were measured using the ion chromatography method. In addition, chromate was tested using a classic spectrophotometry method (UV-6000, Shanghai Metash Instrument Co., Ltd., Shanghai China) at 540 nm. The selectivity of LCB-AE toward different anions was evaluated by adsorption experiment with a multi-component solution of sulfate, chromate, nitrate, and phosphate anions.
RESULTS AND DISCUSSION
Optimization of LCB Preparation Conditions
Sugarcane bagasse was dissolved in an ionic liquid, [Amim]Cl, under different temperature and time conditions. The dissolution-regeneration results are shown in Table S1. The higher LCB yield was obtained at dissolution time of 1 h and temperature of 110 °C under a certain ratio (3%) of sugarcane bagasse to [Amim]Cl. Compared to previous reports (Sun et al. 2012; Chen et al. 2016), the pre-treatment process of raw biomass was simplified in this study, i.e., the extraction step with benzene-ethanol solvent was omitted. The experiment suggests that sugarcane bagasse powder also can be dissolved well in the ionic liquid without extraction pre-treatment, and LCB can be prepared in a relatively green and simply way. Under excessively high temperature or prolonged dissolution time, lignocellulose will degrade, and the color of the resulting solution will be dark brown. Previous studies showed the popular dissolution mechanism of cellulose in liquid solution is deconstruction of hydrogen bond by the cation ions from ionic liquid (Kosan et al. 2008; Li et al. 2018). Dissolution of sugarcane bagasse has a similar mechanism, but it is more difficult. This is because lignocellulose material has a more complicated structure than pure cellulose, since it contains cellulose, lignin, and hemicellulose. Moreover, these components combine into a more complex structure through intermolecular interaction (mainly hydrogen bonds) to form the plant cell wall (Moyer et al. 2018; Huang et al. 2019).
Different coagulation baths including acetone, acetone aqueous solution (1:1), water, cold water, and ethanol (95%) were tested to prepare LCB. Photos were taken immediately after formation of LCBs in a beaker, as shown in Fig. S1. Results show that sugarcane bagasse lignocellulose could not be regenerated into spherical beads in acetone and even in 1:1 acetone water. Loose granular flocs are observed when ethanol was used as coagulation bath. Beads that have pale yellow color, clear border, and solid feel could be obtained by using water or cold water as coagulation bath. Thus, water is a good and economical coagulation bath for preparing LCB. This result also shows that regeneration of lignocellulose is similar to that of pure cellulose, implying that shaping techniques of cellulose, such as droplet and dispersion, can be tried (Gericke et al. 2013).
Basic Physicochemical Properties
Basic physicochemical properties of LCB and LCB-AE are summarized in Table 1, including bead size, color, density, pH point of zero charge (pHpzc), and the contents of cellulose, hemicellulose, and lignin. The prepared LCB is spherical with faint yellow color. It is like a small soybean and has an average diameter of 2.2 mm. The LCB-AE had a light brown color and was the same diameter as the LCB. From the wet density, it can be seen that both LCB and LCB-AE were denser than water when fully wetted. After modification, LCB-AE had a higher pHpzc, which can be attributed to the introduction of protonated ammonium groups, which will be confirmed by subsequent characterization. It is believed that the increase of pHpzc is beneficial for removal of anionic species over a wider range of pH conditions, suggesting LCB-AE is more suitable for anion removal from water. When comparing the components between raw sugarcane bagasse and LCB, it can be found that cellulose and hemicellulose contents slightly increased after dissolution-regeneration process, whereas lignin decreased. This phenomenon is consistent with the findings reported in the previous studies (Zhang et al. 2016). It should be noted that the ash content was relatively high, which may have resulted from the insufficient combustion of the acid insoluble matter in the test.
Table 1. Basic Physicochemical Properties of LCB and LCB-AE
ND: Not determined
Pore Structures Characterization by MIP
To improve the pore structure of LCB, a pore-forming agent was used. During dissolution of sugarcane bagasse, NaHCO3 was added into ionic liquid, and correspondingly the water coagulation bath was replaced with a diluted HCl solution. The purpose was to produce carbon dioxide during regeneration and solidification process of lignocellulose, which is helpful to form a porous structure in LCB material. The MIP characterization results for LCB prepared with and without pore-forming agent are both shown in Table 2. Total intrusion volume of LCB almost doubled using the pore-forming agent, and the porosity and apparent density also increased remarkably. The increase of density may be attributed to the loading of NaHCO3, which has a much higher density than lignocellulose. It was also observed that the average pore diameter increased from 3808 to 7255 nm, indicating that the main effect of pore-forming agent was to produce macropores. According to a general classification, pores in material are distinguished based on their diameter as micropores (< 2 nm), mesopores (2 to50 nm), and macropores (> 50 nm). The LCB in the present work had similar density (g/cm3) and porosity (%) with a reported wheat straw-based lignocellulose bead, but much larger pore diameter and volume (Zhang et al. 2016). It should be noted that LCB-AE had a lower total intrusion volume and pore area than LCB, implying that chemical modification may block the pores and reduce the pore area and volume.
Morphology Observation by SEM
General images of sugarcane bagasse, LCB, and LCB-AE were taken by cell phone, and these are shown in Fig. S2. The SEM images of LCB and LCB-AE with different magnifications are shown Fig. 2. It can be seen from Fig. 2a and 2c that both LCB and LCB-AE have coarse surface that consisted of many thin fragment structures and aperture gaps. The fragments were possibly made of regenerated lignocellulose from dissolved sugarcane bagasse. From higher magnifications, the thin fragments of LCB were smooth and flat, as shown in Fig. 2c, whereas they became rough and covered with microparticles in LCB-AE, as shown in Fig. 2d. The microparticles had diameters in the range of 1 to 3 µm, and their element compositions measured by SEM-EDS (atom fraction) were mainly carbon (71.58%), oxygen (17.53%), nitrogen (9.02%), and chlorine (1.87%).
Fig. 2. SEM images of LCB and LCB-AE
Elemental Composition and TEC of LCB-AE
Total element compositions of LCB and LCB-AE were determined by elementary analysis with CHNS pattern. The results are shown in Table 3. Clearly, LCB-AE has much more nitrogen element than LCB, suggesting N-containing groups were introduced into the material during modification process. Considering the result of SEM-EDS analysis, the introduced groups were mainly concentrated on the surface of LCB-AE. According to nitrogen content, the theoretical anion exchange capacity of anion exchanger can be calculated following Eq. 1,
where N% is the nitrogen content in weight percentage. The TEC of LCB-AE was calculated as 1.63 mEq/g. This value is analogous to TEC of reported lignocellulose-based anion sorbent such as quaternary ammonium modified wheat straw (Gao et al. 2009), rice straw (Cao et al. 2011), sugarcane bagasse (Laszlo 1996; Wen et al. 2018), and soy hulls (Marshall and Wartelle 2004). This result suggests that regeneration and shaping of lignocellulosic biomass did not have much effect on its quaternization modification efficiency. The TEC of LCB-AE was also close to that of pure cellulose-based anion exchangers, such as a diethylamine modified cellulose sorbent (Anirudhan et al. 2009), and commercial products such as Whatman DE series and QA52 that have approximately 1.0 mEq/g anion exchange capacity (Marshall et al. 2004). It needs to be pointed out that LCB-AE had a much lower TEC in contrast to commercial polystyrene-based anion exchangers that generally have exchange capacity of 3 to 4 mEq/g. However, lignocellulosic materials have lower cost and better environmental compatibility compared with petrochemical derivatives.
Table 3. Total and Surface Element Analysis of LCB and LCB-AE
*: Surface element compositions were measured by XPS
The XPS survey spectra provides surface element information of LCB and LCB-AE, which are also summarized in Table 3. It is apparent that nitrogen content increased on the surface of LCB-AE, suggesting N-containing groups had been successfully introduced into LCB surface, as expected. The appearance of chlorine element in LCB-AE originated from the soaking process with HCl solution, which can transform the anion exchanger into Cl–-resident form.
High-resolution XPS C1s and N1s spectra of LCB and LCB-AE were achieved in scanned models. As shown in Fig. 3a and 3c, C1s high-resolution spectra were fitted to three components including an intensive peak at binding energy of 284.5 eV, a peak around 285.8 eV, and a relatively weak band around 287.0 eV. The peak at 284.5 eV is related to C-C/C-H bonds in hydrocarbons, particularly methyl and methylene groups from the lignocelluloses structure (Tang et al. 2016; Gu et al. 2019). The band around 287.0 eV is assigned to carbonyl groups (C=O) and O-C-O linkages in ketones and carboxyl groups in lignocellulose structure (Tang et al. 2016; Gu et al. 2019). The peak around 285.8 eV is usually attributed to C-O bonds in hydroxyl (C-OH) and ether (C-O-C) groups (Tang et al. 2016; Gu et al. 2019). Table 4 shows the identification and quantification of C-groups from the high-resolution XPS C1s spectra. The atom fraction of different C-groups in the surface of LCB changes after quaternization modification, while the corresponding peak positions of binding energy are relatively stable. The proportion of C-C/C-H and C=O/O-C-O bonds decreased after modification, while an increase in the amount of C-N/C-OH/C-O-C linkages was observed. This can be attributed to the formation of epoxypropyl-cellulose in the second step of modification process, which increased the amount of ethers and hydroxyl groups in the surface of LCB-AE. Furthermore, the quaternization reaction introduced many C-N bonds into material surface, which can also enhance the peak around 286 eV in XPS C1s spectra of LCB-AE (Cao et al. 2016).
Fig. 3. The high-resolution XPS C1s spectra of LCB and LCB-AE
Table 4. Identification and Quantification of C- and N-groups in Surface of LCB and LCB-AE
a: binding energy; b: atom fraction
The high-resolution XPS N1s spectra of LCB and LAB-AE and their fitting curves are shown in Fig. 3b and 3d. The XPS N1s spectrum is often fitted into two peaks, which are assigned to free amino groups (-N(CH)2/-NH2) around 399.2 eV and the protonated amino groups (-N(CH3)3 +) at 401.6 eV (Jansen and van Bekkum 1995; Ederer et al. 2017). The atom fraction of free and protonated groups for LCB and LCB-AE are also summarized in Table 4. As shown, most of amino groups in the surface of LCB were free amino groups (-N(CH)2/-NH2), which probably originated from N-containing compounds in sugarcane bagasse such as protein. The proportion of protonated amino groups of LCB-AE increased remarkably in contrast to LCB, due to that quaternization modification of LCB with trimethylamine introduced the protonated amino groups. Protonation of amines also can be resulted from the hydrogen bonding or interaction of free amino groups with lignocellulose substrate (Zafar et al. 2012). It needs to pointed out that amino groups, especially protonated amino groups, are responsible for efficient removal of anionic species. Therefore, LCB-AE with more protonated amino groups shows the potential to be used as an anion exchanger.
Figure 4 shows the solid state 13C-NMR spectra of LCB and LCB-AE. The adsorption bands from 60 to 110 ppm both in LCB and LCB-AE had the same position and similar shape, which are believed to be from carbon atoms in cellulose structure from C1 to C6. According to previous studies (Fu et al. 2015; Kono et al. 2016; Khiari et al. 2017), the sharp peak at 105 ppm is related to C1 of cellulose, and the two peaks at 83 and 88 ppm are respectively attributed to C4 of amorphous and crystalline cellulose. The strongest signal bands from 72 to 75 ppm are attributed to C2, C3, and C5, and the peaks around 64 ppm arise from C6 of crystalline cellulose. Comparing the 13C-NMR spectra of LCB and LCB-AE, it can be seen that the curve profile and peak positions for C1 through C6 were consistent, implying quaternary ammonium modification had little effect on cellulose crystal structure. It is noticed that there was a new peak at 55 ppm in spectra of LCB-AE. This peak originated from carbon atoms linked to quaternary ammonium groups (C-N(CH3)3) according to previous studies (Sun et al. 2006; Cao et al. 2016). This result confirms the presence of quaternary ammonium group in LCB-AE, which is in agreement with characterization result of XPS N1s spectrum.
Fig. 4. The 13C NMR spectra of LCB and LCB-AE
Anions Sorption Performance
Sorption isotherms of sulphate (SO42-), phosphate (PO43-), nitrate (NO3–), and chromate (CrO42-) anions on LCB-AE are shown in Fig. 5. Under experimental conditions, LCB-AE exhibited good removal for the anions with adsorption amounts in the range of 30 to 50 mg/g. The experimental data were further analyzed with Langmuir and Freundlich adsorption models as expressed in Eqs. 2 and 3, respectively (Wang et al. 2019).
where qe (mg/g) and Ce (mg/L) are sorption amount and adsorbate concentration at equilibrium, respectively; qm (mg/g) represents the maximum sorption capacity for Langmuir adsorption model; KL (L/mg) and KF (mg/g) are constants corresponding to Langmuir and Freundlich equations, respectively; and n is a dimensionless constant for Freundlich equation. The Langmuir model is a theoretical equation based on single molecular layer adsorption on homogeneous surface, whereas the Freundlich model is a semiempirical equation for describing heterogeneous surface adsorption (Wang et al. 2019).
As shown in Fig. 5, it is clear that the Langmuir model described the experimental data much better than the Freundlich model for the sorption of sulphate, chromate, and nitrate on LCB-AE, while the Freundlich model achieved a higher coefficient of determination (R2) for phosphate adsorption. The maximum sorption capacities (qm) calculated from Langmuir isotherm model were 47.28, 39.63, 51.60 mg/g for chromate, sulphate, and nitrate, respectively. After converting the unit of sorption amount, the corresponding results were 0.91, 0.41, and 0.82 mmol/g, respectively. Considering that sulphate is a divalent anion and chromate mainly exists as a monovalent anion (HCrO4–) in neutral solution (Wen et al. 2018), it can be estimated that the experimental sorption ability of LCB-AE was approximately 0.8 to 0.9 mEq/g for common anions. This value is comparable to actual exchange capacity of pure cellulose-based anion exchangers, but less than that of commercial polystyrene-based anion exchangers, which is consistent with the comparison result of TEC mentioned in the Elemental Composition and TEC of LCB-AE section.
Fig. 5. Adsorption isotherms of CrO42-, SO42-, NO3–, and PO43- on LCB-AE: (a) Langmuir model and (b) Freundlich model
To examine the selectivity of LCB-AE, competitive sorption experiments were conducted in a multi-component solution system, which contained CrO42-, SO42-, PO3–, NO3–, and Cl–. The experimental results are summarized in Table 5. Although these anions had almost the same initial concentration, their removal percentages differed noticeably, following the order of CrO42- > PO43- > SO42- > NO3–. The best sorption was achieved on CrO42-, probably because of the occurrence of Cr(VI) reduction to Cr(III) by hydroxyl and carboxyl groups in LCB-AE, which enhances Cr(VI) removal. Previous studies have shown that Cr(VI) could be reduced to Cr(III) by lignocellulose-based materials to some extent (Cao et al. 2018; Wen et al. 2018). The removal percentage was close for PO43- and SO42-, whereas it was much lower for NO3–. This result suggests that sorption selectivity of monovalent anion on LCB-AE was weaker than that of divalent/trivalent anion, which is consistent with selectivity order of commercial anion exchangers. Table 5 also shows that Cl– content clearly increased after the sorption experiment, indicating the occurrence of ion exchange between removed anions and Cl–. Based on a rough estimate, the increase of Cl– was comparable with the sum of removal quantity of other anions multiplied by their valence state. Thus, ion exchange can be regarded as the main way for sorption of anions by LCB-AE.
Table 5. Competitive Sorption of Common Anions on LCB-AE in a Multi-component Solution
The symbol ‘Δ (Conc.) ’ means the difference between the final and initial concentration
- The appropriate preparation conditions of LCBs include a 3% dissolution ratio of biomass to ionic liquid, dissolution temperature and time of 110 °C and 1 h, and water as the coagulation bath. The pore formation method of NaHCO3/diluted HCl coagulation bath can effectively improve macropore structure of LCB.
- The prepared LCB-AE had a porous structure, a rough surface, a uniform diameter of 2.2 mm, and a pHpzc value of 5.3. The theoretical TEC of LCB-AE was calculated as 1.63 mEq/g according to nitrogen content.
- Both the XPS and 13C-NMR characterizations showed that quaternary ammonium group and hydroxyl group were the main surface groups of LCB-AE.
- The LCB-AE exhibited good removal to common anions, and sorption selectivity followed the order of CrO42- > PO43- > SO42- > NO3–. The Sorption isotherms agreed well with Langmuir model, and accordingly the experimental exchange abilities of LCB-AE were estimated as 0.8 to 0.9 mEq/g, which is comparable to actual exchange capacity of pure cellulose-based anion exchangers. The LCB-AE, as a spherical biosorbent, has good potential to be used for removal of anionic pollutants from water.
This work was supported by the Fundamental Research Funds for the Central Universities (grant number: ZQN-712), the Natural Science Foundation of Fujian Province (grant number: 2021J01283), and the Science and Technology Planning Project of Xiamen City (grant number: 3502Z20203043).
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Article submitted: March 11, 2022; Peer review completed: April 26, 2022; Revisions accepted: May 4, 2022; Published: May 6, 2022.
Table S1. Effects of Dissolution Time and Temperature on Preparation of LCB
Fig. S1. LCB formation in different coagulation baths
Fig. S2. Material images taken by cellphone camera