Preparition and multiple-dye adsorption of magnetic chitosan/Fe3O4/graphene oxide adsorbent

Ming GAO Tong-Qing ZHANG Jian-Jun LI Jia-Qi HU Ming-Yan JIN Yan ZHAO Hong-Yang WANG Chang-Guo XUE

Citation:  Ming GAO, Tong-Qing ZHANG, Jian-Jun LI, Jia-Qi HU, Ming-Yan JIN, Yan ZHAO, Hong-Yang WANG, Chang-Guo XUE. Preparition and multiple-dye adsorption of magnetic chitosan/Fe3O4/graphene oxide adsorbent[J]. Chinese Journal of Inorganic Chemistry, 2023, 39(4): 723-734. doi: 10.11862/CJIC.2023.042 shu

磁性壳聚糖/Fe3O4/氧化石墨烯吸附剂的制备及其多染料吸附性能

    通讯作者: 李建军, ljjhero4@gmail.com
    赵岩, 363990474@qq.com
  • 基金项目:

    安徽省自然科学基金 1908085ME127

    国家大学生创新创业培养计划 202110361073

    安徽理工大学环境友好材料与职业健康研究所(芜湖)研究基金 ALW2021YF11

摘要: 为了提高壳聚糖的多染料吸附性能并使其便于固液分离,采用共沉淀法制备了壳聚糖、磁铁矿纳米颗粒、氧化石墨烯复合磁性吸附剂(CS/Fe3O4/GO)。系统的结构表征显示,CS包覆的Fe3O4磁性纳米颗粒均匀地分布在GO的表面。CS/Fe3O4/GO具有高达42.5 emu·g-1的室温铁磁性,因此可在外加磁场中实现高效固液分离。研究表明,CS/Fe3O4/GO对亚甲基蓝(MB)、甲基橙(MO)和刚果红(CR)等多种染料具有良好的吸附性能,溶液的pH、初始浓度和吸附时间对其多染料吸附性能具有显著影响。在最佳条件下,CS/Fe3O4/GO对MB、MO和CR的吸附量分别达到210.6、258.6和308.9 mg·g-1。CS/Fe3O4/GO具有优异的循环利用性能,经5次循环后仍能保留90%以上的原始吸附量。采用吸附等温线和吸附动力学对CS/Fe3O4/GO的多染料吸附性能进行了拟合分析,并详细讨论了其吸附机理。

English

  • Synthetic dyes are widely used in leather, textile, printing, cosmetics, food, and other industries, resulting in large amounts of dyes discharged into water[1-3]. The presence of dye molecules can increase water chroma, decrease light transmittance, reduce dissolved oxygen, interfere with the photosynthesis of aquatic plants, and finally destroy the aquatic ecosystem[4]. Furthermore, many kinds of dyes are bio - toxic and easy to gather in organisms, thus posing a high threat to the health of human beings[5]. Eliminating dye from water has become a critical issue. However, most dyes have a stable molecular structure, so they are difficult to be biodegraded. To address this problem, some techniques, including enhanced biodegradation, electrochemical treatment, advanced oxidation processing, membrane separation, and adsorption, have been explored[6-8]. Adsorption is regarded as one of the most promising methods owing to its low price, simple operation, high efficiency, flexibility, and no secondary pollution[9-10]. Various dye adsorbents, including modified polyaniline, bio-based adsorbents, clay-based adsorbents, chitosan (CS), and graphene oxide, were fabricated and studied. Most of the adsorbents were found to be selective to one or two types of dye molecules, which carry the same charge or have similar chemical functional groups. However, few adsorbents have high adsorption to multiple dyes with different charges simultaneously. Considering the complexity of dyes in industrial wastewater, it is necessary to develop multi-dye adsorbents.

    CS is regarded as one of the promising candidates for multi - dye adsorbents, due to its excellent adsorption effect, a wide range of sources, low cost, biodegradability, biocompatibility, non-toxicity, and high chemical reactivity[4, 11-14]. CS has abundant chelating groups, including primary hydroxyl, secondary hydroxyl, and amino groups, thus it has adsorption activity to various dye molecules having different charges and chemical groups[15-16]. However, the solubility of CS in an acidic solution limited its application[17]. To solve this problem, molecular cross - linking was explored to maintain the stability of CS[2]. Nevertheless, molecular crosslinking decreased the number of active sites on the CS surface significantly, which reduced the adsorption performance[18]. It was reported that the molecular stability of CS can be improved by combining CS with other materials[17]. Although the adsorption activity could also be affected since some of the active groups are consumed or covered, the loss of adsorption capacity may be compensated for by the compound synergy. The combination of multiple materials can also introduce more active groups, thereby giving them multi - dye adsorption properties. Therefore, finding a proper combining material for CS is the key factor for the multi-dye composite adsorbent.

    Graphene oxide (GO) is a new carbon material with excellent properties, such as chemical stability, large specific surface area, and rich functional groups[19-22]. By compounding GO with CS, the mechanical, thermal, and chemical stability of CS could be improved[23]. Moreover, the rich functional groups in GO can be used for multi-dye adsorption. Meanwhile, there could be synergistic effects between GO and CS, which would increase the adsorption of the CS composite.

    In this study, a magnetic composite adsorbent composed of CS, magnetite nanoparticles, and GO (CS/Fe3O4/GO) was prepared by a simple co - precipitation method. The addition of Fe3O4 can impart magnetism to the samples, allowing for efficient magnetic separation. The morphology, structure, and magnetism of the samples were characterized. The adsorption capacity, adsorption kinetics, adsorption mechanism, and recycling times of methylene blue (MB), Congo red (CR), and methyl orange (MO) were studied.

    The chemical reagents used in the experiment, including CS (deacetylation degree > 95%), ferric chloride (FeCl3), ferrous chloride tetrachloride (FeCl2· 4H2O), MB, sodium hydroxide (NaOH), and acetic acid, were all purchased from Shanghai McLean Chemical Co., LTD. Ammonia (25%), CR, hydrochloric acid, GO, and MO are from Shanghai Alighting Chemical Co. All reagents were analytically pure grade. All solutions were prepared with laboratory-made deionized water.

    The synthesis of CS/Fe3O4/GO is schematized in Fig. 1[24]. Briefly, a CS solution was prepared by dissolving 0.5 g CS into 100 mL 2% acetic acid. 0.1 g GO was dispersed in 50 mL deionized water by ultrasound, and then added to the CS solution and stirred for 1 h. Meanwhile, 1.07 g FeCl3 and 0.87 g FeCl2·4H2O were dissolved into 50 mL deionized water, followed by dropping dilute ammonia water to adjust the pH to 9.8. Then the prepared iron salt solution was poured into the cooked CS/GO solution, with the careful dropping of ammonia water to keep the pH at 9.8. After vigorously stirring for 2 h, the obtained precipitation was separated with a hand - held magnet and washed to neutral. After vacuum drying at 50 ℃ and grinding, CS/Fe3O4/GO was obtained. For comparison, Fe3O4 nanoparticles, CS/Fe3O4, and Fe3O4/GO were prepared by the same method and parameters.

    Figure 1

    Figure 1.  Schematically illustration of the synthesis process and molecular evolution of CS/Fe3O4/GO
    1.3.1   Plotting of standard absorption curves

    Before adsorption experiments, the standard absorption curves of each dye solution were drawn by spectrophotometry. Taken MO as an example, the standard absorption curve was obtained as follows. MO solutions with concentrations of 0, 2, 4, 6, 8, and 10 mg·L-1 were prepared. The absorbance was measured by an ultraviolet-visible spectrophotometer (UV -5100, China) at 464 nm. After linear fitting of MO concentration and absorbance, a functional relationship can be obtained:

    $ Y = 0.0758X + 0.0254\quad \left( {{R^2} = 0.9996} \right) $

    (1)

    Where, X and Y are the concentration and absorbance of MO, respectively. It can be seen from the value of R2 that there was a good linear relationship between the concentration and absorbance of MO.

    1.3.2   Multi-dye adsorption experiments

    Multi - dye adsorption experiments were carried out in 250 mL beakers at room temperature. Briefly, 0.2 g adsorbent was added into 100 mL (150 mg·L-1) MO solution, and shaken under a shaking speed of 150 r·min-1 for 10 h in a constant temperature shaker. Part of the supernatant was extracted every few minutes, and its absorbance was measured. By substituting the measured absorbance into Eq.1, the current concentration of MO can be calculated. The removal rate (Rr) of MO can be calculated as follows:

    $ {R_{\rm{r}}} = \left( {{c_1} - {c_0}} \right)/{c_0} \times 100\% $

    (2)

    Where, c1 and c0 are the current concentration and initial concentration of MO, respectively. The adsorption experiments of MB and CR solutions were carried out in a similar method. In this study, the effects of the solution pH, adsorption time, as well as dye concentration on the adsorption were investigated.

    1.3.3   Recycling of the adsorbent

    To verify the continuous adsorption of CS/Fe3 O4/GO, the adsorbent was recycled 5 times to estimate its recyclability. All the adsorption was performed at optimal conditions. Firstly, the adsorbed CS/Fe3O4/GO was separated by magnetic separation. Then desorption was carried out in different methods. Put simply, 0.05 g CS/Fe3O4/GO adsorbed with MB was added to 100 mL 1 mol·L-1 hydrochloric acid, and 0.1 g of CS/Fe3O4/GO adsorbed with MO or CR was added to 100 mL 1 mol· L-1 NaOH, respectively. The mixtures were treated in a constant temperature shaker with a shaking speed of 150 r·min-1 for 10 h. After magnetic separation, the desorption adsorbents were added to 100 mL dye solution for the next adsorption cycle.

    The morphology of the samples was characterized by transmission electron microscope (TEM, Tecnai G2 F20, USA), and the acceleration voltage was 200 kV. The structure of the samples was analyzed by X-ray diffractometer (XRD, Bruker D8 Advance, China) and X-ray photoelectron spectroscopy (XPS, Escalab 250 Xi, USA). In XRD investigation, Cu radiation (λ = 0.154 nm) was used to scan at a rate of 5 (°)·min-1 in a 2θ range of 20° to 80°. FT-IR spectra of samples were recorded with a Fourier Transform infrared spectrometer (FT-IR, Nicolet 380, Gre) in a range of 400- 4 000 cm-1. The magnetic properties of the samples were analyzed by vibrating sample magnetometer (VSM, HH-20, China).

    The XRD patterns of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO are shown in Fig. 2a. It is found that all the samples contained a clear Fe3O4 structure. The characteristic peaks at 30.2°, 35.6°, 43.3°, 53.7°, 57.3°, and 62.7° correspond to the (220), (311), (400), (422), (511), and (440) crystal plane diffraction peaks of inverse - spinel Fe3O4 (PDF No.38- 0214), separately. The broad peak at 20.0° presenting in the XRD patterns of CS/Fe3O4 and CS/Fe3O4/GO is the characteristic diffraction peak of crystal CS, indicating the existence of CS. FT-IR investigation provides more information on chemical composition and chemical binding. As illustrated in the FT-IR spectrum of CS/Fe3O4/GO (Fig. 2b), the peak at 565 cm-1 corresponds to the vibration peak of the Fe—O group, confirming the presence of Fe3O4. The adsorption peaks at 3 416, 1 630, and 1 070 cm-1 correspond to O— H stretching vibration[25], C=C skeleton vibration, and the C—O stretching of GO, respectively. The peaks at 3 420, 1 630, and 1 070 cm-1 belong to the N—H stretching vibration of CS, C=O stretching vibration of —NHCO— official energy group, and C—O vibration shrinkage adsorption, separately[26-27]. The FT - IR spectrum not only supports CS, Fe3O4, and GO exist in CS/Fe3O4/GO, but also implies that they are bound together by chemical bonds. The stable chemical structure facilitates the continuous and efficient use of the adsorbents.

    Figure 2

    Figure 2.  (a) XRD patterns and (b) FT-IR spectra of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO; (c) TEM image of CS/Fe3O4/GO; (d) Magnetic hysteresis loops of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO

    Inset in d: Photo of adsorbent separated from solution by magnet

    The TEM analysis was applied to characterize the morphology of CS/Fe3O4/GO. As shown in Fig. 2c, there was a large amount of film-like substance in the field of view. Some of them are curled up. From the morphology contrast, these substances may be GO. A lot of nanoparticles were dispersed on GO membranes. The particles were 5 -24 nm in diameter and some of them had shells with low contrast. Considering CS had a lower contrast compared with Fe3O4, the particles could be CS/Fe3O4. The CS/Fe3O4 particles were relatively evenly distributed on the GO surface and a uniform composite material was formed. The hysteresis loops of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO are shown in Fig. 2d. The specific saturation magnetization of the Fe3O4 particles was up to 58.8 emu·g-1. While that of the CS/Fe3O4 particles decreased to 45.6 emu·g-1. The specific saturation magnetization (Ms) of the CS/Fe3O4/GO composite was further reduced to 42.5 emu·g-1 after the combination of non - magnetic GO. But the magnetism was still strong enough for efficient magnetic separation. As shown in the inset of Fig. 2d, the CS/Fe3O4 /GO adsorbent could be easily separated by a hand magnet.

    To further understand the chemical state of the elements in Fe3O4, CS/Fe3O4, and CS/Fe3 O4/GO, XPS investigations were performed (Fig. 3). The survey spectra (Fig. 3a) clearly showed the presence of Fe, C, and N elements in CS/Fe3O4 and CS/Fe3O4/GO. The C1s XPS spectra are shown in Fig. 3b. To distinguish different chemical states of carbon elements, the C1s peak was fitted. It is seen that the C1s envelope of CS/Fe3O4 contained four peaks, corresponding to four different carbon species (282.5 eV for C—H, 283.5 eV for C=C and C—C, 284.5 eV for C—N, 286.5 eV for C=O). Due to the addition of GO, the corresponding C1s binding energy changed slightly. The C1s peaks of CS/Fe3O4/GO displayed at 283.4, 284.9, 285.5, and 286.8 eV, which are assigned to carbide, non-oxygenated carbon (C—C/C=C), carbons bound to nitrogen (C—N), and carbons bound to oxygen (C=O), respectively[28]. The change of binding energy suggests that CS molecules may chemically bond to GO molecules. The N1s XPS spectra of CS/Fe3O4 and CS/Fe3O4/GO are shown in Fig. 3c. In the N1s spectrum of CS/Fe3O4, there was only one peak at 397.9 eV, which is attributed to the binding energy of N in —NH2 group. While the N1s peak of CS/Fe3O4/GO changed to 398.2 eV, indicating the amide bonding effects (—NH — C) between amino groups of CS and carboxyl groups of GO[12]. It supports that CS molecules are chemically bonded to GO molecules. In the Fe2p spectrum (Fig. 3d), the Fe2p1/2 and Fe2p3/2 are located at 724.3 and 710.6 eV, respectively, evidencing the existence of Fe3+ and the absence of Fe2+ on the surface[29]. The XPS results indicate the presence of CS, Fe3O4, and GO, and their chemical bonding of them, which is consistent with the previous characterization results.

    Figure 3

    Figure 3.  XPS spectra of survey (a), C1s (b), N1s (c), and Fe2p (d) of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO
    2.2.1   Dye adsorption of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO

    The obtained samples were used to adsorb multiple dyes in water. As shown in Fig. 4, the CR adsorption of Fe3O4 was very low, only 4.9 mg·g-1. The adsorption capacity increased to 60.2 mg·g-1 after CS modification. The optimal adsorption condition was at pH=2. The growth of CR adsorption could be due to the hydroxyl group and amino group on the CS molecule, which has a strong affinity for CR molecules. The adsorption capacity of Fe3O4/GO increased to 68.4 mg· g-1, a little larger than that of CS/Fe3O4. It maybe owing to the high surface area of GO and a large number of hydroxyl groups, epoxy carboxyl, and carboxyl groups on it. By contrast, the adsorption capacity of CS/Fe3O4/GO increased significantly. The highest CR adsorption capacity was up to 294.4 mg·g-1. Since the combination of Fe3O4 with CS or GO cannot produce such a high adsorption capacity, the increase in adsorption capacity should be owing to the synergistic effect of GO and CS. Interestingly, the dye adsorption capacity of CS/Fe3O4 /GO was closely related to the pH of the solution. The highest CR adsorption capacity was obtained at pH=2, while the adsorption capacity at high pH was low. This could be owing to the surface charge variation of CS/Fe3O4/GO with pH. It is well known that CR is a cationic dye. Thus, it is positively charged when dissolved in water. Under acidic conditions, the surface charge of the CS/Fe3O4/GO adsorbent tends to be negatively charged[18]. Therefore, the electrostatic attraction between CR and CS/Fe3O4/GO was enhanced under low pH conditions, and the dye adsorption capacity increased.

    Figure 4

    Figure 4.  Effect of pH on CS/Fe3O4/GO, Fe3O4/GO, CS/Fe3O4, and Fe3O4 for CR adsorption
    2.2.2   Multiple-dye adsorption performance

    CS/Fe3O4/GO had high adsorption not only for CR but also for MB and MO. As illustrated in Fig. 5a, when the initial concentration was 150 mg·L-1 and pH=2 and 11, the equilibrium adsorption capacities of CR, MB, and MO were 303, 200, and 167 mg·g-1, respectively. The adsorption capacity of CR was higher than those of MO and MB, which may be due to the different charges after hydrolysis. MO and CR are positively charged, while MB is negatively charged. The multi - dye adsorption was very fast in the first 20 min, then slowed down gradually until the adsorption equilibrium was reached at about 120 min. Few adsorbents can simultaneously adsorb multiple dyes with different charges, therefore, the multi - dye adsorption of CS/Fe3O4/GO should be a big advantage for its application[30]. The multi-dye adsorption of CS/Fe3O4 /GO could be closely related to the multiple chemical functional groups on the surface and the synergistic effect of GO and CS.

    Figure 5

    Figure 5.  Effect of time (a), pH (b), and initial concentration (c) on adsorption capacity using CS/Fe3O4/GO as an adsorbent; Comparison of the adsorption effect of CS/Fe3O4/GO, CS/Fe3O4, and Fe3O4/GO (d)

    The effect of pH on the adsorption of MB, MO, and CR was also studied. As shown in Fig. 5b, when the initial concentration was 150 mg·L-1 and after 180 min of adsorption, the adsorption of MB increased with the increase of solution pH, while those of MO and CR increased with the decrease of pH value. The highest adsorption capacities for MB, MO, and CR were obtained at pH=11 and 2, respectively. The different adsorption capacities could be due to the different charges of the dye molecules after hydrolysis. MB is positively charged, while MO and CR are negatively charged when dissolved in water. Meanwhile, the surface potential of CS/Fe3O4/GO also changed with pH. Under alkaline conditions, the surface potential of CS/Fe3O4/GO is negatively charged, thus the adsorption of MB increases due to the electrostatic attraction. Similarly, under acidic conditions, the surface potential of CS/Fe3O4/GO is positively charged, and the adsorption capacity of MO and CR is enhanced.

    2.2.3   Effect of dye concentration on the multi-dye adsorption

    It is found that the initial dye concentration had a significant influence on the multiple-dye adsorption of CS/Fe3O4/GO, as shown in Fig. 5c. When the pH=2 and 11, after 180 min of adsorption, at low concentrations, the adsorption capacities of all the dyes increased with the increase in dye concentration. But the increasing trend stopped as the dye concentration was enhanced above a certain value. The certain values for MB, MO, and CR were 80, 150, and 180 mg·L-1, respectively. After this, the dye adsorption remained unchanged. The change of adsorption capacity with dye concentration could be understood by the adsorption equilibrium theory[31]. When the concentration of dye was low, the adsorption active sites on the adsorbent were only partly occupied. Thus, more dye molecules could be adsorbed rapidly when the dye concentration increased. When the dye concentration increased to a certain value, most of the adsorption point was occupied, then the dye adsorption capacities cannot continue to increase. In this case, a dynamic balance between adsorption and desorption was achieved near the surface of the adsorbent. Continuing to increase the dye concentration would not increase the amount of adsorption.

    2.2.4   Adsorption isotherm modeling

    The adsorption isotherm model was used to analyze the adsorption mechanism of CS/Fe3O4/GO. The linear formulas of the Langmuir model and Freundlich model are expressed as follows:

    Langmuir equation:

    $ {q_{\rm{e}}} = \frac{{{q_{\rm{m}}}{K_1}{c_e}}}{{1 + {K_{\rm{L}}}{c_{\rm{e}}}}} $

    (3)

    Freundlich equation:

    $ {q_{\rm{e}}} = {K_{\rm{f}}}c_{\rm{e}}^{1/n} $

    (4)

    Where qe is the adsorption amount at adsorption equilibrium, qm is the maximum adsorption capacity, ce is the equilibrium concentration of dye, KL is Langmuir constants, and Kf and n are Freundlich constants. Fig. 6a shows the isotherm simulation of MB, and Fig. 6b displays the isotherm simulation of MO and CR. The parameters of the Langmuir and Freundlich models are shown in Table 1. It is found that for all the multiple - dye adsorption, the R2 values of the Langmuir model were greater than that of the Freundlich model. The adsorption curves simulated by the Langmuir model were also more consistent with actual measurements. This suggests that the multiple - dye adsorption of CS/Fe3O4/GO mainly follows the Langmuir model, implying monomolecular adsorption.

    Figure 6

    Figure 6.  Langmuir and Freundlich isotherm models for the adsorption of (a) MB, (b) MO, and CR onto CS/Fe3O4/GO; (c) Kinetic model adsorption of CS/Fe3O4/GO; (d) Effect of cycle numbers of CS/Fe3O4/GO on MB, MO, and CR adsorption

    Table 1

    Table 1.  CS/Fe3O4/GO adsorption isotherm parameters for MB, MO, and CR
    下载: 导出CSV
    Isotherm Parameter MB (pH=11) MO (pH=2) CR (pH=2)
    Langmuir qmax/(mg·g-1) 210.6 258.6 308.9
    KL/(L·mg-1) 0.354 1 0.224 3 0.261 5
    R2 0.956 9 0.982 59 0.983 58
    Freundlich n 2.736 0 2.263 1 1.825 5
    Kf/(mg1-1/n·g-1·L1/n) 64.064 2 62.676 4 70.939 8
    R2 0.875 75 0.935 99 0.979 05
    2.2.5   Adsorption kinetics

    Pseudo-first-order and pseudo-second-order kinet - ic models were used to fit the time - dependent multi - dye adsorption data of CS/Fe3O4/GO. The adsorption mechanism including chemical reaction, diffusion control, and mass transfer of the dyes on adsorbents was studied. The adsorption kinetic equations are as follows:

    Pseudo-first-order rate equation:

    $ {q_t} = {q_{\rm{e}}}\left( {1 - {{\rm{e}}^{ - {k_1}t}}} \right) $

    (5)

    Pseudo-second-order rate equation:

    $ {q_t} = \frac{{t{k_2}q_{\rm{e}}^2}}{{1 + t{k_2}{q_{\rm{e}}}}} $

    (6)

    Where, qe and qt are the dye adsorption capacity at equilibrium concentration and predetermined time t respectively. k1 and k2 are rate constants of the pseudofirst-order model and pseudo-second-order model respectively.

    Fig. 6c shows the dynamics simulation of the pseudo-first - order model and pseudo-second-order model. It is found that the actual adsorption curves of all three dyes were more consistent with pseudo-secondorder kinetics, after 180 min of adsorption. Detailed parameters are shown in Table 2. The R2 of the pseudosecond-order model was higher than that of the pseudofist-order model. Therefore, the kinetic fitting was better described by pseudo-second-order models, indicating the chemical adsorption of the multidye adsorption. Combining the simulation results of adsorption thermodynamics and adsorption kinetics, it could be concluded that the multi-dye adsorption of CS/Fe3O4/GO belongs to the single - molecular layer chemical adsorption.

    Table 2

    Table 2.  CS/Fe3O4/GO adsorption kinetics parameters for MB, MO, and CR
    下载: 导出CSV
    Method Parameter MB (pH=11) MO (pH=2) CR (pH=2)
    Pseudo-first-order qt/(mg·g-1) 200.00 167.24 303.22
    k1/min-1 0.240 8 0.262 4 0.127 9
    R2 0.982 8 0.991 0 0.984 3
    qe/(mg·g-1) 192.10 161.85 289.02
    Pseudo-second-order k2/(mg·g-1·min-1) 1.48×10-3 2.4×10-3 5.96×10-4
    R2 0.997 7 0.995 6 0.998 6
    qe/(mg·g-1) 207.66 172.81 315.88
    2.2.6   Recycling of the adsorbent

    To test the reusability and stability of CS/Fe3O4/GO, recycling experiments were carried out. After the dye adsorption, CS/Fe3O4/GO was separated magnetically and soaked in certain acidic or alkaline solutions for desorption. Five adsorptiondesorption experiments were performed. As shown in Fig. 6d, although the removal rate decreased slightly after every desorption, the adsorption of each dye was still above 90% after five cycles.

    2.2.7   Mechanism of multi-dye adsorption

    As discussed above, the dye adsorption of CS/Fe3O4/GO was much higher than those of CS/Fe3O4 and Fe3O4/GO. The low adsorption of CS/Fe3O4 could be due to the loss of some active sites and groups after CS cross - linking[18]. The low adsorption capacity of Fe3O4/GO suggests that GO does not have a high chemical affinity for dyes, despite its high specific surface area. Therefore, the significant increase in adsorption of CS/Fe3O4/GO could not come from CS or GO itself. CS and GO may have a synergistic effect on the adsorption, which improves the adsorption and fixation to multi - dyes and expands dye selectivity. The good recyclability and reusability of CS/Fe3O4/GO implied that the stability of CS was greatly improved. It could even be used in acidic conditions, which would expand the application field.

    The difference in adsorption of the three dyes may be caused by their different molecular structures, especially for different chemical groups, which affect the hydrolytic properties and chemical affinity. As illustrated in Fig. 7, the adsorption of MO, CR, and MB had different mechanisms. The dye molecules can form covalent bonds with the amine groups in CS/Fe3 O4/GO. The adsorptions of MO and CR could be due to the combination of the amino group and sulfonic acid group, as well as the existence of π -π interaction and hydrogen bond[32]. The adsorption of MB is owing to the combination of carboxyl and amino groups, as well as the existence of π-π interaction and hydrogen bond[33]. Careful N1s XPS investigation gave clear proof for the mechanism above. The binding energy of the amine group changed differently after various dye adsorptions, as shown in Fig. 8[32]. The N1 s binding energy of the adsorbent after MO, CR, and MB adsorption changed from 398.2 eV to 399.4, 399.4, 399.1 eV, respectively, indicating the presence of —NH—SO3, —NH—SO3, and —NH—COO[34].

    Figure 7

    Figure 7.  (a) Chemical structures of MO, CR, MB; Mechanisms of MO (b), CR (c), and MB (d) adsorption on CS/Fe3O4/GO

    Figure 8

    Figure 8.  Comparison of the N1s XPS spectra of CS/Fe3O4/GO before and after adsorption of MB, MO, and CR

    CS/Fe3 O4/GO composite adsorbent was fabricated by a simple co -precipitation method. Careful investigations suggest that Fe3O4/CS nanoparticles are evenly distributed on the surface of GO. CS/Fe3O4/GO had a 42.5 emu·g-1 of high magnetism, which is strong enough for an efficient solid - liquid magnetic separation.

    Multi - dye adsorption experiments on CS/Fe3O4/GO indicated that the maximum adsorptions of MB, MO, and CR were 210.6, 258.6, and 308.9 mg·g-1, respectively. The influence of pH, initial dye concentration, and contact time on the adsorption performance was carefully investigated. The adsorption kinetics and adsorption isotherm fitting suggest that the multi - dye adsorption of CS/Fe3O4/GO is a single - molecular layer chemical adsorption. The enhanced multi - dye adsorption could be closely related to the synergistic effect of CS and GO. Recycling experiments showed that the multiple-dye adsorption remained more than 90% after five cycles.


    Acknowledgments: This work was financially supported by the Natural Science Foundation of Anhui Province (Grant No. 1908085ME127), the National Undergraduate Innovation and Entrepreneurship Training Program of China (Grant No. 202110361073), and the Research Foundation of the Institute of Environment-friendly Materials and Occupational Health (Wuhu), Anhui University of Science and Technology (Grant No. ALW2021YF11).
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  • Figure 1  Schematically illustration of the synthesis process and molecular evolution of CS/Fe3O4/GO

    Figure 2  (a) XRD patterns and (b) FT-IR spectra of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO; (c) TEM image of CS/Fe3O4/GO; (d) Magnetic hysteresis loops of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO

    Inset in d: Photo of adsorbent separated from solution by magnet

    Figure 3  XPS spectra of survey (a), C1s (b), N1s (c), and Fe2p (d) of Fe3O4, CS/Fe3O4, and CS/Fe3O4/GO

    Figure 4  Effect of pH on CS/Fe3O4/GO, Fe3O4/GO, CS/Fe3O4, and Fe3O4 for CR adsorption

    Figure 5  Effect of time (a), pH (b), and initial concentration (c) on adsorption capacity using CS/Fe3O4/GO as an adsorbent; Comparison of the adsorption effect of CS/Fe3O4/GO, CS/Fe3O4, and Fe3O4/GO (d)

    Figure 6  Langmuir and Freundlich isotherm models for the adsorption of (a) MB, (b) MO, and CR onto CS/Fe3O4/GO; (c) Kinetic model adsorption of CS/Fe3O4/GO; (d) Effect of cycle numbers of CS/Fe3O4/GO on MB, MO, and CR adsorption

    Figure 7  (a) Chemical structures of MO, CR, MB; Mechanisms of MO (b), CR (c), and MB (d) adsorption on CS/Fe3O4/GO

    Figure 8  Comparison of the N1s XPS spectra of CS/Fe3O4/GO before and after adsorption of MB, MO, and CR

    Table 1.  CS/Fe3O4/GO adsorption isotherm parameters for MB, MO, and CR

    Isotherm Parameter MB (pH=11) MO (pH=2) CR (pH=2)
    Langmuir qmax/(mg·g-1) 210.6 258.6 308.9
    KL/(L·mg-1) 0.354 1 0.224 3 0.261 5
    R2 0.956 9 0.982 59 0.983 58
    Freundlich n 2.736 0 2.263 1 1.825 5
    Kf/(mg1-1/n·g-1·L1/n) 64.064 2 62.676 4 70.939 8
    R2 0.875 75 0.935 99 0.979 05
    下载: 导出CSV

    Table 2.  CS/Fe3O4/GO adsorption kinetics parameters for MB, MO, and CR

    Method Parameter MB (pH=11) MO (pH=2) CR (pH=2)
    Pseudo-first-order qt/(mg·g-1) 200.00 167.24 303.22
    k1/min-1 0.240 8 0.262 4 0.127 9
    R2 0.982 8 0.991 0 0.984 3
    qe/(mg·g-1) 192.10 161.85 289.02
    Pseudo-second-order k2/(mg·g-1·min-1) 1.48×10-3 2.4×10-3 5.96×10-4
    R2 0.997 7 0.995 6 0.998 6
    qe/(mg·g-1) 207.66 172.81 315.88
    下载: 导出CSV
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  • 发布日期:  2023-04-10
  • 收稿日期:  2022-08-24
  • 修回日期:  2023-03-09
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