Al-Doped chitosan nonwoven in a novel adsorption reactor with a cylindrical sleeve for dye removal: performance and mechanism of action

Abstract

In order to overcome the inconvenience of solid/liquid separation of powdered adsorbents, a novel adsorption reactor with a cylindrical sleeve was designed to match the textile-pattern of chitosan nonwoven for the sake of easy separation, simple operation and high efficiency. Meanwhile, Al-doped chitosan nonwoven was prepared to enhance the adsorption ability of chitosan nonwoven through the chelation interaction between dyes and metal centers of the Al-doped chitosan nonwoven. In this system, the adsorption of C. I. Acid Red 73 into Al-doped chitosan nonwoven could maintain high efficiency in a wide pH range from 3 to 10, with a high maximum adsorption capacity of 260.03 mg g⁻¹. Additionally, the studies of reusability, pH sensitivity, the effect of the coexisting ions and the adsorption of other organic dyes indicated that the Al-doped chitosan nonwoven fixed in as a cylindrical sleeve within a reactor was technically feasible, highly efficient, and implies a potential of practical application for dyeing effluent treatment.

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1. Introduction

Nowadays, various low-cost materials including bio-chars,¹ agricultural wastes,² industrial wastes,³ sea materials⁴ and soil materials,⁵ have been used to adsorb pollutants from wastewater due to the advantages of inexpensiveness and universal availability.⁶ However, the majority of the adsorbents described above are in the powdered form, which often suffer from the inconvenience of solid/liquid separation.

Among these low-cost adsorbents, chitosan has proved to be a promising material, which is a major component of crustacean shells and one of the most abundant biopolymers in nature. To overcome the separation and recovery difficulties, it has been molded in several shapes, such as membranes, fibers, beads, microspheres, sponges and films.⁷ Additionally, magnetism has been used as an effective means of separating the suspended chitosan from water.⁸ Magnetic chitosan could be rapidly and easily separated from water by the application of an external magnetic field. Nonetheless, in order to further simplify the treatment process, as well as improving the adsorption ability of chitosan, novel methods still need to be explored to separate the adsorbent powders in a cost-effective way.

Recently, with the fast growing nonwoven fabric industry, chitosan has been manufactured into a nonwoven using a variety of techniques including electrospinning, wet-spinning and solution casting.^9–11 Owning to its textile-like pattern, nonwoven is easy to separate and replace from wastewater in a continuous flow system. Chitosan nonwoven is widely used in membrane bioreactors to enhance the antifouling properties based on its advantage of being highly porous and non-toxic.^12–15 Nevertheless, comparatively few studies have been focused on the adsorption performance of chitosan nonwoven. Although the results have shown the promising applicability of chitosan nonwoven in adsorption processes, only small pieces of fiber mats were used to test the adsorption performances.^16–19 The lack of a suitable reactor might be one of the key limitations that impede the practical application of chitosan nonwoven because the textile-like pattern of chitosan nonwoven is entirely different from the various shapes of other common low-cost adsorbents.

Thus, in this study, a novel adsorption reactor with a cylindrical sleeve was designed to match the textile-like pattern of chitosan nonwoven for the sake of easy separation, simple operation and high efficiency. The nonwoven could be fixed without deformation through the glass strips on the inner cylinder and facilitate contact with the solution. All the treatment procedures such as modification, adsorption, desorption and regeneration can be proceeded in the same reactor in a simple way. Additionally, Al-doped chitosan nonwoven was prepared because the advantage of chitosan–metal complexes have been reported to enhance the adsorption ability of chitosan via chelating interactions between pollutants and metal centers of polymer–metal complexes.^4,20–23 Meanwhile, anionic dyes were selected as the target pollutant to investigate the performance of Al-doped nonwoven in a novel adsorption reactor with a cylindrical sleeve because the dye manufacturers and dyeing industry have been largely relocated to developing countries nowadays and industrial dye-laden effluents are an increasingly major concern that need to be effectively treated.

2. Materials and methods

2.1 Chemicals

AlCl₃·6H₂O, Al₂(SO₄)₃·18H₂O, Al(NO₃)₃·9H₂O, HCl, NaOH, NaHCO₃ and ethanol were obtained from National Medicines Corporation Ltd. of China. Chitosan nonwoven (CSNW) was purchased from Youngchito Bio Co. Ltd., C. I. Acid Red 73 (AR73) is a commercial product and its structure is shown in Fig. S1 in the ESI.† Doubly distilled water was used throughout this study. Other chemicals were of laboratory reagent grade and used without further purification.

2.2 Preparation of the Al-doped chitosan nonwoven

Prior to adsorption of the dye, CSNW samples were pretreated via surface reactions with different kinds and concentrations of Al³⁺. CSNW (5 g) was soaked in 2 L of Al³⁺ solutions with stirring for 12 h at room temperature. The reaction time for the preparation of Al-doped chitosan nonwoven (Al-CSNW) varied over several time intervals (1, 2, 3, 4, 6, 9 and 12 h). The nonwoven was then washed with doubly distilled water to remove the excess of Al³⁺ and finally dried in an oven at 40 °C.

2.3 Removal procedures of dyes

Dye removal experiments were performed using the novel nonwoven reactor. The Al-CSNW was sealed and agitated at 100 rpm at room temperature. The typical reaction mixture was initiated with 2 L of dye at 100 mg L⁻¹ and 5 g of Al-CSNW, and all the adsorption process were repeated more than 3 times. During the pH study, the initial pH value of the dye solutions was adjusted from 3 to 10 by addition of 0.1 mol L⁻¹ HCl and 0.1 mol L⁻¹ NaOH as necessary. The reusability experiment of Al-CSNW was performed under the same conditions, and the dyes were added after every adsorption experiment. Dye concentrations were analyzed using a Shimadzu UV-2401PC UV-vis spectrometer (Tokyo, Japan) at its absorbance maximum (510 nm).

2.4 Adsorption isotherms measurements

The adsorption isotherms of AR73 on Al-CSNW in water were carried out using the batch slurry method. The slurry, containing 5 g of Al-CSNW and 2 L of AR73 solution at various concentrations, was agitated at 100 rpm at room temperature until equilibrium. The amount of adsorbed AR73, q_e, was calculated by eqn (1):where q_e is the dye capacity in the sorbent at equilibrium (mg g⁻¹), C₀ is the initial dye concentration in the liquid phase (mg L⁻¹), C_e is the liquid-phase dye concentration at equilibrium (mg L⁻¹), V is the volume of solution (L) and M is the mass of sorbent used (g).²⁴

2.5 Reactor design and configuration

The adsorption reactor (Fig. 1) was made of Plexiglas, comprising an electric motor, an adsorption vessel and a mechanical stirrer. The adsorption vessel was cylindrical with a diameter of 250 mm and a height of 310 mm. The electric motor and controller were placed on the top of the adsorption vessel, and the mechanical stirrer rotates by electric motor. The upper sidewall and the bottom of the adsorption reactor were connected with water inlet and outlet tubes, respectively. Importantly, there was an inner cylinder with a diameter of 160 mm and a height of 310 mm in the absorption vessel. The inner cylinder was full of 30 mm diameter holes. One piece of Al-CSNW (5 g with an area of 0.16 m²) was wrapped around the inner cylinder and fixed with 4 glass strips.

Fig. 1 Schematic diagram of the adsorption reactor with cylindrical sleeve for dye removal.

The reactor in this study could be operated by both directions using both batch and continuous experiments. The water inlet and outlet described in Fig. 1 was designed for continuous experiments. In order to investigate the adsorption ability of Al-CSNW, static adsorption experiments were done in this study. Based on the cylindrical sleeve structure of this reactor, the nonwoven could be fixed without deformation through the glass strips on the inner cylinder in order to facilitate contact with the dye solution, which was the main advantage over normal flasks or other reaction containers. Then, all the treatment procedures such as modification, adsorption, desorption and regeneration can proceed in the same reactor in a simple way. However, the continuous adsorption experiment was also of great significance to evaluate a new adsorption reactor and it should be tested in future studies.

2.6 Characterization

Scanning electron microscope (SEM). Scanning electron microscope (SEM) micrographs were taken on a TM 3030 scanning microscope (Hitachi, Japan) at a voltage of 5.0 kV to test the morphological characterization of Al-CSNW-AR73 samples. The sample surfaces were carbon-coated before analysis.

X-ray diffraction (XRD) patterns. X-ray diffraction (XRD) patterns of samples were measured using a D/MAX-2550/PC diffractometer (Rigaku, Japan) with a Cu K_α ray source (α = 1.54 Å). The scanning the rate was set at 2° min⁻¹ with the spectra being recorded at 40 kV and 36 mA.

X-ray photoelectron spectroscopy (XPS) analyses. X-ray photoelectron spectroscopy (XPS) analyses of the chitosan nonwovens were performed on a Thermo ESCALAB 250 electron spectrometer (Thermo Electron, US). The samples were excited with X-rays over a specific 400 μm area using monochromatic Cu K_α radiation (1486.6 eV) at 150 W. As the maximum measurable depth of XPS is less than 20 nm, it could not detect the background signal in our analysis. The XPS spectra were fitted assuming a Gaussian–Lorentzian distribution for each peak with a linear background in order to determine the binding energy of the various element core levels, and all binding energies were referenced to the neutral carbon peak at 284.8 eV. All of the XPS spectra were normalized.

Fourier-transform infrared spectroscopy (FTIR). Fourier-transform infrared spectroscopy (FTIR) of chitosan nonwovens were recorded by a Shimadzu 8900-FTIR spectrometer (Tokyo, Japan) using KBr pellets containing about 1% (w/w) of samples.

Ultraviolet-visible spectroscopy (UV-vis). Ultraviolet-visible spectroscopy (UV-vis) of Al-CSNW, before and after adsorption, were performed on a Lambda 35 spectrophotometer (America, PerkinElmer).

¹H nuclear magnetic resonance spectroscopy (¹H NMR). ¹H nuclear magnetic resonance spectroscopy (¹H NMR) of chitosan nonwoven and Al-CSNW was performed using a Bruker AV400 instrument. CSNW and Al-CSNW were dissolved in a mixed solvent CD₃COOD/D₂O.

3. Results and discussion

3.1 The performance of nonwoven in the sleeve reactor

To overcome the separation difficulty of low-cost adsorbent, Al-CSNW was prepared and a novel reactor with a cylindrical sleeve structure was designed for this adsorption material. The design of the reactor facilitates contact between the nonwoven and the dye solution completely. More importantly, the procedures of modification, adsorption, desorption and regeneration were easy to operate in the same reactor.

During the modification process, chitosan nonwoven was impregnated in Al³⁺ solution to prepare the Al-CSNW. The critical factors of modification such as different aluminum source, the concentration of Al³⁺ content in the modification solution, modification time and pH of the modification solution were examined. As shown in Fig. S2 in the ESI,† Al-CSNW resulted in an improved adsorption ability compared to initial CSNW at dye removal. The concentration of Al³⁺ and the reaction time of modification are two crucial factors that could significantly improve the adsorption ability of CSNW. The Al-CSNW prepared in 0.5 mmol L⁻¹ Al(NO₃)₃ solution with the reaction time of 12 h exhibited the highest removal efficiency and at a relatively lower material dose. This material was then used for all of the following adsorption experiments unless otherwise specified. It is worth noting that the CSNW modified by 0.5 mmol L⁻¹ Al(NO₃)₃ without pH adjustment (pH was 4.21) could adsorb 99% of AR73 in 120 min. Hence, the Al-CSNW was prepared without pH adjustment in the following experiments.

The kinetic behavior of the adsorption process was studied at different initial AR73 concentrations using Al-CSNW (Fig. 2). It can be observed from the Fig. 2a that the amount of adsorption increased rapidly in the first 40 minutes, contributing to 90% of the ultimate adsorption amount, and then augmented slowly and approached the adsorption equilibrium in about two hours. The total amount of AR73 adsorbed increased with increasing initial dye concentration. The kinetic data of dye adsorption were investigated by pseudo-first-order and pseudo-second-order kinetic models in order to understand the adsorption mechanism. The kinetic models are given in Text S1 in the ESI† material. Table S1† lists the results of rate constant studies for different initial concentrations calculated from the pseudo first-order and pseudo second-order models. Based on the values of regression coefficient (R²), it was found that the pseudo second-order model was better at describing the kinetic data of dye adsorption. This indicates that the Al-CSNW sorption system followed the pseudo second-order rate model predominantly, and the overall process appears to be controlled by chemisorption.^25,26

Fig. 2 Removal kinetics of AR73 (initial concentrations ranged from 20 mg L⁻¹ to 200 mg L⁻¹): (a) time profile with Al-CSNW (5 g); (b) fitting with a pseudo-first-order curve; (c) fitting with a pseudo-second-order curve.

To optimize the design of an adsorption system for dye removal, it is important to establish appropriate correlations for the equilibrium curves. Three isotherm equations including Langmuir, Freundlich and Langmuir–Freundlich, were tested in the present study (Text S2 in ESI†). The results of the experimental data fitted to these three equations and the parameters are shown in Fig. 3 and Table S2.† The Langmuir–Freundlich isotherm best fits with the experimental data for the dye adsorption both on the initial CSNW and the Al-CSNW. The sorption capacity of the Al-CSNW adsorbent was found to be 260.03 mg g⁻¹ according to the fitted parameter of the Langmuir–Freundlich equation, which was much higher than the initial CSNW adsorbent (88.43 mg g⁻¹), indicating the capacity advantage of Al-CSNW.^21,27

Fig. 3 (a) The initial CSNW and (b) Al-CSNW adsorption isotherms fitted to the Langmuir model (the dash line), Freundlich equation (the dash-dotted line) and Langmuir–Freundlich model (the solid line).

3.2 The effect of pH and co-existing ions

The investigation of the pH sensitivity of Al-CSNW is illustrated in Fig. 4a. With the increase of pH, the adsorption rate decreased obviously, but the final adsorption efficiency of AR73 on the Al-CSNW just slightly decreased from 99.8% to 92.1% when the pH increased from 3 to 10. According to published work, electrostatic attraction is now recognized as the main factor in anionic dyes adsorption by chitosan.²⁸ It is hard for dyes to be adsorbed on chitosan in high pH solutions because the free amino groups of chitosan could not be protonated according to its point of zero potential, which lies within 6.5–6.7 (ref. 29) and could not facilitate electrostatic interaction between chitosan and the negatively charged anionic dyes. Thus, in alkaline conditions, the chelation interaction between dye molecules and the Al³⁺ center of Al-CSNW played the leading role instead of electrostatic interactions in AR73 adsorption. This indicates that the AL-modified chitosan nonwoven has more flexible adaption in a wide range of pH values, and the mechanism for dyes adsorption is not only dependent on protonation of the –NH₂ group.

Fig. 4 (a) pH sensitivity of Al-CSNW on dye adsorption (initial concentration: 100 mg L⁻¹, 2 L, adsorbent: 5 g). (b) Effect of the coexisting ions on the removal efficiency of the dye in the presence of Al-CSNW (initial concentration: 50 mg L⁻¹ to 100 mg L⁻¹, room temperature, 2 L, adsorbent: 5 g, pH = 6).

The fact that, practically in industry settings, high concentrations of ions are also applied to enhance bath dye exhaustion, other ions cannot be neglected in this study.³⁰ Different ions were added to the dye solution under optimal experimental conditions (ion concentration: 50 mg L⁻¹ and 100 mg L⁻¹) and the results are shown in Fig. 4b. The influences of Cl⁻, SO₄²⁻, Na⁺, K⁺, Mg²⁺and Ca²⁺ on AR73 removal were negligible, suggesting that these common coexisting ions almost had no negative effect on dye removal by Al-CSNW. However, the removal efficiency decreased obviously in the presence of HCO₃⁻ and CO₃²⁻, reflecting the relative affinity of these anions for the chitosan–alumina complex. Moreover, the same conditions for AR73 removal were applied to three other different dyes. The removal rate of C. I. Reactive Black 5, C. I. Reactive Blue 19 and C. I. Direct Yellow 12 were 99%, 99.9% and 98.39%, respectively. This suggests that the Al-CSNW could efficiently adsorb these dyes.

3.3 The regeneration and reusability of Al-doped chitosan nonwoven

Regeneration and reusability are important properties that cannot be ignored for practical applications and economic demand.³¹ Fig. 5 shows the reusability and regeneration of Al-CSNW for the adsorption of AR73. The results show that the removal efficiency of dyes by Al-CSNW decreased obviously after recycling 5 times due to the limitation of adsorption saturation. Then, basic solutions (NaOH, NaHCO₃, Na₂CO₃ and NH₄OH) and polar solvents (methanol, ethanol and acetone) of different concentrations were tested to find the optimal eluent for desorption. The basic solution of NaOH, Na₂CO₃ and NaHCO₃ can help to desorb the dye more than 90%. Considering the alkalinity of the eluent, 0.05 mol L⁻¹ of NaHCO₃ was selected as the eluent for desorption to avoid alkaline damage of the nonwoven. The desorbed dyes were condensed during the desorption procedure due to the volume of the eluent being only one fifth of the wastewater. Then, the desorbed dyes could be reused after adjusting the pH value. After the desorption process, 2.5 mmol L⁻¹ HCl was applied to regenerate the Al-CSNW complex. Afterwards, the regenerated adsorbent could be reused. The regeneration efficiency indicates that Al-CSNW remained almost constant after desorption and regeneration, indicating that there were no irreversible sites on the surface of the adsorbent.

Fig. 5 Recycling experiment for AR73 removal (dye: 100 mg L⁻¹, 2 L; pH = 6; Al-CSNW: 5 g).

The desorption study indicated that a basic eluent such as NaOH, Na₂CO₃ and NaHCO₃ of 0.05 mol L⁻¹ molarity can help to desorb the dye more than 90%. The high concentration of HCO₃⁻ would break the chelation interaction between dyes and Al-CSNW and replaces the dyes bound to the Al³⁺ center. Then, the 2.5 mmol L⁻¹ HCl wash regenerated the Al-CSNW, and the nonwoven could maintain high removal performance for reuse. Furthermore, Al leaching was also of great importance in the case of repeated runs. According to the results of SEM-EDS, the weight percentage of elemental Al in Al-CSNW was 0.2267%. When the Al-CSNW was used after one adsorption–desorption–regeneration cycle, the weight percentage of Al slightly decreased to 0.2167%. Additionally, the Al content in the aqueous phase was detected by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) after the dye adsorption. The total Al content in the aqueous phase was only 0.088 ppm. There was some Al leaching in this system, but it was just equal to 1.27% of the Al that was supported on chitosan nonwoven. Also, it was observed that the small leaching of Al did not cause an obvious loss to the adsorption performance according to the reusability experiment.

3.4 The proposed mechanism

The initial chitosan nonwoven contains abundant chemically reactive hydroxyl (–OH) and amine groups (–NH₂) that can capture metals efficiently in aqueous solution via chelation. The results of SEM-EDS analysis confirmed that the Al³⁺ ions were successfully chelated by CSNW. SEMs of initial CSNW and Al-CSNW after dye adsorption are shown in Fig. 6 at different magnifications. The nonwoven is composed of plenty of individual fibers with smooth surfaces, and the fibers are long and continuous with a uniform diameter. After the modification with Al³⁺, the nonwoven maintained its fibrous morphology with no obvious diameter change. EDS elemental mapping images of dye-adsorbed nonwoven (CSNW-AR73 and Al-CSNW-AR73) are shown in Fig. 6g–m. The Al-CSNW was composed of C, O, N and Al with the weight percentage of 45.42%, 43.48%, 10.8% and 0.2267%, respectively. The element C and O are mostly derived from chitosan and their mapping results clearly show the fibrous morphology both in CSNW-AR73 and Al-CSNW-AR73. The element S originated from the dye AR73. Due to the relatively lower concentration of AR73, the element S mapping of CSNW-AR73 is obscure. The element S mapping of Al-CSNW-AR73 presents the fibrous morphology obviously, and its outline corresponds well to the mapping of element Al that was derived from the Al³⁺ modification. These results reveal that the Al³⁺ ions were efficiently chelated on the surface of CSNW and the presence of Al³⁺ obviously improved the dye adsorption on CSNW.

Fig. 6 SEM of initial CSNW after dye adsorption (CSNW-AR73, (a–c)) and Al-CSNW after dye adsorption (Al-CSNW-AR73, (d–f)), EDS elemental mapping images of initial CSNW after adsorption (CSNW-AR73, (g–i)) and Al-CSNW after adsorption (Al-CSNW-AR73, (j–m)).

According to a reported study, both –NH₂ and –OH groups bind to the metal ions and more than one polymer chain is involved in the formation of chitosan–metal complexes.³² Based on the results of the kinetic studies, it was found that the pseudo second-order model was better at describing the kinetic data of dye adsorption, indicating that the overall adsorption process was controlled by chemisorption. And the isotherms study showed that the adsorption capacity of Al-CSNW was 260.03 mg g⁻¹, which was two times higher than that of the initial CSNW (88.43 mg g⁻¹). It indicates that the Al-doping on CSNW has a greater concentration of active sites for capturing the dyes.

The XRD patterns of the initial CSNW and Al-CSNW before and after dye adsorption are shown in Fig. 7a. All the cases of CSNW exhibit the main peak at 2θ = 20°, which corresponds to the semi-crystalline structure of chitosan as reported.³³ Yen and Mau³⁴ found that the crystallinity of chitosan increased with the presence of more free amine groups within the molecular structure, which results in better packing of the macromolecular polymeric chains with a consequent increase in the crystallinity. For the initial CSNW, the crystalline peak at 20° significantly decreased after the adsorption of dyes (CSNW-AR73), suggesting the efficient bonding of dyes to the amine groups of chitosan. For the Al-CSNW, although its crystalline peak was weaker than the CSNW because of the chelation between Al³⁺ and amine group of chitosan, the intensity of the crystalline peak almost remained the same after dye adsorption. This result indicated that the mechanism of dye adsorption on Al-CSNW was different from that on the initial CSNW because the Al³⁺ chelated on CSNW performed as the adsorption active site instead of the amine group.

Fig. 7 (a) XRD patterns and (b) N 1s XPS spectra of initial CSNW, initial CSNW after adsorption (CSNW-AR73), Al-CSNW and Al-CSNW after adsorption (Al-CSNW-AR73).

Fig. 7b shows the N 1s XPS spectra of the CSNW with and without the adsorbed dye. The N 1s spectrum of the initial CSNW was assigned to peaks at the binding energies of 399.6 and 402.1 eV for the nitrogen atoms in the –NH₂ and –NH₃⁺ groups, respectively. After the adsorption of AR73, the same two peaks are still evident; however, the intensity of the NH₃⁺ peak obviously decreased, which indicates that the electrostatic adsorption of AR73 onto the –NH₃⁺ sites changed the state of the –NH₃⁺ nitrogen atoms. In the case of Al-CSNW, the N 1s signal was very weak; this is probably due to the amount of Al³⁺ taken up by the CSNW and covers the amino groups from detection at the surface. The peak at the binding energy around 399.6 eV, which refers to the –NH₂ group in chitosan, could still be observed, and it almost remained the same after the adsorption of AR73. These results corresponded with the results of XRD analysis. The electrostatic bonding between –NH₃⁺ groups and anionic dyes played an important role in the dyes adsorption by the initial CSNW. This was replaced by the interaction between Al³⁺ and anionic dyes when the Al³⁺ ions were chelated to the surface of CSNW.

Additionally, FT-IR, UV-vis and ¹H-NMR spectra are shown in Fig. S3–S5 in the ESI.† There was no significant difference between initial CSNW and Al-CSNW before and after the adsorption of dyes. The detailed information is disclosed in Text S3 in the ESI.†

Overall, the main possible reason for dye removal by Al-CSNW with as a cylindrical sleeve is proposed in Fig. 8. During the modification process, the Al³⁺ ions were efficiently chelated on the surface of CSNW through the interaction between Al³⁺ with amine groups and hydroxyl groups of the chitosan polymer (Fig. 8a). When the dye solution was injected into adsorption reactor, the dyes would be adsorbed by chelating with Al³⁺ centers of Al-CSNW and the protonated amine groups (Fig. 8b). After adsorption saturation, the high concentration of HCO₃⁻ would break the chelation interaction between dyes and Al-CSNW and then the dyes could be desorbed. The Al-CSNW could then be regenerated by an acidic solution (Fig. 8c).

Fig. 8 Proposed mechanism between dyes and Al-CSNW fixed in a cylindrical sleeve.

4. Conclusion

In this study, Al-CSNW in a novel adsorption reactor as a cylindrical sleeve was proposed for dyes removal in order to overcome the separation difficulty of low-cost adsorbents. The adsorption reactor was designed to maximize the advantage of high efficiency, easy separation and simple operation. The nonwoven could be fixed without deformation through the glass strips on the inner cylinder in order to facilitate contact with the dye solution. This reactor can conveniently and efficiently remove dyes with a high maximum adsorption capacity of 260.03 mg g⁻¹, which was enhanced by the chelation interaction between the sulfonic group of the dye and the Al³⁺ centers. Meanwhile, the studies of reusability, pH sensitivity, the effect of the coexisting ions and the adsorption of other organic dyes indicated that the Al-CSNW fixed in a cylindrical sleeve reactor was technically feasible and highly efficient. This implies the potential for practical application in waste water treatment.

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (No. 21407021), the Shanghai Yang-Fan Program of Science and Technology Commission of Shanghai (No. 14YF1405000), the National Key Research and Development Program of China (Grant No. 2016YFC0400501), the Fundamental Research Funds for the Central Universities and DHU Distinguished Young Professor Program.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21369c

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