Hierarchically structured, well-dispersed Ti⁴⁺ cross-linked chitosan as an efficient and recyclable sponge-like adsorbent for anionic azo-dye removal

Abstract

Porous macro- and hierarchically structured materials with desired morphologies and high adsorption capacities are of great interest because of their potential applications in realistic environmental pollutant treatment. In this study, sponge-like Ti⁴⁺ cross-linked chitosan (SL-TiCs) was synthesized using a chitosan solution and titanium metal salt as precursors. The as-prepared product was characterized via FE-SEM, HR-TEM, EDS, EMI, FT-IR and XPS. The results demonstrate that SL-TiCs is composed of multiple nano-layer twisted sheets and a high content of well-dispensed Ti⁴⁺ on its surface, which provide plenty channels for mass transfer and active sites for pollutant adsorption. The hierarchical structure of SL-TiCs exhibits a high adsorption capacity for Orange II with a fast adsorption rate and good recyclability. The adsorption performance can be described by the pseudo-second order and Langmuir isotherm models. The maximum adsorption capacity is 1120 mg g⁻¹, which is much higher than that of many other adsorbents. It is speculated that the probable mechanisms involve ligand exchange (44.6%) between Cl⁻ and dye–SO₃⁻ and electrostatic attraction (55.4%) between Ti–OH₃⁺ and dye–SO₃⁻ anions in the adsorption process. This study implies that SL-TiCs can be a potential adsorbent for the adsorption and separation of anionic dyes from large volumes of industrial dye wastewater.

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1. Introduction

Many textile, printing and pigments industries that use dyes release a large amount of highly colored effluent into wastewater, which results in serious environmental pollutant issues. Approaches to rationally and efficiently deal with contaminated effluents before discharge are of great importance.¹ Various techniques have been used to remove dyes from wastewater, including photocatalysis, membrane filtration, chemical oxidation, coagulation, and adsorption.^2–4 Among these methods, adsorption is considered as one of the best available techniques because of its high efficiency, operational simplicity and low energy requirements.^5,6 Numerous adsorbents have been designed and reported for the removal of organic dyes by many researchers, including common adsorbents (activated carbon, clay, rice husk, red mud, and fly ash),⁷ functional composites (graphene, silica, and metal–organic frameworks),^8–12 nano-particles (metal oxides, and nano-porous adsorbents),^13–15 and magnetic nano-materials (Fe⁰, and Fe₃O₄).^16–19 Nevertheless, common adsorbents often only have limited adsorption capacities, costly regeneration and low adsorption rates. Although many nano-materials possess a high adsorption capacity and fast rate, they also have some drawbacks, such as easy aggregation and difficult separation, which thus limit their realistic industrial application. Magnetic adsorbents have provided a good solution for hard separation, however they need an extra magnetic system and consume more energy.

To overcome these disadvantages, macro-sized porous hierarchically structured materials have received attention, and are constantly being designed for wastewater treatment.^20–22 The large size of these materials favors their easy separation from water, and their hierarchical structures can provide high specific surface areas and large surface-to-volume ratios for contact, more active sites for reaction and plenty channels for mass transfer.¹ Therefore, the development of novel porous hierarchical structured adsorbents with high adsorption efficiency and easy separation are still in demand. Chitosan is a biodegradable polysaccharide, which has many specific properties such as easy film-formation, low-cost, non-toxicity, eco-friendliness as well as high adsorption capacity. The abundance of amine (–NH₂) and hydroxyl (–OH) groups on chitosan can provide more adsorption sites for binding organic and inorganic molecules. On the other hand, the Ti⁴⁺ ion is a high valence ion, which can be used as an ion-binding source to prepare functional compounds and adsorbents for the removal of many toxic dyes and ions.^23,24 The incorporation of the film-forming chitosan and high valence Ti⁴⁺ to obtain porous hierarchical structure hybrid adsorbents of sponge-like Ti⁴⁺ cross-linked chitosan (SL-TiCs) may preserve or even improve the major features of each component in the hybrid materials, and furthermore, new properties may come from the synergy of both components. Thus, the use of a certain strategy to prepare the hierarchical structure hybrid material, SL-TiCs, for wastewater treatment is great significance.

Herein, we develop a facile and simple route to synthesize porous hierarchical SL-TiCs based on a chitosan solution and tetrabutyl titanate as the ion precursor. The aim of this work is to investigate the possibility of using the as-obtained SL-TiCs for the removal of organic dye pollutants. Orange II serves as a model compound of the harmful and water soluble azo organic anions dyes, which are inexpensive, widely used in the textile and printing industries, and harmful to the environment. The adsorption performance of Orange II on SL-TiCs is studied in the batch type mode. Isotherm and kinetics models are applied to study its adsorption mechanism. The experimental study of its performance for the Orange II dye indicates that SL-TiCs is a suitable adsorbent for the efficient removal of organic dye pollutants from aqueous solution.

2. Materials and methods

2.1. Materials

Tetrabutyl titanate (analytical grade) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Chitosan (food grade), with a 90% deacetylation degree and average molecular weight of 5.0 × 10⁵ g mol⁻¹, was supplied by Shanghai Weikang Biological Co. (China). Glutaraldehyde (analytical grade) was purchased from Sigma-Aldrich Chemical Co. (China). Acetic acid, hydrochloric acid, sodium hydrate and ethanol were all A.R. grade reagents and also supplied by Sinopharm Chemical Reagent, Ltd (China). The Orange II stock solution (1000 mg L⁻¹) was freshly prepared by dissolving 0.10 g Orange II (analytical grade, Sigma-Aldrich) in 100 mL de-ionized water. All work solutions were derived through the appropriate dilution of the stock solution with de-ionized water. All other reagents used in this study were of analytical grade and used without further purification.

2.2. Synthesis of SL-TiCs

SL-TiCs was synthesized according to a simple complex reaction followed by cross-linking in chitosan solution using tetrabutyl titanate as the ion precursor. In detail, 2.0 g chitosan powder was added to 100 mL 1% acetic acid solution and dissolved completely with magnetic stirring. The Ti⁴⁺ ion precursor solution was obtained by using 4 mL of concentrated HCl to dissolve 3.42 g tetrabutyl titanate and the addition of 16 mL of de-ionized water then removing the upper organic phase. This Ti⁴⁺ solution was added to the chitosan solution with stirring for 2 h at room temperature to obtain a homogeneous solution. After that, a 2.0 mol L⁻¹ NaOH solution was added dropwise to adjust the solution pH to 8. Subsequently, 6 mL of 5% glutaraldehyde aqueous solution, as the cross-linked reagent, was added and the mixture stirred vigorously for another 2 h. The prepared cross-linked gel solution was sprayed with liquid nitrogen and then transferred into 50 mL of 50% ethanol for washing. The obtained homogeneous gel was transferred to a square mold and iced at −20 °C, and afterwards freeze dried to obtain SL-TiCs (1.5 cm × 1.5 cm × 0.3 cm). The as-prepared SL-TiCs was used for Orange II adsorption studies. For comparison, different amounts of titanium salt (1.71 and 5.17 g) were added to prepare SL-TiCs under the same conditions.

2.3. Characterization

The morphology, composition analysis and elemental mapping images (EMI) were studied using an SEM450 field emission scanning electronic microscope (FE-SEM, FEI, NOVA Nano, USA) with energy dispersive X-ray spectroscopy (EDS). Sheet with a fine morphology and hierarchical structure were obtained by high resolution transmission electron microscopy (HR-TEM, Genesis XM2 system, JEOL, JEM-2100). Fourier transform infrared (FT-IR, resolution: 4.0 cm⁻¹; number of scans: 16) spectra of chitosan, and SL-TiCs before and after Orange II adsorption were recorded on a Nicolet 6700 Fourier transform infrared spectrometer (Thermo Electron, USA). X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher Scientific Corp.) with monochromatic Al Kα X-ray radiation was used to evaluate the composition and coupled condition of SL-TiCs before and after Orange II adsorption. The fine-scan XPS spectra were fitted assuming a Gaussian–Lorentzian distribution for each peak. The zeta potential was measured at room temperature on a zeta potential analyzer (Nano-ZS90, English). The sample was prepared using liquid nitrogen to dry SL-TiCs which was then grinded to a powder before testing. The pH values of the solution were measured using a precision pH meter (pHS-3D, Shanghai Precision & Scientific Instrument Co., Ltd.). The concentration of Orange II in solution was determined using a JASCO V-570 UV-vis spectrophotometer at the maximum adsorption (λ = 483 nm).

2.4. Adsorption experiments

All adsorption experiments were carried out using 250 mL glass conical flasks in the batch type mode, with parameters including solution pH, initial Orange II concentration and contact time. For each experiment, a piece of SL-TiCs (about 25 mg) was added to 100 mL Orange II solution. The pH value of the solution was adjusted with 0.5 mol L⁻¹ HCl or NaOH to obtain the desired value. Then the conical flask was placed on a shaker at 150 rpm for a certain time, and subsequently the adsorbent was taken out directly from the flask using a steel tweezers. For the kinetics studies, samples were taken to determine the concentration of Orange II at different time intervals. The residual solution was then determined for the concentration of Orange II in the UV-visible range at the maximum absorption using a UV-vis spectrophotometer. The amount of adsorbed dye on SL-TiCs was calculated according to the following equation:and the dye removal of Orange II was calculated as follows:where, q_e and q_t (mg g⁻¹) are the adsorption capacity values in the solid phase at equilibrium and t time, respectively. C_o, C_t and C_e (mg L⁻¹) are the initial concentration, t time and equilibrium concentration of Orange II in the liquid phase, respectively. V (L) is the volume of the solution and m (g) is the mass of SL-TiCs used.

2.5. Separation and recycle study

In this experiment, one piece of SL-TiCs (about 25 mg) was added to an Orange II solution (100 mL) with an initial concentration of 300 mg L⁻¹ under optimum adsorption conditions. For a certain adsorption time, the residual Orange II was determined using a UV-vis spectrophotometer and the adsorption capacity calculated. After 120 min adsorption, the saturated Orange II absorbed SL-TiCs was directly taken out from the solution and rinsed with water. For regeneration, the adsorbent was immersed in 0.10 mol L⁻¹ of NaOH and stirred for 20 min. At 5 min intervals during desorption, the concentration of Orange II in solution was measured and the desorption capacity calculated. After 20 min desorption, the SL-TiCs were directly taken out from the solution and washed with water. The adsorbent was freeze-dried before being reused in the next cycle. The adsorption–regeneration cycles were repeated three times with the Orange II adsorption capacity analysis.

3. Results and discussion

3.1. Morphology characterization and composition analysis for SL-TiCs

Typically, SL-TiCs was synthesized through a simple cross-linking method in chitosan solution using tetrabutyl titanate as the ion precursor. Obviously, the as-prepared adsorbent is a sponge-like, white color, porous material, as shown in Fig. 1A. The size of the adsorbent was approximately 1.5 cm × 1.5 cm × 0.3 cm, and this large size would allow the adsorbent to be separated easily from water, thus avoiding secondary pollution. SL-TiCs was characterized via the BET surface area and pore size, which was 2.46 m² g⁻¹ and 1.96 nm, respectively. The morphology of SL-TiCs was further characterized via FE-SEM and HR-TEM. According to the FE-SEM image (Fig. 1B), the fine morphology of SL-TiCs was found to consist of dozens of twisted sheets and exhibited a flat surface with a size up to the micrometer scale. The twisted sheets were connected with each other to form a hierarchical structure and each sheet was also tightly overlapped together by many nano-sheets, which can be confirmed by the HR-TEM image (Fig. 1C). Further magnification to 50 000 shows that nanometer particles were well-dispensed on the surface of the twisted sheets, which showed a slightly rough surface (Fig. 1D). After Orange II (Fig. 1E) adsorption, however, a flat surface is displayed due to the accumulation of Orange II by certain adsorption mechanisms. In order to confirm the existence of certain elements in the adsorbent, EDS analysis was used to characterize SL-TiCs before and after Orange II adsorption, which is shown in Fig. 2A and B (EDS), respectively. Peaks and wt% corresponding to the elements of C, N, O, Cl, Ti and S were observed. From the comparison of the EDS spectra of SL-TiCs before and after Orange II adsorption, three points are concluded from this experiment: (i) the existence of the Ti element indicates that Ti⁴⁺ ions are cross-linked into the chitosan matrix; (ii) the disappearance of the Cl element and appearance of the S element demonstrate the occurrence of ligand–exchange reactions between Cl⁻ and dye–SO₃⁻; and (iii) the wt% of adsorbed S is as high as 11.6%, which suggests that the as-prepared SL-TiCs exhibits excellent adsorption capacity for Orange II removal. Furthermore, EMI images of SL-TiCs before and after Orange II adsorption were also obtained to examine the presence and distribution of elements in the adsorbent, as shown in Fig. 2A and B, respectively (EMI). For comparison, the elements of C, N and O were omitted in the images. The EMI images clearly demonstrate the presence of Cl and Ti on the SL-TiCs and that the elemental distribution is uniform and well-dispensed on the twisted sheets. This also clearly confirms the incorporation Ti⁴⁺ ions into the chitosan matrix. After Orange II adsorption, S is presented in the EMI image and Cl almost disappears, which indicate that Orange II adsorption occurred on the adsorbent and a ligand exchange reaction occurred between the coordinated Cl⁻ and dye–SO₃⁻. These results are consisted with the EDS analysis.

Fig. 1 Image of the as-synthesized SL-TiCs (A), SEM (B), TEM (C), and high magnification SEM before (D) and after (E) the adsorption of Orange II by SL-TiCs.

Fig. 2 (A) and (B) EDS and EMI (R replaces the C, O, and N elements) before and after the adsorption of Orange II on SL-TiCs, respectively.

3.2. Orange II adsorption performance on SL-TiCs

Generally, if a material has hierarchical structures and a high content of metal ions, it will possess more available active adsorption sites, efficient transport pathways and may exhibit excellent adsorption performances.¹ Therefore, the as-synthesized hierarchical SL-TiCs was used to absorb the Orange II azo-dye, which was chosen as a model pollutant in water. The efficiency of dye adsorption from aqueous solution using an adsorbent depends on various factors such as pH, initial concentration of dye and equilibrium time of adsorption. Firstly, it is well known that pH plays a significant role in the adsorption of dyes. In this experiment, the effect of pH on the adsorption of the Orange II dye onto SL-TiCs was studied by varying the pH from 2 to 11 at room temperature, since pH not only affects the surface charges of adsorbents, but also the species of dyes ions in solution. The plot of adsorption capacity vs. pH is shown in Fig. 3A. The adsorption capacity of Orange II presents a significant decrease trend from 396 to almost 0.0 mg g⁻¹ with an increase in pH value in the range of 2–11. Also, the Orange II removal efficiency can reach up to 98.9% when the initial pH of the Orange II solution is less than 3. This suggests that SL-TiCs can be a high-efficiency adsorbent for the adsorption of Orange II, and acidic solution benefits Orange II adsorption. On one hand, in acidic solution, Orange II dye can be ionized and converted to anionic dyes ions with sulfate groups (dye–SO₃⁻).²⁵ On the other hand, the element of Ti on SL-TiCs can be protonated to Ti–OH₃⁺ by hydrogen ions (H⁺). The excellent adsorption performance at this condition might be attributed to the electrostatic interaction between the positively charged Ti–OH₃⁺ and negatively charged dye–SO₃⁻ ions on the liquid–solid interface. To further understand the influence of pH on the Orange II adsorption process, we also measured the zeta potential of SL-TiCs and this value was found to be 4.3, which indicates that SL-TiCs carries positive charges at pH below 4.3 and protonated Ti–OH₃⁺ groups. Hence, strong electrostatic forces between the positively charged adsorbent and negatively charged Orange II anions happen at low pH values, which thus lead to the excellent adsorption capacity. However, when pH is above 4.3, a lower adsorption capacity is expected because of the repulsive forces between the negatively charged adsorbent and anions dyes. Additionally, different amounts of titanium salt (1.71, 3.42 and 5.13 g) were employed to synthesize SL-TiCs to remove Orange II, and the data is shown in Table S1 (ESI†). From this table, the maximum adsorption was obtained from 3.42 g titanium salt. Besides, a higher concentration of Ti was leached into solution due to more titanium hydroxide being formed during the preparation, which may result in a decrease in adsorption capacity. Therefore, we chose the amount of 3.42 g tetrabutyl titanate as the adsorbent for Orange II adsorption in the subsequent experiments.

Fig. 3 (A) Effect of pH on Orange II dye adsorption (C_o = 100 mg L⁻¹); (B) adsorption isotherms of Orange II dye adsorption on SL-TiCs; (C) curve of the removal rate; and (D) images of the removal effect with different initial dye concentrations (30–300 mg L⁻¹).

The adsorption capacity of SL-TiCs to different initial concentrations of Orange II can be used to reflect the realistic application of the adsorbent. The variation of Orange II adsorption capacity and removal efficiency with different initial Orange II concentrations were determined in the range of 30–400 mg L⁻¹ with an adsorbent dose of about 0.25 g L⁻¹. Fig. 3B–D record the curves of q_e vs. C_e, removal rate vs. C_o and the pictures the effect of the removal of Orange II at different initial concentrations. From Fig. 3B, it is clear that the adsorption capacity of Orange II on SL-TiCs increased with an increase in initial concentration of Orange II from 30 to 300 mg L⁻¹. This is due to the fact that the higher initial Orange II concentration supplied a stronger driving force for dye–SO₃⁻ ions from solution to the surface of SL-TiCs, which resulted in more chances of contact between the dye–SO₃⁻ ions and active sites. When the initial concentration was higher than 300 mg L⁻¹, the adsorption capacity remained constant at approximately 1120 mg kg⁻¹, which was attributed to the fact that the adsorption of Orange II on SL-TiCs reached the state of saturation. Meanwhile, the dye removal rate decreased with a further increase in initial Orange II concentration (Fig. 3C). Importantly, when the concentration was in the range of 30–300 mg L⁻¹, more than 90% of Orange II was removed (the images of Fig. 3D display the removal effect with different initial dye concentrations), which indicates that SL-TiCs has good potential for dye effluent applications. For further interpretation of the adsorption process, the Langmuir and Freundlich isotherms were employed to describe the experimental data. As shown in Fig. S1A (ESI†), the Orange II adsorption can be well fitted to the Langmuir adsorption model according to the high correlation coefficients (R², Table S2, ESI†), which indicates that the adsorption performance of Orange II on SL-TiCs can be regarded as monolayer adsorption. The maximum adsorption capacity (q_max) of SL-TiCs for Orange II was estimated to be 1120 mg g⁻¹. To evaluate the merits of SL-TiCs for Orange II adsorption, the q_max was compared to other reported materials and summarized in Table 1. It can be clearly seen that SL-TiCs has a 1.97–28.7 times greater q_max than that of other reported adsorbents. The sponge-like material in our work exhibits a superior adsorption efficiency for Orange II, which is ascribed to its hierarchical structures and well-dispersed Ti⁴⁺ ions to increase the active sites after the cross-linking reaction.

Table 1 Comparison of the maximum adsorption capacity of Orange II dye on various adsorbents

Adsorbents	Co (mg L^−1)	Temp. (°C)	pH	qmax (mg g^−1)	Ref.
SL-TiCs	30–300	25	3.0	1120	This work
Porous titania aerogel	50–1500	30	2.0	420	25
activated carbon	0–500	30	7.0	569	26
NH2-MCM-41	50–300	25	3.0	278	27
Sludge adsorbent	30–80	60	5.6	270	28
Rattle-type carbon–alumina core–shell spheres	15–50	25	5.0–6.0	210	29
Phosphonium-modified Algerian bentonites	50–150	20	6.5	53.8	30
Poly(N-isopropylacrylamide) microgel	13–94	50	7.0	49	31
Magnetic graphene/chitosan	0–60	25	3.0	42.7	32
HDTMA-coated zeolite	0–200	30	1.0	38.96	33

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Equilibrium time is another very important parameter that affects the design of dye wastewater treatment systems. Thus, the adsorption performance of SL-TiCs for the removal of Orange II from water was investigated via batch-type adsorption kinetic experiments. Fig. 4A and B show the adsorption kinetics of Orange II on SL-TiCs at an initial Orange II concentration of 30 and 150 mg L⁻¹, respectively. The concentration of Orange II decreased rapidly within the initial 5 min and then became gradually constant. As shown in the abovementioned two figures, adsorption equilibrium can be reached approximately after 40 min and 90 min, respectively. It is noteworthy that the removal rate was higher than 85% in the first 5 min of contact time on both concentrations, which indicates that it is fast enough to meet the industry application. Even more interesting is that the concentration of Orange II was almost 0 mg L⁻¹ (see insert Fig. 4A(a)) after 60 min adsorption when the initial concentration was 30 mg L⁻¹ (250 mL). In order to record the process of adsorption, UV-vis spectroscopy was applied to monitor the adsorption curve of Orange II after the addition of SL-TiCs to the dye solution. As shown in the insert of Fig. 4B(b), the intensity of the adsorption peak at λ_max dropped drastically within 10 min after SL-TiCs was added, and the adsorption peak almost vanished after 20 min, which indicates the rapid removal of Orange II. Additionally, to investigate the adsorption rate and mechanism of the adsorption process, the kinetics of Orange II adsorption on SL-TiCs were analyzed using the pseudo-first order (PF) and pseudo-second order (PS) models. According to Fig. S1B and Table S2, ESI,† we found that the PS order model can best describe the kinetic data for Orange II adsorption on the surface of SL-TiCs because it possesses a higher R² value (R² = 1.000). These results indicate that the rate-limiting step in the present system is chemical adsorption.^29,34

Fig. 4 Effect of contact time at the initial concentration of 30 mg L⁻¹ (A) and 150 mg L⁻¹ (B). Image of the removal effect before and after adsorption (insert a) and UV-vis spectra of Orange II after treatment by SL-TiCs at different contact times (insert b, dilution factor: 3).

3.3. Adsorption mechanism study

The FT-IR spectra of raw chitosan, SL-TiCs and SL-TiCs-OII were obtained and studied to gain insight into the adsorption mechanism, as shown in Fig. 5A. The characteristic peaks of chitosan were at 3446 cm⁻¹ (–OH and –NH₂ stretching vibrations), 2923 and 2879 cm⁻¹ (–CH and –CH₂ stretching vibration), 1653 cm⁻¹ (–NH₂ bending vibration), 1383 cm⁻¹ (–CH symmetric bending vibration), 1083 cm⁻¹ (C–O stretching at C₃) and 1034 cm⁻¹ (C–O stretching at C₆). For the FT-IR spectrum of SL-TiCs, the –NH₂ bending at 1627 cm⁻¹ underwent a shift of 26 cm⁻¹ and the two bands at 1083 and 1034 cm⁻¹ were merged into one band and shifted to 1088 cm⁻¹ when compared with raw chitosan, which indicate that the Ti⁴⁺ ions interacted with the –NH₂ and –OH groups present in chitosan. Besides, the band at 3423 cm⁻¹ also shifted by 23 cm⁻¹ in SL-TiCs, which further confirms Ti⁴⁺ ions bent onto the amino and hydroxyl groups of chitosan. After adsorption, some new bands were obviously presented in the spectrum of SL-TiCs-OII, including vibrations at 1505 cm⁻¹ (–N=N– bending), 1599 and 1452 cm⁻¹ (aromatic bending), 1620 cm⁻¹(C=N stretching),1258 and 1210 (v(C–N) and v(N–N) stretching), and 1210 and 1030 cm⁻¹ (symmetric vibrations of –O–S–(O₂) group).^16,33 These new peaks were due to the characteristic adsorption peaks of Orange II, which suggested that Orange II was successfully adsorbed on the surface of SL-TiCs. To further investigate the adsorption mechanism, XPS analysis was also carried out. The XPS wide scan spectra of SL-TiCs before and after Orange II adsorption are shown in Fig. 5B. In this figure, many peaks, including C 1s, O 1s, N 1s, Ti 2p and Cl 2p are observed. A new peak attributed to S 2p at the binding energy of around 168 eV is clearly presented and the peak of Cl 2p₃ simultaneously almost disappears after Orange II adsorption, which further confirm the adsorption of Orange II on the surface of SL-TiCs through ligand exchange. This result is consistent with the EDS and EMI analyses. Moreover, the high resolution XPS patterns of S 2p were obtained and shown in Fig. 5C. Interestingly, the curve fitting of the S 2p showed that two types of S existed in SL-TiCs-OII. The binding energies of the two S components are at 167.5 eV and 168.6 eV, which might be attributed to R–OH₃⁺–(SO₃⁻–dye) (55.4% w) and R–Ti–(SO₃⁻–dye) (44.6% w), respectively. Coupled with the results of the pH study and EDS analysis, these two types of S partially belong to electrostatic attraction (55.4% w) and ligand exchange interaction (44.6% w), which indicate that the two mechanisms for Orange II have almost the same trend during the adsorption process. Based on the above analysis, the active sites on the surface of SL-TiCs play a more vital role for Orange II dye adsorption. The high content of Ti on SL-TiCs could be protonated to positively charged Ti–OH₃⁺ in acidic solution. One of the mechanisms is based on the electrostatic attraction between Ti–OH₃⁺ cations and the negatively charged sulfonic group of dye–SO₃⁻. On the other hand, the ligand of Cl on SL-TiCs was also exchanged with dye–SO₃⁻ to form the SL-TiCs–SO₃–dye complex. Herein, the adsorption mechanism, including the adsorption process and the interactions between SL-TiCs and Orange II, is Cl ligand exchange and electrostatic attraction of the protonated sites of the Ti⁴⁺ center. The possible adsorption mechanism is schematically illustrated in Fig. 5D.

Fig. 5 FT-IR spectra (A) of chitosan, SL-TiCs and SL-TiCs after adsorption Orange II (SL-TiCs-OII); wide scan XPS spectra (B) of SL-TiCs and SL-TiCs-OII; high resolution S 2p spectrum (C) after Orange II adsorption; and proposed adsorption mechanism (D) of Orange II dyes by using SL-TiCs.

3.4. Recycle and reuse

The economic feasibility of using adsorbents to remove dyes from aqueous solution relies on their regeneration ability during multiple adsorption/desorption cycles. Based on the result of pH effect, SL-TiCs has little adsorption capacity for Orange II in alkaline solution. This is because the occurrence of more competition by OH⁻ ions results in an increase in the coulombic repulsion between dye–SO₃⁻ and the negative surfaces of SL-TiCs. Herein, the desorption process for Orange II was carried out using 0.10 mol L⁻¹ NaOH solution. The adsorption and desorption processes were repeated under optimum conditions and the data are shown in Fig. 6. From this figure, it is noticed that the times of adsorption and desorption are about 120 min and 20 min (C_o = 300 mg L⁻¹), respectively, which suggest that the desorption rate is faster than the adsorption rate. With three continuous adsorption/desorption cycles, the adsorption capacities of SL-TiCs was still maintained above 95%. More interestingly, the SL-TiCs after Orange II adsorption can be directly taken out from the flask using a steel tweezers. Therefore, it is concluded that SL-TiCs can be easily renewed and used repeatedly as a recyclable and efficient adsorbent for practical application in dye effluent treatment.

Fig. 6 Adsorption and desorption processes for Orange II on SL-TiCs.

4. Conclusions

In summary, SL-TiCs was successfully prepared through a simple complex reaction and cross-linking method. The as-synthesized adsorbent displays a porous macro- and hierarchical structure. The resulting SL-TiCs shows excellent adsorption capacity to Orange II with a fast adsorption rate and good recyclable, owing to the cooperative contribution of ligand exchange and electrostatic attraction between the sulfonic group of Orange II and active sites of the adsorbent. The adsorption data of Orange II on SL-TiCs can be described by pseudo-second-order kinetics and the Langmuir isotherm model with a remarkable maximum adsorption capacity of 1120 mg g⁻¹, which is higher than other reported absorbents in the literature. Our findings provide a new method for the design of effective and recyclable adsorbents to improve the adsorption capacity of anionic organic dyes. It is believed that the as-obtained SL-TiCs can be used as a good promising adsorbent for the adsorption and separation of anions dyes from large volumes of industrial dye wastewater.

Acknowledgements

We are grateful for the financial support by National Natural Science Foundation of China (Grant No. 21407050) and the Fundamental Research Funds for the Central Universities (Grant No. 22A201514019).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24446g

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