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Tuesday, 31 May 2016
Removal of direct dyes from aqueous solution by oxidized starch cross-linked chitosan/silica hybrid membrane
Published Date January 2016, Vol.82:174–181,doi:10.1016/j.ijbiomac.2015.11.005 Title Removal of direct dyes from aqueous solution by oxidized starch cross-linked chitosan/silica hybrid membrane
Author
Xuemei He a,,
Mei Du b
Hui Li a
Tianchi Zhou a
aCollege of Textiles and Clothing, Yancheng Institute of Technology, Jiangsu 224003, China
bCollege of Textiles and Clothing, Yancheng Institute of Industry Technology, Jiangsu 224003, China
Received 22 May 2015. Revised 17 October 2015. Accepted 2 November 2015. Available online 4 November 2015.
Abstract
In this research, chitosan/oxidized starch/silica (CS/OSR/Silica) hybrid membrane was prepared by using oxidized starch and 3-aminopropyltriethoxysilane (APTES) as cross-linking agents. The characterizations of the hybrid membrane were investigated by using attenuated total reflection (ATR) spectroscopy, scanning electron microscopy (SEM), thermogravimetry (TG) analysis and swelling measurement. The CS/OSR/Silica hybrid membrane exhibited the improved thermal stability and low degree of swelling in water. The adsorption properties of the CS/OSR/Silica hybrid membrane were studied by using two direct dyes (Blue 71 and Red 31). The results indicated the adsorption capacity of the CS/OSR/Silica hybrid membrane was found optimal at pH 9.82 and temperature 60 °C for Blue 71 and Red 31. The adsorption kinetic data followed pseudo-second order kinetic model and the adsorption behavior of the two dyes on the hybrid membrane fitted well with the Freundlich model. The CS/OSR/Silica hybrid membrane can be used as an appropriate biosorbent for removal of direct dyes from colored wastewater.
Keywords
Chitosan Oxidized starch
Hybrid Direct
dyes
adsorption
1 Introduction
Direct dyes with the presence of one or more azo groups (NN) are one of the most popular types of colorant used for the dyeing and printing of cellulosic fibers and their blends. However, their direct discharge can cause serious environment problems such as contribution of high organic loading, toxicity and esthetic pollution related to color[1]. At present, different techniques including ion exchange, coagulation/flocculation, adsorption, membrane filtration, etc. have been developed and applied for the treatment of dyes in effluents[2]and[3]. Amongst them, adsorption technique was considered economic, simple and effective method[4].
Chitosan (CS), N-deacetylated form of chitin, is the second abundant renewable biopolymer, next to cellulose [5]. In recent years, chitosan has been reported as a bio-sorbent material for the removal of different dyes, such as direct dyes, reactive dyes, acid dyes, disperse dyes from aqueous solutions due to good coagulating properties, excellent forming film characteristics, nontoxic, environment-friendly, high-efficiency [6], [7] and [8]. However, chitosan is highly swollen in water, therefore losses physical structure, and its low mechanical strength and poor acid resistance are also not very satisfactory [9], [10] and [11]. In order to avoid or retard the dissolution/degradation, chitosan is often modified by cross-linking reaction, in which a cross-linking agent (e.g., formaldehyde, glutaraldehyde, etc.) links chitosan chains using covalent bonding that aldehydic function react with the amino groups [12], [13]and [14]. It enhances the mechanical strength and chemical resistance of chitosan against chemicals such as alkali and acid [14]. But the formaldehyde and especially glutaraldehyde are toxic to human tissues, even at small traces [15] and [16].
In an attempt to avoid the commonly used dialdehydes and obtain a more biocompatible material [17] and [18], recently, aldehyde-functionalized anhydroglucose unit have received considerable interest due to minimal toxicity [19]. The preparation of anhydroglucose unit containing aldehyde groups have been achieved by reaction with nitrous acid (HNO2) [20] and periodate (HIO4) [21], [22]and [23]. For example, Pourjavadi et al. designed a new hydrogel based on chitosan using tetra aldehyde molecule as a crosslinker agent, obtained from periodate oxidation of sucrose [24]. The hydrogel swelling showed reasonable pH sensitivity, which the maximum swelling occurred at low pH (below pH 6.5). Similarly, Wang and Hon used oxidized sugars to prepare crosslinked chitosan-PEG membranes and crosslinked N-alkylated chitosan membranes, respectively [25]. Lee et al. reported a cross-linked chitosan/AS layer on the HA surface. The physisorbed chitosan on the AS layer was exposed to 2 wt.% sodium periodate solution, to prevent acid erosion of the model dental hydroxyapatite surface [26].
In this work, using oxidized dialdehyde starch and 3-aminopropyltriethoxysilane (APTES) as cross-linking agents; chitosan/oxidized starch (CS/OSR) and chitosan/oxidized starch/silica (CS/OSR/Silica) hybrid membranes were prepared and characterized respectively. The adsorption properties of Blue 71 and Red 31 onto the hybrid membranes were studied. The effects of different conditions such as adsorption temperature, solution pH, initial dye concentration, and contact time on the adsorption capacities of hybrid membranes were investigated respectively. In addition, the kinetics and adsorption equilibrium of Blue 71 and Red 31 on the hybrid membranes were analyzed by fitting experimental data in various kinetics and isotherm models.
2 Experimental
2.1 Materials
Chitosan (CS, degree of deacetylation 96.31%, MW = 7.9 × 105 Da) was purchased from Zhejiang Jinke Bio-tech Co. Ltd. (Zhejiang, China). Wheat starch (WS) (food grade), sodium periodate (analytical pure) and sodium borohydride were purchased from Guoyao Chemical Co.; 3-aminopropyltriethoxysilane (APTES) was supplied by Yancheng Renbo Chemical Co.; Direct Blue 71 (MW = 1029.7) and Direct Red 31(MW = 713.64) were supplied by Zhejiang Longsheng Dye Chemical Co., Ltd. (China) and used without further purification. The chemical structures of the two direct dyes were shown in Fig. 1. All other chemicals were of analytical grade and purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China).
Fig. 1. Chemical structures of two direct dyes: (a) Direct Blue 71 and (b) Direct Red 31.
2.2 Preparation of 2,3-dialdehyde starch by periodate oxidation
Oxidation of wheat starch is performed following the conditions adopted previously [15]: 3 g of wheat starch are added into 100 mL of 0.3 mol/L periodate sodium aqueous solution under stirring (molar ratio [monosaccharide units]/[periodate] = 1.613:1) at 30 °C in a lightproof condition. After 2 h, 20 mL of 0.1 mol/L solution of ethylene glycol was added to halt reaction and remove the excess of unreacted iodate for 0.5 h. After that, filtrate, then, 100 mL acetone was poured into the liquid filtrate solution to precipitate the product, 2 h. The precipitates were filtrated and dried at room temperature under vacuum. The obtained oxidation dialdehydes starch was denoted as OSR.
2.3 Preparation of hybrid membranes
Using the periodate oxidized starch as cross-linking agent, the chitosan/oxidized starch and chitosan/oxidized starch/silica hybrid membranes were fabricated respectively by means of a casting/solvent evaporation method. Briefly, chitosan (2 g) was dissolved in 100 mL 2% (v/v) aqueous acetic acid solution by stirring for 1 h at 25 °C. The pure chitosan membrane can be attained and denoted as CS. A certain amount of oxidized starch (mass ratio of chitosan:oxidized starch = 2:1) was added into 2% chitosan acetic acid solution with constant stirring for 2 h at 25 °C, then, for the other mass ratio of chitosan:oxidized starch = 2:1 part, 2 mL APTES was dropped slowly into the mixture solution containing chitosan and oxidized starch under stirring for 2 h at 25 °C. After the reaction was stopped, the solutions were degassed and casted onto clean glass plates to form membrane at room temperature. Both hybrid membranes were treated with 0.2 mol/L NaBH4 30 min, then, washed with distilled water several times, finally dried at ambient. The smooth chitosan/oxidized starch and chitosan/oxidized starch/silica hybrid membranes were obtained and denoted as CS/OSR and CS/OSR/silica respectively. The cross-linking reaction route for the hybrid membranes is shown in Scheme 1.
Scheme 1. Preparation of CS/OSR and CS/OSR/Silica hybrid membranes.
2.4 Swelling properties
Swelling studies were performed in water at 25 °C. The dried membranes (dried in a vacuum oven at 50 °C) were weighed before swelling; they were then immersed in water. After the desired time had elapsed, the swollen membranes were weighed. Taking the average value of three measurements for each sample, the swelling ratio (SR) is defined as follows:
equation1
where Ws is the weight of sample in the swollen state and Wd is the weight of the dried membranes.
2.5 Adsorption experiments
Batch adsorption experiments of two direct dyes onto hybrid membranes were conducted by placing 100 mg hybrid membranes into 50 mL of 160 mg/L dye aqueous solution, 60 min. The effect of temperature varying from 30 to 90 °C at pH 7.0 on the adsorption capacities of adsorbents was evaluated. The pH of the solution was adjusted to 7, 8.6, 9, 9.82, 11.2, 11.92 respectively using 0.01 M phosphate buffer. After filtration, dye concentration in the filtrate and initial concentration were determined by UV–vis spectrometer at λmax and the adsorption capacities were calculated as follows:
equation2
where q is the adsorption capacities of hybrid membranes (mg dye/g adsorbent); V is the volume of dyes solution (L), C0 is the concentration of dyes (mg/L), and m is the weight of membranes (g).
2.6 Measurements
The FTIR-ATR spectra of different membranes were recorded using a Fourier transform infrared spectrometer (Thermo Nicolet, USA), equipped with an attenuated total reflectance (ATR) device in the wave number range 500–4000 cm−1 with 20 scanning rate with resolution of 4 cm−1. The SEM images of different membranes were taken using a scanning electron microscope (FEI Quanta 200 scanning electron microscope, USA). Samples were coated an approximate layer thickness of 15 nm with gold, and the SEM images were taken at an accelerating voltage of 10 kV. Thermal stability of the hybrid membranes were analyzed using a thermo gravimetric analyzer (Type STA-449C; NETZSCH Instrument Co. Ltd.; Germany) in N2atmosphere. The temperature was ranged from 20 to 650 °C at a scanning rate of 10 °C min−1.
3 Results and discussion
3.1 FTIR-ATR
The FTIR-ATR spectra of the pure CS, CS/OSR and CS/OSR/Silica membranes are shown in Fig. 2. As can be seen, there was a strong absorption peak at 3432 cm−1 in each spectrum, which was due to the OH stretching vibration of surface OH groups in the materials, and the CH stretch was appeared between 2990 and 2879 cm−1. When two or more substances are mixed, physical blends versus chemical interactions are reflected by changes in characteristic spectra peaks [27]. With addition of OSR, compared to the pure CS membrane, the peaks at 3300–3400 cm−1become weaker and narrower, which revealed the Schiff's bonding interaction between oxidized starch and chitosan. The peak at 700 cm−1 decreased, while the intensity of the peak at 1410 cm−1 after cross-linking was increased substantially, which made the peaks at 1422 and 1381 cm−1 become one peak, due to the formation of the CN in the CS/OSR membrane. Meanwhile, the peak at 1559 cm−1was enhanced with the addition of oxidized starch; a similar phenomenon has been investigated by Xu et al. [28]. As it was expected, the silicon oxide matrix is detected in the CS/OSR/Silica hybrid membrane in Fig. 2. This is evidenced by the characteristic silicon oxide broad bands at 790, 950 and 1070 cm−1 corresponding to symmetric SiOSi bond stretching, SiOH bond stretching and asymmetric SiOSi bond stretching, respectively. The 1070 cm−1 band accounts for condensed polymeric precursors forming oligomeric units typical of SiO2 polymerization [25] and [29].
Fig. 2. FTIR-ATR spectra of different membranes.
3.2 Morphology of hybrid membranes
The morphology of membrane was a very important characteristic because it ultimately determined many of their properties [30] and [31]. SEM micrographs of the CS, CS/OSR and CS/OSR/Silica hybrid membranes are presented in Fig. 3a–c respectively. The surface of the pure CS membrane was found to be relatively smooth and homogenous (Fig. 3a), which was significantly different from the CS/OSR membrane with oxidized starch cross-linking. The CS/OSR membrane exhibited a compact and ordered structure, which suggests oxidized starch and chitosan are compatible polymers, able to form a homogenous structure [32]. In addition, it was also obvious that the CS/OSR/Silica hybrid membrane exhibited a rough and irregular surface morphology in Fig. 3c. The roughened surface with large surface area can be suitable for absorbing the dye molecules in aqueous phase [32] and [33]. There are little pores with pore sizes ranging from 50 to 100 μm on the surface of CS/OSR/Silica hybrid membrane. The micrographs showed that the sol–gel process provided a highly efficient way of producing chitosan–silica hybrid membrane with a porous and uniform structure [34] and [35].
Fig. 3. SEM micrographs of different membranes: (a) pure CS; (b) CS/OSR and (c) CS/OSR/Silica.
3.3 Thermal stability
It is important to carry out studies on the thermal stability properties of the hybrid membranes. The DSC and TGA curves of the pure CS, CS/OSR, and CS/OSR/Silica hybrid membranes are shown in Fig. 4. The pure CS membrane and the hybrid membrane containing oxidized starch and silica exhibited decomposition in three stages. The initial weight loss due to the evaporation of water is 8.92%, 4.95%, 9.13% respectively for the pure CS, CS/OSR and CS/OSR/Silica membranes. Taking the temperature at 5% weight loss (Td) to evaluate the thermal stability of the hybrid materials, it can be seen that the CS/OSR membrane exhibited better thermal stability than did the pure CS [36]. The second decomposition temperature of the pure CS, CS/OSR and CS/OSR/Silica membranes was observed at 236, 246.3 and 241.8 °C respectively. The highest weight loss of the CS, CS/OSR and CS/OSR/Silica membranes is about 61.28%, 55.50% and 50.67% respectively in the decomposition starting from 200–400 °C. The temperatures of rapid weight loss (Tmax) of the samples shifted to higher temperature regions, to further indicate the CS/OSR and CS/OSR/Silica membranes possessed good thermal stability. This was consistent with the results by Tuhin et al. [37].
Fig. 4. Thermal stability of different membranes: (a) DSC curves of membranes and (b) TG curves of membranes.
3.4 Swelling behavior of different membranes
Fig. 5 shows the swelling behavior of different membranes in water. With the time increase, the swelling ratio increases for all samples, and reaches an equilibrium swelling level after 25 h adsorption. All samples absorbed more than 100% of their weight in water at different time interval. Visual inspection of the samples also showed appreciable volume increase. The pure CS membrane had a relatively higher swelling ratio, compared to the CS/OSR and CS/OSR/Silica membranes. The lower water uptake by the CS/OSR membrane could be due to the cross-linking of oxidized starch [28]. The addition of silica can enhance the cross-linking between polysaccharides and oxidized starch through esterification reaction with the hydroxyl groups of chitosan and oxidized starch [38]. The swelling ratio of the CS/OSR/Silica hybrid membrane decreased significantly due to the cross-linking of oxidized starch and the SiOSi network formation of chitosan–silica hybrid materials [39].
Fig. 5. Swelling behavior of different membranes.
3.5 Effect of different factors on adsorption properties of hybrid membranes
3.5.1 Different hybrid membranes
Adsorption capacity of different membranes on Blue 71 and Red 31 were obtained at pH neutral, 30 °C and dye concentration 160 mg/L, as shown in Fig. 6. Obviously, the pure CS membrane had higher adsorption capacity than the CS/OSR membrane because of its abundant NH2 groups; while the adsorption capacity of the CS/OSR membrane decreased owing to the Schiff base formation between the aldehyde groups from oxidized sugar with the amine groups [26]. However, compared with the CS and CS/OSR membrane, the CS/OSR/Silica hybrid membrane had the better adsorption capacity. It could be attributed to the introducing of redundant hydroxyl groups onto the hybrid membrane surface by the silica cross linkages, hydrolysis and condensation, which can greatly increase dye uptake.
Fig. 6. Adsorption properties of different membranes.
3.5.2 Effect of pH
The pH is an important factor that controls the sorption of dyes from aqueous solutions onto solids. Therefore, the effect of pH ranging from 7 to 12 on the adsorption was investigated at 30 °C and 160 mg/L dye concentration (Fig. 7). Studies below pH 7 have not been carried out due to the degradation of membranes in the acidic medium. As shown in Fig. 7, the pH values of the aqueous solutions have considerable effect on the adsorption capacities of the CS/OSR and CS/OSR/Silica membranes. The adsorption capacity of the CS/OSR membrane increased with increasing of pH for two direct dyes, while the maximum sorption of two direct dyes on the CS/OSR/Silica membrane occurred at pH 9.82, and adsorption capacity decreased slowly beyond this pH value. This may be explained on their structures shown in Fig. 1. Direct Blue 71 and Red 31 have planar structures and occupy more space on the surface of the membrane and hinder other molecules to come into contact with the surface of the membrane, hence give high Kd at a comparatively higher pH, i.e., 9.82. At pH 9.82, all the protonated terminal amino groups fixed membrane interact with the dye, no additional dye is adsorbed, and the maximum adsorption capacity was obtained at this value [40].
Fig. 7. Effect of pH on the adsorption capacities of hybrid membranes.
3.5.3 Effect of temperature
The effect of temperature on the adsorption capacities of hybrid membranes for two direct dyes was conducted at different temperatures (30, 45, 60, 75, and 90 °C). As shown in Fig. 8, it is clear that the adsorption capacities of the CS/OSR and CS/OSR/Silica membranes increase with the increasing temperature from 30 to 60 °C and reached the highest at 60 °C for Blue 71 and Red 31 respectively. This can be explained by an increase in the diffusion rate of the dye molecules into hybrid membranes. The increase in temperature would increase the mobility of the large dye ions as well as produce a swelling effect with in the internal structure of the hybrid membranes, thus enabling the large dye molecules to penetrate further. The higher temperature might contribute to the adsorption of dyes. Diffusion is an endothermic process [41]. However, further increase in temperature (>60 °C would cause a decrease in adsorption capacity. This could be due to the more activation of the dye molecules so that they were not allowed to keep on the surface of hybrid membranes [42].
Fig. 8. Effect of temperature on the adsorption capacities of hybrid membranes.
3.5.4 Effect of time and adsorption kinetics
Fig. 9a shows the effect of adsorption time on the adsorption capacities of the CS/OSR and CS/OSR/Silica hybrid membranes at 60 °C, concentration about 160 mg/L for Blue 71 and Red 31 from aqueous solution. The adsorption capacities of two hybrid membranes were found to increase with an increase in adsorption time for Blue 71 and Red 31 respectively. The increase was fast in the beginning, and then tended to slow till equilibrium. The equilibrium was established after 120 min for two direct dyes. Safa et al. observed the similar results about the direct dye absorption onto waste rice husk [1]. Ahmad et al. considered that the absorption rate was rapid in the start due to adsorption of dye molecules on the upper surface of the absorbent [43]. Then it became slow due to slow passing of dye molecules into the inner structure of the absorbent; another reason was that a large number of exchanging sites at the start helped the absorption process and then saturation occurred [40].
Fig. 9. Adsorption kinetics and fitting of hybrid membranes: (a) adsorption kinetics curves; (b) pseudo first-order model and (c) pseudo second-order model.
The amount of dye adsorbed at equilibrium was 75.53 mg/g, 60.19 mg/g on the CS/OSR membrane, and 77.18 mg/g, 61.77 mg/g on the CS/OSR/Silica membrane for Blue 71 and Red 31 respectively. This may be explained by their structure shown in Fig. 1. Direct Blue 71 has more large linear planar structures and more SO3Na groups than Red 31, which lead to better substantivity to the hybrid membranes, so the adsorption capacity of Blue 71 is higher than that of Red 31 onto both hybrid membranes. On the other hand, the introduction of silica couple agent (APTES) by cross-linking, also increase the surface areas of CS/OSR/Silica hybrid membrane, which promote further adsorption on dyes [44].
To examine the adsorption mechanism of the two direct dyes, the kinetics data based on adsorption equilibrium capacities of the CS/OSR and CS/OSR/Silica hybrid membranes were analyzed using pseudo-first-order, pseudo-second-order. The differential equations are described in Eqs. (3) and (4)[45] and [46]:
equation3
equation4
where qe (mg/g) and qt (mg/g) are respectively the adsorption quantity at adsorption equilibrium and the adsorption quantity at time t (min); k1 (min−1) and k2 (g mg−1 min−1) are respectively the kinetics rate constants for the pseudo-first-order equation and the pseudo-second-order equation. The slopes and intercepts of plots of ln(qe − qt) versus t are used to determine the pseudo-first-order rate constant k1 and qe. The slopes and intercepts of plots of t/qt versus t are used to calculate the pseudo-second-order rate constant k2 and qe[47]. The value of k1 and k2, correlation coefficients (R2), qe (calculated) are listed in Table 1.
Table 1. Comparison of the pseudo-first and pseudo-second order constant.
Membranes
Dyes
Pseudo-first order kinetic model
Pseudo-second order kinetic model
k1 (min−1)
R2
k2 × 10−3 (g mg−1 min)
R2
Q (mg/g)
CS/OSR
Blue 71
0.0183
0.949
1.02
0.997
77.9
Red 31
0.0181
0.893
2.14
0.998
60.5
CS/OCS/Silica
Blue 71
0.0236
0.960
1.47
0.998
79.3
Red 31
0.0189
0.922
1.98
0.999
62.5
The applicability of these kinetic models was determined by measuring the correlation coefficients (R2). When the value of R2 is high; the model is best applicable to data. According to Table 1, it is noted that correlation coefficients (R2) of the pseudo-second-order model for Blue 71 and Red 31 on both hybrid membranes are higher than those of pseudo-first-order model, indicating that the pseudo-second-order kinetic model better describes the adsorption process of the two direct dyes onto the CS/OSR and CS/OSR/Silica hybrid membranes.
3.5.5 Effect of initial concentration and sorption isotherms
Fig. 10 shows the plot of the equilibrium sorption capacities, qe (mg/g); versus the liquid phase direct dyes equilibrium concentrations at pH of 9.82 and adsorption time 6 h for various different dyes on the CS/OSR and CS/OSR/Silica hybrid membranes respectively. It is obvious that the CS/OSR/Silica membrane has the higher sorption capacity than the CS/OSR membrane for Blue 71 and Red 31. This demonstrates that the addition of silica in hybrid membrane can affect the sorption properties.
Fig. 10. Sorption plots of two dyes onto different membranes.
The absorption isotherm models can determine the interaction between absorbent and dyes. In this present study, Langmuir and Freundlich models were used to describe the adsorption data. The adsorption data interpretations were done with the non-linear forms of Langmuir, Freundlich isotherm equations [48]. The equations are expressed in the form as Eqs. (5) and (6):
equation5
equation6
where Cf and Cs are the equilibrium concentrations of direct dyes on adsorption; KLand aL are Langmuir constants and KF and n are the Freundlich constants and the heterogeneity factor, respectively [1]. The corresponding isotherm parameters determined from the fitting calculation are listed in Table 2.
Table 2. Parameters of the sorption isotherms obtained for different membranes.
Dyes
Membranes
Langmuir
Freundlich
qmax (mg/g)
KL (L/g)
R2
KF
n
R2
Blue 71
CS/OSR
47.7
3.04
0.769
0.392
2.38
0.993
CS/OSR/Silica
67.2
4.47
0.839
1.067
2.10
0.998
Red 31
CS/OSR
120.6
0.958
0.725
0.251
1.49
0.99
CS/OSR/Silica
94.4
0.952
0.801
0.197
1.59
0.99
From the data given in Table 2, it can be inferred that they were not appropriate for the Langmuir isotherm models because of the low value of R2, while high value of R2for the Freundlich isotherm indicates a very good mathematically fit of the Freundlich isotherm model. The isotherm plots also confirmed that the experimental equilibrium isotherm data fitted well to the Freundlich model under these studied conditions. The application of the Freundlich isotherm equation to the adsorption isotherm implied a multilayer adsorption at lower concentration of two direct dyes and monolayer adsorption at higher concentration onto the two hybrid membranes.
4 Conclusions
In this study, removals of two direct dyes (Blue 71 and Red 31) were studied using the cross-linked chitosan hybrid membranes (CS/OSR and CS/OSR/Silica membrane) with oxidized starch and silica couple agent. By cross-linking, the thermal stability and swelling property of the CS/OSR and CS/OSR/Silica hybrid membranes were improved remarkably. At a fixed membrane dosage and dye concentration, the adsorption capacity of the CS/OSR membrane increased with pH increasing, while the adsorption capacity of the CS/OSR/Silica membrane was higher at pH 9.82 for two direct dyes. Kinetic studies referred that the adsorption behavior of two direct dyes onto the CS/OSR and CS/OSR/Silica membranes respectively follow pseudo-second order model and experimental equilibrium data fitted well Freundlich isotherm model. To compare with the CS/OSR membrane, the CS/OSR/Silica membrane exhibits better performance for adsorption of two direct dyes. Bases on these results, the oxidized starch cross-linked chitosan–silica hybrid membrane with the higher dye adsorption capacity, might be a suitable alternative to remove direct dyes from colored wastewater.
Acknowledgements
The project was supported by the research fund of Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (No. AE201321).
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