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BY-NC-ND 3.0 license Open Access Published by De Gruyter January 29, 2016

Characterization and adsorption performance of chitosan/diatomite membranes for Orange G removal

  • Xiu-Juan Wu , Ji-De Wang and Li-Qin Cao EMAIL logo
From the journal e-Polymers


Novel chitosan/diatomite (CS/DM) membranes were prepared by phase inversion technique to remove anionic azo dyes from wastewater. The fabricated composite membranes exhibited the combined advantages of inorganic substances, diatomites, and polysaccharides. These composite membranes were characterized through Fourier transform infrared spectroscopy, scanning electron microscopy, and X-ray diffraction analysis. The mechanical properties of the membranes were also evaluated. Adsorption experiments were conducted under varied initial dye concentration, solution pH values, contact time, and adsorbent dosage. The results indicate that pH 3 is the optimal pH value for Orange G adsorption. The CS/DM membranes exhibit the highest adsorption capacity of 588 mg g-1 and removal rate of 94% under an initial dye concentration of 200 mg l-1, contact time of 6 h, and membrane dosage of 8 mg. Langmuir, Freundlich and Redlich-Peterson adsorption models were applied to describe the equilibrium isotherms at different dye concentrations. The equilibrium data was found to be fitted well to the Redlich-Peterson isotherm. Pseudo-first-order and pseudo-second-order kinetics models were used to describe the adsorption of membranes. The adsorption data were well explained by pseudo-second-order models, and also followed by the Elovich model. In addition, these membranes display high adsorption capacity and mechanical performance even after reused for seven times.

1 Introduction

Dyes are widely used in several fields, such as textiles, paper, dyestuffs, and leather. With the rapid industrial development, dye wastewater has been a major environmental pollutant source (1). As most dyes, which are visible in water, are highly toxic and can seriously affect human health and marine lives dye wastewater treatment has gained increased research attention. Traditional methods, such as adsorption (2–4), chemical oxidation (5–8), coagulation (9), membrane filtration (10), and ion exchange (11), have been adopted for dye removal from aqueous effluents. Adsorption is usually employed in wastewater treatment because this technique is economical and effective and requires low-cost natural materials.

Chitosan (CS) is one of the most widely used natural polymeric materials for adsorption. CS, the deacetylated form of chitin, generally exists in nature and possess special properties, such as non-toxicity, hydrophilicity, and biodegradability (12, 13). CS, which contains high contents of amino and hydroxyl groups, can be utilized to remove dyes (14, 15), metal ions (16, 17), and proteins (18, 19). In particular, the amino groups of CS can be easily protonated under acidic conditions, thereby imparting CS with capacity to strongly adsorb anionic dyes through electrostatic attraction (20). Nevertheless, CS is pH sensitive and forms gels below pH 5.5, at which dye-binding capacity could not be evaluated. In this respect, several cross-linking reagents (21–26) were applied to enhance CS resistance to acids. Inorganic substances were also introduced to improve the mechanics and adsorption capacity of CS. For example, activated clay can improve CS capacity to remove dyes (27). CS-coated bentonite can tolerate low solution pH and exhibits high adsorption capacity (28). CS and montmorillonite composites demonstrate improved mechanics and adsorption properties (29).

Diatomite (DM) is a siliceous sedimentary rock that consists primarily of the fossilized skeletal remains of the diatom (30). DM is used as an adsorbent because of its unique physical and chemical properties, as well as its abundance and low cost. DM, which is porous and presents high surface area, can promote dye immobilization (30, 31). DM also enhances the mechanical strength of composite membranes when blended with cellulose (28). The silicate hydroxylated end groups of DM can form hydrogen bonds with the amino and hydroxyl groups of natural polymers, resulting in strong interaction between cellulose and DM. However, to the best of our knowledge, little research efforts have been reported on a diatomite/chitosan composite membrane for the removal of anionic azo dyes from aqueous solutions.

In this work, CS/DM membranes were fabricated to improve the mechanical and adsorption properties of CS. The membranes were then used to adsorb Orange G (OG). The effects of different amounts of DM in the composite membrane and various adsorption conditions were studied. The reusability of CS/DM membranes was investigated via recycling for several times under the optimal condition.

2 Experimental

2.1 Materials

CS powder [80%–95% deacetylated, average molecular weight (MW) 9.0×106 Da] was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Solutions of 1 N hydrochloric acid and 4 wt.% sodium hydroxide were used hydrochloric acid and solid sodium hydroxide (AR) were obtained by Baishi chemical Co., Ltd. (Tianjin, China). Glutaraldehyde (AR, 50 vol.% in water) and enthylene glycol (EG) was acquired from Shengao Chemical Reagent Co., Ltd. (Tianjin China). DM (MW=60.89) and OG (C16H10N2Na2O7S2, MW=452.37, λmax=478 nm, as shown in Figure 1) were obtained from Aladdin Chemistry Co. Ltd (Shanghai, China).

Figure 1: The structure of Orange G.
Figure 1:

The structure of Orange G.

2.2 Synthesis of CS/DM membranes

CS/DM membranes were prepared by mixing solutions with different ratios of CS and DM, with DM ranging from 2% to 25% by mass. Briefly, 1.2 g of CS powder was dissolved in 50 ml of 2 wt.% acetic acid solutions. A specific proportion DM and EG was then dispersed in the solution. The mixture was continuously stirred for 8 h and added with 0.5 wt.% glutaraldehyde as the cross-linking agent for 12 h. The solution was cast on a Teflon plate. After drying the plate at 40°C, the composite membranes were immersed in 4 wt.% sodium hydroxide solution and washed with distilled water for several times. Finally, the CS/DM membranes were dried at room temperature.

2.3 Characterization of CS/DM membranes

Scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and wide-angle X-ray diffraction pattern (XRD) measurement were performed to characterize the composite membrane. The FT-IR spectra of CS, DM, and CS/DM membranes were recorded on a BRUKER EQINOX55 FT-IR spectrophotometer (Bruker, Germany) with KBr pellets. The SEM images of the composite membranes were obtained using an LEO 1430VP SEM (Bruker, Germany). The XRD pattern (Bruker D8, Bruker, Germany) was obtained at a scanning rate of 5° min-1.

2.4 Mechanical properties

Tensile strength and elongation-at-break of the membranes in the wet state were measured using a static mechanical tester (H5KT, Tinius Olsen, Horsham, PA, USA). The membrane was cut into 50 mm×10 mm rectangles. The membrane was completely immersed in deionized water for 10 h to reach swelling equilibrium before the tests. And the content of water in CS/DM (10%), CS/DM (15%), CS/DM (25%) were 57.3%, 51.0%, 47.8%, respectively. For tensile test, a crosshead speed of 5 mm min-1 was used. And the average values of tensile strength were obtained from five specimens. All membranes have a thickness of about 0.156–0.194 mm. The tensile test was performed in the wet state to simulate the mechanical properties of the membranes in the aqueous environment of dye removal.

2.5 Dye adsorption experiment

Adsorption experiments were performed on a thermostated shaker (Shanghai Yihen, China) with constant shaking at 30°C. About 20 mg of CS/DM membranes were added into 25 ml of OG solution (200 mg l-1) at pH 3 to determine the effect of different weight ratios of CS and DM on OG adsorption capacity. The effect of initial OG concentration was investigated from 100 mg l-1 to 350 mg l-1 by using 8 mg of the composite membrane at pH 3. The effect of pH was determined by adjusting OG solutions (200 mg l-1) to different pH values (2–6) through addition of 1 N hydrochloric acid. The effect of membrane dosage on the adsorption capacity of OG was assessed by adding 8–25 mg of the membrane into 200 mg l-1 OG solution at pH 3. For kinetic studies, contact times were varied from 20 min to 720 min. Changes in OG concentration were determined using a spectrophotometer (UV-2550 SHIMADZU, Japan). Adsorption capacity of the composite membranes was then calculated according to the following equation:

[1]Q=(Ce-C0)VW [1]
[2]q=C0-CeC0×100% [2]

where Q (mg g-1) is the amount of dye adsorbed on the membrane, C0 (mg l-1) is the initial dye concentration, Ce (mg l-1) is the equilibrium dye concentration in the solution, V (ml) is the volume of the used dye solution, W (g) is the weight of the used membrane, and q is the dye removal rate by the membrane.

2.6 Adsorption equilibrium isotherms

The Langmuir isotherm equation (32) can be used to evaluate monolayer adsorption onto a surface with a finite number of identical sites. The equation can be expressed as:

[3]CeQe=1KLQmax+CeQmax [3]

where Qmax (mg g-1) is the maximum amount of dye (per unit weight of the membrane) capable of forming complete monolayer coverage on the surface at the high equilibrium concentration Ce (mg l-1), Qe (mg g-1) is the amount of dye adsorbed per unit weight of the membrane at equilibrium, and KL (l mg-1) is the Langmuir constant.

Another widely used empirical formula is the Freundlich isotherm equation, which is based on adsorption on a heterogeneous surface. The equation is represented by:

[4]logQe=logKF+1nlogCe [4]

where KF (mg g-1) is the predicted indicator of adsorption capacity and 1/n of the adsorption intensity. A linear form of the Freundlich equation yields the constants KF and 1/n.

Redlich and Peterson isotherm (33) incorporate three parameters into an empirical isotherm. The equation reforms the deficiency of Langmuir and Freundlich isotherm equations in some adsorption systems. The equation can be described as follow:

[5]log(KRCeQe-1)=logaR+βlogCe [5]

where KR (l g-1) and aR (l mg-1) are the Redlich-Peterson isotherm constants, and β is the exponent which lies between 0 and 1. Fitting Eq. [5] to the experimental data to obtain a linear plot of log(KR(Ce /Qe )-1) vs. log(Ce ) is not applicable, because three parameters are unknown. KR value must be tried before the optimum line is gained. Therefore a minimization procedure is a method to solve Eq. [5] by maximizing the correlation coefficient between the theoretical data for Qe predicted from Eq. [5] and experimental data. Thus the parameters of the equation were determined by minimizing the distance between the experimental data and the theoretical model predictions with the solver add-in function of the Microsoft Excel.

2.7 Adsorption kinetics models

Two kinetics models, namely, pseudo-first-order and pseudo-second-order (34), were employed to evaluate the adsorption of OG on CS/DM membranes. The pseudo-first-order equation is stated as:

[6]log(Qe-Qt)=log(Qe)-kf2.303t [6]

where Qt (mg g-1) is the amount of dye adsorbed at time t (min), Qe (mg g-1) is the equilibrium adsorption capacity, kf (min-1) is the pseudo-first-order rate constant. The pseudo-second-order equation is:

[7]tQt=1ksQe2+tQe [7]

where Qt and Qe (mg g-1) are the equilibrium adsorption capacities, and ks (g mg-1 min-1) is the pseudo-second-order rate constant.

Elovich kinetic model (35) was employed to describe the adsorption of OG on the CS/DM membranes. The Elovich equation is stated as:

[8]Qt=1βlog(αβ)+1βlogt [8]

where Qt (mg g-1) is the amount of dye adsorbed at time t (min), α is the initial adsorption rate (mg g-1 min-1), and β (g mg-1) is the constant of Elovich.

2.8 Desorption and reusability of CS/DM membrane

After the adsorption reached saturation, the CS/DM membrane was immersed in 100 ml, 4 wt.% sodium hydroxide solution 1 h for desorption. Then the membrane was taken out and washed with amounts of deionized water to neutral. After that, the adsorbent was re-immersed in 25 ml of dye solution (200 mg l-1, pH 3) for adsorption. To explore the reusability of CS/DM membrane, the adsorption- desorption recycle was repeated 7 times. Desorption efficiency of the dye was analyzed by spectrophotometer.

3 Results and discussion

3.1 FT-IR spectroscopy

Figure 2 shows the FT-IR spectra of CS membrane, CS/DM composite membranes, and DM. The FT-IR spectra of the CS membrane shows an overlapping between the N-H and the -OH stretching vibrations around 3380 cm-1. The band at 1655 cm-1 is related to the stretching vibration of the carbonyl bond C=O of the amide group, and the band at 1595 cm-1 is attributed to the bending vibration of the amine group. The bands at 1153 and 899 cm-1 indicate the characteristic glycoside absorption peak. The FT-IR spectra of the CS/DM composite membrane differed from the base membrane mainly in terms of relative intensity and absorption band width. The peak at 3380 cm-1 shifted to 3439 cm-1, and the bands at 1595 cm-1 disappeared. This finding indicated that the amide and amine groups of CS were bound to or adsorbed by DM. The coated DM contributed to the absorption bands of Si-O of the silanol group at 795 cm-1. This study confirmed that CS was well-integrated with DM.

Figure 2: FT-IR spectra of (A) CS/DM composite membrane, (B) CS membrane, and (C) DM.
Figure 2:

FT-IR spectra of (A) CS/DM composite membrane, (B) CS membrane, and (C) DM.

3.2 SEM

Figure 3(A) and (B) show the SEM images of the CS/DM (10%) and CS/DM (20%) membranes. The surface of the composite membranes set with many DM particles wrapped with chitosan. And the amount of DM particles in Figure 3(B) was more than that in Figure 3(A). From Figure 3(C) and (D), it can be seen clearly that DM (6 μm–8 μm in diameter) with cylindrical shape dispersed in the CS membrane which presented porous structure. This featured morphology reflected that DM was well-coated with CS.

Figure 3: SEM images of CS/DM membrane; (A) CS/DM (10%) membrane, (B) CS/DM (20%) membrane, (C) exterior view of CS/DM (10%) membrane and (D) elevated view of CS/DM (10%) membrane.
Figure 3:

SEM images of CS/DM membrane; (A) CS/DM (10%) membrane, (B) CS/DM (20%) membrane, (C) exterior view of CS/DM (10%) membrane and (D) elevated view of CS/DM (10%) membrane.

3.3 XRD analysis

The XRD patterns of the samples are shown in Figure 4. In Figure 4(A), intense peaks of DM were found at 2θ of 21.8°, 28.2°, 31.3°, and 36.0° (36). In addition, the typical crystalline peak at 2θ of 21.8° reflects the SiO2 of DM. In Figure 4(B), peaks appearing at 2θ of 10.6° and 20.4° are assigned to CS. Although the diffraction peaks of DM did not shift, the peaks in the CS/DM (25%) membrane were weakened. Furthermore, in Figure 4(C), the diffraction peaks of DM diminished when the DM content was decreased to 10%. This phenomenon is probably due to the inorganic and hydrophilic DM was dispersed easily in the chitosan solution to form a favorable interaction with hydrophilic chitosan. These results are in agreement with that of SEM characterization.

Figure 4: XRD patterns of (A) diatomite, (B) CS/DM (25%) membrane, (C) CS/DM (10%) membrane.
Figure 4:

XRD patterns of (A) diatomite, (B) CS/DM (25%) membrane, (C) CS/DM (10%) membrane.

3.4 Mechanical properties

Table 1 illustrates the tensile strength, tensile modulus and elongation-at-break of CS/DM membranes in the wet state. The mechanical properties of the CS membranes changed with the addition of DM. The tensile strength of the composite membrane containing 10% DM was the maximum value of 6.191 MPa. On the other hand, as the DM content was increased, the elongation at break of the membranes exhibited an initial increase followed by a gradual decrease. The maximum elongation at break of 54.40% was obtained at 5% DM, which was higher than that of the chitosan membranes. At a higher loading of 25% DM, the elongation was markedly reduced to 29.9% which may be attributed to the presence of rigid fillers. Besides, tensile modulus of the membranes were tested in the wet state, the composite membranes exhibited a lower tensile modulus and showed a very different variation in comparison to that of incorporation of nanoclays (37).

Table 1

Mechanical properties of pure CS membrane and CS/DM composite membranes in the wet state.

MembraneTensile strengtha (MPa)Elongation at break (%)Modulus of elasticity (MPa)
CS/DM (2%)5.682±0.3650.96±0.250.1115±0.008
CS/DM (5%)6.037±0.2154.40±0.180.1110±0.005
CS/DM (10%)6.191±0.1742.68±0.210.1451±0.026
CS/DM (15%)5.210±0.2349.62±0.620.1050±0.016
CS/DM (20%)5.341±0.1644.47±0.550.1195±0.013
CS/DM (25%)4.601±0.5629.97±0.450.1535±0.020

aStandard deviation of tensile strengeth, elongation at break, modulus of elasticity.

3.5 Adsorption behavior

3.5.1 Effect of weight ratio of CS/DM membranes on adsorption

In Figure 5, the amount of OG adsorption on the composite membranes was slightly higher than that of the pure CS membrane. Adsorption capacity increased with increasing DM content in the membrane. As DM is porous and contains high surface area, addition of this compound could enhance the ability of CS to agglomerate and remove dye. The highest adsorption capacity was observed at 10% DM and was slightly higher than the capacity of membranes with more than 10% DM. This result indicates that the interaction between OG dye with chitosan plays a dominant role in adsorption.

Figure 5: The adsorption ability of CS/DM membrane with different content of diatomite (OG initial concentration=200 mg l-1, contact time=360 min, CS/DM dosage=20 mg/25 ml).
Figure 5:

The adsorption ability of CS/DM membrane with different content of diatomite (OG initial concentration=200 mg l-1, contact time=360 min, CS/DM dosage=20 mg/25 ml).

3.5.2 Effect of pH on adsorption

The pH of a solution plays a vital role in adsorption and significantly affects adsorption capacity. Figure 6 shows the effect of pH on OG adsorption onto the CS/DM membrane. The adsorption capacity of OG was considerably higher at low initial pH values. The maximum value of the adsorption capacity reached 238.2 mg g-1 at pH 3. Under acidic conditions, the amino groups of CS were protonated and formed NH3+, which interacted with the sulfonic groups of OG through electrostatic attraction. In order to study the charge density of CS/DM membrane, its Zeta potential was measured at pH 3, 4 and 5 by Nano ZS90 (Malvern Instruments Corp, UK). And the Zeta potential values were 12.8 mV, 8.9 mV and 7.8 mV, respectively. These results certified that the amino groups of CS molecules will be more available to interact with OG at low pH values. Meanwhile, OG adsorption at pH 2 was lower than that at pH 3, which could be due to the protonation of the composite membranes and OG.

Figure 6: Effect of initial solution pH on OG adsorption onto the CS/DM membrane (OG initial concentration=100 mg l-1, contact time=300 min, CS/DM dosage=10 mg/25 ml).
Figure 6:

Effect of initial solution pH on OG adsorption onto the CS/DM membrane (OG initial concentration=100 mg l-1, contact time=300 min, CS/DM dosage=10 mg/25 ml).

3.5.3 Effect of initial concentration on adsorption

As shown in Figure 7, an increase in the initial dye concentration evidently led to an increase in the adsorption capacity of the CS/DM membrane. However, dye removal rate decreased with increasing dye concentration. At the lowest dye concentration (100 mg l-1), the adsorption capacity was 309.0 mg g-1 and the removal rate was 98.9%, whereas the values were 794.2 mg g-1 and 72.6%, respectively, at the highest concentration (350 mg l-1). This is due to the increase in the driving force of the concentration gradient, as an increase in the initial dye concentration. On the other hand, the amount of dye adsorbed per unit adsorbent was limited, so that the removal rate would decrease in high initial concentration. All the removal rates exceeded 90% at the OG concentration range of 100 mg l-1–200 mg l-1. Overall, 200 mg l-1 OG was the optimal initial concentration for adsorption, under which the adsorption capacity was 588 mg g-1 and the removal rate was 94.0%.

Figure 7: Effect of initial concentration on OG adsorption by CS/DM membrane (contact time=360 min, pH=3, CS/DM dosage=8 mg/25 ml).
Figure 7:

Effect of initial concentration on OG adsorption by CS/DM membrane (contact time=360 min, pH=3, CS/DM dosage=8 mg/25 ml).

3.5.4 Adsorption isotherms

The Langmuir isotherm model (32) is dominated by a monolayer adsorption process, whereas the Freundlich isotherm model is based on heterogeneous surface adsorption, Redlich and Peterson (33) proposed a three-parameter empirial equation, which could be used over a wide concentration range.

Figure 8(A), (B) and (C) show the Langmuir, Freundlich and Redlich-Peterson plots for OG removal by the CS/DM membrane. The equation parameters were shown in Table 2. In this work, the linear R2 coefficients for the Langmuir, Redlich-Peterson and Freundlich isotherms were 0.9984, 0.9986 and 0.9690, respectively. Hence, the Langmuir isotherm model was found to be more suitable than Freundlich isotherm for OG adsorption. As illustrated in Table 2, the Qmax of OG was 819.6 mg g-1, which closely approximates the value obtained from the batch adsorption studies. With the highest R2 coefficient, the Redlich-Peterson isotherm fitted the experiment data better than other isotherms (38).

Figure 8: Adsorption isotherms plot for OG removal by CS/DM membrane; (A) Langmuir isotherm and (B) Freundlich isotherm. (C) Redlich-Peterson isotherm plot for OG removal by CS/DM membrane.
Figure 8:

Adsorption isotherms plot for OG removal by CS/DM membrane; (A) Langmuir isotherm and (B) Freundlich isotherm. (C) Redlich-Peterson isotherm plot for OG removal by CS/DM membrane.

Table 2

Adsorption isotherms studies for OG onto CS/DM membranes.

Isotherm modelsParametersValue
LangmuirKL (l-1 mg)0.2531
qm (mg g-1)819.6
FreundlichKf (mg l-1)316.1
Redlich-PetersonKR (l-1 g)838.9
αR (l-1 mg)1.905

3.5.5 Effect of adsorbent dosage on adsorption

The effect of CS/DM membrane dosage on OG removal is shown in Figure 9. Removal rate increased as CS/DM membrane dosage increased up to a certain limit and then reached a constant value. At a given initial dye concentration, the amount of dye adsorbed decreased with increasing composite membrane dosage. High adsorbent amounts provided numerous active sites for dye adsorption. These results indicated that the optimal CS/DM membrane dosage for OG removal was 8 mg per 25 ml of the solution.

Figure 9: Effect of adsorbent mass on OG adsorption by CS/DM membrane (OG initial concentration=200 mg l-1, contact time=300 min, pH=3).
Figure 9:

Effect of adsorbent mass on OG adsorption by CS/DM membrane (OG initial concentration=200 mg l-1, contact time=300 min, pH=3).

3.5.6 Adsorption kinetics

The effect of contact time on OG adsorption was studied for 720 min at initial dye concentrations of 150 and 200 mg l-1 and solution pH of 3. As shown in Figure 10, OG was rapidly removed within the first 120 min and then gradually removed afterwards. After 360 min of contact, the uptake of the CS/DM membrane was nearly saturated and then remained constant with further time extension. Thus, the adsorption equilibrium was established.

Figure 10: Effect of contact time on OG adsorption onto CS/DM membrane.(OG initial concentration=150 and 200 mg l-1, CS/DM dosage=8 mg/25 ml, pH=3).
Figure 10:

Effect of contact time on OG adsorption onto CS/DM membrane.(OG initial concentration=150 and 200 mg l-1, CS/DM dosage=8 mg/25 ml, pH=3).

Pseudo-first-order, pseudo-second-order (34) and Elovich equations (35) were analyzed to investigate the kinetics of the adsorption process. Figure 11(A) and (B) show the plots of the pseudo-first-order and pseudo-second-order models, and Figure S1 shows the plot of the Elovich model. Table 3 presents the associated parameters. It was found that for the different dye concentrations, the R2 of the pseudo-second-order model (R2>0.98) were higher than those of the pseudo-first-order (R2>0.84) and the Elovich model (R2>0.91). This indicates that the behavior over the whole adsorption process could be predicted by pseudo-second-order model, and it was conducted by chemical sorption which may involve valency forces through sharing or exchange of electrons between dye anions and adsorbent.

Figure 11: Adsorption kinetics model for OG removal by CS/DM membrane; (A) pseudo-first-order and (B) pseudo-second-order.
Figure 11:

Adsorption kinetics model for OG removal by CS/DM membrane; (A) pseudo-first-order and (B) pseudo-second-order.

Table 3

Kinetic parameters for OG adsorption onto CS/DM membrane.

Initial concentrations (mg l-1)Pseudo-first-orderPseudo-second-orderElovich
kf (min-1)R2ks (g mg-1 min-1)R2α (mg g-1 h-1)βR2
150 mg l-17.807×10-20.97573.265×10-50.995241.084.048×10-30.9173
200 mg l-11.133×10-20.84101.327×10-50.983326.952.941×10-30.9335

3.5.7 Reusability of CS/DM membrane

Figure 12 shows the desorption study for dye of the CS/DM membranes. The adsorption saturated membrane was kept in 4 wt.% sodium hydroxide solution for desorption. The membrane separated dye out as soon as it was immersed in sodium hydroxide solution. The solution was dyed red by OG under alkaline condition. Pipette and adjusted to neutral, the color of tiny solution became yellow. The uptake of composite membrane was 611.3 mg g-1. Just 10 min, the desorption efficiency of membrane reached to 98.4%. Figure 13 shows the results of regeneration studies on CS/DM membranes. After reused for seven times, the CS/DM membrane exhibited a reduced adsorption capacity of 6%. Thus, the CS/DM membrane could be an efficient and reusable adsorbent membrane for the removal of anionic azo dyes.

Figure 12: Desorption efficiency of OG from CS/DM membrane (100 ml, 4 wt.% sodium hydroxide solution, CS/DM dosage=8 mg, t=60 min).
Figure 12:

Desorption efficiency of OG from CS/DM membrane (100 ml, 4 wt.% sodium hydroxide solution, CS/DM dosage=8 mg, t=60 min).

Figure 13: Recycle of CS/DM membrane.
Figure 13:

Recycle of CS/DM membrane.

4 Conclusions

This study presented the preparation of CS/DM composite membranes by controlling the weight ratios of CS to DM. The results demonstrated improved mechanical properties of the CS/DM membranes compared with those of pure CS membranes. The process of OG adsorption onto the CS/DM membranes was also analyzed. The adsorption capacities of the composite membranes were found to be dependent on contact time, pH value, initial OG concentration, and adsorbent dosage. Experimental adsorption kinetics and isotherms indicated that the adsorption process best fitted the pseudo-second-order and Redlich-Peterson models. The highest OG uptake of 588 mg g-1 was achieved at pH 3, with an initial dye concentration of 200 mg l-1, adsorbent dosage of 8 mg, and contact time of 6 h. Moreover, the highest removal efficiency of OG reached 94.0% and the desorption efficiency of OG was 98.4%. Importantly, after recycling seven times, the CS/DM membranes still exhibited satisfactory adsorption capacity and mechanical performance.

Corresponding author: Li-Qin Cao, Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education and Xinjiang Uyghur Autonomous Region, Xinjiang University, Urumqi 830046, P. R. China, Tel.: +86 991 8581018, Fax: +86 991 8582807, e-mail:


The authors acknowledge the research grant provided by Natural Science Foundation of China (No. 51263020; No. 51003090).


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Supplemental Material

The online version of this article (DOI: 10.1515/epoly-2015-0218) offers supplementary material, available to authorized users.

Received: 2015-9-30
Accepted: 2015-12-27
Published Online: 2016-1-29
Published in Print: 2016-3-1

©2016 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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