Adsorption of hazardous dyes indigo carmine and acid red on nanofiber membranes

Abstract

Through an electrospinning process, thiol-functionalized polyvinyl alcohol (PVA)/SiO₂ composite nanofiber membranes were synthesized as adsorbents. The adsorption of indigo carmine and acid red onto the nanofiber membranes was investigated. The effect of contact time, initial concentration, pH, dosage, temperature, the adsorption equilibrium and thermodynamic parameters were studied. The adsorption equilibrium was reached within 20 min. The initial concentration, pH and the temperature played important roles on the adsorption capacity onto the nanofiber membranes. Increase of the initial concentration could effectively increase the adsorption capacity. The best adsorption pH was 2 for indigo carmine and acid red. Higher temperatures were favourable for the adsorption. The maximum indigo carmine adsorption capacity was 266.77 mg g⁻¹ for 500 mg L⁻¹ indigo carmine and 211.74 mg g⁻¹ for 315 mg L⁻¹ acid red at pH 2 and 25 °C. The adsorption isotherm studies showed that the adsorption of the two dyes fitted well with Langmuir, Freundlich, and Redlich–Peterson adsorption models, while the Redlich-Peterson model achieved the best results. The kinetic adsorption shows that the pseudo-second-order model fits the adsorption process of indigo carmine while the pseudo-first-order model fits the adsorption process of acid red.

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Introduction

Dyes are widely used in many industries such as dyestuffs, textile, cosmetics, leather, plastic and paper. Very small amounts of dyes in water are highly visible, undesirable and even harmful to mankind.^1–3 Indigo carmine is considered to be a highly toxic dye and the exposure can cause skin and eye irritations to human being.⁴ Acid red is a typical dye which contains nitrogen atoms with azo functional groups [–N=N–] in its chemical structure.³

With complex molecular structures, dyes are resistant to aerobic digestion and are stable to oxidizing agents, so they are very recalcitrant and difficult to be removed and degraded.^1–5

Currently, there are many methods to remove these dyes from polluted water, such as sedimentation, coagulation, flocculation, chemical treatments, oxidation, electrochemical methodology, photodecomposition, biological treatments, adsorption, and membrane process.^1–3 Adsorption techniques have found many practical applications for the separation and purification of liquid and gaseous mixtures in the chemical, biological, pharmaceutical, environmental, and electronics industries.^6,7 Mesoporous nanofibers can remove different types of coloring materials, providing an attractive treatment⁵ and the functionalized mesoporous materials have been regarded as effective adsorbents^6–15 because of their high surface area, large-diameter, large-volume of functionalized pore channels and their narrow pore size distribution. Our research group has successfully synthesized the thiol-functionalized PVA/SiO₂ mesoporous composite nanofiber membranes and used them for the adsorption of heavy metal ions from water successfully.^16–18 However, there is very little research on the use of thiol-functionalized PVA/SiO₂ nanofibers membranes to remove dyes from water.

In this study, mercapto group functionalized PVA/SiO₂ mesoporous composite nanofiber membranes were synthesized as an adsorbent. And then the adsorption performance of indigo carmine and acid red onto the membranes was systemically investigated for the effect of contact time, initial concentration, pH, the adsorption equilibrium, thermodynamic parameters, adsorption isotherms, kinetics, and the adsorption mechanisms. The results showed that the adsorption of indigo carmine and acid red onto PVA/SiO₂ mesoporous composite nanofiber membranes has some advantages such as high adsorption capacity and short adsorption time.^1,2,4,5

Materials and methods

Reagents and materials

Hydrochloric acid and tetraethyl orthosilicate silicate (TEOS) were purchased from Shanghai Chemical Reagent Factory; polyvinyl alcohol 1750 (PVA), cetyltrimethylammonium bromide (CTAB), nitric acid, potassium hydroxide and ethanol were purchased from Sinopharm Chemical Regent Co.Ltd., 3-mercaptopropyltrimethoxysilane (MPTMS,KH-590) was purchased from Jiangsu Jintan Organic Chemical Plants; distilled water was used throughout this work.

The structure of indigo carmine and acid red is shown in Table 1.

Table 1 Structure of two dyes

Name	Molecular formula	Structure	Molecular weight (g mol^−1)	λ ^a max (nm)
a λ max: the maximum absorption wavelength.
Indigo Carmine	C16H8N2Na2O8S2		466.37	611
Acid Red1	C18H13N3Na2O8S2		509.42	531

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Instruments

Electronic Analytical Balance (FA2004, Shanghai Jingke Balance Factory) was used to weigh chemicals; a temperature magnetic stirrer (85–2, Shanghai Siyue Instrument Factory) and electric-heated thermostatic water bath (HWS24, Shanghai Scientific Instrument Co., Ltd.) were adapted to mix solutions under different temperatures; a vacuum oven (DZF-6020, Test Equipment Co., Ltd. Shanghai Jing Hong) was used to dry fibers; FTIR spectra (Nicolet 7199, Nicolet Co., Ltd., U.S.A) , SEM (XL-30E, Philips Co., Ltd., Netherland), specific surface area and pore size distribution analyzer (Tristar 3000, Micromeritics Co., Ltd., U.S.A), TEM (JEM-2011, Japan Electronics Co., Ltd.) were used to characterize the PVA/SiO₂ nanofiber membranes synthesized through electrospinning technology; precision pH meter (PHS-3C, Shanghai Lei Magnetic Co., Ltd.) was used to adjust the pH of solutions. A constant temperature shaker (DHZ-D, Experimental Equipment Factory, Jiangsu Taicang) was used to perform the adsorption experiment and a UV-Vis spectrophotometer (UV1700, Shimadzu Corporation, Japan) was used to measure the concentration of dyes before and after adsorption.

Synthesis of the PVA/SiO₂ mesoporous adsorbents

The PVA/SiO₂ nanofibers were synthesized using electrospinning technology following the method in our previous study.^16–18 First, 2.19 g cetyltrimethylammonium bromide (CTAB) was dissolved in 7.37 g ethanol and vigorously stirred for 0.5 h at 60 °C. Second, 8.64 g distilled water and 1.54 g 3-mercaptopropyltrimethoxysilane (MPTMS) were added, mixed and further stirred for 0.5 h at 60 °C. Then, 6.64 g tetraethyl orthosilicate silicate (TEOS) was slowly added into the solution mixture. Finally, 0.8 mL 2 mol L⁻¹HCl was dropped into the mixture and a silica gel was obtained after reacting for 2 h at 60 °C. 10.0 g of 10 wt.% PVA solution was dropped slowly into the silica gel, and then the reaction proceeded in a water bath at 60 °C for another 4 h. Then the solution was added to a 50 mL syringe with a needle diameter of 1.2 mm, under a voltage of 15 kV and a speed of 0.3 ml h⁻¹. The distance between the needle and the receiving polar was 20 cm and the temperature for the electrospinning was 25 °C. Thus the fibers were collected and dried initially for 12 h at 60 °C under vacuum. Then the electrospun PVA/SiO₂ fibers were refluxed in ethanol/HCl (molar ratio of 70 : 1) for 12 h at 70 °C to remove the template and dried for 6 h at 60 °C under vacuum.^17,18,20

Adsorption study

Adsorption isotherm test. Indigo carmine and acid red were dissolved in deionized water to prepare the initial dye solutions with concentrations 10 mg L⁻¹, 50 mg L⁻¹, 100 mg L⁻¹, 200 mg L⁻¹, 300 mg L⁻¹, 400 mg L⁻¹ and 500 mg L⁻¹. 10 milligrams of adsorbent and 10 mL of dye solutions were placed in stoppered vials, using nitric acid and potassium hydroxide to adjust the pH of the solution, and then oscillation on the shaker with the shaking speed of 180 r/min under four different temperatures (288.15 K, 298.15 K, 308.15 K and 318.15 K). The resulting mixture was filtered through a 0.22 μm Uniflo filter unit and the filtrate was analyzed viaUV-Vis spectrophotometer (UV 1700) to determine the concentration of dyes.

Adsorption mechanism. The experimental method was the same as the adsorption isotherm test, keeping the initial concentration of dye solutions and the temperature constant; adsorption mechanism was studied at different pH. In this experiment, the initial concentration was 20 mg L⁻¹ for indigo carmine and 200 mg L⁻¹ for acid red. The temperature was kept at 25 °C.

Adsorbent dosage study. The effect of dosage amount of nanofibers membrane on the adsorption performance were investigated by the same method for adsorption isotherm test without changing pH. Here, the initial concentration for indigo carmine was 300 mg L⁻¹, and 540 mg L⁻¹ for acid red.

Adsorption kinetics test. 10 mg of adsorbent and 10 mL of 300 mg L⁻¹ (500 mg L⁻¹ for indigo carmine) dye solutions were placed in stoppered vials without changing pH, and then oscillated on the shaker with a shaking speed of 180 r/min for 5, 10, 15, 20, 25, 30, 40, 60 and 80 min under four different temperatures (288.15 K, 298.15 K, 308.15 K and 318.15 K). Our preliminary results indicated that 80 min is enough for the sorption to reach equilibrium. After filtration through a 0.22 μm filter, the filtrate was analyzed viaUV-Vis spectrophotometer. All of the data in this study are the average of triplicate samples. The relative errors in all cases are within 5% of the averages.

Results and discussion

Characterization of adsorbents

The adsorbent was characterized by FTIR to ascertain the modification of –SH. The FTIR patterns present similar location and appearance of the major bands in Fig. 1.

Fig. 1 FTIR spectrum of PVA/SiO₂ nanofibers.

From Fig. 1, we can see that the characteristic bands for the mercapto group vibrational peak appear around 2560.0 cm⁻¹, which means that the mercapto group has been successfully modified to the surface of PVA/SiO₂ nano-fiber membranes through hydrolysis and condensation reactions.^19,20 In Fig. 1, the peaks of V_C–H (2920.8, 2847.2 cm⁻¹), V_C–OH (1617.1 and 591.3 cm⁻¹), V_Si–O–H (3420.3, 1637.5 and 955.0 cm⁻¹) and V_Si–O–Si (1069.4, 791.5 and 456.4 cm⁻¹) are observed.^19,20

SEM and TEM images of PVA/SiO₂ nanofibers were depicted in Fig. 2a and Fig. 2b. From Fig. 2a, it is indicated that the diameter of the nanofibers is around 300 nm to 500 nm. The surface of the fiber was rough with a porous structure. Fig. 2b shows that the PVA/SiO₂ nano-fiber surface has a mesoporous structure with smaller diameter and irregular pores.

Fig. 2 (a, top) SEM image of PVA/SiO₂ nanofibers. (b, bottom) TEM image of PVA/SiO₂ nanofibers.

The surface area and the average pore diameter of the nanofibers was 140.1 m² g⁻¹ and 6.34 nm, respectively, as mentioned in our previously published paper.¹⁸

Dyes adsorption

Adsorption isotherms. Under four different temperatures (288.15 K, 298.15 K, 308.15 K and 318.15 K), the adsorption isotherms of indigo carmine and acid red onto PVA/SiO₂ nanofibers were studied with solution pH of 2. The results are shown in Fig. 3.

Fig. 3 Adsorption isothermal curve of indigo carmine (a) and acid red (b) onto PVA/SiO₂ nanofibers.

From Fig. 3, it can be seen that the equilibrium adsorption amount q_e increased with increasing initial concentration of dye solutions. Some references^7,21 indicated that when the test temperature was within 60 °C, the equilibrium adsorption amount q_e increases with increasing temperature. The mesoporous nanofibers shows similar adsorbent behavior. The q_e–c_e curve ascends as temperature increased, indicating that the adsorption of indigo carmine and acid red was endothermic. It was also found that the q_e of indigo carmine was higher than that of acid red.

Three adsorption models (Langmuir, Freundlich and Redlich–Peterson) were used to estimate the adsorption isotherm. Fig. 4 shows the non-line fitting adsorption isothermal curve of indigo carmine and acid red onto PVA/SiO₂ nanofibers at 298.15 K.

Fig. 4 Non-linear fitting adsorption isothermal curve of indigo carmine (a) and acid red (b) onto PVA/SiO₂ nanofibers at 298.15 K.

Three nonlinear models (eqn (1) to eqn (5)) were used to estimate the adsorption capacity and its adsorption behavior.

(1) Langmuir equation^22,23

where, q_e is the amount of dye on the adsorbent (mmol g⁻¹), Q is the amount of dye for a complete monolayer (mmol g⁻¹), b is a coefficient (L mmol⁻¹) and C_e is the equilibrium concentration of dyes (mmol L⁻¹)

In eqn (1),

where, C₀ is the initial concentration of dye (mmol L⁻¹), V is the solution volume (mL) and m is the dosage of adsorbent (mg).

The Langmuir equilibrium coefficient, R_L, can be obtained by the following equation:

where, C_m is the maximum concentration of dyes before adsorption

(2) Freundlich model:^24,25

where, K is the Freundlich coefficient, n is a coefficient.

(3) Redlich–Peterson model:²⁶

where, K is the α-Redlich–Peterson coefficient and β is a coefficient.

The isotherm equations and the fitting parameters of indigo carmine and acid red adsorption behavior onto adsorbent are depicted in Table 2–4.

Table 2 Non-linear fitting parameters and equations of the Langmuir model

Dyes	T/K	Fitting parameters
		b	Q	R ^2	RL
Indigo carmine	288.15	258.581	0.67183	0.94962	0.00371
	298.15	246.889	0.69845	0.94795	0.00418
	308.15	237.628	0.71236	0.94775	0.00438
	318.15	225.919	0.72925	0.94214	0.00512
Acid red	288.15	86.7496	0.44597	0.92362	0.00288
	298.15	81.7259	0.48198	0.92994	0.00311
	308.15	74.9700	0.52236	0.93802	0.00327
	318.15	71.8572	0.53537	0.94067	0.0033

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Table 3 Non-linear fitting parameters and equations of Freundlich model

Dyes	T/K	Fitting parameters
		K	1/n	R ^2
Indigo carmine	288.15	1.05121	0.22499	0.95180
	298.15	1.09825	0.22885	0.95212
	308.15	1.12523	0.23209	0.95664
	318.15	1.16031	0.23676	0.95952
Acid red	288.15	0.60712	0.2737	0.89305
	298.15	0.65712	0.27843	0.89783
	308.15	0.71407	0.28578	0.90372
	318.15	0.73184	0.28858	0.90573

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Table 4 Non-linear fitting parameters and equations of Redlich–Peterson model

Dyes	T/K	Fitting parameters
		K	α	β	R ^2
Indigo carmine	288.15	457.7047	508.2102	0.8568	0.98866
	298.15	450.4240	478.8218	0.8539	0.98739
	308.15	461.4297	476.0791	0.8481	0.98936
	318.15	487.0656	482.3442	0.8384	0.98842
Acid red	288.15	53.0577	104.8769	0.8998	0.9366
	298.15	53.91888	98.90265	0.9006	0.94197
	308.15	53.41228	90.71756	0.9014	0.9489
	318.15	52.67624	87.43388	0.9013	0.95113

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As can be seen from Table 2 to Table 4, the fitting results of three isothermal models for the adsorption of indigo carmine are as follows: Redlich–Peterson > Freundlich > Langmuir, while the fitting results of the adsorption of acid red followed: Redlich–Peterson > Langmuir > Freundlich.

The Redlich–Peterson isotherm incorporates the features of the Langmuir and the Freundlich isotherms.²⁶ The Redlich–Peterson model contains a description of the non-uniform adsorption, the R² value is higher than the Langmuir model, indicating that the mesoporous adsorbent adsorption of indigo carmine and acid red were not ideal for monolayer adsorption. The adsorption sites and surface pores exhibit heterogeneity. From Table 4, the value of β was near to 1, indicating that the mesoporous adsorbent adsorption of indigo carmine and acid red is dominated by monolayer chemical adsorption. When the Freundlich adsorption model index 1/n is between 0.1–0.5 it means dyes were easily absorbed. If the value is above 2, it means that the adsorption was difficult to carry out. Here, 1/n for indigo carmine and acid red was between 0.1 to 0.5, indicating that the adsorption process occurred easily.

Effect of pH. Generally, the change of pH value would affect the surface charge characteristics of thiol functional adsorbent and also affect the thiol interactions with dye materials. Therefore, here we investigated the effect of pH on the adsorption of dyes onto PVA/SiO₂ nanofibers (see Fig. 5) and studied the adsorption mechanism.

Fig. 5 Effect of pH on the adsorption of indigo carmine (a) and acid red (b) onto PVA/SiO₂ nanofibers.

It can be seen from Fig. 5 that the adsorption capacity of the mesoporous adsorbent is at its minimum near neutral conditions. From pH 2 to 6, the adsorption capacity decreased slightly for the two types of dyes.

There are many factors that may influence the adsorption of the dye such as surface charge and structure of the dye, adsorbent surface properties, hydrophobic and hydrophilic nature, hydrogen bonding, electrostatic interaction, steric effect, van der Waals forces, and so forth.²⁷ Here, the mechanisms for indigo carmine and acid red adsorption include electrostatic interaction and the hydrophobic–hydrophobic interaction between dyes and the adsorbent. Through fitting three isothermal models, we can see that the adsorption was dominated by monolayer chemical adsorption. From Fig. 5 we can see that the adsorption capacity increases at lower pH (although not significantly), which indicated that the electrostatic interaction is not the dominant factor for adsorption. The main mechanism for adsorption was the hydrophobic–hydrophobic interaction between dyes and the adsorbent.

Under acidic conditions, the modified –SH and the –OH on the surface of the adsorbent can be protonated with H⁺ to produce –SH₂⁺ and –OH₂⁺. Meanwhile, the sulfonate groups of the dye (D–SO₃Na) reacted with the H⁺, producing pigment acid (D–SO₃H). The pigment acid and –SH₂⁺ combined through chemical bonding. After reaction with –SH₂⁺via chemical bonding, the sulfonic acid group becomes less hydrophilic, which results in decolorization. The protonated and pigment process were hindered at neutral conditions because of very low H⁺ concentration, so the value of q_e was low. The electrostatic mechanism for the decolorization of indigo carmine and acid red onto the mesoporous adsorbents is shown in Fig. 6 (D in the dye molecular stands for the rest of the dye structure).

Fig. 6 Electrostatic mechanism of adsorption of dyes onto mercapto groups.

Meanwhile, another important factor leading to the adsorption of dyes is the hydrogen bond interaction between dyes and the membrane. Hydrophobic–hydrophobic interactions exist between the hydrophobic part of the dye molecule and the hydrophobic regions of the adsorbent.²⁸ The amine, imine, and hydroxyl groups of the adsorbent form hydrogen bonds with the azo, hydroxyl, and amine groups of dyes, as shown in Fig. 7.

Fig. 7 Hydrogen bonding mechanism of dye adsorption onto mercapto groups ((a) indigo carmine (b) acid red).

Effect of adsorbent dosage. When the adsorption time is 4 h at pH 2 with an adsorption temperature of 298.15 K, the effect of adsorbent dosage is shown in Fig. 8.

Fig. 8 Effect of dosage on the adsorption of indigo carmine and acid red onto mesoporous adsorbent

It can be seen from Fig. 8 that the adsorption capacity increased with increasing adsorbent dosage. For indigo carmine, the adsorption increased slightly in the range of 1 to 2 mg mL⁻¹, and the increase of adsorption amount is almost the same with further increase in the adsorbent dosage. For acid red, the increase trend was great from 1 to 3 mg mL⁻¹, but the increase trend was not obvious with further increase of the adsorbent dosage. The adsorbed amount increased with the increasing adsorbent dosage, due to the increased number of adsorption sites when increasing the adsorbent dosage. With a further increase in adsorbent dosage, the adsorbed amount was almost constant. This was due to a less commensurate increase in adsorption resulting from the lower adsorptive capacity utilization of the adsorbent with the increasing adsorbent;^29–31 the increase of the adsorbent led to the overlap or accumulation of adsorption sites, resulting in the increase of diffusion path.³²

In consideration of the long process for the synthesis of adsorbent, we choose 1 mg mL⁻¹ as the optimum dosage of adsorbent in this study.

Adsorption kinetics. The pseudo-first-order kinetic model and pseudo-second-order kinetic modeling is shown in Fig. 9.

Fig. 9 Non-linear fitting adsorption kinetics curves of indigo carmine (a) and acid red (b) onto PVA/SiO₂ nanofibers at different temperatures.

Fig. 9 shows the adsorption process of indigo carmine and acid red under the temperatures of 288.15 K, 298.15 K, 308.15 K and 318.15 K. At the initial stage of the adsorption (0–20 min), the concentration of indigo carmine and acid red decreased quickly, and the decreasing rate of this phase was the maximum in the whole adsorption process; with the adsorption time going on (20–40 min), the adsorption amount was gradually increased. The concentration of indigo carmine and acid red decreased and the adsorption rate changed relatively slowly, while the adsorption reached equilibrium after 40 min. When the adsorption time was more than 60 min, the change of C_t/C₀ was very small. The contact time of 60 min was determined as the ideal contact time for the adsorption of indigo carmine and acid red onto the mesoporous adsorbent.

The pseudo-first-order and pseudo-second-order kinetic models are commonly used to estimate the rate constants, initial adsorption rates and adsorption capacities of an adsorbent for some adsorbates.²⁰

Pseudo-first-order model:³³

Pseudo-second-order model:³³

where, t is the contact time (min); q_t (mmol g⁻¹) is the amount of dyes onto adsorbent at time t; q_e (mmol g⁻¹) is the adsorption capacity at equilibrium; k₁ (min⁻¹) and k₂ (g mmol⁻¹ min⁻¹) are the rate constants of pseudo-first-order and pseudo-second-order adsorption, respectively.

The fitting kinetic model parameters are listed in Table 5 and Table 6.

Table 5 Fitting parameters of pseudo-first-order adsorption kinetics model of adsorption indigo carmine and acid red onto adsorbent at different temperatures

Dyes	Temperature	Fitting parameters			q e
	(K)	qe	k 1	R ^2	Experimental data
Indigo carmine	288.15	0.59521	0.19621	0.98265	0.5755814
	298.15	0.64867	0.19294	0.98947	0.5922176
	308.15	0.67649	0.18044	0.99119	0.6063276
	318.15	0.70626	0.20265	0.97813	0.6134442
Acid red	288.15	0.37664	0.13497	0.993345	0.380523
	298.15	0.39369	0.15254	0.99154	0.419616
	308.15	0.41405	0.19866	0.99108	0.450067
	318.15	0.4233	0.23929	0.99564	0.464963

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Table 6 Fitting parameters of pseudo-second-order adsorption kinetics model of adsorption indigo carmine and acid red onto adsorbent at different temperatures

Dyes	Temperature	Fitting parameters			qe
	(K)	qe	k 2	R ^2	Experimental data
Indigo carmine	288.15	0.63929	0.52823	0.99908	0.6894469
	298.15	0.69563	0.48362	0.99832	0.7185602
	308.15	0.72618	0.4326	0.99746	0.7199108
	318.15	0.75548	0.47872	0.99737	0.7373222
Acid red	288.15	0.40924	0.53189	0.97894	0.380523
	298.15	0.42414	0.61432	0.98533	0.419616
	308.15	0.44137	0.82737	0.99647	0.450067
	318.15	0.44664	1.07529	0.99784	0.464963

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Table 5 and Table 6 indicate that the pseudo-second-order model fitted the adsorption process of indigo carmine and the pseudo-first-order model fitted the adsorption process of acid red. The value of q_e and k showed that the adsorption amount of dyes increased with increasing temperature. The experimental data fitted well to the kinetic models. The value of q_e obtained by the pseudo-first-order model was near the experimental q_e of the same initial concentrations, while the q_e obtained by the pseudo-second-order model better fitted to the experimental value of q_e, slightly lower than the experimental value.

Conclusions

Based on this study, it can be concluded that the adsorption of indigo carmine and acid red onto the PVA/SiO₂ nanofibers are affected by many factors such as the initial concentration of dyes, pH, dosage of the adsorbent, temperature and contact time. Increasing in the initial concentration could effectively promote the adsorption capacity. The lower the pH, the higher the adsorption capacity for indigo carmine and acid red. Increase in the temperature and the dosage of the adsorbent could effectively enhance the adsorption. The adsorption isotherm studies showed that the adsorption of the two dyes fitted well with Langmuir, Freundlich, and Redlich–Peterson adsorption models, while the Redlich–Peterson model achieved the best fitting. The result of kinetic adsorption shows that the pseudo-second-order model fits the adsorption process of indigo carmine while the pseudo-first-order model fits the adsorption process of acid red.

Acknowledgements

This work was supported in part by the China International Science and Technology Cooperation Fund No.2010DFA92800, 2010DFA92820. The research was also partially supported by the National Natural Science Foundation of China No.51108328.

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