Synthesis and characterization of a starch–AlOOH–FeS₂ nanocomposite for the adsorption of congo red dye from aqueous solution

Abstract

This work described the synthesis and characterization of a starch–AlOOH–FeS₂ nanocomposite for the adsorption of congo red (CR) dye from aqueous solution. The morphology of the starch–AlOOH–FeS₂ was characterized by using scanning and transmission electron microscopy, N₂ adsorption–desorption isotherms, X-ray photoelectron spectroscopy, and fourier transform-infrared spectroscopy. The adsorption of CR onto starch–AlOOH–FeS₂ was evaluated as a function of contact time, solution pH, concentration and temperature. The adsorption results demonstrate that the maximum removal of CR was found to be at pH 5. The adsorption kinetics data fitted well to the pseudo first-order equation whereas the Freundlich equation exhibits better correlation to the experimental data. Thermodynamic parameters, such as the standard free energy change (ΔG°), the standard entropy change (ΔS°) and the standard enthalpy change (ΔH°), were also evaluated. The results suggested that starch–AlOOH–FeS₂ is a potential adsorbent for CR dye removal from aqueous solution.

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Introduction

Organic dyes and pigments discharged from sources such as textile, cosmetic, food, pharmaceutical, and paper industries are serious environmental problems because their unwanted colour and resistance to decomposition. Synthetic dyes are toxic and carcinogenic in nature towards the organism and mammals.^1,2 Congo red (CR) (sodium salt of benzidinediazobis-1-naphthylamine-4-sulfonic acid) is widely used in the textile, leather, paper, and printing industries and around 15% CR is discharged in wastewater during operation. CR is well known carcinogenic and its exposure may cause some allergic responses.^2,3 Moreover, due to its complex aromatic structure, eliminate or degrade (bio/photo) of CR from aqueous medium by conventional methods is difficult. Several methods such as adsorption, oxidation, photocatalysis, membrane filtration, biological degradation etc. have been used for the treatment of dyeing effluents.^1,4–7 Among all of them, adsorption is a very promising method for the removal of dyes from wastewater. Several adsorbents such as carbons, metal oxides, polymers, clays, composites etc. have been used for the removal of CR from aqueous solutions.^1,2,4,8–10

However, low or moderate adsorption capacities of these materials towards the CR removal promote the researchers to develop novel materials with high efficiency and economical removal of CR from dyeing effluents.

The recently discovered hybrid materials showed many advantages in wastewater treatment.³ The adsorption ability of hybrid materials relies on the smart manipulation of the structure of the embedded compounds such as charge, functionality, hydrophobic–hydrophilic nature etc.⁵ Boehmite (AlOOH) is non-hazardous material, exhibit good adsorption capacity (85–99 mg g⁻¹) toward CR from aqueous solution.¹¹ The adsorption of the CR onto AlOOH takes place due to interaction between metal sites and amine group of dye. The adsorption performance of AlOOH can be enhanced by tailoring its surface. The high functionality of starch and pyrite combined with AlOOH can much effectively remove the CR from wastewater. Pyrite and starch are most abundant sulfide mineral and biopolymer, respectively. Starch and starch based adsorbent have been investigated for the removal of various classes of dyes.^12–14 To the best of our knowledge, pyrite or pyrite based adsorbent has not been investigated for the removal of dyes. However, few studies were reported for the removal of metal ions such as molybdate, tetrathiomolybdate,¹⁵ Cu(II), Cd(II) and Pb(II),¹⁶ As(III) and As(V),¹⁷ etc. Pyrite showed good adsorption capacity for negatively charged metal ions. Therefore, it is supposed to be good adsorbent for the removal of negatively charged CR dye. In this work, a simple route for the synthesis of starch–AlOOH–FeS₂ composite and its adsorption efficiency for the removal of CR is reported. The adsorption of CR onto starch–AlOOH–FeS₂ is discussed in terms of kinetics, thermodynamics and isotherm studies.

Materials and methods

Materials

Starch (potato) and aluminum nitrate were purchased from Panreac Quimica S.A. Congo red and Sodium dodecyl sulfate (SDS) were obtained from Techno Pharmachem Haryana, India and Scharlab S.L. Spain. FeSO₄·7H₂O and NH₄HCO₃ were supplied by BDH chemical Ltd and Fisher Scientific Company, New Jersey.

Synthesis of starch–AlOOH–FeS₂nanocomposite

In a typical synthesis, 13.9 g Al₂(NO₃)₃ was dissolved in 25 mL water under sonicated (Elma S30H Elmasonic) for 20 min, and then 1 g starch was added into the solution. After sonication at 40 °C for 30 min, 1.2 g SDS was added into the solution to prevent the agglomeration of particles. Thereafter, 100 mL 2.0 M NH₄HCO₃ was added to the mixture and sonicated for 2 h at 60 °C. Resulting white precipitate was filtered by Buchner funnel with a sintered glass disk and thoroughly washed with hot de-ionized water and ethanol for the removal of excess starch and SDS. The obtained white precipitate was transferred into ethanol solution (200 mL) containing 8.0 g Na₂S₂O₃ and 4.4 g FeSO₄·7H₂O under sonication at 40 °C for 30 min. Thereafter, the mixture was refluxed at 180 °C for 1 h and finally cooled to room temperature. The precipitate was filtered and thoroughly washed with hot de-ionized water and ethanol, and dried at 200 °C for 4 h. Dry sample was collected and stored for further adsorption studies.

Characterization

The morphology of starch–AlOOH–FeS₂ was investigated by field emission scanning electron microscopy (FESEM) (JSM-7500 F; JEOL-Japan) without spurting coated gold or carbon and transmission electron microscopy (TEM) (model Tecnai G2 F20 Super Twin) at 200 kV with LaB₆ emitter. X-ray photoelectron spectroscopy (XPS) measurements were recorded on SPECS GmbH, (Germany) spectrometer operated at a base pressure of 10⁻¹⁰ mbar and step size 1 eV. A non-monochromatic Mg-Kα (1253.6 eV) X-ray source was used to irradiate the sample surface with 13.5 kV, 100 W X-ray power. The nitrogen adsorption–desorption isotherm analysis [Brunauer, Emmett and Teller (BET) and Density Functional Theory (DFT)] of starch–AlOOH–FeS₂ was performed at 77 K Quantachrome Nova Win 2 (Quantachrome instruments) equipment by degassing the sample at 200 °C. FTIR spectra of starch–AlOOH–FeS₂ before and after CR adsorption were recorded using a Perkin Elmer Spectrum 100 FTIR Spectrometer over a range of 500–4000 cm⁻¹.

Adsorption studies

Adsorption of CR was investigated under a wide variety of experimental conditions. A series of experiments were performed by reacting 0.02 g starch–AlOOH–FeS₂ with 20 mL of CR solution (50–500 mg L⁻¹) as a function of solution pH ranging from 5–9. The solution pH was adjusted using 0.1 M HCl and NaOH. Adsorption isotherms were performed at pH 5, 7, 9 and at temperature 30, 40, 50 °C. The effect of contact time was studied in a series of flasks contain 20 mL dye solution of 500 mg L⁻¹ in water bath shaker at 200 rpm at temperature 25, 40, 50 °C. After a fixed time interval, the samples were filtered and the amount of dye remaining in the supernatant solution was analyzed using a HACH LANGES DR-6000 UV-visible spectrophotometer at 495 nm. The amount of CR adsorbed on starch–AlOOH–FeS₂ can be estimated by:

q_e = (C₀ − C_e)V/m (1)

where, q_e is the amount of CR adsorbed per unit mass of starch–AlOOH–FeS₂ (mg g⁻¹), C₀ and C_e is the initial and equilibrium dye concentration (mg L⁻¹), respectively. V is the volume of dye solution (L) and m is the mass of starch–AlOOH–FeS₂ (g).

Results and discussion

Characterization

The morphology of the prepared composite material was investigated by SEM and TEM as shown in Fig. 1. The SEM images (Fig. 1a) shows the needle like clusters of starch–AlOOH–FeS₂. The TEM image (Fig. 1b) also reflects the needle like structure. The high resolution TEM image (Fig. 1c) shows the polycrystalline nature of starch–AlOOH–FeS₂ composite.

Fig. 1 SEM (a), TEM (b) and HRTEM (c) of starch–AlOOH–FeS₂.

The specific BET surface area and pore structure of the starch–AlOOH–FeS₂ was investigated using N₂ adsorption–desorption measurements. Fig. 2 shows the type IV isotherm. The BET, DFT surface area and total pore volume are 40.013, 25.33 and 0.055 m² g⁻¹, respectively. The pore diameter is found to be in the range of 1.348 to 3.39 nm and the total micropore and mesopore are 0.0549 and 0.0484 cm³ g⁻¹, respectively. The surface analysis of starch–AlOOH–FeS₂ by XPS is shown in Fig. 3. The sulfur peak located at 163.82 eV is corresponding to the 2p^1/2 which is identical with the S²⁻ in the pyrite.¹⁸ Broad peaks located at 168.69 and 530 eV are due to the presence of SO₄²⁻ impurities of iron precursor and FeOOH (O 1s). Moreover, the Fe 2p^3/2 and 2p^1/2 peaks are located at 708.2 eV and 719.6 eV are consistent with bulk pyrite.¹⁹ Two peaks appeared at 118.5 eV and 74.68 eV are corresponding to the Al 2s and 2p, and confirm the presence of AlOOH in the composite.²⁰ The binding energies for C 1s of the starch functional groups at 284.8, 286.6 and 288.38 eV are corresponding to the C–C/C–H, C–O (alcohol) and O–C–O (ester), respectively.²¹ In addition, the presence of these functional groups can be confirmed by FTIR analysis of starch–AlOOH–FeS₂ nanocomposite. Fig. 4 shows the FTIR spectra of starch–AlOOH–FeS₂. The broad and strong peaks centered at 3213 and 1633 cm⁻¹ are attributed to the O–H vibrations of starch–AlOOH or water. The broad peak at 1079 cm⁻¹ can be assigned to the C–O–C linkage of the starch.^22,23 Moreover, the broad peak with small shoulders in the range of 1000–1200 may be attributed to the pyrite.²⁴ The strong band at 766 cm⁻¹ may be ascribed to the AlO₆ vibration mode which shifted to 757 cm⁻¹ after adsorption. After dye adsorption new peaks appear in the region 1500–1600 and 1417, 1371, 1040 cm⁻¹ may be assign to the NH, NH₂ and C–H (bending), S–O (stretching) vibrations of the dye.^2,25,26 From the FTIR spectra, it is noted that most of the peaks shifted or diminished after the adsorption of dye. The shift in wave numbers and intensity of the peaks could be due to the interaction of dye functional groups (NH, NH₂, SO₃⁻) with the adsorbent functional groups.²

Fig. 2 Nitrogen adsorption–desorption isotherm plot of starch–AlOOH–FeS₂.

Fig. 3 XPS analysis of starch–AlOOH–FeS₂.

Fig. 4 FTIR spectra of starch–AlOOH–FeS₂ before and after CR adsorption.

Effect of solution pH

CR is highly sensitive to the solution pH and shows colour change from red (λ_max – 496 nm) to blue (λ_max – 580 nm) in the presence of inorganic acids in the pH ranges from 2–4 due protonation of azo group (π–π transition).^2,8 Fig. 5 shows the pH dependent adsorption of CR onto starch–AlOOH–FeS₂ nanocomposite. The adsorption of CR decreases with the increase in the solution pH from 5 to 9 and maximum adsorption is found to be at pH 5 as shown in Fig. 5. The adsorptive removal of CR shows a slight decrease at pH 9 compared to pH 7. Several investigations revealed that adsorption of negatively charged CR usually increases as the solution pH decreased.^2,8,27 This behavior can be explained on the basis of change in the functionality of adsorbent with respect to the solution pH. According to Gissinger et al.,²⁸ S–S–H groups are predominated in acidic medium and pyrite surface shows a deficiency of iron and formation of iron vacancies Fe_1−xS₂, while O–H groups are predominated in basic medium. When the solution pH increases, the pyrite surface shows the excess of FeOOH. Both S–S–H and O–H groups are coexisting over the entire pH range at pyrite surface. Therefore, high adsorption is observed in acidic medium and both groups are thus probably involved in the binding of CR by the surface complexation. The adsorbent shows effective removal of CR at natural and basic condition (pH 9). This may be explain on the basis of positive surface charge of AlOOH (in basic medium at pH < 10) while CR is anionic charged. Thus, CR is electrostatically adsorbed on adsorbent surface (Dye–SO₃⁻ + AlOOH₂⁺ ↔ Dye–SO₃⁻⋯⁺H₂OOAl).^27,29 Furthermore, CR may be adsorbed on the surface of metal oxides due to a coordination effect of metal atoms with –NH₂ and –SO₃⁻ groups of CR. Moreover, the adsorption of CR may be largely ascribed to the strong hydrogen bond formed between the electronegative atoms of dye and adsorbent such as oxygen, nitrogen or sulfur and hydrogen atoms.⁹ From the results mentioned above and previous reports^9,10,27 hydrogen bonding, electrostatic interaction, complexation and coordination effect of metal atoms with –NH₂ and –SO₃⁻ groups are mainly responsible for the adsorption of CR in neural CR solution. Therefore, pH 7 is selected for the further adsorption studies.

Fig. 5 Effect of solution pH on CR adsorption.

Adsorption kinetics

The Effect of contact time on CR adsorption is shown in Fig. 6. The removal of CR is very fast during the first 100 min due to the adsorption of dye molecules onto the vacant adsorbent sites and thereafter rate of dye adsorption decreases due to the saturation of active sites (internal surface adsorption).² The equilibrium is attained within 300 min. Adsorption dynamics of CR removal on starch–AlOOH–FeS₂ are performed to find the rate controlling step. The adsorption data has been analyzed using pseudo first-order and pseudo second-order kinetic models. The linear equations for pseudo first-order³⁰ and pseudo-second order³¹ kinetic models, respectively, are given as:

log(q_e − q_t) = log q_e − (k₁t/2.303) (2)

t/q_t = (1/k₂q_e²) + t/q_e (3)

where and q_e (mg g⁻¹) and q_t (mg g⁻¹) refer to the adsorption capacity at equilibrium and after time t (min) respectively. k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) are the pseudo-first and pseudo-second order rate constant, respectively. The values for pseudo first-order and pseudo second-order kinetic parameters obtained from their respective linear plots log(q_e − q_t) against t and t/q_e against t, respectively are tabulated in Table 1. The calculated adsorption capacities (q^cal_e) from pseudo first order kinetic equation are close to the experimental (q^exp_e) values. Furthermore, R² values obtained for pseudo first-order kinetic equation are higher than pseudo second-order kinetic equation. These results revealed that the ongoing adsorption reaction proceeds via a pseudo-first order kinetic rather than a pseudo second-order kinetic mechanism. Similar type of adsorption behavior for CR adsorption onto bio-polymeric composite was reported by Chatterjee et al.,³ and Du et al.³²

Fig. 6 Effect of Contact time on CR adsorption.

Table 1 Kinetic parameters for CR adsorption onto starch–AlOOH–FeS₂

Temp. (°C)	q^(exp)e (mg g^−1)	Pseudo-first order model			Pseudo-second order model
		q^(cal)e (mg g^−1)	k1 (min^−1)	R^2	q^(cal)e (mg g^−1)	k2 (g mg^−1 min^−1)	R^2
25	261	266.07	11.51 × 10^−3	0.994	333.33	3.474 × 10^−5	0.985
40	313.2	305.49	9.212 × 10^−3	0.996	500	1.550 × 10^−5	0.990
50	346.2	320.62	9.212 × 10^−3	0.987	500	19.704 × 10^−5	0.984

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Adsorption isotherms

The adsorption isotherms analysis has been performed at different concentrations and temperatures as shown in Fig. 7. The removal of CR increases with the increases in solution concentration and temperature. This can be due to the greater driving force through a higher concentration gradient at high concentration.² An increase in the temperature, enhance the mobility of the dye molecule and decrease in the retarding forces acting on the diffusing dye molecules. This results in the increase in the adsorption capacity of starch–AlOOH–FeS₂ for CR.³³

Fig. 7 Effect of concentration of CR adsorption.

In attempting to describe the adsorption isotherm of CR removal by starch–AlOOH–FeS₂, Langmuir, Freundlich and Temkin isotherm models are tested. The linear equation for Langmuir,³⁴ Freundlich³⁵ and Temkin³⁶ isotherm models, respectively are given as:

1/q_e = (1/q_mbC_e) + 1/q_m (4)

ln q_e = ln K_F + (1/n)ln C_e (5)

q_e = B ln A + B ln C_e (6)

where q_e (mg g⁻¹) and C_e (mg L⁻¹) are the CR adsorption capacity and concentration at equilibrium, q_m (mg g⁻¹) and b (L mg⁻¹) are Langmuir constant. Freundlich isotherm constants K_F (mg^1−1/n L^1/ng⁻¹) and n are representing the capacity and intensity of adsorption. Temkin isotherm parameters, A (L mg⁻¹) and B are related to the equilibrium binding constant and the heat of adsorption, respectively. The parameters for Langmuir, Freundlich and Temkin isotherms are calculated for their respective linear plots 1/q_e vs. 1/C_e, ln q_e vs. ln C_e and q_e vs. ln C_e. From the Table 2, it can be seen that the values of correlation coefficient (R²) for the Freundlich isotherm model are higher than the Langmuir and Temkin adsorption models, revealing that experimental equilibrium data fits better with Freundlich isotherm. This reveled that adsorption of CR occur through multilayer formation on the heterogeneous surface of starch–AlOOH–FeS₂composite.³⁷ Moreover, the magnitude of Freundlich constant (n) should be more than one for favorable adsorption process.^38,39 The values of n (Table 2) are greater than one at all the studied temperatures revealing the feasible adsorption of CR onto starch–AlOOH–FeS₂ composite.

Table 2 Adsorption isotherm parameters for CR adsorption onto starch–AlOOH–FeS₂

Temp. (°C)	Langmuir isotherm model				Freundlich isotherm model			Temkin isotherm model
	qm (mg g^−1)	b (L mg^−1)	R^2	RL	KF (mg^1−1/n L^1/ng^−1)	n	R^2	A (L mg^−1)	B	R
30	333.33	0.032	0.951		44.880	2.288	0.970	0.226	78.06	0.955
40	333.33	0.040	0.957		45.422	2.293	0.980	0.274	81.66	0.970
50	333.33	0.166	0.912		70.035	3.246	0.956	1.455	61.65	0.923

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Adsorption thermodynamics

The adsorption of CR onto starch–AlOOH–FeS₂ increases from 298 mg g⁻¹ to 346 mg g⁻¹ with the increase in solution temperature from 30 °C to 50 °C as shown in Fig. 7. These results indicate that the adsorption process is endothermic in nature. Adsorption thermodynamic parameters for CR removal, standard Gibbs free energy change (ΔG°), standard entropy change (ΔS°) and standard enthalpy change (ΔH°) are calculated from following equations:

ΔG° = −RT ln K_c (7)

K_c = (C_ae/C_e) (8)

ln K_c = (ΔS°/R) − (ΔH°/RT) (9)

where, K_c and R (8.314 J mol⁻¹ K⁻¹) are the distribution coefficient and gas constant, respectively. T is the temperature (K). C_ae and C_e are the equilibrium concentration of CR (mg g⁻¹) on adsorbent and in solution, respectively. The value of ΔH° and ΔS° calculated from the slope and intercept of ln K_c verses 1/T are in Table 3. As can be seen from Table 3, the values of ΔG° are negative indicating the feasibility and spontaneous nature of adsorption process for all the studied temperatures. The positive value of ΔH° and ΔS° confirming the endothermic process and increase in randomness at solid–solution interface.⁴⁰ To a certain extent, the forces involve in the adsorption process can be determined by the magnitude of ΔH° and ΔG°. For the physisorption, the ΔG° value should be lower than −20 kJ mol⁻¹ and those for chemisorptions ranges from −80 to −400 kJ mol⁻¹. The ΔG° values for CR adsorption are in the range of −0.891 to −2.172 kJ mol⁻¹, indicating that physical forces are involved in CR adsorption onto starch–AlOOH–FeS₂. The energy (ΔH°) associated with physisorption: van der Waals forces (4–10 kJ mol⁻¹), hydrophobic interaction (5 kJ mol⁻¹), hydrogen bonding (2–40 kJ mol⁻¹), coordination exchange (40 kJ mol⁻¹), dipole bond forces (2–29 kJ mol⁻¹), and electrostatic interaction (20–80 kJ mol⁻¹), while for chemisorptions bond strength can be ranges from 80 to 450 kJ mol⁻¹ (ref. 40 and 41). The value of ΔH° is 17 kJ mol⁻¹ confirming the involvement of physical forces in CR adsorption.

Table 3 Thermodynamic parameters for CR adsorption onto starch–AlOOH–FeS₂

Temperature (°C)	ΔG° kJ mol^−1	ΔH° kJ mol^−1	ΔS° J mol^−1 K^−1	R^2
30	−0.891
40	−1.358	17.002	59.137	0.948
50	−2.173

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Conclusion

This article described the facile method for the synthesis of starch–AlOOH–FeS₂ nanocomposite for the high efficient removal of CR from aqueous solution. The prepared material shows good surface area with the mesoporous structure. FTIR analysis shows that the NH, NH₂ and SO₃⁻ groups are involved in the interaction of dye on the surface of starch–AlOOH–FeS₂. The adsorption results demonstrate that CR removal is highly dependent on the solution pH, contact time, concentration and temperature. Adsorption isotherm study revealed that surface of starch–AlOOH–FeS₂ is heterogeneous and multilayer adsorption of CR supposes to occur on the adsorbent surface. Thermodynamic study suggested that adsorption of CR onto starch–AlOOH–FeS₂ is spontaneous and endothermic in nature. The magnitude of standard Gibbs free energy change (ΔG°), and standard enthalpy change (ΔH°) revealed that the physical forces are involved in the adsorption of CR onto starch–AlOOH–FeS₂ surface. These results therefore suggested that starch–AlOOH–FeS₂ can be considered as a potential material for the scavenging of CR from aqueous solution.

References

1. M. Dong, Q. Lin, D. Chen, X. Fu, M. Wang, Q. Wu, X. Chen and S. Li, RSC Adv., 2013, 3, 11628–11633.
2. R. Ahmad and R. Kumar, Appl. Surf. Sci., 2010, 257, 1628–1633.
3. S. Chatterjee, M. W. Lee and S. H. Woo, Bioresour. Technol., 2010, 101, 1800–1806.
4. W. Cai, Y. Hu, J. Chen, G. Zhang and T. Xia, CrystEngComm., 2012, 14, 972–977.
5. Z. Y. Chen, H. W. Gao and Y. Y. He, RSC Adv., 2013, 3, 5815–5818.
6. M. Solís, A. Solís, H. I. Pérez, N. Manjarrez and M. Flores, Process Biochem., 2012, 47, 1723–1748.
7. E. Forgacsa, T. Cserhati and G. Oros, Environ. Int., 2004, 30, 953–971.
8. M. Toor and B. Jin, Chem. Eng. J., 2012, 187, 79–88.
9. J. Wu, J. Wang, H. Li, Y. Du, K. Huang and B. Liu, J. Mater. Chem. A, 2013, 1, 9837–9847.
10. A. Mahapatra, B. G. Mishra and G. Hota, Ceram. Int., 2013, 39, 5443–5451.
11. C. Wang, S. Huang, L. Wang, Z. Deng, J. Jin, J. Liu, L. Chen, X. Zheng, Y. Li and B. L. Su, RSC Adv., 2013, 3, 1699–1702.
12. V. Janaki, K. Vijayaraghavan, B. T. Oh, K. J. Lee, K. Muthuchelian, A. K. Ramasamy and S. K. Kannan, Carbohydr. Polym., 2012, 90, 1437–1444.
13. P. R. Chang, P. Zheng, B. Liu, D. P. Anderson, J. Yu and X. Ma, J. Hazard. Mater., 2011, 186, 2144–2150.
14. S. Xu, J. Wang, R. Wu, J. Wang and H. Li, Chem. Eng. J., 2006, 117, 161–167.
15. N. Xu, C. Christodoulatos and W. Braida, Chemosphere, 2006, 62, 1726–1735.
16. A. Ozverdi and M. Erdem, J. Hazard. Mater., 2006, 137, 626–632.
17. D. S. Han, J. K. Song, B. Batchelor and A. A. Waha, J. Colloid Interface Sci., 2013, 392, 311–318.
18. S. C. Hsiao, C. M. Hsu, S. Y. Chen, Y. H. Perng, Y. L.Chueh, L. J. Chen and L. H. Chou, Mater. Lett., 2012, 75, 152–154.
19. A. R. Pratt, H. W. Nesbitt and J. R. Mycroft, J. Geochem. Explor., 1996, 56, 1–11.
20. C. Gao, X. Y. Yu, R. X. Xu, J. H. Liu and X. J. Huang, ACS Appl. Mater. Interfaces, 2012, 4, 4672–4682.
21. A. Uliniuca, M. Popa, E. Drockenmuller, F. Boisson, D. Leonard and T. Hamaidea, Carbohydr. Polym., 2013, 96, 259–269.
22. Q. Lin, J. Pan, Q. Lin and Q. Liu, J. Hazard. Mater., 2013, 263, 517–524.
23. L. O. Filippov, V. V. Severov and I. V. Filippova, Int. J. Miner. Process., 2013, 123, 120–128.
24. R. K. Rath, S. Subramanian and T. Pradeep, J. Colloid Interface Sci., 2000, 229, 82–91.
25. Y. Yang, G. Wang, B. Wang, Z. Li, X. Jia, Q. Zhou and Y. Zhao, Bioresour. Technol., 2011, 102, 828–834.
26. F. Zhang, Y. Liu, Y. Cai, H. Li, X. Cai, I. Djerdj and Y. Wang, Powder Technol., 2013, 235, 121–125.
27. Y. Wang, W. Li and X. J. Chen, J. Mater. Chem. A, 2013, 1, 10720–10726.
28. P. B. Gissinger, M. Alnot, J. E. Ehrhardt and P. Behra, Environ. Sci. Technol., 1998, 32, 2839–2845.
29. K. Rezwan, L. P. Meier and L. J. Gauckler, Biomaterials, 2005, 26, 4351–4357.
30. S. Lagergren and K. Sven, Vetenskapsakad. Handl., 1898, 24, 1–39.
31. G. McKay and Y. S. Ho, Process Biochem., 1999, 34, 451–465.
32. Q. Du, J. Sun, Y. Li, X. Yang, X. Wang, Z. Wang and L. Xia, Chem. Eng. J., 2014, 245, 99–106.
33. V. S. Mane and P. V. V. Babu, J. Taiwan Inst. Chem. Eng., 2013, 44, 81–88.
34. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361–1403.
35. M. I. Temkin and V. Pyzhev, Acta Physiochem. USSR, 1940, 12, 217–222.
36. H. Freundlich, Uber die adsorption in loseungen, J. Phys. Chem., 1907, 57, 385–470.
37. L. Yu, W. Xue, L. Cui, W. Xing, X. Cao and H. Li, Int. J. Biol. Macromol., 2014, 64, 233–239.
38. S. Dawood and T. K. Sen, Water Res., 2012, 46, 1933–1946.
39. V. Vimonses, S. Lei, B. Jin, C. W. K. Chow and C. Saint, Chem. Eng. J., 2009, 148, 354–364.
40. S. Liu, Y. Ding, P. Li, K. Diao, X. Tan, F. Lei, Y. Zhan, Q. Li, B. Huang and Z. Huang, Chem. Eng. J., 2014, 248, 135–144.
41. F. M. Machado, C. P. Bergmann, E. C. Lima, B. Royer, F. E. de Souza, I. M. Jauris, T. Calvete and S. B. Fagan, Phys. Chem. Chem. Phys., 2012, 14, 11139–1.

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