Mesoporous carbonaceous materials prepared from used cigarette filters for efficient phenol adsorption and CO₂ capture

Abstract

Mesoporous carbonaceous materials (MCMs) with a 2-D hexagonal (p6mm) mesostructure are synthesized through evaporation induced self-assembly on the surface of cigarette filters by using phenol/formaldehyde resol as a carbon precursor, triblock copolymer F127 as the template and cigarette filters as the matrix scaffold. The obtained MCMs incorporate the advantages of cigarette filters and phenol/formaldehyde resol, and results in enhanced performance for phenol adsorption and CO₂ capture. X-ray diffraction and transmission electron microscopy results indicate that the obtained carbon materials have an ordered p6mm mesostructure and good thermal stability. The MCMs possess a uniform pore size (5.1 nm), large surface area (526 m² g⁻¹) and pore volume (0.39 cm³ g⁻¹), as well as exhibiting a considerable phenol adsorption (261.7 mg g⁻¹) and CO₂ capture (2.48 mmol g⁻¹).

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1. Introduction

A large amount of cigarettes are consumed around the world every year, and in most cases the filters are thrown away. Disposed cigarette filters are one of the biggest solid wastes produced. Much attention has to be paid as around 766 571 metric tons of cigarette filters are produced each year, which pose a significant environmental contamination threat.¹ Environmental hazards may result from the leaching of the toxic components from disposed cigarette filters. This will expose the environment not only to heavy metals but also to ethyl phenol and pesticide residues.² Cigarette filters made of cellulose acetate contain a high degree of carbon atoms, and can potentially be the initial scaffold materials for the preparation of porous carbon.³ Polarz et al. obtained carbon materials by carbonizing cigarette filters at 1000 °C for 7 h with a temperature ramp of 1 °C min⁻¹, and studied the carbonization of cigarette filters in limited carbonization conditions.⁴ Unfortunately, a BET specific surface area of only 262 m² g⁻¹ and a porosity volume of 0.21 cm³ g⁻¹ were obtained.

Ordered mesoporous carbons (OMCs) have received considerable attention owing to their large surface area, tunable pore structure, uniform and adjustable pore size, and mechanical stability.⁵ These properties make them ideal candidates for applications in adsorption,^6–9 supercapacitors,^8,9 and catalysis.¹⁰ In general, a nanocasting strategy is adopted to fabricate ordered mesoporous materials by employing nanosized ordered mesoporous silica, followed by carbonization and removal of the silica template.¹¹ However, this hard-templating synthesis method is time-consuming, costly, and unsuitable for mass production. A reliable and facile strategy to synthesize OMCs without the use of a hard template is desirable. With this in mind, many studies have been conducted to obtain OMCs, for example an organic–organic assembly method has been employed to synthesize OMCs by using amphiphilic triblock copolymers as a soft template and phenolic resol as a carbon source.^12,13 Among these studies, a solvent evaporation induced self-assembly (EISA) process is known as a powerful preparation route for ordered mesoporous carbonaceous films or powders. It can be carried out by solution casting on planar substrates or solid scaffolds by evaporation, thermopolymerization, and carbonization to obtain the carbonaceous materials. In this way, by employing cigarette filters as a solid scaffold, the obtained materials can show a much larger surface area and pore volume than that of scaffold carbon alone, which may result in an enhanced adsorption performance.

Recently, Zhao’s group prepared carbon–silica composite materials by an EISA route using a decomposable polyether polyol-based polyurethane (PU) foam scaffold.¹⁴ Afterwards, this group prepared hierarchically porous carbonaceous monolith materials with 3-D cubic (Im3̄m) and 2-D hexagonal (p6mm) mesostructures using a foam scaffold.¹⁵ It is important to note that a good foam scaffold plays a key role in the preparation of hierarchically ordered porous carbons (HOPC). However, it is time-consuming to obtain the foam scaffold and requires the allocation of large resources to fabricate, and produces lots of unfavourable residues to the environment.^16,17 In this case, it is also not an eco-friendly way to prepare HOPC by using plenty of PU foam. Cigarette filters, one of the most common solid wastes, are often thrown away in most cases. It is an important task to reuse these solid wastes as a new resource. Nevertheless, the carbonaceous materials obtained by carbonizing used cigarette filters couldn’t be widely used due to its lower specific surface area and pore volume. In this paper, a strategy for the preparation of MCMs is presented by using solid waste, i.e. cigarette filters as a scaffold. The resulting materials obtained using an EISA method have the advantages of both cigarette filters and phenol/formaldehyde resol, and possess a uniform pore structure, large surface area and pore volume, as well as exhibiting considerable CO₂ capture (2.48 mmol g⁻¹) and phenol adsorption (261.7 mg g⁻¹).

2. Experimental

2.1. Samples

Smoked cigarettes were collected, and the coating was separated from the remaining filter materials. The filters were washed with deionized water and dried for future use.

2.2. Experimental methods

2.2.1. Preparation of carbonaceous materials.
Preparation of the carbon precursor. The carbon precursor was prepared according to the literature.¹⁸ For a typical procedure, phenol (6.1 g, 65 mmol) was completely melted at 42 °C in a flask, then a 20 wt% NaOH solution (1.3 g, 32.5 mmol) was slowly added with stirring. After that, 37 wt% formalin (10.5 g, 130 mmol), which is an aqueous solution of formaldehyde, was added, and the mixture was heated at 70 °C for 1 h. After cooling to room temperature, the pH was adjusted to 7.0 using 2.0 M HCl solution. After removing the water under vacuum, the product was dissolved in ethanol (40 wt%).
Preparation of the ordered mesoporous carbon (FDU-15). The ordered mesoporous materials were prepared according to the literature method.¹⁸ The typical synthetic procedure was as follows: the triblock copolymer poly(ethyleneoxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO, pluronic 127, 1.0 g) was dissolved in ethanol (20.0 g), then the carbon precursor containing phenol (0.61 g, 6.50 mmol) and formaldehyde (0.39 g, 13.0 mmol) was added into the solution and stirred for 10 min. After that, the solution was poured into a dish to evaporate the ethanol at room temperature over 8 h to obtain a transparent membrane. The membrane was then kept at 100 °C for 24 h to thermopolymerize the phenolic resins. Pyrolysis was carried out in a tubular furnace under a N₂ atmosphere at 350 °C for 2 h and then 600 °C for 2 h with a ramp rate of 1 °C min⁻¹.
Preparation of the mesoporous carbonaceous materials (MCMs). The mesoporous composites were prepared according to the literature method using resol, the F127 triblock copolymer and cigarette filters.¹⁵ In a typical synthetic procedure, F127 (1.0 g) was dissolved in ethanol (8.5 g) and stirred at 40 °C for 1 h to afford a clear solution. A resol solution (2.5 g) of the above precursors was slowly added with stirring over 1 h. After that, an amount of used cigarette filter was immersed in the obtained homogeneous solution. Air bubbles inside the cigarettes filters were removed by squeezing the scaffolds with a glass rod. It took 8 h to evaporate the solvent at room temperature in a drafty closet, and then 24 h at 100 °C in an oven for the following thermopolymerization. Mesoporous materials were obtained by carbonizing the as-made samples at 600 °C for 2 h in N₂ with a ramp rate of 1 °C min⁻¹.
Preparation of the porous carbonaceous materials (PCMs) using used cigarette filters. The PCMs were obtained by carbonizing the used cigarette filters without adding phenolic resin. The filters were carbonized at 600 °C for 2 h in N₂ with a 1 °C min⁻¹ heating rate.

2.2.2. Adsorption performance of the carbonaceous materials.
Phenol adsorption measurements. Since phenol is toxic and corrosive, experiments were conducted in a fume hood while wearing rubber gloves and a gas mask. The adsorption performance of the three adsorbents (MCMs, FDU-15, PCMs) was investigated. In each adsorption experiment, 0.20 g of adsorbent was added to a 100 mL phenol solution with different initial phenol concentrations (100–700 mg L⁻¹). The resulting mixture was continuously stirred at 25 °C for a certain time until equilibrium was reached. The phenol concentration in the supernatant was analyzed using a UV spectrophotometer (UV-1700, Shimadzu), with the wavelength at 270 nm. Prior to analysis, centrifugation was used to avoid potential interference from suspended scattering particles in the UV analysis. The amount of phenol adsorbed q_e (mg g⁻¹) was calculated aswhere C₀ (mg L⁻¹) is the initial concentration of the phenol solution, C_e (mg L⁻¹) is the equilibrium concentration of the phenol solution, V (L) is the volume of the solution, and W (g) is the weight of the adsorbent. To determine C_e, the working curve of the UV absorbency of the phenol standard solution was first measured, then the absorbency of the residual phenol solution was measured and C_e was calculated based on the working curve.
CO₂ capture measurements. CO₂ adsorption measurements were carried out using a Micromeritics TriStar 3020 volumetric adsorption analyzer at 25 °C and pressures of CO₂ from 0 to 101 kPa. The temperature was controlled using a circulating bath. Before measurement the samples were degassed at 120 °C in a vacuum for 24 h to remove any moisture and CO₂ molecules adsorbed in the pores.

2.3. Characterization techniques

Small-angle X-ray diffraction (XRD) patterns were collected using a Rigaku D/MAX-2500 X-ray diffraction system (Cu Kα radiation, λ = 0.15406 nm) operated at 40 kV and 40 mA. Scanning electron microscopy (SEM) was performed on a HITACHI S-4800-I scanning electron microscope. The samples were mounted on an aluminum stub using adhesive carbon tape and SEM images were obtained at different magnifications. High-resolution transmission electron micrographs (HR-TEM) were obtained using a JEOL JEM-2010 electron microscope. Samples for HR-TEM studies were prepared by placing a drop of the suspension of the sample in ethanol onto a carbon-coated copper grid, followed by evaporating the solvent. Nitrogen adsorption–desorption isotherm measurements were performed using a Micromeritics TriStar 3020 volumetric adsorption analyzer at −196 °C. The samples were degassed at 100 °C overnight before the measurement. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area of each sample and the average pore size distribution was derived from the desorption branch of the corresponding isotherm using the Barrett–Joyner–Halenda (BJH) method. The total pore volume was estimated from the N₂ amount adsorbed at a relative pressure of P/P₀ = 0.97. The specific surface area was calculated from the relative pressure of P/P₀ = 0.05–0.35.

3. Results and discussion

3.1. XRD characterization

The small-angle X-ray diffraction (XRD) patterns of the carbon materials are shown in Fig. 1. It shows that FDU-15 has two well-defined diffraction peaks which can be indexed as the (100) and (200) reflections of 2D hexagonal (p6mm) symmetry, indicating a highly ordered mesostructure.¹⁸ In comparison, the MCMs show a less resolved diffraction peak due to the introduction of the disordered cigarette filter carbon. Only one diffraction peak is seen for the MCMs. According to the literature, this diffraction peak could be indexed as the (100) reflection of a 2D hexagonal mesostructure (p6mm space group).^18,19 It suggests that the products obtained are imperfect with degradation of the ordered mesostructure, and may contain ordered mesostructured carbonaceous shells and amorphous carbon of the cigarette filters. It also shows that the ordered mesostructure of the MCMs is thermally stable during the carbonization process.^14,18 The ordered mesostructure is perfectly retained, despite the removal of a large part of the template on calcination.

Fig. 1 XRD patterns of the carbon materials, MCMs, FDU-15 and PCMs.

3.2. SEM and TEM characterization

The scanning electron microscopy (Fig. 2a–c) images show that the struts of the carbonaceous materials are not hollow, but are irregular rods with some grooves. The SEM image of the PCMs (Fig. 2a) fabricated with used cigarette filters shows that several impurities appear on the surface of the irregular rods, which result from residual impurities of tobacco burning. As shown in Fig. 2c, much carbon from phenolic resins (red circle) coats the surface of the MCMs. It is noted that the surface transforms into a folded structure due to the presence of schistose carbonaceous materials, which come from the polymerization and carbonization of the phenolic resins. In addition, it also shows the presence of residual impurities (blue circle). Transmission electron microscopy (TEM) images reveal more refined structural features. The MCMs show large domains of stripe-like patterns (Fig. 2d), further confirming the ordered mesostructure. In addition, lots of micropores appear on the edges of the carbonaceous materials or the walls of the mesoporous channels (Fig. 2e and f), that is to say, the main mesoporous channels are interconnected through micropores inside the walls of the main channels, which is beneficial for CO₂ capture.²⁰

Fig. 2 SEM images of PCMs (a) and MCMs (b and c), and TEM images (d, e, f) of MCMs.

3.3. N₂ adsorption–desorption measurements

N₂ sorption isotherms of the carbon materials are shown in Fig. 3. It can be seen that FDU-15 and the MCMs both exhibit representative type IV curves with obvious capillary condensation steps at P/P₀ = 0.4–0.75 (Fig. 3a), which is consistent with the literature.^15,18 The adsorption and desorption branches of the MCMs deviate at low relative pressure, which is related to the asymmetric shrinkage of the pores.¹⁵ Compared with FDU-15, the capillary condensation step of the MCMs is shifted slightly to a lower relative pressure, which is related to the slight reduction in aperture size to 5.1 nm, resulting from the framework shrinkage.¹⁵ The MCMs show a BET surface area of 526 m² g⁻¹ and a narrow pore size distribution with a mean value of 5.1 nm calculated from the adsorption branch based on the BJH model (Table 1). The H₁ hysteresis loop of the MCMs suggests that it has imperfect cylindrical channels, implying the presence of partial asymmetric pore shrinkage.¹⁴ Compared with the PCMs and FDU-15, an increase in surface area (526 m² g⁻¹) and pore volume (0.39 cm³ g⁻¹) of the MCMs is obtained after the template is removed. This can be attributed to the formation of micropores resulting from the cigarette filter carbonization or the edges of the carbonaceous materials and the walls of the mesoporous channels.²¹

Fig. 3 N₂ sorption isotherms and pore size distribution of the MCMs, FDU-15 and PCMs.

Table 1 Pore structure parameters of the MCMs, FDU-15, and PCMs; S_BET is the specific surface area, D_BJH is the pore diameter, and V_BJH is the total pore volume and the capacity of CO₂ capture

Sample	SBET (m^2 g^−1)	VBJH (cm^3 g^−1)	DBJH (nm)
MCMs	526	0.39	5.1
FDU-15	460	0.22	6.6
PCMs	296	0.18	—

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3.4. Phenol adsorption measurements

3.4.1 Effect of contact time and various concentrations on phenol removal. Fig. 4 depicts the effect of contact time and various phenol concentrations on the removal of phenol over the three adsorbents. In Fig. 4a, the saturation curves rise sharply in the initial stages, indicating that there are plenty of readily accessible sites. Elemental analysis shows that the samples are mainly composed of carbon, with a small amount of hydrogen and oxygen. Such fast adsorption kinetics for phenol removal is related to the hydrogen bonding between the hydroxyl groups of phenol and the porous carbon surface.²² Eventually, a plateau is reached in all the curves, indicating that the adsorbent is saturated at this level. It can be seen from Fig. 4a that the removal curves are single, smooth and continuous, indicating the formation of monolayer coverage of phenol molecules on the outer surface of the adsorbent.²³ In Fig. 4b, the adsorption isotherms are obtained after adsorption for 12 h. The adsorbed phenol capacity of the MCMs is much higher than that of the PCMs and FDU-15 for these experimental conditions, suggesting that the MCMs have a high efficiency, which is attributed to the large surface area and pore volume of the MCMs.²³ After 6 adsorption–desorption cycles, the performance is reduced to 86%. It is noteworthy that there was no significant difference in the adsorption capacity between new cigarette filters and used cigarette filters. In other words, the existence of tar has little influence on the result of the whole adsorption. Furthermore, the filters were washed to remove tiny attached dust, and these show better performance than non-washed filters.

Fig. 4 Effect of contact time (a) and various initial phenol concentrations (b) on the phenol adsorption over MCMs, FDU-15 and PCMs.

3.4.2 Adsorption isotherms. Adsorption isotherms are important for describing how solutes interact with adsorbents, and are critical in optimizing the use of adsorbents. As shown in Fig. 5, adsorption isotherms have been plotted to follow the Langmuir and Freundlich equations. The Langmuir model is based on the assumption of a homogeneous adsorbent surface with identical adsorption sites and no transmigration in the plane of the surface,²⁴ which can be expressed as:where C_e is the equilibrium concentration of adsorption (mg L⁻¹), q_e is the amount of absorbate adsorbed (mg g⁻¹), Q₀ is the Langmuir constant (the maximum adsorption amount), and b is the Langmuir constant (L mg⁻¹ or L mol⁻¹). Therefore, Q₀ can be obtained from the reciprocal of the slope of a plot of C_e/q_e against C_e. The Q₀ values determined from Fig. 5 are summarised in Table 2.

Fig. 5 (a) The Langmuir isotherms of phenol adsorption over the three adsorbents; (b) the Freundlich isotherms of phenol adsorption over the MCMs, FDU-15 and PCMs.

Table 2 Isotherm constants for the adsorption of phenol over the MCMs, FDU-15 and PCMs

Adsorbent	Langmuir			Freundlich
	Q0 (mg g^−1)	b (L mg^−1)	R^2	KF ((mg g^−1) (L mg^−1)^1/n)	n	R^2
MCMs	257.5	0.19	0.998	45.02	3.502	0.947
FDU-15	143.9	0.11	0.994	17.17	2.704	0.920
PCMs	99.1	0.05	0.990	6.39	2.232	0.911

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The Freundlich model is an empirical equation assuming heterogeneous adsorptive energies on the surface, and can be shown as:²⁵

q_e = K_FC^1/n_e (3)

where K_F and n are Freundlich constants related to the adsorption amount and intensity, respectively.

The parameters are also calculated and summarised in Table 2. It can be seen that the Langmuir isotherms of the three adsorbents fit quite well with the experimental data, compared to the Freundlich isotherms which have lower correlation coefficients. This is comparable to the result for phenol adsorption on activated carbon reported in the literature.²⁵ In addition, compared to the other adsorbents, the MCMs exhibit a much larger monolayer adsorption capacity (257.5 mg g⁻¹) of phenol, which is 2.59 and 1.79 times that of the PCMs and FDU-15, respectively. This should be attributed to its large surface area and large pore volume.²³ It should also be noted that the MCMs show the best adsorption capacity of phenol among the adsorbents reported in the literature (1.48–165.80 mg g⁻¹).^26–28

3.4.3 Adsorption kinetics. The adsorption kinetics curves of phenol are shown in Fig. 6. The kinetics are fitted with pseudo-first-order and pseudo-second-order models, which are extensively used in kinetic studies. The pseudo-first-order model can be expressed as:²⁹

ln(q_e − q_t) = ln q_e − k₁t (4)

where k₁ is the first-order rate constant. Values of ln(q_e − q_t) are calculated from the experimental data and plotted against t, and k₁ is the calculated from the slope of this plot.

Fig. 6 (a) Plots of the pseudo-first-order kinetics of phenol adsorption over the three adsorbents, and (b) the curves of the pseudo-second-order kinetics of phenol adsorption over the MCMs, FDU-15 and PCMs (C_i: 400 ppm).

The pseudo-second-order model is shown as:³⁰

where k₂ is the second-order rate constant. Values of t/q_t are plotted against t, and q_e and k₂ are calculated from the slope and intercept of the plot.

The adsorption constants of the two models over the three adsorbents are listed in Table 3. It can be seen that the pseudo-first-order model gives poor fitting with low R² values and notable differences between experimental and theoretical uptakes. The pseudo-second-order fits the experimental data well for all samples. The R² values are close to unity and the experimental and theoretical uptakes are in good agreement, which agrees with the report that pseudo-second-order models are suitable for the adsorption of lower molecular weight adsorption on smaller adsorption particles.³¹

Table 3 Comparison of pseudo-first-order and pseudo-second-order adsorption rate constants, and the calculated and experimental q_e values over the MCMs, FDU-15 and PCMs

Adsorbent	qe,exp (mg g^−1)	Pseudo-first-order			Pseudo-second-order
		k1	R^2	qe,cal (mg g^−1)	k2	R^2	qe,cal (mg g^−1)
MCMs	261.7	0.418	0.952	120.79	0.039	0.997	265.3
FDU-15	148.9	0.327	0.965	93.86	0.026	0.996	152.7
PCMs	72.4	0.172	0.616	25.08	0.019	0.992	74.2

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3.5. CO₂ adsorption measurements

The capture of CO₂ has attracted considerable attention in recent years because it is the main anthropogenic contributor to climate change. A comparative analysis of the CO₂ adsorption isotherms (measured at 25 °C and 1.0 bar) of the three carbonaceous materials is presented in Fig. 7. It can be seen that the CO₂ adsorption capacities of the MCMs with a large surface area (526 m² g⁻¹) is 2.48 mmol g⁻¹. The value is higher than or at least comparable with those of reported N-doped porous carbon for CO₂ adsorption.^6,32 In contrast, the PCMs and FDU-15 only show CO₂ capacities of 1.74 mmol g⁻¹ and 1.95 mmol g⁻¹ with surface areas of 296 m² g⁻¹ and 460 m² g⁻¹, respectively. It should be noted that the MCMs display a much larger pore volume (see Table 1) than that of FDU-15 and the PCMs. This demonstrates that a large surface area and pore volume are beneficial to CO₂ capture. Some previous studies have also reported that microporosity in materials is beneficial for CO₂ capture and storage.^33,34 This is due to micropores having a high adsorption potential that enhances the adsorption performance of CO₂ molecules.²³ In addition, pore channels with a high specific surface area and large pore volume can accelerate the diffusion and interaction of CO₂ molecules during the sorption process.³⁵ This is confirmed by the CO₂ capture performance of the MCMs. In addition, no saturation is observed up to a pressure of 1.0 bar, suggesting that a higher CO₂ adsorption capacity can be achieved at a higher pressure.

Fig. 7 Adsorption isotherms of the MCMs, FDU-15 and PCMs for CO₂ capture at 25 °C.

4. Conclusions

Porous carbon materials with an ordered mesostructure were prepared successfully using the EISA method with phenol/formaldehyde resol as the carbon precursor, triblock copolymer F127 as the mesostructural template, and cigarette filters as the carbon matrix scaffold. The resulting products possess an ordered p6mm mesostructure, large surface area and uniform pore size. In addition, it is noted that the MCMs incorporate the advantages of ordered mesostructured carbon and amorphous carbon of cigarette filters. The obtained materials exhibited a considerable performance for phenol adsorption (261.7 mg g⁻¹) and CO₂ capture (2.48 mmol g⁻¹), and provides a new way of recycling of solid waste and cigarette filters.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (20906019), Science and Technology Research Projects in Hebei Universities (QN20131069, QN2014142, ZD20131032 and Z2013001), and Five Platform Open Fund Projects of Hebei University of Science and Technology (2014PT86).

References

1. E. A. Smith and T. E. Novotny, Tobac. Contr., 2011, 20, 2–9.
2. E. T. Novotny, S. N. Hardin, L. R. Hovda, D. J. Novotny, M. K. McLean and S. Shan, Tobac. Contr., 2011, 20(1), 17–20.
3. Y. S. Kazemi, S. S. Masoudi and S. Hossein, Adv. Mater. Res., 2012, 587, 88–92.
4. S. Polarz, B. Smarsly and J. H. Schattka, Chem. Mater., 2002, 14, 2940–2945.
5. W. Li, Q. Yue, Y. Deng and D. Zhao, Adv. Mater., 2013, 25, 5129–5152.
6. J. Wei, D. D. Zhou, Z. K. Sun, Y. H. Deng, Y. Y. Xia and D. Y. Zhao, Adv. Funct. Mater., 2013, 23, 2322–2328.
7. M. Barczak, K. Michalak-Zwierz, K. Gdula, K. Tyszczuk-Rotko, R. Dobrowolski and A. Dabrowski, Microporous Mesoporous Mater., 2015, 211, 162–173.
8. H. Liu, W. Cui, L. Jin, C. Wang and Y. Xia, J. Mater. Chem., 2009, 19, 3661–3667.
9. Q. Shi, R. Zhang, Y. Lv, Y. Deng, A. A. Elzatahrya and D. Zhao, Carbon, 2015, 84, 335–346.
10. K. Kwon, Y. Sa, J. Cheon and S. Joo, Langmuir, 2012, 28, 991–996.
11. G. Mane, S. Talapaneni, C. Anand, S. Varghese, H. Iwai and Q. Ji, et al., Adv. Funct. Mater., 2012, 22, 3596–3604.
12. C. Liang and S. Dai, J. Am. Chem. Soc., 2006, 128, 5316–5317.
13. F. Zhang, D. Gu, T. Yu, F. Zhang, S. Xie and L. Zhang, et al., J. Am. Chem. Soc., 2007, 129, 7746–7747.
14. C. Xue, B. Tu and D. Zhao, Adv. Funct. Mater., 2008, 18, 3914–3921.
15. C. F. Xue, B. Tu and D. Y. Zhao, Nano Res., 2009, 2, 242–253.
16. G. Behrendt and B. Naber, Metallurgy, 2009, 44, 3–23.
17. J. H. Janik and M. Marzec, Mater. Sci. Eng., C, 2015, 48, 586–591.
18. Y. Meng, D. Gu, F. Zhang, Y. Shi, H. Yang and Z. Li, et al., Angew. Chem., Int. Ed., 2005, 44, 7053–7059.
19. D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548–552.
20. L. Wang and R. Yang, J. Phys. Chem. C, 2011, 115, 21264–21272.
21. G. Chandrasekar, W. Son and W. Ahn, J. Porous Mater., 2009, 16, 545–551.
22. J. He, K. Ma, J. Jin, Z. Dong, J. Wang and R. Li, Microporous Mesoporous Mater., 2009, 121, 173–177.
23. B. Hameed and A. Rahman, J. Hazard. Mater., 2008, 160, 576–581.
24. K. Hall, L. Eagleton, A. Acrivos and T. Vermeulen, Ind. Eng. Chem. Fundam., 1996, 5, 212–223.
25. H. Freundlich, J. Phys. Chem., 1906, 57, 385–470.
26. I. Vázquez, J. Rodríguez-Iglesias, E. Marañón, L. Castrillón and M. Álvarez, J. Hazard. Mater., 2007, 147, 395–400.
27. A. Kumar, S. Kumar and D. Gupta, J. Hazard. Mater., 2007, 147, 155–166.
28. B. Özkaya, J. Hazard. Mater., 2006, 129, 158–163.
29. E. Tutem, R. Apak and C. Unal, Water Res., 1998, 32, 2315–2324.
30. E. Haque, J. Jun, S. Talapaneni, A. Vinu and S. Hung, J. Mater. Chem., 2010, 20, 10801–10803.
31. F. Wu, R. Tseng, S. Huang and R. Juang, Chem. Eng. J., 2009, 151, 1–9.
32. G. Hao, Z. Jin, Q. Sun, X. Zhang, J. Zhang and A. Lu, Energy Environ. Sci., 2013, 6, 3740–3747.
33. V. Zelenak, D. Halamova, L. Gaberova, E. Bloch and P. Llewellyn, Microporous Mesoporous Mater., 2008, 116, 358–364.
34. C. Martín, M. Plaza, S. García, J. Pis, F. Rubiera and C. Pevida, Fuel, 2011, 90, 2064–2072.
35. H. Yang, Y. Yuan and S. Tsang, Chem. Eng. J., 2012, 185–186, 374–379.

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