Article pubs.acs.org/jced
Sepiolite Fiber Oriented-Polypyrrole Nanofibers for Efficient Chromium(VI) Removal from Aqueous Solution Jun Chen,*,† Xiaoqin Hong,† Qingdong Xie,† Diankai Li,‡ and Qianfeng Zhang† †
School of Chemistry and Chemical Engineering, Institute of Molecular Engineering & Applied Chemistry, and ‡School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, China ABSTRACT: Polypyrrole-sepiolite nanofibers were prepared by in situ chemical oxidation polymerization in the presence of sepiolite. The nanostructure of PPy-sepiolite nanofibers was confirmed by field-emission scanning electron microscopy. The adsorption of Cr(VI) onto the PPy-sepiolite nanofibers was highly pH dependent and the adsorption kinetics followed the pseudo-second-order model. The Langmuir isothermal model well described the adsorption isotherm data and the maximum adsorption capacity increased with the increase of temperature. The maximum adsorption capacity of the PPy-sepiolite nanofibers for Cr(VI) was up to 302 mg·g−1 at 25 °C. Thermodynamic investigation indicated that the adsorption process is spontaneous and endothermic. Desorption experiment showed PPy-sepiolite can be regenerated and reused for four consecutive cycles without obvious loss of its removal efficiency. The excellent adsorption characteristic of PPy-sepiolite will render it a highly efficient and economically viable adsorbent for Cr(VI) removal.
1. INTRODUCTION Due to the increased concern about global environmental pollution problems, more and more researchers dedicated their efforts to find available approaches to deal with heavy metal ions. Among those various kinds of heavy metal ions, Cr(VI) ion is one of the most toxic pollutants due to toxicity, carcinogenicity, and mutagenicity. Still the biggest problems regarding chromium come from electroplating and leather chemical tanning. Conventional ion exchangers have been used for such applications.1,2 To date, many approaches including adsorption, chemical precipitation, membrane filtration, and chemical coagulation-flocculation were developed to reduce/ remove Cr(VI) ions from aqueous solutions.3−6 Among these approaches, adsorption is an easy-to-operate and cost-effective technique for the removal of Cr(VI) from water and hence has attracted attention in recent years.7 Different types of adsorbent materials have been widely studied for the removal of Cr(VI) from water including activated carbons,4,8 zeolites,9 clay minerals,10 organic resin,11 and chitosan.12 However, conventional adsorbents often show a limited adsorption capacity or no reduction ability because they do not have enough surface area, functional groups, and hydrophilic surface. Polypyrrole (PPy) carries large amounts of positively charged nitrogen atoms in the polymer chains, which render it a good prospect in adsorption application. Wei et al. demonstrated for the first time that PPy films could be used for the reduction of toxic Cr(VI) at 100% efficiency to the environmentally more tractable Cr(III) species.13 The effect of PPy-film electropolymerization conditions and its efficiencys toward the removal of Cr(VI) has also been studied.14,15 Glycine doped polypyrrole was synthesized by in situ polymerization and had the maximum adsorption capacity of 217 mg/g at pH 2.0. The mechanism of Cr(VI) adsorption was © 2014 American Chemical Society
governed by ionic interaction as well as reduction of Cr(VI) to Cr(III).16 In order to improve adsorption capacity, various kinds of nanostructured PPy with large surface area become a rapidly growing field of research. Li et al. prepared bamboo-like PPy nanotubes via a reactive-template vapor phase polymerization approach and demonstrated that the adsorption capacity of bamboo-like PPy nanotubes was much higher than that of traditional PPy nanoparticles.17 Hierarchical porous PPy nanoclusters composed of small PPy nanospheres were successfully prepared in one step by a reactive-template method without any surfactants and showed an excellent ability to remove Cr(VI) ions in aqueous solution. The maximum removal amount of Cr(VI) ions for PPy nanoclusters was 3.47 mmol·g−1 in aqueous solution at pH 5.0.18 Traditionally, bare PPy nanoparticles are easily aggregated in aqueous solutions, which results in low adsorption efficiency and slow kinetics. In order to avoid aggregation of PPy properly and enhance the adsorption capacity, PPy composites were subject to extensive studies by in situ polymerization of pyrrole in the presence of other materials. Li et al. synthesized PPy/ graphene oxide (GO) composite nanosheets by using sacrificial-template polymerization method; the adsorption capacity of the PPy/GO composite nanosheets is about two times as large as that of conventional PPy nanoparticles based on the synergy effect.19 Exfoliated PPy-organically modified montmorillonite clay nanocomposite was prepared via in situ polymerization for adsorption of toxic Cr(VI) from aqueous solution and they have the maximum adsorption capacity of 119.34 mg·g−1 at 298 K.20 PPy/natural or synthetic fiber Received: April 9, 2014 Accepted: May 16, 2014 Published: May 29, 2014 2275
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Figure 1. SEM image of sepiolite (a), low-magnification SEM (b), high-magnification SEM (C), and EDX (d) images of PPy-sepiolite with the sepiolite/pyrrole feeding ratio of 0.4.
Ltd. (China). Sepiolite (Pangel S9) was purchased by TOLSA, S.A (Spain). All chemicals were of analytical grade and used without further purification. 2.2. Preparation of PPy-Sepiolite Nanofibers and PPy Homopolymer. PPy-sepiolite nanofibers were prepared by the chemical oxidation method as follows: the certain amount of the sepiolite was dispersed in 160 mL of 0.5 mol·L−1 hydrochloric acid. Then, 0.8 g pyrrole monomer was added and stirred for 30 min. The polymerization started by introduction of APS solution (2.722 g APS in 40 mL of 0.5 mol·L−1 hydrochloric acid) dropwise. An overnight reaction was allowed to ensure completion of polymerization. The resultant precipitate was filtered and sequentially washed with copious amounts of 0.1 mol·L−1 hydrochloric acid and industrial alcohol until the filtrate was clear. Finally, the product was dried at 50 °C in an oven. PPy-sepiolite nanofibers with different sepiolite/pyrrole weight ratio were prepared according to the aforementioned method. The preparation procedure of PPy homopolymer was similar to that of PPysepiolite except for the addition of sepiolite. 2.3. Removal of Cr(VI) Ions. An aqueous stock solution of Cr(VI) (1000 mg·L−1) was prepared by dissolving potassium dichromate in deionized water. Aqueous Cr(VI) solutions with different concentrations were obtained by diluting the stock solution with water. The pH values of solution were well adjusted by NaOH and HCl. The concentration of Cr(VI) was analyzed by spectrophotometer using 1,5-diphenylcarbazide as
composite adsorbents were also developed for Cr(VI) removal.21−23 Magnetic PPy nanocomposites have attracted attention as emerging materials for the removal of Cr(VI) from water for the ease of separation by the application of an external magnetic field.24,25 Sepiolite is a naturally occurring fibrous phyllosilicate with a large specific surface area (more than 200 m2·g−1). Because of its natural abundance, the dimensions of the isolated fiber, and the silanol-based chemistry of the surface, sepiolite shows excellent adsorption efficiency for heavy metal ions.26,27 In this paper, we report a facile synthetic method at room temperature and application of PPy-sepiolite nanofibers for the removal of Cr(VI) from aqueous solution. Sepiolite can guide the growth orientation of PPy nanofibers and the resulting PPy-sepiolite nanofibers exhibit high efficiency for Cr(VI) removal. The effect of several parameters namely pH, contact time, temperature, initial concentration of Cr(VI) and dose of adsorbent were tested in batch mode. The removal efficiency of the as-prepared PPy-sepiolite nanofibers for Cr(VI) was also compared to that of PPy homopolymer and sepiolite.
2. EXPERIMENTAL SECTION 2.1. Materials. Pyrrole monomer was purchased from Aladdin and stored at −4 °C prior to use. Ammonium persulfate (APS), 1,5-diphenylcarbohydrazide, potassium dichromate, sodium hydroxide, sulfuric acid, and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., 2276
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Figure 2. SEM images of PPy-sepiolite with different sepiolite/pyrrole feeding ratio: (a) 0.1; (b) 0.2; (c) 0.6.
where m is the adsorbent mass (g) and V is the volume of solution (L), and Ct is the concentration of Cr(VI) at any time t (mg·L−1). Sorption isotherms at three different temperatures (25 °C, 35 °C, and 45 °C) were studied by changing the initial concentration of Cr(VI) from 200 mg·L−1 to 400 mg·L−1 at pH 2 while maintaining the adsorbent amount of 1 g·L−1 and contact time of 6 h. The amount of Cr(VI) adsorbed was calculated using eq 3:
the complexing agent at the wavelength of 540 nm (GB 7467− 87).28,29 In order to investigate the effect of pH on removal of Cr(VI) by the PPy-sepiolite nanofibers, adsorption studies were conducted by varying the pH of the Cr(VI) solution from 2.0 to 10.0. After 6 h of contact the solution was centrifuged, and the supernatant liquid was analyzed for residual Cr(VI). The Cr(VI) removal efficiency was determined using eq 1. ⎛ C − Ce ⎞ %removal = ⎜ 0 ⎟ × 100 ⎝ C0 ⎠
qe =
(1)
where C0 and Ce are the initial and equilibrium concentrations (mg·L−1) of Cr(VI), respectively. At optimum pH, the effect of adsorbent dose on the removal of Cr(VI) was studied by varying the dose of adsorbent from 0.25 g·L−1 to 1.5 g·L−1. For the kinetics study, 250 mL of Cr(VI) solution at pH 2 containing 250 mg of PPy-sepiolite nanofibers was placed in a thermo-static shaker and agitated at 200 rpm. An appropriate amount of the solution was taken out at predetermined intervals and centrifuged quickly. The supernatant liquids were used for analyzing Cr(VI) concentration. The adsorption capacity of the PPy-sepiolite qt (mg·g−1) at time t was obtained from eq 2. qt =
⎛ C0 − Ct ⎞ ⎜ ⎟V ⎝ m ⎠
⎛ C0 − Ce ⎞ ⎜ ⎟V ⎝ m ⎠
(3)
where qe is the equilibrium adsorption capacity (mg/g). 2.4. Regeneration Study. To investigate the regenerability and reusability of PPy-sepiolite nanofibers, desorption experiments were conducted in a batch mode. Initially for adsorption of Cr(VI), 0.05 g of PPy-sepiolite nanofibers was treated with 50 mL of 100 mg·L−1 Cr(VI) at pH 2. Desorption of Cr(VI) loaded PPy-sepiolite nanofibers was performed by using 50 mL of 0.1 M NaOH solution. Thereafter, for regeneration of the sorption sites of adsorbent, PPy-sepiolite nanofibers were contacted with 2 M HCl solution. The regenerated PPysepiolite nanofibers were examined for four consecutive adsorption−desorption cycles to verify the reusability of PPysepiolite nanofibers. 2.5. Characterization. The morphology and distribution of elemental components of PPy-sepiolite nanofibers were
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characterized by scanning electron microscopy (SEM) equipped with an energy dispersive analysis system of X-ray spectrometer (EDX) (Nova NanoSEM 430, FEI Company). The concentration of Cr(VI) ions in aqueous solution was measured by UV−vis spectroscopy (UV3600, Shimadzu Corporation).
3. RESULTS AND DISCUSSION 3.1. SEM Analysis. SEM micrographs of sepiolite and PPysepiolite are shown in Figure 1a−c. It is observed that sepiolite fibers are 1−2 μm in length and 20−30 nm in diameter. After in situ polymerization process, PPy is oriented on the fibrous surface of sepiolite, rendering that the diameter of fibers increases. This finding indicated that sepiolite has a guiding role in the growth of PPy-sepiolite nanofibers. Polypyrrole carries some positive charges on the N atoms when it is propagating in the reaction medium. Due to a large amount of Si−OH groups and the negative charges on the sepiolite, positively charged polypyrrole is oriented on the fibrous surface of sepiolite and polypyrrole-sepiolite nanofibers were formed during in situ oxidation polymerization. The main elemental compositions of PPy-sepiolite composite are shown in the EDX results in Figure 1d. The existence of C, O, Cl, Si, and Mg is verified, suggesting it is a composite of PPy and sepiolite. Figure 2 shows the morphologies of PPy-sepiolite synthesized with different weight ratio of sepiolite/pyrrole. It can be seen that all the PPy-sepiolite obtained were nanofibers with high uniformity and a narrow diameter distribution by increasing the sepiolite/pyrrole weight ratio from 0.1 to 0.6. When the sepiolite/pyrrole weight ratio increased, the diameter of prepared PPy-sepiolite nanofibers decreased. 3.2. Removal of Cr(VI). In order to determine which PPysepiolite nanofibers provide better performance toward Cr(VI) removal, some preliminary adsorption experiments for PPysepiolite nanofibers with different sepiolite/pyrrole weight ratio have been conducted. The results showed that the Cr(VI) removal percentage was almost all more than 95 % except for PPy-sepiolite with sepiolite/pyrrole weight ratio of 0.1 using 200 mg·L−1 of Cr(VI) solution at pH 2. PPy-sepiolite nanofibers with sepiolite/pyrrole weight ratio of 0.4 were chose as a model adsorbent for all experiments. 3.3. Effect of pH on Adsorption of Cr(VI). It is wellknown that solution pH is one of the most important parameters in the heavy metal adsorption process. The sorption efficiency of Cr(VI) onto PPy-sepiolite nanofibers, PPy homopolymer, and sepiolite as a function of pH (2−10) was studied and results are presented in Figure 3. As shown in Figure 3, 96.63 % removal efficiency for PPy-sepiolite nanofibers was observed compared to 61.95 % for PPy homopolymer and 20.97 % for sepiolite at pH 2. This result suggested that PPy-sepiolite nanofibers have high removal efficiency for Cr(VI) compared to PPy homopolymer and sepiolite. It is also noticed that the Cr(VI) ion removal efficiency by PPy-sepiolite nanofibers increased with an decrease in solution pH from 10 to 2. Generally, the predominant species of Cr(VI) in aqueous solutions at lower pH (2−6) is HCrO4− ions. In the case of PPy homopolymer, it has been reported that the adsorption process occurred via the ion exchange property of PPy by replacing the doped Cl1− ions with HCrO4− ions.24 With increase of pH value, the decrease of removal efficiency is due to the competitive interaction between the OH− ions and HCrO4− or CrO42− ions on the adsorbent
Figure 3. Effect of pH on the adsorption of Cr(VI) by PPy-sepiolite nanofibers, PPy homopolymer and sepiolite (adsorbent dose 1g·L−1; initial Cr(VI) 200 mg·L−1; time 6 h).
sites. Based on these observations, all further adsorption experiments were performed at pH 2. 3.4. Adsorption Kinetics. The effect of sorption time on the removal of Cr(VI) by PPy-sepiolite nanofibers are shown in Figure 4a. It clearly indicates that the amount of Cr(VI) removal increases with increasing contact time until equilibrium
Figure 4. (a) Adsorption kinetics curves and (b) pseudo-second-order and pseudo-second-order kinetic plots for the adsorption of Cr(VI) by PPy-sepiolite nanofibers. (PPy-sepiolite dose 1g·L−1; initial Cr(VI) 200 mg·L−1; pH 2). 2278
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Figure 5. (a) Plot of adsorption capacity against initial Cr(VI) concentration; Langmuir adsorption isotherm (b), Freundlich adsorption isotherm (c), and Temkin isotherm (d) for Cr (VI) ions adsorption on the PPy-sepiolite.
Table 1. Parameters of the Langmuir, Freundlich, and Temkin Isotherm Models Langmuir constants
Freundlich constants
Temkin constants
T/K
qm/ (mg·g−1)
b/ (L·mg−1)
R2
Kf/ (mg·g−1)
n
R2
A/ (L·g−1)
B/ (kJ·mol-−1)
R2
298 308 318
302.11 337.84 353.36
0.17 0.48 1.81
0.998 0.998 0.999
148.28 203.66 273.00
6.97 8.68 16.02
0.994 0.992 0.986
127.79 201.06 274.80
34.22 29.97 18.46
0.993 0.989 0.976
with the experimental values of qe (193.26 mg·g−1), indicating that the adsorption kinetics accords well with the pseudosecond-order model. 3.5. Adsorption Isotherm. To investigated the capacity and thermodynamics of Cr(VI) adsorption by PPy-sepiolite nanofibers, adsorption isotherms were studied at three different temperatures (25 °C, 35 °C, and 45 °C) and are shown in Figure 5a. The adsorption capacity of PPy-sepiolite nanofibers increases with increase in temperature, which indicates that the adsorption process is endothermic. Langmuir, Freundlich and Temkin isotherm models were extensively used to investigate the isotherm data. The three linear isotherm equations are expressed as the following:
is established between the solid phase and liquid phase in the adsorption system. In order to understand better the adsorption behaviors, pseudo-first-order and pseudo-secondorder equations were applied to fit the kinetic data: t 1 t = + qt qe k 2qe 2 log(qe − qt) = log qe −
(4)
k1 t 2.303
(5) −1
where k2 is the rate constant (g·mg min), k1 is the pseudofirst-order rate constant (min−1). Figure 4b depicts the plots of the linearized forms of the two kinetic models. It is observed that the correlation coefficient for pseudo-second-order kinetic model (0.998) is greater than that of pseudo-first-model (0.961). Moreover, the calculated values of qe (196.08 mg·g−1) for pseudo-second-order kinetic model are in good agreement
Langmuir: 2279
Ce C C = e + e qe qmb qm
(6)
dx.doi.org/10.1021/je500319a | J. Chem. Eng. Data 2014, 59, 2275−2282
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Article
1 ln Ce n
(7)
Temkin: qe = A + B ln Ce
(8)
where qm and b are the Langmuir constants related to the adsorption capacity (mg·g−1) and the rate of adsorption (L· mg−1), respectively; kF and n are the Freundlich isotherm parameters related to adsorption capacity (mg·g−1) and intensity of adsorption, respectively; A and B are Temkin isotherm constants. The linearized Langmuir, Freundlich, and Temkin isotherms for the three different temperatures are shown in Figure 5b−d. The Langmuir, Freundlich and Temkin isotherm parameters calculated from the intercept and slope of the linear equations are summarized in Table 1. The higher values of correlation coefficient reveal that Langmuir model fitted well the isotherm data compared to other two models, suggesting a monolayer adsorption for the uptake of Cr(VI) on the surface of PPy-sepiolite nanofibers. The maximum adsorption capacity was increased from 302.11 to 353.36 mg· g−1 with an increase of temperature from 25 °C to 45 °C. Table 2 summarizes the maximum adsorption capacity and optimum pH value for the removal of Cr(VI) of various
Figure 6. Plot to determine thermodynamic parameters of Cr(VI) adsorption onto PPy-sepiolite nanofibers.
Table 3. Thermodynamic Parameters for Cr(VI) Adsorption by PPy-Sepiolite Nanofibers
Table 2. Comparison of Adsorption Capacity of the PPySepiolite Nanofibers with Other PPy Based Adsorbents for Cr(VI) Removal qm/(mg·g−1)
pH
T/K
glycine doped polypyrrole porous PPy nanoclusters
217 180.4
2.0 5.0
PPy/Fe3O4 nanocomposites PPy/wood sawdust PPy-OMMT NC3 polypyrrole-polyaniline nanofibers polyacrylonitrile/polypyrrole nanofiber PPy-sepiolite nanofibers
169.4 3.4 119.3 227
2.0 5.0 2.0 2.0
298 not given 298 298 298 298
24 31 20 30
61.8
2.0
298
23
302
2.0
298
present work
adsorbents
ref
T/K
ΔG0/(kJ·mol−1)
ΔH0/(kJ·mol−1)
ΔS0/(kJ·mol−1·K−1)
298 308 318
−5.183 −6.155 −10.195
33.06
0.128
ΔH0 reveals the endothermic nature of adsorption. The positive value of ΔS0 indicates the increase in randomness at the adsorbent -solution interface due to the release of Cl− ions which are present on the surface of PPy-sepiolite nanofibers.30 3.7. Effect of PPy-Sepiolite Nanofibers Dose. Figure 7 indicates the effect of PPy-sepiolite nanofibers dose on the
16 18
Polypyrrole-based adsorbents reported in the literature. It can be found that the adsorption capacity of PPy-sepiolite nanofibers of the present study is higher than that of other adsorbents. The results indicate that PPy-sepiolite can be a promising adsorbent for water treatment contaminated with Cr(VI). 3.6. Thermodynamic Investigations. Thermodynamic parameters for the adsorption such as free energy change (ΔG0), enthalpy change (ΔH0), and entropy (ΔS0) were calculated by the following equations: ΔG 0 = −RT ln Kc
(9)
ΔS 0 ΔH 0 − (10) R RT −1 −1 where R (J·mol ·K ) is the ideal gas constant, T (K) is the absolute temperature and Kc (L·mol−1) is the thermodynamic equilibrium constant. The values of ΔH0 and ΔS0 were calculated from the slope and intercept of the plot (Figure 6) of ln Kc versus 1/T. The values of ΔG0, ΔH0, and ΔS0 are summarized in Table 3. With an increase in temperature negative values of change in ΔG0 are obtained which indicate the spontaneity of the adsorption process. The positive value of
Figure 7. Effect of adsorbent dose on the removal of Cr(VI) by PPysepiolite nanofibers (initial Cr(VI) 200 mg·L−1; pH 2; time 6 h).
ln Kc =
removal efficiency of Cr(VI) from aqueous solution. It is found that the removal efficiency changed from 56.2 % at a dose of 0.25 g·L−1 to 99.4 % at a dose of 1.25 g·L−1. The reason is due to an increase in surface area and availability of active sites for adsorption with the increase of adsorbent dose. Further addition of PPy-sepiolite nanofibers did not significantly change 2280
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the removal efficiency of Cr(VI) because the Cr(VI) concentration after adsorption becomes limiting. 3.8. Regeneration Studies. Regeneration of PPy-sepiolite nanofibers was studied by performing an adsorption− desorption experiment for four consecutive cycles. Only 5.64 % of the adsorbed Cr(VI) was desorbed in the first cycle of desorption. A lower desorption efficiency was also observed for polypyrrole-organically modified montmorillonite clay nanocomposite and polypyrrole/Fe3O4 nanocomposite.20,25 This is due to the reduction of adsorbed Cr(VI) to Cr(III) by PPy which could not be desorbed upon treatment with NaOH solution.20,25 The removal efficiency for Cr(VI) of PPy-sepiolite nanofibers in each cycle of adsorption is shown in Figure 8. It is
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AUTHOR INFORMATION
Corresponding Author
*Tel/Fax: +86 555 2311822 E-mail:
[email protected]. Funding
This work was supported by the National Natural Science Foundation of China (No. 21207001), the Natural Science Fund of Anhui Provincial Education committee of P. R. China (KJ2012Z036), Anhui Province College Outstanding Young Talent Fund of P.R. China (2012SQRL032ZD), and National Undergraduate Training Programs for Innovation and Entrepreneurship (201310360026). Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 8. Regeneration of PPy-sepiolite for four cycles.
observed that the removal efficiency (100 %) of PPy-sepiolite nanofibers remained almost same for the three cycles and in the subsequent forth cycle removal efficiency decreased to 95.5 %. Therefore, the PPy-sepiolite nanofibers can be successfully reused for three adsorption cycles without any loss of its removal efficiency.
4. CONCLUSIONS Sepiolite is a natural resource and has a fibrous structure with a large specific surface area. Due to its unique structure, environmental friendliness, and cost effectiveness, this raw material is combined with PPy by in situ polymerization to obtain a novel adsorbent for efficient Cr(VI) removal from aqueous solution. Sepiolite has a guiding role in the growth of PPy-sepiolite nanofibers. Compared with PPy homopolymer and sepiolite, PPy-sepiolite nanofibers exhibit higher efficiency for Cr(VI) removal. The optimum pH for the maximum removal of Cr(VI) by PPy-sepiolite nanofibers was found at pH 2. The adsorption isotherms were well described by Langmuir models and the maximum adsorption capacity increased with the increase of temperature. Thermodynamic study suggested that the adsorption process is spontaneous and endothermic. Adsorption kinetics followed the pseudo-second-order kinetic model. PPy-sepiolite nanofibers adsorbent can be regenerated and reused for four consecutive cycles without obviously loss of its removal efficiency. Therefore, PPy-sepiolite nanofibers could be useful material in water treatment contaminated with Cr(VI). 2281
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dx.doi.org/10.1021/je500319a | J. Chem. Eng. Data 2014, 59, 2275−2282
