Research Article pubs.acs.org/journal/ascecg
Evaluation of Mechanism on Selective, Rapid, and Superior Adsorption of Congo Red by Reusable Mesoporous α‑Fe2O3 Nanorods Debabrata Maiti,† Soumita Mukhopadhyay, and Parukuttyamma Sujatha Devi* Sensor and Actuator Division, CSIR-Central Glass & Ceramic Research Institute, 196, Raja SC Mullick Road, Jadavpur, Kolkata 700 032, India S Supporting Information *
ABSTRACT: A large scale synthesis of mesoporous hematite (α-Fe2O3) nanorods with a high surface area of 98 m2/g and an average pore size of ∼26 nm was used for adsorption studies for pollutant dye removal. The nanorods exhibited rapid, superior, and selective adsorption efficiency toward Congo red, an organic dye present in wastewater. Highly selective adsorption capability of the mesoporous α-Fe2O3 nanorods has been attributed to the presence of abundant surface active sites with porous networks which make it highly water dispersible facilitating the formation of H-bonding and coordination effect between the -NH2 group of Congo red with its surface -OH groups and Fe3+, respectively. Adsorption studies concerning the effect of contact time, initial dye concentration, dosage of adsorbent, and effect of pH on adsorption kinetics were explored in addition to the desorption process investigation regarding the effect of solution pH from acidic to alkaline. To unravel the unresolved phenomenon toward selective adsorption of Congo red by mesoporous α-Fe2O3 nanorods beyond conventional factors (viz., surface area/porosity, electrostatic interaction, and so on), investigation was also carried out by varying the nature of the different dye molecules as well as the phase and morphology of the α-Fe2O3 nanomaterials. KEYWORDS: α-Fe2O3 nanorods, Mesoporous, Selective adsorption, Congo red (CR), Hydrogen bonding, Coordination effect
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INTRODUCTION Polluting organic dyes commonly used in printing, textile, photographic, paper-pulp, tannery, and paint industries are the major water contaminants which cause severe damage to the environment. A large number of processes are used for wastewater treatment such as chlorination, ozonation, adsorption, reverse osmosis, ultrafiltration, electrochemical process, advanced oxidation processes such as Fenton/photo-Fenton reactions, and photodegradation of dye molecules through photocatalysis, etc.1−3 In our previous studies, we have reported photocatalytic degradation process as a means for the removal of dyes from wastewater.4−7 However, among various processes available for water pollutants removal, adsorption is the most expedient and well-established method because of its ease, high efficiency, and low energy requirements. Various nanomaterials with specific morphology and structure have been extensively studied as adsorbents to remove organic dyes.8−12 In recent years, porous nanostructured metal oxides have been extensively used for wastewater treatment exhibiting interesting adsorption performance13−16 because of their high surface area, large surface-to-volume ratio, and available surface active sites for intimate contact with adsorbates. Adsorption of dyes onto the adsorbents occurs via electrostatic interaction, hydrogen bonding, coordination effect, surface/pore diffusion depending on the functional groups, surface properties, and morphological diversity of both dye and adsorbents. Hematite (α-Fe2O3), a © 2017 American Chemical Society
low band gap semiconducting metal oxide, is the most stable form of iron oxide, which can be synthesized under ambient conditions by nontoxic and ecofriendly methods. There are many reports on the preparation of porous α-Fe2O3 with mesoporous structures using soft17,18 and hard19,20 template methods. However, soft template technique unavoidably resulted in amorphous frameworks, and hard templating techniques are usually obscured and time-consuming, which results in high costs and hinders the practical application of αFe2O3. Nevertheless, nontemplate techniques such as precursor calcination of hydroxides, oxyhydroxides, and carbonates to fabricate porous nanostructures have additional benefits in terms of large scale and cost-effective production. Several highlighted works have been reported on the removal of Congo red (CR) by α-Fe2O3 exhibiting different morphologies. For example, Yu et al. reported template free preparation of mesoporous α-Fe2O3 exhibiting a maximum adsorption capacity of 53 mg g−1.21 Wei et al. developed hollow nest-like α-Fe2O3 spheres which showed a maximum adsorption capacity of 160 mg g−1, where urea was used as a soft template.22 The Fei group prepared hierarchical urchin-like α-Fe2O3, which displayed a maximum adsorption capacity of 66 mg g−1.23 Received: May 28, 2017 Revised: September 7, 2017 Published: November 1, 2017 11255
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solutions were kept in the dark at the same temperature without any perturbation. Subsequent to the adsorption processes, the adsorbents were conveniently separated by centrifugation at 10,000 rpm and the absorbance spectra of the supernatant was immediately checked by UV−vis spectrophotometer (UV-2550, Shimadzu, Japan). To evaluate the adsorption capacity, the dosage concentration of the adsorbent (0.2, 0.5 and 1 g L−1), the concentration of the initial dye solution (20, 40, 60, and 80 mg L−1), and the pH of the dye solution (3, 7.5, and 10) were also varied. The pH of the dye solution was adjusted by means of 0.1 M HCl and 0.1 M NaOH solutions, and the resultant pH of the solution was measured using a pH meter (Sartorious). In order to compare the ability of mesoporousα-Fe2O3 nanorods (NRs) as an adsorbent and the mechanism behind its activity, the adsorption study was also executed using nonporous αFe2O3 nanoparticles (1 g L−1) and α-FeOOH nanomaterial. In order to assess the adsorption capacity toward Congo red, the dosage concentration of the α-FeOOH (1 g L−1) was also assorted by varying the concentration of the initial dye solutions (80, 120, 160, and 200 mg L−1). Moreover, the investigation was further extended to explore its selectivity in dye adsorption by varying the ionic nature of the dye molecules, namely, MB and MO. Desorption Studies. Desorption study was conducted for the Congo red adsorbed hematite porous nanorods (where adsorption study was conducted at pH 6.5) using citrate buffer of pH ∼ 3, phosphate buffer of pH ∼ 7.5, and carbonate/bicarbonate buffer of pH ∼ 10, keeping the other experimental conditions constant. A 15 mL aliquot of each buffer solution was used for the desorption of CR from CR adsorbed NRs (∼15 mg), and the experiments were carried out on an incubator shaker with a shaking speed of 180 rpm for 4 h in the dark. After the desorption processes, the adsorbents were completely separated by centrifugation at 10,000 rpm and the absorbance spectra of the supernatant were instantly checked by UV−vis spectrophotometer. Surface Charge Analysis. The surface charge was measured by ζ potential measurements. The bare hematite nanorods and nanorods after dye desorption were dispersed in aqueous medium to measure the ζ potential. The bare hematite nanorods are designated as Fe-R whereas dye desorbed hematite nanorods are designated as Fe-R3 (dye desorbed at pH 3), Fe-R7 (dye desorbed at pH 7.5), and Fe-R10 (dye desorbed at pH 10), respectively, throughout this work.
Though there are many reports on the adsorption of Congo red by α-Fe2O3nanoparticles,21−23 there is, still, lack of information on the mechanism of adsorption of such nanoparticles toward the selective adsorption of Congo red. Systematic studies on the adsorption characteristics and selectivity toward organic dyes are also technologically promising in water onslaught. It is highly demanding to synthesize a stable efficient nanoadsorbent via cost-effective ecofriendly technique to exert strong selectivity toward environmental remediation applications. In the present work, we have synthesized mesoporous αFe2O3 nanorods with high surface area and well-controlled pore structure by an improved processing condition compared to our previously reported works.24,25 In our previous work, we have reported the formation of various forms of Fe2O3 and αFeOOH by carefully controlling the addition of hydrazine to ferric nitrate.24 After heat treatment of α-FeOOH at 350 °C for 4 h the formation of mesoporous α-Fe2O3 nanorods was noticed. The synthesized porous α-Fe2O3 nanorods exhibited excellent adsorption capacity to remove Congo red from wastewater selectively at a very fast rate. We noticed that the mesoporous α-Fe2O3 nanorods have the ability to adsorb 81% of CR within 2 min. In order to explore this unusual adsorption capability and the mechanism behind the strong adsorption of CR onto mesoporous α-Fe2O3 nanorods, we have systematically investigated the adsorption of another anionic dye, methyl orange (MO), and a cationic dye, methylene blue (MB). Dye adsorbing capability of the porous nanorods was also compared with a nonporous particle such as α-Fe2O3 nanostructure and α-FeOOH nanomaterial. The effect of pH on the desorption of Congo red was also investigated to disclose the adsorption mechanism in addition to the effect of contact time, initial dye concentration, dosage of adsorbent, and effect of pH on adsorption kinetics of mesoporous α-Fe2O3 nanorods. To the best of our knowledge, this is the first report where a mechanism has been proposed for the selective adsorption of Congo red (CR) by a porous oxide nanomaterial. This mechanism will open a new strategy for researchers in designing materials for removing various dye molecules from wastewater to keep the environment clean and healthy.
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CHARACTERIZATIONS In order to confirm the oxide phases formation, X-ray diffraction measurements were carried out on a X’Pert Pro MPD X-ray diffractometer (PANalytical) system with Cu Kα radiation (λ = 1.5406 Å). Raman spectra were acquired using a RenishawInVia Reflex micro-Raman spectrometer with an excitation of argon ion (514 nm) laser, and the spectra were collected with a resolution of 1 cm−1. The morphology was studied on a Supra 35 VP (Carl Zeiss) field-emission scanning electron microscope (FESEM) and FEI Tecnai G2 transmission electron microscope (TEM). The dye removal efficiency was calculated from the UV−visible absorption spectroscopy measurements carried out on a Shimadzu UV− vis−near-IR spectrophotometer, UV-3600. The ζ potential was measured on a Horiba dynamic laser scattering particle analyzer (Model-SZ-1000Z). Nitrogen adsorption−desorption measurements were carried out at 77 K on a Quantachrome (ASIQ MP, Quantachrome, Boynton Beach, FL, USA) instrument. The powders were degassed under vacuum at 250 °C for 4 h for the measurement. The surface area was obtained using Brunauer− Emmet−Teller (BET) method within the relative pressure (P/ Po) range of 0.05−0.20, and the pore size distribution (PSD) was calculated by Barret−Joyner−Halenda (BJH) method. The nitrogen adsorption volume at P/Po = 0.99 was used to determine the pore volume. Fourier transform infrared (FT-IR) spectra were recorded between 4000 and 400 cm−1 on a
EXPERIMENTAL SECTION
Materials. Ferric nitrate nonahydrate [Fe(NO3)3·9H2O] GR and ethanol GR were purchased from Merck, Germany. Hydrazine monohydrate (N 2H4·H2O), hydrochloric acid (HCl), sodium hydroxide (NaOH), citric acid (C 6 H 8 O 7 ), trisodium citrate (Na3H8O7), sodium biphosphate (Na2HPO4), and sodium bicarbonate (NaHCO3) were purchased from Merck, India. Synthesis of Hematite Nanomaterials. The synthesis of goethite and maghemite have been reported in our previous work.24,25 In short, on simultaneous and dropwise addition of N2H4· H2O to100 mL of 0.1 M Fe(NO3)3 aqueous solution, goethite (αFeOOH) and maghemite (γ-Fe2O3) were formed in the absence of any template. The as-prepared α-FeOOH and γ-Fe2O3 powders were calcined at 350 and 500 °C, respectively, in a muffle furnace for 4 h at a heating rate of 1 °C/min in air in separate silica crucibles. After heat treatment, the light yellow goethite and brownish black maghemite were transformed into red powder as shown in Supporting Information Figure S1. Experiments on Adsorption. Adsorption study for the removal of CR, MO, and MB as model organic water pollutants of concentration of 20 mg L−1were carried out by employing hematite porous nanorods (1 g L−1). All the adsorption experiments were performed at 27 ± 3 °C, in an incubator shaker (Rivotek) with a shaking speed of 180 rpm and at the ambient pH of the dye solution (pH ∼ 6.5) without using any external acid/base. Subsequently, all the 11256
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ACS Sustainable Chemistry & Engineering NICOLET 380 FTIR spectrometer. Samples were prepared by mixing the powder with KBr in 1:4 ratios by weight and compressing the mixture into a pellet. The spectra were observed after integration over 200 individual scans.The thermogravimetric analysis (TGA) was carried out on the asprepared samples on a NETZSCH STA409C instrument to determine the decomposition nature of the precursor powder samples in the temperature between room temperature and 1000 °C at a heating rate of 10 °C/min in an air flow. X-ray photoelectron spectroscopic measurements were carried out in a PHI 5000 Versa probe II scanning XPS microprobe (ULVAC-PHI, USA). The measurements were carried out at room temperature and at a base pressure better than 6 × 10−10 mbar. All the spectra were acquired with monochromatic Al Kα (hν = 1486.6 eV) radiation with a total resolution of about 0.7 eV and a beam size of 100 μm.
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RESULTS AND DISCUSSION The crystal phase of the red powder obtained on heat-treating α-FeOOH at 350 °C for 4 h was identified by XRD analysis which confirmed it to be hematite.24,25 The XRD pattern of αFe2O3 nanoparticles obtained from the heat treatment of γFe2O3 are shown in Figure S2. We have also carried out Raman spectroscopic study to further confirm the formation of hematite as shown in Figure S3. The Raman spectra exhibited six peaks at 225, 245, 292, 408, 493, and 608 cm−1 due to transversal optical (TO) modes of hematite.25 The peaks at 245 and 493 cm−1 were attributed to the A1g modes. The rest of the four peaks at 225, 292, 408, and 608 cm−1 were assigned to the Eg mode of the α-Fe2O3 phase.26 To extensively investigate the morphology of the nanostructures, we have carried out FESEM and TEM analyses as shown in Figure 1. The morphological images in Figure 1a−e reveal that phase transformation from α-FeOOH (precursor) to αFe2O3 does not alter the rod-like nanostructures at this temperature. TEM bright field images as shown in Figure 1c−e clearly show that the nanorods were of ∼235 nm average length and ∼40 nm average diameter. The reduced aspect ratio may be due to the shrinkage in both longitudinal and lateral direction during the thermal treatment. In addition, there are no side branches and cross-linking between the nanorods, which is a major advantage of a single nanorod for various specific applications. Moreover, huge numbers of disordered uniform mesopores were generated in nanorods, and this could be due to the removal of water from α-FeOOH as has been found earlier.24 The high resolution TEM (HRTEM) image is presented in Figure 1f. The HRTEM confirms the formation of well-developed lattice fringes confirming the single crystalline nature of the individual particles. The distance between adjacent lattice fringes measured as 0.272 nm (Figure 1f) corresponds to the (104) reflection of α-Fe2O3. The presence of mesopores is also evident in the HRTEM image of nanorods (indicated by circles). FESEM and TEM bright field images of α-Fe2O3 nanoparticles are shown in Figure 2. The porous structures of the as-prepared α-Fe2O3 nanorods were further analyzed by nitrogen adsorption−desorption isotherms and BJH pore size distribution as presented in Figure 3a. The BET surface area and the total pore volume of the nanorods were ∼98 m2 g−1 and 0.652 cm3 g−1, respectively. As observed in the figure, α-Fe2O3 nanorods exhibit type IV isotherms with H3hysteresis loops at the high relative pressure region (P/Po = 0.9−1), implying the origin of mesoporous structure due to the dehydration of water from α-FeOOH nanorods. This result
Figure 1. (a, b) FESEM, (c−e) TEM bright field images, and (f) HRTEM image of α-Fe2O3 nanorods.
Figure 2. (a, b) FESEM and (c, d) TEM bright field images of αFe2O3 nanoparticles.
agrees well with the TEM images also. The sample exhibited a broad distribution of pores with an average pore diameter of 26 nm (inset, Figure 3a). The strong intense XRD peak in the lowangle region (Figure 3b) at 2θ ∼ 1.03° also claims that the material is mesoporous. In contrast, α-Fe2O3 nanoparticles exhibited a surface area and total pore volume of ∼19 m2 g−1 11257
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Figure 3. (a) N2 adsorption−desorption isotherm of α-Fe2O3nanorods and nanoparticles. Inset shows corresponding BJH desorption, dV/dD, pore volume vs pore diameter curves. (b) Low-angle XRD of mesoporous α-Fe2O3nanorods.
Figure 4. Rapid adsorption of CR (20 mg L−1) by α-Fe2O3nanorods (1g L−1).
Figure 5. (a) Removal percentages of different concentrations of CR with time, (b) adsorption isotherm, (c) linearized Langmuir and (d) linearized Freundlich isotherms, (e) pseudo-first-order kinetic plot, and (f) pseudo-second-order kinetic plot for the adsorption of CR by α-Fe2O3 nanorods.
and 0.031 cm3 g−1, respectively, and no hysteresis was observed in the isotherm indicating the absence of any kind of porosity.
It was also confirmed by the BJH pore size measurement shown in the inset of Figure 3a. 11258
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ACS Sustainable Chemistry & Engineering Table 1. Comparison of Adsorption Capacities toward CR on Various Adsorbents
a
sample
BET (m2/g)
adsorption capacity (mg g−1)
ref
mesoporous α-Fe2O3 Fe3O4 hollow nest-like α-Fe2O3 sphere hollow urchin-like α- FeOOH nanostructures hierarchical urchin-like α-Fe2O3 porous hierarchical MgO α-Fe2O3 nanoparticles and nanowhiskers α-Fe2O3 hollow structures carbon nanofiber @graphite felt-240 α-Fe2O3Nanorods magnetic carbon nanosheet α-Fe2O3 porous nanorods α-FeOOH nanorods
111 34.56 152.4 239 69 148 164.1 32.9 144.2 22.78
53 28.46 160 239 66 2340 253.8 93.54 733.2a 78 724 57.2 160
21 8 22 23 23 43 44 45 46 47 48 this study this study
98 39
The unit of adsorption capacity is expressed in kg m−3.
Adsorption of Dye Molecules by α-Fe2O3 Nanomaterials. Owing to the high adsorption capabilities of hematite nanomaterials as reported earlier,27−29 we have evaluated the adsorption capacity of mesoporous α-Fe2O3 NRs for the removal of CR, MO, and MB as model organic water pollutants. The chemical structures of different dye molecules have been shown in Figure S4. Figure 4a shows the UV−visible absorption spectra of Congo red solution (initial dye concentration fixed at 20 mg L−1) as a function of experimental time using 1 g L−1 adsorbent concentration of porous α-Fe2O3 NRs. From the spectra it is clear that the porous α-Fe2O3 nanorods are capable of removing 81% of CR within 2 min; with increasing time, even up to 95% could be removed within 15 min. The kinetic plot for the adsorption of CR (20 mg L−1) by α-Fe2O3 nanorods (1 g L−1) has been shown in Figure 4b. The high adaptability of α-Fe2O3 nanorods in removing CR could be largely attributed to the highly porous 1D structure. The absorption spectra and the inset photograph (inset, Figure 4b), confirm almost complete removal of CR from the water by the porous α-Fe2O3 nanorods with loading amount of 1 g L−1 within 15 min. High dispersibilty of nanorods in CR solution as shown in the digital image implies the occurrence of an excellent chemistry between them. The digital image depicts that, after adsorption of the entire Congo red, the maximum amount of dispersed α-Fe2O3nanorods had settled down. In contrast, very poor adsorption was observed for the cationic dye, MB, and the other anionic dye, MO. Nonporous α-Fe2O3 nanoparticles (1 g L−1) also exhibited poor adsorption capability during the removal of CR (20 mg L−1), keeping the other experimental parameters constant as shown in Figure S5. Moreover, it was interesting to observe that the α-Fe2O3 nanorods were not getting dispersed in the MB and MO solutions (Figure S5d) compared to that in CR solution (inset, Figure 4b). Furthermore, apart from rapid adsorption, α-Fe2O3 nanorods were capable of removing a high concentration of CR as elucidated in Figure 5a. This exceptional behavior of nanorods toward the adsorption of CR motivated us to explore and reveal a mechanism behind their strong affinity. Adsorption Isotherm. Adsorption isotherm studies are essential for the evaluation of adsorption capacity of the adsorbent for a selective contaminant. Thus, a series of experiments were carried out by varying the dose of hematite nanorods with the initial Congo red concentration fixed at 20 mg L−1 as shown in Figure S6. After optimizing the adsorbent concentration (1 g L−1), initial concentrations of Congo red
were varied to determine its maximum adsorption capacity (qm). Both Langmuir and Freundlich adsorption isotherm models were employed for the adsorption analysis. The Langmuir isotherm model was used to describe homogeneous systems (eq 1). Such a model was exploited to represent the relationship between the amount of dye adsorbed at equilibrium (q e , mg g −1 ) and the equilibrium solute concentration (Ce, mg L−1), while monolayer formation occurs on adsorbate surface sites. The equation is as follows: qm = qebCe/(1 + bCe)
(1)
where qm (mg g−1) is the maximum adsorption capacity corresponding to complete monolayer coverage and b is the Langmuir equilibrium constant (L mg−1). The monolayer saturated adsorption capacity (maximum adsorption capacity, Figure 5b) of the α-Fe2O3 porous nanorods was found to be 57.2 mg g−1 (Table 1). As shown in Figure 5c, the experimental data fit well with the Langmuir adsorption isotherm with a correlation coefficient, R2,of 0.99348 (Table S1). The Freundlich isotherm model was employed to describe heterogeneous systems. The mathematical expression (eq 2) of the Freundlich isotherm model is ln qe = 1/n ln Ce + ln KF
(2)
KF and n are Freundlich constants corresponding to adsorption capacity (L g−1) and adsorption intensity, respectively. The values of KF and n can be calculated by a plot of ln qe against ln Ce. The curve-fitting result is shown in Figure 5d, and R2 of the Freundlich equation is 0.91605, where the value of KF is 19.035 and n is 2.96 (Table S1). R2 > 0.99, as shown in Table S1, indicated that the adsorption data fit the Langmuir isotherm model better than the Freundlich isotherm model. The Langmuir equilibrium constant, b, offers information about the attractive interaction between the adsorbent and adsorbate. The adsorbate−adsorbent affinity is quantified using a dimensionless separation factor RL, and it can also be used to define a favorable parameter, Kc0.29,30 It has been established that the adsorption of CR on α-Fe2O3 nanorods follows the Langmuir isotherm model well and the calculated value of b = 0.409 L mg−1 (Table S1). The magnitude of RL and the favorable parameter Kc0 of 0.029 and 32.78, respectively, further ensure the highly favorable nature of the adsorption process.29,30 11259
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concentration of 20 mg L−1, very fast adsorption was observed (within 2 min) and after that qt becomes almost constant, indicating a rapid adsorption in 2 min (Figure 6a, inset). With an increase of the initial dye concentration, it requires more time to reach the equilibrium state. It was noticed that qt increases abruptly within the first 15 min and continued up to 60 min, and then it reaches equilibrium after 2 h. Thus, qt becomes constant for different concentrations of CR after 60 min, beyond which no more dyes were adsorbed even on prolonging the contact time. Surface Diffusion Model Analysis. Adsorption process and attainment of equilibrium state basically occur due to (i) bulk diffusion where adsorbate diffuses from bulk solution to the boundary layer of the liquid film adjacent to the adsorbent, (ii) film/surface diffusion where adsorbate diffuses through the liquid film, and (iii) intraparticle diffusion where adsorbent carry the available pore sites since pores augment the surface area and consequently enhance the film/surface diffusion. Here, bulk diffusion can be neglected as a sufficient speed of shaking was used during the adsorption. Weber and Morris developed a kinetic based model that symbolizes the time dependent intraparticle diffusion of adsorbate.31 The kinetic model is expressed in eq 5,
Adsorption Kinetics. Adsorption kinetics depicts the solute uptake rate prevailing in the residence time of the adsorption reaction and also defines the efficiency of adsorption. In order to investigate the kinetics of adsorption processes of CR on the adsorbents, pseudo-first-order and pseudo-second-order kinetics were tested. Pseudo first-order (eq 3) and pseudo-second-order (eq 4) models are presented as follows: log(qe − qt ) = log qe −
k1 t 2.303
(3)
t 1 t = + 2 qt qe k 2qe
(4) −1
where qe and qt are the amount (mg g ) of Congo red adsorbed on α-Fe2O3nanorods at equilibrium and at different times, t (min); k1 and k2 are the rate constants of the pseudofirst-order and pseudo-second-order models, respectively. For the pseudo-first-order model, the values of k1 and qe are calculated from the slope and intercept of plots of log(qe − qt) against t as shown in Figure 5e. The values of k2 and qe are calculated from the plot of t/qt againstt (Figure 5f). The best-fit values of qe and k2 along with correlation coefficients (R2) of the pseudo-second-order model for the adsorption of various concentrations of CR by mesoporous α-Fe2O3 nanorods are shown in Table 2, and the corresponding plots are given in
qt = kint 1/2 + C
where kin is the intraparticle diffusion rate constant (mg g−1 min−1/2) and C is the intercept (mg g−1), which confers an idea about the thickness of the boundary layer. The larger the intercept C, the greater the contribution of the film/surface adsorption in the rate controlling step. If C = 0, it indicates that film/surface diffusion or boundary layer diffusion is negligible and the intraparticle diffusion is the only rate controlling step. In linear plots qt vs t1/2 for different initial dye concentrations, the values of C ≠ 0 and different values of kin indicate the different diffusion processes (Figure 6b, Table S3). In order to investigate the exact diffusion process, it is useful to quantify the relative intraparticle diffusion parameter (β).31 In all cases we observe that β was more than 1, which demonstrates that the fastest diffusion process corresponds to film/surface diffusion (Table 3). Indeed the large values for β indicate high intraparticle diffusion resistances, and hence high pore resistance in all cases.32 It is observed that the value of β is 66.7 for maximal adsorption. Hence, the observed high β value clearly indicates that the adsorption of CR on α-Fe2O3 nanorods is a surface diffusion phenomenon and not intraporous diffusion.
Table 2. Second-Order Kinetic Parameters for the Removal of CR by α-Fe2O3 Nanorods CR concn (mg L−1) 20 40 60 80
k2 (g mg−1 min−1) 9.36 2.86 2.26 1.55
× × × ×
−2
10 10−3 10−3 10−3
qe, cal (mg g−1)
qe, exp (mg g−1)
R2
19.4 37.4 55.1 56.8
18.7 34.8 49.6 50.4
0.9984 0.9988 0.9908 0.9992
(5)
Figure 5f. R2 values are all above 0.99, indicating a good fit of the pseudo-second-order model compared tothe pseudo-firstorder model (Table S2). Similarly, the values of calculated equilibrium adsorption capacities (qe,cal) showed good agreement with the experimental equilibrium q values (qe,exp). Effect of Initial Dye Concentration and Contact Time. The plot qt vs t indicates that the value of qt increases with an increase of time and reaches a plateau in all cases (Figure 6a). This indicates that a dynamic equilibrium exists between the amount of dye adsorbed and desorbed from adsorbent. For CR
Figure 6. (a) Adsorption performance with contact time and (b) intraparticle diffusion plots for the CR onto α-Fe2O3 nanorods. 11260
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ments (shown in Figure S7). With surface negative charge, nanorods should have exhibited strong affinity toward cationic dye, MB adsorption rather than the anionic dyes (CR and MO) as shown in Scheme 1b. Hence, it could be confirmed that mesoporosity and the surface charge of hematite nanorods are not the sole parameters for the selective adsorption of Congo red. Variations in pH of the CR dye solution have also contributed significantly to postulate the adsorption mechanism. CR is a dipolar molecule which exists as anionic form (deep red color) at neutral or alkaline pH and cationic form at acidic pH exhibiting a dark blue color. This dark blue color appears due to the protonation of the azo (-NN-) group, which leads to the π−π* transition resulting in shifting of absorbance maxima toward higher wavlength.33 The extent of higher dye adsorption at acidic pH between the cationic form of the dye and the anionic surface charged α-Fe2O3 nanorods was accompanied by spontaneous electrostatic attraction which in turn facilitated further chemical interaction. But a lack of such possibility in alkaline pH resulted in slower dye adsorption kinetics. But an extensive amount of CR adsorption selectively onto the porous nanorods at its ambient solution pH (∼6.5) led us to unravel the indecisive phenomenon beyond surface area/porosity effects or electrostatic interactions for the dye adsorption. Therefore, in this case, we explored the possibility of hydrogen bonding34 between the functional groups of Congo red dye and the adsorbed moisture on the adsorbents, as it plays an important role on the adsorption. Another significant factor for the adsorption is coordination effect between the donor entity of dye and the acceptor ion of adsorbent. Here mesoporosity could create more active sites and large surface area for the dye molecules to be adsorbed strongly via H-bonding and coordination effect. Mesoporous hematite nanorods were highly water dispersible owing to the formation of hydrogen bonding between adsorbed water and the solvent water through porous network and led to
Table 3. Different Parameters with Variation of Dye Concentration for the Removal of CR by α-Fe2O3 Nanorods CR concn (mg L−1)
α1
α2
β
20 40 60 80
1.003 1.439 2.56 2.89
0.114 0.034 0.0452 0.0436
9.11 42.3 63.2 66.7
Effect of pH. While studying the effect of pH on the adsorption of CR by α-Fe2O3 nanorods, it was noticed that the color of CR solution (20 mg L−1) turned dark blue at pH ∼ 3 and darkened its red color at pH ∼ 7.5 and 10, respectively. Digital images, as shown in the insets of Figure 7a−c, clearly evidence this phenomenon. Interestingly, it was observed that the dark blue solution of CR (pH ∼ 3) has readily adsorbed onto the α-Fe2O3 nanorods at ∼98% within 2 min (Figure 7a), whereas only 29% and 10% of dyes were adsorbed at pH ∼ 7.5 and 10, respectively, at the same time (Figure 7b,c). Furthermore, no significant amount of dye was removed even after 120 min of treatment at alkaline pH as depicted in Figure 7d. Adsorption Mechanism. To disclose the strong affinity of CR dye toward mesoporous α-Fe2O3 nanorods, the adsorption mechanism was investigated in detail. There are several fundamental factors which are responsible for the adsorption of dye molecules, such as porosity/surface area, pH of the experimental solution, and electrostatic attraction, where the surface charge on the adsorbents is opposite to the charge of the ionized dye involved as adsorbate. If porosity/surface area was the crucial factor, then the porous rods should not have exhibited any selective adsorption toward Congo red (Scheme 1a). On the other hand, we cannot consider electrostatic attraction as the major factor here, as the surface charges of the adsorbents are negative as confirmed by ζ potential measure-
Figure 7. Effect of pH on adsorption of CR by the α-Fe2O3 nanorods at pH (a) 3, (b) 7.5, and (c) 10 and (d) bar diagram for the removal (%) of CR after 120 min incubation. 11261
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ACS Sustainable Chemistry & Engineering Scheme 1. Plausible Factors Responsible for Selective Adsorption
Scheme 2. Schematic Representation for the Adsorption and Desorption of Congo Red
coordination effects between Fe3+ and -NH2 groups (indicated by dotted arrow in Scheme 2). Investigation was also carried out to identify the actual functional group between -NH2 and the azo (-NN-) group present in the CR that is responsible for the formation of hydrogen bonding and coordination effect with surface -OH groups and Fe3+ ions on the adsorbent, respectively. During CR adsorption no red shifting of absorbance maxima (centered at 499 nm) of Congo red33 was observed, suggesting that the -NH2 groups of Congo red are the main entity for the formation of hydrogen bonding and the creation of coordination effect. If the azo group of CR plays a vital role, methyl orange (MO) dye carrying the azo group also should have strongly adsorbed onto the mesoporous αFe2O3 nanorods through coordinative interaction. But a slightly higher extent of adsorption of MO (34%) compared to the MB
the better performance as compared to the nonporous materials. Lack of porosity in α-Fe2O3 NPs resulted in less amount of adsorbed water onto it, which in turn makes it moderately water dispersible owing to the lack of sufficient Hbonding. Hence, the maximum amount of water-soluble dye could not reach to the adsorbent site, which ultimately results in weak coordination effect. It should be mentioned that the surface -OH groups and Fe3+ ions present on the adsorbent anchor the dye molecules firmly via hydrogen bonding and coordinative interaction which in turn resist their intraparticle diffusion. By this, the dye molecules may get more time to interact with the nanorods and consequently this facilitates the formation of H-bonding between -NH2 groups of Congo red and the surface -OH groups of adsorbed water (illustrated by dashed line in Scheme 2) as well as enhance the surface 11262
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Figure 8. (a) Adsorption isotherm and removal percentages of different concentrations of CR and (b) pseudo-second-order kinetic plot (linearized Langmuir isotherm in the inset) for the adsorption of CR by α-FeOOH. Adsorption of (c) methyl orange and (b) methylene blue by α-FeOOH.
Figure 9. (a) XPS survey and core level spectra of (b) Fe 2p, (c) O 1s, and (d) N 1s of α-Fe2O3 nanorods before and after CR dye adsorption.
(27%) for NRs could be due to a minor possibility of coordination effect from the azo (-NN-) group of MO. The red shift of absorbance maxima from 462 to 470 nm suggests the coordination effect as demonstrated in Figure S5a. Aqueous solution of MO exhibits absorbance maxima at 462 nm, which
is associated with the n−π* transition of the nonbonding electrons present in the N atom of the azo group (Figure S4 and Scheme 1c). Coordination effect from the N atom of the azo group possesses partial positive charge onto it which leads to little extent of π−π* transition resulting in the shifting of 11263
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Figure 10. (a) Effect of pH on desorption of dye from dye-loaded adsorbent, (b) TGA of bare α-Fe2O3 nanorods and the same after pH treatment, and (c) recycled adsorption of CR (20 mg L−1) under dark condition (pH ∼ 6.5) over the α-Fe2O3 nanorods after pH treatment.
α-FeOOH,24 the highly selective adsorption further strengthened our proposed mechanism based on H-bonding and coordination effect. Furthermore, X-ray photoelectron spectroscopy (XPS) was also carried out to investigate the chemical state of the α-Fe2O3 nanorod surface before and after CR dye adsorption as shown in Figure 9. The survey spectra shown in Figure 9a clearly demonstrate the existence of Congo red on the surface after dye loading as is clear from the corresponding signature peaks. The absence of Fe 2p corresponding to Fe2O3 sample after dye adsorption may be attributed to the coverage of dye molecules onto the surface, which readily reappears after sputtering. Figure 9b shows the core level spectra of Fe 2p where two predominant peaks are attributed to Fe 2p3/2 (710.2 eV) and Fe 2p1/2 (723.6 eV) with the representative satellite lines present at 718.3 and 732.5 eV, respectively.35 Minute observation reveals the fact that the intensity of the Fe 2p peak was attenuated and slightly shifted after dye adsorption.36 Figure 9c represents the core level O 1s spectra before and after dye adsorption. From the figure it was clearly observed that before dye adsorption the predominant peak at 529.6 eV attributed to Fe−O ebbed out completely after dye adsorption where the persisting broad peak at ∼532.2 eV indicates the presence of hydroxyl (O−H) groups on the dye adsorbed surface. On deconvolution, it gives rise to two peaks which evidently indicate the coexistence of surface O−H groups (532.3 eV) and chemically bound O−H groups (531.6 eV).37,38 It is noteworthy to mention that Fe−O peak after dye adsorption reappears after sputtering with slight increase in binding energy. In another aspect, core level N 1s spectra as observed in Figure
absorbance maxima toward higher wavelength. This phenomenon was also observed for CR in cationic conformation at acidic pH ∼ 3.The selective adsorption of CR on the mesoporous α-Fe2O3 nanorods also satisfies the aforementioned surface diffusion phenomenon occurring through the formation of some chemical interaction. In order to confirm the effect of H-bonding in the proposed mechanism for the adsorption of CR, the adsorption performance was also carried out with α-FeOOH. It exhibited remarkable adsorption capability toward CR (80 mg L−1) as shown in Figure S8. Motivated by this attractive outcome, we have thoroughly investigated the maximum adsorption capacity and the adsorption kinetics for the CR adsorption by αFeOOH. It was observed that CR (concentration of 200 mg L−1) was almost completely removed after 120 min as shown in Figure 8a. From the kinetic study it was observed that R2 values (Table S4 for all of the results are above 0.99, indicating a good fit of the pseudo-second-order model (Figure 8b). The experimental data fit well the Langmuir adsorption isotherm (inset, Figure 8b) with R2 = 0.9998 (Table S4), and the calculated maximum adsorption capacity of the α-FeOOH nanorods was found to be 160 mg g−1 (Table 1). On the contrary, the adsorption of MO and MB by α-FeOOH was almost negligible as demonstrated in Figure 8c,d, respectively. The observed remarkable adsorption performance of αFeOOH toward CR could be due to the effective intermolecular H-bonding among abundant -OH groups of αFeOOH and -NH2 groups of CR simultaneously with a possibility of coordination effect between -NH2 groups of CR and Fe3+ of α-FeOOH. Despite having a lower surface area for 11264
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desorption of CR. The mass loss of ∼5% for Fe-R was due to the removal of adsorbed water on the surface. The mass loss of 11−12% for Fe-R10 and Fe-R7 was due to the decomposition of dye molecules as these were not completely desorbed from the nanorods. A large quantified mass loss of ∼59% for Fe-R3 was due to the degradation of dye molecules. This experimental result strongly supports the aforementioned desorption mechanism in pH 7.5 and pH 10. Reusability Test of Porous Nanorods. The regeneration of adsorbent is considered as an essential factor in sustainable applications. After pH treatment, the regenerated adsorbents were recovered as fresh materials by washing with distilled water several times to neutralize the pH, followed by drying in vacuum oven. Then adsorption/desorption experiment was recycled up to three times as shown in Figure 10c. Excellent recyclability of Fe-R7 and Fe-R10 due to their stable durability with negligible diminished efficiency further supports its future application as a selective adsorbent of Congo red from wastewater. Fe-R3, on the other hand, exhibited insignificant performance toward the removal of Congo red.
9d appear due to the adsorption of CR onto Fe2O3. On deconvolution, three peaks became prominent at 398.7, 399.6, and 400.4 eV attributed to N-, N−N/N−H, and chemically bound -NH2 group, respectively.39,40 This chemical interaction supports the formation of coordination bonding and Hbonding with Fe3+ and surface hydroxyl (O−H) groups,37,40 respectively, which is also reflected in the O 1s and Fe 2p core level spectra (Figure 9b,c) after dye adsorption and validates our proposed mechanism for the strong affinity of porous αFe2O3 nanorods toward CR adsorption. In Figure S9, the FT-IR spectra of α-Fe2O3 nanorods and CR adsorbed α-Fe2O3 nanorods are presented. In both spectra, IR bands at around 642, 554, and 453 cm−1 are observed which are attributed to the stretching vibrations of Fe−O and stretching and bending vibrations of FeO of α-Fe2O3, respectively.41 Along with these bands CR adsorbed nanorods exhibited four more strong bands. The band at ∼1023 cm−1 was due to the stretching vibration of SO, present in Congo red.42 The observed bands in the CR adsorbed α-Fe2O3 nanorods at around 1182, 1380, and 1492 cm−1 were assigned to framework vibrations of benzene rings. The broad common FT-IR band at approximately 3300−3670 cm−1 was due to the stretching vibration of the O−H of adsorbed water. Another common band at around 1630 cm−1 was ascribed to the bending vibration of O−H of adsorbed water. Desorption Mechanism of Congo Red. The adsorption mechanism of Congo red was also demonstrated by the investigation of desorption as shown in Figure 10a. In pH ∼ 7.5 and pH ∼ 10 buffer solutions, phosphate and carbonates/ bicarbonates are present in the solution as the anionic groups, respectively. In alkaline condition, phosphate and carbonate/ bicarbonate ions compete with the -NH2 groups of dye to coordinate with Fe3+ and consequently the coordination effect between Fe3+ and -NH2 groups becomes weaker at the anionic environment (step I, Scheme 2). Since, in pH ∼ 10, the anionic charge effect is higher than that of pH ∼ 7.5 ,the ability for weakening the coordination bond between -NH2 group and Fe3+ is large. As a consequence, 86% and 59.5% of the dye molecules were desorbed from the adsorbents after 4 h of stirring in dark condition at pH ∼ 10 and pH ∼ 7.5, respectively. With desorption study in different pHs, the surface of the adsorbents retained negative charge, as obtained from ζ potential measurements (Figure S7). It has been observed that Fe-R, Fe-R3, Fe-R7, and Fe-R10 exhibited the ζ potential values of −35, −44, −60, and −68 mV, respectively (Figure S7). These high negative ζ potential values for Fe-R7 and Fe-R10 were observed due to the presence of phosphate and carbonate/bicarbonate ions decorated on the surface of the nanorods. Surprisingly, though Fe-R3 (at pH 3) exhibited negative ζ potential value, there was no release of dye molecules as observed in Figure 10a. It may be due to the fact that, in acidic condition, there was no impact on coordination effect to desorb the anionic dye molecules unlike in alkaline condition. In contrast, the negative charge on the FeR3 surface may be due to the presence of sulfonate groups of Congo red in an outer direction, as it was fully adsorbed on the surface of the nanorods through hydrogen bond and coordination effect by -NH2 groups of Congo red (step II, Scheme 2) as discussed earlier. In order to confirm the desorption behavior of Congo red from nanorods in different pHs, TGA was conducted on Fe-R, Fe-R3, Fe-R7, and Fe-R10 as shown in Figure 10b. No effective mass loss was observed for Fe-R7 and Fe-R10 as compared to Fe-R3, indicating a complete
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CONCLUSION A thorough investigation was carried out to explore the selective and rapid adsorption of Congo red by mesoporous hematite (α-Fe2O3) nanorods and to rule out the conventional factors regarding adsorption−desorption phenomenon.The porous α-Fe2O3 nanorods were capable of removing 81% of CR within 2 min; with increasing time, even up to 95% within 15 min. The template free synthesized mesoporous α-Fe2O3 nanorods exerted fast, high, and selective adsorption toward Congo red not only due to its high surface area and porous structure but also due to the formation of hydrogen bonding and coordination effect between -NH2 group in the Congo red with abundant surface -OH groups and Fe3+ ions on the highly dispersible adsorbent. This phenomenon was further confirmed by using α-FeOOH nanomaterial as another adsorbent and the other dye molecules as adsorbate with different ionic structures. The adsorption isotherms fitted well with the Langmuir model, signifying a monolayer adsorption, and the adsorption kinetics follows the pseudo-second-order kinetics. All the results demonstrated here strongly corroborated with rapid adsorption (81% within 2 min) by mesoporous α-Fe2O3 nanorods toward selective removal of Congo red from wastewater. The variations of desorption activities of Congo red from the nanorods at different pH conditions purely support our proposed probable mechanism for the selective and rapid dye adsorption. Consequently, after desorption the regenerated α-Fe2O3 nanorods were successfully reused in dye adsorption with reduced efficiency, which further supports its candidacy as a superior selective adsorbent toward CR removal.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01684. Digital images of samples, XRD data, Raman spectra, chemical structure of dye molecules, adsorption study with different dyes, ζ potential data, FT-IR spectra, and experimental results in tabular form (PDF) 11265
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Tel.: 033-2483-8082. Fax: +91-33-2473-0957. ORCID
Parukuttyamma Sujatha Devi: 0000-0002-6224-7821 Present Address †
Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS P.S.D. acknowledges partial financial support from Council of Scientific and Industrial Research (CSIR) through the network project BIOCERAM, ESC 0103. D.M. acknowledges CSIR, Government of India for the award of a Senior Research Fellowship (SRF). S.M. acknowledges UGC, Government of India for the award of a Senior Research Fellowship (SRF).
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