M. Arshadi*a,
F. SalimiVahidb,
J. W. L. Salvacionb and
M. Soleymanzadehb
aDepartment of Science, Fasa Branch, Islamic Azad University, PO Box 364, Fasa 7461713591, Fars, Iran. E-mail: m-arshadi@ch.iut.ac.ir; mohammadarshadi@yahoo.com; Tel: +989361528179
bMapúa Institute of Technology, Muralla St. Intramuros, Manila 1002, Philippines
First published on 13th February 2014
In this paper, a novel nano-adsorbent containing Mn-nanoparticle decorated organo-functionalized SiO2–Al2O3 mixed-oxide was introduced as a new scavenger of dyes such as methyl orange. The SiO2–Al2O3 mixed-oxide was functionalized with a Schiff base ligand and thereafter, in the next step, Mn-nanoparticles were prepared over the organo-functionalized SiO2–Al2O3 mixed-oxide. The synthesized materials were characterized by several methods, such as FT-IR spectroscopy, UV-vis, CHN elemental analysis, SEM, TEM, ICP-OES, EPR and XPS. The contact time to obtain equilibrium for maximum adsorption was 15 min. EPR and XPS of the Mn ions evidenced that most of the covalently bonded active sites of the nano-adsorbent are in the form of Mn(III) ions. The heterogeneous Mn(III) ions were found to be an effective adsorbent for the removal of methyl orange ions from solution. The adsorption process was spontaneous and endothermic in nature and followed a pseudo-second-order kinetic model.
The following conventional methods are used in dye removal from wastewater: coagulation and flocculation, oxidation or ozonation, membrane separation, and adsorption.2 Adsorption processes have been reported to be low-cost promising alternatives for the treatment of dyes present in wastewater. The use of activated carbons, modified clays, polymeric resins, waste materials, and zeolites as adsorbents have also been described.1,2 For most natural adsorbents, such as clays, zeolites, agricultural waste, and chitosan, their particles show negatively charged surfaces and consequently, excellent adsorption properties for cationic organic compounds can be obtained. However, these systems generally have some disadvantages in dye removal, such as the difficulty in separating these powdery natural adsorbents, except by high speed centrifugation, from the treated effluent, nonresistance against acid solutions, their poor mechanical strengths and that they are less adaptable to a wide range of dye wastewaters limits their practical applications. However, most of these adsorbents either do not have considerable adsorption capacities or need relatively long adsorption contact times, e.g., from several hours to a couple of days (Table 1). It is, therefore, desirable to develop effective adsorbents with short contact times for the removal of dye ions from aqueous solutions. Activated carbon is the most prevailing adsorbent for this process because of its high surface area, high adsorption capacity, and high degree of surface reactivity. However, it is expensive and must be regenerated on a regular basis. Inorganic supports present several advantages with respect to activated carbon, including stability, high surface area, possible reuse, relative rapidity in reaching equilibrium, better high mechanical resistance, easy modifications and a higher concentration of chelating groups on the surface, and they are often much cheaper than their organic counterparts. For comparative purposes, the adsorption capacities of several adsorbents for methyl orange are summarized in Table 1.
Adsorbents | Qmax (mg g−1) | Ref. |
---|---|---|
Ferrocene-Si–Al | 381 | 3 |
λ-Fe2O3/MWCNTs/chitosan | 60.5–66.1 | 4 |
Hypercrosslinked chloromethylated PS adsorbent functionalized with formaldehyde carbonyl groups (HJ-1) | 70.9–76.9 | 5 |
Mesoporous magnetic Co-NPs/carbon nanocomposites | 380 | 6 |
Acid modified carbon coated monolith | 132.7 | 7 |
Calcined layered double hydroxides | 200 | 8 |
Alkali-activated multiwalled carbon nanotubes | 149 | 9 |
Pinecone derived activated carbon | 404.4 | 10 |
Carbon nanotubes | 35.4–64.7 | 11 |
Chitin/alginate magnetic nano-gel beads (MCAs) | 107.5 | 12 |
Porous spongy CSGO monolith | 567.07 | 13 |
Chitosan/MgO composite | 60 | 14 |
Cu@Cu2O | 344.8 | 15 |
Ultrafine coal powder | 5.24 | 16 |
Modified ultrafine coal powder | 5.56 | 16 |
Chitosan | 34.83 | 17 |
Calcined Lapindo volcanic mud | 333.3 | 18 |
Mesoporous Y-Fe2O3/SiO2 nanocomposites calcined | 476 | 19 |
Y-Fe2O3/SiO2/chitosan composite | 34.29 | 20 |
MgNiAl layered double hydroxides | 375 | 21 |
Protonated cross-linked chitosan | 89.30 | 22 |
Mesoporous carbon CMK-3 | 294.1 | 23 |
De-oiled soya | 16.66 | 24 |
Bottom ash | 3.61 | 24 |
Metal–organic framework (MOF) | 194 | 25 |
Ni-containing ordered mesoporous carbons | 107.1 | 26 |
Hypercrosslinked polymer | 72.9 | 27 |
Banana peel | 21 | 28 |
Ammonium-functionalized silica nanoparticle | 105.4 | 29 |
Palygorskite clays | 39 | 30 |
700 °C treated palygorskite clays | 98 | 30 |
Mn@Si/Al | 571 | This work |
In this paper, the application of stable, covalently immobilized Mn-nanoparticles (manganese is one of the most versatile metals, playing a central catalytic role in the environment) on the surface of a SiO2–Al2O3 mixed-oxide host by a linker approach has been studied as a novel nano-adsorbent for capturing methyl orange from aqueous solution. Methyl orange (MO), with an IUPAC name of sodium 4-[(4-dimethylamino) phenyl diazenyl] benzene sulfonate, is a typical water-soluble anionic dye and has a harmful effect on living organisms within a short period of exposure.
The adsorbent containing Mn-nanoparticles was obtained by stirring 0.5 g of the hybrid material, the Si–Al mixed-oxide-Schiff base ligand, with Mn(OCOCH3)2·4H2O (5.4 mmol), and LiCl (8.5 mmol) in 30 mL of ethanol at reflux for 24 h. Then, the resulting material (brown powder) was filtered off, washed with copious amounts of ethanol and methanol and dried under vacuum at 60 °C.
The amount adsorbed was calculated as:
qe = V(Co − Ce)/m | (1) |
Catalyst | Elemental analysesb (wt%) | Organic functional group (mmol g−1 mixed-oxide)c | Immobilized Mn–Schiff base-complex (mmol g−1 mixed-oxide)d | Structural parameterse | |||
---|---|---|---|---|---|---|---|
N | Mn | Surface area (m2 g−1) | Pore volume (cm3 g−1) | Pore diameter (Ao) | |||
a Molar ratio of Si–Al was 60:40, determined from EDX analysis.b Nitrogen was estimated from the elemental analyses. Mn content determined from ICP analysis.c Determined from the N-content.d Determined from the Mn-content.e The pore size calculated using the BJH method. | |||||||
SiO2–Al2O3 mixed-oxidea | — | — | — | — | 243 | 0.028 | 20 |
Si/Al-pr-NH-et-N = methyl-2-pyridylketone | 6.2 | — | 4.4 | — | 131 | 0.014 | 16 |
Si/Al-pr-NH-et-N = methyl-2-pyridylketone-Mn | 5.7 | 3.0 | 4.1 | 0.56 | 114 | 0.014 | 16 |
In the infrared spectrum of the pristine SiO2–Al2O3, the structural bands at 780 cm−1 correspond to the stretching vibration modes of the M–O, M–O–M, and O–M–O bonds, where M means metal atoms, and O means oxygen atoms. Furthermore, it can be seen that there are complex bands in the 780 cm−1 region, attributed to ring vibrational modes. It is interesting to note that the bands related to the M–O lattice vibrations changed in intensity at around 780 cm−1, which further indicated that an organo-functional group was successfully immobilized on SiO2–Al2O3. However, for the spectrum of Mn@Si/Al, it can be easily seen that the bands are similar to those of SiO2–Al2O3 in the range from 900 to 600 cm−1, indicating that the M–O lattice vibrations showed a small change. Further evidence for this coordination mode was provided by the ν(Mn–N) band at ca. 412 cm−1.34 The FT-IR results demonstrate the formation of the Mn-complex, immobilized on the Si–Al mixed-oxide through the 2-AE-3-APTMS linker (Fig. 1).
The X-band EPR spectrum of the immobilized Mn-nanoparticles on the functionalized SiO2–Al2O3 mixed-oxide measured at room temperature is indicated in Fig. 5. The EPR spectrum shows that some of the Mn ions are in other oxidation states, because Mn(III) exhibits no EPR signal. The observed EPR signals, characteristic of Mn(II)/Mn(IV), indicate that, at least, part of the Mn ions are present in the +2 or +4 oxidation state upon immobilization on the functionalized SiO2–Al2O3 mixed-oxide. Thus, next to Mn(III), both Mn(II) and Mn(IV) species must be present, and the observed EPR spectrum must be due to one or both of these species (the reported g values for Mn(IV) are slightly less than 2, whereas those of Mn(II) are slightly above 2).
The EPR spectrum appears to have a broad feature centered at around g = 2, with wide wings (such wings are typical for a distribution of the zero-field splitting parameters, and have been described for Mn(II) in glass environments,37 or for frozen solutions of Mn(II) phosphate complexes).38 In fact, the peak broadening and wings are due to dipole–dipole interactions,39 and consequently, they are predominant in the spectra of higher loaded samples (samples containing more than 1.5 wt% Mn).40
The EPR spectrum of Mn@Si/Al has a lower intensity in comparison with the other spectra reported in the literature. On the other hand, the inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis of the samples indicates that the nanoadsorbent has a high loading of manganese. This contradiction might be explained by stating that Mn(III) species (expected to be EPR silent under these conditions) are probably the most likely product of the reaction between Mn(II) and Si/Al-pr-NH-et-N = methyl-2-pyridylketone.
Therefore, Mn(III) is the main oxidation state which is immobilized on the functionalized SiO2–Al2O3 mixed-oxide and probably MO was mainly captured by Mn(III). It is believed that during the immobilization of Mn(II) over the modified SiO2–Al2O3 under air conditions (in the presence of atmospheric oxygen), the oxidation state of Mn(II) was easily oxidized and began to prefer the Mn(III) state (in fact, oxidation of Mn(II) gives a stable cation: Mn(III)). The EPR peak broadening of Mn@Si/Al could be due to the presence of Mn(II) and Mn(IV) in its structure (g = 1.95). Microcrystalline nano-catalysts often yield characteristic EPR spectra with features distinctly different from those observed for magnetically isolated ions. The detailed features of the resonances depend strongly on the shape, size distributions and on the magnetic properties of the particles. Consequently, with regard to the EPR spectrum and TEM photo, the broad feature around gav = 2.03 is probably characteristic of super paramagnetic relaxation as it is concerned with the magnetic moment of the whole particle and either may be due to the highly dispersed Mn species on the nano-adsorbent (Fig. 5).
Fig. 6 XP spectroscopy of Mn@Si/Al. The Mn 2p3/2 component is fit by multiplet peaks labeled 1, 2 and 3, with peak 3 corresponding to the shake-up satellite. The Mn 2p1/2 component is fit by two peaks and a satellite peak.42 |
As shown in Fig. 7, the modified Si–Al nano-sized mixed-oxide with organometallic groups indicates higher MO capturing capacity than the unmodified Si–Al at various pH values. The results indicate that the pH has a significant influence on the adsorption of MO onto Mn@Si/Al. A similar observation was published in the literature for different adsorbents.10–30 As the initial pH increased to 6.8, the sorption capacity increased to more than 97.3% and decreased at the initial pH values above 6.5 (>48.6%). The lowest MO sorption capacity of Mn@Si/Al (48.6%) was found at an initial solution pH of 13.0, which may be due to partial dissolution of the immobilized Mn ions at this solution pH (data not published).
At acidic pH, protons can combine with one of the nitrogens in a nitrogen–nitrogen double bond, which can generate a reddish colored MO dye solution, that is, MO is protonated and hence the electrostatic repulsion interaction between protonated MO and the positively charged Mn@Si/Al active sites results in a decline in the percentage of adsorption (the dissociation constant pKa for MO is 3.46, so MO molecules were predominantly present as monovalent anions at this equilibrium pH). However, decreasing MO adsorption in basic pH may be attributed to the competition of OH− with the MO ions for the adsorption sites on the Mn@Si/Al (the active sites, Mn(III) ions, are closely associated with hydroxyl ions OH−, that is, restricting the approach of MO ions as a result of the repulsive force) and therefore fewer groups are available for MO to bind with. Therefore, the possible mechanism of MO adsorption may be considered as the strong electrostatic interaction between the positive adsorption active sites of the adsorbent and the negatively charged MO (Scheme 2). This pH-dependent trend has also been observed for the adsorption of similar anionic dyes onto TiO2,43 activated carbon modified with Y(III) ions44 and the copper(II) complex of dithiocarbamate-modified starch,45 confirming the interaction of the anionic dyes with metal centers, which directs the adsorption process.
To further study the oxidation state of the immobilized Mn ions at different pH values, the ESR spectrum of Mn@Si/Al at pH 2.0 was measured (Fig. 8) and compared with those recorded at pH 7.0 (Fig. 5). Among the methods used in this study [Fourier transform infrared (FT-IR), diffuse reflectance UV-Vis (DRS) and electron paramagnetic resonance (EPR)], EPR was found to be sensitive to the oxidation state of the manganese atom at different pH values. The ESR spectra of Mn@Si/Al at pH values lower than 4.0 show similar features to that of Mn@Si/Al at pH values higher than 4.0 but the intensity of the ESR spectra of Mn@Si/Al at pH > 4.0 is less than that recorded at pH < 4.0. The intensity of the ESR spectrum of Mn@Si/Al at pH 2.0 shows a high intensity with a g value of about 2 compared to those prepared at pH > 4.0, which is consistent with Mn(II) (g = 2.0) in the environment of distorted octahedral symmetry. Therefore, for the sample at pH > 4.0, there might be more oxidation of Mn(II) to Mn(III). Indeed, Mn(III) sites are also on Mn@Si/Al as they are more inclined to be incorporated into the framework than Mn(II).46 In order to introduce a greater number of Mn(III) sites on Mn@Si/Al, the conversion of Mn(II) to Mn(III) is required. Since this oxidation can be carried out by dissolved O2, pH values higher than 4.0 might be more effective than pH 2.0 or 3.0 to dissolve more oxygen and to increase oxidation. However, these results revealed that better heterogenization of Mn(II) on the active sites of Schiff base ligands can be obtained at pH values lower than 4.0. Thus, increasing the concentration of Mn(III) on Mn@Si/Al is feasible if the experimental pH is raised to 4.0. All the above experiments clearly reveal that the acidity of the solution influences the electronic structure of the immobilized Mn-nanoparticle.
However, the higher adsorbent dose results in a lower removal capacity of Mn@Si/Al (94.6%). It is believed that at the low adsorbent dosage, the dispersion of Mn@Si/Al nanoparticles in an aqueous solution is better, that is, all of the active sites on the adsorbent surface are entirely uncovered, which could accelerate the approachability of MO molecules to a large number of the adsorbent active sites. Thus, the adsorption on the active sites is saturated quickly, resulting in a high removal capacity. On the other hand, at higher adsorbent dosages, the accessibility of adsorbent active sites with higher energy decreases and a larger fraction of the active sites with lower energy become occupied, leading to a decrease in the adsorption capacity. Furthermore, increasing the adsorbent dosage enhances the chance of collision between the adsorbent nanoparticles and hence creates particle aggregation, inducing a decline in the total surface area and an increase in the diffusion path length, which both result in a decrease in the amount of removal capacity of MO from aqueous solution. Therefore, a 0.07 g adsorbent dosage was chosen as the optimal dosage for the rest of the study.
Fig. 10 Equilibrium absorption of MO by Si–Al and Mn@Si/Al at 25 °C. Dashed line represents the fitting curve using the Langmuir adsorption model. |
When the initial MO concentrations were increased in the presence of Mn@Si/Al, the removal efficiency of the dye reached 98.0% and 53.5% for 150 and 1000 mg L−1 initial dye concentrations, respectively. The relative increase in the loading capacity of the sorbent with increasing MO concentrations is probably due to the interaction between the dye and the adsorbent, which provides the vital driving force to defeat the resistance to the mass transfer of MO ions between the aqueous solution and Mn@Si/Al. The observed enhancement in the MO uptake on increasing the initial dye ion concentration could be due to an increase in electrostatic interactions (relative to covalent interactions), which involves active sites of progressively lower affinity for MO up to the saturation point. On the other hand, the higher removal efficiency of Mn@Si/Al at a low MO initial concentration could be related to the high ratio of initial mole numbers of MO to the available active sites on the surface area and therefore, the fractional adsorption is dependent on the initial concentration.
qe = qmKLCe/1 + KLCe | (2) |
qe = KFCe1/n | (3) |
System | qm (mg g−1) | KL (L mg−1) | KF (mg g−1 (mg L−1)n) | n | R2 | Sorption model |
---|---|---|---|---|---|---|
Mn@Si/Al | 571.2 | 2.73 × 10−2 | 0.9927 | Langmuir | ||
0.448 | 0.294 | 0.6927 | Freundlich |
The Langmuir equation relates the coverage of molecules on a solid surface to the concentration of a medium above the solid surface at a fixed temperature. Adsorption is limited to monolayer coverage, and intermolecular forces decrease with the distance from the adsorption surface, whereas the Freundlich model supposes that the adsorption surface is heterogeneous, that interaction between adsorbed molecules can occur, and that multilayer adsorption is possible. The Langmuir and Freundlich adsorption isotherms exhibit an approximately linear relationship for Mn@Si/Al. The data obtained from Mn@Si/Al revealed that the Langmuir isotherm model correlated better (R2 > 0.9827) than the Freundlich isotherm (Table 3 and Fig. 10), using the experimental data for the adsorption equilibrium of MO ions by the modified SiO2–Al2O3 mixed-oxide. Thereby, it is realistic to infer that the adsorption active sites on the modified Si–Al are not mostly energetically heterogeneous (non-uniform surface), and this is a major aspect in this model. Likewise, the maximum adsorption capacity (qm) for Mn@Si/Al (571.2 mg g−1), as obtained using the Langmuir isotherm, was much higher than the values found for most other adsorbents reported in the literature,3–30 thereby also suggesting that there is a high thermodynamic stability for MO heterogenization at the active sites of Mn@Si/Al (Table 3).
Fig. 11 The adsorption kinetics of Mn@Si/Al for MO at room temperature. The inset shows the pseudo-second order plot for the adsorption. |
In order to determine and interpret the mechanisms of the MO adsorption processes and the main parameters governing sorption kinetics, empirically obtained kinetic sorption data were fitted to the pseudo-first-order, pseudo-second-order, and intra-particle diffusion models shown in eqn (4) and (5), and (6), respectively. Each of these models has been widely used to describe metal and organic sorption on several sorbents.1,2 From the linear form of these three models, equations can be written as follows:
Pseudo-first-order equation: qt = qe[1 − exp(−k1t)] | (4) |
Pseudo-second-order equation: qt = k2qe2t/1 + k2qet | (5) |
Intra-particle diffusion equation: qt = kintt1/2 | (6) |
The initial adsorption rate (h) can be determined from the k2 and qe values using
h = k2q2e | (7) |
The calculated kinetics parameters for the adsorption of MO dye ions onto Mn@Si/Al at an initial concentration of 150 mg g−1 are presented in Table 4, where it can be seen that the pseudo-second-order equation appeared to be the best-fitting model compared to those for the other two equations (the correlation coefficient is extremely high for the pseudo-second-order equation of Mn@Si/Al; R2 > 0.999). The value of qe,cal also appeared to be very close to the experimentally observed value of qe,exp. The plot of the linear form of the pseudo-second-order for the adsorption of MO ions is shown in the inset of Fig. 11. Similar results have been reported for the adsorption of dyes onto different adsorbents in the literature.1–10
Pseudo first order constants | Pseudo second order constants | Intra-particle diffusion constants |
---|---|---|
k1 (min−1) = 0.623 | k2 (g mg−1 min−1) = 0.0065 | kint (mg g−1 min−1/2) = 1.7743 |
q1 (mg g−1) = 144.9 | q2 (mg g−1) = 149.2 | R2 = 0.7486 |
R2 = 0.8563 | h (mg g−1 min−1) = 144.9 | |
R2 = 1 |
This consistency in the experimental data with the pseudo-second-order kinetic model indicates that the rate limiting step for the adsorption of MO ions on the organometallic-functionalized SiO2–Al2O3 mixed-oxide is chemical adsorption (Scheme 2). As a result, the adsorption of the MO dye onto the nano-adsorbent may be considered to involve two processes with initial adsorption rate of 144.9 mg (g min)−1 over Mn@Si/Al. Although this adsorption rate is related to the content and type of active adsorption site on the matrix of the adsorbent, Mn(III) sites are the main reactive groups for the removal of MO ions from aqueous solution. A larger advantage of the pseudo-second-order model is that it predicts the behavior over the whole range of the adsorption process.
Fig. 12 Effect of temperature on the adsorption of MO by Mn@Si/Al at different initial concentrations. |
To gain a better understanding of the effect of rising temperature on the adsorption of the dye ions onto Mn@Si/Al, three basic thermodynamic parameters were studied: the Gibbs free energy of adsorption (ΔG°), the enthalpy change (ΔH°), and the entropy change (ΔS°).
The thermodynamic parameters ΔG°, ΔS° and ΔH° for this adsorption process were determined using the following equation.
ΔG = −RTlnK | (8) |
dlnK/dt = ΔH°/RT2 | (9) |
The ΔH° and ΔS° values were calculated from the slope and intercept of the linear plot of lnK vs. 1/T, as shown in Fig. 13.
lnK = ΔS°/R − ΔH°/RT | (10) |
ΔG° = ΔH° − TΔS° | (11) |
The corresponding values of the thermodynamic parameters are presented in Table 5. This shows that ΔH° and ΔS° are positive for all the experiments and ΔG° is negative in all the systems. The positive values of ΔH° reveal that the adsorption process is endothermic in nature and thus, the removal of MO increases with increasing temperature. The positive value of ΔS° reveals an increase in randomness and an increase in the degrees of freedom at the Mn@Si/Al–solution interface during the immobilization of the MO dye ions on the active sites of the adsorbent, where methyl orange is a bulk molecule in comparison with H2O, so some water molecules could be desorbed during the adsorption of a methyl orange molecule. This process led to a partial liberation of the solvated MO ions from the solvent molecules before adsorption (liberation of water molecules from solvated-MO), thereby enabling commonality of randomness and spontaneity in the system.1,2 The necessity of a large amount of heat to remove the MO dye ions from the solution makes the sorption process endothermic. A positive ΔS° (disorder of the system) was observed on Mn@Si/Al, which indicates that the MO ions lose most of their water of solvation. This is also supported by the positive ΔH° value of MO sorption onto Mn@Si/Al. The positive value of the standard enthalpy change for MO ion sorption indicates the endothermic nature of the adsorption. It was also observed that with an increase in temperaturem, the value of ΔG decreases, which indicates that the sorption process is spontaneous and thermodynamically favorable by an increase in temperature (Table 5). In fact, the value of ΔG° for physisorption is in the range −20 > ΔG° < 0 kJ mol−1, but chemisorption is between −400 > ΔG° < −80 kJ mol−1.47 The calculated ΔG° values based on eqn (14) were more than −40 kJ mol−1 for most cases (Table 5). Therefore, the ΔG° values suggest that the adsorption of MO ions onto Mn@Si/Al is a chemisorption process, as the adsorption mechanism is proposed in Scheme 2.
Initial MO concentration | ΔH° | ΔS° | ΔG°/kJ mol−1 | ||||
---|---|---|---|---|---|---|---|
mg L−1 | (J mol−1) | (J mol−1 K−1) | 288 | 298 | 313 | 333 | 353 |
150 | 95.45 | 348.11 | −100.16 | −103.64 | −108.86 | −115.82 | −122.79 |
250 | 80.00 | 298.58 | −85.911 | −88.896 | −93.375 | −99.347 | −105.31 |
350 | 67.25 | 240.98 | −69.334 | −71.744 | −75.359 | −80.179 | −84.998 |
450 | 55.18 | 199.84 | −57.498 | −59.497 | −62.494 | −66.491 | −70.488 |
550 | 32.84 | 119.45 | −34.368 | −35.563 | −37.355 | −39.744 | −42.133 |
650 | 31.66 | 113.26 | −32.587 | −33.719 | −35.418 | −37.683 | −39.949 |
750 | 23.15 | 82.66 | −23.782 | −24.609 | −25.849 | −27.502 | −29.155 |
850 | 20.93 | 73.30 | −21.089 | −21.822 | −22.921 | −24.387 | −25.853 |
1000 | 16.73 | 57.33 | −16.494 | −17.067 | −17.927 | −19.074 | −20.220 |
To prove the proposed mechanism in Scheme 2, FT-IR spectra of Mn@Si/Al and MO-loaded Mn@Si/Al samples, before and after the adsorption process, were recorded in the range 4000–400 cm−1 (Fig. 14). FT-IR is often used to study the active sites of the adsorbents and to identify the interactions of those groups responsible for the dye adsorption. The adsorption ability of Mn@Si/Al for MO dye ions from aqueous solution results from the strong interaction of the active sites of the immobilized Mn-nanoparticles towards the dye. In the equilibrated sample of Mn@Si/Al with a MO solution, extra bands at 1729, 1606, 1447, 1421, 620 and 577 cm−1 were observed, showing the presence of MO anchored to the active sites of the modified nano-adsorbent. The peak at 1729 cm− 1 is enhanced in Mn@Si/Al–MO. This may belong to a coordinated sulfonate group, and generally appears when the bond between the metal and oxygen is strong (Mn–O–SO2–R, Scheme 2).48 The modes of MO ion coordination with the immobilized Mn(III) is evidenced through the O atoms of the sulfonate group appearing as a new band at around 577 cm−1 (Fig. 14). Indeed, the new absorption peaks observed in the range of 560 to 660 cm−1 are the characteristic absorption peaks of a “Metal–O” bond, which demonstrates the existence of the Mn–O bond (Mn–O–SO2–R, Scheme 2).49 In addition, the kinetics results proved that the adsorption obeys a pseudo second-order kinetics model, which indicates that the adsorption involves chemisorption.
Fig. 14 FT-IR spectra of Mn@Si/Al before and after adsorption of MO from aqueous solution. Legends: Mn@Si/Al (A), Mn@Si/Al after MO adsorption (B). |
In order to provide further clarification into the mechanistic aspects of the MO adsorption onto Mn@Si/Al, a desorption study was investigated. The use of water and ethanol solutions for the MO desorption are ineffective, even after 48 h. Very low desorption (does not exceed 7% of the presorbed amount) with these solutions suggests that metal–dye complexes were formed on Mn@Si/A. The high basic solution (above 0.01 mol L−1) did help in breaking the chelating interaction, in which the hydroxyl groups replaced the MO to bind Mn@Si/A.
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