Process for the Separation of Antimony(III) - American Chemical Society

Article pubs.acs.org/jced

New “Magmolecular” Process for the Separation of Antimony(III) from Aqueous Solution Ali Asghar Rooygar,† Mohammad Hassan Mallah,*,‡ Hossein Abolghasemi,† and Jaber Safdari‡ †

School of Chemical Engineering, College of Engineering, University of Tehran, P.O. 47771-16796, Tehran, Iran Nuclear Fuel Cycle Research School, Nuclear Science & Technology Research Institute, End of North Karegar Ave., Tehran, Iran

‡

ABSTRACT: In this work, the separation potential of a new “magmolecular” process for ions of antimony(III) from wastewater was studied. The magmolecular process is a technique in which a high gradient magnetic field is applied to eliminate heavy metal from wastewater. In this process, specifically we have synthesized two nanoadsorbent magnetic materials, MnFe2O4 nanoparticles both uncoated (MNPs) and amino-modified coated (AS-MNPs), and used them to remove antimony(III) from synthetic wastewater. Several variables affecting the adsorption behavior such as pH, contact time, temperature, amount of MNPs and AS-MNPs, initial concentration of antimony(III), and influences of co-ions were studied. The kinetics was exanimated using the Lagergren pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion methods. The equilibrium results were evaluated by Langmuir, Freundlich, and Dubinin−Radushkevich equations. The uptake was relatively rapid and followed pseudo-second-order kinetics in a manner suggesting chemical sorption. The best commentary for the adsorption results was obtained by the Langmuir model, and the maximum uptake capacity of MNPs and AS-MNPs was found to be (10.66 and 7.78) mg of antimony(III) per gram adsorbent at conditions of pH 2 and temperature of 298 K, respectively. Thermodynamic factors indicated that the uptake process was endothermic, spontaneous, and chemical in nature. After five uptake−desorption cycles, results showed that uptake−desorption efficiencies of AS-MNPs were better than that of MNPs.

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INTRODUCTION The use of magnetic nanoparticles as titled of “magmolecules” to reduce environmental difficulties is a new technique.1−3 In comparison to other separation techniques, the advantages of magnetic separation method are due to its high speed and great convenience. Magmolecules can be applied to remove pollutants from wastewater, and after the sorption is done completely, the sorbent is separated from the aqueous effluents using an implementing magnetic field. Nevertheless, the majority of the adsorbents employed in this method have the drawback of a low sorption capability or slow kinetics because of their small active sites or their poriferous characteristics. These drawbacks restrict the use of these adsorbents.4 Magmolecules surmount these major difficulties and can provide a larger sorption capability for the elimination of heavy metal ions.5 Manganese ferrite nanoparticles (MnFe2O4) with spinel structure are a renowned magnetic substance. It is anticipated for the adsorbent to be very effective for metal binding due to the functional groups of active sites, which usually create a large adsorption capacity. The pure magnetic nanoparticles might have problems such as stability of these pure particles for a long time without agglomeration or precipitation, alteration of magnetic properties, and their toxicity. Therefore, it is vital to develop efficient strategies to © XXXX American Chemical Society

coat a protective layer to ensure chemical stability and modify the surface of magnetic nanoparticles for the adsorption. One straightforward method is to improve the nanoparticles with a thin silica layer. Using silica coatings as the stabilizer has a number of advantages. Silica is chemically inert and has no effect on the redox reaction at the surface of nanoparticles. Also, the silica shell prevents or at least minimizes the effect of the outside environment and oxidation on the core nanoparticles. Moreover, the −OH groups on the active site of the magmolecule can easily bond with the functional amino group (−NH2). Amino-functionalized nanoadsorbents are expected to be effective and efficient for the reduction of heavy metal ions. For the present study, in this regard, the influences of solution pH, temperature, and contact time on the antimony(III) adsorption capacity onto MNPs and AS-MNPs, initial concentration of antimony(III), amount of adsorbent, and effects of co-ions were examined. In order to evaluate the equilibrium results, Langmuir, Freundlich, and Dubinin− Radushkevich (D−R) equations have been utilized. The reusability of MNPs and AS-MNPs during the successive Received: May 28, 2014 Accepted: September 16, 2014

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dx.doi.org/10.1021/je500462p | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Basic Information on the Sample Used in the Present Study chemical name

source

Mw/g·mol−1

ρ/g·cm−3

purity

linear formula

(3-aminopropyl) triethoxysilane tetraethyl orthosilicate manganese(II) chloride tetrahydrate iron(III) chloride hexahydrate sodium hydroxide hydrochloric acid fuming ammonia solution ethanol antimony(III) chloride

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Merck Merck Sigma-Aldrich Merck

221.37 208.33 197.91 270.30 40.00 36.46 35.05 46.07 228.12

0.946 0.933 2.01 1.82 2.13 1.19 0.903 0.789 3.14

99 % ≥ 99.0 % ≥ 99 % ≥ 99 % ≥ 98 % 37 % ≥ 25 % in H2O ≥ 99.8 % ≥ 99.0 %

H2N(CH2)3Si(OC2H5)3 Si(OC2H5)4 MnCl2·4H2O FeCl3·6H2O NaOH HCl NH4OH CH3CH2OH SbCl3

cooled and diluted to 1000 mL with reagent water. Working solutions in this study were made by the dilution of stock solution. In each adsorption experiment, an appropriate dosage of adsorbent was added to 20 mL of wastewater with suitable concentration of antimony(III) ions. The mixture was shaken at 150 rpm and 25 °C using a thermostatic shaker for a specific period of time. The pH amounts were measured using a digital pH meter (uncertainty of ± 0.05). After removal of the adsorbent by permanent magnet, the remaining antimony(III) concentrations in the solution were determined using the inductively coupled plasma (ICP) spectrometry. The batchwise adsorption tests were carried out as follows: the initial antimony(III) concentration (0.411 to 7.391) mmol·kg−1, the contact time (2 to 240) min, the adsorbent dosage (1 to 32) g· kg−1, pH (1 to 12), and the temperature (25 to 55) °C. The percentage of adsorbed antimony(III) was determined as follows:

uptake−desorption cycles has been examined. Moreover, the kinetics and thermodynamics of magmolecular sorption process of removal of antimony(III) have been investigated in detail.

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EXPERIMENTAL SECTION Materials. Table 1 demonstrates main characteristics of the chemicals used in this study, including their supplier, purity, molecular weight (Mw), density (ρ), and linear formula. No further purification was done before the experiments. Preparation of MnFe2O4 Nanoparticles. MnFe2O4 nanoparticles were synthesized via the coprecipitation process.6 Manganese ferrite nanoparticles were produced by using the following reaction: MnCl 2 + 2FeCl3 + 8NaOH → MnFe2O4 (s) + 8NaCl + 4H 2O

(1)

At the beginning, 200 mL of distilled water was deoxygenated for 30 min with nitrogen flow, and 0.02 mol of MnCl2·4H2O and 0.04 mol of FeCl3·6H2O were completely dissolved in distilled water under vigorous stirring with a constant speed. Then, NaOH of 1.5 mol·kg−1 was poured into the solution to achieve pH 11. The mixture continuously reacted at 90 °C for 2 h, and the resulting black precipitate was separated under magnetic field and washed with ionized water four times and ethanol once. Finally, the black MNPs were procured after freeze-drying. Preparation of MnFe2O4 at SiO2−NH2 Nanoparticles. The MNPs were then functionalized successively with tetraethyl orthosilicate and (3-aminopropyl) triethoxysilane to produce functional amino groups. Typically, 1 g of MnFe2O4 nanoparticles was dispersed in 20 mL of water, and 80 mL of ethanol was homogenized using ultrasonication for 15 min before the addition of 4 mL of NH3·H2O. After stirring vigorously for 30 min with a mechanical stirrer, 1.5 mL of tetraethyl orthosilicate was poured into the mixture. The interaction proceeded for 24 h, and then 0.2 mL pf aminopropyltriethoxysilane was added and the reaction continued for another 24 h at room temperature. The magmolecule was collected by permanent magnets, washed multiple times with ethanol and water to eliminate the impurities, and then dried at temperature of 110 °C for 24 h. Batch Adsorption Procedure. Adsorption tests were carried out in batch mode. Antimony(III) stock solution, 8.213 mmol·kg−1, was made from SbCl3. First 1.874 g of SbCl3 was dissolved in 0.5 mL of concentrated hydrochloric acid and 2 mL of nitric acid. The mixture was heated to effect solution and then cooled. A portion of 20 mL distillated water and 0.15 g of tartaric acid was added to the mixture while heating to disappear the white precipitates. Finally, the solution was

adsorption(%) =

(C i − Ce) ·100 Ci

(2)

where Ci and Ce are the initial and equilibrium concentrations of antimony(III) in wastewater, respectively. Reusability Test Procedure. Reusability is a crucial parameter for an efficient sorbent. To observe the reusability of MNPs and AS-MNPs, a reusability experiment consisting of subsequent five cycles of uptake−desorption was performed by utilizing 20 mL of HCl of 0.5 mol·kg−1 as eluent. The concentration of antimony(III) in wastewater was analyzed using ICP. The recovered magmolecules were thoroughly washed with the extra amount of HCl of 0.5 mol·kg−1 and ionized water to reutilize for the further test.

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RESULTS AND DISCUSSION Characterization of Magmolecules. The X-ray diffraction (XRD) pattern of magmolecules indicates six characteristic peaks at (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 0 0) revealing the cubic spinel structure of magmolecules according to the joint committee on powder diffraction standards. The median size of MNPs (≈ 6 nm) is estimated using the Debye−Scherrer equation to (3 1 1) XRD peaks. The prepared magnetic nanoparticles was further characterized by FTIR spectra. The vibration bands with peaks at (587 and 457) cm−1 are due to the presence of Fe−O and Mn−O bonds. The other two characteristic peaks at (3421 and 1630) cm−1 can be attributed the bending and stretching vibrations of H2O molecules and −OH, respectively.7 Both spectra of MNPs and AS-MNPs show a broad adsorption band in the range from (900 to 1100) cm−1, which can be assigned to Si−O stretching, and the bands at 803 cm−1 observed in the spectrum of AS-MNPs are corresponding to the bending B



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vibration of the −NH2 group.8 These results prove that tetraethyl orthosilicate and aminopropyltriethoxysilane were successfully bonded to the surface of the nanoparticles. Influence of pH. Since the wastewater pH plays a crucial role in the adsorption methods, the influence of wastewater pH on the removal of antimony(III) using MNPs and AS-MNPs was examined in a range of (1 to 12). The wastewater pH affects not only the degree of ionization but also active site charge of the adsorbent, and degree of dissociation of functional groups on the surface of the sorbent.9 Therefore, the influence of pH on adsorption of antimony(III) onto adsorbents was investigated at 25 °C by changing the pH of 0.411 mmol·kg−1 of antimony(III) wastewater for fixed adsorbent concentrations (16 and 8) g·kg−1 for MNPs and AS-MNPs, respectively. The results are presented in Figure 1.

wastewater is economically essential. For this purpose, the influence of the amount of adsorbent MNPs and AS-MNPs on the removal of antimony(III) adsorption was studied using shaking 0.411 mmol·kg−1 wastewater of antimony(III) ions employing the optimum factors. The amount of MNPs and ASMNPs was altered from (40 to 640) mg to a 20 mL of antimony(III) of wastewater. As is seen in Figure 2, an initial

Figure 2. Effect of magmolecule concentration (m) on the adsorption percent (AP) of antimony(III) onto MNPs and AS MNPs (antimony(III) concentration, 0.411 mmol·kg−1; temperature, 25 °C; pH, 2): ▲, MNPs; ●, AS-MNPs.

strong dependence of percentage adsorption is acquired up to about 16 g·kg−1 for both MNPs and AS-MNPs, and then a plateau portion is achieved, and the removal rate of antimony(III) ions did not remarkably increase. It means that an adsorbent dose of 16 g·kg−1 is the appropriate concentration for the uptake of antimony(III) from the used wastewater. However, the adsorbent concentration was used to be (16 and 8) g·kg−1 for MNPs and AS-MNPs for further investigations, respectively. Effects of Contact Time and Temperature. The contact time and temperature are key factors in practical sorption processes. The influence of the contact time on the adsorption of antimony ions onto MNPs and AS-MNPs under the conditions of following: antimony(III) concentration, 0.411 mmol·kg−1; adsorbent concentration, (16 and 8) g·kg−1 for MNPs and AS-MNPs, respectively; and pH, 2, are shown in Figures 3 and 4. It can be seen that the adsorption of antimony(III) onto MNPs is very faster than AS-MNPs and the antimony(III) uptake by MNPs and AS-MNPs was completed within (10 and 120) min which reveals that the functionalization of MNP surface affected the sorption equilibration time. Also, it is seen that the adsorption of antimony(III) on AS-MNPs took place quickly within the first 30 min (63 % at 25 °C), and a significant uptake of antimony(III) ions (80 % at 25 °C) occurred at 120 min. After this period of time, a considerable increase in the values of the removal percentage of antimony(III) ions did not happen, and thus (10 and 120) min times were selected as the optimal contact times for MNPs and ASMNPs, respectively. This finding is very reassuring because the contact time is a crucial factor which can be used to design an optimal treatment plant of wastewater. The influence of temperature on the uptake of antimony(III) onto AS-MNPs

Figure 1. Effect of pH on the adsorption percent (AP) of antimony(III) onto MNPs and AS-MNPs (antimony(III) concentration, 0.411 mmol·kg−1; magmolecules concentration, 16 g·kg−1 for both MNPs and AS-MNPs, respectively; temperature, 25 °C): ▲, MNPs; ●, AS-MNPs.

In wastewater, the antimony(III) is presented as [SbO]+ and [Sb(OH)2]+ form at pH < 2, while [HSbO2] and [Sb(OH)3] species are abundant at pH (2 to 10). At pH > 10 the concentration of [SbO2]− increases in solution.10 In this study, the maximum adsorption was given at pH 2 for both MNPs and AS-MNPs. At pH < 2, the H3O+ ions exist abundantly in the wastewater and compete strongly with antimony species, [SbO]+ and [Sb(OH)2]+, for the active sites, thus leading to a lower adsorption of antimony ions. At higher pH (pH > 2), competitive adsorption probably occurred between OH− ions and the antimony species with a negative charge, [SbO2]−, resulting in a low level of antimony adsorption. Therefore, the optimum pH 2 was utilized in further tests unless otherwise mentioned. In some industries such as lead−acid batteries, the wastewater pH is about 2, and the antimony concentration is approximately 0.616 mmol·kg−1.11 So this condition is like this study. Furthermore, antimony solubility is very high at low pH. Therefore, our adsorbent is excellent for these conditions. Similar observations were previously presented for the antimony adsorption on goethite (α-FeOOH),10 rice husks,12 metal-loaded saponified orange waste,13 and activated alumina.14 Effect of Adsorbent Concentration. Determining the best amount of adsorbents for removal heavy metal ions from C



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Dubinin−Radushkevich (D-R) equations have been selected to analyze the adsorption data. The Langmuir isotherm assumes single layer uptake on a homogeneous active site with a finite amount of surface area and has obtained significant applications in many single layer uptake processes. The nonlinear Langmuir equation can be presented as:16 qe =

qmKLCe 1 + KLCe

(3)

where qe is the equilibrium uptake capacity per unit weight adsorbent, qm is the maximum adsorption capacity per unit weight adsorbent, Ce is the equilibrium concentration of the solution, and KL is the Langmuir adsorption constant. Nonlinear regression analysis is performed to calculate KL, qm, and the standard error. As indicated in Figure 5 and Tables 2 and 3, the correlation coefficient (R2) was determined to be (0.8321 and 0.9905) for MNPs and AS-MNPs, respectively.

Figure 3. Effect of contact time (t) on the adsorption percent (AP) of antimony(III) onto MNPs (antimony(III) concentration, 0.411 mmol· kg−1; temperature, 25 °C; pH, 2).

Figure 4. Effect of magmolecule concentration of AS-MNPs in the adsorption percent (AP) of antimony(III) (antimony(III) concentration, 0.411 mmol·kg−1; pH, 2): ○, T = 25 °C; ◊, T = 35 °C; △, T = 45 °C; □, T = 55 °C.

was also examined implementing the optimum parameters. As also seen from Figure 4, with the rise in temperature from (25 to 55) °C, the uptake capacities of antimony(III) ions onto ASMNPs increased from (80 to 90) %. Such behavior is expected of an endothermic adsorption method. The increase in temperature increased the percent of adsorption of antimony(III), which is due to an increase in the number of active sites attainable for uptake of antimony(III) ions on MNPs and ASMNPs and the decrease in the thickness of the mass transfer boundary layer surrounding the sorbents leading to a decline in the mass transfer resistance. Inasmuch as diffusion is an endothermic process, the rate of mass transfer increases with temperature.15 Adsorption Isotherm Model. The adsorption isotherm model explains how adsorbates generally interact with adsorbents, and therefore it is crucial in optimizing the design of adsorbents and adsorption conditions. Also, the isotherm models can be applied to make a comparison of the sorption capabilities of the adsorbents for contaminants in wastewaters. In this research, therefore, Langmuir, Freundlich, and

Figure 5. Langmuir isotherm curves observed for the uptake of antimony(III) onto magmolecules of MNPs (a) and AS-MNPs (b) (magmolecule concentration, (16 and 8) g·kg−1 for MNPs and ASMNPs, respectively; contact time, 120 min; pH, 2): ○, T = 25 °C; ◊, T = 35 °C; △, T = 45 °C; □, T = 55 °C.

The antimony(III) maximum uptake capacity on MNPs and AS-MNPs at room temperature (25 °C) was found to be (10.66 and 7.78) mg·g−1 for MNPs and AS-MNPs, respectively. The KL values was found as (0.7665 and 0.3744) kg·mmol−1 for MNPs and AS-MNPs. It is clear that the adsorption capacities of AS-MNPs compared with MNPs have decreased. The D



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Table 2. Isotherm Constants for the Adsorption of Antimony(III) onto Magmolecules of MNPs at Various Temperaturesa Langmuir T

qm

K

mg·g

298.15 308.15 318.15 328.15 a

Freundlich

KL −1

kg·mmol

10.66 11.69 13.06 19.84

Dubinin−Radushkevich (D−R)

Kf −1

0.7665 1.3106 0.8631 0.393

RL 0.8392−0.9631 0.7532−0.9385 0.8225−0.9586 0.9105−0.9807

2

R

0.8321 0.8642 0.7854 0.8947

−1 1−1/n

n

R

(mg·g )

8.29 9.09 7.32 6.66

β

qm

6.1029 7.1975 6.9818 9.4980

2

0.9711 0.9135 0.9687 0.9611

mg·g

−1

16.55 19.09 21.82 35.13

E −2

mol ·kJ 2

−3

1.12·10 1.06·10−3 1.14·10−3 1.27·10−3

R

2

0.9931 0.9788 0.9930 0.9923

kJ·mol−1 20.43 21.68 20.97 19.83

Standard uncertainty u is u(T) = 0.05 K.

Table 3. Isotherm Constants for the Adsorption of Antimony(III) onto Magmolecules of AS-MNPs at Various Temperaturesa Langmuir

a

Freundlich

Dubinin−Radushkevich (D−R)

T

qm

KL

qm

β

K

mg·g−1

kg·mmol−1

RL

R2

n

(mg·g−1)1−1/n

R2

mg·g−1

mol2·kJ−2

R2

kJ·mol−1

298.15 308.15 318.15 328.15

7.78 8.18 9.07 9.24

0.37 0.42 0.35 0.56

0.9144−0.9816 0.9059− 0.9796 0.9204−0.9830 0.8778−0.9729

0.9905 0.9890 0.9904 0.9901

9.77 9.45 7.95 9.25

4.85 5.00 5.06 5.71

0.9708 0.9718 0.9728 0.9796

12.11 12.77 15.74 16.02

1.13·10−3 1.15·10−3 1.40·10−3 1.28·10−3

0.9463 0.9505 0.9438 0.9282

21.07 20.85 18.89 19.76

Kf

E

Standard uncertainty u is u(T) = 0.05 K.

disparity can come from the amino groups on the active site of MnFe2O4 at SiO2, indicating amino groups acted as inefficient chelating surface for sorption of antimony(III) under studied conditions. If we define q0 as the value of q in equilibrium with a solution having solute concentration C0, we can define dimensionless concentrations: y=

q q0

x=

C C0

(4)

And then write the Langmuir isotherm in dimensionless form as x y= RL + (1 − RL)x (5) The parameter RL is usually called the separation factor, and for the Langmuir isotherm it can be shown to be equal to17 RL =

1 1 + KLC0

(6)

In most adsorption situations with solids like our adsorbents, the solute strongly prefers the solid phase over the fluid phase and RL < 1. If RL is greater than 1, the isotherm has a concave shape, indicating that the solute prefers the fluid phase over the solid phase. The case of RL = 1 corresponds to a linear isotherm (y = x). The RL values in this investigation were in the range from (0.7532 to 0.9807) and (0.8778 to 0.9830) for MNPs and AS-MNPs, respectively (Tables 2 and 3), showing a desirable adsorption process in removal of antimony(III). The Freundlich model is suitable to both single layer and multilayer sorption. This model assumes that the active site of the adsorbent is heterogeneous and the adsorption is a nonuniform distribution of heat of adsorption. The Freundlich equation is18 qe = K f Ce1/ n

Figure 6. Freundlich isotherm curves observed for the uptake of antimony(III) onto magmolecules of MNPs (a) and AS-MNPs (b) (magmolecule concentration, (16 and 8) g·kg−1 for MNPs and ASMNPs, respectively; contact time, 120 min; pH, 2): ○, T = 25 °C; ◊, T = 35 °C; △, T = 45 °C; □, T = 55 °C.

(7)

The Kf and n isotherm constants are evaluated from nonlinear regression. As shown in Tables 2 and 3, the Kf parameters were found to be (6.103 and 4.855) for MNPs and AS-MNPs, respectively; also the n values were estimated to be

where Kf and n are the Freundlich constants related to uptake capacity and uptake intensity, respectively. Figure 6 presents the nonlinear Freundlich isotherm plots. E



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Figure 7. Fitting of different kinetic equations for antimony(III) uptake onto AS-MNPs magmolecules at various temperatures (antimony(III) concentration, 0.411 mmol·kg−1; pH, 2; magmolecule concentration, 8 g·kg−1): ○, T = 25 °C; ◊, T = 35 °C; △, T = 45 °C; □, T = 55 °C: (a) pseudo-first order ; (b) pseudo-second order; (c) Elovich kinetic model; (d) intraparticle diffusion.

uptake energy (E) can be calculated from eq 10 using the D−R parameter β (Tables 2 and 3):

(0.121 and 0.102), respectively. The Freundlich constant 1/n was smaller than 1, implying a desirable adsorption process of antimony(III) onto MNPs and AS-MNPs. Moreover, the experimental equilibrium data have been employed to the Dubinin−Radushkevich (D−R) isotherm. The D−R isotherm is linearized as19 ln qe = ln qm − βε 2

E=

(10)

The mean sorption energy gives useful information about the type of uptake such as physical and chemical sorption. If E < 8 kJ·mol−1, physical uptake occurs. If E > 16 kJ·mol−1, chemisorption is the dominant factor.20 These values are greater than 16 kJ mol−1 showing the uptake of antimony(III) on MNPs and AS-MNPs to be dominated by chemical forces. The Langmuir, Freundlich, and D−R constants for the sorption of antimony(III) onto ASMNPs has been listed in Table 3. These results confirmed that the Langmuir equation fitted the sorption data better than the Freundlich and Dubinin−Radushkevich equations on the basis of the R2 values in Table 3. Kinetics of Adsorption. The study of chemical kinetics is economically very important in designing sorption process and selecting optimum operating conditions. Thus, the kinetics of

(8)

where qe is the equilibrium metal ion concentration on sorbent, β is the constant related to the adsorption energy, qm is the highest sorption capacity, and ε is the Polanyi constant, which is computed from the following equation:

⎛ 1⎞ ε = RT ln⎜1 + ⎟ Ce ⎠ ⎝

1 2β

(9)

The sorption data have been examined by plotting ln qe against ε2 to determine the amount of qm and β from the intercept and slope, respectively (figures not shown). The F



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(Figure 7b). The rate constants k2, the estimated equilibrium sorption capacities qe(calc.), and the coefficients of determination values R2 obtained by linear regression are summarized in Table 4. The Elovich equation is often employed to elucidate the kinetics of adsorption and effectively explain the predominantly chemical uptake on highly heterogeneous adsorbents. The Elovich equation was expressed as

antimony(III) adsorption onto AS-MNPs were analyzed using pseudo-first order,21 pseudo-second order,22 Elovich kinetic model,23 and intraparticle diffusion,24 where qe and qt are the uptake capacity at equilibrium and at time t, h is the initial sorption rate in pseudo-second order kinetics, and k is the rate constant. The sum of squared errors (SSE) between the theoretical amounts and the empirical data can be computed using the following relation: SSE =

n ∑i = 1 (qexp

qt =

2

− qcalc)

where qexp is experimental sorption capacity of antimony(III) on AS-MNPs, qcalc was obtained from the kinetic models, and N is the number of data points. The pseudo-first-order model is expressed as follows: ln(qe − qt ) = ln qe − k1t

(12)

The Lagergren-first-order rate constants k1 were determined from the slope of Figure 7a. The coefficients of determination (R2) were estimated in the range of (0.9531 to 0.8335) shown in Table 4. The pseudo-second-order model is expressed as follows: t 1 t = + 2 qt qe k 2qe

h = k 2qe2

qt = k intra(t )0.5 + C

(13)

Table 4. Kinetic Constants for the Adsorption of Antimony(III) onto Magmolecules of AS-MNPs at Various Temperaturesa T/K

qe(exp.)/mg·g−1

298.15

308.15

318.15

4.6250 4.8125 5.1697 Pseudo-First-Order Equation qe(cal.)/mg·g−1 2.7782 2.4198 1.6313 k1/min−1 0.0238 0.0247 0.0258 R2 0.9531 0.9322 0.8626 SEE 0.1910 0.2410 0.3718 Pseudo-Second-Order Equation qe(cal.)/mg·g−1 4.8876 5.0176 5.2826 k2/g·mg−1·min−1 0.0332 0.0436 0.0832 h/mg·g−1·min−1 0.7920 1.0985 2.3229 R2 0.9987 0.9994 0.9999 SEE 0.2649 0.1765 0.0616 Elovich Equation qe(cal.)/mg·g−1 4.7201 4.9885 5.4373 α/mg·g−1·min−2 2.8391 5.1830 36.3547 β/g·mg−1·min−1 1.2158 1.2816 1.5716 R2 0.9958 0.9764 0.9122 SEE 0.0790 0.1800 0.2932 Intraparticle Diffusion Equation qe(cal.)/mg·g−1 4.6429 4.8335 5.1866 kintra/mg·g−1·min−1/2 0.1725 0.1415 0.0854 C 3.0064 3.4911 4.3764 R2 0.9969 0.9940 0.9912 SEE 0.0279 0.0326 0.0258

(15)

where kintra is the intraparticle diffusion rate constant and C is the intercept. The value of C relates to the thickness of the boundary layer. If adsorption is controlled by the intraparticle diffusion process, the plot should be linear when plotting qt as a function of t−1/2. As shown in Figure 7d, it can be seen that the points are not linear rate for the whole contact time but give two straight lines with two different slopes, which indicates that the intraparticle diffusion is not the only controlling stage and other mechanisms may dominate.25 The first steeper region could be due to surface adsorption, whereas the second region was related to intraparticle diffusion to reach the equilibrium. The intercept of the plot gave an estimate of the thickness of the boundary layer; that is, the larger the intercept value, the greater the boundary layer effect was. The lower SSE and higher coefficients of determination (R2) values for pseudosecond-order and intraparticle diffusion models indicated that the adsorption followed pseudo-second order and intraparticle diffusion mechanisms.26 Thermodynamics of Adsorption. Thermodynamic analyses have major importance for practical use of an adsorption process. An increase in temperature led to an increase in sorption rate, implying that the adsorption of antimony(III) on MNPs and AS-MNPs is endothermic. Parameters of thermodynamic comprising the standard Gibbs free energy (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°) associated with the adsorption process were calculated by using eq 16:

The pseudo-second order model parameters k2 and qe can be determined from the intercept and slope of the plots of t/qt vs t

kinetic models and parameters

(14)

where α is the initial adsorption rate and β is the desorption constant. Here the relationships between qt and ln(t) were illustrated in Figure 7c. Therefore, the constants can be obtained from the slope and the intercept of a straight line plot of qt against ln(t) and are given in Table 4. In order to investigate the controlling stage of sorption rate, further investigation is needed to find out the effect of intraparticle diffusion to the entire sorption process. The intraparticle diffusion model is given as the following:

(11)

N

1 1 ln(αβ) + ln(t ) β β

328.15 5.3114 1.3883 0.0215 0.8335 0.3466 5.3879 0.1114 3.2352 0.9999 0.0689 5.5180 280.5976 1.9589 0.8995 0.2534

ΔG° = RT ln KD

(16)

where R is the universal gas constant, T is the temperature, and KD (qe/Ce) is the distribution coefficient (or the equilibrium constant).27 The enthalpy (ΔH°) and entropy (ΔS°) parameters were estimated from eq 17:

5.3172 0.0658 4.6930 0.9979 0.0094

ln KD =

ΔS o ΔH o − R RT

(17)

According to eq 17, the ΔH° and ΔS° parameters were determined from the slope and intercept of the plot of ln KD versus 1/T, respectively (Figure 8 and Table 5).

a

Standard uncertainties u are u(T) = 0.05 K and u(qe(exp)) = 0.02 mg·g−1. G



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⎛ E ⎞ k 2 = k exp⎜ − a ⎟ ⎝ RT ⎠

(18)

where k is the independent of temperature parameter, Ea is the activation energy of adsorption, R is the universal gas constant, and T is the solution temperature. A plot of ln k2 versus 1/T yields a straight line, with slope Ea/R (Figure 9) and a high correlation coefficient of 0.994.

Figure 8. Plot of ln KD vs 1/T observed for the uptake of antimony(III) onto MNPs and AS-MNPs magmolecules (pH, 2; magmolecule concentration, (16 and 8) g·kg−1 for MNPs and ASMNPs, respectively; contact time, 120 min): ▲, MNPs; ●, AS-MNPs.

The enthalpy change, ΔH°, and the entropy change, ΔS°, for the sorption of antimony(III) were estimated to be 18.61 kJ· mol−1 and 99.45 J·mol−1·K−1 for MNPs, respectively, while these parameters were obtained to be 16.80 kJ·mol−1 and 88.83 J·mol−1·K−1 for AS-MNPs, respectively. The positive values of ΔH° indicate that the sorption of antimony(III) on MNPs and AS-MNPs was carried out as endothermic between (25 and 55) °C. The increase of antimony(III) uptake on AS-MNPs with the rise of temperature from (25 to 55) °C prove these findings. The positive values of ΔS° suggested the increasing fortuitousness at the solid−liquid interface at the time of antimony(III) adsorption on adsorbents because of the affinity of MNPs and AS-MNPs toward antimony(III) and some structural alterations in adsorbate and adsorbent. The negative ΔG° values indicated that the sorption of antimony(III) was feasible and spontaneous, and the lowering of ΔG° with rising temperature signified that the sorption process was more desirable at high temperatures. Also, these amounts (lower than −20.0 kJ·mol−1) suggest that the electrostatic reaction may play an important role in the sorption of antimony(III) on MNPs and AS-MNPs. Activation Energy of Adsorption. The rate constants of antimony(III) adsorption were calculated at several temperatures from equilibrium results supposing a pseudo-secondorder model. The activation energies for the adsorption processes are determined using the Arrhenius equation, as shown below:

Figure 9. Determination of the activation energy (E) for antimony(III) uptake onto AS-MNPs magmolecule.

The magnitude of the activation energy is important in any of adsorption process because it gives information about the mechanism of the reactions. Because the sorption of antimony(III) increased with temperature, the slope of Arrhenius plot yields a negative value, and the activation energy is calculated to be positive. The activation energy for the sorption of antimony(III) on AS-MNPs was 32.95 kJ·mol−1 calculated from the slope of this plot indicating that the adsorption of antimony(III) onto AS-MNPs is a chemically controlled process. These results showed that the antimony(III) sorption process by AS-MNPs is endothermic and involves chemical sorption.28 Effects of Competing Ions. The presence of competing ions can dramatically alter metal eliminate in comparison with the metal-only system. In systems with more than one adsorbate, the competition between the adsorbates for active sites of adsorbent may depend on the number of surface sites, the kind and concentration of the competing adsorbates, and the affinity of the adsorbent toward the adsorbate. To investigate the effects of competing ions, selenium were selected as co-ions on antimony removal. The initial concentration of antimony was fixed at 0.411 mmol·kg−1,

Table 5. Thermodynamic Parameters of Antimony(III) Adsorption onto Magmolecules at Different Temperaturesa MNPs T K 298.15 308.15 318.15 328.15 a

ln KD L·kg

−1

4.46 4.60 4.85 5.19

ΔG° kJ·mol

−1

−11.05 −11.78 −12.82 −14.15

AS-MNPs ΔH° −1

kJ·mol

18.61

ΔS° −1

ln KD −1

J·mol ·K 99.45

L·kg

−1

3.88 4.05 4.31 4.53

ΔG° kJ·mol

−1

−9.61 −10.37 −11.03 −12.38

ΔH° −1

kJ·mol

16.80

ΔS° J·mol−1·K−1 88.83

The standard uncertainty u is u(T) = 0.05 K. H



Article

that the sorption of antimony(III) on MNPs and AS-MNPs may be construed as chemisorption. Thermodynamic analysis demonstrated that the uptake process was endothermic and spontaneous. The highest desorption efficiency ((90 and 80) % for MNPs and AS-MNPs, respectively) was achieved using 0.5 mol·kg−1 HCl. After five uptake−desorption cycles, the reusability of MNPs and AS-MNPs reduced to (14 and 7) % for uptake and (32 and 10) % for desorption, respectively. This process is able to surmount the separation problems related to uptake-based remediation methods with its advantages of high capacity, high speed, and high performance. Additional studies to confirm the capability of this adsorbent with regard to the uptake of antimony(III) in a continuous process can be worth extending treatment approaches for wastewater with toxic metal ions.

whereas the selenium initial concentration varied between (0.063 and 1.266) mmol·kg−1. Varying the concentration of selenium ions from (0.063 to 1.266) mmol·kg−1 did not influence on the adsorption process of antimony onto MNPs and AS-MNPs. This may be due to antimony having a higher complexing ability with MNPs and AS-MNPs than selenium as a co-ion. Reusability. In numerous processes, reutilizing the adsorbents is economically essential. With the rising prices of materials, the regeneration processes have received considerable attention. In this study, the sorption of antimony(III) on the MNPs and AS-MNPs was extremely dependent on pH. Therefore, the uptake of antimony(III) can be accomplished by decreasing the pH of solution. The desorption efficiency was calculated as follows: desorption efficiency(%) =

CeVd ·100 qeM

■

(19)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98(021)82064483. Fax: +98(021)88221113. Address: P.O. Box 14395-836, Tehran, Iran.

where Vd is the volume of the eluent, M is the mass of the magmolecule, Ce is the equilibrium concentration, and qe is the sorption capacity. The uptake−desorption cycles, utilizing the same magmolecule and 0.5 mol·kg−1 HCl as eluent, were performed five times to evaluate the regeneration capability of these nanoadsorbents. After the adsorption of antimony(III) on MNPs and AS-MNPs was completed, the magmolecules were separated using a permanent magnet and then added to 20 mL of 0.5 mol·kg−1 HCl and shaken for 3 h. The magmolecules were thoroughly washed with distillate water and reconditioned for adsorption in the succeeding cycle. The capacity of MNPs and AS-MNPs for the readsorption of antimony(III) in consecutive cycles was examined, and a comparison was made. After the fifth cycle, the antimony(III) adsorption efficiency for MNPs and AS-MNPs was reduced to (14 and 17) %, respectively. Also, the desorption efficiency was reduced to (32 and 10) % for MNPs and AS-MNPs during five cycles, respectively. These findings show that the desorption efficiency AS-MNPs is better than MNPs because the shell of silica around AS-MNPs protects it in acidic solution. These findings indicate that the stability and durability of MNPs and ASMNPs are almost acceptable for practical uses.

Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS In this work, a new process combining magnetic nanoadsorbent and magnetic separation, as titled the “magmolecular process”, was developed for the uptake and recovery of antimony(III) from aqueous solution. Two kinds of magnetic nanoadsorbent MnFe2O4 nanoparticles (MNPs) and amino-modified silicacoated MnFe2O4 nanoparticles (AS-MNPs) had been prepared, characterized, and evaluated as magmolecules of antimony(III) removal. Several parameters affecting the adsorption behavior such as temperature, contact time, pH, amount of MNPs and AS-MNPs, initial concentration of antimony(III), and influences of co-ion have been examined. The sorption of antimony(III) on adsorbent was rather quick, and the highest uptake of antimony(III) took place at pH 2 for both magmolecules. The sorption results were studied applying the Langmuir, Freundlich, and Dubinin−Radushkevich equations. It has been seen that the adsorption of antimony(III) was well-fitted to the Langmuir isotherm with maximum uptake of ((10.66 to 19.84) and (7.78 to 9.24)) mg·g−1 at (298 to 328) K, respectively. The mean adsorption energies ((20.43 and 21.07) kJ·mol−1 for MNPs and AS-MNPs, respectively) demonstrated I



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Process for the Separation of Antimony(III) - American Chemical Society

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