Schiff Base Ligands Immobilized on a Nanosized ... - ACS Publications

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Schiff Base Ligands Immobilized on a Nanosized SiO2Al2O3 Mixed Oxide as Adsorbents for Heavy Metals Mohammad Arshadi,† Mehran Ghiaci,*,† and Antonio Gil‡ † ‡

Department of Chemistry, Isfahan University of Technology, Isfahan, Iran, 8415683111 Department of Applied Chemistry, Los Acebos Building, Public University of Navarra, Campus of Arrosadia, E-31006, Pamplona, Spain ABSTRACT: Three adsorbents have been synthesized by immobilization of Schiff base ligands on the surface of SiO2Al2O3 mixed oxides and evaluated for the removal of Cd(II) and Pb(II) from aqueous solutions. The contact time to attain equilibrium for maximum adsorption was 120 min. These heterogeneous Schiff base ligands were found to be effective adsorbents for the removal of heavy metal ions from solution, with Si/Al-pr-N=salicylaldehyde having a high adsorption capacity for Cd(II) ions and Si/Al-prN=pyridine-2-carbaldehyde having a high adsorption capacity for Pb(II) ions. The adsorption of heavy metal ions has been studied in terms of pseudo-first-order and pseudo-second-order kinetics, and the Freundlich, Langmuir, and LangmuirFreundlich isotherm models have also been applied to the equilibrium adsorption data.

1. INTRODUCTION The release of various harmful heavy metal ions into the environment has attracted great attention worldwide in recent years because of their toxicity and widespread use. In light of this, it is vital to develop simple, rapid, and efficient methods for monitoring metal ions in the environment. In particular, many attempts have been made to develop materials to remove Pb(II) and Cd(II) from drinking water.1,2 Adsorption processes have been reported to be low-cost alternatives for the treatment of heavy metals present in wastewater. The use of activated carbons, modified clays, polymeric resins, waste materials, and zeolites as adsorbents has also been described.35 Activated carbon is the most popular adsorbent for this process due to its high surface area, high adsorption capacity, and high degree of surface reactivity, although it is expensive and must be regenerated on a regular basis. Inorganic supports6,7 offer several advantages with respect to activated carbon, including better mechanical stability 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 modified inorganic adsorbents for Pb(II) and Cd(II) are summarized in Table 1. Due to the toxicity of Pb(II) and Cd(II), the Agency for Toxic Substances and Disease Registry of the U.S. Department of Health and Human Services has designated these chemicals as priority pollutants.17 The maximum permissible limit established by the U.S. Environmental Protection Agency (USEPA) for Pb(II) is 0.05 mg/dm3 in wastewater (0.015 mg/dm3 in drinking water). The relevant EU Directive, as well as the USEPA and the FAO/WHO, have established a maximum contaminant level of 5 mg/dm3 for Cd(II) cations in drinking water. Pb(II) is a highly toxic substance that can produce a wide range of adverse health effects in both adults and children upon exposure. Thus, very low levels of exposure can result in reduced intelligence quotient, learning disabilities, attention deficit disorders, behavioral problems, stunted growth, impaired hearing, and kidney damage in children under the age of six years. At high levels of exposure, such children may become mentally retarded, r 2011 American Chemical Society

fall into a coma, and even die from lead poisoning. In adults, lead can increase blood pressure and cause fertility problems, nerve disorders, muscle and joint pain, irritability, and memory or concentration problems.17 The health effects of Cd(II) on humans include nausea, vomiting, diarrhea, muscle cramps, salivation, loss of calcium from bones, yellow coloration of teeth, reduction of red blood cells, bone marrow damage, hypertension, kidney failure following oral ingestion, lung irritation, chest pain, and loss of sense of smell after inhalation. Chronic cadmium poisoning produces proteinuria and affects the proximal tubules in the kidney, thereby resulting in the formation of kidney stones.17 The aim of the present study was to apply three Schiff base ligands immobilized on the surface of SiO2Al2O3 mixed oxides using a heterogeneous method. SiO2Al2O3 mixed oxides, or composites of them, are widely used as catalysts and ceramic materials.1821 Clay minerals,18 zeolites,19,22 mullite,20 and SiO2Al2O3 catalysts21,23 are examples of materials where the existence of AlOSi bond structures or Al2O3SiO2 interfaces controls their final performance for the desired application. The surface formed contains multidentate ligands that were inspired by their previous application in bioinorganic chemistry (enzyme modelization) and in medical imaging, such as a heteroatom in the organic chain. Furthermore, the high content of amino and oxygen groups allows chemical modification of SiO2Al2O3 mixed oxides in order to improve their adsorbent properties, such as selectivity and adsorption capacity.

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents (AR grade) were purchased from Merck and Fluka and used without further purification, except for solvents, which were treated according to standard methods. Received: July 14, 2011 Accepted: November 12, 2011 Revised: October 7, 2011 Published: November 12, 2011 13628

dx.doi.org/10.1021/ie2015153 | Ind. Eng. Chem. Res. 2011, 50, 13628–13635

Industrial & Engineering Chemistry Research

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Table 1. Adsorption Capacities of Pb(II) and Cd(II) on Various Adsorbents amount adsorbed (mg/g) adsorbent

Pb(II)

Cd(II)

reference

256

1

143 54

8 9

PSDCc

39

10

modified γ-alumina

96

11

8-hydroxyquinolineimmobilized bentonite grafted silicaa SG-PS-azo-IMb

2.3. Adsorption Measurements. Adsorption experiments were carried out in batch conditions: 0.1 g of immobilized Schiff base ligand was shaken with 30 cm3 of the inorganic pollutant, at a concentration of between 5 and 100 mg/dm3, at a controlled temperature of 25 °C. The time required to reach equilibrium conditions was determined by preliminary kinetic measurements. After centrifugation at 3000 rpm for 30 min, the liquid phase was separated and the solute concentration was determined by atomic absorption spectrophotometry using a Perkin-Elmer 2380 instrument. The amount adsorbed was calculated as

qe ¼ V ðC0 Ce Þ=m

nanoparticlesd S8-1Ne f

8.4

9.2

12

8.3

12

S8-3N

9.3

chitin

13

13

hybrid macroporous materialg

39

14

Sil-SSHh

496

246

15

T-DTPMPAi

140

890

Si/Al-pr-N=pyridine-

88

3.8

16

91

72

where C0 and Ce are the initial and equilibrium liquid-phase concentrations (mg/dm3) of adsorbates, V is the volume of the solution (dm3), and m is the amount of adsorbent (g). Equation 1 assumes that the change in volume of the bulk liquid phase is negligible as the solute concentration is small and the volume occupied by the adsorbent is also small. The amount of heavy metals adsorbed on the sample was calculated using a previously determined calibration curve.

this work

2-carbaldehyde Si/Al-pr-N=salicylaldehyde

ð1Þ

this work

a

Silica is grafted with N-[3-(trimethoxysilyl)propyl]ethylenediamine. Silica gel microspheres encapsulated by imidazole functionalized polystyrene. c N,N-di(carboxymethyl)dithiocarbamate chelating resin. d Nano alumina modified with 2,4-dinitrophenylhydrazine. e Porous silica modified with 3-aminopropyltriethoxysilane. f Porous silica modified with N1-(3-trimethoxysilylpropyl)diethylenetriamine. g TiO2O3S PrSH. h Silica gel surface modified with 3-mercaptopropyltrimethoxysilane. i Porous titania modified with diethylene triamine pentamethylene phosphonic acid. b

The SiO2Al2O3 mixed oxide was prepared according to a previously reported procedure.23 SiO2Al2O3-supported 3aminopropyl (see Scheme 1) was prepared by refluxing 5.2 g of SiO2Al2O3, previously activated by heating at 550 °C for 6 h under air, with 3.5 cm3 (0.0195 mol) of 3-aminopropyl-trimeth oxysilane (3-APTES) in dry dichloromethane (100 cm3) for 24 h. The resulting solid was filtered, washed with dry methanol and dry dichloromethane, and dried under vacuum at 100 °C for 6 h. The functionalized SiO2Al2O3 mixed oxides are hereafter referred to as Si/Al-pr-NH2. The appropriate aldehyde or ketone (salicylaldehyde, methyl-2-pyridylketone, or pyridine-2-carbaldehyde) was then added to a suspended solution of Si/Al-pr-NH2 in dry methanol and the mixture was refluxed for 24 h to prepare a Schiff base (bidentate ligand; see Scheme 1) on the surface of the mixed oxide. 2.2. Characterization Techniques. Elemental analyses were performed using a Heraeus CHN-RAPID elemental analyzer. Nitrogen (99.999%) adsorption experiments were performed at 196 °C using a volumetric apparatus (Quantachrome NOVA automated gas sorption analyzer). All samples were degassed under vacuum at 120 °C for 16 h before the adsorption experiments. The specific surface areas were calculated using the BET method and the total pore volume from the nitrogen adsorbed at a relative pressure of 0.95.24 IR spectra were recorded using a Jasco FT/IR-680 plus spectrophotometer and KBr pellets. Diffuse reflectance spectra were recorded using a JascoV550 UV/vis spectrophotometer.

3. RESULTS AND DISCUSSION Heterogenized Schiff base ligands are characterized by their high percentage of nitrogen, present in the form of imine, and hydroxyl groups, which are responsible for metal ion binding through chelate formation. The decrease of the surface area after functionalization is due to the presence of bidentate groups, which block the access of nitrogen molecules into the structure of the SiO2Al2O3 mixed oxide (Table 2). The infrared absorption values (cm1) of significant valence vibrations, which are collected in Figure 1 and Table 3, are helpful for identification of the immobilized ligands. Thus, the band around 1050 cm1 is due to the asymmetric stretching vibration of (Si/Al)O4 units of the SiO2Al2O3 mixed oxide, whereas the bands at 29212943 and 28452877 cm1 are assigned to the stretching mode of the CH2 groups. The presence of these bands strongly suggests that the SiO2Al2O3 mixed oxide has successfully been modified by amine spacer groups. Likewise, the NH deformation peak at 15401560 cm1 confirms the successful functionalization of the SiO2Al2O3 mixed oxide with 3-APTES. In accordance with previously reported data,25,26 the CdN (Schiff base) absorptions appear in the region 16311639 cm1, the skeletal vibration of benzene appears in the range 1590 1600 cm1, and the CdN vibration of the pyridine groups appears in the range 15691571 cm1.27 The peaks in the range 30523070 cm1 are attributed to the CH stretching vibrations of the phenyl group, and the peaks at 14341437 cm1 can be assigned to the CdC stretching vibration of the phenyl group. Finally, the Fourier transform infrared (FT-IR) spectra of all Schiff bases clearly show the CH vibrations of phenyl and pyridine groups at 30523070 cm1, thus further confirming the presence of phenyl and pyridine groups on the SiO2Al2O3 mixed oxide after immobilization of the ligands. Anchoring of ligands on the solid surface was also followed by diffuse reflectance UVvis spectroscopy of the resulting adsorbents.25,27 Thus, although the UVvis spectrum of the SiO2 Al2O3 mixed oxide only had a sideband adsorption close to 244 nm, the spectra of the immobilized Schiff base ligands were characterized by strong absorptions in the 230360 nm region due to the π f π* and n f π* transitions of the ligands. 13629

dx.doi.org/10.1021/ie2015153 |Ind. Eng. Chem. Res. 2011, 50, 13628–13635


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Scheme 1. Procedure Followed To Immobilize Schiff Base Ligands on SiO2Al2O3 Mixed Oxide

Table 2. Chemical Composition and Textural Properties of Schiff Base Ligands Immobilized on SiO2/Al2O3 Mixed Oxide textural parameters system

a

a

SBET (m /g)

Vp (cm3/g)

dpb (Å)

2

N (wt %)

SiO2/Al2O3 mixed oxide

243

0.028

20

Si/Al-pr-N=salicylaldehyde

3.9

102

0.014

17

Si/Al-pr-N=methyl-2-pyridylketone

3.4

116

0.022

19

Si/Al-pr-N=pyridine-2-carbaldehyde

3.2

134

0.018

18

Nitrogen content was estimated from the elemental analyses. b The pore size was calculated using the BJH method.

Figure 1. IR spectra of Schiff base ligands immobilized on SiO2/Al2O3 mixed oxide in the region 4004000 cm1. Red spectrum, Si/Alpr-N=methyl-2-pyridylketone; blue spectrum, Si/Al-pr-N=pyridine-2carbaldehyde; green spectrum, Si/Al-pr-N=salicylaldehyde; brown spectrum, Si/Al-pr-NH2.

The adsorption capacities for target heavy metal ions from aqueous solution were measured to assess the surface reactivity of the SiO2Al2O3 mixed oxide modified with three Schiff base ligands prepared by condensation of methyl-2-pyridylketone,

pyridine-2-carbaldehyde, and salicylaldehyde with an amine linker, namely 3-APTES. The metals chosen for investigation in the single-component study were Pb(II) and Cd(II). The effect of shaking time (0300 min) on the adsorption of Pb(II) and Cd(II) (5.0 mg/dm3) by Si/Al-pr-N=pyridine-2carbaldehyde and Si/Al-pr-N=salicylaldehyde (0.1 g), respectively, at 25 °C, in a solution with pH 6.5, are shown in Figures 2 and 3, from which it can be seen that the amount of adsorption increases with increasing contact time. Studies of the adsorption kinetics of Pb(II) and Cd(II) removal revealed that the majority of metal ions were removed within the first 120 min of contact with the adsorbents. The percentage of maximum adsorption was 94.0% for Pb(II) and 93.6% for Cd(II) at 120 min. This initial rapid adsorption gives way to a very slow approach to equilibrium. Indeed, the fast adsorption during the initial stages is probably due to the high concentration gradient between the adsorbate in solution and that on the adsorbent as there are a high number of vacant sites available during this period.28 The removal of both Pb(II) and Cd(II) from solution was complete within 300 min. Therefore, in order to optimize the adsorption process, the adsorption isotherms for the remaining initial concentrations were obtained for a time of 120 min. 13630



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Table 3. Infrared Assignments for the Most Significant Absorptions of the Organo-Functionalized SiO2/Al2O3 Mixed Oxide FT-IR data (cm1) system

ν(CH) aliphatic

ν(CH) aromatic

ν(NH)

ν(CdN)

ν(CdC)

ring

ν(CO)

Si/Al-pr-NH2

2933, 2853

15451560

Si/Al-pr-N=salicylaldehyde

2925, 2854

3068, 3056

15451560

1633

1540

1457, 760

1278

Si/Al-pr-N=methyl-2-pyridylketone

2927, 2852

3066

15451560

1635, 1571

1590, 1434, 1418

1458

Si/Al-pr-N=pyridine-2-carbaldehyde

2925, 2852

3064, 3052

15501560

1633, 1569

1540, 1591, 1437

1457, 750

Figure 2. Adsorption kinetics of Pb(II) ions on Si/Al-pr-N=pyridine-2carbaldehyde at room temperature.

Figure 4. Effect of pH on adsorption of Pb(II) ions by Si/Al-pr-N= pyridine-2-carbaldehyde.

Figure 3. Adsorption kinetics of Cd(II) ions on Si/Al-pr-N=salicylaldehyde at room temperature.

Figure 5. Effect of pH on adsorption of Cd(II) ions by Si/Al-pr-N= salicylaldehyde.

In order to evaluate the influence of pH on the adsorption of Pb(II) and Cd(II), experiments were carried out in the pH range 1.09.0. Figures 4 and 5 show the varying Pb(II) and Cd(II) uptake capacity for Si/Al-pr-N=pyridine-2-carbaldehyde and Si/Al-pr-N=salicylaldehyde, respectively, at various pH values. The observed lower uptake in an acidic medium may be due to partial protonation of the functional groups and the competition between H+ and metal ions for binding to adsorption sites on the immobilized Schiff base ligands. Thus, removal of Pb(II) and Cd(II) increases with increasing solution pH, reaching a maximum value at an optimal equilibrium pH of around 6.5. The results obtained for the three adsorbents upon varying the initial Cd(II) ion concentration (5.0100 mg/dm3) are illustrated

in Figure 6, where it can be seen that an increase in the initial Cd(II) ion concentration from 5 to 100 mg/dm3 results in an increase in the amount of metal cations adsorbed from 5.0 to 77.32 mg/g at 25 °C after 300 min. The removal of heavy metals in this study followed the order Cd(II) < Pb(II), thus mirroring the trend of the hydrated radius (0.230 and 0.265 nm for Cd(II) and Pb(II), respectively.29 The order of adsorption on heterogenized Schiff base ligands for Pb(II) was Si/Al-pr-N= salicylaldehyde > Si/Al-pr-N=pyridine-2-carbaldehyde > Si/Alpr-N=methyl-2-pyridylketone, whereas for Cd(II) it was Si/Alpr-N=salicylaldehyde > Si/Al-pr-N=methyl-2-pyridylketone > Si/Al-pr-N=pyridine-2-carbaldehyde. 13631



Figure 6. Effect of equilibrium concentration on adsorption of Cd(II) ions by Schiff base ligands immobilized on SiO2/Al2O3 mixed oxide at 25 °C.

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Figure 8. Effect of temperature on adsorption of Cd(II) ions by Si/Al-pr-N=salicylaldehyde at several initial concentrations.

adsorption occurs solely due to the complexation reactions of the chemically immobilized Schiff base ligands. In order to determine and interpret the mechanisms of metal 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 intraparticle diffusion models shown in eqs 2, 3, and 4, respectively. Each of these models has been widely used to describe metal and organic sorption on several sorbents.31 pseudo-first-order equation: qt ¼ qe ½1 expð k1 tÞ

ð2Þ

pseudo-second-order equation: qt ¼ Figure 7. Effect of temperature on adsorption of Pb(II) ions by Si/Alpr-N=pyridine-2-carbaldehyde at several initial concentrations.

k2 qe2 t 1 þ k 2 qe t

ð3Þ

intraparticle diffusion equation: The equilibrium adsorption capacities of Pb(II) and Cd(II) ions on the favored ligands, namely Si/Al-pr-N=pyridine-2carbaldehyde and Si/Al-pr-N=salicylaldehyde, were studied at higher temperatures of 50 and 80 °C at pH 6.5 (Figures 7 and 8, respectively). This temperature increase led to an increase in the adsorption capacity of the immobilized Schiff base ligands, thereby indicating that adsorption of metal cations onto the active sites of the Schiff base ligands is endothermic, possibly due to the availability of more such sites and the enlargement and activation of the adsorbent surface at higher temperatures. It could also be due to an increased mobility of metal cations from bulk solution toward the adsorbent surface, thereby enhancing the accessibility of the active sites.30 It should be noted that, besides adsorption on the Schiff base ligand, metal ions may also interact with the surface SiOH and AlOH groups of the SiO2Al2O3 mixed oxide structure. To determine whether this was the case, the nonfunctionalized SiO2 Al2O3 mixed oxide was also used in the adsorption experiments. However, the results of this study showed no significant concentration change for any of the metal ions tested (approximately 0%). Unmodified SiOH and AlOH groups (unmodified SiO2 Al2O3 mixed oxide) therefore make essentially no contribution to the removal of heavy metal ions, thereby indicating that

qt ¼ kint t 1=2

ð4Þ

The initial adsorption rate (h) can be determined from k2 and qe values31 using h ¼ k2 qe2

ð5Þ

where k1, k2, and kint are the adsorption rate constants of the firstand second-order kinetics and intraparticle diffusion models, in min1, dm3/(mg 3 min), and mg/(g 3 min1/2), respectively, and qe and qt, in mg/g, are the equilibrium adsorption uptake (at time t = ∞) and adsorption uptake (at time t), respectively. The calculated kinetic parameters for adsorption of Pb(II) and Cd(II) on Si/Al-pr-N=pyridine-2-carbaldehyde and Si/Al-prN=salicylaldehyde, respectively, at an initial concentration of 5.0 mg/g are presented in Table 4, where it can be seen that the second-order equation appears to be the best-fitting model (R2 > 0.999). This consistency of the experimental data with the pseudo-second-order kinetic model indicates that the ratelimiting step for the adsorption of Pb(II) and Cd(II) ions on the organo-functionalized SiO2Al2O3 mixed oxide is chemical adsorption, which involves valence forces through sharing or exchange of electrons between adsorbent and adsorbate, with no involvement of a mass transfer in solution.32 Consequently, 13632



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Table 4. Pseudo-First-Order, Pseudo-Second-Order, and Intraparticle Rate Parameters for Pb(II) and Cd(II) Adsorption by Schiff Base Ligands Immobilized on SiO2Al2O3 Mixed Oxidea first-order model 1

k1 (min )

system

R2

q1 (mg/g)

Pb(II) on Si/Al-pr-N=pyridine-2-carbaldehyde

18.63

5.81

0.98

Cd(II) on Si/Al-pr-N=salicylaldehyde

32.74

6.07

0.993

second-order model q2 (mg/g)

k2 (g/(mg 3 min))

system

R2

h (mg/(g 3 min))


0.0154

5.23

0.422

0.9996


0.0072

5.47

0.218

0.9992

interparticle model kint (mg/(g 3 min1/2))

R2


0.19

0.69


0.25

0.84

system

a

C0 = 5 mg/dm3, T = 25 °C, pH 6.5.

Table 5. Langmuir, Freundlich, and LangmuirFreundlich Parameters for Pb(II) and Cd(II) Adsorption by Schiff Base Ligands Immobilized on SiO2Al2O3 Mixed Oxidea qm

KF

n

R2

0.998

Langmuir

0.332

0.35 0.88

0.995 0.9992

Freundlich Langmuir Freundlich

0.998

Langmuir

0.293

0.38

0.97

Freundlich

1.01

0.998

Langmuir Freundlich

0.9991

Langmuir

KL

model

Si/Al-pr-N=salicylaldehyde Pb(II)

Cd(II)

588

0.0019

283

0.0025

212

0.0048

237

0.0047

5673

0.00015

Si/Al-pr-N=pyridine-2-carbaldehyde Pb(II)

0.318 Cd(II)

1982 8.79

0.00045 0.0084 0.044

5.29

0.0041

0.36

0.9990

Freundlich

1.0

0.995 0.98

Langmuir Freundlich Langmuir

1.0

0.90

Freundlich

0.70

0.98

Langmuir Freundlich

0.991

Langmuir

0.37

0.98

Freundlich

0.76

0.994

Langmuir Freundlich

0.42

0.98 0.96

Langmuir Freundlich

0.60

0.991

Langmuir Freundlich

Si/Al-pr-N=methyl-2-pyridylketone Pb(II)

395

0.0024 0.298

Cd(II)

151

0.00023

194

0.0038 0.241

68.7 a

0.001.4

T = 25 °C, pH 6.5.

adsorption of these heavy metals on the organo-functionalized SiO2Al2O3 mixed oxide can be considered to involve two processes with initial adsorption rates of 0.422 and 0.218 mg/(g 3 min) for Pb(II) and Cd(II) on Si/Al-pr-N=pyridine-2-carbaldehyde and Si/Al-pr-N=salicylaldehyde, respectively (see Table 4). Although this adsorption rate is related to the content and type of active adsorption site on the adsorbent matrix, imine (CdN) and hydroxyl (OH) sites are the main reactive groups for the removal of heavy metal ions from aqueous solution (see Scheme 1).

Equilibrium adsorption isotherms are known to be very important when it comes to understanding adsorption mechanisms. Thus, the experimental adsorption equilibrium data for heavy metals on the immobilized Schiff base ligands were fitted by applying the Langmuir, Freundlich, and LangmuirFreundlich isotherm models, which are typically used for aqueous-phase adsorption.33 These adsorption models give a representation of the adsorption equilibrium between an adsorbate in solution and the surface of the adsorbent. The Langmuir, Freundlich, and 13633



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LangmuirFreundlich adsorption isotherms can be expressed by eqs 6, 7, and 8, respectively. qe ¼

q m KL C e 1 þ K L Ce

qe ¼ KF Ce 1=n qe ¼

qm KL Ce 1=n 1 þ KL Ce 1=n

ð6Þ ð7Þ ð8Þ

where qe (mg/g) is the specific equilibrium amount of adsorbate, Ce (mg/dm3) is the equilibrium concentration of adsorbate, qm is the maximal adsorption capacity, and K (KL and KF) (dm3/mol) and n are empirical constants that indicate the extent of adsorption and the adsorption effectiveness, respectively. The constant n gives an idea of the degree of heterogeneity in the distribution of energetic centers and is related to the magnitude of the adsorption driving force.34 High n values therefore indicate a relatively uniform surface, whereas low values mean high adsorption at low solution concentrations. Furthermore, low n values indicate the existence of a high proportion of high-energy active sites. The Langmuir equation relates the coverage of molecules on a solid surface to the concentration of a medium above that surface at a fixed temperature (adsorption is limited to monolayer coverage), whereas the Freundlich model supposes that the adsorption surface is heterogeneous, that interactions between adsorbed molecules can occur, and that multilayer adsorption is possible. The Langmuir and LangmuirFreundlich isotherms correlated better (R2 > 0.988) than the Freundlich isotherm for the three systems studied (see Table 5), using the experimental data for the adsorption equilibrium of metal ions by the modified SiO2Al2O3 mixed oxides, thereby suggesting a monolayer adsorption. Likewise, the maximum adsorption capacity (qm) for Pb(II), as obtained using the Langmuir isotherm, was higher than that for Cd(II), thereby also suggesting a high thermodynamic stability for Pb(II) chelation at the active sites of the Schiff base ligands. The maximum adsorption capacity (qm) for Pb(II) increased in the order Si/Al-pr-N=pyridine-2-carbaldehyde > Si/Al-pr-N=salicylaldehyde > Si/Al-pr-N=methyl-2-pyridylketone, whereas that for Cd(II) increased in the order Si/Al-pr-N= salicylaldehyde > Si/Al-pr-N=methyl-2-pyridylketone > Si/Al-pr-N= pyridine-2-carbaldehyde. Furthermore, the adsorption ability (KL) for Pb(II) increased in the order Si/Al-pr-N=methyl-2-pyridylketone > Si/Al-pr-N=salicylaldehyde > Si/Al-pr-N=pyridine-2-carbaldehyde, whereas that for Cd(II) increased in the order Si/Al-prN=pyridine-2-carbaldehyde > Si/Al-pr-N=salicylaldehyde > Si/Alpr-N=methyl-2-pyridylketone (see Table 5). The adsorbents used in the present study and the other adsorbents used for removal of Pb(II) and Cd(II), as reported in the pertinent literature, are compared in Table 1. When the proposed methods in this work were compared with other published data, it can be observed that the adsorbents employed here for removal of Pb(II) and Cd(II) behave in a comparable way, or even better, in most cases with respect to other adsorbents. The results demonstrated that Cd(II) ion in comparison with Pb(II) interacts more effectively in binding to the pendant groups which contain oxygen atom as a basic center than others with nitrogen ones. This behavior reflects the high affinity of the hydroxyl basic centers of Si/Al-pr-N=salicylaldehyde for Cd(II) ion. This process clearly expresses the high tendency of hard

acid/hard base interactions,35 as a consequence of the larger population of the hard base centers on the pendant group.

4. CONCLUSIONS This work shows that mixed oxides modified with Schiff base ligands could find applications as adsorbents for the removal of heavy metal ions from aqueous solutions. The presence of a high percentage of nitrogen (imine) and hydroxyl groups on the surface of the modified adsorbents is responsible for metal ion binding through complexation reactions. The adsorption capacities of Cd(II) and Pb(II) ions on the adsorbents were studied at various pH conditions and several temperatures. The removal of heavy metal ions was found to increase with increasing solution pH, with maximum values being reached at an optimal equilibrium pH 6.5. The adsorption capacity of the modified adsorbents also increased with temperature. Finally, of the modified adsorbents studied, Si/Al-pr-N= pyridine-2-carbaldehyde shows a high adsorption capacity for Pb(II) ions and Si/Al-pr-N=salicylaldehyde shows a high adsorption capacity for Cd(II) ions. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: +98311 391 3254. Fax: +98311 391 3250. E-mail: mghiaci@ cc.iut.ac.ir.

’ ACKNOWLEDGMENT Thanks are due to the Iranian Nanotechnology Initiative, the Research Council of Isfahan University of Technology, and the Center of Excellence at the Chemistry Department of Isfahan University of Technology for their support for this work. ’ REFERENCES

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Schiff Base Ligands Immobilized on a Nanosized ... - ACS Publications

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