Application of functionalized nano HMS type mesoporous silica with N-(2-aminoethyl)-3-aminopropyl methyldimethoxysilane as a suitable adsorbent for removal of Pb (II) from aqueous media and industrial wastewater Academic research paper on "Chemical sciences"

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Application of functionalized nano HMS type mesoporous silica with N-(2-aminoethyl)-3-aminopropyl methyldimethoxysilane as a suitable adsorbent for removal of Pb (II) from aqueous media and industrial wastewater

Hamedreza Javadian, Behrouz Babzadeh Koutenaei, Robabeh Khatti, Mohammadreza Toosi

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S1319-6103(14)00017-9 http://dx.doi.org/10.1016/j.jscs.2014.01.007 JSCS 619

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Journal of Saudi Chemical Society

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20 November 2013 16 January 2014 20 January 2014

Please cite this article as: H. Javadian, B.B. Koutenaei, R. Khatti, M. Toosi, Application of functionalized nano HMS type mesoporous silica with N-(2-aminoethyl)-3-aminopropyl methyldimethoxysilane as a suitable adsorbent for removal of Pb (II) from aqueous media and industrial wastewater, Journal of Saudi Chemical Society (2014), doi: http://dx.doi.org/10.1016/jjscs.2014.01.007

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Application of functionalized nano HMS type mesoporous silica with N-(2-aminoethyl)-3-aminopropyl methyldimethoxysilane as a suitable adsorbent for removal of Pb (II) from aqueous media and industrial wastewater

Hamedreza Javadiana*, Behrouz Babzadeh Koutenaeib, Robabeh Khattic, Mohammadreza Toosid a Department of Chemical Engineering, Shahrood Branch, Islamic Azad University, Shahrood,

b Department of Chemical Engineering, Mahshahr Branch, Islamic Azad University, Mahshahr,

cDepartment of Chemistry, Sari Branch, Payame noor University, Sari, Iran. dDepartment of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran. Corresponding author. Tel.; +98-911-3235538 E-mail address: Hamedreza.Javadian@yahoo.com.

medreza

Application of functionalized nano HMS type mesoporous silica with N-(2-aminoethyl)-3-aminopropyl methyldimethoxysilane as a suitable adsorbent for removal of Pb (II) from aqueous media and industrial wastewater

Hamedreza Javadiana*, Behrouz Babzadeh Koutenaeib, Robabeh Khattic, Mohammadreza Toosic a Department of Chemical Engineering, Shahrood Branch, Islamic Azad University, Shahrood,

b Department of Chemical Engineering, Mahshahr Branch, Islamic Azad University, Mahshahr,

cDepartment of Chemistry, Sari Branch, Payame noor University, Sari, Iran. dDepartment of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran. Corresponding author. Tel.; +98-911-3235538 E-mail address: Hamedreza.Javadian@yahoo.com.

uthor. T medreza

ABSTRACT

In this work, adsorption characteristics of nano NH2-HMS (functionalized HMS type mesoporous silica with amine groups) for the removal of Pb (II) ions from aqueous solutions

were investigated. The adsorbent was characterized using FE-SEM, TEM, XRD, FTIR and BET. The adsorption of Pb (II) ions by batch method was applied and the optimum conditions were investigated. In optimum experimental conditions, removal efficiency was 99.08 %. It was found that temperature has a positive effect on the removal efficiency. The thermodynamic parameters were evaluated, as well. The evaluated thermodynamic parameters (AG, AH and AS) showed that the adsorption of Pb (II) ions onto nano NH2-HMS was feasible, spontaneous and endothermic in nature. It can be concluded that NH2-HMS is potentially able to remove Pb (II) ions from aqueous solutions. Also, more than 90% of desorption efficiency was achieved using 0.01M HNO3.

MS, Kinetic, Isotherm.

1. Introduction

One of the most integral environmental problems that derived so much attention of researchers to itself is heavy metal pollution that accompanies industrial development and population growth [Idris et al., 2012; John Thomas and Crittenden, 1998; Ho et al., 2005; Butter et al., 1998; Yoon et al., 2006]. However heavy metals are very beneficial for human beings but spreading in the environment can cause one of environmental problems, surface and underwater pollutions in many countries [Hutton and Symon, 1986; Dogan Uluozlu et al., 2008]. One of the primary problems of heavy metals is that they are not metabolized in the human's body. Actually, the heavy metals are not easily voided from the body but accumulate in body tissues like fat, muscles, bones and joints and cause variety of diseases. They also can take up salts and other minerals placed in the body [Gupta et al., 2001]. On the other hand, heavy metals toxicity, accumulative characteristics and their entrance in the food chain of plants and animals manifold their dangerous impact and induce several ecological effects. These metals inter waterways through varieties of industrial processes like melting, leather and plastic manufacturing, battery production, fossil fuel combustion, medical supplies, dyes, catalysts, electrical industries, metallurgy, garbage dumps, extraction and purification of metals through dispersing polluted gas and industrial wastewaters [Butter et al., 1998; Barid, 1995; Wilde and Benemann, 1993, Sari and Tuzen, 2008]. Via these industrial developments, polluted water of these industries enters into the environment without purification, and as a result rivers and waterfalls gradually get polluted.

Out of so-called heavy metals, lead is a metallic and soft element in bluish white color that is extraordinarily poisonous. The most important consumers of lead and its compounds are

industries such as battery manufacturing, dye production, chemical and plastic industries. In two ways lead enters human and animals' body and causes poisoning, one through entering into food cycle and the other is through breathing the air which is polluted with lead. The tolls of lead consumption in the human body are like as follows: rising of lead in the blood that resulting of

poisoning in two ways of acute and chronic. As a result of chronic poisoning, diseases like lead colic, nerve palsy, kidney inflammation, anemia, rises of uric acid in blood, saturnine gout and abortion will emerge in pregnant human and animals. The most important diseases of acute poisoning are Encephalopathy and Brain lesions [Bailey et al., 1999]. Consequently, declining and controlling these metal ions have been considered by many nations all around the world.

The most prevailed method to remove heavy metals is precipitation that has some disadvantages such as incomplete removal and bulking sludge production, considering the fact that, using precipitation is not economical as well [Prakasham et al., 1999]. Methods like ion exchange and reverse osmosis are very expensive as far as their operational and maintenance expenses are concerned [Hu et al., 2009]. Other methods like evaporation, solvent extraction and membrane process seem to be very expensive if the concentration of metal in solution is very low [Heidman and Calmano, 2008].

Considering the higher performance and ease of use, adsorption method is introduced as one of the widely used methods. In this method heavy metals are superficially adsorbed on the surface of the pores of the adsorbents, which are insoluble in water. The simplicity of adsorption technique for performance and no need to complicated production and breeding processes as well as higher performance in surface adsorption and being selective for heavy metals are some of the advantages of these adsorbents [Singh et al., 2008].

>rous in oposed

Porous solids have a lot of commercial applicability as an absorbent, catalyst and catalyst base because of their very high specific surface. According to IUPAC definition, porous solids are divided into three classes are called micro-porous, meso-porous and macro-porous in accordance with pore diameter [Edwards and Petersen, 1936]. Pinnavaia and Tanev providing a neutral casting mechanism based on hydrogen bond between primary amines and inorganic neutral species [Dunnick et al., 1995]. Molecular sieves such as HMS, MCM-41, SBA-15 are called mesoporous or hexagonal mesoporous silica. Work done based on hexagonal mesoporous silica performance like MCM-41 type mesoporous silica is indicated as based catalyst and also referred to as absorbent in wastewater treatment [Shrikant et al., 2009; Heidari et al., 2009].

In this study, nano NH2-HMS type mesoporous silica is synthesized and its capacity for the removal of Pb (II) from aqueous solutions is investigated. The effect of different parameters including pH of the solution, reaction time, adsorbent dosage, initial concentration of Pb (II) on removal are investigated in detail. Also, adsorption kinetics, adsorption isotherms and thermodynamic parameters are obtained. 2. Materials and methods 2.1. Reagents andstandard solutions

Tetraethyl ortho silicate (TEOS, SiC8H20O4), ethanol (C2H5OH), hydrochloric acid (HCl), dodecylamine (C12H25NH2) is used as surfactant, N-(2-aminoethyl)-3-aminopropyl methyldimethoxysilane (C8H22N2O2SO (Zhejiang Feidian Chemical, China), normal hexane (n-hexane (C6H14)), HNO3 and NaOH for pH adjustment. Pb (II) solution is prepared using Pb(NO3)2. All the chemicals and reagents used throughout this study are analytically graded from Merck (Merck, Darmstadt, Germany) except N-(2-aminoethyl)-3-aminopropyl

methyldimethoxysilane which are obtained from Zhejiang Feidian Chemical Co., Ltd. Distilled deionized water is used throughout this work.

2.2. Synthesis of nano HMS

HMS nanoparticles were provided using the method first described by Pinnavia et al

[Tanev et al., 1995]. Dodecyl amine was used as a surfactant. First, 1.4 mL HCl, 3.5002 g dodecyl amine and 46.0011 g distilled deionized water was stirred for 5 min. Then this solution was added to another solution consisting of 15.0426 g TEOS and 21.0033 g ethanol which also stirred for 30 min. The resulting mixture was stirred for 5 h and then left for 24 h. All steps were performed at room temperature (25oC). After 24 h the obtained gel was filtrated, washed with distilled deionized water and dried at 100oC for 6 h. The dried gel was then calcined in a furnace at 550oC for 6 h.

2.3. Synthesis of nano NH2-HMS

Nano NH2-HMS was sy d according to the method of Ho et al. [Ho et al.,

. The dr

2003]. In a typical, 2.5 g of HMS calcined nanoparticles were refluxed in 50 mL of n-hexane containing 2.5 g of N-(2-aminoethyl)-3-aminopropyl methyldimethoxysilane for 6 h. Then the product was filtered, washed with 20 mL of n-hexane and dried at room temperature overnight. The schematic route of functionalized HMS mesoporous silica is shown in Fig. 1. 2.4. Instrumentation

The surface morphology was obtained using scanning electron micrograph (FE-SEM, S-4160, HITACHI, Japan). Transmission Electron Microscopy (TEM) was performed using a field emission TEM (TEM, CM120, PHILIPS, Holland) operating at 120 kV. The functional groups were tested by the technology of Fourier transform infrared spectrometry (FTIR, 8400S, SHIMADZU, Japan) in the wavelength of 4000-400 cm-1. BET (Brunauer-Emmet-Teller)

atomic ab

specific surface area was determined by fitting the linear portion of the BET plot to BET equation. X-ray diffraction (XRD) patterns of samples were obtained using X-ray diffractometer (XRD, PHILIPS instruments, Australia). The operating conditions were 35kV and 28.5mA, using Cu, Ka radioactive source. The sample was scanned in the range of 0 to 80 . Before and

after the adsorptive reaction, samples of solutions were analyzed by atomic absorption spectrophotometer (Model GBC 302, Australia). 2.5. Batch sorption tests

Pb (II) removal experiments with the synthesized nano NH2-HMS were carried out by batch tests in 250 mL beaker under magnetic stirring. Whole tests contained of transferring 50 mL of Pb (II) solution prepared from the dilution of 1 g.L-1 stock solutions with a desired pH and initial concentration into the beaker on the magnetic stirrer. Then a desired amount of NH2-HMS (NH2-HMS dosage) was added to the solution and the resulting suspension was instantly stirred for a predefined time. After the mixing time ended, samples were taken at predetermined time intervals, centrifuged at 4000 rpm for 15 min and residual Pb (II) in solution was analyzed using an atomic absorption spectrophotometer.

The adsorption percent of Pb (II), i.e., Pb (II) removal efficiency, was determined using the following expression:

Removal efficiency (%) = (Ci - Ct / Ci) x 100 (1)

where Ci and Ct represent the initial and final (at any time t) Pb (II) concentrations, respectively. The adsorption capacity at the time t, qt (mg.g-1), was obtained as follows: qt = (Ci - Ct) x V / M (2)

where Ci and Ct (mg.L-1) represent the liquid-phase concentrations of solutes at initial and a given time t, V is the volume of the solution and M the mass of NH2-HMS (g). The amount of adsorption at equilibrium, qe, was calculated using: qe = (Ci - Ce) x V / M

Here Ce (mg.L-1) was the ion concentration at equilibrium. 3. Results and discussion

3.1. Characterization of adsorbent

Figs. 2a and 2b show the FE-SEM and TEM images o particles, roughly uniform in distribution.

The diffraction pattern of X-ray for HMS (Fig. 3a) shows an intense reflection near 20

= 2 corresponding to the (100) plane [Damyanova et al., 2003]. It is similar to the observation of

Tanev et al. that the formation of HMS is confirmed by the single low angle diffraction pattern

[Tanev et al., 1995, Tanev et al., 1994]. A relatively broad band in the range of 20 to 30 (Fig. 3b) can be attributed to the amorphous nature of SiÜ2 [Gac et al., 2007, Zhao et al., 2006]. The existence of two peaks shows a favorable formation of HMS structure in the prepared sample in this research.

FTIR spectra in the range of 4000-400 cm-1 were used to confirm the functionalizing of HMS with aminosilane groups. Figs. 4a and 4b are for HMS and NH2-HMS, respectively. In Fig. 4a, broad band in the range of 3750-2800 cm-1 is related to reflection of structure of Si-OH groups with absorbed water molecules and destroyed places, the observed peak at 1095 cm-1 is related to vibrations of Si-O-Si, the peaks at 802 cm-1 and 462 cm-1 are related to symmetric stretch and bending stretch, respectively [Sarkar et al., 2003; Luo et al., 2001]. In the spectrum of NH2-HMS (Fig. 4b), a broad band is observed at 1596 cm-1 related to bending vibration of N-H

hese fiss depict small

groups. Stretch bands of N-H and C-N are also observed in the range of 3750-3000 cm-1 and 1350-850 cm-1, respectively. Also, reduction in peak intensity in the range of 3750-3000 cm-1 is

HMS is

as a result of functionalizing HMS with aminosilane groups that confirms nano NH2-HMS is formed.

The specific surface area was determined using the BET equation applied to the

adsorption data. The BET surface area and the average pore size of the samples are shown in

Table 1. When aminosilane groups were grafted on HMS sample, the BET surface area of the

composite material was decreased from 987 to 89 m .g" . This can be attributed to the occupancy

of aminosilane groups into the structure. Reduction of the pore sizes from 0.926 to 0.374 cm g

of the meso pores and 0.56 to 0.009 cm .g" of the micro pores was attributed to the presence of the propyl chains of aminosilane groups in the pores of HMS. Also, there is a significant increase in meso and micro pores diameter that the reason was the entry of the aminosilane groups into the pores and extending of them. 3.2. Effect of pH

Solution pH is one of the integral parameters in controlling adsorption seems very important since it influences on the solubility of metal ions, concentration of ions on the surface of the functional group and the ionization degree of adsorbent during the reaction. By changing pH, the chemical characteristics of adsorbent and adsorbate change. The available functional group on adsorbent is severely affected by the pH of the solution [Vasconcelos et al., 2008]. Results about the effect of pH on the removal efficiency (%) of Pb (II) adsorption are illustrated in Fig. 5. Owing to the fact that in pH less than 2, removal efficiency is very low and in pH higher than 5, metal ions deposit as hydroxide because of high concentration of OH-. Consequently, pH range is considered from 2 to 5 to adsorbing metal ions. In fact, adsorption in

different pH depends on both functional group and the chemistry of the metal in the solution. The results show that by increasing pH from 2 to 5 for Pb (II) the removal efficiency increased from 15.74% to 99.08% and the adsorption capacity increased from 7.86 to 49.41 mg.g-1 that the most percentage of adsorption related to pH 5 and 1 g.L-1. These outcomes illustrate that in lower

pH, amino group existed in adsorbent has been protonated and lose their ability as a legend for making a complexion of metal ions [Sohn et al., 1986].

3.3. Effect of adsorbent dosage

As can be seen in Fig. 6, by increasing the amount of adsorbent, the removal efficiency (%) of metal ions decreased from 99.08% to 84.08% when the amount of adsorbent increased from 1 to 3 g.L-1. Also, the amount of adsorption capacity decreased from 49.41 to 38.68 mg.g-1. By increasing the amount of adsorbent the amount of adsorbed material decreases, that is because the active sites can be efficiently applied when the amount of adsorbent is low. Whereas by increasing the amount of adsorbent in case of constant initial concentration, a considerable amount of sites will remain intact. By comparing the amount of adsorbent in various dosages for adsorption of Pb (II), 1 g.L-1 was selected as the optimum dosage for all further experiments.

3.4. Effect of contact time and initial concentration on adsorption capacity

The effect of contact time on Pb (II) adsorption on NH2-HMS is shown in Fig. 7. From Fig. 7, it can be seen that the adsorption capacity of Pb(II) ions on NH2-HMS increased as the contact time increased. The adsorption of Pb(II) onto NH2-HMS was reached to equilibrium within 80 min. Hence, in the present study, 80 min was chosen as the equilibrium time. The effect of initial concentration indicates that the removal efficiency increased by increasing the initial concentration of metal ions from 30 mg.L-1 to 50 mg.L-1. By increasing the concentration

from 50 mg.L-1 to 70 mg.L-1 the removal efficiency of metal ions decreased from 99.08% to 94.75%. The decrease in the removal efficiency of Pb(II) can be attributed to the lessen of available active sites above a certain concentration of Pb(II). Beside, the capacity for adsorption of Pb (II) in 50 mL of solution increased from 27.55 mg.g-1 to 63.28 mg.g-1 (Fig. 7). The increase

in adsorption capacity may be due to the higher adsorption rate and the utilization of all available active sites for adsorption at higher Pb(II) concentration. In other words, the adsorption capacity increases by increasing the initial concentration of metal ions. In addition, by increasing the initial concentration, the ions and adsorbent collision increases that this phenomenon increases the rate of adsorption process. Also, an increase in the initial concentration of metal ions in the solution causes increasing in electrostatic interaction that provides places with low affinity for reacting with metal ions. 3.5. Kinetics of sorption

The adsorption kinetic parameters were explained by the pseudo-first-order and pseudo-second-order models are presented below [Lagergren, 1898; Ho and McKay, 1999; Altun Anayurt et al., 2009]:

log (qe -qt) = log qe - №303) t (4)

t/qt= 1/(k2qe2)+ (1/qe)t (5)

where qe and qt are the amount of Pb (II) adsorbed (mg.g-1) at equilibrium and at any time t, k1 (min-1) and k2 (g.mg-1min-1) are the pseudo-first-order and pseudo-second-order rate constants, respectively.

The obtained statistical data and kinetic constants are presented in Table 2. For Pb (II) adsorption, the obtained kinetic data proved that the adsorption process was controlled by pseudo second-order model.

are preser

of adsórbate 2-HMS was

3.6. The isotherm model

An adsorption isotherm is a graphical representation showing the relationship between the amounts adsorbed by a unit weight of adsorbent and the amount of adsorbate remaining in a test medium at equilibrium. The Pb (II) uptake capacity of NH2-evaluated using the Langmuir [Langmuir, 1916; Gundogdu et al., 2009; Sari and Tuzen, 2009] and Freundlich [Javadian et al., 2013a] and Tempkin [Tempkin and Pyzhev, 1940] adsorption isotherms. The Langmuir model assumes that the adsorptions occur at specific homogeneous sites on the adsorbent and is used successfully in many monolayer adsorption processes [Langmuir, 1918]. The Freundlich model can be applied for non-ideal adsorption on heterogeneous surfaces and multilayer adsorption [Freundlich, 1907]. The derivation of the Temkin isotherm assumes that due to adsorbate/adsorbent interaction, the heat of adsorption decrease linearly rather than logarithmically, as implied in the Freundlich equation [Tempkin and Pyzhev, 1940]. These isotherm models are often offered in the form of the following equations: Ce/qe = 1/bqm+ Ce/qm (6)

log (qe) = log (Kf) + (1/n) log (ce) (7)

qe = B ln Kt + B ln Ce (8)

where qe is the amount of Pb (II) adsorbed per specific amount of adsorbent (mg.g-1), Ce is the equilibrium concentration of the solution (mg.L-1), qm is the maximum amount of adsorption metal ions (mg.g-1), b is the Langmuir constant related to the free energy or net enthalpy of adsorption, Kf and (1/n) are the Freundlich constant and adsorption intensity, respectively, B = (RT / At), At (J.mol-1) is constant related to heat of sorption and Kt (L.g-1) is Tempkin isotherm equilibrium binding constant.

Kt = 1.075 and At = 32.29 which is an indication of the heat of sorption indicating a physical adsorption process.

The statistical results and also the isotherm constants are offered in Table 3. As it can be inferred from the results, Tempkin and Freundlich isotherm models are capable of

representing the data better than Langmuir model with regression coefficient 0.998 and 0.997, respectively.

3.7. Effect of temperature on adsorption of Pb (II)

The effect of temperature on the sorption of Pb (II) by NH2-HMS was investigated

at a constant concentration (50 mg.L-1), temperature (15-450C), pH 5, stirring rate 200 rpm and adsorbent dosage 1 g.L-1. The results are shown in Fig. 8. The thermodynamic parameters were calculated from the slope and intercept of ln Kc against 1/T (Fig. 9) by using the equation [Javadian et al., 2013b]:

ln Kc = (AS / R) - (AH / RT) (9)

where Kc is the standard thermodynamic equilibrium constant, AH, AS, R, and T are the

enthalpy change, entropy change, gas constant (8.314 J.mol-1.0K -1), and absolute temperature

(0K), respectively.

ange (av 6

Gibbs change (AG) of sorption was calculated from the following equation [Javadian et al., 2013b]:

AG = - RT ln Kc (10)

The values of thermodynamic parameters for the sorption of Pb (II) on NH2-HMS are given in Table 4. The enthalpy change of sorption suggests the possibility of strong bonding between sorbate and sorbent. The positive value of AH indicates the endothermic nature of the sorption process. The removal of water molecules from the solid/solution interface and from the

sorbing cations may be the reason for endothermicity of the heat of sorption. This dehydration process of the ions requires energy [Shah et al., 2009]. This energy of dehydration supersedes exothermicity of the ions getting attach to the surface [Abasi et al., 2011]. The Gibbs free energy change (AG) is negative, indicating that the sorption process is more favorable temperatures and spontaneous in nature. The positive value of AS suggests some structural changes in the sorbate and the sorbent and indicates the increased randomness at the solid/solution interface during the sorption process.

Desorption study as a function of pH is for obtaining recycling capacity of adsorbent and adsorbate. The process of adsorption and desorption was done in 50 mL of solution with

free energy e at higher

initial concentration of 50 mg.L-1. In adsorption stage, increasing of adsorption capacity with increasing of pH from 2 to 5 range was observed, however, the best operation of desorption process was in acidic media. In acidic pH, metal ions are replaced by protons and adsorbent surface is protonated. So a kind of repulsive force is formed between positively charged (protonated) surface and metal cations that by decreasing pH this repulsive force increases. As it can be observed in Fig. 10, the metal ions are desorbed in low pH values. It is observed that the highest amount of desorption of the adsorbent happens in acidic media (0.01M HNO3). Since Pb (II) is adsorbed in a relatively basic media, consequently, it returns to the solution in acidic media.

After finding suitable conditions for desorption, adsorption-desorption cycle with alike preparations was done to ascertain the reusability of NH2-HMS. The achieved results are presented in Table 5. As it can be inferred from the data offered in this table, the removal efficiency was not changed to a significant amount in the process of desorption and modified

nano HMS can be reused for three times without a remarkable loss in adsorption efficiency (lower than 10%).

3.9. Application of NH2-HMS for removal of the Pb (II) from real wastewater sample

To investigate the application of the NH2-HMS for the removal of Pb (II) ions, industrial wastewater containing Pb (II) from battery manufactory, Isfahan, Iran was tested at 25oC in 50 mL of wastewater with rotating speed 200 rpm, pH 5 and the NH2-HMS dose of 1 and 2 g.L-1. The initial concentration of Pb (II) in wastewater was 141 mg.L-1. After batch sorption of Pb (II) with 1 and 2 g.L-1 of adsorbent, removal efficiencies (%) were found to be 76.59% and 97.74%, respectively. The results of the application studies showed that Pb (II) concentration in the treated wastewater samples were 33 and 3.18 mg.L-1, therefore sorption of Pb (II) was favorable.

3.10. Comparison of various adsorbents

The removal of Pb (II) by different adsorbents has been studied extensively, and Pb (II) adsorption capacities were reported in literatures. Table 6 compares the adsorption capacities of the composite obtained in this work with different adsorbents previously used for removal of Pb (II). It can be seen from Table 6 that the adsorption capacities of the synthesized adsorbent for Pb (II) are much higher than that of many other previously reported adsorbents, indicating that the as-prepared adsorbent has great potential application in Pb (II) removal from aqueous solution. It must be mentioned that the amount of maximum capacity can not utterly be specify via NH2-HMS because the adsorption isotherm was fitted better by Tempkin and Freundlich isotherm models. Therefore it can only be a comparison with other adsorbents in term of adsorption capacity. 4. Conclusions

The potential use of NH2-HMS as an adsorbent for cadmium was studied. This new adsorbent is able to remove the Pb (II) ions from aqueous solutions, and the sorption capacity was dependent on the adsorbent nature, dosage, initial metal ions concentration and initial pH. The op"

concentration of 50 mg.L-1.The experimental data well fitted to the Tempkin and Freundlich isotherm models, with good correlation coefficients. The experimental data also showed that intra-particle diffusion is significant in the sorption rate determination. The pseudo-second-order coefficient values obtained in this study for Pb (II) sorption onto NH2-HMS confirm the feasibility and the spontaneous nature of the sorption process for Pb (II) ions. The negative value of AG indicates the spontaneity of the process, positive value of AH shows the process is endothermic and positive values of AS implies affinity of adsorbent towards Pb (II) ions. Desorption of Pb (II) from the NH2-HMS was investigated using HNO3 from 0.01 to 1 M and desorption efficiency was more than 90% using 0.01 M HNO3. After desorption, NH2-HMS showed considerable removal efficiency for Pb (II) adsorption and reduction of removal efficiency was lower than 10%. Results showed that NH2-HMS can be used as a potential sorbent to remove Pb (II) ions from aqueous solutions and industrial wastewater. Results also

removal efficiency occurs at pH 5, adsorbent dose of 1 g.L-1 and 50 mL of solution w

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Legend for the Figures

Fig. 1. Schematic functionalization route of HMS mesoporous silica. Fig. 2a. FE-SEM image of HMS. Fig. 2b. TEM image of HMS. Fig. 3a. X-ray diffraction patterns of HMS (Reflective peak related to reflection from dioo plane in areas of 0.5° to 6°).

Fig. 3b. X-ray diffraction pattern of HMS (Graph related to amorphous in 20 to 30 area).

Fig. 4. FTIR spectrum of (a) HMS and (b) NH2-HMS.

Fig. 5. The effect of pH on the removal efficiency and sorption capacity.

(Adsorbent dosage: 1 g.L-1; contact time: 80 min; initial concentration: 50 mg.L-1; stirring rate:

200 rpm; temperature: 25 °C.)

Fig. 6. The effect of adsorbent dosage on the removal efficiency and sorption capacity. (pH: 5 ; contact time: 80 min; initial concentration: 50 mg.L-1; stirring rate: 200 rpm; temperature: 25 °C.)

Fig. 7. The effect of contact time and initial concentration on the sorption capacity. (pH: 5; adsorbent dosage: 1 g.L-1; stirring rate: 200 rpm; temperature: 25 °C.) Fig. 8. Effect of temperature on sorption of Pb(II)

(pH: 5; adsorbent dosage: 1 g.L-1; initial concentration: 50 mg.L-1; stirring rate: 200 rpm.) Fig. 9. ln Kc versus 1/T for enthalpy and entropy change of the sorption process.

Fig. 10. Desorption efficiency of Pb(II) from the NH2-HMS adsorbent in solutions with different HNO3 concentrations.

»»SlSdi

m * . A ^ »__ ^

\r r ' r ^flr^

W vTj •

Sf ^ ^

: l • 0 a

Table 1. The BET surface area and the average pore size of the samples

Sample Sbet (m2.g-1) Vp(meso) (cm3.g-1) Vp(micro) (cm3.g-1) dp(meso) (nm) dp(micro) (nm)

HMS 945 0.926 0.56 5.29 2.25

NH2-HMS 89 0.374 0.009 15.65 2-86

Table 2. Kinetic constants for Pb (II) adsorption.

Lagergren

Pseudo second order

Initial concentration

(mg.L-1) (mg.g-1) (1.min-1) (mg.g-1)

30 27.766 0.09 21.1

40 38.461 0.0128 1.181

50 44.319 0.0244 1.871

60 55.726 0.0246 4.666

70 63.283 0.0368 1.486

(mg.g"1.min"i) (mg.

mg.g"1)

0.9906

.999 9949

29.239 0.0289 0.0253 99 0.0465

0.0092 38.459

45.662 56.818 63.694

0.9992 0.9998 0.9999 0.9997 0.9999

Table 3. Isotherm constants for Pb (II) adsorption.

b (L.mg-1 ) qm (mg.g-1) Rl R2 >

Langmuii Ce/qe =1/bqm+ Ce/qm 0.313 119.047 0.06 0.585

Freundlich K ( L.mg-1 ) 1/n R2

36.618 0.438 0.997

Kt B R2

Tempkin 1.329 76.73 0.998

Table 4. Thermodynamic parameters for Pb (II) adsorption Pb (II) AH AS AG (j.mol-1)

mg.L-1 (j.mol-1) (j.mol-1.K-1)

288UK 298UK 308UK

50 106153.67 414.095 -13167.8043 -17308.754 -21449.704 -25590.654

Table 5. Reusing the NH2-HMS after desorption Removal efficiency (%) First time Second time Third time

Pb (II) 99.08 96.23 91.57

Table 6. Comparison of the maximum monolayer adsorption (qm) of Pb (II) onto various adsorbents.

Adsorbent

Maximum monolayer adsorption capacity (qm (mg/g))

Reference

Turkish kaolinite (Bandirma region) 31.75 [Sari et al., 2007]

Tree fern 39.80 [Ho et al., 2002] Jh

Biosorbent (Cephalosporium aphidicola) 0.924 [Tunali et al., 2006]

Pine cone activated carbon 27.53 [Momcilovic et al., 2011]

Activated carbon-zeolite composite 2.65 [Kumar Jha et al., 2008]

Activated carbon (PASBAC) 51.81 [Mohammadi et al., 2010]

Agricultural waste aV 20 [Mohamad Ibrahim et al., 2010]

Heartwood of Areca catechu powder 11.72 [Chakravarty et al., 2010]

Seed powder of Prosopis juliflora DC 45.45 [Jayaram and Prasad, 2009]

Acid-activated clay (AGC) 40.75 [Eloussaief and Benzina, 2010]

NH2-HMS 119.047 This research