JOURNAL OFENVIRONMENTALSCIENCES

ISSN 1001-0742

CN 11-2629/X

www.jesc.ac.cn

Available online at www.sciencedirect.com

Journal of Environmental Sciences 2013, 25(6) 1252–1261

Chemically modified silica gel with1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: Synthesis,

characterization and application as an efficient andreusable solid phase extractant for selective

removal of Zn(II) from mycorrhizal treated fly-ash samples

R. K. Sharma1,∗, Aditi Puri1, Anil Kumar1, Alok Adholeya2

1. Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi-110007, India2. Biotechnology and Management of Bioresources Division, The Energy and Resource Institute, New Delhi-110003, India

Received 11 September 2012; revised 24 December 2012; accepted 21 January 2013

Abstract1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone functionalized silica gel was synthesized and used as a highly efficient, selectiveand reusable solid phase extractant for separation and preconcentration of trace amount of Zn(II) from environmental matrices. Theadsorbent was characterized by fourier transform infrared spectroscopy (FT-IR), elemental analysis,13C CPMAS NMR spectroscopy,scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and BET surface area analysis. The dependence of zincextraction on various analytical parameters such as pH, type and amount of eluent, sample flow rate and interfering ions wereinvestigated in detail. The material exhibited superior adsorption efficiency for Zn(II) with high metal loading capacity of 1.0 mmol/gunder optimum conditions. After adsorption, the recovery (> 98%) of metal ions was accomplished using 1.0 mol/L HNO3 as aneluent. The sorbent was also regenerated by microwave treatment in milder acidic environment (0.1 mol/L HNO3). The lower detectionlimit and preconcentration factor of the present method were found out to be 0.04 µg/L and 312.5 respectively. The modified silicasurface possessed excellent selectivity for the target analytes and the adsorption/desorption process remained effective for at least tenconsecutive cycles. The optimized procedure was successfully implemented for the extraction of Zn(II) from mycorrhizal treated flyash and pharmaceutical samples with reproducible results.

Key words: solid phase extraction; silica gel; preconcentration; fly-ash; zinc

DOI: 10.1016/S1001-0742(12)60173-9

Introduction

Zinc has a fundamental role in the structure and function ofnumerous proteins, including metalloenzymes, transcrip-tion factors and hormone receptors. The widespread role ofzinc in metabolism is also accentuated by the occurrenceof zinc in all tissues, organs and fluids of the humanbody (DeMartino et al., 2010). In addition to this, sincethe industrial revolution, the use of zinc has increasedexponentially due to its presence in every area of modernconsumerism: from construction materials to cosmetics,medicines to processed foods and appliances to personalcare products (Perez-Quintanilla et al., 2009; Yu and Li,2011). The extensive utilization and application of zinc invarious industrial and commercial activities necessitates its

* Corresponding author. E-mail: rksharmagreenchem@hotmail.com

accurate analytical determination and recovery for regu-lating and minimizing its discharge into the environmentfrom the view point of safety. This is because elevatedquantities of zinc in living organisms have been reportedto cause various acute and chronic adverse effects, reduc-tion in growth reproductive and developmental defects,electrolyte imbalance, nausea lower levels of high-densitylipoprotein cholesterol, intracellular production of reactiveoxygen species (ROS), and in consequence, oxidativestress or death of cells (Environmental Health CriteriaDocument 221 Zinc, 2001; US EPA, 2005). To conquer therising concern of zinc toxicity, various preconcentrationtechniques such as ion pair extraction (Malvankar andShinde, 2007), precipitation (Lenz and Martins, 2007)and liquid-liquid extraction (Shukla and Rao, 2002) arebeing practiced over decades. But, these methods are

No. 6 Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······ 1253

not economical and eco-friendly and suffer from variousdrawbacks like lack of sensitivity and selectivity and use oflarge amount of toxic organic solvents which have delete-rious effect on human health and environment. Thus, solidphase extraction (SPE) has come to the forefront in recentyears for selective preconcentration and separation oftrace amounts of elements as it simplifies labour intensivesample preparation, lessens the cost and time, eliminatesthe clean-up step, provides high enrichment factors, andminimizes costs due to low consumption of reagents thusproviding an economic, persuasive and greener alternativeto other traditional methodologies (Sharma et al., 2003)

In continuation of our research work (Sharma et al.,2012b, 2005, 1999a; Garg et al., 1999b; Sharma andDhingra, 2011; Sharma and Pant, 2009a, 2009b; Sharmaand Goel, 2005; Sharma, 2001) on synthesis of efficientand cost effective solid adsorbents having specific metalion binding affinity against background constituents, thepresent work is focused on the synthesis of schiff’s basefunctionalized silica gel with a view of finding out asimple and extremely selective solid phase extractant forZn(II) ions. In fact, a comparative data of the presentwork with some literature precedents, based on ligand-functionalized amorphous and mesoporous silica materialsfor zinc adsorption has been compiled in Table 1. It isevident from comparison that the synthesized adsorbentexhibits enhanced analytical characteristics with respectto most of the chelating resins based on different sile-nous matrix. Although, MTTZ-MCM-41 was able to bindquantitatively more Zn(II) ions from aqueous solution,the preconcentration factor was low. Moreover, adsorbentcould not be reutilized for more than three successiveadsorption-desorption cycles.

The applicability of the present method is judged byinvestigating the uptake behavior of the adsorbent for

zinc in fly ash samples from two major fertilizer andthermal power plants of North India. Indian coal usedin fertilizer or thermal power plants generates massivecontents of fly ash which are generally being considered adeadly source of health hazards because of the presence ofpotentially toxic concentrations of heavy metals (Hansen etal., 2002). Land fill disposal of fly ash has adverse impactson terrestrial and aquatic ecosystems due to leaching oftoxic metal ions into soil and groundwater. Therefore, theextraction of metals from fly ash is of utmost concernto cease the environmental transitions caused by thesemetal loaded dumps. Major initiatives have been taken upby TERI (The Energy and Resources Institute) to ecore-store these ash containing lands. These hazardous siteshave been reclaimed to reduce the leachable content ofheavy metals through the implementation of mycorrhizalfungi-based technology (Pandey et al., 2009; Gaur andAdholeya, 2004; Sharma et al., 2012a). We have examinedand compared the concentration of zinc left in the areasafter mycorrhizal treatment with non treated ash dumps.Subsequently, we have implemented the outlined methodfor effective and selective uptake of residual zinc contentfrom these samples. To best of our knowledge, this type ofapplicability has not been elucidated before for solid phaseextraction of zinc.

1 Materials and methods

1.1 Reagents

4-Amino acetophenone, silica gel and salicylaldehydewere procured from Sisco Research Laboratory andused as received without further purification. 3-Aminopropyltriethoxysilane (APTES) was purchasedfrom Sigma Aldrich. Working solutions were preparedby appropriate dilution of the stock standard solutions.

Table 1 Comparison of important analytical characteristics of various chelating matrices used for the separation and preconcentration of Zn(II) ions

Immobilized ligand Support Adsorption capacity Preconcentration Referencematerial (mmol/g) factor

5-Mercapto-1-methyltetrazole MSU-2 and HMS 0.94 and 0.72 200 Perez-Quintanilla et al., 2010Sulfanilamide Silica Gel 0.292 100 Zou et al., 20095-Mercapto-1-methyltetrazole SBA-15 0.96 200 Perez-Quintanilla et al., 20093-Aminopropyltriethoxysilane Mesoporous silica 0.36 – Yang et al., 20085-Mercapto-1-methyltetrazole MCM-41 1.59 100 Perez-Quintanilla et al., 2007Polyamidoamine and EDTA-polyamidoamine SBA-15 0.21 and 0.15 – Jiang et al., 2007Curcumin Silica Gel 0.37 75 Zhu et al., 20072,3-Dihydroxybenzaldehyde Silica Gel 0.133 – Alan et al., 2007Cyanex 272 SBA-15 0.111 – Northcott et al., 20062-Aminomethylpyridine Silica Gel 0.22 – Sales et al., 2004o-Dihydroxybenzene Silica Gel 0.168 – Venkatesh et al., 2004PEI Silica Gel 0.82 – Ghoul et al., 20038-Hydroxyquinoline Silica Gel 0.177 200 Goswami et al., 2003Resacetophenone Silica Gel 0.191 150 Goswami et al., 2002a1,4-Bis-[3-(trimethoxysilyl)propyl]ethylenediamine HMS 0.0011 – Hossain et al., 2002Cyanex 272 Silica Gel 0.31 – Chah et al., 20021,8-Dihydroxyanthraquinone Silica Gel 0.18 – Goswami et al., 2002b1-{4-[(2-Hydroxy-benzylidene)amino]phenyl}ethanone Silica Gel 1.004 312.5 This work

1254 Journal of Environmental Sciences 2013, 25(6) 1252–1261 / R.K. Sharma et al. Vol. 25

Double deionized water was used throughout theexperiment. The pH of the solutions was adjusted usingthe following buffers: (a) potassium chloride/hydrochloricacid for pH 2, (b) sodium acetate/acetic acid for pH 3–6,(c) disodium hydrogen phosphate/hydrochloric acid forpH 7–8 and ammonia/ammonium chloride for pH 9.

1.2 Instruments

Infrared spectra (4000–400 cm−1) were recorded on FT-IRspectrophotometer (Perkin Elmer, USA) using KBr pellets.The contents of carbon, hydrogen and nitrogen wereanalyzed with Elementar Analysenysteme GmbH VarioELanalyser, Germany. The pH measurements were carriedout using ELICO LI 120 pH meter, India. LABINDIAAA 7000 Flame atomic absorption spectrophotometer(LABINDIA, India) was employed for the determinationof metal ion. 13C CPMAS NMR spectra were recorded onDSX-300 NMR spectrometer (Bruker, Germany) at 75.47MHz equipped with a commercial 4 mm MAS NMR probe(magnetic field 7.04 T, pulse delay of 5 sec, contact time 3ms). Thermogravimetric analysis was performed on DTG-60 instrument (Shimadzu, Japan), equipped with TG unitsat a heating rate of 10°C/min from 25 to 900°C in N2 atmo-sphere. Digestions and elutions were performed in AntonPaar multiwave 3000 microwave reaction system (PerkinElmer, USA), equipped with temperature and pressuresensor. Scanning electron microscopy (SEM) images wereobtained using an EVO 40 instrument (ZEISS, Germany).The samples were placed on a carbon tape and thencoated with a thin layer of gold using a sputter coater.Qualitative analysis of metal sorbed onto the soild phasewas performed by X-Ray XAN-FAD BC ED-XRF spec-trometer equipped with a tungsten anode (Fischerscope,Netherlands). Surface area analysis was carried out at 77K on Gemini-V2.00 instrument (Micromeritics InstrumentCorp., USA).

1.3 Synthesis of 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone (HBAPE)

The schiff’s base was synthesized according to the reportedmethod (Yuce et al., 2004) with minor modifications.Equimolar quantities of 4-aminoacetophenone and salicy-laldehyde were sonicated in ethanol for 5 min to affordthe desired yellow product, which was recrystallized twicewith ethanol. The properties are as follows: yield: 82%;melting point: > 300°C; anal. calc. for C15H13NO2 (Mol.wt.=239.27): C 75.42, H 5.54, N 5.60; found C 75.30, H5.58, N 5.85; IR ν(cm−1): 1710 (C=O), 1650 (C=N), 1449(N–H), 1287 (phenolic C–O) (Fig. S1).

1.4 Synthesis of 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone functionalized silica gel (HBAPE-APSG)

To increase the number of silanol groups on the surfaceof the silica gel, it was activated by drying in oven at

423 K for 18 hr. Then the reaction between the silylatingagent (APTES) and the silanol groups on activated silicasurface was performed using greener protocol to obtainaminopropylated silica gel (APSG) (Atia et al., 2009).Subsequent functionalization of APSG with the schiff’sbase was performed by refluxing a mixture of 4 g ofHBAPE (an excess of 2.0 equiv., approximately 17 mmol)and 5 g of APSG (approximately 8.5 mmol-NH2) in 50mL of ethanol for 2 hr for condensation of carbonylgroup of organic moiety with terminal amine of APSG.The yellow colored solid obtained (HBAPE-APSG) wasfiltered, washed copiously with alcohol to rinse away anysurplus ligand and dried under vacuum at 110°C for 4 hr(Scheme 1).

1.5 Analytical procedures

1.5.1 Batch methodA dose of 50 mg of dry resin was weighed accuratelyand introduced directly into 100 mL stoppered conicalflask. Then, 10 mL of metal ion solution solution (5.0µg/mL) maintained at optimum pH was added to theflask. The mixture was shaken vigorously for 30 min tofacilitate adsorption of the metal ions onto the sorbent.After filtration, the concentrations of the metal ions in thefiltrate were directly determined by FAAS using optimumparameters. In addition to this, the metal ions complexedto the organic phase of the solid sorbent were elutedwith 8 mL HNO3 under optimum concentration (either1.0 mol/L under normal conditions or 0.1 mol/L undermicrowave irradiation) and their concentrations in eluentwere analyzed.

1.5.2 Column methodA total 50 mg of sorbent was fed into a glass column (15× 2 cm). It was washed and conditioned to the desiredpH with 10 mL of buffer. After conditioning, 10 mLaliquots (5.0 µg/mL) of sample solutions were passedthrough the column at specified flow rate and the boundmetal ions were stripped off from the column with HNO3using pre-mentioned optimal conditions (Section 1.5.1).The desorbed analyte content in the eluting solvent wasdetermined by FAAS. Before using for the successive run,double deionized water was repeatedly passed through thecolumn in order to equilibrate, clean and neutralize thesolid matrix.

1.6 Sample collection and digestion

To preserve our ecosystem and to achieve the sustainabil-ity, we have built a joint collaboration with TERI. Thisjoined venture has supported many industrial projects andrecently we have developed an optimized procedure forlarge scale online recovery of palladium using a newlydesigned reactor (Sharma et al., 2012b; Adholeya andSharma, 2010). This time our focus is to completelyremove traces of zinc left in fly ash after mycorrhizal treat-

No. 6 Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······ 1255

HO

APSG HBAPE HBAPE-APSG

Scheme 1 Preparation of HBAPE-APSG adsorbent.

ment using newly synthesized adsorbent. The pretreatedash samples were procured from TERI and out of them;the control sample signifies the ash from the portion of thedump land that has not been reclaimed.

Two tablets of each pharmaceutical formulation (Zevit,Remidex Pharma Pvt. Ltd. and Becosules Z Pfizer Ltd.obtained from local pharmacy with the reported Zinccontent of 15.08 mg) were decomposed separately with 6mL of aqua regia under microwave irradiation (operatingconditions: power of 400 W for 15 min). Similar conditionswere applied to the cumulated ash samples (0.5 g) fordigestion. The obtained mixtures were filtered and bufferedto the optimum pH. Appropriate aliquots from the workingsolution were taken and aforementioned preconcentrationprocedure was applied for the determination of Zn(II).The fly ash samples were further spiked with appropriatecontents of target analyte (1.0 µg/mL) for validation of themethod by analyzing the quantitative recoveries.

2 Results and discussion

2.1 Surface coverage and characterization

2.1.1 Fourier transform infrared spectroscopyFT-IR spectrum of the silica sample exhibits the charac-teristic bands of the polysiloxane framework, i.e., Si–O–Siasymmetric stretching (1084 cm−1 with a shoulder around1220 cm−1), Si–O stretching of Si–OH groups on the sur-face (967 cm−1), Si–O–Si symmetric stretching (800 cm−1)and Si–O–Si bending vibrations (469 cm−1). Additionally,the spectrum presents a band at 1653 cm−1 associatedwith the H–O–H bending vibrations of physically adsorbedwater. A broad band centered around 3476 cm−1 is dueto O–H stretching vibrations of hydrogen-bonded surfacesilanol groups and physisorbed water (Fig. 1). The post-grafting of APTES on the silica surface is confirmed bythe emergence of new C–H weak bands in the rangeof 2928–2855 cm−1 and N–H vibrations around 1595cm−1. Moreover, the absence of the silanol stretchingoriginally present at 967 cm−1 assures the covalent linkageof aminopropyl groups to the silenous matrix (Fig. 1). In

4000 3200 2400 1800 1600 1200 800

SG

APSG

HBAPE-APSG

νOH

νN-HνC-H

νC=CνC=N

νSi-OH

νSi-O-Si

Wavelength (cm-1)

Fig. 1 FT-IR spectra of SG, APSG, HBAPE-APSG.

the spectrum of the final material (HBAPE-APSG), thereis appearance of a new band at 1583 cm−1 resulted fromC=N, which testifies the reactivity of the primary amine(–NH2), along with a characteristic band around 1468cm−1 due to C=C vibrations of the aromatic region of theorganic moiety (Abdel-Fattah and Mahmoud, 2011) (Fig.1).

2.1.2 Elemental analysisThe surface immobilization of silica gel was confirmedby the presence of carbon and nitrogen in the modifiedmaterials, which were primarily absent in activated silica.The nitrogen content reveals that APTES is successfullyintroduced on surface of silica with a loading capacity of1.7 mmol/g (nitrogen 2.38 wt.%, carbon 6.98 wt.%, hydro-gen 2.78 wt.%). The fragment of the ligand immobilizedper gram of silica was also determined and found out to

1256 Journal of Environmental Sciences 2013, 25(6) 1252–1261 / R.K. Sharma et al. Vol. 25

be 1.2 mmol/g (nitrogen 3.40 wt.%, carbon 20.20 wt.%,hydrogen 4.50 wt.%).

2.1.3 13C CPMAS NMR spectroscopy13C CPMAS NMR spectrum of APSG (Fig. 2a) exhibitsthree resonance peaks at δ = 9.2, 22.9 and 43.2 ppm,which are associated to the three carbon atoms fromaminopropyl groups, –Si–CH2–, –CH2– and –N–CH2–groups respectively. It substantiates the grafting of APTESover parental backbone. Besides this, the covalent bindingof ligand with APSG (HBAPE-APSG) has been proved byshifting of –N–CH2- peak to 60.9 ppm whereas, the peak at41.6 ppm is assigned to the uncomplexed –N–CH2- group.Peaks in the range of 114–131 ppm are referred to carbonatoms of the aromatic region of HBAPE. In addition tothis, peak at 160.4 ppm signifies the presence of carbon

atom of the functionalized moiety attached to oxygen atom(–C–OH) and schiff’s base condensation is confirmed bythe presence of peak at 164.4 ppm (C=N) (Fig. 2b).

2.1.4 Scanning electron microscopySEM micrographs were displayed to clarify the changein morphological features after functionalization of thepolysiloxane surface. The images captured at high mag-nification (Fig. 3b and d) revealed that the surface of theskeletal silica was smooth initially and turned out to berough after treatment to the support material. However,no agglomeration has been witnessed during the modifi-cation process which is responsible for its high sorptionefficiency. In fact, the particle size and appearance of themodified phase was found to be analogous to the parentalbackbone (Fig. 3a and c), which inferred that silica gel has

a b

260 220 180 140 100 60 20 0 260 220 180 140 100 60 20 0ppm ppm

163.628

43.24222.989

9.232

160.460130.494

117.671115.686

60.970

41.617

22.929

9.128

Fig. 2 13C solid state NMR of APSG (a); HBAPE-APSG (b).

100 μm EHT = 20.00 kV

WD = 11.5 mm

Date: 12 Jan 2012

Mag = 353× AIRF, JNU

a b

c d

10 μm EHT = 20.00 kV

WD = 11.5 mm

Date: 12 Jan 2012

Mag = 4.22 k× AIRF, JNU

2 μm EHT = 20.00 kV

WD = 11.5 mm

Date: 12 Jan 2012

Mag = 17.85 k× AIRF, JNU

100 μm EHT = 20.00 kV

WD = 11.5 mm

Date: 12 Jan 2012

Mag = 393× AIRF, JNU

Fig. 3 SEM micrograph of silica gel at low magnification (a), high magnification (b); and HBAPE-APSG at low magnification (c); high magnification(d).

No. 6 Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······ 1257

good mechanical strength and is not affected by conditionsencountered during preparative route.

2.1.5 Thermal analysisThe thermogravimetric curves reflect the thermal stabilityof the synthesized materials. The quantitative decomposi-tion in each stage confirms the existence of the peripheralfunctionalities grafted onto silica surface. APSG showstwo step degradation: the first stage mass loss of 1.9% iscorresponding to physisorbed water while the second oneof 9.9% is assigned to degradation of aminopropyl groupsattached to silica support (Fig. S2). In addition, the curveinvolving HBAPE-APSG is presenting a pronounced massloss of 26.9%, after the exclusion of physisorbed water. Itis allotted to the decomposition of the immobilized organicfraction together with the condensation of the remainingsilanol groups (Fig. S3).

2.1.6 BET surface area analysisThe presence of organic moieties covalently attached tothe silica backbone reduces the access of nitrogen tothe skeleton base, which results in gradual decrease inits surface area. Thus, as anticipated, the initial specificsurface area of silica matrix (235.6 m2/g) reduces to 148.8m2/g upon modification with silylating agent. Not onlythis, a further drop in S BET of HBAPE-APSG (73.2 m2/g)verifies the successful immobilization of organic moietiesover solid surface.

The average surface density, d, of the attached moleculeand the average intermolecular distance, l, of the modifiedsilica (HBAPE-APSG) has also been calculated by apply-ing the following equation (Dias-Filho, 1998):

d = NL

S BET(1)

l =

√1d

(2)

where, N is the Avogadro’s number and L is the proportionof functional groups attached on the surface. Results ob-tained (d = 0.987 molecule/nm2 and l = 1.006 nm) confirman efficient functionalization of the support material.

2.2 Influences of pH

The pH of the aqueous phase is an important factorfor analyte sorption due to changes in the protona-tion/deprotonation equilibrium of the complexing moiety.So, the effect pH on the adsorption performance of solidmatrix for Cu(II), Zn(II), Cd(II), Cr(III), Fe(II), Co(II),Al(III) and Mo(VI) ions was systematically investigated inthe pH range of 2–9. Experimentation was carried out bypassing 10 mL of test solution containing 5 µg/mL of eachanalyte at different pH values and the results are depictedin Fig. 4. As can be seen, the maximum enrichment ofthe modified silica surface was attained with zinc in the

100

80

60

40

20

0

Adso

rpti

on (

%)

1 2 3 4 5 6 7 8 9 10

pH

Fe(II)

Cr(III)

Cu(II)

Cd(II)

Co(II)

Mo(VI)

Zn(II)

Al(III)

Fig. 4 Influence of pH on adsorption of various metal ions on the solidadsorbent.

pH range of 6–7. At higher pH, lower loadings efficienciesof metal ion were observed which could be attributable tothe precipitation of targeted ion. Consequently, in order tomaximize the sorption strength and to avoid precipitationof Zn(II) ions, further experiments were conducted at thesolution pH value of 6.0.

2.3 Selection of eluting agent

To increase the economic viability of the adsorbent, it isimportant to reutilize it. Therefore, in order to optimizethe system aiming the quantitative recovery of zinc, twoeluents (HCl and HNO3) were evaluated with fixed amountof sorbent (0.1 g), in concentrations that varied from 0.1to 1.0 mol/L. The volume of acid required to extract theanalyte from the column was also investigated in the rangeof 6–10 mL (Table 2). The quantitative recovery of Zn(II)was obtained by using 8 mL of 1.0 mol/L HNO3 as aneluent. Therefore it was selected as an appropriate eluentfor further applications.

An alternative methodology for recovery of zinc fromloaded sorbent was also adopted using microwave assisteddigestion (Idris et al., 2011). A 0.1 g of solid sorbent wasdigested with 8 mL of HNO3 with varying concentrationranging from 0.05 to 0.5 mol/L (microwave operatingconditions: heating rate-ramped to 90°C for 5 min heldat 90°C for 10 min). Analysis of the filtered nitric acidsolutions collected after microwave treatment indicatedthat 0.1 mol/L HNO3 was sufficient for the recovery ofapproximately 98% of Zn(II) ions complexed to the solidmaterial.

2.4 Effect of flow rate of sample and eluent solutions

To verify the influence of the loading and elution flow rateson the recovery of metal ion, column experiments werecarried out employing different flow rates (in the range of2.0–15.0 mL/min) at optimum conditions. It was observedthat the adsorption efficiency of the sorbent was not

1258 Journal of Environmental Sciences 2013, 25(6) 1252–1261 / R.K. Sharma et al. Vol. 25

Table 2 Effect factors on elution solutions on the recovery of Zn(II)

Eluent type Concentration Volume Recovery(mol/L) (mL) (%)

HNO3 0.2 6 19.78 28.310 33.2

0.4 6 22.08 37.110 40.4

0.6 6 48.38 54.110 59.0

0.8 6 71.78 79.510 89.9

1.0 6 92.68 98.510 98.9

HCl 0.8 6 58.78 63.210 78.3

1.0 6 82.38 89.610 92.3

Amount of sorbent: 0.1 g; metal ion solution passed: 10 mL of 5 µg/mL.

altered upto a sample flow rate of 12.0 mL/min. However,at higher run, there was a reduction in the percentageadsorption of metal ion. This could be probably due to theinsufficient contact time between the sample solution andsolid sorbent. Moreover, there was a substantial decrease inrecoveries of the analyte ions when the eluent flow rate wasover 6.0 mL/min. Hence, for all subsequent experiments,sample solution and eluent were surged at a flow rate of12.0 and 6.0 mL/min, respectively.

2.5 Adsorption capacity

To determine the maximum amount of analyte resin canuptake, 0.1 g of resin was shaken with 50 mL of analytesolution having different concentration (50, 100, . . . , 200µg/mL) under optimum conditions. Initially with contin-uous increase in amount of zinc ion, the plateau values(adsorption capacity values) increased and later on nochange was observed even with high concentration of zincions, representing the saturation of active binding sites.Adsorption capacity was calculated using the followingequation (Sharma and Pant, 2009a):

Q =(C0 −Ce)V

w(3)

where, Q (mg/g) represents adsorption capacity, C0 (mg/L)and Ce (mg/L) are the initial and final concentrations ofmetal ion, respectively, w is the weight of the resin and Vis the volume of metal ion solution. So, the capacity of thesorbent to uphold maximum amount of zinc was found outto be 65.65 mg/g (1.004 mmol/g). The column method wasalso used to determine the sorption capacity. The result wasfound to be conducive with batch method.

The qualitative analysis of the solid sorbent was alsoperformed by energy dispersive X-ray fluorescence and the

40

35

30

25

20

15

10

5

00 200 400 600 800 1000

Total: 1093 cps

t = 34 sec

cps

Channel

Fig. 5 ED-XRF spectrum of metal loaded solid sorbent.

spectrum is shown in Fig. 5, which further confirmed theadsorption of zinc on the synthesized solid phase.

2.6 Influence of interfering ions

In order to assess the ability of the synthesized solid sur-face for separation of zinc from common interfering ionsin environmental samples, extraction experiments wereperformed with the aforementioned optimized conditions.A fixed amount of analyte (10 mL of 5.0 µg/mL) was takenwith different amounts of foreign ions and recommendedprocedure was followed. As can be seen in Table 3, largenumbers of ions used have no considerable effect on thedetermination of analyte ions upto a reasonable amount.

2.7 Effect of sample volume, preconcentration factorand regeneration capacity

For dealing with real samples containing very low con-centrations of trace metal ions, the maximum applicablesample volume and preconcentration factor must be de-

Table 3 Effect of interfering ions on the recovery of Zn(II) metal ion

Ion Amount Recovery Ion Amount Recovery(µg) (%) (µg) (%)

CH3COO− 1200 89.1 Cu2+ 1000 92.81000 97.2 800 97.3800 99.0 480 99.1

NO3− 480 83.6 Cd2+ 1200 89.1

240 97.7 1000 98.2120 97.9 Cr3+ 1800 65.7

NO2− 480 78.6 1400 91.4

240 92.4 1200 98.8120 98.6 800 98.9

SO42− 800 56.3 Co2+ 1800 56.5

480 97.0 1400 74.2240 99.1 1200 88.8

PO43− 1200 85.9 1000 92.3

800 92.3 Al3+ 1800 87.3480 98.6 1400 94.9240 98.9 1000 96.8

Cl− 800 82.9 Fe2+ 1200 87.6480 96.3 1000 98.1240 97.5 800 98.2

I− 1200 65.9 Mo6+ 1800 91.81000 87.0 1400 94.6800 95.4 1200 95.5

1000 96.0

Amount of sorbent: 0.05 g; aqueous volume: 20 mL; 10 mL of 5.0 µg/mLmetal ion + 10 mL of foreign ion solution.

No. 6 Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······ 1259

termined. The effect of the sample solution volume inthe range of 500–3000 mL was investigated under theoptimum conditions with 0.1 g sorbent, keeping the totalamount of loaded metal ion constant to 50 µg. The recov-eries of the target analyte were quantitative upto 2500 mL.So, under optimum conditions, the preconcentration factorwas found to be 312.5 for the sample volume of 2500 mL.To study the regeneration capacity of the material, columnprocedure was applied and the adsorption capacity of thematerial for Zn(II) was determined for various loading andelution cycles. Sorption efficiency showed reproducibleresults up to 10 cycles of continuous usage with a minorreduction of < 10% in Q (adsorption capacity) value.

2.8 Analytical performance

The precision for analysis of trace amount of Zn(II)ions in aqueous media was evaluated under the optimumexperimental conditions. For this purpose, eight portionsof standard solutions were enriched and analyzed simul-taneously by following the recommended procedure. Therecoveries were found to be greater than 97% with lowrelative standard deviation values (RSD < 3%) for eachsolution. The detection limit (DL) found as the ratio ofthe three standard deviations of the blank to the slope ofplot was 0.04 µg/L (10 replicates, R2 = 0.9990, calibrationequation A = 0.5705C + 0.0407, where A is absorbanceand C (µg/mL) is the Zn(II) concentration (Matos et al.,2009). The results indicate that the proposed method issensitive and suitable for determination of trace Zn(II) ionin ecological samples.

2.9 Application of the modified solid surface

The applicability of solid sorbent for preconcentrationof trace level of Zn(II) was tested using fly ash andpharmaceutical samples. For carrying out the preconcen-tration procedure, digested samples were passed throughthe column charged with 0.1 g of matrix after adjustingthe pH to an optimum value. The sorbed metal ionswere eluted with eluting agent and their concentrationswere determined by FAAS. In pharmaceutical samples,the recoveries were ranged from 98.7% to 100.1% forthe mean of three replicate determinations. These resultsconfirmed the accuracy of the proposed method as theanalyte contents established with the present procedureagreed very well with the values as per USP standards. Forash sample solutions of both the locations, the analyticalresults are arranged in Table 4. As predicted, the leftover content of zinc in all the samples after mycorrhizalreclamation were found to be less as compared to thesample of non reclaimed portion of the dump sites. Thevalidity of the proposed method was examined by spikingthe known concentration of Zn(II) metal ions (1.0 µg/mL)into the solutions. A good concurrence between the addedand measured analyte amounts with low RSD values ofless than 3.0% has been observed. The encountered results

Table 4 Analytical results for Zn(II) ions in fly ash samples

Sample Added metal ion Found (µg/mL) Recovery (%)(µg/mL) Reading RSD

Site 1a, – 0.3884 2.6 –Control 1.0 1.3690 2.9 98.61 – 0.2295 1.5 –

1.0 1.2035 2.3 97.92 – 0.2692 1.9 –

1.0 1.2332 0.8 97.23 – 0.2417 0.5 –

1.0 1.2227 2.7 98.54 – 0.1917 0.7 –

1.0 1.1921 2.5 100.05 – 0.2407 1.4 –

1.0 1.2203 2.2 98.46 – 0.3498 0.5 –

1.0 1.3147 1.0 97.47 – 0.1882 1.8 –

1.0 1.1799 1.1 99.3Site 2b, – 0.1990 2.0 –Control 1.0 1.1840 0.9 98.71 – 0.1193 0.7 –

1.0 1.1224 1.7 100.32 – 0.0734 1.6 –

1.0 1.0605 1.2 98.83 – 0.1724 2.8

1.0 1.1669 0.9 99.54 – 0.1882 2.1

1.0 1.1698 2.2 98.45 – 0.0826 2.7

1.0 1.0701 1.1 98.86 – 0.1902 1.2

1.0 1.1712 2.7 98.47 – 0.0949 2.6

1.0 1.0759 2.5 98.3a Fly ash samples from Thermal Power Plant.b Fly ash samples from Fertilizer Plant.Sample volume: 100 mL, eluent: 8 mL, N (replicate determinations): 5.–: signifies without metal ion addition

concluded that the outlined procedure is highly efficientfor complete and selective removal of the remaining targetanalyte and hence can be perceived as a viable way out forsustainable ecosystem.

3 Conclusions

The proposed method exhibits commendable sorption ca-pacity level, improved detection limit and good toleranceto the interfering ions with the concomitant benefits ofhigh selectivity and preconcentration factor. Moreover, theoutlined protocol has been proved as a promising, versatile,simple, inexpensive and environmentally benign approachto lower the leachable metal content of fly ash to such anextent that the ecological demands are obeyed.

Acknowledgments

One of the authors, Aditi Puri expresses her gratitudeto University Grant Commission, Delhi, India for theaward of junior research fellowship and also acknowledgesTERI, Delhi, India for their significant contribution. Also,

1260 Journal of Environmental Sciences 2013, 25(6) 1252–1261 / R.K. Sharma et al. Vol. 25

due thanks to DRDO, Delhi, India for BET surface areaanalysis, AIRF, JNU, Delhi, India for SEM analysis andIISc, Bangalore, India for solid state NMR measurements.

Supporting materials

Supplementary data associated with this article can befound in the online version.

References

Abdel-Fattah T M, Mahmoud M E, 2011. Selective extractionof toxic heavy metal oxyanions and cations by a novelsilica gel phase functionalized by vitamin B4. ChemicalEngineering Journal, 172(1): 177–183.

Adholeya A, Sharma R K, 2010. Patent: CBR 41711146/del/2010.

Alan M, Kara D, Fisher A, 2007. Preconcentration of heavymetals and matrix elimination using silica gel chemicallymodified with 2,3-dihydroxybenzaldehyde. Separation Sci-ence and Technology, 42(4): 879–895.

Atia A A, Donia A M, Al-Amrani W A, 2009. Effect of aminetype modifier on the uptake behaviour of silica towardsmercury(II) in aqueous solution. Desalination, 246(1-3):257–274.

Chah S, Kim J S, Yi J, 2002. Separation of zinc ions from aqueoussolutions using modified silica impregnated with Cyanex272. Separation Science and Technology, 37(3): 701–716.

DeMartino M G, Macarovscha G T, Cadore S, 2010. The use ofzincon for preconcentration and determination of zinc byflame atomic absorption spectrometry. Analytical Methods,2(9): 1258–1262.

Dias-Filho N L, 1998. Adsorption of copper(II) and cobalt(II)complexes on a silica gel surface chemically modified with3-amino-1,2,4-triazole. Colloids and Surfaces A: Physico-chemical and Engineering Aspects, 144(1-3): 219–227.

EPA/635/R-05/002, 2005. Toxicological Review of Zinc andCompounds, U.S. Environmental Protection Agency. Wash-ington DC, USA.

Garg B S, Sharma R K, Bhojak N, Mittal S, 1999. Chelatingresins and their applications in the analysis of trace metalions. Microchemical Journal, 61(2): 94–114.

Gaur A, Adholeya A, 2004. Prospects of arbuscular mycorrhizalfungi in phytoremediation of heavy metal contaminatedsoils. Current Science, 86(4): 528–534.

Ghoul M, Bacquet M, Morcellet M, 2003. Uptake of heavy metalsfrom synthetic aqueous solutions using modified PEI-silicagels. Water Research, 37(4): 729–734.

Goswami A, Singh A K, 2002a. Silica gel functionalized withresacetophenone: synthesis of a new chelating matrix andits application as metal ion collector for their flame atomicabsorption spectrometric determination. Analytica ChimicaActa, 454(2): 229–240.

Goswami A, Singh A K, 2002b. 1,8-Dihydroxyanthraquinoneanchored on silica gel: synthesis and application as solidphase extractant for lead(II), zinc(II) and cadmium(II) priorto their determination by flame atomic absorption spectrom-etry. Talanta, 58(4): 669–678.

Goswami A, Singh A K, Venkataramani B, 2003. 8-Hydroxyquinoline anchored to silica gel via new moderate

size linker: synthesis and applications as a metal ioncollector for their flame atomic absorption spectrometricdetermination. Talanta, 60(6): 1141–1154.

Hansen Y, Notten P J, Petrie J G, 2002. The environmentalimpact of ash management in coal-based power generation.Applied Geochemistry, 17(8): 1131–1141.

Hossain K Z, Mercier L, 2002. Intraframework metal ion adsorp-tion in ligand-functionalized mesoporous silica. AdvancedMaterials, 14(15): 1053–1056.

Idris S A, Harvey S R, Gibson L T, 2011. Selective extrac-tion of mercury(II) from water samples using mercaptofunctionalised-MCM-41 and regeneration of the sorbent us-ing microwave digestion. Journal of Hazardous Materials,193: 171–176.

IPCS (The International Programme on Chemical Safety), 2001.Environmental Health Criteria Document 221 Zinc. WorldHealth Organization, Geneva.

Jiang Y J, Gao Q M, Yu H G, Chen Y R, Deng F, 2007.Intensively competitive adsorption for heavy metal ions byPAMAM-SBA-15 and EDTA-PAMAMSBA-15 inorganic-organic hybrid materials. Microporous and MesoporousMaterials, 103(1-3): 316–324.

Lenz D M, Martins F B, 2007. Lead and zinc selective precipita-tion from leach electric arc furnace dust solutions. RevistaMateria, 12(3): 503–509.

Malvankar P L, Shinde V M, 2007. Ion-pair extraction anddetermination of copper(II) and zinc(II) in environmentaland pharmaceutical samples. Analyst, 116(10): 1081–1084.

Matos G D, dos Reis E B, Costa A C S, Ferreira S L C, 2009.Speciation of chromium in river water samples contam-inated with leather effluents by flame atomic absorptionspectrometry after separation/preconcentration by cloudpoint extraction. Microchemical Journal, 92(2): 135–139.

Northcott K, Kokusen H, Komatsu Y, Stevens G, 2006. Synthesisand surface modification of mesoporous silicate SBA-15for the adsorption of metal ions. Separation Science andTechnology, 41(9): 1829–1840.

Pandey V C, Abhilash P C, Singh N, 2009. The Indian perspectiveof utilizing fly ash in phytoremediation, phytomanagementand biomass production. Journal of Environmental Man-agement, 90(10): 2943–2958.

Perez-Quintanilla D, Sanchez A, del Hierro I, Fajardo M, SierraI, 2010. New hybrid materials as Zn(II) sorbents in watersamples. Materials Research Bulletin, 45(9): 1177–1181.

Perez-Quintanilla D, Sanchez A, del Hierro I, Fajardo M, SierraI, 2009. Preconcentration of Zn(II) in water samples using anew hybrid SBA-15-based material. Journal of HazardousMaterials, 166(2-3): 1449–1458.

Perez-Quintanilla D, Sanchez A, del Hierro I, Fajardo M,Sierra I, 2007. Preparation, characterization, and Zn2+ ad-sorption behavior of chemically modified MCM-41 with5-mercapto-1-methyltetrazole. Journal of Colloid and In-terface Science, 313(2): 551–562.

Sales J A A, Faria F P, Prado A G S, Airoldi C, 2004. Attachmentof 2-aminomethylpyridine molecule onto grafted silica gelsurface and its ability in chelating cations. Polyhedron,23(5): 719–725.

Sharma R K, 2001. Design, synthesis, and application of chelat-ing polymers for separation and determination of trace andtoxic metal ions. A green analytical method. Pure and

No. 6 Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······ 1261

Applied Chemistry, 73(1): 181–186.Sharma R K, Adholeya A, Puri A, Das M, 2012a. Bioextrac-

tion: The Interface of Biotechnology and Green Chemistry.In: Biomass Conversion: The Interface of Biotechnology,Chemistry and Materials Science (Baskar C, Baskar S,Dhillon R S, eds.). Springer, New York. 435–457.

Sharma R K, Dhingra S, 2011. Designing and Synthesis ofFunctionalized Silica Gels and their Applications as MetalScavengers, Sensors, and Catalysts: A Green ChemistryApproach. LAP Lambert Academic Publishing, Germany.

Sharma R K, Garg B S, Bhojak N, Bist J S, Mittal S, 1999.Separation and preconcentration of Metal ion and theirestimation in vitamin, steel and milk samples using o-vanillin-immobilized silica gel. Talanta, 48(1): 49–55.

Sharma R K, Goel A, 2005. Development of a Cr(III)-specific po-tentiometric sensor using aurin tricarboxylic acid modifiedsilica. Analytica Chimica Acta, 534(1): 137–142.

Sharma R K, Mittal S, Azmi S, Adholeya A, 2005. Surface mod-ified silica gel for extraction of metal ions: an environmentfriendly method for waste treatment. Surface Engineering,21(3): 232–237.

Sharma R K, Mittal S, Koel M, 2003. Analysis of trace amountsof metal ions using silica-based chelating resins: A greenanalytical method. Critical Reviews in Analytical Chem-istry, 33(3): 183–197.

Sharma R K, Pandey A, Gulati S, Adholeya A, 2012b. Anoptimized procedure for preconcentration, determinationand on-line recovery of palladium using highly selectivediphenyldiketone-monothiosemicarbazone modified silicagel. Journal of Hazardous Materials, 209-210: 285–292.

Sharma R K, Pant P, 2009a. Preconcentration and determinationof trace metal ions from aqueous sample by newly de-veloped gallic acid modified Amberlite XAD-16 chelatingresin. Journal of Hazardous Materials, 163(1): 295–301.

Sharma R K, Pant P, 2009b. Solid phase extraction and deter-mination of metal ions in aqueous samples using quercetinmodified Amberlite XAD-16 chelating polymer as metalextractant. International Journal of Environmental Analyti-cal Chemistry, 87(7): 503–514.

Shukla R, Rao G N, 2002. Solvent extraction of met-als with potassium-dihydro-bispyrazolyl-borate. Talanta,57(4): 633–639.

Venkatesh G, Singh A K, Venkataramani B, 2004. Silica gelloaded with o-dihydroxybenzene: design, metal sorptionequilibrium studies and application to metal enrichmentprior to determination by flame atomic absorption spec-trometry. Microchimica Acta, 144(4): 233–241.

Yang H, Xu R, Xue X M, Li F T, Li G T, 2008. Hybrid surfactant-templated mesoporous silica formed in ethanol and itsapplication for heavy metal removal. Journal of HazardousMaterials, 152(2): 690–698.

Yu J X, Li T H, 2011. Distinct biological effects of differentnanoparticles commonly used in cosmetics and medicinecoatings. Cell and Bioscience, 1(1): 19–27.

Yuce S, Ozek A, Albayrak C, Odabasoglu M, Buyukgun-gor O, 2004. A redetermination of 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone. Acta Crystallograph-ica Section E, E60: o718–o719.

Zhu X B, Chang X J, Cui Y M, Zou X J, Yang D, Hu Z, 2007.Solid-phase extraction of trace Cu(II) Fe(III) and Zn(II)with silica gel modified with curcumin from biologicaland natural water samples by ICP-OES. MicrochemicalJournal, 86(2): 189–194.

Zou X J, Cui Y M, Chang X J, Zhu X B, Hu Z, Yang D, 2009.Silica gel surface modified with sulfanilamide for selectivesolid-phase extraction of Cu(II), Zn(II) and Ni(II). Inter-national Journal of Environmental Analytical Chemistry,89(14): 1043–1055.