Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: Synthesis, characterization and application as an efficient and reusable solid phase extractant for selective removal of Zn(II) from mycorrhizal treated fly-ash samples

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JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X

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

www.jesc.ac.cn

Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: Synthesis, characterization and application as an efficient and reusable 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, India 2. 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

Abstract 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone functionalized silica gel was synthesized and used as a highly efficient, selective and reusable solid phase extractant for separation and preconcentration of trace amount of Zn(II) from environmental matrices. The adsorbent was characterized by fourier transform infrared spectroscopy (FT-IR), elemental analysis,13 C CPMAS NMR spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and BET surface area analysis. The dependence of zinc extraction on various analytical parameters such as pH, type and amount of eluent, sample flow rate and interfering ions were investigated in detail. The material exhibited superior adsorption efficiency for Zn(II) with high metal loading capacity of 1.0 mmol/g under optimum conditions. After adsorption, the recovery (> 98%) of metal ions was accomplished using 1.0 mol/L HNO3 as an eluent. The sorbent was also regenerated by microwave treatment in milder acidic environment (0.1 mol/L HNO3 ). The lower detection limit and preconcentration factor of the present method were found out to be 0.04 µg/L and 312.5 respectively. The modified silica surface possessed excellent selectivity for the target analytes and the adsorption/desorption process remained effective for at least ten consecutive cycles. The optimized procedure was successfully implemented for the extraction of Zn(II) from mycorrhizal treated fly ash 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 of numerous proteins, including metalloenzymes, transcription factors and hormone receptors. The widespread role of zinc in metabolism is also accentuated by the occurrence of zinc in all tissues, organs and fluids of the human body (DeMartino et al., 2010). In addition to this, since the industrial revolution, the use of zinc has increased exponentially due to its presence in every area of modern consumerism: from construction materials to cosmetics, medicines to processed foods and appliances to personal care products (P´erez-Quintanilla et al., 2009; Yu and Li, 2011). The extensive utilization and application of zinc in various industrial and commercial activities necessitates its * Corresponding author. E-mail: [email protected]

accurate analytical determination and recovery for regulating and minimizing its discharge into the environment from the view point of safety. This is because elevated quantities of zinc in living organisms have been reported to cause various acute and chronic adverse effects, reduction in growth reproductive and developmental defects, electrolyte imbalance, nausea lower levels of high-density lipoprotein cholesterol, intracellular production of reactive oxygen species (ROS), and in consequence, oxidative stress or death of cells (Environmental Health Criteria Document 221 Zinc, 2001; US EPA, 2005). To conquer the rising concern of zinc toxicity, various preconcentration techniques such as ion pair extraction (Malvankar and Shinde, 2007), precipitation (Lenz and Martins, 2007) and liquid-liquid extraction (Shukla and Rao, 2002) are being practiced over decades. But, these methods are

Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······

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not economical and eco-friendly and suffer from various drawbacks like lack of sensitivity and selectivity and use of large amount of toxic organic solvents which have deleterious effect on human health and environment. Thus, solid phase extraction (SPE) has come to the forefront in recent years for selective preconcentration and separation of trace amounts of elements as it simplifies labour intensive sample preparation, lessens the cost and time, eliminates the clean-up step, provides high enrichment factors, and minimizes costs due to low consumption of reagents thus providing an economic, persuasive and greener alternative to other traditional methodologies (Sharma et al., 2003) In continuation of our research work (Sharma et al., 2012b, 2005, 1999a; Garg et al., 1999b; Sharma and Dhingra, 2011; Sharma and Pant, 2009a, 2009b; Sharma and Goel, 2005; Sharma, 2001) on synthesis of efficient and cost effective solid adsorbents having specific metal ion binding affinity against background constituents, the present work is focused on the synthesis of schiff’s base functionalized silica gel with a view of finding out a simple and extremely selective solid phase extractant for Zn(II) ions. In fact, a comparative data of the present work with some literature precedents, based on ligandfunctionalized amorphous and mesoporous silica materials for zinc adsorption has been compiled in Table 1. It is evident from comparison that the synthesized adsorbent exhibits enhanced analytical characteristics with respect to most of the chelating resins based on different silenous matrix. Although, MTTZ-MCM-41 was able to bind quantitatively more Zn(II) ions from aqueous solution, the preconcentration factor was low. Moreover, adsorbent could not be reutilized for more than three successive adsorption-desorption cycles. The applicability of the present method is judged by investigating the uptake behavior of the adsorbent for Table 1

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zinc in fly ash samples from two major fertilizer and thermal power plants of North India. Indian coal used in fertilizer or thermal power plants generates massive contents of fly ash which are generally being considered a deadly source of health hazards because of the presence of potentially toxic concentrations of heavy metals (Hansen et al., 2002). Land fill disposal of fly ash has adverse impacts on terrestrial and aquatic ecosystems due to leaching of toxic metal ions into soil and groundwater. Therefore, the extraction of metals from fly ash is of utmost concern to cease the environmental transitions caused by these metal loaded dumps. Major initiatives have been taken up by TERI (The Energy and Resources Institute) to ecorestore these ash containing lands. These hazardous sites have been reclaimed to reduce the leachable content of heavy metals through the implementation of mycorrhizal fungi-based technology (Pandey et al., 2009; Gaur and Adholeya, 2004; Sharma et al., 2012a). We have examined and compared the concentration of zinc left in the areas after mycorrhizal treatment with non treated ash dumps. Subsequently, we have implemented the outlined method for effective and selective uptake of residual zinc content from these samples. To best of our knowledge, this type of applicability has not been elucidated before for solid phase extraction of zinc.

1 Materials and methods 1.1 Reagents 4-Amino acetophenone, silica gel and salicylaldehyde were procured from Sisco Research Laboratory and used as received without further purification. 3Aminopropyltriethoxysilane (APTES) was purchased from Sigma Aldrich. Working solutions were prepared by appropriate dilution of the stock standard solutions.

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

Immobilized ligand

Support material

Adsorption capacity (mmol/g)

Preconcentration factor

Reference

5-Mercapto-1-methyltetrazole Sulfanilamide 5-Mercapto-1-methyltetrazole 3-Aminopropyltriethoxysilane 5-Mercapto-1-methyltetrazole Polyamidoamine and EDTA-polyamidoamine Curcumin 2,3-Dihydroxybenzaldehyde Cyanex 272 2-Aminomethylpyridine o-Dihydroxybenzene PEI 8-Hydroxyquinoline Resacetophenone 1,4-Bis-[3-(trimethoxysilyl)propyl]ethylenediamine Cyanex 272 1,8-Dihydroxyanthraquinone 1-{4-[(2-Hydroxy-benzylidene)amino]phenyl}ethanone

MSU-2 and HMS Silica Gel SBA-15 Mesoporous silica MCM-41 SBA-15 Silica Gel Silica Gel SBA-15 Silica Gel Silica Gel Silica Gel Silica Gel Silica Gel HMS Silica Gel Silica Gel Silica Gel

0.94 and 0.72 0.292 0.96 0.36 1.59 0.21 and 0.15 0.37 0.133 0.111 0.22 0.168 0.82 0.177 0.191 0.0011 0.31 0.18 1.004

200 100 200 – 100 – 75 – – – – – 200 150 – – – 312.5

P´erez-Quintanilla et al., 2010 Zou et al., 2009 P´erez-Quintanilla et al., 2009 Yang et al., 2008 P´erez-Quintanilla et al., 2007 Jiang et al., 2007 Zhu et al., 2007 Alan et al., 2007 Northcott et al., 2006 Sales et al., 2004 Venkatesh et al., 2004 Ghoul et al., 2003 Goswami et al., 2003 Goswami et al., 2002a Hossain et al., 2002 Chah et al., 2002 Goswami et al., 2002b This work

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

Double deionized water was used throughout the experiment. The pH of the solutions was adjusted using the following buffers: (a) potassium chloride/hydrochloric acid for pH 2, (b) sodium acetate/acetic acid for pH 3–6, (c) disodium hydrogen phosphate/hydrochloric acid for pH 7–8 and ammonia/ammonium chloride for pH 9. 1.2 Instruments Infrared spectra (4000–400 cm−1 ) were recorded on FT-IR spectrophotometer (Perkin Elmer, USA) using KBr pellets. The contents of carbon, hydrogen and nitrogen were analyzed with Elementar Analysenysteme GmbH VarioEL analyser, Germany. The pH measurements were carried out using ELICO LI 120 pH meter, India. LABINDIA AA 7000 Flame atomic absorption spectrophotometer (LABINDIA, India) was employed for the determination of metal ion. 13 C CPMAS NMR spectra were recorded on DSX-300 NMR spectrometer (Bruker, Germany) at 75.47 MHz equipped with a commercial 4 mm MAS NMR probe (magnetic field 7.04 T, pulse delay of 5 sec, contact time 3 ms). Thermogravimetric analysis was performed on DTG60 instrument (Shimadzu, Japan), equipped with TG units at a heating rate of 10°C/min from 25 to 900°C in N2 atmosphere. Digestions and elutions were performed in Anton Paar multiwave 3000 microwave reaction system (Perkin Elmer, USA), equipped with temperature and pressure sensor. Scanning electron microscopy (SEM) images were obtained using an EVO 40 instrument (ZEISS, Germany). The samples were placed on a carbon tape and then coated with a thin layer of gold using a sputter coater. Qualitative analysis of metal sorbed onto the soild phase was performed by X-Ray XAN-FAD BC ED-XRF spectrometer equipped with a tungsten anode (Fischerscope, Netherlands). Surface area analysis was carried out at 77 K on Gemini-V2.00 instrument (Micromeritics Instrument Corp., USA). 1.3 Synthesis of 1-{4-[(2-hydroxy-benzylidene)amino] phenyl}ethanone (HBAPE) The schiff’s base was synthesized according to the reported method (Yuce et al., 2004) with minor modifications. Equimolar quantities of 4-aminoacetophenone and salicylaldehyde were sonicated in ethanol for 5 min to afford the desired yellow product, which was recrystallized twice with ethanol. The properties are as follows: yield: 82%; melting point: > 300°C; anal. calc. for C15 H13 NO2 (Mol. wt.=239.27): C 75.42, H 5.54, N 5.60; found C 75.30, H 5.58, 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 (HBAPEAPSG) To increase the number of silanol groups on the surface of the silica gel, it was activated by drying in oven at

Vol. 25

423 K for 18 hr. Then the reaction between the silylating agent (APTES) and the silanol groups on activated silica surface was performed using greener protocol to obtain aminopropylated silica gel (APSG) (Atia et al., 2009). Subsequent functionalization of APSG with the schiff’s base was performed by refluxing a mixture of 4 g of HBAPE (an excess of 2.0 equiv., approximately 17 mmol) and 5 g of APSG (approximately 8.5 mmol-NH2 ) in 50 mL of ethanol for 2 hr for condensation of carbonyl group of organic moiety with terminal amine of APSG. The yellow colored solid obtained (HBAPE-APSG) was filtered, washed copiously with alcohol to rinse away any surplus ligand and dried under vacuum at 110°C for 4 hr (Scheme 1). 1.5 Analytical procedures 1.5.1 Batch method A dose of 50 mg of dry resin was weighed accurately and introduced directly into 100 mL stoppered conical flask. Then, 10 mL of metal ion solution solution (5.0 µg/mL) maintained at optimum pH was added to the flask. The mixture was shaken vigorously for 30 min to facilitate adsorption of the metal ions onto the sorbent. After filtration, the concentrations of the metal ions in the filtrate were directly determined by FAAS using optimum parameters. In addition to this, the metal ions complexed to the organic phase of the solid sorbent were eluted with 8 mL HNO3 under optimum concentration (either 1.0 mol/L under normal conditions or 0.1 mol/L under microwave irradiation) and their concentrations in eluent were analyzed. 1.5.2 Column method A total 50 mg of sorbent was fed into a glass column (15 × 2 cm). It was washed and conditioned to the desired pH with 10 mL of buffer. After conditioning, 10 mL aliquots (5.0 µg/mL) of sample solutions were passed through the column at specified flow rate and the bound metal ions were stripped off from the column with HNO3 using pre-mentioned optimal conditions (Section 1.5.1). The desorbed analyte content in the eluting solvent was determined by FAAS. Before using for the successive run, double deionized water was repeatedly passed through the column in order to equilibrate, clean and neutralize the solid matrix. 1.6 Sample collection and digestion To preserve our ecosystem and to achieve the sustainability, we have built a joint collaboration with TERI. This joined venture has supported many industrial projects and recently we have developed an optimized procedure for large scale online recovery of palladium using a newly designed reactor (Sharma et al., 2012b; Adholeya and Sharma, 2010). This time our focus is to completely remove traces of zinc left in fly ash after mycorrhizal treat-

Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······

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HO

APSG

HBAPE-APSG

HBAPE Scheme 1 Preparation of HBAPE-APSG adsorbent.

ment using newly synthesized adsorbent. The pretreated ash samples were procured from TERI and out of them; the control sample signifies the ash from the portion of the dump 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 Zinc content of 15.08 mg) were decomposed separately with 6 mL of aqua regia under microwave irradiation (operating conditions: power of 400 W for 15 min). Similar conditions were applied to the cumulated ash samples (0.5 g) for digestion. The obtained mixtures were filtered and buffered to the optimum pH. Appropriate aliquots from the working solution were taken and aforementioned preconcentration procedure was applied for the determination of Zn(II). The fly ash samples were further spiked with appropriate contents of target analyte (1.0 µg/mL) for validation of the method by analyzing the quantitative recoveries.

HBAPE-APSG ν νC=NC=C

APSG

νC-H

νN-H

SG νSi-OH νOH

2 Results and discussion 2.1 Surface coverage and characterization 2.1.1 Fourier transform infrared spectroscopy FT-IR spectrum of the silica sample exhibits the characteristic bands of the polysiloxane framework, i.e., Si–O–Si asymmetric stretching (1084 cm−1 with a shoulder around 1220 cm−1 ), Si–O stretching of Si–OH groups on the surface (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 associated with the H–O–H bending vibrations of physically adsorbed water. A broad band centered around 3476 cm−1 is due to O–H stretching vibrations of hydrogen-bonded surface silanol groups and physisorbed water (Fig. 1). The postgrafting of APTES on the silica surface is confirmed by the emergence of new C–H weak bands in the range of 2928–2855 cm−1 and N–H vibrations around 1595 cm−1 . Moreover, the absence of the silanol stretching originally present at 967 cm−1 assures the covalent linkage of aminopropyl groups to the silenous matrix (Fig. 1). In

νSi-O-Si

4000

3200

2400

1800

1600

1200

800

Wavelength (cm-1) Fig. 1 FT-IR spectra of SG, APSG, HBAPE-APSG.

the spectrum of the final material (HBAPE-APSG), there is appearance of a new band at 1583 cm−1 resulted from C=N, which testifies the reactivity of the primary amine (–NH2 ), along with a characteristic band around 1468 cm−1 due to C=C vibrations of the aromatic region of the organic moiety (Abdel-Fattah and Mahmoud, 2011) (Fig. 1). 2.1.2 Elemental analysis The surface immobilization of silica gel was confirmed by the presence of carbon and nitrogen in the modified materials, which were primarily absent in activated silica. The nitrogen content reveals that APTES is successfully introduced on surface of silica with a loading capacity of 1.7 mmol/g (nitrogen 2.38 wt.%, carbon 6.98 wt.%, hydrogen 2.78 wt.%). The fragment of the ligand immobilized per gram of silica was also determined and found out to

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

1256

be 1.2 mmol/g (nitrogen 3.40 wt.%, carbon 20.20 wt.%, hydrogen 4.50 wt.%). 2.1.3 13 C CPMAS NMR spectroscopy 13

C CPMAS NMR spectrum of APSG (Fig. 2a) exhibits three resonance peaks at δ = 9.2, 22.9 and 43.2 ppm, which are associated to the three carbon atoms from aminopropyl groups, –Si–CH2 –, –CH2 – and –N–CH2 – groups respectively. It substantiates the grafting of APTES over parental backbone. Besides this, the covalent binding of ligand with APSG (HBAPE-APSG) has been proved by shifting of –N–CH2 - peak to 60.9 ppm whereas, the peak at 41.6 ppm is assigned to the uncomplexed –N–CH2 - group. Peaks in the range of 114–131 ppm are referred to carbon atoms of the aromatic region of HBAPE. In addition to this, peak at 160.4 ppm signifies the presence of carbon a

atom of the functionalized moiety attached to oxygen atom (–C–OH) and schiff’s base condensation is confirmed by the presence of peak at 164.4 ppm (C=N) (Fig. 2b). 2.1.4 Scanning electron microscopy SEM micrographs were displayed to clarify the change in morphological features after functionalization of the polysiloxane surface. The images captured at high magnification (Fig. 3b and d) revealed that the surface of the skeletal silica was smooth initially and turned out to be rough after treatment to the support material. However, no agglomeration has been witnessed during the modification process which is responsible for its high sorption efficiency. In fact, the particle size and appearance of the modified phase was found to be analogous to the parental backbone (Fig. 3a and c), which inferred that silica gel has b

9.232 43.242 22.989

9.128 117.671115.686 160.460 130.494

220

180

22.929 41.617

60.970

163.628

260

Vol. 25

140

100 Fig. 2

60 13 C

20

0

ppm

260

220

180

140

100

60

20

0

ppm

solid state NMR of APSG (a); HBAPE-APSG (b).

a

100 μm EHT = 20.00 kV Date: 12 Jan 2012 AIRF, JNU WD = 11.5 mm Mag = 353× c

100 μm EHT = 20.00 kV Date: 12 Jan 2012 AIRF, JNU WD = 11.5 mm Mag = 393×

b

10 μm

EHT = 20.00 kV Date: 12 Jan 2012 WD = 11.5 mm Mag = 4.22 k× AIRF, JNU

d

2 μm

EHT = 20.00 kV Date: 12 Jan 2012 WD = 11.5 mm Mag = 17.85 k× 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).

Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······

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

100

2.1.5 Thermal analysis

2.1.6 BET surface area analysis The presence of organic moieties covalently attached to the silica backbone reduces the access of nitrogen to the skeleton base, which results in gradual decrease in its surface area. Thus, as anticipated, the initial specific surface area of silica matrix (235.6 m2 /g) reduces to 148.8 m2 /g upon modification with silylating agent. Not only this, a further drop in S BET of HBAPE-APSG (73.2 m2 /g) verifies the successful immobilization of organic moieties over solid surface. The average surface density, d, of the attached molecule and the average intermolecular distance, l, of the modified silica (HBAPE-APSG) has also been calculated by applying the following equation (Dias-Filho, 1998): d=N √ l=

L S BET

1 d

Fe(II) Cr(III) Cu(II) Cd(II) Co(II) Mo(VI) Zn(II) Al(III)

(1)

(2)

where, N is the Avogadro’s number and L is the proportion of functional groups attached on the surface. Results obtained (d = 0.987 molecule/nm2 and l = 1.006 nm) confirm an efficient functionalization of the support material. 2.2 Influences of pH The pH of the aqueous phase is an important factor for analyte sorption due to changes in the protonation/deprotonation equilibrium of the complexing moiety. So, the effect pH on the adsorption performance of solid matrix for Cu(II), Zn(II), Cd(II), Cr(III), Fe(II), Co(II), Al(III) and Mo(VI) ions was systematically investigated in the pH range of 2–9. Experimentation was carried out by passing 10 mL of test solution containing 5 µg/mL of each analyte at different pH values and the results are depicted in Fig. 4. As can be seen, the maximum enrichment of the modified silica surface was attained with zinc in the

Adsorption (%)

80

The thermogravimetric curves reflect the thermal stability of the synthesized materials. The quantitative decomposition in each stage confirms the existence of the peripheral functionalities grafted onto silica surface. APSG shows two step degradation: the first stage mass loss of 1.9% is corresponding to physisorbed water while the second one of 9.9% is assigned to degradation of aminopropyl groups attached to silica support (Fig. S2). In addition, the curve involving HBAPE-APSG is presenting a pronounced mass loss of 26.9%, after the exclusion of physisorbed water. It is allotted to the decomposition of the immobilized organic fraction together with the condensation of the remaining silanol groups (Fig. S3).

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60

40

20

0 6 7 8 9 10 pH Fig. 4 Influence of pH on adsorption of various metal ions on the solid adsorbent. 1

2

3

4

5

pH range of 6–7. At higher pH, lower loadings efficiencies of metal ion were observed which could be attributable to the precipitation of targeted ion. Consequently, in order to maximize the sorption strength and to avoid precipitation of Zn(II) ions, further experiments were conducted at the solution pH value of 6.0. 2.3 Selection of eluting agent To increase the economic viability of the adsorbent, it is important to reutilize it. Therefore, in order to optimize the system aiming the quantitative recovery of zinc, two eluents (HCl and HNO3 ) were evaluated with fixed amount of sorbent (0.1 g), in concentrations that varied from 0.1 to 1.0 mol/L. The volume of acid required to extract the analyte from the column was also investigated in the range of 6–10 mL (Table 2). The quantitative recovery of Zn(II) was obtained by using 8 mL of 1.0 mol/L HNO3 as an eluent. Therefore it was selected as an appropriate eluent for further applications. An alternative methodology for recovery of zinc from loaded sorbent was also adopted using microwave assisted digestion (Idris et al., 2011). A 0.1 g of solid sorbent was digested with 8 mL of HNO3 with varying concentration ranging from 0.05 to 0.5 mol/L (microwave operating conditions: heating rate-ramped to 90°C for 5 min held at 90°C for 10 min). Analysis of the filtered nitric acid solutions collected after microwave treatment indicated that 0.1 mol/L HNO3 was sufficient for the recovery of approximately 98% of Zn(II) ions complexed to the solid material. 2.4 Effect of flow rate of sample and eluent solutions To verify the influence of the loading and elution flow rates on the recovery of metal ion, column experiments were carried out employing different flow rates (in the range of 2.0–15.0 mL/min) at optimum conditions. It was observed that the adsorption efficiency of the sorbent was not

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

1258 Table 2

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

Eluent type

Concentration (mol/L)

Volume (mL)

Recovery (%)

HNO3

0.2

6 8 10 6 8 10 6 8 10 6 8 10 6 8 10 6 8 10 6 8 10

19.7 28.3 33.2 22.0 37.1 40.4 48.3 54.1 59.0 71.7 79.5 89.9 92.6 98.5 98.9 58.7 63.2 78.3 82.3 89.6 92.3

0.4

0.6

0.8

1.0

HCl

0.8

1.0

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 percentage adsorption of metal ion. This could be probably due to the insufficient contact time between the sample solution and solid sorbent. Moreover, there was a substantial decrease in recoveries of the analyte ions when the eluent flow rate was over 6.0 mL/min. Hence, for all subsequent experiments, sample solution and eluent were surged at a flow rate of 12.0 and 6.0 mL/min, respectively.

cps 40 35 30 25 20 15 10 5 0 0

Vol. 25

Total: 1093 cps t = 34 sec

200

400

800 600 1000 Channel Fig. 5 ED-XRF spectrum of metal loaded solid sorbent.

spectrum is shown in Fig. 5, which further confirmed the adsorption of zinc on the synthesized solid phase. 2.6 Influence of interfering ions In order to assess the ability of the synthesized solid surface for separation of zinc from common interfering ions in environmental samples, extraction experiments were performed with the aforementioned optimized conditions. A fixed amount of analyte (10 mL of 5.0 µg/mL) was taken with different amounts of foreign ions and recommended procedure was followed. As can be seen in Table 3, large numbers of ions used have no considerable effect on the determination of analyte ions upto a reasonable amount. 2.7 Effect of sample volume, preconcentration factor and regeneration capacity For dealing with real samples containing very low concentrations of trace metal ions, the maximum applicable sample volume and preconcentration factor must be deTable 3 Effect of interfering ions on the recovery of Zn(II) metal ion

2.5 Adsorption capacity To determine the maximum amount of analyte resin can uptake, 0.1 g of resin was shaken with 50 mL of analyte solution having different concentration (50, 100, . . . , 200 µg/mL) under optimum conditions. Initially with continuous increase in amount of zinc ion, the plateau values (adsorption capacity values) increased and later on no change was observed even with high concentration of zinc ions, representing the saturation of active binding sites. Adsorption capacity was calculated using the following equation (Sharma and Pant, 2009a): (C0 − Ce )V (3) w where, Q (mg/g) represents adsorption capacity, C0 (mg/L) and Ce (mg/L) are the initial and final concentrations of metal ion, respectively, w is the weight of the resin and V is the volume of metal ion solution. So, the capacity of the sorbent to uphold maximum amount of zinc was found out to be 65.65 mg/g (1.004 mmol/g). The column method was also used to determine the sorption capacity. The result was found to be conducive with batch method. The qualitative analysis of the solid sorbent was also performed by energy dispersive X-ray fluorescence and the Q=

Ion

Amount (µg)

Recovery (%)

Ion

Amount (µg)

Recovery (%)

CH3 COO−

1200 1000 800 480 240 120 480 240 120 800 480 240 1200 800 480 240 800 480 240 1200 1000 800

89.1 97.2 99.0 83.6 97.7 97.9 78.6 92.4 98.6 56.3 97.0 99.1 85.9 92.3 98.6 98.9 82.9 96.3 97.5 65.9 87.0 95.4

Cu2+

1000 800 480 1200 1000 1800 1400 1200 800 1800 1400 1200 1000 1800 1400 1000 1200 1000 800 1800 1400 1200 1000

92.8 97.3 99.1 89.1 98.2 65.7 91.4 98.8 98.9 56.5 74.2 88.8 92.3 87.3 94.9 96.8 87.6 98.1 98.2 91.8 94.6 95.5 96.0

NO3 − NO2 − SO4 2− PO4 3−

Cl− I−

Cd2+ Cr3+

Co2+

Al3+

Fe2+

Mo6+

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

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Chemically modified silica gel with 1-{4-[(2-hydroxy-benzylidene)amino]phenyl}ethanone: ······

termined. The effect of the sample solution volume in the range of 500–3000 mL was investigated under the optimum conditions with 0.1 g sorbent, keeping the total amount of loaded metal ion constant to 50 µg. The recoveries of the target analyte were quantitative upto 2500 mL. So, under optimum conditions, the preconcentration factor was found to be 312.5 for the sample volume of 2500 mL. To study the regeneration capacity of the material, column procedure was applied and the adsorption capacity of the material for Zn(II) was determined for various loading and elution cycles. Sorption efficiency showed reproducible results up to 10 cycles of continuous usage with a minor reduction of < 10% in Q (adsorption capacity) value.

Table 4

Added metal ion (µg/mL)

Found (µg/mL) Reading RSD

Recovery (%)

Site 1a , Control 1

– 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0 – 1.0

0.3884 1.3690 0.2295 1.2035 0.2692 1.2332 0.2417 1.2227 0.1917 1.1921 0.2407 1.2203 0.3498 1.3147 0.1882 1.1799 0.1990 1.1840 0.1193 1.1224 0.0734 1.0605 0.1724 1.1669 0.1882 1.1698 0.0826 1.0701 0.1902 1.1712 0.0949 1.0759

– 98.6 – 97.9 – 97.2 – 98.5 – 100.0 – 98.4 – 97.4 – 99.3 – 98.7 – 100.3 – 98.8

2 3 4 5 6

The precision for analysis of trace amount of Zn(II) ions in aqueous media was evaluated under the optimum experimental conditions. For this purpose, eight portions of standard solutions were enriched and analyzed simultaneously by following the recommended procedure. The recoveries were found to be greater than 97% with low relative standard deviation values (RSD < 3%) for each solution. The detection limit (DL) found as the ratio of the three standard deviations of the blank to the slope of plot was 0.04 µg/L (10 replicates, R2 = 0.9990, calibration equation A = 0.5705C + 0.0407, where A is absorbance and C (µg/mL) is the Zn(II) concentration (Matos et al., 2009). The results indicate that the proposed method is sensitive and suitable for determination of trace Zn(II) ion in ecological samples.

7

The applicability of solid sorbent for preconcentration of trace level of Zn(II) was tested using fly ash and pharmaceutical samples. For carrying out the preconcentration procedure, digested samples were passed through the column charged with 0.1 g of matrix after adjusting the pH to an optimum value. The sorbed metal ions were eluted with eluting agent and their concentrations were determined by FAAS. In pharmaceutical samples, the recoveries were ranged from 98.7% to 100.1% for the mean of three replicate determinations. These results confirmed the accuracy of the proposed method as the analyte contents established with the present procedure agreed very well with the values as per USP standards. For ash sample solutions of both the locations, the analytical results are arranged in Table 4. As predicted, the left over content of zinc in all the samples after mycorrhizal reclamation were found to be less as compared to the sample of non reclaimed portion of the dump sites. The validity of the proposed method was examined by spiking the known concentration of Zn(II) metal ions (1.0 µg/mL) into the solutions. A good concurrence between the added and measured analyte amounts with low RSD values of less than 3.0% has been observed. The encountered results

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

Sample

2.8 Analytical performance

2.9 Application of the modified solid surface

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Site 2b , Control 1 2 3 4 5 6 7

2.6 2.9 1.5 2.3 1.9 0.8 0.5 2.7 0.7 2.5 1.4 2.2 0.5 1.0 1.8 1.1 2.0 0.9 0.7 1.7 1.6 1.2 2.8 0.9 2.1 2.2 2.7 1.1 1.2 2.7 2.6 2.5

99.5 98.4 98.8 98.4 98.3

a

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

concluded that the outlined procedure is highly efficient for complete and selective removal of the remaining target analyte and hence can be perceived as a viable way out for sustainable ecosystem.

3 Conclusions The proposed method exhibits commendable sorption capacity level, improved detection limit and good tolerance to the interfering ions with the concomitant benefits of high selectivity and preconcentration factor. Moreover, the outlined protocol has been proved as a promising, versatile, simple, inexpensive and environmentally benign approach to lower the leachable metal content of fly ash to such an extent that the ecological demands are obeyed. Acknowledgments One of the authors, Aditi Puri expresses her gratitude to University Grant Commission, Delhi, India for the award of junior research fellowship and also acknowledges TERI, Delhi, India for their significant contribution. Also,

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

due thanks to DRDO, Delhi, India for BET surface area analysis, AIRF, JNU, Delhi, India for SEM analysis and IISc, Bangalore, India for solid state NMR measurements. Supporting materials Supplementary data associated with this article can be found in the online version.

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