research article amino-functionalization of multiwall...

Hindawi Publishing CorporationISRN NanotechnologyVolume 2013, Article ID 674289, 8 pageshttp://dx.doi.org/10.1155/2013/674289

Research ArticleAmino-Functionalization of Multiwall CarbonNanotubes and Its Use for Solid Phase Extraction ofMercury Ions from Fish Sample

Majid Soleimani, Majid Ghahraman Afshar, and Arman Sedghi

Department of Chemistry, Imam Khomeini International University (IKIU), P.O. Box 288, Qazvin 34149-16818, Iran

Correspondence should be addressed to Majid Soleimani; [email protected]

Received 11 May 2013; Accepted 7 June 2013

Academic Editors: G. Alfieri, Y. A. Koksharov, W. Mamdouh, X. Qin, Z. Shi, and H. Tang

Copyright © 2013 Majid Soleimani et al.This is an open access article distributed under theCreativeCommonsAttribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

We describe here the amino-functionalization of multiwall carbon nanotubes (MWCNTs) and also its application as an adsorbentof solid phase extraction (SPE).The amino-functionalized MWCNTs have a good capacity to retain Hg2+, but the raw and purifiedMWCNTs are found not to adsorb Hg2+ ions. The amino-functionalized MWCNTs are prepared with amino-functionalizationof purified MWCNTs by ethylenediamine. The physicochemical properties of purified and amino-functionalized MWCNTs arecharacterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and the Boehm titration. The amino-functionalizedMWCNTs are selected as novel sorbents for the solid phase extraction of Hg2+.The amino-functionalizedMWCNT-SPEmethod is used for the determination of Hg2+ from complex matrix including fish and real water samples. Effective parameterson Hg2+ retention such as pH, flowrate, nature of the eluent, the ionic strength, selectivity coefficient, and retention capacity areinvestigated.The enrichment factor andmaximum capacity of the sorbent are 100mL and 11.58mg/g, respectively.The linear range,limit of detection, and relative standard deviation of the proposed method are 0.003 to 0.3 𝜇g/L, 1.25 × 10−3 𝜇g/L, and 2.23%,respectively. Selectivity experiments show that the adsorbents have a stronger specific retention for Hg2+ than Fe3+, Cu2+, Pb2+,Ni2+, Mn2+, Ca2+, and Mg2+.

1. Introduction

Mercury presents as different species in different environ-mental media including atmosphere, soil, sediment, naturalwaters, waste water, and water body. Mercury pollution inwater has a negative impact on the aquatic organism andhuman. Speciation of mercury in water is very important forevaluation of mercury pollution [1, 2]. Mercury is also a kindof environmental hormone, which can destroy the functionsof natural metabolism and incretion, procreate for organism,and lead tomaladjustment in hormone secretion of organism.At the same time, mercury is widely dispersed in naturalworld because of its extensive applications [3]. It is wellknown that mercury is one of the most toxic heavy metals.In contaminated environment, different forms of mercurycan be accumulated in animals and plants and also enterinto human body by food cycle, resulting in the damage

of central nerve. Because of mercury accumulative andpersistent character in the environment and living organisms,lethal concentrations of mercury salts range from less than0.1 ng/mL to more than 200.0 ng/mL for marine species andfreshwater organisms [4]. Mercury is a nonessential toxicelement and is known to affect the central nervous system ina number of ways. Food is a major source of mercury intake,especially in areas where fish and other seafood are the maincomponents of the diet.

It is therefore clear that total mercury and organomer-cury, particularly methylmercury, levels in food should bedetermined [5]. Because of low concentration of mercuryspecies and complexity of the environmental samples matrix,an enrichment step is usually necessary before separation anddetection [1]. The determination of Hg2+, presents at trace orultratrace levels especially in naturalmatrices needs two basicsteps. The first is a selective separation step to remove matrix

2 ISRN Nanotechnology

interference, and the second is an efficient preconcentrationto meet the detection limits required for its determinationmode.

Generally, several methods have been used for precon-centration of trace amounts of metal ions in aqueoussolutions including liquid-liquid extraction, solid phaseextraction (SPE), solid phase micro-extraction, liquid phasemicroextraction [1], cloud point extraction, precipitation,membrane filtration, and floatation separation [3].

In fact, SPE has become known as a powerful tool forseparation and enrichment of various inorganic and organicanalytes. The basic principle of SPE is the transfer of theanalyte from the aqueous phase to bind to active sites ofthe adjacent solid phase. It has several advantages over othertechniques, including stability and reusability of the solidphase, reach of high preconcentration factors, easiness ofseparation and enrichment under dynamic conditions, noneed for organic solvents which may be toxic, and minimalcosts due to low consumption of reagents. Accordingly,several selective solid phase extractors have been preparedeither by physical loading or chemical binding of selectedchelating reagents to different solid supports such as silicagel, activated carbon, cellulosic derivatives, polyurethanefoam, chelating ion exchange resins, naphthalene alumina,activated carbon, fly ash, peat, sewage sludge ash, zeolite, bio-materials, recycled alum sludge, manganese oxides, peanuthulls, kaolinite, and resins [6]. However, these adsorbentssuffer from low adsorption capacities or removal efficienciesof metal ions.Therefore, researchers carried out investigationfor new promising adsorbents.

Carbon nanotubes had been discovered by Iijima. Thehexagonal arrays of carbon atoms in graphite sheets of carbonnanotubes surface are ideal for strong interactions withother molecules [7]. The advantages of carbon nanotubes areunique structural, electronic, optoelectronic, semiconductor,metallic, mechanical, and chemical large specific surfacearea, hollow and layered structures, and physical properties.Carbon nanotube would impact future nanoscience andnanotechnology since it is a highly potential material and hasbecome available in macroscopic quantities.

Carbon nanotubes are divided into two types: (1) single-walled carbon nanotubes and (2) MWCNTs. Carbon nano-tubes walls are not reactive, but their fullerene-like tipsare known to be more reactive, so end functionalizationof carbon nanotubes is used relatively often to generatefunctional groups (e.g., –COOH, –OH, or –CO). Carbonnanotubes have attracted great attention in latent applica-tions such as nanodevices, field emission, gas adsorption,composite reinforcement, metal (ion) nanocomposites, andcatalyst supports as they possess exceptional mechanicalproperties, unique electrical properties, high chemical andthermal stability, and a large specific surface area. Theapplications of carbon nanotubes have been extensivelyemployed in the various detection devices, for example, inelectrochemical detectors, gas sensors, and biosensors withimmobilized biomolecules.Their applications are favorable involtammetric and electrochemical stripping methods [8–13].

Because of their highly porous and hollow structure,large specific surface area, and light mass density, carbon

nanotubes have been investigated for the removal of organicpollutions, metal ions, and some radionuclides such as Pb2+[5, 6, 14–16], Cd2+ [9, 16], Cu2+ [8, 11, 16, 17], Ni2+ [10, 17],Zn2+ [17–19], Sr2+, Eu3+ [13], and Co2+ [20].

In this paper, MWCNTs are oxidized by nitric acid, andthe obtainedMWCNTs are amino-functionalized by ethylenediamine. Amino-functionalized MWCNTs are characterizedby many techniques such as Fourier transform infraredspectroscopy (FTIR), thermogravimetric analysis (TGA) andBoehm titration. The Amino-functionalized MWCNTs areemployed as new sorbents of SPE. Solid phase extractionof Hg2+ is carried out in real water and fish samples. Thesesorbents are also conducted to determine the optimal chem-ical modification of MWCNTs in water and wastewatertreatment.

2. Experimental

2.1. Reagent. MWCNTs (OD 10–20 nm, ID 5–10 nm, length5–15 𝜇m, and purity 95%) were purchased from Shenzhen(China). Ethylenediaminetetraacetic acid (EDTA), metha-nol (HPLC grade), ethanol, propanol, HgCl

2, NaOH, NaCl,

Cu(NO3)2⋅4H2O, Pb(NO

3)2, Cd(NO

3)2⋅4H2O, Ca(NO

3)2⋅

4H2O, MgCl

2⋅6H2O, DMF, THF, SOCl

2, HCl (35%), HNO

3

(65%), andThiourea were of the highest purity available fromMerck AG (Darmstadt, Germany). All the other chemicalswere of analytical reagent grade. Stock solution of Hg2+(1000 𝜇g/mL) was prepared by dissolving a proper amount ofHgCl2in 50mL of double distilled water.

2.2. Apparatus. A pH is measured with a B2000 pH meterequipped with a GCFC 11 combination glass electrode.Determination of mercury is conducted with a flame atomicabsorption spectrometer (FAAS) GBC model 902AA (GBCScientific Equipment, Dandenong, VIC, Australia) usingan air-acetylene flame in the optimization steps and anatomic absorption spectrometer model GBC 902AA withcontinuous flow hydride generator HG3000 in quantitativeand real sample analysis. GBC HG 3000 continuous-flowvapor system equipped with a gas-liquid separator is used forHgH2generation. A mercury hallow cathode lamp is used as

a light source at 354 nm and 0.5 nm bandwidth. FTIR spectraare recorded in the range of 400–4000 cm−1 using BrukerTensor 27 FTIR spectrometer (Germany) by KBr pellets. Thethermal analysis studies are carried out using TG/DTA-SII(Perkin Elmer, USA). A syringe is available from BD, USA(5 cm × 0.9 cm i.d.). All glassware containers are carefullytreatedwith 2.0Mnitric acid (guaranteed reagent) and rinsedwith double distilled water.

2.3. Preparation of Oxidized MWCNTs. The oxidized MWC-NTs are prepared by oxidizing of raw MWCNTs. MWCNTs(0.3 g) are dispersed into a flask containing 70mL HNO

3

(65%).Themixture is refluxed at 100∘C for 120min to oxidizeMWCNTs (Figure 1(a)). After cooling to room temperature,the mixture is filtered through a 0.45 𝜇m PTFE filter, andthe solid is washed with deionized water until the pH of thefiltrate is 7. The filtered solid is then dried at 70∘C for 2 h.

ISRN Nanotechnology 3

M

W

C

N

T

M

W

C

N

T

M

W

C

N

T

M

W

C

N

T

OH

O

OH

O

NH

O

NH

O

NH

O

Oxidation

Amino-functionalization

SPE

(a) Chemical reaction (b) Chemical reaction (c) Supramolecular interaction

NH

O

NH2

NH2 NH2

NH2

2) C2H4(NH2)21) SOCl2

HgCl2 (aq)HNO3

Hg2+

Hg2+

Figure 1: Schematic illustration of amino-functionalization of MWCNTs (a and b) and solid phase extraction step (c). In the first step,MWCNTs are oxidized by HNO

3(a), and subsequently the oxidized MWCNTs are amino-functionalized by ethylenediamine (b). In the

solid phase extraction step, mercury ions are adsorbed on the surface of amino-functionalizedMWCNTs, and this is a kind of supramolecularinteraction.

The obtained solid is used as purified MWCNTs in the nextstep [21].

2.4. Preparation of Acyl Chloride MWCNTs. To prepare theacyl chloride MWCNTs, the obtained oxidized MWCNTs(0.3 g) are suspended in 150mL of SOCl

2and 50mL tetrahy-

drofuran (THF). The suspension is treated with ultrasonicbath (40 kHz) for 30min and then stirred at 70∘C for 24 hto convert the surface-bound carboxyl groups into acylchloride groups. The solid is then filtered and washed withanhydrous THF. Subsequently, it is dried under vacuum atroom temperature for 2 h [22, 23]. The obtained solid isapplied in the amino-functionalization step.

2.5. Preparation of Amino-Functionalized MWCNTs. Theamino-functionalized MWCNTs are prepared with amino-functionalization of purified MWCNTs by ethylenediamine(see, Figure 1(b)). MWCNTs suspensions are prepared bydispersing 0.3mg of MWCNTs-COCl material in 50mL ofN,N-dimethylformamide (DMF) with the aid of ultrasonicagitation for 1 h. The mixture is added with 5mL of ethylene-diamine under nitrogen; then it is heated at 70∘C understirring for 72 h. Aliphatic multiamines are grafted ontoMWCNTs through amide linkage. The resulting reactionmedium is vacuum filtered through a 0.45𝜇m PTFE mem-brane, then it is washed with acetone. The sample is driedat 80∘C for 24 h and denoted as amino-MWCNTs, whichis a mixture of mainly the amino-functionalized MWCNTs[24–27].

2.6. Preparation of Amino-Functionalized MWCNTs Column.The determination of Hg2+ is performed with continuoussolid phase extraction with column. Amino-functionalizedMWCNTs columns are prepared by packing the dry amino-functionalized MWCNTs particles (500mg) in a 5mL emptysyringe. The syringe is attached with a stop cock and twosieve plates at the bottom end and the top end of amino-functionalized MWCNTs packed particles. The sieve plates

are obtained from commercial SPE cartridges. The amino-functionalized MWCNTs columns are washed with 10mL0.1M EDTA for removing impurities. Before loading samplesolutions, the polymer should be activated through 5mLwater rinse.

2.7. Solid Phase Extraction Step. The aqueous sample solu-tions (10mL, 50mg/L) of mercury ion are passed throughamino-functionalized column at a flow rate of 1mL/min byvacuum system. As we observed in Figure 1(c), the mercuryion is adsorbed by amino-functionalized MWCNTs, and thisis a kind of supramolecular interaction. After extracting, theextracted mercury ion is stripped from the column usingEDTA solution (2.5mL, 0.1M). Then, the concentration ofmercury ion in effluent and eluent is determined by atomicabsorption spectroscopy (a mercury hallow cathode lamp isused as a light source at 354 nm and 0.5 nm band width).

3. Results and Discussion

3.1. Amino-Functionalized MWCNTs Characterization

3.1.1. IR Spectra. Figure 2 illustrates the IR spectra of raw,oxidized, and amino-functionalized MWCNTs. All of theIR spectra of MWCNTs are recorded by using KBr pelletmethod. The IR spectra of raw, oxidized, and amino-fun-ctionalized MWCNTs showed similar characteristic peaks,indicating the similarity in the structure of the MWCNTs.Further, there are peaks at ∼1159 and 1620 cm−1 in the IRspectra of oxidized MWCNTs corresponding to stretching ofcarbon-oxygen bond of lactone and asymmetric stretchingof groups in the surface, respectively. In the raw MWCNTs,these peaks are not available. These results indicate thatafter the oxidation step, the oxygen functional groups suchas carboxylic and lactone are formed on the surface ofpurified MWCNTs. There are peaks at ∼1159, 1400, and1620 cm−1 in the IR spectra of amino-functionalized MWC-NTs corresponding to stretching of carbon-oxygen bond


Raw

Oxidized

Amidized

0500 1000 1500 2000 2500 3000 3500 4000

Wavenumber (1/cm)

Tran

smitt

ance

(%)

Figure 2: IR spectra of raw, oxidized, and amino-functionalizedMWCNTs.

of lactone, carbon-nitrogen bond of amide, and asymmetricstretching of carboxylic groups of amide in the surface,respectively.There is no peak at 1400 cm−1 in the IR spectra ofoxidized MWCNTs. These results indicate that, after amino-functionalization step, the acyl chloride group is replacedwith the amine group and also the nitrogen functional groupsare formed on the surface of purified MWCNTs.

3.1.2. Thermogravimetric Analyses. It is useful to comparethe TGA pattern of raw, oxidized, and amino-functionalizedMWCNTs because in the functionalization process the func-tional groups are formed on the end and defect sites. Theraw, oxidized, and amino-functionalized MWCNTs showsimilar TGA pattern, indicating the analogous thermal sta-bility of these MWCNTs. The raw, oxidized, and amino-functionalized MWCNTs are stable up to 650, 450, and 250,respectively.The rawMWCNTs aremore stable than oxidizedMWCNTs, and oxidized MWCNTs are more stable thanamino-functionalized MWCNTs (Figure 3). The observedstabilized temperature is known to be strongly influenced bythe step of the functionalization.

3.1.3. Boehm Titration. Oxygen functional groups are quan-titatively evaluated using the Boehm titration method. Theamount of carboxyl and total acidic site on the carbon surfaceis quantitatively chemically analyzed by titration.The amountof carboxylic and total acidic sites is determined by additionof 50mL, 0.05N NaHCO

3, Na2CO3, and NaOH, respectively

into 50mg purified and amino-functionalizedMWCNTs andthen stirred for 48 hours to reach an equilibrium state.Thereafter, the solution is filtrated to remove MWCNTsfrom clear solution. The removal of the MWCNTs fromthe titrated solution is essential to avoid HCl reacting withthe deprotonated groups at MWCNTs surface, which wouldinduce errors as well as slow down the titration. A partof filtrated solution (10mL) is selected, and the titration isperformed by (0.1 N) HCl solution to determine the excessof the base until the neutral solution is reached [29, 30].

Raw

Amidized

Oxidized

00 100 200 300 400 500 600 700 800

Temperature (∘C)

Wei

ght (

%)

Figure 3: TGA plots of raw, oxidized, and amino-functionalizedMWCNTs.

Table 1: Boehm titration of acidic group on the surface of purifiedand amino-functionalized MWCNTs.

Kind of MWCNTsAcidic group (mmol/g)

Total Carboxyl Lactone PhenolPurified MWCNTs 1.04 0.66 0.21 0.16Amino-functionalizedMWCNTs

1.04 0.66 0.21 0.16

As seen in Table 1, the result of purified and amino-functionalized MWCNTs are similar. The carboxyl group onthe surface of purified and amino-functionalized MWCNTsis equal because amide groups like carboxylic group are astrong base, and they titrated with Na

2CO3. It is found that

the amount of carboxylic groups is not changed in amino-functionalization step, but hydroxyl groups are changed toamide groups.

3.2. Optimization of Solid Phase Extraction. The percentagerecovery is known to be strongly influenced by the manyparameters.The effect of pH (1.5–7.5), samplematrix (acetate,formate, citrate, and phosphate buffers), flow rate of samplesolution and eluent (1–10mL/min), aqueous phase and eluentvolume (10–300mL), nature of eluent (CH

3OH, C

2H5OH,

EDTA, thiourea, acetone, and HCl), and concentration ofeluent on percentage recovery is examined. All Hg2+ samplesolutions which are used in the optimization step are pre-pared by diluting of stock solution.

3.2.1. Effect of pH. The pH of aqueous phase is an importantfactor in a solid phase extraction procedure. Changes ofpH can influence the reaction between Hg2+ and the aminegroup on the surface of amino-functionalized MWCNTs.To determine the effect of pH, pH of solutions containing50𝜇g/mL Hg2+ onto amino-functionalized MWCNTs col-umn is changed within the range of 1 to 7. Figure 4 shows thatmercury ions are effectively adsorbed at pH between 1 and 3.


The recovery of the adsorption decreases at pH higherthan 3 due to the formation of mercury hydroxides. Atstrong acidic pH, it is possible that the protonated aminegroup on the surface of functionalized MWCNTs and thepercentage recovery decreases. Hence, subsequent extractionexperiments are carried out at pH 3.

3.2.2. Effect of Sample Matrix. It is anticipated that thesample matrix can affect the percentage recovery; so it isvery necessary to examine the matrix effect in this work.The effect of sample matrix on the retention of mercury isinvestigated by adding 60mM acetate, citrate, formate, andphosphate buffers into the sample solution. The percentagerecovery of the mercury ion for citrate, formate, phosphate,and acetate buffer is 91.48, 95.12, 75.23, and 73.02, respectively.The percentage recovery of the mercury ion decreases withadding buffers because the tendency of acetate, citrate, andphosphate anions for the formation of noncharge ion pairwith Hg2+ is strong and noncharge ion pair of Hg2+ cannotretain on the amino-functionalized MWCNTs column. Asseen, the percentage recovery of Hg2+ cannot change withadding formate buffers, and the recoveries of Hg2+ decreasewith adding acetate, citrate, and phosphate buffers. Hg2+solutionwithout any buffer is chosen as the optimal conditionto separate analyte for all experiments.

3.2.3. Effect of Flow Rate of Sample. The effect of flow rateis an important parameter in the SPE method due to thetime of analysis and contact time. To ensure quantitativerecovery (>95%), the effect of flow rate of sample solutionis investigated. The flow rate of 1–5mL/min is found tobe suitable for optimum loading of Hg2+ on the amino-functionalized MWCNTs column. At higher flow rates (5–10mL/min), there is a reduction in the percentage adsorptionof mercury. This could be probably due to the insufficientcontact time between the sample solution and column. A flowrate of 1mL/min is maintained as an optimal condition toanalysis.

3.2.4. Effect of Sample Volume. Due to the low concentrationof Hg2+ in real samples, these analytes should be taken intosmaller volumes for high preconcentration factor by usingsample solutions with large volumes. Therefore, the maxi-mum applicable sample volume is determined by increasingthe dilution of Hg2+ solution, while keeping the total amountof loadedHg2+ fixed at 0.67mg.Different feed volumes variedbetween 10 and 250mL. The results are given in Figure 5.As can be seen the recoveries of Hg2+ are quantitative up to250mL of sample volume. At volumes higher than 100mL,probably the analyte is not retained effectively because ofpassing excess of water through the column. So the 100mLsample volume is selected as an optimum condition.

3.2.5. Effect of Choice of Eluent. The nature of the eluent is ofprime importance and should meet three criteria: efficiency,selectivity, and compatibility [4]. Several eluent solutionssuch as propanol, ethanol, acetone, thiourea, EDTA, HNO

3,

pH

Reco

very

(%)

100

90

80

70

60

50

400 1 2 3 4 5 6 7 8

Figure 4: Effect of the pH on extraction efficiency.

Volume of sample (mL)

Reco

very

(%)

100

95

90

85

80

75

70

65

600 50 100 150 200 250

Figure 5: Effect of volume of sample on extraction efficiency.

H2SO4, and HCl are tested. As it is observed in Figure 6,

a solution of EDTA at concentration of 0.1M is a propersolvent to elute Hg2+ from the column with 95% recovery.Higher concentration of EDTA has no effect on the recovery.The effectiveness of EDTA as an eluent may come from bothits ability to produce stable complexes of Hg2+ that is notretained on the column and that Hg2+ is eluted from thecolumn.

3.2.6. Effect of Volume of Eluent. According to the low con-centration of mercury ion in real samples, these analytesmust be taken into smaller eluent volume. The effect ofeluent volume on the recovery of Hg2+ is investigated in therange from 0.5 to 10.0mL. The recoveries of Hg2+ increasesignificantly with the increasing eluent volume.The recoveryof Hg2+ reached 95% by using only 2.5mL eluent. At lowereluent volume, elution of Hg2+ decreases. Increasing the elu-ent volume further will result in dilution of the analytes.Thus,the volume of eluent is optimized as 2.5mL.


Kind of eluent

Reco

very

(%)

100

90

80

70

60

50

40

30

20

10

0

2-Pr

opon

al

Etha

nol

Acet

one

EDTA

0.1

M

Sulfu

ric ac

id 1

M

Nitr

ic ac

id 1

M

Chlo

ridric

acid

1 M

Thio

urea

0.1

M

Figure 6: Effect of type of eluent on extraction efficiency.

Table 2: Selectivity test, recovery of different cations with respect toHg+2.

Metal ion(M𝑛+)

Concentration(𝜇g/mL)

Recovery ofM𝑛+(%)

Recovery ofHg2+(%)

Fe3+ 100 68.64 95.01Cu2+ 100 25.78 94.48Ni2+ 100 22.75 85.43Ca2+ 100 59.67 74.83Mg2+ 100 45.32 63.29Mn2+ 100 83.32 93.76Condition: the sample solution is 5mL, 50𝜇g/mL Hg+2, and M𝑛+.

3.2.7. Effect of Flow Rate of Eluent. The eluent flow ratethrough the column is a very important parameter, since thisis one of the steps that control the time of analysis.Therefore,the effect of the flow rate of eluent is examined under theoptimum conditions (pH, the sample volume and eluent,etc.). The flow rate is adjusted in a range of 1–5mL/min.It is found that the analyte retention is not changed up to5mL/min flow rate. Thus, a flow rate of 1.0mL/min is em-ployed as an optimal condition.

3.2.8. Results of Selectivity Experiments. The effects of matrixions for cold vapor atomic absorption spectrometric determi-nations on the recoveries of Hg2+ on amino-functionalizedMWCNTs column are also investigated.The results are giveninTable 2. A fixed amount ofmetal ions is takenwith differentamounts of foreign ions, and the recommended procedureis followed. The recovery of the analytes from the amino-functionalized MWCNTs column is not affected by thesolution containing the high concentrations of matrix ions.The tolerance limit of foreign ions is taken as that value whichcaused an error of notmore than±30% in the absorbance.Theions normally present inwater do not interfere under the usedexperimental conditions. Also, some of the transition metalsat mg/L did not interfere with the recoveries of the analyteions on the amino-functionalized MWCNTs column. These

Table 3: Comparison of the analytical results on different CNTs ad-sorption methods.

Metal Kind of sorbent Adsorptioncapacity (mg/g) Reference

Hg2+ Amino-functionalizedMWCNTs 11.58 This method

Pb2+ MWCNTs 5.50–16.40 [6]Pb2+ Oxidized MWCNTs 22.5, 59 and 11.2 [14–16]Cd2+ Oxidized MWCNTs 1.1–11 [16, 18]Ni2+ Oxidized MWCNTs 9–47.85 [10]Zn2+ Oxidized MWCNTs 10.21–32.68 [18, 19]Cu2+ Oxidized MWCNTs 7.5–11.3 [11, 19]Sr2+& Eu2+ Oxidized MWCNTs — [13]

Co2+ L-Tyrosine onoxidized MWCNTs 1.18–2.21 [28]

results show that the proposed preconcentration/separationmethod could be applied for mercury species to the highlysaline samples and the samples that contain some transitionmetals.

3.2.9. Analytical Performance. At volumes higher than100mL, probably the analyte is not retained effectively sothe break-through volume for the method is greater than100mL. After eluting the Hg2+, the final solution volume of2.5mL is achieved; therefore, the preconcentration factorsof 40 are obtained. The maximum capacity of the cartridgeis investigated by passing 5mL of sample solutions con-taining 50–3000𝜇g/mL of Hg2+ ion through the columnand followed by determination of the retained ions usingatomic absorption spectrometry. The maximum capacity ofthe sorbent is 11.58mg/g. Calibration graphs are constructedwith five standard solutions containingHg2+ ion according tothe general procedure. The linear range, regression equation,and correlation coefficient of the proposed method are 0.003to 0.3 ng/mL,𝐴 = 6.94𝑋−0.0042 (ng/mL), and 0.983, respec-tively. The limit of detection and relative standard deviation(RSD) of the proposedmethod are 1.25×10−3 𝜇g/L and 2.23%,respectively.

3.2.10. Comparison of the Analytical Results on DifferentCNTs Adsorption Methods. Comparison of functionalizedMWCNTs for adsorption of Hg2+ with published papersincluded MWCNTs as a sorbent for adsorption of othermetals which showed that the adsorption capacity of thesorbent is better than the other method (Table 3). Thecolumn packed with the amino-functionalized MWCNTs issuitable enough to separate Hg2+ from the complex matrixescontaining interference ions such as Fe3+, Cu2+, Pb2+, Ni2+,Mn2+, Ca2+, and Mg2.

4. Application

The experimental protocol can be used for the determina-tion of mercury ions in real samples with complex matrix.


Table 4: Determination of Hg2+ in real samples.

Sample Added Hg2+

(ng/mL)Found Hg2+(ng/mL) RSD (%)

Tap water (Karaj)

0 0.08 2.140.25 0.32 3.320.5 0.58 2.280.75 0.84 2.421 1.13 3.34

Seawater (Khazar)

0 0.06 3.760.25 0.28 3.450.5 0.62 2.760.75 0.86 4.321 1.07 5.12

Fish 0 4.12 (𝜇g/g fishsample) 4.81

Condition: the measurement is performed at optimum effective parametersand p.f. = 40.

To validate the methodology, the proposed method formercury determination is applied to analyze fish sample andnatural waters. Trout fish from Karaj River, Seawater fromKhazar Lake, and tap water from Karaj in Iran are subjectedto the recommended procedure.

4.1. Analysis of Real Water Samples. It is anticipated that thismethod can eventually form a platform to measure mercuryion in water sample directly. The water samples are filteredthrough 0.45𝜇m pore size millipore membrane filters; thenthe pH of these solutions is adjusted to the optimum level.This protocol is performed after standard addition of Hg2+.In the standard addition method, 100mL of the samples isspiked with 0-1 ng/mL of Hg2+ ions before loading onto thecolumn. Trace mercury in samples is preconcentrated byusing this procedure. Upper aqueous solution is used for thedetermination of mercury by cold vapor atomic absorptionspectrophotometry. The results are reported in Table 4.

4.2. Analysis of Fish Samples. The key relevant application ofdetecting mercury ions is for food analysis. The detection ofHg2+ in fish samples is explored. The proposed method isapplied to determine Hg2+ from fish sample. To digest thisspecies, 500mg dried sample is placed in a digesting vessel,and 5mL HNO

3(70%) and 6mL H

2O2(30%) are added

to the vessel. The vessel is immediately assembled, gentlyswirled, and placed in the preheated oven at 180∘C for about1.5 hours. About 6.0mL of 1M K

2SO4is added to the vessel;

then the vessel is heated for 30min. The digested fish sampleis cooled at room temperature. Appropriate amount of 2MNaOH is added to neutralize the excess of HNO

3and adjust

pH at the optimized value (Table 4). The determination ofmercury ions in the obtained solution is followed by thisprotocol [31, 32].

5. Conclusion

The properties of MWCNTs such as purity, structure, andnature of the surface are greatly improved after amino-functionalization which made them more hydrophilic andsuitable for the adsorption of Hg2+ from aqueous solution.The functional groups are easily formed on the ends and atthe defect sites. The Hg2+ adsorbed by amino-functionalizedMWCNTs aremainly aggregated on the ends and at the defectsites on the amino-functionalized MWCNTs. The sorptionmechanism appears mainly attributable to supramolecularinteraction between the mercury ions and the surface func-tional groups of amino-functionalizedMWCNTs which havea negative charge. Sorption/desorption study elucidated thepossibility to reuse the spent MWCNTs for the removal ofdivalent Hg2+ in fish and real water samples.

Acknowledgment

The authors thank Imam Khomeini International University(IKIU) research council for financial support of this work.

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