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Journal of Hazardous Materials 166 (2009) 1383–1388

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

pplication of cotton as a solid phase extraction sorbent for on-linereconcentration of copper in water samples prior to inductivelyoupled plasma optical emission spectrometry determination

ohammad Faraji a, Yadollah Yaminia,∗, Shahab Shariatib

Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, IranDepartment of Chemistry, Faculty of Sciences, Islamic Azad University, Rasht Branch, Rasht, Iran

r t i c l e i n f o

rticle history:eceived 21 October 2008eceived in revised form 10 December 2008ccepted 11 December 2008vailable online 24 December 2008

eywords:opper

a b s t r a c t

Copper, as a heavy metal, is toxic for many biological systems. Thus, the determination of trace amountsof copper in environmental samples is of great importance. In the present work, a new method wasdeveloped for the determination of trace amounts of copper in water samples. The method is basedon the formation of ternary Cu(II)–CAS–CTAB ion-pair and adsorption of it into a mini-column packedwith cotton prior applying inductively coupled plasma optical emission spectrometry (ICP-OES). Theexperimental parameters that affected the extraction efficiency of the method such as pH, flow rate andvolume of the sample solution, concentration of chromazurol S (CAS) and cethyltrimethylammonium

n-line solid phase extractionCP-OESASater samples

bromide (CTAB) as well as type and concentration of eluent were investigated and optimized. The ion-pair(Cu(II)–CAS–CTAB) was quantitatively retained on the cotton under the optimum conditions, then elutedcompletely using a solution of 25% (v/v) 1-propanol in 0.5 mol L−1 HNO3 and directly introduced into thenebulizer of the ICP-OES. The detection limit (DL) of the method for copper was 40 ng L−1 (Vsample = 100 mL)and the relative standard deviation (R.S.D.) for the determination of copper at 10 �g L−1 level was found tobe 1.3%. The method was successfully applied to determine the trace amounts of copper in tap water, deep

two d

well water, seawater and

. Introduction

Copper is a heavy metal extensively examined in environmental,ndustrial and biological applications. Copper is vital and toxic for

any biological systems [1,2], so that its determination in wateramples is warranted by the narrow window of concentrationetween essentiality and toxicity [3,4]. On the other hand, copper

s an important element in geochemistry. It can be easily releasedrom silicates, sulfites and oxides after some physical and chemicaleathering and then transferred by water into soil and sediments

5]. Thus, the determination of trace amounts of copper in differentatrices is of great importance.Despite the sensitivity and selectivity of analytical techniques

uch as flame atomic absorption spectrometry (FAAS) and induc-

ively coupled plasma optical emission spectrometry (ICP-OES),here is a great necessity for preconcentration of copper prioro its determination, basically due to its low concentration orhe effects of matrix in aqueous samples. Preconcentration pro-

∗ Corresponding author. Fax: +98 21 88006544.E-mail address: yyamini@modares.ac.ir (Y. Yamini).

304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2008.12.063

ifferent mineral waters, and suitable recoveries were obtained (92–106%).© 2008 Elsevier B.V. All rights reserved.

cedures such as liquid–liquid extraction [6], ion-exchange [7],cloud point extraction [8-10], coprecipitation [6], adsorptive strip-ping voltametric [11] or solid phase extraction [9] have beenapplied to extract copper ions from aqueous samples. Solid phaseextraction (SPE) is an attractive method that reduces consump-tion of and exposure to solvent, disposal costs and extraction time[12]. The nature and the properties of the sorbent materials areof prime importance for effective retention of materials in SPE[13].

Ion-exchange resins, Chelex-100 and resin 122 [7]; octade-cyl bonded silica gel, C18 [14–16]; modified silica gel [17,18];polystyrene-divinilbenzene polymer (PS-DVB), Amberlite XAD-2[19], XAD-4 [20], XAD-2010 [21], PS-DVB functionalized [22];coated alumina [23]; biopolymer chitosan [24]; polyurethane foam,PUF [25]; polytetrafluoroethylene polymer, PTFE as turnings [26];polychlorotrifluoroethylene, PCTFE [27]; oxidized multi-walledcarbon nanotubes [28,29]; activated carbon [30,31] and double-imprinted polymer [32] have been used as SPE sorbents to extract

copper ions from different water samples.

Choi et al. demonstrated that cotton, milkweed and kenaf have1.5–3 times better sorption properties than polypropylene fibers[33,34]. Because of their excellent oil sorption properties andhigh biodegradability, wool-based non-woven materials [35] and

1 dous Materials 166 (2009) 1383–1388

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ydrophobic cotton fibers [36] have been adopted to remove oilrom water. Recently, cotton column has been applied to selectiveetaining of synthetic colorants [37] and preconcentration enrich-ent of polycyclic aromatic hydrocarbons (PAHs) [38]. To the best

f our knowledge, cotton fibers have not been employed previouslyor the extraction and preconcentration of hydrophobic ion-pairf copper with chromazurol S (Cu–CAS) and cationic surfactant ofethyltrimethylammonium bromide (CTAB).

In the present study, feasibility of cotton (as a column packingaterial) was investigated for on-line preconcentration followed

y ICP-OES determination of the trace amounts of copper.

. Experimental

.1. Apparatus

A simultaneous ICP-OES (Varian Vista-Pro, Springvale, Australia)oupled to a V-groove nebulizer and equipped with a charge cou-led device (CCD) was applied for determination of the tracemounts of copper. The operation conditions and the wavelength ofhe analytical line are summarized in Table 1. A two-channel peri-taltic pump model Ultra Voltametry (Farayand Gostar Company,ehran, Iran) was applied to pump the sample solution through aome-made polyethylene mini-column. The mini-column (25 mm

ength × 4 mm i.d.) packed with cotton was used in the manifold forxtraction/preconcentration process. The pH of the solutions wasdjusted and measured by a WTW pH meter (Inolab, Germany) sup-lied with a combined electrode. A six-way two-position injectionalve (Tehran University, Iran) was applied in the preconcentra-ion/elution process.

.2. Reagent

All of the reagents used were of analytical grade.u(NO3)2·3H2O, chromazurol S (CAS), CTAB, ammonium acetatend KI were purchased from Merck Company (Darmstadt, Ger-any). The stock solution of copper (1000 mg L−1) was prepared

y dissolving an appropriate amount of Cu(NO3)2·3H2O in doubleistilled water. Working solutions were prepared by appropri-te dilution of the stock solution with buffer solution. Doublyistilled water was used throughout the work. Acetate bufferolution (0.01 mol L−1) was prepared by dissolving sufficientmount of ammonium acetate in water and adjusting the pHith 0.5 mol L−1 nitric acid or 0.5 mol L−1 sodium hydroxide

olutions. Stock solutions of CTAB (0.1%, w/v) and 0.15 mol L−1

I were prepared in double distilled water. In the former case, atock solution of CAS with a concentration of 0.01 mol L−1 wasrepared in distilled water. The chemical structure of CAS is shown

n Fig. 1.

.3. Column preparation

The preconcentration column was made from a polyethyleneyringe tube with an effective length of 25 mm and inner diameterf 4 mm. 150 mg of natural cotton (Kave Company, Iran) was firmly

able 1CP-OES operating conditions and analytical line for copper.

lasma gas Argon

lasma gas flow rate 15 L min−1

uxiliary gas flow rate 1.5 L min−1

requency of RF generator 40 MHzF generator power 1.55 kWbservation height 6 mmebulizer pressure 130 kPaavelength 324.754 nm

Fig. 1. Chemical structure of the CAS.

packed into the column and blocked by two polypropylene filtersat the ends. The column was then connected to the injection valvewith PTFE tubing to form preconcentration system. The stability andpotential regeneration of the column were investigated. The columncan be reused after being regenerated first with 2 mL of methanoland then with 15 mL of distilled water. The column was stable upto 30 adsorption–elution cycles without an obvious decrease in therecovery of copper ions.

2.4. Preconcentration procedure

A schematic diagram of the extraction apparatus is shownin Fig. 2. Twenty-five millilitres of the sample solution contain-ing 50 �g L−1 of copper and buffered at pH 4.6 was transferredinto a 100 mL beaker. After successive addition of 200 �L of the0.02 mol L−1 CAS and 0.6 mL of the 0.1% (w/v) CTAB solutions, theobtained solution was stirred to form ion-pair (Cu–CAS–CTAB).After addition of 2 mL of the 0.15 mol L−1 KI solution, as phaseseparation reagent, the injection valve (V) was located at “loadposition” and pump P1 (peristaltic pump) was activated. Thenthe mixture solution was passed through the mini-column at theflow rate of 6.5 mL min−1. After completion of loading of the sam-ple solution, the valve, V, was turned to the injection positionand the retained ion-pair was eluted using a 0.2 mL of solutionof 25% (v/v) 1-propanol in 0.5 mol L−1 HNO3 at the flow rate of1.2 mL min−1 using ICP peristaltic pump (P2). The eluent was thentransferred directly into the nebulizer of the ICP-OES. For mini-mum dispersion, the eluent was passed through the mini-columnin reverse direction than that of the sample solution. The peakheight of the signal was proportional to copper concentration in thesample, which was used in all of the quantitative measurements.The recorded peak was sharp (width ∼10 s) and the baseline wasstable.

2.5. Sample preparation

Tap water sample was collected from our laboratory (TarbiatModares University, Tehran, Iran) and well water sample wascollected from a deep well water in Tarbiat Modares Univer-sity. The natural mineral waters were collected from CheshmeAla (Damavand, Tehran) and Cheshme Ghale Dokhtar (Damavand,Tehran). The seawater sample was collected from the Caspian Sea(Noor, Iran). The samples were collected in cleaned polyethylenebottles and only the seawater was filtered through a 0.45 �m

pore size membrane filters immediately after sampling. The sam-ple’s pH were adjusted to 4.6 using 0.5 mol L−1 nitric acid or0.5 mol L−1 sodium hydroxide solutions. Finally the proposedmethod, was applied to extraction of copper ions from the watersamples.

M. Faraji et al. / Journal of Hazardous Materials 166 (2009) 1383–1388 1385

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the results are shown in Fig. 3. The presence of surfactantfavors the formation of hydrophobic complexes [41]. The emis-sion intensities increased by increasing of CTAB concentrationup to 2.4 × 10−3% (w/v) because of the formation of hydrophobic

ig. 2. Schematic diagram of the extraction set up: S, stirrer; M, mixture solution [cond V, six-port two-position injection valve ((a) load position; (b) injection position

. Results and discussion

.1. Selection of elution reagent and its flow rate

Organic solvent like ethanol, methanol and MIBK have beenxtensively used as effective eluents in on-line solid phase extrac-ion preconcentration systems. In this work, based on our previousork [8], the solutions with different concentrations of nitric acid in-propanol were used as eluent. On the basis of the obtained exper-

mental results, the solution of 25% (v/v) 1-propanol in 0.5 mol L−1

NO3 was chosen as the eluent.The effect of the eluent’s flow rate on ICP-OES signal was stud-

ed within the range of 0.8–2.5 mL min−1. Maximum intensity wasbtained at the flow rate of 1.2 mL min−1. Above that flow rate, themission intensity of copper decreased mainly due to decreasing ofhe nebulization efficiency or incomplete elution of the retainedons. Thus, the flow rate of 1.2 mL min−1 was applied in furtherxperiments.

.2. Effect of the pH

Among the chemical variables, pH was the most critical param-ter for effective formation and retention of the ternary ion pairsu(II)–CAS–CTAB on the cotton. In order to evaluate the effect ofH on the extraction efficiency, the pH of the sample solutionsontaining 50 �g L−1 of copper ions was adjusted in the range of.0–7.0 and the recommended procedure was applied. Accordingo obtained results, the maximum intensity was obtained at pH 4.6.t lower pHs (<4.0), a competition occurred between protons and

he copper ions for occupying the ligand active sites, while at higherHs >5.5, the effective charge of CTAB (N-base) decreased. Thus, theH of the sample solutions was adjusted at 4.6 on the subsequentorks.

.3. Effect of CAS concentration

The use of potentiometric and spectrometric methods have beeneported in a study of the complex formation between copper(II)

ons and different ligands such as CAS [39]. At the pH range of–7, two complexes with the composition of Cu(H2O)2HCAS andCu(H2O)2)2CAS were detected and the stability constants werealculated as log K = 4.02 ± 0.05 and log K = 13.7 ± 0.1, respectivelyat 25 ◦C and the ionic strength of 0.1 M (KCl)) [40]. Depend-

CAS + CTAB + KI]; E, eluent; W, waste; P1 and P2, peristaltic pumps; C, mini-column

ing on the concentrations of the components, ternary ion pairs(Cu(II)–CAS–CTAB) with stoichiometries of 1:1:1, 2:1:1 and 1:2:2were formed. In all of these cases, anionic Cu(II)–CAS complexeswere formed [41], to which the CTAB cation is bounded probablyfrom the SO3

− group. Also, the UV–Vis spectrum of the complexesin the presence and absence of CTAB have already been reported[41].

The effect of CAS concentration on the extraction efficiencywas studied in the range of 0.0–7.5 × 10−3 mol L−1. The emissionintensities increased by increasing of CAS concentration up to5.0 × 10−3 mol L−1, while at higher concentrations, the intensitieswere decreased. Therefore, a 5.0 × 10−3 mol L−1 solution of CAS wasselected for further experiments.

3.4. Effect of CTAB concentration

The effect of CTAB concentration on the extraction effi-ciency was investigated in the range of 0.0–1.2 × 10−2% and

Fig. 3. Effect of CTAB concentration on the extraction of 50 �g L−1 of Cu(II) ionsfrom 25 mL of the sample solution. Conditions: CCAS = 5 × 10−3 mol L−1, sample pH4.6, CKI = 4 × 10−3 mol L−1 and CNH4CH3COO− = 0.01 mol L−1.

1 dous Materials 166 (2009) 1383–1388

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Table 2Effect of interference on preconcentration and determination of Cu(II) ions.

Interference Added as Interference to metalion ratio

Recovery (%)

Na+ NaCl 2000 107K+ KNO3 2000 108Mg2+ Mg(NO3)2·6H2O 2000 109Ca2+ CaCl2 2000 92Ba2+ BaCl2 2000 103Co2+ CoCl2·6H2O 1000 96Mn2+ MnCl2·4H2O 750 93Zn2+ Zn(NO3)2 500 107Pb2+ Pb(NO3)2 500 96Cd2+ Cd(NO3)2·4H2O 500 108Ni2+ Ni(NO3)2·6H2O 300 93Hg2+ Hg(CH3COO)2 200 102Cr3+ Cr(NO3)3·6H2O 100 98

a better sensitivity, higher enhancement factor and lower detec-tion limit. Moreover, in comparison with the commonly used C18sorbents, cotton is very cheap and it can be very easily packed intothe column.

Table 3Analytical performance of the proposed method for determination of Cu(II) ions.

Sampling frequency (h−1) 2Preconcentration time (min) 7.5Sample flow rate (mL min−1) 6.5Breakthrough volume (mL) 100Enhancement factor 680

386 M. Faraji et al. / Journal of Hazar

ernary ion pairs (Cu(II)–CAS–CTAB) that increases the retentionf copper ternary ion pairs on the cotton. But at higher concen-rations, the emission intensities decreased due to competitionf CTAB with Cu(II) to form an ion-pair with CAS (Cu(II)–CAS)41].

.5. Effect of KI concentration

Increasing of the ionic strength of the solution by addition ofI, does not affect the formation of the reaction products or theinetics of the reaction [41]. The effect of salt addition on thextraction efficiency was investigated by the addition of KI in theoncentration range of 0.0–1.8 × 10−2 mol L−1. The results showedhat by increasing of KI concentration up to 1.2 × 10−2 mol L−1, thextraction efficiency increased, may be due to the effect of salt onhe phase separation and retention of the ion-pair on the cotton.onsequently, a 1.2 × 10−2 mol L−1 of KI was selected for furthertudies.

.6. Sample flow rate

The flow rate of the sample solution in on-line SPE is one ofhe most important parameters affecting both the retention effi-iency of the analytes and the extraction time. The effect of sampleow rate on the extraction efficiency was studied in the range of.2–9.6 mL min−1. The results showed that at the flow rates greaterhan 6.5 mL min−1, the emission intensity of Cu(II) ions decreasedecause of incomplete retention of the formed ion-pair on the SPEolumn. Thus, the flow rate of the sample solution was adjusted at.5 mL min−1 for further studies.

.7. Effect of sample volume

Breakthrough volume is another parameter that influenceshe preconcentration factor and reliability of analytical results ofn on-line SPE system. It is very important to get satisfactoryecoveries for the analytes from a large volume of the sampleolutions. The effect of sample volume on the retention of cop-er from the sample solution was investigated. For this purpose,5, 50, 75, 100 and 150 mL of the sample solutions contain-

ng 1.25 �g of copper were passed through the mini-column athe optimum flow rate. The results showed that quantitativextraction of Cu(II) ions (>90%) were obtained up to 100 mL ofhe sample solution. Above 100 mL, the recoveries of Cu(II) ionsecreased.

.8. Interference studies

The effect of potential interfering, occurring in the envi-onmental samples, on the on-line SPE extraction and ICP-OESetermination of Cu(II) ions was investigated. Solutions containing0 �g L−1 Cu(II) ions and different concentrations of the interfer-

ng ions were treated according to the proposed procedure. Theolerance limits of the coexisting ions are defined as the largestmount of the ions in the solution that decrease the recovery ofhe Cu(II) to less than 92% (Table 2). Most of the cations and anionsxamined did not interfere with the extraction of Cu(II) ions. Sincehe chloride and nitrate salts were employed in this study withoutny interference, their respective anions could pose no interfer-nce either. However, some of the species tried, such as Al(III) and

e(III), interfered with the determination of Cu(II) ions. The inter-erences of Al(III) and Fe(III) ions were eliminated in the presence of000 �g mL−1 of F− and 1000 �g mL−1 of SCN−, as masking agents,espectively. Moreover, the potential interferences from some com-on matrix anions such as I− and SO4

2− were also investigated.

Al3+ AlCl 10a 107Fe3+ Fe(NO3)3·9H2O 10a 96

a In the presence of masking agent (1000 �g mL−1).

They also did not interfere with the extraction of Cu(II) ions at leastup to 200 mg L−1.

3.9. Analytical performance of the method

Under the optimum conditions described above, the figures ofmerit of the proposed method were investigated (Table 3). Dynamiclinear range (DLR) was calculated using 10 spiking level of Cu(II)ions in the concentration range of 0.5–100 �g L−1. For each spikinglevel, three replicates of analyses were performed and the calibra-tion curve with the correlation coefficient better than 0.998 wasobtained. The detection limit (DL) is obtained from CDL = kSb/m,where, k = 3, Sb is the standard deviation of six replicate blank mea-surements, and m is the slope of calibration curve. The DL of theproposed method for determination of Cu(II) ions under the opti-mum conditions was 0.04 �g L−1. On the other hand, the proposedmethod revealed good reproducibility with the relative standarddeviation (R.S.D.) of 1.3% (six replicate measurements at 10 �g L−1

Cu(II)). The sample throughput was 2 sample h−1 at the sampleloading rate of 6.5 mL min−1 for 100 mL of sample uptake volume.The experimental enhancement factor, calculated as the ratio of theslopes of the preconcentration and direct calibration equations, was680. The theoretical preconcentration factor, calculated as the ratioof the sample volume (100 mL) to the peak volume (0.2 mL), was500.

According to the figures of merit of the proposed method incomparison with other reported on-line solid phase extractionmethods, the proposed method showed very good sensitivity andprecision for determining of Cu(II) ions (summarized in Table 4).Although this method has lower sampling frequency, but it showed

Linear range (�g L−1) 0.5–100Regression equation (�g L−1) I = 782.4 C + 907.8Correlation coefficient (r2) 0.9981Detection limit (3Sb/m) (�g L−1) 0.04Precision (R.S.D., n = 6) (%) 1.3

M. Faraji et al. / Journal of Hazardous Materials 166 (2009) 1383–1388 1387

Table 4Comparison of the figures of merit of the proposed method with the other reported solid phase extraction methods in the literature to determine the Cu(II) ions.

Sorbent material Reagent PT (s)a SV (mL)b f (h−1)c DL (�g L−1) R.S.D.% EF Ref.

Chelex-100 or 122 resin – 100 10 60 0.07 2.2 88 [11]C18 DDC 20 1.4 120 0.2 1.3 19 [12]C18 DDPA 20 2.9 d 1.4 1.5 35 [13]C18 Phenathroline 30 1.6 90 0.3 3.0 32 [14]SiO2-modified – 90 11.2 27 0.2 1.4 40 [15]Gallic acid-modified – d 2000 d 0.86 4.31 200 [16]Amberlite XAD-2 (functionalized) – 180 24 18 0.54 6.1 35 [17]Amberlite XAD-4 (load.) – 3000 25 d 0.06 1.2 300 [18]Amberlite XAD-2010 (load.) – 500 d d 0.05 d 82 [19]PS-DVB (functionalized) – 240 26.4 13 0.93 5.3 43 [20]Alumina (coated) – 480 40 d 0.3 4.5 100 [21]Chitosan (modified) 8-Hydroxyquinoline (derive.) 90 10.8 40 0.2 0.7 19.1 [22]Polyurethane APDC 60 12 36 0.2 2.8 170 [23]PTFE-turnings APDC 60 12 40 0.05 1.5 340 [24]PCTFE-beads DDPA 90 11.6 30 0.07 1.8 250 [25]Oxidized MWCNTs – d d d 0.32 2.88 d [26]Double-imprinted polymer Chitosan-succinate 3600 100 1 0.83 6.8 196 [27]Cotton CAS 1800 100 2 0.04 1.3 680 This work

a Preconcentration time.b Sample volume.c Sampling frequency.d Data not available.

Table 5Determination of copper in different water samples by applying the proposedmethod.

Sample Real (�g L−1) Added(�g L−1)

Found (�g L−1) Recovery%

Tap water 7.23 ± 0.10 6.0 14.05 ± 0.18 106Well water 1.45 ± 0.05 2.0 3.54 ± 0.15 103Mineral water (1) 3.82 ± 0.06 4.0 7.62 ± 0.24 97Mineral water (2) 1.06 ± 0.02 2.0 3.18 ± 0.17 103S

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eawater 2.57 ± 0.12a 2.0 4.22 ± 0.15 92

a Determination of copper in seawater was performed using standard additionethod.

.10. Analytical application

The proposed method was applied to determine Cu(II) ions in theatural water samples (tap, deep well, sea and two different min-

ral waters). The accuracy of the method was investigated with thenown amounts of Cu(II) ions. The obtained results are presentedn Table 5. The recoveries of Cu(II) ions from the spiked samples var-ed in the range of 92–106%. The R.S.D. for copper determination inhe examined samples varied in the range of 1.3–4.5%. The recorded

ig. 4. Recorded peaks from the seawater sample analysis. Conditions: CCAS =× 10−3 mol L−1, sample pH 4.6, CCTAB = 2.4 × 10−3% (w/v), CKI = 4 × 10−3 mol L−1

nd CNH4CH3COO− = 0.01 mol L−1.

[

peaks for copper determination using standard addition method inthe seawater sample are shown in Fig. 4.

4. Conclusion

In the present work, a simple, sensitive and reliable on-line SPE-ICP-OES method was developed for the preconcentration of copperin different water samples using cotton as solid phase extractionsorbent. Compared with the commonly used C18 sorbents, this sor-bent is very cheap and it can be easily packed into the column.In addition, the retained ion pairs of Cu(II) on cotton can be eas-ily desorbed and no carry-over is observed in the next analysis.The developed method in the present research is characterized bygood precision and accuracy. The method was successfully appliedto determine Cu(II) ions in the water samples and the recovery per-centages for different samples were more than 90%. Accordingly, itis an easy, safe, rapid and inexpensive method for the preconcentra-tion and determination of trace amounts of Cu(II) ions in aqueoussolutions.

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