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Spectrochimica Acta Part B 57 (2002) 21752180

0584-8547/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S058 4-8 547 0 2.0 0 1 7 9 - 9

Technical note

Determination of lead in seawater by inductively coupled plasmaoptical emission spectrometry after separation and pre-concentration with cocrystallized naphthalene alizarin

Marcia Sousa Bispo , Maria das Gracas Andrade Korn *, Elane Santos da Boa Morte ,a a, a Leonardo Sena Gomes Teixeiraa,b

Universidade Federal da Bahia, Instituto de Qumica, Campus Universitario da Federacao, Salvador, Bahia 40170-290, Brazila

Departamento de Engenharia e Arquitetura, Universidade Salvador Unifacs, Av. Cardeal da Silva 132, Salvador,b

Bahia 40220-141, Brazil

Received 17 May 2002; accepted 20 August 2002

Abstract

An analytical method for separation and pre-concentration of lead in seawater for determination by inductivelycoupled plasma optical emission spectrometry has been investigated. Lead was retained in the solid phase (0.5 g)

composed of co-precipitated naphthalene and alizarin red. The solid phase quantitatively sorbs Pb (II) at pH 89, andthe metal was eluted using 5.0 ml of 2 mol l nitric acid. The effect of NaCl, KCl, BaCl , CaCl , Na SO , MgCly1 2 2 2 4 2and Na PO on the sorption of Pb(II) in the solid phase was studied. A set of solutions containing varying amounts3 4of electrolytes (0.5; 1.0; 3.0 and 5.0% myv) with Pb (50 mg) was prepared and the recommended procedure applied.The Na PO was found to interfere; the other electrolytes did not interfere up to 5% myv. A pre-concentration factor3 4of 40 was obtained in this analytical procedure. The limit of detection and limit of quantification for Pb (II) were 53and 176 mg l , respectively. Lead was determined in seawater samples collected in Salvador city, Bahia, Brazil. They1

precision, expressed as R.S.D., was 1.84.6%, and the recovery of lead added to seawater samples was 9597%. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Lead; Naphthalene; Alizarin; ICP OES; Saline samples

1. Introduction

Determination of trace metals in environmental

This paper was presented at the 7th Rio Symposium onAtomic Spectrometry, held in Florianopolis, Brazil, April 2002

and is published in the Special Issue of Spectrochimica ActaPart B, dedicated to that conference

*Corresponding author. Fax: q55-71-2374117.

E-mail address: [email protected] (M.G.A. Korn).

samples is a subject of considerable interest

because trace metals play important roles in bio-

logical processes both as essential components and

toxins w1x. Among these, lead, even at very low

concentrations, is a well-known element with toxic

effects for animals and humans. Cumulative poi-

soning effects of lead are serious hematological

damage, anemia, kidney malfunctioning and brain

damage w2,3x. For humans, the main lead sources

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2176 M.S. Bispo et al. / Spectrochimica Acta Part B 57 (2002) 21752180

are water and food, and therefore rapid and sensi-tive methods must be accessible for its determi-nation in these samples w4x.

Inductively coupled plasma optical emissionspectrometry (ICP OES) can be used to determi-nate lead at low concentration levels in salinematrices, but previous separation and pre-concen-tration steps are usually required. There are severalpre-concentration methods for Pb(II) described inthe literature, such as liquidliquid extraction, ionexchange and solid phase extraction (SPE). SPEhas a number of attractive features compared withtraditional extraction techniques. It is fairly simple,inexpensive, can be used in the field, needs rela-

tively little toxic solvents, and can be easilyautomated w2,5,6x.

As an effective technique for separation andpre-concentration of trace elements, SPE withnaphthalene has received great attention in recentyears. Due to its simple procedure, rapid phaseseparation and high extraction capacity, SPE withnaphthalene has great prospect in trace and ultra-trace analysis w6x.

Alizarin (1,2-dihydroxyanthraquinone) is anochre-yellow powder that melts at 290 8C and

sublimes above 110 8C. Alizarin is soluble in 300parts of water, it is moderately soluble in alcohol,but is quite soluble in hot methanol, benzene,ether, glacial acetic acid, acetone, carbon disulfide,and pyridine. Due to its structure, alizarin reactswith many metallic ions to form stable complexes.Other elements that may be detected or determinedwith the aid of alizarin are copper, indium, urani-um, platinum, bismuth, chromium, scandium, mer-cury and titanium w7x. The complexation reactionbetween alizarin complexan and Pb(II) has been

studied by spectrophotometric method w8x and pre-concentration methods with the associationbetween alizarin and different solid supports havebeen investigated w9,10x.

The association of complexing agents and naph-thalene has been applied to the extraction of metalcomplexes. In these systems the metal ions reactwith the chelating agent to form a complex, whichis extracted with naphthalene. In this way, amethod for the determination of lanthanum, euro-pium and ytterbium after solid liquid extraction

with microcrystalline naphthalene was developedw11x. Zinc, cadmium and lead were determined bya spectrophotometric method after extraction of

their morpholine-4-carbodithioates into moltennaphthalene w12x. Another procedure describesatomic absorption spectrometric determination ofzinc after pre-concentration with the ion pair of 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol andammonium tetraphenylborate on naphthalene w13x.

In this work, a new pre-concentration and sep-aration method has been developed using alizarinred (1,2-dihydroxyanthraquinone) as lead chelat-ing agent and naphthalene as the solid phaseextractor. The developed method was applied for

the determination of lead in seawater samples byICP OES.

2. Experimental

2.1. Apparatus

An Applied Research Laboratories model 3410minitorch sequential inductively coupled plasmaspectrometer (Valencia, CA), equipped with anIBM PC-AT computer (Armonk, NY) was usedfor lead determination. The emission intensitymeasurements were made under the conditionsshown in Table 1.

A DIGIMED DM-20 digital pH meter (Sao

Paulo, Brazil) was used to measure the pH values.

2.2. Reagents and solutions

All reagents were of analytical grade unlessotherwise stated. Ultrapure water from Milli-Q

water purification system (Millipore, Bedford,MA) was used to prepare all solutions. The labo-ratory glassware was kept overnight in a 10%nitric acid solution. Before use, the glassware waswashed with deionized water and dried in a dust-free environment.

The lead stock solution was prepared fromTitrisol concentrates (Merck, Darmstadt, Germa-ny) to a final concentration of 1000 mg ml . They1

working solutions were obtained by dilution of thestock solution with deionized water.

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2177M.S. Bispo et al. / Spectrochimica Acta Part B 57 (2002) 21752180

Table 1Operating parameters for the inductively coupled plasma spectrometer

Incident output power 650 W

Reflected power -5 WNebulizer Glass (Meinhard Associates, Santa Ana, CA)Plasma gas flow rate 7.5 l miny1

Auxiliary gas flow rate 0.8 l miny1

Aerosol carrier gas flow rate 0.8 l miny1

Solution uptake rate 2.5 ml miny1

Wavelength 220.353 nmSignal integration time 5 sIntegration for determination 3

The pH of the working solution was adjusted topH values of 3, 6 and 810 using buffer solutions

of glycine, Tris(hydroxymethyl)aminomethane andammonium, respectively. The glycine buffer wasprepared by diluting 50 ml of 0.2 mol l glyciney1

solution and 1.14 ml of 2 mol l HCl to 1 l withy1

deionized water. The Tris buffer was prepared bydiluting 50 ml of 0.2 mol l Tris solution to 200y1

ml. The ammonium buffer was prepared using53.5 g of NH Cl and 500 ml of 2 mol l NHy14 3solution diluted to 1 l with deionized water.

Sulfuric acid, nitric acid and hydrochloric acidwere of suprapure quality (Merck). The alizarin

red (Merck) was used as received.The samples analyzed were collected from sea-water in Salvador, Bahia and filtered through 0.45mm Millipore membrane and acidified with 1%vyv HNO .3

2.3. Preparation of alizarin red-naphthalene solid

phase

Naphthalene (80 g) was impregnated with aliza-rin red (0.8 g) in the presence of 1 l of acetone(Merck) for 30 min by using a magnetic stirrer.

The mixture was transferred to a beaker withdeionized water in order to initiate the naphtha-lene alizarin red cocrystallization process. Thesolid phase was vacuum filtered, dried and storedfor further use.

2.4. Procedure for the sorption of lead in the solid

phase

Ten to two hundred milliliters of sample con-taining up to 20 mg of lead were placed in conical

flasks, and 10 ml of ammonium buffer solutionpH 8.1 was added, followed by 0.5 g of the solid

phase. The flask was closed with a stopper andmechanically shaken for 5 min. The Pb(II) wasquantitatively retained, and the solid mass, con-sisting of naphthalene and metal complex, wasseparated by filtration on filter paper and washedwith 5 ml of 2 mol l HNO to desorb the leady1 3from the phase. The resulting solution was ana-lyzed by ICP OES at a wavelength of 220.353nm.

3. Results and discussion

Several parameters were studied to determinethe optimum conditions for quantitative recoveryof lead by using naphthalenealizarin red as solidphase: pH, reaction time, mass of solid phase,eluent type, sample volume, and sorption capacity.

3.1. Effect of pH on lead sorption

Extraction was carried out by the general pro-cedure at different pH values, keeping other con-

ditions constant. The effect of pH on the recoveryof Pb(II) was examined and the experimentalresults indicated that lead recovery is maximumand quantitative ()95%) within a pH range from8.1 to 9.1. A pH value of 8.1, controlled withammonium buffer, was selected for subsequentexperiments.

3.2. Effect of mass of solid phase

The mass of solid phase was varied between 0.1

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and 1.0 g under optimum conditions. Resultsdemonstrated that 0.5 g of solid phase was suffi-cient for quantitative retention of Pb(II).

3.3. Effect of shaking time

The effect of extraction time between lead andsolid phase was studied by varying the shakingtime from 1 to 30 min. It was observed thatquantitative lead recovery ()95%) was obtainedafter a shaking time of 5 min. This shaking timewas therefore chosen in all future experiments.

3.4. Effect of volume of aqueous phase

As the volume of solid phase is small comparedwith that of the aqueous phase, it was importantto study the effect of the latter on the extractionof Pb(II). The volume of the aqueous phase wasvaried from 10 to 1000 ml, and the extraction wasfound to be quantitative when the volume of theaqueous phase did not exceed 200 ml.

3.5. Retention capacity of the solid phase

The sorption capacity of the naphthalenealiza-rin solid phase for lead was determined using 50ml of deionized water, 10 ml of ammonium buffer(pH 8.1), 0.5 g of solid phase and a set of solutionscontaining varying amounts of lead in the rangeof 51000 mg. Results demonstrated that 0.5 g ofsolid phase had a capacity to retain up to 200 mgof Pb(II), hence, the phase had a sorption capacityof 1.93 mmolPb g of solid phase.y1

3.6. Choice of eluent

Sulfuric acid, nitric acid and hydrochloric acidin different concentrations (2.0; 3.0; 4.0 and 6.0mol l ) and volumes (18 m l) were tried toy1

desorb the lead from the solid phase. It wasessential to select an eluent that could also be usedfor ICP OES measurements without problems.Hence, organic solvents could not be used for thatpurpose. The best results for recovery of lead wereobtained using nitric acid as eluent. It was found

that 5 ml of 2.0 mol l HNO was sufficient toy1 3desorb quantitatively the metal.

3.7. Effect of electrolytes

The effect of NaCl, KCl, BaCl , CaCl ,2 2Na SO , MgCl and Na PO on the sorption of2 4 2 3 4Pb(II) in the solid phase was studied. A set ofsolutions containing varying amount of electrolyte(0.5; 1.0; 3.0 and 5.0%) with Pb (50 mg) wastaken and the recommended procedure applied.Phosphate interferes in the sorption of lead in allproportions; the other electrolytes did not interfereup to 5% myv. The effect of phosphate is due to

precipitation losses of lead.

3.8. Procedural blanks and detection limit

The accurate determination of lead at low con-centration levels in seawater after pre-concentra-tion requires low and reproducible blanks.Absolute blanks, based on the analysis of separateconcentrates from different volumes of ultrapurewater were submitted to the pre-concentration

procedure.The limit of detection (3s of 30 measures ofblank, where s is the S.D. of blank) for thedetermination of Pb was 53 mg l , and the limity1

of quantification (10s of 30 measures of blank)was 176 mg l . This limit remained unchangedy1

varying the volume of blank solution between 20and 200 ml.

3.9. Applications

The experimental procedure was applied for leaddetermination in seawater samples (saline level;3.5%) collected from different places of Salva-dor city, Bahia, Brazil. The samples were collectedin urban beaches near to wastewater effluents. Theresults, as means of three determinations on eachof four samples, are shown in Table 2 and exhibita good precision (R.S.D. 1.84.6%). The methodvalidation was made through spike recovery testswith the addition of 20 mg of lead, and therecovery was between 95 and 97%.

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2179M.S. Bispo et al. / Spectrochimica Acta Part B 57 (2002) 21752180

Table 2Determination of lead in seawater samples

Sample Added lead Found leada Recovery

(mg ml )y1 (mg ml )y1 (%)

Ondina seawater 0 0.014"0.002 960.100 0.109"0.002

Rio Vermelho seawater 0 0.011"0.002 970.100 0.108"0.005

Barra seawater 0 0.012"0.002 950.100 0.106"0.002

Ipitanga seawater 0 0.007"0.001 950.100 0.102"0.002

At 95% confidence level.a

4. Conclusion

This study demonstrated that the use of solidphase naphthalenealizarin red for the extractionof lead from seawater presents some advantagesdue to rapid kinetic, low consumption of reagentsand eluent, good stability and a good enrichmentfactor (40=). The naphthalene used to prepare thesolid phase was of commercial grade and the lowcost of this phase is an additional advantage.

Furthermore, the elution step does not involve theuse of organic solvents as other procedures do.The spike recovery showed that the proposedprocedure had good accuracy. The proposed meth-od is simple and suitable for the separation anddetermination of lead in saline water samples.

Acknowledgments

The authors acknowledge the grants from CT-PETROyCNPq, SEPLANTECyCADCT-BAHIA,PIBICyCNPq, CNPq, FINEP and CAPES.

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