Evaluation of sorption and desorption

Evaluation of sorption and desorption

Evaluation of sorption and desorption characteristics of cadmium, lead and zinc on Amberlite IRC-718

iminodiacetate chelating ion exchanger Mo´nica E Mallaa, Mo´nica B Alvareza, Daniel A Batistonib,c,*

aDepartment of Chemistry and Chemical Engineering, Uni 6ersidad Nacional del Sur,(8000)Bahı´a Blanca,

Pro 6incia de Buenos Aires, Argentina

bChemistry Unit, Constituyentes Atomic Center, Comisio´n Nacional de Energı´a Ato´mica, A 6enida General Paz 1499 ,

(1650)San Martı´n, Pro 6incia de Buenos Aires, Argentina

cINQUIMAE, School of Sciences, Uni 6ersidad de Buenos Aires, Buenos Aires, Argentina

Received 27 July 2001; received in revised form 19 December 2001; accepted 8 January 2002

Abstract

A chelating type ion exchange resin (Amberlite IRC-718), containing iminodiacetate groups as active sites, has been characterized regarding the sorption and subsequent elution of Cd, Zn and Pb, aiming to metal preconcentration from solution samples of different origins The methodology developed is based on off-line operation employing mini columns made of the sorbent The eluted metals were determined by flame atomic absorption spectrometry The effect

of column conditioning, influent pH and flow rate during the sorption step, and the nature of the acid medium employed for desorption of the retained metals were investigated Working (breakthrough) and total capacities were measured under dynamic operating conditions and the results compared with those obtained with Chelex-100, a resin extensively employed for analytical preconcentration Structural information on the complexation of metals by the chelating groups was obtained by Fourier Transform infrared spectrometry The analytical response of the Amberlite sorbent was assessed for the analysis of water samples and digestates of marine sediments © 2002 Elsevier Science B.V All rights reserved

Keywords: Ion exchange; Chelating resin; Atomic absorption; Cadmium; Lead; Zinc

www.elsevier.com/locate/talanta

1 Introduction

Solid organic and inorganic ion exchangers

constitute the basis of widely employed chemical

separation procedures, with applications ranging

from analytical and environmental chemistry re-search to water purification, waste management and material technologies (such as in the nuclear and electroplating industries) [1 – 4] The principal interest of their use in trace analytical chemistry lies on the design of methods for separation, preconcentration and, more recently, speciation of metals and non-metals In the first two cases,

* Corresponding author Fax: + 54-11-6772-7886.

E-mail address: batiston@cnea.gov.ar (D.A Batistoni).

0039-9140/02/$ - see front matter © 2002 Elsevier Science B.V All rights reserved.

PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 0 3 4 - 6

procedures are aimed to enhancing the sensitivity

of the method through analyte enrichment and

simultaneously to lessen the influence of sample

macrocomponents able to interfere with the

sub-sequent (off-line or on-line) measurements [5 – 7]

Among the numerous types of organic ion

ex-changers (anionic, cationic, weak base anionic),

chelating ion exchange resins have ionogenic

groups that can form coordination bonds with

many metals Such donor groups are principally

constituted by oxygen, nitrogen, sulfur or a

com-bination of these elements in the same functional

group A very comprehensive and detailed review

covering theoretical aspects and the principal

characteristics and methods of synthesis of this

type of resins has been published by Sahni and

Reedijk [8]

A great deal of materials based on chelating

groups bound to polymeric cross-linked chains

have been synthetized and characterized in

con-nection to their ability to selectively adsorb

ele-ments or groups of elements, particularly

transition and heavy metals [5 – 11] The weakly

acidic nature of the chelating groups makes the

desorption of metals by the action of acids

rela-tively straightforward after removing alkaline and

alkaline earth ions, enabling the subsequent

re-generation of the resin with an appropriate

medium

Applications of polymeric resins containing

iminodiacetate groups as active sites are well

doc-umented in the literature Among the

commer-cially available products, Chelex-100 (Bio-Rad) is

one of the most well characterized regarding its

applications [3,12 – 15] Less information exists,

however, on the behavior and practical analytical

aspects of similar resin types which may

poten-tially be employed for trace metal

preconcentra-tion [16 – 20]

In this paper we present an evaluation of the

sorption – desorption properties of Amberlite

IRC-718 chelating resin, in connection with its use

for the separation, matrix elimination and

precon-centration of Cd, Pb and Zn present at trace

levels in natural aquatic systems This resin is

claimed to present a macroporous

(macroreticu-lar) structure that provides high resistance to

os-motic shock and short diffusional paths that may

result in improved operation kinetics The methodology is based on off-line operation by employing a micro column filled with the sorbent for deposition of the metals, followed by desorp-tion – eludesorp-tion and subsequent measurement by flame atomic absorption spectrometry (FAAS) In establishing the dynamic operation conditions for the studied material, some measurements were also performed with Chelex-100 for comparative purposes

2 Experimental

2.1 Chemicals

Deionized water (ultra pure type II) was em-ployed throughout The acids emem-ployed were high purity HCl, HClO4 and HNO3 (Erbatron RSE, Carlo Erba) Other chemicals were of analytical reagent grade Stock solutions (1.000 mg ml− 1) of

Cd and Zn were prepared from Riedel-de Haen Fixanal, and appropriately diluted to the required concentrations A Pb solution of similar concen-tration was prepared in 1% (v/v) HNO3 in water from analytical reagent grade Pb(NO3)2 The con-centration of this stock solution was verified by titrimetry Multielement calibrant solutions of the metals were prepared in 1% (v/v) HNO3 by daily dilution of the corresponding stock solutions Ammonium acetate buffer solution (1 M) was prepared by mixing appropriate amounts of am-monia solution (28%) and glacial acetic acid, fol-lowed by dilution with water (final pH 9.5) Required pH adjustments were performed with ammonia or HCl Amberlite IRC-718 resin in the sodium form, 16 – 50 mesh, (equivalent particle size 300 – 900 mm) made by Rhom & Haas, Philadelphia, PA, was obtained from Biosix SA (Argentina) Chelex-100 resin (sodium form, 100 –

200 mesh, equivalent particle size 75 – 150mm) was from Bio-Rad Laboratories (Richmond, CA)

2.2 Apparatus

FAAS measurements were performed with a Hitachi Z 6100 flame atomic absorption spec-trometer equipped with single element

hollow-cathode lamps The instrument was operated at

maximum sensitivity with an air-acetylene flame

Analytical wavelengths (nm) and instrumental

de-termination limits (mg ml− 1) were: Cd 228.8/0.01;

Pb 283.3/0.34; Zn 213.9/0.01 No background

cor-rection was required Data reduction and

calibra-tion calculacalibra-tions were performed by employing

the standard software provided by the

manufac-turer Measurements of pH were performed with

a conventional pH meter (glass electrode)

Infrared spectra of the Amberlite resin in the

protonated and contacted with the metal forms

were obtained with a Nicolet 510 P Fourier

trans-form infrared spectrometer (KBr pellets)

Elemen-tal analyses of the sodium and protonated forms

of the resin were performed with a Carlo Erba EA

1108 microanalyzer

2.3 Procedures

An amount equivalent to 2 ml of resin was

packed in a glass mini column (8 cm length, 0.5

cm internal diameter) The circulation of liquid

through the resin bed was driven by gravity

Conversely, a peristaltic pump was employed to

deliver the influent solution at a fixed rate Flow

rate was generally 1 ml min− 1 during the

condi-tioning step, 3 ml min− 1 for the deposition –

re-tention step and 2 ml min− 1for analyte recovery

by elution A washing step with pure buffer

solu-tion previous to metal elusolu-tion, at the same flow

rate, was employed for elimination of

concomi-tant elements that could be partially retained by

the column Column blanks were obtained by the

same procedure with no analyte added

Recovery studies were performed on river a

seawater samples by spiking with amounts of the

analytes equivalent to those tested with synthetic

samples Generally, between 200 and 500 ml

sam-ple volumes containing 25mg of Cd, Pb and Zn,

with the addition of the appropriate volume of

acetate buffer (final pH: 9.0), were employed in

the deposition step Desorption of metals was

achieved with HNO3 and the eluted solutions

analyzed by FAAS

For the analysis of sediment digestates,

por-tions of 0.50090.001 g of dry sediment were

digested in a PTFE container with 12 ml of a

(5:5:2) mixture of concentrated HF/HCl/HNO3, heating to near dryness Then 2 ml of HClO4were added, repeating the heating until white fumes, and the residue was redissolved with concentrated HNO3, diluting to 200 ml with water The result-ing solution, buffered to pH 8.5 – 9.0 was passed through the column Elution of the analytes was performed, after a washing step, with 50 ml of 1

M HNO3, and the solution analyzed for Cd, Pb and Zn by FAAS To correct for fractional recov-ery from the column the procedure was applied in parallel to a similarly treated sample spiked with

25 mg of each metal

For dynamic capacity measurements, appropri-ately buffered solutions of each analyte at fixed

pH were continuously passed at a given flow rate through the column Successive fractions of 10 ml

of the effluent were collected and metal concentra-tions determined by FAAS Calculation of the dynamic working and total capacities were per-formed as described by Wang and Barnes [5] For the FT-IR studies, portions of about 2 g of the resin were contacted in batch during several hours respectively with 25 ml of 50% HCl (v/v) solution and with 200 ml of 1.0 g ml− 1buffered solutions of the individual analytes The resulting samples were air dried under an infrared lamp before preparing KBr pellets for FT-IR spectroscopy

3 Results and discussion

3.1 Purification of the resin

Preliminary tests demonstrated that erratic and abnormally high blank levels were observed when the resin was employed as received In conse-quence we employed a previous purification step based on washing with ethanol followed by suc-cessive portions of 5 M HNO3, water, 5 M HCl, and water, stirring in each case during 15 min This procedure left the resin in protonated form

No significant changes in the position and shape

of IR bands of the resin were observed by FT-IR spectrometry in the 4000 – 500 mm range when compared with the original non-purified material (also in protonated form)

3.2 Effect of column conditioning and influent

pH

Because the active iminodiacetate groups of the

Amberlite IRC-718 are weak acids, the degree of

protonation will critically affect the ability of the

resin to retain metal cations This situation should

be similar with that observed for the Chelex-100

resin In this resin protonation of the carboxylates

and the donor N atom are reported to be

com-plete at pH 2.21 [21] A comcom-pletely deprotonated

form is reached at pH = 12.30 To evaluate the

effect of the influent pH on retention of the

analytes, a column 2-ml of IRC-718 resin was

conditioned by passing 10 ml of ammonium

ace-tate buffer A volume of analyte solution at a

given pH containing the equivalent to 25 mg of

each element was passed through the column and

the amount of non-retained metal measured in the

percolated solution The total amount of analyte

deposited was subsequently estimated after

des-orption by employing 50 ml of 1 M HNO3

solu-tion Analyte recovery data, corresponding

respectively to the descending curve (non retained

metal) and ascending curves (eluted metal), are

graphically depicted in Fig 1 A consistent trend

is observed, indicating that the retention may be

more favorable at pH higher than around 8.0

These results differ from those reported for the

Chelex-100 resin, for which near quantitative

re-covery of similar metals is reported at pH as low

Fig 2 Effect of HNO3concentration on desorption of metals Circles: Cd; filled squares: Pb; triangles: Zn.

as 5.0 [12 – 15] In consequence, further analyte enrichment experiments with Amberlite IRC-718 were performed in the 8.5 – 9.0 pH range Higher

pH values were avoided to prevent the precipita-tion of metal hydroxides, particularly in the case

of Pb However, as described in the corresponding section, because resin capacity measurements re-quired a relatively high concentration of this ele-ment in the influent solution, capacity measurements for Pb were carried out at pH 7.5

3.3 Acid elution of metals

Desorption of electrostatically bound metals is expected to be achieved by proton exchange from acidic solutions After deposition of the metals following the procedure described in the prece-dent section, desorption was tested by employing

25 ml total volumes of HNO3 and HCl solutions

of increasing molarity Results are depicted in Figs 2 and 3 as % recovery vs acid molar con-centration In the case of HNO3similar recoveries were obtained with acid concentrations between 1 and 2 M However, recoveries are lower than 100%, particularly for Cd and Pb, suggesting that the employed volume may not be suited for com-plete elution of the analytes Elution of Pb seems

to be weakly influenced by the molarity of HCl, but the maximum recovery is only about 70% Furthermore, in the case of Zn and Cd the

mea-Fig 1 Effect of influent solution pH on deposition of metals.

Circles: Cd; filled squares: Pb; triangles: Zn.

sured recovery decreases with increasing acid

mo-larity Formation of stable complexes of the

ana-lytes in the presence of chloride ions could explain

these results At the very low pH values

corre-sponding to higher chloride concentrations, the

coordinating resin groups become fully

proto-nated and the resin behaves as a weak anion

exchanger regarding the negatively charged

com-plexes (i.e MCl3−, MCl4−,… with M = metal)

Consequently these species will tend to be

strongly bound, lowering the efficiency of the HCl

eluent for metal desorption The observed

behav-ior of the sorbent concurs qualitatively with that

reported for chloride metal complexes in an anion

exchange resin The tabulated log Dmaxvalues [22]

for Cd, Pb and Zn are respectively 3.5 (2 M HCl),

1.5 (1 M HCl) and 3.2 (2 M HCl), corresponding

to the trend observed in the elution curves: Pb

tends to be the most easily lost, while Cd seems to

be the strongly retained Additionally, a

mecha-nism of this sort has also been invoked by

Hashemi and Olin [23] for a FIA-ICP-AES system

based on preconcentration to explain the low

exchange rate of Cd retained in Chelex-100 and in

an iminodiacetate based sorbent (Novarose®)

when relatively high HCl concentrations are

em-ployed for desorption Similarly, Knezˇevic´ et al

[24] attributed the non-quantitative recovery of

Pb, Zn and Cr from Chelex-100 to the formation

of neutral and negatively charged complexes of

the metals with chloride and acetate ions

Fig 4 Elution curves for different concentrations of HNO3in the eluent Squares: 0.1 M; circles: 1 M; triangles: 2 M (a) Cd; (b) Pb; (c) Zn.

Fig 3 Effect of HCl concentration on desorption of metals.

Circles: Cd; filled squares: Pb; triangles: Zn.

Preconcentration factors for a given volume of the sample solution passed through the column depend upon the original sample volume and the volume of acid solution required to quantitatively elute the metal sorpted onto the resin We ob-tained elution curves for the studied metals em-ploying different HNO3 eluent concentrations by measuring the amount of analytes in successive 10

ml fractions of the percolated solution collected after previous deposition of 25 mg of each metal Results are shown in Fig 4(a) – (c) It is observed that with 1 M HNO3, the curves leveled off at an eluted volume of 50 ml Assuming a maximum

volume of 500 ml of sample originally passed

through the column, an enrichment factor of ten

times could be achievable However, were the

recoveries not quantitative, this could result in a

degradation of the enrichment factor

Conse-quently recovery factors should be taken into

consideration for more accurate estimation of the

analyte concentrations

3.4 Dynamic capacity measurements

One parameter that describes the operational

characteristics of a ion exchanger is the capacity,

resulting from the effective number of functional

active groups per unit of mass of the material

The theoretical value depends upon the nature of

the material and the form of the resin When the

column operation mode is employed, the

opera-tional capacity is usually lower than the available

capacity, and depends on several experimental

factors such as flow rate, temperature, particle

size and concentration of the feeding solution

Besides, the ‘breakthrough’ of solution from the

column defines a working capacity, which is lower

than the total capacity [5] The defined working

capacity corresponds to the maximum amount of

analyte that is retained with minimum leakage of

the element from the influent solution The

vol-ume of solution percolated from the breakthrough

point to the point of leveling of the loading curve

for a given solution flow rate also depends upon

the kinetics of exchange

We obtained saturation curves by circulating

analyte containing solutions at a predetermined

pH and collecting successive volumes of 10 ml of

effluent The concentration (Ci) of metal in each

fraction was determined by FAAS, and the ratio

of each concentration to the concentration of the

influent feeding solution (C0) was plotted vs the

effluent volume

The behavior of Amberlite IRC-718 resin is

depicted in Figs 5 and 6 Analogous plots,

ob-tained with a column of similar dimensions

con-taining Chelex-100 are presented, for comparative

purposes, in Fig 7 The shapes of the dynamic

capacity curves were found to depend upon

sev-eral experimental parameters Fig 5 graphically

depicts the variation observed in the case of Zn

Fig 5 Cd and Zn dynamic capacity curves for Amberlite IRC-718 at different effluent flow rates Metal initial

concen-tration (C0): 600 mg ml − 1 ; pH: 9.0 Circles: Cd (1 ml min − 1 ); squares: Zn (2 ml min − 1 ); filled squares: Zn (0.8 ml min − 1 ).

for the same concentration of element in the influent at two different elution ratios (0.8 and 2

ml min− 1) Although the leveling of the curves at

Fig 6 Pb dynamic capacity curves for Amberlite IRC-718 for two different influent concentrations and flow rates, at pH 7.5 Circles: 2 ml min − 1, C0: 250 mg ml − 1 ; filled squares: 3

ml min − 1, C: 125 mg ml − 1

Fig 7 Dynamic capacity curves for Chelex-100 Effluent flow

rate: 2 ml min − 1 , pH: 5.6 Filled circles: Pb (375 mg ml − 1 );

squares: Cd (600 mg ml − 1 ); filled squares: Cd (350 mg ml − 1 );

triangles: Zn (400 mg ml − 1 ).

(7.5), indicating that weak active sites may be involved in the deposition of Pb at higher flow rates and lower influent pH

Results are qualitatively similar for Chelex-100,

as presented in Fig 7 In the case of the plotted curves the operating pH was 5.5 and flow rates were maintained at a constant value of 2-ml min− 1

for variable concentrations of the metals

Calculated capacities and distribution

coeffi-cients (KD) for both sorbents are presented in Table 1 Assuming that equilibrium is attained when the effluent volume reaches the curve

plateau, KDvalues were calculated as:

KD = (mmol of element/g of resin)

/(mmol of element/ml of solution) The approximate location of the initial points of

the plateaus at C/C0= 1 were estimated, when necessary, by extrapolation of the ascending region

of the capacity curves

In general, total dynamic capacities are similar

or slightly higher for Chelex-100 This could be attributed, in part, to the smaller particle size of that resin As already mentioned the Zn capacity for Amberlite IRC-718 is particularly dependent

on the solution flow rate, pointing to significant kinetics effects Such effects are not absent in this type of resins The effectiveness of flow rates as low

as 0.2 ml min− 1 for the retention of metals by Chelex-100 resin has been reported by Paulson [26]

The differences in capacities observed among the metals retained by the sorbent materials tested

C/C0= 1 is not reached, the working capacity

results noticeably higher at the lower flow rate,

suggesting that strong retention sites in the resin

are more favorably involved in deposition when

the interaction sorbent – solution is longer [25] An

acceptable breakthrough point is observed for Cd

at a flow rate of 1 ml min− 1 The slope of the

ascending portion of the curve also suggests a

higher exchange rate Similar trends in the shape of

the loading curves were obtained for Pb with

different combinations of metal concentration in

the influent and flow rates (Fig 6) The

break-through points are not well defined at the pH tested

Table 1

Estimated dynamic capacities

Sorbent Element Working capacity (mmol g −1 ) Total capacity (mmol g −1 ) KD (ml g −1 )

Pb c

1.00

Cd Chelex-100

61

Influent flow rate: 2 ml min −1 (otherwise indicated); a 1 ml min −1 ; b 2 ml min −1 , pH: 7.5; c 3 ml min −1 , pH: 7.5; d 0.8 ml min −1

may evidence a negative steric effect on

coordina-tion with the iminodiacetate groups The effective

ionic radius of Pb(II) is 119 pm, compared to 95

pm for Cd(II) and 74 pm for Zn(II) [27] Stability

of the chelate is expected to be less favorable for

ions of larger size Formation of strong chelate

bonds for metals with smaller ionic radii may

explain the values obtained for the working and

total capacities in the case of Chelex-100 The

same qualitative correlation regarding ionic size

was found for Cd and Pb in the IRC-718 resin,

but the measured working capacity for Zn is lower

than anticipated from the above considerations

3.5 FT-IR absorption spectra

In order to further characterize the active sites

responsible of the binding of metals in the

Amber-lite IRC-718 resin, we obtained infrared spectra

under different conditions of saturation

Sepa-rated 2 g portions of the sorbent were equilibSepa-rated

for 72 h in batch (stirring occasionally) with 200

ml of solutions of the metals of concentration 1.0

mg ml− 1, buffered with ammonium acetate at pH

9.0 for Cd and Zn, and at pH 7.5 for Pb

Simi-larly, the resin in the protonated form was

ob-tained by contacting it with a 50% (v/v) solution

of HCl In all cases the sorbent was separated by

filtration, washed with water and air dried under

IR lamp for several hours before preparing the

potassium bromide pellets for IR spectrometric

analysis

The bands recorded in the 4000 – 500 cm− 1

wavenumber range are compared in Fig 8 The

spectra for the forms protonated and saturated

with Pb are noticeably similar The absorption

features near 1730, 1220 and 1396 cm− 1

corre-spond respectively the two first to carbonyl

stretching and the third to OH bending [28],

indicating the presence of protonated carboxylic

groups A band of significant intensity at about

1100 cm− 1 (tertiary amine) [21] is conspicuously

absent, indicating that the nitrogen atom in the

imino group is still protonated at the working pH

[8], and suggesting a lower involvement of the

group in the chelation of Pb This may further

explain the relatively low resin capacity observed

for this metal

Fig 8 IR absorption bands of Amberlite IRC-718 in the forms protonated and contacted with metals (a) Cd; (b) H + ; (c) Pb; (d) Zn.

The spectra produced by the resin contacted with Cd and Zn at a higher pH differ from that recorded from the protonated form The absorp-tion bands due to carboxylic acid are not ob-served, being in turn substituted by strong bands

at 1600 and 1400 cm− 1, attributable to the pres-ence of carboxylate anions These groups present two strongly coupled band, arising the more in-tense one from the asymmetric stretching in the

1550 – 1650 cm− 1region and the weaker one from the symmetric stretching near 1400 cm− 1[28] In addition, the band at about 1100 cm− 1 is clearly observed for the Cd and Zn saturated resin, pointing to the presence of deprotonated nitrogen

In consequence coordination with the metals through the nitrogen atom of the imino group is favored, allowing the resin to behave as a triden-tate ligand This behavior, generally accepted for

Table 2

Chemical analysis of Amberlite IRC-718

7.61

6.28

for the sodium form, indicating that, apart from the H attached to the N and the carboxylates of the imino group, a significant amount of excess H

is present Although the oxygen content was not measured, the excess H may be attributable to the presence of several water molecules associated to the resin in the sodium form

3.7 Analyte reco 6ery studies

In order to evaluate the response of Amberlite IRC-718 resin in real analytical situations such as those in which an enrichment step, prior to the determination by FAAS is involved, we carried out recovery studies on tap, stream and sea water samples Variable volumes were spiked with known amounts of the analytes and passed through the column at optimized conditions of

pH and flow rate Subsequent elution of the metals was carried out with 50 ml of 1 M HNO3 Obtained results, expressed as % recoveries of the added amounts of analytes, are presented in Table

3 An overall consideration of the figures without taking into account the different origin of the samples indicates that near quantitative recovery (between the limits of experimental error), is ob-served for about 40% of the data In addition, recoveries of 90% or higher were obtained for about 70% of the samples tested About 25% of the measurements gave recoveries lower than 80%, suggesting that a recovery factor should be

this type of chelating resin, may justify the

rela-tively higher capacity observed for Cd and Zn It

is worth mentioning, however, that the results for

Pb do not allow to rule out the possibility of

formation of weaker 1:2 type metal – ligand

associ-ations with the resin groups, as proposed by

several authors for Chelex-100 [15,20] Further

experiments will be required to univocally clarify

this situation for the Amberlite IRC-718 resin,

which if confirmed, would be an additional

expla-nation of the lack of effectiveness of the sorbent

for retention of Pb

3.6 Elemental composition of the resin

The analysis of the elemental composition of

the resin for the content of C, N and H in both

the original sodium form and in the protonated

form (the last prepared as previously described for

FT-IR analysis) was carried out, and the results

presented in Table 2 It was found that the C/N

ratio has a constant value of about 12 However

the C/H and N/H ratios are substantially lower

Table 3

Analyses of spiked water samples

% Recovery ( 9SD) a

Water sample Sample volume (ml) Concentration of metal added ( mg ml −1 )

type

79.2 96.7

Tap

101.2 97.1

91.5 95.7 83.5 91.0

63.0 92.4

87.0 97.4 97.5 95.0

84.0 92.8 95.0 97.1

86.5 93.5 0.062

400

a Average of three determinations.

Table 4

Analyses of sediment digestates

Found Reported

0.48 90.01

2.9 90.2 79.3 93.3 88.8 96.4

1.05 90.01 23.2 90.9 25.0 91.8 507 915

Values in mg g −1 ( 9SD).

employed in particular cases to reach more

accu-rate analytical estimations

In addition to the above described experiments,

we assessed the applicability of the Amberlite

resin to the preconcentration of metals in

solu-tions arising from the acid digestion of marine

sediments These samples are usually complex and

contain relatively large amounts of alkaline and

alkaline earth concomitants, as well as other

sili-cate components The materials employed in our

study include a Certified Standard Reference

Ma-terial (MURST-ISS-A1, Antarctic bottom

sedi-ment), and two surface sediment samples

prepared in our laboratory The mineralogical

and chemical composition of the latter regarding

the elements of interest have been reported

else-where [29] Spiked samples were employed to

account for the partial recovery of the analytes

after the preconcentration step A comparison of

certified (or reported) and obtained concentration

values is presented in Table 4

4 Conclusions

The results reported in the present study

demonstrate the applicability of the chelating

resin Amberlite IRC-718 for off-line enrichment

of trace metals from relatively complex water and

sediment samples, prior to the spectrometric

de-termination by FAAS If extreme enrichment

fac-tors are not required, the sorbent compares

favorably with the widely employed Chelex-100

resin Apart from a significantly low recovery rate

of Pb in one of the sea water samples that may be

ascribed to analyte losses during operation, the

larger departures from 100% are observed in

gen-eral for Cd Acceptable recoveries were obtained for Pb and Zn, but the efficiency of retention for

Zn seems to be affected by the original sample volume: the recovery decreases with the influent sample volume It is worth mentioning that the metals considered are prone to strong complexa-tion by organic species frequently present in natu-ral water systems Because the complexes are in many cases more stable that the associations of the metal with the iminodiacetate groups of the resin, particularly in the case of Cd, the deposi-tion may be seriously impaired Also, saturadeposi-tion

of the active groups with weakly adsorbed alka-line and alkaalka-line earth metals due to a mass effect could lessen the retention efficiency of trace metals Acceptable agreement between known and found concentration values was achieved in the analysis of sediment digestates that involves a preconcentration step, providing that recovery factors derived from the analysis of analyte spiked samples are employed to account for the frac-tional recovery of metals from the column The studied sorbent may show also utility for on-line concentration of metals prior to their determina-tion by atomic spectrometric methods

Acknowledgements

The authors are indebted to Myriam Crespo (CERZUS, CONICET) for her collaboration in performing the atomic absorption analyses, to Mireille Perec (INQUIMAE) for obtaining and helping in the interpretation of the FT-IR spectra, and to Marı´a dos Santos Afonso (INQUIMAE) for performing the resin microanalyses This work was carried out as part of CNEA-CAC Projects