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Evaluation of sorption and desorption
Talanta 57 (2002) 277–287 Evaluation of sorption and desorption characteristics of cadmium, lead and zinc on Amberlite IRC-718 iminodiacetate chelating ion exchanger Mo´nica E. Malla a ,Mo´nica B. Alvarez a , Daniel A. Batistoni b,c, * a Department of Chemistry and Chemical Engineering, Uni6ersidad Nacional del Sur, (8000) Bahı´a Blanca, Pro6incia de Buenos Aires, Argentina b Chemistry Unit, Constituyentes Atomic Center, Comisio´n Nacional de Energı´a Ato´mica, A6enida General Paz 1499 , (1650) San Martı´n, Pro6incia de Buenos Aires, Argentina c INQUIMAE, School of Sciences, Uni6ersidad 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: S0039-9140(02)00034-6 M.E. Malla et al. / Talanta 57 (2002) 277 – 287 278 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– elution 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 employed were high purity HCl, HClO 4 and HNO 3 (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) HNO 3 in water from analytical reagent grade Pb(NO 3 ) 2 . The con- centration of this stock solution was verified by titrimetry. Multielement calibrant solutions of the metals were prepared in 1% (v/v) HNO 3 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 75–150 mm) 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- M.E. Malla et al. / Talanta 57 (2002) 277 – 287 279 cathode lamps. The instrument was operated at maximum sensitivity with an air-acetylene flame. Analytical wavelengths (nm) and instrumental de- termination limits (mgml −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 calculations 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 −1 for analyte recovery by elution. A washing step with pure buffer solu- tion previous to metal elution, 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 25 mg 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 HNO 3 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/HNO 3 , heating to near dryness. Then 2 ml of HClO 4 were added, repeating the heating until white fumes, and the residue was redissolved with concentrated HNO 3 , 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 HNO 3 , 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 −1 buffered 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 HNO 3 , 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). M.E. Malla et al. / Talanta 57 (2002) 277 – 287 280 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 completely 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 mgof 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 of1MHNO 3 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 HNO 3 concentration 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 HNO 3 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 HNO 3 similar 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. M.E. Malla et al. / Talanta 57 (2002) 277 – 287 281 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. MCl 3 − , MCl 4 − ,… 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 D max values [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 HNO 3 in 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 HNO 3 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 HNO 3 , the curves leveled off at an eluted volume of 50 ml. Assuming a maximum M.E. Malla et al. / Talanta 57 (2002) 277 – 287 282 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 (C i ) of metal in each fraction was determined by FAAS, and the ratio of each concentration to the concentration of the influent feeding solution (C 0 ) 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 (C 0 ): 600 mgml −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 , C 0 : 250 mgml −1 ; filled squares: 3 ml min −1 , C 0 : 125 mgml −1 . M.E. Malla et al. / Talanta 57 (2002) 277 – 287 283 Fig. 7. Dynamic capacity curves for Chelex-100. Effluent flow rate: 2 ml min −1 , pH: 5.6. Filled circles: Pb (375 mgml −1 ); squares: Cd (600 mgml −1 ); filled squares: Cd (350 mgml −1 ); triangles: Zn (400 mgml −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 (K D ) for both sorbents are presented in Table 1. Assuming that equilibrium is attained when the effluent volume reaches the curve plateau, K D values were calculated as: K D=(mmol of element/g of resin) /(mmol of element/ml of solution) The approximate location of the initial points of the plateaus at C/C 0 =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/C 0 =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 Working capacity (mmol g −1 ) Total capacity (mmol g −1 ) K D (ml g −1 )Element Cd a 1981.06Amberlite IRC-718 0.65 0.096 80Pb b 0.01 – 0.048 80Pb c Zn 0.10 1.04 113 Zn d 0.71 1.77 193 1.000.67 321CdChelex-100 61Pb 0.081 0.11 Zn 2081.270.99 Influent flow rate: 2 ml min −1 (otherwise indicated); a 1mlmin −1 ; b 2mlmin −1 , pH: 7.5; c 3mlmin −1 , pH: 7.5; d 0.8 ml min −1 . M.E. Malla et al. / Talanta 57 (2002) 277 – 287 284 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 equilibrated 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 −1 region 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 M.E. Malla et al. / Talanta 57 (2002) 277 – 287 285 Table 2 Chemical analysis of Amberlite IRC-718 Resin form C (%) H (%) N (%) 7.61Sodium 2.9334.62 6.28Proton 5.4363.29 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 reco6ery 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 of1MHNO 3 . 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 (9 SD) a Water sample Sample volume (ml) Concentration of metal added (mgml −1 ) type Cd Pb Zn 79.29 6.7200 96.49 1.6 104.0 9 3.60.125Tap 101.29 7.1500 99.09 6.20.05 96.29 2.2 91.59 5.783.59 1.063.09 2.4Stream 0.05500 87.09 7.4 97.59 5.0Sea c 1 200 0.125 65.59 4.2 200 0.125Sea c 2 81.29 4.5 63.69 5.1 99.19 8.6 84.09 2.895.09 7.186.59 3.50.062400 500 0.05 91.09 2.8 94.09 5.7 84.09 5.7 a Average of three determinations. M.E. Malla et al. / Talanta 57 (2002) 277 – 287 286 Table 4 Analyses of sediment digestates Pb ZnSample Cd Found ReportedReported Found Reported Found 0.489 0.01MURST-ISS-A1 21.09 2.10.549 0.02 20.89 0.2 51.99 3.2 54.89 0.2 2.99 0.2 79.393.3 88.896.43.19 0.2 1079 4PG-1 1229 3 1.059 0.01 23.29 0.9 25.09 1.8 507 9 15PG-2 4629 341.169 0.03 Values in mgg −1 (9 SD). 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, saturation of the active groups with weakly adsorbed alka- line and alkaline 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 [...]...M.E Malla et al / Talanta 57 (2002) 277–287 95-Q-02-01 and 02– 03 Financial support was provided by Agencia Nacional de Promocion Ci´ entıfica y Tecnologica (Project PICT-06-00000´ ´ 0354) and SGCYT, Universidad Nacional del Sur (Projects 24M052 and 068) References [1] B.L Karger, L.R Snyder, C Howath, An Introduction to Separation Science, Wiley, New... 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