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Study of sorption of copper(II) on complexing resin columns by solid phase extraction
Analytica Chimica Acta 389 (1999) 59±68
Study of sorption of copper(II) on complexing resin columns by solid phase extraction Maria Pesaventoa,*, Enrica Baldinib a
Dipartimento di Chimica Generale, UniversitaÁ di Pavia, V. Taramelli 12, 27100 Pavia, Italy b Istituto di Scienze Mat. Fis. Chim., UniversitaÁ di Milano, V. Lucini 3, Como, Italy
Received 7 August 1998; received in revised form 18 January 1999; accepted 25 January 1999
Abstract The position of the breakthrough pro®le of a substance sorbed on solid phase columns is given by the central point of the breakthrough curve, VC. This is the volume of eluted solution for which the concentration of the substance is half of that present in the original solution. VC depends on the characteristics of the column (volume of the stationary and mobile phase) and on the distribution ratio KD. In the case of the sorption of metal ions on complexing resins, the distribution ratio can be evaluated from the exchange coef®cients of the metal ion on the resin, which in turn depend on the conditions considered. In the present paper the dependence of the exchange coef®cients on the conditions (acidity and composition of the solution, composition of the resin phase) is accounted for by the Gibbs±Donnan model for the ion-exchange resin. In this work it is shown that KD calculated on the basis of the model is suf®ciently accurate to allow the prediction of VC. The sorption of copper(II) on column of complexing resin was studied by determining the breakthrough curves, whose position on the eluted volume axis was evaluated by the central point VC. Two complexing resins were considered, with different sorbing properties towards copper(II), Chelex 100 and Amberlite CG50, containing iminodiacetic and carboxylic groups, respectively. It was found that the values of VC calculated from the model are in acceptable agreement with those obtained experimentally under different conditions. The presence in solution phase of substances able to complex copper(II) was also examined. They have an in¯uence on KD and VC as expected on the basis of the model, considering the values of the side reaction coef®cients. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Copper; Solid phase extraction; Amberlite CG50; Chelex 100
1. Introduction The solid phase extraction (SPE) of trace metals is often carried out using resins containing chemically immobilized complexing groups because of welldocumented advantages over simple ion-exchange *Corresponding author. Tel.: +39-382-507-580; fax: +39-382528-544; e-mail: [email protected]
materials or hydrophobic phases on which ionic or neutral complexes are sorbed [1]. Column or batch procedures can be used for the extraction, which is characterized by the distribution ratio KD, i.e. the ratio of the concentration of the metal ion in the two phases. It has been recently shown that this quantity can be predicted using the Gibbs±Donnan model for the resin [2±4], when the ``intrinsic complexation constants'' of the metal ion on the resin
0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 1 2 4 - 5
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M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
Fig. 1. Breakthrough profiles of copper(II) on Chelex 100 (0.25 g dry resin); CCu9.010ÿ5 mol kgÿ1; 0.1 mol kgÿ1 NaNO3; flow rate 1 ml minÿ1; curve 1, pH1.3; curve 2, pH1.5.
are known. They are independent of the experimental conditions, while not being thermodynamic quantities, and can be used to characterize the sorption of a metal ion on a given resin. The distribution ratio at particular conditions can be evaluated from the intrinsic complexation constants on the basis of the Gibbs±Donnan model of the resin [2±4], and in this way the time consuming experimental determination can be avoided. The validity of this approach in the case of simple solutions not containing any ligand has been previously shown in the case of particular complexing resins, i.e. anion exchange resins loaded with anionic ligands [5], and a resin with ®xed carboxylic groups, Amberlite CG50 [6]. On the other hand, the column procedure is more widely used in SPE of metal ions particularly since it is easily implemented in on-line preconcentration apparatuses [7,8]. Thus in the present paper an attempt is described of characterizing the sorption of metal ions on complexing resins by column procedure by the same model previously used for the batch procedure. Two resins are examined. One of them, Chelex 100, is widely used because it strongly sorbs transition metal ions due to the presence of iminodiacetic groups. For
comparison a second resin is also examined containing the less strongly complexing carboxylate group, Amberlite CG50, which is also commercially available and used for particular separation purposes. The sorption properties of both the resins for copper(II) are known from previous investigations [2±4,6]. The sorption of metal ions on the column can be well characterized by the breakthrough pro®le, i.e. the graph reporting the concentration of the metal ion in the eluate in function of its volume. An example obtained in the case of copper(II) on Chelex 100 is reported in Fig. 1. The volume of eluate corresponding to a concentration in the eluate which is half of the total concentration is indicated by VC, and characterizes the position of the breakthrough curve. It must be noticed that VC is different from the breakthrough volume, Vb, i.e. the volume for which the metal ion appears in the eluate, which depends also on the shape of the breakthrough pro®le. VC and Vb depend on the acidity, and on the composition, of the solution, and in the cases presented in Fig. 1 neither of them correspond to the total number of complexing groups present in the resin. When the number of mmoles of metal in VC is lower than the number of mmoles of active groups in the resin it has been demonstrated [9]
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
tion ratio is
that VC Vm KD Vs ;
(1)
where Vm and Vs indicate the volume of the mobile and stationary phases, in ml, respectively. The distribution ratio is KD cs =cm ;
(2)
where cs and cm are the concentration of the metal ion in the stationary and mobile phases, respectively. In the case of resins containing complexing groups, as Chelex 100 and Amberlite CG50, it is possible to evaluate KD from the intrinsic complexation constants of the metal ion on the resin [2±4,6], as will be shown in the next paragraph. Thus, also the position of the breakthrough curve of a metal ion on a given resin can be calculated by Eq. (1). Of course, this is of interest when performing SPE of metal ions on complexing resins, since for example the volume of solution which can be passed through the column with quantitative sorption can be evaluated at least approximately. In the present paper simple solutions were considered as well as solutions containing complexing substances. This is important when the resins are used for real samples, where the metal ions are often complexed, particularly in the case of natural waters [10]. 1.1. Model for the sorption of a metal ion on a complexing resin The value of KD which is required for calculating VC by Eq. (1) can be experimentally obtained from a determination in batch exactly at the same conditions, and for each set of conditions. However, in the case of sorption of metal ions on a resin containing complexing groups HrL, KD can also be calculated for each particular set of conditions from known quantities as described here. It has been demonstrated [2±4,6] that the sorption equilibrium in this case is M nHr L $ MHp Ln sH and the corresponding exchange coef®cient is MHp Ln Hs n : 1npex M Hr L
61
(3)
KD 1npex Hr Ln w=Hs Vs ;
where w indicates the grams of water sorbed in the stationary phase. When more than one complex is formed in the resin it is convenient to indicate the ratio of the concentration of the metal sorbed on resin to that of the free metal in solution by K, which is given by the following equation [3,11]: n P X 1npex Hr L : (6) K Hs 0 The exchange coef®cients strongly depend on the composition of the resin and of the solution phase. It has been demonstrated [3±6] that the exchange coef®cient 1npex is related to the thermodynamic complexation constant inside the resin, T(1np), by the following relationship, derived from the Gibbs± Donnan model: 1npex T 1np
The barred species are those inside the resin and all the concentrations are in mol kgÿ1. Thus the distribu-
g fC
mÿs
M Hn L r
fC g mÿs H MH nrÿs Ln
:
(7)
The quantity T 1np = Hn L = MH nrÿs Ln is the r intrinsic complexation constant, indicated by 1np, which is independent of the experimental conditions [3,4]. The counter ion, C, is a monovalent cation, m is the charge of the metal ion and indicates the activity coef®cients of the species. Eq. (7) shows that the value of 1npex varies with the activity of the counter ion both in the solution and in the resin, and it can be calculated for the conditions considered. When one or several (N) complexing substances are present in solution, each with the following complexation equilibrium: M gHz Y $ MHy Yg xH;
(8)
the side reaction coef®cient, , i.e. the fraction of total to free metal ion in solution [3,11], is given by
(4)
(5)
N X 0
N X Hz Yg MHy Yg =M gy ; Hx 0
(9)
where gy is the complexation constant in solution. In this case the distribution ratio of the metal ion, KD, depends also on the complexation in solution
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M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
2. Experimental
A GBC 908 graphite furnace-based AA spectrophotometer was used for copper determinations when the metal was present at trace level. The analyses were carried out under standard conditions. The linearity range was from 5 to 50 mg lÿ1 and the detection limit was around 0.4 mg lÿ1. All the samples and the calibration solutions were supplied with nitric acid to a ®nal concentration of 0.1 mol kgÿ1. In samples containing 10ÿ5±10ÿ4 mol kgÿ1 copper(II) the metal concentration was sometimes measured by a colorimetric procedure [13] using a Jasco 7800 spectrophotometer, by the standard additions method.
2.1. Materials
2.3. Determination of the breakthrough profile
Complexing resins. Chelex 100 was obtained from Bio-Rad, Richmond, CA, in the sodium form (particle size 100±200 mesh), whereas Amberlite CG50 was obtained from Aldrich, Steinheim, in the acidic form. Both the resins were cleaned by treating with 0.1 mol kgÿ1 HNO3, and Chelex 100 was then transformed into ammonium form. Their sorption properties have been previously investigated [2±4,6,12]. The total capacity of the resins, i.e. the number of active groups per g of dry resin, is 2 and 10 for Chelex 100 and Amberlite CG50, respectively. All other reagents were of analytical reagent grade. Columns. Pyrex columns with an internal diameter of 0.5 and 1 cm, equipped with a PTFE plug and a removable PTFE stopcock, were used. The column with é1 cm was also equipped with the EconoColumn ¯ow adaptor. The ¯ow rate was always 1 ml minÿ1. Different amounts of the resins were inserted in the columns (0.2±1.25 g) giving different bed heights from 1.5 to 8.5 cm depending also on the size of the column and the composition of the mobile phase. Vm was determined by passing in the column a solution containing a negatively charged azo-dye (Chromotrope 2B) which is not sorbed by the cationic exchange resins considered here. Vs was obtained by difference from the total bed volume.
The breakthrough curves (Figs. 1 and 4, see below) were obtained by plotting the copper(II) concentration in the eluate against the volume of eluate. Before each experiment the column was conditioned with a solution at the same composition of that eluted, but not containing the metal ion. For a good conditioning 100±150 mobile phase volumes were required. The fractions of eluate were collected and then analyzed separately by graphite furnace atomic absorption spectroscopy (GF-AAS) or colorimetric method.
according to the following relationship:
KD K w=Vs :
(10)
The lower is, the higher should be VC, and thus the volume of solution which can be passed through the column with quantitative sorption of metal ion. On the contrary the sorption of the metal ions is practically zero from solutions in which the side reaction coef®cient is higher than 10 Kw/Vs. In the absence of any ligand, when only the hydrated ion is present in solution, is equal to 1.
2.2. Apparatus The pH of the solutions was measured with a combined Orion glass electrode 9102 SC.
2.4. Determination of the sorption curves The sorption curves were obtained by plotting the amount of copper(II) sorbed on the column (qads, mmol) from different volumes of solution passed through the column, against the volumes themselves (in ml). Increasing volumes of solution, under the selected conditions, were passed through the column, which was then washed with water and eluted with 15 ml of nitric acid 0.2 mol kgÿ1, in order to recover the copper(II) sorbed. This was determined in the eluate by one of the methods described above. A few of these graphs are reported in Fig. 2. Two straight lines can be detected. The ®rst one, which is obtained at low volumes of sorbed solution, has an intercept equal to zero and a slope corresponding to the concentration of copper(II) in the sample (CTOT), whereas the second one, obtained when the volumes of solution passed through the column (V) are high, is independent of the volume.
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
63
Fig. 2. Sorption curves of copper(II) on Chelex 100 (0.25 g of dry resin); flow rate 1 ml minÿ1, CCu1.010ÿ4 mol kgÿ1; curve 1, obtained in the absence of ligands, pH1.3, 0.1 mol kgÿ1 NaNO3; curve 2, obtained in the presence of IDA 0.35 mol kgÿ1, pH3.016, 0.1 mol kgÿ1 NaNO3; curve 3, obtained in the presence of EDTA 1.010ÿ3 mol kgÿ1, pH8.95, 1 mol kgÿ1 NaNO3.
The ®rst straight line is obtained when solution volumes smaller than the breakthrough volume Vb are passed through the column. Here the sorption of copper(II) on the column is quantitative and takes place according to the following equation: qads CTOT V:
(11)
The maximum amount of the metal ion which can be sorbed on resin, qM, at given conditions, is as follows: qM CTOT VC :
(12)
For volumes of solution passed through the column much higher than VC the amount of metal ion sorbed on resin, qads, is always constant and equal to qM. From the intercept of the two straight lines, when qadsqM, VC can be evaluated. 3. Results and discussion 3.1. Position of the breakthrough curves of copper(II) on complexing resins In Fig. 1 the breakthrough curves of copper(II) on a Chelex 100 column from 0.1 mol kgÿ1 (H,Na)NO3 at
different acidity, pH1.3 and pH1.5, are presented as an example. Other systems were examined as well and the results are reported in Table 1, in terms of VC experimentally obtained in the different experiments. They are compared to VC calculated by Eqs. (1) and (10), with 1, since no ligand is present in the solutions considered here. This comparison is made to show that VC can be effectively predicted by the proposed model. Columns containing two resins, i.e. Chelex 100 and Amberlite CG50, were examined. A constant copper(II) concentration in the eluate, corresponding to the total concentration in the original solution, was always obtained with a suf®ciently large solution volume passed through the column. From the experimental breakthrough curves the central point VC was obtained. Two concentration levels of copper(II) were considered. In the experiments with Chelex 100 the total original metal in the synthetic solution was 9.010ÿ5 mol kgÿ1 and copper(II) in the eluate was detected by the spectrophotometric method reported in Section 2. In the determinations with Amberlite CG50 the copper(II) concentration was
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Table 1 Elution of copper(II) solutions not containing any ligand on columns of complexing resins Column
Solution pH
Ionic composition of solution
VC experimental (ml)
VC calculated (ml)a
K
0.2 g of dry Amberlite CG50, Vm0.17 ml 0.5 g of dry Amberlite CG50, Vm0.47 ml 1.25 g of dry Chelex 100, Vm0.40 ml 0.25 g of dry Chelex 100, Vm0.17 ml 0.25 g of dry Chelex 100, Vm0.17 ml 0.25 g of dry Chelex 100, Vm0.17 ml 0.25 g of dry Chelex 100, Vm0.17 ml
3.95 3.95 1.5 1.5 1.3 1.3 1.5
1 mol kgÿ1 NaNO3 1 mol kgÿ1 NaNO3 0.1 mol kgÿ1 NaCl 0.1 mol kgÿ1 NaCl 0.1 mol kgÿ1 NaCl 0.1 mol kgÿ1 NaNO3 0.1 mol kgÿ1 NaNO3
24 52 750 250 81 41 101
26.2 55.7 542 108 43 43 106
130.5 130.5 434 434 173 173 434
CCu9.010ÿ5 mol kgÿ1 for experiments with Chelex 100. CCu5.010ÿ6 mol kgÿ1 for experiments with Amberlite CG50. Calculated using K values reported in the last column.
a
5.010ÿ6 mol kgÿ1, and the metal was determined by GF-AAS. Similar results were obtained, independent of the concentration of metal ion, as expected from the model. In any case the active groups in the resin were in large excess compared to the amount of metal introduced in the column and the position of the breakthrough curve should depend on KD according to Eq. (1). Moreover, the active groups complexed by the metal ions were always negligible with respect to the total, and their concentration was constant and easily evaluated. For instance in 0.25 g of Chelex 100 there are 0.5 mmol of active groups, while in 0.5 l of a 1.010ÿ4 mol kgÿ1 only 0.05 mmol are present, so that the active groups complexed are only a 10% of the total. In Table 1, last column, the values of K, calculated from the intrinsic complexation constants of copper(II) for Chelex 100 and Amberlite CG50 by Eqs. (6) and (7), are reported. The sorbing properties of these resins have been previously investigated by a batch procedure and the values of the intrinsic protonation constants, Ka, and the intrinsic complexation constants of copper(II), 1np, are as follows: for Chelex 100 [4], log Ka19.1, log Ka23.2, log 110ÿ0.68, log 120ÿ6.00, and for Amberlite CG50 [6,12], log Ka4.8, log 110ÿ3.0, log 120 ÿ7.7, log 12ÿ1ÿ13.4. The exchange coef®cients required for calculating K under the given conditions depend both on the concentration of the counter ion inside the resin and on that in the external solution according to Eq. (7). The concentration of the counter ion, Na, inside the resin was evaluated as previously described in the case of
Chelex 100 [2]. At the pH of the experiments reported in Table 1, the concentration of the counter ion Na inside the resins is mainly determined by the diffusion from the solution since the active groups are completely protonated. This means that Na fNa g, and that the exchange and protonation coef®cients are equal to the intrinsic constants. Once K is known, VC can be evaluated according to Eqs. (1), (6) and (10). This is also reported in Table 1, and it is in acceptable agreement with that experimentally obtained in the case of NaNO3 solutions, showing that the model is valid for the prediction of the position of the breakthrough curve in the absence of ligands. Values somewhat higher than predicted, about twice, were obtained when 0.1 mol kgÿ1 NaCl solutions were eluted. This can be ascribed to the inaccuracy in the evaluation of the exchange coef®cients at these conditions, or to a different sorption mechanism in the resin phase in the presence of chloride ions. Anionic complexes can be formed, for example MClÿ 3, directly inside the resin or in solution phase. This can be sorbed in the resin phase since at the acidity considered (pH 1.3±1.5) Chelex 100 can also be an anionic exchanger. As a matter of fact the aminogroups are protonated at these acidities. 3.2. Comparison of the breakthrough and elution curves: evaluation of the preconcentration factor As previously observed [5,6] Eq. (1) is similar to that for the maximum elution volume, VE. To con®rm this point an example is reported in Fig. 3. Five ml of a
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
65
Fig. 3. Elution curve of copper(II) on Chelex 100 (0.25 g of dry resin); amount of copper(II) sorbed: 4.510ÿ4 mmol, pH1.5, C(H,Na)Cl0.1 mol kgÿ1; flow rate 1 ml minÿ1. Eluting solution: pH1.5, C(H, Na)Cl0.1 mol kgÿ1.
0.1 mol kgÿ1 (H,Na)Cl solution at pH1.5 containing copper(II) 9.010ÿ5 mol kgÿ1 was sorbed on a column containing 1.25 g of dry Chelex 100. These conditions are equal to those of an experiment reported in Table 1, for which VC was found to be 750 ml. Thus it is expected that copper(II) is quantitatively sorbed when only 5 ml of solution is sorbed on the column. The column was washed with water and eluted with the same (H,Na)Cl 0.1 mol kgÿ1 solution at pH1.5, the fractions of eluate were collected and the concentration of metal ion in each was determined. The elution curve reported in Fig. 3 shows that the maximum elution volume is at 710 ml, corresponding to the center of the breakthrough curve reported in Table 1. Copper(II) was recovered quantitatively. In other experiments copper(II) sorbed in the column was eluted with more acidic solutions at different compositions to recover the metal into a lower volume, so eventually obtaining a preconcentration. An example is that obtained from the last experiment reported in Table 1 (0.25 g Chelex 100, pH1.5, 0.1 mol kgÿ1 (H,Na)NO3), for which VC101 ml was obtained. The amount of copper(II) sorbed was 9.110ÿ3 mmol. The column was eluted with 0.2 mol kgÿ1 HNO3, obtaining VE3.5 ml in agree-
ment with that calculated at these conditions (2.7 ml, K being 10.9). The total recovery of copper(II) was obtained with only 13 ml of eluting solution. The best concentration factor obtained in the experiments was 40, obtained as following: a copper(II) solution 2.010ÿ4 mol kgÿ1 (400 ml, 0.1 mol kgÿ1 (Na,H)Cl, pH1.5) was passed on a Chelex 100 column (1.25 g of dry resin). A quantitative sorption of the metal should be obtained since the breakthrough volume was not exceeded, as seen from VC reported in Table 1 for an experiment carried out at the same conditions. Copper(II) sorbed on resin was eluted with hydrochloric acid 0.2 mol kgÿ1. The quantitative recovery of the metal was obtained with 10 ml of acidic solution, resulting in a good concentration factor. 3.3. Evaluation of VC from the sorption curves The sorption curves were obtained by plotting the amount of copper(II) sorbed on the resin against the volume of solution passed in the column, as described in Eq. (11). Some examples are reported in Fig. 2. Curve 1, for example, is the sorption curve of copper(II) on Chelex 100 under the same conditions of curve 1 in Fig. 1, i.e. that with pH1.3 and
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M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
Fig. 4. Breakthrough profiles of copper(II) in the presence of ligands on Amberlite CG50 (0.5 g of dry resin); 0.1 mol kgÿ1 NaNO3, CCu5.010ÿ6 mol kgÿ1, flow rate 1 ml minÿ1; curve 1: oxalic acid 5.210ÿ4 mol kgÿ1, pH 4.58; curve 2: ethylenediamine 1.010ÿ3 mol kgÿ1, pH 3.97; curve 3: glycine 5.010ÿ4 mol kgÿ1, pH6.70.
(H,Na)NO30.1 mol kgÿ1. The value of VC obtained from the intersection of the two straight lines, according to Eqs. (11) and (12), is the same as that evaluated from the center of the breakthrough curve (VC40 ml; Fig. 1, curve 1). This method for evaluating VC is useful when the breakthrough curve cannot be obtained directly, because the metal ion concentration is too low or interfering substances are present. 3.4. Breakthrough curves of copper(II) on complexing resins in the presence of ligands The effect of the presence of complexing substances in solution was examined. In Fig. 4 the breakthrough curves of copper(II) from 0.1 mol kgÿ1 NaNO3 solutions at different pH, and in the presence of some ligands at known concentration, glycine, oxalic acid and ethylenediamine, are reported. In all the assays the same resin was used, i.e. Amberlite CG50, and the metal in the different fractions of eluate was determined by GF-AAS. The copper(II) concentration was 5.010ÿ6 mol kgÿ1 and the ligand concentration was
never lower than 5.010ÿ4 mol kgÿ1. This is important because when the ligand is in large excess compared to the metal ion the side reaction coef®cient is easily calculated by Eq. (9), where the total and free ligand concentration are equal. Fig. 4 shows the expected effect of a ligand in solution on VC, since that calculated assuming 1 has a value different from that experimentally obtained. For example curve 3 in Fig. 4, obtained in the presence of 5.010ÿ4 mol kgÿ1 glycine, in which copper(II) has a side reaction coef®cient as high as 5.6102, shows VC429 ml, while under the same conditions VC should be 2.36105 ml, if not any ligand were present ( 1). Notice that, given the concentration of copper(II) in the sample here considered, the total capacity of the resin is not exceeded neither in the presence of ligand nor in the absence, so that Eq. (1) holds for VC. Thus the model proposed is particularly useful when metal ions at trace levels are considered. Of course, the presence of a weak ligand as ethylenediamine which produces a side reaction coef®cient not different from 1 does not have any effect on the position of the breakthrough pro®le as seen in curve 2. In Table 2 the values of K, ,
Column
Solution Ligand pH
0.2 g of dry Amberlite CG50, Vm0.3 ml 0.5 g of dry Amberlite CG50, Vm0.47 ml 0.5 g of dry Amberlite CG50, Vm0.47 ml 0.256 g of dry Chelex 100, Vm0.24 ml 1.25 g of dry Chelex 100, Vm1.2 ml 0.259 g of dry Chelex 100, Vm0.19 ml 0.25 g of dry Chelex 100, Vm0.17 ml 1.25 g of dry Chelex 100, Vm1.37 ml 1.25 g of dry Chelex 100, Vm1.37 ml 1.25 g of dry Chelex 100, Vm1.57 ml 0.25 g of dry Chelex 100, Vm0.19 ml 0.25 g of dry Chelex 100, Vm0.14 ml 0.25 g of dry Chelex 100, Vm0.27 ml
6.700 3.970 4.580 3.020 3.014 3.009 5.050f 5.020f 5.020f 7.200 7.040 6.700 8.950
Glycine 510ÿ4 mol kgÿ1 Ethylenediamine 10ÿ3 mol kgÿ1 Oxalic acid 5.210ÿ4 mol kgÿ1 IDA 0.35 mol kgÿ1 IDA 0.35 mol kgÿ1 IDA 0.1 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1 EDTA 510ÿ4 mol kgÿ1 EDTA 510ÿ4 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1
Ionic composition of solution NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3
VC experimental (ml)
1 mol kgÿ1 429 0.1 mol kgÿ1 72 0.1 mol kgÿ1 6 1 mol kgÿ1 28 1 mol kgÿ1 128 1 mol kgÿ1 108 0.1 mol kgÿ1 0.62 0.1 mol kgÿ1 2.53 0.1 mol kgÿ1 2.26 0.1 mol kgÿ1 2.41 1 mol kgÿ1 0.45 1 mol kgÿ1 0.6 1 mol kgÿ1 8
CCu9.010ÿ5 mol kgÿ1 for experiments with Chelex 100. CCu5.010ÿ6 mol kgÿ1 for experiments with Amberlite CG50. a Calculated by Eq. (9) from the literature data [23,24]. b Calculated by Eq. (9) from the literature data [21,22]. c Calculated by Eq. (9) from the literature data [20]. d Calculated by Eq. (9) from the literature data [14±16]. e Calculated by Eq. (9) from the literature data [17±19]. f pH in the presence of 0.01 mol kgÿ1 CH3COOH/CH3COOÿ.
VC calculated (ml)
K
calculated
420 76 7 27 129 109 0.34 1.54 3.02 11.9 1.6 0.92 82.6
1.18106 158 2446 2.02106 1.95106 1.87106 9.781010 8.371010 8.371010 2.511015 1.711015 2.501014 2.841018
5.63102a 1.04b 1.85102c 1.93104d 1.90104d 4306d 1.451011e 1.451011e 6.331010e 3.051014e 2.011014e 8.001013e 8.631015e
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
Table 2 Elution of copper(II) solutions containing ligands on columns of complexing resins
67
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M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
experimental VC, and calculated VC are reported for a number of different solution eluted. K is calculated by the method previously described (Eqs. (6) and (7)), while is evaluated from the complexation and protonation constants of the ligands reported in the literature [14±24]. The experimental VC was obtained from the breakthrough curves in the assays with Amberlite CG50, but in all other experiments of Table 2 with Chelex 100 it was evaluated from the sorption curves, some examples of which are reported in Fig. 2. Here curve 2 was obtained in the presence of iminodiacetic acid (IDA) and curve 3 in the presence of EDTA. This procedure is much more time consuming than that based on the direct determination of the breakthrough curve. However, it allows the separation of the metal ion from the matrix and its preconcentration. This can be of great help when the detection limits must be improved. Furthermore the removal of the matrix allows the best determination of the metal in solution because of the elimination of some potential interferences. The results reported in Table 2 show that the values of VC calculated by Eqs. (1), (6) and (10), almost always agree with the experimental values, with the exception of the assay in which the ligand in solution is EDTA at basic pH. Values of VC much lower than those calculated were obtained for instance at pH8.95. This is the maximum difference found in all the experiments. In some of the assays in the presence of EDTA very low values of VC were obtained, 1±2 ml. The experimental uncertainty in this case is remarkable, but the results seem to be in fairly good agreement. The difference found in the case of basic EDTA solutions can be ascribed to an inaccurate knowledge of the EDTA complexation at high pH. The results here obtained are in agreement with those reported by other authors in a work on the heavy metal ions speciation in the land®ll leachates [25]. In that work different amounts of Chelex 100 were used to sorb copper(II) by a batch procedure from a model solution at pH8, containing EDTA 1.0 10ÿ3 mol kgÿ1, and at ionic strength 0.25 mol kgÿ1. Treating those data according to a method previously proposed [9] the value of was found to be 1.241016, one magnitude order higher than that
evaluated from the complexation constants reported in the previous literature, 1.891015. Acknowledgements This work was ®nancially supported by the National Research Council (CNR) of Italy. References [1] C. Kantipuly, S. Katragadda, A. Chow, H.D. Gesser, Talanta 37 (1990) 491. [2] M. Pesavento, R. Biesuz, M. Gallorini, A. Profumo, Anal. Chem. 65 (1993) 2522. [3] M. Pesavento, R. Biesuz, React. Polym. 36 (1998) 135. [4] M. Pesavento, R. Biesuz, Anal. Chim. Acta 346 (1997) 381. [5] M. Pesavento, A. Profumo, A. Sastre, Talanta 41 (1994) 1689. [6] M. Pesavento, R. Biesuz, J.L. Cortina, Anal. Chim. Acta 298 (1994) 225. [7] Y. Liu, J.D. Ingle, Anal. Chem. 61 (1989) 525. [8] K.A. Das, R. Chakrabotry, Fresnius' J. Anal. Chem. 357 (1997) 1. [9] F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962, p. 466. [10] W. Stumm, J.J. Morgan, Aquatic Chemistry, Chapter 6, Wiley, New York, 1996. [11] M. Pesavento, R. Biesuz, Anal. Chem. 67 (1995) 3558. [12] M. Pesavento, unpublished results. [13] T. Soldi Fulle, C. Riolo Bertoglio, G. Gallotti, M. Pesavento, Gazzetta Chim. Ital. 107 (1977) 347. [14] A. Hulanicki, T. Kranczykeld, Anal. Chim. Acta 58 (1984) 343. [15] B.E. Leach, R.J. Angelici, Inorg. Chem. 8 (1969) 907. [16] K. Venkatachalapathi, M.S. Nair, D. Ramaswany, M. Santappa, J. Chem. Soc., Dalton Trans. (1982) 291. [17] G. Anderegg, S.C. Malik, Helv. Chim. Acta 59 (1976) 1498. [18] E.W. Baumann, J. Inorg. Nucl. Chem. 36 (1974) 1827. [19] S. Yamaguchi, N. Oyama, K. Ikeda, Bull. Chem. Soc. Jpn. 54 (1981) 373. [20] M. Grasso, S. Musumeci, E. Rizzarelli, S. Sammartano, Ann. Chim. (Rome) 70 (1980) 193. [21] G. Anderegg, Inorg. Chim. Acta 111 (1986) 25. [22] M. Branca, G. Micera, T. Kiss, M. Sinibaldi, J. Chem. Res. (1990) 392. [23] T. Kiss, I. Sovago, A. Gergely, Pure Appl. Chem. 63 (1991) 597. [24] G. Berthon, M.J. Blais, M. Piktas, K. Houngbossa, J. Inorg. Biochem. 20 (1984) 113. [25] M. Majone, M.P. Papini, E. Rolle, Environ. Technol. 17 (1996) 587.
Study of sorption of copper(II) on complexing resin columns by solid phase extraction Maria Pesaventoa,*, Enrica Baldinib a
Dipartimento di Chimica Generale, UniversitaÁ di Pavia, V. Taramelli 12, 27100 Pavia, Italy b Istituto di Scienze Mat. Fis. Chim., UniversitaÁ di Milano, V. Lucini 3, Como, Italy
Received 7 August 1998; received in revised form 18 January 1999; accepted 25 January 1999
Abstract The position of the breakthrough pro®le of a substance sorbed on solid phase columns is given by the central point of the breakthrough curve, VC. This is the volume of eluted solution for which the concentration of the substance is half of that present in the original solution. VC depends on the characteristics of the column (volume of the stationary and mobile phase) and on the distribution ratio KD. In the case of the sorption of metal ions on complexing resins, the distribution ratio can be evaluated from the exchange coef®cients of the metal ion on the resin, which in turn depend on the conditions considered. In the present paper the dependence of the exchange coef®cients on the conditions (acidity and composition of the solution, composition of the resin phase) is accounted for by the Gibbs±Donnan model for the ion-exchange resin. In this work it is shown that KD calculated on the basis of the model is suf®ciently accurate to allow the prediction of VC. The sorption of copper(II) on column of complexing resin was studied by determining the breakthrough curves, whose position on the eluted volume axis was evaluated by the central point VC. Two complexing resins were considered, with different sorbing properties towards copper(II), Chelex 100 and Amberlite CG50, containing iminodiacetic and carboxylic groups, respectively. It was found that the values of VC calculated from the model are in acceptable agreement with those obtained experimentally under different conditions. The presence in solution phase of substances able to complex copper(II) was also examined. They have an in¯uence on KD and VC as expected on the basis of the model, considering the values of the side reaction coef®cients. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Copper; Solid phase extraction; Amberlite CG50; Chelex 100
1. Introduction The solid phase extraction (SPE) of trace metals is often carried out using resins containing chemically immobilized complexing groups because of welldocumented advantages over simple ion-exchange *Corresponding author. Tel.: +39-382-507-580; fax: +39-382528-544; e-mail: [email protected]
materials or hydrophobic phases on which ionic or neutral complexes are sorbed [1]. Column or batch procedures can be used for the extraction, which is characterized by the distribution ratio KD, i.e. the ratio of the concentration of the metal ion in the two phases. It has been recently shown that this quantity can be predicted using the Gibbs±Donnan model for the resin [2±4], when the ``intrinsic complexation constants'' of the metal ion on the resin
0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 1 2 4 - 5
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M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
Fig. 1. Breakthrough profiles of copper(II) on Chelex 100 (0.25 g dry resin); CCu9.010ÿ5 mol kgÿ1; 0.1 mol kgÿ1 NaNO3; flow rate 1 ml minÿ1; curve 1, pH1.3; curve 2, pH1.5.
are known. They are independent of the experimental conditions, while not being thermodynamic quantities, and can be used to characterize the sorption of a metal ion on a given resin. The distribution ratio at particular conditions can be evaluated from the intrinsic complexation constants on the basis of the Gibbs±Donnan model of the resin [2±4], and in this way the time consuming experimental determination can be avoided. The validity of this approach in the case of simple solutions not containing any ligand has been previously shown in the case of particular complexing resins, i.e. anion exchange resins loaded with anionic ligands [5], and a resin with ®xed carboxylic groups, Amberlite CG50 [6]. On the other hand, the column procedure is more widely used in SPE of metal ions particularly since it is easily implemented in on-line preconcentration apparatuses [7,8]. Thus in the present paper an attempt is described of characterizing the sorption of metal ions on complexing resins by column procedure by the same model previously used for the batch procedure. Two resins are examined. One of them, Chelex 100, is widely used because it strongly sorbs transition metal ions due to the presence of iminodiacetic groups. For
comparison a second resin is also examined containing the less strongly complexing carboxylate group, Amberlite CG50, which is also commercially available and used for particular separation purposes. The sorption properties of both the resins for copper(II) are known from previous investigations [2±4,6]. The sorption of metal ions on the column can be well characterized by the breakthrough pro®le, i.e. the graph reporting the concentration of the metal ion in the eluate in function of its volume. An example obtained in the case of copper(II) on Chelex 100 is reported in Fig. 1. The volume of eluate corresponding to a concentration in the eluate which is half of the total concentration is indicated by VC, and characterizes the position of the breakthrough curve. It must be noticed that VC is different from the breakthrough volume, Vb, i.e. the volume for which the metal ion appears in the eluate, which depends also on the shape of the breakthrough pro®le. VC and Vb depend on the acidity, and on the composition, of the solution, and in the cases presented in Fig. 1 neither of them correspond to the total number of complexing groups present in the resin. When the number of mmoles of metal in VC is lower than the number of mmoles of active groups in the resin it has been demonstrated [9]
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
tion ratio is
that VC Vm KD Vs ;
(1)
where Vm and Vs indicate the volume of the mobile and stationary phases, in ml, respectively. The distribution ratio is KD cs =cm ;
(2)
where cs and cm are the concentration of the metal ion in the stationary and mobile phases, respectively. In the case of resins containing complexing groups, as Chelex 100 and Amberlite CG50, it is possible to evaluate KD from the intrinsic complexation constants of the metal ion on the resin [2±4,6], as will be shown in the next paragraph. Thus, also the position of the breakthrough curve of a metal ion on a given resin can be calculated by Eq. (1). Of course, this is of interest when performing SPE of metal ions on complexing resins, since for example the volume of solution which can be passed through the column with quantitative sorption can be evaluated at least approximately. In the present paper simple solutions were considered as well as solutions containing complexing substances. This is important when the resins are used for real samples, where the metal ions are often complexed, particularly in the case of natural waters [10]. 1.1. Model for the sorption of a metal ion on a complexing resin The value of KD which is required for calculating VC by Eq. (1) can be experimentally obtained from a determination in batch exactly at the same conditions, and for each set of conditions. However, in the case of sorption of metal ions on a resin containing complexing groups HrL, KD can also be calculated for each particular set of conditions from known quantities as described here. It has been demonstrated [2±4,6] that the sorption equilibrium in this case is M nHr L $ MHp Ln sH and the corresponding exchange coef®cient is MHp Ln Hs n : 1npex M Hr L
61
(3)
KD 1npex Hr Ln w=Hs Vs ;
where w indicates the grams of water sorbed in the stationary phase. When more than one complex is formed in the resin it is convenient to indicate the ratio of the concentration of the metal sorbed on resin to that of the free metal in solution by K, which is given by the following equation [3,11]: n P X 1npex Hr L : (6) K Hs 0 The exchange coef®cients strongly depend on the composition of the resin and of the solution phase. It has been demonstrated [3±6] that the exchange coef®cient 1npex is related to the thermodynamic complexation constant inside the resin, T(1np), by the following relationship, derived from the Gibbs± Donnan model: 1npex T 1np
The barred species are those inside the resin and all the concentrations are in mol kgÿ1. Thus the distribu-
g fC
mÿs
M Hn L r
fC g mÿs H MH nrÿs Ln
:
(7)
The quantity T 1np = Hn L = MH nrÿs Ln is the r intrinsic complexation constant, indicated by 1np, which is independent of the experimental conditions [3,4]. The counter ion, C, is a monovalent cation, m is the charge of the metal ion and indicates the activity coef®cients of the species. Eq. (7) shows that the value of 1npex varies with the activity of the counter ion both in the solution and in the resin, and it can be calculated for the conditions considered. When one or several (N) complexing substances are present in solution, each with the following complexation equilibrium: M gHz Y $ MHy Yg xH;
(8)
the side reaction coef®cient, , i.e. the fraction of total to free metal ion in solution [3,11], is given by
(4)
(5)
N X 0
N X Hz Yg MHy Yg =M gy ; Hx 0
(9)
where gy is the complexation constant in solution. In this case the distribution ratio of the metal ion, KD, depends also on the complexation in solution
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M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
2. Experimental
A GBC 908 graphite furnace-based AA spectrophotometer was used for copper determinations when the metal was present at trace level. The analyses were carried out under standard conditions. The linearity range was from 5 to 50 mg lÿ1 and the detection limit was around 0.4 mg lÿ1. All the samples and the calibration solutions were supplied with nitric acid to a ®nal concentration of 0.1 mol kgÿ1. In samples containing 10ÿ5±10ÿ4 mol kgÿ1 copper(II) the metal concentration was sometimes measured by a colorimetric procedure [13] using a Jasco 7800 spectrophotometer, by the standard additions method.
2.1. Materials
2.3. Determination of the breakthrough profile
Complexing resins. Chelex 100 was obtained from Bio-Rad, Richmond, CA, in the sodium form (particle size 100±200 mesh), whereas Amberlite CG50 was obtained from Aldrich, Steinheim, in the acidic form. Both the resins were cleaned by treating with 0.1 mol kgÿ1 HNO3, and Chelex 100 was then transformed into ammonium form. Their sorption properties have been previously investigated [2±4,6,12]. The total capacity of the resins, i.e. the number of active groups per g of dry resin, is 2 and 10 for Chelex 100 and Amberlite CG50, respectively. All other reagents were of analytical reagent grade. Columns. Pyrex columns with an internal diameter of 0.5 and 1 cm, equipped with a PTFE plug and a removable PTFE stopcock, were used. The column with é1 cm was also equipped with the EconoColumn ¯ow adaptor. The ¯ow rate was always 1 ml minÿ1. Different amounts of the resins were inserted in the columns (0.2±1.25 g) giving different bed heights from 1.5 to 8.5 cm depending also on the size of the column and the composition of the mobile phase. Vm was determined by passing in the column a solution containing a negatively charged azo-dye (Chromotrope 2B) which is not sorbed by the cationic exchange resins considered here. Vs was obtained by difference from the total bed volume.
The breakthrough curves (Figs. 1 and 4, see below) were obtained by plotting the copper(II) concentration in the eluate against the volume of eluate. Before each experiment the column was conditioned with a solution at the same composition of that eluted, but not containing the metal ion. For a good conditioning 100±150 mobile phase volumes were required. The fractions of eluate were collected and then analyzed separately by graphite furnace atomic absorption spectroscopy (GF-AAS) or colorimetric method.
according to the following relationship:
KD K w=Vs :
(10)
The lower is, the higher should be VC, and thus the volume of solution which can be passed through the column with quantitative sorption of metal ion. On the contrary the sorption of the metal ions is practically zero from solutions in which the side reaction coef®cient is higher than 10 Kw/Vs. In the absence of any ligand, when only the hydrated ion is present in solution, is equal to 1.
2.2. Apparatus The pH of the solutions was measured with a combined Orion glass electrode 9102 SC.
2.4. Determination of the sorption curves The sorption curves were obtained by plotting the amount of copper(II) sorbed on the column (qads, mmol) from different volumes of solution passed through the column, against the volumes themselves (in ml). Increasing volumes of solution, under the selected conditions, were passed through the column, which was then washed with water and eluted with 15 ml of nitric acid 0.2 mol kgÿ1, in order to recover the copper(II) sorbed. This was determined in the eluate by one of the methods described above. A few of these graphs are reported in Fig. 2. Two straight lines can be detected. The ®rst one, which is obtained at low volumes of sorbed solution, has an intercept equal to zero and a slope corresponding to the concentration of copper(II) in the sample (CTOT), whereas the second one, obtained when the volumes of solution passed through the column (V) are high, is independent of the volume.
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
63
Fig. 2. Sorption curves of copper(II) on Chelex 100 (0.25 g of dry resin); flow rate 1 ml minÿ1, CCu1.010ÿ4 mol kgÿ1; curve 1, obtained in the absence of ligands, pH1.3, 0.1 mol kgÿ1 NaNO3; curve 2, obtained in the presence of IDA 0.35 mol kgÿ1, pH3.016, 0.1 mol kgÿ1 NaNO3; curve 3, obtained in the presence of EDTA 1.010ÿ3 mol kgÿ1, pH8.95, 1 mol kgÿ1 NaNO3.
The ®rst straight line is obtained when solution volumes smaller than the breakthrough volume Vb are passed through the column. Here the sorption of copper(II) on the column is quantitative and takes place according to the following equation: qads CTOT V:
(11)
The maximum amount of the metal ion which can be sorbed on resin, qM, at given conditions, is as follows: qM CTOT VC :
(12)
For volumes of solution passed through the column much higher than VC the amount of metal ion sorbed on resin, qads, is always constant and equal to qM. From the intercept of the two straight lines, when qadsqM, VC can be evaluated. 3. Results and discussion 3.1. Position of the breakthrough curves of copper(II) on complexing resins In Fig. 1 the breakthrough curves of copper(II) on a Chelex 100 column from 0.1 mol kgÿ1 (H,Na)NO3 at
different acidity, pH1.3 and pH1.5, are presented as an example. Other systems were examined as well and the results are reported in Table 1, in terms of VC experimentally obtained in the different experiments. They are compared to VC calculated by Eqs. (1) and (10), with 1, since no ligand is present in the solutions considered here. This comparison is made to show that VC can be effectively predicted by the proposed model. Columns containing two resins, i.e. Chelex 100 and Amberlite CG50, were examined. A constant copper(II) concentration in the eluate, corresponding to the total concentration in the original solution, was always obtained with a suf®ciently large solution volume passed through the column. From the experimental breakthrough curves the central point VC was obtained. Two concentration levels of copper(II) were considered. In the experiments with Chelex 100 the total original metal in the synthetic solution was 9.010ÿ5 mol kgÿ1 and copper(II) in the eluate was detected by the spectrophotometric method reported in Section 2. In the determinations with Amberlite CG50 the copper(II) concentration was
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Table 1 Elution of copper(II) solutions not containing any ligand on columns of complexing resins Column
Solution pH
Ionic composition of solution
VC experimental (ml)
VC calculated (ml)a
K
0.2 g of dry Amberlite CG50, Vm0.17 ml 0.5 g of dry Amberlite CG50, Vm0.47 ml 1.25 g of dry Chelex 100, Vm0.40 ml 0.25 g of dry Chelex 100, Vm0.17 ml 0.25 g of dry Chelex 100, Vm0.17 ml 0.25 g of dry Chelex 100, Vm0.17 ml 0.25 g of dry Chelex 100, Vm0.17 ml
3.95 3.95 1.5 1.5 1.3 1.3 1.5
1 mol kgÿ1 NaNO3 1 mol kgÿ1 NaNO3 0.1 mol kgÿ1 NaCl 0.1 mol kgÿ1 NaCl 0.1 mol kgÿ1 NaCl 0.1 mol kgÿ1 NaNO3 0.1 mol kgÿ1 NaNO3
24 52 750 250 81 41 101
26.2 55.7 542 108 43 43 106
130.5 130.5 434 434 173 173 434
CCu9.010ÿ5 mol kgÿ1 for experiments with Chelex 100. CCu5.010ÿ6 mol kgÿ1 for experiments with Amberlite CG50. Calculated using K values reported in the last column.
a
5.010ÿ6 mol kgÿ1, and the metal was determined by GF-AAS. Similar results were obtained, independent of the concentration of metal ion, as expected from the model. In any case the active groups in the resin were in large excess compared to the amount of metal introduced in the column and the position of the breakthrough curve should depend on KD according to Eq. (1). Moreover, the active groups complexed by the metal ions were always negligible with respect to the total, and their concentration was constant and easily evaluated. For instance in 0.25 g of Chelex 100 there are 0.5 mmol of active groups, while in 0.5 l of a 1.010ÿ4 mol kgÿ1 only 0.05 mmol are present, so that the active groups complexed are only a 10% of the total. In Table 1, last column, the values of K, calculated from the intrinsic complexation constants of copper(II) for Chelex 100 and Amberlite CG50 by Eqs. (6) and (7), are reported. The sorbing properties of these resins have been previously investigated by a batch procedure and the values of the intrinsic protonation constants, Ka, and the intrinsic complexation constants of copper(II), 1np, are as follows: for Chelex 100 [4], log Ka19.1, log Ka23.2, log 110ÿ0.68, log 120ÿ6.00, and for Amberlite CG50 [6,12], log Ka4.8, log 110ÿ3.0, log 120 ÿ7.7, log 12ÿ1ÿ13.4. The exchange coef®cients required for calculating K under the given conditions depend both on the concentration of the counter ion inside the resin and on that in the external solution according to Eq. (7). The concentration of the counter ion, Na, inside the resin was evaluated as previously described in the case of
Chelex 100 [2]. At the pH of the experiments reported in Table 1, the concentration of the counter ion Na inside the resins is mainly determined by the diffusion from the solution since the active groups are completely protonated. This means that Na fNa g, and that the exchange and protonation coef®cients are equal to the intrinsic constants. Once K is known, VC can be evaluated according to Eqs. (1), (6) and (10). This is also reported in Table 1, and it is in acceptable agreement with that experimentally obtained in the case of NaNO3 solutions, showing that the model is valid for the prediction of the position of the breakthrough curve in the absence of ligands. Values somewhat higher than predicted, about twice, were obtained when 0.1 mol kgÿ1 NaCl solutions were eluted. This can be ascribed to the inaccuracy in the evaluation of the exchange coef®cients at these conditions, or to a different sorption mechanism in the resin phase in the presence of chloride ions. Anionic complexes can be formed, for example MClÿ 3, directly inside the resin or in solution phase. This can be sorbed in the resin phase since at the acidity considered (pH 1.3±1.5) Chelex 100 can also be an anionic exchanger. As a matter of fact the aminogroups are protonated at these acidities. 3.2. Comparison of the breakthrough and elution curves: evaluation of the preconcentration factor As previously observed [5,6] Eq. (1) is similar to that for the maximum elution volume, VE. To con®rm this point an example is reported in Fig. 3. Five ml of a
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
65
Fig. 3. Elution curve of copper(II) on Chelex 100 (0.25 g of dry resin); amount of copper(II) sorbed: 4.510ÿ4 mmol, pH1.5, C(H,Na)Cl0.1 mol kgÿ1; flow rate 1 ml minÿ1. Eluting solution: pH1.5, C(H, Na)Cl0.1 mol kgÿ1.
0.1 mol kgÿ1 (H,Na)Cl solution at pH1.5 containing copper(II) 9.010ÿ5 mol kgÿ1 was sorbed on a column containing 1.25 g of dry Chelex 100. These conditions are equal to those of an experiment reported in Table 1, for which VC was found to be 750 ml. Thus it is expected that copper(II) is quantitatively sorbed when only 5 ml of solution is sorbed on the column. The column was washed with water and eluted with the same (H,Na)Cl 0.1 mol kgÿ1 solution at pH1.5, the fractions of eluate were collected and the concentration of metal ion in each was determined. The elution curve reported in Fig. 3 shows that the maximum elution volume is at 710 ml, corresponding to the center of the breakthrough curve reported in Table 1. Copper(II) was recovered quantitatively. In other experiments copper(II) sorbed in the column was eluted with more acidic solutions at different compositions to recover the metal into a lower volume, so eventually obtaining a preconcentration. An example is that obtained from the last experiment reported in Table 1 (0.25 g Chelex 100, pH1.5, 0.1 mol kgÿ1 (H,Na)NO3), for which VC101 ml was obtained. The amount of copper(II) sorbed was 9.110ÿ3 mmol. The column was eluted with 0.2 mol kgÿ1 HNO3, obtaining VE3.5 ml in agree-
ment with that calculated at these conditions (2.7 ml, K being 10.9). The total recovery of copper(II) was obtained with only 13 ml of eluting solution. The best concentration factor obtained in the experiments was 40, obtained as following: a copper(II) solution 2.010ÿ4 mol kgÿ1 (400 ml, 0.1 mol kgÿ1 (Na,H)Cl, pH1.5) was passed on a Chelex 100 column (1.25 g of dry resin). A quantitative sorption of the metal should be obtained since the breakthrough volume was not exceeded, as seen from VC reported in Table 1 for an experiment carried out at the same conditions. Copper(II) sorbed on resin was eluted with hydrochloric acid 0.2 mol kgÿ1. The quantitative recovery of the metal was obtained with 10 ml of acidic solution, resulting in a good concentration factor. 3.3. Evaluation of VC from the sorption curves The sorption curves were obtained by plotting the amount of copper(II) sorbed on the resin against the volume of solution passed in the column, as described in Eq. (11). Some examples are reported in Fig. 2. Curve 1, for example, is the sorption curve of copper(II) on Chelex 100 under the same conditions of curve 1 in Fig. 1, i.e. that with pH1.3 and
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M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
Fig. 4. Breakthrough profiles of copper(II) in the presence of ligands on Amberlite CG50 (0.5 g of dry resin); 0.1 mol kgÿ1 NaNO3, CCu5.010ÿ6 mol kgÿ1, flow rate 1 ml minÿ1; curve 1: oxalic acid 5.210ÿ4 mol kgÿ1, pH 4.58; curve 2: ethylenediamine 1.010ÿ3 mol kgÿ1, pH 3.97; curve 3: glycine 5.010ÿ4 mol kgÿ1, pH6.70.
(H,Na)NO30.1 mol kgÿ1. The value of VC obtained from the intersection of the two straight lines, according to Eqs. (11) and (12), is the same as that evaluated from the center of the breakthrough curve (VC40 ml; Fig. 1, curve 1). This method for evaluating VC is useful when the breakthrough curve cannot be obtained directly, because the metal ion concentration is too low or interfering substances are present. 3.4. Breakthrough curves of copper(II) on complexing resins in the presence of ligands The effect of the presence of complexing substances in solution was examined. In Fig. 4 the breakthrough curves of copper(II) from 0.1 mol kgÿ1 NaNO3 solutions at different pH, and in the presence of some ligands at known concentration, glycine, oxalic acid and ethylenediamine, are reported. In all the assays the same resin was used, i.e. Amberlite CG50, and the metal in the different fractions of eluate was determined by GF-AAS. The copper(II) concentration was 5.010ÿ6 mol kgÿ1 and the ligand concentration was
never lower than 5.010ÿ4 mol kgÿ1. This is important because when the ligand is in large excess compared to the metal ion the side reaction coef®cient is easily calculated by Eq. (9), where the total and free ligand concentration are equal. Fig. 4 shows the expected effect of a ligand in solution on VC, since that calculated assuming 1 has a value different from that experimentally obtained. For example curve 3 in Fig. 4, obtained in the presence of 5.010ÿ4 mol kgÿ1 glycine, in which copper(II) has a side reaction coef®cient as high as 5.6102, shows VC429 ml, while under the same conditions VC should be 2.36105 ml, if not any ligand were present ( 1). Notice that, given the concentration of copper(II) in the sample here considered, the total capacity of the resin is not exceeded neither in the presence of ligand nor in the absence, so that Eq. (1) holds for VC. Thus the model proposed is particularly useful when metal ions at trace levels are considered. Of course, the presence of a weak ligand as ethylenediamine which produces a side reaction coef®cient not different from 1 does not have any effect on the position of the breakthrough pro®le as seen in curve 2. In Table 2 the values of K, ,
Column
Solution Ligand pH
0.2 g of dry Amberlite CG50, Vm0.3 ml 0.5 g of dry Amberlite CG50, Vm0.47 ml 0.5 g of dry Amberlite CG50, Vm0.47 ml 0.256 g of dry Chelex 100, Vm0.24 ml 1.25 g of dry Chelex 100, Vm1.2 ml 0.259 g of dry Chelex 100, Vm0.19 ml 0.25 g of dry Chelex 100, Vm0.17 ml 1.25 g of dry Chelex 100, Vm1.37 ml 1.25 g of dry Chelex 100, Vm1.37 ml 1.25 g of dry Chelex 100, Vm1.57 ml 0.25 g of dry Chelex 100, Vm0.19 ml 0.25 g of dry Chelex 100, Vm0.14 ml 0.25 g of dry Chelex 100, Vm0.27 ml
6.700 3.970 4.580 3.020 3.014 3.009 5.050f 5.020f 5.020f 7.200 7.040 6.700 8.950
Glycine 510ÿ4 mol kgÿ1 Ethylenediamine 10ÿ3 mol kgÿ1 Oxalic acid 5.210ÿ4 mol kgÿ1 IDA 0.35 mol kgÿ1 IDA 0.35 mol kgÿ1 IDA 0.1 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1 EDTA 510ÿ4 mol kgÿ1 EDTA 510ÿ4 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1 EDTA 10ÿ3 mol kgÿ1
Ionic composition of solution NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 NaNO3
VC experimental (ml)
1 mol kgÿ1 429 0.1 mol kgÿ1 72 0.1 mol kgÿ1 6 1 mol kgÿ1 28 1 mol kgÿ1 128 1 mol kgÿ1 108 0.1 mol kgÿ1 0.62 0.1 mol kgÿ1 2.53 0.1 mol kgÿ1 2.26 0.1 mol kgÿ1 2.41 1 mol kgÿ1 0.45 1 mol kgÿ1 0.6 1 mol kgÿ1 8
CCu9.010ÿ5 mol kgÿ1 for experiments with Chelex 100. CCu5.010ÿ6 mol kgÿ1 for experiments with Amberlite CG50. a Calculated by Eq. (9) from the literature data [23,24]. b Calculated by Eq. (9) from the literature data [21,22]. c Calculated by Eq. (9) from the literature data [20]. d Calculated by Eq. (9) from the literature data [14±16]. e Calculated by Eq. (9) from the literature data [17±19]. f pH in the presence of 0.01 mol kgÿ1 CH3COOH/CH3COOÿ.
VC calculated (ml)
K
calculated
420 76 7 27 129 109 0.34 1.54 3.02 11.9 1.6 0.92 82.6
1.18106 158 2446 2.02106 1.95106 1.87106 9.781010 8.371010 8.371010 2.511015 1.711015 2.501014 2.841018
5.63102a 1.04b 1.85102c 1.93104d 1.90104d 4306d 1.451011e 1.451011e 6.331010e 3.051014e 2.011014e 8.001013e 8.631015e
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
Table 2 Elution of copper(II) solutions containing ligands on columns of complexing resins
67
68
M. Pesavento, E. Baldini / Analytica Chimica Acta 389 (1999) 59±68
experimental VC, and calculated VC are reported for a number of different solution eluted. K is calculated by the method previously described (Eqs. (6) and (7)), while is evaluated from the complexation and protonation constants of the ligands reported in the literature [14±24]. The experimental VC was obtained from the breakthrough curves in the assays with Amberlite CG50, but in all other experiments of Table 2 with Chelex 100 it was evaluated from the sorption curves, some examples of which are reported in Fig. 2. Here curve 2 was obtained in the presence of iminodiacetic acid (IDA) and curve 3 in the presence of EDTA. This procedure is much more time consuming than that based on the direct determination of the breakthrough curve. However, it allows the separation of the metal ion from the matrix and its preconcentration. This can be of great help when the detection limits must be improved. Furthermore the removal of the matrix allows the best determination of the metal in solution because of the elimination of some potential interferences. The results reported in Table 2 show that the values of VC calculated by Eqs. (1), (6) and (10), almost always agree with the experimental values, with the exception of the assay in which the ligand in solution is EDTA at basic pH. Values of VC much lower than those calculated were obtained for instance at pH8.95. This is the maximum difference found in all the experiments. In some of the assays in the presence of EDTA very low values of VC were obtained, 1±2 ml. The experimental uncertainty in this case is remarkable, but the results seem to be in fairly good agreement. The difference found in the case of basic EDTA solutions can be ascribed to an inaccurate knowledge of the EDTA complexation at high pH. The results here obtained are in agreement with those reported by other authors in a work on the heavy metal ions speciation in the land®ll leachates [25]. In that work different amounts of Chelex 100 were used to sorb copper(II) by a batch procedure from a model solution at pH8, containing EDTA 1.0 10ÿ3 mol kgÿ1, and at ionic strength 0.25 mol kgÿ1. Treating those data according to a method previously proposed [9] the value of was found to be 1.241016, one magnitude order higher than that
evaluated from the complexation constants reported in the previous literature, 1.891015. Acknowledgements This work was ®nancially supported by the National Research Council (CNR) of Italy. References [1] C. Kantipuly, S. Katragadda, A. Chow, H.D. Gesser, Talanta 37 (1990) 491. [2] M. Pesavento, R. Biesuz, M. Gallorini, A. Profumo, Anal. Chem. 65 (1993) 2522. [3] M. Pesavento, R. Biesuz, React. Polym. 36 (1998) 135. [4] M. Pesavento, R. Biesuz, Anal. Chim. Acta 346 (1997) 381. [5] M. Pesavento, A. Profumo, A. Sastre, Talanta 41 (1994) 1689. [6] M. Pesavento, R. Biesuz, J.L. Cortina, Anal. Chim. Acta 298 (1994) 225. [7] Y. Liu, J.D. Ingle, Anal. Chem. 61 (1989) 525. [8] K.A. Das, R. Chakrabotry, Fresnius' J. Anal. Chem. 357 (1997) 1. [9] F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962, p. 466. [10] W. Stumm, J.J. Morgan, Aquatic Chemistry, Chapter 6, Wiley, New York, 1996. [11] M. Pesavento, R. Biesuz, Anal. Chem. 67 (1995) 3558. [12] M. Pesavento, unpublished results. [13] T. Soldi Fulle, C. Riolo Bertoglio, G. Gallotti, M. Pesavento, Gazzetta Chim. Ital. 107 (1977) 347. [14] A. Hulanicki, T. Kranczykeld, Anal. Chim. Acta 58 (1984) 343. [15] B.E. Leach, R.J. Angelici, Inorg. Chem. 8 (1969) 907. [16] K. Venkatachalapathi, M.S. Nair, D. Ramaswany, M. Santappa, J. Chem. Soc., Dalton Trans. (1982) 291. [17] G. Anderegg, S.C. Malik, Helv. Chim. Acta 59 (1976) 1498. [18] E.W. Baumann, J. Inorg. Nucl. Chem. 36 (1974) 1827. [19] S. Yamaguchi, N. Oyama, K. Ikeda, Bull. Chem. Soc. Jpn. 54 (1981) 373. [20] M. Grasso, S. Musumeci, E. Rizzarelli, S. Sammartano, Ann. Chim. (Rome) 70 (1980) 193. [21] G. Anderegg, Inorg. Chim. Acta 111 (1986) 25. [22] M. Branca, G. Micera, T. Kiss, M. Sinibaldi, J. Chem. Res. (1990) 392. [23] T. Kiss, I. Sovago, A. Gergely, Pure Appl. Chem. 63 (1991) 597. [24] G. Berthon, M.J. Blais, M. Piktas, K. Houngbossa, J. Inorg. Biochem. 20 (1984) 113. [25] M. Majone, M.P. Papini, E. Rolle, Environ. Technol. 17 (1996) 587.