Anal. Chem. 1993, 65, 2522-2527
2522
Sorption Mechanism of Trace Amounts of Divalent Metal Ions on a Chelating Resin Containing Iminodiacetate Groups Maria Pesavento,' Raffaela Biesuz, Mario Gallorini,t and Antonella Profumo Dipartimento di Chimica Generale, Uniuersitd di Pavia, Viale Taramelli, No. 12-27100 Pauia, Italy
The sorption of metal ions on chelating resins is believed to take place through complexation by the active groups. Thus the selectivity of the resin for different metals is expected to be comparable to the complexing properties of a monomer having a structure similar to that of the active groups. This correlation can be done in a quantitative way on the basis of the Gibbs-Donnan model, which allows prediction of the extraction coefficients of metal ions on a chelating resin from the complexation constants, and other quantities, such as the concentration of the counterion in solution. I n the case of chelating resins containing iminodiacetic groups, the selectivity toward some divalent metal ions is much worse than expected from the complexation by iminodiacetate in aqueous solution. Calcium for instance is sorbed at much lower pH than anticipated. Its sorption on acommercial chelating resin, Chelex 100, was examined by the Gibbs-Donnan model, and a complex Ca(HL)2 has been found to be formed, with a complexation constant log 8 2 e x ~ i= -5.1. Similar findings were obtained for zinc and cadmium. I n this case, the complexes ML and ML2, analogous to those formed in aqueous solution, are not negligible. The equilibrium constants of M(HL)2 was found to be log 8 2 e x ~ i= -3.6 and -3.5, respectively. I n the case of copper and nickel, the sorption mechanism involves only the formation of the complex ML and the extraction coefficients are in good agreement with those predicted from the complexation constants of the hydrosoluble analogue by the GibbsDonnan model, respectively, log B l e x L = -0.75 and log @lexL = -2.90, while log K1 = -0.68 and log K2 = -3.05 in aqueous solution. INTRODUCTION The chelating resins are ion-exchange resins containing groups which are also able to complex metal ions. Their sorption mechanism is supposed to be through chelation instead of simple ion exchange, and as a consequence, they should be muchmore selective than ion-exchangeresins. They are widely used for separation and preconcentration of metal ions particularly from complicated matrices, where high amounts of interfering ions are present. Resins containing iminodiacetic chelating groups are particularly popular and are commercially distributed from many companies. It has been claimed that their selectivity is a t least qualitatively in agreement with the complexation constants of similar chelating monomers with metal ions in aqueous solution. This is true only very roughly. It has been shownl-l that the sorption t
CentrodiRadiochimicaeAnalisiper Attivazione,UniveraitAdi Pavia,
Viale Taramelli,No. 12-27100Pavia, Italy.
0003-2700/93/0385-2522$04.00/0
Table I. Sorption of Divalent Metal Ions on Chelex 100 by a Batch Procedure, Predicted from the Gibbs-Donnan Model, Considering Methyliminodiacetic Acid as a Model* dry log log metal resin (g) Klb Kzb p h C c pHfouadC copper nickel zinc cadmium calcium
0.1879 0.8977 0.3900 0.2907 0.2246
-0.68 -3.04 -4.11 -4.93 -8.02
-5.62 -7.59 -9.45 -11.04
1.5 2.2 2.9 3.4 6.1
1.49 2.10 2.60 2.72 3.50
Conditions: 25 mL of 0.1 mol kgl NaNOs aqueous solution, active groups in large excess with respect to metal ions. Complexation constants in aqueous solution:'K~ = [MLI[H12/[M1[H2Ll;Kz = [MLzI[HlY[M] [HzLIz. c pH values at which 50% of total metal ion is sorbed on the resin. pHdc are the estimated values, pHfouad those experimentally obtained.
*
of metal ions on chelating resins can be accurately described by means of the Gibbs-Donnan mode1.616 It was previously used to describe and predict the sorption of metal ions on some particular resins, the loaded chelating resins,14 in which the active groups were introduced into strong base anion-exchange resins by anion exchange. The only possible sorption process for metal ions was through chelation by the active groups, according to the equilibrium
M +n
e
-
MH,,,L,
+ SH
where bars indicate species in the resin phase. In such systems the predictions made by considering that the same complexation equilibrium was set up in aqueous phase and in the resin were well verified in practical batch separations. The same model is now applied to the sorption of metal ions on a commercially available and widely used chelating resin, containing iminodiacetic groups, Chelex 100 (Bio-Rad Laboratories). Table I reports the pH values at which 50% of metal ion should be sorbed by Chelex 100under typical batch conditions if the same complexes as in aqueous solution were formed inside the resin, accordingto the Gibbs-Donnan model. These values are compared with those experimentally obtained from the sorption isotherms. The agreement is good in the case of copper and nickel, not good in the case of zinc and cadmium, and very bad in the case of calcium. Actually, even if the selectivity of Chelex 100 is qualitatively in agreement with the complexation constant values in aqueous solution, which are also reported in Table I, it is much worse than expected. For instance, one could expect from the complexation (1) Pesavento,M.; Profumo,A.; Biesuz,R. Talanta 1988,35,431-437. (2)Pesavento, M.; Profumo,A.; Riolo, C.; Soldi,T. Analyst 1989,114, 623-626. (3)Pesavento, M.; Biesuz, R. React. Polym. 1991,14,239-260. (4)Pesavento, M.; Soldi, T.; Profumo, A. Talanta 1992,39,943-951. (5)Catterjee,A,; Marinsky, J. A. J. Phys. Chem. 1963,67,41-50, (6)Merle, Y.;Marinsky, J. A. Talanta 1984,31, 199-204. (7) Schwarzenbach, G.;Anderegg, G.; Schneider, W.; Senn, H. Helu. Chim. Acta 1956,132,1147-1170. 0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 19Q3 2523
constants in aqueous solution that calcium is excluded from the resin at p H = 4, while in practice it is quantitatively sorbed. This means that a different complexation mechanism is involved. The aim of this investigation is the elucidation of the sorption mechanism on Chelex 100, trying to quantitatively explain the observed selectivities, and making predictions about the distribution equilibria of metal ions between aqueous solutions of different composition and Chelex 100. EXPERIMENTAL SECTION Apparatus and Reagents. A PHM 84 Research pH meter, Radiometer Copenaghen,with ABU 80autoburet and a combined Orion glass electrode (91-02) were used for the acid-base titrations. A Perkin-Elmer ll00B flame AAS was used for metal ionsdeterminations. All chemicalswere analytical-reagentgrade. Preparation of Chelex 100 in E+Form. Chelex 100 (BioRad Laboratories, Richmond, CA) with a particle size range of 100-200mesh,obtained in sodiumform,was washed with distilled water and convertedto the hydrogen form by a column procedure, by passing 10 bed volumes of 0.1 M HCl. Then the column was washed with bidistilled water to neutrality and air-dried. Known amounts were sealed in tubes, and the water content was determined by drying a known amount in an oven at 80 O C to constant weight. Acid-Base Titration and Determination of the Capacity of the Resin The capacity of an ion exchangeris the number of ion-exchange sites per unit mass or volume. In the case of Chelex 100, the total capacity was defied as the amount of iminodiacetic groups per gram of dry resin obtained from acidbase titration. This was performed in a conventional way: a known amount of Chelex 100 in H+form was dispersed in a fixed volume of aqueous solution at fixed composition, containing a known amount of acid. The temperature was kept constant at 25 OC, and a small nitrogen overpressure was applied. The equilibrium was reached usually after 1-2 h, and then known amounts of NaOH were added and the equilibrium pH was measured by a glass electrode standardized in H+concentration.8 Each titration was carried out up to pH = 11and included about 60 points. The capacity was obtained from the milliequivalents of base required to neutralize the resin. The equivalence point on the basic site was determined by the Gran's method. From the titrationcurvesthe protonation equilibria can also be investigated by the usual methods, examining the functions pH vs a,the deprotonation degree, Le., the ratio of the concentration of the deprotonated form of the active group to its total concentration in the resin.1Ai8 Determination of the Water Content and Counterion Concentration in Chelex 100. Accordingto the Gibbs-Donnan model, the counterion concentration in the resin is in moles per kilogram of water (molality). It depends on the deprotonation degree cy, on the imbibition, and on the water content, which is related to the deprotonation degree, as demonstrated by previous researchers.9JO The water content p (grams of water per gram of dry resin) was determined at different protonation degrees by a previously described method.1' The amount of counterion in the resin is the summation of the cations which neutralize the deprotonated active groups c, and those which enter by diffusion,' together with an equilalent amount of co-ions,c d . The former is evaluated from the deprotonation degree by the relationship
(e).
[e,] = aQ/q
(2)
and the latter from the Gibbs-Donnan equilibrium:' [CI[Bl = (IC,] + [c,I)[cdl (3) For the sake of simplicity, in eq 3 both C and B are monovalent, and the activity coefficient ratio is assumed to be 1. (8) Pesavento, M.; Profumo, A,; Bieeuz, R.; Hdgfeldt, E. Ann. Chim. (Rome) 1990,80,265-270. (9) Krasner, J.; Marinsky, J. A. J. Phys. Chem. 1963,67, 2559-2561. (10) Szabadka, 6. Talanta 1982,29,183-187. (11)Pepper, K. W.; Reichenberg,D.; Hale, D. K. J . Chem. SOC.1952, 3129-3136.
Experimental Determination of Sorption Isotherms of Trace Metals on Chelex 100 by Batch Procedure. The ratio ofsorbedtototalmetalionisreportedvspHtoobtainthesorption isotherms. A fiied volume of aqueous solution containing a known amount of metal ion at fixed ionic composition was equilibrated with a fixed amount of Chelex 100.1-' Only systems containing a large excess of chelating groups compared to a trace amount of metal ions were considered here in order to reproduce the analytical conditions. The pH was changed by addition of smallamounts of concentrated acid or base. After the equilibrium was reached (usually 1-2 h at 25 "C), the pH of the aqueous solution was measured potentiometrically with a glass electrode standardized in H+concentration, and the concentration of metal ion in the aqueous phase was determined by AAS. The sorbed amount was calculated by difference. Determination of the Exchange Coefficients. The exchange coefficientsare obtained from the experimental sorption isotherms. In the case of some chelating resin previously investigated,l4 the sorption takes place through complexation accordingto eq 1,i.e., by forming only one complex (MH(,)L,) in the resin phase. Thus f, the fraction of metal ion sorbed at each pH value is given by the relationship
where Vindicates the grams of water in the aqueous phase and w the grams of water in the r e phase. Since a large excess of active groups was present, [HAL]was evaluated from the total concentration of active groups inside the resin, EL, according to the following relationship:
[@I = EJSJHlr/~B,[Hlx (5) where the summation is extended from x = 1 to x = N, the maximum number of protons linked to the active group. are the globalx-protonation Coefficients of the group inside the resin, pertinent to the following equilibrium:
n
The product is extended from j = 1to j = x , and Kgi are the protonation coefficients referred to each protonation step: (7)
When more than one complex is formed inside the resin, a relationship similar to eq 4 can be used to describe the sorption. However, in this case more than one sorption mechanism must be considered. Suppose that one complex is MH,,L, and the second one is MHu,,,,,Y,, formed according to the following equilibrium:
-
M +I H Y MH,-p,Y, with the extraction coefficient
+ pH
(8)
In this case the fraction of sorbed metal ion is given by
1 1+
1
(10)
[GI"w/CHI' V + 81exy1-1 'w/[HIPV HrL and H,Y can also be the same chelating group, which may be able to form complexes with different stoichiometry. If the composition and the extraction coefficient of the complex MH,,,L, are known, the formation of MH,,,-,,Y, may be investigated by the following relationship:
Equation 10 is easly extensible to the case of the formation of more than two complexes inside the resin.
2524
ANALYTICAL CHEMISTRY, VOL. 05, NO. 18, SEPTEMBER 15. 1993
11-
Table PI. Titration Data of Chelex 100 at Different Concentrations of NaNOp l_og 1% a qb [Nal PH K.2 0.013 mol kg-1 NaNOs
I
lo
i
91
@i I
7;
E i
1
0.00
0.20
0.40
0.60
0.80
1.00
1.20
4.01 4.04 4.13 4.21 4.30 4.42 4.54 4.68 4.84 5.05 5.29
0.236 0.286 0.335 0.381 0.433 0.479 0.534 0.590 0.641 0.692 0.745
3.62 3.65 3.69 3.76 3.82 3.95 4.04 4.13 4.23 4.35
0.322 0.359 0.397 0.428 0.461 0.534 0.573 0.610 0.647 0.689
2.86 2.90 2.95 3.00 3.05 3.09 3.13 3.17 3.22 3.26 3.30 3.35 3.45 3.51 3.58 3.65 3.72 3.81
0.467 0.482 0.497 0.515 0.535 0.547 0.620 0.578 0.595 0.613 0.632 0.651 0.689 0.709 0.729 0.750 0.771 0.793
mL NaOH 0.1972 mol kg-1
Fburr 1. Tkratlon curve of Chelex 100 wRh NaOH (0.1972mol kg-l). [NaN03],1 mol kg-l; V = 25 mL; 0.03177 g of dry resin; excess of protons, 0.074 mmol.
COndRlOnS:
Prediction of the Protonation and Extraction Coefficients on the Basis of the Gibbs-Donnan Model. The Gibbs-Donnan model for the ion-exchange resins states that when an electrolyte CbB, distributes between a resin and an aqueous solution the following equilibrium is set up: (C}*{B)"= (C}*{B)"
(12)
where symbols between the braces indicate activities and the same standard state (1 m solution) is selected for the aqueous solution and the resin phase. By applying eq 12,the protonation (K,) and the extraction Coefficients (B,,.,.L) of the resin can be expressed as follows:
(14)
where m is the charge of the metal ion and y y is the activity coefficient of species Y. The ratios of the activities in the resin phase are equal to the thermodynamicconstants in the aqueous and were substituted by them in eqs 13 and phase,Kui and firnu, 14,because the same reference and standard states are chosen for aqueous solution and resin, according to the Gibbs-Donnan model. To be able to apply eqs 13 and 14 in a practical way, a simplifying assumption was made about the value of the ratio of the activity Coefficients in the resin phase, which was assumed to be unity, in analogywith other systems.'-' In this way the extraction coefficientsmay be evaluated in asimpleway and allow prediction of the sorption isotherms, if the complexations inside the resin are the same as in aqueous solution.
RESULTS AND DISCUSSION Protonation and Capacity of Chelex 100. The protonation was studied in order to characterize the resin and to determine its capacity. The iminodiacetic groups are twice protonated at pH = -2, and these protons can be neutralized by addition of a base, up to pH = 11. A typical titration curve is reported in Figure 1. From the inflection point at pH = 6.5it can be calculated that 1.6 protons per g of dry resin are titrated in the acidic region, which is the capacity in terms of iminodiacetic chelating groups consideringthat one proton per iminodiacetic group is titrated in this acidity range. The second proton is neutralized at pH = -9. The protonation of some iminodiacetic resins has been previously investigated by other researchers,QJOwho were able to show that the protonation constants of the active groups
0.97 -0.4025 1.07 -0.3618 1.17 -0.3311 1.26 -0.3084 1.37 -0.2870 1.46 -0.2716 1.57 -0.2560 1.68 -0.2426 1.78 -0.2323 1.88 -0.2231 1.99 -0.2148 0.10mol kg-1 NaNO3 1.14 -0.3518 1.22 -0.3341 1.29 -0.3181 1.36 -0.3067 1.42 -0.2959 1.57 -0.2759 1.65 -0.2670 1.72 -0.2596 1.79 -0.2527 1.88 -0.2457 1.00 mol kg-l NaNOs 1.43 0.1981 1.46 0.2001 1.49 0.2021 1.53 0.2043 1.56 0.2064 1.59 0.2081 1.62 0.2097 1.66 0.2114 1.69 0.2130 1.73 0.2148 1.76 0.2165 1.80 0.2182 1.88 0.2214 1.92 0.2229 1.96 0.2244 2.00 0.2258 2.04 0.2273 2.09 0.2287
log Kazi
4.52 4.44 4.43 4.42 4.42 4.45 4.48 4.52 4.59 4.69 4.82
3.12 3.00 2.95 2.93 2.90 2.93 2.94 2.97 3.03 3.13 3.25
3.94 3.90 3.88 3.88 3.89 3.89 3.91 3.94 3.97 4.00
3.30 3.25 3.20 3.20 3.19 3.18 3.19 3.21 3.23 3.26
2.92 2.94 2.96 2.98 2.99 3.01 3.02 3.03 3.05 3.06 3.07 3.08 3.11 3.13 3.14 3.14 3.19 3.22
2.72 2.73 2.75 2.77 2.79 2.80 2.81 2.82 2.83 2.84 2.85 2.86 2.88 2.90 2.91 2.94 2.96 2.90
a Conditions: V = 25 mL; [NaOHl = 0.1972 mol kgl. 0.013 mol k g l NaN03: 0.01720g of dry resin, excess of protons, 0.067 mmol. 0.10mol kg-1 N d O 3 : 0.03177 g of dry resin, excess of protons, 0.074 mmol. 1.00mol kg-1 NaNOa: 0.05230g of dry resin, e x c w of protons, -0.019 mmol. q (grams of water per gram of dry resin), calculated from the experimentally obtained relationship q = 0.5 2a.
*
+
in the resin were related to that of some model monomers, respectively, benzyliminodiacetic and iminodiacetic acid, in aqueous solution. The equations they proposed were similar to eq 13. Marinskys derived it from the Gibbs-Donnan equilibrium, and Szabadkalo from the ion-exchange equilibrium. In the present investigation the second protonation was investigated, because it prevails in the pH range 2-6 here considered:
-
HL + H
-H,L
- -
K, = [H,Ll/[HLl [HI
(15)
where HL is negatively charged but charges are omitted for sake of simplicity. Some results are reported - in Table 11, where the deprotonation degree a = [HL]/E, is reported as a function of the pH of the aqueous solution for different concentrations of sodium nitrate. The protonation coefficients, the amount of sorbed water, and the concentration of sodium in the resin are also given. This last quantity was that evaluated as described above (eqs 2 and 3). This is very convenient because the experimental determination of the counterion concen-
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
Table 111. Capacity of Chelex 100 for Different Metal Ions in NaNOs (0.01 mol k g l pH = 5, T = 25 "C) dry resin proton (mol-') metal ion (mmol g-1)
copper nickel zinc cadmium calcium
1.60 1.60 1.48 1.46
1.8
0.80
'.OO
1 0.5
0.70 at pH = 5.0
0.6 0
0.60
1.52 at pH = 8.2
tration in the resin is difficult and time consuming. The agreement between the counterion concentration estimated in this way, and, for instance, the values experimentally found by Szabadkalo is satisfactory. The intrinsic protonation constant Kdi is evaluated by eq 13 and is reported in Table 11. The value obtained is in good agreement with those previously reportedQ-10and similar to the protonation constant of iminodiacetic acid in aqueous solution, as expected from the Gibbs-Donnan model. It is interesting to notice that the amount of sorbed water q (grams of water per gram of dry resin) linearly depends on the deprotonation degree and is practically independent of the ionic concentration of the external solution. The corresponding experimental equation, obtained at different sodium nitrate concentrations, is reported in Table 11. The capacity of the resin can also be evaluated by determining the amount of metal ion sorbed from solutions containing an excess of metal ion. Some previous observations showed that a number of divalent metal ions are quantitatively sorbed at pH = 4-5, and C N ~ O= ~0.01 mol k g l , where the iminodiacetate is monoprotonated, HL. Thus the total capacity for these metal ions may be determined a t pH = 5, by contacting the resin with an excess of metal ion, under stirring, at 25 "C. The equilibrium was reached in 2 h. Table I11 shows the results: the capacity is always around 1.5 mmol of ion per g of dry resin, in agreement with the number of iminodiaceticchelating groups titrated by acid-base titration. Only the capacity for calcium is 0.7 mmol g1of dry resin at pH = 5, while 1.52 mmol g1at pH = 8. This suggests that a complexation mechanism involving two iminodiacetic groups per one calcium is involved at the lower pH. Moreover, the number of protons eliminated for each metal ion sorbed a t pH = 5 is zero in the case of calcium and can be explained by the following sorption mechanism:
-
Ca + 2 m Ca(HL), (16) Copper and nickel are sorbed at pH = 5 through chelation by the iminodiacetate according to the equilibrium
M+EL-GL+H
(17)
while in the case of zinc and cadmium the two complexes probably coexist. At sufficiently high pH (pH = 81, the capacity for calcium reaches the value corresponding to the number of chelating groups, showing that the sorption mechanism a t this acidity is probably that of eq 17.
Sorption Isotherms of Divalent Metal Ions on Chelex 100. In Figure 2 some sorption isotherms of trace amounts
of copper and nickel on Chelex 100 from solution at different NaN03 concentrations are reported. The continuous curves were calculated from eq 4 by considering the extraction equilibrium
-
M + H,L E + 2H (18) which is that expected on the basis of the complexation equilibria in aqueous solution. The extraction coefficients
2525
'i
1 .IA P E ! , ,, ,,
0.80
1.30
1.80
,I,,
2.30
, , , , ,, , , ,, , , , , , , , I , , , , 2.80
3.30
p 1-1 Figure 2. Sorption Isotherms of nickeland copper on Chelex 100 from NaNOS solutlons: (0)[NaN03] 1.00 mol kg-l, V = 26 mL; Ni 0.099 mmol, 0.5966 g of dry resin. (A)[NaN03] 1.00 mol kg-l, V = 25 mL; Cu 0.030 mmol, 0.1879 g of dry resin. (0)[NaN03] 0.10 mol kg-l, V = 26 mL; NI 0.150 mmol, 0.8977 g of dry resin. (A)[NaNOS] 0.10 mol kg-I, V = 25 mL; Cu 0.030 mmol, 0.2178 g of dry resin. Contlnuow curves are calculated from of eq 4.
were evaluated from the formation constants of the complexes metal-methyliminodiacetic acid7 by eq 14 and the protonation coefficients from eq 13. The concentration of the counterion in the resin was obtained from the values reported in Table I1 by interpolation. The formation of the 1:2 complex is expected to be negligible. Thus the sorption isotherms should be independent of the ionic composition of the aqueous solution, according to eq 14, since m - s = 0. Actually the concentration of the counterion in aqueous solution does not have any influence on the sorption of copper and nickel. The sorption isotherms predicted by the model and those experimentally obtained are in good agreement. The equilibrium coefficients of eq 18 are respectively log Bled = -0.75 for copper and log @led = -2.90 for nickel. In contrast, the concentration of the ionic medium does affect the sorption isotherms of zinc, cadmium and calcium, as is seen in Figures 3-5. This is particularly impressive in the case of calcium. Moreover its sorption isotherm, calculated on the hypothesis of the chelation reported in eq 18, is located at acidities much lower than the experimental ones. This indicates that an equilibrium different from eq 18 is involved. The sorption data were analyzed graphically by using eq 11, where the term pertinent to equilibrium 18 is neglected since the extraction coefficient evaluated on the basis of the model is very low, log P l e d = -8.02 (see Table I). As no other donor groups are present in the resin, it was assumed that this complexation was also by iminodiacetate. The sorption equilibrium has been found to be Ca + 2 m
-
Ca(HL),
+ 2H
(19)
with an extraction coefficient j32e~= 106.l. According to eq 14 it corresponds to the complexation constant, j3ze&, because in this case m = s. The sorption isotherms calculated on the basis of this sorption mechanism are shown in Figure 5. They are affected by the ionic composition of the aqueous phase through the protonation coefficients, whose value decreases with the concentration of the counterion in solution. The complex Ca(HL)Zhas never been reported before for iminodiacetate in aqueous solution; however, it is not completely unexpected considering that calcium is complexed by
2526
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
I
I
f
060
/
:
'
r '
c
I 0.10
-
, - I
000 200
/* 300
400
5do
,
,,,,,,,,I 600
700
aoo
PI1
PH Flguro 5. Sorption Isotherms of zinc on Chelex 100 from NaN03 solutlons: (m) [NaN03] 1.00 mol kg-l, V = 25.5 mL; Zn 0.016 mmol, 0.0944 g of dry resin. ( 0 )[NaN03] 0.10 mol kg-l, V = 26.0 mL; Zn 0.064 mmol, 0.6046 g of dry resin. (0)[NaN03] 0.01 mol kg-l, V = 25.5 mL; Zn 0.016 mmol, 0.0972 g of dry resln. Continuous curves are calculated from eq 10 on the hypothesls of formatlon of ML and ML2. Curve 1 Is for 0.10 mol kg-1NaN03, curve 2 is for 1.OO mol kg-' NaN03, and curve 3 1s for 0.01 mol kg-1 NaN03. Dotted curve 3' is that calculated from eq 10 for the complexes ML, ML2,and M(HL)2with & . A of Table I V for 0.01 mol kg-1 NaN03. 1.00 :
Flgure 5. Sorptlon Isotherms of calcium on Chelex 100 from NaN03 solutions: (X) [NaN03] 1.00 mol kg-l, V - 30.5 mL; Ca 0.122 mmol, 0.6179 g of dry resin. (+) [NaN03] 0.10 mol kg-l, V = 26.6 mL; Ca 0.037 mmol, 0.2240 g of dry resin. (*) [NaN03] 0.01 mol kg-l, V = 30.5 mL; Ca 0.128 mmol, 0.5851 g of dry resln. Continuous curve Is calculated from eq 4. Dotted curves are calculated from eq 10 with 822.Areported In Table I V .
Table IV. Extraction Coefficients of Some Divalent Metal Ions Chelex 100 (T= 26 "C) by the Equilibrium M + -on 2HIL M(HL), + 2H
-
0.01mol kg-1
Nd03 zinc
cadmium calcium 0.80 1
0.60 1
f 0.40
/I
1
0.00 ' I I I 1.40
If
I
a
, ?%
2.50
I
I
3.00
3.50
4.00
p 1-1
Figure 4. Sorption Isotherms of cadmlum on Chelex 100 from NaN03 solutions: (0)[NaN03] 0.98 mol kg-l, V = 25 mL; Cd 0.053 mmol, 0.3424 g of dry resln. (0)[NaN03] 0.10 mol kg-l, V = 25 m k Cd 0.053 mmol, 0.3036 g of dry resln. (0)[NaN03] 0.004 mol kg-l, V = 25 m k Cd 0.053 mmol, 0.3294 g of dry resln. Continuous curve is calculated from eq 4. Dotted curves are calculated from eq 10 with P22.d reported In Table I V .
carboxylic donors, like acetate, and that the ligand is very concentrated in the resin phase, with values often higher than 2 mol kg-1, so the formation of complexes with molar compositions of 1:2 may be promoted.3 Some sorption isotherms of zinc and cadmium are reported in Figures 3 and 4, together with those calculated on the hypothesis of chelation by iminodiacetic groups, with the complexation constants obtained from Schwarzenbach.7 In the case of zinc, three calculated curves are reported, one for each ionic strength, because the formation of the 1:2 complex is not negligible, at least at some conditions, as it is seen from the complexation constants values log ,??le&= -4.11 and log
-3.64 -3.47
-5.11
log BZLXL 0.10mol kg-1 NaNOs -3.67 -3.40 -5.08
1.00 mol kg-1 NaNOa -3.44 -3.74
-5.00
8 2 e= ~ -9.54 (in 1.00 mol kg-1 NaNOs), log 8 %= -11.54 ~ (in 0.10 mol k g l NaNOs), and log &.L = -13.54 (in 0.01 mol k g l NaN03). Thus, according to eq 14, the sorption isotherms must depend on the ionic compositionof the external solution, because m - 8 = -2. The curves a t 1.00 and 0.10 mol kg-1 agree with the points experimentally obtained, while that in 0.01 mol kg-1 does not: the estimated sorption isotherm is at an acidity lower than the experimental one. In the case of cadmium, the formation of the complex ML2 is negligible (log fils& = -5.00, log 8 %= -11.00 ~ (in 1.00mol kg1 NaN03), log = -13.00 (in 0.10 mol k g l NaNOa), and log 8 %= ~ -15.00 (in 0.01 mol kg-1 NaNOs), but even if the difference is less striking than for calcium, its sorption also takes place at an acidity higher than that calculated. Thus, probably in the case of zinc and cadmium too, a different sorption mechanism is of importance. As the chelation by iminodiacetate is not negligible, the unknown complexation reactions were evaluated by eq 11. The stoichiometry of the unknown complexes was found to be the same as that of calcium (eq 19). The values of the complexation coefficients are reported in Table IV. They are near to those evaluated from the complexation constants of zinc and cadmium with acetate in aqueous solution,12 respectively, -3.9 and -3.3 for the 1:2 complexes. This confirms the sorption equilibrium proposed, involving complexation by two carboxylic groups from two different iminodiacetate groups. At lower acidities the chelation by one iminodiacetate prevails over this complexation.
CONCLUSIONS When metal ions are sorbed on Chelex 100 from aqueous solution, the chelation mechanism by iminodiacetate clearly prevails if strong complexes are formed, as is the case for (12) Kolat, R. S.; Powell, J. E.Znorg. Chern. 1962,1,293-236.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
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copper and nickel, while complexation by carboxylate prevails when metal ions with lower complexation constants with the iminodiacetate, such as zinc, cadmium, and calcium, are involved. The discriminating value of the formation constant seems to be at around log B l e d = -3.5. The formation of M(HL)p inside the iminodiacetate resin was never considered before, even if it can explain that the selectivity of Chelex 100 is lower than expected, considering only the complexation constants of the iminodiacetate group. Indeed, the complexes of divalent metal ions with carboxylicdonors, such as acetate, have formation constants very near to each other, not so different as those with iminodiacetate. It must also be observed that the effect of the global ionic compositionof the aqueous solution on the sorption isotherms depends on the sorption mechanism. It can be negligible in some cases, or high in other cases. A good example is given in a previous paper,la where the sorption isotherms of some metal ions on Chelex 100from freshwater and from seawater are compared. In the case of copper and nickel, they are very similar, because a strong complex ML is formed, in bothcases, with low competition from the other ions which are present
in the seawater. In contrast, the sorption of zinc and cadmium takes place at pH values higher in seawater than in freshwater. On the basis of the findings of the present investigation, this can be ascribed to the fact that calcium and magnesium, which are present a t high concentrations in seawater, compete with the considered heavy metal ions for the formation of the complexes M(HL)2 inside the resin. It is interesting to note that sometimes such differences are ascribed to the presence of complexing substances in seawater. This seems not to be correct when the sorption mechanism is examined more closely, and the effect of the different conditions is properly considered. To correctly decide about the presence of different chemical forms in aqueous solution,the experimental isotherms must be compared with the theoretical ones, which can be calculated on the basis of the model here presented.
(13)Pai, S. C.; Whung, P. Y.; Lai, R. L. Anal. Chim. Acta 1988,211,
RECEIVED for review November 25, 1992. Accepted June 7, 1993.
257-270.
ACKNOWLEDGMENT Financial support from the Italian Ministry of University and Scientific and Technological Research (MURST, 40 7%) is gratefully acknowledged.
