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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
Use of Chelex Resin for Determination of Labile Trace Metal Fractions in Aqueous Ligand Media and Comparison of the Method with Anodic Stripping Voltammetry Paul Figura and Bruce McDuffie' Laboratory for Trace Methods & Environmental Analysis, Department of Chemistry, State University of Binghamton, New York 13901
The use of Ca-Chelex to determine labile fractions of Cd(II), Cu( 11), Pb( 11), and Zn( 11) in the presence of the model ligands nitrilotriacetic acid (NTA), ethylenediaminetetracetic acid (EDTA), glycine, and humic acid was investigated. Under conditions where resin kinetics are fast and batch uptake is quantitative, the slow dissociation of soluble metal-ligand complexes can limit trace metal uptake by Chelex columns, as with EDTA and with an NTA impurity whose Cd complex dissociates two orders of magnitude slower than the corresponding Cd-NTA complex. Labile fractions determined by the Chelex method are generally larger than the fractions obtained by anodic stripping voltammetry, reflecting different time scales of measurement of the two techniques. The concept of a metal binding spectrum for trace metals in aqueous media is proposed.
Chelating ion-exchange resins, such as Chelex-100, have been utilized by a number of researchers for the preconcentration of trace metals (TMs) from natural water samples ( 2 - 4 ) , and Chelex in t h e Ca-form has been recently characterized ( 5 ) . Also it has been observed that the uptake of T M s by Chelex columns from a number of natural water samples is incomplete under conditions favorable for the rapid uptake of free T M ions (5--7). Apparently significant fractions of many of the T M s found in these samples exist in forms which render t h e m inert t o t h e column exchange reaction, i.e., as thermodynamically stable metal complexes, as relatively nonlabile metal complexes, or in the form of colloidal matter. T h e effect of complexing agents on the uptake of TMs by Chelex columns has not been investigated thoroughly. In several studies ( & I O ) , batch procedures with relatively long equilibration times were used; thus no kinetic data were obtained. Stolzberg a n d Rosin ( 1 1 ) studied the effect of nitrilotriacetic acid (NTA) and ethylenediaminetet,racetic acid (EDTA) on TM uptake by a Na-Chelex column, in developing a procedure for measuring t h e complexing capacity of seawater; when samples containing excess Cu were passed through t h e resin bed at a fast flow rate (30 mL/min/cm2), all t h e excess Cu(1I) was taken up, but the Cu-EDTA and most of the Cu-NTA complexes passed through the column without dissociation. T h e purposes of the present work are threefold: (1) to investigate t h e equilibria and kinetics of T M uptake by Ca-Chelex in the presence of chelating agents, (2) to measure t h e "labile" fraction of metal retained by Chelex columns under various conditions, and (3) t o contrast the "labile" fractions found in this way with those measured by anodic stripping voltammetry (ASV). As model ligands for surface waters, t h e chelating agents N T A (impure and purified), EDTA, humic acid (HA),and glycine are used. Under solution conditions where batch uptake of' TMs from the model ligands is quantitative, the kinetics of dissociation of these metalligand complexes is investigated utilizing the column technique. T h e results are compared to those obtained by direct 0003-2700/79/0351-0120$01 OO/O
New
York at Binghamton,
ASV analysis of the solutions, and are interpreted on the basis of the differing time scales of measurement of the two techniques. Finally, a scheme is proposed for classifying soluble TM-complexes in aqueous media according t o their relative labilities, using ASV data as well as Chelex column and batch uptake characteristics.
EXPERIMENTAL Reagents. All chemicals used were certified ACS reagent grade unless otherwise noted. Analytical grade Chelex-100, Na-form, 100-200 mesh, was obtained from Bio-Rad Laboratories (lot no. 14930). Samples of NTA from the J. T. Baker Chemical Co. (lot no. 1-4485, minimum 99% pure) and from the Sigma Chemical Co. (lot no. 1166-0275, 99.570 pure) were used; "purified NTA" was prepared from Sigma KTA by recrystallizing four times from 1 mmol Cd(I1) solutions, using the procedure of Shuman and Shain (12). Fisher reagent grade EDTA and glycine were used without purification. Redistilled HNOB( G . F. Smith Chemical Co.) and isothermally distilled ",OH were used to obtain low T M blanks. Buffers and salt solutions were run through a column containing the appropriate form of Chelex-100 to remove T M impurities. A standard TM medium, prepared fresh as needed, contained 25 wg/L each of Cd, Cu, P b and Zn, along with 2.3 X M Ca(NOJ2, in 0.02 M tris(hydroxymethy1)aminomethane (Tris)-hydrochloricacid at pH 7.8. (NaC1 was added to the buffer in all experiments to give a total C1- concentration of 0.02 M). Free TMs are completely taken up by Ca-Chelex columns under these conditions, and the pH chosen is close to that of many natural waters. A stock HA solution was prepared by dissolving 0.100 g of technical grade HA (Aldrich Chemical Co.) in 1 L ofO.l M NaOH, then filtering the sample through a 0.40-~mNuclepore filter; the residue (-30%) was discarded. Then a TM-free solution of HA (3.5 mg/L) in 0.02 M Tris pH 7.8 buffer medium was prepared as follows: a quantity of 150 mL of the stock solution in NaOH was combined with 120 mL of 0.5 M Tris pH 6 buffer and 1 2 g of Na-Chelex, then diluted to 3 L with distilled water; the covered sample was allowed to equilibrate for four days with continuous magnetic stirring, then the solution phase was separated from the resin by filtration through a fritted column. Electrochemical Measurements. Differential pulse anodic stripping voltammetry (DPASV) was performed with a 1I.W Model 174 Polarographic Analyzer using a hanging mercury c!, . l j i electrode (HMDE) as reported previously (5). An effective diffusion layer thickness of 2 X 10 ' cm was caluilated for typical plating conditions (13). All HMDE potentials were measured vs. the SCE reference electrode. Ca Chelex Columns. The normal column parameters 1.3 g of resin, 0.8-cm i.d. columns, 2 - 3 mL/min flow rate) were identical to those reported previously (5).as was the preparation uf the Ca-form of the resin with the exceptions that doubly(i.e., distilled water passed through a column of' ",-Chelex DDXW) was used, and 1 M ",OH was substituted for 2 M NaOH in order to obtain a lower reagent hiank. The columns were preconditioned by passage of 30 mL of the appropriate buffer medium. Under these conditions, the void volume at pH 7.8 was 0.3 mL. Thus the time'of contact, t,: of increments of solution with the resin phase, calculated by dividing the void volume by the flow rate, was normally 6- 9 s. Procedure for Elution and Analysis of Microgram or Sub-Microgram Quantities of TMs from Resin. All columns c 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 1. JANUARY 1979 were eluted with 10 mL of 2 M H N 0 3 and washed with 10 mL of DDXW. Then, to minimize dilution, the combined eluate and washings were neutralized to pH 2 with 6 M ",OH and adjusted to a volume of 25.0 mL with DDXW. A sample volume of 1 L thus would give a preconcentration factor of 40. This HNOq/ ",OH solution was analyzed for Cd, Cu, and P b by DPASV, then for Zn after addition of 2 mL of p H 6 buffer (1 M NH40Ac/HOAc) to 8 mL of sample. Typical reagent blanks found for this procedure (av. f std. dev.) were 0.0075& 0.0015 pg Cd, 0.062 f 0.012 pg Pb, 0.087 f 0.015pg Cu, and 0.060 f 0.012 p g Zn, low enough for accurate determination of ultra-trace amounts of metals in natural water samples after preconcentration by the resin. Effect of Complexing Agents on TM Uptake, Using Column and Batch Techniques. Batches of the standard T M medium, with varying concentrations of NTA or EDTA, were equilibrated for at least 2 h in 2-L polyethylene containers which had been pre-equilibrated with the identical sample medium to minimize adsorption losses. Portions of these samples (250 m1,) were then passed through Ca-Chelex columns at the desired f l o ~ , rate. Identical portions were also added to 100-mL beakers containing 1.3 g of Ca-Chelex, the beakers capped with Parafilm. and the contents stirred continuously for 4 days. after which the phases were separated, the resin being collected in a fritted column. The resins from the column and batch experiments (each done in triplicate) were eluted with redistilled HNOBand analyzed for TMs as described above. Identical procedures were also followed for samples containing 1X M glycine, and for samples containing 3.5 mg/L HA separately spiked with 5 X M of each of the above four metals. except that no Ca(NO,), was added to the standard T M medium. DPASV Titration of Humic Acid with Various TMs. Samples containing 3.5 mg/L HA in 0.02 M Tris pH 7.8 buffer were titrated separately with Cd, Cu, Pb, and Zn using DPASV to measure the concentrations of free and releasable TMs. A 10-min equilibration period and a 3-min plating time were used after each microaddition of metal titrant. (No significant change in peak current occurred when the equilibration period was extended to 2 h). In these experiments. the plating potentials (E,) were --1.X V for Zn, 0.9 V for Cd. ~ 0 . 7V for Pb. and -0.4 V for Cu. DPASV Method for Labile TM Fractions. The Tbl-EDTA. -NTA, -HA, and -glycine complexes were analyzed for TMs by DPASV in media identical to those used in the column experiments. For each metal. the labile fraction is operationally defined as that fraction of total metal measured by the stripping peak. The E,'s used in this 0.02 M Tris pH 7.8medium were as follows: Zn, 1.2 V;Cd, -4.8 V: Pb, 4.65 V; and Cu. 4 . 3 7 V. (These values represent the minimum E,'s required in this medium t o ohtain maximum plating efficiency for the respective free metals.) Construction of DPASV Pseudo-Polarograms. The ASV characteristics of Cd-NTA, Pb-NTA. free Cu, Cu-NTA and Cu-EDTA solutions were examined by constructing pseudopolarograms (14). Solutions were plated for I20 s at a desired E , with stirring. followed by a 15-s quiescent period and a rapid anodic scan (50 mV/s) to reach -0.8 V for Cd- and Pb-NTA. -0.1 V for Cu. Cu-NTA. and Cu-EDTA in Tris medium. and 0.3 \for the three Cu solutions in 0.02 M NaOAc/HOAc p H 6.1 medium. (These potentials. i.e., -0.8, -0.4, and -0.3 1.. will he referred to as the equilibration potentials, E,,). When E,, was reached, the cell was then placed on HOLD, both the scan rate and current range were readjusted, and at the 180-smark (total elapsed time) the anodic scan at 2 m V / s was started and the stripping peak recorded. This process was repeated at other E,'s to obtain the complete polarogram. For ED'sanodic of Eeq. the potential was changed manually to E,, after the plating a n d quiescent period, and held there until the 180-smark. Corrections were made for metal plated during the equilibration step. Calibrations were made on solutions containing no complexing agent, using the identical procedure or by comparing peak heights to those obtained at E,'s well beyond the E,,,'s for the various complexes. ~
RESULTS AND DISCUSSION Effect of NTA Reagents on TM Uptake by Ca-Chelex Columns. Using solutions prepared from Baker N T A , re-
?
S
121
401
I
I
2oL
0
Cd
C" Pb
z I,
Figure 1. Effect of NTA concentration Ion TM uptake by Ca-Chelex. Standard TM medium, with Baker NTA added. (*) av. f std. dev. of 3 determinations. (--) column experiments, (- - -) batch experiments 100.
-~
Q!
i n
05
a -
0 5
Log ( F l o w R a t e , mL;minl
Figure 2. Effect of flow rate on column uptake of TMs from NTA medium. Standard T M medium, with 5 X M Baker NTA added. (@) av. k std. dev. of 3 determinations
tention of TMs by Ca Chelex columns was found t,o be incomplete. For example with 1.6 X 10 M NTA in t h e standard TM medium. the ret,entirin of Cu, Pb. Zn, and Cd were only 82, 7.7. 75, and 12% c-omplete. reqpectively. I~Jnder these same conditions, hatch uptake was 100 k 2% complete for all TMs. In support of this work. Pakalns et al. ( 7 5 ) noted incomplete uptake of a number of T M s hy Na-Chelex columns in the presence of excess NTA. T h e effect of varying the Raker NTA concentrations on column and batch uptake of TMs i:; shown in Figure 1. At concentrations above 10 M, the column technique gave lower values than t h e hatch method. particularly for Cd. T h u s it, appeared that slow solution kinetics, i.e.. slow dissociation of T M complexes into species that c a n exchange rapidly with the resin, was responsihle for the low cohimn retention values. Several other experiments were done t o eluc%iate this phenomenon. using the same NT.4 mt,dium: 11) r o l u m n s of resin in the ",-form were substituted for Ca- Chelex. since the ".,-form has larger selectivity constants for TM ions and has been reported to exchange faster than the Ca-form with simple metal ions ( 1 6 ) .b u t no improvement. in column retention was noted. (2)T h e t , for solution species with the resin phase was increased hy utilizing 3x longer columns, hut no increase in retention was observed: slower flow rates were used to increase t,. further. and increased retention was in fact observed at extremely low flow ( 36 s) as shown in Figure 2 . (3) Standard T M medium. ;i x 10 IvT in NTA, was allowed to equilibrate several hours after passage through a column of Ca-Chelex. and yielded additional T M uptake when passed through a fresh column of Ca-~Chelex. Other portions of the equilibrated column effluent were passed through NTA-conditioned resins (both freshly prepared and
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
Table I. Uptake of TMs by Ca-Chelex Columns in Presence of Different Lots of NTAa % uptake (av. i std. dev.)b metal Baker NTA Sigma NTA Purified NTA Cd 425 2 73 * 2 99 * 3 cu 82i 2 91 t 3 98 t 3 89 i 3 97 i 4 Pb 75 * 4 Zn 77 i 2 90 i 2 1ooi 2 a All experiments done in triplicate a t a flow rate of 2-3 mL/min, in standard TM medium (see text) with M NTA added. Initially 25 ug/L of each 1.6 X metal was present.
I -1 2
-1 6
Plating Potential. V
aged); identical results were obtained in all cases, indicating t h a t exposure to NTA does not interfere with the ability of the resin to pick u p TMs. (4)An "interruption test" (17) conducted in a batch experiment with an interruption period of 38 h , indicated t h a t particle diffusion in the resin phase was not the limiting factor in slow uptake of TMs by the resin. These results all were consistent with the hypothesis that slow solution kinetics was responsible for the incomplete column uptake. T h e dissociation of CdNTA- can be represented by the equation: k
' O 0I
+
C d N T A - d,Cd2+ NTA3(1) where NTA3- will be 100% protonated at pH 7.8 and where hd is a first-order or pseudo-first-order rate constant, reported t o have t h e value 1-3 s-l in the p H range 2.5-6.0 (18, 19). Raspor and Branica (20) found h d to be 1.6 s-l in 0.03 M CaC1, a t p H 8. Based on this value, the time for 99.9% dissociation of CdNTA- is calculated to be 4.3 s. Thus in a column experiment with t, of 6-9 s, there is sufficient time for complete dissociation of the CdNTA- complex, and the uptake of Cd should approximate t h a t from batch equilibration. T h e fact t h a t our experiments showed otherwise led us to suspect the presence of an impurity (or impurities) in the NTA used. Shuman and Shain (12)had noted t h a t various lots of NTA contained an impurity which complexed Cd to a higher extent than NTA. I n Table I, results are presented for the column uptake of T M s from standard T M medium containing three different samples of NTA-Baker, Sigma, and purified. The differences in percentage uptake are striking, with the results for purified NTA agreeing with the kinetic calculations, Le., uptake being essentially complete for Cd in a column with t , of 6-9 s. Assuming 1:l complexes, with the impurity ligand, X , (as yet unidentified), the total concentration of TM-X species left in solution from the 1.6 X M Baker NTA medium (see Table I) is -3 X lo-' M. Therefore, at NTA concentrations less than M, and corresponding lower impurity levels, a negligible fraction of metal in our experiments would he chelated with X (see Figure 1). I n Figure 2, the plateau region observed a t flow rates from 7.5 t o 1.0 m L / m i n was followed by an increase in uptake a t flow rates less than 1 mL/min. T h u s an initial portion of column uptake was relatively fast, controlled perhaps by the dissociation of M-NTA complexes, but the increased uptake a t distinctly lower flow rates reflects a much slower dissociation reaction. Assuming first-order kinetics, a rate constant of 0.01 s-l was calculated from the slow-flow Cd retention data of Figure 2, assuming t h a t all Cd not taken up by the resin a t various t , values was undissociated CdX. P s e u d o - P o l a r o g r a m s for C h a r a c t e r i z i n g NTA Rea g e n t s . T h e three samples of NTA were also characterized by the DPASV pseudo-polarograms for Cd and P h (Figures 3a and 3b, respectively). The CdNTA- chelate has been shown to be irreversibly reduced at -0.9 V (20). T h u s one would
-
b
2a
t 10
-1 4
-1 8
Plating Potential, V
Figure 3. DPASV pseudo-polarograms of Cd- and Pb-NTA solutions. Standard TM medium, with 5 X M NTA. (I) Baker NTA, (11) Sigma NTA, (111) Purified NTA. (a) Cd-NTA, (b) Pb-NTA Table 11. Determination of Labile TMs in Presence of Model Ligands Using Ca-Chelex Column Methoda % TM uptake (av. f std. dev.) TMI E D T A ~ ,N ~TA~,C,=' ligand 5 X 1.6 x HAP glycineb concn M M 3.5 mg/L M Cd 17k 2 99.3 74f 7 loo? 3 Cu 61 t 5 982 3 56i 4 100 t 6 Pb 13i 1 97i4 81i9 99 t 3 Zn 9 t 1 look2 98*2 104.7 All experiments done in triplicate a t a flow rate of 2-3 mL/min, in 0.02 M Tris pH 7.8 buffer; ligands used at concentrations giving 100% TM uptake in batch 25 pg/L each of Cd, Cu, Pb, and Zn experiments. M in Ca(NO,),. added. Solution was 2.3 X =' Recrystallized 4 X from Sigma NTA, see text. e Individually spiked with 5 X 10.' M Cd, Cu, Pb, or Zn. expect maximum deposition of CdNTA to be achieved a t an E , of -1.1 V. However, as polarograms I and I1 of Figure 3a indicate. only a small percentage of the total Cd is reduced a t t h a t potential. Another wave begins to appear a t an E , of -1.6 V, indicating a Cd complex with a higher stability constant than CdNTA , or one more irreversibly reduced. The percentages of Cd found a t -1.1 V (i.e., Curve I, 7 % ; Curve 11, 28%; Curve 111, 89%) reflect an increasing purity of NTA, with the 4 X recrystallized product being nearly free of the Cd-inhibiting impurity. For PhNTA- also, the pseudo-polarograms of Figure 3b indicate the existence of two or more species: a first wave at -0.9 V corresponding to PbNTA- ( 2 1 ) ,and a second wave a t E , beyond -1.0 V corresponding to an impurity whose relative magnitude decreases as the purity of NTA is increased. Column U p t a k e f r o m EDTA, HA, a n d Glycine Media. T h e effects of three other complexing agents on the column
ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
123
Table 111. Comparison of DPASV and Chelex-Column Labile Fractions of TMs in Model Ligand Mediau 3’6 DPASV-labile metal ( ( 6 Chelex-labile metal) TM/ EDTA ligand 5 x M concn Cd Cu
0 (17) 50(61) O(13) 0(9)
NTAb 1.6 X l o - ‘
HA
M
3 5mg/L
1x
8 (99) 94(98) 11 (97) O(100)
66 (74) 0 (54) 74 (81) lOO(98)
100 (100) 98 (100) 1 0 2 (99) 100 (104)
glycine M
Pb Zn (I See Table I1 for solution conditions and text for Recrystallized .4X from Sigma NTA. DPASV details. 0
4
8
12
Metal Titrant Added, motes x 10”
Figure 4. DPASV titration of humic acid with specific TMs. 10 rnL of solution, containing 3.5 mg/L humic acid in 0.02 M Tris pH 7.8 buffer
uptake of TMs are presented in Table 11,along with the results from purified NTA. In parallel experiments, it was found that the batch uptake of Cd, Cu, Pb, and Zn was quantitative under these conditions. Thus, from a thermodynamic standpoint, none of these complexing media should prevent the quantitative preconcentration of T M s by Chelex-100. T h e T M uptake was lowest from the EDTA system,
