Environ. Sci. Technol. 1997, 31, 3081-3085
Microcolumn Ion-Exchange Method for Kinetic Speciation of Copper and Lead in Natural Waters JESU Ä S RODRIGUEZ PROCOPIO,* MARIA DEL MAR ORTIZ VIANA, AND LUCAS HERNANDEZ HERNANDEZ Department of Analytical Chemistry and Instrumental Analysis, Autonoma University of Madrid, Science Faculty, E-28049 Madrid, Spain
A scheme for metal speciation based on an ion-exchange column procedure has been developed. The dissociation kinetics of metal complexes were studied using a Amberlite XAD-8 microcolumn technique, covering a time scale of measurement between 0.02 and 3 s. Application of this procedure to copper and lead complexes permits their characterization in four lability categories: labile, quasilabile, slowly labile, and inert. This procedure has been applied to species of copper and lead in natural waters (e.g., Jarama River).
Introduction In recent years, several speciation schemes have been proposed for the determination of chemical forms of the metals in seawaters and freshwaters, many of them involve a series of sample treatment, separation, and measurement steps (1-3). As these speciation procedures change the equilibria of the water samples, the classification of metal species is only operationally defined. Davison (4) has recommended that the effective time of measurement should be reported in the speciation scheme so that measurements performed by different workers under different experimental conditions could be compared. Kinetic studies of metal species have emerged as a powerful approach to chemical speciation (2, 5-10). Such kinetic studies have a great potential because the dissociation kinetics of metal complexes can be related to the bioavailability and toxicity of the metal under study. A more important fact is that information about the kinetics allow a detailed modeling of natural systems, and in several cases, kinetic properties of metal species predominate over equilibrium considerations (10). The classical speciation method described by Figura and Mc Duffie (11) subdivides the dissolved metal into four categories: labile, moderately labile, slowly labile, and inert, based upon the concentrations measured by anodic striping voltammetry and Chelex ion-exchange column and batch procedures. The technology they used implies a simple kinetic basis for the discrimination between detectable vs undetectable metal. Each fraction is defined by a measurement time, but no adequate information can be related to the chemical species. In kinetic speciation, the discriminatory criteria between labile and inert species is the characteristic time scale of the measurement: time during the measurement that the metal complex is permitted to dissociate. So more information about actual chemical or physical metal species in water can be obtained by changing the time scales along the experiment. Muller and Kester (6) combined anodic stripping voltammetry, using a rotating disk electrode, along with ion-exchange
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1997 American Chemical Society
procedures in the column and batch modes to study the lability characteristics of zinc and cadmium in seawater. By combining the three techniques, they can change the effective time of measurement between 10-2 and 106 s. Burba et al. (10) have characterized the lability of heavy metals bound to aquatic humic substances by means of ligand exchange with cellulose-immobilized triethylenetetraaminepentaacetic acid in a flow system. They could vary the effective time of measurement between 0.1 and 1000 s. In this paper, we report on the use of an ion-exchange column procedure to characterize the kinetics of dissociation of several complexes types of copper and lead. At fast flow rates, a kinetically limited uptake of free metal incomplete exchange is achieved (6). To obtain lower contact times with flow rates lower than 10 mL/min, it is necessary to reduce the column size and to use an ion exchanger providing fast kinetics of exchange. In this first study, we show the capabilities of the proposed method, using two kinds of microcolumns containing 30 and 5 mg of exchanger, respectively. These column sizes allow contact time between 0.024 and 3 s. As an exchanger, the adsorbent Amberlite XAD-8 (polyacrilic acid ester) conventionally applied for the separation of organic substances (e.g., humic substances) (12) was used. This resin is a cationic exchanger of low capacity but with a fast kinetic of exchange.
Experimental Section Reagents. Ultrapure water obtained with a MilliRo-MilliQ system (Millipore, USA) and analytical-grade reagents (Carlo Erba, Italy) were used throughout. Humic acids (HA, Fluka, Switzerland, 600-1000 g/mol) were dissolved in a minimum amount of 10-3 M KOH until no pH variation was observed and then filtrated over a 0.45-µm membrane filter (Durapore, Waters, USA). The 0.01 M stock standard solutions of glycine (Gly), L-cysteine (Cys), salicylic acid (SA), nitrylotriacetic acid (NTA) (Merck, Germany), reduced glutathione (Glu, Sigma, USA), ethylenediaminetetraacetic acid (EDTA, Carlo Erba, Italy), and ammonium pyrrolidinedithiocarbamate (PDTC, Fluka, USA) were prepared in water. All solutions were stored in polyethylene containers at 4 °C in the dark. Amberlite XAD-8 resin (Carlo Erba, Italy) supplied as 20-50 mesh was ground and sieved to 100-120 µm. The resin was further purified by successive soaking with 1.0 M HCl, 1.0 M KOH, and methanol (24 h, each). Between each reagent, the resin was washed with copious water. Apparatus. Trace metals in the eluates were determined by graphite-furnace atomic absorption spectrometry (GFAAS) using a Hitachi Zeeman Z-8200 spectrophotometer (Japan), equipped with a Model SSC 300 autosampler. The flow system was composed for a dual piston pump (Waters Model 47) and a precolumn Upchurch Scientific Model C-133B (USA) of 20 mm long and 2 mm i.d. filled with XAD 8. All tubes were made of Teflon. Microcolumn Procedure for Kinetic Speciation Studies. Solutions of model complexes containing 100 µg/L lead or 50 µg/L copper, respectively, 0.01 M KNO3, and the selected ligand concentration were prepared. All studied ligands were 10-5 M in concentration, except HA, bicarbonate, and chloride, with concentrations of 10 mg/mL, 5 × 10-4 M and 10-4 M, respectively. These solutions were pH adjusted between 6.8 and 7.0 by using KOH or HNO3 solutions and allowed to equilibrate for 24 h. Before each sample run, the adsorbent in the column was changed and, for column preconditioning, 8.0 mL (2 mL if the small column is used) of sample solution was passed through the column at a flow rate of 0.5 mL/min. This step was carried out to equilibration of the resin with respect to
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TABLE 1. Summary of Speciation Scheme Proposed labile fraction quasi-labile fraction slowly labile fraction inert fraction
major cations of water as well as pH to avoid pH and compositional changes upon passage through the column. Then the sample was passed through the column at several flow rates over the range of 0.3-10 mL/min, and three subsamples of eluate for each rate were collected. The 1.00 mL of subsample solutions, each of which resulted from a particular contact time between 0.024 and 3 s, was acidified with 10 µL of 68% HNO3; metal determination was carried out by GFAAS according to standard conditions recommended by the manufacturer. Detections limits of 0.5 and 1.0 µg/L for copper and lead, respectively, were achieved in these conditions (signal to noise, 3:1). The overall standard deviations, obtained through experiments were within 2-5%. Samples. River water samples were collected into 5-L polyethylene bottles. After a thorough rinse with river water, the bottle was uncapped ≈15-20 cm below the surface and capped again while still immersed. The sample was then brought to the laboratory, filtered through 0.45-µm cellulose acetate filters using 1-µm prefilters, and kept at 4 °C in the dark. Measurement of these samples was achieved within 48 h. The speciation information was obtained by making measurements in a filtered subsample and in another filtered subsample that was UV-irradiated. The UV photooxidation of the filtered sample was performed with a Osram 500-W Hg vapor lamp (Spain). The water sample was irradiated for 8 h in a quartz vessel cooled by circulation of cold water. The differences in results between UV-irradiated and filtered subsamples provide an estimate of “organically bound” metal.
Kinetic Model The dissociation of a 1:1 complex formed between a divalent metal ion M (M ) Cu, Pb) and the ligand L and the subsequent adsorption of M at the resin R can be described by the overall reaction (charges omitted for simplicity): kd
ML w\ xM+L k f
ka
KR + M \ w x MR + K
(1) (2)
where kd, kf, and ka are the rate constant for dissociation and formation of ML and for formation of resin product, MR, respectively. The latter step is mostly controlled by fast film diffusion in the outer sphere of ion-exchanger bead (13). Assuming that the complex ML is not directly adsorbed and ka if fast as compared with kd, the adsorption of M on the resin is only affected by slow kinetics of dissociation of ML. In the present work, the term “availability” of metal, X, as the ratio [M]/[M]total, is employed (6); where [M] is the metal concentration retained in the column and [M]total is the total metal concentration. That is to say, X is the molar fraction of dissociated ML at time t; t being the characteristic contact time of the measurement. In this specification model, we can typify the different ligands according to their lability degree into labile, and inert. Each fraction is operationally defined by a time scale of measurement, t. Then the retained metal fraction, X, at time t can be related with the dissociation rate constant, kd, by means of
1/(1 - X) ) exp(kdt)
(3)
The labile fraction is the fraction of metal that is retained at a contact time of 0.024 s, the shortest contact time
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kd > 200 s-1 0.329 s-1 < Kd < 200 s-1 3 × 10-3 s-1 < Kd < 0.329 s-1 Kd < 3 × 10-3 s-1
metal fraction retained within 0.024 s metal fraction totally retained within >0.024 and
