Environ. Sci. Technol. 2002, 36, 914-920
Species Kinetics and Heterogeneous Reactivity of Dissolved Cu in Natural Freshwaters E R I C P . A C H T E R B E R G , * ,† JOHANNES T. VAN ELTEREN,‡ AND ZVONIMIR I. KOLAR‡ Department of Environmental Sciences, Plymouth Environmental Research Centre, University of Plymouth, PL4 8AA Plymouth, U.K., and Interfaculty Reactor Institute, Delft University of Technology, 2629 JB Delft, The Netherlands
Using high specific activity 64Cu2+ as radiotracer, the distribution kinetics among Cu species were established in natural organic-rich freshwaters under steady-state conditions, i.e., with minimal disturbance of existing equilibria. Study sites with contrasting suspended particulate matter (SPM) characteristics were investigated. Our analytical protocol allowed the differentiation between the following Cu species: SPM associated Cu, dissolved reactive (free and labile) Cu, and organically complexed Cu. The data obtained were successfully evaluated by compartmental analysis, which showed the importance of organically complexed Cu in freshwaters, and the dominant role of the interactions between organically complexed Cu and SPM in a SPM-rich water. The kinetic 64Cu measurements indicated that the attainment of equilibrium between dissolved reactive and organically complexed Cu took ca. 90%) of Cu in freshwater (4, 5) and seawater (6-8) is complexed by organic ligands. * Corresponding author phone: 44-1752-233036; fax: 44-1752233035; e-mail:
[email protected]. † University of Plymouth. ‡ Delft University of Technology. 914
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 5, 2002
Partitioning of Cu between the various chemical fractions in natural waters may be slow. It is known that the attainment of equilibrium between added ionic Cu and dissolved Cu complexing organic ligands may take several hours (6, 9). Furthermore, the interaction of Cu between the dissolved and particulate phases can be kinetically limited (10). However, many studies assume the presence of a chemical equilibrium between the various Cu species, and this is exemplified by the use of conditional distribution coefficients (KD) in water quality models (e.g. EcoS, 11). KD is expressed as the ratio of the concentration of the element considered in the particulate and dissolved phases, respectively (differentiation between these phases is typically performed using membrane filtration (0.4-0.45 µm cutoff)). The KD concept assumes that the particulate phase is easily exchanged with the aqueous phase. Data on the rate of complex formation, i.e., equilibration, are scarce although slow kinetic processes may have important biogeochemical consequences in environmental situations with rapidly changing physicochemical conditions, as for example in wastewater discharges, algal blooms, or estuarine mixing processes. Many of the trace metal speciation studies reported in the literature have been undertaken using stable (nonradioactive) metals. However, the use of radiotracers for the study of trace metal speciation and sorption kinetics in natural waters is advantageous due to the specificity of the methods, lack of contamination problems and sensitivity of the counting techniques (12). Ample studies using radiotracers have been published showing the importance of kinetics in dissolved-particulate trace metal interactions (13-15). The radiotracer techniques can be used for kinetic studies utilizing “isotopic exchange”, whereby the rate of homoionic exchange among the species participating in equilibria can be followed, with no net mass transport (i.e. steady-state). Radiotracers with a high specific activity (Bq mol-1) are required for the isotopic exchange studies. The final concentration of added trace metal, i.e., the radiotracer plus the accompanying stable metal, will have to be much lower than the natural water concentration of the metal in order not to disturb the chemical equilibria. Moreover, the radiotracer should be added in the proper chemical form, which is as a free metal ion or a complex, depending on the tracee of interest. Applied to natural waters, this approach may give information about the distribution of the radiotracer among the SPM, dissolved complexes and free ions in water. A recent study (15) reported on the kinetics of complexation of radiolabeled Mn, Co, Fe, Cd, Zn, Ag, and Cs with SPM and dissolved ligands in freshwater. For the present study we employed the short-lived radioisotope 64Cu to investigate Cu partitioning in natural waters utilizing isotopic exchange. Differentiation between SPM and the dissolved phase was made by online filtration, and online separation of dissolved Cu species was achieved by Chelex-100. The chromatographic approach of dissolved Cu species separation resulted in the following operationally defined fractions: Chelex-labile and organically complexed Cu (16). The Cu species separation approach is simple and rapid, which is important in the case of kinetic radiotracer studies. The application of the 64Cu isotope for trace metal speciation and kinetics investigations using environmentally realistic conditions is uncommon, because of the short half-life (12.7 h) of this isotope. Only a few studies in natural waters using 64Cu tracers have been reported, and all of these dealt with Cu uptake mechanisms by aquatic organisms (3, 17, 18). A unique feature of our study is the application of adsorptive cathodic stripping voltammetry (AdCSV) for the 10.1021/es010109m CCC: $22.00
2002 American Chemical Society Published on Web 01/29/2002
characterization of the equilibrium (stable) Cu speciation in the natural waters, in addition to the kinetic radiotracer experiments. In AdCSV, ligand competition between an added AdCSV ligand and natural metal complexing ligands for available metal is used, and this allows the determination of concentrations of electrochemically labile and free aqueous metal and natural metal complexing organic ligands with their conditional stability constants (6, 19, 20, 21). The aims of our study are to apply a dual experimental approach to natural waters, using 64Cu for short-term laboratory kinetic speciation experiments and AdCSV for equilibrium determinations, and to investigate the effectiveness of this approach in terms of a geochemical interpretation of the kinetics and heterogeneous reactivity of Cu species.
Experimental Section and Methods Sampling Sites and Chemicals. Freshwater samples were collected (January 1997) in acid-cleaned High Density Polyethylene (HDPE, Nalgene) sample bottles from North Canal in Rotterdam and City Canal in Delft (The Netherlands) and the river Scheldt at Hemriksem (Belgium). There was extensive ice cover on the Rotterdam and Delft canals, whereas the Scheldt was not covered by ice during this period. The waters in Rotterdam and Delft are rather stagnant due to the presence of sluices, whereas the Scheldt river feeds a macrotidal estuary and has a high water flow (average yearly flow 104 m3 s-1 (22)). Sample filtration (where appropriate (see below)) was performed immediately upon sample collection, and all critical speciation analyses were undertaken within 48 h upon collection. All reagents and wash solutions were made up in water purified by reverse osmosis (Milli-Ro, Milli-Pore) followed by ion-exchange (Milli-Q, Milli-Pore). Reagents were purchased from Merck unless stated otherwise. All labware (HDPE sample bottles and reagent containers) were first cleaned using detergent and then soaked in 2 mol L-1 HNO3 for 48 h. They were subsequently rinsed using Milli-Q water and stored inside resealable polyethylene bags. Samples were stored at 4 °C in the dark prior to electrochemically labile Cu analyses and natural Cu complexing ligand measurements (stable Cu) and radiotracer experiments (64Cu). Filtration of samples for dissolved total and electrochemically labile Cu analysis and for dissolved Cu complexing natural ligand measurements was performed online using Swinnex filterholders (25 mm diameter, Milli-Pore) with polycarbonate membrane filters (0.4 µm pore size; Nuclepore). Samples for subsequent analysis of total dissolved Cu were acidified to pH 2.2 using concentrated HCl (Aristar). Membrane filters were retained (stored frozen) for analysis of particulate Cu. 64 Cu (t1/2 ) 12.7 h; Eγ ) 0.511 MeV) was obtained by irradiation of 3 mg of Cu wire (99.99%; Ventron, Karlsruhe) in the Delft nuclear reactor in a thermal neutron flux of 2‚ 1017 s-1 m-2 for 12 h. After 3 h of “cooling”, the irradiated wire was dissolved in 25 µL of concentrated HNO3 (Aristar) and diluted with Milli-Q water to make a 10-5 mol L-1 Cu2+ stock solution. The activity concentration of this solution directly after “cooling” was 150 GBq L-1, and thus the specific activity (i.e. activity/quantity) amounted to 15 TBq mol-1. Spikes to natural waters were minimized with regard to the amount of Cu added (ca. 5 nmol L-1) in order to reduce the disturbance of the chemical equilibria in the sample, but at the same time measurable 64Cu quantities and reasonable counting times had to be realized. By initiating the experiments immediately after “cooling”, we were able to perform measurements for 2 days (approximately 2 half-lives). Radiotracer Equilibration Experiments. The radiotracer experiments were performed using unfiltered natural waters. A spike of 250 µL of the radioactive Cu stock was added to sample volumes (V) of ca. 0.5 L in acid-cleaned HDPE bottles (increase of total Cu ca. 5 nmol L-1). To reduce loss of Cu
FIGURE 1. Experimental setup for investigation of 64Cu2+ exchange with Cu species present in natural waters. [64CuT(t)] is total Cu activity concentration, 64CuP(t) or [64CuP(t)] active Cu associated with particulate matter, [64CuD(t)] dissolved Cu activity concentration, 64Cu (t) or [64Cu (t)] active Cu associated with Cu2+ and ChelexR R labile Cu, and [64CuC(t)] organically complexed Cu activity concentration. FR1 and FR2 are flow rates for online filtration and dissolved phase separation, respectively. to the walls, the HDPE bottles had previously been equilibrated for 2 h with the sample, which was subsequently discarded. Samples were buffered at pH 7.6 using monosodium N-hydroxyethylpiperazine-N′-2-ethanesulfonate (HEPES, 5 mmol L-1, final concentration) and stirred using a magnetic stirrer (Ikamag, Rec-G) with a Teflon coated magnetic bar. Experiments were performed at ambient laboratory temperature (ca. 19-20 °C). Figure 1 shows the experimental setup for the online filtration and dissolved Cu species separation scheme. Peristaltic pumps were used for propulsion of the sample. Online filtration was performed using Swinnex filterholders (25 mm diameter; Millipore) and polycarbonate membrane filters (0.4 µm pore size, Nuclepore). Dissolved Cu species were separated using Chelex100 columns (200-400 mesh; Biorad), which remove labile Cu from solution and allow strongly complexed Cu to pass (16). Chelex-100 columns have a styrene lattice with iminodiacetic acid exchange groups and were prepared in the laboratory using a polyethylene column (0.5 mL) and ca. 175 mg of Chelex-100 material. Prior to use Chelex-100 was wetted overnight in Milli-Q water, packed in the column, and subsequently rinsed using 10 mL of HNO3 (1 mol L-1) followed by 10 mL of Milli-Q water, and 10 mL of NH3 (0.1 mol L-1), and then Milli-Q water until neutral pH was obtained. All experiments were performed using one filtration unit followed by three Chelex-100 columns in parallel, resulting in triplicate measurements of the dissolved Cu speciation. The relative standard deviation of the triplicate 64Cu measurements was typically smaller than 3%. Immediately upon addition of the 64Cu2+-spike to the water sample, an aliquot was taken for activity determination, i.e., counting (t ) 0). Samples were taken at strategic time intervals t up to 1500 min. The waters were subjected to the VOL. 36, NO. 5, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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experimental procedure involving filtration and dissolved Cu species separation. The flow rate used for the online filtration (FR1) was ca. 10 mL min-1 and for the dissolved phase separation (FR2) ca. 1 mL min-1. The pumping times t′ and t′′ were adjusted to ca. 0.5 and 5 min, respectively, resulting in pumped volumes (FR1‚t′ and FR2‚t") of ca. 5 mL. Each of the experimental procedures resulted in the following fractions to be processed for 64Cu measurement, i.e., counting (see Figure 1): unfiltered sample, membrane filters, filtrates from membrane filters, and eluents from Chelex-100 columns. Counting yielded the following data: total Cu activity concentration ([64CuT(t)]), active Cu associated with particulate matter (64CuP(t) or [64CuP(t)]), dissolved Cu activity concentration ([64CuD(t)]), active Cu associated with Cu2+ and Chelex-labile Cu (64CuR(t) or [64CuR(t)]), and organically complexed Cu activity concentration ([64CuC(t)]). The activity balances are as follows:
[64CuT(t)] ) [64CuP(t)] + [64CuD(t)]
(1)
7.40 6.63 95.6 10.8 166 3.53 × 21.9
Delft
Rotterdam
7.69 21.05 3.26 25.7
7.55 17.88 2.56 47.7
71 10-15
3.3 190 ( 0.5
2.16 × 9.85
103 4.44 × 10-12 6.48
10-15
3.3 82.6( 2
3.0 104 ( 3
15.3 ( 0.5
15.4 ( 0.7
13.4 ( 0.04
99.99
99.99
99.9
CuL is Cu complexed by organic ligands. All Cu analyses were performed using voltammetry. 64
[64CuP(t)] )
CuP(t) in FR1‚t′ (Bq L-1) FR1‚t′
(2)
and
[64CuD(t)] ) [64CuR(t)] + [64CuC(t)]
(3)
where 64
[64CuR(t)] )
CuR(t) in FR2‚t′′ (Bq L-1) FR2‚t′′
(4)
The fraction of active Cu associated with particulate matter was obtained directly through measurement ([64CuP(t)]/ ([64CuT(t)]) and indirectly through calculation (([64CuT(t)][64CuD(t)])/[64CuT(t)]). There was a good correlation between these two data sets indicating that no losses occurred during the filtration step. Seven sets of samples were obtained during the experimental period. All samples were collected in glass counting vials and counted for 10 min using an automatic γ-counter (NaI detector; Wallac 1480 Wizard); the counting error was e 5%. Corrections were made for background radioactivity and 64Cu decay. The activity balance calculations indicated that the loss of 64Cu to the reaction vessel walls was negligible. Voltammetric Analysis. Concentrations of stable Cu in water samples were determined using AdCSV. The voltammetric analyses were performed using an µAutolab voltammeter (EcoChemie) interfaced to a hanging mercury drop electrode (HMDE; 663 VA Stand, Metrohm). Electrochemically labile Cu was measured directly in filtered water samples using salicylaldoxime (SA; 8 µM, final concentration) as added AdCSV ligand (6) and HEPES (final concentration 5 mM, pH 7.8) as pH buffer. Total Cu concentrations in filtered water samples were obtained after UV-digestion of acidified samples in the presence of H2O2 (10 mM), using a 400 W mercury vapor lamp (Photochemical Reactors Ltd) (23). Total dissolved Cu was analyzed following neutralization of the sample using NH3 (purified using isopiestic distillation), and addition of SA (20 µM, final concentration) and HEPES buffer (10 mM, final concentration). Voltammetric conditions used for the determination of total and labile Cu in water samples were as follows: deaeration of an aliquot of 10 mL for 4 min using N2, adsorption of the Cu-SA complex onto the HMDE at a potential of -0.18 V for 1 min, followed by a potential scan using square wave scan modulation (50 Hz) from -0.2 V to 9
Scheldt pH DOC (mg L-1) SPM (mg L-1) labile dissolved Cu (nmol L-1) total dissolved Cu (nmol L-1) Cu2+ (M) SPM leachable Cu (µg g-1) Log KD (L kg-1) Cu complexing ligand (L, nmol L-1) conditional stability constant (log K′CuL) CuL (%) a
where
916
TABLE 1. Characteristics of Water Samples and Results from Stable Cu Analysesa
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 5, 2002
-0.5 V. The peak potential for the Cu-SA reduction appeared at a potential of ca. -0.3 V. Quantification was performed using the standard addition method. Natural Cu Complexing Ligands. Dissolved natural ligand concentrations in filtered freshwater samples were determined by Cu titrations utilizing ligand competition for Cu between an added AdCSV ligand (SA) and natural ligands. For this purpose, approximately 100 mL of water sample was transferred into a PTFE bottle, and 5 mM HEPES and 8 µM SA (final concentrations) were added. The use of HEPES resulted in a pH of 7.6, which was close to the values observed in the natural waters (see Table 1). Copper (stable form) was added to 10 polystyrene vessels giving a concentration range between 20 and 200 nM in 10 steps; 10 mL aliquots of sample were then pipetted into the vessels, and these were covered and equilibrated overnight (ca. 14 h). The electrochemically labile Cu concentration in each vessel was determined using AdCSV. The titration data was linearized using the Van den BergRuzick approach (19), to obtain the total concentration of natural Cu complexing ligands (CL) and the conditional stability constant (K′CuL; CuL is the organically complexed Cu fraction) for the interaction between Cu and natural ligands (L). This mathematical data treatment approach has been described in detail in the literature (4, 25, 26, 27). The following specific values of variables were used for our calculations: log RCuSA ) 5.44 using 8 µM SA (RCuSA is the ratio of CuSA over the free copper ion (24)) and R′Cu′ ) 0.48 in freshwater at pH 7.6 (buffered by HEPES), ionic strength ) 0.02 (R′Cu′ is the R-coefficient for inorganic complexation of Cu2+). Log RCuSA was determined using log K′CuSA and log β′CuSA2 (9.74 and 15.56, respectively; obtained from ref 6), which are the conditional stability constants for the formation of Cu-SA+ and Cu(SA)20 complexes, respectively (in freshwater at the pH used during the analysis), and log R′Cu′ was calculated using an ion-pairing model. The concentration of Cu2+ was calculated using
[Cu2+] ) {-b + (b2 - 4ac)1/2}/2a
(5)
where a ) R′Cu′‚K′CuL, b ) R′Cu′ + CL‚K′CuL - Cutot‚K′CuL, and c ) - Cutot. The dissolved Cu concentration (Cutot) was obtained after UV-digestion of the sample. Additional Measurements. Dissolved organic carbon (DOC) concentrations in filtered samples were analyzed using a high-temperature catalytic oxidation carbon analyzer (Shimadzu TOC 5000) (28). Leachable particulate Cu was
determined in the SPM using an ammonium acetate (1 mol L-1) leaching procedure (2 h incubation time) followed by Cu analysis of the leachate using AdCSV.
Results and Discussion Physico-Chemical Water Characteristics. The natural waters studied were all organic rich and the highest DOC concentrations were observed in the Rotterdam and Delft canals (Table 1). The low SPM levels in the Rotterdam and Delft canals, compared with the Scheldt, can be explained by the stagnant nature of these waters and the decreased wind mixing as a result of ice cover. The Scheldt river showed the highest dissolved and particulate Cu concentrations, which can be attributed to important domestic and industrial wastewater inputs into this system (29-31). The fraction of particulate Cu (as part of total dissolved + particulate Cu) in the Scheldt was ca. 16% and in the other waters
