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Marine Chemistry, 29 (1990) 221-233 221 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands Lead Speciation in Surface Waters of the Eastern North Pacific GABRIELE CAPODAGLIO Department of Environmental Sciences, University of Venice, 30123 Venice (Italy) KENNETH H. COALE* and KENNETH W. BRULAND Institute of Marine Sciences, University of Cali/ornia at Santa Cruz, Santa Cruz, CA 95064 (U.S.A.) (Received November 28, 1988; revision accepted November 20, 1989) ABSTRACT Capodaglio, G., Coale, K.H. and Bruland, K.W., 1990. Lead speciation in surface waters of the eastern North Pacific. Mar. Chem., 29: 221-233. Titrations using differential pulse anodic stripping voltammetry (DPASV) to detect electroactive lead were carried out on fresh seawater samples and on samples stored acidified to determine the extent of lead complexation in the eastern North Pacific. Results of these analyses on surface water samples indicate total dissolved lead concentrations between 17 and 49 pM. The inorganic or DPASV labile fraction is 30-50%. Titration with lead yielded data consistent with one class of organic ligand(s), present at low concentration {between 0.2 and 0.5 nM) with a mean value for a conditional stability constant with respect to inorganic lead of log K'~ond=9.7.The presence of this ligand, together with the various inorganic ligands in seawater, gives rise to a concentration of free ionic lead of ~ 0.4 pM. INTRODUCTION Chemical properties of heavy metals with respect to their interaction with suspended particles, sediments and aquatic organisms are strongly affected by the chemical forms in which dissolved metals exist in natural waters (Salomons and FSrstner, 1984). The inorganic speciation of heavy metals such as lead has been studied extensively, although the results are at times conflicting (Sipos et al., 1980; Byrne, 1981; Turner et al., 1981). To understand the toxicity, bioavailability and geochemical reactivity with respect to particle scavenging in the water column, the organic complexation of trace metals also needs to be known. Although we are beginning to understand more about the organic *Present address: Moss Landing Marine Laboratories, P.O. Box 450, Moss Landing, CA 95039O45O, U.S.A. 0304-4203/90/$03.50 © 1990 Elsevier Science Publishers B.V. 222 G. CAPODAGLIO ET AL. complexation of some metals (e.g.copper, Fiirstner and Salomons, 1983; van den Berg, 1984; Kramer, 1986; Coale and Bruland, 1988), relatively littleis known about lead. Lead is a highly toxic element and the predominant source of lead to the ocean is anthropogenic (Schaule and Patterson, 1981; Flegal and Patterson, 1983; Boyle et al.,1986). Boyle et al. (1986) have shown that different geochemical processes act on lead at differentdepths. A residence time of ~ 2 years was observed in surface waters where the largest biological activityis concentrated, whereas in deep waters a residence time of a few hundred years was estimated. In the last decade the application of differentialpulse anodic stripping voltammetry ( D P A S V ) has been introduced to study speciation of heavy metals in natural waters (Nurnberg et al.,1976; Plavsic et al.,1982). This technique provides the possibilityof defining two fractions of dissolved lead: (1) the A S V electroactive or labilefraction (which includes inorganic or weakly complexed species) and (2) the inert fraction (strongly complexed metal species that are not electroactive). Titrations of seawater samples with lead, followed by adequate equilibration and D P A S V analysis, allows estimation of the concentration of ligands which have affinity for lead and the conditional stabilityconstant for the lead-ligand association (Ruzic, 1982; Plavsic et al.,1982). The abilityto conduct trace metal speciation experiments in the open ocean poses unique problems that have only recently been overcome. A prerequisite for speciation studies is the collection and processing of samples uncontaminated for the element of interest as well as other potentially competing metals and organic contaminants. It is imperative in such studies that the abilityto obtain the proper total metal value can be demonstrated, especiallyfor metals such as lead, a ubiquitous and contamination-prone element. M a n y speciation experiments have been jeopardized or rendered meaningless because of inadequate processing techniques and inadvertent contamination. In this study, great care was taken to ensure that a high degree of sample integritywas maintained throughout the course of sample collectionand analysis. CONCEPTAND TERMS If we consider a complexation equilibrium between lead ion and a class of natural organic ligands, [L n- ],with a stoichiometric ratio of I: 1,we can define a thermodynamic equilibrium constant K = [PbL 2-n ]/ [Pb 2+ ] [L ~- ]. Thermodynamic constants can be corrected by the use of activity coefficients (7) of the various species in solutions of high ionic strength such as seawater. W e can then define K' = K/(~PbL/~Pb~L). Other corrections can be made ifwe consider the complexation side-reactions of lead and organic ligands with inorganic species. Metal cations can be complexed by inorganic anions such as CO~-, O H - and CI-, and the organic ligand (s) may be coordinated by major LEAD SPECIATION IN THE EASTERN NORTH PACIFIC 223 cations such as Ca 2+ and Mg 2+. These side-reactions can strongly affect the free metal ion and free organic ligand concentrations that enter in the thermodynamic stability constant. The side-reaction coefficient (aM and ~L) (Ringbom and Harju, 1972; Ringbom and Still, 1972) provides an expression for these side-reactions. The free lead ion and free ligand concentrations are, respectively, [Pb 2+ ] = [Pb' ]/aPb' and [L"- ] = [L' ]/C~L,, where [Pb'] is the concentration of dissolved lead present in all the inorganic forms and [L' ] is the total concentration of the organic ligand present not bound with lead. For seawater conditions the complexation equilibrium can be expressed as a conditional stability constant (K'cond), related to K' by side-reaction coefficients: K'cond= [ P b L ] / [ P b ' ] [L'] = [ P b L ] / [ P b 2+ ]o~pb,[Ln- ]O~L, K'cond=g'/oLpb, OLL,. With DPASV, [Pb'] is the inorganic lead fraction detected, [PbL] is calculated by difference between total Pb and [Pb' ], and K'condis calculated. As ~L' for natural ligand is not known, the K'condcannot be converted to a thermodynamic stability constant; however, K'condand [L'] for the Pb-ligand interaction in seawater allow an estimation of the extent of lead complexation with organic ligands. METHODS The samples collected for this study were obtained during the August 1987 leg of the VERTEX VII cruise in the eastern North Pacific (see Fig. 1; for coordinates, see Table 1 }. Seawater was collected in the mixed layer (10 m) using internally Teflon-coated 30-1 Go-Flo bottles (General Oceanic) deployed on Kevlar hydrowire (Philadelphia Resins) and tripped with a Teflon messenger. Great care was taken to avoid contamination from the ship. Upon recovery, the Go-Flo bottles were pressurized at 0.5 atm with filtered nitrogen gas and the samples were collected in 2-1 FEP bottles. Samples for total lead anal! 60°N • oo i °e°° 40ON J ~ Cruz Pacific Ocean 20°N i 180°W I60°W i i 140°W I i 120°W I lO0°W Fig. 1. Locationof samplingstations in the eastern North Pacific. 224 G. CAPODAGLIOET AL. TABLE 1 Location,temperatureand chlorophyllfor the stations occupiedduringthe August1987VERTEX VII cruise Position Coordinate Temperature (°C) Chlorophyll (,ugl -~) T8 T7.5 T7 TA TB TC TD TE TF TG TH 55 ° 26' N 52°39'N 50° 19'N 48°03'N 46°52'N 45°25'N 44°11'N 42°44'N 41°24'N 39°51'N 38°31'N 13.4 12.1 12.4 14.1 14.7 15.7 16.1 17.1 17.7 16.9 16.9 0.350 -0.598 0.258 0.173 0.176 0.048 0.068 0.092 0.078 -- 147 ° 14'W 146°39'W 144°42'W 140° 19'W 138 ° l l ' W 135°37'W 133°29'W 131°06'W 129°00'W 126°46'W 124°42'W ysis were collected in 500-ml polyethylene bottles and acidified with 1 ml of quartz-distilled HC1. The samples collected at stations T7 and T8 (see Fig. 1 ) were filtered by forcing the seawater from the Go-Flo bottle (0.5 atm) through an acid-cleaned polycarbonate membrane filter (142-mm diameter, 0.3-/lm pore size, Nuclepore) in a Teflon filter-holder (Millipore). All manipulations of open samples at sea took place within a positive-pressure clean-air sampling van or under a Class 100 laminar flow hood within a positive-pressure cleanair analytical van. An Adriatic Sea sample was collected about 3 miles offshore of the Lagoon of Venice for the production of UV-oxidized seawater, and to examine the kinetic lability of metal-organic complexes. DPASV M E A S U R E M E N T Measurements on the North Pacific samples were carried out by DPASV using the apparatus and instrumental modifications described earlier (Mart et al., 1980; Bruland et al., 1985; Coale and Bruland, 1988). Measurements on Adriatic Sea samples were carried out with an EG&G Model 264A Polarographic Analyzer, using a rotating glassy carbon disk electrode (EG&G Rotel 2). All potentials reported are relative to Ag/AgC1,KC1 saturated. The sensitivity of the commercial apparatus was ~ 15% lower, primarily because its maximum electrode rotation speed was 4000 r.p.m. The procedure for the preparation of the mercury film and blank control was as described by Coale and Bruland (1988). Measurements were carried out by deposition at - 0 . 7 5 V and 5000 r.p.m, for 15 min, followed by a quiescent LEAD SPECIATION IN T H E EASTERN NORTH PACIFIC 225 period of 30 s. The potential scan was then started at 10 mV s- 1 in the positive direction until a final potential of - 0 . 1 5 V was reached. The electrode was rotated and held at this potential for 5 min to strip out any residual metals from the film. All iead additions were allowed to equilibrate for 15 rain before the next deposition step. For the zero-addition point, DPASV analyses were carried out using the same operating conditions, except that the deposition time was increased to 30-60 min. This improved the precision in the Pb' determination. The currents were normalized to 15 min of deposition time in accordance with the rest of the titration. Upon completion of a titration the sample volume was measured. The detection limit, expressed as three times the noise, using 15 rain of deposition time, was 10 pM. Detection limits were proportionally reduced to 5 and 2.5 pM by increasing the deposition time to 30 and 60 rain, respectively. Two initial lead additions each increased the lead concentration in the cell by ~ 40 pM, then larger additions were carried out until a final lead concentration of ~ 3 nM was obtained. An equilibration experiment was performed using seawater from station T7.5 to examine potential kinetic effects on the complexation between lead and natural ligands. In this experiment ~ 60-ml aliquots of fresh sample were transferred to a series of cleaned 60-ml F E P bottles and each aliquot was spiked with lead to an amount equivalent to the standard additions used in a normal lead titration. This set of bottles, representing a single titration, was allowed to equilibrate for 24 h before analysis. These samples were analysed using the same analytical procedure as for a normal titration except that the electrode was turned off at the end of each deposition/stripping cycle and the next degassed aliquot transferred to the analytical position. The total initial concentration of lead was determined by DPASV at the UCSC Laboratory on samples acidified to pH 2 with hydrochloric acid and stored in clean polyethylene bottles for at least I week. The method of standard additions was used and at least four spike additions were made (correlation coefficients were always > 0.999). The relative standard deviation (RSD) for the inorganic fraction, for the zero addition measurement, was 20% ( n = 5 ) for measurements carried out on an Adriatic Sea sample (lead concentration 40 pM) and with Pacific Ocean samples replicated. A more extensive statistical evaluation of the repeatability and significance of parameters available from titrations was carried out on samples collected in the Antarctic Ocean to study speciation of lead (Capodaglio et al., 1990); the RSD for the total initial concentration was 7% for a concentration of 78 pM, the RSD for the labile fraction in the initial sample was 20% for a concentration of 40 pM, for a ligand concentration of 0.37 nM the RSD was 10%, and for the conditional stability constant, calculated by linearization of the titration point with the Ruzic and van den Berg method (Ruzic, 1982; van den Berg, 1982 ), it was 30%. 226 G. C A P O D A G L I O E T AL. LABORATORY R E S U L T S The application of ASV techniques to the study of metal-organic ligand interactions has raised a certain amount of controversy and criticism. The principal uncertainties arise from the possible reduction of metal-organic complexes at the potential applied in the deposition step and from the kinetic lability of metal complexes with organic ligands in the diffusion layer (Tuschall and Brezonik, 1981 ). Both processes would lead to an underestimation of the conditional stability constant and of the amount of ligands present. In this study, an effort was made to minimize these potential sources of error. To determine whether there was an effect due to kinetic lability of metalorganic complexes, titration experiments were carried out at three different rotation rates on an Adriatic Sea water sample. No significant differences were observed in the conditional stability constant using rotation speeds of 4000, 3000 and 2000 r.p.m. (see Table 2 ). This indicates that the Pb-organic ligand complexes were kinetically inert with respect to the electrochemical techniques. However, to minimize the potential for a kinetic contribution and to maximize sensitivity, the highest rotation speed was used. To minimize any possible reduction of organically complexed lead, a relatively weak deposition over-potential (for high-chloride media) was applied ( - 0 . 7 5 V). To determine the optimum deposition potential for this study, a pseudo-polarogram was made (curve obtained by plotting the oxidation current vs. the deposition potential). For deposition potentials more negative than -0.72 V, no increase in peak current was observed. A potential of -0.75 V was selected because it is the weakest possible potential in the reproducible portion of the pseudopolarogram plateau. Analogous to the work of other authors for copper (Plavsic et al., 1982; Coale and Bruland, 1988 ), an attempt was made to verify better the DPASV technique by testing it against well-characterized model ligands. EDTA {28.1 nM) was added to UVSW and allowed to equilibrate with lead additions for at least 1 h before analysis. These titrations were modelled according to the linear transformation method used by Ruzic (Ruzic, 1982; van den Berg, 1982 ). Lead TABLE2 Ligand concentration and conditional stability constant measured by titration with lead at three different rotation speeds Rotation speed (r.p.m.) Concentration (nM) K' (1 tool -1) 4000 3000 2000 7.2 8.1 7.9 3.8 × lOs 3.7 X 10 s 4.4 × 10 s LEAD SPECIATION IN THE EASTERN NORTH PACIFIC 227 was added until at least four sequential additions gave a linear response. The sensitivity, obtained from the slope of the response to the last additions, was used to calculate the inorganic fraction observed for each addition. If lead and the organic ligand form a complex whose binding stoichiometry is 1:1, plotting the ratio of inorganic lead to organically complexed lead [Pb' ]/[PbL] vs. inorganic lead [Pb' ] should yield a straight line. The slope and the intercept of this line allow calculation of the concentration of the ligand and the conditional stability constant (Ruzic, 1982; Coale and Bruland, 1988). The experimental results yielded values very close to those expected: the EDTA concentration measured was 27.3 nM against 28.1 nM added and the measured log K'condwas 8.6, which is in good agreement with the result calculated for seawater at our experimental conditions (at pH =7.8 and low CO2 content, a log apb, = 0.9 was calculated) of log K'cond= 8.5. These results further support the observations of others using DPASV with model ligand systems (Raspor et al., 1980). To observe the DPASV response to lead additions in the absence of complexing organic ligands, UV-oxidized seawater (UVSW) (Coale and Bruland, 1988) was titrated according to the procedure outlined above. Results show a linear relation between anodic current and lead concentration; the final concentration was 4.3 nM (correlation coefficient 0.9996 ). To investigate the specificity of the natural ligands which show affinity for lead, some tests were performed using seawater samples. Iron and copper were added in large excess (20 nM) with respect to the lead complexing ligand. After 30 min equilibration and subsequent deposition, the lead signal during the stripping cycle showed no change, indicating no significant competition between lead and the added metals. In a second experiment, two aliquots of the same sample were prepared. To one aliquot 0.83 nM of Fe (III) was added and equilibrated. Both aliquots were then titrated and analysed. Results of the two samples yielded ligand concentrations and a conditional stability constant of [ L ] = 0.49 nM, K'cond= 2.4 × 109 1 mol- 1 and [ L ] = 0.56 nM, K'~ond= 3.3 × 109 l mol-1, respectively, indicating little, if any, competition by iron for the ligands which show affinity for lead. FIELD RESULTS AND DISCUSSION Samples for Pb complexation analysis were collected at 11 locations in the eastern North Pacific (see Fig. 1 ). The titrations were carried out on the ship within 3 days of collection. Results including total lead, ASV labile lead, ligand concentrations and conditional stability constants are presented in Table 3. The ligand concentrations and the relative conditional stability constant were calculated by the graphical linearization illustrated in the laboratory results section. Figure 2 shows a typical titration curve and the corresponding linearization plot according to the Ruzic and van den Berg method. 228 G. CAPODAGLIO E T AL. TABLE 3 Concentrations of lead species, ligand and conditional stability constants for the samples collected in the North Pacific Ocean Position [ Pb ] t (pM) [ Pb' ] (pM) [ Pb e + ] (pM) [L ] (nM) log (K'cond) T8 T7.5 T7 TA TB TC TD 17 31 30 49 27 25 25 4 15 8 11 . 10 22 0.1 0.5 0.2 0.3 0.22 0.24 0.31 0.51 10.0 9.8 9.4 9.6 TE 30 10 TF TG TH 30 41 37 13 24 29 0.24 0.24 0.54 0.56 0.41 0.47 9.8 9.9 9.6 9.6 9.8 9.7 . . 0.3 0.7 0.3 0.4 0.7 0.9 . 1.5 o < ::L 4 1.0 C 5 -Q \ "U 2 o.s g g cL 0 /yl p / // 0D I 1 Pb concentration, 2 nM Fig. 2. Lead titration of station TE; ( O ) current vs. total concentration, ( O ) inorganic lead/ organically complexed lead ratio vs. inorganic lead. Graphical linearizations of inorganic/organically complexed lead vs. inorganic lead showed a kinetic effect (Ruzic and Nicolic, 1982). To examine this phenomenon, the sample collected at T7.5 was titrated using a 10-min equilibration time (5 rain less than the normal time of 15 min) between standard additions, and the results were compared with a titration equilibrated for 24 h. The kinetic effect was not apparent in the sample equilibrated for 24 h (see Fig. 3 ). The ligand concentrations and conditional stability constants obtained 229 LEAD SPECIATION IN THE EASTERN NORTH PACIFIC 15 I 0 10 min. • 24 hr. i ~ 0 o o 0 I 1 I 2 3 Pbinorg '(nM) Fig. 3. Linear t r a n s f o r m a t i o n of lead t i t r a t i o n of t h e T7.5 sample using ( O ) 10 rain a n d ( Q ) 24 h of equilibration time. by linear transformation of the experimental data were [L']=0.23 nM, Kcond=5.5X 109 1 tool-1 and [L'] =0.24 nM, K'cond=6.0X !09 1 tool-I with 10 rain and 24 h of equilibrationtime, respectively.For the titrationequilibrated for only 10 rain,the zero-addition point and the last six points of the titration were used to estimate [L' ] and K'co,d.Thus, even though an apparent kinetic effect was observed, the results obtained by the linear transformation of the data and the interpretationof Ruzic and Nicolic (1982) were not significantly different.Using the 10-rain equilibratedtitrationand the kinetictreatment of titrationdata presented in Ruzic and Nicolic (1982), a complexation rate constant of kf= 5 × 104 was estimated. The bulk of the samples were equilibratedfor 15 rain and exhibited a slight kinetic effect in the initialadditions,as a resultof lack of complete equilibration of the added lead with the organic ligands. However, estimates of the ligand concentrations and conditional stabilityconstants could stillbe readily made by linearization of the zero-addition and the last six titration points. Good precision was obtained in the measurement of ligand concentrations; the R S D s were ~ 15%, calculated from the slope of the straightline obtained from the graphical linearization.Lower precision was observed in the measurement of the conditional stabilityconstant obtained from the intercept ( R S D was 35%). The total lead concentrations vary between 17 and 49 pM and show little spatial structure (see Fig. 4 ). The horizontal distribution of ligand concentration and associated conditional stability constants are plotted in Fig. 5A and B. The ligand concentration varies between 0.2 and 0.5 nM. Although there appears to be a possible increase in ligand concentration within the California Current, no apparent trends in the stability constants for this association are evident. The ASV labile fraction that we consider as the inorganic fraction, 230 G.CAPODAGLIOETAL 80 50 O 0~0 Total Pb @--@ I n o r g a n i c Pb " o /-o / 30 0.._..0__0/0-0 o. so . / @\ / lo 0 ~o~o ~ @/0 / _ ~O I I t I I i I I t I I T-8 T-7.6 T-7 T-A T-B T-C T-D T-E T-F T-G T-H Station Fig. 4. Geographical d i s t r i b u t i o n o f total lead ( O ) a n d inorganic fraction ( • ). 11 "a co %O ko 0 .j 9 A 0.6 t I I I t I I I I I I I I I I J I I I I I I 7.5 7 A B C D E F G H ~' ¢- 0.4 ¢C:~ 0.2 .J B 0.0 8 T-Station Fig. 5. Geographical distribution of (A) log K'condand (B) lead-complexing ligand concentration. [Pb' ], represents an average of 48% of the total. The free lead fraction was calculated by the application of side-reaction coefficients, as previously described. A value for the inorganic side-reaction coefficient of log ~Pb' = 1.51 (for p H = 8 . 1 ) was used (Turner et al., 1981). Free lead concentrations obtained are shown in Table 3 and vary between 0.1 and 0.5 p M in the Subarctic Gyre, and from 0.3 to 0.9 p M in the California Current. The free lead fraction LEAD SPECIATION IN THE EASTERN NORTH PACIFIC 231 represents ~ 1.4% of the total lead concentration, about half the value expected if only inorganic ligands were considered (Turner et al., 1981 ). No appreciable differences were noticed in lead speciation between filtered samples (T7 and T8) and an unfiltered sample collected between those positions (T7.5) (see Table 3). This is consistent with results observed for copper (Coale and Bruland, 1988). Thus, the bulk of the organic complexation exists in the dissolved ( < 0.3 nM) phase. However, possible complexation due to colloids cannot be ruled out. For elements naturally present in biological systems as co-factors, e.g. copper and iron, ligands with high selectivity might be expected in natural systems. However, there is no apparent explanation for the high specificity observed for the lead complexing ligands. The high value of the inorganic sidereaction coefficient of Fe(III) in surface seawater (log C~Fe=12.0; Turner et al., 1981 ) might be the reason for the low competition of this element for the lead complexing ligands; however, this does not explain the low competition from copper. More extensive studies will be needed to address the apparent selectivity of this class of ligands for lead. A prerequisite for accurate speciation studies is the ability to collect, process and store samples without contamination. Even picomolar levels of inadvertent lead contamination could significantly alter the results obtained for lead speciation. Our total dissolved lead concentrations are consistent with those previously determined in the California Current waters (Bruland et al., 1985) and in eastern North Pacific samples (Schaule and Patterson, 1981; Flegal et al., 1986). We can therefore be confident that underestimations of the complexing ligands have not occurred as a result of trace metal contamination. CONCLUSIONS According to this study, the organically complexed lead fraction contributes a significant portion of the total (50%) and must be considered in speciation models for lead in surface seawater. Comparison between samples equilibrated with lead for 10 rain and 24 h shows that the kinetics of the association of lead with ligands is not as fast as observed for copper (Plavsic et al., 1982; Coale and Bruland, 1988). We are unable to define the exact nature and origin of this class of complexing ligands. 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