The influence of temperature on PbCO30 formation in seawater

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Marine Chemistry 110 (2008) 1 – 6 www.elsevier.com/locate/marchem

The influence of temperature on PbCO30 formation in seawater Alan L. Soli a , Zachary I. Stewart a , Robert H. Byrne b,⁎ b

a Department of Chemistry, Eckerd College, 4200 54th Ave. S., St. Petersburg, FL 33711, USA College of Marine Science, University of South Florida, 140 7th Ave. S., St. Petersburg, FL 33701, USA

Received 13 September 2007; received in revised form 14 January 2008 Available online 26 January 2008

Abstract UV spectrophotometry was used to directly observe the partitioning of lead(II) between PbCO03 and the forms of lead in natural seawater (S = 35.4) that are dominant at low pH (Pb2+ and complexes with Cl− and SO2− 4 ). Lead carbonate formation constants were determined in the form, β1 =MPbCO3 (MPbMCO3)− 1, where MPbCO3 represents the concentration (mol/kg) of PbCO03, MPb represents the sum concentrations of free lead (Pb2+) and its complexes with chloride and sulfate, and MCO3 represents the sum concentration of free and ion paired carbonate in seawater. Over a range of temperature between 15 and 35 °C, the reaction enthalpy appropriate to β1 was calculated as ΔH = −1.4± 0.6 kJ/mol. This is sufficiently small that, in the context of the uncertainties in β1 measurements at each temperature, the PbCO03 formation constant over the 20 °C range in temperature is effectively constant (β1 = (1.27 ± 0.02) × 104 (mol/kg)− 1). For total dissolved inorganic carbon concentrations in the order of 2 mm (millimolal), PbCO03 is the dominant inorganic form of lead(II) when pH is greater than 7.6. © 2008 Elsevier B.V. All rights reserved. Keywords: Carbonate; Chemical speciation; Complexation; Lead; Seawater; Spectrophotometry

1. Introduction The marine chemistry of lead is distinctive in a number of respects. In the recent past, anthropogenic levels of lead in the North Atlantic appeared to exceed those of any other trace metal (Schaule and Patterson, 1981). Currently, distributions of lead in the ocean are exhibiting rapid changes in response to decreasing anthropogenic emissions (Boyle et al., 1986; Wei and Boyle, 1997; Boyle, 2001; Reuer, 2002; GEOTRACES Planning Group, 2006). The observed evolution of lead distributions in the oceans is influenced not only by changes in input characteristics, but also by the behavior ⁎ Corresponding author. Tel.: +1 727 553 1508; fax: +1 727 553 1189. E-mail address: [email protected] (R.H. Byrne). 0304-4203/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2008.01.004

of natural removal processes. As is the case for many elements in the oceans (Jannesch et al., 1996), removal of lead from seawater is significantly influenced by sorption onto sinking biogenic particles. It is generally appreciated that this process, termed scavenging (Balistrieri et al., 1981), involves an interplay of surface complexation and solution complexation. The influence of solution complexation on lead speciation is unique in that lead appears to be the only element whose speciation in seawater is strongly influenced by natural organic ligands, chloride ions and carbonate ions in similar proportions. Capodaglio et al. (1990) concluded that lead is complexed in North Pacific surface waters by a single class of organic ligands. Although it was not possible to specify the exact nature and origin of the organic ligands, measured conditional stability constants (log K′cond = 9.7) and

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ligand concentrations (0.2–0.5 nM) indicated that the organic and inorganic fractions of lead were quite similar. Direct observations in natural seawater at 25 °C indicated (Byrne, 1981) that the inorganic lead is partitioned between chloride and carbonate species, with carbonate complexes dominant above pH 7.85. Through recent work on lead complexation (Luo and Millero, 2007), it is now possible to describe the intensity of lead chloride complexation as a function of temperature. However, the influence of temperature on lead carbonate complexation in seawater, and thereby the comparative importance of chloride and carbonate complexation, is unknown. The present work investigates the influence of temperature on the partition of lead between lead chloride complexes and lead carbonate complexes in natural seawater. Through this work, and the work of Luo and Millero (2007), it is possible to model the complexation intensity of inorganically associated lead (total inorganic lead/free Pb2+) over a range of temperature. This work also provides insight into the quality of previous assessments of lead carbonate complexation in synthetic solutions (NaClO4–NaHCO3) at 25 °C and 0.7 M ionic strength (Sipos et al., 1980a,b). 2. Methodology 2.1. Theory The complexation of free lead(II) ions with carbonate can be represented by the equilibrium 0 Pb2þ þ CO2 3 ²PbCO3

ð1Þ

with a formation constant, β1, defined as b1 ¼

MPbCO3 MPb MCO3

lead in seawater, lead speciation is independent of MPb over a wide range of concentrations. The formation constant, β1, then provides the means of partitioning dissolved inorganic lead into a form, PbCO30, that is dominant in seawater at high pH, and an ensemble of forms, represented as MPb, that are dominant at low pH. PbCO30, Pb2+, and lead chloride complexes absorb strongly in the ultraviolet. A representative example of lead absorbance data in seawater is shown in Fig. 1. Of the 33 spectra obtained at the alkalinity and temperature appropriate to Fig. 1, only spectra at the lowest and highest pH values and three spectra at pH values intermediate to the highest and lowest values are shown. Absorbances were corrected for small variations in baseline by monitoring nonabsorbing wavelengths. Data that showed baseline changes in excess of 0.01 absorbance units over approximately two hours were not used for quantitative analysis. On this basis, approximately one third of the data sets at each temperature were excluded from analysis due to excessive baseline drift. Consistent with the observations of Byrne (1981), Fig. 1 shows a strong pH dependent absorbance, with lead carbonate complexation dominant at high pH and lead chloride complexation dominant at low pH. The welldefined isosbestic point at 237 nm suggests that the equilibrium investigated in this work involved a transition between an ensemble of low pH species, present at constant relative proportions, and a high pH complex that contained a single carbonate ligand. The lead absorbance data were analyzed in the same manner as Byrne (1981). Carbonate ion concentrations were determined from alkalinity and pH on the total hydrogen ion concentration scale using the CO2SYS computer routine (Lewis and Wallace, 1998). The

ð2Þ

where MPbCO3 is the concentration of PbCO30, MCO3 represents the sum concentrations of free plus ion paired carbonate in seawater, and MPb is the sum concentration of all forms of lead exclusive of PbCO30, PbOH+ and, of course, organic complexes. In seawater, MPb dominantly consists of free hydrated Pb2+, chloride complexes (PbCl+, PbCl20, PbCl3−, etc), and PbSO40 at levels that are significantly lower than those of Pb2+. Therefore, Eq. (1) represents a substitution reaction where carbonate replaces one or more chloride ions associated with Pb2+. It should be noted that the relative concentrations of chemical forms that constitute MPb are independent of pH. Furthermore for lead concentrations well in excess of the concentrations of the natural organics that complex

Fig. 1. Absorbance of lead(II) in natural seawater (with enhanced alkalinity) as a function of wavelength and pH (25 °C, Alkalinity= 3789 μmol/kg).

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absorbance of lead in seawater can be described by Eq. (3) k A¼

k e0

þ k e1 b1 d MCO3 1 þ b1 d MCO3

ð3Þ

where λA is the absorbance at wavelength λ, MCO3 is the concentration of carbonate (mol/kg), λε1 is the absorbance due to the PbCO30 complex, and λε0 is the absorbance due to the lead(II) species which predominate at low pH. 2.2. Materials and instrumentation All experiments used unfiltered seawater obtained from a depth of five meters in the Gulf of Mexico (35.78 salinity). This seawater was initially acidified and sparged with nitrogen gas to remove all dissolved CO2. The pH of this sparged seawater was typically 3.7–3.9. Solution pH was determined with a Ross-type combination pH electrode (Orion #800200) and an Orion Model 720A pH meter in the absolute mV mode. The electrode was calibrated on the total hydrogen ion concentration scale using a Tris buffer prepared following the method of Dickson (Dickson, 1993; DelValls and Dickson, 1998). Nernstian behavior of the electrode was verified by titrations of 0.725 m NaCl at 25 °C with a standard HCl solution. A Nernstian dependence of slope on temperature was assumed for temperatures greater and less than 25 °C. Dry Na2CO3 was added to a weighed portion of the sparged seawater to bring the alkalinity to a well-defined value within the range 2300–6000 μmol/kg. Immediately prior to each experiment, the pH of this seawater was lowered to about 5.7–5.9 by sparging with a 30% CO2 gas standard. Absorbance measurements were made with a Hewlett Packard 8453 UV–visible spectrometer. The linear array detector of this spectrometer allows measurements across entire UV spectral range (220–400 nm) on a 1 Hz time scale. One hundred and fifty mL of seawater that was prepared as described above was transferred by pipette into a 10-cm open top quartz spectrophotometric cell. The calibrated pH electrode, an overhead stirrer, a temperature probe, and a fritted sparging tube were then inserted in the cell. While the seawater was continuously sparged with CO2, a baseline absorbance spectrum was obtained. Addition of 1.50 mL of acidified Pb(II) solution (0.001024 M in 0.0010 M HCl) to the seawater produced a lead(II) concentration equal to 10.2 μM. Subsequently changing the sparging gas to N2 caused the seawater pH to gradually increase. Approximately 30–35 spectra with corresponding pH values were recorded until the pH reached approximately 7.9–8.1.

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Possible baseline shifts were monitored at absorbances between 370–390 nm, where lead species are nonabsorbing. In each experiment, the temperature of the spectrophotometer cell was held constant to ± 0.1 °C with a Lauda K-2R refrigerated thermocirculator. After correcting both the salinity and the alkalinity of the seawater for addition of the acidified Pb(II) solution (final salinity equal to 35.43), the carbonate ion concentration for each spectrum was determined from the measured pH and known constant alkalinity using CO2SYS (Lewis and Wallace, 1998). The CO2 system constants used in this work (total hydrogen ion concentration scale) were those of Mehrbach et al. (1973) as modified by Dickson and Millero (1987). 3. Results and discussion Lead-absorbance data at each temperature were obtained at three or four different alkalinity values and were analyzed at seven different wavelengths (240, 245, 250, 255, 260, 265, and 270 nm). A superimposed set of data, obtained at 25 °C and λ = 250 nm is shown in Fig. 2. For each individual analysis, λA data were fit (Eq. (1)) via non-linear least-squares analysis with MCO3 as the independent variable (Byrne, 1981). Determinations of β1 for each experiment are shown in Table 1. Eq. (1) was also modified to account for the potential formation of PbHCO3+ and Pb(CO3)22−. The concentration of bicarbonate is much higher than that of MCO3 in the experimental pH range and remained relatively constant during the course of each experiment. Nonlinear analysis including terms for PbHCO3+ and Pb(CO3)22− resulted in formation constants that were poorly defined and far too small to have any impact on lead(II) speciation in seawater.

Fig. 2. Absorbance at 250 nm expressed as a logarithmic function of the carbonate ion concentration (25 °C).

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Table 1 Replicate values for lead(II)–carbonate formation constants, β1, at each temperature and alkalinity Temperature (°C) Alkalinity (μmol/kg) Wavelength (nm)

15

20

25

30

35

2282 4002 4840 3923 5504 2282 2768 4275 5204 3789 4612 5489 4551 2737 5186 4598 6000 2504 5660

Average (±S.E.) ln(average) (± S.E.)

240

245

250

255

260

265

270

11,550 12,650 11,620 12,190 11,930 12,190 10,420 11,900 12,850 11,950 11,200 15,030 10,460 12,670 10,900 12,320 12,110 11,970 10,420

12,440 13,140 12,580 13,080 12,130 12,470 12,210 12,400 12,710 12,310 11,930 13,920 10,760 12,950 11,470 12,690 12,450 11,950 10,480

12,790 13,480 13,080 13,400 12,630 12,650 12,790 12,830 13,040 12,610 12,350 14,140 11,360 13,160 11,930 13,110 12,950 12,060 10,800

12,940 13,660 13,390 13,620 13,050 12,830 13,030 13,130 13,360 12,810 12,610 14,490 12,110 13,240 12,320 13,490 13,400 12,110 11,120

12,980 13,760 13,520 13,680 13,380 12,860 13,030 13,290 13,590 12,900 12,720 14,790 11,840 13,300 12,560 13,710 13,670 12,180 11,340

12,670 13,710 13,370 13,520 13,560 12,730 12,750 13,360 13,680 12,810 12,660 15,080 11,400 13,230 12,680 13,690 13,780 12,200 11,500

11,840 13,420 12,970 13,130 13,610 12,280 11,950 13,210 13,570 12,530 12,270 15,500 11,900 12,980 12,640 13,380 13,680 12,140 12,140

A critical review of hydrolysis constants (Baes and Mesmer, 1976) for formation of PbOH+ in NaClO4 at 25 °C indicated that ⁎β1 = [PbOH+][H+] / [Pb2+] is on the order of 10− 7.8 or less (Olin, 1960a,b) for ionic strengths between 0.3 and 3 M. In view of the extent of lead chloride and lead sulfate complexation in seawater, whereby MPb / [Pb2+] ~ 11.5 (Byrne et al., 1988), the lead (II) hydrolysis constant defined in terms of MPb would become [PbOH+][H+] / MPb ~ 10− 8.9. Using a formation constant of this magnitude at a pH as high as 8.1, calculations reveal that [PbOH+] could represent on the order of 4.5% of the total dissolved lead in seawater. For our seawater samples that were enriched in carbonate, this fraction would be reduced by more than a factor of two. Inspection of the formation constant data shown in Table 1 demonstrates no systematic dependence of β1 on seawater alkalinity as alkalinity ranged between 2282 and 6000 μmol/kg. Thus, no influence of PbOH+ on our formation constant determinations could be discerned within the uncertainties of our measurements. Furthermore, as noted previously, observations of well-defined isosbestic points further add to the conclusion that Pb2+ hydrolysis exerted, at most, a very minor influence on our data interpretations. The temperature dependence of the formation constant of PbCO30 is shown in Fig. 3 in the form of a van't Hoff plot. Error bars reflect the standard error of the central estimate at each temperature. The superimposed linear fit has a very small slope. Considering the very small slope and the uncertainty of β1 at each

13,006 (± 118)

9.4720 (±0.0093)

12,664 (± 125)

9.4452 (±0.0103)

12,668 (± 128)

9.4474 (±0.0103)

12,815 (± 258)

9.4529 (±0.0201)

12,387 (± 190)

9.4212 (±0.0156)

temperature, it is seen that β1 is essentially constant over the experimental temperature range. b1 ¼ ð1:27F0:02Þ  104

ð4Þ

The previous result obtained by Byrne (1981) at 25.0 °C and 34.7 salinity was β1 = (10.0 ± 0.3) × 103. The difference between this value and the result shown in Eq. (4) may be attributable to the use of the NBS pH scale by Byrne (1981). Use of modern seawater pH scales, such as the total hydrogen ion concentration scale, obviates potential problems associated with residual liquid junction potentials (Dickson, 1993).

Fig. 3. Temperature (van't Hoff) relationship for lead(II)–carbonate complexation in natural seawater.

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The van't Hoff relationship appropriate to Fig. 3 is given as ln b1 ¼ ð8:89F0:23Þ þ ð167F69Þð1=T Þ

ð5Þ

where the slope in Eq. (5) is equal to −ΔH /R, and R is equal to 8.3145 J/mol∙K (Stumm and Morgan, 1996). Estimation of the reaction enthalpy from this relationship yields ΔH = −1.4 ± 0.6 kJ/mol (−0.33 ± 0.13 kcal/mol). This very small value, with considerable relative uncertainty, reinforces the idea that β1 is essentially invariant over the temperature range employed in this work. The small reaction enthalpy obtained in this work implies that the influence of temperature on the formation of PbCl+, PbCl20 and PbCl3− in seawater is very similar to the influence of temperature on PbCO30 formation from Pb2+ and CO32−. Complexation of lead(II) by CO32− in seawater is a substitution reaction with the carbonate replacing one or more chlorides. If the bond energies of the lead carbonate and lead chloride are quite similar, replacement of lead chloride complexes with lead carbonate species will result in a very small overall reaction enthalpy in seawater, and substantial constancy of β1 over the experimental temperature range. This work indicates that PbCO30 represents more than 50% of the total dissolved inorganic lead in seawater when CO32− concentrations are greater than approximately 80 μM. For total dissolved inorganic carbon concentrations on the order of 2000 μm, it then follows that PbCO30 is the dominant inorganic form of lead(II) when pH, on the total hydrogen ion concentration scale, is greater than about 7.6 (as shown in Fig. 4). Very few observations of lead carbonate complexation have been obtained at ionic strengths relevant to seawater. The formation constants of Sipos et al. (1980b), based on the results of Sipos et al. (1980a),

5

were obtained in NaClO4–NaHCO3 mixtures at 25 °C and an ionic strength (0.725 m) appropriate to seawater. Sipos et al. (1980b) reported   PbCO03 b 1V ¼  2þ  2  ¼ ð4:19F0:22Þ  105 ð6Þ CO3 T Pb where [CO32−]T = [CO32−] + [NaCO3−]. After corrections are made for CO32− ion pairing and Pb2+ complexation by Cl− and SO42− in seawater, the result reported by Sipos et al. (1980b) can be directly compared with the result in Eq. (4). In a comparison of CuCO30 formation constants in 0.725 m NaClO4 and seawater at 25 °C and S = 36.1, Byrne and Miller (1985) estimated MCO3 / [CO32−] = 7.14 and [CO32−]T / [CO32−] = 2.23. As such, the relationship between the MCO3 and [CO32−]T parameters in Eqs. (2) and (6) is MCO3 = 3.20 [CO32− ] T. The relationship between the MPb and [Pb2+] in Eqs. (2) and (6) can be calculated using the lead chloride and lead sulfate results summarized by Byrne et al. (1988): MPb = 11.8 [Pb2+]. Substitution of these MCO3 / [CO32−]T and MPb / [Pb2+] relationships into Eq. (6) results in the estimate   PbCO03 ¼ 1:11  104 : ð7Þ MPb MCO3 This result is in reasonable agreement with the direct measurements reported in Eq. (4). It is likely that the concordance between the two results (Eqs. (4) and (7)) would improve somewhat if it were possible to quantitatively account for the contribution of the mixed ligand species, PbClCO3−, to direct measurements of β1 (Eq. (4)) in seawater. The results obtained in this work indicate that the partition of PbII between carbonate and chloride complexes is not sensitive to temperature. The quantitative characterization of lead chloride complexation (Luo and Millero, 2007), and the comparative carbonate/chloride speciation of lead in seawater, stand in contrast to what is known about the organic speciation of PbII in seawater. While the inorganic speciation of PbII is relatively well characterized as a function of temperature and pH, the influence of temperature and pH on lead complexation by natural organics in seawater is unknown. Acknowledgements

Fig. 4. Species distribution plot for inorganic lead in seawater (t = 25 °C, S = 35) as a function of pH.

This work was supported in part by the Office of Naval Research: contract N00014-02-1-0823. We also gratefully acknowledge the reviewers for their helpful suggestions.

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