Palladium Chemistry in Seawater Lee R. Kumpt and Robert H. Byme'
Department of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg, Florida 33701 The solution chemistry of palladium has been examined directly in seawater by ultraviolet absorbance spectroscopy. Our results indicate that chloride complexation strongly suppresses the hydrolysis of Pd2+in seawater. At 25 "C and salinity equal to 36, hydroxy complexes account for 18% of the total palladium and the remainder is present principally as PdCl,". Strong similarities previously reported for Pd and Ni distributions in seawater appear to have dissimilar origins at the molecular level. The enormous reactivity of Pd2+ relative to Ni2+ appears to be moderated, in seawater, by extensive complexationof Pd2+ with chloride ions.
-
Introduction The development of modern, clean analytical techniques has produced remarkably rapid advances in trace-metal geochemical investigations. Recent advances are particularly notable in the case of the six platinum group metals, Ru, Rh, Pd, Os, Ir, and Pt. Until quite recently the platinum metals were the only group of metals in the periodic table to have escaped systematic investigation by marine scientists (I). Beginning with analyses of Pd distributions in the northeast Pacific (2) and continuing with studies of Pt, Ir, and Ru (1,3-5), knowledge of Pt group ocean chemistry has been markedly improved. In contrast to the growing understanding of platinum group sources, sinks, and environmental distributions, important fundamental aspects of platinum group geochemical behavior remain quite poorly understood. In a recent review of trace element seawater chemistry (6),the platinum metals were the only elements among the 76 considered for which no seawater speciation assessment could be provided. As a first step toward elucidating the nature of platinum group interactions in seawater, we have undertaken investigations of the solution chemistry of palladium. It is generally well-known that Pd has an extraordinary affdty for halide ions (7). Although this observation provides a basis for the presumption ( I ) that chloride complexes dominate the seawater speciation scheme of Pd, detailed assessment of Pd chemistry in chloride media suggests (8) that the species Pd(OH)20is dominant over the major chloride complex PdCL2- within the normal range of seawater pH. In order to directly resolve this issue, we have conducted Pd speciation investigations in natural seawater. Following the observation that PdC12(solid)additions to seawater give rise to a strong absorbance band centered Present address: Department of Geosciences, Pennsylvania State University, 210 Deike Building, University Park, PA 16802.
at 280 nm, our analyses were conducted by means of ultraviolet absorbance spectroscopy.
Methods Our spectrophotometric measurements were conducted at 25.0 f 0.1 "C in S = 36 surface seawater taken from the Gulf of Mexico. Use of an open-top 10 cm path length cell housed in the thermostating well of a Cary 17D spectrophotometer permitted simultaneous measurement of both absorbance and pH (9). Solution pH measurements were obtained on the NBS scale with a Ross combination pH electrode (Orion No. 810200) and a Corning Model 130 pH meter. The absorbances of solutions containing Pd were recorded versus a palladium-free seawater reference solution. Experiments were initiated by dissolving PdClz in C02-free seawater at pH 3.5. Total palladium concentrations were 10 pM (experiments 1 and 2) or 5 pM (experiments 3 and 4). Absorbance observations were begun after raising the seawater pH to 8.9 with a 0.1 M solution of Na2B40,-10H20.Absorbance vs pH observations within the range 4.0 IpH I8.7 were obtained with a 1.0 M HC1 titrant solution. Under the conditions of our experiments the total chloride concentration, [c1]T = 0.558 M, varied by less than 0.3%. Results and Discussion The results of one of our four spectrophotometric titrations are shown in Figure l. Our results indicate that, within the normal range of ocean pH, palladium in seawater does not exist exclusively as chloro complexes. At a pH of 8.0, for example, the palladium absorbances in our experiments at 290 nm are approximately 85% of the readings observed at low pH. We have quantitatively modeled our results in terms of palladium chloride and palladium hydroxide complexation. A t constant chloride ion concentration it is possible to express total dissolved palladium, [PdIT, in the following form: 3
4
[pd]= ~ [Pd2+](1+ CPn[C1-In+ C P1n*[C1-]n[H+]-l+ n=l
n-0
2
C P~n*[Cl-l"[H+l-~)(1)
n=O
where p,, Pin* and PZn* are formation constants with the general forms [PdC1n2-n] [PdOH(C1),l-n][H+] Pin* = P n = [Pd2+][C1-]n [Pd2+][C1-In (2) [Pd(OH)2C1,,-"] [H+] P2n* = [Pd2+][Cl-]"
0013-936X/89/0923-0663~01.50/0 0 1989 American Chemical Society
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663
Table I." 0.3
0
t
\w
'
z a
model eq 5, five parametersAo, Ai, A29 &, Bz
0.2
$ m a
0.1
av eq 5, four parametersAoV Ai, BI, Bz
310nm
t
0.01
expt
'0.
320nmz
7
0
I
9
8
PH
Flgurr 1. Absorbance of 5 I.IM palladium in C0,-free seawater (25 'C, S = 36) as a function of pH. The absorbances shown between 290 and 320 nm are atblbutable to palladium CMOride complexes. Due to hydrolysis, the abswbnce slgnal at each wavelength decreases by -50% between pH 7.0 and 8.7.
and brackets denote the concentration of each indicated chemical species. At constant chloride concentration, eq 1 can be expressed in the form [ p d ]= ~ [Pd2+](Bo+ B1'[H+]-' + B2'[H+]-2) (3) where Bo, B,', and BP) are constants. Similarly (gll), at constant chloride ion concentration it is possible to show that the absorbance of our solutions a t wavelength X can be expressed as follows: AA = [Pd2+](iAo' + xA1'[H+]-' + AAP)[H+]-~) (4) where AAd represents the molar absorptivity contributions of Pd2+plus palladium chloride complexes, and AA1' and AAP) are constants representing the molar absorptivity contributions of one-hydroxy and two-hydroxy palladium species. Equations 3 and 4 can be combined to provide the equation used in our data analyses:
av eq 5, three parametersAo, BI, Bz av eq 5, two parametersA09 B1
[P~IT, NM -log B1 -log Bz
1 2 3 4
10 10
1
10 10
2
8.75 8.64 8.81 8.81 8.75 8.74 8.64 8.43 8.67 8.62 8.75 8.66 8.78 8.81 8.75 8.62 8.59 8.72 8.73 8.67
5
5
3 4
5
1 2 3 4
10 10 5 5
1 2 3 4
10 10 5 5
5
av
17.63 17.78 17.63 17.94 17.75 17.66 17.79 17.29 17.71 17.61 17.69 17.90 18.16 18.16 17.98
RSS 9.0 X lo6 3.1 X lo-' 2.4 X 1.8 X 9.2 X 3.1 X lo4 2.9 X 1.8 X 1.1 X lo-' 3.3 X lo4 3.6 X lod 7.4 X 1.4 x 8.1 X 1.2 x 2.1 x
"The formation constant results obtained with eq 5, and successively truncated versions of eq 5, are shown for each of our four experiments. The final column in this table provides the minimized residual sum of squares obtained in each analysis. In logarithmic form, the formation constants B1and Bz can be expressed as 4
3
X [PdC1,2-"]) + log [H+]
log B1 = log ( E[PdOHCl,'-"]/
n-0
n=O
2
4
n=O
n-0
log Bz = log (C [Pd(OH)zCln-"]/ X [PdCln2-"])+ 2 log [H+] 5 was used in four decreasingly complex versions through successive omission of the parameters AA2,AA1, and BO. The results of our analyses are shown in Table I. A salient conclusion of our analyses is that, for all models considered, the result
B, = (2.1 f 0.6) x 10-9 M where AAo = AA,,'[Pd]~Bo-l, AA1 = AA,'[Pd]TBO-l,and AA2 = AAP)[Pd]~Bo-'.Expressed in this form, B1 and B2 are formation constants that can be used to directly calculate the ratio of hydrolyzed species concentrations to the sum concentration of free Pd2+plus palladium complexed solely with chloride ions. B1' - = B1 = ([PdOH+] + [PdOHClO]+ [PdOHC12-] + BO [PdOHC132-])[H+]/([Pd2+] + [PdCl+] + [PdClt] + [PdC13-] + [PdC142-]) (6)
([Pd(OH),O] + [Pd(OH)&l-] ([Pd2+]+ [PdCP] + [PdCl:]
+ [Pd(OH)2C122-])[H+]2 + [PdC13-] + [PdC1d2-])
(7) Our data analyses entailed the use of eq 5 in fits of absorbance, AA, versus [H+] data at four wavelengths (290, 300,310, and 320 nm) simultaneously. Our analyses provided parameten B1,B2, AAo, &A1and AA2,which minimized the residual sum of squares function RSS = C(Ai(observed) - Ai(predicted))2 (8) i
Since we did not know, a priori, which terms in eq 5 are essential for obtaining good descriptions of our data, eq 664
Envlron. Scl. Technol., Vol.
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10-3 lo4 10-3 10-4
(9)
provides a good description of our data. The range 1.5 X 10" I B1I 2.6 X lo4 encompasses the results we obtained in 15 of 16 analyses. The term B2 is not well-defined in our data analyses. Although comparisons of the RSS results of our analyses indicate that the term B2 is significant in obtaining good fits to our data, two-hydroxy palladium species are generally minor species under the conditions of our experiments. In 11of the 12 analyses that assessed the magnitude of B2, our results indicated that B2 I 2.4
(10)
X
Consequently, with this result as a probable upper bound estimate for B2,two-hydroxy species are unlikely to comprise more than 2% of the total Pd in seawater at pH 8.0. In contrast, our best estimate for B1 indicates that, at pH 8, one-hydroxy Pd species in seawater comprise 16% of the total palladium, and -82% is present exclusively as chloro complexes. Using our best estimates for B1 and B2, Figure 2 provides a probable speciation scheme for Pd in S = 36 seawater at 25 OC. Our analyses were conducted at boron concentrations elevated approximately 30 times above natural seawater levels. Although boron is generally a very weakly complexing ligand (12,13), in an additional set of analyses we considered the possibility that our results could be described in terms of Pd2+ complexation by borate ions. Using a model similar to eq 5, wherein [H+]-' is replaced by [B(OH),-], we obtained much larger RSS values than
-
(In,
100-
a "7
8
9
10
PH Flgure 2. Solutlon speciation of Pd2+ in seawater between pH 7 and 10. By use of the chloride complexation constants of Elding ( 7 ) 3proprlate at 25 "C and 1 M ionic strength, the ratios [PdCI,*-]/[Pd
and [PdCg-]/\%f+] are calculated (at [Cr] = 0.558 M) as 3.4 X 101 i and 2.6 X 10 . Lower order chloride complexes are of slight significance relative to PdCi,- and PdCit-. By using these results and our hydrolysis observations,the ratio of total to free Pd2+ at pH 8 and 25 "C in normal seawater should be approximately [Pd],/[Pd2+]z 4.5 x 1010.
those produced by all but the simplest (two-parameter) hydrolysis model. Even with a seven-parameter model, which accounted for the possibility of Pd(B(OH),),- and PdC1(B(OH)4)a2-complexes, RSS improvements relative to our five-parameter hydrolysis model were obtained in only two of our four experiments. We conclude that our results are best explained in terms of chloride complexation and hydrolysis. Our results indicate that palladium hydrolysis is much less significant than has been supposed (8) based on the works of Nabivanets et al. (14) and Izatt et al. (15). Furthermore, to the extent that borate complexation may have influenced our absorbance data to some degree, even our own assessments (eq 9 and 10) should be regarded as upper bound values. In view of our results, and the observation that hydrolysis should be suppressed with decreasing temperature (16),it is probable that hydrolyzed palladium species are of minor importance within nearly all of the ocean's volume. Palladium and nickel exhibit a strong covariance in ocean waters (2). The similarity in biogeochemical pathways implied by this covariance was attributed in the pioneering work of Lee (2)to the metals' "... similar ionic potential, their strong association in minerals, and the fact that they belong to the same group in the periodic table". To the contrary, considering (17) the extreme degree of Pd2+complexation in seawater ( [ p d ] ~ / [ P d ~5+ ]) ' .o l and the quite minor extent ([NiIT/[Ni2+]S 2) of Ni2+complexation (In,the observed covariance of Pd and Ni in oceanic waters could also be viewed as surprising. Rather than rationalizing Pd geochemical behavior in terms of direct similarities with Ni, it would appear useful to consider Pd behavior in terms of the moderating influence of chloride complexation on strong tendencies toward Pd2+ hydrolysis and surface complexation: (I) In the absence of chloride complexation, Pd2+ speciation, within the normal range of seawater pH, would be very strongly influenced by hydrolysis (8). As such, were it not for the effect of chloride complexation, it is likely that the chemistry of dissolved Pd2+in seawater would be highly pH and temperature sensitive. Our observations in this work,
combined with ancillary thermodynamic data indicate that the ratios [ p d ] ~ / [ P d ~and + ] [ N i l ~ / [ N i ~are + ] only mildly influenced by temperature and pH in the ocean. (11)Available thermodynamic data (18-20) indicate that Pd2+exhibits remarkable affinities for organic ligands. As an example, while the stability constant for Ni2+glycine complexation is approximatelyl@at 25 "C and 0.1 M ionic strength, under the same conditions the Pd2+ glycine stability constant is approximately 10l6 (18). In the absence of extensive complexation of Pd2+by chloride ions in seawater, it is likely that, relative to Ni, palladium would exhibit a remarkable affinity for the organic rich surfaces (21)of marine particulates. Since the scavenging of metals (22)from ocean waters by marine particulates is thought to be a dominant mechanism influencing trace-metal oceanic residence times, the strong chloride complexation of Pd2+relative to Ni2+is an important factor giving rise to observed similarities (2,23)in Pd and Ni distributional characteristics. Registry No. Pd, 7440-05-3. Literature Cited (1) Hodge, V. F.;Stallard, M.; Koide, M.; Goldberg, E. D. Earth Planet. Sci. Lett. 1985, 72, 158-162. (2) Lee, D.S.Nature (London) 1983,305, 47-48. (3) Hodge, V.; Stallard, M.; Koide, M.; Goldberg, E. D. Anal. Chem. 1986,58,616-620. (4) Goldberg, E. D.;Hodge, V.; Kay, P.; Stallard, M.; Koide, M. Appl. Geochem. 1986,1,227-232. ( 5 ) Koide, M.; Stallard, M.; Hodge, V.; Goldberg,E. D. Neth. J. Sea Res. 1986,20,163-166. (6) Bruland, K.W. In Chemical Oceanography; Riley, J. P., Chester, R., Eds.; Academic Press: London, 1983;Vol. 8, Chapter 45. (7) Elding, L. I. Znorg. Chem. Acta 1972,6,647-651. (8) Baes, C. F.,Jr.; Mesmer, R. E. The Hydrolysis of Cations; Wiley-Interscience: New York, 1976;pp 264-266. (9) Byrne, R. H.; Miller, W. L. Geochim. Cosmochim. Acta 1985,49,1837-1844. (10) Byme, R. H.; Kester, D. R. J. Solution Chem. 1981,10, 51-67. (11) Byme, R.H. Nature (London) 1981,290,487-489. (12) Turner, D.R.; Vukadin, I. Mar. Chem. 1983,14,133-139. (13) Elrod, J. A.; Kester, D. R. J. Solution Chem. 1981, 9, 885-894. (14) Nabivaneta, B. I.; Kalabina, L. V. Russ. J. Inorg. Chem. (Engl. Transl.) 1970,15,818-821. (15) Izatt, R. M.; Eatough, D.; Christensen, J. J. J. Chem. SOC. A 1967,1301-1304. (16) Baes, C. F.;Mesmer, R. E. Am. J. Sci. 1981,281,935-962. (17) Byrne, R.H.; Kump, L. R.; Cantrell, K. J. Mar. Chem. 1988, 25, 163-181. (18) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1974; Vol. 1, pp 1-3. (19) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1977;Vol. 3, pp 244, 298-314. (20) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1982;Vol. 5, pp 59-147. (21) Balistrieri,L.;Brewer, P. G.; Murray, J. W. Deep Sea Res. 1981,28A,101-121. (22) Goldberg, E. D.J. Geol. 1954,62,249-265. (23) Goldberg, E. D.Mar. Chem. 1987,22,117-124. Received f o r review July 8, 1988. Accepted January 18, 1989. The work was supported by the National Science Foundation under Contract No. OCE-83-08890.
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