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Stamper, J. H.; Nigg, H. N.; Allen, J. C. Enuiron. Sci.
Anderson,J. J.; Dulka, J. J. J.Agric. Food Chem. 1985,33,
Technol. 1979,13, 1402. Hamaker, J. W.; Goring,C. A. I. In Bound and Conjugated Pesticide Residues; Kaufman, D. D., Still, G. G., Paulson, G. D., Eds.; ACS Symposium Series 29; American Chemical Society: Washington, DC, 1976; p 219. Scow, K. M.; Alexander, M. In Reactions and Movement of Organic Chemicals in Soils; Sawhney, B. L., Brown, K.,
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Eds.; Soil Science Society of America and American Society of Agronomy: Madison, WI, 1989, p 243.
Fletcher, C.; Hong, S.; Eiden, C.; Barrett, M., Environmental Fata and Effects Division, Standard Evaluation Procedure, Terrestrial Field Dissipation Studies; United
States Environmental Protection Agency: Washington,DC, 1989.
Nose, K. Nippon Noyaku Gakkaishi 1987, 12, 505. Thompson, D. G.; Stephenson, G. R.; Solomon, K. R.; Skepasts, A. V. J. Agric. Food Chem. 1984, 32, 578. Wood, L. S.; Scott, H. D.; Marx, D. B.; Lavy, T. L. J. Environ. Qual. 1987, 16, 251.
Sadeghi,A. M.; Kissel, D. E.; Cabrera, M. L. Soil Sci. SOC. A m . J . 1989, 53, 15. Carsel, R. F.; Smith, C. F.; Mulkey, L. A,; Dean, D.; Jowise, P. User's Manual for the Pesticide Root Zone Model (PRZM),Release I; United States Environmental Protection Agency: Athens, GA, 1984. Johanson, R. C.; Imhoff, J. C.; Davis, H. H.; Kittle, J. L.; Donigian,A. S. User's Manual for Hydrological Simulation Program-Fortran (HSPF) (Release 7.0);United States Environmental Protection Agency: Athens, GA, 1981. Gustafson, D. I. Environ. Toxicol. Chem. 1989, 8, 339. Nijpels, E. H. T. M. Environmental Criteria Concerning Substances for the Protection of Soil and Water; The Netherlands Ministry for Housing, Regional Development and the Environment: The Hague, 1989. Kopelman, R. Science 1988,241, 1620. Negre, M.; Gennari, M.; Cignetti, A.; Zanini, E. J. Agric. Food Chem. 1988,36, 1319.
Hill, B. D.; Schaalje, G. B. J . Agric. Food Chem. 1985,33, 1001.
Johnson, N. L.; Kotz, S. Continuous Univariate Distributions-1; John Wiley & Sons: New York, 1970, p 166. SAS Users Guide: Statistics, Version 5 Ed.; SAS Institute: Cary, NC, 1985; p 575. Seber, G. A. F.; Wild, C. J. Nonlinear Regression; John Wiley & Sons: New York, 1989. Smith, A. E. J. Agric. Food Chem. 1988, 36, 393. Bull, D. L.; Ivie, G. W.; MacConnell, J. G.; Gruber, V. F.; Ku, C. C.; Arison, B. H.; Stevenson, J. M.; VandenHeuvel, W. J. A. J. Agric. Food Chem. 1984,32,94. Walker, A.; Brown, P. A. Bull. Environ. Contam. Toxicol. 1985, 34, 143. Carsel, R. F.; Nixon, W. B.; Ballentine, L. G. Environ. Toxicol. Chem. 1986,5, 345. Walker, A. Pestic. Sci. 1976, 7, 50. Smith, A. E.; Walker, A. Pestic. Sci. 1977, 8, 449. Technical Aspects of Herbicides Containing Chlorimuron Ethyl; E. I. du Pont de Nemours & Co., Inc.: Wilmington, DE, 1987; p 3. Bowman, B. T. J. Environ. Qual. 1988,17, 689. Szeto, S. Y.; Brown, M. J.; Mackenzie, J. R.; Vernon, R. S. J. Agric. Food Chem. 1986, 34, 876. Lee, P. W. J. Agric. Food Chem. 1985,33,993. Jones, R. L.; Homsby, A. G.; Rao, P. S. C. Pestic. Sci. 1988, 23, 307. Bowman, B. T. Environ. Toxicol. Chem. 1989,8, 485. Steinberg, S. M.; Pignatello, J. J.; Sawhney,B. L. Environ. Sci. Technol. 1987, 21, 1201. Calderbank, A.; Slade, P. In Herbicides: Chemistry, Degradation and Mode of Action; Kearney, P. C., Kaufman, D. D., Eds.; Marcel Dekker, Inc.: New York, 1969; Vol. 2, p 510. Received for review August 17, 1989. Accepted March 12, 1990.
Amino Acid Complexation of Palladium in Seawater Jln-He LI and Robert H. Byme'
Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701 Complexation experiments using glycine in natural seawater demonstrate significant conversion of PdC1:- to the mixed species PdC12G- at very low concentrations of free unprotonated glycine. A t 25 "C and salinity 35.3 the PdC12G- formation constant is given as P2: = [PdC12G-][Pd2+]-1[Cl-]-2[G-]-1= 1019.6. Our results indicate that PdC12G- dominates all inorganic complexes of Pd(I1) at NH2CH2COO- concentrations somewhat greater than 3 nM. Pd(I1) and Pt(I1) have similar equilibrium properties with a variety of ligands. However, the potentially analogous oceanic behavior of these elements may be somewhat decoupled by the very slow ligand-exchange rates of Pt(I1) compared to Pd(I1).
Introduction Although relatively little is known about the environmental chemistry of palladium and platinum, the fundamental chemical properties of these metals suggest that some aspects of their environmental behavior should be remarkable, if not unique. The affinity of palladium and platinum for amino carboxylic acids, ammonia, and a variety of other nitrogenous ligands (1-4)is unmatched by other divalent metals, and perhaps by other metals in 1038
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general. In view of the general importance of solution complexation in controlling geochemical behavior ( 5 , 6 ) , and the likelihood of increased Pd and Pt inputs in marine and estuarine systems (7, 8), we have undertaken direct examinations of platinum group solution complexation in seawater (9). In this work we present evidence that Pd(I1) chemistry in the surface ocean is strongly influenced by amino acids and note that analogous behavior should be expected for Pt(I1).
Methods Palladium complexation can be conveniently examined in seawater by ultraviolet absorbance spectroscopy (9). Palladium chloride complexes, which are dominated in seawater by PdCld2-,exhibit a strong absorbance peak at 279 nM. Figure la shows a series of palladium absorbance spectra obtained in natural seawater over a range of free glycine concentrations up to 3 nM. Figure l b shows qualitatively similar palladium behavior in a simple salt solution in which chloride ion serves as the only inorganic ligand. Spectrophotometric measurements were conducted at 25.0 f 0.1 " C in surface seawater taken from the Gulf of Mexico (salinity 35.3) and additionally in a 1 M ionic
0013-936X/90/0924-1038$02.50/0
0 1990 American Chemical Society
o.6
t
a o.6 0.5
i
NaCI-NaC104
0.4
0.3
0.2
0.1
0.0L
I
I
I
I
260
280
300
320
'
0.0
WAVELENGTH (nanometers)
I
1
I
1
260
280
300
320
WAVELENGTH (nanometers)
Flgure 1. Absorbance of Pd(I1) in seawater and NaCI-NaCIO,. The total glycine concentration in each of the experiments shown in this figure was 1 X M. In (a) the solution pH ranged between 2.23 (top curve) and 4.08 (bottom curve); in (b) the solution pH ranged between 2.22 and 4.29.
strength synthetic solution (0.55M NaCl + 0.45 M NaClod). Use of an open-top 10-cm cell housed in the thermostating well of a Cary 17D spectrophotometer permitted simultaneous measurement of both absorbance and pH. Each test solution was made 5 X lo4 M in PdC12 by using an acidic palladium chloride stock solution. The total glycine concentrations in three seawater experiments were 5 X lo4, 7.5 X lo4, and 1 X 10" M. This range of total glycine concentrations was also used in six experiments conducted in the 1M total ionic strength synthetic solution. Palladium absorbances were referenced to palladium-free seawater and palladium-free synthetic solutions. Solution pH was determined on the free hydrogen ion concentration scale (10,11) with a Ross combination electrode (Orion No. 810200) and a Coming Model 130 pH meter. Experiments were performed within the pH range 2.2 IpH I 4.3 while the solutions were gently bubbled with N2. Each solution was initially acidified with 1 M HC1. Subsequently, the pH of each test solution was raised by using 0.55 M NaCl 0.45 M NaOH (C02 free). All titrants (PdC12, glycine, HCl, NaOH) were added from calibrated Gilmont microburets. Our absorbance data, obtained at constant chloride ion concentration, can be quantitatively described by the following model:
+
A = A&
+ H&'[HGOI + cPi'[G-I + GPZ'[G-I~)-'
HGP'
GP2'
GPl'
=
[PdGZl = [PdCI,] [G-I2
[PdGI [PdCl"I [G-I
25.0 "C
formation constant medium seawater (S = 35.3) 0.55 M NaCl + 0.45 M NaCIOI
HGBll
G81'
08;
78 f 10 (3.17f 0.13) X lo8 11.4 X 10ls 136 f 13 (4.18f 0.12) X lo8 11.1 X 10l6
sum concentration of all palladium complexed with a single G- ligand, and [PdG2]represents the concentration of all Pd2+complexed with two G- ligands. Our data analyses using eq 1 were conducted through least-squares minimization of the residual sum of squares function:
RSS = CIAi(observed) - Ai(predicted)12 i
(3)
The independent variables [G-] and [HGO] were calculated from pH and total glycine ( [GIT)by using the following equations: [G]T = [G-](1 + K1[H+] + K,K2[H+l2) where
(1)
where A is absorbance per molar centimeter of palladium, A. is absorbance per molar centimeter of palladium in the absence of glycine, [HGO] represents the concentration of NH3+CH2COO-,and [G-] represents the concentration of free unprotonated glycine, NH2CH2COO-. The conditional &', and are defined as formation constants H&', [PdHG] = [PdCl,][HGo]
Table I. Palladium-Glycine Conditional Stability Constants in Seawater and Synthetic Chloride Solution at
(2)
where [PdCl,] represents the sum concentrations of Pd2+ plus palladium complexed solely with chloride ions, [PdHG] represents the sum concentration of all palladium complexed with a single HGO ligand, [PdG] represents the
(4) Complexation of glycine by Mg2+and Ca2+in seawater can be ignored under the conditions of our experiments. Since our experiments were conducted at pH I 4.3 the contributions of [MgG+]and [CaG+] to the total glycine concentration are trivial compared to [HGO] and [H2G+]. Results and Discussion The results obtained in least-squares analyses of six experiments at 1 M ionic strength (0.55 M NaCl + 0.45 M NaC104) and three experiments in natural seawater are shown in Table I. The conditional stability constant obtained for HG&' is sufficiently small that complexation of Pd2+by protonated amino acids (e.g., HGO) should be insignificant in natural solutions that have chloride conEnviron. Sci. Technol., Vol. 24, No. 7, 1990
1039
-'-
I
I
I
,
I
equilibrium constants for the reactions PdC1d2- + PdGzO e 2PdCl2GPdC12(H20)2 + PdGzO F! 2PdClG(HZO)O
0 2.0
t
0.0 1 0
i
,
1.o
1 I
1
2.0
3.0
4.0
NH CH2 COO- CONCENTRATION (nanomoles/liter)
Figure 2. The reclprocel of palladkm absorbance per molar centimeter shown as a function of NH,CH,COO- concentration. Equation 1 shows that A-' vs NH,CH,COO- will be very nearly linear if palladium complexation in our solutlons is dominated by a single glycine palladium complex, PdG. The slight negatlve curvature seen in this figure at low NH,CH,COO- concentrations Is due to a small but significant extent of palladium complexation at iow pH by protonated glycine, NH,+CH,COO-.
centrations as high as seawater. Our conditional stability constant result GOl' = 3.2 X lo8, obtained directly in seawater, indicates that, at G- concentrations greater than or equal to 3.1 nM, the concentration of palladium complexed with glycine in seawater should exceed the sum concentration of free Pd2+plus palladium complexed solely with chloride ions. Since amino acid stability constants, for any given metal, are quite similar (I),our &' results indicate that Pd2+complexation by free unprotonated amino acids should be significant at very low amino acid concentrations. The complex P d G t was not significant under the conditions of our experiments. Our least-squares analyses provided only upper bound estimates for &'. The conclusion that PdG20complexes are significant only at Gconcentrations greater than those employed in our investigation is directly supported by our absorbance observations. Plots of A-' vs [G-] (Figure 2) exhibit linear behavior at NH2CH2COO-concentrations as high as 4 nM. Consequently, our analyses indicate that at namomolar concentrations of dissolved, unprotonated amino acids, palladium is partitioned between chloride complexes and a 1:l palladium-amino acid complex. Speciation calculations demonstrate that the 1:l palladium-glycine complex responsible for palladium complexation in seawater cannot be the simple species Pd(NH2CH2COO)+. Previous determinations of the Pd(NH2CH2COO)+stability constant (I) produced the result [Pd(NH&H&OO)+] [Pd2+]-'[NH2CH2COO-]-1 = 1015.25 Since [PdCl,][Pd2+]-l N 10'o.w in seawater (13,it follows that [Pd(NH&H2COO)+][PdCl,]-'[NH&H~COO-]-'
N
104.75
This result, which is very much smaller than the result ,${ = 3.2 X lo8 (Table I), demonstrates that the conditional stability constant, &', in seawater does not describe the formation of Pd(NH2CH2COO)+.Since Pd2+should have a maximum coordination number of 4, alternative possibilities to explain our absorbance observations include only PdC12G- and PdClG(H20)O. The relative concentrations of these complexes can be predicted from theoretical considerations. From statistical arguments (12-16), 1040
Environ. Sci. Technol., Vol. 24, No. 7, 1990
should be nearly identical. PdC12G- and PdCIG(H20)O concentrations are then related approximately as follows: [PdC12G-]/[PdC1G(H20)o]= ([PdCl~-]/[PdC12(H20),])'/2. Since the concentration of PdCld2-is nearly 3 orders of magnitude greater than the concentration of PdC12(H20)t in seawater (13,PdC1G(H20)Oshould be of slight significance in our solutions compared to PdCI2G-. Consequently, our analyses indicate that [PdG] in eq 2 should be identified with the concentration of PdC12G- in our solutions. Presuming that the glycine complexation observed in our experiments is attributable to the species PdC12G-,our GPl' results in Table I can be used to obtain direct estimates of PdC12G- formation constants in terms of free Pd2+,C1-, and G- concentrations. The stability constant, P21for the reaction Pd2++ 2C1- + G- e PdC12Gcan be written as
Since &' is defined (eq 2) as &' = [PdG]/([PdCl,I[G-]), if [PdG] is identified as [PdC12G-] then PZl and Gfll' are related as follows:
As log ([PdC1,]/[Pd2+]) is approximately equal to 10.53 in our 1 M ionic strength synthetic solution, log PZl calculated by using eq 6 is log PZl = 19.7. In seawater, through a similar calculation we obtain the result log PZl = 19.6. Pd(I1) complexation in natural seawater is likely to involve a wide variety of amino acids in addition to glycine. Although direct observations of Pd(I1) complexation by amino acids are quite scarce, abundant Cu(I1) complexation data suggest that the complexation properties of glycine and other amino acids should be quite similar. Since total amino acid concentrations in seawater are strongly dominated by the protonated amino acid form, HAo, it is convenient to examine metal complexation in terms of the exchange equilibrium, M2++ HAo 2 MA+ + H+. Equilibrium constants for the above reaction can be conveniently constructed from conventional amino acid = [MA+][M2+]-'[A-]-l) and Aformation constants protonation constants (AK1 = [HAo][H+]-'[A-]-'). Observations (1)of equilibrium constants of the form &/AK1 = [CuA+][H+][HA0]-'[Cu2+]-'indicate that Apl/AKI values for most amino acids are comparable to or larger than the A P 1 / A K 1 value observed ( I ) for glycine. Consequently, available evidence suggests that if metal complexation by glycine is atypical of metal-amino acid complexation in general, it is atypical only in the sense that complexation by glycine is somewhat unfavorable compared to complexation by most other amino acids. Dissolved free amino acids have been observed in surface seawater at concentrations on the order of 30 nM or more (18,19). In this case the concentration of unprotonated dissolved amino acids in surface seawater should be approximately 1 nM, and the concentration of palladiumamino acid complexes will be about 30% as large as the sum concentrations of all Pd2+ complexed solely with chloride ions. Mopper and Lindroth (19) observed dis-
solved amino acid concentrations, [HAo], as high as 200-400 nM in the Baltic Sea. In this case, it would be expected that the extent of palladium complexation by amino acids should substantially dominate the complexation of palladium by chloride ions. It is, furthermore, interesting to note that since amino acid concentrations in the Baltic were observed to undergo diel cycles (19), similar diel cycles would be expected for palladium complexation. Pd(I1) and Pt(I1) exhibit similar equilibrium properties with a variety of ligands (20).Consequently, amino acid complexation may play a significant role in the ocean chemistry of both Pt(I1)and Pd(I1). However, in contrast to expectations based solely upon equilibrium data, it is important to note that the generally slow ligand-exchange kinetics of Pt(I1) (21) may create substantial dissimilarities in the oceanic behaviors of Pd(II) and Pt(II). In particular, Pt(I1)should be much less responsive than Pd(I1) to dissolved and particulate biogenic transients in the upper ocean. As such, the oceanic scavenging of Pd(II) and Pt(II) may exhibit considerable differences. The complexation of Pd(I1) by amino acids apparently constitutes the only reported instance where an identified class of natural organic ligands appears to play a significant role in a metal’s solution chemistry. The remarkable extent of Pd(I1) complexation by an identified class of natural ligands at low concentrations in seawater indicates that the extent of complexation of Pd(I1) by uncharacterized organic ligands in seawater may be extraordinary. Environmental inputs of palladium and platinum appear to have grown quite rapidly within the past 15 years (7, 8). Hodge and Stallard (7)observed Pd and Pt in roadside dust at concentrations comparable to relatively rich ores and noted the likelihood of these metals finding their way to coastal waters via storm runoff. Our analyses indicate that amino acids, and perhaps other nitrogenous ligands, may be quite significant in influencing Pd(I1) and Pt(I1) biogeochemical behavior in ligand-rich estuarine and coastal systems. Registry No. Pd,7440-05-3; glycine, 56-40-6.
Literature Cited (1) Martell, A. E.;Smith, R. M. Critical Stability Constants; Plenum: New York, 1974; Vol. 1. (2) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum: New York, 1975; Vol. 2. (3) Martell, A.E.;Smith, R. M. Critical Stability Constants; Plenum: New York, 1977; Vol. 3. (4) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum: New York, 1976; Vol. 4. (5) Garrels, R. M.;Christ, C. L. Solutions, Minerals, and Equilibria; Harper & Row: New York, 1965. (6) Stumm, W.; Morgan, J. J. Aquatic Chemistry; John Wiley & Sons: New York, 1981. (7) Hodge, V. F.;Stallard, M. 0. Environ. Sci. Technol. 1986, 20, 1058-1060. (8) Lee, D. S. Nature 1983,305, 47-48. (9) Kump, L. R.; Byrne, R. H. Environ. Sci. Technol. 1989,23, 663-6 65. (10) Ramette, R. W.; Culberson, C. H.; Bates, R. G. Anal. Chem. 1977, 49, 867-870. (11) Khoo, K. H.; Ramette, R. W.; Culberson, C. H.; Bates, R. G.Anal. Chem. 1977,49, 29-34. (12) Byrne, R. H. Mar. Chem. 1980,9, 75-80. (13) Byrne, R. H. Mar. Chem. 1983,12, 15-24. (14) Dynsen, D.; Jagner, D.; Wengelin, F. Computer Calculation of Ionic Equilibria and Titration Procedures; Almquist & Wiksell: Stockholm, 1968. (15) Sigel, H. In Metal Ions in Biological Systems; Sigel, H., Ed.; Dekker: New York; 1973; Vol. 11, Chapter 2. (16) Watters, J. I.; DeWitt, R. J . Am. Chem. SOC.1960, 82, 1333-1339. (17) Byrne, R. H.; Kump, L. R.; Cantrell, K. J. Mar. Chem. 1988, 25, 163-181. (18) Lee, C.; Bada, J. L. Limnol. Oceanogr. 1977,22,502-510. (19) Mopper, K.; Lindroth, P. Limnol. Oceanogr. 1982, 27, 336-347. (20) Hancock, R. D.; Finkelstein, N. P.; Even,A. J.Inorg. Nucl. Chem. 1977,39, 1031-1034. (21) Cotton, F.A.;Wilkinson, G. Advanced Inorganic Chemistry; John Wiley and Sons: New York, 1966.
Received for review October 4,1989. Accepted February 27,1990. This work was supported in part by the Chemical Oceanography Program of the National Science Foundation.
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