Solvent extraction of copper acetylacetonate in studies of copper(II) speciation in seawater

Marine Chemistry, 21 (1987) 301 313

301

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

S O L V E N T E X T R A C T I O N OF C O P P E R A C E T Y L A C E T O N A T E IN S T U D I E S OF COPPER(II) S P E C I A T I O N I N S E A W A T E R

JAMES W. MOFFETT* and ROD G. ZIKA

University of Miami, Rosenstiel School of Marine and Atmospheric Science, Division of Marine and Atmospheric Chemistry, 4600 Rickenbacker Causeway, Miami, FL 33149-1098 (U.S.A.) (Received April 4, 1986; revision accepted April 9, 1987)

ABSTRACT Moffett, J.W. and Zika, R.G., 1987. Solvent extraction of copper acetylacetonate in studies of copper (II) speciation in seawater. Mar. Chem., 21:301 313. A liquid liquid partition, ligand exchange procedure involving the formation of copper(II) complexes with acetylacetone is presented for the determination of stability constants and concentrations of copper chelators in seawater. Acetylacetone competes with natural ligands for copper, and the equilibrium concentration of the copper acetylacetonate complex is used in speciation calculations. The concentration of the complex is calculated by partitioning a fraction of it into an organic phase and determining the total Cu concentration in that phase by back extracting with acid, and analyzing by flameless atomic absorption spectroscopy. The concentration of Cu acetylacetonate in seawater in equilibrium with the organic phase is calculated from the partition coefficient. The simple, thermodynamically well characterized procedure offers several advantages over previous techniques. Studies using organic free seawater and model ligands show good agreement between experimental and calculated conditional stability constants. Studies from seawater in Biscayne Bay, Florida, indicate two ligand types are present; type 1, K 1 = 1.2 × 1012,CL1 = 5.1 × 10 9M;type2, K2 = 2.8 × 101°,CL2 - 1.1 × 10-TM. Speciationis dominated by ligand type 1. Depth profiles of [Cu(II)]free/[Cu(II)]totaI measured with the procedure at ambient copper concentrations show an increase from < 5 × 10 5 at 5 0 ~ 0 m to > 1 × 10 3 at the surface at two stations off the Florida coast. INTRODUCTION

A number of procedures have been used by various workers to evaluate copper activity and complexation capacity in natural waters. These include ion selective electrodes (Zirino et al., 1983), anodic stripping voltammetry (Huizenga and Kester, 1983), amperometry (Waite and Morel, 1984), and ligand exchange techniques (Hirose et al., 1982; van den Berg, 1984). However, there is a wide disagreement between these various procedures. Many electrochemical procedures lack the sensitivity to detect electroactive forms of copper at natural copper concentrations in the presence of strong chelators. Other techniques involve separation and preconcentration steps where shifts in the * Present Address: Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 U.S.A.

0304-4203/87/$03.50

© 1987 Elsevier Science Publishers B.V.

302

prevailing equilibrium induced by the separation step may be difficult to quantify. These problems result in an underestimation or overestimation of the conditional stability constants and probably contribute to discrepancies in the literature. This indicates the need for a simple thermodynamically well characterized procedure which involves a minimum of operationally defined or empirical parameters. In this work a procedure is presented for the investigation of copper(II) speciation using a ligand exchange/solvent extraction technique. It involves the formation of a Cu(II) complex with acetylacetone in the presence of competing natural chelators, and the partitioning of this complex into an organic solvent. This procedure has been used extensively to investigate metal complexation equilibria, and is ideal for systems where the total metal concentration is low (Rossoti and Rossoti, 1961; Dyrssen, 1963). The application of the procedure to studies of Cu(II) speciation in seawater was investigated using model ligands and natural organic and inorganic chelators in seawater. Concentrations and conditional stability constants of natural organic chelators were determined in a sample from Biscayne Bay, Florida. In addition, [Cu(II)]free/[Cu(II)]totaI as a function of depth in the upper water column was determined at two stations off the Florida Coast. EXPERIMENTAL

SECTION

Choice of ligand Acetylacetone (AA) was chosen as the ligand for several reasons. Stability constant data for its complexes with copper have been determined at a variety of temperatures and ionic strengths (Liljenzin et al., 1969). The Cu(AA)2 complex is stable enough to compete with natural chelators at moderate ( < 10 -3 M) AA concentrations and the complex partitions into a variety of organic solvents. Stability constants used in this work are shown in Table I.

Choice of solvent The principal criterion in selection of a solvent was the partition coefficient of the Cu(AA)2 complex between the seawater and organic phases, [Cu(AA)2]org/[Cu(AA)2]aq. This had to be large enough that a measurable fraction of the complex partitions into the organic phase, but not so large that it is completely extracted. Carbon tetrachloride and toluene were found to be the TABLEI S t a b i l i t y c o n s t a n t d a t a for c o p p e r a c e t y l a c e t o n a t e fll = 1.65 _+ 0.12 × l 0 s f12 = 6.5 _+ 0.4 × 1014 25 ° I = 1.0 M f r o m L i l j e n z i n e t al. (1969)

complexes

303 most suitable. Furthermore, both solvents are sparingly soluble so they do not affect the electrostatic properties of the aqueous phase. As they are highly nonpolar, n a t u r a l l y occurring organic compounds important in Cu(II) speciation are unlikely to be extracted. This was confirmed over the concentration range 5 × 10 -9 M to 1 x 10 -7 M Cu in coastal and oligotrophic waters. Carbon tetrachloride was used in this study, but toluene has been used for subsequent work (Moffett, 1986).

Procedure Acetylacetone was added to the aqueous system under study followed by addition of solvent equivalent to 10% of the aqueous phase volume. Vigorous shaking for several minutes in a separatory funnel was performed and the phases allowed to separate. In experiments involving longer equilibration time, the solutions were shaken periodically. The concent rat i on of copper in the organic phase was determined by back extraction with 2 ml of 1% nitric acid. This was shown to remove all copper from up to 100ml of CC14 or toluene. Copper in this sample was determined by graphite furnace atomic absorption spectrophotometry using a Perkin Elmer 403 instrument. All experiments were carried out at 25°C. Small changes in temperature of +_I°C did not appreciably affect the results. For seawater studies and in model studies at low metal concentrations trace metal clean procedures were used and determinations carried out in a vertical flow laminar flow hood equipped with a duct at the ent rance to exhaust solvent vapors. At sea, samples were collected in teflon coated GO-Flo bottles (General Oceanics, Miami, FL) and transferred to the laminar flow hood via teflon tubing. Irradiation of seawater to destroy natural organic chelators was carried out in a batch irradiation system with a 1000 W HgXe lamp for 3 h. Total copper in seawater samples was determined using the APDC/DDDC solvent extraction procedure described by Danielsson et al. (1978). RESULTS

Cu(AA)e distribution The distribution constant for the Cu(AA)2 complex between seawater and solvent, defined as ~Cu(AA)2 = [Cu(AA)2]org/[Cu(AA)2]aq, was measured in artificial seawater containing 10-3M AA. At this concentration, virtually all the Cu(II) was present as Cu(AA)2. The value for ~Cu(AA)2, is 6.9 + 0.1 for CCI4 and 6.2 + 0.I for toluene, in reasonable agreement with earlier results in 1 M perchlorate media (Allard et al., 1974). The distribution constant of the ligand, [AA]org/[AA]aq was also measured and was 0.25 at pH 8.0 for CCIt. The value is low because of the formation of a complex with Mg 2÷, which is not extracted. Generally, when the volume of solvent was ~<10% of the volume of seawater, this partitioning was neglected.

304 At greater solvent to seawater ratios, however, the partitioning consideration to determine the aqueous AA concentration.

was taken into

Ligand exchange studies using model ligands I t is n e c c e s s a r y

aAA-

-

[AA ] [AA]t

-

t o k n o w t h e v a l u e f o r anA , d e f i n e d a s

-

[AA-]

[AA- ] + [ H A A ] + [ M g A A ÷]

(1)

w h e r e [ A A ] t = [AA]total i n t h e a q u e o u s p h a s e . This could not be calculated from literature stability constant data, because d a t a f o r M g ( A A ) ÷, t h e m a j o r A A c o m p l e x i n s e a w a t e r , w a s o n l y a v a i l a b l e a t low ionic strengths. Copper distribution between AA and competing model l i g a n d s w a s s t u d i e d i n s e a w a t e r , i n o r d e r t o a s s i g n a c o n d i t i o n a l v a l u e f o r aAA-" F o r t h e m o d e l l i g a n d s , w e l l k n o w n s t a b i l i t y c o n s t a n t s w i t h C u 2÷, C a 2÷ a n d M g 2÷ a t o r n e a r s e a w a t e r i o n i c s t r e n g t h w e r e e s s e n t i a l , s o N T A a n d E D T A were chosen. Stability constant data for these ligands were taken from Martell a n d S m i t h (1977). S t a b i l i t y c o n s t a n t d a t a f o r t h e i n o r g a n i c l i g a n d s i n s e a w a t e r w a s t a k e n f r o m Z u e h l k e a n d K e s t e r (1983). S e a w a t e r f r o m t h e F l o r i d a C u r r e n t was UV irradiated to destroy organic ligands. Copper and model ligands were a d d e d a n d a l l o w e d t o e q u i l i b r a t e f o r 24 h. A A w a s a d d e d a t v a r i o u s c o n c e n t r a tions and extraction carried out. T h e r e s u l t s , i n T a b l e II, s h o w 0~Cu(AA)2 VS. [ A A ] t, w h e r e

aCu(AA)2

-

-

[ C u ( A A ) 2 ]aq [Cu(II)~ -

f12 [ h A - ]2 1 + 2:Ki[Li] + f l l [ A A ] + f l 2 [ A A - ] 2

(2)

TABLE II Measurement of ~Cu(AA) 2 and calculation of ~AA- at different [AA]t in U V irradiated seawater with and without added ligands, p H = 8.15, T = 25°C

[AA]t(M) I.[NTA] = 2 x 10-6M 5 × 10 ~ 1 × 10 4 1.5 × 10 _4

aCu(AA)2 [Cu(II)~t = 1.1 × 10 0.08 0.20 0.37

VM + 0.005 + 0.01 + 0.02

II.[EDTA] = 4 × 10 7molL 1 [Cu(II)~t = 2 x 10-SM 2 x 10 4 0.10 + 0.005 4 x 10 4 0.30 + 0.01 8 x 10 4 0.60 +_ 0.03 III. 2 5 1 5 1.5

No added ligands [Cu(II)~t = 2 x 10-~M x 10 5 0.15 x 10 s 0.35 × 10 4 0.50 x 10 4 0.87 × 10 _3 1.00

_+ 0.01 _+ 0.02 + 0.02 + 0.02

~AA-( × 103)

5.2 _+ 0.1 4.5 + 0.2 4.8 + 0.2 4.8 _+ 0.2 4.7 _+ 0.2 4.3 _+ 0.3 7.6 6.6 4.6 4.0

_+ 0.3 _+ 0.3 _+ 0.3 _+ 1.5

305

where [Cu(AA)2](aq)

=

V(acid) 1 [Cu(II)](acid) × - × V(CC14) ~Cu(AA)2

L i = all ligands p r e s e n t (in this i n s t a n c e i n o r g a n i c ligands + NTA or EDTA) [Cu(II)~t = t o t a l Cu(II) in aqueous phase (including AA complexes). To c a l c u l a t e aAA-, eq. 2 was r e a r r a n g e d to form a q u a d r a t i c ((XCu(AA) 2 --

1)fl2[AA ]2 + ~Cu(hA)2fl,[A A ] + (~Cu(AA)2(1 + Z K i L i ) = 0

(3)

The e q u a t i o n was used to solve for [AA ] at each [AA]t a n d UAA- c a l c u l a t e d from eq. 1. Results are shown in Table II. T h e r e is good a g r e e m e n t b e t w e e n the two ligand systems, yielding a v a l u e of aAA = 4.7 × 10 -3 . T h e r e is some d e v i a t i o n from the m e a n at the highest and lowest AA values. However, a g r e e m e n t is good from 1 to 4 × 10 4 M AA, the r a n g e of c o n c e n t r a t i o n s generally used in s e a w a t e r studies. E s t i m a t e s of aAA_ were also made in UV i r r a d i a t e d s e a w a t e r with no added chelator. T h e results, in Table II, show g r e a t e r variability. At 1 0 - ' M A A , a g r e e m e n t is good. At 5 × 10 4M, a g r e e m e n t is r e a s o n a b l e given the large e r r o r associated with c a l c u l a t i n g gAA- w h e n only a small f r a c t i o n of the copper is not p r e s e n t as Cu(AA)2. At low c o n c e n t r a t i o n s of AA, however, the values of aAA_ are c o n s i d e r a b l y higher. This could be due to using an e r r o n e o u s v a l u e for fl, w h i c h is v e r y i m p o r t a n t at these low AA levels, w h e r e Cu(AA) + is more i m p o r t a n t t h a n Cu(AA)2, or c o e x t r a c t i o n of some Cu(AA) + with a c o u n t e r i o n such as O H - . H o w e v e r , this was not a significant problem because s e a w a t e r studies were c a r r i e d out at h i g h e r [AA]t ( > 10 -4 M). A l t e r n a t i v e l y , the stability c o n s t a n t for the copper c a r b o n a t e complex derived by Zuehlke and K e s t e r (1983) m a y be too high, as i n d i c a t e d by B y r n e and Miller (1985), which might a c c o u n t for the discrepancies in the organic-free seawater. Using aAA = 4.7 × 10 -3, an estimate of log K~+~ was made and c o m p a r e d with the l i t e r a t u r e value. F r o m eq. 1, log K~+g = 3.6, c o m p a r e d to the v a l u e of 3.63 d e t e r m i n e d at 0.02 M ionic s t r e n g t h (Izatt et al., 1955). N o t e t h a t because the [HAA] term is of m i n o r i m p o r t a n c e in the d e n o m i n a t o r in e q u a t i o n (1), ~AA is i n d e p e n d e n t of pH in the r a n g e n o r m a l l y e n c o u n t e r e d in seawater.

Ligand exchange studies in seawater The p r o c e d u r e was applied to s e a w a t e r collected in B i s c a y n e Bay, Florida. Aliquots of AA were added to 800 ml of s e a w a t e r in a Teflon s e p a r a t o r y funnel. T h e n 80ml of solvent were added. Time d e p e n d e n c e studies indicated t h a t equilibrium was a t t a i n e d rapidly, in a couple of minutes, e x c e p t w h e n e n o u g h AA was added to c h e l a t e all the copper. U n d e r these conditions, equilibrium was a t t a i n e d o v e r several hours. A d s o r p t i o n of Cu(II) o n t o the walls of the teflon c o n t a i n e r in the p r e s e n c e of AA was a problem, but this could be p r e v e n t e d by adding the solvent and periodically s h a k i n g d u r i n g equilibration.

306 A v a l u e for UCu2+was c a l c u l a t e d u s i n g the following e q u a t i o n s XiKi[L1]

-

fl2[AA ]2

(1 + f l , [ A A - ] + fl2[AA-] 2)

(4)

{XCu(AA)2

~Cu2 +

--

[Cu(II)]f

I

[Cu(II)]t

1 + ZK~[L~]

(5)

Li = all n a t u r a l s e a w a t e r ligands [Cu(II)]f = free Cu 2÷ ion [Cu(II)]t =

[Cu(II)]f + Z[CuLi] ~

Z[CuLi]

Optimum acetylacetone concentration In theory, m e a s u r e m e n t s of ~Cu~+ s h o u l d n o t be a f u n c t i o n of a c e t y l a c e t o n e c o n c e n t r a t i o n . I n model systems this was g e n e r a l l y the case, a l t h o u g h t h e r e was some d e v i a t i o n at the lowest A A c o n c e n t r a t i o n . These discrepancies were n o t observed in m e a s u r e m e n t s at c o n c e n t r a t i o n s of AA used in n a t u r a l seaw a t e r studies. P r e c i s i o n in the m e a s u r e m e n t of ~cu~+ is limited by the precision in a t o m i c a b s o r p t i o n d e t e r m i n a t i o n in the acid e x t r a c t ( + 1 × 10 8 M) and by precision in the t o t a l copper m e a s u r e m e n t s (+ 1.5 x 10-1°M). H i g h A A conc e n t r a t i o n s are less s a t i s f a c t o r y b e c a u s e as the f r a c t i o n of copper a s s o c i a t e d with the n a t u r a l ligands becomes small, the errors a s s o c i a t e d with the above m e a s u r e m e n t s lead to large e r r o r s in the d e t e r m i n a t i o n of ~cu2+• Therefore, AA c o n c e n t r a t i o n s should be as low as possible w i t h o u t c o m p r o m i s i n g a n a l y t i c a l precision. Generally, c o n c e n t r a t i o n s of 1-4 x 10 4 M A A were used. U n d e r these conditions, the relative s t a n d a r d e r r o r was found to be equal to the relative s t a n d a r d e r r o r for total a q u e o u s copper m e a s u r e m e n t plus the relative s t a n d a r d e r r o r in the acid e x t r a c t m e a s u r e m e n t .

Calculation of conditional stability constants This p r o c e d u r e was used for the c a l c u l a t i o n of c o m p l e x i n g c a p a c i t i e s and c o n d i t i o n a l stability c o n s t a n t s in seawater. S t a n d a r d additions of copper to aliquots of a sample collected from B i s c a y n e B a y were performed. The samples were allowed to equilibrate for 2-4 h p r i o r to AA addition. To d e t e r m i n e if the TABLE III Determination of acu2~ versus time for Biscayne Bay samples with added Cu [Cu] added 6 × 10 9M

Time (h) 0.25 2 24

3.3 + 0.2 0.15 2.8 i 0.15 2.6 + 0.15

1 3.5 14

39 i 3 44 _+ 3 41 _+ 3

1

7.5 × 10 SM

~cu2+( × 104)

2.9 +

307 TABLE IV Results from a standard addition study to Biscayne Bay water used to compile Scatchard plots and calculate conditional stability constants. Concentrations in nM [Cu(II)]0

aCu(AA)2

Z[CuLi]

Z[CuLi]/[Cu(II)]~

3.5 5.8 7.3 11.5 13.9 16.6 19.9 27.3 36 50.9 81.3 120.9 230.9 436

0.082 0.23 0.27 0.27 0.29 0.27 0.27 0.34 0.31 0.37 0.40 0.39 0.46 0.56

3.1 3.9 4.5 7.1 8.1 10.2 12.2 14.0 20.0 24.0 34.8 53.5 79.2 86.1

2411~ 653 508 509 451 509 509 338 402 284b 239b 253b 166b 79b

[Cu(II)]0 = total aqueous copper before extraction. aData point used to calculate KLI. bData points used to calculate KL2. samples had equilibrated, we studied the time dependence of complexation of a d d e d c o p p e r b y m e a s u r i n g ~cu2+ v e r s u s t i m e i n s p i k e d s e a w a t e r s a m p l e s . T h e r e s u l t s i n T a b l e I I I s h o w t h a t e q u i l i b r a t i o n is a c t u a l l y c o m p l e t e by 2 h a t b o t h high and low Cu additions. A f t e r e q u i l i b r a t i o n , 1.25 × 10 4 M A A w a s a d d e d t o t h e s p i k e d s a m p l e s . T h e r e s u l t s a r e s h o w n i n T a b l e IV. A d d i t i o n a l p o i n t s w e r e o b t a i n e d a t c o p p e r l e v e l s b e l o w t h e a m b i e n t c o n c e n t r a t i o n b y i n c r e a s i n g t h e A A c o n c e n t r a t i o n a n d by i n c r e a s i n g t h e r a t i o o f s o l v e n t t o s e a w a t e r ( T a b l e V). S c a t c h a r d p l o t s , as d e s c r i b e d b y M a n t o u r a a n d R i l e y (1975), w e r e c o m p i l e d f r o m t h e d a t a . F o r e a c h TABLE V Additional speciation data for water collected February 12, 1986 obtained at various AA concentrations and solvent to seawater ratios I. AA concentration varied [AA]t(M × 104)

Z[CuL~](nM)

Z[CuLi]/[Cu(II)] ~

1.5 2.5 5.0

4.3 3.5 2.8

1580 1899 3347

II. Ratio of solvent to seawater varied, [AA]t = 2 × 10-4M CC14:seawater

Z[CuLi](nM)

Z[CuLi]/[Cu(II)]f

2

1.3

4718

These data used in calculating CL, and KL~.

308

ligand in the system, the following equation is valid [CuLl]

K jILl(total) ] - K i [ C u L i ]

[Cu(II)]~

(6)

A plot of [CuLi]/[Cu(II)]f versus [CuLi] gives a straight line with slope - K i . For a system of several chelators such a plot will give several lines or form a curve. The data, shown in Fig. 1, indicate a mixture of at least two chelators. From the figure, stability constants for two ligand sites were estimated from the slopes lines generated by linear regression through data points at the two ends of titration. Points used are indicated in Tables IV and V. The ligand concentrations were calculated from the following equations eL2 :

~2~[CuLi]~

_ Kinorgainorg

\ [Cu(II)]f ][CuLil=0 K2 (l~[Cuai]~

CL, = \

/Eo.,il=0

(7) _ K2CL2-

KinorgLinorg

K2

f

where

. 1Z[CuLi].

(8) and

are the y axis intercepts of the lines generated for ligands 1 and 2, respectively. Results are shown in Table VI. A curve calculated from these values gives a good fit to the data. In addition, the data in Table V confirm that measurements

i

0

20

40

60

80

I00

Ecuy,] (nM)

Fig. 1. Scatchard plot of the data compiled in Table IV. Curve is computed from stability constant estimates in Table VI.

309 T A B L E VI E s t i m a t e d v a l u e s for c o n d i t i o n a l s t a b i l i t y c o n s t a n t s a n d c o n c e n t r a t i o n s of n a t u r a l l i g a n d s in Biscayne Bay sample Ligand

KL

CL

1. 2.

1.0 _+ 0.2 x 1012 2.8 _+ 0.7 x 10 TM

5.1 i 0.3 x 10-SM 1.1 _+ 0.1 x 10 VM

made at different AA levels yield consistent results in seawater as all points fall, within error, on the Scatchard plot.

O~c.~+ versus depth Depth profiles of [Cu(II)]f/[Cu(II)]t (~cu2+) in the photic zone were measured on the R.V. "Cape Florida" during cruise III of the SOLARS (Studies of Light Activated Reactions in Seawater) program in September, 1985 at two stations o f f t h e Florida coast. Station locations are shown in Fig. 2. Samples drawn from different depths were allowed to equilibrate with the lab t em perat ure (25°C) and then AA was added for a final c onc e nt r at i on of 1.25 x 10-tM. This led to a small but measurable fraction of the copper forming Cu(AA)2. In no case was [Cu(II)] t at equilibrium less t han 35% of the initial nat ural value. The results were calculated from the data using eqs. 2, 4 and 5 and are shown in Table VII. Profiles are plotted in Fig. 3, and show very similar characteristics despite different locations, exhibiting a surface maximum which declines with depth to a minimum value at about 5 m, which is then i nvari ant t hrough the rest of the photic zone. 30

28'.

LORI

i I

I

24t

STN8o . . . .

,o. t "

| 84 °

82 °

Fig. 2. M a p s h o w i n g s a m p l i n g locations.

80 °

310 TABLE VII Determination

o f acu2+ a t a m b i e n t C u a t t w o s t a t i o n s o f f t h e F l o r i d a c o a s t Depth

[Cu(II)]0

UCu(AA)2

=cu2 +

Z[CuLi]

Stn. 2

2 12 25 40 70 90

2.7 2.7 1.8 1.8 1.7 1.8

0.24 0.20 0.11 0.02 0.025 0.02

1.6 1.25 5.8 9.2 1.2 7.2

× × x × ×

10 -3 10 3 10 -4 10 5 10 -4 x 10 5

1.7 1.7 1.4 1.7 1.7 1.7

Stn. 8

10 20 40 70 120 180

1.3 1.2 1.0 1.0 1.2 1.3

0.21 0.22 0.08 0 0 0

1.3 1.4 4.03 < 5 <5 <5

× × × x × x

0.8 0.8 0.9 1.0 1.2 1.3

[Cu(II)] 0 = t o t a l C u b e f o r e e x t r a c t i o n ( n M ) . Z [ C u L i ] --~ [Cu(I[)] t = t o t a l a q u e o u s C u a f t e r e x t r a c t i o n

10 -3 10 -3 10 -4 10 -5 10 5 10 -3

(nM).

[Cu(ll)]f/[Cu(ll)lt (x I04) 0

4

8

i

i

12

16

20

1~

•

~ 0 - -

0

40 •- 0

CP~

4:)-

•

80 -0 ..c: (3. o~ Eb

120

160

O-

20C F i g . 3. D e p t h p r o f i l e s o f [ C u ( I I ) ] f / [ C u ( I I ) ] t (acu2+) a t t w o s t a t i o n s o f f t h e F l o r i d a c o a s t . S t a t i o n 2, ( * ) , S t a t i o n 8, (O).

311 DISCUSSION

The results indicate t h a t the procedure is valid for studies of Cu(II) speciation in seawater, as the distribution can be well characterized by thermodynamic parameters both for the ligand exchange and the solvent partitioning. Because the concentration of Cu(AA)2(aq) in equilibrium with the organic phase is determined, there are no artefacts associated with re-equilibration occurring in the separation step. A second advantage is that ligand exchange and Cu(AA)2 formation occurs rapidly, requiring only a few minutes to reach equilibrium. This enables greater temporal resolution of chelation processes than is possible with chelators such as EDTA, which exchanges very slowly (Hirose et al., 1982). Recently van den Berg (1984) has used a similar ligand exchange technique involving the formation of a Cu(II) catechol complex. Detection of the adsorbed complex is by cathodic stripping voltammetry. The results indicate two ligand classes, as observed in this work and values of conditional stability constants and ligand concentrations are surprisingly similar using both techniques, given the different sampling locations and treatment. Anodic stripping voltammetry (ASV) has been used by numerous workers to study Cu(II) speciation in seawater. However, the ASV labile copper measurable in seawater containing strong chelators is often below the limit of detection of the procedure at natural levels. For instance, Huizenga and Kester (1983) were unable to obtain quantitative information about Cu(II) speciation in the photic zone at stations in the Northwest Atlantic for this reason. In addition, dissociation or reduction of organic complexes may occur during the plating period, which leads to an underestimation of stability constants. This is evident from ASV measurements of Spencer (1984) of conditional stability constants and chelation capacities in Biscayne Bay, Florida. Values for CL average about 7 × 10 s, in reasonable agreement with CL2 in this work. However, the stability constants are one to two orders of magnitude lower than those estimated for K2 (except when a correction was made for the reduction of organic compounds, in which case the agreement is reasonable). Also, the ASV measurements do not reveal the existence of ligand class 1. Recent observations obtained from a comparison of ASV and the CSV technique in the North Atlantic support these conclusions (Buckley and van den Berg, 1986). The use of Scatchard plots has been criticized by Klotz (1983) on the grounds that extrapolation of the plot to determine complexation capacity leads to large errors, particularly when a number of different binding sites with different stability constants are involved. This indicates a large potential error in the concentration of ligand class 2, the most abundant ligand class and any weaker, more abundant ligands which may be present but were not detectable in this study. However, the data indicate t h a t speciation is dominated by ligand class 1 at natural copper levels. Accurate evaluation of ligand class 2 or the total complexation capacity is less important in determining copper speciation. Extrapolation of the initial slope to yield the concentration and stability

312 constant of ligand class I can be made with greater confidence, because the plot is linear in this region and the difference in slope between ligand class 1 and ligand class 2 is so large. Alternative treatments, such as that suggested by Klotz, offer no advantages in this region. The primary limitation, regardless of numerical treatment, is obtaining accurate data around the ambient concentration. The stability constants derived for ligand class 2 are in reasonable agreement with the values derived by Zuehlke and Kester for copper complexes isolated using a C-18 Sep Pak cartridge. They did not isolate chelators with properties of ligand type 1, indicating such ligands are not retained by the Sep Pak cartridges, in agreement with other work (van den Berg, 1984; Donat et al., 1986). The depth profile characteristics of [Cu(II)]f/[Cu(II)]t indicate that a variety of processes influence Cu(II) speciation in the upper water column, evidently on a rapid time scale compared with mixing processes. The increase in free Cu(II) towards the surface may be the result of photolysis of metal complexes and/or chelators in surface waters. Indeed the profiles show similar characteristics to photochemically dependent species such as H202. Recently the sunlight-induced increase in free (Cu(II) has been demonstrated using this technique (Moffett and Zika, 1987). The source of the chelators may be in situ biological production in the region of the productivity maximum. The profiles are consistent with this source but cannot confirm it. Few detailed profiles of Cu(II) speciation in the upper 100m have been reported in the literature for comparison. Results obtained by Buckley and van den Berg (1986) show an increase in free Cu(II) towards the surface at a subtropical station in the eastern Atlantic (21°59'N, 2°39'W), and the values of ~Cu2+calculated from their data are comparable. However, at another station further north (49°N, 8°50'W), no such trends were observed. This may be due to less sunlight or greater mixing at the more northerly station. There is a general consensus that the bioavailability and toxicity of copper depends on the level of free copper, not the total concentration. Therefore the observation t h a t the free copper changes by up to a factor of 20 between the surface and 50m, and is increased by sunlight irradiation, has important implications for the ecological role of copper in the upper marine water column. ACKNOWLEDGEMENTS The assistance of Dr. David Dyrssen in this work is gratefully acknowledged. Thanks also to Dr. Ken Mopper, Chief Scientist on SOLARS III, and the Captain and crew of the R.V. "Cape Florida". This research was supported by the Office of Naval Research, contract number N00014-85C-0020.

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