Electrochemical speciation of trace metals in sea water

The Science o f the Total Environment, 37 (1984) 41---60 Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands

41

E L E C T R O C H E M I C A L SPECIATION OF T R A C E METALS IN SEA WATER

LUTZ BRUGMANN Academy of Sciences of the G.D.R., Institute of Marine Research, G.D.R.-2530 RostockWarnemiinde (German Democratic Republic)

ABSTRACT Various approaches, such as bioassays, computer-assisted modelling and direct measurement following physical separation are currently being used to investigate the speciation of trace metals in sea water. The electrochemical techniques used successfully as a necessary prerequisite for the experiments involved include conventional polarography, anodic stripping voltammetry (ASV) and potentiometry. Differential pulse anodic stripping voltammetry (DPASV) using mercury film electrodes enables direct studies in ultratrace levels present in non-contaminated ocean waters. By varying the conditions of the sample chemistry and electroanalysis it is possible to characterize metal-organic interactions. This is demonstrated in the case of natural sea water samples subjected to ASV diagnosis. Differences in the Pb and Cu values yielded for Baltic waters by two methods based on DPASV and AAS are discussed with regard to speciation. An analysis of the existing literature is used to briefly summarize the needs of future research. Important problems requiring a more precise quantitative analysis include the adsorption of organics on electrodes and the kinetics and thermodynamic constants of chelates with special regard to the physico~hemical nature of metal-humic substances. The introduction of new and improved electroanalytical techniques and equipment for speciation studies is strongly recommended. INTRODUCTION E n v i r o n m e n t a l c h e m i s t s are c o n s t a n t l y being c o n f r o n t e d with the necessity o f f i n d i n g m o r e reliable m e t h o d s f o r i d e n t i f y i n g a n d q u a n t i f y i n g p h y s i c a l a n d c h e m i c a l f o r m s o f t r a c e metals in sea water. T h e k n o w l e d g e gained h i t h e r t o regarding t h e t o t a l c o n c e n t r a t i o n s o f these e l e m e n t s is n o l o n g e r s u f f i c i e n t t o p e r m i t d e s c r i p t i o n or i n t e r p r e t a t i o n o f their b e h a v i o u r in m a r i n e e c o s y s t e m s . This applies in p a r t i c u l a r to the e f f e c t o f metals o n organisms a n d t h e i r b i o g e o c h e m i s t r y in fluxes and cycles o f m a t t e r in t h e o c e a n a n d its interfaces with t h e a t m o s p h e r e , t h e sea b o t t o m a n d the continents. T h e b i o t o x i c i t y o f various f o r m s o f c o p p e r in t h e sea was first investigated over 10 years ago [ 1 - - 5 ] . T h e results s h o w e d t h a t ionic f o r m s o f c o p p e r , for e x a m p l e in u p w e l l i n g d e e p water, can inhibit p r i m a r y p r o d u c t i o n signific a n t l y . S u b s e q u e n t research led to t h e d i s c o v e r y o f c o n t r o l loops, w h i c h 0048-9697/84/$03.00

© 1984 Elsevier Science Publishers B.V.

42 obviously lead to detoxification by organic algal excretions [6--8]. The active fractions of the dissolved organic matter (DOM) involved in this process were isolated [9--11] and analyzed for copper [12--13]. The flux and circulation of trace metals in the ocean are sometimes characterized by drastic changes in their speciation. The metals, imported by surface run-off and atmospheric fall-out, by hydrothermal solutions and by the interstitial water of the sediment are present in genuinely dissolved, colloidally dispersed, or particulate form [14]. The dissolved fraction consists of both a multitude of inorganic forms, such as hydratized metal ions and charged or neutral complexes with anionic main components of the sea salt, and organic forms such as organic salts, ~r-complexes, covalent organometal compounds and coordination complexes [15]. The chelates, in which a central metal ion is surrounded by several functional donor groups, are a special form of coordination complex that is important for the speciation of m a n y metals. These compounds can be so stable that, in the sediment, t h e y are only limited subject to diagenetic change in the course of thousands of years [16]. The principal natural ligands for these complexes are humic substances (HS) and biologically synthesized enzymes, nucleic acids and peptides [ 15 [. The purpose of this contribution is to describe briefly the conventional approaches to trace metal speciation in sea water. In view of their particular importance in this respect, it will concentrate on the application of electrochemical methods as a means of implementing these approaches. It will also discuss practical trace metal speciation examples based on the results of investigations into Baltic Sea water.

APPROACHES TO TRACE METAL SPECIATION

Bioassays Table 1 presents results yielded by investigations into the toxic effects of ultratrace ionic copper fractions on marine organisms. Assuming that only ionic forms are present, these results imply that oceanic water with pCu values of only 8.5 to 10 [24--26] will have a negative effect on primary production. The effects caused by other potential pollutants in natural samples make biotoxicity tests useless as a speciation tool in all but exceptional cases.

Computer modelling T h e r m o d y n a m i c models have been developed on the basis of knowledge gained regarding the chemistry of particular metals and potential ligands, and they have been checked experimentally by means of synthetic solutions containing, usually, unrealistically high trace metal concentrations. These have yielded information regarding equilibrium concentrations of different fractions under given conditions, such as pH value, salinity, temperature and concentration of the reaction partners [27--41].

50% loss of motility

10.4

20% inhibition of 14C uptake

a p c u -- negative 10-based log of free copper ion activity. bTotal concentration in coastal water.

Phytoplankton (natural oceanic community)

9.3 8.4

23

22 Up to 50% inhibition of amino acid uptake 15% inhibition of amino acid uptake

8.7

21

Onset of growth inhibition Complete growth inhibition

Bacteria (5 natural oceanic communities)

20

19

18

17

References

50% inhibition of the assimilation rate

9.0--11.5 8.5

~7 b

8.3

50% inhibition of glucose uptake 100% inhibition of glucose uptake

Onset of growth inhibition Complete growth inhibition

10.4 8.7

9.1

Cu deficit Onset of growth inhibition Complete growth inhibition

Effects

14.6 10.6 8.3

pCu a

Phytoplankton (11 species from four classes)

Phytoplankton (natural estuarine community)

Bacteria (gram-negative, euryhaline

( Gonyaulax tamarensis )

Dinoflagellates

(Nannochloris atomus Butcher)

Green algae

(Thalassiosira pseu donana)

Diatoms

Organisms

TABLE 1 EFFECTS OF LOW COPPER ACTIVITIES ON MARINE ORGANISMS

¢.O

44 Although the first models did not take interactions with the DOM into account, all of the known competing equilibria were taken into account in later ones. The main obstacle to this approach is our lack of knowledge regarding the kinetics and t h e r m o d y n a m i c constants of complexes with natural macromolecules. In other words, the equilibrium condition is, probably, often rather doubtful [42].

Analytical method Another approach aims at directly measuring the natural metal forms by appropriate methods of physical and chemical analysis [13, 14, 43--49]. Such methods involve the solution of complicated analytical problems. The sufficiently accurate and precise measurement of trace metals in sea water has become possible only in recent years, and even this has n o t permitted the qualitative and quantitative estimation of different species, i.e. of fractions of the total quantities involved. Information regarding these separate fractions yielded by the introduction of more sophisticated methods [50] has, of course, been taken into account in model calculations, in which the earlier consideration of only inorganic forms such as CuCO ° [27, 31, 37] or Cu(OH) ° [28, 34, 36] is gradually giving way to increased consideration of natural organic complexes [40].

ELECTROCHEMICALMETHODS OF TRACE METAL SPECIATION Ever since speciation studies on sea water were first undertaken, t h e y have been closely associated with the application of electrochemical techniques, and in some cases they would have been inconceivable w i t h o u t them [51, 52]. This applies to: -- Conventional polarographic or anodic stripping voltammetric (ASV) techniques for verifying t h e r m o d y n a m i c speciation models by measuring the stability constants of complexes with anionic main components [53--55]. -- ASV techniques for the direct identification of metal fractions and for characterizing their properties, for indicating labile fractions during titration and for metal analysis in previously separated elutions, extracts, dialysates, filtrates, etc. -- Ion selective electrodes (ISE) for the direct measurement of metal activities and as potentiometric indicators during titrations for measuring the complex-binding capacity of sea water [ 56--58]. Sea water is an almost ideal natural supporting electrolyte of high ionic strength, which, moreover, contains surfactants that suppress maxima. Electrochemical techniques are in some cases superior to other analytical methods for metal investigations in sea water because analysis need not be preceded by separation of the trace and the matrix, and only small amounts of sample are needed for field measurements. The introduction of ASV with a stationary hanging mercury drop electrode (ASV/HMDE) or quasi-stationary mercury electrode (ASV/SDME) to instrumental analysis was therefore

45 TABLE 2 E L E C T R O A N A L Y T I C A L T R A C E M E T A L STUDIES IN SEA WATER, 1963/82 References

Year

Methods a

Trace metals

59 60 61 53 72 73 74 75 63 54 76 77 65 64 78 79 66 80 81 82 83 84 35 85 47 86 87 88 71 89 90

1963 1964 1965 1967 1969 1971 1971 1972 1973 1973 1973 1974 1974 1974 1975 1975 1975 1975 1976 1976 1977 1977 1977 1978 1978 1979 1980 1981 1981 1982 1982

ASV 'HMDE ASV 'HMDE ASV/SDME Pol. 'DME ASV 'HMDE ASV 'HMDE ASV 'HMDE ASV 'HMDE ASV 'MCGE ASV 'MCGE ASV 'HMDE ASV 'HMDE ASV 'MCGCE ASV 'MCGE ASV 'MCGE ASV 'MCGCE ASV 'MCGCE DPASV/MCGE ASV/GE DPASV/MCGCE ASV/HMDE DPASV/MCGCE DPASV/Au DPASV/MCGCE ASV/MCGE PSA/MCGCE DPP/DME DPASV/HMDE DPASV/MCGCE DPASV/MCGCE PSA/MCGCE

Zn~ Cd Cu Zn Cd, Zn Zn, Cd, Pb, Cu Zn~ Cd, Pb, Cu Zn Zn, Cd, Pb, Cu Zn, Cd, Pb, Cu Zn Zn, Cd, Pb, Cu Zn, Cd, Pb Sn Zn Zn, Cd, Pb, Cu Zn, Cd, Pb, Cu, Bi Cd, Pb, Cu Zn, Cd, Pb, Cu Hg Cd, Pb, Cu Zn, Cd, Pb, Cu Cu Hg Pb Pb, Cu, (Cd) Zn, Cd, Pb, Cu Mn Zn Cu Mn

1970 1975 1976 1977 1977 1979 1982

ASV/HMDE ASV/MCGE ASV/MCGCE DPASV/HMDE ASV/MCGCE DPASV/MCGCE DPASV/HMDE

Zn Zn Cd, Zn, Zn, Zn, Cd,

Bi

Speciationschemes 91 92 45 46 67 14 93

Pb, Cd, Cd, Cd, Pb,

Cu Pb, Cu Pb, Cu Pb, Cu Cu

aMethods/electrodes: Pol. -- Polarography; DME -- Dropping Mercury Electrode; SDME -- Slowly Dropping Mercury Electrode; HMDE -- Hanging Mercury Drop Electrode; GE -- Graphite Electrode; MCGE -- Mercury Coated Graphite Electrode; MCGCE -- Mercury Coated Glassy Carbon Electrode; ASV -- Anodic Stripping Voltammetry; PSA - - Potentiometric Stripping Analysis; DPP -- Differential Pulse Polarography, DPASV -- Differential Pulse Anodic Stripping Voltammetry.

46 quickly followed by application of the m e t h o d in marine chemistry [ 59-6 1 ] . This also happened later in the case of mercury film electrodes on waximpregnated graphite (ASV/MCGE) [62--64] and on glassy carbon (ASV/ MCGCE) [ 6 5 - 6 7 ] . The film required was often deposited with the amalgamforming c o m p o n e n t s to be accumulated, as described in [ 6 8 ] . The combination of these efficient accumulation techniques with powerful detection modes like differential pulse polarography (DPASV/MCGCE) led to a renaissance of electroanalytical m e t h o d s in marine environmental chemistry because it permitted extreme detection limits with low blank values to be obtained for several elements, such as Pb, Cd, Zn and Cu, with relatively simple and cheap instruments [ 6 9 ] . Due to their utilisable signals in the sea water matrix some 14 metals (Bi, Cd, Co, Cr, Cu, Hg, In, Mn, Ni, Pb, Sb, Sn, T1, Zn) can be investigated, after their previous accumulation. However, Table 2 shows that investigations hitherto have concentrated on Cd, Cu, Pb and Zn. These elements occur in ultra trace quantities and so interference due to intermetal c o m p o u n d s is scarcely a problem, although mercury film electrodes are used. Interference caused by the formation of a Cu/Zn c o m p o u n d during zinc analysis can be avoided by adding gallium ions. Although Ni and Zn have almost the same peak potentials, zinc analysis in this case will, due to the extreme difference in sensitivity, be disturbed only if nickel concentrations are unusually high [ 7 0 ] . Cu peaks produced by DPASV/MCGCE can become successively smaller in the case of multiple measurements. This p h e n o m e n o n is explained both by the a m o u n t exceeding the solubility limit for copper in a Hg film being stepwise reduced by calomel formation and by increasing amounts of Cu(I) [71]. Many of the electroanalytical investigations concerning the metal content of sea water that have been performed hitherto can be regarded as speciation studies. The authors (Table 2) [35, 47, 53, 54, 59--61, 63--66, 71--90] generally distinguish between several different fractions, for instance the "labile", "non-labile" or "particulate" fractions, and their dependence on the variables involved in chemical sample preparation and electroanalysis. Some authors have also proposed schemes along the lines of chemical separation operations for the experimental classification of metals [14, 45, 46, 67, 9 1 - - 9 3 ] . However, the following must be taken into account when interpreting data yielded in this way: due to contamination and adsorption effects can scarcely be avoided during extensive sample preparation in the ultra trace range. For instance, strong adsorption of lead as Pb(OH) +, a form believed to be c o m m o n in sea water, onto the DPASV/MCGE measuring set-up including Teflon and quartz glass parts, has been observed [ 9 4 ] . The ion-exchange resins often used to separate ionic fractions and labile complexes possibly bind other metal forms as well, such as non-labile complexes and colloidally dispersed material [ 1 5 ] . DOM with metal binding properties is sometimes stable to the UV irradiation techniques used to liberate the fractions being investigated [14, 6 7 ] . - - E r r o r s

--

--

47 TABLE 3 DIAGNOSIS OF T R A C E METAL SPECIATION IN SEA WATER BY ASV [51 ] Vary

1

A S V characteristics

1.1

Pre-electrolysis potential (E) Pre-electrolysis time (t) Stripping scan rate (v)

1.2 1.3

Plot a

Information

ip--E

Strength of organic sequestering

ip--t

Kinetics of dissociation of non-labile complexes

ip--V

(MeNL) Rate of formationof labilecomplexes(MeLL)

Ep--CMe

Extent of complex formation Total concentration of non-labile ligands (NL) Stability constants of complexes Effect of ligands on availability of electroactive metal Release of metals from organic complexes and colloids Release of metals from organic complexes and colloids

2

Chemistry o f sample

2.1

Concentration of electroactive metal (cMe)

ip--CMe

2.2

Concentration of ligands ( CL)

Ep--C L ip--C L

2.3

pH

ip--pH

2.4

Duration of UV irradiation ( t u v )

ip--tuv

aip __ Peak current resulted in stripping scan. Ep -- Peak potential resulted in stripping scan.

The time factor, which cannot be kept constant during the different operations of the analytical procedure, often leads to problems in interpreting the measured results if the complexes involved have different stabilities [ 9 5 ] . Moreover, changes in pH during passage through ionexchange resins and the influence of competing ligands, such as acetate from the buffer, and metal ions, such as Hg(II) during in situ film formation, must also be taken into account.

--

ASV DIAGNOSIS IN N A T U R A L SEA WATER

ASV is used not only for analytically orientated work but also for investigating the properties of metal complexes in sea water. Several i m p o r t a n t items of information can be obtained by changing the variables in the sample chemistry and the ASV characteristic (Table 3) [51]. If, as described in item 1.1 in Table 3, the electrolysis potential (E) is made more anodic, the plot of iF vs. E yields pseudo-polarographic curves like those recorded previously, for example, for Pb [62] and Cd [96]. The resultant "half-wave potentials" are used to calculate the stability of the complexes in the normal way. Examples of corresponding curves for Cu and Pb in sea water from the Baltic are shown in Fig. 1. The number of waves was reduced appreciable by intensive UV irradiation.

48

10-gA 6-

° Cu

5-

Pb

.._ . . . . .

432-

.o

o

o-O-O

I0

-017

-0',9

-111

-1:3

-1:5 V

Fig. 1. Relationship between peak current (ip) and electrolysis potential (E) for copper and lead in filtered Baltic Sea water [14]. / ip ~

ip

b /

CMe

CMe

ip ~

cj

ip

cM e

//

/

J

CMe

Fig. 2. Typical ASV titration curves of ligands with metal ions [97] ; ( a ) o n l y Me and/or MeLL, (b) NL surplus, formation of "strong" MeNL, (c) presence of "weak" MeNL, NL surplus, (d) presence of "strong" MeLL, of LL and "weak" NL. ( ) Theoretical titration curve, (. . . . . . ) experimental titration curve. I f t h e ligands c a p a b l e o f f o r m i n g e l e c t r o a n a l y t i c a l l y n o n - l a b i l e c o m p l e x e s in sea w a t e r are d e n o t e d b y N L , a n d t h e l i g a n d s o f labile c o m p l e x e s b y L L , t h e f o l l o w i n g cases are p o s s i b l e u n d e r n a t u r a l c o n d i t i o n s [ 6 2 ] :

49

I0"$A 10.

/o /

/

//

/

~-

//

os

//i/°//

6-

o.

4-

/ o~°~°

0L6

½

" .... 4

;

;

1'0

.

17

pg'l't

O'

Fig. 3. T i t r a t i o n of Baltic coastal water with c o p p e r and lead solutions [ 14 ]. (c~---- - -- ---~) Unfiltered, (e "-) filtered (0.45 ~m), (± A) after U V irradiation.

1 Excess of metal ions (Me) 1.1 Me ÷ MeLL 1.2 Me ÷ MeNL 1.3 Me + MeLL + MeNL 2 Excess of ligands 2.1 LL ÷ MeLL ÷ MeNL 2.2 LL + NL ÷ MeNL (MeNL "stronger" than MeLL) 2.3 NL + MeLL + MeNL 2.4 LL ÷ NL ÷ MeLL (MeLL "stronger" than MeNL). The terms "labile" and "non-labile" above are used relative to the time constant of electrolytic reduction of the metal at the electrode (t E ). MeLL therefore dissociate one order of magnitude faster, a n d . M e N L correspondingly slower, than tE. Knowledge of the NL concentration in a water sample is of major practical importance because NL can dampen the biotoxic effect of metal import into the marine environment like a buffer. The curves yielded by ip vs. CMe diagnosis as described in item 2.1 (Table 3) can be expected theoretically to be of four different types (Fig. 2) [97]. However, since natural samples always contain a mixture of different LL, NL, MeLL and MeNL the curves are difficult to analyze and interpret (Fig. 3). Sea water with an Me surplus is suitable for titration with appropriate ligands (item 2.2 of Table 3). Humic extracts are usually used as ligands because other fractions of the DOM that m a y potentially form complexes, such as amino acids, bind trace metals only in exceptional cases [99], due to competition by ions of the alkaline earth metals [98]. Even the addition of humic substances at concentrations typically present in the oceans [100] induces a distinct reduction in ii, for Pb and Cu (Fig. 4), whereas the Cd

50 pg.t q

0.5-

0,4-

0,3-

0,2-

0.1

o

½

~

1'2

3'2

7'2 1½2 2~2 15~2h0~21

:"

f)

1'2

3'2

7'2 122 2;2 1552h0~ 392 712 1/,32pgHS't -1

392 712 1432pgHS'tq

pg.I q

2+0'

1,5-

1.0-

0,5

0

i

2

i

I

i

Fig. 4. D e p e n d e n c e o f ASV-labile lead a n d c o p p e r c o n c e n t r a t i o n s o n t h e a m o u n t o f h u m i c s u b s t a n c e s (HS) a d d e d ; m a t r i x : A t l a n t i c w a t e r f r o m a d e p t h o f 2 , 0 0 0 m , filtered, 12 h U V i r r a d i a t e d , [ 1 4 ] .

peak is n o t noticeably affected [ 1 4 ] . The results of similar investigations using marine fulvic acid extracts [101] have been applied to calculate stability constants for corresponding Cu and Zn complexes. The pK values ranging from 10 I° to 1012 yielded by the calculations are in rough agreement with the values derived from ligand exchange experiments with EDTA [ 1 0 2 ] . Another criterion often used for diagnostic purposes is the ip vs. pH curve. The gradual realization of metals from complexes and colloids with decreasing pH can be easily followed (Fig. 5). In unfiltered samples, b o u n d particulate fractions are desorbed between pH 5.5 and 3.3. During these measurements the electrolysis potential must be kept sufficiently negative

51

I pg-I"~ 2.8-

2.4-

pg-1-1 1.4-

i

'

Cu

1.2-

2,0-

1.0-

1.6-

0,8-

1,2-

0.6-

0.8-

~

°~

°~.

0,4-

°

\\\~--o

0,4-

0,2-

0

0

0

1

½

3

4

5

6 pH

~-T~---o

0

Fig. 5. Dependence of ASV-labile copper and lead concentrations in in-shore Baltic Sea water on the pH value [14 ]. ( ~ - - - - - - - ~ ) Unfiltered, (-" ~) filtered (0.45 pro).

to ensure that the region of ip vs. E dependence is avoided. This means that problems may arise, for instance, when interpreting pH-dependent speciation measurements for zinc. It must also be remembered that not all MeNL dissociate when acidified [ 9 3 , 1 0 3 ] . The information obtained by ASV diagnosis regarding, for example, the stoichiometry and pK values of MeNL and MeLL or the NL concentration is, for a variety of reasons, often considered in the literature to be of only semiquantitative value. The reasons given for this view include the following: -- During titration with substances of the humic t y p e (HS), obtained from DOM extracts for instance, the organic ligands are irreversibly changed b y hydrolysis or oxydation in the course of accumulation and purification [ 104]. Moreover, if the Me:HS ratio is greater than unity, Pb/HS complexes [ 1 0 5 ] , for example, can be precipitated while taking up additional metal ions. Attention has also often been drawn to the influence of HS adsorption at the electrode [47, 103, 1 0 6 - - 1 0 9 ] . The resultant effect on the ASV peaks depends on the pH value [109] ; it is most distinct in the cases of Cu and Pb, b u t is less noticeable in the cases of Cd and Zn [47, 1 0 9 ] . The use of AC polarographic techniques can in addition induce tensammetric waves, which excessively increase the Faraday current of the above elements [ 1 1 0 ] . -- The kinetic effects involved in the titration of metal/organic ligand systems also await a satisfactory explanation and treatment [97, 1 0 7 ] . Natural samples must also be expected to contain ligands, for example, with dissociation constants of the same order as the time constant of the electrode, and so t h e y cannot be handled by the experimental and mathematical procedures generally used for MeNL and MeLL. The patchy distribution of labile ligands in sample solutions after the MeLL have been reduced at the electrode can also affect the results yielded by electrochemical speciation studies [ 1 1 1 ] . Kinetic effects can be expected primarily in connection with Pb and Cu, which associate with HS despite the fact that

52 calcium is more c o m m o n in sea water -- by a factor of 106 to 108 [108, 112]. The interaction between cadmium and HS is weaker and is affected by Ca [47, 112]. On the other hand, it is theoretically easier to interpret because it is less affected by kinetic effects [107]. In view of the above problems it has been recommended to use the appropriate ISE in addition to ASV for titration in metal/organic ligand systems [113]. Unfortunately, appropriate ISE have so far been f o u n d only for copper, and these do n o t exhibit an ideal Nernstian behaviour; moreover, due to Cu ° impurities, they act like a polarised sensor, which can be influenced by oxidants [ 114].

TRACE METAL SPECIATION IN BALTIC SEA WATER In September/October, 1980 and May, 1981 the Cd, Pb and Cu concentrations were measured in ca. 250 samples taken from 25 stations in the Baltic Sea. The samples were analyzed by DPASV/MCGCE on board, and by AAS ashore, after extraction of the dithiocarbamates [26]. The results, shown in Table 4, prove that in terms of reproducibility, DPASV is comparable with AAS. The good degree of agreement between the mean CdAA s and CdD1,ASv values shows t h a t in the pH range used, this element is present in equally labile form for both extraction and electroanalysis. The significant differences between the PbAA s and PbDPAS v values in May, 1981, on the other hand, indicate that fractions which are electrolabile at pH values less than 2 cannot be completely extracted at pH values between 3 and 4. This is completely consistent with the quantitative information conveyed by Fig. 5, which shows that the distinct dissociation of MeNL and the realization of Me from colloids does n o t appear to set in until pH values lower than 2.5 are reached. In the a u t u m n of 1980 measurements, the PbDPAS v values are only slightly higher than those obtained by AAS. The more significant difference between them, in May 1981, may be explained by the lead fraction being more strongly bound as PbNL following the spring plankton bloom. Analysis of the substantial differences between the Cu values yielded by the different methods relative to the investigation area, depth of water and redox conditions (Fig. 6) reveals the following four distribution patterns: -- The concentration of NL (CuNL) in the water column is obviously slight in the relatively shallow salt- and oxygen-rich water of the Western Baltic. Cu, NL and/or CuNL are obviously re-emitted from the oxic surface layer of the sediment after mineralization of the particulate organic material (POM) (Fig. 6a). In regions with less saline surface water, Cu is possibly imported, p r e d o m i n a n t l y as CuNL, with fresh water. In deep water, where conditions are sometimes anoxic, the differences between the AAS and DPASV values are less pronounced because the realization of NL under such conditions

-

-

53 TABLE 4 C A D M I U M , L E A D A N D C O P P E R I N B A L T I C S E A W A T E R S , 1 9 8 0 A N D 1 9 8 1 (ng1-1)

Mean

Range

R.S.D. a

(-+%)

Sept./Oct. 1980 Cd -- A A S Cd -- D P A S V Pb -- A A S Pb--DPASV Cu - - A A S Cu -- D P A S V

55 53 46 51 860 490

~ 2-- 200 ~ 2-- 240 5-- 330 ~ 2 - - 220 39--2800 190--1500

100 73 100 82 52 35

66 67 30 54 950 530

~2-- 540 2-- 500 3-- 220 7-- 340 46--2900 100--1900

110 100 108 102 51 50

May 1981 Cd Cd Pb Pb Cu Cu

-------

AAS DPASV AAS DPASV AAS DPASV

aR.S.D.

=

Relative standard deviation.

takes place more slowly due to the delayed mineralization of the POM (Fig. 65). -- The NL and CuNL concentrations increase over almost the whole water column as the latitudes of the stations increase. When anoxic layers of deep water are reached, however, the extractable amounts of Cu decrease to a minimum, whereas DPASV meaurements above the b o t t o m yield m a x i m u m values (Fig. 6c). This could be explained by the occurrence of electrochemically accumulated polysulfide complexes [115] which were n o t extractable under the experimental conditions. -- The Kattegat and Skagerrak are characterized by low copper concentrations in the salt-rich deep water and a four times higher Cu level in the outflowing Baltic water that, with a lower salinity, also contains higher CuNL concentrations (Fig. 6d). It has been observed that in productive regions of the North Atlantic, in contrast to the Baltic Sea, copper concentrations in the euphotic layer are reduced by the p h y t o p l a n k t o n . Differences between the extractable fractions and the fractions that are electrolabile at pH ~ 2 are n o t significant in this case (Fig. 6e).

CONCLUSIONS

The great importance still attached to speciation of trace metals in the ocean as an aid to assessing their toxicity and biogeochemistry is reflected by the positions allotted to this problem in the lists of priorities drawn up by c o m p e t e n t marine scientists [116--121]. It can be expected that further

54 0 m

b

O

\

I

0

f

--m

20

-/,0

40

- 80

\ I

0 .ug-1-1

I

f

I

0.5

I

110

0 )Jg.1-1

I

I

0.5

1.0 0 m

-20 I00 -

-

I

0

I

I

/

I

~

3Jg.1-1 1.0

210

I

I

40 60

I

3Jg. 1-1 0./.

0.8

0 rn

I

200 -

J DPASV

I 400

-

AAS

600 --

8OO

i

I

~Jg. 1-1 005

i

0.1

Fig. 6. Vertical c o p p e r profiles in the Baltic Sea and the A t l a n t i c in 1 9 8 0 / 8 1 as measured by A A S and D P A S V ; (a) Western Baltic, (b) B o r n h o l m Deep, (c) G o t l a n d Deep, (d) Kattegat/Skagerrak, (e) N o r t h Atlantic.

clarification of the structure of the DOM (90% of which still awaits identification [ 1 5 ] ) and of potential NL such as HS and biochemicals in particular, together with a better insight into the interactions taking place b e t w e e n dissolved metal forms and the dispersed and sedimented material covered by NL or LL will bring further progress in providing answers to, for example, the following questions: Does oceanic water contain metal-specific ligands, say of the siderochrome (trihydroxamate) t y p e that form chelates which give complexes with pK values of a b o u t 102° or above? Is the formation of Me/organic complexes a secondary process that takes place in the sea water, or are thermodynamically or kinetically stable complexes mainly produced biosynthetically and then injected into the medium? -

-

-

-

55 Electrochemical methods, coupled with appropriate separation techniques s u c h as H P L C , can in s o m e cases m a k e an o u t s t a n d i n g c o n t r i b u t i o n t o solving t h e s e a n d o t h e r p r o b l e m s . F u t h e r i m p r o v e m e n t o f t h e limits o f d e t e c t i o n a n d t h e r e p r o d u c i b i l i t y o f results y i e l d e d b y A S V t e c h n i q u e s c o u l d lead t o a r e d u c t i o n in e l e c t r o l y s i s times. This in t u r n w o u l d r e d u c e t h e effect on the natural balance between different metal forms and permit m e a s u r e m e n t s t o be m a d e in r e s p e c t o f t h e k i n e t i c s o f c o m p l e x e s in a b r o a d e r range. T h e p r o b l e m o f i n t e r f e r e n c e b y a d s o r p t i o n / d e s o r p t i o n p r o c e s s e s c o u l d b e i n v e s t i g a t e d m o r e closely b y t h e p u r p o s e f u l a p p l i c a t i o n o f AC t e c h n i q u e s . M o r e o v e r , g r e a t e r use s h o u l d b e m a d e o f t h e p o t e n t i a l o f f e r e d b y relatively n e w e l e c t r o a n a l y t i c a l t e c h n i q u e s like p o t e n t i o m e t r i c s t r i p p i n g analysis (PSA) [ 8 6 , 1 2 2 ] . In PSA, t h e s t r i p p i n g process, f o r e x a m p l e , is a f f e c t e d m u c h less b y organic m a t e r i a l . O n t h e o t h e r h a n d , t h e a p p l i c a t i o n o f d i f f e r e n t o x i d a n t s c o u l d yield a d d i t i o n a l i n f o r m a t i o n . T h e use o f f u r t h e r d e v e l o p e d I S E s e e m s p r o m i s i n g as a s u p p l e m e n t t o A S V d u r i n g s p e c i a t i o n studies a n d in c o n n e c t i o n w i t h t o x i c i t y e s t i m a t i o n . I m p r o v e d w o r k i n g c o n d i t i o n s o n b o a r d r e s e a r c h vessels are m a k i n g it increasingly p o s s i b l e t o p e r f o r m d i r e c t e l e c t r o a n a l y t i c a l m e a s u r e m e n t s o n b o a r d w i t h p r a c t i c a l l y c o m p l e t e p r o t e c t i o n against c o n t a m i n a t i o n , t h u s h e l p i n g t o a v o i d f a l s i f i c a t i o n o f s p e c i a t i o n results d u e t o s a m p l e preserv a t i o n a n d storage.

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