Chemical speciation of trace metals in seawater: Implication of particulate trace metals

Marine Chemistry, 28 (1990) 267-274

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

Chemical Speciation of Trace Metals in Seawater: Implication of Particulate Trace Metals KATSUMI HIROSE

Geochemical Laboratory, Meteorological Research Institute, Nagamine 1-1, Tsukuba, Ibaraki 305 (Japan) (Received October 10, 1988; revision accepted May 23, 1989)

ABSTRACT Hirose, K., 1990. Chemical speciation of trace metals in seawater: implication of particulate trace metals. Mar. Chem., 28: 267-274.

Chemical speciation of particulatemetals in seawater was examined theoretically.Mass balance considerations showed that the apparent conditional stabilityconstant, defined for organically binding metals in suspended particles,coincides with the conditional stabilityconstant determined for the corresponding metal-organic complexes dissolved in seawater. This hypothesis suggests that some metals, which are present as organic complexes (e.g.copper), are directlyassociated with particulate organic matter. Metals, whose free ion is buffered by organic and/or inorganic ligands, may be used as indicators of the presence of particulate organic matter in the marine environment.

INTRODUCTION

The dominant mechanisms in control of concentrations of most trace metals in seawater appears to be physical and chemical adsorption on biologically produced particulate matter (Wangersky, 1986). The chemical speciation of trace metals in particulate matter is important in the understanding of the geochemical cycle of trace metals in the marine environment. Suspended particles in seawater are traditionallydefined by a physical separation, that is filtrationusing a membrane filterwith a pore size of 0.45 #m. A physical classificationbased on particle size,however, does not define clear boundaries between dissolved components and suspended particles. O n the other hand, a filtrationtechnique is effective as a kind of speciation of trace metals in the aqueous environment (van de Meent et al.,1982), although its chemical implication is not understood. W h e n measured concentrations of certain metals in seawater are compared with concentrations predicted from thermodynamic solubility calculations, 0304-4203/90/$03.50

© 1990 Elsevier Science Publishers B.V.

26S

K, mR()SE

these metals are found to be undersaturated (Goldberg, 1954; Krauskopf, 1956 ). Nevertheless, most of the trace metals in seawater are also found in suspended particles. These observations have led to the hypothesis that sorption (adsorption, absorption and surface precipitation) of metals by particles is an important and efficient process. Suspended particles in seawater, which chemically interact with metals, can originate from a variety of sources. The largest of these sources is the formation of phytoplankton biomass by photosynthetic processes in the euphotic zone and its decomposition by biological processes such as zooplankton grazing. A major part of suspended particles, in fact, consists of organic materials (Chester and Stoner, 1974; Tanoue et al., 1982 ). Taking into consideration the chemical interaction between suspended particles and trace metals, the role of organic materials in suspended particles is more important than that of inorganic suspended matter because concentrations of inorganic suspended adsorbers such as oxyhydroxides of Fe and Mn are usually very low in open-ocean waters (Landing and Bruland, 1987; Martin and Gordon, 1988). Recently, Chester et al. (1988) found that ~ 50% of the total copper in suspended materials from Atlantic Ocean surface waters is held in organic associations. In this paper, we, using our knowledge of coordination chemistry (Stumm and Morgan, 1981 ), introduce a new equilibrium model to discuss theoretically the implication of particulate metals in seawater, and will suggest that some particulate metals associate directly with binding sites in particulate organic matter. THEORY

Suspended particles, which mainly consist of organic materials (Tanoue et al., 1982), have a continuous size spectrum (McCave, 1984). Tanoue et al. (1986) have reported that a major part of particulate organic matter is composed of amino acid, lipid, carbohydrate and so on. These components have also been found in dissolved organic matter associated with metals, which shows a relatively high molecular weight (Sugimura and Suzuki, 1985). The complexation properties of binding sites in suspended particles, therefore, are similar to those in dissolved organic matter in seawater. Figure 1 gives a scheme of a new equilibrium model containing both dissolved and particulate phases. When organic matter containing similar binding sites is physically divided into two fractions, e.g. by filtration, the apparent mass balance for each fraction can be represented as follows M ~+

+L1 ~ML~

M ~+ +

L2

(1)

~ML2

where M n+ is a free metal ion and L1 and L2 are the organic ligands separated

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TRACE METALS IN SEAWATER

DissoLved phase H

ParticuLate phase

H

H

Mix ~

M""

"-~

MH

-0 H

MH J~/~' /H~~ - J Freebindingsite

OM

Metal compLex(cheLate)

H Inorganic

~--

H

FiLtration

Organicpolymer

complexes

Ligand Lz

~

LigandEl

(I= OH-,CO3Z-,...)

Fig. 1. Equilibrium model containing both dissolved and particulate phases. This model implies that the ratio of the free binding site to the complexed metal in the particulate phase is equal to that in the dissolved phase.

physically, which have the same complexation properties. Apparent conditional stabilityconstants for the equilibria (1) are defined as follows K . I -- [ML~] [Mn+ ]-~[Lt] -1 , K..2 = [ML2] [M"+] -1 [L2]-1

(2)

where K.,, and K.,2 are the apparent conditional stability constants corresponding to ligands 1 and 2, respectively. In seawater, overall equilibrium between a metal and the organic ligand consisting of ligands 1 and 2 is established. A 'true' conditional stability constant is defined as follows

Kt-- [ML] [Mn+ ] - ~ [ L ] - I

(3)

where ML and L are the metal complex and the organic ligand, the concentrations of which are represented by [L] = [LI] + [L2]

(4)

[ML] = [MLi] + [ML2]

(5)

Introducing A as a ratio of the ligand LI to L2, from eqns. ( 2 ) - ( 5 ) we obtain

Ka,1 =Kt + (Kt - K , , 2 ) A -1

(6)

In the case of L2 > L1, the apparent conditional stability constant for a metal

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K. HIROSE

complex of a major fraction, i.e. L~, is expected to coincide with the true conditional stability constant because the complexation properties of binding sites cannot be considered to differ seriously between major and minor fractions. If the true conditional stability constant is high enough to form complexes in the conditions of seawater, then the second term of eqn. (6) can be ignored. The apparent conditional stability constant for a minor fraction, therefore, is equal to the true conditional stability constant, represented by The result implies that a trace metal associated with the organic fraction in suspended particles has the same conditional stability constant as that determined for the corresponding dissolved fraction when a trace metal is separated between dissolved components and suspended particles by physical processes such as filtration. I)ISCUSSION

Organic particulate metals Several metals (e.g. copper, cobalt, nickel, zinc) in suspended particles are significantly enriched relative to the crust and are not directly related to the presence of aluminosilicates (Buat-Menard and Chesselet, 1979). These metals are also enriched in maritime aerosols (Arimoto et al., 1987). Atmospheric deposition of these metals on the ocean surface, however, cannot be the cause of enrichment of these metals in oceanic suspended particles because most of the enriched metals are inorganically undersaturated and, on the contrary, reference metals, e.g. aluminium and iron, are rather insoluble in seawater (Moore et al., 1984). Recently, Maring and Duce (1989) suggested that dissolved organic matter and possibly cations in seawater may play an important role in increasing the solubility of aerosol copper. The result of this theoretical consideration indicates that the conditional stability constant of a trace metal complexing with an organic binding site in suspended particles coincides with that of the corresponding metal-organic complex dissolved in seawater. This supports the hypothesis that metals (e.g. copper) with high conditional stability constants, of which the main species dissolved in seawater are metalorganic complexes (Hirose et al., 1982; Sunda and Hanson, 1987; Hering et al., 1987), are associated with the organic binding site in suspended particles by complexation. If the conditional stability constant and the free ion concentration for a metal are known, the concentration of its metal binding with particulate organic matter, Cp.M,can be estimated from the following equation Cp, M

=KMI,[M n+ ] [S]

(8)

where KML is the conditional stability constant of the dissolved metal-organic

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TABLE 1 Total ligand concentration, conditional stability constants and calculated ambient pCu Sampling location

pH pCu L1 (nM)

L2(nM)

log K1

Western North Pacific Cape SanBlas, FL Mississippi river plume Narragansett Bay Narragansett Bay Coastal Peru Christiansen Basin Montauk Point

8.1 8.2 8.1 8.0 8.0 8.2 8.0 8.2

80 130 100 100 70 68 50

11.8 11.2 11.1 12.4 12 12.3 11.7 11.7

11.5 11.5 11.3 12.5 12.1 11.4 11.8 12.2

21 13 20 50 20 4.5 50 20

log K2

Reference Hirose et al. ( 1982 )

9.0 8.9 10 10 9.2 9.1 9.1

Sunda and Ferguson (1983) SundaandHanson (1987)

t

Heringetal. (1987)

complex and [S] is the concentration of the free binding site in suspended particles. For copper, the term, KML [ M n + ], is calculated to be 0.5-8 based on previous studies. Table 1 lists the conditional stability constants of organic copper complexes and free copper ion concentrations (pCu = - l o g [Cu 2+ ] ) in seawater. Unfortunately, we have no information on the concentration of the free binding site in suspended particles. Its level, however, can be estimated by assuming that the ratio of the binding site to particulate organic matter is nearly equal to that for dissolved organic matter. The level of particulate organic matter in open ocean waters is about two orders of magnitude lower than that of dissolved organic matter (Williams, 1971; Chester and Stoner, 1974; Whittle, 1977; Tanoue et al., 1982; Sugimura and Suzuki, 1988). As the concentration of nonbinding organic ligand dissolved in seawater ranges from 3 to 30 nmol 1-1 (Hirose, 1988), the concentration of the free binding site in suspended particles is calculated to be ~ 0.03-0.3 nmol l-1. The level of organically bound copper in suspended particles is then estimated to be in the range 0.02-3 nmol 1-1. The observed values for particulate copper (range 0.04-0.9 nmol 1-1; BuatMenard and Chesselet, 1979; Chester et al., 1988) fall within our estimates. Furthermore, this hypothesis is strongly supported by the fact that ~ 50% of the copper in the surface water particulates is held in organic associations as shown by a sequential leaching technique for the Atlantic Ocean surface suspended particles (Chester et al., 1988). These findings suggest that the chemical states of particulate metals reflect those of the dissolved species of the corresponding metal. Particulate metals as an indicator of suspended particles

When metals in suspended particles form complexes with particulate organic matter, the concentration of organic binding site in suspended particles, Cp,s, is expressed as

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Cv,s =KMI,~S [M n+ ] -1Cp,M

K HIROSE

(9)

where c~s is the side reaction coefficient for the organic binding site in suspended particles. Equation (9) implies that particulate metal concentrations are linearly related to the concentration of the organic binding site in suspended particles if the concentration or activity of the corresponding free metal ion is maintained at a constant level in the marine environment. Hirose and Sugimura (1985) suggested that the activity of the free copper ion is maintained at a constant level by buffer action of the organic ligand dissolved in seawater. The activity of the free uranium ion may also be at a constant level because dissolved uranium is mainly present as carbonato complexes (Djogic et al., 1986) and has a uniform distribution in open ocean waters (Sugimura and Mayeda, 1980). Anderson (1982) reported that uranium in suspended particles is associated with particulate organic matter. These findings suggest that the concentration or activity of the free ion for some metals is almost constant, as a result of the buffer action of inorganic and/or organic ligands dissolved in seawater. These metals in suspended particles, accordingly, are linearly related to the concentration of binding sites in particulate organic matter. In the case where the proportion of the organic binding site in particulate organic matter is approximately constant in the marine environment, the concentration of organic binding sites can be replaced by the amount of particulate organic matter. Particulate metals, therefore, are expected to be linearly related to particulate organic matter. These metals may be used as indicators of the presence of particulate organic matter in seawater. CONCLUSIONS Mass balance considerations indicate that the conditional stability constant defined on organically binding metals in suspended particles has the same value as that determined for the corresponding dissolved metal-organic complex if the binding site in particulate organic matter is chemically similar to that of dissolved organic matter. Metals forming stable complexes with the organic ligand in seawater, e.g, copper, are not adsorbed onto suspended particles as inorganic salts, e.g. hydroxides, but are directly associated with the binding site in particulate organic matter. This is supported by the results for the chemical speciation of particulate copper (Chester et al., 1988). The direct association between metals and particulate organic matter suggests that the removal of these metals from the water column is closely related to the formation, decomposition and downward transport of particulate organic matter. In the case where a free metal ion is m a i n t a i n e d at a c o n s t a n t l e v e l as a r e s u l t

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of the buffer action by inorganic and/or organic ligands dissolved in seawater, the concentration of particulate metal is linearly related to that of the binding site in particulate organic matter, and may reflect the amount of particulate organic matter. As some free metal ions, e.g. copper and uranium, are expected to have a constant activity because of the buffer action of organic and/or inorganic ligands in the marine environment, the concentration of such metals in suspended particles is linearly related to the presence of particulate organic matter. Therefore, the relationship between particulate metals and particulate organic carbon or particulate organic matter, which can be measured directly, will give a method of finding the speciation of trace metals in suspended particles. We believe that, in future, it will be more important experimentally to study the chemical speciation of particulate metals because they play a significant role in metal scavenging and reflect the chemical states of the dissolved species. ACKNOWLEDGEMENTS

The author thanks Dr. Y. Sugimura (Director of Geochemical Laboratory, MRI) for his support of this research, and Dr. E. Tanoue (MRI) for his useful discussion.

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