Metal–organic complexation in the marine environment
George W Luther III1, Timothy F Rozan1 , Amy Witter2 and Brent Lewis3
1College of Marine Studies, University of Delaware, Lewes, DE 19958, USA
2Chemistry Department, Dickinson College, Carlisle, PA 17013, USA
3Science & Math Department, Kettering University, Flint, MI 48504, USA
We discuss the voltammetric methods that are used to assess metal–organic complexation in seawater. These consist of titration methods using anodic stripping voltammetry (ASV) and cathodic stripping voltammetry competitive ligand experiments (CSV-CLE). These approaches and a kinetic approach using CSV-CLE give similar information on the amount of excess ligand to metal in a sample and the conditional metal ligand stability constant for the excess ligand bound to the metal. CSV-CLE data using different ligands to measure Fe(III) organic complexes are similar. All these methods give conditional stability constants for which the side reaction coefficient for the metal can be corrected but not that for the ligand. Another approach, pseudovoltammetry, provides information on the actual metal–ligand complex(es) in a sample by doing ASV experiments where the deposition potential is varied more negatively in order to destroy the metal–ligand complex. This latter approach gives concentration information on each actual ligand bound to the metal as well as the thermodynamic stability constant of each complex in solution when compared to known metal–ligand complexes. In this case the side reaction coefficients for the metal and ligand are corrected. Thus, this method may not give identical information to the titration methods because the excess ligand in the sample may not be identical to some of the actual ligands binding the metal in the sample.
Geochemical Transactions 2001, 2:65. Open Access Article.
In the last two decades, our knowledge of trace metal speciation has grown tremendously. With the advent of trace metal clean sampling techniques and sensitive voltammetric techniques, [2-4] the marine community now recognizes that metal speciation in seawater and estuarine waters is dominated by complexation with organic compounds of unknown composition and origin. [5-12] Recent culture work [13-18] has shown that microorganisms produce a variety of low molecular weight organic compounds that complex metals with high stability constants. These compounds have a variety of functional groups that include phosphate, carboxylic acids, amines, thiol and hydroxy groups. Specific functional groups such as hydroxamate, catecholate and β-hydroxyaspartate are bidentate groups and organisms make molecules with three bidentate groups in a molecule.[14,19-21] In addition, plant degradation products [22-30] such as porphyrins are significant organic ligands that bind metals through four N atoms in a square planar arrangement. These latter multidentate molecules have very high stability constants with metals and are also kinetically inert to metal–ligand dissociation processes. [31-34] For this reason, organisms generally uptake the free metal ion rather than a metal–ligand form.[35,36] Thus, an understanding of metal–organism interactions requires an understanding of the amount of dissolved free ion present relative to the total dissolved metal concentration as well as the metal acquisition methods that an organism can use. [35-37]
In this paper we review and compare the principal voltammetric methods, which provide evidence for metal–organic complexes. Most voltammetric work is performed with the hanging mercury drop electrode (HMDE) or the rotating disk electrode (RDE) with a thin mercury film (TMF) because these permit the measurement of metal–organic complexation at (sub)nanomolar levels directly in the solution of interest. The actual experimental methods can be broken into two broad categories and are based on the electrochemical behavior of the metal bound to an organic ligand.
The first method consists of titration experiments that measure the amount of ligand in excess to the metal in the solution [38-44] and the conditional stability constant, Kcond M’L , for the excess ligand with the metal. The Kcond M’Lis generally assumed to be a 1:1 metal–ligand complex and is given by
Kcond M’L= [ML]/([M’] [L’])
where M’ and L’ are the concentrations of the metal and ligand that are not bound to each other. These are related to the total metal [M]T and [L]T via
[M’] = [M]T – [ML] and [L’] = [L]T – [ML].
The free metal [Mn+] plus the metal bound to other inorganic ligands equals [M’],
[M’] = [Mn+] + Σ MXi
and the fraction of free metal in the solution without the organic ligand is given by
[Mn+] = [M’] αM
αM = 1/(1 + Σ , [X]i)
This has also been expressed as the side reaction coefficient for M’, αM’, which is the reciprocal of αM or
αM’ = [M’]/[Mn+]
The conditional constant for M’L is related to Mn+L by
Kcond ML= [ML]/([Mn+] [L’]) = Kcond M’L (αM’)
Similar equations can be written for the organic ligand to give a thermodynamic constant,
Ktherm = [ML]/([Mn+] [Ln–]) = Kcond M’L (αM’) (αL’)
but in environmental samples the interactions of H+, Ca2+ and Mg2+ with the ligand are unknown.
The titration experiments include (1) anodic stripping voltammetry (ASV), which is useful for metals that react at the electrode directly (Cu2+, Zn2+, Cd2+, Pb2+), and (2) cathodic stripping voltammetry/competitive ligand exchange[3,8,9] (CSV-CLE) which is useful for metals that do not react at the electrode directly but have a metal–ligand complex that does (Fe3+, Co2+). The CSV-CLE method depends on the measurement of a known metal–ligand complex (the competing ligand), that adsorbs to the mercury electrode. In addition, a kinetic CSV-CLE approach [10-12] for excess ligand binding a metal has been used to measure the metal organic formation rate constant (kf), dissociation rate constant (kd), the half-life or residence time (t1/2) of the complex and Kcond M’L(from kf/kd). The second type of voltammetry method involves the breakdown of the actual complex in situ and is termed pseudovoltammetry, [45-48] which is useful for metals that react at the electrode directly. This method gives information on the amount of ligand binding to a specific complex with a thermodynamic constant, Ktherm, that differs from Kcond ML. Kcond ML is corrected for the side reaction coefficient of the metal but not the ligand whereas Ktherm is corrected for the side reaction coefficients of the metal and ligand via comparison to metal–ligand complexes of known Ktherm (chelate scale).
We describe the use of these methods for unknown ligands in seawater as well as with model ligands in UV irradiated seawater for the metals Cu(II), Zn(II) and Fe(III). In the case of CSV-CLE, we show for known Fe(III)-organic complexes that the use of different ligands [1-nitroso-2-napthol, or 1N2N, and salicylaldoxime, or SAL) gives comparable K and ligand concentration data.