Environ. Sci. Technol. 2000, 34, 4969-4973
Analysis of the Cu2+-Soil Fulvic Acid Complexation by Anodic Stripping Voltammetry Using an Electrostatic Model J O S EÄ L . C A R B A L L E I R A , JUAN M. ANTELO, AND FLORENCIO ARCE* Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago de Compostela, E15706 Santiago, Spain
An electrostatic model has been used in the analysis of the fulvic acid-copper ion complexation with the aim of estimating the contribution of the electrostatic effect to metal binding and calculating the intrinsic complexation parameters in solutions with a dissolved organic carbon (DOC) analogous to that in natural media such as aqueous environments and soil solution. For this purpose solutions of a soil extracted fulvic acid (FA) at concentrations 30, 40, 50, and 60 mg L-1 and ionic strengths 0.001, 0.005, 0.01, 0.05, and 0.1 were titrated at pH 6.5 with copper ion by using differential pulse anodic stripping voltammetry. By means of a 1:1 nonelectrostatic model conditional complexation parameters (apparent stability constant and complexation capacity) were calculated, and their dependence on the FA concentration and ionic strength was demonstrated. Consideration of a second binding site and assumption of bidentate binding sites do not significantly affect the results. The application of a 1:1 electrostatic model with the geometric parameters derived from a previous study of the proton binding reaction of the fulvic acid allowed us for the obtention of the intrinsic stability constants which do not exhibit any dependence on the FA concentration nor on the ionic strength. Comparison of the apparent and intrinsic stability constants shows a significant contribution of the electrostatic effect on the copper complexation.
Theory The master curve approach is described elsewhere (9), and it has been previously applied to the analysis of proton binding by the same soil fulvic acid studied in the present work (11). In this procedure, electrostatic effects are considered via the diffuse double-layer model. The concentration of metal ion in the double layer close to the fulvic acid surface, [Ms], is calculated from
[Ms] ) [Mn+] exp(nFψs/RT)
Introduction Binding of cations by humic substances (humic and fulvic acids) may have significant effects on metal speciation, and so it influences the mobility of metal ions in the soil and aqueous environments (1, 2), the solubility of the heavy metals in soil and the plant uptake, which depend on the ionic activities of heavy metals in soil solution, rather than on their total concentration (3). Humic substances are polydisperse mixtures of organic polyelectrolytes bearing different functional groups that act as binding sites for cations, and, as a result of this heterogeneous nature of humic substances, each specific binding site type exhibits a characteristic chemical affinity for cations. On the other hand, due to ion binding humic molecules exhibit an electric charge that creates an electric field which influences further ion binding. * Corresponding author phone: 00 34 981 547 145; fax: 00 34 981 547 079; e-mail:
[email protected]. 10.1021/es001036u CCC: $19.00 Published on Web 11/01/2000
This combination of chemical heterogeneity and electrostatic effect entails using models involving various simplifications to interpret the binding of cations. A useful model for metal ion binding should be able to describe ion binding over a wide range of conditions with respect to pH, solution concentration, and ionic strength. Initially, cation binding was described in terms of discrete site and continuous distribution models (4, 5) that include no electrostatic effects. These early methods were of limited use as they described cation binding under specific experimental conditions and raised the need for models including electrostatic effects which arise from the large charge on the fulvic molecules. For example, Marinsky and Ephraim (6) and Benedetti et al. (7) used a Donnan equilibrium phase model; Bartschat et al. (8) and de Wit et al. (9) developed and oligoelectrolyte model by applying the Poisson-Bolztmann equation to impenetrable spherical fulvic acid particles; and Tipping and Hurley (10) reported an empirical description of the electrostatic term. According to the master curve approach initially developed by de Wit et al. (9), the inclusion of a simple electrostatic double layer model allows one to analyze chemical heterogeneity independently from any electrostatic effect. Recently, we obtained the intrinsic parameters of proton binding by a soil fulvic acid (11) by using an electrostatic discrete site model where the contribution of the electrostatic term was calculated in terms of the geometry of the fulvic acid particles determined in constructing the master curve. In the present paper we analyzed the binding of Cu2+ by the previously studied fulvic acid. To this end we carried out copper titrations of fulvic acid solutions under different experimental conditions which resemble a natural matrix like aqueous environments or soil solutions. The intrinsic stability constants of copper binding were obtained in the basis of the ionic strength effect on the titration curves. The interpretation of such effect was made by using an electrostatic discrete site model and assuming, for the geometric parameters of the fulvic acid particles, the values derived from the study of the proton binding reaction.
2000 American Chemical Society
(1)
where [Mn+] is the metal ion concentration in the bulk solution, ψs is the mean electric potential of the fulvic acid particle at the binding sites, n is the charge of metal ion, and F, R, and T have their usual meaning. To calculate the electrical potential at the surface, one must know the surface charge density, σs, which is related to the net surface charge, Q. In the presence of metal ions, the surface charge is determined by the number of protons dissociated from the humic substance and the amount of bound metal. For a single divalent cation
Q ) -Qmax + QH +2QM
(2)
where Qmax is the surface charge when all the acidic functional groups are dissociated, QH is a function of the amount of proton binding, and QM is related to the amount of metal ion VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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binding. The term (-Qmax + QH) represents the number of protons dissociated and is derived from previously carried out acid-base titrations (11).
Methods Reagents. The extraction procedure and characterization of the soil fulvic acid studied, FA, were described elsewhere (11). All the other chemicals were Merck suprapur, and solutions were made by dissolving in Milli-Ro-Milli-Q (Millipore) water. Voltammetric Titrations. Copper titrations were performed in FA solutions by using a standard copper solution. The copper concentration range studied varied with the FA concentration and ionic strength; however, all titrations were started at p[Cu2+]total ∼ 7 (the lowest value whose peak current intensity corresponding to the nonorganically complexed copper,was reliable and reproductible) and finished at p[Cu2+]total ∼ 4-5. Titrations were carried out at pH 6.5, using 30, 40, 50, and 60 mg‚L-1 fulvic acid solutions in 0.001, 0.005, 0.01, 0.05, and 0.1 M NaNO3 as background electrolyte. The lowest FA concentration studied (
