Environ. Sci. Technol. 2002, 36, 3109-3113
Analysis of the Effect of pH on Cu2+-Fulvic Acid Complexation Using a Simple Electrostatic Model M A R IÄ A A . R A M O S , S A R A H F I O L , R O C IÄ O L O Ä PEZ, JUAN M. ANTELO, AND FLORENCIO ARCE* Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago de Compostela, E15782 Santiago de Compostela, Spain
A simple electrostatic model was used to study the effect of pH on the binding of Cu2+ to fulvic acid in solutions containing similar amounts of dissolved organic carbon (DOC) as natural media, such as aquatic environments and soil solutions. Complexation behavior was affected by increased pH because of changes in the electrostatic interaction resulting from an increase in the negative charge on the fulvic acid molecule. Solutions of soil-extracted fulvic acid (FA), at concentrations of 25 and 35 mg L-1, ionic strength 0.005 M, and pH 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5, conditions that simulate those of natural freshwaters, were titrated with copper ion using differential pulse anodic stripping voltammetry. Assuming the formation of 1:1 complexes, the conditional binding parameters (stability constant and binding capacity) were calculated for each pH value. Use of a 1:1 electrostatic model allowed estimation of the contribution of the electrostatic effect to the ion binding reaction, at each pH value, as well as calculation of a binding constant that was not dependent on pH and which thus represented the contribution of the chemical heterogeneity. Furthermore, it was found that only a small proportion of the FA acid functional binding sites are involved in the formation of copper complexes.
Introduction The total aqueous concentration of a metal ion is not a good predictor of its bioavalability, because the metal speciation will greatly affects its availability to aqueous organisms. Humic substances (humic acids and fulvic acids) play an important role in the speciation, fate, and transport of metal ions in many aquatic environments, where they are ubiquitous, because they possess numerous functional groups capable of binding metals (1). Humic substances are polydisperse mixtures of organic polyelectrolytes; therefore, to be able to understand their properties, in particular how they affect metal behavior, simplified models must be used. Reasonably successful models of proton binding have been developed, but for metal ion binding the situation is more complex. A useful model of metal binding should allow interpretation and prediction of the effect of certain variables, such as pH or ionic strength, on ion-humic substance binding reactions. Due to the high degree of chemical heterogeneity of humic substances, ion binding can only be satisfactorily described * Corresponding author phone: 00 34 981 547 145; fax: 00 34 981 547 079; e-mail:
[email protected]. 10.1021/es0101734 CCC: $22.00 Published on Web 06/17/2002
2002 American Chemical Society
using a distribution of affinity constants. Two main approaches have been described in the literature: continuous affinity distributions (2-4) and discrete sites, each with its own affinity and abundance (5, 6). Furthermore, because of ion binding, humic molecules have a variable electric charge that creates an electric field, which in turn influences ion binding. Many studies have been carried out with the aim of finding ways of taking this effect into account in the analysis of ion binding (5-9). Carboxylic and phenolic groups are the two main types of sites responsible for the formation of metal-humic substances complexes. At naturally occurring pH values many of these groups will be ionized; therefore, the humic molecules have a charge that varies with pH and that creates an electric field, which in turn influences ion binding. Cabaniss and Shuman (10) assumed that the effect of pH on Cu-FA binding is due to competition between H+ and Cu2+ for binding sites and formation of mixed ligand species, and they used a five-site model for quantitative modeling of copper binding by Swannee FA and several other fulvic acids (11). These authors did not include electrostatic effects of pH on the ligand in their model, and they found that the pH dependence of Cu-FA formation is not simply inverse first order in pH (10). Another approach used in modeling the effect of pH on metal binding is to consider the explicit Coulombic contribution to the binding strength, which arises from the charge on a humic molecule (6). De Wit et al. (8) suggested that proton and metal ion binding can be considered as monocomponent adsorption processes, even though the two processes are linked by the electrostatic field around the charged humic particles. In this case the pH dependence basically arises from the effect of the pH on the electric charge of the humic molecules, and the aim of the present study was to test the validity of this approach by analyzing the binding of Cu2+ with soil fulvic acid in the pH range 4.5-7.0, using a double layer model, first developed by de Wit et al. (8), to quantify the electrostatic effect. The starting point in developing a physicochemical model for the description of metal ion binding to natural organic matter is the description of proton binding as a function of pH in a 1:1 electrolyte; therefore, in the present study we used a soil fulvic acid for which the proton binding reaction has already been studied (12).
Theory Humic substances are considered to possess a limited number of chemically different types of binding sites for proton and metal ions; therefore, the binding of a metal species to such a heterogeneous ligand system may be considered as the sum of the contributions of the different types of site present. The binding of a metal ion M to a particular type i site on a humic molecule at constant pH can be written as
M + Li T MLi and the corresponding stability constant may be expressed as
Kapp,i ) [MLi]/[Li][M]
(1)
where [M] is the concentration of free metal ion in solution and [MLi] and [Li] are the concentrations of bound and free type i positions, respectively. The constant Kapp,i is a conditional constant because its value depends on experimental conditions, such as ionic strength and pH. The fraction VOL. 36, NO. 14, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of type i sites occupied by metal ion is
TABLE 1. Characteristics of Fulvic Acid
θi ) [MLi]/([MLi]+[Li]) The total fraction of sites occupied by M can be expressed as
θT ) [ML]T/[L]T where [ML]T ) Σ[MLi] is the total concentration of metal ion bound to the fulvic acid and [L]T ) Σ([Li]+[MLi]) is the total concentration of binding sites, i.e., the maximum binding capacity. Experimental measurement of the concentration of free metal ions (or even of the total fraction of occupied binding sites) for an interval of total concentration of metal ions allows calculation of the complexation parameters, Kapp,i and [Li]T, which characterize the metal-fulvic acid binding reaction. When, as a result of ionization, the fulvic acid molecules are negatively charged, the concentration of metal ion in solution at the location of the metal binding sites can be defined, according to the Boltzmann distribution law, as
[M]S ) [M] exp(-nFΨS/RT)
(2)
where, as previously mentioned, [M] is the free metal ion concentration in the bulk solution, n represents the charge number of the cation, ΨS is the mean electrostatic potential at the fulvic acid surface, F is the Faraday constant, R is the gas constant, and T is the temperature. Taking the previous eq 1 into consideration, the apparent conditional binding constant can be resolved as
Kapp,i ) [MLi]/[Li][M]S exp(nFΨS/RT)
(3)
In electrostatic models used to describe metal cationfulvic acid complexation it is assumed that the conditional stability constant includes both the contribution made by specific interaction between the metal and fulvic acid, Khet (which depends on the chemical heterogeneity of the ligand, i.e., it reflects the chemical properties of the different binding sites), and also the contribution made by nonspecific interaction of an electrostatic nature, Kelec (which is caused by the interaction of metal cation charge and the negative charge generated on the fulvic acid molecules as a consequence of ionization of acid groups, mainly carboxylic and phenolic groups). In accordance with this Kapp,i can be expressed as
Kapp,i ) Khet,iKelec,i
(4)
Taking into account that the fraction of acid groups ionized and therefore the size of the charge on the fulvic acid molecules depend on pH, any variation in pH will give rise to a change in the electrostatic interaction between the metal ion and the fulvic acid, which will affect the value of Kapp and be reflected in the term Kelec. Using a model that allows calculation of the contribution of the electrostatic term to the apparent conditional stability constant, we can obtain the value of the term Khet, whichsas previously mentioneds represents the chemical heterogeneity of the sample, allowing verification of whether this is dependent on pH and the fulvic acid concentration. To describe the contribution made by the electrostatic term to metal ion binding by fulvic acid we have used the approach described by de Wit et al. (8) and used in the analysis of metal ion binding by a peat humic acid (13). The surface potential was evaluated using the double layer model selected. In the present study, we restricted our attention to the spherical model (i.e. it was assumed that the FA molecules can be described as uniform, spherically shaped particles) 3110
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N (%)
C (%)
H (%)
E4/E6
ca. mol wt (g mol-1)
carboxylic content (mequiv g-1)
1.13
45.40
3.80
3.53
1900
5.7
because this was used in a previous study of the proton binding reaction in the same fulvic acid sample (12). To apply the diffuse double-layer model the surface charge density, σS, must be known and the appropriate Poisson-Boltzmann equation must be numerically solved, to calculate ψS (3, 6, 14). The relationship between σS and the measurable charge per unit mass, Q, is provided by S, the specific surface area of the fulvic acid particle (σS ) Q/S). For uniform particles, S can be estimated from the geometry and the particle density, F, or the molecular weight of fulvic acid. When titrating a metal at constant pH the overall charge of the fulvic acid molecule, Q (C g-1), for each point of the experimental curve, will be determined by the protons ionized from the weakly acid functional groups, QpH, and the extent of metal binding for each value of the total concentration of metal added. Therefore in the presence of a dissolved cation
Q ) QpH + n[ML]F/[FA]
(5)
where [ML] is the concentration of the metal-fulvic acid complex, in mol L-1, [FA] is the concentration of fulvic acid, in g L-1, and F is the Faraday constant in C mol-1. Once the charge Q is known, the surface potential can be calculated from the double layer model, as previously outlined, and once ψS is known, [M]S can be obtained from eq 2. By including [M]S in eq 3 and taking eq 4 into account we can estimate the term Kelec and obtain Khet, a value of the complexation constant that is independent of pH.
Methods Reagents. Fulvic acid was extracted following the IHSS procedure (15). The elemental composition, the ratio between the absorbances at 465 and 665 nm (E4/E6) obtained following the procedure of Chen et al. (16), an empirical estimation of the molecular weight obtained from the extinction coefficient at 280 nm (17), and the content in carboxylic groups obtained according to the Ca-acetate method (18) for fulvic acid appear in Table 1. Titrant copper solutions were prepared from Aldrich standard copper solution for atomic absorption, all the other chemicals were Merck suprapur, and solutions were made by dissolving reagents in Milli-Ro-Milli-Q (Millipore) water. Voltammetric Titrations. The range of concentrations of copper studied varied with FA concentration and pH; however, all titrations were started at p[Cu2+] = 6.5-7 (the lowest value whose peak current intensity corresponding to the electrolabile copper ion was reliable and reproducible) and finished at p[Cu2+] = 4.5-5. Titrations were carried out at pH 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5, using 25 and 35 mg L-1 fulvic acid solutions. A small volume of a solution of NaOH 0.3 M was initially added until the required pH was reached, and then a solution of 0.1 M NaNO3 was added until the ionic strength of the solution was 0.005 M. The initial volume of the sample was 30 mL, and the maximum volume of the titrant added was 0.75 mL. The pH value was adjusted on each addition of titrant, using a stream of CO2/N2. Electrolabile copper ion concentrations were measured by Differential Pulse Anodic Stripping Voltammetry (DPASV), using a Metrohm E-506 polarograph with a 663VA stand. As described in a previous paper (19) during the deposition step, a potential value far from the zero charge potential
FIGURE 1. Effect of pH on the experimental titration curves. [FA] ) 25 mg L-1. ([) pH ) 5.0, (9) pH ) 5.5, (2) pH ) 6.0, (×) pH ) 6.5, (/) pH ) 7.0, (b) pH ) 7.5. s Model fit.
FIGURE 2. Effect of pH on the concentration of copper-fulvic acid complex. [FA] ) 35 mg L-1. ([) pH ) 5.0, (9) pH ) 5.5, (2) pH ) 6.0, (×) pH ) 6.5, (/) pH ) 7.0, (b) pH ) 7.5. s Model fit.
(-0.8 v vs Ag/AgCl) and a low deposition time ( 5.5, the number of binding sites per molecule of fulvic acid is slightly greater than 1.
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Received for review June 21, 2001. Revised manuscript received March 27, 2002. Accepted May 15, 2002. ES0101734
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