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  • 19 Effects of Humic Substances on Metal Speciation

    E. Michael Perdue

    School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, GA 30332

    This chapter addresses some of the problems that must be understood and solved before the effects of metal-humic substance complexation on water-treatment processes can be quantitatively addressed. The heterogeneity of ligands in a humic substance not only complicates the mathematical description of equilibrium data, but also makes the complexation capacity of a humic substance almost impossible to determine accurately. Complexation capacities (meq/g) of humic sub-stances are widely reported to vary with pH, ionic strength, concen-tration of the humic substance used in the measurement, and nature of the metal being studied. By analogy with the behavior of a simple ligand (citrate), this chapter demonstrates that the reported effect of humic-substance concentration on complexation capacity is probably an artifact and that other experimental parameters affect conditional concentration quotients for metal complexation reactions. These ef-fects create the illusion that complexation capacity is a function of pH, ionic strength, and nature of the added metal ion.

    HUMIC SUBSTANCES ARE UBIQUITOUS in the aquatic environment, and their ability to form complexes with metal ions is well documented by many experimental and modeling studies. The interaction of humic substances with metal ions has been the subject of several recent review papers (1-6). In the context of water-treatment chemistry, the interaction of humic substances with metal ions can potentially affect removal of humic substances by coagulation-flocculation processes, removal of toxic heavy metals from polluted waters, and rates and products of reaction of humic substances with disinfectants.

    0065-2393/89/0219-0281$06.00/0 1989 American Chemical Society

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 282 AQUATIC HUMIC SUBSTANCES

    Rather than process-oriented aspects of metal-humic substance complexation, the focus here is on describing some of the experimental and conceptual pitfalls that undermine our efforts to quantitatively describe metal-humic substance complexation in a predictive manner. Until such problems are clearly understood, the effects of metal-humic substance complexation on water-treatment processes wil l remain obscure, and trial-and-error wil l continue to be the most common approach toward the development of water-treatment methodologies.

    Experimental studies can be conducted to examine metal complexation by humic substances at any p H , ionic strength, or combination of competing metal ions. Quantitative modeling of metal-humic substance complexation has not advanced, however, beyond single-metal complexation at constant p H and ionic strength. Even in such relatively simple systems, proper recognition of the effects of p H , ionic strength, nature of the metal, and the concentration of humic substances used in an experiment on metal-humic substance complexation equilibria is needed for interpretation of complex-ation-capacity measurements and interpretation of thermodynamic data on metal-humic substance complexation.

    Overview of Metal Complexation Equilibria This section presents an overview of the pertinent equations that describe metal-ligand complexation equilibria, both for a simple ligand and for a complex mixture of ligands. The distinction between concentrations and activities must be clearly developed and maintained. Equilibrium constants depend only on temperature and pressure; concentration quotients depend on temperature, pressure, and ionic strength; and conditional concentration quotients depend on temperature, pressure, ionic strength, p H , concentrations of competing metals, and ligands.

    Complexation by a Single Ligand. The reactions between a metal ion (M) and a single binding site (Lt) can be described by either overall or stepwise formation constants. For example, for the 1:1 metal-to-ligand complexation reaction, M + Lj = M L 4 , the overall and stepwise formation constants are the same (the stepwise wi l l be used):

    { M L } [ML,] 7ML, = R M * { M R U [M][LJ * 7M7L{ 1 ' U

    where M is a metal aqueous ion; L* is a fully deprotonated binding site; ML{ is the complex formed from 1 mol each of M and L f ; braces { } and square brackets [ ] denote activities and concentrations, respectively; and 7-values are activity coefficients. K{ is a true thermodynamic constant, but the concentration quotient and the activity coefficient ratio ( are complementary

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 19. PERDUE Humic Substances and Metal Speciation 283

    functions of ionic strength. For most simple metal-ligand complexes, basic electrostatic considerations (Debye-Hiickel theory) indicate that the activity coefficient ratio (,) equals 1 at zero ionic strength and increases with increasing ionic strength. values therefore equal K{ values at zero ionic strength and tend to decrease with increasing ionic strength. In a given solution of metal and ligand, the concentration of the complex M L 4 is thus expected to decrease upon addition of a background electrolyte. Experimental studies generally yield concentrations, rather than activities, of reac-tants and products. Consequently, Kt values cannot be directly measured, but must be obtained either by estimation of < values or by extrapolation of Ki values (obtained at several ionic strengths) to zero ionic strength.

    Another factor that affects the extent of complexation of M by Lj is competition from side reactions, especially the hydrolysis of the metal ion to produce hydroxy complexes and the protonation of the ligand to produce its conjugate acid(s). These reactions do not actually change K(c as we have defined it, but they do affect the degree of complexation of M by L f . As a general rule, ligands tend to form protonated ligands at low p H and metal ions tend to form hydroxy complexes at high p H . Consequently, the reaction between the metal ion and the ligand is often most favorable at intermediate p H values.

    For mathematical convenience, a conditional concentration quotient K{* is often defined, in which the precise terms in equation 1 are replaced by more convenient terms:

    = [M^bound)]

    ^ [ M ( M W M ]

    In this equation, M (free) represents all forms of the metal ion that are not bound to the ligand of interest, L,(free) represents all forms of the ligand that are not bound to the metal ion, and ML ((bound) represents all complexes of 1:1 metal-to-ligand stoichiometry. Unlike K ^ , which is a function only of ionic strength, K ( * is a function of ionic strength, p H , concentrations of competing metal ions and ligands, and so on. If all side reactions are well understood, K f * is a useful parameter that can be directly related to Kf.

    Complexat ion by a Mul t i l i gand M i x t u r e . In the previous section, the use of K,* instead of Kf was a matter of mathematical and experimental convenience. In the study of metal binding by a multiligand mixture such as humic substances, however, there is no choice. It is simply not possible to fully describe the side reactions of a ligand mixture whose individual components are unknown. Conditional concentration quotients or related hybrid expressions are used exclusively, even though the users of such expressions may not always recognize their limitations. In extending the concept of a conditional concentration quotient for metal complexation by a

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 284 AQUATIC HUMIC SUBSTANCES

    single ligand to multiligand mixtures, an expression that formally resembles equation 2 is usually written

    X [Ml^bound)] K* = (3)

    [M(free)] Wfree)] 1 '

    where 2[ML t(bound)] is the sum of the concentrations of all complexes formed between M and the multiligand mixture, X[Lf(free)] is the sum of the concentrations of all binding sites that are not associated with M , and [M(free)] is the sum of metal species that are not associated with the multiligand mixture.

    The experimental methods that are used to study metal complexation by humic substances directly provide either none or, at best, one of the three terms in equation 3. The missing terms are always calculated fr

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