Effects of Humic Substances on Metal Speciation - Advances in

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Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch019

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 substances are widely reported to vary with pH, ionic strength, concentration 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 effects 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 sub stances 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

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


282

AQUATIC H U M I C SUBSTANCES

Rather than process-oriented aspects of metal-humic substance com plexation, 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 com plexation on water-treatment processes will remain obscure, and trial-anderror will 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 rec ognition 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 complexation-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 , concentra tions of competing metals, and ligands. Complexation by a Single Ligand. The reactions between a metal ion (M) and a single binding site (L ) can be described by either overall or stepwise formation constants. For example, for the 1:1 metal-to-ligand com plexation reaction, M + L j = M L , the overall and stepwise formation constants are the same (the stepwise Κ will be used): t

4

{ML} *

{MRU

[ML,]

7ML,

[M][LJ * 7M7L

=

R

1

M

'

{

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 ; braces { } and square brackets [ ] denote activities and concentrations, respectively; and 7-values are activity coefficients. K is a true thermodynamic constant, but the con centration quotient and the activity coefficient ratio Γ are complementary {

f

{

(


19.

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283

Humic Substances and Metal Speciation

functions of ionic strength. For most simple metal-ligand complexes, basic electrostatic considerations (Debye-Hiickel theory) indicate that the activity coefficient ratio (Γ,) equals 1 at zero ionic strength and increases with in creasing 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 is thus expected to decrease upon addition of a background electrolyte. Experi mental studies generally yield concentrations, rather than activities, of reactants and products. Consequently, K values cannot be directly measured, but must be obtained either by estimation of Γ< 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 L j 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 as we have defined it, but they do affect the degree of complexation of M by L . 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: {

4


t

c

(

f

{

=

^

[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 * is a useful parameter that can be directly related to Kf. (

(

f

C o m p l e x a t i o n b y a M u l t i l i g a n d 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


284


single ligand to multiligand mixtures, an expression that formally resembles equation 2 is usually written X [Ml^bound)] K* = [M(free)] £

(3) '

1

Wfree)]

where 2[ML (bound)] is the sum of the concentrations of all complexes formed between M and the multiligand mixture, X[L (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 from the experimental data and some stoichiometric assumptions about the system being studied. The most common assumptions involve the neglect or in vocation of the existence of simple inorganic complexes (hydroxy and_carbonato complexes) in the system under investigation. For example, K * is often calculated directly from experimental data as t


f

K* =

C

" - M [ M ] ( C - C + [M]) L

(4) W

M

where C and C are the total stoichiometric concentrations of metal and ligand in the system under study and [M] is the concentration of free metal ion. In calculating £[ML,(bound)] as ( C - [M]), the presence of inorganic complexes of the metal ion has been neglected. In calculating Σ [L/free)] as ( C - C + [M]), an average 1:1 metal-to-ligand stoichiometry has been assumed for the mixture of binding sites. It is also assumed that C is known. Most of the remainder of this chapter will address the experimental deter mination of C from metal-binding data. Although an expression can be written for K * in equations 3 and 4 that formally resembles the conditional concentration quotient in equation 2 (Κ·*), Κ * is not a constant at a given p H and ionic strength. Rather, K * will decrease steadily as_the total metal-to-ligand ratio ( C / C ) increases. The functional nature of K * arises from preferential reactions of stronger ligands at low metal-to-ligand ratios and has been discussed by several investigators (1-11). Nevertheless, the variation of K* with the total metal-to-ligand ratio has often erroneously been cited as evidence for the existence of two binding sites in a humic substance, one reacting more favorably than the other with the metal ion. Average K* values are ultimately functions of ionic strength, p H , and the degree of saturation of the multiligand mixture with metal ion. This latter term is loosely reflected in the C / C ratio. Reported stability M

L

M

L

M

L

L

M

M

L

L


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285


"constants" for metal-humic-substance complexation are not actually con stant and should be viewed with skepticism. In simple systems containing one metal ion and one ligand, K * , Kf, and K | values can be interconverted, as described in the preceding section. In metal-multiligand mixtures, however, similar interconversions of K * , K , and Κ values are not practical at all. For example, to remove the p H dependence of K * values, it would be necessary to treat all pH-dependent side reactions of the metal ion and all components of the multiligand mixture quantitatively. Although such corrections are practical for metal-ion hy drolysis, there is not yet a rigorous treatment of the acid-base chemistry of humic substances (12). Even if the corrections could be made, the resulting K values would still be functions of ionic strength and the degree_of saturatiorurf the multiligand mixture with metal ion. The conversion of K values into Κ values could theoretically be accomplished by extrapolation to zero ionic strength of K values obtained at constant degree of saturation of the multiligand mixture with metal ion and variable ionic strength. The resulting Κ values would still be functions of the degree of saturation of the multiligand mixture with metal ion. The remaining functional dependence is a funda mental characteristic of mixtures, and it cannot be eliminated by any ex perimental method short of total fractionation of the mixture into pure compounds that could be studied separately. f


c

c

c

c

Complexation Capacity: Definitions and Measurements Definition of Complexation Capacity. For a pure ligand reacting with divalent or trivalent metal ions, even though complexes of higher stoichiometry (1:2, 1:3, etc.) may form at low levels of bound metal, 1:1 com plexes predominate at higher levels of added metal. Thus, the complexation capacity of the ligand is usually about 1 mol of metal per mole of ligand. The important point is that complexation capacity is a compositional, rather than thermodynamic, parameter. The complexation capacity of citrate ion (Cit ), for example, is about 1 mol of metal per mole of citrate, regardless of p H , ionic strength, nature of the metal, or the concentration of citrate ion used in the measurements. Theoretically, the complexation capacity (CC) of a humic substance or other complex mixture is, to a good approximation, a weighted average of the complexation capacities of the individual ligands in the mixture: 3-

2 (CCMweightL 2, [weight], where (CC) is the complexation capacity of the ith ligand in the mixture and [weight], is a weighting factor that reflects the relative abundance of f


286


that ligand in the multiligand mixture. The nature of the weighting factor depends on the dimensional units of C C , commonly given in milliequivalents per gram. If (CC) values are also in milliequivalents per gram, then [weight] is the mass of the ith ligand in the mixture. If the mixture is not fractionated, C C will be an average constant from which total ligand concentrations (C ) can be computed for use in equilibrium expressions (equation 4). The metal-humic substance literature suggests that the complexation capacity of a humic substance varies considerably with almost every conceivable ex perimental variable: increasing at higher p H (13-18), decreasing at higher ionic strength (13, 16), increasing at higher humic-substance concentrations (13-15, 19, 20), and generally varying with the nature of the added metal ion (13, 14, 17, 21). The differences between these reported results and theoretical expectations must be resolved. t

4


L

Major Misconceptions Concerning Complexation Capacities. Two fundamental misconceptions are responsible for much of the confusion over complexation capacities and their reported dependence on experi mental conditions (13-21). First, the effects of experimental conditions on conditional concentration quotients (K*) are erroneously interpreted as mod ifications of the complexation capacity (CC) of the humic substance. Com plexation capacity data are usually interpreted with no regard for the fact that K * values are affected by p H , ionic strength, and the nature of the reacting metal. It is simply easier to saturate ligands with metal at nearneutral p H , low ionic strength, and with a strongly binding metal such as Cu than otherwise. This greater ease of formation of metal-humic sub stance complexes leads directly to apparently higher C C values under these optimum conditions. Second, the effect of simple dilution on the position of the equilibrium in reactions of the type M + L , M L , is erroneously interpreted as a variation of C C with humic-substance concentration. Be cause metal complexation equilibria shift toward free metal and ligand with dilution, it is more difficult (at a constant C / C ratio) to saturate a dilute ligand mixture than a concentrated ligand mixture, which gives rise to the illusion that the C C of a humic substance decreases with dilution. These apparent effects on C C values will be demonstrated in a later section of the chapter. A potential source of confusion in reported C C values for metal-humic substance complexation is dimensional units. A typical humic substance has a carboxylic acid content of about 5 meq/g and contains about 50% carbon. Suppose that the C C of that humic substance is also 5 meq/g for divalent and trivalent metal ions. Then, depending on the choice of dimensional units and the charge of the metal ion, C C might be reported as 10 meq/g of C, 5.0 mmol/g of C , 3.3 mmol/g of C , 5 meq/g, 2.5 mmol/g, or 1.67 mmol/g. A l l of these dimensional units can be found in the literature on metal-humic substance complexation, as well as much poorer choices 2 +

M

L


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287


such as milligrams per gram or milligrams per gram of C . This potentially confusing problem is not particularly serious and, if recognized, it is easily corrected. If C values in K * expressions such as equation 4 are obtained from C C values, C is actually a binding-site concentration, where a binding site is defined as a group of one or more donor atoms in the humic substance that can bind a single metal ion. It is implicit in this definition that all metal-ligand complexes are of 1:1 stoichiometry (one metal ion per binding site). If a total ligand concentration is defined in this manner, metal com plexation data should not be interpreted in terms of a mathematical model that assumes the occurrence of complexes of greater than 1:1 stoichiometry. L


L

Simulation of Complexation Capacity Titrations.

The integrated

effects of ionic strength, p H , nature of the added metal ion, and ligand concentration on apparent C C values can be demonstrated by computer simulations of C C titrations for a system of one metal and one ligand. The additional inherent complications that are due to the multiligand nature of humic substances could be demonstrated only by analogous computer sim ulations of multiligand mixtures. In this chapter, complexation capacity ti trations have been simulated for citrate ion, with C u and C a as titrants at several p H values (4, 5 , 6 , 7), ionic strengths (0, 0 . 1 ) , and total ligand concentrations [log(C ) = - 4 , - 5 , - 6 ] . Thermodynamic data were obtained from a recent tabulation (22) and are given in Table I. Experimental param eters for 1 0 simulated titrations are given in Table II. In all cases, titrations were simulated over a C / C range of 0 . 0 - 3 . 0 . The choice of an upper C M / ^ L t i ° of 3 . 0 was based on this value as typical of most complexation 2 +

2 +

L

M

L

r a

Table I. Typical Thermodynamic Data for Metal-Citrate Complexation Reactions at Zero Ionic Strength and 298 Κ Reaction +

log K

eq

3

2

H + Cit "

Effects of Humic Substances on Metal Speciation - Advances in

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