Analytica Chimica Acta 402 (1999) 211–221
Effect of metal/fulvic acid mole ratios on the binding of Ni(II), Pb(II),
Cu(II), Cd(II), and Al(III) by two well-characterized fulvic acids in
aqueous model solutions
Amina L.R. Sekaly a , R. Mandal a , Nouri M. Hassan a , J. Murimboh a , C.L. Chakrabarti a,∗ ,
M.H. Back a , D.C. Grégoire b , W.H. Schroeder c
a
Ottawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa,
K1S 5B6, Ont., Canada
b Geological Survey of Canada, 601 Booth Street, Ottawa, K1S 0E8, Ont., Canada
c Atmospheric Environment Service, Environment Canada, 4905 Dufferin Street, Downsview M3H 5T4, Ont., Canada
Received 22 March 1999; received in revised form 25 June 1999; accepted 7 July 1999
Abstract
The kinetics of dissociation of Ni(II), Pb(II), Cu(II), and Cd(II)–fulvic acid (FA, Armadale), and Al(III)–FA (Suwannee
River) complexes in aqueous model solutions was studied by the competing ligand exchange method (CLEM). Chelex-100
cation exchange resin was used as the competing ligand and inductively-coupled plasma mass spectrometry (ICP-MS) or
graphite furnace atomic absorption spectrometry (GFAAS) was used to determine the rate of dissociation of the metal–FA
complexes in the model solutions. At low metal concentrations and at very low [metal]/[FA] mole ratios, the metals formed
inert complexes (dissociation rate coefficient kd ≈ 10−5 s−1 ) with the FAs. The percentage of inert complexes increased
as the [metal]/[FA] mole ratio was decreased at a constant concentration of the metal. The percentage of labile metal–FA
complexes (dissociation rate coefficient kd ≈ 10−2 –10−3 s−1 ) increased as the [metal]/[FA] mole ratio in the model solutions
was increased. The results were similar for all the metals studied. The two well-characterized FAs showed similar behavior
despite their marked differences in the binding capacity and in the source and nature of the two FAs — the Armadale FA
is pedogenic, and the Suwannee River FA is aquogenic. The results have special significance for the natural environment in
which potentially toxic metals are often present at trace levels. In such a situation, experiments at the environmentally relevant
[metal]/[FA] mole ratios are necessary to determine the effect of the [metal]/[FA] mole ratios on dissociation of metal–FA
complexes and release of metal–aquo complexes which are reported to be toxic. ©1999 Elsevier Science B.V. All rights
reserved.
Keywords: Fulvic acid complexes of Ni(II), Pb(II), Cu(II), Cd(II), Al(III); Armadale soil fulvic acid; Suwannee River fulvic acid; Dissociation
rate coefficients
1. Introduction
∗
Corresponding author. Tel.: +1-613-520-2600 x3839;
fax: +1-613-520-3749 x3830
E-mail address: chuni [email protected] (C.L. Chakrabarti)
Complexation reactions of potentially toxic metals
by naturally-occurring organic complexants such as
humic substances, which are ubiquitous in fresh-
0003-2670/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 5 3 4 - 6
212
A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
waters, are increasingly being recognized as important
factors in many natural systems because these reactions determine, to a large extent, the metal speciation
and bioavailability of the metal species [1]. Metal speciation also determines the mobility of trace metals
in the natural environment. For example, complexation of metal ions onto insoluble organic compounds
strongly reduces their mobility, whereas the formation
of soluble metal complexes with dissolved organic
compounds enhances their mobility in the natural
environment [2–4].
Dissolved organic compounds may have terrestrial
or aquatic origin, and may degrade or condense in
the water column. Their composition varies spatially
and temporally, and differences in their elemental
composition and spectroscopic properties have been
well-documented [4]. Humic substances, such as fulvic acids (FA) and humic acids (HA), represent a major fraction of dissolved organic compounds present
in freshwaters. FAs generally have a lower molecular
weight than HAs, higher oxygen content, higher carboxylic content and higher acidity [5]. Hence, FA is
the more water-soluble of the two. HA is the fraction
that is soluble in only alkaline solutions.
Humic substances are polyfunctional (i.e. each
molecule may have a large number of different complexing sites, e.g. carboxylic, phenolic, phthalic, salicylic and amine functional groups) [4,6]. They are also
oligoelectrolytic [6]. The total charge on the molecule
depends on the pH and environmental conditions [6].
At low pH, the functional groups are protonated and
uncharged; at higher pH, the functional groups dissociate and become negatively charged. Around the
charged molecules, a diffuse double layer develops
[7]. The double layer screens the charge so that the
effect of electrostatic interactions is decreased. In the
case of a negatively charged molecule, the concentration of the positively charged metal ions is larger in
the double layer than in the bulk of the solution. Consequently, the amount of background electrotrolyte
ions determines the efficiency of the screening of
the surface charge. At low ionic strength, the electric
field around the charged molecule extends relatively
far in the solution and the double layer is thick. At
high ionic strength, a strong screening results in a
thin double layer. The presence of ions in the electrical double layer can affect the effective charge of
the charged surface of humic substances, resulting in
an ionic strength dependency of metal ion binding
[8]. Futhermore, the effective charge and the extent
of hydrogen bonding can also affect the conformational properties of humic substances. The degree of
aggregation and coiling of the various molecules of
humic substances increase with ionic strength, and as
a result, the accessibility of some ligand sites to larger
metal ions may be reduced [6,9,10]. It has been reported that, at low pH, humic substances have tightly
coiled, cross-linked (via H-bonding) conformation
where metal binding sites are not readily available,
but at higher pH, they have a more open conformation
[11,12]. Consequently, metal ion binding to humic
substances is affected by both physical and chemical
heterogeneity [2,8,13,14].
The chemical equilibria involved in metal complexation reactions have been studied in great detail, and
numerous thermodynamic and mathematical models
have been proposed [15–19]; however, the kinetics
of metal complexation reactions has received less attention. Several studies investigating the kinetics of
metal ion–humic substance interactions have recently
been reported [20,21]. Langford et al. [22] developed
a ligand exchange probe for kinetic speciation. Shuman et al. [23] used a rotating disk electrode (RDE)
in combination with anodic stripping voltammetry
(ASV) to estimate the dissociation rate coefficients
of metal complexes of humic substances. Olson et al.
[24,25] studied the dissociation of copper complexes of estuarine humic substances. They used
4-(2-pyridylazo) resorcinol (PAR) as the competing
ligand and monitored the dissociation reaction by
measuring the concentration of the metal–PAR complex spectrophotometrically. Lavigne et al. [26] also
used PAR in their study of the dissociation of nickel
complexes of a well-characterized soil FA. Hering
and Morel [27] studied the rates of ligand exchange
reactions of copper–nitrilotriacetic acid complexes
and copper–humate complexes. In their work, Hering and Morel [27] found that the rate coefficients
of ligand exchange were pH-dependent and reported
conditional rate coefficients that depended not only
on the properties of the humic substance but also on
the experimental conditions.
The complexing sites of heterogeneous, macromolecular, organic complexants can be classified into
major and minor sites types [28]. The major sites are
weak binding sites and are the ones present in large
A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
proportion [29]. There are only a few types of major
sites, including carboxylate and phenolate functional
groups. The range of the free energy of formation of
the complexes with the major sites does not exceed
the equivalent of 1–2 log K units because the sites are
chemically homogeneous. The minor sites are strong
binding sites and represent a small fraction of the
total sites, but they consist of a very large number of
sites types. These sites form strong complexes with
metals. Examples of minor sites include nitrogen and
sulphur bearing functional groups. The distribution of
equilibrium (formation) constants for the minor sites
is larger (at least several log K units) because of not
only the physical factors mentioned earlier but also of
the wide variety in the chemical nature of these sites
[10].
This paper forms a part of our research program
for the development of a comprehensive scheme of
chemical speciation for metals and metalloids in freshwaters, rain and snow samples based on kinetic approaches [30–40]. The objective of this research is to
investigate the progressive occupation of strong and
weak binding sites of two well-characterized FAs by
selected metals. This paper shows how kinetic data
lead to differentiation between strong and weak binding sites in FA. The metals selected for the study were
Ni(II), Pb(II), Cu(II), Cd(II) and Al(III).
2. Theory
2.1. CLEM
The method used for the determination of dissociation kinetics of metal complexes was the CLEM,
in which Chelex-100 cation exchange resin was used
as a solid-phase competing ligand. The kinetic model
proposed by Olson et al. [24,25] was adopted [36]
to study the dissociation kinetics of metal complexes,
MLi , where M is a metal ion and Li is the ith binding site on a polyfunctional, macromolecular, organic
complexant, such as FA. For simplicity, the charges
on M and Li have been omitted.
Consider an aqueous mixture of n components in
which each component, designated MLi , undergoes a
first-order or pseudo-first-order reaction and exists in
equilibrium with its dissociation products:
k1
MLi M + Li (slow)
k−1
213
(1)
where k1 and k−1 are the rate coefficients for the forward and the backward reaction, respectively.
The Chelex-100 reacts with M as follows:
k2
Chelex + M Chelex − M (fast)
k−2
(2)
where Chelex represents Chelex-100 cation exchange resin, and k2 and k−2 are first-order (or
pseudo-first-order) rate coefficients for the forward
and the backward reaction, respectively.
The model is based on two assumptions: (i) that
reaction (2) is much faster than reaction (1); and
(ii) that [Chelex] [M]. As [Chelex] is large and
[Chelex] [M], [Chelex] can be considered as constant and reaction (2) as pseudo-first-order. Since k2
is large, as has been determined by ICP-MS, and
with Chelex added in large excess, the condition k2
[Chelex] k−1 [Li ] holds and the overall reaction
Chelex + MLi → Chelex − M + Li
(3)
is assumed to be irreversible, and the rate of dissociation of MLi is
d[MLi ]
d[M − Chelex]
=−
= k1 [MLi ]
dt
dt
(4)
The two assumptions made above have been verified as follows: assumption no. 1, by our previous
studies using ICP-MS, and assumption no. 2, by taking a large excess of Chelex-100 resin over the very
low concentrations of metal ions present in the test
samples.
If each complex, MLi , undergoes, independently
and simultaneously, a first-order or pseudo-first-order
dissociation reaction, the sum, C(t), of the concentrations of all components remaining undissociated in the
test solution at time t can be described as
n
X
Ci0 exp(−ki t)
C(t) =
(5)
i=1
where Ci0 is the initial concentration of MLi , the
ith component. C(t) is determined by ICP-MS or by
GFAAS, by measuring the decrease in the amount of
metal ions in the test solution due to their binding by
the Chelex-100, as a function of time [32].
214
A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
3. Experimental
3.1. Chemicals and reagents
Stock solutions containing (1000 ␮g ml−1 ) of
Pb(II), Cu(II), Cd(II) and Ni(II) (ICP-MS-2 high-purity
standard Delta Scientific) were used to prepare
the model solutions. The Al(III) stock solution
(1000 ␮g ml−1 ) was prepared by dissolving an appropriate amount of aluminium metal (SPEX 99.99%)
in ultrapure nitric acid (ULTREX II, J.T. Baker Inc.,
Phillipsburg, NJ, USA) with heating, and diluting to
the appropriate volume with ultrapure water. Ultrapure water having a resistivity of 18.2 M cm was
used to prepare all standards and test solutions. The
ultrapure water was obtained from a Milli-Q-Plus
water purification system (Millipore Corporation).
Analytical grade (minimum 99% pure), Chelex-100
(100–200 mesh), a styrene-divinylbenzene co-polymer
with iminodiacetate functional groups, was supplied by Bio-Rad Laboratories. The Chelex-100
cation-exchange resin was pre-treated for this work
as follows: it was equilibrated with a sodium
acetate–acetic acid (NaOAc–HOAc) buffer solution
(pH 5.0 ± 0.1) by soaking it in the buffer solution
for 24 h. The quantity of cations exchanged by the
Chelex resin is a function of the pH and is very small
at any pH below 2.0; the quantity is a maximum at pH
7.4. Dryness of the Chelex-100 affects its properties
and changes the pH of the test solution during kinetic
runs. Hence, after the above pre-treatment, the Chelex
resin was kept in the buffer solution till it was used.
The pH of the test solutions was adjusted to pH
5.0 ± 0.5 using 1% NaOH. The sodium hydroxide solution was purified by electrodeposition at –1.2 V (versus Esce ) for at least 48 h immediately prior to use.
All the standard solutions were acidified to make them
1% (v/v) using HNO3 (ULTREX II, J.T. Baker Inc.,
Phillipsburg, NJ, USA). Standard buffers (supplied by
Fisher Scientific) were used for the calibration of the
pH-meter.
3.2. FAs used as complexants
Two well-characterized FAs were used as heterogeneous, macromolecular, organic complexants in
this study. The FAs used in this study were Armadale
soil FA supplied by Dr. D.S. Gamble of Agriculture
Canada, Ottawa, Canada [41–45], and the other was
the Standard Suwannee River aquatic FA obtained
from the International Humic Substance Society
(IHHS) [46].
The total amounts of phenolic OH and carboxyl
groups determined by potentiometric titration of
the Armadale FA were 3 mmol g−1 [41,42] and
7.71 mmol g−1 [41,43,44], respectively. Hence, the
bidendetate complexing capacity of the Armadale soil
FA was estimated to be approximately 5.4 mmol g−1
[45]. The complexing capacity for Al(III) of the
Standard Suwannee River FA was reported to be
390 ␮mol g−1 at pH 4.7 [47].
An estimate of the metal background concentration
in the Armadale FA was obtained by us by using
ICP-MS. The concentrations of all detectable trace
metals were estimated semi-quantitatively by using a
single-standard analytical calibration curve. The background concentrations of the metals of interest were
found to be Ni(II) 5.2 ␮g g−1 , Cu(II) 18.4 ␮g g−1 ,
Pb(II) 6.5 ␮g g−1 , Al(III) 524.8 ␮g g−1 ; Cd(II) was
not detectable.
Taylor and Garbarino reported the occurrence and
distribution of trace metals in the IHSS’s FA isolated from Suwannee River [48]. The concentration
of Al(III) was estimated semi-quantitatively to be
30 ␮g g−1 , by using ICP-MS and a single-standard analytical calibration curve. Isotope-dilution mass spectrometry was used for accurate determination of the
concentration of Ni(II), Cu(II), Pb(II) and Cd(II) and
some other elements. The concentrations of Ni(II),
Cu(II), Pb(II) and Cd(II) in the Standard Suwannee
River FA were reported to be 0.92 ± 0.06, 1.7 ± 0.08,
1.2 ± 0.06, and 0.06 ± 0.8 ␮g g−1 , respectively.
3.3. Instrumentation
The kinetics of decrease in the amounts of nickel,
lead, cadmium, copper and aluminium in the model
solutions due to binding by Chelex-100 as a function
of time was determined by ICP-MS, and by GFAAS.
The ICP-MS used was a Perkin–Elmer SCIEX Elan
5000, fitted with a pneumatic nebulizer as the sample
introduction system. The instrument operating conditions and the data acquisition protocol are described
in Table 1. The graphite furnace atomic absorption
A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
Table 1
Instrumental operating conditions and data acquisition protocol for
the ICP-MS
Inductively-coupled plasma
RF power
Coolant argon flow rate
Carrier argon flow rate
Auxiliary argon flow rate
1 kW
15 l min−1
0.90 l min−1
0.85 l min−1
Data acquisition protocol
Dwell time
Scan mode
Signal measurement
Points/spectral peak
Resolution
100–1000 min
peak hop
counts per second
1
normal
215
ratios of [metal]/[FA] of 0.0018, 0.018, 0.18. It should
be noted that, for naturally occurring heterogeneous
complexants, the molar mass is not directly accessibile [4,10]. Nevertheless, a molar concentration has
been calculated, based on the bidentate complexing
capacity, mmol g−1 , of FA, as determined by Gamble et al. [45] for the Armadale FA, and by Hawke
et al. [47] for the Suwannee River FA, respectively.
The pH of the test solutions was then adjusted to
5.0.
3.6. Analytical procedure
spectrometer used was Perkin–Elmer model 5000,
fitted with a Zeeman-Background Correction System.
Table 2 shows the instrumental parameters used in
the GFAAS determination of the metals Ni(II), Pb(II),
Cu(II), Cd(II) and Al(III). For the determination of
the metal concentrations, 10 ␮l of the solution was
injected into the graphite furnace, where it was dried,
ashed and atomized. The signal was measured in the
peak area mode. An Accumet 20 pH/mV/conductivity
meter (Fisher Scientific), fitted with an Accuplast combination glass electrode, was used for pH
measurements.
3.4. Cleaning procedures for containers
All containers used were made of Teflon, and they
were cleaned as follows. First, they were completely
filled with 10% HNO3 (AR grade) and allowed to
stand at room temperature for 1 week. Then, they were
rinsed with ultrapure water (having a conductivity of
18.2 m cm), and then filled with ultrapure water and
allowed to stand until they were used; the filling water
was renewed periodically to ensure purity of the filling
water.
3.5. Samples
Stock solutions of FAs (1.000 ± 0.001 g l−1 ) were
prepared from the solid FAs. The model solutions were
prepared by adding known amounts of the FA to a
known volume of the metal test solutions to give mole
The kinetics of decrease in the amounts of nickel,
lead, cadmium, copper and aluminium in the model
solutions due to binding by Chelex-100 as a function
of time was determined by ICP-MS, and by GFAAS.
One percent (w/v) of Chelex-100 resin was added to
the test solution placed in a Teflon Reactor (cylindrical, 500 ml capacity) and was stirred continuously
with a Teflon-coated stirring bar. Data acquisition was
initiated from the instant the Chelex-100 resin was
added to the Teflon Reactor. For ICP-MS, the test solution was delivered continuously to the plasma torch
through the solution nebulizer using a peristaltic pump
at a flow rate of 1 ml min−1 . The interval between the
data points and the total time for data acquisition was
set for each experiment based on the rate of decrease
in the amount of metal ions due to binding by the
Chelex-100 resin. In the case of rapid decrease in the
amount of metal ions, both the time interval and the total time for data acquisition were short. Longer times
for data acquisition were required for slowly dissociating metal–FA complexes, with corresponding longer
time intervals.
3.7. Data treatment
The experimental data were analyzed for discrete
values of the dissociation rate coefficients by a method
that was based on non-linear regression analysis. This
method analyzes the data assuming the decrease in the
amount of the metal, which represents dissociation of
the complexes as a sum of exponential terms. All the
experimental data were used in the fitting.
216
A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
Table 2
Analysis lines and instrumental conditions for Zeeman GFAAS
Element
Wavelength (nm)
Pyrolysis temperature (◦ C)
Atomization temperature (◦ C)
Atomization time (s)
Ni
Pb
Cu
Cd
Al
232.2
217.1
325.0
228.5
217.1
1000
700
900
250
700
2300
2300
2500
2100
2300
6
6
6
6
6
4. Results and discussion
4.1. Effect of the [metal]/[FA] mole ratio on the rate
of dissociation of FA complexes of Ni(II), Pb(II),
Cu(II) and Cd(II)
The Armadale FA was used for all studies of Ni(II),
Pb(II), Cu(II) and Cd(II). Figs. 1–4 present the effects
of the [metal]/[FA] mole ratios on the rates of dissociation of the FA complexes of Ni(II), Pb(II), Cu(II)
and Cd(II). The [metal]/[FA] mole ratio was varied by
keeping the metal concentration constant (∼10−8 M)
and varying the FA concentration. For the sake of
simplicity of calculating the [metal]/[FA] mole ratio,
the FA will be considered to have ∼1% strong binding sites, and ∼99% weak binding sites. If the FA
had ∼1% strong binding sites, then the [metal]/[FA]
mole ratio required for the saturation of strong sites
of the FA was ∼0.01. To be on the safe side (i.e. to be
sure of the saturation of strong sites), we started with
a [metal]/[FA] mole ratio of 0.001. The kinetic data
were obtained using the CLEM Chelex-100 cation exchange resin in the batch mode. The kinetics of dissociation of the metal–FA complexes was measured by
ICP-MS and GFAAS for quantitative determination of
the metals.
In Figs. 1–4, the top curves represent the [metal]/[FA]
mole ratio of 0.0018. At this mole ratio, the rate of
dissociation of the FA complexes is very slow and
hardly detectable over the entire period of the measurement. All of the metal complexes are bound to
strong binding sites and the complexes may be described as inert. When the [metal]/[FA] mole ratio is
increased to 0.018, the rate of dissociation is initially
rapid, but becomes slow again at longer times. At the
highest mole ratio of [metal]/[FA] of 0.18, the initial
rapid decrease is more evident, but again, a leveling off to a slow rate of dissociation at longer times
Fig. 1. Nickel remaining bound to Armadale FA as a function
of time in model solutions, measured by ICP-MS. pH = 5.0; ionic
strength ∼0. Chelex-100 cation exchange resin was the competing
ligand. Concentration of nickel was held constant and equal for
all curves. 䊐 : [Ni(II) 4.3 × 10−8 M]/[FA 2.3 × 10−5 M] = 0.0018;
䊊: [Ni(II) 4.3 × 10−8 M]/[FA 2.3 × 10−7 M] = 0.18.
was observed. These curves are in conformity with
the postulation that as the [metal]/[FA] mole ratio
is increased, the strong binding sites become saturated and the excess metal then binds to the weaker
sites, forming weak complexes which are labile. This
observation was common for all the metals studied.
Results of the dissociation rate coefficients of the
metal–FA complexes are presented in Table 3. For the
lowest mole ratio of 0.0018, the rate of metal–complex
dissociation was very slow; hence, only an estimate of
the rate coefficient was possible (kd < 10−5 s−1 ). The
kinetic data at the mole ratios 0.018 and 0.18 were
analyzed using the method described above. The fraction of labile metal species (Figs. 1–4, the second and
the third curve from the top) increase with increasing
[metal]/[FA] mole ratios. Two rates of dissociation appear to be involved for the [metal]/[FA] mole ratios of
A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
Fig. 2. Lead remaining bound to Armadale FA as a function of
time in model solutions, measured by ICP-MS. pH 5.2; ionic
strength ∼0. Chelex-100 cation exchange resin was the competing
ligand. Concentration of lead was held constant and equal for
all curves. 䊊: [Pb(II) 4.3 × 10−8 M]/[FA 2.3 × 10−5 M] = 0.0018;
5: [Pb(II) 4.3 × 10−8 M]/[FA 2.3 × 10−6 M] = 0.018; 䊐: [Pb(II)
4.3 × 10−8 M]/[FA 2.3 × 10−7 M] = 0.18.
Fig. 3. Copper remaining bound to Armadale FA as a
function of time in model solutions, measured by Zeeman
GFAAS. pH = 5.1; ionic strength ∼0. Chelex-100 cation exchange resin was the competing ligand. Concentration of copper
was held constant and equal for all curves. 䊊: [Cu (II)
4.8 × 10−8 M]/[FA 2.6 × 10−5 M] = 0.0018; 䊐: [Cu(II) 4.8 ×
10−8 M]/[FA 2.6 × 10−6 M] = 0.018; 4: [Cu(II) 4.8 × 10−8 M]/[FA
2.6 × 10−7 M] = 0.18.
0.018 and 0.18. In addition, the fraction of the metal
bound to the strong sites decreases, and the dissociation rate coefficients of the strongly-bound complexes
increase with increasing [metal]/[FA] mole ratios. This
217
Fig. 4. Cadmium remaining bound to Armadale FA as a function of time in model solutions, measured by Zeeman GFAAS.
pH 5.2, ionic strength ∼0. Chelex-100 cation exchange resin was
the competing ligand. Concentration of cadmium was held constant and equal for all curves. 䊊: [Cd(II) 3.1 × 10−8 M]/[FA
1.7 × 10−5 M] = 0.0018; 䊐: [Cd(II) 3.1 × 10−8 M]/[FA 1.7 ×
10−6 M] = 0.018; 4: [Cd(II) 3.1 × 10−8 M]/[FA 1.7 × 10−7 M] =
0.18.
is in conformity with the postulation that the strong
binding sites are occupied first by the metal ions. After the strong binding sites are filled completely by
the metal, the remaining metal binds to the weak sites,
forming weak complexes that are labile. The rate of
dissociation of Ni–FA complexes was measured by
both ICP-MS and GFAAS, and the rate coefficients for
dissociation of the strongly-bound complexes were in
reasonable agreement (Table 3). The relatively poor
agreement between the dissociation rate coefficients
given by GFAAS and ICP-MS for the weakly bound
complexes was probably due to the much shorter time
scale of measurement by ICP-MS. This shorter time
scale of measurement by ICP-MS allowed much better resolution of the quickly dissociating complexes.
The measurement by GFAAS takes a longer time,
and hence, GFAAS cannot measure the rapidly dissociating metal complexes which can be measured by
ICP-MS. Hence, the dissociation rate coefficients of
the first labile complexes observed by GFAAS probably represent a weighted average dissociation rate coefficients of the labile complexes and of the moderately labile complexes observed by ICP-MS. Only one
set of values (those of Ni(II) complexes) by ICP-MS
have been presented in Table 3 to show that these
values are similar to those obtained by GFAAS.
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A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
Table 3
Effect of the [metal]/[FA] mole ratio on the dissociation rate coefficients of metal–FA complexes in aqueous model solutions. pH = 5.0 ± 0.5,
ionic strength ∼0
[Metal]/[FA] mole ratio
Kinetically distinguishable components
C1 (%)
k1 × 104 (s−1 )
C2 (%)
k2 × 105 (s−1 )
Complexes with Ni(II); [Ni(II)] = 4.3 × 10−8 M, measured by ICP-MS
0.0018
–
–
0.018
18 ± 1
8.7 ± 0.5
0.18
49 ± 2
10 ± 5
98 ± 2
82 ± 1
51 ± 1
0.6 ± 0.1
3.0 ± 0.1
18 ± 1
Complexes with Ni(II); [Ni(II)] = 5.2 × 10−8 M, measured by GFAAS
0.0018
–
–
0.018
15 ± 1
0.4 ± 0.1
0.18
40 ± 2
10.2 ± 0.3
100 ± 2
85 ± 1
60 ± 1
0.2 ± 0.1
0.6 ± 0.5
23 ± 1
Complexes with Pb(II); [Pb(II)] = 2.4 × 10−8 M, measured by GFAAS
0.0018
–
–
0.018
7.8 ± 0.2
0.2 ± 0.1
0.18
18 ± 0.5
8.5 ± 0.5
100 ± 2
92 ± 2
82 ± 1
0.2 ± 0.1
0.4 ± 0.5
20 ± 1
Complexes with Cu(II); [Cu(II)] = 4.8 × 10−8 M, measured by GFAAS
0.0018
–
0.018
9±2
0.2 ± 0.1
0.18
20 ± 1
9.0 ± 0.3
100 ± 1
91 ± 1
80 ± 1
inerta
0.5 ± 0.1
21 ± 1
Complexes with Cd(II); [Cd(II)] = 3.1 × 10−8 M, measured by GFAAS
0.0018
–
–
0.018
9±2
6.0 ± 0.3
0.18
30 ± 1
9.5 ± 0.3
99 ± 1
91 ± 1
70 ± 1
0.3 ± 0.1
1.1 ± 0.1
22 ± 1
Inert k2 ≤ 10−5 s−1 . k1 , k2 are the dissociation rate coefficients of the first (the faster), and the second (the slower) component,
respectively. Values after ± signs are standard deviations of non-linear regression analysis.
a
Table 3 shows that a minimum of two components
were required to describe the dissociation kinetics of
the metal–FA complexes at the [metal]/[FA] mole ratios of 0.018 and 0.18 for the four different metals.
This suggests the presence of two distinct classes of
binding sites in the Armadale FA. Sunda and Hanson, using an initial metal concentration of 10−8 M,
reported, for the first time, the existence of the strong
sites of humic substances, whereas the earlier workers had failed to detect their existence because they
had used at least one order of magnitude higher concentrations of the metals in their titration experiments
[49,50].
In Table 3, the rate coefficients (measured by
GFAAS), either for the strong complexes or for the
weak complexes, do not show much difference between the four metals, Ni(II), Pb(II), Cu(II) and
Cd(II). Such a lack of difference in the rate coefficients raises the serious question as to whether the
rate coefficients for the water exchange (k−w ) of the
different metals are indeed the determinant of the
strength of the metal’s affinity for a particular binding
site. Since the k−w for Pb(II) and Cu(II) is larger than
the k−w for Ni(II) by four orders of magnitude ([51],
p. 400), similar difference in the rate coefficients
for dissociation between the Pb(II)–FA complex, the
Cu(II)–FA complex and the Ni(II)–FA complex should
be observed if the k−w is indeed the determinant of
the complex’s stability. However, no significant difference in the rate coefficients was observed (Table
3), particularly in the values obtained by GFAAS. In
predicting the stability of the metal complex in Table
3, the following other factors that also contribute to
the stability of metal–FA complexes should be considered: effective nuclear charge, polarizability of the
metal ions and the ligand, the ligand field stabilization energy (LFSE) for the transition metals, Ni(II),
Cu(II), and Cd(II), and the polyelectrolyte effect
(electrostatic interactions between the charges on the
FA and those on the cations and other ions). The polyelectrolyte effect is the same for all the above metal
ions and can, therefore, be left out of consideration
A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
as a factor responsible for the above-noted lack of
difference in the rate coefficients for the dissociation
of the metal–FA complexes. The above factors do
not apply to Pb(II), which is the only non-transition
metal considered, and for which the kw is the only
determinant of the Pb(II)–FA complex’s stability. For
d8 (Ni2+ ) configuration, it seems that the low value
for the water exchange rate coefficient (3 × 104 s−1 ),
caused by its highly unfavorable change in LFSE
during the complex formation ([51], p. 398) is more
than compensated for by the other factors mentioned
above (effective nuclear charge, polarizability of the
d8 (Ni2+ ) ion and of the FA ligand), resulting in the
formation of an Ni(II)–FA complex of a stability similar to that of the other three metal–FA complexes,
and hence, in a relatively inert Ni(II)–FA complex.
The rate coefficients for both strong and weak complexes show a significant increase at the highest mole
ratio of [metal]/[FA] listed in Table 3. The binding
sites in FA are known to involve a range of binding energies [52]. Even among the strong and weak binding
sites, as revealed by the present experiments, a distribution of sites stronger or weaker than the experimentally measured averages really exists even though
Table 3 presents the average values. As more metal becomes available for binding, a broader range of binding sites becomes occupied and the rate coefficients
for dissociation should show a corresponding distribution. Caution should be exercised when considering
such dissociation rate coefficients as they are derived
from a minimum set of parameters to fit the decay
curves within the experimental error. There may well
be a more complicated distribution of dissociation
rate coefficients than the minimum set [53].
No account was taken of the naturally-occurring
metals in the FAs used, either in computing the experimental results or in their interpretation.
4.2. Effect of the [metal]/[FA] mole ratio on the rate
of dissociation of Al(III)–FA complexes
The effect of [Al(III)]/[FA] mole ratio on the lability of the Al–FA complexes is presented in Fig.
5. For aluminium, the Standard Suwannee River FA
was employed using a fixed concentration of Al(III)
[7.4 × 10−7 M]. In order to ascertain whether there
would be any precipitation of Al(OH)3 at pH 5 which
219
Fig. 5. Aluminium remaining bound to Standard Suwannee
River FA as a function of time in model solutions, measured
by ICP-MS. pH 5.2; ionic strength ∼0. Chelex-100 cation
exchange resin was the competing ligand. Concentration of
aluminium was held constant for all curves. 4: [Al(III)
7.1 × 10−7 M]/[FA 4.1 × 10−5 M] = 0.018; 䊐: [Al(III) 7.1 ×
10−7 M]/[FA 4.1 × 10−6 M] = 0.18; 䊊: [Al(III) 7.1 × 10−7 M]/[FA
4.1 × 10−7 M] = 1.8.
would affect the kinetic experiments, we carried out
equilibrium calculations using the MINEQL+ computer equilibrium program and found that there was
no significant formation of Al(OH)3 at pH 5 at the Al
(III) concentration used in the kinetic experiments.
The [Al(III)]/[FA] mole ratio was calculated based on
the aluminium complexation capacity for the Standard
Suwannee River FA, which had been reported to be
390 ␮mol g−1 [47]. The Al(III)–FA model solutions
were studied by both ICP-MS and GFAAS, using the
three mole ratios of [Al(III)]/[FA], 0.018, 0.18 and
1.8, for the experiments performed by ICP-MS, and
the two mole ratios of 0.0033, and 0.33 for the experiments performed by GFAAS. The decay of Al(III) for
the three ratios, as measured by ICP-MS, is presented
in Fig. 5 and the data analysis is given in Table 4.
At the [Al(III)]/[FA] mole ratio of 0.018, virtually all
of the Al(III)–FA complexes were inert (Fig. 5, top
curve), suggesting that only the strong binding sites of
the FA were occupied by Al(III), forming strong complexes that are inert. The dissociation rate coefficients
of the strongly-bound Al(III)–FA complexes was kd
∼10−5 s−1 , which was comparable to the dissociation rate coefficient of the strongly-bound Ni(II)–FA
complexes measured by ICP-MS and GFAAS. The
fraction of the labile Al-complex increased with
220
A.L.R. Sekaly et al. / Analytica Chimica Acta 402 (1999) 211–221
Table 4
Effect of the [metal]/[FA] mole ratio on the dissociation rate coefficient of Al(III)–FA complexes in model solutions. pH = 5.0 ± 0.5, ionic
strength ∼0
[Metal]/[FA] mole ratio
FA (M)
Kinetically distinguishable components
k1 × 104 (s−1 )
C2 (%)
k2 × 105 (s−1 )
Complexes with Al(III); [Al (III)] = 7.1 × 10−7 M, measured by ICP-MS
0.018
4.1 × 10−5
–
0.18
4.1 × 10−6
52 ± 1
1.8
4.1 × 10−7
78 ± 1
–
5.1 ± 0.5
11 ± 1
97 ± 3
49 ± 1
32 ± 1
inerta
5.9 ± 0.7
8.9 ± 0.5
Complexes with Al(III); [Al (III)] = 7.1 × 10−7 M, measured by GFAAS
–
0.0033
5.6 × 10−6
0.33
5.6 × 10−8
42 ± 4
–
27 ± 1
93 ± 7
59 ± 3
inerta
1.3 ± 0.1
C1 (%)
Inert k2 ≤ 10−5 s−1 . k1 and k2 are the dissociation rate coefficients of the first (the faster), and the second (the slower) component,
respectively. Values after ± signs stand are standard deviations of non-linear regression analysis.
a
the increasing [Al(III)]/[FA] mole ratio. In addition,
the fraction of Al(III) bound to the strong sites decreased and the dissociation rate coefficients of the
strongly-bound Al(III)–FA complexes increased with
increasing [Al(III)]/[FA] mole ratios. These trends
were observed in both GFAAS and ICP-MS results,
which suggest the presence of two distinct classes of
binding sites. The strong binding sites are occupied
first, and the weak sites become occupied only after the strong sites are occupied fully. It seems that
the Suwannee River FA and the Armadale FA have
similar metal binding characteristics.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
Acknowledgements
[10]
The authors are grateful to the Nickel Producers
Environmental Research Association, USA, International Nickel Company Ltd. Canada, and Falconbridge
Nickel Ltd. Canada, and to the Natural Sciences and
Engineering Research Council of Canada, for research
contracts and research grants. The authors are grateful
to Dr. D.S. Gamble for supplying the Armadale FA.
One of the authors, Nouri M. Hassan, is also grateful
to the Government of Libya for providing him with a
graduate scholarship for the Ph.D. degree program at
Carleton University.
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
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