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J. Electroanal. Chem., 115 (1980) 293--308
293
© Elsevier Sequoia S.A., Lausanne - - P r i n t e d in The Netherlands
K I N E T I C S A N D M E C H A N I S M O F T R A C E M E T A L C H E L A T I O N IN
SEA WATER *
B. R A S P O R
Institute of Chemistry, Institute for Applied Physical Chemistry, Nuclear Research Center
(K.F.A.) Juelich, (F.R.G.) and Center for Marine Research Institute "Rudjer Bo~kovi6"
Zagreb, Croatia (Yugoslavia)
H.W. N O R N B E R G and P. V A L E N T A
Institute of Chemistry, Institute for Applied Physical Chemistry, Nuclear Research Center
(K.F.A.), Juelieh (F.R.G.)
M. B R A N I C A
Center for Marine Research, Institute "'Rudjer Bo~kovi~ ", Zagreb, Croatia (Yugoslavia)
(Received 18th F e b r u a r y 1 9 8 0 ; i n revised f o r m 28th July 1980)
ABSTRACT
The kinetics of chelation of Pb(II), Zn(II) and Cd(II) with the ligand E D T A have been
f o l l o w e d at the realistic trace c o n c e n t r a t i o n level 10-8--10 -7 M for b o t h reactants in sea
water and m o d e l solutions o f its major salinity c o m p o n e n t s by differential pulse stripping
v o l t a m m e t r y . In this m a n n e r the specific influences of the salinity c o m p o n e n t s on the form a t i o n rate constants [having in sea water for P b E D T A the order of 3 X 103 and for the
E D T A chelates of Zn(II) and Cd(II) of 3 X 1021 M -1 s -1 ] could be determined. The measurements emphasize the p r o n o u n c e d specific influences of Ca(II) on the kinetics and
course of the trace metal chelation in media where this alkaline earth ion is present in substantial excess to organic chelating agents. The experiments with E D T A are to be regarded
as a close simulation of the chelation processes occurring for the trace metals studied by
c o m p o n e n t s with suitable chelation p o w e r of dissolved organic m a t t e r (DOM) in the sea
and the resulting conclusions on the m e c h a n i s m are thus of general significance.
INTRODUCTION
The chelation of heavy metal traces by dissolved organic ligands is an area
of particularly fundamental interest in marine and aquatic trace metal chemistry. It might affect significantly the kind of distribution over the various species of the total trace metal levels dissolved in various types of natural waters
and this will be of considerable importance for the fate of trace metals in
aquatic ecosystems [1--6]. There is general trace analytical evidence in estuaries, in m a n y but n o t all regions of coastal waters [7,8] and in some oceanic
areas with high biological activity, as well as in m a n y rivers and lakes, that
* Presented in part at the Vth International S y m p o s i u m , Chemistry of the Mediterranean,
Rovinj, 8 t h - - 1 2 t h May, 1978.
294
certain fractions of the dissolved trace metal content are rather strongly
bound b y certain components of dissolved organic matter (DOM) [9]. The
exact chemical nature of the relevant components of DOM has to a large
extent still to be identified before experiments with particular natural organic
ligands can be designed. Nevertheless, much information of general significance
on the stability of organic chelates of heavy metals and on the rate and mechanism of their formation can be obtained under quite realistic conditions from
voltammetric studies in various types of natural waters with organic chelating
agents of defined and known coordination chemistry properties, such as NTA
and EDTA [10--17]. Both ligands, NTA and EDTA, are to be regarded in this
context not only as model ligands with high chelation efficiency, b u t as also
occurring as pollutants in rivers, lakes, estuaries and regionally in coastal
waters [18].
Owing to their general potentialities, high sensitivity and the fact that they
provide in principle a species-sensitive approach, suitable modes of voltammetry [4,19,20] are very illuminating and convenient for this kind of speciation
study, as we have shown previously by investigations on the stability of the
chelates of the toxic metals Cd [10,11] and Pb [12] in sea water.
GENERAL METHODOLOGICAL ASPECTS
Use is made of the fact that there exists a substantial difference (often
several hundred millivolts) between the reversible reduction of the unchelated
trace metal Me{II) and the irreversible reduction potential of chelated species
MeL. In/~atural waters, particularly sea water, the overwhelming amount of
the unchelated trace metals, e.g. Cd, Pb, Zn, etc. will not exist as free hydrated cations but as labile complexes with inorganic ligands, as CI-, OH-,
CO~ w, etc. Nevertheless, these labile complex species MeX~ of a given trace
metal will undergo a reversible electrode process at a mercury electrode at a
virtually c o m m o n potential. Thus, the whole unchelated amount of the trace
metal studied will produce a c o m m o n reversible voltammetric response at the
potential characteristic for the respective metal Me while the chelate yields
an irreversible response at a significantly more negative potential.
This irreversible response is less well defined at the ultra trace level than
the reversible response occurring at more positive potential values for the unchelated form of the trace metal studied. Therefore the corresponding
decrease of the unchelated a m o u n t of the trace metal after addition of certain amounts of an organic chelating agent to the solution is determined by
voltammetry to elucidate the chelation degree [11,12]. If these measurements
are performed under equilibrium conditions for the respective chelate formation the conditional stability constants can be evaluated from the relation
determined between the remaining concentration of unchelated trace metal
and the adjusted concentration of the organic ligand L [11].
In this context it should be added that this evaluation requires the knowledge of the parameters of all significant relevant labile complex equilibria of
the various species MeXI involved in the total unchelated amount of the
trace metal Me studied in the respective aqueous system. If the data are not
known they can be determined equally as well by voltammetry, as has been
295
described for various trace metals elsewhere [19,21--23]. However here it
should be emphasized that the methodological approach is quite different
for labile complexes MeXj. In this case, as is usual in polarography [24], the
shift of the reversible reduction potential as a function of the ligand concentration Cx has to be determined and evaluated for the conditional stability
constants and the ligand numbers ].
Investigations on the kinetics of the formation of strong trace metal
chelates are also conveniently performed by voltammetry [16,17]. The
approach is quite analogous to the above-mentioned determination of the
degree of chelation via the monitoring of the amount of unchelated trace metal present in sea water predominantly in the form of labile inorganic complexes MeXj.
The kinetics of the attainment of the chelation equilibrium are followed
by voltammetric monitoring of the decrease of the unchelated trace metal
level with the time elapsed subsequent to the addition of a certain concentration of organic chelating agent to the solution (see Fig. 1). From those
kinetic measurements the conditional rate constant of chelate formation and
the type of reaction mechanism can be found. Thus, voltammetry offers a
comprehensive experimental access to the chelation and complexation of
trace metals in natural waters. The high sensitivity of voltammetry permits
those studies under quite realistic conditions with respect to the concentration o f the respective trace metal. Specific influences by certain excess substances in a given natural water type, e.g. the salinity components in sea
016
012
.<
008
v4
B
004.
/
000
~_
-08
i
-06
~
I
-04
Potenhal / V vs Ag/AgCI sat
~
J
-02
KCI
Fig. 1. DPASV-responses of unchelated PI~(II) as function of time elapsed after addition of
chelating ligand Na3HEDTA. Medium: 0.45 p filtrated Ligurian sea water, pH 8.0, spiked
with 4 pg 1-1 Pb(II) corresponding to 2 X 10 -8 M (viz. voltammogram 1). Subsequent to
addition of 2 X 10 -8 M Na3HEDTA after 30 min and 20 s, peak 2, and peak 3 after 71 rain
and 50 s. (For clarity the voltammograms have been artificially shifted along the current
ordinate. )
296
water or the components of hardness in a river water, frequently require, if
they are to be disentangled, additional measurements in appropriate artificial
model media.
According to the general methodological principles outlined, our systematic
studies on the general aspects of the chelation of trace metals in sea water have
been extended from Cd to Pb and Zn. Corresponding studies with the chelating
agent NTA under equilibrium conditions devoted to stability constants are
reported elsewhere [10--15]. This paper deals with the determination of the
kinetics and the mechanism of the formation of PbEDTA and ZnEDTA and
the comparison of the data obtained with those previously obtained [16,17]
for CdEDTA. Since in sea water Pb and Zn always occur at least at comparable levels, and in coastal waters Zn may even be present in substantial excess
to Pb, it is relevant to study both trace metals in common.
These kinetic studies on the formation rate of trace metal chelates in
natural waters are a necessary, logical and informative extension of investigations on chelate stability under the specific influences of excess salt components [11]. The kinetic parameters will provide information on the time
pattern these processes have at the trace concentration levels typical for
natural waters of the substances involved as reactants. The formation rate
constants determined further reflect specific influences of the medium, allow
the conclusive definition of the operative mechanism and indicate the reaction path alternative taken under the respective medium conditions. Thus,
these studies contribute considerably to a more detailed understanding of the
involved physicochemical behaviour of trace metals in natural waters.
EXPERIMENTAL
The kinetic measurements were performed by differential pulse anodic
stripping voltammetry (DPASV) [19,20,25] at the a u t o m a t e d hanging mercury drop electrode (HMDE), P.A.R. 314, which was connected to the combination of a P.A.R. polarograph 174 and the a u t o m a t e d control device
315 A. The automated control unit, P.A.R. 315 A, was readjusted in such a
way t h a t the sequence of voltammetric steps could be repeated as m a n y
times as desired with an excellent precision concerning the duration of each
step and of the whole cycle. The trace metals investigated, Pb and Zn, were
accumulated in the first step of each cycle at their deposition potentials
(--0.80 V for Pb and --1.20 V for Zn). The deposition time td was 60 s for
the trace metal concentration level of about 10 -s M and 30 s for concentrations of the 10 -7 M level.
During the accumulation of unchelated trace metal in the mercury drop,
the solution was stirred with a magnetic stirrer at a constant rotation speed
of 800 rpm. Subsequent to a cathodic deposition period td the solution was
allowed to come to rest for 15 s (tr). Then with a rate of 2 mV s-1 the potential was scanned into the anodic direction in the differential pulse mode. The
amplitude of the pulses was 50 mV, the duration 57 ms and the clock time
0.5 s. With the a u t o m a t e d P.A.R. control unit 315 A the duration of each
cycle could be strictly controlled. A cycle represents the sum of deposition
t i m e t d at the deposition potential Ed, the rest time t~, the stripping time and
297
lOO
80
60
Pb×1 ! %
40
20
D-
0
20
60
~
1 0 '0 0 Z& ,
z~'
1400
,Z~
2200
1800
T~me / s
Fig. 2. Time function for concentration of unchelated PbX~ in 0.55 M NaC1. 2 × 10 -8 M
PbX 1 and N a 3 H E D T A 1:1; (A) 1.3 pg 1-1 Zn(II) and (o) 2.2 pg 1-1 Zn(II) present.
the time (3 s) required for the automatic extrusion of a fresh mercury drop
before the subsequent cycle. In this manner the time function of the
decrease of the unchelated concentration of the trace metals subsequent to
the addition of the chelating reactant EDTA could be recorded (see Figs. 2
and 3).
All measurements were performed in a quartz cell containing 50 ml of the
electrolyte investigated and were thermostated to 25 + 0.4°C. A three-electrode system was applied with the HMDE as working electrode, an Ag/AgC1
sat. KC1 electrode with a salt bridge filled with 0.7 M NaC1 {Merck, Suprapur)
as reference electrode and a platinum wire as auxiliary electrode. The solution
in the cell was deaerated with 99.999% argon passed, before entering the cell,
through a washing bottle containing a suspension of magnesium hydroxide
carbonate in borate buffer of pH about 8 in order to prevent changes of the
carbonate concentration in the solution [26].
The chemicals for the preparation of the model solutions were of Suprapur
grade (E. Merck, Darmstadt) except MgC12 which was of p.a. grade. In order
to keep the levels of the constituents of the model solutions constant and as
close as possible to natural sea water, besides adjustment of appropriate chloride concentration, ionic strength and pH, the concentration of the buffer
components was kept constant throughout all measurements. Therefore
0.43 mM H3BO3 and 2.35 mM NaHCO3 was present in each model solution
loo
D
[3
60
PbXj / %
40
20
i
0
J
800
i
i
1600
i
i
J
2400
3200
i
4000
i
48O0
Time / s
Fig. 3. Time function for concentration o f unchelated PbXj in various media. 2 x 10 - s M
PbX i and N a 3 H E D T A 1:1; (z~) 0.0536 M MgC12 containing 1.2 X 10 -8 M Zn(II), (o) 0;01 M
CaC12 containing 6.3 X 10 -9 M Zn(II) and (~) artificial sea water containing 1.6 X 10 -8 M
Zn(II).
298
and the artificial sea-water sample. Artificial sea water was prepared according to the recipe of L y m a n and Fleming [27]. The sea-water samples, taken
from the North Sea near Heligoland and from the Ligurian Sea, were filtered
through a cleaned and subsequently conditioned 0.45 pm Sartorius membrane
filter [7,28].
The pH was controlled with an Orion pH meter, Model 701, using an Ingold
glass electrode, Type 405 NS, which was standardized with borate buffer of
pH 8.0, manufactured by E. Merck, Darmstadt.
RESULTS
The c o m m o n characteristics of the solutions investigated, which remained
constant throughout the measurements were 0.55 M chloride concentration,
a pH value between 7 and 8 and a total ionic strength of 0.7 M, if necessary
adjusted by appropriate additions of NaC104. The concentration range of the
total dissolved Pb over which the kinetics of Pb(II)--EDTA chelate formation
were measured was 2 X 10-SM to 1 X 10-TM. Compared with the average
value of the dissolved Pb level in Ligurian and Tyrrhenian coastal waters
[7,8,29,30] these concentrations are still a factor 15--70 higher, but they
come quite close to more heavily Pb-polluted local areas, e.g. the port region
of Genoa [7,8] or certain coastal areas of the Irish Sea [31]. The concentration ratio of Pb or Zn to EDTA was always 1 : 1. Owing to this adjustment
the rate of chelate formation could be studied under conditions of strictly
second-order kinetics. Therefore, in this kinetic study only the very strong
chelating agent EDTA was used as organic ligand. With a less strong chelating
agent, such as NTA, a ligand excess would be required to achieve a measurable
chelate formation and thus the chelate formation could not be easily investigated under conditions of second-order kinetics for the small concentration
levels relevant for the actual situation in sea water. The concentrations of Zn(II)
studied also usually corresponded to the range 10-s--10-TM, i.e. a Zn(II) level
of 0.65--6.5 pg 1-1. They are thus within the range reported for coastal waters
[31--33].
In this study the formation kinetics of the EDTA chelates of Pb and Zn have
been considered, because under the natural conditions both trace metals are
present in sea water, and in fact Zn can be present in coastal waters at even
higher concentrations than Pb. Owing to the fact that both trace metal chelates
have different stability constants (viz. Table 1) and are present at different concentration levels it is of interest to investigate how the rate of their chelate formation will be influenced when Pb and Zn are present together in a sea-water
sample.
In Table 1 the stability constants of various MeEDTA chelates are listed. The
data from Sillen and Martell [34] refer to 0.1 M ionic strength. The numerical
values will be somewhat different in sea water having an ionic strength of 0.7 M,
but the relative differences between the stability constants of the chelates will
remain about the same.
The principle of the measurements consists in following the change of the
peak height corresponding to the unchelated part of the trace metals, Pb(II)
and Zn(II), with the time elapsed after the addition of a similar concentration
of the chelating agent, Na3HEDTA. The stripping response, which is propor-
299
TABLE 1 a
Stability constants of various m e t a l - E D T A chelates at 0.1 M ionic strength and t e m p e r a t u r e
of 20°C
Metal
Ionic strength/M
Temperature/°C
Stability c o n s t a n t
Mg
Ca
Zn and Cd
Pb
0.1
0.1
0.1
0.1
20
20
20
20
2.5
2.0
3.2
1.0
X
X
X
×
108
101°
1016
10 is
a After Sillen and Martell [34].
tional to the unchelated part of Pb(II) and Zn(II), decreases if the formation
of PbEDTA and ZnEDTA chelate occurs (see Fig. 3). The chelates are not
electroactive in the reduction potential range of the unchelated part of the
trace metals. At the small trace metal chelate concentrations the dissociation
rate has a negligible effect within the applied deposition times of 30 or 60 s
plus the rest time tr of 15 s on the concentration of the unchelated trace metals,
as becomes immediately evident applying the criteria discussed by Davison [39].
The previous study [16,17] on the kinetics of CdEDTA chelate formation
indicated that in measurements in the m u l t i c o m p o n e n t system, sea water alone
will n o t immediately give an illuminating answer on the mechanism of chelate
formation. Hence, at first, measurements were performed in appropriately
selected model solutions, having in c o m m o n with sea water 0.55 M chloride
concentration, a pH between 7 and 8 and the ionic strength 0.7 M.
In this manner the individual specific influences of the major salinity components were studied at first in appropriate model solutions of 0.55 M NaC1,
0.01 M CaC12 and 0.0536 M MgC12. The cumulative effects of the major salinity
components were then investigated in artificial sea water, prepared according
to L y m a n and Fleming [27] and finally in natural sea-water samples taken from
the North Sea and Ligurian Sea.
The experimental results on the kinetics of PbEDTA formation in the NaC1
model solution are presented in Fig. 2. It is obvious that the formation of
PbEDTA occurs so rapidly that the kinetics are hardly measurable with the
applied voltammetric procedure. The same is valid for the formation rate of
ZnEDTA in this medium.
From Fig. 2 it is also evident that the a m o u n t of PbEDTA chelate formed
depends on the concentration of the dissolved Zn(II). At a concentration of 2.2
pg 1-1 Zn(II) about 10% of Pbwot remains unchelated. Conversely, if the Zn(II)
concentration is about 1 pg 1-1 or less, the concentration of Pbwo t ( 2 X 1 0 -s M )
is completely chelated.
Different results are obtained in CaC12 and/or MgC12 model solutions having
the same concentration of the alkaline earth cations as sea water. In Fig. 3 experimental results determined in 0.0536 M MgC12, 0.01 M CaC12 and artificial
sea water are summarized. A continuous decrease with time is observed in these
media for unchelated Pb(II) after addition of the chelating agent Na3HEDTA.
It can be noticed t h a t PbEDTA is more rapidly formed in the MgC12 solution
300
than in the CaC1: solution or in artificial sea water. The reason is the significantly lower stability constant of MgEDTA in comparison to that of the Ca
chelate, as the data in Table 1 reflect.
In artificial sea water, which contains CaC1: and MgCI: and all the other salinity components at the same levels as in genuine sea water, the time dependence
of PbEDTA chelate formation corresponds to t h a t measured in CaCI: solution.
This finding emphasizes that the rate-determining step in the reaction of
PbEDTA chelate formation is primarily connected with the Ca(II) content of
sea water.
The conditional rate constant kt of chelate formation in the respective medium follows from eqn. (1), corresponding to the case of a second-order reaction
with equal reactant concentrations [Me(II)] : [EDTA] = 1 : 1).
[MeEDTA]/[Me(II)] • [Me(II) -- MeEDTA] = k~ t
(1)
where [MeEDTA] is the chelate concentration formed at time t, [Me(II)] is the
initial and consequently the total trace metal concentration and [Me(II) -MeEDTA] is the concentration of both unchelated reactants at time t. In
Table 2 the kt values evaluated for PbEDTA formation in various model solutions and two natural sea-water samples from different seas are summarized.
Besides the concentration of the reactant investigated that of total dissolved
Zn(II) is also mentioned. It was experimentally observed that the kinetics of
PbEDTA formation can be measured only if the Zn concentration is the same
or lower than the Pb(II) concentration.
The rate of PbEDTA formation in 0.55 M NaC1 solution is too high to be
TABLE 2
C o n d i t i o n a l r a t e c o n s t a n t kf f o r P b E D T A c h e l a t e f o r m a t i o n in m o d e l s o l u t i o n s a n d n a t u r a l
sea-water samples a
Electrolyte/M
Reactant conc./M
k f / m o 1 - 1 1 s -1
Zn coac./M
0.55 NaC1
2 × 10 -8
u n m e a s u r a b l y fast
1 . 3 - - 2 . 2 × 10 -8
0 . 0 5 3 6 MgCI:
2 × 10 -8
1 X 1 0 -7
(3.08 +_ 0 . 3 8 ) × 104
5.05 x 104
1.2 × 10 -8
1 . 1 × 10 -8
0.01 CaC12
2 X 10 -8
1 × 10 -7
2.47 × 103
(1.37 -+ 0 . 0 9 ) × 103
6.3 X 10 -9
6.2 X 10 -9
Artificial
sea w a t e r
2 × 10 -8
1 × 10 -7
(1.71 +- 0 . 4 0 ) × 1 0 3
3 . 9 5 × 103
1.6 X 1 0 - 8
1.3 × 10 -8
N o r t h Sea w a t e r
Heligoland
1 X 10 -7
3.86
X 103
1.3 x 10 -7
Ligurian
Sea w a t e r
2 × 10 -8
1 × 10 -7
2.05
1.71
× 103
× 103
2.6 × 10 -8
2.1 × 10 -8
a T h e e r r o r m a g n i t u d e f o r s o m e m e d i a o b t a i n e d b y several r e p e t i t i o n s o f t h e m e a s u r e m e n t s
p r o v i d e s an i m p r e s s i o n o f t h e t y p i c a l p r e c i s i o n o f t h e v a l u e s f o r t h e f o r m a t i o n r a t e c o n s t a n t
kf.
301
measured with the applied DPASV m e t h o d . The rate constant kf determined
in 0.0536 M MgC12 is about one order of magnitude higher than in 0.01 M
CaC12 solution. The kf values determined in artificial sea water and in natural
sea-water samples (taken at Heligoland and in Ligurian coastal waters) are,
within the experimental error, the same as those determined in CaC12 solution.
The coincidence of the kf values within the experimental error in artificial and
natural sea water emphasizes that further ingredients of the sampled natural
sea water add no further influences on the kinetics of trace metal chelation.
The values of the conditional rate constant kf determined in various electrolytes for ZnEDTA formation are listed in Table 3. The specific effects of the
macrocomponents of the respective electrolyte on the rate of chelate formation, which became evident for Pb (and have been previously observed for cadmium [17]) also exist for ZnEDTA-chelate formation. Again the kf values in
0.01 M CaC12 model solution and in artificial and natural sea water are of the
same magnitude. Below 10 -7 M Zn(II) and Na3HEDTA no formation of
ZnEDTA chelate was observed within the overall experimental time of about
1.5h.
It is to be expected t h a t the kf values for ZnEDTA and CdEDTA formation
have about the same magnitude in similar media as the stability constants for
both chelates are equal (see Table 1). This has been experimentally confirmed
by extending the previous study [16,17] on the kinetics of CdEDTA formation
to the lower reactant concentration range (~<2 × 10 -7 M). From the sea-water
sample used as medium the Zn(II) had been virtually eliminated by preelectrolysis to avoid competition with Cd(II) for the chelating agent EDTA. The
kf values obtained are given in Table 4 and they are of the same magnitude as
previously determined at equal concentrations of Cd(II) and Na3HEDTA above
2 × 10 -7 M. Furthermore, they have the same magnitude as the kf value for
ZnEDTA formation in CaC12, artificial and natural sea water (see Table 3).
In natural waters the most probable situation will be n o t the presence of
just one trace metal. Therefore, the rate of simultaneous formation of PbEDTA
and ZnEDTA chelates was studied in the sample of sea water taken at
TABLE 3
C o n d i t i o n a l rate c o n s t a n t kf for Z n E D T A c h e l a t e f o r m a t i o n in m o d e l s o l u t i o n s and natural
sea w a t e r
Electrolyte/M
Reactant conc./M
kf/mo1-11 s-1
0.55 NaC1
2.4 x 10 -8
u n m e a s u r a b l y fast
0.0536 MgC12
1
3.28 X 1 0 3
0.01 CaC12
7.5 x 10 -9
1 X 10 -7
u n m e a s u r a b l y slow
1.89 X 1 0 2
Artificial
sea w a t e r a
2.2 X 10 -8
1 X 10 -7
u n m e a s u r a b l y slow
(3.17 -+0.40) X 102
Ligurian
Sea w a t e r
1
2.69 X 102
X 1 0 -6
X 10 -7
a The e r r o r given for artificial sea w a t e r is typical o f t h e precision o b t a i n a b l e
302
TABLE 4
C o n d i t i o n a l rate c o n s t a n t kf for C d E D T A c h e l a t e f o r m a t i o n in N o r t h Sea w a t e r p r e e l e c t r o l y z e d for p u r i f i c a t i o n f r o m Z n ( I I ) ; s a m p l e d near Heligoland
Reactant concentration/M
kf/mo1-1 1 s -1
2
1
1
5
(3.01
(4.94
(2.41
(5.10
X
x
x
×
10 -7
10 -7
10 -7
10 -s
-+ 0 . 9 8 )
+- 3.55)
+_ 1.36)
+_ 6 . 8 1 )
x
×
x
×
102
102
102
102
Heligoland. The sea water had been previously filtrated through an 0.45 pm
membrane filter to separate it from suspended particulate matter. The concentration of the reactants and the respective kf values are presented in scheme (I).
1.3 × 10-TM Zn(II) ]
1.0 X 10-TM Pb(II) /
+ 1.0 × 10-7M Na3HEDTA
~,kf = (6.28 -+3.70) X 102M-' s-'
"~k, = (3.86 -+ 1.22) X 103M -' s-'
(I)
While the concentration of dissolved Zn(II) had an original value of 1.3 X 10-TM
in the sample of sea water of pH 7.53 investigated, the concentration of Pb(II)
had to be adjusted to a similar level, i.e. to 1 X 10-7M. Otherwise, if Zn(II) had
remained in excess no formation of PbEDTA chelate could be observed. The
concentration of Na3HEDTA was adjusted to the same concentration to maintain a ratio of 1 : 1 for both reactants, trace metals and ligand.
The rate of simultaneous formation of CdEDTA and PbEDTA chelates was
also studied, and is shown in scheme (II).
2 × 10-SM Cd(II)
t + 4 × 10-8M Na3HEDTA ~"unmeasurably slow
5 X 10-SM Pb(II) )
"*ks = (5.8 -+1.90) × 103M -' s-'
(II)
The experiment was performed in a sample of Heligoland sea water with pH
7.84 which had been previously preelectrolyzed to remove the existing natural
Zn(II) concentration of 1.3 × 10 -7 M from the solution investigated. The respective concentrations of Cd(II) and Pb(II) had been artificially adjusted. Under
the adjusted conditions the formation of the Cd chelate was no longer measurable owing to an at least 30 times smaller rate compared with the chelation
of the present concentration of Pb(II) as one is at the lower limit of the kinetic
window for the applied DPASV procedure (see below).
DISCUSSION
General methodological aspects
The kinetic window within which the applied DPASV m e t h o d permits the
kinetics of chelate formation to be followed becomes evident from the following consideration of its time constant summarized in Table 5. For a range of kf
values the reaction half-times tin resulting for the corresponding initial reactant
concentration cl, presuming equal values for both reactants of a second-order
303
TABLE 5
Kinetic w i n d o w of the applied D P A S V m e t h o d ( . . . .
k ~ / 4 M -1 s -1
tln/S
1 X 10 -8 M
109
108
107
106
0.1
1.0
10
100
- - o - - e - - . - - t - - o
lO s
a p p r o x i m a t e boundaries)
- - o
103
2 X 10 -8 M
1 X 10 -7 M
0.05
0.5
5
50
4
•
0.01
0.1
1
10
•
500
104
5 X 103
0.001
0.01
0.1
1
•
[
100
"__,
104
1 X 10 -6 M
10
• ---%
103
I
i-_o__
103
10 2
1__01
[
I
i
lO s
5 X 104
104
10 6
5 X IO s
IO s
10 7
5 × 106
106
100
•
_
_
a
- -
•
103
[
i
g . ~ . - - o
104
l0 s
- - W
- -
Q - - I I - -
O m l l
--
reaction (1 : 1 reactant ratio), have been calculated according to eqn. (2):
tin = 1/c~k~
(2)
Depending on the cathodic deposition time td adjusted according to the initial reactant concentration either to 60 or 30 s, the kinetic time constant of the
applied DPASV m e t h o d amounting to the sum of t d and the 15 s rest time tr
becomes 75--45 s respectively. Consequently, for all tz/2 values and the corresponding reactant concentrations a b o v e the d o t t e d line in Table 5, the reaction
proceeds too fast for a determination of kf. For a tin below 500 s a reliable
measurement hardly seems feasible, because the basic supposition that over the
kinetic time constant of the m e t h o d the changes in reactant concentrations
remain negligible is unduly violated. The lower limit of the kinetic window is
set for practical experimental reasons. The maximal permissible tin value lies
in this respect between 10 s and 104 s, corresponding to a tin between about
30 and 3 h. Consequently, within the adjusted maximal experimental overall
observation time of 1.5 h there will occur for a tl/2 of l 0 s s an alteration of the
reactant concentration of only a few percent and thus the precision limit of
the m e t h o d is reached. The resulting lower limit for the kinetic window of the
approach used agrees well with the experimental observation that the measurement of Zn(II) and Cd(II) chelation kinetics below an initial concentration of
5 × 10 -8 M is n o t possible. The rather poor precision in the k~ value measured
in sea water for Cd(II) at the 5 X 10-8M reactant level also emphasizes that the
lower limit of the kinetic window for the applied m e t h o d is approached (see
Table 4). However, for Pb(II) with an about one order of magnitude higher k~
value measurements could be extended down to 2 X 10 s M Pb(II) with satisfactory precision.
304
Mechanism
The k~ values in Tables 2, 3 and 4 for the chelation of Pb(II), Zn(II) and
Cd(II) in all media containing alkaline earth ions at the sea-water levels are
within the kinetic window of the applied method. However, in the absence of
Ca(II) and Mg(II), i.e. in pure NaC1 model solution, the chelate formation rate
is so fast that the kinetics are n o t measurable with the applied DPASV method.
The simplest formulation for the mechanism of chelation is obviously a
recombination mechanism according to
Me(II) + HEDTA 3- -~
kf MeEDTA:- + H+
(III)
As the rate of such a recombination mechanism depends completely or at least
predominantly on the diffusional approach of the oppositely charged reactants,
kf values of 109--101° 1 mole -1 s-1 may be expected [35--37]. If to the control
of the overall recombination rate furthei- stages contribute such as the release
of water from the primary hydration shell of the trace metal ions Me :+ or the
release of labile inorganic ligands X from trace metal species MeX;, somewhat
lower k~ values will occur. But the resulting k~ values would still remain well
above the limits of the kinetic window of the DPASV m e t h o d according to the
existing knowledge from fundamental studies [35] on the kinetics of this type
of reaction.
In this context it is concluded that a recombination according to eqn. (III)
is the mechanism in 0.55 M NaC1 model solution. The trace metals will participate partially in the diffusional encounter with the chelating agent EDTA
according to eqn. (III) as chlorocomplexes MeC1j. But as these chloro species
are labile "and exist in a very mobile equilibrium with free hydrated Me 2+ ions,
this will cause no significant contribution to the control of the overall recombination rate and the k~ value will remain virtually unaffected and consequently
be diffusion controlled.
However, the situation becomes somewhat different in alkaline earth model
solutions and in sea water. If the concentration of Ca(II) and Mg(II) is in substantial excess to that of a chelator able to form very stable chelates with the
alkaline earth metals, e.g. EDTA (see Table 1), then virtually the whole chelating agent concentration will be present as alkaline earth chelate. Owing to the
about 80 times larger stability constant the predominant alkaline earth chelate
in sea water will be CaEDTA 2-. In a similar manner the Ca(II) excess and to a
smaller extent the Mg(II) excess will chelate in the sea with the chelating components of DOM and thus determine the prevailing speciation of these natural
organic chelating agents in the sea.
In analogy to eqn. (III), one has for the Ca(II) model medium and sea water
the following reaction as the overall process of trace metal chelation with the
model ligand EDTA:
-~ MeEDTA 2- + Ca 2+ + jXMeX; + CaEDTA:- kf
The kf values determined for the chelation of Pb(II), Zn(II) and Cd(II) (see
Tables 2, 3 and 4) reveal, however, that the rate-determining step is n o t the
diffusional approach of MeX~ and CaEDTA 2- as in a simple recombination
(IV)
305
mechanism. In this context it is emphasized that the coincidence of the measured data for the Ca(II) model medium and sea water (see Fig. 3) clearly
show that the chelating agent -- EDTA in this case -- is offered in the form
of CaEDTA 2- to the labile inorganic trace metal species MeXj. This refers to
the conditions studied which resemble the situation in the sea with respect
to the concentrations of the involved heavy trace metals, chelating components o f DOM modelled by EDTA and the excess level of 1 X 10 -2 M Ca(II).
It turns out that formally the trace metal chelation occurring in an overall
reaction according to eqn. (IV) can still be treated in terms of recombination, if the side reaction coefficient concept [40] is applied. One then obtains:
kf = kf(d)/OiMeXj 0~L
(3)
where kf(d) is the rate constant value for chelate formation by diffusioncontrolled recombination and aMeXi and aL are the side reaction coefficients
for the inorganic trace metal species and the chelating agent, in this case
EDTA. Owing to the very mobile complex equilibria for MeX~ the side reaction
coefficient 0tMeXj Can be set with sufficient approximation for this estimate
equal to 1 while for aL one has:
aL = 1 + ([H+]/KHEDTA 3- ) -I-•CaEDTA2- [Ca 2+] + /~MgEDTA2_[Mg2+])
(4)
The most significant term in sea water is that referring to CaEDTA 2-. At pH 8,
typical for sea water, one obtains for aT. a value of 1.5 × 106. Assuming for kf(d)
values between 10 TMand 1091 mole -~ s-~ and inserting aL into eqn. (3a),
kf ~ kf(d)/a L
(3a)
one obtains for k~ values of 6.7 × 103 and 6.7 X 10:1 mole -1 s-1 respectively.
As comparison with the kf values determined (see Tables 2, 3 and 4) for the
formation of MeEDTA 2- with Pb(II), Zn(II) and Cd(II) in sea water shows, the
application of the side reaction coefficient concept to the diffusion-controlled
values of the chelate formation rate constant kf(d) yields the same order of magnitude for the actually observed chelate formation rate constant k~. Thus these
estimates, based on the side reaction coefficient concept, emphasize again the
conclusion that in sea water [as well as in the appropriate Ca(II) model solution]
the studied chelation of the trace metals Pb(II), Zn(II) and Cd(II) with EDTA
occurs according to eqn. (IV) with CaEDTA 2- as the chelating reactant. This
will be generally the case in the sea for the chelation mechanisms with other
natural organic chelating agents, i.e. the components of DOM, usually present
in sea water in concentrations orders of magnitude smaller than Ca(II), which
has an overall concentration of 1 × 10-: M in the sea.
In terms of reaction mechanisms the overall reaction for trace metal chelation in the sea according to eqn. (IV) can be termed a ligand exchange mechanism [17]. Virtually the total chelating agent concentration is present as
CaEDTA:-, and in an encounter with a labile inorganic trace metal species MeXj
the higher stability of the trace metal chelate MeEDTA ~- causes the transfer of
the chelating agent, EDTA in this case, to the trace metal Me(II) under simultaneous release of ]X- according to eqn. (IV).
The kf values determined are conditional data related to the ionic strength
and its composition in the respective media. They contain the activity coeffi-
306
cients of the reactants. Inspection of Fig. 3 and Tables 2 and 3 further reveals
that the trace metal chelation by ligand exchange with alkaline earth chelates
at a concentration level similar to sea water proceeds faster in a medium containing only Mg(II), while for 0.01 M Ca(II) and sea water the k~ values and
consequently the trace metal chelation rate coincide within the experimental
error. This finding indicates in more detail the prevailing reaction path of trace
metal chelation in the sea. It will usually occur via ligand exchange with the
Ca(II) chelate -- in the present study Ca(II) EDTA. Although the ligand
exchange with the Mg chelate would be faster, this will hardly occur, if trace
concentration levels for the chelating agent prevail, as adjusted in this study for
EDTA. Under these circumstances the available chelating agent will be bound
virtually only by Ca(II) because usually the Ca(II) chelates have a higher stability constant than those of Mg(II), as Table 1 shows for the EDTA chelates. In
other words, initially after the addition of EDTA formed MgEDTA will convert
by a rapid ligand exchange largely into CaEDTA [ 17]. If the discrepancy
between the stabilities of the chelates of Ca(II) and Mg(II) will be smaller for
other ligands than EDTA, then a certain a m o u n t of chelated Mg(II) will be
available as well, but an equilibrium between both alkaline earth chelates will
virtually always exist in the sea and corresponding model media.
A consequence of ligand exchange between Ca chelates and trace metals prevailing as the mechanism of trace metal chelation in sea water is also the fact
that the k~ values for the respective trace metal chelates parallel the stability of
the chelates formed (see Table 1), i.e. the higher the stability of the formed
trace metal chelate the higher also is its formation rate constant k~, while as the
data for Cd and Zn show, similar stabilities also cause similar formation rate
constants (see Table 1--4).
For t h e r m o d y n a m i c reasons the significantly higher stability of Pb(II) chelates
would deinand their predominant formation in the sea, if the concentration of
the chelating agent is of the same order as the trace metals, because true thermodynamic equilibrium would exist only when the demand of Pb(II) has been
completely satisfied. However, it has to be borne in mind that the statistical
probability to achieve this state is rather low at the trace level, because that
would require that further ligand exchange occurs between the less stable chelates
of Cd, Zn, etc. formed and the a m o u n t of Pb(II) not yet chelated by ligand
exchange with Ca chelates. The probability and rate for this will certainly be
rather low in the sea due to the small reactant concentrations involved (see Table 5).
The whole behaviour described will prevail only as long as there is a medium
in which the alkaline earth ions are in substantial excess over the concentration
of the chelating agent. Yet this is exactly the situation encountered in sea water
with respect to the natural chelating components of dissolved organic matter
(DOM) or pollution by small amounts of EDTA and similar anthropogenic
chelating agents in coastal waters. Thus it is to be emphasized that in a general
sense our study, although restricted to EDTA, has been performed under quite
realistic conditions with respect to the chelation processes going on with trace
metals such as Pb, Zn and Cd, etc. in the sea.
The chelation mechanism of trace metals Me in the sea thus seems to follow
the scheme (V), which is up to now experimentally confirmed for three im-
307
portant marine trace metals, i.e. Cd [17], Zn and Pb, and seems to have general
validity.
M g E D T A 2~i"
CaEDTA
kf
2- + M e X j
~ MeEDTA
Mg 2+
2- +
+ iX-
(v)
Ca 2+
with X- --- CI-, OH-, HCO~, CO~-, SO~- etc. This emphasizes the key role of
Ca(II) among the salinity components exerting specific influences on the stability and formation of trace metal chelates with organic materials dissolved in the
sea.
ACKNOWLEDGEMENT
This study is part of the joint research project, "Environmental Research in
Aquatic Systems", of the Institute of Applied Physical Chemistry, Nuclear Research Center (K.F.A.), Juelich, and the Center for Marine Research, Institute
"Rudjer Bo§kovid", Zagreb, in the bilateral German--Yugoslavian agreement.
Financial support by the International Bureau of K.F.A., Juelich, is gratefully
acknowledged.
The authors also express their thanks to Dr. L. Sipos for the rearrangement
of the P.A.R. 315 A automatic control unit, which enabled them to perform
the kinetic measurements with the automated HMDE by deliberate repetition
of the whole voltammetric cycle.
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