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Pergamon
0043-1354(94)00205-3
Wat. Res. Vol. 29, No. 3, pp. 803-809, 1995
Copyright© 1995ElsevierScienceLtd
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SPECIATION OF DISSOLVED CADMIUM:
INTERPRETATION OF DIALYSIS, ION EXCHANGE
AND COMPUTER (GEOCHEM) METHODS
PETER E. HOLM 1., SJUR ANDERSEN2~ and THOMAS H. CHRISTENSEN ~
qnstitute of Environmental Science and Engineering/Groundwater Research Centre, Technical University
of Denmark, Building 115, DK 2800 Lyngby, Denmark and :JORDFORSK, Centre for Soil and
Environmental Research, 1432 As, Norway
(First received February 1994; accepted in revisedform July 1994)
Abstract--Equilibrium dialysis and ion exchange methods, as well as computer calculations
(GEOCHEM), were applied for speciation of dissolved cadmium (Cd) in test solutions and leachate
samples. The leachate samples originated from soil, compost, landfill waste and industrial waste. The ion
exchange (IE) method separates dissolved Cd into free divalent Cd (Cd2+) and complexed Cd and
furthermore separates the latter into the operationally defined forms: labile, slowly labile and stable
complexes. The dialysis (ED) method determines high molecular weight Cd complexes (above
1000mol. wt). For both methods the reproducibility was good. By combining the results of the
GEOCHEM calculations in terms of the inorganic complexes, and the IE results, the fractions of free and
inorganically complexed Cd were estimated. The IE and ED results furthermore provided information
about the organic complexes. Selected environmental leachates showed different Cd speciation patterns
as expected. Some leachates were dominated by free divalent Cd (1-70%), some by inorganic complexes
(1-87%), and some by organic complexes (7-98%).
Key words---cadmium, speciation, leachate, complexes, ion exchange, dialysis, GEOCHEM
INTRODUCTION
The availability and mobility of cadmium (Cd) in
terrestrial environments, e.g. in soils and waste disposal sites, are not only related to concentrations in
solution but also to the Cd species present in the
solution, i.e. free divalent Cd (Cd z÷), inorganic and
organic complexes. The analytical determination of
dissolved species is still in its infancy and no method
is generally accepted, though many approaches are
used. Speciation methods in general, advantages and
problems have been reviewed by Florence and Batley
(1980).
Berggren (1989) studied cadmium species in humic
soil solutions by equilibrium dialysis. High molecular
weight complexes accounted for 16--43% of dissolved
Cd in soil solutions containing dissolved organic
carbon (DOC) concentrations of 30 and 160 mgC/1.
Lun and Christensen (1989) determined Cd species in
solid waste leachates using ion exchange methods.
Free divalent Cd accounted for less than 10% of
dissolved Cd. In most of the leachates, the operationally defined fractions "labile" and "slowly
labile" complexes accounted for 69-100% of Cd in
solution. Morrison et aL 0990) found maximum Cd
concentrations of 4--13/~gCd/l in urban runoff water,
and anodic stripping voltammetry showed that
*Author to whom all correspondence should be addressed.
wa 29/3-o
cadmium was predominantly ionic and inorganically
complexed. DOC concentrations in solution ranged
up to 41 mgC/l. These results from the literature
support the need for speciation methods, but none of
the methods alone provides enough information to
evaluate the significance of inorganic complexes and
the size and apparent stability of the naturally occurring organic complexes. Such information is in particular needed for evaluation of the mobility of Cd in
the terrestrial environment, since the majority of the
information available only relates to the mobility of
free divalent Cd. As shown above, free divalent Cd
may only constitute a small fraction of dissolved Cd
in the environment.
The purpose of this study was to show, by combining the results of existing speciation methods applied
to selected solution samples, that it is possible to
determine the fraction of free divalent Cd, the fraction of the inorganic and the organic complexes, and
to evaluate the size and stability of the organic Cd
complexes. The three speciation methods applied
were: (i) equilibrium dialysis (referred to as the ED
method), which separates complexes according to
their size, (ii) ion exchange (referred to as the IE
method), which determines free divalent ions and
categorizes the complexes according to their apparent
stability, and (iii) GEOCHEM computer calculations
of the ratio between inorganic complexes and free
divalent Cd.
803
804
Peter E. Holm et al.
Table 1. Characteristicsof samples used for speciation
Solution
pH
Spcc. cond.
(/tS/cm)
CaCI2 solution (10 -2 M)
Humic acid solution
Soil solution
Compost leachate
Polluted groundwater
Industrial leachate 1
Industrial leachate 2
Industrial leachate 3
7.0
7.0
6.6
6.6
6.8
7.2
7.0
6.5
2300
275
2000
1050
4940
12000
16300
2330
DOC
(mgC/l)
CdT
(/~g/l)
0
25
64
13
264
79
34
3200
Mg
(mg/I)
Ca
(mg/I)
0
I
23
12
36
90
80
10
400
50
400
119
57
300
270
90
9.0
10.1
6.3
8.7
9.2
71
8.9
660
Na
(rag/l)
CI
(mg/l)
0
-24
23
332
2200
2400
40
710
4
45
56
626
2900
3200
60
SO4
(mg/I)
0
l0
90
30
<1
15
20
9
- - N o t analyzed.
MATERIALS AND METHODS
General
All chemicals used were analytical grade (Merck, pro
analysis) unless otherwise stated. All plastic and glassware
were cleaned and soaked in 2 M HNO3 for at least 12 h, then
rinsed with deionised distilled water and dried at 45°C in
a convection oven. The dialysis tubing (Spectra/Por® 7
Membranes, molecular weight cutoff of 1000, 45 mm flat
width, 6.42 ml/cm) was soaked in a 0.05% sodium azide
solution, acid washed in 0.1 M HNO 3 and rinsed with
deionised water.
When specific or constant pH was needed, adjustments
were made by means of small additions of HNO3 or NaOH.
ED method principles, calculations and design
The ED method separates dissolved Cd in solution (CdT)
according to the size of the complexes. The sample constitutes the inner solution in the dialysis bag, and free divalent
Cd and complexes with a molecular weight cutoff (MWCO)
smaller than 1000, equilibrate over the dialysis tubing
membrane into an outer solution identical to the sample
with respect to dissolved Cd and major inorganic ions.
Complexes larger than 1000 in molecular weight are retained
inside the dialysis tubing. At equilibrium, the activities of all
ions and complexes smaller than 1000 in molecular weight
are identical in the sample (inner solution) and the outer
solution.
The concentration ofdaigh molecular weight organic metal
complexes Cd*Mwin the inner solution at equilibrium equals
the difference between the total metal concentration in the
inner (Cd0 and in the outer solution (Cdo) at equilibrium:
[Cd~Mw] =[Cdl] - [Cdo]
[CdT]
[Cd~]
[CdT]
[Cd[]
t
I
I
!
Identical:
pH
A 2+ M 2,
Cd, ua , g ,Na*
48 hours
I
I I
cf, SO~ and NO3
activity
!
(2)
Therefore, the fraction of high molecular weight Cd complexes of the original sample solution is calculated as
[CdnMw] [ C d l ] - [Cdo]
Outer solution
Initial
(1)
where [ ] is concentration and * refers to the equilibrium
situation.
Assuming that the concentration of high molecular
weight ligands substantially exceeds the concentration of
dissolved Cd, the fraction of high molecular weight Cd
complexes is the same in the sample solution and in the inner
solution at equilibrium
[CdnMw] [Cd~Mw]
experimental design and allows for the use of relative small
sample solutions.
The dialysis tubing containing 50 ml sample (inner solution) was suspended in 4.01 solution (outer solution) in a
51 polyethylene bucket with lid. The outer solution was
identical to the sample solution with respect to major
inorganic ions and also approximately to inorganic Cd
(Table 1). Organic matter and, probably, carbonate contributed significantly to the charge balance in some of the
leachates. These charge contributions were not quantified,
but additional ions were supplied to reach identical specific
concentrations in the two solutions. Cd was added to the
outer solution in the same concentration as the total dissolved Cd concentration (Cdr) in the sample (inner solution) given in Table 1. The dialysis tubing was fixed with
(0.30 mm diameter nylon) lines and a 75 mm tubing closure
(Spectra/Por Closures, Spectrum Medical Industries Inc.) at
the upper end, and with a 75 mm weighted closure at the
lower end. The outer solution was continuously stirred
(magnetic stirrer), thus minimizing the dialysis time needed
to obtain equilibrium. The dialysis time was 48 h.
At equilibrium, the inner solution and 3 subsamples of the
outer solution were transferred to polyethylene bottles and
prepared for analysis of dissolved Cd and dissolved organic
carbon (DOC). The ED method design is shown in Fig. 1.
(3)
The complementary fraction is defined as the low molecular weight fraction and contains free ions and small complexes. The ED calculations are summarized in Fig. 1.
Presumably, the complexing effect of low molecular weight
organic ligands is of less importance because the concentrations of these are diluted 80 times in the outer solution.
Berggren (1989) presented the experimental design involving
the dilution of low molecular weight organic ligands in the
outer solution, which is an improvement of the classic ED
~,alculations
Highmolecularweight
Low molecular weight
CdHaw =
Cxlr
c~
~
Cd~- Cd 0
Cd=
=cdo
Cd~
Fig. 1. Equilibrium dialysis method (ED): outline of experimental procedures and calculations for determination of
the high molecular weight (> 1000 MW) Cd.
Speciation of dissolved cadmium
Swaae
Chelex
(2nYJmln)
Rataranea
50.400mgAmbedite
50-400mgAmbedite
IdenUcal:
Ca2÷,Mg2'"
ionicstrength
Cd<lOOOl,tg/I
~,,,,,..,~_.~'~
lOrrL: ~
IOOrng
48.,~u~
I'~
805
The second part of the method operationally separates the
Cd complexes into three fractions, but does not provide a
• theoretical measure of the stability of the complexes. The
experimental calculations and procedures are summarized in
Fig. 2. The method is based on 50 ml samples for the full
speciation. Free divalent ions and complexes defined as
labile are retained in the column (solution retention time
is approximately 2 min in the column). Complexes not
redistributed, therefore defined as stable, are in solution
after the final batch procedure (48 hours equilibration).
However, complexes in solution after the column procedure,
but not after the final batch procedure, are defined as slowly
labile complexes. Further details on the IE method are given
in Christensen and Lun (1989) and Holm et al. (1995).
G E O C H E M calculations
Cabu~r~
Free divalentions
Labilecomplexes
Slowlylabilecomplexes
Stablecomplexes
~._~+_- cdt-~l. ~ :
~t
e.xl, e_~ - Cd;
Cd 'C , -cd÷
CdT
Cd~c" Cd~- Cd=
Car
Cdt
Cdt
CdT Cdt
Fig. 2. Ion exchange method (IE): outline of experimental
procedures and calculations for determination of the Cd
species: free divalent Cd 2+, labile, slowly labile and stable
complexes.
IF, method principles, calculations and design
The IE method separates dissolved Cd into free divalent
Cd and 3 complexed fractions. The determination of the free
divalent Cd concentration is based upon the equilibrium
established between Cd 2+ in solution and Cd 2+ ions that are
exchanged onto a weak cation exchange resin (Ca saturated
Amberlite). The procedure for the Cd 2+ determination
involves a reference experiment. The complexed Cd fraction
is separated according to the ability of the metal-ligand
equilibria to redistribute, allowing the released Cd 2÷ ions to
exchange onto a strong cation exchange resin (Ca saturated
Chelex).
The procedure for the Cd 2+ determination involves an
initial batch experiment together with a reference experiment, both shown in the upper part of Fig. 2. Details can
be found in Holm et al. (1995). The reference experiment
contains Cd solely added as Cd 2÷. All other conditions
(resin weight/sample volume, Ca and Mg concentrations,
and ionic strength), except the presence of any ligands, are
identical in the sample batch experiment and in the reference
experiment. The reference experiment provides the required
information on the distribution of free Cd 2+ ions between
the resin and the solution under conditions comparable to
those of the sample. Assuming that Cd 2+ has the same
affinity for the resin in both the sample and the reference
experiment, the ratio of Cd 2+ to total Cd in solution can be
calculated from the equation:
[Cd 2+] [CdT]- [Cds]
[Cd~']
(4)
[Cd~]
[Cds]
[Cd~] - [Cd~']
where * refers to the reference experiment and all concentrations refer to dissolved Cd concentrations that can be
measured directly (see Fig. 2). Note that this approach is
only accurate if the ligand concentration exceeds the total
Cd concentration by at least a factor of 5. This assumption
is considered fulfilled for most leachates, see also Holm et al.
(1995).
Inorganic complexes in the samples were calculated with
the thermodynamic speciation program GEOCHEM
(Mattigod and Sposito, 1979). The program is used in this
study only to account for inorganic complexes, as general
stability constants for dissolved organic carbon are not
available. Input parameters are major anions and cations,
total dissolved Cd and the pH (Table 1). The program
calculates the ionic strength of the solution and corrects the
stability constants to this ionic strength. Output values of
the program are the equilibrium concentrations of all
species, given as prow of each metal complexed with each
anion. The program output was used for calculating the
ratio between the inorganically complexed Cd fraction and
the fraction of CA2+ in the sample.
Samples
We selected samples to cover different characteristics with
regard to the content of Cd, DOC and inorganic ligands.
Both test solutions and leachate samples were considered.
The characteristics of all employed solutions and samples
are found in Table 1. Apart from the humic acid solution,
all solutions and samples containing organic carbon were
centrifuged (3500 rpm, 30 min) and decanted to yield solutions free of suspended matter. The humic acid solution
was filtered through a 0.2 #m pore size filter (Sartorious
Minisart ® NML). Test solutions and those leachates originally containing no Cd were spiked with Cd [stock solutions
of Cd(NO3)2] and equilibrated for at least 2 days before
centrifugation.
Test solutions
A 2.10-2M C1- solution was prepared from a 1.0 M
CaCI2 stock solution. The solution was spiked with Cd and
pH adjusted to 7.
Humic acid solution was prepared by dissolving a
commercially available humic acid (Roth Chemicals,
Humussaure No. 7821) in distilled water. The solution was
predialysed (with tubing MWCO 1000) in deionised water
under continued stirring for 24 h to remove low molecular
weight organic compounds. The predialysed solution was
spiked with Ca and Cd.
Leachates
Leachate representing soil solution was collected from
plant pots each containing 1 kg soil. The soil was a sandy
loam from the Woburn Market Garden Experiment, U.K.,
which contained elevated levels of trace metals arising from
previous amendments with sewage sludge (McGrath, 1984).
Soil solution was collected by leaching with distilled water
for 2 days.
A compost leachate was generated in the laboratory.
About 1 kg of household compost was soaked in 21 of water
and equilibrated for 2 days. The leachate was spiked with
Cd.
Polluted groundwater was collected from the leachate
pollution plume a few metres downgradient of the Vejen
806
Peter E. Holm et al.
Landfill, Denmark (Lyngkitde and Christensen, 1992). The
sample was spiked with CA.
Leachates were collected from a waste site containing
industrial paint waste. The sea-side site, located in
Sandefjord, Norway, reflects the characteristics of the
leachates (Andersen, 1991). The three leachate samples from
the waste site represent different characteristics: high Cd
concentration and high sea salt concentration (Industrial
leachate 1), low Cd concentration and high sea salt concentration (Industrial leachate 2), high Cd concentration and
low sea salt concentration (Industrial leachate 3). Industrial
leachate 2 was spiked with Cd.
Analytical methods
Soluble Cd was determined by solvent extraction and
graphite furnace atomic absorption spectrophotometry
(Perkin-Elmer 5000, deutrium background correction, HGA
400 graphite furnace, AS-1 automatic sample injection
system). Before analysis all samples were acidified to 10-2 M
HNO3.
Ca, Mg and Na were determined by flame atomic absorption spectrophotometry (Perkin-Elmer 370). Before analysis, all samples were acidified to 10-2 M HNO3. Ca, Mg and
Na in the industrial waste leachate samples were analyzed
by inductively-coupled plasma emission spectroscopy
(Thermo Jarrel Ash ICAP 1100 with simultaneous detection). CI and SO4 analysis was performed by standard
autoanalyzer routine (Technicon Autoanalyzer II). CI and
SO4 in the industrial waste leachate samples were analyzed
by ion chromatography (Dionex QIC analyzer with AS4ASC column and 1.8mM Na2CO3/1.7mM NaHCO 3 as
eluent at a flow rate of 2 ml/min).
Dissolved organic carbon (DOC) analysis was performed
by standard procedures on a total organic carbon analyzer
(Dohrmann DC-80 TOC analyzer).
pH was measured by a pH Meter (Hanna Instruments DP
7916R) with a pH combination electrode (Radiometer GK
2401C).
Specific conductivity was measured by a conductivity
meter (Radiometer CDM 83 Conductivity Meter) with
electrode (Radiometer Type CDC 104).
RESULTS
Reproducibility
Reproducibility of the two methods was studied by
replicate analysis of four of the solutions: the two test
solutions (calcium chloride solution and the humic
acid solution) and two leachates (compost leachate
and the polluted groundwater). Replicate speciations
of all four samples were performed at the same time.
Results of these speciations are presented in Table 2.
For both methods, the reproducibility was considered
good because the determined Cd fractions were the
same in the replicates. All results from the replicate
speciations are presented as average values of the two
determinations in Fig. 3.
Test solutions
The results of the two speciation methods applied
to test solutions and theoretically computed inorganic speciations are shown in Fig. 3.
The speciation of the 10 -2 M CaCI 2 solution gave
the expected results. The ED method determined
100% low molecular weight Cd. This is in accordance
with the fact that the Cd-chloro complexes are
smaller than the 1000 MWCO of the dialysis membrane. The IE method yielded 47% Cd 2÷ and 53%
labile complexes, which is in close agreement with the
computer predictions (49% Cd 2÷, 51% Cd-chloro
complexes).
The speciation of the predialyzed humic acid solution by the ED method identified 52% low molecular weight Cd which was consistent with the 45%
Cd 2÷ determined by the IE method. The difference
between the 52% low molecular weight Cd and 45%
Cd 2+ is within the uncertainty of the experimental
approach but may be explained by the inorganically
(3%) and low molecular weight organic complexed
Cd fractions. Although the solution was predialyzed,
a difference of 2 mgC/l was observed between the
inner solution, initially and at equilibrium (Table 3).
Thus, the size distribution of humic acids may change
upon removal of the smaller size fraction as suggested
by Berggren (1989). The IE method identified a
significant fraction of labile Cd complexes (47% of
the dissolved Cd) but only a minor fraction of slowly
labile (9%) and stable (0%) complexes indicating that
the major part of the 48% high molecular weight Cd
complexes were labile.
The speciation of the 10 : M CaCI 2 solution and
the predialyzed humic acid solution suggests that the
two speciation methods yield consistent results with
respect to solutions containing both inorganically
and organically complexed Cd.
Leachates
The computed speciation of the soil solution
Table 2. Results of replicate speciations of Cd in four samples by the IE method and the ED method
IE method
% of dissolved Cd
Replicate
Cd 2÷
Labile
complexes
CaCI 2 solution (10 -2 M)
A
B
45
49
54
51
0
0
1
0
99
100
I
0
Humic acid solution
A
B
44
45
48
45
8
10
0
0
51
52
49
48
Compost leachate
A
B
43
47
45
41
11
I1
1
1
77
78
23
22
Polluted groundwater
A
B
1
0
17
17
56
55
26
28
33
31
67
69
Sample
Slowly labile
complexes
ED method
% of dissolved Cd
Stable
complexes
MW < 1000
MW > 1000
Speciation of dissolved cadmium
0
10
I
20
30
40
50
60
70
80
90
100
.
IEa
I
,
¢
,
,
,
,
|
t
1
0
10
,
20
30
40
50
60
70
80
90
100
i
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
go
100
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
gO
100
80
gO
100
Eo'
.
EO
_iE
.
.
.
' "
-
~
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
807
indicated that the inorganic complexes amounted to
only 11% of the concentration of Cd ~+ and inorganically complexed Cd. The ED speciation showed
almost no high molecular weight complexes, while
the IE method showed 30% labile complexes
and identified the remaining part as Cd 2+. These
results indicate that the labile complexes consisted
of both inorganic and low molecular weight organic
complexes. During the dialysis, 50% of the dissolved
organic carbon permeated into the outer solution
(Table 3), confirming that low molecular weight
organic compounds were present in the soil solution.
The speciation results obtained for the compost
leachate showed that about half of the dissolved Cd
was present as free divalent Cd, tess than one tenth
as inorganic complexes, about one fourth as low
molecular weight organic complexes and one fourth
as high molecular weight organic complexes. However, the complexes were all categorized as labile or
slowly labile. The stable complex fraction amounted
to only 1%. The substantial organic complexation
was caused by only 13 mgC/l in the leachate.
The computed speciation of Cd in the polluted
groundwater, paying attention to only the inorganic
ligands, showed that the fraction of inorganic complexes was comparable to the fraction of free divalent
Cd. The computed ratio of these two fractions should
apply also to the real sample and indicates that the
inorganic complexes in the leachate polluted groundwater was as insignificant as the free divalent Cd
fraction, which by the IE method was quantified to
only 1%. The substantial Cd complexation observed
(99%) is, therefore, ascribed exclusively to organic
complexes, of which 70% was categorized as high
molecular weight complexes (ED) and about 25% as
stable complexes (IE).
The speciation results for industrial leachates 1 and
2 were very similar. The composition of the solutions
were very similar, but Cd was 10 times higher in
industrial leachate I than in industrial leachate 2.
However, from a theoretical point of view (see for
example Christensen et al., 1994), the ratio between
complexed and free divalent Cd is independent of the
metal concentration, when the ligand is much more
abundant than the metal. This relation is convincingly demonstrated by these two industrial leachates,
ED
80
90 100
% ot dissolved Cd
CAL [] free divalentions Q inorganiccomplexes
J
ED m low MW<1000 • high MW>1000
J
IE [ ] freedivalent
Im labile.
• slowly • stable [
ions
complexes labile
complexesJ
Table 3. Dissolved organic carbon concentration of the ED
inner solutions, initially and at the end of the experiment
(equilibrium)
DOC inner solution
(mgC/I)
I
complexes
I
Fig. 3. Cd speciation results for the samples characterized in
Table 1. CAL refers to GEOCHEM calculations, ED to the
equilibrium dialysis method and IE to the ion exchange
method, aaverage of two determinations.
Solution
Initially
Humic acid solution
Soil solution
Compost leachate
Polluted groundwater
Industrial leachate 1
Industrial leachate 2
Industrial leachate 3
25
64
13
264
79
34
3200
At equilibrium
23
32
7.5
80
7.3
6.1
104
808
Peter E. Holm et al.
Table 4. InterpretedCd speciesin 6 leachate samples(% of dissolvedCd)
Soil Compost Polluted Industrial Industrial Industrial
Cd species
solution leachate groundwater leachateI leachate2 leachate3
Free divalentCd
70
45
I
19
23a
33
InorganicCd complexes
9
6
1
64
87a
4
Organic Cd complexes
21
49
98
17
7a
63
Organic Cd complexesb
--labile
20
37
16
13
6
0
--slowly labile
0
11
55
3
0
63
--stable
1
1
27
I
1
0
Organic Cd complexes¢
--low molecularweight
19
26
30
7
0
40
--high molecularweight
2
23
68
10
7
23
•The sum of these percentagesexceeds 100% (117%), reflectingthe uncertaintyof this approach.
bCharacterized with respect to stability/redistributability.
CCharacterized with respect to size.
where chloride was the dominating ligand and present in similar concentrations in the two leachates.
Industrial leachate 3 was characterized by high
concentrations of Cd (660 gg/l) of which two thirds
was complexed by organic ligands. The dissolved
organic carbon content of this leachate was very high
(3200 mgC/1), but only 3% of this was characterized
as high molecular carbon (see Table 3). However,
apparently 23% of the dissolved Cd was associated
with this high molecular fraction. The IE method
categorized all complexes as slowly labile.
DISCUSSION
The results of the three speciation methods applied
to the selected environmental samples confirm the
need for methods to characterize dissolved Cd
species. The fraction of free divalent Cd varied from
1 to 70% and the complexes varied between predominantly inorganic and organic complexes. Furthermore the organic complexes varied substantially in
terms of size (smaller or larger than 1000 mol. wt) and
ability to redistribute upon changes in solution composition (labile, slowly labile, stable).
GEOCHEM speciation was performed to evaluate
the ratio between the inorganically complexed Cd
fraction and the fraction of free divalent Cd. This
ratio was applied to the IE results for identifying
the significance of the inorganic complexes in the
real sample, including the organic ligands. The
GEOCHEM calculations and the IE method are
useful together because they both identify the fraction
of free divalent CD, which is the basis for applying
the computer speciation results for the inorganic
solution composition to the real sample. Knowing
both the free and the inorganically complexed fraction of Cd in the sample, the remaining part must be
organically complexed. The organic complexes may
in addition be characterized in terms of high or low
molecular weight or as stable, slowly labile or labile.
This approach has been applied to the speciation
results obtained for the environmental samples and
the outcome is presented in Table 4.
The interpreted speciation results presented in
Table 4 reveal that inorganic complexes of Cd are
only significant in industrial leachate 1 and 2. In both
cases the major ligand is chloride originating from sea
water intrusion into the waste disposal site. Very few
other inorganic ligands are believed to be of environmental significance at acid to neutral pH values.
Table 4 also shows that organic complexes are
dominating in the compost leachate, the polluted
groundwater and the industrial leachate 3. The
characteristics of the organic complexes are, however,
different and no simple correlation between the concentration of dissolved organic carbon and the significanoe of organic complexes exists (refer to Table 3
with respect to DOC of the solutions). The organic
complexes seem to be fairly labile, or may easily
redistribute and lose the complexed Cd upon changes
in the solution composition. A substantial fraction of
stable Cd complexes was only identified in the polluted groundwater. In this context, a stable complex
is defined as a complex that does not lose its Cd
within 48 h exposure to an excessive amount of a
strong cation exchange resin. In an environmental
context, this does not indicate that this fraction of the
dissolved Cd is irreversibly bound to the organic
compound: the fraction only indicates that the complex is slowly reacting upon changes in the solution
composition, and that it may facilitate the transport
of Cd in terrestrial environments.
Acknowledgements--This project was supported financially
by the Norwegian Agricultural Research Council, Norsk
Hydro and the Norwegian Ministry of Agriculture and the
Technical University of Denmark. The technical assistance
of Jakob Futtrup and the review of the manuscript by
William S. Warner is gratefully acknowledged.
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