Download Sodium acetate method for determining CEC of cadmium

85
Sustain. Environ. Res., 22(2), 85-89 (2012)
Sodium acetate method for determining CEC of
cadmium-contaminated soil
Shu-Fen Cheng,1,* Chin-Yuan Huang2 and Min-Siou Lin1
1
Department of Environmental Engineering and Management
Chaoyang University of Technology
Taichung 41349, Taiwan
2
Department of Bioinformatics
Asia University
Taichung 41354, Taiwan
Key Words: Soil, cation exchange capacity, cadmium, remediation
ABSTRACT
Ion exchange is commonly adopted to measure the cation-exchange capacity (CEC) of soil and
to remediate soil that is contaminated by heavy metals. The sodium acetate-method is the most
common method for determining soil CEC in Taiwan. Based on ion exchange theory, Na+ is not an
ion with high exchange potential, which in fact raises a question regarding the effectiveness of the
sodium acetate method to determine the CEC of soils that are contaminated by heavy metals. This
investigation utilizes the chlorides and acetates of metal ions Na+, K+, NH4+, Mg+2, Ca+2, Al+3 and Fe+3
to extract cadmium from Cd-contaminated soils. The results indicate that sodium acetate extracts
only 55% of exchangeable Cd, whereas FeCl3 and AlCl3 extract 86 and 83% respectively. These
results indicate that a method in which FeCl3 is used to determine the Cd-contaminated soil CEC
outperforms the sodium acetate method. For contaminated soils with a large amount of Cd that are
bound to a fraction of Fe-Mn oxides, preliminary results suggest the use of AlCl3 and CaCl2 for
sequential washing remediation can provide a high washing efficiency and avoid soil acidification. .
INTRODUCTION
Cation-exchange capacity (CEC) is an important
factor in the characterization of contaminated soil and
the selection of remedial technology. CEC is the
amount of exchangeable cations that can be retained
by a given mass of soil [1]. Two common methods for
measuring CEC exist. In the first, the soil sample is
saturated with a selected cation solution; then, a second
selected cation solution is used to exchange and remove the first exchangeable cation from the soil.
Finally, the amount of the first selected cation that is
removed by the second solution is measured as the
CEC of the soil sample. In the second method, a particular species of cation is used to replace the cations
in the soil. The amounts of the exchanged cations,
including Ca, Mg, and K, are measured individually
.
[1-4].
Some of the chemical/physical properties of cations reveal its exchangeability. Highly charged cations
are retained more tightly than less strongly charged
cations; for example, Al+3 > Ca+2 > Na+. For a given
*Corresponding author
Email: [email protected]
positive charge, cations with a smaller hydration
radius are retained more tightly [1,5-7]. For example,
hydration radii of K+ = NH4+ (0.331 nm) < Na+ (0.358
nm); Ca+2 (0.412 nm) < Mg+2 (0.428 nm) [8]. Potassium
ions have a greater exchange potential than sodium
ions. Several factors influence the measured CEC,
including the nature of the cation that is being
exchanged, the ionic strength of the reagent solution,
the pH of that solution, the contact time, and the
soil:solution ratio [9,10]. The major components of
soil are clay mineral, organic matter, carbonate and
Fe/Mn oxides; the first two components dominate the
CEC [1,7]. Schnitzer and Hansen [11] ranked the
stability constants of metal-humic substance complexes as Fe+3 > Al+3 > Cu+2 > Ni+2 > Co+2 > Pb+2 > Ca +2 >
Zn+2 > Mn+2 > Mg+2. Yong [6] noted that a highly
charged cation has a great replacing capacity, which
follows the order Na+ < Li+ < K+ < Rb+ < Cs+2 < Mg+2
.
< Ca+2 < Ba+2 < Cu+2 < Al+3 < Fe+3 < Th+4.
Ammonium acetate and sodium acetate are two
common CEC test reagents, and recommended by the
Environmental Protection Administration of Taiwan
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Cheng et al., Sustain. Environ. Res., 22(2), 85-89 (2012)
[12,13]. According to ion exchange theory and the
results of Yong's investigation [6], Na+ and NH4+ can
exchange or replace a small proportion of the heavy
metals in soil samples. The present investigation aims
to evaluate the effectiveness of these two salts in
measuring the CEC of heavy metal-contaminated
soils. This investigation also evaluates the exchangeability of numerous extractants for remedying Cdcontaminated soils.
.
MATERIALS AND METHODS
Natural soil was impregnated with CdCl2 solution
to form the Cd-contaminated soil samples for testing.
Natural soil was collected from an uncontaminated site
in Dali, Taichung, Taiwan. It was passed through a 2
mm sieve to remove pebbles and impurities, and then
ground and homogenized to measure pH, particle size
distribution, CEC, amount of organic matter, total Cd
concentration, and Cd content in various forms. The
total amount of heavy metal was determined using the
aqua regia digestion method. A sample of 3 g of dried
soil was digested by slowly adding 21 mL concentrated
hydrochloric acid and 7 mL nitric acid. After mixing,
the sample was left undisturbed for 16 h at room
temperature, and then slowly heated to boiling for 2 h.
The amount of exchangeable, carbonate, Fe-Mn oxides
and organic fractions of Cd in the soil was determined
using the sequential extraction approach proposed by
Tessier et al. [14]. Finally, the residual fraction of Cd
was extracted using the aqua regia digestion method
[15]. Flame atomic absorption spectrometry (Perkin
Elmer, AAnalyst 400) was used to determine the heavy
.
metal concentration.
Preparation of Cd-contaminated soil: First, dissolve 65.2 mg CdCl2 in 1 L water; pour into 1 kg of
uncontaminated natural soil and mix well; stir once
every two days for 2 wk, and dry in air in a natural
environment. Second, place 100 g of this soil in a centrifuge tube; add 100 mL deionized water; shake at
200 rpm at room-temperature for 2 min; centrifuge for
10 min at 5,000 rpm until the Cd concentration in the
upper clear solution is less than 0.01 mg L-1 (repeat
rinsing five times); bake the soil sample; grind it
.
evenly, and pack it in a sealed bag for later use.
The aqueous reagent was utilized to prepare 12
extractants, which were sodium acetate (NaOAc),
NaCl, magnesium acetate (Mg(OAc)2), MgCl2, CaCl2,
calcium acetate (Ca(OAc)2), KCl, potassium acetate
(KOAc), ammonium acetate (NH4OAc), NH4Cl, AlCl3
and FeCl3. All these were in 1 M solution but their pH
values varied, as shown in Table 1. The procedures for
soil washing by single extraction and sequential extraction were following the sodium acetate method
[13] for determining CEC, a 4 g sample of Cdcontaminated soil in a centrifuge tube was mixed with
33 mL of extractant. After 5 min of shaking at 200
rpm, the mixture was centrifuged for 5 min, its super-
natant and extracted soil after dried at 105 °C were
then subjected to measure the Cd content. For a sequential extraction, the extraction took place in three
stages; each with an extractant to completion. A sample of 2 g of this dried extracted soil was analyzed by
the aqua regia digestion method and the sequential
extraction procedure to identify the total Cd content
and forms of bonding of Cd. The above procedures
were performed on three 4 g soil samples.
.
Table 1. Cd extraction efficiency
Extractant
NaOAc
NaCl
Mg(OAc)2
MgCl2
Ca(OAc)2
CaCl2
KOAc
KCl
NH4(OAc)
NH4Cl
AlCl3
FeCl3
Extractant pH
8.2
6.85
7.89
6.34
7.19
9.2
7.84
7.04
7.09
5.7
1.56
0.38
Extraction efficiency (%)
48.7 1.3
68.8 1.4
51.5 1.7
75.3 0.4
58.5 1.4
73.5 0.6
37.2 1.8
70.7 0.8
45.6 1.5
67.6 1.1
80.8 1.0
81.0 0.7
Cd in sample 37.6 mg kg-1; sample size n = 3.
RESULTS AND DISCUSSION
1. Properties of Soil
.
The soil was sandy loam. Its pH was 5.95; organic
matter content was 5.36%; CEC before contamination
was 14.0 cmol kg-1 and that after impregnation was
13.7 cmol kg-1. The Cd content of the Cd-contaminated
.
soil sample was 37.6 mg kg-1.
2. Capacity of Each Extractant to Extract Cd from
.
Cd-contaminated Soil
Table 1 presents data on the extraction of Cd using
12 salt extractants at 1 M concentration. For the same
metal ion, chloride outperforms acetate. For example,
NaCl extracted 69% of the total Cd from the soil, 20%
more than 49% that was extracted by NaOAc. MgCl2
extracted 75% of the Cd whereas Mg(OAc)2 extracted
52% , yielding a difference of 23%. The largest difference was between KCl (71%) and KOAc (37%) about
34%. Dermont et al. [16] reported that chloride salts
were effective in extracting cationic metals Pb+2 and
Cd+2, which react with chloride ions to form stable and
soluble metal chloro-complexes, such as CdCly2-y [1618]. Among the 12 extractants, FeCl3 and AlCl3 were
the most effective at about 81%. At pH < 2.2, the dissolution process replaces the ion exchange in metal
extraction. Hence, low pH of FeCl3 and AlCl3 extract-
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Cheng et al., Sustain. Environ. Res., 22(2), 85-89 (2012)
25
Original
FeCl3
AlCl3
CaCl2
MgCl2
KCl
NaCl
NH4Cl
20
mg kg-1
10
5
0
Exchangeable Carbonate Fe/Mn-oxide Organic
Residual
Cd fractionation
Fig. 1. Cd content in five soil fractions before and after
extraction using seven chloride extractants.
25
Original
Ca(CH3COO)2
Mg(CH3COO)2
K(CH3COO)
Na(CH3COO)
NH4(CH3COO)
15
mg kg-1
ant solutions may cause dissolution of soil substrate
and release of Cd, and result in higher extraction efficiency [17]. To prevent the ionic concentration of the
extractant from having any effect, pH was not adjusted.
As indicated in Table 1, the pH values of chlorides,
ammonium chloride was lower than MgCl2, NaCl, KCl
and CaCl2 with the Cd extraction efficiency of NH4Cl
the poorest, indicating that pH is unlikely to be responsible for the poor performance of Cd extraction.
Except FeCl3 and AlCl3, the pH and Cd extraction of
other 10 extractants show no discernable consistent
.
relation in extraction efficiency.
To elucidate the differences in Cd extraction using
various extractants, Cd in soil was fractionated into
five fractions
exchangeable, carbonate, Fe-Mn
oxides, organic and residual for sequential extraction.
In Cd-contaminated soil, the exchangeable fraction
dominated at 61.5%; the carbonate and Fe-Mn oxides
fractions represented 16% each; the organic fraction
represented 2.3% and the residual fraction represented
3.6%. Organic matter was the main contributor to
CEC; but in this investigation, the organic bonding
fraction represented only 2.3%. Most of the Cd that
was bound to organic matter was plausibly extracted in
the exchangeable fraction. Figures 1 and 2 present the
distributions of each fraction of Cd in soil before and
after extraction using seven chloride and five acetate
extractants. In the exchangeable fraction, chlorides
outperformed acetates. Among chlorides, FeCl3 is the
best, with an extraction efficiency of around 86%,
followed by AlCl3 at 83%, CaCl2 82% and NH4Cl 67%,
which was the lowest of all extraction efficiencies. The
capacities of chlorides to extract cadmium in the
exchangeable fraction follow the order Fe+3 > Al+3 >
Ca+2 > Mg+2 > K+ > Na+ > NH4+, revealing a positive
relationship between extraction capacity and strength
of charge. For cations of a given charge, a smaller
hydration radius corresponds to better extraction capacity, as in the pairs Ca+2 > Mg+2 and K+ > Na+. The
efficiencies of sodium acetate and ammonium acetate
in extracting cadmium in the exchangeable fraction
were only 55 and 46%, respectively. According to data
on the total Cd extraction efficiency in Table 1 and the
data on fractionated Cd extraction in Figs. 1 and 2,
NaOAc and NH4OAc extractants are clearly outperformed by the other extractants, of which the most
effective was FeCl3. Historically, NaOAc and NH4OAc
methods were developed for use in agricultural
science; the results herein cast doubt on their applicability to Cd-contaminated soil. The results also indicate
that FeCl3 is the most efficient extractant among those
studied for determining the CEC of soils that are contaminated with heavy metal Cd. These results strongly
support further studies to identify efficient extractants
and operating conditions, and establishing a method
for determining CEC that is suitable for soil that is
contaminated with heavy metals.
.
10
5
0
Exchangeable Carbonate Fe/Mn-oxide Organic
Residual
Cd fractionation
Fig. 2. Cd content in five soil fractions before and after
extraction using five acetate extractants.
3. Efficiencies of Washing out Cd
.
Soil washing is a commonly used method for
remediating heavy metal-contaminated soils. It uses an
acid solution or chelating reagent as the washing agent.
Acid washing destroys the properties of the soil;
EDTA washing causes the accumulation of EDTA,
which degrades poorly in soil, and metal leaching
threatens human and animal health. Recently, many
have suggested using nontoxic salts with high ion
exchange capacity, such as Ca+2, Mg+2 and Fe+3, as
washing agents for remediating cultivated land [17,1920]. This investigation also evaluated the feasibility
of using these 12 extractants to remediate Cdcontaminated soils. The results indicated that chlorides
were more effective than acetates in washing; among
the chlorides, FeCl3 was the most effective, and the
washing efficiencies of chlorides followed the order
Fe+3 > Al+3 > Mg+2 > Ca+2 > K+ > Na+ > NH4+. This
pattern also held true for the efficiencies of removal
of the exchangeable Cd, with the exception Ca+2 >
Mg+2, which was consistent with the relative exchange
potentials of Ca+2 and Mg+2. The leaching of carbonate-
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Cheng et al., Sustain. Environ. Res., 22(2), 85-89 (2012)
bound fraction of Cd was strongly sensitive to pH [14].
These results reveal that, for a given valence of the
extracting ions, a lower-pH extractant had a higher
efficiency; accordingly, Mg+2 is better than Ca+2 and
NH4+ is better than Na+ which is better than K+. This
pattern is also observed in extracting the Fe-Mn
oxides-bound fraction of Cd, with the exception of
FeCl3, which has the lowest pH but not the capacity of
AlCl3. Currently, agricultural experts suggest FeCl3
washing for Cd-contaminated agricultural soil [20].
This investigation found that AlCl3 is similarly or
slightly less effective than FeCl3 in Cd extraction, but
the former has some advantages in soil remediation.
First, at a given concentration, the pH of AlCl3 extractant is not as low as that of FeCl3, so the former
acidifies the soil less. The original pH of this study soil
is 5.95; after three extractions, its pH is 2.38 when
AlCl3 is used, 0.97 when FeCl3 is used, and 7.33 when
CaCl2 is used (Table 2). Second, AlCl3 is more efficient
than FeCl3 in the remediation of the Fe-Mn oxidesbound fraction of Cd. To prevent soil acidification,
AlCl3 and CaCl2 can be used in that order for washing. .
Table 2. Soil pH and Cd removal efficiency after
sequential extraction
Soil pH after
Cd removal
extraction
efficiency (%)
First/Second/Third First Second Third
AlCl3/AlCl3/AlCl3
97.1 0.2
2.47
2.43
2.38
FeCl3/FeCl3/FeCl3
97.9 0.4
1.08
1.02
0.97
CaCl2/CaCl2/CaCl2 6.56
88.8 0.7
6.88
7.33
AlCl3/CaCl2/CaCl2 2.47
95.6 0.2
3.33
4.08
FeCl3/CaCl2/CaCl2 1.08
97.1 0.1
1.85
2.53
pH of original soil is 5.95; extractant concentration: 1 M
Extract sequence
CONCLUSIONS
This investigation reveals that sodium acetate and
ammonium acetate cannot effectively extract an
exchangeable fraction of cadmium, in which property
is closely related to the CEC. A new effective method
must be devised to determine CEC, since the sodium
acetate and ammonium acetate methods significantly
under-estimate its value. FeCl3 is the most efficient
extractant identified for measuring the CEC of contaminated soils. In the remediation of Cd-contaminated
soils, when the Cd content in the Fe-Mn oxides-bound
fraction is high, sequential application of AlCl3 and
CaCl2 is favored to achieve high efficiency and prevent
soil acidification.
.
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
.
Manuscript Received: July 19, 2011
Revision Received: October 21, 2011
and Accepted: November 21, 2011