electrokinetic study on copper contaminated soils
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Electrokinetic study on coppercontaminated soilsJy‐Gau Sah a & L. Yu Lin b
a Department of Environmental Science and Engineering ,National Pingtung University of Science and Technology ,Pingtung, Taiwan E-mail:b Department of Civil and Environmental Engineering , ChristianBrothers University , Memphis, Tennessee, U.S.A.Published online: 15 Dec 2008.
To cite this article: Jy‐Gau Sah & L. Yu Lin (2000) Electrokinetic study on copper contaminatedsoils, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances andEnvironmental Engineering, 35:7, 1117-1139, DOI: 10.1080/10934520009377023
To link to this article: http://dx.doi.org/10.1080/10934520009377023
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J. ENVIRON. SCI. HEALTH, A35(7), 1117-1139 (2000)
ELECTROKINETIC STUDY ON COPPER CONTAMINATED SOILS
Key Words: Electrokinetic technology, soil contamination, heavy metal,
copper, pollutant transport
Jy-Gau Sah1,* and L. Yu Lin 2
1Department of Environmental Science and Engineering,National Pingtung University of Science and Technology,
Pingtung, Taiwan2Department of Civil and Environmental Engineering
Christian Brothers UniversityMemphis, Tennessee, U.S.A.
ABSTRACT
Electrokinetic technology was conducted on three copper contaminated
soils to investigate the potential use of this technology for soil remediation.
Several variables, such as adsorption capacity of the soils, fractions of copper
in the soils, reaction time, pH and injection of conducting solutions into the
soils that may affect the removal efficiency of electrokinetic process were
studied. The results showed that the electrokinetic process has the potential to
remove carbonate and Fe-Mn oxides' copper in contaminated soils, which
accounts for 70-85% of copper in the soils. With 8 volts of electrification for
30 days, the highest removal efficiency was found in an acidic clay soil mixed
with 0.1 N of HC1 conducting solution. This study suggests that the higher
adsorption capacity and the lower saturated basic soils produce lower removal
* Corresponding author: e-mail: [email protected]
1117
Copyright © 2000 by Marcel Dekker, Inc. www.dekker.com
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1118 SAH AND LIN
efficiency. In order to increase removal efficiency, injection of strong acid into
the specimen seems to be a promising solution. Strong acid solutions prevent
the precipitation of copper hydroxide in the specimen and support the ion
desorbed from the soils, which result in increasing the removal efficiency by
40%.
INTRODUCTION
Improper disposal of wastewater and hazardous materials has seriously
contaminated Taiwan's soil in recent years. According to an Environmental
Information Study in the Taiwan area (Taiwan EPA, 1995), more than 50,000
hectare of farmland have been contaminated to the higher contamination level
and more than 790 hectare of farmland to the highest contamination level by
heavy metals. These contaminated lands represent 5.63% and 0.90%,
respectively, of the farmlands in the total survey area. This study reports that
Taiwan's soil environment has been gradually declining. In order to clean up
heavy metals from contaminated soils, recent attention has focused on the
development of more cost-effective techniques both in situ and off site
treatment.
Most heavy metals in soils are in salt forms that can be easily uptaken by
the organisms. The movement of the organisms causes the secondary pollution
in the soils (Haan et al., 1976). When heavy metals are presented in ionic
forms, they are attracted to the soils by the negative static electrical forces of
the soil colloids. The attraction of metal ions to the soil depends primarily on
soil electronegativity and the dissociation energy of the ions. Soil colloids are
also selective in attracting different metals. One metal species that is difficult
to be adsorbed can be substituted for another metal ions that are more easily
adsorbed to the soil colloids' surface (Swift and McLaren, 1991). In general,
cation with a higher charge is more easily attracted to soils than soils with a
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COPPER CONTAMINATED SOILS 1119
lower cation; i.e. Pb is more easier adsorbed by soil than Cd (Baker et al.,
1990). Under alkaline conditions, most heavy metal ions in soils may become
hydroxides (Cu(OH)2, Pb(OH)2) or carbonates (CuCO3, PbCO3) complexes,
which will become precipitation and will deposit into soils.
In recent years, there has been a growing interest in using low-level direct
current (DC) in metal removal from the soils. Although these processes,
including electrokinetic, electroreclamation, electrophoresis, electrolysis have
different operations, the fundamental concepts are very similar. Electrokinetic
process is a controlled application of electrical migration and electroosmosis
with the electrolysis reactions at the electrodes (Lageman, 1993; Khan and
Alam, 1994; Reed, 1995; Acar and Alshawabkeh, 1996). When low DC
current is applied to the porous medium, the electric current leads to
electrolysis reactions at the electrodes, which generate an acidic medium at the
anode and an alkaline medium at the cathode. The H+ generated at the anode
advances through the soil toward the cathode by ion migration, pore fluid
flow, pore fluid advection, and diffusion. On the other hand, reduction
reaction at the cathode area dissociates water to form H2 and OH" during
electrolytic dissociation. Consequently, the pH value near the cathode
increases. The H+ and OH" ions generated from the electrolytic dissociation
are moved across the pore fluid within soil particles toward either the anode or
the cathode (Reed, 1995). A situation in which the attraction of the soil to H+
exceeds that of heavy metal ions causing the exchange of metal ions from the
diffuse electrical double layer on soil' surface to the solution. These metal ions
move to the cathode through migrational flux, electroosmosis flux or diffusive
mass flux (Acar et al., 1993). As a result, metals are deposited at the cathode
and anions at the anode.
Both soil pH and electrolysis reactions at the electrodes play a critical
role in the electrokinetic process. The increase of pH from the anode to the
cathode is a consequence of the advancement of the acid front by migration,
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1120 SAH AND LIN
diffusion and advection. The acid front is generated at the anode across the
soil specimen and neutralizes the base generated at the cathode. Acar et al.
(1990) hypothesized that the movement of the acid front would cause
desorption of cations from the soil surfaces and would facilitate their release
into the pore fluid. This reaction associated with the concurrent electro-
osmostic flow reinforces metal removal from the soils. They suggested that
there is a need to use different enhancement techniques to overcome
precipitation in electrokinetic process. The use of a weak organic acid such as
acetic acid to neutralize and depolarize the hydroxyl ions generated can
alleviate the precipitation problem as the metals transport to the cathode
compartments. Other potential techniques such as an increase in water content
in soil and injection of species at or between the electrodes have been
recommended. In addition, the movement of the acid front in the soils directly
causes desorption of cations from soil surface. This reaction, associated with
the concurrent electro-osmotic flow, enforces removal of metal from
contaminated soils. Hamed (1991) and Gray (1970) suggested that
electroosmotic flow be promoted at a high water content's soil. From these
references, it is worthwhile to investigate each enhancement technique in
order to improve the performance of the electrokinetic process in soil
remediation.
Electrokinetic technique was initially applied to soil dehydration or slop
stabilization. It has also been utilized to remediate contaminated soil in situ
and off site. This technique has been applied in some countries and
demonstrated to economically treat a wide range of contaminants (Trombly,
1994). European and American research institutes have begun to
collaboratively develop this soil remediation technique (Lageman, 1993; Khan
and Alam, 1994). Pilot studies have shown that inorganics, such as arsenic,
cadmium, chromium, copper, iron, lead, mercury, nickel and zinc can be
removed from soils using electrokinetic process (Pamukcu et al 1991;
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COPPER CONTAMINATED SOILS 1121
Banerjee et al. 1990; Acar et al. 1993; Runnels and Wahli 1993; Probstein,
1994; Acar and Alshawabkeh 1996; Sah and Chen 1998). This technique has
also shown the potential removal of organic species, such as benzene, toluene,
phenol, acetic acid, hexachlorobutadiene and hexachlorobenzene (Shapiro et
al. 1989; Segal and Bruell 1992; Shapiro and Probstein, 1993). However, most
former electrokinetic studies showed pollutants were removed from synthetic
soil materials in laboratory scale. A few researchers have investigated real
soils and potential variables that may affect the electrokinetic process. Sah and
Chen (1998) found that Cd and Pb can be transported to cathode in spiked acid
soils. In Taiwan, effective and complete implementation of electrokinetic
technology to remediate pollutants in a contaminated site has not yet been
established.
This study attempts to contribute the application of this technique on Cu
removal from real soil medium. The objectives of this study are to: (1)
investigate the potential use of electrokinetic process for Cu removal from
three contaminated soils; (2) examine several variables that may affect the
removal efficiency in electrokinetic process, including adsorption capacity of
the soils, fractions of copper in the soils and reaction time; and (3) study the
enhancement of the electrokinetic process by injection of strong acids into the
specimen field.
MATERIALS AND METHODS
Three soils collected from Pingtung County and Kaohsung County in
Taiwan were used in this study. The samples were air-dried, coarsely
grounded and sieved with a 20-mesh sieve. Soil' pH, cation exchange
capacity, organic contents, conductivity, and copper concentration were
measured. Following the standard procedure, 500mg Cu/kg soil of Cu-spiked
samples was prepared in the laboratory. Copper solutions obtained from
MERCK'S standard stock solution (CuSO4- 5H2O) were diluted with distilled
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1122 SAH AND UN
water to 250mg/l. 200 ml of diluted Cu stock solution was then spiked into
each 100g of soil samples. The resulting slurry was constantly stirred to allow
the Cu stock solution to balance within the soil samples. The slurry was air-
dried for at least 7 days in order to have an even distribution of Cu in the
entire sample. To ensure that the spiked concentration remained in the
samples, the total Cu concentrations were measured again prior to replacing
the samples into the testing module.
The fractions of Cu in the soils were tested by the sequential chemical
extraction procedure (Tessire, 1982). This method provides not only a good
identification of metal fraction in soils, but also a logical estimation of metal
mobility in soils. According to this procedure, the fractions of Cu can be
classified into five portions: exchangeable, carbonate, Fe-Mn oxides, organic,
and residual fraction. For this study, soil samples were placed in a tube with
MgCl2 (pH=7.0). After shaken and separated, the extracts were of
exchangeable forms. The remained portion were added NaOAc (pH=5). After
shaken and centrifuged, the extracts contained carbonate forms. A further
process of adding NH2OH-HC1 (in 25% HOAC) into the sample that was
used to determine Fe-Mn oxide forms. The samples were then heated with
HN03, H2O2, and NH4OAC (in HN03) to find the organic forms. Finally, the
residual forms were obtained by using HF and HCIO4 digestion procedure.
It is clear that pH in soils and the reaction time affect the removal
efficiency of metal in eletrokinetic process. Reaction time can be simply
defined as the time required for metal to migrate from the porous medium to
the cathode. According to previous study, the soil pH and the metal species
itself directly affected the length of reaction time in the electrokinetic process
(Sah and Chen, 1998); e.g., Cd in acidic soil can be completely removed to the
cathode in 7 days. Due to the dimensions of the testing modules being
different, the considered reaction time was also different (Sah and Lin, 1999).
In this study, the reaction time ranging from 5 to 30 days was used. At each
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COPPER CONTAMINATED SOILS 1123
five-day interval, soil samples were collected from the batch module and
tested for the remaining copper concentrations in the soils.
Despite the pH and the reaction time, metal adsorbed to the soil surface
would decrease the removal efficiency in electrokinetic process. To
understand the adsorption capacity of the three soils, the adsorption isotherm
was conducted through a batch study. Following the experiment procedures
suggested by Yousef and Lin (1992), Lin et al. (1994) and Liu et al. (1995),
200 mg/1 to 8,000 mg/1 of Cu stock solutions were spiked into 100 grams of
soil samples. With a rotation speed of 200 rmp, the supernatant was collected
in each 4-hour period. The samples were filtered and acidified to pH less than
2.0. Finally, aqua regia digestion procedure was used to analyze copper
concentrations in the solutions.
A small acrylic cell was assembled and used in the batch experiments.
The unit was made from plastic, which can prevent corrosion and resist
compact pressure from the soils. The cell is composed of one main
compartment 18cm in length, 5cm in width, and 3 cm in depth (in Figure 1). A
pair of electrodes with a low direct current (DC) provides 8 volts power
strength and a constant current strength. To avert corrosion between the
electric wire and graphite rod, the junctions in both cells were covered with
water-resistant glue. For this study, sampling locations are located at A (0-3.6
cm), B (3.6-7.2 cm), C (7.2-10.8 cm), D (10.8-14.4 cm), and E (14.4-18.0 cm).
After each reaction time was expired, several grams of soil samples were
collected from each sample location. Copper concentrations in the soil were
tested by aqua regia digestion procedure.
The enhancement of electrokinetic process was conducted by placing soil
samples into the reaction module separately. In each soil, 100 ml of distilled
water, 0.1 N HC1, or 0.01N HC1 was added to the soils to increase water
content and pH in the soils. The top cells were covered with plastic wrap to
prevent water vapor and to sustain temperature as a constant. A constant 8
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1124 SAH AND LIN
Anode
B D
Cathode
w 3cm
J -J-L5 cm
18cm
DC Power8V
FIGURE 1Batch pilot module of electrokinetic technique
volts DC was maintained throughout all of the experiments. The current and
the pH values were monitored at a fixed time interval. When the prescribed
time period had expired, acrylic separators were used to collect soil samples
from each sample location. Soil samples were then oven-dried to determine
the remaining copper concentrations.
RESULTS AND DISCUSSION
The soil characteristics of three soil samples collected from contaminated
sites in the Southern portion of Taiwan are summarized in Table 1. From the
soil classification, Soil 1, Soil 2, and Soil 3 can be classified as acidic clay,
alkaline silt clay loam and neutral silty clay. Because Soil 1 and Soil 3 were
both collected from Pingtung County, the texture of these soils is Diluvium
Red Soil and the other is Slate Noncalcareous Older Alluvial Soil. With a
different geological formation, Soil 2 obtained from Kaohsung County is
Mudstone Lithosol Soil, i.e. a silt clay soil. Among these three contaminated
soils, Soil 1 has the highest cation exchange capacity and the lowest organic
matters. Soil 2 has the highest pH and conductivity. The higher pH value
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COPPER CONTAMINATED SOILS 1125
TABLE 1Summary of Soil Characteristics
Material Tested
Soil Type
PH
Cation ExchangeCapacity (CEC)
(cmol(+)/kg)
Organic Content(%)
Cu Concentration(mg/kg)
Conductivity(mmhos/cm)
SoilClassification
CopperConcentration in
Soil(mg/kg)
Soill
Diluvium Red Soil
4.6
3.2
0.77
27
97
Clay
490
Soil 2
Mudstone LithosolSoil
8.1
3.0
1.5
23
11,000
Silt Clay
530
Soil 3
SlateMoncalacreousOlder Alluvial
Soil6.4
2.1
2.2
29
530
Silt Clay Loam
520
References ofTesting Methods
.
Taiwan EPA,(1997)
Taiwan EPA,(1997)
Nelson et al.(1982)
Taiwan EPA,(1997)
Rhoades,(1982)
Gee,(1986)
Taiwan EPA,(1997)
found in Soil 2 implies that the hydroxyl in the soil can neutralize the acid
front in the specimen during the eletrokinetic process, which could reduce the
removal efficiency of metal from the soils in the electrokinetic process.
Different stock metal solutions spiked into the same soil result in
different distributions of metal in the soils. Because the large portion of
MERCK's stock solution was spiked into the contaminated soil samples, two
largest forms of Cu found in the soils are the carbonate and the Fe-Mn oxides'
copper (in Figure 2). Despite the small variations of the metal distribution, the
dominant contaminant fractions are very consistent. Overall, more than 70%
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1126 SAH AND LIN
soil 3
D Exchangeable
• Carbonate
El Fe-Mn oxides
S Organic
E3 Residual
FIGURE 2Fractional distribution of Cu in the soils.
of carbonate and Fe-Mn fractions of Cu were detected in the soils. Among
these three soils, the majority of Cu is in the Fe-Mn forms, followed by the
carbonate, the exchangeable, the organic and the residual forms. With an
average, the soils contain 44% of carbonate fractions, 43% of the Fe-Mn
fractions, 8% of the exchangeable fractions, 3% of the organic fractions, and
2% of the residual fractions. No exchangeable fractions were found in Soil 2
because the exchangeable fractions only exist in acidic and neutral soils. As
mentioned earlier, the original fraction distributions of Cu have been changed
because the larger amount of Cu in the Fe-Mn oxides and the carbonate form
was attributed from the spiked stock solution. The intention of this sequential
chemical extraction study is not to determine the Cu removal from the spiked
solution, but to estimate the removal efficiency from the copper fractions in
electrokinetic process. According to the sequential extraction analysis, the
preliminary assumption suggests that the electrokinetic process has the
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COPPER CONTAMINATED SOILS
300
1127
200
&100
0
/ Soü3:y =
/ t.t C * % t
• ^ ^ — " '^^SOUL 2: v = 74LnM - 407
R2 = 0.9524
8.75Ln(x)-3Z0
= 0.9065 fim11-y-3fi7Fi<x).13.7
R2-0.9126
• Soill
• Soil 2
* Soil 3
0 2,000 4,000 6,000 8,000 10,000
Cu Concentration in Solution (mg/kg)
FIGURE 3Langmuir adsorption isotherms of three studied soils.
potential to remove more than 70% of Cu from contaminated soils. Although
the organic and the residual Cu are not as easy to remove in the electrokinetic
process, long-term operation of the electrokinetic process could change metal
fractions. As a result, the removal efficiency would improve. If the
exchangeable forms of Cu are included, more Cu ions are expected to be
removed. It is interesting to know how these processes operate and how the
physical and chemical variables will improve the removal efficiency in the
electrokinetic process.
Among the adsorption isotherms, the Langmuir sorption isotherm is the
best-fit isotherm to the data (in Figure 3). The Cu adsorption capacity of Soil
1, Soil 2 and Soil 3 is 13 mmole/kg, 250 mmole/kg, and 32 mmole/kg,
respectively. It indicated that Soil 2 has the highest adsorption capacity among
the studied soils. Metal retention by natural soils is highly dependent on the
strong pH and cation exchange capacity. In a study of metal adsorption by two
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1128 SAH AND LIN
ferruginous subsoils, Elliott et al. (1986) found that the heavy metal adsorption
became greater as the pH increased, particularly when the pH was above 4.5.
From Table 1, all soil samples have a pH above 4.5, indicating that the higher
dependence of pH and adsorption is most likely shown in these soils. Because
Soil 2 has the strongest pH value and moderate cation exchange capacity, its
adsorption capacity is the highest among these investigated soils.
Inconsistently, the cation exchange capacity and the soil organic matters
apparently had little correlation with adsorption in this study. Several similar
findings had been reported. Anderson and Christensen (1988) conducted batch
adsorption for low concentrations of Cd, Co, Ni, and Zn in thirty-eight soils.
Statistical results indicated that metal sorption onto the soils was influenced by
the presence of clays, pH and hydrous oxides of Fe and Mn. The high
correlation between metal adsorption and CEC only appeared in clay soil with
a higher concentration of Fe and Mn hydroxides. The soil organic matter also
showed little influence on the distribution of metals onto the soils. Among the
investigated parameters, organic matter had the least influence on metal
adsorption. Other studies (Christensen, 1987, Lin et al., 1994) also found a
small reduction in the adsorption capacity followed by removed organic matter
from the soils. These studies suggested that inorganic colloids, such as clay,
contribute more to the metal adsorption than organic matters.
Electric current conductivity is inversely related to the resistance of
current flow. The resistance of current flow in the soil comes from soil texture,
pore sizes, tortuosity in the porous medium, and variations in pore-fluid and
double layer electrolyte concentrations. The higher conductivity, the lower
resistance of current flow is presented. Among three soils, Soil 2 has the
highest conductivity, so that it has the higher potential to permit current flow
transported through the soil specimen. However, it is not necessary to predict
that Soil 2 can achieve the highest Cu removal in these three soils. Lageman
(1989) processed metal removal from soils using electro-reclamation. In order
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COPPER CONTAMINATED SOILS 1129
to achieve a high percentage of removal in a short period of time, he used a
very high current density to treat the soils with the soil having conductivity
ranging from 2,000 to 3,000 lasiemens/cm. The result found that the higher
current density with the high range of conductivity in electro-reclamation
process caused higher resistance and higher cost. Hamed (1991) studied Pb
removal from low conductivity soils. In his study, Hamed suggested that the
removal process is not necessarily made more efficient by increasing current
densities. With high levels of conductivity in soils, it needs a lower current
density to achieve a high percentage of removal. From these previous
investigations, it is difficult to predict the potential efficiency of metal removal
from soils based only on the soil conductivity or the used current density.
However, the control of current density can ensure success in the
electrokinetic process, as well as improve in the efficiency of metal removal.
Since higher conductivity may require higher current density, three levels of
current density were employed in this study. Figure 4 shows changes of
electric current density during the course of this study. Similar trends had been
observed among three soils, except for the first few days in Soil 2. Soil 2 had a
slightly different pattern because Soil 2 had the highest conductivity and
highest Na+ contents. In the first few days, the increase in the current density
comes from the Na+ ions increase ion migration in the specimen. All electric
current densities began with a high value, then dropped slowly. The reasons
for the current density to drop are as follows: (1) activation polarization:
visible gaseous bubbles, such as O2 and H2, were formed around the electrodes
during the electrokinetic process. The bubble is a good insulator to reduce
electrical conductivity. As a result, the current density is reduced; (2)
resistance polarization: After the electrokinetic process, a white layer was
developed on the cathode surface. This thin film consists of insoluble salts and
other impurities, which would attract the cathode and inhibit conductivity in
the specimens; and (3) concentration polarization: The H+ generated at the
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1130 SAH AND LIN
2.0
1.6
1.2
Q 0.8
0.4
0.0
.m. m
10 15 20 25Time (days)
30
FIGURE 4Distribution of electric current density in electrokinetic process.
anode is attracted to the cathode, and the OH" generated at the cathode is
attracted to the anode. When the acid and alkaline cannot be quickly
neutralized, the current density is then reduced.
Because of the electrolysis reaction, H* is produced by the oxidation
reaction at the anode and OH" is produced by reduction reaction at the anode.
pH increases from the anode to the cathode that directly reflects the
migrational flux of H+in the specimen. Table 2 compares the pH-distributions
of three soils across the specimens. In general, the pH gradient increases in the
specimen from acid range in Zone A to the base range in Zone E. When
cations and anions move to the opposite electrode in the specimen, they met
midway and became insoluble salts that cause a significant pH change. During
this study, most insoluble salts formed in copper hydroxides appear near or at
Zone C, except Soil 1. Soil 1 is an acidic soil and OH" cannot exist in the
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COPPER CONTAMINATED SOILS 1131
TABLE 2pH Distribution of Three Contaminated Soils across the Specimen
Distance Soil 1 Soil 2 Soil 3
5d lOd 20d 30d ^ 5d lOd 20d 30d ^ 5d lOd 20d 30d *"£pH pH. pH
1.4 1.6 1.3 1.4 2.0 1.9 2.0 2.0
4.1 7.3 2.8 3.9 3.9 2.6 2.3 2.3
} 12.012.112.611.7 7.8 6.5 5.5 4.2 3.0 5.7
12.7 12.6 12.8 12.3 6.9 11.0 6.7 6.9
13.2 13.1 12.9 12.8 11.6 11.6 11.6 11.4
0-3.6
3.6-7.2
7.2-10.8
10.8-14.4
14.4-18
3.3
4.3
4.7
4.7
5.2
3.1
4.4
4.7
4.7
5.1
3.3
4.6
4.7
4.8
5.6
3.1
3.8
4.6
5.4
9.2
specimen that makes Cu hydroxides only appear at cathode or at Zone E. Soil
2 has the highest Na+ contents, pH and current conductivity. After 30 days of
electroreaction, the pH in Soil 2 dropped from the initial 7.8 to 1.4 near the
anode and 12.8 at the cathode. This further explains that the removal
efficiency in Soil 2 is less than Soil 1 and Soil 3.
Under 8 volts, the batch-pilot module was first saturated with distilled
water. Cu horizontally moved across the module from the anode to the cathode
by electrification. The accumulated Cu concentration (C) in the specimens as a
percentage of the initial concentration (Co) (or the relative Cu concentration)
was plotted in Figure 5. The results indicated that a small amount of
contaminant be transported from the anode to the cathode in the first 5 days.
As the reaction time increases, the removal efficiency also increases. This
suggested that electrokinetic process is a time-correlated process. In other
words, it is insufficient to operate the electrokinetic process in a short
duration. In Soil 1, the relative Cu concentrations increase from 27% in Zone
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1132 SAH AND LIN
300
S?
ô200
Ô 100
h j i L . j i. j]10 20 30soill
10 20 30 5soil 2
Reaction Time (days)
10 20 30soil 3
OA,0-3.6cm from anode
• B,3-6-7.2cm from anode
• C,7.2-10.8cm from anode
HD,10.8-14.4cm from anode
HE,14.4-18cm from anode
FIGURE 5Relative Cu concentration (C/Co) of three soils after electrification.
A (anode) to 200% in Zone E (cathode) within 30 days. A similar result was
found in Soil 3. A strong elongation of the electrification improved the
transportation efficiency in Soil 3. It resulted in a 280% relative removal
percentage in Zone D after 30 days. In Soil 2, Cu concentrations dropped from
150 % in zone A to 30 % in zone B. Because of the polarization in Soil 2 and
high iron hydroxide contained in Zone B, Cu cannot more any further to the
cathode. Therefore, the removal efficiency of Cu in Soil 2 appears to be lower
than that in Soil 1 and Soil 3.
After 10 days of operation, no more copper ions in Soil 2 migrated to the
cathode. Although Soil 2 has the most abundance of carbonate and Fe-Mn
oxides' coppers, there is a high ion affinity between copper and soils bounds
copper ions in the soil. Despite this, there are two reasons for the low
percentage of copper removed from Soil 2. These are: (1) Soil 2 is a basic soil
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COPPER CONTAMINATED SOILS 1133
that can hinder the acid front moving through the specimen. At 10 days, the
pH at Zone B was 7.3, indicating that the acid front be neutralized. The
majority of the copper oxide has been precipitated in Zone B. As the
electrification continuously operated in the specimen, no significant H + and
copper ions can migrate to the cathode; and (2) the use of high conductivity
and high current density produced the final removal in a short period of time.
As compared to the current density applied in Soil 1 and Soil 3, more than six
folders of current density were used in Soil 2 during the electrokinetic process.
The result showed that most copper ions quickly move to Zone B within 5 to
10 days. Although the electrification continuously processes, no copper ions
are able to migrate further in the specimen. While Soil 2 had the lowest
removal efficiency, the operation cost was the highest among these three soils.
When high current density (0.8 mA/cm 2) was used in an electrokinetic study,
Lageman (1989) found that the operation expenses were increased to 288
kWh/m3 in eight weeks of operation. It is very clear that high conductivity and
high current density used in the electrokinetic process does not secure removal
efficiency, but increase on the operation cost.
The enhancement of the electrokinetic process was conducted to decrease
the soil pH and improve the degree of saturation in the soils by adding 100 ml
of distilled water, 0.1 N HC1 or 0.0 IN HC1 into each soil. Figure 6 presents the
relative concentrations in the three soils. Regardless of what solutions were
used, the Soil 2 sample still has the lowest copper removal in all tested soils.
After 30 days, high moisture contents in Soil 2 did not favor the electrokinetic
process. As compared to Figure 5 and Figure 6, only a slight increase of
relative concentration was detected in all soils, which were soaked in 100 ml
of distilled water solution. Since Gray (1970) suggested that the
electroosmotic flow can be promoted by a higher water content, there are no
other data to support the benefit of soil saturation in the electrokinetic process.
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1134 SAH AND LIN
o
cuu
sUu
450
400
350
300
250
200
150
100
50
0
r i
jjj â •
= !I S
1; ,ILrj
•mm
IIIIIIIII
" "
s r : !
: J! !;
D A,0-3.6cm from anode
• C.7.2-10.8cm from anode
BD,10.8-14.4cm from anode
. HE,14.4-18cm from anode
0.1NHC1 001NHC3 H2O 0.1NHO
soil 1M2O 0INHQ 0 01NHQ H2O
soil 2 SOU 3
Soil Type
FIGURE 6Relative Cu concentration of three soils after injection of conducting
solutions.
In this study, the assumption of increasing saturation used to enhance the
electrokinetic process was a good strategy, but not the best.
The best solutions came from adding an acid conducting solution into the
specimens. The increases in H+ concentrations across the specimens during the
electrolytic reactions support metal removal from contaminated soils was
suggested by Hamed (1991) and Acar et al. (1995). However, injection of
strong acid solutions in the specimen during electrification has not yet been
studied. Theoretically, high concentrations of H+ can be used in neutralizing
soil's OH" and in increasing the desorption of Cu"1"1" from the soils. As 0.01N
and 0.1N of HC1 conducting solutions were injected into the soils, a significant
improvement of copper removal was detected in the electrokinetic process.
Figure 6 presents the results of these solutions and the distilled water solution.
After 30 days under 8 volts of electrification, overall relative copper
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COPPER CONTAMINATED SOILS 1135
concentrations increased; no matter whether 0.01 N or 0.1N of HC1 was
injected into the specimen. The relative copper concentrations of Soil 1, Soil 2
and Soil 3 with 0.1 N HC1 had the best results, increasing the relative
concentrations to 390%, 190% and 410%, respectively. A very interesting
phenomenon was found in Soil 1. As 0.1N HC1 conducting solution was added
into the specimen, the copper flux totally migrated to Zone E and finally
caught by the cathode. This is evidence that the electrokinetic process was
fully completed in the Soil l 's module. Even though Soil 1 is an acidic soil, it
still needs a strong conducting acid to promote the H + across the specimen.
For Soil 3, 0.1N HC1 conducting solution also assists the copper flux in
reaching the cathode. However, as compared to 0.01 N HC1 that was also
injected into the module, 0.0IN HC1 did not supply enough buffering capacity.
As a result, the majority of copper flux only reaches Zone D. From this
enhancement study, the result suggests that the injection of high
concentrations of acid solution into the specimens would be the most
promising technology to improve efficiency for copper removal in the
electrokinetic process.
CONCLUSION
Electrokinetic technology was used for soil remediation on three copper
contaminated soils. In a batch-scale module, the electrokinetic process
demonstrated to be a promising technology in soil remediation. Overall, 70-
85% of copper in contaminated soils was removed. However, copper ions
were not totally transported to the cathode compartment due to soil properties
and electrification in the specimens. Experimental tests also found that the
electrokinetic process can remove copper from both acidic and basic soils. The
acidic soil seems to have a higher removal proportion than the basic soil. Most
coppers removed are most likely in the carbonate and Fe-Mn fractions.
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1136 SAH AND LIN
It is obvious that removal efficiency in the electrokinetic process is
thoroughly influenced by soil properties, physicochemical reactions,
electrolytic reaction/dissociation across the soils, migration of electro-osmotic
flow in the specimens, electrification and pH condition. Several variables
which may impact the electrokinetic process were examined in this study. The
results obtained from this investigation are: (1) pH is a major parameter in the
eletrokinetic process; not only because electro-osmotic across the soil may be
affected by the change in pH, but also desorption of copper ions from the soil
could promote copper ions migrated to the cathode; (2) despite pH, the
adsorption capacity between soil and copper is another parameter that
influences the electrokinetic process for metal removal. High adsorption
capacity of metal means less metal can be migrated by electro-osmotic flow;
(3) higher water contents promote electro-osmotic flow in the specimen.
However, a slight increase in water content by adding 100 ml of distilled water
into the soils did not significantly improve the overall removal efficiency; (4)
injection of high concentrations of strong acid into the pilot module to prevent
precipitation in the specimen seems to be the most promising technology to
improve the removal efficiency. The improvement also depends on the pH
distribution in the specimen; and (5) high current density and high
conductivity in soils did not promise a better removal efficiency. It only
reduces the operation time in a short period of time.
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Received: December 8, 1999
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