An Experimental Study of Heavy Metal Extraction, Using Various Concentration of EDTA in a Sandy Loam Soils
D. Naghipoor Khalkhaliani,
A.R. Mesdaghinia ,
A.H. Mahvi ,
This study provides an evaluation of EDTA solution for the removal of lead, zinc and cadmium from a contaminated soil. The field soil contained 68% sand, 12% clay and 20% silt. The performance of EDTA for the treatment of soil contaminated with heavy metals was evaluated in this study. Soil samples containing variable levels of Pb, Zn, Cd were subjected to Ethylene Diamin Tetra-acetic Acid (EDTA) treatment and the extraction of heavy metals was found to vary, ranging from 54.5 to 100%. Thus the feasibility of soil washing for the decontaminated sandy-loam soil with single and several metals were evaluated in laboratory-scale batch experiments. Of the washing reagent test, Na2- EDTA 0.1 M solutions were generally more effective for removing heavy metals from soils. Na2-EDTA 0.1M preferentially extracted lead over cadmium and zinc. The most efficient washing occurred using the 0.1M EDTA at the lowest pH.
Soils are effective agents for metal sorption because of their high surface
area and the presence of various surface function groups (Gong and Donahoe,
1997). Soil properties, especially the content of Fe, Al and Mn oxides and hydroxides
and the content of clay and organic matter, are believed to be the main controls
of heavy metal sorption (Gong and Donahoe, 1997; Tessier et al., 1995;
Young and Harvey, 1992). Heavy metals in soils are receiving increasing attention
due to greater understanding of their toxicology importance in ecosystem and
for human health (Ottsen et al., 2001). Heavy metals will continue to
be an environmental concern for a long time unless they are taken out from the
ecosystem (Chen et al., 2000). Heavy metals can be transferred by the
intake of vegetation and human health can be impacted through ingestion of both
water and foods that have been contaminated by the soil. Heavy metals are toxic
to human as well as to other organisms (Chen et al., 2000). Contaminated
sites are also pose a threat to groundwater supplies if the metals are not properly
contained and treated. Heavy metals interact with soil matrix and may persist
for a long period of time creating long-term hazards to the environment and
human health. A promising method for removing heavy metals from soil is chelating
extraction (Lo and Yang, 1997). Metals are particularly troublesome because
they can accumulate in soils through adsorption, precipitation and other physico-chemical
processes and the presence of very small concentrations can sometime pose a
serious health threat to living organisms. Clean up of soils contaminated with
trace metals is one of the most difficult and expensive goals in environmental
engineering. Soil washing is considered as one of the most suitable on-site
techniques for removing trace metals. Recently increased attention has been
focused on the use of acidic or chelating agents to dislodge and dissolve trace
metals from solids into solution. Citrat acid, Nitrilotree Acetic Acid (NTA)
and EDTA are the common alternatives suggested for chelating polluted soils
(Zeng et al., 2005; Papassipi et al., 1997). Clearly, action should
be taken to control the contamination of soils (i.e. particle size or gravity
separation, flotation, etc.) or to employ chemical methods involving the application
of appropriate leaching solution to the contaminated soil. Metals can be removed
by physical methods. These techniques is to separate the metals from soil by
using chelating agents to form soluble metal-chelate complexes. Technologies
available for re-mediating metal contaminated soils can be divided mainly into
two groups, namely, immobilization methods and separation/concentration methods.
In the first type of remediation, contaminants are immobilized thereby preventing
the leaching of contaminants into the groundwater. The second type of remediation
deals with separating the containment from the soils or reducing the volume
of contaminated soil (Khodadost et al., 2005). Soil washing is a variable
treatment alternative for metal contaminated sites. Chemical extractions are
some times introduced in the washing fluid to enhance the efficiency of heavy
metal extraction. These extractions include acids, bases, chelating agents,
electrolytes and redox reagents. Process parameters in soil washing include
the mode of extraction (e.g., batch or column), extractant type and concentration,
pH, electrolyte concentration, liquid -to- solid ratio (L/S) and retention time.
The soil related parameters are pH, particle size distribution and mineral type
of metal to be extracted and their concentration, distribution and physicochemical
forms in the soils. The kinetics of metal desorption/dissolution is also a crucial
parameter as it can affect the treatment duration and cost (Tandy et al.,
2004; Lim et al., 2004; Reed et al., 1996). Using EDTA to remove
heavy metals has been proven to be an effective method due to strong complexing
ability of EDTA (Samani et al., 1998). Even though EDTA has shown to
be a suitable chelating agent for remediation of heavy metal, not much information
is available with high heavy metal removal efficiency. The objectives of this
research were to further investigate the feasibility of EDTA to achieve high
removal efficiency of heavy metals from polluted soils. The performance was
evaluated for different soil samples containing variable levels, from 500-1000
mg kg-1 in Pb, Cd and Zn. The advantages of using chelating agents
in soil washing is high efficiency of metal extraction, high thermodynamic stabilities
of the metal complexes formed, good solubility of metal complexes and normally
low adsorption of the chelating agents and their metals complexes on soils (Abumaizar
and Smith, 1999; Peters, 1999; Tandy et al., 2004).
MATERIALS AND METHODS
The Sandy-Loam soil, which has been sampled from roadside along agricultural lands of Gilan province in Iran, was selected for this study. This research was conducted for a year in the Department of Environmental Health Engineering and Department of Chemistry of Tehran University in 2004. The surface soils, (0-20 cm) were sampled, air dried, ground, passed by a 2 mm sieve and stored in plastic bags prior to laboratory analysis and use in batch experiments. Soil pH was determined using glass electrodes in a soil: water ratio of 1:1 and the particle size distribution of soils were analyzed by the Pipete method (Gee and Bauder, 1986). Organic carbon was determined by the Walkley-Black wet combustion method (Rhoades, 1982). Exchangeable cations and Cation Exchange Capacity (CEC) were determined using ammonium acetate at pH 7. (Rhoades, 1982).
Selected properties and concentration of the Cd, Zn and Pb
in unpolluted field soil
Soil preparation: Since the concentration of heavy metals in soil samples is usually less than100mg kg-1, hence the soil samples have been contaminated artificially (Table 1).
Several solutions containing Zn, Cd and Pb were prepared by using acetate of these metals. These solutions were added to a portion of the mentioned soil sample at a rate of 100 mL/10 g (solution/solid) in centrifuge tubes. Therefore, the soil sample received 500 mg kg-1 or 1000 mg kg-1 heavy metals. The suspension samples were then placed on a mechanical shaker operated at 180 rpm and at a room temperature (27-30°C) for 48 h.
The wet-aging stage was essential to ensure complete and even exposure of every soil particle to contamination. At the end of the 48 h wet-aging period, the suspension was centrifuged to separate the solid phase from solution. The supernatant liquid phase from the centrifuge tubes was filtered and the equilibrium concentration of heavy metals in the liquid was measured using Atomic Absorption Spectrometer (AAS). The pH of the solution was measured and all the contaminated samples pH was about 7. The contaminated soil was then washed with DI water to remove the entrapped water in the soil. So an artificially contaminated soil sample was used in this study. The advantage of deliberately contaminating the soil is that a rather homogeneous test sample, with consist heavy metal concentration and speciation, soil composition, contamination process and contamination period can be obtained. This would minimize ambiguity in the extraction of potential results arising from sample heterogeneity (Elliot and Shasteri, 1999).
Batch extraction with EDTA: Batch extraction experiments were conducted
using an EDTA solution at various concentrations and contact times to determine
appropriate range of concentration and dosage of the washing solution to achieve
high heavy metal extraction efficiencies.
|| Remediation efficiency vs. reaction time of contaminated
Sandy -loam soil with 500 mg kg-1 Conc.
The extraction solution was prepared from reagent-grade disodium ethylenediaminetetraacetat
(Na2 EDTA). The EDTA concentration was 0.005, 0.01 and 0.1 M and
the pH values of the three solutions were 4.7, 4.53 and 4.46, respectively.
The contact time for metal extraction was 2 h. All the experiments were performed
at room temperature (27-30°C). A soil sample of Teng used with a 2.5:1 liquid/solid
ratio was employed for all soil extraction experiments. Ten gram of soil sample
and 25 mL of EDTA solution were added to a polyethylene centrifuge tube. The
tube was sealed with a lid and then placed on a mechanical shaker operated at
180 rpm at room temperature for the desired contact period. Two hours reaction
time was deemed sufficient based upon batch rate desorption tests conducted
over 48 h period. After mixing, the samples were allowed to settle for about
15 min and then centrifuged and filtrated through a 0.45 μm member filter.
The pH of the washing solution before contact with the soil and the pH of the
filtrate were measured and recorded. Following filtration, the filtrate was
acidified to a pH of <2.0 with 1:1 HNO3 for heavy metals analysis.
It was assumed that the metal concentration of the filtrate was released from
the contaminated soil. Remediation efficiencies were determined by dividing
the heavy metal release quantities to the initial quantity in the soil. All
heavy metal analysis performed by using a Perkins-Elmer Atomic Absorption Spectrometer
(AAS). DI Water was performed to provide a baseline for the removal obtained
by chemical washing.
RESULTS AND DISCUSSION
Based on this study and the results shown in Table 2 a reaction
time of 2 h was considered suitable since the curves illustrate that no appreciable
amount of metal was extracted beyond this time. Table 2 represents
the remediation efficiency of the contaminated soils in 0.5, 1, 2, 3, 4, 6,
12, 18, 24, 36 and 48 h time periods for three types of metals Cd, Zn and Pb.
The vacillation of remediation process results usually occurs in initial hours
and then the efficiency will follow a constant rate, therefore the optimum time
period, which is used as a bench mark for comparison, was 2 h. Apparently Pb,
Zn,and Cd release were very rapid such that reached an equilibrium within 1-2
h of extraction time. Table 3 and 4 represent
the remediation efficiency by various rates of EDTA concentrations at pH original
(about 7) on a Sandy-Loam soil samples with the contamination rate of 500 mg
Remediation efficiency (%) of single metal-contaminated soil
(Sandy-loam) with Conc. 500 mg kg-1 using various M EDTA
Remediation efficiency (%) of multi metal-contaminated soil
(Sandy-loam) with Conc. 500 mg kg-1 using various M EDTA
The results represent the cardinal effect of EDTA on Pb; meanwhile the increase
of the EDTA concentration in Zn and Cd had not a considerable effect on the
remediation efficiency (Table 3 and 4).
According to Table 3 the remediation efficiency for Pb with
the concentration of 0.1 M EDTA was 100% and for the EDTA with the concentration
of 0.005 M the efficiency was just 71.8%. A 0.1 M EDTA exhibited complete removal
of Cadmium and Lead for a soil to solution ratio of 1:12.5 (Abumaizar and Smith, 1999). Soil flushing tests performed (Palma et al., 2003) on
a sandy loam showed that complete Pb removal can be achieved by using four pore
volume (4PV) of a solution containing 0.01 M EDTA., Other research results showed
that a higher concentration of EDTA and PV were necessary because of the high
organic content of soil used in the experimentation. Heavy metal retention by
soil is in fact, strongly affected by its natural organic matter concentration
(Palma et al., 2003).
Also the effect of EDTA concentration on remediation process of mixed soil (multimetal-soil) was investigated and the results showed that the maximum and minimum efficiencies for the Pb were with EDTA 0.1 M and 0.005 M, respectively, (Table 4). It means that EDTA in single-metal contaminated soils is more efficient than in a multi-metal contaminated soil which this is possibly because of the greater ratio of the EDTA to the metal. Kim et al. (2003) reported that lead extraction efficiency was a function of the stoichiometric ratio of the applied EDTA concentration to the total lead concentration in the soil sample.
The excess dosage would ensure that there were always sufficient molecules
of chelating agents available to these heavy metals, even though some molecules
of the chelating agent might be precipitated, adsorbed by the soil, or might
form complexes with other cations (Ca, Mg, Fe, Al and other trace heavy metals)
that originally present in the soil.
Remediation efficiency (%) of single metal-contaminated soil
(Sandy-loam) with Conc.500 mg kg-1 using 0.1 M EDTA
It shows that Pb, Cd and Zn could be easily extracted by 0.1 M EDTA. It was
believed that the high Pb, Cd and Zn extraction efficiencies demonstrated by
the chelating agent could be attributed to the formation of strong and stable
metal complexes which could overcome the adsorption intraction between the heavy
metals and soil solids (Lim et al., 2004). The organic matter in the
soil and sludge also might play some part in retaining the metal by copmlexation.
Many other studies have reported a significant influence of soil properties
such as pH, organic matter and cation exchange capacities on metal retention
by the soil (Surampalli et al., 1998). Tyler and McBride (1992) indicated
that the organic matter can retard heavy metal migration in soils. Elliot et
al. (1986) also reported that the addition of organic matter would enhance
retention of most heavy metals. In the next stage of this study the effect of
pH on soil remediation was investigated. The remediation was not considerably
affected by pH. The remediation efficiency in pH 3 for the Pb was approximately
10% more than in pH 9 for a Clay-Sandy-Loam soil but this increase for Sandy-
Loam was not significant (Table 5).
Masky and Calver (1990) concluded that pH was a very important factor for determination the adsorption behavior of metals on soils. This finding is supported by Ottsen et al. (2001) who found that Zn is desorbed at a higher pH than Cu which again it desorbed at higher pH than Pb. An important observation from recent soil washing research is that removal eficiencies of metals from artificially contaminated soils via metal adsorption were significantly greater than those for soils from actual waste sites. As observed in Table 1 the initial Zn contaminated soil is higher than Pb and Cd and as a result the removal efficiency of Zn is lower than those of Pb and Cd. Tuin and Tels (1990) found that metal removals from actual waste-site soils were not as high as from an artificially contaminated soil when both soils were washed under similar conditions.
With progressively alkaline conditions, the ability of chelats to enhance solubility
of oxides and other solid phase decreases because hydrolysis ( the attachment
of -OH as ligand ) is favoured over completion by EDTA (Ottsen et al.,
Remediation efficiency (%) of single metal-contaminated soil
(Sandy-loam) with Conc. 1000 mg kg -1 using various M EDTA
Remediation efficiency (%) of single metal-contaminated soil
(Sandy-loam) with Conc. 500 mg kg-1 using DI water
Such metal ions can be immobilized in the soil by the formation of insoluble
precipitates, incorporation into the crystalline structure of clay particles
and metal oxides and/or by physical entrapment in the immobile water surrounding
the micro and macropores of soils (Peters, 1999).
Moreover the effect of initial metal concentration in soil for the remediation
rate has been investigated by Na2 EDTA. When the initial metal concentration
was increased two times (1000 mg kg -1) remediation efficiency by
0.1 M EDTA decreased approximately 9% for Pb and 1% for Cd and 4% for Zn, but
when the contamination rate was twice and the 0.005 M EDTA was used, a 45% decrease
was observed for lead remediation efficiency which is completely considerable,
whereas this decrease for Cd and Zn were about 108% Table 3
and 6. As another part of the study the remediation process
has been done on some samples with the contamination rate of 500 mg kg -1
by DI. The results represented poor remediation efficiency especially for the
lead contaminated soil. An insignificant amount of metals removal by this method
resulted in a poor removal efficiency that was less than 22% for Zn, 18% for
Cd and 9% for Pb, indicating that the adsorbed HMs could not be readily removed
by rising along even though the soil were artificially contaminated in the laboratory.
Table 7 represents the removal efficiency with DI water in
As observed, efficiency is different in the mentioned soil with the different concentration of EDTA especially for Pb, however, the remediation efficiency is maximum for lead. Table 5 provides the remediation efficiency at various pH values that it is nearly independent to pH. The influence of pH and concentration of EDTA on the solubilisation of certain metals in contaminated sediments and found that, in all cases, the lower the pH and the higher the EDTA concentration the greater the extraction recovery. The vacillation of remediation process results usually occurs in initial hours. A rapid uptake of lead from contaminated soil by Na2 EDTA was observed by Evangelista and Zownir (Fisher et al., 1998). The possible disadvantages of chelating agent include: 1- EDTA is not easily biodegradable and pose a potential environmental hazard if they remain in the treated soil and 2- chelating agents are relatively expensivechemical compound. The results of batch washing experiments completed in this study indicated that Pb, Cd and Zn can be extracted from artificially contaminated soil using a chelating agent solution. There are significant decrease in the extraction of Pb with low concentration of EDTA (0.005 M). By using a washing solution at a concentration of 0.1 M of EDTA, extraction yields of 100% for Pb, 79.3% for Cd and 84.7% for Zn were obtained. Metal extraction kinetic was found to be fast, a reaction time of 2 h was deemed sufficient contact time between the contaminated soil and the washing solution based upon batch desorption tests conducted over a 48 h period. Result presented in Table 5 are in aggrement with Lim et al. (2004) who found at the pH 4.7, the amounts of Pb and Cd extracted were slightly higher than those achieved at higher pH values, which were belived to be partly due to dissolution of soil minerals that caused simultaneous release of the adsorbed metals.
The authors would like to express theirs gratefully acknowledgements to Prof. M.B. Kirkham from Department of Agronomy, Kansas State University for his valuable advise during this research.
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