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 1. Introduction
Heavy metals contaminations are produced by human activities such as industrial and agricultural
activities and the heavy metals in soil are not degraded by the natural ecosystem; therefore,
hazardous matters accumulate in the soil [1, 2]. They are transported to plants or groundwater, and
are toxic to the biological systems of human and animal environments via the food chain [3]. In
certain cases, the heavy metals should be remediated using an appropriate method.
Remedial technologies for heavy metals-contaminated soils include containment methods such as
physical, encapsulation, and vitrification; ex situtreatment methods such as physical ethylene
separation, soil washing, and pyrometallurgical; and in situ treatment methods such as soil flushing,
electrokinetic, and phytoremediation by plants [4, 5]. Among the ex situ technology, soil washing is
a sufficient method for the removal of heavy metals using various forms of solvent solutions, which
are chelating agents including: ethylenediaminetetraacetic acid (EDTA), nitrilotriacetate,
diethylenetriaminepentaacetic acid, and inorganic acids; sulfuric and hydrochloric acids and organic
acids; and acetic and citric acids [6, 7]. These agents extract heavy metals from soil by dissolving,
complexation, and chemical reaction such as cation exchange. In addition, the soil washing method
is sufficient in terms of cost effectiveness (25？300 US$/ton) compared to other ex
situ technologies (60？1,000 US$/ton) [4].

EDTA, a representative chelating agent, can extract heavy metals from contaminated soils with
high efficiency [8]. However, with EDTA it is difficult to treat the effluent solution after treatment
due to low biodegradability, biological toxicity, and high cost for treatment [7, 9, 10]. Non humified
organic acid, and weak organic acid such as citric, tartaric, oxalic, formic and fumaric acids are
natural products of root exudates and plant and animal residue decomposition that can be used as a
washing solvent with biodegradable characteristics [11-14]. Strong inorganic acid can be used for
useful washing solutions in terms of reasonable cost and simple handling of the effluent solution.
Even though the strong acid causes soil acidification, it is an effective solvant due to high its
removal effciency on heavy metal extraction, especially hydrochloric acid [10].
The effectiveness of soil washing is dependent on process parameters: the extracting mode such as
batch or column, solvent type and concentration, extracting time, solid to liquid (S/L) ratio, and soil
physicochemical characteristics such as pH, organic content, particle size distributions, as well as
contaminated metal type and its concentration [7].
In this study, characteristics of Pb desorption for remediation of the Pb-contaminated soil were
evaluated using hydrochloric acid (HCl) by a washing method. Batch experiments were performed
to identify the factors influencing extractive decontamination such as the S/L ratio, extracting time,
particle size distributions and multi-stage washing. In addition, the Pb desorption was characterized
by applying it to a kinetics model.
2. Materials and Methods
2.1. Soil Sampling and Characterization
Soil samples were collected from two different areas: a slope area (SA) and a land area (LA) at a
clay shooting range in Korea, which was highly contaminated with lead (Pb). The soil was sampled
at a depth below 20 cm from the surface of each area. The collected soil samples were air-dried and
analyzed for the following characteristics: initial pH, water content, specific weight, organic carbon
content and Pb concentration for each sample. The lead bullet was removed from the initial soil
sample for the accurate measurement of the contamination concentration of lead in the soil. Pb was
pre-treated by the EPA Method 3050B [15] and analyzed using an atomic adsorption spectrometer
(AA-6300; Shimadzu, Tokyo, Japan). All other tests followed the Korean standard method for soil
pollution [16]. Particle size distribution was performed using the wet method by spraying with
water (about 150 mL/900 g of soil). Soil was separated using 5 steps with the following particle
sizes: >2.8, 2.8？0.71, 0.71？0.25, 0.25？0.075, <0.075 mm. Lead particles larger than 1 mm were
manually removed for the entire examination.
2.2. Sequential Extraction of the Soil

Soil samples with particle sizes of 2.8？0.075 were analyzed by the sequential extraction
method [17] to identify the metal combined property in the soil. Heavy metals in the soil were
sequentially extracted in different forms including exchangeable (EX), carbonate-bound (CB),
Fe/Mn oxides-bound (OX), organic matter-bound (OM), and residual (RS). The designated portion
of the soil was then examined using the sequential extraction method (Table 1).
2.3. Batch and Multistage Washing
Each soil sample separated according to particle size was examined for washing efficiency using
HCl as a washing solution. Thirty grams of each soil sample was placed into three different
Erlenmeyer flasks with a 0.1 M HCl solution with specific solid
[Table 1.] Sequential extraction procedure using 5 steps for heavy metals in soil

to liquid ratios of 1:2, 1:3, and 1:4 (g of soil to mL of HCl). The samples were mixed in a shaker at
300 rpm for 10 min. Multistage washing tests (3 stages) were sequentially examined with the most
efficient mixing rate obtained from the above washing test at the same condition. The sample was
settled down for a few hours until the soil and liquid phase became separated. After the phase
separation, the liquid was replaced with a new washing solution. The lead concentration in the
residual soil was analyzed at each stage. The added volume of the washing solution was adjusted
according to the existing soil mass in the multistage washing test. The final efficiency and necessity
of the multistage washing were determined throughout the test.
3. Results and Discussion
3.1. Physicochemical Properties of Soil
The properties of the soil examined are presented in Table 2. The initial lead concentrations
contaminated at the slope and land areas were 2,575 mg/kg and 730 mg/kg, respectively. The pH
ranged from 6 to 7.3 and the water contents were similar to those in the general range regarding
sandy loam (10% to 18%). Organic carbon content was roughly 18%, indicating a strong potential
for oxidization by acid to remove the heavy metals in the soil. The soil contents examined were
separated in gravel (>2.8 mm), sand (2.8？0.075 mm), as well as silt and clay (<0.075 mm).
Approximately 70% of the soil consisted of sand. The characteristics of the tested soil were adapted
in order to use chemical washing as a method to remove the lead. The lead concentration in soil was
permitted to be below 400 mg/kg by Soil Environmental Conservation Act (SECA) in Korea [18].
However, the lead concentration below a particle size of 2.8 mm exceeded the standard criteria in
Korea. The lead concentration contaminated in the soil at the slope and land areas was significant in
smaller sized particles. The soil with smaller sized particles provided more specific surface area that
can combined with heavy metals. Therefore, the soil type and distribution characteristics are
important process parameters.
The soil samples were extracted by the sequential method in order to identify the metal combined
property. Extraction efficiency is not only affected by extraction parameters but also a combined
form of heavy metals. In addition, the reaction rate between solution and metals, and movement
characteristics in the soil are dependent on the combined metal property. Heavy metals are divided
by non-detrital and detrital fractions. The non-detrital form of heavy metal can be easily removed
by the washing method [19, 20]. In this study, 92.4% of the heavy metals in the soil at the SA
existed in non-detrital forms (74.5% of EX, 17.2% of CB, and 0.7% of OM), and 7.7% of that in
detrital forms (5.5% of OX and 2.2% of RS). Furthermore, 83% of the heavy metals in the soil at
the LA existed in non-detrital forms (58.6% of EX, 23.4% of CB, and 1.0% of OM), and 17.1% of
that in detrital forms (10.5% of OX and 6.6% of RS). Most heavy metals in soil generally existed in
detrital forms, while most of the Pb in the examined soil existed in non-detrital forms, which can be
easily removed by the soil washing method. Therefore, the analysis revealed that the washing
method using solvent would be effective for remediation of Pb-contaminated soil in this study. In
addition, the low rate of OM and RS forms of soil implies that the heavy metals were influenced
from an external source, not from the soil mineral portion.
3.2. Washing Efficiency in Batch
Pb-contaminated soil at the SA and LA was examined depending on the soil particle sizes and SL
ratios. The Pb removal efficiency would be affected by the S/L ratio because when strong acid is
used as a washing solvent, the form of Pb in the soil can be changed [21]. In addition, an optimal
S/L ratio is required in terms of the treatment of wastewater produced after the washing event.
Additionally, the removal efficiency would also be affected by the soil particle size because the
surface area, where is possibly combinable with heavy metals, is high as particle size smaller. 0.1 M
of HCl was used as a solvent and its efficiency was compared to that of water. The removal
efficiency differed depending on the particle sizes and S/L ratios (Fig. 1). For the SA, with a soil
particle size greater than 2.8 mm, the Pb removal efficiency was significant with a low S/L ratio
(1:2). In contrast, the efficiency was significant with a high S/L ratio having a soil particle size
below 0.075 mm. The optimal S/L ratios were 1:2 (>2.8 mm), 1:3 (2.8？0.075 mm), and 1:4
(<0.075 mm). At the LA, Pb was totally removed in the gravel sized soil (>2.8 mm) and the optimal
S/L ratios were 1:4 in other particle sizes. When the HCl
[Table 2.] Soil characteristics and lead concentration contaminated in the soil by the soil particle
sizes
[Fig. 1.] Pb extraction efficiency in the soil depending on soil particle size and different solid to
liquid (S/L) ratios at slope area (a) and land area (b). H: hydrochloride, W: water.
washing solution was used, the removal efficiency was greater than 54%, which was high compared
to that of the water. Water also presented an average removal efficiency of 18？25% resulting from
the low coherence between soil and Pb. This is because the soil was originally contaminated by lead
bullets and in the soil, the lead exists in the particle phase by being crushing to a small size rather
than by being strongly adsorbed in the soil.
3.3. Multistage Washing
Sequential extraction via 3 steps was conducted with appropriate S/L ratios using 0.1 M of HCl and
water. The experiments were achieved at a soil size of 2.8？0.075 mm because most of the Pb in
the soil with particle sizes of below 2.8 mm and very small particle size soil (<0.075 mm) is
difficult to handle in the field. In addition, the extraction efficiency was comparatively low in the
batch experiment, requiring multi-stage treatment to meet standard criteria [16]. Therefore, the
2.8？0.075 mm of soil was examined at a different S/L ratio, which is the optimal ratio resulting
from the batch experiment using HCl or distilled water (Fig. 2). When the soil was treated with
HCl, approximately 65% to 80% of Pb was extracted at the first washing. The extraction efficiency
decreased compared to the first washing event, but the final extraction efficiency reached around
90% at the following washing event. The highest extraction efficiency was present at the first
washing event due to the low coherence between Pb and the soil while the extraction tendency was
unclear with water as a washing solution.
3.4. Effect of Extraction Time
The optimal extraction time was determined depending on the soil particle size with the proper S/L
ratio. The extraction time was set at 5, 10, 20, 30, and 60 min and the results are presented in Fig. 3.
A particle size greater than 2.8 mm presented high removal efficiency in both testing soil samples
resulting from the initially low contamination. The extraction time was insignificant to the
efficiency for the soil with soil particles sized
[Fig. 2.] Effect of the multi-washing event on Pb extraction. (a) Pb concentration in the soil and (b)
removal efficiency by the washing event. 1:2, 1:3, and 1:4 are optimal solid to liquid ratio. SH:
slope area with HCl, LH: land area with HCl, SW: slope area with water, LW: land area with water,

[Fig. 3.] Pb leaching properties according to washing time for each soil particle size with optimal
solid to liquid ratio.
2.8 to 0.075 mm, which is a practical target for treatment at the SA in Fig. 3(a). The extraction time
was insignificant for soil particles smaller than 0.075 mm and removal efficiency averaged roughly
66%.

The optimal extraction time was 5 min for the soil (2.8？0.075 mm) from the LA while the removal
efficiency with the soil (<0.075 mm) was slightly increased until 30 min and dramatically increased
at 60 min. This is because the initially low Pb concentration (922 mg/L) could contribute to higher
removal efficiency by increasing the extraction time compared to that from the SL soil (3,797
mg/L).
[Fig. 4.] Model fit to the extraction data of Pb from contaminated soil by the two-reaction (TR)
model and the two-constant (TC) model; (a) slope area (b) land area.
 3.5. Desorption Kinetics
The desorption rate of Pb from the soil depending on the extraction time for different soil particle
sizes was determined by the kinetics model. Desorption kinetic analysis was conducted using two
reaction models and two constant kinetic models, which are general equations used to describe the
characteristics of the desorption mechanism in soil [22, 23]. The two-reaction model (Eq. 1) and
two-constant model (Eq. 2) equations are as follows:

where C is the Pb concentration extracted by time (t), Co is the initial Pb concentration in the
soil, a is the ratio of extracted Pb during the initial rapid reaction step, and k1 and k2 are the reaction
constants at rapid and slow extraction steps (min-1), respectively. A is the initial Pb desorption
reaction constant (mg Pb/ kg/min), and B is a desorption reaction coefficient (mg Pb/kg).
Both the two-reaction model and the two-constant model fitted well to the data in this study (Fig.
4). Especially, the Pb desorption characteristic was adequately described by the two-reaction model
with a higher coefficient of determination. Hwang
[Table 3.] Parameters of kinetic models for lead extraction from soil

et al. [22] also presented similar results demonstrating that the desorption characteristic of Pb-
contaminated soil was well described with the two-reaction model. The specific parameters of each
kinetic model are presented in Table 3. Both soil samples with particle sizes greater than 2.8 mm
were rapidly extracted within a short reaction time. The results were associated with the high ratio
of extracted Pb at the initial rapid reaction step, which is a value by kinetics of the two-reaction
model and the high initial Pb desorption reaction constant (A) by kinetics of the two-constant
model. This is because Pb in the examined soil existed in the granule phase rather than being firmly
fixed on the soil. In addition, these parameter values were associated with the combined metal
property results. Tested soils had a high nondetrital portion, which can be easily extracted from the
soil. The exchangeable portion for the SA soil was higher than for the LA soil. These results
correlated well with the kinetic analysis.
4. Conclusions
The Pb-contaminated soil at a clay shooting range was remediated by the soil washing method
using 0.1 M HCl solvent caused by the weak coherence between the soil and Pb. The Pb in the soil
mostly existed in an exchangeable form that could be easily released from the soil. The Pb was
removed from the soil within 5 to 10 min with 65？80% of the removal efficiency occurring during
the first washing event. The soil washing efficiency was significantly affected by the S/L ratio and
soil particle distribution. The characteristic of desorption was well described by the two-reaction
kinetic model. The acidification of the soil by the washing event using a strong acid would possibly
influence the increase of movement of Pb in the soil. Therefore, continuous observation and
management in regards to the contaminated soil would be necessary.