10th Asian Regional Conference of IAEG (2015)

Performance of Sorption Layer Using Ca/Mg Immobilizing Agent against NaturalContamination
Jialin MO(1), Toru INUI(1), Takeshi KATSUMI(1), Atsushi TAKAI(1), Kenji KUNINISHI(2),and Shintaro HAYASHI(2)(1) Graduate School of Global Environmental Studies, Kyoto University, Japan
Email: mo.jialin.85c@st.kyoto-u.ac.jp(2) Sumitomo Osaka Cement, Co., Ltd., Japan
Abstract
Soils and rocks containing heavy metals and metalloids derived from nature were often
generated in huge volumes from excavation works. Without proper treatment, these soils may
generate hazardous leachate and cause environmental contamination. Sorption layer method,which has been developed recently, utilizes a permeable soil layer with sorption capacity lying
beneath piled soil and rock in embankments to retain the chemicals of concern dissolved in theleachate. Immobilizing agent and mineral are used solely or by mixing with a clean host soil tostrengthen the sorption capacity. Permeability and equilibrium sorption characteristics of thesorption layer material are fundamental factors affecting its sorption. Thus, this paperaddresses experimental results on permeability and equilibrium sorption characteristics of thedecomposed granite soil enhanced by the Ca/Mg composite immobilizing agent. Experimental
results showed that higher immobilizing agent content exhibited the higher sorption capacity,
both curing period and mixing ratio has no obvious effect on hydraulic conductivity of themixture.
Keywords: natural contamination, immobilizing agent, hydraulic conductivity, sorption layer
1.

Introduction

the leachate into the surrounding environment.
However, the potential slope failure due to lowinterface friction between piled soils/rocks andgeomembrane sheets is a major concern. In addition,
its execution cost is hard to afford when treating ahuge amount of excavated soils. Insolubilization
methods apply solidification and/or stabilization
agents to fix the heavy metal contained incontaminated soils and reduce the leaching potential.
Its long-term performance under field conditions is
not fully verified and has been a main concern.
Sorption layer method is a newly developed
technology for in situ containment of excavated soils
and rocks (Technical Committee of Environmental
Conservation Technology Association, Hokkaido2012). It utilizes a permeable soil layer with sorption
capacity for contaminants of concern underlying thepiled soils and rocks in embankment (Fig. 1(a)). The
composition of this layer can be natural material (soil,minerals), artificial material (immobilizing agents) or
the mixture of them. Rainfall infiltration is limited bysoil cover to avoid generating more leachate.
The sorption layer should have a sorption capacity
high enough to retain the leachable amount ofSoils and rocks containing heavy metals (e.g. lead,cadmium) and/or metalloids (e.g. arsenic, selenium)
derived from nature due to their geologic histories arewidely distributed in Japan. Once they are excavatedin construction works such as tunnel construction,leaching of these metals may be triggered due tooxidization and decomposition of sulfide minerals
contained in some soils and rocks. Without proper
managements and/or treatments for them, they will
generate acid leachate containing unacceptableconcentrations of heavy metals and/or metalloids,
which are higher than the environmental standards.
Remediation of soils containing natural derived
heavy metals has become a subject under the controloftheAmending
Soil
Contamination
Countermeasures Act issued by the Japan Ministry of
the Environment in 2010, which proposes several
countermeasures for these soils including containment
method,insolubilization
method,andetc.
Containment method uses seepage control works to
reduce generation of leachate, and low-permeable soil
layers or geomembrane sheets to prevent migration of1

10th Asian Regional Conference of IAEG (2015)

contaminants to the acceptable level. The process issimilar to that of a horizontally placed permeable
reactive barrier, where the leachate can pass thesorption layer under a certain velocity andcontaminants of concern is decomposed or sorbed by
the composition material of the layer during
permeation. The concentration of contaminant in theleachate should be lowered, and the time when thecontaminants reach the surrounding environment
should also be retarded. The effluent flowing out of
the sorption layer is expected to meet theenvironmental standard (Fig. 1(b)).

continuously as expected, since and flow conditions(flow rate, retention time, etc.) are known as key
issues for the sorption performance.
This study was conducted to evaluate theperformance of the sorption layer, which wasprepared by mixing a typical soil, decomposed granite
soil, with the Ca/Mg composite immobilizing agent(The Society of Material Science, Japan 2014). The
authors verified that this Ca/Mg composite
immobilizing agent was able to immobilize both
metal cations and anions in water-soluble forms(Bobea et al. 2012). Thus, the agent is expected toeffectively immobilize lead, arsenic, and fluoride,
which are typically contained in the leachate of soils
and rocks. The specific objective of this paper is toobtain both hydraulic and equilibrium sorption
performances of the soil-immobilizing agent mixture,influenced by mixing ratio of the agent and curing
period after mixing. As a trace metal, arsenic wasselected, since it is most frequently contained innaturally contaminated soils and rocks found in Japan.

(a) Schematic diagram of the sorption layer method

2.

Materials and Methods

2.1 Materials
Ca/Mg composite immobilizing agent, which wasmanufactured through calcination of selected naturalminerals by Sumitomo Osaka Cement Co., Ltd., Japan,was used. Properties of the material are shown in
Table 1 (The Society of Materials Science, Japan,2014).

(b) Effect of the sorption layer (Revised from Igarashiand Mikami (2004))

Table 1 Properties of Ca/Mg composite immobilizing
agent
Items
Standard/Method
Value
Bulk density
JIS R 9301(untamped) 0.90 g/cm3
1 g in 100 mL waterpH (25 °C)
10.3
Mixing 1 hour
Blaine’s specific
JIS R 5201
4200 cm2/g
surface area
Water content
5g, Dried under 110°C 0.21 wt%
Main component JIS R 9011
CaO
35.6 %
MgO
19.4 %
3.98 %
SO3
3.73 %
Fe2O3
1.52 %
SiO2
0.068 %
Al2O30.035 %P 2O 5
Ignition loss
33.5 %

Fig.1 Conception of sorption layer method
The definitive aim of this method is not tocompletely block the contaminants, but to lower theirconcentrations to an acceptable level. When certain
working conditions for the sorption layer method canbe met, its advantage of easy construction, low costand good stability make this method act as an
economical countermeasure for huge amount ofexcavated soils and rocks, although some technicalissues should be addressed.
An Immobilizing or stabilizing agent is often
employed to strengthen the sorption capacity as wellas the bearing capacity of the sorption layer by mixing
with a clean host soil. In conventional immobilization
orstabilization
practices,performanceofimmobilizing agents has been evaluated after mixing
it directly with contaminated soils. However theprocess of the sorption layer is quite different because
the contaminants dissolved in the leachate contact thesoil-agent mixture. Thus, how this different process
affects the sorption performance is needed to bestudied. Particularly, the hydraulic performances
should be preliminarily evaluated to ensure whether
the leachate flows through the sorption layer

Commercially available decomposed granite soil(particle density, s = 2.70 Mg/m3) was used as a hostsoil. Soil particles which passed a 2 mm-opening
sieve were used. The particle size distribution curveof decomposed granite soil after sieving is shown inFig. 2. The pH of the suspended soils in the liquid tosolid ratio (L/S) = 5 according to JGS 0211 (2009)210th Asian Regional Conference of IAEG (2015)

was 9.5.

(2) Batch sorption test
The preparation of specimens was similar to thatfor the hydraulic conductivity test, except that thespecimen dimension was 5 cm in diameter and 10 cm
in height, and the specimens were cured after taking
them out of the mold. After curing, specimens werecrushed (≤ 2mm) and mixed with the NaAsO2
solution in L/S = 20. The concentration of the solution
is 0.1, 0.5, 1, 5 and 10 mg/L. The samples weresubjected to 24 hours horizontal shaking at 150 rpm,
then left for 15 minutes. Solid-liquid separation was
done by centrifugation under 3,000 rpm for 20
minutes and filtering using a 0.45μm-opening
membrane filter. Arsenic concentrations before andafter the sorption test were analyzed by an atomic
absorption spectrometry (AAS) method (SHIMAZU,AA-6800).

Fig. 2 Particle size distribution of the decomposed
granite soil after sieving
2.2 Methods(1) Hydraulic conductivity test
Hydraulic conductivity tests were conductedaccording to ASTM D 5084 “Standard Test Methods
for Measurement of Hydraulic Conductivity of
Saturated Porous Materials Using a Flexible Wall
Permeameter” (Fig. 3). The hydraulic gradient is set
to 5 and the cell pressure of 30 kPa was applied.
Distilled water was used as permeant solution.

3.

Results and Discussion

3.1 Hydraulic conductivity
For each mixing ratio (0, 3, and 5%) and curing
period (0 and 7 days), the tests were conducted induplicate.
The variation of average hydraulic conductivity
values with different mixing ratios and curing periods
was presented in Fig. 4.

Fig. 3 Schematic view of a flexible-wall permeameter
The immobilizing agent was mixed with theair-dried soil by 0%, 3% and 5% of its dry weight.
Water content of the mixture was adjusted to its
optimum water content (10.1 to 11.3%), which wasdetermined by the A-a method of JIS A 1210 (2009),
by adding distilled water. Then, the mixture wascompacted in a cylindrical mold in three layers. The
dimension of the mold is 6 cm in diameter and 7 cmin height. The compaction degree was set to 95%.
After compaction, cling film was used to wrap themold, and the specimen was kept in a fully-sealed
plastic box to be cured for 0 or 7 days.
After curing, the specimen was submerged in
distilled water and saturated by vacuum deaeration for
at least 24 hours. Then, the specimen was set on theflexible-wall permeameter. It was sandwiched withfilter papers, filter clothes, porous caps and pedestals,
confined by a latex membrane on the sides.
Permeation
was conducteduntil hydraulicconductivity value was stable and 5 or more porevolumes of flow were permeated.

Fig. 4 Hydraulic conductivity variation among
different cases
The hydraulic conductivity of 0-day and 7-day
curing specimens were almost same, which indicatesthat 7-day curing has no obvious effect, although theunconfined compression strength increased by 7-daycuring (Mo et al. 2014). Considering that thehydraulic conductivity tests lasted for several days,
hydration reactions could happen during permeationeven in 0-day curing specimens. Thus, themicrostructure of the specimens with or without
curing, and accordingly hydraulic conductivity values
became similar.
The hydraulic conductivity of 0% case wasrelatively lower than 3% and 5% cases for mostsamples. This is probably due to aggregation after
addition of the immobilizing agent. However, thesimilar results between 3% and 5% cases indicate thatmixing ratio has no obvious effect on the hydraulic3

10th Asian Regional Conference of IAEG (2015)

cases is within one order between 1×10-6 m/s to
1×10-7 m/s and almost constant throughout the test
period, which indicates that the addition of the Ca/Ma
immobilizing agent does not significantly decrease
the hydraulic conductivity. This trend is preferable forthe sorption layer application, since the leachateshould flow through the sorption layer preferentiallyfor the metals of concern to be sorbed. If the hydraulicconductivity is decreased with time by adding theagent, the leachate is unlikely to flow through thesorption layer and the sorption capacity cannot be
fully mobilized.

conductivity and the extent of hydration reactions wassimilar. This is probably because the total amount ofsilicate offered by the decomposed granite soil was
almost same in both 3% and 5% cases, and thehydration reactions were limited to the same extent.
The variation of hydraulic conductivity with porevolumes of flow and the average hydraulicconductivity value obtained for each case were shownin Fig. 5.

3.2 Sorption performance
Sorption isotherms were determined by anempirical relation between the equilibrium
concentration of the solute and the sorbed amount,based on Freundlich equation, which is expressed asfollows:
S = kCeqn(1)where S is the sorbed amount (mg/g), Ceq is theequilibrium concentration of the solute (mg/L), k andn are constants.
Sorption isotherms in linear scales for all samples
with the mixing ratio of 0%, 3% and 5%, which were
cured for 0 or 7 days, with the equilibrium
concentrations ranging from 0 to 1 mg/L aresummarized in Fig. 6.

(a) Hydraulic conductivity of samples with mixing
ratio of 0% (decomposed granite soil aftercompaction)(b) Hydraulic conductivity of samples with mixing
ratio of 3%
Fig. 6 Comparison of sorption isotherms of different
cases
Sorption isotherms of different cases in log scales
are shown in Fig. 7, which includes the sorbed
amount (S) for the equilibrium concentration (Ceq) of0.01 mg/L, which equals to the environmental
standard of arsenic for soil and groundwater quality,Japan.
There are three possible mechanisms responsible
for the sorption of arsenic by the Ca/Mg immobilizing
agent. First is the precipitation between calcium andarsenic. Second is the surface attraction of arsenic
with magnesium hydroxide. Third is the surface
attraction of arsenic on the hydration products.
It can be observed from Fig. 6 and 7 that adding
the agent clearly strengthens the sorption ability of themixture. This is because the higher immobilizing(c) Hydraulic conductivity of samples with mixing
ratio of 5%
Fig. 5 Hydraulic conductivity of samples with mixing
ratio of 0%, 3% and 5%
Variation of the hydraulic conductivity of different

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10th Asian Regional Conference of IAEG (2015)

agent content provides more available calcium andmagnesium that promote the sorption of arsenic.
However, the effect of curing time is not obvious,
which implies that the sorption effect of hydration
products is not obvious.

(e) Sorption isotherm for sample with mixing ratio of5% and 7-day curing
Fig. 7 Sorption isotherms for samples with different
mixing ratio and curing periods(a) Sorption isotherm for sample with mixing ratio of0%

The less effect of curing time could be explainedby two possible reasons. The first one is related to theexperimental procedure of batch sorption test.
Hydration reactions and crystallization that occurred
during curing and the generated calcium/magnesium
silicate hydrates were damaged by crushing andshaking, and the effect of high specific surface area ofthe hydrates and the positive charge of the surface tocatch arsenic may be offset. The second reason is that
there is not enough Ca2+ to either overcompensationof the hydrate surface or participate in the chemical
reaction. Surface of calcium/magnesium silicate
hydrate remains negatively charged, which are unable
to attract arsenic oxyanion onto its surface.
Thus, sorption onto the surface of hydration
products is not the main mechanism of arsenic
sorption. The precipitation of arsenic with calcium
and sorption onto the surface of magnesiumhydroxide should have the major effect.
Electrical conductivity (EC) variations of thesolution before and after sorption under differentinitial NaAsO2 concentration are shown in Fig. 8.

(b) Sorption isotherm for sample with mixing ratio of
3% and 0-day curing(c) Sorption isotherm for sample with mixing ratio of
3% and 7-day curing

Fig. 8 Variation of EC before and after sorption
EC value is relatively low for initial solution.
After sorption, EC values of 3% and 5% caseincreased greatly compared to 0% case, whichindicates that adding the agent provides moreavailable calcium and magnesium and promotes thesorption of arsenic. The higher equilibrium EC value(d) Sorption isotherm for sample with mixing ratio of
5% and 0-day curing

510th Asian Regional Conference of IAEG (2015)

of 5% case indicates that more available calcium
remaining in the solution, which corresponds to its
stronger sorption performance. Thus, the sorption
performance of the mixture was strengthened withaddition of the agent and more calcium offered.
EC values of 0 day curing case are higher than 7days curing case, which is probably caused by thehydration process that consumed more calcium andmagnesium. Thus, their concentrations in the final
solution decreased, which can also be reflected by thepH variation shown in Fig. 9.

4.

Conclusions

This research was conducted to evaluate theeffectiveness of the soil improved by the Ca/Mg
composite immobilizing agent as a sorption layer
against natural contamination. Particularly, thepermeability and equilibrium sorption characteristicsof the sorption layer were studied.
Hydraulic conductivity of the soil-immobilizing
agent mixture was around 1×10-6 to 1×10-7 m/s.
Adding the agent with 5% in dry weight basis in
maximum has no obvious effect on the hydraulicconductivity. Higher immobilizing agent content
corresponds to higher sorption capacity. However,curing periods has a minor effect in batch test.
Formation of precipitation between arsenic withcalcium and surface attraction on magnesiumhydroxide is the major mechanism of sorption.
Future researches should evaluate the performanceof sorption layer affected by leachate pH, co-existingions, and dynamic sorption characteristics. An
accurate and reliable design method considering
complicated and filed conditions such as unsaturated
flow needs to be proposed in order to promote theapplication of sorption layer.

(a) Variation of pH before and after sorption ofsamples with mixing ratio of 0%

References
Bobea, J.C., Kikuchi, S., Kuninishi, K., Yamashita, S.,
Katsumi, T., and Inui, T. (2012): Performance ofimmobilization materials against lead and arsenic
contaminated soils. Proc. 10th National Symposium
on Ground Improvement, The Society of Material
Science, Japan, 129-134.
JGS 0211 (2009): Test Method for pH of Suspended
Soils, Japanese Geotechnical Society.
JIS A 1210 (2009): Test Method for Soil Compaction
Using a Rammer, Japanese Industrial Standards
Committee.
Igarashi, T. and Mikami, T. (2004): Utilization ofsurrounding clayey soils as sealing material formucks rich in heavy metal. Soils and Foundations,Vol. 52, No. 9, pp. 22-24.
Mo., J., Seo, A., Inui, T., Katsumi, T., Takai, A.,
Kuninishi, K., and Hayashi, S. (2014): Performanceof Ca/Mg agent stabilized soil used for sorption
layer against natural contamination, 11th JGS
Symposium on Environmental Geotechnics,
Japanese Geotechnical Society, submitted.
Technical Committee of Environmental Conservation
Technology Association, Hokkaido (2012): Manualfor Designing Sorption Layer Method, Technical
Report of Hokkaido Environmental Conservation
Technology Association No.5, in Japanese.
The Society of Material Science, Japan (2014): Heavy
metal insolubilization agent: Magic Fix. Report on
Ground Improvement Technology Review andCertification, No.1015, in Japanese.

(b) Variation of pH before and after sorption ofsamples with mixing ratio of 3%(c) Variation of pH before and after sorption ofsamples with mixing ratio of 5%
Fig. 9 Variation of pH before and after sorption
The pH of final solution of 0 day curing cases ishigher than 7 days curing cases, which is caused byhigher concentration of calcium and magnesium.

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