10th Asian Regional Conference of IAEG (2015)

Effects of adsorption and covering-soil layers
on arsenic leaching from excavated rocks
Jin OTA (1), Toshifumi IGARASHI(2), Shuichi TAMOTO(3), and Takahiro YAMAMOTO(4)(1) Graduate School of Engineering, Hokkaido University, Sapporo, Japan
E-mail: jin@trans-er.eng.hokudai.ac.jp(2) Faculty of Engineering, Hokkaido University, Sapporo, Japan(3) Civil Engineering Research Institute for Cold Region, Sapporo, Japan(4) Pacific Consultants Co., Ltd., Sapporo, Japan
Abstract
Leachate containing arsenic (As) was observed from excavated rocks of tunnels
located in the north of Hokkaido, Japan. The leachate could potentially contaminate thesurrounding soil and groundwater environment. Thus, column leaching experiments using
the excavated rocks with adsorption and covering-soil layers both consisting of a river
sediment were carried out to identify the leaching mechanism of As and to decrease theleaching concentration of As from the rocks. The leachate from the column packed onlywith the excavated rocks showed alkaline pH and As concentration exceeding theenvironmental standard of Japan. The As concentration was correlated with iron and sulfate
concentrations in the leachate. This indicates that the As leaching results from oxidation ofpyrite contained in the rocks. The column having an adsorption layer showed that theadsorption layer reduced pH of the leachate near neutral and that the leaching concentrationof As decreased. In particular, As concentration dramatically decreased to approximately 2μg/L when the column had both adsorption and covering-soil layers. This means that bothlayers can reduce oxygen concentration in the rock layer, which reduce the rate of oxidationand the leaching of As.
Keywords: Arsenic, column experiments, pyrite, adsorption, covering-soil layer
1.

Introduction

layer were conducted. In addition, a column
experiment with both adsorption and covering-soil
layers were carried out to understand effects of theselayers on As leaching.

Arsenic (As) is well known as one of toxic
elements, which affects human health. It has beenreported that hydrothermally altered rocks andmudstone of marine origin have potential hazard
because they release a variety of toxic elements such
as As (Tabelin and Igarashi, 2009; Tabelin et al.,2010; Tabelin et al., 2013). When such rocks aredisposed, the hazardous elements may be leached out
and contaminate surrounding environment andgroundwater. To solve this issue, these rocks havebeen covered with sealing sheet or geomembrane toisolate the rocks from rainwater and groundwater.
However, this method is expensive and impracticaldue to a huge volume of excavated rocks produced ina short term. Thus, reasonable methods to dispose therocks are required.
Recently, the adsorption layer method has beenreported as one of the reasonable countermeasures(Tabelin et al., 2013; Tatsuhara et al., 2012).
Therefore, column experiments with an adsorption

2.

Materials and Methods

2.1 Sample collection
An excavated rock sample used in this study wascollected from a tunnel (OT tunnel) located in thenorth of Hokkaido, Japan. The rock sample wascollected from a storage site of As-rich excavatedrocks. The sample was brought back to the laboratory,
and crushed by a jaw crusher, air dried in room
temperature, and sieved through a 20 mm aperturescreen. The <20 mm fraction was used for theexperiments.
A river sediment used in this study was takenfrom a river near the tunnel construction site by usinga shovel at random points of the impoundment, andair dried at room temperature, lightly crushed usingmortar and pestle, and sieved through a 2 mm

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10th Asian Regional Conference of IAEG (2015)

aperture screen. The <2 mm fraction was used asadsorption and covering-soil layers in theexperiments.

adsorption layers. Case 3 had both covering-soil andadsorption layers on and under the excavated rocks.
Distilled water was poured at 50 mL/week in Cases
1-1 and 2-1, and 200 mL/week in Cases 1-2, 2-2, and3. Each layer was separated by a PVC net and No. 5Cfilter.
After collecting the effluent, temperature, pH,
oxidation reduction potential (ORP), and electricalconductivity (EC) were measured. The effluent
samples were filtrated through 0.45 μm Millex®
filters and provided for chemical analysis.

2.2 Chemical and mineralogical analyses of thesamples
To measure the chemical and mineralogicalproperties of the rock sample and river sediment, an
X-ray fluorescence spectrometer (XRF) and X-ray
diffractometer (XRD) were utilized. In thepreparation for the analyses, the excavated rocks andriver sediment were crushed to <50 μm. Spectro
Xepos (Rigaku corporation, Japan) and Multi Flex
(Rigaku corporation, Japan) were used for chemical
and mineralogical analyses, respectively. In addition,a scanning electron microscope (SEM: ShimadzuCorporation, Japan) and energy dispersive X-ray
spectrometry (EDX: Shimadzu Corporation, Japan)
were used to identify trace minerals in excavatedrocks. In this analysis, the excavated rocks of theother tunnel located in the same geological formationnear OT tunnel were used because the fresh
excavated rocks of OT tunnel could not be collected.

2.4 Chemical analysis of effluent
Concentration of dissolved As higher than 0.1
mg/L was analyzed by using an inductively coupledplasma atomic emission spectrometer (ICP-AES)(ICPE-9000,Shimadzu
Corporation,Japan).
Concentration of dissolved As less than 0.1 mg/L wasanalyzed by using the ICP-AES connected with ahydride vapor generator. Before analyzing As,
effluent samples were filtrated through Sep-pak®
cartridge (Waters Corporation, USA) to remove As(V). Thus, total As and As(Ⅲ) were analyzed directly
by using ICP-AES. As(V) concentration wascalculated by the following equation.

2.3 Column experiments
The details of columns used are depicted in Fig. 1.
In the experiments, five columns were constructed tounderstand the leaching and adsorption behaviors by
the effects of an infiltration rate and covering-soillayer. Cases 1-1 and 1-2 had only excavated rocklayer. Cases 2-1 and 2-2 had excavated rock andAs(V) = Total As – As(Ⅲ)
Concentrations of coexisting ions were determinedby using cation and anion chromatographs (ICS-90
and ICS-1000, Dionex Corporation, USA).
3.

Results

3.1 Properties of samples
Mineralogical and chemical compositions of theexcavated rock and river sediment were listed inTables 1 and 2, respectively. The excavated rock
Table 1 Mineralogical properties of the rockand river sedimentFig. 1 Schematic diagram of columns used in theexperiments

Minerals

QuartzAlbiteSiderite

OT
River sediment++++++++++++++: Strong, ++: Medium, +: WeakTable 2 Chemical composition of the rock and river sediment
Sample

SiO2

TiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

SO3

As

(wt.%)(wt.%)(wt.%)(wt.%)(wt.%)(wt.%)(wt.%)(wt.%)(wt.%)(wt.%)(wt.%) (mg/kg)

OT

61.3

0.59

12.3

5.94

0.05

1.87

1.13

1.80

2.92

0.06

0.17

6.3

River sediment

55.3

0.81

15.2

6.97

0.16

2.02

1.75

1.35

1.73

0.07

-

0.9-: Under the detection limit

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contains 6.3 mg/kg of As whereas the river sedimentcontains As less than 1 mg/kg.
Figure 2 shows the SEM image of sulfur and ironin the excavated rock of the other tunnel located in
the same geological formation near OT tunnel. Tracepyrite was observed in this figure. The results of thechemical analysis of the point showed 55wt.% of Feand 45wt.% of S. This means that the excavated rockof OT tunnel contains pyrite. Since pyrite inhydrothermally altered rocks contains As (Tabelin etal., 2012), As is likely to be contained in the rock.

in Case 1-2 and the environmental standard of As in
Japan. The highest total As concentration in Case 2-2
was 17 μg/L at the 4th. week whereas that in Case 1-2
was 30 μg/L at the 1st. week. In particular, total As
concentration in Case 3 remained almost constantaround 2 μg/L and the highest As concentration wasdecreased to 3 μg/L.
From these results, the river sediment reveals asignificant performance of As adsorption. In addition,
the covering-soil layer influenced reducing Asleaching.

Fig. 2 SEM image of the rock
3.2 As concentration of effluent from columns
Figure 3 shows total As concentration of effluent
from the columns in Cases 1-1 and 1-2 at an irrigation
rate of 50 mL/week. Except the As concentration at
the 4th. week, total As concentration in Case 2-1 wasless than that in Case 1-1. Furthermore, total As
concentration in Case 2-1 was less than As
environmental standard in Japan (10 μg/L) except at
4th. and 5th. weeks whereas that in Case 1-1 exceeded
the standard for all weeks. The highest total As
concentrations in Cases 1-1 and 2-1 were 50 and 36
μg/L, respectively.

Fig. 4 Total As concentration vs. time(200 mL/week)4.

Discussion

To evaluate the effects of adsorption andcovering-soil layers on As release, the amount of As
released from columns was calculated by thefollowing equation.
As leaching amount = ΣCiVi
where, C is As concentration (μg/L), V is the volumeof leachate (L), and i is the number of effluent
collected.

Fig. 3 Total As concentration vs. time
(50 mL/week)
Figure 4 shows total As concentration of effluent
from the columns in Cases 1-2, 2-2, and 3 at anirrigation rate of 200 mL/week. Total As
concentration in Case 2-2 was generally less than thatFig. 5 The amount of As released by consideringchemical species

310th Asian Regional Conference of IAEG (2015)

Figure 5 shows the amount of total As released
from columns by considering chemical species. Theamount of total As released in Cases 2-1 and 2-2
decreased by 74% and 54%, compared with those inCases 1-1 and 1-2, respectively. The amount of As
released in Case 3 decreased by 90% compared withthat in Case 1-2. In addition, not only As(V) but alsoAs(III) was adsorbed by the river sediment although
the major chemical species of As was As(V) in anycase. Furthermore, the leaching of both As(V) andAs(III) from the rock layer was reduced by being
covered with the river sediment. Therefore, bothadsorption and covering-soil layers have significant
effects on reducing As leaching.
Figure 6 shows the relationship between As andSO42- in Cases 1-1 and 1-2. A positive correlation
between As and SO42- was observed. Trace pyrite
was also observed as shown in Fig. 2. These indicatethat As leaching results from oxidation of pyrite
containing As. The data in Cases 2-1 and 2-2 are alsoplotted in Fig. 6. Only an adsorption layer is noteffective in reducing nonreactive SO42- concentration
because the concentration range in SO42- were almost
the same between Cases 1-1 and 2-1, and between
Cases 1-2 and 2-2.
By considering the above results, adsorption layer
retains As leached from the crushed rock. Moreover,both adsorption and covering-soil layers may reduce
oxygen concentration in the rock layer, leading to
reducing the rate of oxidation and the leaching
concentration of As.

5.

Conclusions

The effects of adsorption and covering-soil layers
on As leaching were examined. The findings are
summarized as follow:(1) The bottom adsorption layer adsorbed Asleached from the excavated rock, and reduced
the As concentration in the effluent fromcolumns.
(2) Arsenic concentration in effluent from thecolumn with both adsorption and covering-soillayers was dramatically decreased comparedwith that without a covering-soil layer.
(3) Arsenic leaching from the excavated rock resultsfrom oxidation of pyrite in the rock used.
(4) Both adsorption and covering-soil layers have an
important role in reducing As release.
References
Tabelin, C.B. and Igarashi, T. (2009): Mechanism ofarsenic and lead release from hydrothermally alteredrock, Journal of Hazardous Materials, Vol. 169, pp.
980-990.
Tabelin, C.B., Igarashi, T. and Tamoto, S. (2010):
Factors
affectingarsenicmobility
fromhydrothermally altered rock in impoundment-type insitu experiments, Minerals Engineering, Vol. 23, pp.
238-248.
Tabelin, C.B., Igarashi, T., Tamoto, S. and Takahashi,
R. (2012): The roles of pyrite and calcite in themobilization of arsenic and lead fromhydrothermally altered rocks excavated in Hokkaido,
Japan, Journal of Geochemical Exploration, Vol.
119, pp. 17-31.
Tabelin, C.B., Igarashi, T., Yoneda, T. andTamamura, S. (2013): Utilization of natural andartificial adsorbents in the mitigation of arsenic
leached from hydrothermally altered rock,
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Tatsuhara, T., Arima, T., Igarashi, T. and Tabelin,
C.B. (2012): Combined neutralization-adsorption
system for the disposal of hydrothermally alteredrock producing acidic leachate with hazardous
elements, Engineering Geology, Vol. 139-140, pp.
76-84.

Fig. 6 Total As concentration vs. SO42concentration in effluent
The pH values of effluent from the columns inCases 1-1 and 1-2 were alkaline ranging from 7.7 to
9.7 whereas those from the columns in Cases 2-1 and2-2 were neutral ranging from 6.9 to 8.0. This meansthat the adsorption layer has an effect of decreasing
pH from alkaline to almost neutral.

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