Download Mineralogy and Leaching Behavior of Mineralized Rocks Excavated

Paper ID 50
Mineralogy and Leaching Behavior of Mineralized Rocks Excavated
Near Tunnel Construction Sites
Miyuki HIROTA1, Nohara YOKOBORI1, Toshifumi IGARASHI2
and Tetsuro YONEDA2
1
Graduate school of Engineering, Hokkaido University, Sapporo, Japan
2
Faculty of Engineering, Hokkaido University, Sapporo, Japan
e-mail: [email protected]
ABSTRACT
Several tunnels for the Hokkaido Bullet Train Line are being planned for construction in mineralized
areas widely distributed in Hokkaido, Japan. Mineralized rocks when exposed to surface oxidizing conditions
are potential sources of acid rock drainage (ARD) and heavy metals/toxic metalloids. Mineralogical analyses
and leaching experiments using the mineralized rocks collected near the construction sites were carried out to
characterize the rock samples. The results showed that they contained substantial amounts of sulfide and zinc
minerals, and the leaching concentrations of Cd and Pb were related to zinc or sulfur content. This indicates
that zinc or sulfur will be a good indicator of leaching of hazardous elements.
KEY WORDS: Leaching / Mineralized rocks / Mineralogy
1. INTRODUCTION
Contamination of soil and groundwater by toxic
elements, such as arsenic (As), cadmium (Cd) or lead
(Pb), is a serious problem around the world. Chronic
exposure to As may cause skin pigmentation,
hyperkeratosis, cardiovascular disease and cancer
[1][2]. The mining and metal processing industry is a
major anthropogenic contributor of As to the
environment. Lead contaminations of soils have also
been reported in mining areas, and chronic Pb
poisoning causes paralysis of the peripheral motor
nerves, anemia, kidney damage, abnormal fetal
development,
and
abnormal
neurological
development
and
function
[3][4].
The
biogeochemical and ecological impacts of Cd have
been two of the fastest growing areas of research
since the 1960s, because of the discovery of a painful
bone disease (itai-itai disease) caused by Cd
pollution in Fuchu, Toyama Prefecture, Japan [5][6].
Anthropogenic sources of Cd include ash from coal
combustion, atmospheric fallout, urban refuse,
fertilizer, smelting and refining of non-ferrous
metals, and manufacturing processes of metals and
chemicals [6]. Cadmium is toxic because it causes
irreversible damage to most cell types via the
inhibition of cell respiration and some key enzymedependent processes. The specific target systems
include the lungs, erythrocytes, spleen, endocrine
glands, liver and kidneys [7].
These toxic elements are relatively rare in
nature, but could be found concentrated into soils or
rocks due to certain natural phenomena, such as
weathering of sulfide-rich parent materials, volcanic
activity and forest fires. Mineralized rocks formed
due to active volcanic strata are important sources of
these toxic elements. These rocks contain sulfide
minerals, such as pyrite (FeS2), which is of primary
interest because of its importance in the generation of
acid rock drainage (ARD) as well as a source of
heavy metals/toxic metalloids. Sulfide minerals are
unstable under oxidizing conditions so that once
these rocks are exposed to surface conditions, ARD
and heavy metals/toxic metalloids are simultaneously
released into the surrounding environment.
Hokkaido in northern Japan has many active
volcanoes, mineralized areas and abandoned mine
sites. Extension of the Hokkaido Bullet Train Line
from Honshu to Hokkaido requires the excavation of
several new tunnels. Many of the tunnels being
planned for excavation would transect across some of
the mineralized areas of the island. To understand the
leaching behavior of these heavy metals/toxic
metalloids from the rocks, we collected mineralized
rock samples near the tunnel construction site and
investigated the relation between the mineralogy of
the rocks and leaching behavior of the toxic trace
elements contained. Furthermore, the information
obtained here will be useful to evaluate the risks
involved in disposing of heavy metals/metalloid-rich
excavated rocks.
Environmental Concerns 33
2. MATERIALS AND METHODS
3. RESULTS
2.1 Materials
3.1 Mineralogical analysis
Nine mineralized rock samples were collected
in areas close to tunnel construction site. M-samples
(M1 and M2), K-samples (K1, K2, K3, K4 and K5),
O-sample (O1) and T-sample (T1) were collected.
These nine rocks were formed during the Middle
Miocene. The surface of the rocks was weathered
since the rocks were collected near the ground
surface. Thus, the oxidized surface of the rocks were
removed by washing with deionized water.
Table 1 lists the identified mineralogical
composition of the rock samples by XRD. Sample
M1 contains pyrite while those from K-site have
pyrite, galena (PbS) and/or sphalerite (ZnS). Figure 1
shows photomicrographs of a sphalerite crystal using
polarized light microscopy, indicating the presence
of significant impurities in sphalerite.
2.2 Methods
2.2.1 Mineralogical analysis
The rocks collected were air dried, crushed
using a jaw crusher or agate mortar and pestle, and
sieved through a 2 mm aperture screen. The crushed
rocks passing the screen were further ground to <50
μm in preparation for the mineralogical and chemical
analyses. Polished thin sections of the rock samples
were also prepared for the microscopic analysis using
a microscope and an electron probe micro analyzer
(EPMA). Microscopic observation and X-ray
diffraction spectrometry (XRD) were carried out to
characterize the rock samples. On the other hand, Xray fluorescence spectrometry (XRF), scanning
electron microscopy/energy dispersive spectroscopy
(SEM-EDS) and EPMA were conducted to determine
the relative abundance of the major and trace
elements in the samples.
2.2.2 Batch leaching experiments and chemical
analyses
Batch leaching experiments using crushed rock
samples (<2 mm) were carried out. These were done
by mixing 5 grams of the rock samples and 50 ml of
deionized water for 6, 24 and 168 h at 120 rpm. After
the predetermined shaking time, the pH, electrical
conductivity (EC) and oxidation-reduction potential
(Eh) of the suspensions were measured. This was
followed by filtration of the suspensions through 0.45
μm Millex® membrane filters (Merck Millipore,
USA).
Dissolved As, Cd, Pb and Zn in the filtrates
were determined using an inductively coupled
plasma atomic emission spectrometer (ICP-AES)
(ICPE-9000, Shimadzu Corporation, Japan) or an
inductively coupled plasma mass spectrometer (ICPMS) (iCAP Qc, Thermo Scientific, USA). Sulfate
(SO42-) concentration was quantified by anion
chromatography (ICS-1000, Dionex Corporation,
USA).
Table 1 The mineralogical properties of the rock
samples by XRD
Samples
M1
M2
K1
K2
K3
K4
K5
O1
T1
Identified minerals
Quartz, pyrite
Quartz
Sphalerite, galena, anglesite, susannite, barite
Quartz, pyrite, muscovite
Quartz, sphalerite, barite
Sphalerite, galena, cerussite, barite
Quartz
Quartz, muscovite
Quartz, hedenbergite
impurities
impurities
sphalerite
Figure 1 Photomicrographs of a sphalerite crystal of
sample K4 under polarized light
The sulfuer (S) and heavy metal/metalloid
contents of the rock samples are summarized in
Table 2. Although some samples contained
substanial amounts of As and Cd, As-bearing and
Cd-bearing minerals were not detected by the XRD
analysis. This suggests that these toxic elements are
found in the rocks most probably as in the crystal
matrix of other minerals like sphalerite or pyrite.
Table 2 Sulfur and heavy metal/metalloid contents of
the rock samples
M1
S
(wt %)
2.82
M2
0.171
113
13.0
4.82
7.43
K1
33.5
287,000
254,000
1.00
1,120
K2
0.925
124
690
7.20
5.30
K3
13.5
456
270,000
438
919
K4
22.2
42,600
418,000
1,580
3,510
K5
1.15
2,890
4640
16.8
33.8
O1
1.34
4,720
14,600
32.0
44.4
T1
0.108
309
523
5.14
5.68
Samples
Pb
(mg/kg)
22.0
Zn
(mg/kg)
48.0
As
(mg/kg)
32.0
Cd
(mg/kg)
6.20
Environmental Concerns 34
Figure 2 shows the elemental maps of one
sphalerite crystal of sample K4. The results show that
Cd is concentrated inside the sphalerite crystal. In
addition, As is also found in the sphalerite crystal,
but in contrast to Cd, a part of As may exist as the
mineral tennantite ((Cu, Fe, Zn)12As4S13).
4. DISCUSSION
The rock samples with sulfide minerals released
a significant amount of heavy metals. Among sulfide
minerals, sphalerite had the strongest effect on the
leaching concentration of Cd. On the other hand,
although some samples considerably contained
amounts of As, its leaching concentration was very
low. This could be attributed to the As
immobilization via coprecipitaion with ferric
hydroxide (Fe(OH)3), which starts to precipitate
above pH 3 or 4 [8].
The Zn content of the rocks correlated well with
the S content except for samples with pyrite (Figure
3). Moreover, the leaching concentrations of Zn had
a positive correlation with those of SO42- for most of
the rock samples while the correlation of Zn and
SO42- concentrations in samples with pyrite were not
statistically significant (Figure 4). This suggestes that
the oxidation of sphalerite is the primary source of
Zn and SO42- in most of the rock samples.
The results of batch leaching experiments
showed that the pH of the leachate of most of the
rock samples was around neutral, except for those
samples containig pyrite (Table 3).
The concentrations of Cd leached from Ksamples were significantly higher than the
environmental standards of Japan (10 µg/L) (Table
3). However, those from the other sites were lower
than the K-samples. The concentrations of Pb in the
leachate of M-, K-samples and O1 were higher than
that of T1. In contrast to Cd and Pb, the
concentrations of As were very low in all samples
and were not detected by ICP-MS. The
concentrations of heavy metals in the leachate did not
change dramatically with time, reaching apparent
equilibrium after just 6 h.
100000
10000
1000
with pyrite
without pyrite
100
10
Samples
M1
M2
K1
K2
K3
K4
K5
O1
T1
pH
3.94
4.95
5.18
4.15
6.00
6.06
5.29
7.21
7.62
As
(mg/L)
ND
ND
ND
ND
ND
ND
ND
ND
ND
Cd
(mg/L)
0.0037
0.0016
0.243
0.0275
0.0187
0.436
0.0252
0.0051
0.0048
Pb
(mg/L)
1.06
0.198
7.78
0.255
0.126
7.53
4.42
0.119
ND
1000
10000
100000
1000000
Figure 3 Zn content vs. S content of the rock
samples
1000
with pyrite
without pyrite
100
10
1
0.1
10
Table 3 Leachate chemistry of the rock samples
100
S (mg/L)
Zn (mg/L)
3.2 Leaching behaviour of arsenic, cadmium and
lead
Zn content (mg/kg)
1000000
Figure 2 Elemental maps of S, As, Cd and Zn of the
rock sample (K4) at a magnification of 50 μm (The
sphalerite grain is the part of the elemental map of Zn
with color.)
100
1000
SO42- (mg/L)
Figure 4 Zn concentration vs. SO42- concentration in
the leachate
The concentrations of Cd and Zn had a strong
positive correlation (Figure 5), indicating that the
processes responsible for their release are similar.
Since majority of Zn came from the oxidation of
sphalerite and Cd was mostly found as an impurity in
sphalerite crystal, it follows that the main release
mechanism of Cd is the oxidation of sphalerite under
oxidizing conditions.
Environmental Concerns 35
5. CONCLUSION
1
R2 = 0.84742
Cd (mg/L)
0.1
0.01
1E-3
0.1
1
10
100
Zn (mg/L)
Figure. 5 Cd concentration in the leachate vs. Zn
concentration in the leachate. R2 is the determination
coefficient.
The total amounts of Pb and Cd leached from
the rocks strongly depended on their initial contents
of these elements (Figures 7 and 8). The moderate
positive correlations between the dissolved
concentrations of Pb and Cd to their initial content of
the rock indicate that the leaching concentrations of
these toxic elements would increase proportional to
the their initial contents in the solidphase.
The mineralogical properties of several
mineralized rock samples from Hokkaido as well as
the leaching behaviors of As, Cd and Pb found in
these rocks were elucidated. The findings are
summarized as follows:
(1) The leaching of heavy metals/metalloid was
caused by the oxidation of sulfide minerals,
especially sphalerite.
(2) We can roughly estimate the leaching
concentrations of Cd and Pb from the Cd and Pb
contents of the rocks.
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Environmental Concerns 36