3037
Evaluating the long-term leaching characteristics of heavy metals in excavated
rocks
Évaluation des caractéristiques de lixiviation à long terme de métaux lourds dans les roches
excavées.
Inui T., Katsumi T., Takai A.
Kyoto University, Kyoto, Japan
Kamon M.
Kagawa National College of Technology, Takamatsu, Japan,
ABSTRACT: When excavation works are executed at stratums that naturally contain heavy metals due to their geologic histories,
time-saving and reliable assessment of contamination potential by these metals is a current geotechnical challenge. This paper
addresses the long term leaching characteristics of arsenic and lead in several rock materials, which were weathered in outdoor for
more than two years. In addition, applicability of several time-saving laboratory test methods were verified as a tool to estimate the
actual leaching behaviour of heavy metals.
RÉSUMÉ :
Lorsque des travaux d’excavation sont effectués dans des strates contenant naturellement des métaux lourds en raison de
leurs antécédents géologiques, une évaluation fiable et économe en temps du potentiel de contamination par ces métaux pose à l’heure
actuelle un problème géotechnique. Ce document traite des caractéristiques de lixiviation à long terme de l’arsenic et du plomb dans
plusieurs matériaux rocheux, ces derniers ayant été exposés à l’extérieur aux intempéries pendant plus de deux ans. En outre,
l’applicabilité de plusieurs méthodes de test en laboratoire économes en temps a été vérifiée comme outil d’évaluation du
comportement de lixiviation réel des métaux lourds.
KEYWORDS: natural contamination, rock, arsenic, leaching test, outdoor exposure test
1. INTRODUCTION
When excavation works are executed at stratums that naturally
contain heavy metals due to their geologic histories, time-saving
and reliable evaluation to judge the contamination potential by
these metals and metalloids is required for proper management
of geomaterials with unacceptable contamination potentials.
Heavy metals existing naturally in stratums are fundamentally
stable and immobile under the anaerobic depositional
environment. However, in some cases, leaching of these
constituents is triggered when exposed to water and oxygen
after excavation works. Acid drainage, which comes mainly
from the interactions between water, oxygen, and sulfide
minerals in soils and rocks, often produces sulfide acid and
increases the leaching concentrations of metals Thus,
development of testing methodologies to assess their long-term
leaching potential and behavior has been a great
geoenvironmental issue in Japan (e.g. Hattori et al. 2003,
Okumura et al. 2007). Generally, conventional batch and/or
column leaching tests have been widely employed for
evaluating the leaching potential of contaminated soils and
waste-based geomaterials. However, when they are employed
for rock materials, sample preparation including sampling and
crushing is more likely to influence the leaching amount of
trace elements (Inui et al. 2010). In addition, effects of
oxidation after excavation on the leaching behavior should be
considered since the acid rock drainage due to dissolution of
sulfide minerals in rock is one of the main mechanisms which
promote the leaching of heavy metals.
This study firstly presents the results of more than 27-month
outdoor exposure tests conducted for five rock samples to assess
their in situ leaching characteristics. They contain certain
concentrations of natural-derived lead and arsenic, which are
expected to be released if they are exposed to water and oxygen.
The main focus is placed on the comparison of leaching
characteristics of trace metals in outdoor exposure tests with
those in various conventional laboratory tests, which include the
total trace metal content test, conventional batch leaching test,
accelerated oxidation tests and column leaching test. Laboratory
leaching tests were validated as an index of the long term
leaching characteristics of trace metals by comparatively
assessing the results with the outdoor leaching behaviors
2 MATERIALS AND METHODS
2.1
Materials
Five different rocks materials were used. They were generated
in construction works such as excavation and tunneling, which
were executed in mountainous areas far from industrial and
urban areas in Japan. Thus, it can be considered that heavy
metals contained are derived from natural resources. Five rock
samples consist of black shale, andesite, and three mudstones
(MS-1, 2 and 3). They were all stored with their natural water
content under the temperature of approximately 20 ºC in sealed
condition to prevent the exposure to oxygen and water. Table 1
shows their appearances and the total contents of As and Pb as
well as chemical compositions which were determined by the
X-ray fluorescence analysis. According to the Japanese
guideline, excavated materials containing more than 9 mg-
As/kg or 23 mg-Pb/kg should be assessed in terms of their
contamination potential (Ministry of Land, Infrastructure,
Transport and Tourism, Japan 2010). All the samples used in
this study contains more than 9 mg-As/kg. Pb higher than this
standard was detected only in black shale.
Figure 1 shows pH values of rock samples for distilled water
and H
2
O
2
solutions with 3% and 30% concentrations. pH
measurement using H
2
O
2
solutions were employed to evaluate
the possible pH change of geomaterials when they are oxidized
under weathering conditions. Geomaterials with pH < 3.5 for
30% H
2
O
2
solution are considered to have acidification
potentials (Japanese Geotechnical Society 2009). In this study,
3% H
2
O
2
solution was also used to evaluate the acidification
potential against weaker oxidization effects. Black shale and
