4.2 Soil texture and chemical properties
Tin mining activities have drastically altered soil texture, resulting
negative impact attributed mainly by the presence of high volumes of
tailings (Nouri et al., 2011). In the present study, sand fraction
increased from 57-69% in native soils to 82-96 % in soil tailing
(mostly > 90 %) implying that the ability of soil tailing
to retain water was very low, leading to extreme limiting factors for
crop growth. Similar results reported by Ashraf et al (2010) in post-tin
mining in Malaysia containing both gravel and sand as much as 95%. In
addition, the high sand fraction of tailing causes high soil temperature
during the day time, rapid drainage (high pores), intensive nutrient
leaching, low cation exchange capacity to retain nutrients, leading to
water stress and insufficient nutrients for crops.
Total SiO2 content of tailings is very high (92-96%)
and agrees well to the predominance of quartz mineral constituent (70-95
%), implying that silica derived mainly from quartz minerals, thereby
native nutrient sources for crops are negligible (Table 1). Predominant
quartz accompanied by some opaque, zircon and tourmaline minerals, which
are mineral resistant to chemical weathering and bear trace if any
nutrient for crops, clearly indicated that native nutrient sources were
negligible. The low nutrient content is supported by XRF analysis
showing extremely low concentrations of total elemental oxides of Ca,
Mg, K, P, and S (< 0.2% altogether) in each layer of tailing
profiles (Table 2). The implication was many nutrients (if not all)
became serious limiting factors for crop growth. Hence for reclamation
purposes, the high rate and many types of input (fertilizers) were
required to allow plant growth in the sandy tailings.
The exception is the TBB3 spoil profile contains lower
SiO2 in the topsoil (43%) but is high in the subsoil
(93%) owing to the presence of significant amounts of silt and clay
fractions (their summation was 19%) deriving from native topsoil and
mixed with tailing during mining processes. The mixture of tailing and
native topsoil contains high oxides of
Al2O3 (36 %) and
Fe2O3 (5 %) (Table 2). This observation
provides an advantage in tailing management by adding some native
topsoil to promote water and nutrient retentions.
The comparison of tailings deriving from different parent materials
showed tailing from granite contained higher total K2O
(0.2%) than from sandstone (< 0.05%). The high
K2O in tailing derived from granite was related to the
presence of K-bearing minerals (orthoclase, biotite and sanidine) in the
granite, while K-bearing minerals were absent in sandstone parent
materials. This statement was supported by total elemental analysis of
granite rocks that showed the high K2O content (5.2%,
Table 2). This observation is valuable for tailing deriving from granite
material and might have more inherent K nutrient source in a long-term
period of time.
The heavy metals in tailing were mainly
Cr2O3 and its concentration is lower in
native soils than tailing, leading to point out that Cr element was
occluded in mineral host structures, i.e., garnet (uvarovite species,
Ca3Cr2(SiO4)3) (Table 1). In addition, epidote
mineral (Hancockite,
CaPb2Al2Fe(SiO4)3(OH))
may host Pb. The dominance of Cr heavy metal may achieve 4695 mg
kg-1 in serpentinite rocks, corresponding to 533-633
mg kg-1 in soil develop on it (Anda, 2012). According
to Hudson-Edwards (2003) the information of the mineralogy of heavy
metals bearing phases is important in (i) understanding their stability,
solubility, mobility, bioavailability and toxicity; (ii) modelling their
future behaviour; and (iii) developing remediation strategies. Hence the
Cr and Pb heavy metals in this study may have been preserved in mineral
structure of garnet and epidote, respectively and led to minimum
solubility in soils, corresponding to minimal health risk.
For the Sn heavy metal, it is anticipated to have a high concentration
in post-tin mining areas. In fact, Sn concentration is low and only
occurs in a few layers of tailings. The low concentration could be
explained by Sn-bearing mineral (cassiterite) has been mined as a target
tin ore but was not completely removed from the sand fraction during the
separation of tin ores. The interesting finding was the much higher
concentration of SnO in the topsoil than the subsoil of native soils
(TBB4 and TBB5 profiles, Table 3). This trend indicated that Sn
host-mineral (cassiterite) was resistant to chemical weathering and
immobile in soils, which according to Smeck et al. (1994), the highest
concentration of mineral resistant to weathering at the soil surface was
due to maximal losses of components susceptible to weathering. The
resistance of cassiterite mineral against chemical weathering was
evaluated using a scanning electron microscope (SEM), and the result
showed grain morphological features with fresh, clean, and smooth
surfaces (Figure 1a). These morphological mineral features are
indicators of mineral resistant to chemical weathering in the
environment (Anda et al., 2009). According to Aleva (1985) cassiterite
barely experiences weathering and the solution of cassiterite in surface
and soil waters is slight. Cassiterite (Sn02), the main
tin mineral of ores, is both heavy and chemically resistant against
weathering, leading to the formation of large deposits or residual
concentrations (Sainsbury, 1969; Gama-Castro et al., 2000). The minimal
Sn2+ translocation and uptake by plants associated
with low solubility in soils were also reported by Nakamaru and Uchida
(2008) in tin Japanese agricultural soils.
In Chile, Ramirez et al. (2005) reported that Cd, Fe, Mn, Ni, and Pb
were mostly occluded in the mineral structure, corresponding to their
minimal availability in soils. According to Tessier et al. (1979) the
occluded heavy metals were mainly associated with crystal structure of
primary and secondary minerals. Therefore, the health risk of Cr, Pb and
Sn heavy metals should not be of major concern in the post-tin mining
areas, especially in the short-term period. To support this statement
the available heavy metals were measured using CaCl2 and
the results showed that the heavy concentration is very low in all
tailings and native soils, namely (mg kg-1)
< 0.5 for Pb, < 0.2 for Cr and not detected for Sn
(Table 4).
The elemental composition of tailing also showed the high Cl content in
sandy tailing (370-970 mg kg-1) may suggest that Cl
element was occluded in the primary mineral structure, probably in
quartz or pyroxene mineral. In addition, Cl content was much higher in
native soil profiles (1270-1380 mg kg-1) than tailing
(370-970 mg kg-1) which is associated with the high
clay content to hold Cl, releasing from host-minerals during soil
formation processes.
The drastic decrease in soil cation exchange capacity (CEC) less than 2
cmoc kg-1 attributed by tin mining was
mainly related with the loss of soil clay fraction during washing to
separate tin ores from other refused materials and left behind the
accumulation of sand fraction with very low or negligible CEC. However,
there is an interesting observation in respect to rehabilitation as
revealed by TBB6 tailing profile, which has been reclaimed since 1990
and showed considerably higher CEC (varying from 1.6 to 2.2
cmolc kg-1) among other profile
tailings (< 1.3 cmolc kg-1).
This indicates reclamation practice used Acacia mangeum was
successful in improving soil CEC by increasing soil organic C and clay
content (from 8 to 13%), especially in the uppermost part of the
tailing. The low CEC of tailing (< 2 cmockg-1) was also reported in tin mining in Malaysia
(Madjid et al., 1998).