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).