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\begin{document}

\title{Strategic reclamation of post-tin mining areas based on soil mineralogy,
heavy metal and particle size constituent of refused materials}



\author[1]{Markus Anda}%
\author[2]{Nurhayati Purwantari}%
\author[3]{Erna Suryani}%
\author[4]{Husnain Husnain}%
\author[5]{F. Agus}%
\affil[1]{Ministry of Agriculture Republic of Indonesia}%
\affil[2]{Indonesian Research Institute for Animal Production}%
\affil[3]{Indonesian Centre for Agricultural Land Resource Research and Development}%
\affil[4]{Indonesian Centre for Agricultural land Resouce Research and Development}%
\affil[5]{Indonesian Soil Research Institute}%


\vspace{-1em}



  
  \date{May 08, 2024}


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\selectlanguage{english}
\begin{abstract}
Characteristics of refused materials are prerequisite information
required to determine the strategic reclamation of extreme land
degradation in post-tin mining areas. The objective of the study was to
evaluate mineralogical, chemical, and physical properties and heavy
metals of spoil and tailing as the basis for reclamation measures. Seven
representative soil profiles were made and sampled to a depth of 130 cm
for various soil analyses. Results showed that tin mining has
drastically altered the soil texture from sandy clay loam under native
conditions to loamy sand and sand under post-tin mining. Mineralogical
constituents of refused materials were mainly mineral resistant to
chemical weathering, consisting of predominant quartz with small amounts
of tourmaline, opaque, zircon and garnet. Total X-ray fluoresce
elemental analysis showed extreme high SiO2 content (92-96\%) associated
mainly with quartz mineral, and extremely low oxides of Ca, Mg, P, K and
S (\textless{} 0.2\% altogether). This suggests all nutrients are severe
problems for crops. Type of total heavy metals showed the Cr2O3 was high
in sandy tailing (204 - 286 mg kg-1), while the SnO was low (0 -153 mg
kg-1) and they were preserved in the structure of host minerals, thereby
the health risk is negligible. Based on many serious constraints of
soils, the strategic reclamation to recover soil productivity and
ecological function was the building up soil organic matter,
establishing ``pot planting point'' technique, complete fertilizer
application, and selection of crops with an ability to fix N nutrient
from the atmosphere, and adaptive to low soil nutrients.%
\end{abstract}%



\sloppy


\textbf{Abstract}

Characteristics of refused materials are prerequisite information
required to determine the strategic reclamation of extreme land
degradation in post-tin mining areas. The objective of the study was to
evaluate mineralogical, chemical, and physical properties and heavy
metals of spoil and tailing as the basis for reclamation measures. Seven
representative soil profiles were made and sampled to a depth of 130 cm
for various soil analyses. Results showed that tin mining has
drastically altered the soil texture from sandy clay loam under native
conditions to loamy sand and sand under post-tin mining. Mineralogical
constituents of refused materials were mainly mineral resistant to
chemical weathering, consisting of predominant quartz with small amounts
of tourmaline, opaque, zircon and garnet. Total X-ray fluoresce (XRF)
elemental analysis showed extreme high SiO\textsubscript{2} content
(92-96\%) associated mainly with quartz mineral, and extremely low
oxides of Ca, Mg, P, K and S (\textless{} 0.2\% altogether). This
suggests all nutrients are severe problems for crops. Type of total
heavy metals showed the Cr\textsubscript{2}O\textsubscript{3} was high
in sandy tailing (204 - 286 mg kg\textsuperscript{-1}), while the SnO
was low (0 -153 mg kg\textsuperscript{-1}) and they were preserved in
the structure of host minerals, thereby the health risk is negligible.
Based on many serious constraints of soils, the strategic reclamation to
recover soil productivity and ecological function was the building up
soil organic matter, establishing ``pot planting point'' technique,
complete fertilizer application, and selection of crops with an ability
to fix N nutrient from the atmosphere, and adaptive to low soil
nutrients.

\textbf{KEYWORDS:} Post-tin mining, soil characteristics, heavy metals,
tailing, spoil, strategic reclamation

\textbf{1 INTRODUCTION}

Mining left the legacy of environmental problems attributed by disposal
of refused materials. Global land use for mining between 1976 and 2000
is cumulatively about 3,700,000 ha or about 0.2\% of earth's land
surface (Barney, 1980). Mines produce large amounts of refused-materials
because the ore constituent of the total volume of the mined materials
occurs only in a small fraction (Duhka and Adriano, 1997). For tin
mining, the separation of tin ores, the heavy sand fraction (density
\textgreater{} 4 g cm\textsuperscript{-3}), from the clay, silt and
light sand fractions leads to the dumping of large amounts of spoils and
tailings, and their cumulative amounts increase with time. The disposal
of tailings is a major environmental issue that has become more serious
with the increased exploration for metals and the exploitation of lower
grade mineral deposits (Ozkan and Ipekoglu, 2002).

Southeast Asian countries (Indonesia, Malaysia, Thailand and Myanmar)
produced 9.6 million tonnes of tin, equivalent to 54\% of the world's
tin production since 1800 (Schwartz et al., 1995). In 2017, Indonesia is
the second larger world producer (50,000 metric tonnes) of tin after
China (100,000 metric tonnes) with total world production of 290,000
metric tonnes (USGS, 2018). Further, tin world reserve is 4,800,000
metric tonnes in which China and Indonesian reserves are 1,100,000 and
800,000 metric tons, respectively (USGS, 2018). The major locations of
tin producer in Indonesia are Bangka, Belitung and Singkep islands. Open
pit mining of tin is destructive due to the complete removal of
vegetation and topsoil. In addition, tin mining processes involving the
dispersion and washing to separate heavy sand fraction of ores from
refused materials could result huge piles of sandy tailing. The large
idle bare land (abandoned tailings or waste piles) is massively
occurred. The high economic return from tin mining activities are not
reconciled with the existing environmental management.

In this study, tailing was referred to dumped materials left over after
the process of separating the valuable heavy sand fraction ores from the
uneconomic light sand fraction. Likewise, spoil was referred to an
accumulation of displaced earthy material or other waste material
removed during mining or excavation. Refused materials (e.g. spoils,
tailings and slime) are mine's byproducts accumulated from the
excavation, washing, concentration or treatment of ground ores to
extract valuable ores. The Bangka island has been exploited for tin
since 1711 (Ko, 1986), resulting a widespread of huge spoil and tailing
disposal as the extreme threat to the environment. Hence,
characterization of spoil and tailings in terms of chemical, physical,
mineralogical properties and heavy metal contents should be done to
obtain reliable information as a basis for reclamation measures.

Generally, mining is considered to be one of the most significant
sources of heavy metal contamination (Acosta et al., 2011; Dudka and
Adriano, 1997; Fryer et al., 2006). Ores and mineral extraction have
inflicted serious environmental damage, especially in the heavy metal
pollution (Acosta et al., 2011; Komnitsas and Modis, 2006; Zhou et al.,
2007). Metals of major interest in bioavailability studies, as listed by
the U.S. Environmental Protection Agency (EPA), are Al, As, Be, Cd, Cr,
Cu, Hg, Ni, Pb, Se, and Sb (McKinney and Rogers, 1992). Heavy metals
such as Pb, Zn, Cd, Hg and Cr generally refer to metals and metalloids
having densities greater than 5 g cm\textsuperscript{-3} (Oves et al.,
2012). Li et al., (2014) reviewed the soil heavy metal pollution from
mines in China and concluded that the mean concentrations of As, Cd, Cr,
Cu, Ni, Pb, Zn, and Hg in soils are higher (varying from 0.4 to 36.5
times greater, depended on a heavy metal type) than the Grade II
environmental quality standard for soils in China. The geochemical flux
explained the lithological origin of potential toxic elements (As, Cd,
Co, Cr, Cu, Fe, La, Mn, Ni, Pb, Sc, Th, V) (Pinto et al., 2017)

Mineralogical composition plays a crucial role in understanding native
nutrient sources for crops and heavy metals both short and long-term
period of time. However, mineralogical composition of refused materials
has been ignored in post-tin mining areas, and more attention is given
to chemical properties and heavy metals. Hence, we investigated mineral
composition of refused materials deriving from different parent
materials in post-tin mining areas and relating the minerals to soil
chemistry, nutrients and heavy metals. The objective of the study was to
evaluate mineralogical constituents, particle sizes, chemical and
physical properties and heavy metal contents of refused materials in
post-tin mining areas as the strategic basis for reclamation of soil
productivity and ecological function.

2 \textbf{MATERIALS AND METHODS}

\textbf{Study site and field samplings}

The study areas locate in Bangka Island, Kepulauan Bangka Belitung
province, Indonesia between X 626183 -Y 9757090 and X 632834 -Y 9752946
UTM grid, Zone 48 M with the elevation of 27 to 60 m above sea level.
The areas were selected based on their lithological type that represents
most of the post-tin mining areas in Kepulauan Bangka Belitung province.
Lithologically, Bangka Island occurred on granite and sandstone parent
rocks. The granitoid plutons of the Indonesian Tin Islands was Triassic
age (193-251 Ma) (Aleva,1985; Ko, 1986). It was expected that the
similarity in lithology of the areas would produce a similar property of
tailings and spoils. The process of tin mining and deposition of refused
material were mainly gravitational separation without any chemical
treatment. The companies having tin mining land concession extracted the
tin-host ores by dry-excavating of soils and the underlain deposit
layers, mostly down 5 to 50 m. The companies used conventional
bulldozers, loaders and haulers for the excavation of deposit ores. The
separation of non-tin layer sediment such as white clay was directly
dumped as a spoil, during the excavation. Hydraulic mining by spraying
the ore deposits associated soils to form a slurry and pumped to the wet
separation (concentrator) facility. The slurry was then passed through
sequences of cyclones and separator in which clays, silts, and some very
fine sands (collectively referred to as slimes) were separated
gravitationally from the sand fractions. The heavy mineral sand of ores
(particle density \textgreater{} 4.0 g/cm\textsuperscript{3}) was
further separated from the light sand fraction by cyclones water. The
refused fractions consisting of light sand were dumped as tailings.

The characteristics of spoils (mostly a mixture of native soil, light
sand and white clay) and sandy tailing piles of post-tin mining were
assessed and evaluated as a basis for reclamation measures. Tailings and
spoils deriving from sandstone and have been mined from 2000 to 2015
were represented by two sandy tailing profiles (TBB1 and TBB2), and a
spoil profile (TBB3). The topsoil of TBB3 profile consisted of initial
native soil with some tailing (0-30 cm), while the underlying layers
consisted of a mixture of white clay and sandy tailing. For tailings
resulting from granite materials and mining from 1960 to 1998 were
represented by TBB6 and TBB7 profiles. The native soil profiles adjacent
to post-tin mining areas were included for each type of parent materials
(TBB4 for sandstone, and TBB5 for granite materials) as a comparison to
spoils and tailings. We excavated a soil profile with the dimensions of
100 \selectlanguage{ngerman}× 200 × 120--130 cm (width × length × depth) for morphological
property observation and soil sampling. We collected a composite soil
sample for each soil layer from all sides of the profile; the sample was
thoroughly mixed prior to subsampling \textasciitilde{}1 kg for
mineralogy, chemical and heavy metal analyses.

\textbf{2.2 Laboratory analyses}

The potential of soil nutrient sources in spoils and tailings was
assessed by analyzing the primary mineralogical compositions of 50-250
\selectlanguage{greek}µ\selectlanguage{english}m sized sand fraction. Mineral types were identified using a polarizing
microscope (Carl Zeiss Microscopy, Germany). The number of each mineral
type was counted from 300 grains by line count traverses.

X-ray diffraction (XRD) analysis for tin ores and white clay sediment
was carried out using a Rigaku (SmartLab, Japan). Tin ores and white
clay were analyzed to determine host-mineral of Sn and that white clay
for type of clay minerals. The refused white clay layer during tin
mining could be potential to be used as ameliorant for sandy tailing.
The tin ores were collected from re-mined tailing by local miners nearby
the study site. The tin ores were dried and finely ground as powder to
pass through a 50 \selectlanguage{greek}µ\selectlanguage{english}m sieve and kept for XRD analysis. For white clay
sediment, it was collected from the spoil profile, dried and finely
ground as powder to pass through a 50 \selectlanguage{greek}µ\selectlanguage{english}m sieve. The powder specimen was
mounted on glass slides and run on X-ray diffractometer, using Cu-alpha
radiation target, operated at 40 kV and 25 mA. The powder specimens were
scanned from 3 to 45\selectlanguage{ngerman}º 2\selectlanguage{greek}θ \selectlanguage{english}at 1\selectlanguage{ngerman}°/min. XRD data were collected and stored
by IBM compatible PC.

The scanning electron microscope (SEM) analysis was performed for finely
ground tin ores (\textless{} 50 \selectlanguage{greek}µ\selectlanguage{english}m) using an EVO MA10 (Carl Zeiss
Microscopy, Cambridge, England) to observe their morphological features
and resistance to weathering processes. The specimens were oven dried
prior to gluing into an aluminum stub and then coated with carbon and
gold/palladium in a vacuum evaporator. The specimens were viewed at 11.0
and 15.0 kV, using a secondary electron detector. The chemical surface
compositions of specimens were analyzed using energy dispersive X-ray
(EDX).

The particle sizes of soil were determined by the pipette method (Soil
Survey Staff, 1992). The sand fraction was separated from the clay and
silt fractions by wet sieving. The silt- and clay-sized fractions were
measured by sedimentation according to Stokes law. The pH and electrical
conductivity (EC) were measured in water with a 1:5 soil:solution ratio
using an Orion pH meter and a conductivity meter, respectively. The
total organic C content was measured according to Walkley and Black wet
oxidation method (Soil Survey Staff, 1992). Total N was determined by
the Kjeldhal method (Bremner and Mulvaney, 1982). Soil cation exchange
capacity (CEC) was determined using a leaching column. The soil of 2 g
was transferred into a leaching column followed by leaching with 50 ml
of 1 M NH\textsubscript{4}OAc at pH 7.0 for an hour, then the cations
were measured in the supernatant (Soil Survey Staff, 1992) using an
atomic absorption spectrometer (AAS). The CEC was determined in 1 M
NH\textsubscript{4}OAc (buffered at pH 7.0) after extraction of
NH\textsubscript{4}\textsuperscript{+} as a measure of CEC by 10\% NaCl.
The Bray 1 method was used to determine soil available P, and that
exchangeable Al was extracted by 1 M KCl as described by Van Reeuwijk
(1993).

Total elemental analysis of native soil, spoil and tailing was
determined using X-ray fluorescence (XRF). Soil, spoil and tailings were
finely ground using a ball mill/ pulverizer to pass through a 100-200
mesh sieve as powder. Measurements of major and trace elements were
carried out on pressed pellets, prepared following the procedure of
Norrish and Chappell (1977). The sample powder was mixed with carboxyl
methyl cellulose (CMC) as a binder by lightly ground. Samples were
pressed into pellets with boric acid backing in a ring stainless steel,
and oven dried at 55\selectlanguage{ngerman}°C for about half hour. Elemental composition was
determined using X-Ray Fluorescence (Thermo Scientific ARL 9900 Series,
2011 Germany) with a Be tube operating at 60 kV, 40 mA for Ba,Sn, Cd,
Ag, Rb,Mo, Zr, Y, Pb, As, Se, Hg, Zr and Y; at 40 kV, 60 mA for Ni, Cu,
Co, Si, Al, Fe, Mn, V, Cr, Ti, Ca, K; and at 30 kV, 80 mA for Mg and Na
using scintillation counter. The available heavy metals were extracted
by 0.05 M CaCl\textsubscript{2}. The use of CaCl\textsubscript{2} in
place of 1 M MgCl\textsubscript{2} as used by Tessier et al. (1979) was
due to native conditions of tailing contain higher Ca than Mg, hence the
use of CaCl\textsubscript{2} extractant solution is more closely
represent the native conditions.

\textbf{3 RESULTS}

\textbf{3.1 Morphological features of tailing profiles}

Mining activities drastically altered soil morphological features
(Figures S1 and S2). In tailing derived from sandstone materials, tin
mining activities altered the soil morphological features from sandy
clay loam texture, dark brown topsoil and yellowish brown subsoil in
native soil (TBB4 profile) to sandy texture with stratification of light
grey and yellowish grey (TBB1) or of yellowish grey and reddish grey
(TBB2) colour throughout profiles (Figure S1). Further, a spoil profile
with a mixture of native topsoil and sandy tailing showed a mixture of
brown and greyish colours in the topsoil (TBB3 profile), which was
underlain by greyish sand and whitish kaolin colours. The presence of
yellowish and reddish colours in sandy tailing profiles indicated the
initiation of soil formation in post-tin mining areas.

For post-tin mining soils developed from granite parent materials,
morphological properties drastically altered from deep solum
(\textgreater{} 140 cm), dark greyish brown topsoil and yellowish brown
subsoil with sandy clay loam texture (TBB5; Figure S2) in the native
soil to loamy sand and grey colour in post-tin mining. These native soil
properties disappeared after mining was related to sluice and washing
during the separation of heavy tin ore from light sand fractions. The
profile of a sandy tailing (TBB6), which has been reclaimed for 26 years
using~\emph{Acasia mangeum} ~started a weak soil development as
indicated by yellowish colour (iron mobilization and precipitation), the
formation of fine soil granular structure, and the growth of many fine
roots downward to 60 cm deep (Figure S2). Unfortunately, the formation
of fine soil granular structure was not shown by TBB7 profile, a
counterpart of TBB6 and just 6 m away. The absence of fine soil granular
structure in the TBB7 profile was due to re-mined activities by local
traditional miners and abandoned as bare land by 2008 (8 years old of
post tin mining), leading to reset back the new start of soil
development. The interlayer variations with different colours within a
profile indicated the initial accumulation of iron with yellowish grey
colour (probably goethite) and it's leaching down the profile.

\textbf{3.2 Mineralogical composition of tailings}

Mineralogical composition of soils at post- tin mining areas was
assessed to get an idea of potential inherent nutrient source and heavy
metals in weathereable minerals (host-nutrients). Tailings and native
soils developing from sandstone contained 70-95\% and 81-84\% quartz,
respectively (Table 1). The corresponding values for tailings and native
soils developed from granite were lower, namely 48-73\% and 77-80\%,
respectively. In native soils (TBB4 and TBB5 profiles), the turbid
quartz was higher than transparent quarts, irrespective of parent
material types, indicating transportation was a dominant process during
soil formation.

In contrast to quartz mineral, the tourmaline mineral was considerably
higher in tailing deriving from granite (12-22\%) than tailing from
sandstone (2-10\%). The corresponding values in native soil profiles
were 5-7\% and 2-3\%, respectively. Both quartz and tourmaline minerals
are well known as mineral resistant to weathering. Opaque mineral was
also higher in tailing derived from granite (mostly 9-15\%) than tailing
from sandstone (1-7\%). Further, the tailing profiles deriving from
granite (TBB6 and TBB7) contained a small number of orthoclase and
sanidine, which were absent in tailing profiles from sandstone (TBB1 and
TBB2). Other minerals in small amount were zircon, garnet, epidote and
rutile/anatase in all tailings and native soils.

\textbf{3.3 Soil chemical properties of post-tin mining areas}

\textbf{3.3.1 Total major elemental oxides}

Total major and minor elemental compositions (expressed as elemental
oxides) as measured by XRF of selected layers of tailing, spoil and
native soil profiles are given in Tables 2 and 3. The composition of
major elemental oxides of tailing showed much higher
SiO\textsubscript{2} content (86-96\%) than the native soils (68-76\%).
The exception for the topsoil of TBB3 spoil profile contained much less
SiO\textsubscript{2} (43\%) associated with some native topsoil
containing 15\% clay. In tailings, the oxides of potential major
nutrients for crops such as Ca, Mg, K, P, S were very low in each soil
layer (\textless{} 0.2\% altogether), indicating these nutrients are
major problems in post-tin mining reclamation.

\textbf{3.3.2 Total and available heavy metals}

The content of total heavy metals showed the presence of
Cr\textsubscript{2}O\textsubscript{3} in all samples (native soils,
tailing and spoil), and SnO in all native soils and some layers of
tailing profiles (Table 3). The total concentration of
Cr\textsubscript{2}O\textsubscript{3} toxic heavy metal was considerably
higher in tailing (sand fraction) (204-286 mg kg\textsuperscript{-1})
than the native soils (149-190 mg kg\textsuperscript{-1}), indicating
that Cr was occluded in the host-mineral structure. On the other hand,
the SnO heavy metal was much higher in the native soil and spoil than in
tailings. In addition, the SnO was higher in a native soil deriving from
sandstone than the soil from granite.

Total concentrations of other heavy metals such as Pb, Cd, Ni and Hg
were not detected in native soils and tailings, except for the TBB3
spoil profile, where Pb was present in the topsoil. The non-heavy metal
(trace element) present in high amount was Cl and its concentration was
much higher in native soils (1270-1380 mg kg\textsuperscript{-1}) than
tailing (370-970 mg kg\textsuperscript{-1}).

The available heavy metals as measured by 0.05 M
CaCl\textsubscript{2}showed the presence of Pb, Cr and Hg in all native
soils and tailings, and few samples contained As (Table 4) but their
concentration was very low. Although total concentrations of Pb and Hg
were not detected in native soils and tailings, they present in small
amounts in available forms. The available Pb of soils deriving from
sandstone was similar in native and tailing conditions (\textless{} 0.30
mg kg\textsuperscript{-1}), which in turn was lower than Pb in native
soils and tailings deriving from granite materials (\textless{} 1.20 mg
kg\textsuperscript{-1}). The higher available Pb in tailings than native
soil deriving from granite, indicating that the Pb was inherited from
host rocks. This was supported by the total PbO in the granitic rocks
was 48 mg kg\textsuperscript{-1} (Table 3).

For Sn available form, it was not detected in both native soils and
tailings (Table 4), although its total concentration as measured by XRF
was high in native soils (203-418 mg kg\textsuperscript{-1}) (Table 3).
This further indicates that the Sn-bearing mineral (cassiterite) was
resistant to chemical weathering and Sn heavy metal should not raise
concern for health risk in the post- tin mining areas. Other heavy
metals such as Cd, Ni, and Co were not detected in both total and
available forms of native soils and tailings.

\textbf{3.3.3 Particle size, organic matter and nutrient availability of
post-tin mining areas}

Particle size in the fine earth (\textless{} 2 mm) of post-tin mining
areas was dominated by sand fraction that sharply increased from 57-69\%
in native soils to 82-96 \% in soil tailing (mostly \textgreater{} 90
\%) (Table 5). On the other hand, the clay fraction sharply decreased
from 27-38\% in native soils to less than 14\% in tailings and spoil
(Table 5). In other words, the soil texture changed from sandy clay loam
to sand. Further, organic matter (C and N) and potential
P\textsubscript{2}O\textsubscript{5} and K\textsubscript{2}O were very
low status in tailings and spoil, and their content was lower than
native soils.

In all cases, the layers of tailing profiles have higher pH (varying
from 5.2 to 6.3) than the corresponding layer of native soil profiles
(varying from 4.7 to 5.2) deriving from a similar parent material (Table
5). The higher pH of soil tailings than the native soils was associated
with the lower exchangeable Al in the former owing to sandy texture,
leading to very low ability to retain Al. In native soils, the high clay
fraction corresponded to the high exchangeable Al and low pH (Table 6).
The available P\textsubscript{2}O\textsubscript{5} in tailing was very
low (\textless{} 4 mg
kg\textsuperscript{-1}P\textsubscript{2}O\textsubscript{5} according to
Bray 1), suggesting the crops would experience a serious P deficiency.

Although potential K\textsubscript{2}O as extracted by 25\% HCl was very
low status in all tailings, there is a good trend for its higher
concentration in soils derived from granite parent materials compared to
tailings derived from sandstone. This is consistent with the high total
K\textsubscript{2}O in granite rocks (5.2\%) (Table 2). K-bearing
minerals were not observed in soil tailings deriving from sandstone.

All exchangeable cations, Ca, Mg, K and Na were very low with the sum of
\textless{} 0.8 cmol\textsubscript{c} kg\textsuperscript{-1} for each
layer of soil tailing, and the order of decreasing content was Ca
\textgreater{} Mg \textgreater{} Na \textgreater{} K (Table 6). The
exchangeable cations in tailings were only slightly lower compared to a
corresponding layer of native soil profiles. This indicates exchangeable
cations are serious limiting factors for crops in both native soils and
tailings. Cation exchange capacity (CEC) drastically decreased from
3.8-6.8 cmol\textsubscript{c} kg\textsuperscript{-1} in native soil
profiles to 1.0 - 2.3 cmol\textsubscript{c}
kg\textsuperscript{-1}(mostly \textless{} 1.5 cmol\textsubscript{c}
kg\textsuperscript{-1}) in tailing profiles.

\textbf{3.4 Characteristics of tin ores and white clay spoil}

Properties of tin-host minerals and white clay were assessed using
scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and
X-ray diffraction (XRD) analyses. The large ore grain of tin (Figure 1a)
showed a fresh, clean and smooth surface with elongated grain shape and
euhedral crystal edges. The EDX surface chemical composition analysis
showed the presence of triple peaks of Sn in the order of decreasing
intensity at 3.45, 3.70 and 3.90 KeV, and another Sn peak at 0.5 KeV,
indicating the Sn-bearing grain is cassiterite (Sn0\textsubscript{2})
mineral (Figure 1c). Further, the XRD analysis of the ore grains
indicated the presence of three very sharp and slender peaks in the
order of decreasing intensity at 3.33, 2.63, and 2.36 Å, confirming the
very well crystal structure consisting of cassiterite mineral (Figure
1d). Therefore, the tin ores in this study consisted of mainly
cassiterite mineral.

In Bangka, some of the spoils showed greyish white colour with fine
texture and sticky consistency, while information of its mineral
composition was not available. Hence, it is necessary to examine the
mineral constituent of whitish clay and its relation to management as
crop media with respect to reclamation measures. The XRD analysis showed
the presence of prominent sharp and slender peaks at 7.15 and 3.57 Å,
accompanied by less peak intensity at 4.47, 2.56, 2.49, and 2.34Å
(Figure 2), indicating the kaolinite mineral. There is also evidence for
the kaolinite impurity due to the presence of goethite in the mineral,
which is shown by XRD peaks at 4.16 and 2.19 Å. The SEM showed the
micro-pores occurring within kaolinite layer arrangement, which are able
to store water (data not shown).

\textbf{DISCUSSION}

\textbf{4.1 Weatherable minerals of post-tin mining areas}

The number of weatherable minerals (including feldspar and
ferromagnesian minerals) as native nutrient sources in post-tin mining
areas is practically depleted, while the resistant minerals (eg. quartz,
opaque and zircon) containing no essential crops are predominant. The
number of quartz mineral is higher in soil tailings developing from
sandstone (70-95\%) than tailing from granite parent materials (48-73\%
quartz) (Table 1). The higher quartz mineral in sandstone materials is
related to the nature of fluvial sediment containing higher sand
fraction with high quartz composition (Ko, 1986). Other resistant
minerals that bear no plant nutrients but occur in considerable amounts
are opaque and zircon, further exacerbate nutrient problems. Given that
tailing deriving from granite contains some K-bear minerals (orthoclase
and sanidine), the K nutrient is slightly better compared to tailing
derived from sandstone materials.

In the Dachang district, western China, the tin-cassiterite bearing
mineral was formed in a granite-related magmatic-hydrothermal and the
rock-forming minerals consisted of quartz (30-32\%), K-feldspar
(35-40\%), plagioclase (20-25 \%), biotite (3-6 \%) and muscovite
(1-2\%) (Guo et al., 2018). In native soils (TBB4 and TBB5 profiles) of
the present study, the turbid quartz was higher than transparent quarts,
irrespective of parent material types, indicating transportation was a
dominant process during soil formation. The higher transparent quartz
than turbid quartz minerals in tailings suggests that the light sand
fractions experienced less transportation. The exception was tailing
TBB7 profile deriving from granite has higher turbid quartz than
transparent quartz due to several times of re-mining (reworking
practices at a tailing pile could mimic transportation processes).

\textbf{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 \textgreater{} 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 SiO\textsubscript{2} 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 (\textless{} 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
SiO\textsubscript{2} 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
Al\textsubscript{2}O\textsubscript{3} (36 \%) and
Fe\textsubscript{2}O\textsubscript{3} (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 K\textsubscript{2}O
(0.2\%) than from sandstone (\textless{} 0.05\%). The high
K\textsubscript{2}O 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 K\textsubscript{2}O 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
Cr\textsubscript{2}O\textsubscript{3} 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,
Ca\textsubscript{3}Cr\textsubscript{2}(SiO\textsubscript{4})\textsubscript{3})
(Table 1). In addition, epidote mineral (Hancockite,
CaPb\textsubscript{2}Al\textsubscript{2}Fe(SiO\textsubscript{4})\textsubscript{3}(OH))
may host Pb. The dominance of Cr heavy metal may achieve 4695 mg
kg\textsuperscript{-1} in serpentinite rocks, corresponding to 533-633
mg kg\textsuperscript{-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 (Sn0\textsubscript{2}), 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
Sn\textsuperscript{2+} 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 CaCl\textsubscript{2} and
the results showed that the heavy concentration is very low in all
tailings and native soils, namely (mg kg\textsuperscript{-1})
\textless{} 0.5 for Pb, \textless{} 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\textsuperscript{-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\textsuperscript{-1}) than tailing
(370-970 mg kg\textsuperscript{-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
cmo\textsubscript{c} kg\textsuperscript{-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
cmol\textsubscript{c} kg\textsuperscript{-1}) among other profile
tailings (\textless{} 1.3 cmol\textsubscript{c} kg\textsuperscript{-1}).
This indicates reclamation practice used \emph{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 (\textless{} 2
cmo\textsubscript{c}kg\textsuperscript{-1}) was also reported in tin
mining in Malaysia (Madjid et al., 1998).

\textbf{4.3 Strategy for agro-ecological reclamation of post-tin mining
areas}

Evaluation of soil characteristics showed the current post-tin mining
areas have many severe constraints for crop growth and environmental
reclamation. Major constraints included sandy soil texture, the absence
or limited weatherable minerals (potential native nutrient sources), the
dominance of quartz mineral in tailings, the low water holding capacity,
the extreme low in soil organic matter and all nutrients, and nutrient
susceptible to leaching. Reclamation of soil capability to retain
nutrients and water was prerequisite. We proposed the strategic approach
in the view of long-term investment in agroecological reclamation as
follows: \textbf{(i)} levelling measures of on-site mounds resulted from
refused material dumping; \textbf{(ii)} building up soil organic matter
content (SOM) of sand tailing using green manure or compost to increase
soil organic C, supply various nutrients, promote microbial activities
and aggregate formation, reduce soil temperature, and increase soil
ability to hold water and nutrients; \textbf{(iii)} establishing a
special cultivation technique. Direct planting on soil tailings with
sandy texture dominated by quartz minerals is unlikely to allow crop
growth as shown in Fig.3a. Hence, the ``pot planting point'' technique
could be introduced to overcome the unfavourable conditions for crop
development. The technique is experimentally being tested in the field
and is still collecting more data (Fig.3c). Crop performance using pot
planting point technique showed rigorous growth of Paspalum grass
(\emph{Paspalum atratum} ) (Fig. 3d) and various legumes, namely
Gliricidia (\emph{Gliricidia sepium} , Jacq.) (Fig.3b, background),
Sesbania (\emph{Sesbania grandifora} (L) (Fig.3b, front), Indigofera
(\emph{Indigofera zollingeriana} ) (Fig. 3e), Stylosanthes
(\emph{Stylosanthes quianensis} ) (Fig. 3f) and Lab lab (\emph{Lab lab
purpureus} ) (Fig.3g). This technique is highly promising to restore
soil productivity and ecological recovery in post tin mining areas;
and\textbf{(iv)} complete fertilizer application and selection of crops
with an ability to fix nutrient from the atmosphere. The suggested
adaptive legumes with nodules on roots to fix atmospheric N are
Indigofera (Figure S3a) and Lab lab (Figure S3b).

\textbf{Conclusions}

Mineral composition of post-tin mining provides strategic roles in
providing the potential amount and type of native nutrient sources and
heavy metals which are basic information required to determine soil
nutrient management in supporting rehabilitation measures.

Post-tin mining areas have many severe constraints for crop development
and ecological reclamation. The characteristics of tailings are a
dominant sand fraction (up to 96\% sand), limited clay fraction
(\textless{} 14\%), dominant quartz minerals, low water holding
capacity, extremely low organic matter and all nutrients (especially N,
P, K, Ca, Mg, S), slightly acid, and the limited number of weatherable
minerals (potential native nutrient sources). Total elemental oxide
composition was dominated by SiO\textsubscript{2} (92-96\%) deriving
from quartz minerals.

Heavy metals were occluded in structures of hot-minerals meaning that
they were less availability and low health risk. Heavy metals consisted
of mainly Cr and small amounts of Sn, Pb and Hg. Sn availability was
minimum in all cases since it was preserved in host-mineral structure
(cassiterite) which was resistant to chemical weathering.

Without technology intervention, the reclamation of post-tin mining
areas was unlikely and was uneconomically feasible in the short-term.
Strategic reclamation includes building up soil organic matter content,
establishing pot planting point technique, thoroughly incorporated
initial topsoil if it is available, complete fertilizer application and
selection of adaptive crops with an ability to fix nutrient from the
atmosphere.

\textbf{Author contributions}

M.Anda. designed the research and led the field soil sampling, soil
analysis, and writing the manuscript as the main contributor.
N.D.Purwantari, E.Suryani, and F. Agus assisted in field soil samplings,
designed field rehabilitation techniques and drafting the manuscript,
Husnain assisted in designing the field research, managing the financial
support, and revising the manuscript.

\textbf{Acknowledgements}

We would like to thank Agency of Agricultural Research and Development,
Ministry of Agriculture, Indonesia, for financial and technical support.
We thank Mr. Martinus Halim, Mr. M. Abu, Mr. Yace Hidayat and Ms Lenita
(Centre for Agricultural Land Resource Research and Development) for
their field soil sampling and laboratory assistance. The data used to
produce the results in this paper can be obtained from the first author
by request.

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