Distributions of Chromium, Copper, and Cadmium in the estuary-coastal surface sediment of Tin-Rich Island

Belitung Island known as one of the tin producer in the world, as it lied in the South East Asian tin belt. The study was conducted in the North Belitung Island to investigate some metal elements concentration of sediment along the estuary river and coastal area near to carbonate reef environment system. Thirteen sediment samples were collected from estuarine riverbed, shoreline, and offshore of North Belitung. The concentrations of metal elements were analysed to assess the environmental pollution characteristics by using flame AAS (Atomic Absorption Spectrophotometry) method. The result indicated that the average concentrations of chromium (Cr), copper (Cu), and cadmium (Cd) were 1.62-18.26, 5.72-80.07, and 1.4-4.59 mg/kg dry weight, respectively. The calculation of geo-accumulation index (Igeo) was adopted in this study to evaluate the heavy metal pollution. The Igeo values revealed that this research area has been moderately polluted by Cr and Cu, and unpolluted-moderately polluted by Cd. Based on the contamination level to adverse biological effect, the heavy metals concentration in this research area has the low potential to adverse biological effect by Cr and Cu, and low-moderately potential by Cd. This information is important to lead the study about any other elements and heavy metals to evaluate the risk assessment in the tropical area.


Introduction
Tin (Sn) is a precious metal that resists corrosion and is non-toxic, which is why it is often used as a protective coating for other metals.As a result, tin is a key industrial metal in global markets [1], [2].Due to its high demand, some production countries of tin, mine the metal extensively.Around 38% of the tin offered worldwide comes from the region known as the 'Southeast Asian Tin Belt' [3].The Southeast Asian Tin Belt is a north-south elongate zone 2,800 km long and 400 km wide, extending from Burma (Myanmar) and Thailand to Peninsular Malaysia and the Indonesian Tin Islands (Fig. 1) [3].Belitung island, with a land surface area of about 5000 km 2 , forms the easternmost expression of the Southeast Asian tin belt [4] and known as the Indonesian Tin Island altogether with Bangka Island 1245 (2023) 012015 IOP Publishing doi:10.1088/1755-1315/1245/1/012015 2 (Fig. 1).While global demand for tin is rising, the mines in Belitung are becoming increasingly exhausted, increasing the pressure on the environment.Figure 1.South-East Asian Tin Belt, and Belitung Island is at the end of it [5] cited in [4] The excessive accumulation of heavy metals in mining areas may threaten the surrounding ecological environment and the health of surrounding residents [6] [7].In addition, the excessive amounts of heavy metals are introduced to estuarine and coastal environments through river, runoff, and land-based point sources [8].Many metals can be incorporated into marine ecosystems as the result of intense industrial development along the coastline [8].Marine sediments have often been regarded as the ultimate reservoir for heavy metals in the coastal environment [9], [10].These metals are associated with the sediment via adsorption in surface particles, precipitation, and incorporation into biogenic material [11], [12].As a result, sediment contamination poses one of the worst environmental problems in marine ecosystems, acting as sinks and sources of contaminants in aquatic systems [13], [14], [15].
Belitung Island is not only as Indonesia's Tin Island, but also an island with high diversity coastal ecosystem, including 244 different coral genus from 15 families [16].As one of the carbonate reef Island in Indonesia, Belitung's reef is strategically connected to the South China Sea and Java Sea, due to seasonally reversing monsoon currents [17].In addition, Belitung is less exposed by anthropogenic warming of sea surface temperatures, marine heat waves and destructive cyclones [17].As a result, Belitung Island could potentially provide coral larvae to replenish other reef areas in Indonesia following disturbances such as mass bleaching events [17].Therefore, Belitung Island is important to be protected.

3
Heavy metals gained importance due to their known detrimental environmental effects [18].But, the study of heavy metal accumulation in Belitung Island is still rarely done previously, especially around marine/reef sediment.The analysis of marine sediment is an important area of environmental analytical chemistry [18].Based on the ecological risks that may occur due to mining activity in Belitung Island, it is crucial to provide some information about heavy metals distribution in this island.The objective of this study is to provide the information of the heavy metal distribution in the Belitung Island affected by tin mining activity, and to evaluate the contamination level of the heavy metals towards the marine ecosystems.

Sample Collection and Analysis
Figure 2. Study area, marked with the red square This study investigates the distribution of metal concentrations in surface sediments of the eastuarycoastal in Belitung Island, close to reef environment.The study area lies in the north of Belitung Island (Fig. 1).The samples were collected from the estuarine river (n=6), shoreline (n=5), and offshore (n=2) (Fig. 2).The river is natural (Penyairan River) and artificial (Tambak Udang River) ones, ex mining dredging activity.The sediment samples were selected to represent both proximal and distal sites.Sampling was conducted in August 2022, and 13 samples from different sites were collected using plastic shovel and stored in polyethylene plastic bag.The sediment samples collected were undisturbed with 0-5 cm depth.The sampling locations were recorded by Avenza Maps Apps.

Figure 3. Sampling location of surface sediment in North Belitung
In the laboratory, the sediments were dried using a Memmert TM oven set to 80° C for 8 hours.Afterward, the sediment samples were sieved through a 125 µm sieve using a Durham Geo Slope Indicator (DGIS) sieve shaker.The remaining portion of the coarse fraction was then sieved through 250 µm, 500 µm, 1 mm, and 2 mm sieves.Each individual grain-size fraction was digitally weighed.The sediment from the respective location was digitally weighed around 0.5000-0.5010gram in 100 mesh (0,147 mm) grain size of sediment.
For the analysis of solid samples such as sediment, a dissolution step must precede the analysis step, as solutions are most easily analyzed by atomic absorption [18].The digestion process in this study following the procedure from Ozkan et al., 1980 for heavy metal extraction from marine sediment for AAS, with a little modifications.The mixture acids used to dissolve the undesired material were HNO3, H2SO4, HF, and HCl.The dissolved sediment was then diluted with ultrapure deionized water and filtered with filter paper Whatman 41, 100 cycles, diameter 125 mm (CAT No. 1441-125) and made up to 100 ml with ultrapure deionized water.The duplicate sample was performed for every samples respectively.
Flame Atomic Absorption Spectrophotometry (AAS) analysis was performed on all samples to obtain the concentration of heavy metals (Cr, Cu, and Cd), using the AAS machine AA-7000 by Shimadzu (Serial No. A30694901634) and an auto sampler machine ACS-7000 by Shimadzu (Serial No. A30694901446).

Result and Discussion
All sediment samples in the study area are grain supported.Mean grain size sediments are mostly fine (n=5) to medium (n=4) sand, although very coarse sand (n=3) and coarse sand (n=1) also occurred.The sediment from the estuarine river zone (STB.1,STB.2, STB.5, SPB.4,SPB.7, and SPB.8) and coastal area (STB.3,STB.4,SPB.4,SPB.5, and SPB.6) was vary, ranging from very coarsefine sand, while the sediment from the offshore (SPB.1 and SPB.2) area was very coarse -medium sand.The range of the grain size in this study area might be due to the different energy of the wave around this water body from river, coastal to the offshore.
The sediment in the North Belitung are primarily from runoff by around small streams (Penyairan Beach and Tambak Udang Beach), and transported of suspend particulates from open sea by tidal current.Grain size analysis indicates that composition of the surface sediments in North Belitung is relatively very vary from fine-very coarse sand.The average Igeo values of heavy metals in surrounding area in North Belitung were in order of Cd<Cu<Cr.The Igeo class of this study area is class 1-2 for Cr and Cu, and class 1 for Cd.Based on the Igeo class assigned in this study, the sediment quality in this study area can be determined.Sediment quality is the ability of sediment to maintain a healthy benthic population [21].This area is moderately polluted by Cr and Cu, and unpollutedmoderately polluted by Cd.The ANZECC classification [22] following Default Guideline Value (DGV) for toxicant sediment was also assigned in this study to know the contamination level to adverse biological effect around the study area.Based on the DGV, the Cr and Cu has the low risk contamination level to adverse biological effect, but Cd has low-moderate risk contamination level to adverse biological effect. .Distribution of tin-tungsten deposits and granitoid in Belitung Island [3], red square indicated the study area.Tin mining activity has developed in Belitung since 1850 until today (Fig. 5) [23].Nowadays, up to 80% of Indonesian tin exports are produced by artisanal and small-scale mining (ASM) [24], [1], [2].ASM has a large share in Belitung's tin production and is of great importance for the economic and social development on the island [1].However, while ASM mining increases the wealth of Belitung's population, good mining practices, safety standards and land reclamation are neglected [24], [1].Therefore ASM has serious consequences for the environment [24], [25], [26], [27].Tin mining in Belitung has been extensively performed that remains some tailings for the environment.The tailings generated from tin mining activity as such Pb, Cd, Zn, Cu, and Cr as well.
In the study area, the source of tin deposit was from biotite granite and tin placer deposits (Fig. 5).According to extensive distribution of tin deposits and the main economic activity in this island, the major pollution generated to environment mostly possible from the tin mining itself rather than other anthropogenic activities.
One of the estuary river in the study area, which is Tambak Udang River was generated due to tin mining dredging activity.It also can be distinguished from the river's meandering among these two rivers.The natural river will commonly have meanders in its river body, and vice versa.In this case, Penyairan River has some meanders in its body, while Tambak Udang River does not.It also enhanced the argument that in the study area, the tin mining activity was extensive.
Toxicants adsorb to fine-grain organic and inorganic materials and thus sediments play a major role in the transport of contaminants as suspended particles in the water column [21].This finelydisseminated matter influences the feeding habits of benthic and pelagic animals and is part of the uptake process [21].Since the study area is located near to carbonate reef environment, the sediment quality therefore has a strong influence to the biodiversity and ecological health of marine ecosystems [21] including the coral and reef ecosystems.
The sampling location is spotted around the estuary river zone, shoreline and offshore area that close to the marine environment where carbonate reef and marine aquatic lied (Fig. 2).Carbonate reef ecosystem where coral reefs live, protect coastlines from the damaging effects of wave action and tropical storms; provide habitats and shelter for many marine organisms; and are vital for fisheries and tourism.Many factors may be detrimental to coral health, but in East Belitung the main factor compromising coral health is sedimentation [28].
Mining leads to the exposure of soils to the erosive forces of rain and runoff [29], therefore, the sedimentation rate may increase due to intensive mining.Sedimentation will cause a decrease in live coral cover, growth, and recruitment [30], [31], [32], and lead to a decline in the condition of coral reefs particularly in inshore or near-shore settings.In addition, suspended sediment inhibits the sun's energy from being used for photosynthesis.Therefore, due to the intensive mining, the near-shore coral reefs of Belitung Island are under pressure from sediment runoff [33], [34].

Conclusions
The sediments in the North Belitung consisted mainly of fine sand and minor part of coarse sand that indicated similar feature among different sites, showing a stable hydrodynamic setting.Based on the concentrations Cu, Cd, and Cr in the surface sediments in the North Belitung, the highest heavy metal concentrations as follow Cu>Cr>Cd.The highest heavy metal concentrations is located in the offshore and estuarine river zone compare to shoreline area.Sediments quality at most of sites in this study area met Igeo class 1-2 for Cr and Cu, and class 1 for Cd.It suggested that the sediment quality in this surface sediment in North Belitung is moderately polluted by Cr and Cu, and unpolluted to moderately polluted by Cd.The contamination level of Cr and Cu is in low level to adverse biological effect, but for Cd it has low-moderate contamination level to adverse biological effect.
Based on the contamination level and the environmental assessment, North Belitung started to have the ecological risk due to the tin mining activity according to the development of tin mining activity in Belitung since 1850.That is why the further and more detail study in this area should be paid more attention on monitoring of heavy metals variation in the future.

Figure 4 .
Figure 4. Cr, Cu, and Cd concentrations respectively from estuary river-coastal surface sediment in North of Belitung.The Cd concentrations that has been analysed from surface sediment in the North Belitung is relatively low and stable in each particular sampling site.But, Cu and Cr have different patterns from Cd concentrations.They have vary concentrations in each particular sampling site.The graphic shown that the highest concentrations value for Cu is located in the offshore area (SPB.1 and SPB.2) and in the estuarine river zone (SPB.1,SPB.8, and STB.1).The concentration of Cr is also vary in each particular sampling site.Likewise concentrations of Cu, Cr also has the highest concentrations value in offshore (SPB.1) and estuarine river area (SPB.4,SPB.8, and STB.1).The highest concentrations value of Cu is in the very coarse sand and the highest concentration value for Cr is in the medium sand.

7 Figure 5
Figure5.Distribution of tin-tungsten deposits and granitoid in Belitung Island[3], red square indicated the study area.Tin mining activity has developed in Belitung since 1850 until today (Fig.5)[23].Nowadays, up to 80% of Indonesian tin exports are produced by artisanal and small-scale mining (ASM)[24],[1],[2].ASM has a large share in Belitung's tin production and is of great importance for the economic and social development on the island[1].However, while ASM mining increases the wealth of Belitung's population, good mining practices, safety standards and land reclamation are neglected[24],[1].Therefore ASM has serious consequences for the environment[24],[25],[26],[27].Tin mining in Belitung has been extensively performed that remains some tailings for the environment.The tailings generated from tin mining activity as such Pb, Cd, Zn, Cu, and Cr as well.In the study area, the source of tin deposit was from biotite granite and tin placer deposits (Fig.5).According to extensive distribution of tin deposits and the main economic activity in this island, the major pollution generated to environment mostly possible from the tin mining itself rather than other anthropogenic activities.One of the estuary river in the study area, which is Tambak Udang River was generated due to tin mining dredging activity.It also can be distinguished from the river's meandering among these two rivers.The natural river will commonly have meanders in its river body, and vice versa.In this case, Penyairan River has some meanders in its body, while Tambak Udang River does not.It also enhanced the argument that in the study area, the tin mining activity was extensive.Toxicants adsorb to fine-grain organic and inorganic materials and thus sediments play a major role in the transport of contaminants as suspended particles in the water column[21].This finelydisseminated matter influences the feeding habits of benthic and pelagic animals and is part of the uptake process[21].Since the study area is located near to carbonate reef environment, the sediment quality therefore has a strong influence to the biodiversity and ecological health of marine ecosystems[21] including the coral and reef ecosystems.The sampling location is spotted around the estuary river zone, shoreline and offshore area that close to the marine environment where carbonate reef and marine aquatic lied (Fig.2).Carbonate reef

Table 1 .
Summary of heavy metal concentrations in dry weight, from surface sediment in North of Belitung analyzed by AAS.

Table 1 .
The concentrations of Cr ranges from 1.62-18.26mg/kg, Cu ranges from 5.72-20.07mg/kg, and Cd ranges from 1.4-4.59mg/kg in dry weight, respectively.The average concentrations for Cr, Cu, and Cd were 7.232, 8.612, and 2.415 mg/kg, respectively.The highest heavy metal concentration is Cu, and the lowest is Cd in North Belitung Island.

Table 2 .
Environmental Assessment of Heavy Metals (Cr, Cu, and Cd) from estuary-coastal surface sediment in North Belitung