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ABSTRACT

Arsenic (As) is a poisonous metalloid that causes illnesses to millions of individuals around the world. It is vital to track the source of As in soils and sediments because this is from where As enters groundwater, food and ultimately to human body. Arsenic can accumulate in soils and sediments from both natural and man-made sources. Majority of global As pollution stems from geogenic sources. Anthropogenic, primarily point-specific sources are prominent in specific parts of the world. This information will be crucial in developing site-specific viable remediation approaches against this metalloid.

Keywords: Geogenic sources, Anthropogenic sources, Bengal Delta Plain, Mining, Smelting

 

 

Arsenic is the 20^th most abundant element with an average crustal concentration of 1.8 ppm and is classified as a class-I carcinogen. The presence of As in groundwater with a concentration beyond 10 ppm has been reported in more than 108 countries threatening the health of 230 million people worldwide (Shaji et al., 2021). Studies so far have emphasised drinking water as the major source of As. Arsenic in groundwater, in most cases, comes from the solubilisation of As-bearing minerals present in aquifer sediments. Contaminated surface soils also contribute significantly to As intake by humans through food materials grown on them or from direct dermal contact, ingestion and inhalation of As-laden soils and dust. The complex and dynamic interplay between inputs and outputs result in elevated As concentrations in the environment. Source of As in soils and sediments may be geogenic or anthropogenic.

Natural sources of arsenic in soils and sediments

In the Earth’s crust, approximately 568 minerals have been reported to contain As as a critical component and 10 of them are very commonly found in sediments, soils and rocks. Around the globe, more than 90% of As pollution is geogenic in nature, where contaminated alluvial sediments play the most pivotal role. Groundwater As contamination in Bengal Delta Plain (BDP) of India and Bangladesh is a typical example of such geogenic As-pollution, with at least 36 million people exposed to risk over a 0.173 million square kilometres. According to different hypotheses, As in BDP might have been fluvially transported by Ganga and its tributaries, but different possible sources have been held responsible in different postulates. The possible source areas of As are the Gondwana coal region in the Rajmahal trap area, Gorubathan base-metal deposits in the eastern Himalayas, and, most acceptingly, the Himalayas. Mechanical weathering caused by Pleistocene tectonic uplift in the Himalayas is thought to be the most critical process for exposing previously inaccessible mineral surfaces in the Himalayan region, leading to the higher solubilisation of As in the Holocene aquifers. Two major hypotheses have been suggested for describing the probable mechanism of As mobilisation in groundwater with the particular reference to Holocene aquifers like in West Bengal and Bangladesh (Acharyya et al., 1999). Among these two hypotheses, namely the arsenopyrite oxidation and the ferric oxyhydroxide reduction, the former suggests that insoluble As-bearing minerals, such as arsenopyrite (FeAsS), get converted to soluble sulphate compounds upon rapid oxidation due to recession of groundwater, releasing arsenate, sulphate (SO₄^2-), and ferrous iron (Fe²⁺). 

FeAsS + 13 Fe³⁺ + 8 H₂O →14 Fe²⁺ + SO₄^2- + 13 H⁺ + H₃AsO₄ (aq.)

Whereas the latter theory, which is more consistent with the experimental observations reported for As contamination in the aquifer sediments and water of the BDP, suggests that As comes into the solution due to reductive dissolution of As-adsorbing iron oxyhydroxides (FeOOH), while fermentation of excess organic matter and peat under the inbuilt water stagnation for lowland paddy cultivation promotes reducing environment.

 8 FeOOH-As(s) + CH₃COOH + 14 H₂CO₃ → 8 Fe²⁺ +As(d) +16 HCO₃^- + 12 H₂O 

Volcanic eruptions are another principal means for enriching soils and sediments with As. In Latin America, soils and sediments engendered from the weathering of young Cenozoic volcanic rocks in the vicinity of active or former volcanoes are rich in As. Even in areas where these rocks are buried deep, As is transported to the surface through ascending geothermal fluids. Other prominent geological As sources near volcanic belts are volcanic emissions, such as volcanic ash. Two critical concerns regarding volcanic ash as a source of As are its transportability to long distances and relatively high water solubility owing to its amorphous glassy structure. Volcanic ash produced and accumulated over millions of years due to prolonged periods of Andean volcanism (Cenozoic to recent) is found in scattered or discrete layers in loess-type As-enriched sediments in the Chaco-Pampean Plain (Argentina) (Bundschuh et al., 2021).

In the oceans, some animals and plants make organic As compounds. After the death of these organisms, As can be deposited on ocean sediments. In arid regions, surface evaporation of water is another minor natural mechanism that can lead to As augmentation in the upper sediment from the draw up of contaminated subsurface water.

Anthropogenic sources of arsenic in soil

Several anthropogenic activities are responsible for enriching soils and sediments with As. Most anthropogenic sources act as a point source of As, which means their effects are presumably restricted to an area nearby, making them identifiable. Following are the notable man-made sources of As in soils and sediments:

Mining and smelting activity

Mining and smelting of copper, gold, lead, zinc and iron ores generate a considerable amount of As. Arsenic is found in ores as sulphide [arsenopyrite (FeAsS), orpiment (As₂S₃), realgar (α-As₄S₄) etc.] and oxide minerals [arsenolite (As₂O₃)] or as sorbed and occluded species on various metal (hydrous) oxides. Some As is refined for other purposes, while the rest is left in the waste rock. The waste rocks are not a concern unless they are biochemically weathered to release the toxic metalloid into the environment. Mining and smelting operations generally give rise to localised soil and aquifer pollution or may contaminate river sediments if the mines are located on or near riverbanks. In the Australian state of Victoria, mining, smelting and processing of sulphidic gold ores has given rise to elevated soil-As concentration around former mine-waste disposal areas to a level ranging from 280 to 15000 mg kg^-1.

Coal combustion for energy

Coals may contain several pollutant elements and As is one of those. Coal fly ash, generated after coal combustion, is often sluiced into settling basins, and because As in fresh ash is quite soluble, wastewater As concentrations can consequently be relatively high. Arsenic can build up in the sediments of coal fly ash settling basins and reach concentrations of over 1000 mg kg^-1. India is highly susceptible to fly ash borne As contamination since it is the third-largest coal-producing country. India’s energy production primarily depends on the combustion of sub-bituminous coals, with ash percentages ranging from 35 to 50. There is a well-founded concern that As from coal combustion wastes can contaminate soil and lead to elevated As concentrations in soils, especially if they are applied unprocessed in agricultural fields as a source of micronutrients (boron and molybdenum).

Application of arsenic containing pesticides to agricultural soils 

Agricultural uses account for roughly 22% of global As consumption. Arsenical agrochemicals (many older insecticides, rodenticides, herbicides, and fungicides containing inorganic as well as organic forms of As) are responsible for the build-up of high As concentration in instances. For ages, arsenical pesticides have been employed in agriculture in many countries, including Australia, Canada, New Zealand and the USA. The use of As sulphides dates back to 900 AD in China. Lead arsenate was first manufactured as an insecticide in 1892 for use against the gypsy moth (Lymantria dispar) in Massachusetts, USA. It was applied extensively until getting banned in 1988 from the entire USA. The use of lead arsenate plummeted in Australia after the introduction of DDT in 1950. Organic arsenicals, such as dimethylarsinic acid (DMA), have also been used as herbicides on agricultural lands, golf courses and orchards until recently phased out of the market. Cotton crops still receive organic As-containing pesticides, and poultry feed contains organic As for enhancing growth. Present and past use of arsenical pesticides have resulted in highly contaminated agricultural soils in several areas of these countries. Former orchard sites in Washington State have been reported to contain 2353 mg As kg^-1. Soils surrounding cattle dip sites in South Australia, which received sodium arsenate as tickicide decades ago, still contain an absurdly high As concentration, which may reach up to 3000 mg As kg^-1 (Smith et al., 2003).

Disposal of contaminated biosolids and manures

Arsenic concentration in sludge may reach up to 45 mg kg^-1. Land application being the primary means of disposal of sewage sludge, acts as a significant conduit for As pollution in soils. Considering the potential threat that can arise from its application, the maximum As concentration in sewage and sludge has been set at 75 mg kg^-1 dry weight (Prasad et al., 2019). Arsenic concentration may reach up to 20 mg kg^-1 in soils surrounding sewage treatment plants. Arsenic occurs in animal wastes primarily because of the former use of arsenical antibiotics in poultry, turkey and pig feed. These compounds are not readily absorbed or metabolised and thus occur at concentrations up to 40 mg kg^-1 in animal manures.  

Irrigation with contaminated groundwater and wastewater

Every year a significant amount of As is brought onto arable lands, which then enters the food chain due to agricultural irrigation with contaminated groundwater or wastewater. With time, in BDP, As concentration in groundwater has increased with uncontrolled extraction of aquifer and application of that As-laden groundwater for irrigation has spiked up the soil-As level. In this region, 1200 to 1400 mm contaminated water in Boro rice adds 400 to 420 mg As per m², making the soil a secondary source of As pollution. Municipal and industrial wastewater and related effluents have been applied to land for over 400 years, and it is now routine practice in many parts of the world. According to studies, wastewater irrigation agriculture provides 50% of the vegetable supply to metropolitan regions in numerous Asian and African cities (Wuana & Okieimen, 2011). Even though As concentrations in wastewater effluents are typically low, long-term irrigation of land can lead to its accumulation in the soil.

Wood preservation 

Earlier, woods were pressure treated with greenish chromated copper arsenate (CCA) to prevent deterioration from fungal attacks. Eventually the metals in the preservative may leach out of the wood and accumulate in the soil. Despite the fact that CCA has been prohibited in several countries for many years and major attempts have been made to mitigate its environmental impact, its constituents still persist in soil.

Semiconductor, glass, pigment, bullet and pharmaceutical industry

Arsenic and gallium arsenide are fundamental to semiconductor manufacturing. This toxic metalloid is also used as a decolourising agent in the glass industry. It is used in small quantities to improve the structure and sphericity of bullets. Arsenic-containing drugs are being used to treat leukaemia. The pharmaceutical use of As as antibiotics in the form of Fowler’s solution, Salvarsan etc., was way more prevalent in the past when its toxic effects were little known.

Chemical warfare

During the American Vietnam War (1961-1971), the US and South Vietnamese militaries broadcasted a massive load of As (1,132,400 kg) in the form of herbicide Agent Blue (cacodylic acid, C₂H₂AsO₂) over approximately 300,000 ha rice paddies and 100,000 ha Mangrove forests in the Mekong Delta and Central Highlands environments to desiccate paddy before maturity. For the past 60 years, highly variable levels of trace amounts of As have lingered in the Mekong Delta ecosystem, contributing to ongoing As pollution in water supplies, sediments, and soils.

Conclusion

Knowledge about the source of As in soils and sediments is critical because these are the primary retainer of As in the environment from where arsenic enters into food and groundwater. Sources of arsenic could be geogenic or anthropogenic. Geogenic sources like alluvial sediments, volcanic eruptions etc., account for 90% of global As pollution. However, anthropogenic, mostly point specific, sources are also important in specific regions around the world which may shoot up the level of As in the environment absurdly high. This information regarding sources of As in soils and sediments will be instrumental in devising location specific regulatory measures against the carcinogen.

References

Acharyya, S. K., Chakraborty, P., Lahiri, S., Raymahashay, B. C., Guha, S., & Bhowmik, A. (1999). Arsenic poisoning in the Ganges delta. Nature, 401(6753), 545-545.

Bundschuh, J., Armienta, M. A., Morales-Simfors, N., Alam, M. A., López, D. L., Delgado Quezada, V., Dietrich, S., Schneider, J., Tapia, J., Sracek, O., & Ahmad, A. (2021). Arsenic in Latin America: New findings on source, mobilization and mobility in human environments in 20 countries based on decadal research 2010-2020. Critical Reviews in Environmental Science and Technology, 51(16), 1727-1865.

Prasad, M. N. V., de Campos Favas, P. J., Vithanage, M., & Mohan, S. V. (Eds.). (2019). Industrial and Municipal Sludge: Emerging Concerns and Scope for Resource Recovery. Butterworth-Heinemann.

Shaji, E., Santosh, M., Sarath, K. V., Prakash, P., Deepchand, V., & Divya, B. V. (2021). Arsenic contamination of groundwater: A global synopsis with focus on the Indian Peninsula. Geoscience Frontiers, 12(3), 101079.

Smith, E., Smith, J., Smith, L., Biswas, T., Correll, R., & Naidu, R. (2003). Arsenic in Australian environment: an overview. Journal of Environmental Science and Health, Part A, 38(1), 223-239.

 

Wuana, R. A., & Okieimen, F. E. (2011). Heavy metals in contaminated soils: A review of sources, chemistry, risks and best available strategies for remediation. International Scholarly Research Notices, 2011.

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8. Sources of arsenic in soils and sediments_compressed.pdf