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Arsenic Toxicity in Water-Soil-Plant System: An Alarming Scenario and Possibility of Bioremediation

Introduction

Arsenic is present in soil and water by natural means like volcanic eruption, weathering, leaching and by antluopogenic activities like smelting of ores, mining, binning of coal, and manufacturing of herbicides and pesticides (Cavalca et al. 2013). Arsenic was first isolated by Albert Magnus, a German Alchemist, in 1250 AD via heating of soap with orpiment (arsenic trisulphide). By the 18th century, arsenic was well known as a unique toxic metalloid and was used as poison. It was named as ‘Poison of Kings’ for its use to kill several kings in 17th and 18th centuries. In agriculture, arsenic was mainly used to manufacture insecticides by Chinese. Arsenic containing insecticide such as sandarach (realgar, AsS) was effective for protection of grapes. Widespread use of another arsenic containing insecticide (Paris Green) was done to control Colorado potato beetle. This was followed by the use of London Purple (a mixture of calcium arsenate and arsenite with some or ganic matter) as an hrsecticide.

Arsenic is not abundant in the Earth’s crust (0.0001%) and its normal concentration in soils is considered as < 15 mg kg'1 (Orentl and Stolz 2003). It is present in soil in inorganic forms combined with other elements but without carbon and also in organic forms containing carbon. Inorganic arsenic compounds are more toxic and soluble in soil system than its organic forms. Inorganic arsenic compounds are present in the Earth’s crust mostly as metal arsenic, arsenic sulphide and arsenides and also as insoluble sulphide compounds and sulfosalts such as arsenopyrite (FeAsS), lollingite (FeAs2), realgar (As4S4), orpiment (As2S3) and enargite (Qr3AsS4), etc. Arsenic usually exists in soil in four oxidation states, viz. As(JII) (arsine), As0 (elemental arsenic), As(in) (arsenite) and As^ (arsenate) of which arsenite is the most toxic and mobile but arsenate is the most abundant species in groundwater. Toxicity of arsenite predominantly occurs due to its binding ability to the sulfhydryl groups, present in proteins (Gebel 2002). On the contrary, arsenate is present in the environment as a toxic analog of inorganic phosphate. It is generally introduced into the living cell by mimicking the same phosphate transport mechanism through the cell membrane and also interferes in phosphorylating metabolisms occurring in living systems (Shrestha et al. 2008). Arsenic is also present in atmosphere as minute particles and can stay there for a long time as a mixture of both arsenite and arsenate. Arsenic compounds are not biodegradable, get converted from organic to inorganic form or vice-versa and easily react with other chemicals.

Large doses of arsenic have been known to be a human poison for centimes. However, it is creating alarming environmental concern in relatively low doses also in recent times. The United States Environmental Protection Agency (USEPA) declared arsenic a Class A carcinogen. Presently, high toxicity of arsenic and its increased appearance in the biological system has sparked public and political concern. Presence of arsenic in groundwater, above the World Health Organization (WHO) recommended maximum permissible limit for drinking purpose, has been found in several affected zones of the world. About 296 million people, spread in more than 100 countries, are at risk from drinking groundwater contaminated with arsenic above the permissible limit (Chakrabarty et al. 2018). South and South-East Asian countries are facing the worst scenario with more than 187 million people exposed to arsenic contamination. Groundwater lifted from shallow tube wells, especially in lean peiiod (December to April), is used as the major source of irrigation for growing rice and vegetables in these countries. This irrigation water with severe arsenic contamination may lead to the accumulation of arsenic in soil and plant system. Eventually, high accumulation of arsenic in the top soil causes substantial build-up of arsenic in crops as well as its entiy in food chain with potential biomagnifications. Magnified arsenic concentration in the biological food chain through crops or fodder is considered as more dangerous for humans than arsenic intake through drinking water. The chronic exposure to arsenic causes severe toxic effects on human health, which is defined by the generic term ‘arsenicosis’. The common symptoms of arsenic toxicity are weight loss, loss of appetite, weakness, skin itching, skin cancer, vomiting, diarrhoea, dryness and burning of mouth and throat, dysphasia, fatigue, moderate to severe anaemia, chronic respiratory disorder, and gastrointestinal disorders (Banerjee et al. 2011). Considering its harmful effects, remediation of arsenic toxicity in water-soil-plant system has become a great concern.

Several physical, chemical and biological approaches have been attempted for arsenic removal by researchers worldwide. The physical approaches like adsorption, excavation, coagulation, filtration, solidification, stabilization, solid liquid separation, etc. and chemical approaches like acid washing, absorption, oxidation, precipitation, ion-exchange, chelation, reverse osmosis, coagulation with metal salts, iron filing, photo oxidation, etc. were not found too effective to necessarily reduce the risk and are quite expensive. Therefore, the biological approaches are now being tested as potential arsenic remediation measures. Use of plants and microorganisms for arsenic remediation is considered as a stable, natural and economical solution (Hadis 2011). The interactions between micro-organisms and plant roots are also considered as effective biological mechanism for removal of arsenic. The present chapter overviews the sources and distribution of arsenic in soil-water-plant system and explores the bioremediation measures to reduce its toxicity in food-chain.

 
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