Origin of arsenic contamination in soil and aquatic environment
The reason of arsenic contamination in soil is still a debated issue. It is believed that arsenic has entered into the environment either through geogenic (geological) or anthropogenic (human activities) sources. In addition, small amount of arsenic also enters in the soil and water from arsenic-rich biological (biogenic) sources. The detailed sources and mechanisms of arsenic release are presented in Figure 1.
The geogenic arsenic contamination in soil and aquatic systems has primarily been derived from volcanic eruption and by release of arsenic containing minerals. Arsenic is present in almost all the geological substances with varying degree of concentration. The average concentration of arsenic is 1.5 to 2.0 mg kg'1 in the continental Earth’s crust, with presence of 245 arsenic beating
Figure 1. Sources of arsenic in soil and aquatic system (adopted from Mahimairaja et al. 2005, Hasanuzzaman et al. 2015).
minerals. These minerals are mostly sulphide-containing ores of copper, nickel, lead, cobalt, zinc, gold or other base metals. The most predominant sulphide ores of arsenic include arsenopyrite (FeAsS), realgar (AS4S4) and orpiment (As2S3). Arsenopyrite (FeAsS) is the most abundant arsenic containing mineral, which exists in anaerobic environments as well as in the ciystal lattice structure of various sulphide minerals, as the substitute of sulphur. Arsenolite (As203) is the oxidized form of arsenic present in soil while common reduced forms are orpiment (As2S3) and realgar (As4S4). The dissolution of arsenic containing minerals was observed to follow the order: native-arsenic > arsenolite > orpiment > realgar > arsenopyrite > termantite. The release of arsenic in soil and groundwater in deltaic and alluvial plains is mainly attributed to dissolution and desorption of naturally occurring arsenic bearing minerals followed by subsequent leaching and runoff. The geogenic arsenic contamination in soil and groundwater has been observed in different parts of the globe, and the Ganga-Brahmaputra-Meglma fluvial plains of West Bengal, India and adjacent Bangladesh are the typical examples of it.
Based on arsenic geochemistry, three natural phenomena have been believed to be responsible for geogenic arsenic release in groundwater. They are (i) oxidation of arsenic-bearing primary minerals, (ii) dissolution of sorbed arsenic fr ont iron oxyhydroxides and (iii) release of arsenic by competitive ion exchange with phosphates (Bose and Shartna 2002, Hasanuzzaman et al. 2015). (i) Oxidation of arsenic-bearing pyrite minerals is believed to mobilize arsenic in soil. When exposed to atmosphere, insoluble arsenopyrite (FeAsS) rapidly oxidises and releases soluble arsenite, sulphate and divalent ferrous iron (Fen). The oxidation of arsenopyrite, however, depends on oxygen availability and oxidation of sulphide (S2“) to sulphate (S04~). The released arsenite is further oxidised to arsenate by microbially mediated reactions, (ii) In oxidized condition, the iron arsenic may be sorbed by goethite (FeOOH) coatings, present on the surface of soil particles. Dissolution of this goethite coating, in waterlogged reduced condition, liberates arsenic. The process is further driven by fermentation of peat in the subsurface layer, releasing organic acids such as acetic acid (CH3COOH), resulting in release of arsenite and arsenate from FeOOH coatings, (iii) The third geogenic mechanism of arsenic release in soil is competitive ion exchange with phosphates (H2P04), added through the application of phosphatic fertilizers in the cropped fields. Migration of these phosphates’ ions to the subsurface aquifers leads to release of sorbed arsenic from aquifer minerals. Among these three geogenic processes, dissolution of FeOOH under reduced conditions is considered as the most probable cause for arsenic accumulation in groundwater.
Anthropogenic and biogenic origin
Substantial amount of arsenic is added in soil and in aquatic systems by various human (anthropogenic) activities. The major anthropogenic activities responsible for arsenic release are ore dressing, smelting of non-ferrous metals, mining, electronic industries, chemical industries, dye industries, tanning industries, burning of fossils fuels, glass and mirror manufacture, wood processing and preservation, production and use of arsenic containing pesticides, paints, pigments, cosmetics, fungicides, insecticides, etc. The arsenic released from these various anthropogenic activities differs greatly in chemical nature and bioavailability (Mahimairaja et al. 2005). Anthropogenic activity is considered as the predominant sources of arsenic contamination in industrial zones of developed countries as well as in the mega-cities of the developing countries (Cullen and Reimer 1989). A study in the landfill area near Kolkata, India exhibited high arsenic load due to continuous reception of urban and industrial waste (Chakraborty et al. 2017). Industrial wastewater is also responsible for arsenic pollution in urban agglomerations (Deb and Dutta 2017).
Arsenic has widely been used in agriculture since long. However, rapid increase of fertilizers and pesticide application after the green revolution accelerated the use of arsenic. The field application of arsenic containing pesticides causes contamination in soil and water bodies. Sodium arsenite was a known fungicide for protection of grapevines from excoriosis and was in use until 2001. Several arsenic containing compounds such as zinc arsenite, zinc arsenate, lead arsenate, calcium arsenate, magnesium arsenate, Paris Green, etc. are still being used as pesticides in several parts of the world. Disposal from pesticide manufacturing industries is also responsible for arsenic contamination in soil and water system. Herbicides are also repoxted to release arsenic in soil. Monosodium methanearsonate and disodium methanearsonate are two examples of such herbicides which were used since mid-70s (Smith et al. 1998). Sodium arsenite was used to control aquatic weeds and was reported to be responsible for contamination of lakes and small fish ponds in many areas in United States of America (USA) (Adriano 2001). Foitunately, there has been a gradual decline in the use of arsenicals in agriculture. On the contrary, emission of arsenic oxide through burning of arsenic- rich fossil fuels is considered as one of the important anthropogenic reasons for atmospheric arsenic contamination. Mining and smelting of metals, like gold, copper, lead and zinc, etc. also releases a large amount of arsenic because it is widely present in the sulphide ores of these metals.
A small amount of arsenic can be added in the enviromnent by some plants and microorganisms. However, biological activities affect the redistribution of arsenic by several mechanisms like bioaccumulation (e.g., biosoxption), biotransformation (e.g., biomethylation) and transfer (e.g., volatilization). The bioaccumulated arsenic can be transferred from soil to plant, plant to animal and then ultimately to the human through food-chain. The arsenic in the food-chain gets biomagnified in concentration several times than that in water and affects animal and human health. Organisms living in aquatic environment also accumulate higher concentration of arsenic than the water in which they live and subsequently become a source of contamination.