LITERATURE REVIEW
2.1. Introduction, Source and Environmental Impact/Health Effects of Contaminants
2.1.1 Arsenic
CHAPTER 2
acids (H3AsO4, H3AsO4−, H3AsO42−), arsenites, arsenates, methylarsenic acid, dimethylarsinic acid, arsine, etc. (Smedley & Kinniburgh, 2002). In natural waters arsenic exists in oxyanionic form of trivalent arsenite or pentavalent arsenate. In reducing anoxic conditions of groundwater arsenite predominates while arsenate is majorly found in oxygen rich aerobic water environments (Greenwood & Earnshaw, 1984). The mobilisation of arsenic species in groundwater under reducing and oxidising conditions is pH dependent (pH 6.5-8.5). Arsenic speciation in natural waters is strongly controlled by redox potential (Eh) and pH (Fig.2.1) (Wang & Mulligan, 2006). At pH values less than 9.2 and in reducing conditions, the uncharged arsenite (H3AsO30) will be the dominating species. At low pH (< pH 6.9) and oxidising conditions, H2AsO4- is dominant, while HAsO42- dominates at higher pH values (> pH 6.9). In highly acidic and alkaline conditions, H3AsO40 and AsO43−are representing species respectively (Yan et al., 2000).
Arsenic has been found to be a common contaminant of fresh water and sea water in many parts of the world. It can exist both in organic and inorganic forms.
(Ref.: Wang and Mulligan 2006)
Figure 2.1 Eh–pH diagram for arsenic at 25°C and 101.3 kPa.
2.1.1.1 Sources and Occurrence in Water Environment
Arsenic distribution and mobilisation in environment is complex process, occurs through continuous cycling of different forms of arsenic through air, soil and water. It is introduced in soil and groundwater during weathering of rocks followed by leaching and run off. Natural processes of arsenic transport includes mineral weathering, biologically aided mineralisation, and volcanic emissions (Sharma et al., 2014a). However, it can also be introduced as a result of many anthropogenic activities such as wood preservatives, paints, alloys, semi-conductors, fossil fuel combustion, mining wastes, smelting operations, landfilling, sewage, and agricultural applications (pesticides and fertilizers) (Mondal et al., 2013; Singh et al., 2015). Contribution of anthropogenic sources to groundwater arsenic contamination is much less compared to the natural sources, still their contribution cannot be ignored. Most of the arsenic transport in natural aquifers is due to the physical, geochemical conditions (especially reducing conditions) and water–
rock interactions. That‘s why most of the reported arsenic poisoning, throughout the world is due to groundwater As exposure rather than surface water (Smedley &
Kinniburgh, 2002).
Geogenic presence of arsenic in groundwater ranging from 0.5 to 5,000 μg/L (Smedley & Kinniburgh, 2002) has been reported around the world (Ravenscroft et al., 2009). The source of As in the groundwater may be volcanic ash as in the southern Gulf Coast aquifer system in Texas (USA) (Gates et al., 2011), reductive dissolution of Fe- oxyhydroxide minerals as in lower Fraser River Delta, British Columbia, Canada (Bolton
& Beckie, 2011) and Bengal Basin (Uddin et al., 2011), weathering of ultramafic rocks as in bedrock aquifers of Rowe-Hawley Belt of northern Vermont, (USA), Ryan et al.
(2011). In a geochemical and hydrological study of groundwater in Bihar, India, high As is attributed to recharging of deep aquifers through the Pleistocene deposits (Saha et al., 2011).
High As Concentrations are reported in groundwater from many parts of the world such as Africa, Bangladesh (up to 1000 μg/L), Brazil, China (up to 850 μg/L), India (up to 23mg/L), Italy, Mexico, Chile (up to 770 μg/L), Argentina (up to 3810 μg/L), Latin America, Canada, Germany, Ghana, Greece, Korea, Mexico, Mongolia (up to 1800 μg/L), Nepal (2660 μg/L) Poland, South Thailand, UK, USA ( >3000 µg/L), Vietnam (up to 3050 μg/L) and Zimbabwe etc. (Singh et al., 2015; Smedley & Kinniburgh, 2002;
Smedley et al., 2003; Van Halem et al., 2009)
2.1.1.2 Environmental Impact
Arsenic is extremely poisonous. International Agency for Research on Cancer has classified arsenic as a human carcinogenic substance, group 1.(IARC, 2004) According to the United Nations Synthesis report, arsenic poisoning is the second most important health hazard in the world related to drinking water after pathogenic contamination (Singh et al., 2015). More than 150 million people all over the world, including nearly 110 millions of South and South-east Asian countries are at risk of arsenicosis due to arsenic contamination in drinking water (Ravenscroft et al., 2009). Irrigation with As-enriched groundwater increases inorganic arsenic exposure through food, especially rice and vegetables thus lead to enter As in human food chain (Bhattacharya et al., 2012).
Arsenic poisoning in humans may cause melanosis, leuco-melanosis, keratosis, hyperkeratosis, dorsum, nonpetting edema, gangrene and skin problems including cancer (Singh et al., 2015). As(III) is more toxic because it can bind to sulfhydryl groups of cysteine residues in proteins and inactivates them (Cavalca et al., 2013). Long term high arsenic intake can cause peripheral vascular disease, gastrointestinal disturbances, and possibly diabetes, high blood pressure and reproductive disorders including cancers of lung, kidney, liver and bladder (WHO, 2011).
Scientific studies from China and Bangladesh have shown neurological problems, mental retardation and developmental disabilities such as physical, cognitive, psychological, sensory and speech impairments among arsenicosis patients. Moreover, arsenicosis not only affect its victims but also their families by social instability, social discrimination, marriage-related problems, refusal of victims by community and families (Brinkel et al., 2009). The drinking water standard for arsenic is reduced to 10 ppb from 50 ppb by most of the developed nations as WHO directive, because of serious health impacts on humans. However, some countries including Bangladesh and China still following the earlier WHO guideline of 50 ppb (Sharma et al., 2014a; WHO, 2011).