Sources and Occurrence of Arsenic in the Environment

Yellow arsenic has a density of 2.03 gcm 3 at 18 C and is unstable. The nonmetallic form of arsenic can be produced by a sudden cooling of arsenic vapor, and it consists of tetra atomic molecules, As 4 . In its yellow form arsenic is more volatile than in the gray form. Arsenic and its compounds occur in crystalline, amorphous or vit reous forms and usually can be found in t race quantities in all kinds of rocks, soils, waters and even in air Leonard, 1990 Elemental arsenic is not soluble in water. The salts of arsenic widely diffe r with respect to their solubilities in water, which depend on the pH and the ionic environment. In its compou nds arsenic can have four valence states, namely –3, 0, +3 and +5. In its –3 state a rsenic can be p resent as poisonous compound arsenic trihydride arsine, AsH 3 . It is a colorless, ext remely poisonous, neut ral gas with a cha racteristical and disagreeable garlic odor. Its melting and boiling points are -117 C and -55C, respectively. This hyd ride is a powerful reducing agent, even for fairly weak oxidizing agents. Arsine is used in the determination of arsenic with several methods in both atomic and molecular spectromet r y Bombach et al, 1999. Under moderately reducing conditions, arsenic mainly occu rs in the t rivalent state, whereas in oxygenated environments AsV thermodynamically is mo re favored. Because of its ability to form complexes with certain co-enzymes, AsIII is more toxic to animals and plants than AsV. Therefore, the speciation of arsenic is important fo r understanding both the biological and the geochemical behavio r of this element Wrobel et al, 2002

2. Sources and Occurrence of Arsenic in the Environment

The most common mineral of arsenic is arsenopyrite. The natural occu rring a rsenic accounts for about one-third of the atmospheric flow of arsenic. It has been estimated that the atmospheric flow of arsenic amounts to about 73540 tonnesyear of which 60 is of natu ral origin and the rest items from anthropogenic sou rces. Volcanic action is a fu rther most important natu ral source of arsenic, and on a local scale it even may be the dominant atmospheric source Bombach et al, 1994 Inorganic arsenic of geological origin is found in groundwate r, which often are used as drinking water in several pa rts of the world, for example in Bangladesh. Organic arsenic comp ounds such as a rsenocholine, tet ramethylarsonium salts, a rsenosugars and arsenic-containing lipids are mainly found in marine organisms. Elemental a rsenic can be produced by a reduction of arsenic trioxide As 2 O 3 with charcoal. As 2 O 3 is p roduced as a by- product in metal melting processes. Mining, melting of non-fer rous metals and bur ning of fossil fuels are the major industrial p rocesses that contribute to anthropogenic arsenic emissions in air, water and soil. Historically, the use of arsenic-containing pesticides has caused a lot of contamination of agricultu ral land with a rsenic. Furthe r, the use of arsenic for the preservation of timber also has led to contamination of the environment. Arsenic is a main component in more than 200 minerals, of which about 60 a re arsenate, 20 sulfides and sulfosalts and the remaining 20 include arsenides, arsenites, oxides and elemental arsenic. The most common arsenic mineral is arsenopyrite FeAsS and a rsenic is found together with many types of mineral deposits especially with those that stem from sulfide mineralization. The mean total arsenic concentrations in air from remote and ru ral areas ra nge from 0.02 to 4 ngm 3 where in u rban ar eas it ra nge from 3 to about 200 ngm 3 . Much higher concent rations 1000 ngm 3 , however, have been found in the vicinity of industrial sources, although in some areas this is decreasing because of pollution abatement measures. Concent rations of arsenic in open ocean seawater are typically 1- 2 µgL. Arsenic is widely distributed in surface freshwaters, and concentrations in rivers and lakes are generally below 10 µgL, although individual samples near ant hropogenic sources may have arsenic concent rations up to 5 mgL. Arsenic levels in grou ndwater at the average abou t 1- 2 µgL except in areas with volcanic rocks and sulfide mineral deposits, where arsenic concent ration can be up to 3 mgL. In sediments the mean arsenic concent rations range from 5 to 3000 mgkg, and higher levels are found in areas with contamination. Backgrou nd concentrat ions in soil range from 1 t o 40 mgkg, while mean values often arou nd 5 mgkg. Elevated levels of natu r al arsenic in soils may be due to associations with geological substrates such as sulfide ores. In anthropogenically contaminated soils concent rations of arsenic may be up to several grams per 100 mL. Marine organisms normally contain arsenic between 1 to mo re than 100 mgkg. This is predominantly present as organic arsenic species such as arsenosuga rs macroalgae and arsenobetaine in invertebrates and fish. The concent ration of arsenic in va rious types of igneous rocks range from 1 to 15 mg Askg, and the value is about 2 mg Askg. Similar concent rations 1-20 mg Askg are found in sandstone and limestone. Significantly highe r concent rations of up to 900 mg Askg are found in argillaceous sedimenta r y rocks including shales, mudstone and slates. Up to 200 mg Askg can be present in phosphate rocks. Concent rations of a rsenic in open ocean water are typically 1- 2 µgL. The concentrations of arsenic in unpolluted surface water and grou ndwater are typically in the range of 1- 10 µgL. Elevated concentrations in su rface water and groundwater of up to 100- 5000 µgL can be found in areas of sulfide mineralization. Indeed, in natu re arsenic containing minerals u ndergo oxidation and release arsenic to water. This could be one explanation for the occu rence of arsenic in the groundwater of West Bengal and Bangladesh, where the grou ndwater usage is very high. Ma rine organisms accumulate considerable quantities of organic a rsenic compou nds. In marine animals the larger pa rt of this a rsenic is p resent as arsenobetaine AsB, whereas ma rine algae contain most of the arsenic in the form of dimethylarsinoylribosides. Humans are therefore exposed to these arsenic compou nds through all diets that include seafood.

3. Human Exposure