Page 150 - Soil and water contamination, 2nd edition

Heavy metals 137

7.9 ARSENIC
Arsenic is a semi-metal which may form metal arsenides in which the oxidation state is
negative. In addition, it may form sulphides that can be found in sulphide ore minerals.
In aqueous solutions, arsenate (5+ oxidation state ) and arsenite (3+ oxidation state) are the
thermodynamically stable forms. Under mildly reduced conditions, the arsenite uncharged
ion HAsO (aq) is the predominant form. Under oxidising conditions, the monovalent
2
-
arsenate anion H AsO predominates in the pH range between 3 and 7, whereas the
2 4
2-
divalent species HAsO predominates between pH 7 and pH 11 (Hem, 1989). Arsenic
4
compounds can adsorb to oxides and hydroxides of Fe(III), Al(III), Mn(III/IV), to humic
substances , and clay minerals (Bissen and Frimmel, 2003). Like phosphate, arsenate is
strongly adsorbed by ferric oxyhydroxides . Furthermore, ferric and other metal arsenates
are poorly soluble. This allows arsenates to accumulate in zones of iron precipitation when
reduced groundwater seeping upwards crosses the oxidation–reduction interface. In aerated
surface water, adsorption by hydrous iron oxides, or precipitation or coprecipitation with
sulphide in reduced bottom sediments maintain the arsenic concentrations in water at
-1
very low levels (usually below 10 μg l ). Arsenic is also involved in biochemical processes.
Biologically mediated methylation produces organic complexes, such as dimethyl arsenic
acid ((CH ) AsOOH) and methyl arsonic acid (CH AsO(OH) ). Dimethyl arsenic acid
3 2 3 2
is difficult to oxidise and may constitute the main arsenic species in surface water. The
importance of the redox state and the formation of soluble organic complexes of arsenic
result in the arsenic concentrations in natural water varying over several orders of magnitude
(see Table 7.1). Some arsenic compounds are relatively volatile, which is an important factor
in the natural circulation of arsenic in the environment.
’
Arsenic is a toxic, non-essential element. As arsenic s toxicological importance is
partly attributable to its chemical similarity to phosphorus (though the oxidation state of
phosphorus is not sensitive to the redox potential), it can readily disrupt the metabolic
pathways involving phosphorus (Alloway and Ayres, 1997). Organic forms of arsenic are
usually less toxic than the inorganic forms. The largest source of arsenic intake for humans
is food (generally about 25 to 50 μg is ingested per day), with smaller amounts coming from
drinking water and air. However, in areas with naturally elevated arsenic concentrations in
groundwater used for drinking water, such as in southern Bangladesh and West Bengal,
India, the principal source of arsenic intake is drinking water. Some edible fish and shellfish
contain elevated levels of arsenic, but this is predominantly in less toxic organic forms. Small
children who swallow small amounts of soil while playing, may be exposed to arsenic if
they ingest soil containing elevated levels of arsenic. Most arsenic that is absorbed into the
body is converted by the liver to a less toxic form that is efficiently excreted in the urine.
Consequently, arsenic does not have a strong tendency to accumulate in the body except at
high exposure levels. Inorganic arsenic has been recognised as a human poison since ancient
-1
times. Large doses can produce death: a daily intake of 5 to 50 mg d is considered to be
-1
toxic and above 50 mg d as lethal. Long-term exposure to lower levels of arsenic may cause
decreased production of red and white blood cells, abnormal heart function, blood vessel
damage, liver and/or kidney injury, and impaired nerve function. Arsenic has also been
associated with skin cancers (ATSDR, 2013).
Arsenic is present in volcanic gases and is a common constituent of geothermal waters. In
some parts of the world, natural mineral deposits contain large quantities of arsenic ; this may
result in elevated levels of inorganic arsenic in soil and water. Enhanced arsenic concentrations
-1
of 50 μg l or more have been reported in groundwater from a number of large aquifers in
various parts of the world, including Bangladesh, South-east Asia, inner Mongolia, Hungary,
south-west USA, central Mexico, and Argentina. However, these enhanced concentrations
have different causes: as well as occurring in geothermal waters (as mentioned above), they

10/1/2013 6:44:34 PM
Soil and Water.indd 149
Soil and Water.indd 149 10/1/2013 6:44:34 PM