Guidelines for Metals and Metalloids in Ambient ... - ARCHIVE: Defra

Chapter 2
Arsenic
2.1 Background
2.1.1 Basic chemical information
56. Arsenic is one of those chemical elements known as a metalloid, which
reflects the fact that it exhibits properties intermediate between those
of the metals and the non-metals. It occurs naturally in the
environment within the earth’s crust, having a relatively low average
abundance of around 5 mg/kg. Its distribution is, however, uneven and
there are some locations, most notably Bangladesh, where high
naturally occurring concentrations can leach into groundwater, posing
a serious human health risk.
57. Arsenic can exist in four oxidation (valence) states, summarised in the
Table below:
Table 2.1: The different oxidation states of arsenic.
Oxidation state
0
-
3
+
3
+
5
Chemical name
arsenic (free element)
arsine
arsenite (AsIII)
arsenate (AsV)
Within the gas arsine (AsH 3 ) arsenic is in the -3 oxidation state; the free
element (0 oxidation state) is very unlikely to be observed in the
environment. Most commonly arsenic is in the environment in its +3
oxidation state as arsenite – abbreviated to As(III) – or in combination
with other elements such as chlorine and sulphur, or as arsenate (+5
oxidation state) – abbreviated to As(V). Arsenic in the atmosphere is
most commonly present as mixtures of arsenites and arsenates. The
proportion of As(III) to As(V) varies from 27% to over 50% (European
Commission, 2000). Because of the relatively high vapour pressure of
certain As(III) compounds such as arsenic trichloride, arsenic is present
both as vapour and particulate matter in the atmosphere.
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