Subsurface Iron and Arsenic Removal

1 Introduction
Box 1.1
Arsenic speciation, including an Eh-pH diagram for aqueous arsenic species in the system As-O 2
-H 2
O at 25°C and 1 bar total
pressure (Smedley and Kinniburgh, 2002)
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Arsenic is stable in four oxidation states (+5, +3, 0, -3)
under redox conditions occurring in aquatic systems
(Furguson and Gavis, 1972). However, in the aqueous
environment, arsenic occurs mainly as two species; trivalent
arsenic, arsenite or As(III) and pentavalent arsenic, arsenate
or As(V). Under oxidizing and aerated conditions, the
predominant form of arsenic in water is As(V) (as HAsO 4
2-
or H 2
AsO 4
-
around neutral pH), whereas if reducing
conditions prevail, As(III) (around neutral pH as H 3
AsO 3
) is
the predominant arsenic compound (WHO, 2001). As(III)
is thermodynamically unstable in oxic environments and
oxidizes to As(V); however studies have shown that this
reaction proceeds slowly when oxygen is the only oxidant
(AWWARF, 2000; Kim et al., 2000; Stollenwerk, 2003).
Therefore both As species can be found in natural systems.
The speciation of arsenic is controlled by pH within a
particular oxidation state, while the redox potential controls
the distribution of arsenic species between the two oxidation
states (Masscheleyn et al., 1999; Bose and Sharma, 2002).
Concentrations up to 1,800 μg.L -1 have been measured in
Inner Mongolia, a northern province of China (Smedley
et al., 2003). In Vietnam and Cambodia, arsenic
concentrations were also observed to be high (up to 1,340
μg.L -1 ) due to reductive dissolution of young sediments
(Buschmann et al., 2007; Buschmann et al., 2008).
Arsenic mobilization caused by mineral dissolution
has been found in active volcanic areas of Italy (Aiuppa
et al., 2003) and inactive volcanic regions in Mexico
(Armienta and Segovia, 2008). Volcanism in the Andes
has lead to arsenic contamination of groundwater in
Chile and Argentina (Smedley and Kinniburgh, 2002).
Also mining activities have been found to contribute to
arsenic contamination in Latin American groundwater
(Smedley and Kinniburgh, 2002). Mining activities may
cause the oxidation of arsenic-bearing sulphide minerals
resulting in the release of arsenic into groundwater.
Smedley and Kinniburgh (2002) listed cases of arsenic
contamination caused by mining activities in Canada,
Germany, Ghana, Greece, Mexico, South Africa,
Thailand, UK, USA and Zimbabwe. In the past years,
more and more countries have found their waters to
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