Geoscience Reference
In-Depth Information
1.3
REMOVAL OF ARSENIC FROM GROUNDWATER USING
REACTIVE GEOCHEMICAL BARRIERS
1.3.1
General
For some conditions of soils and when the emission sources are not very diffuse, it is possible to
eliminate As using PRBs. As said before, according to the redox conditions of the water, As can
occur as arsenite or arsenate anions. It is possible to use PRBs acting according to two possible
mechanisms (Blowes
et al
., 2000; Lackovic
et al
., 1999; Gubert
et al
., 2004; Younger
et al
.,
2002):
•
Adsorption and/or coprecipitation of the anionic As species. In this case, it is possible to
use mixtures of low cost metallic oxides, such as iron oxides. Within the barrier, these
metallic oxides are the minor component (10%), mixed up with silica (50%) and calcite
(30-50%).
•
A mechanism inherent to the used material, allowing the reduction or formation of a solid
phase, such as As(0) or arsenic sulfides, depending on the presence of sulfur within the
barrier. In this case, metallic iron is the active element (10%), and the remaining material was
the one stated before. The active barrier would take advantage of the redox properties of the
As(III)/As(V) system, as it was studied for other metals and metalloids with high oxidation
number such as Cr(VI), Mo(VI), U(VI), Se(VI) or organic compounds in oxidized forms
(Bianchi-Mosquera
et al
., 1994; Blowes
et al
., 1997; Deng and Hu, 2001; Fryar
et al
., 1994;
Guillham and O'Hannesin, 1992; Joshi and Chaudhuri, 1996; Ptacek
et al
., 1994). Some
materials have been tested with high success such as siderite (FeCO
3
), pyrite (FeS
2
) and Fe(0)
in the zerovalent form of granular fillings [Fe(s)]. Generally, the reactions that have been
considered responsible for the process are the reduction of As(III) and As(V) to the zerovalent
form or to a sulfide, while Fe(II) is oxidized to an oxide-hydroxide form that can originate
coprecipitation mechanisms and/or As adsorption. These mechanisms are not completely clear.
1.3.2
PRB types for treating arsenic in groundwater
Several materials have been used in PRBs for treating As in groundwater. The most common
are: (i) elemental iron, (ii) slag from iron works, (iii) sorbent materials such as mixtures of iron
hydroxides and activated alumina, (iv) multifunctional barriers, either multiple or composed,
consisting in a first barrier of compost or another organic material that promotes the microbial
reduction of sulfates, followed by a second barrier or another sorbent material.
1.3.2.1
PRBs with Fe(0)
Most of this type of barriers uses metallic iron [Fe(0)] as reactive medium to convert the pollutants
into non-toxic species or species with low mobility. These barriers take advantage of oxidation-
reduction processes where the pollutant is reduced and the medium is oxidized. Zerovalent metals
such as iron, tin and zinc, that are moderately strong reducing agents, have been used as reactive
media (Powell
et al
., 1995). Iron is the most studied one, and its efficiency was proved for several
types of pollutants.
Technologies based on zerovalent iron (ZVI) are considered by the US Environmental Protection
Agency as a valuable method to eliminate traces of organic pollutants, and it has also been
considered as potentially adequate to eliminate As and metallic species. Its efficiency as a reducing
agent for several organic and inorganic pollutants was also demonstrated. Among the organics, the
following can be mentioned: aliphatic chlorinated compounds, nitroaromatics, some pesticides
and azo-dyes; among the inorganics, metallic species of high valence such as chromium(VI),
uranium(VI), technetium(VII), mercury(II), molybdenum(VI) and inorganic non-metallic anions
such as nitrate, nitrite, selenate, selenite, arsenite and arsenate, among others (Alvarez, 2000;
Deng and Hu, 2001; Guillham and O'Hannesin, 1992; Lien and Wilkin, 2005).The transformation
process is a surface reaction that requires an intimate contact between the reactive medium and the
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