Geoscience Reference
In-Depth Information
Both the reactions mediated by oxygen or by water result in a pH increase, although the effect
is more pronounced in anaerobic conditions because corrosion occurs faster.
From this description, it can be concluded that there are three main reducing agents in a water-
iron system: metallic iron, ferrous iron and hydrogen resulting from the corrosion (in anaerobic
conditions). The relative intensity of each one in the process depends on the compound to be
reduced. The corrosion of Fe(0) particles or aggregates produces Fe(II), Fe(III) and OH ions,
which promote in turn the precipitation of the Fe(II) hydroxide [Fe(OH) 2 (s)] and several Fe(II/III)
oxyhydroxides and Fe(OH) 3 (s). These precipitation reactions may favor both As coprecipitation
with iron minerals as well as sorption of As on the corroded iron particles, thus contributing to the
elimination of As from the solution (ZVI) (Beak and Wilkin, 2009; Burghardt et al ., 2007; Su and
Puls, 2003). This mechanism is considered as a complex process, not completely known in terms
of the involved reactions, especially at the surface of the precipitated solids, whose chemical
composition is not known, and dependent on pH and on the aqueous phase composition. The
knowledge of the involved reactions, if possible, would allow a better design of the As removal
processes. So far, most studies have been carried at laboratory scale with zerovalent iron materials
for As removal (Gu et al ., 1999; Morgada et al ., 2009; Su and Puls, 2003; Triszcz et al ., 2009)
and only a few experiments have been reported at a full barrier scale.
When the water reacts with Fe(0), besides the increase in pH, the redox potential decreases and
the oxygen concentration increases. The higher pH favors the precipitation of calcium and iron
carbonates, as well as the insoluble metallic hydroxides. The decrease in potential originates the
reduction of metals and metalloids. Finally, the increase in the oxygen partial pressure supports
the activity of chemotrophic microorganisms that use hydrogen as energy source, especially
sulfate and iron reducing bacteria. As(V) anion in water bounds iron originating its oxidation to
ferrous iron through aerobic or anaerobic mechanisms.
Kinetics is fast: McRae (1999) observed As(V) removal from concentrations from 1000 to less
than 3 µ gL 1 in about two hours. Kinetics is also very fast with mixtures of As(III) and As(V).
Mineralogical studies show that As(V) reduces and coprecipitates with iron, which transforms
into goethite over the elemental iron particles.
The reduction yield depends on the superficial area of iron; in many cases, it is possible to
observe a linear relationship between both parameters (O'Hannesin and Gillham, 1998; Puls
et al ., 1999), even though the yield seems to stabilize for high superficial areas (Johnson, 1996;
CL:AIRE, 2001).
Bang et al . (2005) refer that As removal is dramatically affected by DO concentration and
the pH, once high DO concentrations and low pH increase iron corrosion (McRae, 1999). They
found that under oxic conditions As(V) removal is considerably faster than that of As(III). At pH
6, more than 99.8% As(V) was removed compared to 82.6% As(III) after 9 h contact time. When
DO was eliminated by purging with nitrogen, total As removal was below 10%.
1.3.2.2 Barriers with iron slag
Baker et al . (1998) used slag composed of a mixture of iron oxides, calcium oxides and limestone
as reactive medium. This medium proved its ability to remove not only As(V) but also mixtures
of As(III) and As(V) from concentrations up to 1000
gL 1 .
McRae (1999) tested mixtures of slag from steel production (BOFS - Basic Oxygen Furnace
Slag) as material for possible use in PRBs. The material promotes oxidation of As(III) to As(V),
and it is used together with activated alumina, which adsorbs As in both oxidation forms. The
tested mixture had 10% of slag and 20% of activated alumina mixed with limestone and silica
sand. In contrast, the use of slag without other components showed less satisfactory results, and
could not be considered a viable option.
This alternative was immediately implemented in real scale. The “Dupont Site” barrier, built
in Eastern Chicago in 2002, uses BOFS for the remediation of groundwater contaminated with
As. The slag is rich in iron and in calcium oxyhydroxides. The system comprises two permeable
barriers placed at a distance of 5 m. The reactive medium oxidizes As(III) to As(V), which is then
sorbed onto the slag surface. pH increases during the process, reaching values as high as 12.
gL 1
µ
to concentrations below 3
µ
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