In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 12

more directly responsible for the reduction of a contaminant. Moreover, the strength of ferrous
iron as a reductant could be significantly affected by the ligands present in the system, including
organic matter and metal oxides that form complexes with ferrous iron (Beak and Wilkin, 2009;
Herbert et al ., 2000; Roh et al ., 2001).
The technological challenge is to reduce or eliminate the problems of possible obstruction
of the pores or those that can be a physical barrier for the active sites. Alternative strategies
to remove oxides include ultrasound technologies and pH control. In addition, there may be
microbial processes that help to alleviate the reduction in pore volume due to the formation and
trapping of hydrogen gas produced by the anaerobic corrosion of iron (Phillips et al ., 2003). As
already stated, removal of metals and metalloids through ZVI is a combination of transformation
and immobilization processes, in which the species is reduced to a less soluble form. The main
challenge of the method is to avoid the potential risk of remobilization due to dissolution of the
compounds formed (Lo et al ., 2007).
Three important factors that determine the usefulness of a metallic iron material for in-situ
decontamination have been determined (Phillips et al ., 2003; Wilkin et al ., 2003). One is the iron
content, since it must at least be sufficient to stoichiometrically react with the contaminant. For
many contaminated aquifers, the overall concentration is small, in the order of mg L − 1 or gL − 1 .

However, because of the slow flow of groundwater and the division of contaminants within and
outside the solid phase, a typical plume may require between 100 and 200 years to cross a certain
point (Wilkin et al ., 2002).
There is also uncertainty about the limiting step of the elimination process. In general, rates
of transformation in a reactive barrier may be controlled by transport to the surface or by the
surface reaction. Interactions between abiotic and biotic processes represent a particularly difficult
research challenge because of the complexity of the potential synergy or antagonistic effects.
On one hand, in short-term studies, the microorganisms seem to improve the kinetics and the
distribution of final products; however, the effect of microorganisms on permeability and long-
term reactivity is less well known.
In addition, it is important to note that, apart from the limitations of the barrier and the medium,
there are also important costs involved. Although the total cost of an iron reactive barrier is lower
than a pump-and-treat system, the initial costs of installing the barrier is much higher, considering
that several long-term data are still unknown and it is difficult to overcome the reluctance to the
use of this technology for commercial purposes.
The second factor is how efficiently the reducing agent is used. Excavation is the most eco-
nomical on-site decontamination method. The ability to reduce a greater number of moles of
contaminant with the same number of moles of iron means that it is likely that a smaller volume
of material barrier may be required for a specific application and, therefore, the total volume to
be excavated can be reduced (Mackenzie et al ., 1997).
The third factor is the rate of iron corrosion, as the corrosion reactions of iron with oxygen and
water are thermodynamically favored. It is important that these reactions are sufficiently slow in
the time scale of the decontamination process.
Based on the existing technology in the different sites, it is doubtful that the barriers are cost-
effective for the treatment of deep aquifers (deeper than 30 m), or in places geologically difficult
to access. Injection of a reactive material (e.g., colloidal iron) by hydraulic fracturing, mixture in
depth or injection under pressure can be an alternative that overcomes the limitations associated
to the commercial excavation technologies. Thus, a remediation proposal could be the use of
colloidal iron or iron nanoparticles as substitutes for iron particles, usually in the millimeter scale
(3-6 mm). The conceptual change of this application is described in Figure 1.4 (Kaplan et al .,
1996). When the choice of treatment is the one described above, the objective is the introduction
of colloidal or nanometric particles in areas of the aquifer that can capture the contamination
plume. This type of applications has been recognized in the literature as reactive zones (Diels and
Vanbroekhoven, 2008). This implies, as described in Figure 1.4 , that it is not necessary to drill
the trench in which the reactive material is introduced, but it is introduced using injection wells
from the surface.

µ