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contaminants but also immobilize inorganic anions such as arsenic or chromium and can even
be used to recover/remove dissolved metals from solution (Müller and Nowack, 2010). nZVI
has been found to be effective also against PCB and organochlorine pesticides (Zhang, 2003).
In Europe, most remedial actions with nZVI (70%) addressed chlorinated ethenes (PCE, TCE,
DCE) or other chlorinated hydrocarbons (e.g., PCB). A few pilot remediation tests (20%) of
other carbonaceous materials (BTEX, HC, VC) were additionally carried out. Ten percent of the
remediation projects involved the immobilization of metals (Cr, Ni) and one pilot application
also targeted nitrate. For 15 field-scale applications in the USA, nZVI was in most cases used to
treat a source zone of TCE and daughter products, and some of the sites were contaminated with
Cr(VI) (USEPA, 2005). It has also been promoted the treatment of source zones contaminated by
dense non-aqueous phase liquids (DNAPL) especially chlorinated alkenes such as PCE and TCE
(US Navy, 2010).
1.6.7
Summary of advantages of the use of metal nanoparticles
The advantages of nZVI may be summarized as follows (Müller and Nowack, 2010):
•
Fast reaction: (i) short treatment time; (ii) less cost; (iii) less exposure for workers, fauna and
flora.
•
Complete reduction pathway to non-toxic byproducts: (i) less exposure for workers, fauna
and flora.
•
In-situ
treatment: (i) less equipment and aboveground structures required; (ii) less costs.
1.7
PROBLEMS TO BE SOLVED IN THE TECHNOLOGY OF PERMEABLE
REACTIVE BARRIERS WITH ZVI
Implementation of the technology of reactive permeable barriers with metallic iron faces still
several challenges: (i) the production and accumulation of by-products generated by the chemical
reactions involved in the elimination of contaminants due to the low reactivity of iron for the
pollutants; (ii) the decline of the reactivity of iron with time, probably due to the formation of
passive layers or to the precipitation of metallic hydroxides and carbonates, and (iii) engineering
problems in the construction of metal barriers in deep aquifers.
A way to overcome some of these problems is to increase the reactivity of the active material
of the barrier, thus increasing the rate of the reaction and retarding the obstruction of the pores.
The surface area of the solid iron has a direct influence on the number of active surface sites faced
to the contamination plume. By reducing the size of the iron particles, the surface area increases
and consequently the rate of reaction. Increasing the specific surface area should also involve an
increase in the fraction of iron atoms present in the surface of the particle, thus creating a more
reducing ability per unit of mass. This could allow the use of smaller quantities of iron in the
treatment of the contamination plume. The ability to reduce the volume of excavation by using
barriers of less thickness and lower volumes of excavation is an important factor, because the
excavation results in a greater economic cost.
As mentioned before, several laboratory batch studies have shown that supported iron nanopar-
ticles are better than the commonly used commercial iron to eliminate various contaminants,
although studies in real long-lasting conditions are still necessary. In terms of molar ratio, iron
nanoparticles supported on polymer resins reduce the pollutant between 20 and 30 times more
than commercial iron samples. Tests carried out for 60 days show that 90% of the reduction occurs
during the first 48 hours, for supported or unsupported nanoparticles and also for the conven-
tional granular size (Environmental KTN, 2008). The support itself disperses the iron particles,
thereby increasing the total specific surface area, and provides a higher hydraulic conductivity
avoiding the agglomeration of nanoparticles. Reduction by borohydride produces similar metallic
iron nanoparticles in terms of size and surface area, regardless the material of the support used.
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