In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 14

of the material can be increased, the necessary amount will decrease and, consequently, the final
volume of the barrier. A way to improve the reactivity consists in increasing the specific surface
area of the material, because of the increase of the area/volume ratio; i.e., with the same total
mass of the reactive material a higher available mass for the reaction can be obtained. The amount
of available surface area is among the most significant experimental variables that affect the rate
of reduction of pollutants. To attain higher specific surface areas, different techniques have been
developed in the case of iron and other metal nanoparticles (Ponder, 2003; Zhang, 2003).
Nanotechnology has special relevance because of the potential for injecting nanosized (reactive
or adsorptive) particles into contaminated porous media such as soils, sediments, and aquifers.
Many different nanoscale materials have been explored for remediation, such as nanoscale zeo-
lites, metal oxides, carbon nanotubes and fibers, enzymes, various noble metals, and titanium
dioxide. Of these, nanoscale zerovalent iron (nZVI), and more generally, iron-based (e.g., oxides,
salts) nanoparticles are currently the most widely used for the in-situ remediation of soils from
a variety of toxic pollutants (e.g., immobilization of heavy metals, reduction of chlorinated
hydrocarbons, sorption/geochemical trapping of heavy metals, etc.).
Particles of nZVI may range from 10 to 100 nanometers in diameter or slightly larger. The
larger surface area of nZVI provides more reactive sites, allowing for more rapid degradation of
contaminants when compared to macroscale ZVI.
In addition, methods of preparation of nanoparticles are not expensive and, together with the
high reactivity of the particles, they can be competitive with commercial iron particles. It is also
important to take into account that the material usually used in reactive barriers is not pure iron,
but a commercial product, consisting of metal wastes, mostly molten iron, or light alloy material,
and it is coated by a thick oxide layer, having a lower reactivity than that of pure iron (Westall,
1986).
Due to its reduced particle size and high reactivity, metal nanoparticles can be useful in a large
variety of environmental applications such as treatment of soil and sediments and decontamination
of groundwater (Westall, 1986; Zhang, 2003).
Compared with conventional particles of higher size, colloidal and subcolloidal metal particles
offer several potential advantages. Among them, in addition to a high specific surface area, with
the consequent increased surface, the flexibility for its application is included (Gavaskar et al .,
2005; Tratnyek and Johnson, 2006).
Theoretical calculations indicate that for colloidal particles of less than 1 µ m gravity has a
very low effect in the transport and deposition of colloidal particles in a porous medium, and the
Brownian movement (thermal movement) tend to dominate. In aqueous solution, iron nanoparti-
cles can be kept suspended under a very mild stirring. In consequence, it can be feasible to inject
subcolloidal metal particles in contaminated soils, sediments and aquifers for in-situ decontami-
nation, offering an alternative with good cost-effectiveness ratio to the conventional technologies
such as pump-and-treat systems, air stripping or even reactive barriers themselves, allowing in
addition their application in aquifers that cannot be treated by other methods due to is high depth
(Ponder et al ., 2000).
The global performance of a nanoparticle system could be potentially thousands of times better
than that using commercial iron. This is especially important for the injection of iron particles in
groundwaters, because to avoid obstructions it is beneficial to inject only a low amount of very
reactive metal particles. Thus, in respect to its application in removal of pollutants, nanoparticles
have two main applications: (i) direct injection to the polluted medium, or (ii) to be supported in
some type of material for use as reactive barriers.
1.6.2 Preparation of iron nanoparticles
There are several forms of preparation of nanoparticles, which can be divided in two main groups:
physical methods and chemical methods. In the first group, the metal nanoparticles are formed
from atoms in the process of vaporization of the metal and subsequent condensation in various