Geoscience Reference
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for chlorine, which is liberated as a gas. An advantageous consequence is that dichloroethelyenes
(DCE) and vinyl chloride (VC), which are generated by TCE breakdown, are rapidly hydrogenated
at the particle surfaces and do not accumulate in the reaction (Schrick et al ., 2002). In the US,
approximately 50% of all nZVI remediation projects use standard nZVI and 40% use bimetallic
nZVI; in Europe, no field application using bimetallic particles is yet to take place (Müller et al .,
2012).
Bimetallic nZVI reactivity depends on a range of factors, including nanoparticle size, physic-
ochemical properties, and choice and quantity of the nobler metal. Minimal improvement with
respect to their monometallic counterpart has been documented in some studies have (Dickinson
and Scott, 2010) whilst others report an enhancement by several orders of magnitude (Tee et al .,
2009). Bimetallic nZVI could be used in preference to monometallic nZVI if they offer signif-
icantly improved performance at a competitive price (Pt, Pd and Ag are too costly), and if they
guarantee that they do not add ecotoxicity by the inclusion in the treated systems. Because of
corrosion, longevity of the materials can be poor. Consequently, bimetallic nZVI may be best
suited for remediation applications where only short migration times to the contaminant plume
are required.
1.6.4 Stability of metal nanoparticles
One of the most frequently questions in technology of nanoparticles is their long-term perfor-
mance and reactivity. Due to their reduced size and high surface area, nanoparticles react very
rapidly with a large variety of oxidants in groundwater, including dissolved oxygen, natural
organic matter and water. It is speculated that the nanoparticles can have a limited lifetime in the
subterranean environment (Chatterjee, 2008; Kanel et al ., 2006). In experiments carried out with
iron nanoparticles, significant surface oxidation after reduced periods were observed, changing
their color from dark to pale brown. Some experiments have demonstrated that the reactivity
decreases substantially after air exposure for some days.
However, a change of color much more reduced has been observed in bimetallic nanoparticles
of iron and palladium. ZVI nanoparticles underwent surface oxidation within a few hours (black to
reddish-brown), whereas Pd modified iron particles did not undergo an observable color change in
air suggesting stability (Zhang et al ., 1998). Correspondingly, ZVI particles lose reactivity within
a few days, while Fe/Pd particles remain active for at least two weeks (Lien and Zhang, 2001).
These bimetallic particles have demonstrated enough stability under environmental conditions,
and it is expected that they can keep their reactivity for extended periods in the subterranean
environment (USEPA, 2007).
Studies performed about the content of water of the iron particles suggest that there is a limit
of life for the nanoparticles. Due to the corrosion produced by the medium, the estimated lifetime
for metal zerovalent nanoparticles will be more limited than that of the iron particles of lower
surface area. Iron nanoparticles contain considerably higher amount of water, linked physically
and chemically, than iron particles of larger size. Whereas the nanoparticles do not lose their
reducing power for a period of one year or more, a long-term exposure to air can finally produce
dehydration forming less porous and less reactive surface oxides (SenGupta et al ., 2003).
Although it is generally recognized that iron nanoparticles are powerful in removal of pollutants,
the colloidal chemistry of these particles makes them prone to agglomeration. For this reason,
much work is being done on the immobilization of these particles to avoid agglomeration. A
disadvantage of nZVI is the agglomeration of particles to each other and fast attachment of
agglomerates to the soil surface. Agglomeration may be caused by groundwater conditions (pH,
ionic strength), surface properties of the particles, the age of the materials, or shipping conditions
(Saleh et al ., 2008; Zhang, 2003). Modifications to nanoscale iron particles have been made by
using polymers or surfactants to enhance their mobility, reactivity, or stability. Surface modifiers
increase the surface charge of the nanoparticles thereby providing electrostatic stabilization. They
can also create a surface brush layer that engenders long-range strong steric repulsion forces,
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