2.3 Modification of nZVI Particles and Enhancement
of Their Reactivity
In spite of their high surface activity and remarkable environmental uptake capacity,
most synthetic ZVI nanoparticles display some significant disadvantages from the
technological/chemical engineering and cost/benefit perspectives. Therefore, drawbacks such as the strong tendency towards aggregation, fast oxidation, and rapid
sedimentation can eliminate their long-term reactivity and decrease their maximum
capacity.
Generally, it has been reported that the limited mobility of nZVI particles in
saturated porous media is attributed to two reasons; firstly, nZVI can be filtered from
the solution by attaching to aquifer materials and secondly, agglomeration and
aggregation remarkably immobilize the particles (Phenrat et al. 2007). Agglomeration and aggregation phenomena decrease the specific surface area, and in turn,
affects the reactivity and mobility in the subsurface and porous media, such as sand
and soil (Ponder et al. 2000; Sun et al. 2007). The strong tendency towards nZVI
aggregation is to a large extent attributed to their unstable colloidal nature and longranged attractive magnetic interactions between the particles (Phenrat et al. 2009).
Gravitational sedimentation of nZVI particles is a result of the aggregation effect
and can be a good indicator of the colloidal stability of the particles. When
nanoparticles are dispersed in a media, they can remain stable for very long time
under some conditions. These conditions demand that the diffusion flux of
nanoparticles overcomes the sedimentation flux. The diffusion flux of nanoparticles,
that opposes gravity, is inversely proportional to the particle size, while the sedimentation flux is proportional to the square of the particle radius (Phenrat et al.
2008). When nanoparticles start to aggregate to bigger clusters in the range of
micrometers, they settle as a result of the fact that the sedimentation flux turns to
be bigger than the diffusion flux. Thus, their delivery and mobility is limited.
The effects of the fast oxidation of nZVI particles include a gradual loss of
reactivity and permeability (Keum and Li 2004). While metallic iron exists in an
aqueous environment, dissolved oxygen (DO) and water initialize its oxidationdissolution. The whole reaction involves the formation of soluble ionic products or
insoluble oxides/hydroxides. In other words, in aqueous environment, the nZVI
oxidation creates species such as soluble Fe
2+
(aq) , H 2 and various precipitates, e.g.,
Fe(OH) 2 , Fe(OH) 3 , Fe 3 O 4 , Fe 2 O 3 , FeOOH, Fe 5 HO 8 Á4H 2 O, and green rusts (Crane
and Scott 2012). In this manner, when the precipitates are formed as a layer on the
surface of nZVI and reach a critical thickness, the reactivity of nZVI is inhibited. In
this point, the reactivity is eliminated while the surface of metallic iron is blocked to
interact with any media or with the pollutants. These factors concerning the oxidation of the particles can be critical for the overall efficacy of this material since nZVI
can react, be oxidized, consumed, or blocked before it reaches the target and
remediates a desired site.
For all the above-mentioned reasons, the modification and/or stabilization of the
nZVI nanoparticles, which can lead to steady or even enhanced remediation ability
30
T. Phenrat et al.
of Their Reactivity
In spite of their high surface activity and remarkable environmental uptake capacity,
most synthetic ZVI nanoparticles display some significant disadvantages from the
technological/chemical engineering and cost/benefit perspectives. Therefore, drawbacks such as the strong tendency towards aggregation, fast oxidation, and rapid
sedimentation can eliminate their long-term reactivity and decrease their maximum
capacity.
Generally, it has been reported that the limited mobility of nZVI particles in
saturated porous media is attributed to two reasons; firstly, nZVI can be filtered from
the solution by attaching to aquifer materials and secondly, agglomeration and
aggregation remarkably immobilize the particles (Phenrat et al. 2007). Agglomeration and aggregation phenomena decrease the specific surface area, and in turn,
affects the reactivity and mobility in the subsurface and porous media, such as sand
and soil (Ponder et al. 2000; Sun et al. 2007). The strong tendency towards nZVI
aggregation is to a large extent attributed to their unstable colloidal nature and longranged attractive magnetic interactions between the particles (Phenrat et al. 2009).
Gravitational sedimentation of nZVI particles is a result of the aggregation effect
and can be a good indicator of the colloidal stability of the particles. When
nanoparticles are dispersed in a media, they can remain stable for very long time
under some conditions. These conditions demand that the diffusion flux of
nanoparticles overcomes the sedimentation flux. The diffusion flux of nanoparticles,
that opposes gravity, is inversely proportional to the particle size, while the sedimentation flux is proportional to the square of the particle radius (Phenrat et al.
2008). When nanoparticles start to aggregate to bigger clusters in the range of
micrometers, they settle as a result of the fact that the sedimentation flux turns to
be bigger than the diffusion flux. Thus, their delivery and mobility is limited.
The effects of the fast oxidation of nZVI particles include a gradual loss of
reactivity and permeability (Keum and Li 2004). While metallic iron exists in an
aqueous environment, dissolved oxygen (DO) and water initialize its oxidationdissolution. The whole reaction involves the formation of soluble ionic products or
insoluble oxides/hydroxides. In other words, in aqueous environment, the nZVI
oxidation creates species such as soluble Fe
2+
(aq) , H 2 and various precipitates, e.g.,
Fe(OH) 2 , Fe(OH) 3 , Fe 3 O 4 , Fe 2 O 3 , FeOOH, Fe 5 HO 8 Á4H 2 O, and green rusts (Crane
and Scott 2012). In this manner, when the precipitates are formed as a layer on the
surface of nZVI and reach a critical thickness, the reactivity of nZVI is inhibited. In
this point, the reactivity is eliminated while the surface of metallic iron is blocked to
interact with any media or with the pollutants. These factors concerning the oxidation of the particles can be critical for the overall efficacy of this material since nZVI
can react, be oxidized, consumed, or blocked before it reaches the target and
remediates a desired site.
For all the above-mentioned reasons, the modification and/or stabilization of the
nZVI nanoparticles, which can lead to steady or even enhanced remediation ability
30
T. Phenrat et al.