Page 323 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

308 Environmental Applications of Nanomaterials

dissolving Fe-oxides or Fe-oxyhydroxides formed at the water/particle
interface (Gerlach et al. 2000), or by generating reactive surface-
associated Fe(II) species (Williams et al. 2005). Homoacetogens are
another group that could enhance TCE removal—either directly through
cometabolism (using H as primary substrate), or indirectly by stimu-
2
0
lating heterotrophic activity through acetate production (4Fe 2CO
2
+2
5H O h CH COO 4Fe 7OH ) (Oh and Alvarez 2002).
2
3
Microbially induced corrosion could also ensure the localized dissolution
of the iron nanoparticles, thereby eliminating possible concerns from off-
site migration and risk. The data collected thus far on the fate of nano-
iron have been largely collected in the laboratory in deionized water.
Porewater constituents such as carbonate, sulfate, and chloride may
inhibit or promote nanoiron corrosion (Agrawal and Tratnyek 1996;
Bonin et al. 1998; Phillips et al. 2000; Vogan et al. 1999). It is critical to
know what the end product of nanoiron oxidation is under real envi-
ronmental conditions, but a well controlled in situ study of the fate of
nanoiron in the subsurface has yet to be conducted.

Effect of pH. The role of pH is an important one for nanoiron lifetime.
As seen in Eq. 5, decreasing the pH should increase the rate of H evo-
2
+
lution from nanoiron if the reduction of H at the iron surface is the rate
controlling step. According to Eq. 5, in the absence of any other oxidants
and assuming that the reaction is first order with respect to the specific
surface area of the iron, the rate of H evolution is given by Eq. 6.
2

dH 2 # #
1 b
1 b
5 k [SA] [H ] 5 k H 2 obs [H ] (6)
dt H 2
k H is the first-order reaction rate constant for H 2 evolution and [SA]
2
is the specific surface area for the reaction, k H obs , is the observed pseudo-
2
first-order reaction rate constant. The H 2 evolution from nanoiron in
solution at pH ranging from 6.5 to 8.9, the equilibrium pH for an
iron/water system (Pourbaix 1966), is shown in Figure 8.5a. Indeed, as
evolution increases. For pH ranging
the pH decreases, the rate of H 2
from 6.5 to 8.0, there is a log linear relationship between the rates, indi-
cating that b in Eq. 6 is ≈1 (Figure 8.5b). Thus H evolution from RNIP
2
+
is pseudo-first-order with respect to H . At pH greater than 8, the mech-
+
0
anism for Fe oxidation by H appears to change, although some H 2
evolution is occurring at pH > 8. This agrees with published reports for
hydrogen evolution from zerovalent metals at near neutral pH where
k (Eq. 5) has been shown to be the rate controlling step (Wang and
1
Farrell, 2003; Bockris et al., 1987). Most importantly is the effect of pH
on the lifetime of RNIP. Using the data in Figure 8.5a, at pH > 8, the