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Fig. 16.8 Change in the
first-order rate constants k A
and k B for reduction of
C 6 Cl 5 NO 2 and C 6 Cl 5 OH,
respectively, and change in
sorbed Fe(II), [Fe(II)] s ,asa
result of varying pH in media
with 473 lM Fe(II),
100 ± 11 mg/L goethite and
200 mM NaCl. Error bars
indicate 95 % confidence
intervals. When not shown,
error bars are smaller than
symbols. Data at the lower
pH values are also plotted in
the inset graph, showing that
all rate constants are
statistically greater than zero.
Reprinted with permission
from Klupinski et al. ( 2004 ).
Copyright 2004 American
Chemical Society
zero-order kinetic description, Klupinski et al. ( 2004 ) suggest that degradation in
these systems in fact is a surface-mediated reaction. They note that, in the reaction
system, trace amounts of O 2 oxidize Fe(II), which form in situ suspended iron
oxide nanoparticles that serve as surface catalysts for C 6 Cl 5 NO 2 . This behavior is
supported
by
time
profile
results
(Fig. 16.10 )
that
show
the
reaction
to
be
autocatalytic.
Reduction by Fe(II) results in an increase in the amount of iron oxides, which
favor further reaction. Such autocatalytic behavior characterizes the oxidation of
Fe(II) by O 2 and explains C 6 Cl 5 NO 2 reduction by Fe(II) in the absence of an iron
mineral phase. Generalizing this behavior, it can be assumed that Fe(III) colloids
derived from Fe(II) oxidation in subsurface anoxic systems, together with other
colloids, affect the environmental persistence of nitroaromatic contaminants.
Colon et al. ( 2006 ), for example, elucidate factors controlling the transformation of
nitrosobenzenes and N-hydroxylanilines, which are the two intermediate products
formed during the reduction of nitroaromatics in ferric oxide systems. Nitroso-
benzenes are reduced by both Fe(II) solution and Fe(II)-treated goethite suspen-
sions at pH 6, while N-hydroxyl anilines are reduced only by Fe(II)-treated
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