Geoscience Reference
In-Depth Information
Table 16.2
Characteristics of common iron(hydr)oxide minerals
p
H
ZPC
Name
(formula)
BET
surface
(m
2
/g)
[=FeOH]
t
(nm
-2
)
XRD
SEM
Goethite (a-
FeOOH)
7.8
17.5
5.5
H
Acicular; needles with *0.3-1.5 lm
length and *0.1 lm diameter
Hematite (a-
Fe
2
O
3
)
8.3-9.3
13.7
0.7
H
Hexagonal platy; single crystals form
larger aggregates
Lepidocrocite
(c-FeOOH)
7.3
17.6
1.67
H
Irregular plates
Magnetite
(Fe
3
O
4
)
6.4-6.9
8.1
9.4
H
Partly rounded octahedral crystals
Hematite (a-
Fe
2
O
3
)
8.48
109
2.07
H
Spherical particles, 10-12 nm
Reprinted with permission from Pecher et al. (
2002
). Copyright 2002 American Chemical Society
goethite. The nitrosobenzene reactivity trend indicates that electron-withdrawing
groups in the para position increase their rate of reduction.
Pecher et al. (
2002
) show how the uptake of ferrous iron from aqueous solution,
by iron oxides, leads to the formation of a variety of reactive surface species that
are capable of reducing polyhalogenated methanes (PHMs). The iron oxides used
in the experiments and their characteristics are shown in Table
16.2
. The PHMs
studied include bromodichloromethane (CHBrCl
2
), chlorodibromomethane
(CHBr
2
Cl), bromoform (CHBr
2
), tetrachloromethane (CCl
4
), hexachloroethane
(HCE), fluorotribromomethane (CFBr
3
), bromotrichloromethane (CBrCl
3
), and
dibromodichloromethane (CBr
2
Cl
2
).
Reduction of these PHMs took place in experiments containing both Fe(II) and
iron oxide minerals, under anoxic conditions. The transformation of PHMs by
surface-bound
Fe(II)
generally
follows
a
pseudo-first-order
kinetic
rate
law,
expressed by
ln(c
=
co)=
k
obs
t
ð
16
:
1
Þ
where k
obs
is the pseudo-first-order rate constant. Figure
16.11
shows an evalua-
tion of the rate law for the reduction of CBr
2
Cl
2
by Fe(II), in the presence and the
absence of goethite. This iron mineral was considered as the ''master system,'' and
assays with other mineral oxides were performed to obtain complementary data.
The values of the reaction rate, k
obs
, for polyhalogenated alkanes in Fe(II)/
goethite suspensions are noted in Table
16.3
together with their pseudo-rate
constants and half-lives. The reaction rates are affected by contact time, sorption
density, and solution pH. Pecher et al. (
2002
) note that a contact time of 20 h is
necessary to achieve equilibrium for the uptake of Fe(II) by iron oxides, the uptake
being formed by a fast initial step followed by a slower second phase. An increase
in the density of Fe(II) at the surface of the mineral, especially after saturation of
available surface sites, is the determining factor in quantifying transformation rates
