Geoscience Reference
In-Depth Information
Table 16.3 Names, abbreviations, pseudo-first-order rate constants, and half-lives of polyhalo-
genated alkanes in Fe(II)/goethite suspension
Compound
K
obs
(h
-1
)
s
a
n
b
Abbreviation
t
(h)
Bromodichloromethane
CHBrCl
2
0.0013
0.0002
4
533
Chlorodibromomethane
CHBr
2
Cl
0.0029
0.001
4
239
Bromoform
CHBr
3
0.0048
0.002
2
144
Tetrachloromethane
CCl
4
0.016
0.005
7
43
HCE
c
Hexachloroethane
0.0501 1 13.8
Fluorotribromomethane CFBr
3
0.506 0.06 2 1.37
Bromotrichloromethane CBrCl
3
3.577 1.52 2 0.19
Dibromodichloromethane CBr
2
Cl
2
11.296 3.28 8 0.06
Reprinted with permission from Pecher et al. (
2002
). Copyright 2002 American Chemical Society
Experimental conditions: 25 m
2
/L goethite, pH 7.2, t
eq
[ 24 h. Fe(II)
tot
= 1mM
a
Standard deviation
b
number of replicates
c
t
eq
= 5h
Fig. 16.12 Pseudo-first-order rate constants, k
obs
, for the transformation of CFBr
3
(c
0
= 6.5 lM)
in suspensions of goethite (25 m
2
/L; Fe(II)
tot
of 1 mM; at 25 C; ionic strength = 20 mM) as a
function of solution pH (filled square box). The contact time of Fe(II) with iron oxide before
addition of CFBr
3
was [24 h. Also shown are k
obs
values for the control experiments in the
absence of goethite (filled circle). The precipitate formed in the pH 8.9 control (arrow) was
identified as a form of green rust. Reprinted with permission from Pecher et al. (
2002
). Copyright
2002 American Chemical Society
of PHMs by Fe(II) in iron oxide systems. The effect of pH on k
obs
is illustrated in
Fig.
16.12
for the reduction of CFBr
3
in an iron(II)-goethite suspension. We see
that reaction rates increase almost exponentially with pH. Also, surface-bound
Fe(II) species formed at high pH, under conditions favoring the surface precipi-
tates, are more reactive than isolated surface complexes at lower pH or low surface
coverage. Pecher et al. (
2002
) suggest that a variety of ferrous iron species coexist
as potential reductants for PHMs at iron oxide surfaces. The reactivity of these
species with respect to transformation of PHMs depends primarily on the sorption
