Geoscience Reference
In-Depth Information
(a)
Microbial
(labile DOM)
Site B pond
Other SW
11m GW
14m GW
1.7
8m GW
40m GW
52m GW
1.6
1.5
y
= -1.1
x
+ 2.0
R
2
= 0.74
Terrestrial
(sedimentary humics)
1.4
0.3
0.4
0.5
Oxidized
Redox index
Reduced
(b)
Oxidized
humics
Labile
DOM
Fe(II)
Free
As
e-
Sedimentary
humics
Fe-and
humics-
reducing
bacteria
?
CO
2
Reduced
humics
Fe mineral + As
Figure 9.9. Figures showing the relationship between the McKnight et al. (
2001
) FI and
(a)
redox
index in surface water (SW) and groundwater (GW) samples from Bangladesh. Samples with a more
oxidized RI are from an oxidizing surface water environment containing low dissolved arsenic while
the deeper samples are more reduced and come from high arsenic environments.
(b)
A proposed
mechanism for microbially mediated electron shuttling cascade in which humic material acts as an
electron shuttle between labile DOM in the substrate and iron oxide, ultimately resulting in mobilized
iron and arsenic and a change in the redox index of the humic material. [Modified from Mladenov
et al. (
2010
), where more details of the figure symbols can also be found.]
polycondensation than soil extracts from extensively used or unused land. Zsolnay et al.
(
1999
) proposed the HIX
em
to measure the effects of soil drying on DOM quality, demon-
strating that drying released a greater degree of non-humified material, and Agaki et al.
(
2007
) utilized HIX
EM
to assess the effects of two different soil sample treatments.
Several studies have used and compared both humification indices (i.e., both HIX
SYN
and
HIX
EM
). Kalbitz et al. (
2003
) investigated the impact of biodegradation on the properties of
dissolved organic matter extracted from 13 different soil samples. With increasing biodeg-
radation both HIX
EM
and HIX
SYN
decreased, indicative of a smaller degree of humification.
This decrease in humification was also associated with a decrease in aromatic H and an
