7
The Biogeochemistry of Iron
264
the same extraction conditions (reagent composition, sediment : solution ratio, contact time)
must be applied.
Table 7.4 gives an overview of experimentally
derived dissolution behavior of iron bearing
minerals under some selected extraction conditions. The investigation of total-Fe from ascorbate and dithionite solution can be determined by
ICP-AES or flame-AAS. Ferrous and ferric iron
from non-reducing or non-oxidizing extractions
can be determined by polarographic methods
(Wallmann et al. 1993) or colorimetrically with and
without the addition of a reducing agent (e.g.
hydroxylamine hydrochloride, Kostka and Luther
1994). During the acidic extractions evolving
sulfide can be trapped in a separate alkaline
solution (e.g. Sulfur Antioxidant Buffer, SAOB,
Cornwell and Morse 1987) where it can be
determined polarographically, by precipitation
titration with Pb or by a standard ion sensitive
electrode. Sulfide evolving from HCl extraction is
called Acid Volatile Sulfur (AVS).
The leaching with HF/H 2 SO 4 as described
above and the subsequent polarographic
determination of ferrous and ferric iron is based
on work by Beyer et al. (1975) and Stucki (1981) in
order to quantify the silicate bound ferrous and
ferric iron. This extraction has hardly been
applied with respect to questions of early
diagenesis so far, yet, the silicate bound iron
fraction is quantitatively very important in marine
sediments and even a small reactive fraction of
this pool may be of overall significance for the
iron reactivity. As a complementary method to the
commonly applied extractions (Table 7.4) it
renders the calculation of the total iron speciation
in the sediment which may then be compared to
Mössbauer-spectroscopic results (Haese et al.
1997; Haese et al. 2000).
One of the pitfalls in the interpretation of
extraction results from natural sediments is caused
by the fact that the presence of Fe
2+
complexed by
carboxylic acid catalyzes the reduction of crystalline iron oxides such as hematite (Sulzberger et al.
1989), magnetite (Blesa et al. 1989) and goethite
(Kostka and Luther 1994). In order to avoid this
catalytic dissolution of well-crystallized iron oxides
by Fe
2+
during the oxalate extraction Thamdrup
and Canfield (1996) air-dried the sediment in
advance, thereby oxidizing FeS and FeCO 3 to
ferrihydrite. In addition, they applied the anoxic
oxalate extraction and subtracted the released
amount of Fe
2+
from the amount of Fe
3+
determined
from the oxic extraction to calculate the poorly
crystallized iron oxide fraction as intended
according to Table 7.2.
Acknowledgements
I wish to thank Tim Ferdelman, Bo Thamdrup and
Caroline Slomp for their critical reviews in the year
2000 leading to the first edition of this manuscript.
The recent edition was improved by comments by
Lynda Radke and Emmanuelle Grosjean, and it
received permission for publication by the Chief
Executive Officer of Geoscience Australia.
7.6
Problems
Problem 1
The Burdekin River is large river in northwestern Australia, which discharges about 3.4 million
tons of sediment into the coastal sea.
a) Estimate how much total and highly reactive
iron is discharged from the catchments. Assume
that the sediment is of average continental crust
composition. b) Estimate how much highly reactive
iron remains in the estuary and in the near-coastal
zone. c) Which other source of iron must be
considered off-shore of northern Australia?
Problem 2
a) List 3 mineral properties of iron oxyhydroxides which influence the rate of microbial iron
reduction. b) Explain why wet-chemical extractions are not mineral specific.
Problem 3
Explain why bioturbation is important for the
rate of dissimilatory iron reduction in sediments.
Problem 4
Which are important variables affecting the
importance of dissimilatory Fe- and Mn-reduction
relative to other metabolic pathways?
Problem 5
a.) Define and explain the abbreviation DOP.
b.) Which extractions are used to determine
DOP.
The Biogeochemistry of Iron
264
the same extraction conditions (reagent composition, sediment : solution ratio, contact time)
must be applied.
Table 7.4 gives an overview of experimentally
derived dissolution behavior of iron bearing
minerals under some selected extraction conditions. The investigation of total-Fe from ascorbate and dithionite solution can be determined by
ICP-AES or flame-AAS. Ferrous and ferric iron
from non-reducing or non-oxidizing extractions
can be determined by polarographic methods
(Wallmann et al. 1993) or colorimetrically with and
without the addition of a reducing agent (e.g.
hydroxylamine hydrochloride, Kostka and Luther
1994). During the acidic extractions evolving
sulfide can be trapped in a separate alkaline
solution (e.g. Sulfur Antioxidant Buffer, SAOB,
Cornwell and Morse 1987) where it can be
determined polarographically, by precipitation
titration with Pb or by a standard ion sensitive
electrode. Sulfide evolving from HCl extraction is
called Acid Volatile Sulfur (AVS).
The leaching with HF/H 2 SO 4 as described
above and the subsequent polarographic
determination of ferrous and ferric iron is based
on work by Beyer et al. (1975) and Stucki (1981) in
order to quantify the silicate bound ferrous and
ferric iron. This extraction has hardly been
applied with respect to questions of early
diagenesis so far, yet, the silicate bound iron
fraction is quantitatively very important in marine
sediments and even a small reactive fraction of
this pool may be of overall significance for the
iron reactivity. As a complementary method to the
commonly applied extractions (Table 7.4) it
renders the calculation of the total iron speciation
in the sediment which may then be compared to
Mössbauer-spectroscopic results (Haese et al.
1997; Haese et al. 2000).
One of the pitfalls in the interpretation of
extraction results from natural sediments is caused
by the fact that the presence of Fe
2+
complexed by
carboxylic acid catalyzes the reduction of crystalline iron oxides such as hematite (Sulzberger et al.
1989), magnetite (Blesa et al. 1989) and goethite
(Kostka and Luther 1994). In order to avoid this
catalytic dissolution of well-crystallized iron oxides
by Fe
2+
during the oxalate extraction Thamdrup
and Canfield (1996) air-dried the sediment in
advance, thereby oxidizing FeS and FeCO 3 to
ferrihydrite. In addition, they applied the anoxic
oxalate extraction and subtracted the released
amount of Fe
2+
from the amount of Fe
3+
determined
from the oxic extraction to calculate the poorly
crystallized iron oxide fraction as intended
according to Table 7.2.
Acknowledgements
I wish to thank Tim Ferdelman, Bo Thamdrup and
Caroline Slomp for their critical reviews in the year
2000 leading to the first edition of this manuscript.
The recent edition was improved by comments by
Lynda Radke and Emmanuelle Grosjean, and it
received permission for publication by the Chief
Executive Officer of Geoscience Australia.
7.6
Problems
Problem 1
The Burdekin River is large river in northwestern Australia, which discharges about 3.4 million
tons of sediment into the coastal sea.
a) Estimate how much total and highly reactive
iron is discharged from the catchments. Assume
that the sediment is of average continental crust
composition. b) Estimate how much highly reactive
iron remains in the estuary and in the near-coastal
zone. c) Which other source of iron must be
considered off-shore of northern Australia?
Problem 2
a) List 3 mineral properties of iron oxyhydroxides which influence the rate of microbial iron
reduction. b) Explain why wet-chemical extractions are not mineral specific.
Problem 3
Explain why bioturbation is important for the
rate of dissimilatory iron reduction in sediments.
Problem 4
Which are important variables affecting the
importance of dissimilatory Fe- and Mn-reduction
relative to other metabolic pathways?
Problem 5
a.) Define and explain the abbreviation DOP.
b.) Which extractions are used to determine
DOP.