『Abstract
The pool of iron oxides, available in sediments for reductive
dissolution, is usually estimated by wet chemical extraction methods.
Such methods are basically empirically defined and calibrated
against various synthetic iron oxides. However, in natural sediments,
iron oxides are present as part of a complex mixture of iron oxides
with variable crystallinity, clays and organics etc. Such a mixture
is more accurately described by a reactive continuum covering
a range from highly reactive iron oxides to non-reactive iron
oxides. The reactivity of the pool of iron oxides in sediment
can be determined by reductive dissolution in 10 mM ascorbic acid
at pH 3. Parallel dissolution experiments in HCl at pH 3 reveal
the release of Fe(II) by proton assisted dissolution. The difference
in Fe(II)-release between the two experiments is attributed to
reductive dissolution of iron oxides and can be quantified using
the rate equation J/mo =k'
(m/mo)γ, where J
is the overall rate of dissolution (mol s-1), mo the initial amount of iron oxide, k'
a rate constant (s-1), m/mo
the proportion of undissolved mineral and γ a parameter describing
the change in reaction rate over time. In the Romo（両方のoの/が付く)
aquifer, Denmark, the reduction of iron oxides is an important
electron accepting process for organic matter degradation and
is reflected by the steep increase in aqueous Fe2+
over depth. Sediment from the Romo（両方のoの/が付く)
aquifer was used for reductive dissolution experiments with ascorbic
acid. The rate parameters describing the reactivity of iron oxides
in the sediment are in the range k' = 7・10-6
to 1・10-3 s-1 and γ= 1 to 2.4. These values
are intermediate between a synthetic 2-line ferrihydrite and a
goethite. The rate constant increases by two orders of magnitude
over depth suggesting an increase in iron oxide reactivity with
depth. This increase was not captured by traditional oxalate and
dithionite extractions.』
1. Introduction
2. Methods
2.1. Groundwater and sediment sampling
2.2. Water analysis
2.3. Sediment analysis and reduction experiments
3. Results
3.1. Groundwater chemistry in the Romo（両方のoの/が付く)
aquifer
3.2. Reactive iron extracted by oxalate and dithionite
3.3. Reductive dissolution of iron by ascorbic acid
4. Discussion
4.1. Reactive Fe(III) in the sediments
4.2. Reactivity of Fe(III) in sediments and of synthetic Fe-oxides
4.3. Importance of sediment Fe(III) reactivity
5. Conclusions
References