Groundwater arsenic in the Red River delta, Vietnam ... - Fiva

Postma et al., submitted to Geochim. Cosmochim. Acta
and in consequence concluded that reductive dissolution of Fe-oxides probably played a
minor role.
Mössbauer spectroscopy is an extremely sensitive technique that allows
the identification of even small amounts of iron containing minerals in sediments as
well as determining their relative speciation (Murad and Cashion, 2004). By means of
Mössbauer spectroscopy we were able to confirm the presence of goethite with
subordinate amounts of hematite in the sediments of both the Red River and the Dan
Phuong aquifer (Fig 1). Overall, there was no major difference in the iron speciation
between the different sediments. However, line broadening indicates that the goethite
present in river sand and mud is more disordered. Less stable iron oxides such as
ferrihydrite or lepidocrocite were not identified in the bulk and fine fraction samples.
The < 2 m fraction of the river mud, which contains most of the iron oxides, was
investigated by Mössbauer spectroscopy to obtain an even higher resolution. But also in
this case only goethite and hematite were identified. The presence of the more stable
iron oxides like goethite and hematite, and particularly the absence of ferrihydrite, in the
Dan Phuong aquifer sediment is in good agreement with the modelling work of Postma
et al. (2007), who found that the observed sequence of redox processes with
methanogenesis proceeding concomitantly with iron reduction in the Dan Phuong
aquifer thermodynamically only is possible in the presence of the more stable iron
oxides.
Figure 10 compares the reactivity of the Fe(III) pool in the sediments with
the reactivity determined for synthetic Fe-oxide minerals (Larsen and Postma, 2001;
Pedersen et al., 2005; Petersen et al., 2006). In the diagram the rate law (Eq. (1)) for
each mineral or sediment is displayed as a straight line where the intersection with Y-
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