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

Postma et al., submitted to Geochim. Cosmochim. Acta
Evaluating the effect of acetate addition, it is clear that in all incubations,
except for the river mud, the redox processes are rate limited by the availability of
reactive organic carbon since the presence of acetate as additional substrate for the
micro organisms did stimulate the processes. Only with the river mud did the presence
of acetate have little effect. In this case abundant organic carbon is apparently available
and instead the availability of reactive iron oxide seems to be rate limiting.
4. DISCUSSION
4.1 Iron oxide mineralogy, reactivity and reductive dissolution
One of the problems in assessing the role of reductive dissolution of iron oxides as a
mechanism for the mobilization of arsenic in the Holocene aquifers of Bangladesh and
Vietnam has been to identify and characterize the Fe-oxides present in the sediments.
Different approaches have been attempted ranging from mineralogical methods like X-
ray diffraction (XRD) and transmission electron microscopy (TEM) (Akai et al., 2004),
and indirect methods like chemical extractions (Dowling et al., 2002; Akai et al., 2004;
Swartz et al., 2004; Horneman et al., 2004; Postma et al., 2007), and reflectance and
magnetic properties (Horneman et al., 2004). There are only very few studies that have
positively identified iron oxides minerals in the reduced Holocene aquifer sediment. In
Bangladesh, Akai et al. (2004) identified ferrihydrite, goethite and hematite by TEM in
Holocene aquifer sediments, while Rowland et al. (2008) identified goethite by XRD. In
contrast, Polizzotto et al. (2006) used both extended X-ray absorption fine structure
(EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy but failed to
identify any Fe-oxide minerals in Holocene reduced aquifer sediment from Bangladesh
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