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

Postma et al., submitted to Geochim. Cosmochim. Acta
difference between ascorbic acid and HCl can be attributed to reductive dissolution of
Mn-oxides (van der Zee and van Raaphorst, 2004). For both the river sand and the
oxidized aquifer sediment (Figs. 4-5) about 1 mol/g Mn is released by HCl while
ascorbic acid releases an additional 1 mol/g Mn, suggesting the presence of a small
amount of reducible Mn, present either as a separate phase or as a constituent of the
dissolving Fe-oxides. In the river mud (Fig. 3) most of the Mn is already released by
HCl but ascorbic acid creates a small initial excess of Mn. Since the Mn concentration
decreases at longer times, either readsorption or some precipitation of Mn must occur.
In the reduced aquifer (Fig. 6) sediment the amounts of Mn extracted by HCl and
ascorbic acid are about the same.
During reductive dissolution of the As(V) containing Fe-oxides with
ascorbate, arsenate is not reduced to arsenite (Jung and Zheng, 2006; Pedersen et al.,
2006). Assuming that the same is true for arsenic associated with the iron oxides in the
sediments, the leaching experiments can provide information concerning the arsenic
redox speciation in the sediments. The river sediments (Figs. 3-4) show a release pattern
of arsenic which resembles that of iron. For both mud and sand, HCl liberates only a
small portion of the As that is released by ascorbic acid and exclusively as As(III). The
mud sample leached by ascorbic acid, shows a strong initial increase in total As with a
higher As/Fe ratio than later in the experiment. This initial peak is present as As(V) and
constitutes about 30% of the total arsenic release. Later on in the experiment As(V)
appears to become readsorbed by the sediment and As(III) becomes the predominant
redox species. Generally the ratio of released As/Fe in the mud is much higher than
found in the other samples. From the river sand (Fig. 4), ascorbic acid releases both
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