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

Postma et al., submitted to Geochim. Cosmochim. Acta
Fe(II) and total As in a similar pattern. In this case roughly half of the arsenic is found
as As(V).
The behaviour of the aquifer sediments (Figs. 5-6) is different in the
oxidized and the reduced zones. In the oxidized zone there is very little release of
arsenic, even when ascorbic acid is reducing a considerable amount of iron oxide. In the
reduced aquifer sediment (Fig. 6) we noted earlier that ascorbic acid and HCl release
more or less the same amount of Fe (II) indicating the dissolution of a Fe(II)-phase.
Apparently, this Fe(II) phase also contains As(III) since a similar release pattern is
observed, while no As(V) is leached from the sediment.
The release pattern of phosphate (Figs. 3-6) resembles that of iron. The
presence of ascorbic acid seems to be required to mobilise phosphate. Therefore
phosphate must be associated with the iron oxides and first during reductive dissolution
of the iron oxides, phosphate is released. The exception is again the reduced aquifer
sediment (Fig. 6) where, similar to the behavior of iron, phosphate already comes out
with HCl which could support the dissolution of a vivianite type of phase. The amount
of released phosphate is, however, much smaller than that of Fe and therefore if
vivianite dissolution takes place then it can only constitute a very small fraction of the
dissolving Fe(II)-phase.
3.2.1 Reactivity of iron oxides in sediments
The reactivity of the pool of Fe-oxide in the sediment was determined using the
approach of Larsen et al. (2006) where the rate of reduction of iron oxides is described
by the rate law:
J m
= k´
( )
(1)
m0
m0
14