Subsurface Iron and Arsenic Removal

Subsurface iron and arsenic removal for drinking water treatment in Bangladesh
2
Successive cycles
As reported before (Boochs and Barovic, 1981; Braester
and Martinell, 1988; Grombach, 1985; Hallberg and
Martinell, 1976; Mettler, 2002; Rott, 1985; van Beek,
1985) the iron removal efficacy is known to improve
with every injection-abstraction cycle. Figure 2.7
illustrates that such an efficacy increase is also observed
during the first cycles at WTP Lekkerkerk. Several
mechanisms have been proposed to be responsible for
this observation:
• Incomplete Fe breakthrough enhances removal in future
cycles (Appelo et al., 1999);
• Pyrite oxidation during initial cycles;
• Growth of iron oxidizing bacterial communities in the
oxidation zone (Hallberg and Martinell, 1976);
• Increased adsorptive surface area on the soil material
[Fe] mmol.L -1
through accumulation of precipitated iron oxides (Rott,
1985; van Beek, 1985).
0.12
0.08
0.04
0.00
0 2 4 6 8 10 12 14 16
V/Vi
Cycle 1
Cycle 7
Cycle 22
Figure 2.7 Iron measurements during cycle 1, 7 and 22 at WTP
Lekkerkerk
A combination of above mechanisms may contribute
to the improved efficacy of SIR and it is difficult
to differentiate between them with field data.
The first hypothesis, regarding the incomplete Fe
breakthrough, is based on the assumption that lower
Fe 2+ concentrations are present around a tube well at
the end of the abstraction phase. These lowered Fe 2+
concentrations are subsequently pushed into the aquifer
during injection, displacing the original groundwater
even further. During abstraction these lowered Fe 2+
concentrations will load the oxidation zone to a lesser
extent than the original groundwater, resulting in
better retardation of Fe. As can be seen Figure 2.7, the
subsurface iron removal well at WTP Lekkerkerk has
been operated with complete breakthrough of Fe from
the start. Nevertheless, the Fe removal efficacy increases
with every cycle, right from the beginning.
The second hypothesis may be responsible for
that. The occurrence of pyrite oxidation (FeS 2
), can be
determined based on field data as sulphate is released in
the reaction (Appelo and Postma, 2005):
Equation 2.4
15 7
2
FeS 4
4 2
− +
2
+ O2
+ H
2O
→ Fe(
OH )
3
+ 2SO4
+ H
This equation summarizes the oxidation of both
disulfide and ferrous iron, and subsequent hydrolysis
of Fe 3+ to ferrihydrite. The oxidation of pyrite results in
the mobilization of sulphate and a drop in pH. Sulphate
concentrations have been monitored during the first 18
cycles at WTP Lekkerkerk and after SIR was temporary
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