Subsurface Iron and Arsenic Removal

Subsurface iron and arsenic removal for drinking water treatment in Bangladesh
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this a mass balance can be calculated and, for this cycle,
was calaculted to be an approximate removal of 0.66
mmol and 0.77 µmol for iron and arsenic, respectively.
One isolated cycle does however not describe the
potential of subsurface iron and arsenic removal, since
this technology is known for its increasing efficacy with
every successive cycle.
Influence of Fe:As ratio in the groundwater
Subsurface iron and arsenic removal is applied directly
at the shallow tube well and therefore depends greatly
on the occurring groundwater geochemistry. One
of the parameters that may enhance the effective coremoval
of arsenic with the iron is the presence of
significant iron levels in the groundwater. Figures 3.6
and 3.7 show the breakthrough of arsenic and iron for
cycle 6 at locations A and B. At location A, the initial
iron concentration was on average 0.018 mmol.L -1
(molar Fe:As = 9) and the dominant arsenic species
in the groundwater was As(III), measured with anion
exchange resin to be 80-83% of total arsenic. Under
anoxic conditions, with a pH of 6.8 and temperature
of 25°C, arsenic predominantly occurs as H 3
AsO 3
in
the aquatic environment (Ferguson and Gavis, 1972).
The injection water (657L for cycle 6) contained >90%
arsenic(V) in concentrations below 0.1 μmol.L. -1 in all
cycles. Subsurface iron removal at location A shows for
cycle 6 that after V/Vi=6 the measured concentration
was still 65% lower than the initial iron level. Although
iron levels are significantly reduced in the subsurface,
(C/C0)Fe
1
0.8
0.6
0.4
0.2
0
location A
location B
0 1 2 3 4 5 6 7 8
V/Vi
Figure 3.6 Measurements of iron for cycle 6 at locations A
(molar Fe:As = 9, Vi= 657L) and B (molar Fe: As =
140, Vi=744L)
(C/C0)As
1
0.8
0.6
0.4
0.2
0
0 1 2 3 4 5 6 7 8
V/Vi
location A
location B
Figure 3.7 Measurements of arsenic for cycle 6 at locations A
(molar Fe:As = 9, Vi= 657L) and B (molar Fe: As =
140, Vi=744L)
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