Subsurface Iron and Arsenic Removal

6 Characterization of accumulated deposits
%mol ratio was on average 4.8 (±1.65) and 5.4 (±1.26)
near well 04 and the reference well, respectively. The
precipitates in Figure 6.7(c) and (d) have been found
in the deeper layer at 23-24m. ESEM-EDX showed
that the Ca:S ratio in coating (c) is 0.85, which roughly
corresponds to CaSO 4
(gypsum). Calcium sulfate has
not been observed in any of the samples taken between
19 and 22m depth, neither near well 04 nor in the
reference well. The same was the case for the occurrence
of the framboidal pyrite (FeS 2
, (d)). In the presence of
oxygen, pyrite is known to oxidize and release sulphate
and iron into the groundwater. In addition, pyrite
oxidation is an acidifying reaction and therefore not
promoting the (rapid) oxidation of Fe 2+ . The presence
of sulphide minerals is a vital difference between the
depths with iron accumulation (19-22m) and without
iron accumulation (deeper than 22m).
X-Ray Powder Diffraction and 57 Fe
Mössbauer Spectroscopy
The same ten soil samples (