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Gotkowitz, Madeline B.; Roden, Eric E.; Schreiber, Madeline E.; Shelobolina, Evgenya S. / Mineral transformation and release of arsenic to solution under the oxidizing conditions of well disinfection
[DNR-192] (2007)

Results and discussion,   pp. 7-12 PDF (3.2 MB)

Page 9

hours, then increase to 22 tg/L at 15 hrs and 33 [tg/L at 24 hours (Figure 6). Analysis of
iron concentrations in the high chlorine experiment is shown in Figure 7. The pattern of
iron release to solution mimics that of arsenic release - with the initial peak concentration
at 5 minutes, declining to below detection until 15 hours, and then increasing to 24 hrs.
        During the high chlorine experiments, we observed the formation of iron oxides at
approximately 5 minutes after the experiment began. The iron oxides continued to
increase during the experiment, forming a fluffy layer on top of the sulfide material. The
formation of oxides was not observed during the reaction of the LM material with
nanopure water. This observation is significant because Fe oxides play an important role
in adsorbing arsenic. If the formation of Fe oxides is not favored in an aquifer or well
bore, arsenic is more likely to remain in solution.
        The similar patterns of As and Fe release during the high chlorine experiment,
combined with knowledge of Fe solubility, suggest that the initial increase in As and Fe
during the initial 5 minutes likely results from release of ferrous iron and associated As
from pyrite oxidation (reaction 1). Observations of Fe oxide formation and measured
decrease in As and Fe concentrations after five minutes of reaction are consistent with
nucleation and precipitation of Fe oxides and their concomitant adsorption of As.
        The increase of Fe and As in solution at 15 hours may be a result of
deflocculation of the ferrihydrite into nanoparticles, which can pass through the 0.2
micron filter. We have observed this phenomenon in previous studies of the As-Fe
system (Tadanier et al. 2005). Further experiments using ultrafiltration or
ultracentrifugation to remove small particles are necessary to test this hypothesis. An
alternative hypothesis that attributes the increase of Fe and As to reductive dissolution of
the recently-formed iron oxides is not plausible because oxidizing conditions were
maintained throughout these experiments.
Field study of in situ disinfection
       Arsenic concentrations generally increased during non-pumping periods, reaching
a maximum concentration of 13.7 ig/L in well water (table 3). An exception to this was
the pre-disinfection non-pumping period, during which arsenic did not increase. Results
of well water analysis during the pre-disinfection period suggested leaching of dye from
rope was used to suspend the Hydrolab in the well during this period. The rope was
subsequently tested by placing a portion of it in deionized water for several weeks. This
water had elevated concentrations of aluminum, chromium, lead, copper, nickel and zinc
compared to a field blank of deionized water. A different type of rope was used in the
well during all other experimental phases.
        Arsenic consistently decreased to 6 pig/L or less under the control and post-
disinfection pumping periods (table 3 and Figure 8). This is consistent with previous
results from this well that show higher arsenic concentrations in water with a long
residence time in the well bore and lower concentrations of arsenic in groundwater that is
representative of aquifer water (meaning that the well is fully purged when the sample is
collected). Arsenic is primarily dissolved and present as arsenite in samples of well water
with a long residence time in the borehole (stagnant conditions) and samples collected
under fully or partially purged conditions that are representative of aquifer water.

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