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