|Oxidation of As (III) by Aeration and Storage (52 pp, 1.49 MB) (EPA/600/R-01/102) January 2002
A study of the effects of aeration and storage on the oxidation of arsenic (III) was undertaken at three utilities in the United States. to establish the engineering significance of aeration as a potential pre-treatment method for arsenic removal. Aeration has been referred to in the literature as a possibly useful pre-treatment method to ensure that arsenic is in the arsenic (V) state before subsequent removal by any of several treatment processes. Because aeration is a common process for treating ground water for iron oxidation, radon, volatile organics, carbon dioxide, and hydrogen sulfide, it is reasonable to investigate its effectiveness for arsenic (III) oxidation.
The results of this study clearly establish that aeration and aerobic storage do not oxidize arsenic (III). The major conclusion is that aeration is not effective for this purpose and should not be relied on or expected to contribute to the oxidation of arsenic (III).
One of the test sites in this study clearly showed that arsenic (III) is significantly removed by the oxidation and precipitation of iron, but this should not be attributed to an oxidation of arsenic (III) to arsenic (V) by dissolved oxygen. Past research has established that iron precipitation can be partially effective for the adsorptive removal of arsenic (III), and this is the likely explanation for the apparent drop in arsenic (III) at the site that had high iron.
The effect of iron precipitation on the removal of arsenic was also present in the long-term storage of aerated water in this study. When all of the iron (initial iron at 2.7 milligrams per liter) precipitated from the quiescent storage water, the remaining aqueous total arsenic was entirely dissolved and in the arsenic (V) state. The aqueous arsenic (III) was below detection and apparently completely removed or converted by the insoluble iron. Even in this case, it is doubtful that dissolved oxygen was responsible for any oxidation of arsenic (III) because the loss directly correlated to the loss of iron precipitate and no other instance of arsenic (III) oxidation occurred at the other sites.
In summary, the data supported the fact that iron is extremely important in the removal of arsenic (III) but did not support the idea that arsenic (III) is oxidized by aeration. This is true at least for the conditions used in this study.
While the subtleties of the results are interesting, especially for the site with high iron, it is important to emphasize the original objective of this study, which was to establish whether typical aeration and storage methods could oxidize arsenic (III). Based on the results of this study, it is concluded that aeration does not oxidize arsenic (III) and that subsequent storage for up to five days does not result in arsenic (III) oxidation. Dissolved oxygen should not be considered a candidate for arsenic (III) oxidation; however, aeration will continue to be considered a very effective process for the oxidation of iron. In that way, aeration can be said to be effective in bringing about the removal of arsenic via the oxidative precipitation of iron.
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