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  Laboratory Study on the Oxidation of Arsenic (III) to Arsenic (V) (100 pp, 998 KB) (EPA/600/R-01/021) March 2001

A one-year laboratory study was performed to determine the ability of seven oxidants to oxidize arsenic (III) to arsenic (V). The oxidants were chlorine, permanganate, ozone, chlorine dioxide, monochloramine, a solid-phase oxidizing media, and 254 nanometers (nm) of ultraviolet (UV) light.

Chlorine and permanganate rapidly oxidized arsenic (III) to arsenic (V) in the pH range of 6.3 to 8.3. Dissolved manganese, dissolved iron, sulfide, and total organic carbon (TOC) slowed the rate of oxidation slightly, but essentially complete oxidation was obtained in less than one minute with chlorine and permanganate under all conditions studied.

In the absence of interfering reductants, ozone rapidly oxidized arsenic (III). Although dissolved manganese and dissolved iron had no significant effect on arsenic (III) oxidation, the presence of sulfide considerably slowed the oxidation reaction. The presence of TOC had a quenching effect on arsenic (III) oxidation by ozone, producing incomplete oxidation at the higher TOC concentration studied.

Only limited arsenic (III) oxidation was obtained using chlorine dioxide, which was probably due to the presence of chlorine (as a by-product) in the chlorine dioxide stock solutions. The reason for the ineffectiveness of chlorine dioxide was not studied.

Preformed monochloramine was ineffective for arsenic (III) oxidation, whereas limited oxidation was obtained when monochloramine was formed in situ. This showed that the injected chlorine probably reacted with arsenic (III) before being quenched by ammonia to form monochloramine.

Filox, a manganese dioxide-based media, was effective for arsenic (III) oxidation. When dissolved oxygen (DO) was not limiting, complete oxidation was observed under all conditions studied. However, when DO was reduced, incomplete oxidation was obtained in the presence of interfering reductants. The adverse effect of interfering reductants was completely eliminated by supplying enough DO or increasing the contact time. In addition to oxidizing arsenic (III), the Filox media also removed some arsenic by adsorption, which diminished greatly as the media came into equilibrium with the arsenic (III)-spiked synthetic water.

UV light alone was not very effective for arsenic (III) oxidation. Significant oxidation was observed only at very low flow rates, representing 0.6–2.5 percent of the rated capacities of the two UV sterilizer units tested. However, as reported in a patented process, complete oxidation by UV light was observed when the challenge water was spiked with 1.0 milligrams per liter sulfite.


Thomas J. Sorg

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