Conventional treatment for arsenic removal involves co-precipitation with an iron (III) salt to yield insoluble ferric arsenate. However, this process alone cannot achieve drinking water standards (< 5 ppb As). An enhancement to the process involves overdosing the iron (at a >5:1 weight ratio of Fe:As) at elevated pH to allow surface complexation and electrostatic attraction (co-precipitation) to occur. Often this will provide an acceptable quality of water. For those other cases, supplemental treatment will be needed. In this area, hydrogen peroxide has a useful role.

The enhancement involving H2O2 was described in the November, 1992 issue of Water Environment & Technology , titled "Applications of Chemical Oxidation and Electrochemical Iron Generation for Removing Arsenic and Heavy Metals from Water", authored by Michael Brewster of Andco Environmental Processes, Inc. (of Amherst, NY). Mr. Brewster notes that in most groundwaters, arsenic exists as soluble arsenite (As3+) ions. The adsorption curve for arsenite is such that its removal efficiency is 50% less than that of arsenate (As5+) - the reasons being the lower solubility of ferric arsenate and the lower dissociation constants for ferric arsenite (related to potential for surface complexation). Adsorption of both arsenate and arsenite is further inhibited by the presence of other anions such as sulfate and chloride.

Andco's process is unique in two aspects: 1) it uses electrochemically-generated ferrous ions so that no inhibitory anions are added to the water; and 2) it uses H2O2 to oxidize the As3+ to As5+ and the Fe2+ to Fe3+. The H2O2 is critical in that the rapid oxidation overcomes any existing equilibrium conditions thus ensuring that insoluble hydrous ferric oxide will form and be in contact with the more efficiently adsorbed arsenate. Andco offers both stand-alone package plants (including clarifier, solids press, and controls) as well as retrofits (electrochemical cells).

For the same reasons as above, the use of H2O2 to pre-oxidize As3+ to As5+ may have benefit to conventional arsenic removal processes using iron co-precipitation. The oxidation technology is well established and is used commercially to produce CCA (copper-chrome-arsenate) wood preservatives. And the use of FMC's OxyPure H2O2 (NSF-approved) will ensure that no extraneous contaminants will be added in the process.

Other references:

1. Ghosh, M.M. and J.R. Yuan. "Adsorption of Inorganic Arsenic and Organoarsenicals on Hydrous Oxides" in Environmental Progress, August 1987, p. 150.

2. Rissmann, E.F. and S.M. Schwartz. "Treatment of Wastes Containing Arsenic, Selenium, Thallium, and Mercury Compounds." 44^th Purdue Industrial Waste Conference Proceedings, 1990, pp. 643-648.

3. Benjamin, M.M., K.F. Hayes, and J.O. Leckie. "Removal of Toxic Metals from Power-Generation Waste Streams by Adsorption and Coprecipitation." J. of the Water Pollut. Control Fed., Nov. 1982., p. 1473.

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