Arsenic as a Water Contaminant
Main Reference: Water Technology Volume 31, Issue 11 - November 2008
Facts about arsenic:
Arsenic (chemical symbol As, Atomic Number 33) is an odorless, tasteless, semi-metal element. It enters water naturally through erosion of natural earth deposits or through runoff from agricultural and industrial sources.
In groundwater, arsenic generally occurs in two forms: trivalent arsenic (As+3, or arsenite) or pentavalent arsenic (As+5, or arsenate). Although both forms are harmful to human health, trivalent arsenic is more harmful and more difficult to remove from water. Trivalent arsenic can be converted into pentavalent arsenic in the presence of an effective oxidant such as free chlorine. Treatment with chloramines will not ensure a complete conversion of trivalent arsenic to pentavalent arsenic. Other useful oxidants are described below.
Sources: US EPA, industry sources.
More Information about Arsenic removal from water.
Arsenic III and Arsenic V
A common practice in the treatment of Arsenic is to convert Arsenic III to Arsenic V by means of an oxidizing agent, usually chlorine. This is done because As V is easy to remove, while As III is difficult by all currently used Arsenic reduction strategies for point-of-entry or point-of-use treatment.
As stated, chlorine is the most commonly used oxidizer, but other oxidizers have been tested. Some are effective and others aren’t.
Here’s a brief look at the results of tests of the oxidizing agents most commonly used for such standard treatments as iron and hydrogen sulfide reduction. The test looked at these oxidizers in terms of their ability to convert As III to As V. The tests were performed with and without the interference of competing items (reductants) like iron and hydrogen sulfide.
Here, briefly, are the results:
Aeration: Generally ineffective.
Chlorine: Successful. Iron and Manganese had little effect on the ability to treat Arsenic. Sulfide slowed the process a bit, but complete oxidation was obtained in one minute.
Permanganate: Just as successful as chlorine under all conditions, and slightly faster under some conditions.
Ozone: Completely successful and fast under all test conditions.
Chlorine Dioxide: Unsuccessful.
Filox (a solid filtration medium commonly used for iron, manganese, and hydrogen sulfide reduction): Very successful (95% oxidation) when tested without interference from other reductants, in both low and high dissolved oxygen waters and with as little as 0.75 minutes empty-bed-contact time. However, “As III oxidation by Filox was slowed considerably in the presence of all the interfering reductants tested in low-DO water at a contact time of 1.5 min. with sulfide exhibiting the greatest effect. The effects of interfering reductants were eliminated either by increasing the contact time to six min. or increasing the DO to 8.2 mg/L.” (See reference below.)
Ultraviolet. Not effective, even at extremely high dosage levels.
Conclusion: Chlorine, permanganate, ozone, and Filox work well as oxidizers for As III, but aeration, chlorine dioxide, UV, and monochloromine are generally ineffective.
Reference: Oxidizing Arsenic III to Arsenic V for Better Removal by Dr. Dennis Clifford and Ganesh Ghurye, University of Houston