What it is:

• Chemical symbol: As
• Atomic number: 33
• Odorless, tasteless semi-metal element
• Can enter drinking water naturally through the erosion of natural deposits in the earth
• Also enters drinking water through agricultural and industrial runoff.

Ionic states:

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.

Where found:

• Arsenic is abundant in the Earth’s crust. It is present in many different minerals, the most common of which is arsenopyrite.
• Arsenic is also found in the atmosphere. One-third has entered naturally, most from volcanic eruption. The rest is from industrial emissions.
• Geological inorganic arsenic is especially present in Taiwan, Bangladesh and India.
• Organic arsenic is mainly found in sea-dwelling creatures.

Health effects:

• Stomach pain, nausea, vomiting, diarrhea
• Partial paralysis, numbness in hands and feet, blindness, thickening and discoloration of the skin
• Cancer of the bladder, lungs, skin, kidneys, nasal passages, liver and prostate.

Regulation:

• US Environmental Protection Agency (EPA) maximum contaminant level (MCL) = 10 micrograms/liter (parts per billion). This enforceable MCL became effective January 23, 2006, for both organic and inorganic forms.
• EPA MCL goal = zero

Water treatment:

• Ion exchange
• Reverse osmosis
• Distillation.

Sources: US EPA, industry sources.

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.

Monochloramine: 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