Arsenic

Arsenic is a tasteless and odorless semi-metal. It is a potent poison in small amounts and can enter the water supply from natural deposits in the earth or though industrial or agricultural runoff.

In groundwater, arsenic usually occurs in two forms: trivalent arsenic (As+3, or arsenite) or pentavalent arsenic (As+5, or arsenate). Both are harmful to humans, but trivalent arsenic is more harmful and more difficult to remove from water. AS+3 can be converted into pentavalent arsenic in the presence of an effective oxidant such as free chlorine. Treatment with chloramines, however, will not ensure a complete conversion of trivalent arsenic to pentavalent arsenic.

Health effects of arsenic include

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.
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.

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: Arsenic is most effectively removed from water by ion exchange, reverse osmosis, and distillation.

"Filtration through activated carbon will reduce the amount of arsenic in drinking water from 40-70%. Anion exchange can reduce it by 90-100%. Reverse Osmosis has a 90% removal rate, and distillation will remove 98%. If the arsenic is present in organic form, it can be removed by oxidation of the organic material and subsequent coagulation." -- Enting Corp. Engineering Handbook.

WQA Technical Bulletin: Current technology suggests that several techniques may be used for removing arsenite, arsenate, and organic forms of arsenic from drinking water including activated carbon filtration, submicron filtration, anion exchange, distillation, and reverse osmosis. Arsenic in colloidal form can be removed by sub-micron filtration or solid block and precoat adsorption filters. Also effective: weak and strong base anion exchange resins (90 to 100 percent removal), Distillation and reverse osmosis are effective to 90% plus reduction. Deep backwashable beds of granular activated carbon can be 40 to 70 percent effective.

Sources: US EPA, industry sources, Enting Corp. Engineering Handbook, and Water Technology Volume 31, Issue 11 - November 2008

More Detailed 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.

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

Additional Reference:

A comprehensive overview of treatment methods developed by the EPA can be referenced here.

Here are some highlights:

The technologies under review perform most effectively when treating arsenic in the form of As(V). As (III) may be converted through pre-oxidation to As(V). Data on oxidants indicate that chlorine, ferric chloride, and potassium permanganate are effective in oxidizing As(III) to As(V). Pre-oxidation with chlorine may create undesirable concentrations of disinfection by-products. Ozone and hydrogen peroxide should oxidize As(III) to As(V), but no data are available on performance.

Coagulation/Filtration (C/F), is an effective treatment process for removal of As(V) according to laboratory and pilot-plant tests. The type of coagulant and dosage used affects the efficiency of the process. Within either high or low pH ranges, the efficiency of C/F is significantly reduced. Alum performance is slightly lower than ferric sulfate. Other coagulants were also less effective than ferric sulfate. Disposal of the arsenic-contaminated coagulation sludge may be a concern especially if nearby landfills are unwilling to accept such a sludge.

Lime Softening (LS) operated within the optimum pH range of greater than 10.5 is likely to provide a high percentage of As removal for influent concentrations of 50 µg/L. However, it may be difficult to reduce consistently to 1 µg/L by LS alone. Systems using LS may require secondary treatment to meet that goal.

Activated Alumina(AA) is effective in treating water with high total dissolved solids (TDS). However, selenium, fluoride, chloride, and sulfate, if present at high levels, may compete for adsorption sites. AA is highly selective towards As(V); and this strong attraction results in regeneration problems, possibly resulting in 5 to 10 percent loss of adsorptive capacity for each run. Application of point-of-use treatment devices would need to consider regeneration and replacement.

Ion Exchange (IE) can effectively remove arsenic. However, sulfate, TDS, selenium, fluoride, and nitrate compete with arsenic and can affect run length. Passage through a series of columns could improve removal and decrease regeneration frequency. Suspended solids and precipitated iron can cause clogging of the IE bed. Systems containing high levels of these constituents may require pretreatment.

Reverse Osmosis (RO) provided removal efficiencies of greater than 95 percent when operating pressure is at ideal psi. If RO is used by small systems in the western U. S., 60% water recovery will lead to an increased need for raw water. The water recovery is the volume of water produced by the process divided by the influent stream (product water/influent stream). Discharge of reject water or brine may also be a concern. If RO is used by small systems in the western U. S., water recovery will likely need to be optimized due to the scarcity of water resources. The increased water recovery can lead to increased costs for arsenic removal.

Electrodialysis Reversal (EDR) is expected to achieve removal efficiencies of 80 percent. One study demonstrated arsenic removal to 3 µg/L from an influent concentration of 21 µg/L.

Nanofiltration (NF) was capable of arsenic removals of over 90%. The recoveries ranged between 15 to 20%. A recent study showed that the removal efficiency dropped significantly during pilot-scale tests where the process was operated at more realistic recoveries. If nanofiltration is used by small systems in the western U. S., water recovery will likely need to be optimized due to the scarcity of water resources. The increased water recovery can lead to increased costs for arsenic removal.

Point of Use/Point of Entry (POU/POE)The 1996 SDWA amendments specifically identify Point-of-Use (POU) and Point-of-Entry (POE) devices as options that can be used for compliance with NPDWRs. POU and POE devices can be effective and affordable compliance options for small systems in meeting a new arsenic MCL. A Federal Register notice is being prepared by EPA to delete the prohibition {§141.101} on the use of POU devices as compliance technologies. Because of this prohibition, few field studies exist on the application of POU and POE devices. One such case study was performed by EPA, in conjunction with the Village of San Ysidro, in New Mexico (Rogers 1990). The study was performed to determine if POU Reverse Osmosis (RO) units could satisfactorily function in lieu of central treatment to remove arsenic and fluoride from the drinking water supply of a small rural community of approximately 200 people. A RO unit, a common type of POU device, is a membrane system that rejects compounds based on their molecular properties and characteristics of the reverse osmosis membrane. The RO units removed 86% of the total arsenic.

Also discussed:

Conventional Iron/Manganese (Fe/Mn) Removal Processes. Iron coagulation/filtration and iron addition with direct filtration methods are effective for arsenic (V) removal. Source waters containing naturally occurring iron and/or manganese and arsenic can be treated for arsenic removal by using conventional Fe/Mn removal processes. These processes can significantly reduce the arsenic by removing the iron and manganese from the source water based upon the same mechanisms that occur with the iron addition methods. The addition of iron may be required if the concentration of naturally occurring iron/manganese is not sufficient to achieved the required arsenic removal level.