As+3
Arsenite
Trivalent. Difficult to remove.
01 Metalloid · Primary Contaminant
Arsenic
Where arsenic comes from
Arsenic occurs widely in the earth’s crust and, as a result, in groundwater in many parts of the world; it can also enter drinking water through commercial and industrial uses such as pesticides. Arsenic is often found alongside iron and manganese, and the presence of iron can interfere with arsenic removal — so the two are typically addressed together. Arsenic content in a given well can also vary over time and with the well’s rate of draw.
02 Maximum Contaminant Level
A limit that reshaped the industry
The EPA regulates arsenic under the National Primary Drinking Water Standards’ Chemical Contaminants Rules, with an MCL of 0.01 mg/L, lowered from 0.05 mg/L in 2001.
0.01 mg/L
Current EPA maximum contaminant level
0.05 mg/L
The previous limit, before 2001
One third
Of soluble arsenic in U.S. well water occurs as arsenite
The EPA lowered the maximum contaminant level (MCL) for arsenic in drinking water from 0.05 mg/L to 0.01 mg/L in 2001, a change that pushed a significantly larger number of public water systems out of compliance and spurred considerable research into arsenic treatment design. As a result, many technologies are now available for arsenic removal.
03 Speciation
Arsenic 3 and Arsenic 5
Arsenic in water takes different forms. For potable treatment, the key distinction is between trivalent arsenite (As+3, or Arsenic 3) and pentavalent arsenate (As+5, or Arsenic 5). Arsenite is difficult to remove and must first be oxidized to arsenate, which carries a negative charge and can be readily removed by coagulants in an arsenic filtration system. In the U.S., roughly a third of soluble arsenic in well water occurs as arsenite.
As+5
Arsenate
Pentavalent. Carries a negative charge.
Removed
Arsenic filtration system
Readily removed once oxidized.
This oxidation from Arsenic 3 to Arsenic 5 can be achieved through several means, including chlorine, potassium permanganate, ozone, hydrogen peroxide, and ultraviolet light.
04 Co-treatment
An affinity for iron oxides
Arsenic is removed to some extent in iron and manganese treatment plants, since it has an affinity for insoluble iron oxides — iron oxide-coated sand, in fact, has proven an effective adsorbent for arsenic and other metals and metalloids. However, effective system design depends on several other factors (for instance, adsorption decreases as pH increases), so application-specific testing is typically required.
Monitoring
Arsenic has no detectable odor or taste, making it essential to properly monitor treated effluent to confirm removal performance.
05 Public Health
Public Health Concerns
Exposure to arsenic at elevated levels poses serious chronic and acute health effects. Acute effects include nausea, neurological effects, and fatigue; chronic effects include several types of cancer and skin discoloration, with risk increasing alongside exposure level and duration.
Why speciation matters
Both arsenite and arsenate are readily absorbed in the stomach, but arsenite clears from the body far more slowly — making it the more persistent toxin and the greater biological threat.
06 WETS Treatment
Modified coagulation / filtration
There are many technologies available for arsenic removal. In our applications, WETS has most often used modified coagulation/filtration (MCF) technology, which addresses iron and manganese together and offers strong economics for many clients. Our pilot studies help economize chemical dosing and support TCEQ approval — and because we don’t offer an off-the-shelf product, every solution is tailored to the specific plant.
07 Get Started
Solutions to Your Water Treatment Needs
Whether you want a new water treatment system installed, need your current one inspected and repaired, or just want a free quote, contact the WETS LLC. With years of experience in the industry, our skilled team members are trained and knowledgeable with a variety of leading water equipment and products. Our certified water purification experts are here to help.
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