Introduction to Arsenic Removal
By Greg Reyneke, CWS-VI
Arsenic has been recognized and used by humans for almost five hundred years. Frequently misunderstood, arsenic is a highly toxic element that has a long-term and significant environmental impact. Alchemists believed it could be used with sulfur compounds to raise higher states of consciousness; early physicians used compounded arsenic medicines to cure disease and vain European women even used arsenic to make their skin pale and avoid any impression that they worked outside in the sun.
Modern man uses arsenic currently in agriculture, semiconductor manufacture, corrosion control, ammunition, pharmaceuticals and metallurgy. In fact, in the United States, agriculture accounts for over 75 percent of arsenic usage as pesticides, insecticides and herbicides. As the 20th most common element found in the earth’s crust, it is found on every continent and in almost all groundwater. Arsenic levels will be naturally higher in areas with sandstone shale and coal. Since arsenic is frequently found compounded naturally with lead, silver, zinc, copper and sulfide ores—from which it has to be removed in the smelting process—it occurs at high levels in areas where such mining and processing has been performed. Anywhere that arsenic has been used in industry and discharged to the environment, elevated levels in groundwater will be found.
Considering the above, it can be safely assumed that water quality improvement specialists will encounter arsenic removal/remediation issues at some point in their careers. This article will serve as an introduction to how arsenic should be addressed. We’ll delve into technologies and pilot testing strategies in more detail in the next installment.
Know your enemy
As a metalloid, arsenic can be found in four states:
- Elemental arsenic: As0
- Arsine gas: As3-
- Arsenite: As(III)
- Arsenate: As(V)
Waterborne arsenic is most commonly encountered as arsenate or As(V). While arsenite, As(III), is also found, it is usually oxidized to arsenate in waters with dissolved oxygen and pH above 7.0. This process can of course be reversed when acidity is introduced to the water. The current US EPA maximum contaminant level (MCL) for arsenic is 0.010 mg/L (milligrams per liter) or 10.0 μg/L (micrograms per liter). This strict level is primarily due to arsenic’s cumulative nature; long-term exposure to as little as 10-100 mcg/Kg per day leads to neurological, dermatological and hepatic toxicity, as well as carcinogenity under certain circumstances. Naturally US EPA’s maximum contaminant level goal (MCLG) is zero; this certainly makes sense, because we’re already consuming arsenic in most foodstuffs already. Fixing the water Arsenic treatment should be taken very seriously and sold with appropriate limitations of liability. Before selecting any treatment strategy, an industry best practice is as follows:
- Needs analysis and site survey
a) Determine what your clients want
their water to do for them.
b) Does the potential treatment plan need to be approved by
c) Is the system large enough to require additional
regulatory compliance and
d) How much space is available
for potential treatment
e) What water flows/pressures,
electrical and drainage
resources are available?
f) Will weather conditions complicate transportation/
installation (snow/monsoon season, etc.)?
- Raw-water testing. Testing for the following will help in selecting
a treatment technology.
a) Arsenic total, arsenate, arsenite
b) Cations: calcium (Ca), magnesium (Mg), manganese (Mn),
sodium (Na), potassium (K), iron (Fe), lead (Pb), mercury
(Hg), selenium (Se), silver (Ag), zinc (Zn), copper (Cu)
c) Anions: carbonate (HCO3), sulfate (SO4), chloride (Cl),
nitrate (NO3), fluoride (F), silica (Si)
d) Total alkalinity as calcium carbonate (CaCO3)
e) pH, hydrogen sulfide (H2S)
f) Phosphate and orthophosphate (PO4)
g) Turbidity, TDS, TOC
Always perform tests for volatiles like pH and H2S at the
wellhead to improve accuracy.
- Technology selection. Selecting a potential technology can be
difficult, as the most effective technologies are usually the most
expensive. US EPA has identified the following technologies
as being effective in reducing/removing arsenic from drinking
a) Activated alumina
b) Coagulation-assisted filtration
d) Granular ferric hydroxide
e) Ion exchange
f) Lime softening
h) Reverse osmosis
Each technology has relative advantages and disadvantages, with membrane separation, activated alumina and ferric hydroxide being most familiar to current practitioners. Regardless of whether you select a sorbent or physical separation, remember that you’ll need to pre-oxidize with chlorine, potassium permanganate, ozone or a solid-phase oxidant to ensure proper treatment and to maximize return on investment.
- Pilot testing. Potential technologies can be tested on the bench
or in a small-scale pilot plant. Take advantage of these small-scale
tests to investigate important issues like:
a) Size of treatment equipment requiredb) Wastewater/discharge requirements
c) Consumables used
d) Frequency of maintenance/consumable maintenance
e) Level and type of pre-oxidation required
- Installation and commissioning. The best-designed systems
inevitably fail when improperly installed. Always follow the
consultant/engineer and equipment manufacturer instructions
explicitly when installing equipment. Remember to observe and
follow prevailing plumbing codes as well.
- Post-installation testing and calibration. Once the system
has been properly installed and commissioned, additional water
tests should be performed to validate efficacy and to begin the
documentation process of the viable solution. Since groundwater
is constantly changing, it is prudent to frequently retest water
before and after treatment to ensure efficacy of your solution.
Fools rush in…
Arsenic is a potentially life-threatening contaminant and shouldn’t be taken lightly. Addressing arsenic is not like installing a residential water softener; there are significant consequences from failure. Be smart, learn more and listen to your vendors on how to deploy arsenic removal technologies before attempting to do it yourself.
About the author
Greg Reyneke is Managing Partner at Red Fox Advisors, a multidisciplinary research, development and consulting company with a strong emphasis on water, air, microbiology and energy projects. He also serves as an advisor to the ProFlow Dealer Network, a Pentair Platinum Partner and is a member of the WC&P Technical Review Committee.