Arsenic as a Water Contaminant
When pH = 6-10
The presence of arsenic (As) in nature is due mainly to natural deposits of metalloids in the earth’s
crust and usually in ancient rock formations. Arsenic enters ground water through erosion or from manmade sources such as wood preservative, petroleum production, semi-conductor manufacture or due to
misuse of animal feed additives and arsenic-containing pesticides (e.g. Paris green). Since soluble
arsenic is tasteless and colorless, a chemical water analysis is necessary to detect its presence.
Higher levels of arsenic tend to be found more in ground water sources than in surface water
sources (lakes and rivers) of drinking water.
In ground water, arsenic can combine with other elements to form inorganic as well as organic
compounds; the inorganic derivatives are considered more toxic than the organic forms. The inorganic
forms usually exist in potable water in two chemical valence states: as arsenite (As III) or arsenate (As
V). The arsenite species exists in anaerobic/anoxic (reduced or low oxygen) waters as H3AsO3 or
H2AsO31-. The specific form depends on the pH of the water. At pH 9.2, arsenite exists as a 50-50
mixture of H3AsO3 and H2AsO3 1-. At less than pH 8.5, arsenite exists primarily as the neutral (nonionized) species, H3AsO3 (can be written as HAsO2). The arsenate species exists in aerobic (oxidized)
waters up to pH 9 as a mixture of H2AsO4 1- and HAsO4 2- , and as a 50-50 mixture at pH 7.0.
As mentioned above, two conditions that dominate the behavior of arsenic in water are its state of
oxidation (valence) and the pH of the water. Generally, aerated surface waters contain arsenate (As V)
while the reductive well waters contain arsenite (As III). Municipally treated waters containing free
available chlorine (FAC) will generally oxidize arsenite (As III) to the arsenate (As V) form. NOTE:
Chloraminated water utilizing only monochloramine (NH2Cl) will not completely oxidize As III to
As V. Generally, negatively charged (ionized) As V (as H2AsO4 1- and HAsO4 2-) is much easier to remove
than uncharged As III (as H3AsO3 or HAsO2). Tests to determine the concentration of each form, also
known as speciation, must be performed in order to choose the proper removal technique. Current
technology suggests that several techniques may be used to remove the arsenate, arsenite and
organic forms of arsenic from drinking water. These include iron oxide/hydroxides and activated
alumina media filtration, manganese greensand filtration, strong base anion exchange resins,
distillation, and reverse osmosis. Some specialty media include iron oxide/hydroxide-impregnated or
coated activated alumina and anion exchange resins, as well as titanium oxy/hydroxide
As(III), arsenite as H3AsO3 and H2AsO3 1-
As(V), arsenate as H2AsO41- and HAsO4 2-
Sources of Contaminant:
Leaching from natural deposits
Wood preservatives, pesticides, industrial deposits
Coal power plants
Potential Health Effects:
Serious skin problems, endocrine disruptor
Causes Cancer –skin, bladder, lung, kidney, liver, prostate
Harms cardiovascular &nervous systems
Toxicity of arsenic to humans is well known, and ingestion of as little as 100 milligrams (mg) can
result in severe poisoning. Amounts in water are normally much lower, but low concentrations still can
lead to chronic symptoms. The effect of arsenic, when ingested in small amounts, appears very slowly.
In fact it may take several years for the poisoning to become apparent. Chronic arsenosis can in its
most extreme form, cause death. Inorganic arsenic is absorbed readily from the gastrointestinal track
and becomes distributed throughout the body tissues and fluids. Ingestion of inorganic arsenic leads to
a number of health effects as follows:
Cancerous effects: skin, bladder, lung, kidney, nasal passages, liver and prostate cancer
Non-cancerous effects: cardiovascular, pulmonary, immunological, neurological and endocrine
The US EPA’s final arsenic rule was issued in 2001. It revises the Maximum Contaminant Level
(MCL) from 50 µg/L to 10µg/L and sets the Maximum Contaminant Level Goal (MCLG) to zero in the
drinking water. Both community water systems (CWS) and non-transient, non-community water
systems (NTNCWS) are required under this rule to reduce the arsenic concentration in their drinking
water to10 µg/L or lower.
Anion exchange in a fixed bed (requires regeneration)
Manganese greensand (requires regeneration)
Iron-doped anion resin and activated alumina
Activated alumina with or without iron oxide coating
Reverse osmosis (RO)
Anion exchange in a fixed bed (requires regeneration) Coagulation
with iron hydroxide (initially added as FeCl3) followed by filtration
though a bed of adsorptive granular media.
Iron oxide, iron hydroxide and iron coated activated alumina filtration media have shown
effectiveness in removing both arsenite (As III) and arsenate (As V) from levels of over 50 parts per
billion (ppb) or µg/L to effluent levels below 5 ppb (µg/L) for greater than 10,000 bed volumes before
exhaustion. These removal systems operate best at pH less than 8. Iron-based technologies can be
susceptible to competitive adsorbates, such as silicates, vanadates (e.g., VO4
3-) and phosphate. It has
be demonstrated that each 0.5 mg/L increase in phosphate above 0.2 mg/L will reduce adsorption
capacity by roughly 30% (US EPA, 2014). Therefore, a water analysis should be conducted prior to the
selection of a removal technology. For reductive water from wells, where As III is expected to
predominate, an oxidation step such as chlorination or ozonation is necessary if a technique other than
iron based systems are used (e.g., RO and anion exchange). Potassium permanganate (KMnO4) is
also very effective in oxidizing As III to As V. Passing aqueous solutions of As III through a fixed-bed
column of manganese dioxide (MnO2) also has been shown to oxidize As III to As V and, at the same
time, provide some removal capability.
Iron based specialty media are also easy to operate and have a higher capacity than AA media.
Some media can remove both arsenite and arsenate even though capacities may differ. The pH can be
as high as 8.5 for many of these media, however the lower the pH the better the removal performance.
Activated alumina (AA) has easy maintenance and potential for non-hazardous waste disposal after
exhaustion. It is highly selective for As V , so arsenites (As III) must be oxidized to arsenate (As V) and
the pH lowered, preferably to <6.5 or even lower if silica is present. Distillation is useful when only small quantities of drinking water are involved. It has been shown to reduce arsenic to less than 2 ppb (µg/L). Strong base anion (SBA) type I & type II resins in Clform can effectively remove As V (arsenate, as H2AsO4 1- or HAsO4 2- ). However sulfate, selenium, fluoride and nitrate ions present in water will compete with As V ions (H2AsO4 1- and HAsO4 2- ) for exchange sites and may produce earlier exhaustion. Anion exchange is not effective to remove As IIIi (arsenite, as H3AsO3 or HAsO2), since they are not charged (ionized), so it is necessary to oxidize it to As V by an acceptable technique (chlorination, permanganate or ozonation). Optimum pH for anion exchange removal is approximately >7.
Regeneration can be done with brine (NaCl) solution.
Manganese greensand is best used for only As V reduction, so if As III is present, it should first be
oxidized to As V.
NOTE: Prior to installing and utilizing regenerative treatment technologies or bulk disposable
adsorptive media, it is important to contact the local, regional or state regulatory authorities to
determine proper disposal requirements.
Thin film composite (TFC) Reverse Osmosis membranes are best used for only As V reduction, so,
again, As III should be oxidized by chlorination or ozonation, or other oxidation technique acceptable for
the specific application that will not harm the membrane.
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