Arsenic

What is arsenic?

Arsenic is a chemical element whose symbol is As. It has a silvery-white crystalline and semi-metallic color.
Atomic weight 74.922 g / mole.
Atomic number 33.

It is odorless and tasteless. It enters drinking water from natural reservoirs in the ground or from agricultural and industrial practices.

Non-cancerous effects may include thickening and discoloration of the skin, stomach pain, nausea, vomiting; diarrhea; numbness in the hands and feet; partial paralysis; and blindness. Arsenic has been linked to cancers of the bladder, lungs, skin, kidneys, nasal passages, liver and prostate.

The U.S.Environmental Protection Agency (EPA) has set the drinking water arsenic standard at 0.010 parts per million (10 parts per billion) to protect consumers in public systems from the long-term effects of chronic arsenic exposure.

Arsenic poisoning in water.

Inorganic arsenite (arsenic (III)) in drinking water has a much higher acute toxicity than organic arsenate (arsenic (V)). The minimum acute lethal dose of the element in adults is estimated at 70 to 200 mg or 1 mg/kg/day.

Chronic intoxication from drinking contaminated well water over a long period of time. Many aquifers contain high concentrations of salts of the element. The World Health Organization recommends a limit of 0.01 mg/l (10 ppb) of arsenic in drinking water. This recommendation was established based on the detection limit for most equipment testing laboratories at the time of publication of the WHO water quality guidelines. More recent findings show that consumption of water with levels as low as 0.00017 mg/l (0.17ppb) over long periods of time can lead to arsenicosis.

Symptoms of poisoning include abdominal pain, diarrhea, vomiting, dark urine, dehydration, vertigo, delirium, shock and death.

One of the worst incidents of arsenic poisoning through water occurred in Bangladesh, which the World Health Organization called the “largest mass poisoning of a population in history”.

Mining techniques such as hydraulic fracturing can mobilize arsenic in groundwater and aquifers due to enhanced methane transport and the resulting changes in redox conditions, and inject additional arsenic-containing fluid.

What are the methods used to remove arsenic from water?

EPA has not yet determined the best technology for removing arsenic in water. But some studies have been conducted to reduce arsenic concentrations to below levels of 0.20 µg/l.

There are six possible treatments under certain water characteristics that can lower arsenic levels:

Coagulation with filtration.

The coagulation process consists of adding a metal-based coagulant, such as ferric chloride (FeCl3), to water contaminated with arsenic. FeCl3 hydrolyzes in water to form positively charged ferric hydroxide [Fe (OH)3]. Arsenic must be in oxidized form to be effectively removed [As(V)]. Therefore, if arsenite [As(III)] is present, it may be necessary to oxidize it to As(V) using chlorine. Arsenate [As(V)] is a negatively charged anion and absorbs the positively charged Fe(OH)3 particles or flocs. Sedimentation and filtration processes then remove the arsenic particles.

Activated alumina.

The adsorption process with activated alumina is very sensitive to the pH of the water, the highest efficiency is obtained at pH values between 5.5 and 6.0. For this reason, it is required to adjust the pH of the water to be treated to a value in the above mentioned range. On the other hand, contaminants such as silica, fluorine, selenium, and sulfates at high concentrations block the active adsorption sites and cause rapid saturation of alumina, reducing its efficiency with respect to arsenic removal. The water produced by this technique has a pH of <6.5, so it needs to be readjusted with an alkaline reagent.

In the regeneration of activated alumina, the adsorbed arsenic is recovered only 50 to 70%, so the capacity of the adsorbent medium decreases by 5 to 10% in each operation cycle. After a period of use, it is necessary to replace it completely.

Ion exchange.

In anion exchange, two types of resin are needed, one anionic and one cationic. These resins also remove sulfates, nitrates, arsenic and regulate the pH of the water. The cost of operation and maintenance is comparatively low when compared to reverse osmosis (RO) or ultrafiltration (UF). However, in the case that there are other compounds such as those mentioned above and only arsenic needs to be removed, it would not be the best alternative.

Reverse osmosis.

In addition to ion exchange, arsenic is also removed by a reverse osmosis system. Reverse osmosis systems can retain up to 99% of total dissolved solids (TDS), including arsenic in water. The pore size of the RO is so fine that only water molecules pass through. This system is perfect for potable water application or any other application that requires the most purified form of water, as it removes all the contaminants that the water contains, making this purification method a more comprehensive solution. In terms of operation, this is one of the simplest and safest systems to operate, as it does not require the handling of acids or hazardous chemicals.

Electrodialysis.

It is a purification technique that can remove ionic compounds from liquid solutions using selective permeable membranes in a constant electric field (Guastalli, 2004).

This technique has the ability to remove charged contaminant ions down to 0.0001 µm, using membranes or porous membranes of ion exchange resins with a low relative permeability to water. This method, like reverse osmosis, is not selective to only remove arsenic.

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Source:

US EPAOrganization.