Arsenic with symbol and Atomic Number 33 is a metallic element, Atomic Weight 74.92159; forms a number of poisonous compounds, some of which are used in medicine. Arsenic can exist in a metallic state in three forms (yellow, black, and gray; with gray predominating) and in ionic forms.
Elementary arsenic is fairly insoluble, whereas arsenic compounds may readily dissolve. Arsenic is mainly present in watery solutions as HAsO42-(aq) and H2AsO4- (aq), and most likely partially as H3AsO4 (aq), AsO43-(aq) or H2AsO3-(aq).
Examples of solubility of arsenic compounds: arsenic(III)hydride 700 mg/L, arsenic(III)oxide 20 g/L, arsenic acid (H3AsO4.1/2 H2O) 170 g/L, and arsenic(III)sulfide 0.5 mg/L.
Arsenic is widely distributed throughout the earth's crust and is present in a number of minerals and occurs in trace quantities in rocks, soil, water and air. It is usually found in conjunction with sulphur and metals, and also as a pure elemental crystal. It is odourless and tasteless and was first documented by Albertus Magnus in 1250. Around a third of atmospheric arsenic is derived from natural sources, e.g. volcanoes and the remaining two-thirds occur through man-made sources.
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Arsenic poisoning typically affects the skin, liver, lungs and kidneys - hence, the severity of the symptoms. The final stage of the poisoning causes the patient to suffer seizures and go into shock, this could lead to death or coma (and likely subsequent death).
High levels of arsenic can be found in drinking water which has been collected from deeply drilled wells or indeed as a natural phenomenon where ground water has been contaminated by minerals dissolving from rocks and soils. It can enter drinking water supplies from natural deposits in the earth or from agricultural and industrial practices.
Arsenic is introduced into water through the dissolution of minerals and ores, and concentrations in groundwater in some areas are elevated as a result of erosion from local rocks.
Symptoms as a result of arsenic exposure include pain or dizziness, while other signs include a rash, pallor, or swelling. Arsenic poisoning can cause major health complications if not treated, including death. Due to the risks involved, some precautions are required to protect the populations and workers at risk of arsenic poisoning.
If arsenic has been ingested orally, the first signs and symptoms of arsenic poisoning will appear within thirty minutes, and may include some of the following: Drowsiness, headaches, state of confusion and diarrhoea. If the arsenic has been inhaled, or a less concentrated amount has been ingested, symptoms may take longer to emerge. As the arsenic poisoning develops, the patient may start suffering convulsions and their fingernail pigmentation may change (leukonychia).
The following signs and symptoms are associated in more severe cases of arsenic poisoning:
metallic taste in the mouth
mouth produces excess saliva
blood in the urine
loss of hair
breath smells like garlic
Non-cancer effects can include thickening and discoloration of the skin, stomach pain, nausea, vomiting; diarrhoea; numbness in hands and feet; partial paralysis; and blindness. Arsenic has been linked to cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate.
Arsenic contamination of groundwater is often due to naturally occurring high concentrations of arsenic in deeper levels of groundwater. In addition, mining techniques such as hydraulic fracturing mobilize arsenic in groundwater and aquifers due to enhanced methane 28 transport and resulting changes in redox conditions and inject fluid containing additional arsenic.
A 2007 study found that over 137 million people in more than 70 countries are probably affected by arsenic poisoning of drinking water.
World Health Organisation's norms for drinking-water quality go back to 1958. The International Standards for Drinking-Water established 0.20 mg/L as an allowable concentration for arsenic in that year. In 1963 the standard was re-evaluated and reduced to 0.05 mg/L. In 1984, this was maintained as WHO's "Guideline Value"; and many countries have kept this as the national standard or as an interim target, according to the last edition of the WHO Guidelines for Drinking-Water Quality (1993).
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EPA has set the arsenic standard for drinking water at .010 parts per million (10 parts per billion) to protect consumers served by public water systems from the effects of long-term, chronic exposure to arsenic. [10 parts per billion (ppb) of arsenic in water means that there are 10 molecules of arsenic for every 999,999,990 molecules of water. That is roughly equivalent to a few drops of ink in an Olympic-sized swimming pool].
The technology for arsenic removal for piped water supply is moderately costly and requires technical expertise. It is inapplicable in some urban areas of developing countries and in most rural areas world-wide.
Ion exchange - using a resin to remove anionic As species,
â€¢ Coagulation/Micro-filtration - Adding Fe(III) or Al(III) salts to form arsenic-sorbing flocs which are subsequently removed from solution by granular media or membrane filtration,
â€¢ Fixed bed adsorption - Removal of arsenic with an adsorbent, typically a metal (hydr)oxide such as ferric hydroxide or AA (activated alumina),
â€¢ Lime-softening - Adding lime to soften water (remove Ca and Mg) often removing appreciable amounts of arsenic in the process through sorptive uptake by metal carbonates and hydroxides,
â€¢ Iron removal - Oxidizing reduced iron to remove arsenic through sorption/co-precipitation/coagulation,
â€¢ Physical filtration - To remove colloidally-bound arsenic,
â€¢ Membrane processes - Membrane removal of arsenic by Reverse Osmosis (RO) or nanofiltration (NF)
Arsenic removal involves far more than removing arsenic from water. It also requires disposal of waste solids (e.g. spent filter material, coagulant sludges) and residual fluids likely to contain high levels of arsenic and possibly other hazardous or radioactive (e.g. uranium, radon) constituents. If the water requires pre-treatment (e.g. lowering of pH for adsorption processes, or raising it for lime-softening) the waste situation becomes more complicated. At the same time, most treatments tend to affect effluent levels of dissolved components other than arsenic - sometimes in a deleterious fashion. For example, removal of arsenic through ion exchange also tends to lower bicarbonate levels increasing the corrosivity of the effluent. Coagulation using Al(III) or Fe(III) salts can result in increased concentrations of these metals in the treated water, as well as the anions (e.g. sulfate) from the salts used in the process. Most arsenic treatment technologies work better for arsenate than arsenite, so a preoxidation step is often used.
Arsenic may be measured in blood or urine to monitor excessive environmental or occupational exposure, confirm a diagnosis of poisoning in hospitalized victims or to assist in the forensic investigation in a case of fatal over dosage. Some analytical techniques are capable of distinguishing organic from inorganic forms of the element.
There are tests available to diagnose poisoning by measuring arsenic in blood, urine, hair, and fingernails. The urine test is the most reliable test for arsenic exposure within the last few days. Urine testing needs to be done within 24-48Â hours for an accurate analysis of an acute exposure. Tests on hair and fingernails can measure exposure to high levels of arsenic over the past 6-12 months. These tests can determine if one has been exposed to above-average levels of arsenic. They cannot predict, however, whether the arsenic levels in the body will affect health.
Chronic arsenic exposure can remain in the body systems for a longer period of time than a shorter term or more isolated exposure and can be detected in a longer time frame after the introduction of the arsenic, important in trying to determine the source of the exposure.
The Water and Sewerage Authority of Trinidad and Tobago (WASA) is the sole water and sewerage provider in Trinidad and Tobago. Several communities are serviced with ground water from wells that are maintained by WASA. It is important that WASA regularly monitor and test the well water for high arsenic levels that could lead to arsenic poisoning.