Water Pollutants Arsenic

Origin: Arsenic is a natural occurring substance found in the abundance of the earth's crust, soil, water and air. Arsenic is a poisonous substance, which is released both from certain human activities, and naturally from the earths crust. Arsenic is found in compounds present in many different minerals. The most common compound or form of arsenic is found as arsenopyrite a hard, metallic, opaque, steel grey to silver white mineral. In inorganic arsenic of geological origin is found in ground water used in drinking water. Organic compounds of arsenic that of those containing carbon, are found mostly among sea-living creatures, even some on land.

Arsenic Facts: There are trace amounts of arsenic in all living matter. For most Canadians, the primary source exposure to arsenic is food, followed by drinking water, soil and air. Arsenic is a tasteless, odorless substance so if any trace amounts are in your drinking water you won't be able to tell whether or not you have consumed small traces of arsenic.

Arsenic's Purpose: Arsenic is a well-known poison. Arsenic compounds are sometimes used as rat poisons and insecticides but their use is strictly controlled. Surprisingly, arsenic can also have medicinal applications. In Victorian times, Dr. Fowler's solution (potassium arsenate dissolved in water) was a popular cure-all tonic. Today, organoarsenic compounds are added to poultry feed to prevent disease and improve weight gain. It is even used to make certain types of glass, preserve wood, and as a doping agent in semiconductors for solid state devices.

Physical and Chemical Properties: Arsenic is a silver-grey brittle, metallic looking substance which exists in three allotropic form: yellow, black and grey. A melting point of 817 degrees Celsius, boiling point of 612 degrees Celsius. Arsenic tarnishes rapidly in air, and at high temperatures burns a white cloud of arsenic trixoxide. Arsenic is part of group 15 in the periodic table, which combines readily with elements.

To the left is arsenic trisulfide (yellow) and to the right is an example of black arsenic.

Allowed Concentration In Drinking Water: The MAC (maximum acceptable concentration) of arsenic in drinking water is 10mg/L. This was first enacted in the Safe Drinking Water Act of 2002. The set guideline is based off identified health risks associated with contaminants, new studies, and limitation of available treatment technology. However, Ontario failed to establish the provincial standard in 2006 having doubled the allowed concentration; 0.025mg/L, and Ontario has now set their guideline concentration to 0.025mg/L instead of 0.010mg/L.

Human Effects: Arsenic is a human carcinogen, which means that exposure to any level in drinking water may increase the risk of cancer. Human exposure can causes several health problems: cancer, liver damage, dermatosis (a skin disease), and nervous system disturbances; polyneuropathy (degeneration of peripheral nerves that spreads toward the center of the body), EEG abnormalities, and in extreme cases hallucinations, disorientation, and agitation.

Left photo shows arsenic poisoning, where the individual consumed a extreme amounts of arsenic. The right is a diagram showing what health effects are caused by arsenic.

Arsenic Effects on Animals: Arsenic compounds cause short-term and long-term effects in individual plants and animals and in populations and communities of organisms. These effects are evident, for example, in aquatic species at concentrations ranging from a few micrograms to milligrams per litre. The nature of the effects depends on the species and time of exposure. The effects include death, inhibition of growth, photosynthesis and reproduction, and behavioral effects. Environments contaminated with arsenic contain only a few species and fewer numbers within species. If levels of arsenate are high enough, only resistant organisms, such as certain microbes, may be present.

Environmental Effects: Arsenic occurs in soils and natural waters; thus, plants have obviously evolved in the presence of arsenic ions. Arsenic is chemically similar to phosphorus, an essential plant nutrient. That it can substitute for phosphorus, in plant nutrition however, is doubtful; in some soils, the application of phosphate fertilizer increases arsenic toxicity.

As a sapling part of most plants absorb arsenic. The diagram top left had shown arsenic in part of the sapling.

Waste water treatment for arsenic: A wide range of technologies has been developed for the removal of high concentration of arsenic from drinking water. There are several methods available for removal of arsenic from water: oxidation; covert arsenite into arsenate. Coagulation, filtration, and precipitation. In the process of coagulation, arsenic is removed from solution through three mechanisms Precipitation:the formation of insoluble compounds. Co-precipitation:the incorporation of soluble arsenic species into a growing metal hydroxides phases (e.g. co-precipitation with Fe(III); Adsorption: the electrostatic binding of soluble arsenic to external surfaces of the insoluble metal hydroxide. And Ion Exchange.

Solar oxidation and precipitation of Fe(III)-Oxides with Adsorbed As(V)

Epidemiology: Geological contamination of underground water supplies occurs in parts of: Bangladesh (79.9 million people) and west Bengal, India (42.7 million); "largest mass poisoning of population in history." Chile, Argentina, Mexico and the USA 13 million people. China, Taiwan, Thailand.

Bangladesh: In 1993 Bangladesh faced serious threat to the public health, with 85 million people at risk for arsenic contamination in their drinking water and food crops. In Bangladesh, the groundwater As contamination problem is the worst in the world. Ninety-seven percent of the population in the country uses groundwater for drinking and domestic purposes as surface water is mismanaged. High levels of arsenic in groundwater caused widespread poisoning. A survey looked at the As concentrations of drinking water from deep wells in 64 districts in the country and found that 59 had concentrations >10 μg L−1 and 43 had concentrations >50 μg L−1. Arsenic concentration is higher in Bangladeshi soils, groundwater and plants (data based on 4% area of the country) than the permissible limits or normal range reported. This situation poses a serious threat on human and livestock health and highlights the need for scientific studies that would better describes the fate of As in the natural environment and identify all potential routes of exposure.

Map showing extreme amounts of arsenic exposure in Bangladesh.


Created with images by James St. John - "Antimony (South Riverside, California, USA)"

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