An element has caused much more harm to humankind throughout history than lead. Arsenic, one of the metalloids in the periodic table, is an extremely dangerous chemical that is far more common than most people think. It exists throughout the surface of the Earth in many different forms.
Arsenic Chemistry, Mineralogy and Occurrence in Natural Waters
The most common natural forms of arsenic are oxides and sulfides. Due to its chemical similarity to phosphorus, arsenic can readily “replace” phosphorus in many reactions. Given that phosphorus is such an important element to biology, exposure to arsenic can be quite dangerous. Arsenic compounds were used in insecticides and wood treatment chemicals for many years in the United States and Europe before being banned.
It seems intuitive that something so poisonous to insects would also be poisonous to larger life forms, but the effects were thought to be negligible until many cases of brain damage occurred in people working with these compounds. Its use continues in many parts of the world today and, while arsenic poisoning is now somewhat rare in the US and Europe, it continues in many under-developed nations.
The sulfide minerals orpiment, realgar, and arsenopyrite are the most prevalent forms of arsenic on the surface. These often occur in areas rich with volcanic or hydrothermal activity, as the more exotic elements tend to increase in concentration as more stable minerals crystallize out of solution. Therefore, arsenic can be quite common in wells drilled into deposits associated with volcanic activity. This presents a problem for people who rely on well water for drinking.
While there are many recognizable symptoms of acute arsenic poisoning, including severe headaches, hair loss, stomach pain, and convulsions, these can usually be treated before coma or death occurs. Long term exposure to arsenic is more troubling in many ways. Arsenic is thought to play a role in the development of diabetes, cancer, stroke, heart disease, and respiratory ailments; the five leading natural causes of death in the world.
The lack of clean drinking water in the developing world has recently led to further occurrence of arsenic poisoning. In an attempt to provide water free of bacteriological contamination, some governments, most notably Bangladesh, instituted shallow well drilling programs. Due to its concentrations in volcanic rocks and subterranean sediments near the surface, this led to increased exposure to arsenic in exchange for reduced exposure to bacterial pathogens. While the concentrations are often small, extended exposure can lead to a variety of skin diseases including melanoma.
Many municipal water supplies contain small amounts of arsenic. Public water supplies derived from rivers contain arsenic as industrial byproducts associated with wood treatment, mining, and agricultural application. Those public systems affiliated with wells often contain arsenic from natural sources. The EPA requires arsenic to be below 0.01 mg/L in public water supplies, but is attempting to get this number to zero if possible.
Therefore, most municipal water is considered safe and is monitored regularly. Those with private wells on their own property, however, should test for arsenic concentrations at least yearly, possibly more frequently if a problem arises. Local, independent water quality laboratories have extensive information on arsenic tests.
There are many filtration systems available that can remove arsenic and other toxic metal compounds from drinking water. They usually work with an iron oxide cartridge that “absorbs” trivalent and pentavalent arsenic through ion exchange. The most effective ways to remove arsenic, although more tedious and expensive, are by reverse osmosis or distillation. Given the many problems associated with arsenic exposure, it is more than worth it to invest in these technologies.
Sources: Technologies and Costs for Removal of Arsenic from Drinking Water, EPA,