Wednesday, July 14, 2010
Arsenic in the environment
Arsenic exists in the environment in many forms which vary in toxicity and mobility. They constantly change in forms due to environmental changes or biological activity, which explains why there are a wide variety of arsenic species found in the environment. Some of the environmental changes include the change in oxidation-reduction potential and pH.
In order for us to determine the potential transformation and risk of arsenic in the environment, we should identify and quantify both the quantity of arsenic present and the specific chemical forms present. This procedure is known as speciation.
However, speciation by a laboratory is expensive and the sample collection methods to ensure the preservation of in situ conditions are difficult and expensive.
The main species of arsenic found in the environment are the arsenic (III) and arsenic (V) oxyacids. Arsenates, arsenate anions, along with neutral arsenite constitute the main targets for field analytical assays.
In contaminated soils, inorganic arsenate is the predominant species.
In general, the arsenate and other arsenic (V) species are immobilized on geologically available surfaces, usually as iron oxides. Although arsenic (V) compounds are considered a low risk, bacterial and other environmental activities can readily convert them back into more mobile and more toxic forms of arsenic.
Organoarsenic species are found in groundwater and soil. Organoarsenic are generally compounds that contains a chemical bond between arsenic and carbon. They are less toxic than their corresponding oxyacids. The sulfur species may also be found in the environment but they are less common and less toxic than the arsenic oxyacids. However, the presence of this species should not be neglected in field measurements as they constitute a sizable fraction of the naturally occurring arsenic.