Arsenic cycle

Reservoirs and fluxes

Lithosphere

Arsenic's largest reservoir on Earth is the lithosphere. Earth's crust contains more than 200 mineral types containing As, including many sulfide minerals. Arsenic is abundant in ore deposits containing arsenopyrite (FeAsS) and tennantite. Sedimentary rocks bearing coal and shale may also contain high As. Major fluxes of As from the lithosphere to the atmosphere are volcanic emissions.

Soil is the second largest global reservoir of As Under oxic conditions, As is present in soils as arsenate (As(III)), which can bind to Fe(III) hydroxides. The speciation of As in soil depends on soil pH and other factors. Acidic soils may contain arsenate bound to aluminium and iron, while basic soils may contain calcium-bound arsenate. The residence time for As in soils depends on the climate type, ranging from 1,000 to 3,000 years for moderate climates.

Hydrosphere

Freshwater and groundwaters commonly contain <1 ppb of As. The concentration of As is pH dependent; acidic conditions mobilize As at pH <5. Oxic seawater contains As(III) as arsenate (average of 1.7 ppb). Major sinks include sedimentation and subduction.

Biosphere

Arsenic is naturally present in the biosphere, with highest concentration in plant roots. Terrestrial plants can contain up to 200 ppm (parts per million) As. Marine organisms (e.g. Annelida and Echinodermata) contain 6-8 ppm. The human body also contains trace As with highest concentrations in the kidneys and liver (up to ~1.5 ppm).

Anthropogenic emissions

Human use arsenic in pesticides, wood preservatives, metal treatment, paint, and coal-based power plants. Anthropogenic residues and discharges from coal-based power plants, mining, and smelting can contaminate rivers, lakes, streams and soil. Anthropogenic As emissions originate from steel and glass production, and forest and grassland burning. In the atmosphere, As is mainly present in particulates such as dust, with a residence time of 7 to 10 days.